Publikacje

Bisallyl diradicals are central intermediates in allene dimerization, yet they are normally too short-lived to observe under thermal conditions. Here, π-extension of Staudinger-type allene dimers weakens the central cyclobutane bond sufficiently to make reversible ring opening accessible near room temperature. Variable-temperature NMR spectroscopy and ROESY exchange correlations reveal repeated cleavage and reformation of this bond, with activation free energies of 15.77 and 13.79 kcal mol⁻¹ for the Staudinger dimer and its π-expanded analogue, respectively. Calculations reproduce this trend and identify low-population open-shell singlet bisallyls as the intermediates, while explaining the absence of neutral-state EPR signals. Reduction reverses the relative stability of the closed and open valence isomers. Voltammetry and chemical reduction support a sequence of ring-opened radical-anion, dianion, and tetraanion states. The dianion is characterized by NMR spectroscopy and x-ray diffraction, while spectroscopy and calculations reveal progressive bond redistribution from mixed-valence allyl-radical/allyl-anion character to two allyl-anion units and, finally, four fluorenyl-anion-like termini. These results connect hidden neutral diradicals with persistent reduced bisallyl frameworks through charge-controlled covalent-bond reorganization.

Two π-extended core-modified porphyrinoids were synthesized and structurally characterized to probe boron-induced geometry switching and modulation of electron delocalization. trans-Vinylene-bridged hexathiadodecaphyrin forms the largest fully characterized porphyrinoid BODIPY-type complex while retaining the parent open-form geometry, whereas octathiahexadecaphyrin does not support boron coordination. Notably, a smaller figure-of-eight tetrathiaoctaphyrin analogue exhibits unusual direct C–B bond formation, highlighting the pronounced structural sensitivity of expanded porphyrinoids and their utility in accessing unconventional bonding and coordination modes.

Surface-assisted reactions on nonmetallic substrates remain much less developed than their counterparts on metals, especially for heteroatom-doped precursors and intermolecular coupling processes. Here we show that TiO₂(110) can support not only cyclodehydrogenation of a nitrogen-rich nanographene precursor but also annulative intermolecular coupling on a semiconducting oxide surface. Under ultrahigh vacuum, hexapyrrolylbenzene (HPB) undergoes cyclodehydrogenation on rutile TiO₂(110) to give unsubstituted hexapyrrolohexaazacoronene (HPHAC, 1). Upon further annealing, 1 undergoes dehydrogenative dimerization to form a cyclooctatetraene-annulated HPHAC dimer (2), as supported by scanning tunneling microscopy and spectroscopy (STM/STS), density functional theory (DFT), and simulated STM images. The dimer, but not the monomer, undergoes single-electron transfer to the substrate to form [2]^•+, consistent with STM/STS observations, its lower ionization threshold, and computed frontier orbital energies. Gas-phase calculations are consistent with a plausible arenium-type pathway for annulative dimer formation and further indicate that oxidation increases global aromatic character across the fused framework.

Acid–base equilibria can be shifted when binding events alter the local electrostatic and hydrogen-bonding environment of an ionizable site. Implementing such binding-linked acidity changes in a discrete synthetic system would enable regulation of proton activity without changing molecular composition or solvent. Here we describe a new naphthalimide-fused analogue of cyclo[8]pyrrole in which anion recognition dramatically shifts protonation equilibria. Whereas established cyclo[8]pyrroles preferentially form globally aromatic dications, the electron-deficient macrocycle shows an unusual preference for a persistent neutral redox state, while simultaneously revealing exceptionally high Brønsted acidity in organic solvents. The neutral macrocycle retains key anion-recognition characteristics of dicationic cyclo[8]pyrroles, including strong sulfate affinity. Sulfate binding increases the effective first pK_a in DMSO by roughly four units, providing an orthosteric, anion-actuated acid switch and expanding the design space of switchable acidity beyond host–guest pK_a shifts and light-addressable photoacids.

Indene-annulated perylenes merge cyclopentadiene-like benzylic acidity with the multielectron redox chemistry of rylene carbonyl scaffolds, creating discrete redox–protonation (“Pourbaix”) spaces in which electron transfer and acid–base processes become intrinsically coupled. Here we map the accessible states of doubly and singly pentannulated perylenediimides and perylene tetraesters using absorption/emission spectroscopy, electrochemistry and spectroelectrochemistry, and DFT analysis.

Voltammograms of all derivatives show two quasi-reversible, largely core-centered reductions, yet the pentannulated tetraesters uniquely undergo electrochemically induced γ-deprotonation prior to core reduction, consistent with electrogenerated-base chemistry. Chemical stimulation reveals pronounced chemodivergence: potassium tert-butoxide readily produces deprotonated “dienolate” NIR chromophores in the pentannulated tetraester series, whereas the corresponding diimides first form perylene-centered radical anions/dianions that evolve into benzylic anions and higher multianions in a cation-dependent manner, with crown ether sequestering K+ and reshaping reaction trajectories.

Strong reductants enable entry into highly reduced charge states, including an X-ray-verified salt of a pentannulated diimide trianion displaying diverse potassium binding motifs, underscoring the non-innocent role of ion pairing. Oxidation of deprotonated dianions furnishes weakly coupled neutral diradicaloids with NIR absorption. These results establish indene-annulated rylenes as multistate NIR chromophores whose experimentally accessible Pourbaix subsets can be programmed by redox bias, base strength, and cation coordination, with direct implications for organic electrochromism and rylene-based energy-storage chemistries.

43,44,47,48-Tetrathiaoctaphyrin(2.1.1.1.2.1.1.1), obtained in a concise synthesis, is found to adopt a C₂-symmetric figure-eight conformation in solution and in the solid state. The macrocycle is globally nonaromatic but exhibits strong absorption in the visible and near-infrared regions. Metalation of the octaphyrin with [Rh^I(CO)₂Cl]₂ produced an unprecedented dinuclear complex featuring two octahedral Rh^III centers and four Rh–C bonds. The latter species is globally aromatic and has a distinct, quasi-rectangular shape. Using an in-depth NMR and computational analysis, the Rh₂ complex is found to exhibit an unusual case of isomerism resulting from the combined effect of local helicity, conformationally locked trans-vinylene bridges, and polar axial ligation.

The development of new π-conjugated motifs opens pathways to previously unexplored classes of organic semiconductors and functional dyes. In this study, five- and seven-membered carbocycles were fused at the ortho and bay regions of electron-deficient perylenes, starting from a common dialdehyde precursor. Structural analysis of the resulting perylene tetraesters, dianhydrides, and diimides (PDIs) revealed three distinct ring-fusion patterns and defined stereochemistry. The fused PDI cycloheptatrienes demonstrated susceptibility to acid‐catalyzed transarylation, involving tropylium cation intermediates, which can be used preparatively. Under superacidic conditions, the PDI tropylium cations were directly observed and shown to undergo hydride‐transfer reductions. Additionally, a fused PDI bis(heptafulvene) was synthesized by dehydrogenating a suitably substituted PDI cycloheptatriene. The final system contains two quinomethane units, which can be protonated to yield a stable tropylium-like dication.

Two viologen-like macrocyclic receptors, [1]²⁺ and [2]²⁺, merging a bent oligophenylene loop with a (para-xylylene)bispyridinium moiety were designed and synthesized. Both dications were derived from the same precursor: the more strained cycloparaphenylene analogue [1]²⁺ was obtained using a reductive aromatization, whereas the less curved, meta-phenylene-containing [2]²⁺ was generated using a double eliminative rearrangement. [1]²⁺ is active as a receptor toward pyrene-1,3,6,8-tetrasulfonate, displaying a 2:1 binding interaction, with K₁₁ = 7.4(3) × 10³ M⁻¹. In the solid state, [1][TFA]₂ shows differential sorption of C₆ hydrocarbons and shows reversible sorption of water. [1]²⁺ displays yellow emission in solution (λ_em,max = 544 nm in DMF), which is considerably red shifted in the solid state (λ_em,max = 644 nm for dry samples). Both [1]²⁺ and [2]²⁺ undergo electrochemical reduction, yielding transient viologen-like radical cations and diradicals.

Pentacosacyclene (PC) and pentacosacyclene tetraimide (PCTI) were obtained in concise syntheses involving radial extension of tridecacyclene. PC is an electron-rich hydrocarbon with a C₈₈ π-conjugated framework, whereas PCTI is electron-deficient and contains a C₉₆N₄ core. PC and PCTI both have non-planar saddle-shaped conformations, and PC was found to self-assemble with C₆₀ to produce a uniquely structured supramolecular crystalline phase. In solution, PCTI undergoes eight single-electron reductions, while PC exhibits two reversible oxidations and three reversible reduction events. Chemically generated anions of PC and PCTI showcase extended near-infrared to infrared absorptions, with the lowest energy bands observed at >3200 nm for the PCTI monoanion and ca. 2800 nm for the PCTI dianion. The electronic and redox properties of pentacosacyclenes can be explained using molecular orbital and valence bond theories as originating from changes in the local aromaticity of five- and eight-membered rings.

A triskelion-shaped triradical triindeno[1,2-a:1′,2′-g:1′′,2′′-m]triphenylen-7-yl (1) and its internally fused derivative (2) obtained by oxidative cyclization were prepared in a straightforward synthetic sequence. Both compounds were confirmed to be triradicals and to possess intramolecular antiferromagnetic exchange interactions between spins, displaying a spin-frustrated doublet ground state with doublet-quartet energy gaps of −0.14 kcal/mol for 1 and −0.06 kcal/mol for 2. Despite their open-shell character, they were sufficiently stable to be handled under ambient conditions on a timescale of days. Both compounds could be reversibly reduced to mono-, di-, and trianions and oxidized to 1+ and 22+, with strong NIR absorptions (1800 to over 3200 nm) observed for all open-shell ions.

