A Combined Experimental and Theoretical Study on the Circular Dichroism of Staggered and Eclipsed Forms of Dimethoxy[2.2]‑, [3.2]‑, and [3.3]Pyridinophanes and Their Protonated Forms

The circular dichroisms
(CDs) of dimethoxy[2.2]-, [3.2]-, and [3.3]­pyridinophanes
and their protonated forms were investigated experimentally and theoretically.
Characteristic multisignate Cotton effects (CEs), typical for planar
chiral cyclophane derivatives, were observed. The CD spectral pattern
was quite comparable for the staggered forms of [2.2]-, [3.2]-, and
[3.3]­cyclophanes, but significantly differed for the eclipsed forms.
More interestingly, the patterns resembled, but the CE signs were
practically opposite between staggered and eclipsed [2.2]­pyridinophanes.
Upon protonation, the signs of most CEs were inverted in both forms
of cyclophanes, due to the reversal of dipole moment in the pyridine
against the pyridinium moiety. Such a change in CD spectrum upon protonation
was not apparent in [3.2]­pyridinophane, and the CD spectral behavior
was more complex in [3.3]­pyridinophanes. The variation of CD caused
by the protonation/deprotonation process was temperature-dependent
and hence utilized as a thermal sensor. The protonated forms of the
homologous pyridinophanes with different tether lengths in staggered
and eclipsed forms served as a model system for systematically studying
the cation−π interaction and its effects on chiroptical
properties. A steady increase of electronic interaction became apparent
for the smaller-sized cyclophanes from the increased excitation energy
and electronic coupling element of the charge-transfer (CT) band,
while the observed CE at the CT band was a more complex function of
the original transition dipole of donor/acceptor pair and linker atoms,
as well as the strength of the electronic interaction.