Protonation-Induced Sign Inversion of the Cotton Effects of Pyridinophanes. A Combined Experimental and Theoretical Study

Long-range Through-space Charge Transfer in a pH-responsive Mixed Cyclophane of Pyridine and Teropyrene

A large, strained (SE=44.2 kcal/mol) and conformationally flexible mixed cyclophane of pyridine and teropyrene was synthesized using two intramolecular Wurtz coupling reactions and an unprecedented Scholl reaction between the unreactive 2 positions of the pyrene systems in a triply bridged pyrenophane. Protonation of the pyridine unit results in a greatly enhanced preference for nesting in the cavity of the highly bent teropyrene system (θcalc =162.6°) and emergence of a charge transfer absorption band (λmax =592 nm) due to a long range (5.0-5.5 Å), through-space intramolecular transition between the teropyrene and pyridinium units, which does not exist in the neutral cyclophane.

Solvent and Temperature Effects on Dynamics and Chiroptical Properties of Propeller Chirality and Toroidal Interaction of Hexaarylbenzenes

Because of the unique interaction of radial aromatic blades, propeller-shaped hexaarylbenzenes (HABs) attract much research interest and find various practical applications. By introducing a small point-chiral group at the tip of aromatic blade(s), HAB becomes propeller-chiral to exhibit strong Cotton effects. Because of the dynamic nature of propeller chirality, the chiroptical properties of HAB critically responded to minute changes in the environment. Using a series of chiral HABs with one to six ( R)-1-methylpropyloxy substituent(s) introduced at the blade tip, we elucidated how the smallest chiral auxiliary at the HAB periphery progressively and cooperatively boosts the overall chiroptical properties and also how subtle changes in temperature and solvent structure affect the propeller dynamics and thus the chiroptical responses. The unique features of propeller-chiral HABs further enabled us to switch on/off their circularly polarized luminescence.

Circular Dichroisms of Mono- and Dibromo[2.2]paracyclophanes: A Combined Experimental and Theoretical Study

Circular dichroisms (CDs) of planar chiral 4-bromo[2.2]paracyclophane (1) and three isomeric dibromo[2.2]paracyclophanes (p-2, m'-2, and o'-2) were investigated experimentally and theoretically. They all exhibited strong multisignate Cotton effects (CEs) at the ¹L_b, ¹L_a, and ¹B transitions of the component (bromo)benzene chromophore and were comparable to each other. For all of the cyclophanes examined, the enantiomer that eluted earlier from a chiral high-performance liquid chromatography column (Chiralcel IA or IB) exhibited negative and positive CEs at the ¹L_b and ¹L_a bands, respectively, which were followed by a more complicated pattern of CDs at the higher-energy bands. These CD features were well reproduced by quantum chemical calculations, allowing us to unambiguously assign the absolute configurations of the first-eluted enantiomers as R _p in all of the cases examined. Interestingly, the CDs of 1 and 2, although largely comparable in shape, were still sensitive to the number and pattern of bromine substitution, showing closer resemblance between m'-2 and o'-2 and between p-2 and 1. The theoretical calculations also reproduced successfully these spectral resemblance between them. The anisotropy (g) factors for the ¹L_b bands of these cyclophanes were considerably large (∼10^-2), whereas those for the ¹L_a band were conventional in the order of 10^-3. In addition, a weak CE was observed in the low-energy region at around 320 nm, which turned out to originate from the interplanar interaction and is hence assigned to the "cyclophane band". The experimental g factors of this band were fairly large in the order of 10^-2, but the computation turned out to be quite challenging and were less well reproduced theoretically, ascribable to the forbidden nature of the transition.

Chiroptical properties of dithia[3.3]cyclophanes composed of anthracene and pyridine/pyridinium moieties: A combined experimental and theoretical study

Circular dichroisms (CDs) of neutral and protonated [3.3]anthracenopyridinophane (1 and 1-H⁺ ) were investigated experimentally and theoretically. Introducing an anthracene moiety with extended conjugation affected the cyclophane structure with the bent angles being appreciably  reduced from those of parent [3.3]pyridinophane. The Cotton effects (CEs) observed at the ¹ B_b band for both 1 and 1-H⁺ were fairly strong and apparently bisignate, which, however, turned out not to be a simple exciton couplet but to be composed of multiple transitions. In contrast, the CEs were much weaker in the ¹ L_a band region. The spectral changes upon protonation were less significant compared with the parent pyridinophane, being dominated by the local transitions of anthracene. Nevertheless, the CD spectra of 1 and 1-H⁺ were well reproduced by theoretical calculations to allow us an unambiguous absolute configuration determination of the first high-performance liquid chromatography (HPLC) elute (from Chiralcel IB column) as S_p . The transannular interactions between the anthracene and pyridine/pyridinium units were examined by UV-vis and fluorescence spectroscopy to reveal a charge-transfer (CT) band in the low-energy region, particularly for 1-H⁺ . Despite the comparable CT interactions, the CE at the CT band was much stronger for the anthracenopyridinophane than for the parent pyridinophane, affording an anisotropy (g) factor as large as 4 × 10^-3 .