Two families of difluorenoheterole diradicaloids were synthesized, featuring isomeric ring systems with distinct conjugation topologies. The two types of difluorenoheteroles contain, respectively, a Chichibabin-like motif (CH) and a newly introduced heteroatom-linked triphenylmethyl dyad (TD-X). Combined experimental and theoretical investigations show that the TD-X systems have reduced quinoidal character but the interaction between formal spin centers is sufficiently strong to ensure a singlet ground state. The singlet–triplet energy gaps in the TD-X difluorenoheteroles are strongly affected by the heterocyclic ring, with values of −4.3 and −0.7 kcal mol⁻¹ determined for the pyrrole- and thiophene-containing analogues, respectively. In cyclic voltammetry experiments, the TD-X systems show diminished energy gaps and superior reversibility in comparison with their CH counterparts. The radical anions and cations obtained from these diradicaloids show extremely red-shifted bands, occasionally with λ_max > 3500 nm. Computational studies show that some of these ions adopt distonic structures and may be characterized as class-II mixed-valence species.

A saddle-shaped π-extended zinc porphyrin containing a peripheral pyridyl ligand undergoes quantitative self-assembly into a cyclic trimer. The trimer has a prismatic structure with negatively curved side walls, which promote the formation of supramolecular organic frameworks stabilized by dispersion interactions. The first framework type, UWr-1, has the npo topology, with a hexagonal structure analogous to the Schwartz H triply periodic minimal surface. Co-crystallization of the trimer with either C₆₀ and C₇₀ produces the isomorphous cubic UWr-2 and UWr-3 phases, characterized by the ctn network topology and a structural relationship to the Fischer-Koch minimal surface S. All three phases contain complex labyrinths of solvent-filled channels, corresponding to very large probe-accessible volumes (68% to 76%). The UWr-2 network could be partly desolvated while retaining its long range dimensional order, indicating remarkable strength of the dispersion interactions in the crystal. A theoretical analysis of noncovalent interactions shows the role of geometrical matching between the negatively curved porphyrin units and positively curved fullerenes.

A family of propeller-shaped donor-acceptor hexapyrrolylbenzenes (HPBs) were designed and synthesized by sequential nucleophilic substitution of hexafluorobenzene with π-extended pyrroles. In particular, four hybrids were obtained, containing various combinations of electron-rich and electron-poor acenaphthylene-fused pyrroles. Additionally, to probe the efficiency of ortho transfer interactions, a system was designed containing unique donor and acceptor subunits spatially separated with four unfunctionalized pyrroles. DFT calculations showed propeller-shaped geometries of all HPB molecules and separation of frontier molecular orbitals between donor and acceptor subunits. Steady-state and time-resolved photophysical measurements revealed charge-transfer (CT) character of the emission with strong positive dependence on solvent polarity. The principal CT pathway involves ortho-positioned pairs of donors and acceptors and requires bending of the acceptor in the excited state.

Diradicaloid helicenes constructed formally by non-benzenoid double π-extension of phenanthrene were synthesized by a common strategy involving double electrophilic benzannulation. Steric effects in the second benzannulation step led to considerable structural diversity among the products, yielding a symmetrical dinor[7]helicene 1 and two isomeric unsymmetrical double helicenes 2 and 3, containing a nor[5]helicene and [4]helicene fragment, respectively, in addition to a common nor[6]helicene motif. Geometries, configurational dynamics, and electronic structure of these helicenes were analyzed using solid-state structures, spectroscopic methods, and computational analyses. The open-shell character of the singlet states of these helicenes increases in the order 3<1<2, with strongly varying diradicaloid indexes and singlet–triplet gaps. Compounds 1–3 displayed narrow optical gaps of 0.79–1.25 eV, resulting in significant absorption in the near infrared (NIR) region. They also exhibit reversible redox chemistry, each of them yielding stable radical cations, radical anions, and dianions, in some cases possessing intense NIR absorptions extending beyond 2500 nm.

Electronic and steric effects are known to greatly influence the structure, characteristics and reactivity of organic compounds. A typical π bond is weakened by oxidation (corresponding to the removal of electrons from bonding orbitals), by reduction (through addition of electrons to antibonding orbitals) and by unpairing of the bonding electrons, such as in the triplet state. Here we describe tetrafluorenofulvalene (TFF), a twisted, open-shell alkene for which these general rules do not hold. Through the synthesis, experimental characterization and computational analysis of its charged species spanning seven redox states, the central alkene bond in TFF is shown to become substantially stronger in the tri- and tetraanion, generated by chemical reduction. Furthermore, although its triplet state contains a weaker alkene bond than the singlet, in the quintet state its bond order increases substantially, yielding a flatter structure. This behaviour originates from the doubly bifurcated topology of the underlying spin system and can be rationalized by the balancing effects of benzenoid aromaticity and spin pairing.

The composition of hemicelluloses differ considerably in extant hardwood and softwood. In this study, we demonstrate that there are also significant differences between the hemicellulose composition of angiosperm and gymnosperm fossil wood (Eocene to Miocene). Because of the excellent preservation and high hemicellulose contents, Fourier-transform infrared (FT-IR) spectroscopy of xylites shows bands that are characteristic of various hemicelluloses and pectin (despite the overlapping of cellulose and lignin IR bands). In particular, fossil softwood samples show IR bands typical of mannose-containing hemicellulose. The composition of hemicellulose is determined through methanolysis gas chromatography–mass spectrometry (GC–MS). The results correspond with those of extant softwood and hardwood hemicelluloses because of the excellent preservation of 13–35 Myr fossil wood deposited under reducing marine conditions. The main saccharide building blocks found in angiosperm Eocene and Miocene wood are glucose and xylose. In contrast, mannose, galactose, and glucose are dominant in gymnosperms. The relationship among xylose, mannose, and rhamnose suitably differentiates both fossil and extant woods. Moreover, the relatively high contents of rhamnose and galacturonic acid identified mainly in angiosperms indicate the presence of preserved pectin, which has not been previously documented in fossil organic matter. Nuclear magnetic resonance (NMR) spectroscopy reveals the presence of xylogalacturonan fragments in a separate fraction of the angiosperm Eocene wood. The presence of pectin, a significantly labile carbohydrate polymer, confirms the perfect preservation of fossil wood in marine deposits. This study sheds new light on the identification of xylite affinities based on their hemicellulose composition and promotes research toward the palaeochemotaxonomy of fossil plants.

Tridecacyclene tetraimide, TCTI, an electron-deficient non-benzenoid nanocarbon with a C56N4 polycyclic framework was obtained in a concise synthesis. TCTI has a non-planar structure and forms π-stacked dimers in the solid state. In solution, it undergoes eight single-electron reductions, yielding a range of negatively charged states up to an octaanion. Except for the latter species, which has a remarkably large electronic gap, the anions feature extended near-infrared absorptions, with a particularly strong band at 1692 nm observed for the dianion. A computational analysis of the TCTI anions shows that their stability originates from the combined effects of electron-deficient imide groups and the local aromaticity of reduced acenaphthylene units. The properties of TCTI make it potentially useful in electrochromic and charge storage applications.

Circularly polarized luminescence (CPL) and Raman optical activity (ROA) were observed in a single spectroscopic experiment for a purely organic molecule, an event that had so far been limited to lanthanide-based complexes. The present observation was achieved for [16]cycloparaphenylene lemniscate, a double macrocycle constrained by a rigid 9,9′-bicarbazole subunit, which introduces a chirality source and allows the molecule to be resolved into two configurationally stable enantiomers. Distortion of oligophenylene loops in this lemniscular structure produces a large magnetic transition dipole moment while maintaining the π-conjugation-induced enhancement of the Raman signal, causing the appearance of the CPL/ROA couple. A two-photon mechanism is proposed to explain the population of the lowest-energy excited electronic state prior to the simultaneous emission-scattering event.

A 1,3-cycloaddition reaction of 2-(tert-butyl)-8H-isoquinolino[4,3,2-de]phenanthridin-9-ium chloride to Ni^II norcorrole in the presence of base is shown to produce a family of chiral derivatives of polycyclic system(s) fused with pyrrole subunit(s) of the macrocycle. Dehydrogenation of the cycloaddition products gave rise to dibenzoullazine ortho-fused antiaromatic porphyrinoids.

π-Extended dibenzoullazines containing an acenaphthylene subunit were designed and synthesized. Two different synthetic strategies were employed: route A, based on Pd-catalyzed cyclodehydrohalogenation of α,α-disubstituted N-arylpyrroles, and route B, using a dipolar 1,3-cycloaddition reaction of azomethine ylides (PAMYs) to functionalized acenaphthylenes. Molecules of the resulting ullazines are almost flat, leading to strong π–π interactions in the solid state. The new ullazines are highly fluorescent (with a quantum yield of up to 0.89 for the naphthalimide-fused system), and show moderate solvatochromism with no fluorescence quenching in polar solvents. Stepwise two-electron oxidation of the ullazines is possible, yielding reversibly the corresponding ullazine radical cations and dications. Edge expansion of the ullazine core with methylene bridges is additionally shown to produce an ullazine analogue containing two seven-membered rings in its structure, which is characterized by axial chirality and can be resolved into enantiomers.

