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

The circular dichroisms (CDs) of planar chiral [2.2]- and [3.3]-pyridinophanes were investigated experimentally and theoretically. Strong multisignate Cotton effects, typical for cyclophane derivatives, were observed. The CD spectra of [2.2]- and [3.3]-paracyclophanes closely resembled in pattern each other, despite the much greater conformational variations in the latter. Upon protonation, both of the cyclophanes suffered dramatic CD spectral changes with accompanying complete sign inversion, which was attributed to the reversal of diploe moment of pyridinium versus pyridine moiety. This chiroptical property switching driven by protonation/deprotonation was temperature-dependent and hence applicable to thermal sensing. The protonated forms of pyridinophanes served as ideal model systems for studying the cation-π interactions and their effects on chiroptical properties. Thus, the molar CD (Δε) of the charge-transfer band of protonated [2.2]pyridinophane was 10-fold larger than that of protonated [3.3]pyridinophane, which exceeds the increased interplane electronic interactions assessed from the electronic coupling element values.