From Fundamental Photophysics to Photocatalysis: Energy Gap Law Analysis of Anion Radical Excited States

Excited-state ion radicals have recently attracted attention as powerful reductants or oxidants for electron-transfer photocatalysis. A key challenge in their application as photocatalysts is the short lifetime of their excited states, which directly impacts the efficiency of bimolecular photoinduced electron transfer. Here, we present a comprehensive photophysical study aimed at elucidating the factors that govern the lifetime of the doublet excited state in a series of 10 anion radicals derived from structurally related π-conjugated donor-acceptor-donor (DAD) molecules. In these systems, the acceptor is a 2,1,3-benzothiadiazole (BTD) unit flanked by arylene or oligo-(arylene) groups. The anion radicals, generated by chemical reduction in deoxygenated DMF, are stable and were characterized using UV-visible-near-IR absorption/emission spectroscopy and femtosecond transient absorption spectroscopy. Remarkably, the doublet excited state lifetimes vary by nearly 200-fold across the series, ranging from 2.9 to 565 ps. Nonradiative decay rates increase as the excited-state energy decreases, in a manner that is quantitatively consistent with the energy gap law for nonradiative decay. Faster decay is observed for anion radicals with highly delocalized π-systems, reflecting their lower excited-state energies. Finally, we demonstrate the catalytic utility of DAD anion radicals with long-lived doublet excited states in a consecutive photoinduced electron transfer (ConPET) process, using the photodebromination of 4-bromoacetophenone as a model reaction.