Selective self-assembly of heterodimers consisting of two non-identical subunits plays important roles in Nature but is rarely encountered in synthetic supramolecular systems. Here we show that photocleavage of a donor–acceptor porphyrin complex produces an heterodimeric structure with surprising selectivity. The system forms via a multi-step sequence that starts with an oxidative ring opening, which produces an equimolar mixture of two isomeric degradation products (zinc(II) bilatrien-abc-ones, BTOs). These two isomers are susceptible to water addition, yielding the corresponding zinc(II) 15-hydroxybiladien-ab-ones (HBDOs). However, in the photocleavage experiment only one HBDO isomer is formed, and it quantitatively combines with the remaining BTO isomer. The resulting heterodimer is stabilized by a Zn–O coordination bond and extended dispersion interactions between the overlapping π-surfaces of the monomers. The observed selectivity can be seen as a case of completive self-sorting, simultaneously controlled by three types of complementary interactions.

Cyclobuta[1,2-b:3,4-b′]ditetracene – an analogue of nonacene with a cyclobutadiene unit embedded in the central part has been synthesized by the combination of solution and on-surface chemistry. The atomic structure and electronic properties of the product on Au(111) have been determined by high resolution scanning tunnelling microscopy/spectroscopy corroborated by density functional theory calculations. Structural and magnetic parameters derived from theoretical calculations reveal that π conjugation is dominated by radialene-type contribution, with an admixture of cyclobutadiene-like antiaromaticity.

Peripheral substitution of a π-extended porphyrin with bulky groups produces a curved chromophore with four helical stereogenic units. The curvature and stereochemistry of such porphyrins can be controlled by varying the substituents, coordinated metal ions, and apical ligands. In particular, when the achiral saddle-shaped free bases are treated with large metal ions, i.e., Cd^II or Hg^II, the resulting complexes convert to chiral propeller-like configurations. X-ray diffraction analyses show that apical coordination of a water molecule is sufficient to induce a notable bowl-like distortion of the cadmium complex, which however retains its chiral structure. For phenyl- and tolyl-substituted derivatives, the conversion is thermodynamically controlled, whereas complexes bearing bulky 4-(tert-butyl)phenyl groups transform into their chiral forms upon heating. In the latter case, the chiral Hg porphyrin was converted into the corresponding free base and other metal complexes without any loss of configurational purity, ultimately providing access to stable, enantiopure porphyrin propellers.

A family of tunable halochromic switches is developed using a naphthalimide-fused dipyrrin as the core π-conjugated motif. Electronic properties of these dipyrrins are tuned by substitution of their alpha and meso positions with aryl groups of variable donor–acceptor strength. The first protonation results in a conformational change that enhances electronic coupling between the dipyrrin chromophore and the meso substituent, leading to halochromic effects that occasionally exceed 200 nm and switch the absorption between the near-infrared (NIR)-I and NIR-II ranges. A NIR-II photothermal effect, switchable by acid–base chemistry is demonstrated for selected dipyrrins. Further protonation is possible for derivatives bearing additional amino groups, leading to up to four halochromic switching step. The most electron-rich dipyrrins are also susceptible to chemical oxidation, yielding NIR-absorbing radical cations and closed-shell dications.

π-Extended acenaphtho[1,2-d][1,2,3]triazoles, the unsubstituted Anta-H and its di-tert-butyl derivative Dibanta-H, as well as 5,6,7,8-tetrahydro-1H-naphtho[2,3-d][1,2,3]triazole Cybta-H were obtained in concise syntheses. In the solid state, Dibanta-H forms an unprecedented hydrogen-bonded cyclic tetrad, stabilized by dispersion interactions of the bulky tBu substituents, whereas a cyclic triad was found in the crystal structure of Anta-H. These cyclic assemblies form infinite slipped stacks in the crystals. Evidence for analogous hydrogen-bonded self-assembly in solution was provided by low-temperature NMR spectroscopy and computational analyses. Kuratowski-type pentanuclear complexes [Zn5Cl4(Dibanta)6] and [Zn5Cl4(Cybta)6] were prepared from the respective triazoles. In the Dibanta complexes, the π-aromatic surfaces of the ligands extend from the edges of the tetrahedral Zn5 core, yielding an enlarged structure with significant internal molecular free volume and red-shifted fluorescence.

A hexaradicaloid molecule with alternating Kekulé and non-Kekulé connectivities between adjacent spin centers was obtained by fusing two conjugation motifs in Chichibabin and Schlenk hydrocarbons into a coronoid structure. ¹H NMR, ESR, and SQUID experiments and computational analyses show that the system has a singlet ground state with a significant hexaradicaloid character (γ0=0.826, γ1=γ2=0.773). It has multiple thermally accessible high-spin states (up to the septet), with uniform energy gaps of ca 1.0 kcal mol⁻¹ between consecutive multiplicities. In line with its open-shell character, the coronoid has a small electronic band gap (ca. 0.8 eV) and undergoes two consecutive one-electron oxidations at low potentials, yielding cationic forms with extended near-infrared absorption. The hexaradicaloid, which combines open-shell and macrocyclic contributions to its π conjugation, is an example of a design strategy for multistate spin switches and redox-amphoteric NIR dyes.

Cellulose has been identified in fossil material, but the preservation potential of hemicelluloses, which are less resistant to decomposition than cellulose, is generally considered very low. Here we present the discovery of hemicellulose in Miocene xylites (fossil wood) and Cretaceous xylo-detritic coals from Poland. The main building blocks of hemicelluloses in softwood are mannose, glucose, xylose with lesser amounts of galactose and arabinose. These saccharides were detected in the coals and xylites using independent geochemical methods. Based on chemical analysis, the lignites contained significant holocellulose (22–37%), α-cellulose (8–29%) and hemicellulose (7–13%). In the smoke from a xylite burn test, levoglucosan and mannosan were dominant, the latter a specific hemicellulose alteration product. Glucose and mannose products dominated after methanolysis, with minor galactose and xylose. The main hemicellulosic polysaccharides in lignite appear to be glucomannan and/or galactoglucomannan but with a lower mannose content, possibly connected to wood degradation by fungi. The preservation of hemicelluloses in fossil material may be due to structural interconnection between lignin, cellulose and hemicellulose (i.e. lignocellulose), common in extant wood. This is the first documentation of hemicelluloses in fossil material. Our results show that not only cellulose, but also hemicelluloses can persist for millions of years under favorable conditions with only minor structural changes due to slow microbial and/or diagenetic decay. In fossil wood, types of hemicellulose can help assess whether the ancient plants were related to gymnosperms or angiosperms.

A hybrid nanocarbon receptor consisting of a calix[4]arene and a bent oligophenylene loop (“molecular squid”), was obtained in an efficient, scalable synthesis. The system contains an electron-rich cavity with an adaptable shape, which can serve as a host for electron deficient guests, such as diquat, 10-methylacridinium, and anthraquinone. The new receptor forms inclusion complexes in the solid state and in solution, showing a dependence of the observed binding strength on the shape of the guest species and its charge. The interaction with the methylacridinium cation in solution was interpreted in terms of a 2:1 binding model, with K₁₁ = 5.92(7) × 10³ M⁻¹. The solid receptor is porous to gases and vapors, yielding an uptake of ca. 4 mmol/g for methanol at 293 K. In solution, the receptor shows cyan fluorescence (λ_max,em = 485 nm, Φ_F = 33%), which is partly quenched upon binding of guests. Methylacridinium and anthraquinone adducts show red-shifted emission in the solid state, attributable to the charge-transfer character of these inclusion complexes.

Unsymmetrically fused porphyrins containing one or two naphthalimide subunits were prepared in modular syntheses relying on electron-rich and electron-poor pyrrole building blocks. These new chromophores show progressive changes in their electron-deficient character, while retaining comparably small optical and electrochemical band gaps. The intrinsic curvature and extended optical absorption of these systems make them of interest as mono- and difunctional components of multichromophoric assemblies.

Energy-transfer cassettes consisting of naphthaleneimide-fused metalloporphyrin acceptors (M=Zn and Pd) and BODIPY donors have been designed and synthesized. These systems have rigid pseudo-tetrahedral structures with a donor-acceptor separation of ca. 17.5 Å. Spectroscopic investigations, including femtosecond transient absorption measurements, showed efficient excitation energy transfer (EET) occurring according to the Förster mechanism. Strong fluorescence of the donor units and significant spectral overlap of the donor and acceptor subunits are prerequisites for the efficient EET in these systems.

Conductive π-conjugated polymers were electrochemically generated from a variety of donor–acceptor pyrroles containing fused perimidine subunits. Alternating oxidative and reductive polarization of these precursors induces multidirectional coupling involving both perimidine and acenaphtho[1,2-c]pyrrole moieties, producing cross-linked polymeric structures. Chemical oxidation experiments and computational analyses of spin densities in the charged pyrrole intermediates indicate that the formation of inter-perimidine linkages occurs prior to pyrrole–pyrrole couplings. In the resulting polymers, a very stable dication state is achievable with the positive charge located on the bis-perimidine linker. These polymers do not show any stable neutral state and can be easily reduced. Cationic and anionic states of these electrogenerated films were investigated in the solid state by UV–vis–NIR and EPR spectroelectrochemistry, assisted by DFT calculations.

Boomerang-shaped bipyrroles containing donor–acceptor units were obtained through a tandem palladium-mediated reaction consisting of a cyclization step, involving double C–H bond activation, and a double α-oxygenation. The latter reaction can be partly suppressed for the least reactive systems, providing access to α-unsubstituted boomerangs for the first time. These “α-free” systems are highly efficient fluorophores, with emission quantum yields exceeding 80% in toluene. Preliminary measurements show that helicene-like boomerangs may be usable as circularly polarized luminescent materials.

