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.