Resonance-Enhanced Charge Delocalization in Carbazole–Oligoyne–Oxadiazole Conjugates

There
are notably few literature reports of electron donor–acceptor
oligoynes, even though they offer unique opportunities for studying
charge transport through “all-carbon” molecular bridges.
In this context, the current study focuses on a series of carbazole−(CC)n−2,5-diphenyl−1,3,4-oxadiazoles
(n = 1–4) as conjugated π-systems in
general and explores their photophysical properties in particular.
Contrary to the behavior of typical electron donor–acceptor
systems, for these oligoynes, the rates of charge recombination after
photoexcitation increase with increasing electron donor–acceptor
distance. To elucidate this unusual performance, we conducted detailed
photophysical and time-dependent density functional theory investigations.
Significant delocalization of the molecular orbitals along the bridge
indicates that the bridging states come into resonance with either
the electron donor or acceptor, thereby accelerating the charge transfer.
Moreover, the calculated bond lengths reveal a reduction in bond-length
alternation upon photoexcitation, indicating significant cumulenic
character of the bridge in the excited state. In short, strong vibronic
coupling between the electron-donating N-arylcarbazoles
and the electron-accepting 1,3,4-oxadiazoles accelerates the charge
recombination as the oligoyne becomes longer.