A 139-π-electron nanographenoid radical was obtained by expanding the periphery of a naphthalimide–azacoronene hybrid with a methine bridge. The radical was isolated in the form of its σ-dimer, which was shown to possess a conformationally restricted two-layer structure both in the solid state and in solution. The dimer is cleaved into its parent radicals when exposed to ultraviolet or visible radiation in toluene solutions but is resistant to thermally induced dissociation. Under inert conditions, the radicals recombine quantitatively into the σ-dimer with observable kinetics, but they are oxidized into a ketone derivative in the presence of atmospheric oxygen. Combined structural, spectroscopic, and theoretical evidence shows that the σ-dimer contains a weak C(sp3)–C(sp3) bond, but is stabilized against thermal dissociation by a very strong dispersive interaction between the overlapping π surfaces.

Free base, zinc and palladium π-extended porphyrins containing fused naphthalenediamide units were employed as photosensitizers in antimicrobial photodynamic therapy (aPDT). Their efficacy, assessed by photophysical and in vitro photobiological studies on Gram-positive bacteria, was found to depend on metal coordination, showing a dramatic enhancement of photosensitizing activity for the palladium complex.

Oxidative transformation of hexapyrrolylbenzenes into azacoronenes using bromine electrophiles as alternative coupling agents is shown to occur according to a distinct mechanism, different from those proposed for typical high-potential oxidants. It is shown that consecutive cyclizations do not involve brominated intermediates and can be rationalized by assuming a relayed arenium cation attack followed by deprotonation and conjugate elimination. The final cyclization is incompatible with this mechanism and is found to involve double-electrophilic bromination followed by thermal elimination of dibromine. These findings provide insight into the reactivity of sterically congested aromatic systems and may help in designing new methods of C–C bond formation.

Supramolecular nanopatterns of arylene–alkynylene squares with side chains of different lengths are investigated by scanning tunneling microscopy at the solid/liquid interface of highly oriented pyrolytic graphite and 1,2,4-trichlorobenzene. Self-sorting leads to the intermolecular interdigitation of alkoxy side chains of identical length. Voids inside and between the squares are occupied by intercalated solvent molecules, which numbers depend on the sizes and shapes of the nanopores. In addition, planar and non-planar coronoid polycyclic aromatic hydrocarbons (i.e., butyloxy-substituted kekulene and octulene derivatives) are found to be able to intercalate into the intramolecular nanopores.

A cycloparaphenylene-based molecular lemniscate (CPPL) was obtained in a short synthesis involving masked p-phenylene equivalents. The strained figure-eight geometry of CPPL is sustained by the incorporated 9,9′-bicarbazole subunit, which also acts as a stereogenic element. The shape of the distorted [16]cycloparaphenylene nanohoop embedded in CPPL is accurately approximated with a Booth lemniscate. The structure of CPPL, investigated using NMR and Raman spectroscopic methods, revealed strain-dependent features, consistent with the variable curvature of the ring. The electronic and optical properties of CPPL combine features more characteristic of smaller cycloparaphenylenes, such as a reduced optical bandgap and red-shifted fluorescence. CPPL was resolved into enantiomers, which are configurationally stable and provide strong chiroptical responses, including circularly polarized luminescence.

A diindenophenanthrene biradicaloid, formally derived from Chichibabin's hydrocarbon, is obtained in a short, scalable synthesis. The present system is electron-rich and devoid of conjugated substituents, and still exhibits very good stability under ambient conditions. The introduction of the diindeno[1,2-a:2′,1′-i] phenanthrene ring framework results in a singlet biradicaloid system with an easily accessible triplet state (ΔES–T = −1.30 kcal mol⁻¹) and a small electronic bandgap (1.39 V). The stability limits of the title hydrocarbon were explored systematically in the solid state, to reveal an unusual thermally initiated hydrogen-scrambling oligomerization process.

Chiral heteroaromatic propellers based on radially π-extended hexapyrrolohexaazacoronenes were obtained in a concise synthesis from suitably functionalized donor–acceptor monopyrroles. To overcome steric hindrance, a new cyclodehydrogenation method was developed, and it uses bromine electrophiles as oxidative coupling agents instead of the commonly employed high-potential oxidants. The new reaction offers high yields of propeller-shaped targets, even for electron-deficient precursors, and shows electrophile-dependent stereoselectivity, with N-bromosuccinimide and dibromine yielding, respectively D₆- and C₂-symmetric products. The propeller azacoronenes are chiral and can be separated into configurationally stable enantiomers. In addition to providing steric bulk, peripheral functionalization considerably affects the electronic properties of the propellers, which exhibit reduced optical and electrochemical band gaps, and a more clearly defined electroreduction behavior.

[4]Chrysaorene, a fully conjugated carbocyclic coronoid, is shown to be a low-bandgap π-conjugated system with a distinct open-shell character. The system shows good chemical stability and can be oxidized to well-defined radical cation and dication states. The cavity of [4]chrysaorene acts as an anion receptor toward halide ions with particular selectivity toward iodides (K_a = 207 ± 6 M⁻¹). The interplay between anion binding and redox chemistry is demonstrated using ¹H NMR analysis in solution. In particular, a well-resolved, paramagnetically shifted spectrum of the [4]chrysaorene radical cation is observed, providing evidence for inner iodide binding. The radical cation–iodide adduct can be generated in thin solid films of [4]chrysaorene by simple exposure to diiodine vapor.

Recent advances in the chemistry of electron-poor polycyclic aromatic molecules are reviewed with a focus on new structural designs and emerging synthetic methods. We review various classes of building blocks, notably those based on rylene imides, and highlight the application of the most successful C–C bond-forming reactions, e.g., tandem Pd-catalyzed couplings, photocyclizations, and the glyoxylic Perkin condensation. These advances are presented in the context of the electronic properties of the resulting materials, which are of high current interest as n-type organic semiconductors.

1,n-Dipyrrolylalkanes can be efficiently converted into extensively π-conjugated bipyrroles by Pd^II-mediated annulative double C–H activation, and this approach might be further developed into tandem processes involving further cyclization of substituents or oxygenation of pyrrolic α-positions. Herein, the mechanism of these transformations is explored using NMR spectroscopy and DFT calculations. The kinetics of the annulation are found to depend on the conjugation extent and donor–acceptor character of the pyrroles, as well as on substitution and the linker length. Combined experimental and theoretical evidence indicates that a change of the rate-determining step occurs for the most electron-deficient substrates. The unprecedented double α-oxygenation of bipyrroles is found to be a stepwise process, involving α-acetoxylated intermediates.

Pd^II-mediated annulative double C−H activation is shown to efficiently convert 1,n-dipyrrolylalkanes into extensively π-conjugated bipyrroles not accessible by conventional oxidative coupling protocols. This approach is applicable to both electron-rich and electron-deficient systems, and has been further developed into tandem processes involving further cyclization of substituents or oxygenation of pyrrolic α-positions. The new bipyrrole intermediates show enhanced fluorescence as well as tunable optical properties controlled by the alignment of chromophore subunits. Photophysical data, including femtosecond transient absorptions, reveal solvent-induced intramolecular charge transfer in their excited states, dependent on the polarity of the medium.

The antiaromatic ring of norcorrole, a contracted tetrapyrrolic porphyrinoid, was subjected to [2+3] dipolar cycloaddition of iminonitriles. The paratropic character of the resulting chiral chlorins was retained. The chlorins were easily dehydrogenated in the presence of air, yielding pyrazole-fused norcorroles with markedly enhanced paratropicity.

A non-catalytic condensation of Ni^II β-aminonorcorrole with aryl aldehydes is shown to produce a family of pyrromethane dimers that undergo deaminative cyclization to yield pyridine-fused bis(norcorrole)s comprising two antiaromatic macrocycles communicating by an aromatic moiety. The new compounds were characterized by spectroscopic, structural, and electrochemical methods supported by DFT calculations, all of which revealed unexpected antiaromaticity enhancement in the fused system.

An octaphyrin(1.2.1.1.1.2.1.1) containing two conjugated ethynylene bridges has been synthesized and characterized. The macrocycle reveals complex conformational dynamics dependent on its protonation and oxidation state. The [40]annulenoid macrocycle and its [38]annulenoid oxidized form display residual macrocyclic ring currents. In spite of its low apparent aromaticity the new octaphyrin is a potent chromophore with a vis-NIR absorption profile strongly influenced by the redox and acid–base chemistry.

Two hitherto unknown core modified hexaphyrin analogues have been synthesized and characterized where the conformational dynamics of these macrocycles in the free base form is achieved by the rotation of thienothiophene units. Further unique property of these macrocycles is the Hückel-Möbius topological switching. The thermodynamic equilibrium and kinetics of the interconversion leading to Hückel-Möbius switches have been triggered by external stimuli, such as protonation and/or temperature. We have provided a thorough solution-state spectroscopic characterization, solid-state structural evidence combined with in-depth theoretical calculations to investigate the crucial factors involved in such interconversion between Hückel and Möbius topologies for these hexaphyrins which will be useful in designing future new members to expanded porphyrin chemistry.

A hexapyrrolohexaazacoronene derivative containing 37 fused rings, the largest such system to date, was obtained from a naphthalenomonoimide–pyrrole hybrid in a concise and efficient synthesis. This large heterocycle is electron-deficient and shows extended redox activity, spanning at least 13 oxidation levels, but is otherwise chemically stable. Radial expansion of the π system creates a chromophore characterized by strong fluorescence and solvatochromism in the neutral state, and strong near‐infrared absorption in the charged states. Additionally, the enlarged and ruffled aromatic surface supports a unique self‐assembly mode in the crystal, leading to the formation of highly solvated organic clathrates.

