Tictoid Expanded Pyridiniums: Assessing Structural, Electrochemical, Electronic, and Photophysical Features

In regard to semirigid donor–spacer–acceptor
(D–S–A)
dyads devised for photoinduced charge separation and built from an
unsaturated spacer, there exists a strategy of design referred to
as “geometrical decoupling” that consists in introducing
an inner-S twist angle approaching 90° to minimize adverse D/A
mutual electronic influence. The present work aims at gaining further
insights into the actual impact of the use of bulky substituents (R)
of the alkyl type on the electronic structure of spacers (S) of the
oligo-p-phenylene type, which can be critical in
the functioning of derived dyads. To this end, a series of 12 novel
expanded pyridiniums (EPs), regarded as model S–A assemblies,
was synthesized and its structural, electronic, and photophysical
properties were investigated at both experimental and theoretical
levels. These EPs result from the combination of 4 types of pyridinium-based
acceptor moieties with the three following types of S subunits connected
at position 4 of the pyridinum core: xylyl (X), xylyl-phenyl (XP),
and xylyl-tolyl (XT). From comparison of collected data with those
already reported for eight other EPs based on the same A components
but linked to S fragments of two other types (i.e., phenyl, P, and
biphenyl, PP), the following quantitative order in regard to the pivotal
S-centered HOMO energy perturbation was derived (sorted by increasing
destabilization): P < X ≪ PP ≈<
XP ≈< XT. This indicates that spacers (S) are primarily
distinguished on the basis of their mono- or biaryl composition and
secondarily by their number of methyl substituents (R). The electron-donating
inductive contribution of methyl substituents (HOMO destabilization)
more than counterbalances the effect of conjugation disruption (HOMO
stabilization). This “compensation effect” suggests
that mildly electron-withdrawing hindering groups are better suited
for “geometrical decoupling”, given that high-energy
S-centered occupied MOs can assist charge recombination within D–S–A
dyads.