Senthilkumar Kabali

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.

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 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.