Octulene, the higher homologue of kekulene and septulene, was synthesized using the fold-in method. This new hydrocarbon macrocycle contains a large 24-membered inner circuit, which is peripherally fused to 24 benzene rings. Such an arrangement produces considerable hyperbolic distortion of the π-conjugated surface. The consequences of distortion in octulene were explored using photophysical methods, which revealed a reduced electronic band gap and greater flexibility of the π system. Octulene contains a functional cavity with a diameter larger than 5.5 Å that is capable of efficiently binding the chloride anion in a nonpolar solvent (K_a = 2.2(4) × 10⁴ M⁻¹, 1% dichloromethane (DCM) in benzene). The octulene–chloride interaction is stabilized by eight weak C(sp²)H⋅⋅⋅Cl bonds, providing the first example of a hydrocarbon-based anion receptor.

Reversible homolytic dissociation of the bis(pyrimidinenorcorrole) σ-dimer was elucidated by means of variable temperature ESR and ¹H NMR spectroscopy, mass spectrometry, and optical spectrocopy. Dehydrogenation of the σ-dimer yielded another dimer displaying a singlet–triplet equilibrium in solution, strong NIR absorption (1570 nm), and a narrow electrochemical HOMO–LUMO gap (0.74 V).

A family of bandgap-tunable pyrroles structurally related to rylene dyes was computationally designed and prepared using robust, easily scalable chemistry. These pyrroles show highly variable fluorescence properties and can be used as building blocks for the synthesis of electron-deficient oligopyrroles. The latter application is demonstrated through the development of π-extended porphyrins containing naphthalenediamide or naphthalenediimide units. These new macrocycles exhibit simultaneously tunable visible and near-IR absorptions, an ability to accept up to 8 electrons via electrochemical reduction, and high internal molecular free volumes. When chemically reduced under inert conditions, the most electron-deficient of these macrocycles revealed reversible formation of eight charged states, characterized by remarkably red-shifted optical absorptions, extending beyond 2200 nm. Such features make these oligopyrroles of interest as functional chromophores, charge-storage materials, and tectons for crystal engineering.

The purpose of this study was to investigate 3,4-diarylpyrrole electropolymerization and simultaneous or subsequent intramolecular oxidative coupling leading to poly(phenanthropyrrole). Small amounts of π-conjugated products were obtained only under the conditions of increased concentration of the monomer and increased rate of potential sweep, which enabled both processes − monomer oxidation and deprotonation of the σ-dimer. The combined electrochemistry, UV-vis-NIR, ESR and TD-DFT results show, that the α,α'-coupling of 3,4-diarylpyrrole derivatives is inhibited by the stability of the σ-dimer dication, inhibition of the deprotonation and the reversal of the σ bond formation resulting in regeneration of the monomer. The addition of pyridine did not result in σ–dimer deprotonation in 2- and 2'-position.

A one-pot reaction of 5,14-bis(mesityl)-norcorrolatonickel(II) with isoamyl nitrite under mild reaction conditions resulted in the consecutive formation of 3-nitro-, 3,12-dinitro- and 3,16-dinitro-, 3,7,12-trinitro-, and 3,7,12,16-tetranitro-norcorrolatonickel(II) in 50–80% yield. The substituted macrocycles retained their antiaromatic character. The observed regioselectivity of the substitution was analyzed by comparing the relative energies of the DFT energy-optimized models of the radical or arenium cationic intermediates that can be formed upon reaction with NO2. The nitrated systems were characterized by high-resolution mass spectrometry, NMR and UV/Vis spectroscopy, X-ray diffraction analysis, cyclic voltammetry, and DFT calculations. A significant and systematic cathodic shift of the redox couples was observed to correlate with an increasing number of the NO2 group. A decrease of the LUMO energies in the tri- and tetra-nitrated products stabilizes mono- and bis-reduced complexes of these ligands. The reduction takes place on the macrocycle rather than on the metal ion leading to the consecutive formation of stable paramagnetic monoanion radicals and water-soluble diamagnetic dianions with an aromatic character, which were revealed by ESR and ¹H NMR measurements, respectively. The electronic structures of the reduced forms were analyzed by extensive TD-DFT calculations.

A heteroaromatic nanographene containing a unique assembly of five-, six- and seven-membered rings is synthesized using oxidative coupling of an indole-containing precursor. Near-infrared absorption and emission properties of the nanographene core are enhanced by peripheral expansion and ring fusion at all oxidation levels. The dicationic state shows distinct aromaticity originating from a peripheral π-conjugated circuit. A partially coupled intermediate, trapped in the synthesis of the 5-6-7 nanographene, is explored as a reference system, showing an unexpected reduction of the optical band gap due to intramolecular charge transfer.

Two coronoid hydrocarbons, [3]- and [4]chrysaorene were synthesized from fluorenophane precursors using the fold-in strategy. [3]Chrysaorene is a bowl-shaped fragment of the C₂₄₀-I_h fullerene, whereas [4]chrysaorene is planar and contains a unique large 24-membered internal ring. The chrysaorenes show geometry-dependent magnetic properties and are strongly fluorescent.

Z-Selective Wittig olefination was applied to the synthesis of large carbazolophanes containing up to eight heteroaromatic subunits. A number of strategies were devised and tested, showing that cyclooligomerization yields can be significantly improved by using one-component schemes involving heterobifunctional reactants. [4]⁻ and [6]Carbazolophanes were characterized in the solid state, revealing compact, highly folded structures. Electronic and steric effects of substitution and chain length on the Wittig olefination rates and Z-selectivities were explored theoretically using DFT calculations.

5,14-Dimesitylnorcorrolatonickel(II) was hydrogenated under mild conditions (room temperature, 1 atm H₂, THF solution, 5 min.) in the presence of Raney nickel to yield nonaromatic derivatives that were isolated and characterized by NMR spectroscopy, UV/Vis spectrophotometry, HRMS, cyclic voltammetry, and X-ray diffraction analysis. The major hydrogenation product, 1,2,3,7,8,9-hexahydronorcorrolatonickel(II), underwent dimerization in the presence of p-chloranil to give a nonsymmetrically linked 2,3′-bis(norcorrole) system that can adopt eight different oxidation states over a redox potential window of 3 V and has a HOMO–LUMO gap of 0.92 V.

The symmetry of the arrangement of objects has fascinated philosophers, artists and scientists for a long time, and still does. Symmetries often exist in nature, but are also created artificially, for instance by chemical synthesis of novel molecules and materials. The one-sided, non-orientable Möbius band topology is a paradigm of such a symmetry-based fascination. In the early 1960s, in synthetic organic chemistry the interest in molecules with Möbius symmetry was greatly stimulated by a short paper by Edgar Heilbronner. He predicted that sufficiently large [n]annulenes with a closed-shell electron configuration of 4n π-electrons should allow for sufficient π-overlap stabilization to be synthesizable by twisting them with a 180° phase change into the Möbius symmetry of their hydrocarbon skeleton. In 2007, the group of Lechosław Latos-Grażyński succeeded in synthesizing the compound di-p-benzi[28]hexa-phyrin(1.1.1.1.1.1), compound 1, which can dynamically switch between Hückel and Möbius conjugation depending, in a complex manner, on the polarity and temperature of the surrounding solvent. This discovery of “topology switching” between the two-sided (Hückel) and one-sided (Möbius) molecular state with closed-shell electronic configuration was based primarily on the results of NMR spectroscopy and DFT calculations. The present EPR and ENDOR work on the radical cation state of compound 1 is the first study of a ground-state open-shell system which exhibits a Hückel–Möbius topology switch that is controlled by temperature, like in the case of the closed-shell precursor. The unpaired electron interacting with magnetic nuclei in the molecule is used as a sensitive probe for the electronic structure and its symmetry properties. For a Hückel conformer with its higher symmetry, we expect – and observe – fewer ENDOR lines than for a Möbius conformer. The ENDOR results are supplemented by and in accordance with theoretical calculations based on density functional theory at the ORCA level.

Nonclassical nanotube end-caps have been constructed from strain-free heterocyclic precursors using a one-step synthetic procedure, involving multiple nickel-mediated Ullmann couplings. These systems consist of tubular macrocyclic sections that are tightly capped on one side with a bridging benzene ring, forming deep, chemically accessible cavities. The end-caps are characterized by exceptionally high internal strain energies reaching 144 kcal/mol. The optical absorption and emission properties of these molecules show a marked dependence on conjugation length and geometrical factors. The mechanism of end-cap formation, investigated using DFT calculations, relies on precise timing of transmetalation and reductive elimination events.

The adduct formation of protonated bis(N-confused porphyrin) (BNCP, 3,3′-bis(meso-tetratolyl-2-aza-21-carbaporphyrin) with chiral anions, carboxylic acids, and alcohols was studied in solution by means of ¹H NMR and circular dichroism (CD) spectroscopic analysis and DFT methods. The addition of enantiopure guests to the acidified BNCP resulted in optical activity that vanished after neutralization. Pairs of the ¹H NMR-distinguishable diastereomers were formed when enantiopure guests were applied, although a single form was observed upon the addition of the racemic mixtures in each case. Unidirectional configuration change that led to diastereomeric excess was observed in several instances. Such an excess was memorized by metalation of the adducts with AgBF4, thus resulting in optically active silver(III) complexes of BNCP with some enantiomeric excess. Absolute configurations of BNCP cations and bis(zinc) and bis(silver(III)) complexes were determined on the basis of time-dependent (TD)-DFT calculations of their CD spectra. It was shown that some of the chiral carboxylates induced opposite directions of enantiomerization of di- and tetracations or di-/tetracation and bis(zinc) complexes. The source of the optical activity of the equimolar diastereomeric mixture of adducts is discussed.

A family of azacoronenes containing up to two saturated bridges at the periphery was synthesized from substituted hexapyrrolylbenzenes using a two-step condensation–aromatization procedure. The introduction of peripheral bridges provides access to nonplanar, sterically crowded systems that display complex reactivity patterns, involving stereospecific aromatization of bridges and nucleophile additions. Despite the interrupted conjugation on the periphery, the new azacoronenes have easily accessible higher oxidation levels, and a quadruply charged species was chemically generated by reaction with SbCl₅. These oxidized species show extensive π-electron conjugation and are efficient UV/vis/NIR absorbers, active up to ca. 2400 nm. Interruption of peripheral conjugation is shown to induce a tendency toward biradicaloid electron configurations in doubly oxidized species.

Direct alkylation of 9,9′,9′′-triethyl\[2.2.2\](2,7)carbazolophane with dimethoxymethane or paraformaldehyde affords a belt-like heteroaromatic structure, which forms as a kinetic product in acid-catalyzed condensations. In a competing, thermodynamically favored process, polymeric structures are formed by a largely regioselective condensation of stereochemically rigid “semi-belts”. The relationship between these reactivity routes is rationalized in terms of strain release and differential reversibility of consecutive condensation steps.

In a series of thiaaceneporphyrinoids, their conformers exhibit macrocyclic π-conjugation pathways controlled by a dihedral angle between the porphyrin framework and acene planes. Conformational equilibria significantly affect the photophysical properties of these macrocycles.

Reaction of paraformaldehyde with meso-tetraaryl-2-aza-21-carbaporphyrins (NCP) in the presence of a basic catalyst afforded fused lactam derivatives comprising a >C=O bridge linking the internal carbon C21 with one of the internal nitrogens. The isomer 2 with C21–C(O)–N24 bridge is formed with about 9-fold molar excess over that with C21–C(O)–N22 bridge (3). The ¹H NMR and UV/vis spectral characteristics indicate aromatic character of the derivatives. For meso-tetrakis(3′,5′-dimethoxy)- and meso-tetrakis(3′,4′,5′-trimethoxy)-21,24-carbonyl-NCP an efficient external ring fusion by linking C3, i.e., the external carbon of the confused pyrrole, with an ortho carbon of the adjacent aryl was observed under acidic conditions, yielding derivatives 4c and 4d, comprising a linear system of five fused rings. The chirality and configurational stability of these carbonylated systems were established by a chiral stationary phase HPLC and circular dichroism. The interaction of 2 with chiral acids and alcohols leading to the formation of diastereomers was observed by ¹H NMR. Slow racemization of 2 under acidic conditions was established by HPLC and ¹H NMR and a mechanism for this process was proposed.

The four expanded p-benziporphyrins A,C-di-p-benzi[24]pentaphyrin(1.1.1.1.1), N-fused A-p-benzi[24]pentaphyrin, A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1), and A,C-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) were obtained in three-component Lindsey-type macrocyclizations. These compounds were explored as macrocyclic ligands and as potential aromaticity switches. A BODIPY-like difluoroboron complex was obtained from the A,C-di-p-benzi[24]pentaphyrin, whereas A,C-di-p-benzi[28]hexaphyrin yielded a Möbius-aromatic Pd^II complex containing fused pyrrole and phenylene subunits. Conformational behavior, tautomerism, and acid–base chemistry of the new macrocycles were characterized by means of NMR spectroscopy and DFT calculations. Free base N-fused A-p-benzi[24]pentaphyrin showed temperature-dependent Hückel–Möbius aromaticity switching, whereas the A,C-di-p-benzi[28]hexaphyrin formed a Möbius-aromatic dication.

Density functional theory calculations combined with a distortion/interaction energy analysis show that the anomalous Z selectivity observed in Wittig reactions of ortho-substituted benzaldehydes is not caused by phosphorus–heteroatom interactions in the addition transition state, as assumed in earlier work, but is predominantly steric in nature. The calculations reproduced correctly the stereoselectivity preferences for a wide range of reactant pairs as well as relative reactivities for different substituent types, providing deeper structural insight into the mechanism of Wittig olefination.

A 3-phosphonated N-confused phlorin 3 was synthesized by the reaction of N-confused porphyrin 1 and trimethyl or triethyl phosphite 2 in the presence of acetic acid in good yield. The presence of hydrogen and aryl substituents in one of the meso positions (C5) generates a stereogenic center, resulting in configurationally stable enantiomers. The enantiomers were separated by HPLC and characterized by the circular dichroism method for the first time in the case of phlorin. Further oxidation of 3 by DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) afforded the achiral 3-phosphonated N-confused porphyrin 4. Chiral chlorozinc 4–Zn and chlorocadmium 4–Cd, as well as achiral nickel(II) complexes 4–Ni were also characterized. For 4–Cd in the solid state, formation of a dimer consisting of heterochiral subunits joined by two H-bonds was established by a single crystal X-ray analysis. For 4–Cd, separation of enantiomers was achieved. Slow racemization of 4–Cd in solution prevented the absolute configuration determination by the X-ray method indicating the labile character of the complex. The relationship between circular dichroism and absolute configuration of 3a and 4–Cd was established on the basis of TD-DFT calculations.

An electron-rich 3,4-diarylpyrrole is shown to undergo a tandem inter- and intramolecular Scholl oxidation, providing access to bis(phenanthropyrroles), a new class of bipyrrole derivatives with extended π conjugation. Intermolecular coupling of two pyrrole molecules is found to be the initial step of this reaction. Bis(phenanthropyrroles) are characterized by restricted rotation around the α–α bond and exhibit strong blue fluorescence and large Stokes shifts.

Oxidative coupling of activated aryl groups attached to β-positions of the porphyrin ring provides convenient access to derivatives containing peripherally fused phenanthrene and benzo[g]chrysene units. Tetra(benzochryseno)porphyrin, reported here for the first time, contains a nonplanar, sterically locked π system and shows very intense electronic absorptions in the Q range of the electronic spectrum. Tetraphenanthroporphyrins show a tendency to aggregate in solution. In one case, a discrete dimer is formed, whose structure was investigated spectroscopically and theoretically. Derivatives bearing long alkyl chains are mesomorphic and exhibit columnar phases (tetraphenanthroporphyrins) and a monoclinic 3D phase (tetrabenzochrysenoporphyrin). The symmetry of column packing in the columnar phases is dependent on the number of alkyl chains per molecule. X-ray diffraction measurements show that, in spite of their nonplanarity, the aromatic cores in the mesophases are tightly stacked within the column. The corresponding stacking patterns were derived from the structure of the dimer, on the basis of geometrical analysis and molecular modeling.

Setting the 'cene: The structural features of ferrocenophanes and porphyrinoids are combined in ferrocene–porphyrin hybrids. Ferrocenothiaporphyrin (see picture, green) and dihydroferrocenothiaporphyrin (red) are macrocyclic antiaromatic and aromatic systems, respectively. These two systems provide evidence for direct transmission of π-electron conjugation across a d-electron metallocene.

Möbius π-conjugation in cyclic molecules leads to the reversal of Hückel aromaticity rules and affects the electronic and magnetic properties of these systems. We found the first example of a medium-sized macrocyclic structure that is sufficiently flexible to switch between three distinct π-conjugation topologies, planar (T0), Möbius (T1), and twisted Hückel (T2), without changing its oxidation level. The switching is under thermodynamic and kinetic control and can be realized in a three- or four-step cycle. On titration with trifluoroacetic acid (TFAH) or dichloroacetic acid (DCAH), the Möbius free base (T1-H₂), which is the preferred structure in dichlorofluoromethane at 150 K, undergoes a series of acid–base reactions involving changes of the π-conjugation topology. The forms observed in the course of titration involve a Möbius aromatic monocation ([T1-H₃]⁺), an antiaromatic twisted Hückel species ([T2-H₄(A)]⁺) containing a coordinated carboxylate anion (A = TFA, DCA), and two additional Möbius forms ([T1-H₄(A)(HA)_n]⁺ (n = 1, 2)), containing complex carboxylate anions. The protonated forms undergo a thermally activated ring planarization to yield an antiaromatic quasi-planar dication [T0-H₄]²⁺, characterized in the solid state as a TFA salt. The corresponding free base (T0-H₂) is metastable but can be trapped by addition of triethylamine at low temperatures.

The use of 1,4-phenylene-containing tripyrrane analogs provides a general route to expanded p-benziporphyrins. The course of macrocyclization shows a striking dependence on the steric bulk of meso substituents.

Ferrocene-containing porphyrinoids have been synthesized in which ferrocene-1,1′-diyl units are linked to a dipyrrin or thiatripyrrin to form macrocyclic structures. NMR spectroscopic evidence shows that these new systems adopt helical conformations that undergo an inversion process in solution. In addition, small amounts of unexpected scrambling products have been isolated and characterized, namely a dipyrrin-bisferrocenophane and two expanded bis(ferrocene) macrocycles. The formation of these systems, which contain macrocyclic rings of different sizes, is a consequence of the structural flexibility of the ferrocene unit. Cyclic voltammetry shows that ferrocene oxidation is reversible in all the systems reported here, and that it is finely tuned by the properties of the ring system.

A facile synthesis of a functionalized cyclononatripyrrole derivative is described. It involves the iodine-catalyzed cyclotrimerization of ethyl 3,3-dimethyl-5,7-dihydro[1,3]dioxepino[5,6-c]pyrrole-6-carboxylate in methanol. The product obtained in this way exists solely in the crown conformation typical of cyclononatripyrroles. However, in contrast to previously reported compounds of this class, it possesses three labile methoxymethyl arms that can be further functionalized under acidic conditions.

The diprotonated form of an organic-solubilized cyclo[8]pyrrole derivative, bearing eight undecyl chains on the β-pyrrolic positions, was found to extract sulfate anion effectively from neutral aqueous media into a toluene organic phase. The kinetics of sulfate anion exchange between the two phases were found to be exceedingly slow in the absence of the phase-transfer catalyst, Aliquat 336-nitrate (A336N), but appreciable in its presence. The bisnitrate anion bound form of this cyclo[8]pyrrole could be generated in situ by subjecting the toluene phase containing initially 0.5 mM of the sulfate anion bound form and 0.1 mM trioctylamine (TOA) to successive equilibrations with aqueous 0.1 M HNO3 until sulfate was no longer detected in the aqueous phase. This bisnitrate complex, when studied as a 0.5 mM solution in toluene in the presence of 0.1 mM (TOAH)+(NO3-), was also found to be an effective extractant for sulfate anion. DSO4 values of 0.001 and 1000 were observed at 1 M NaNO3(aq) and 0.3 mM NaNO3(aq), respectively, and the logarithm of the conditional exchange constant, log, was calculated to be 4.9 ± 0.4. The present cyclo[8]pyrrole system is thus noteworthy as being the first synthetic receptor that displays a high selectivity for sulfate anion in the presence of excess nitrate under conditions of solvent extraction.

Boron(III) has been inserted into N-confused porphyrin, (NCPH)H₂ (1), and N-fused porphyrin, (NFP)H (2). The reaction of dichlorophenylborane and 1 yields σ-phenylboron N-confused porphyrin (4). The boron atom is bound by two pyrrolic nitrogen atoms and the σ-phenyl ligand. The N-confused pyrrole ring is not involved in direct coordination because the C(21)–H fragment remains intact. A reaction between PhBCl₂ and N-fused porphyrin produces σ-phenylboron N-fused porphyrin (3⁺). 4 converts quantitatively into 3⁺ upon protonation. In σ-phenylboron N-fused porphyrin [(NFP)BPh]Cl, the coordinating environment of boron(III) resembles a distorted trigonal pyramid, with the nitrogen atoms occupying equatorial positions and the phenyl ligand lying at the unique apex. Boron(III) is displaced by 0.547(4) Å from the N₃ plane. The B–N distances are as follows: B–N(22), 1.559(4) Å; B–N(23), 1.552(4) Å; B–N(24), 1.568(4) Å; B–C_ipso(Ph), 1.621(4) Å. 3⁺ can be classified as a boronium cation considering a filled octet and a complete coordination sphere. 3⁺ is susceptible to alkoxylation at the inner C(9) carbon atom, yielding 5-OR. The addition of acid results in protonation of the alkoxy group and elimination of alcohol, restoring the original 3⁺. Density functional theory has been applied to model the molecular and electronic structure of 4, 3⁺, and syn and anti isomers of methoxy adducts 5-OMe.

A number of substituted zinc(II) porphycenes and porphyrins have been synthesized as potentially mesogenic materials. One of the resulting porphycenes, bearing eight decyloxy chains, exhibits two mesophases, a transient lamellar phase (Lam) and a highly ordered lamello-columnar phase (LCol), with remarkably different structural characteristics. The same zinc(II) porphycene also forms an electron donor–acceptor (EDA) complex with tetracyanoquinodimethane (TCNQ), generating a hexagonal columnar mesophase (Colh) that is thermally stable up to ca. 200 °C. The EDA interaction between porphycene and TCNQ has been probed using electronic and vibrational spectroscopy. A mixture of zinc(II) porphyrins, isomeric with the above porphycene complex, forms a rectangular columnar mesophase (Colr).

22-Hydroxybenziporphyrin, a porphyrin analogue containing a phenol moiety, has been shown to exist as an equilibrium mixture of two distinctly different tautomers. One of them actually contains the hydroxy group and shows the local [6]annulene aromaticity in the phenol fragment. The other tautomer contains a keto group and exhibits a [20]annulenoid structure characterized by macrocyclic antiaromaticity. The tautomerization process has been investigated in detail using variable-temperature ¹H NMR spectroscopy. The process is very fast, with an estimated activation energy of ca. 30 kJ/mol. Further insight into the energetics of the tautomerization is obtained from density functional (DFT) calculations. Surprisingly, the estimated energy of the antiaromatic keto species is 3−5 kcal/mol lower than the energy of the phenolic tautomer. The geometric and magnetic manifestations of aromaticity and antiaromaticity in the two tautomers are probed using a number of computational devices, including Wiberg bond indices, resonace weights derived from the harmonic oscillator model, and nucleus-independent chemical shifts. It is shown that mixing of phenolic and keto contributions in both tautomers is stronger than that in related tautomers of phenol. This effect is caused by extensive conjugation with the tripyrrolic unit of 22-hydroxybenziporphyrin and, to a lesser extent, by intramolecular hydrogen bonding.

The synthesis of an inverted, methoxylated sapphyrin derivative is described. This system, wherein inversion of pyrrolic nitrogen atoms is configurationally enforced via the use of a 3,3‘-bipyrrolic precursor, displays what is best described as “weak aromaticity” as judged from its spectroscopic features and supporting theoretical calculations.

A series of expanded porphyrins, incorporating biimidazoles and bipyrroles within their macrocyclic framework, has been synthesized. Insights into the complex conformational characteristics of these ems were obtained from two-dimensional NMR spectroscopic studies. The relative energy values for the various asymmetric structures inferred from these analyses were compared using DFT molecular modeling calculations.Key words: macrocycles, expanded porphyrins, imidazoles, pyrroles, biimidazoles, supramolecular chemistry, heterocycles.

¹H NMR spectra of three paramagnetic Ni^II complexes of benziporphyrins have been investigated in a broad temperature range. For the m- and p-benziporphyrin complexes, the line widths of certain signals exhibit an unusual temperature dependence characteristic of a dynamic process. This behavior is interpreted in terms of an equilibrium in which one of the forms is present at a very small concentration and cannot be observed directly. For the benziporphodimethene complex, the two exchanging forms are present at comparable concentrations and can yield separate signals in the slow exchange limit. To account for the observed exchange process, a mechanism is proposed involving the motion of the phenylene moiety. This hypothesis is further explored with DFT modeling, which indicates that the postulated conformers are thermally accessible.

Iron and copper complexes of tetraphenyl-m-benziporphyrin (TPmBPH)H have been prepared and structurally characterized. The iron system, (TPmBPH)Fe^IIBr, contains a high-spin Fe^II center. In the solid state the complex forms dimeric units linked by weak CH···Br hydrogen bonds. The Cu complex contains a tetrameric copper cluster with a Cu₂Cl₄²⁻ unit bridging two [(TPmBPCl)Cu^II]+ fragments. The formation of (TPmBPCl)H represents an example of copper-catalyzed chlorination on the internal carbon atom of (TPmBPH)H.

Weak metal−arene interactions have been investigated in Zn, Cd, Hg, and Ni complexes of meso-tetraaryl m- and p-benziporphyrin (1 and 2) and of the new compound, m-benziporphodimethene (3). Compounds 1−3 incorporate the phenylene moiety into a macrocyclic structure so as to facilitate the interaction between the arene and coordinated metal ion. X-ray studies performed on Cd(II) and Ni^II complexes show that the arene fragment approaches the ion at a distance much shorter than the sum of van der Waals radii. In chloronickel(II) m-benziporphyrin, a weak agostic bond is actually formed. In the NMR spectra of the Cd(II) and Hg(II) species, unusual ¹H−M and ¹³C−M scalar couplings have been observed that are transmitted directly between the metal and the arene. DFT calculations performed for two Cd(II) species and subsequent AIM analysis show that the accumulation of electron density between the metal and arene necessary to induce these couplings is fairly small and the interaction is steric in nature. In the paramagnetic Ni^II complexes of 1 and 3, the agostic proton of the m-phenylene exhibits large downfield ¹H NMR shifts (386 and 208 ppm at 298 K, respectively). An agostic mechanism of spin density transfer is proposed to explain these shifts as resulting from electron donation from the CH bond to the metal. In chloronickel(II) p-benziporphyrin, the inner protons of the p-phenylene have a contrastingly small shift (0.0 ppm at 298 K), indicating that in this case the agostic interaction is inefficient, in agreement with the X-ray data.

Coordinating properties of acetoxybenziporphyrin, (TPBPOAc)H, have been investigated for a number of metal ions. Insertion of Ni, Pd, and Fe results in the cleavage of the acetoxy group leading to complexes (TPBPO)Ni^II, (TPBPO)Pd^II, and (TPBPO)Fe^IIIX containing a M−O bond. No cleavage is observed with Zn^II and Cd(II), which form complexes (TPBPOAc)M^IICl, where M = Zn, Cd. (TPBPO)Ni^II can also be obtained from the dication of hydroxybenziporphyrin, [(TPBPOH)H₃]Cl₂, which is prepared by acid hydrolysis of the acetoxy compound. The diamagnetic (TPBPO)Ni^II can be transformed into the paramagnetic (TPBPOAc)Ni^IICl in a reaction with acetyl chloride. X-ray structures have been determined for (TPBPO)Pd^II and (TPBPOAc)Zn^IICl. In the palladium species, the phenolate moiety forms a strong bond to the Pd ion and an unusual interaction geometry is observed, enforced by the macrocyclic environment. Association of a TFA molecule to the phenolic oxygen does not cause significant structural changes in the (TPBPO)Pd^II molecule. In (TPBPOAc)Zn^IICl, the metal ion weakly interacts with the phenolic fragment. The paramagnetic Fe^III complexes, (TPBPO)Fe^IIIX, have been investigated with ¹H NMR spectroscopy. The observed spectral patterns are consistent with the presence of a high-spin Fe^III center and π delocalization of spin density onto the phenoxide fragment. Each of the compounds (TPBPO)Fe^IIIX exists in solution as a mixture of two isomers, which for X = I are shown to remain in a temperature-dependent equilibrium. The observed isomerism results from two nonequivalent orientations of the axial halide with respect to the puckered macrocyclic ring.

Reaction of tetraaryl-m-benziporphyrin with pyridine and silver tetrafluoroborate yields 22-pyridiniumyl-m-benziporphyrin as the only substitution product. This compound is further rearranged to a fused m-benziphlorin containing a 4a-azafluorene fragment. A mechanism involving a high-valent silver complex is proposed for the pyridination reaction.

Replacement of one of the pyrrole rings with a p-phenylene unit transforms porphyrin into p-benziporphyrin (1), an aromatic carbaporphyrinoid that locates two connected carbon atoms in the coordination core. p-Benziporphyrin forms a complex with cadmium(II) (2) with an unprecedented η² Cd(II)-arene interaction.

The reaction of [(TrPP)Fe^II(py)₂] (TrPP = dianion of 5,10,15-triphenylporphyrin) with hydrogen peroxide results in oxygenation of the (porphyrin)iron and the formation of (10,15,20-triphenyl-5-oxophlorin)iron [(5-O-TrPP)Fe(py)₂]. Coupled oxidation of [(TrPP)Fe^IIICl] in the presence of ascorbic acid and potassium cyanide in methanol leads to oxidative hydroxylation of the porphyrin ring to form a cyclic dimer [(5-O-TrPP)Fe^III]₂. Three out of four pyrrole ¹H NMR resonances of the dimeric complex present the characteristic displacement with respect to the typical position for the high-spin (tetraphenylporphyrin)iron(III) complexes. The linewidths of the β-H pyrrole resonances correlate with specific locations of the respective protons within the dimeric structure. Addition of HCl to [(5-O-TrPP)Fe^III]₂ results in cleavage of the dimer producing [(5-OH-TrPP)Fe^IIICl]. [(5-O-TrPP)Fe^III]₂ is also split with acetic anhydride to give (5-acetoxy-10,15,20-triphenylporphyrin)iron(III). In the presence of [D₅]pyridine or a large excess of cyanide ion, [(5-O-TrPP)Fe^III]₂ undergoes cleavage to form air-sensitive [(5-O-TrPP)Fe(py)₂] or [(5-O-TrPP)Fe(CN)₂]²⁻. The pyrrole pattern observed for [(5-O-TrPP)Fe(py)₂] is similar to that determined on the basis of an analysis carried out for four (oxophlorin)iron regioisomers obtained from hemes. For comparison, the characteristic ¹H NMR features of the typical (5-substituted-10,15,20-triphenylporphyrin)iron(III) complexes have been investigated for the series of high-spin and low-spin complexes. The spin-density distribution in the (oxophlorin)iron skeleton is dominated by the radical-like electronic structure.

Introduction of a single meso substituent into ClFe^III(OEP) or K[(NC)₂Fe(OEP)] results in significant changes in the geometric and/or spectroscopic properties of these complexes. The mono-meso-substituted iron(III) complexes ClFe^III(meso-Ph-OEP), ClFe^III(meso-n-Bu-OEP), ClFe^III(meso-MeO-OEP), ClFe^III(meso-Cl-OEP), ClFe^III(meso-NC-OEP), ClFe^III(meso-HC(O)-OEP), and ClFe^III(meso-O₂N-OEP) have been isolated and characterized by their UV/vis and paramagnetically shifted ¹H NMR spectra. The structures of both ClFe^III(meso-Ph-OEP) and ClFe^III(meso-NC-OEP) have been determined by X-ray crystallography. Both molecules have five-coordinate structures typical for high-spin (S = 5/2) iron(III) complexes. However, the porphyrins themselves no longer have the domed shape seen in ClFe^III(OEP), and the N₄ coordination environment possesses a slight rectangular distortion. These high-spin, mono-meso-substituted iron(III) complexes display ¹H NMR spectra in chloroform-d solution which indicate that the conformational changes seen in the solid-state structures are altered by normal molecular motion to produce spectra consistent with C_s molecular symmetry. In pyridine solution the high-spin six-coordinate complexes {(py)ClFe^III(meso-R-OEP)} form. In methanol solution in the presence of excess potassium cyanide, the low-spin six-coordinate complexes K[(NC)₂Fe^III(meso-R-OEP)] form. The ¹H NMR spectra of these show that electron-donating substituents produce an upfield relocation of the meso-proton chemical shifts. This relocation is interpreted in terms of increased contribution from the less common (dxz,dyz)4(dxy)1 ground electronic state as the meso substituent becomes more electron donating.

Treatment of the mono-meso-substituted iron(II) octaethylporphyrin complexes, (py)₂Fe^II(meso-NO2-OEP), (py)₂Fe^II(meso-CN-OEP), (py)₂Fe^II(meso-HC(O)-OEP), (py)₂Fe^II(meso-Cl-OEP), (py)₂Fe^II(meso-OMe-OEP), (py)₂Fe^II(meso-Ph-OEP), and (py)₂Fe^II(meso-n-Bu-OEP), with hydrogen peroxide in pyridine-d5 at −30 °C in the strict absence of dioxygen has been monitored by ¹H NMR spectroscopy. The product oxophlorin complexes are stable as long as the samples are protected from exposure to dioxygen. Hydrogen peroxide reacts cleanly with mono-meso-substituted iron(II) porphyrins in pyridine solution under an inert atmosphere to form mixtures of three possible oxygenation products, (py)₂Fe(cis-meso-R-OEPO), (py)₂Fe(trans-meso-R-OEPO), and (py)₂Fe(OEPO). The yields of (py)₂Fe(OEPO), which results from replacemement of the unique meso substituent, as a function of the identity of the meso substituent decrease in the order NO2 > HC(O) ∼ CN ∼ Cl > OMe > Ph, Bu, which suggests that the species responsible for attack on the porphyrin periphery is nucleophilic in nature. A mechanism involving isoporphyrin formation through attack of hydroxide ion on a cationic iron porphyrin with an oxidized porphyrin ring is suggested. The identity of the unique meso functionality also affects the regiospecificity of substitution when the unique meso group is retained. Although random attack at the two different meso sites is expected to yield a cis/trans product ratio of 2, the observed ratios vary in the following order: cyano, 5.0; n-butyl, 4.9; chloro, 3.2; formyl, 2.6; methoxy, 1.9; phenyl 1.4.

8,19-Dimethyl-9,13,14,18-tetraethyloxybenziporphyrin coordinates palladium(II) to form the four-coordinate anionic complex [(OBP)Pd^II]-. The NMR data provide evidence for the retention of macrocyclic aromaticity and coordination via a carbon σ-donor. Protonation of the external oxygen atom to give [(HOBP)Pd^II] switches the molecule to a less aromatic phenol-like state, which is manifested by a significant reduction of the macrocyclic ring current. [(AcOBP)Pd^II] and [(TsOBP)Pd^II], two ester derivatives of [(OBP)Pd^II]-, are similar to the protonated species, and their benzenoid character is more pronounced. However, reaction of [(OBP)Pd^II]- with methyl iodide leads to selective methylation of the coordinating C(22) atom to form a novel organopalladium complex (OBPMe)Pd^II. The strong shielding of the inner Me(22) (δ(1H) −2.00 ppm in CDCl3) indicates that the aromaticity of the macrocycle has been retained. At the same time the ¹³C chemical shift of C(22) (44 ppm) shows that the palladium-bound carbon has undergone a drastic hybridization change. Alkylation with n-BuI yields a mixture of the O-substituted [(n-BuOBP)Pd^II] and the C-substituted [(OBP-n-Bu)Pd^II], which confirms the ambident nucleophilicity of [(OBP)Pd^II]-. DFT calculations carried out for six tautomers of oxybenziporphyrin and the 22-methylated palladium species provide further insight into the electronic structure of the ligand and its complexes. Relative energies of the tautomers, increasing in the order [CH,NH,N,NH,O] < [CH,N,NH,N,OH] < [CH2,N,NH,N,O] < [CH2,N,N,N,OH], have been used to estimate the accessibility of four limiting delocalization modes postulated for oxybenziporphyrin and its derivatives. The state of macrocyclic aromaticity observed experimentally in the free base and the phenolic aromaticity of the O-protonated tautomer are the most favorable, and the latter has its energy higher by only 13 kcal/mol. The peculiar bonding situation in (OBPMe)Pd^II, which can be inferred from the NMR data, is also predicted by the DFT methods, which show a strongly distorted tetrahedral environment of C(22).

6,11,16,21-Tetraphenylbenziporphyrin (TPBPH)H, an analogue of tetraphenylporphyrin with one of the pyrrole groups replaced by a benzene ring, is formed in good yield in the condensation of the appropriate precursor with pyrrole and benzaldehyde. (TPBPH)H gives organometallic complexes with palladium(II) and platinum(II), [(TPBP)Pd^II] and [(TPBP)Pt^II], in which the metal ion is bound in the macrocyclic cavity by three pyrrolic nitrogen atoms and a carbon atom of the benzene ring. In the reaction with silver(I) acetate benziporphyrin does not yield a stable complex but undergoes selective acetoxylation at the internal carbon atom. (TPBPH)H is reversibly reduced to 6-benziphlorin and reacts with a water or methanol molecule to give 6-hydroxy- or 6-methoxy-6-benziphlorin, respectively.