Electroabsorption Spectroscopy of BOPHY Derivatives: Probing Charge-Transfer Character in the Excited State

BOPHYs are excellent fluorophores exhibiting high fluorescence quantum yield, strong visible light absorption, tunable emission colors, controllable electron-accepting properties, and high photostability. These properties make BOPHYs attractive as potential nonfullerene electron acceptors (NFAs). Recent studies on Y6, a benchmark NFA, have collectively suggested that charge-transfer character in the excited state promotes efficient charge generation. Here, the charge-transfer character in the excited state of a series of BOPHYs is investigated using electroabsorption and Stark fluorescence spectroscopies. Methyl-substituted, ester- and phenyl-substituted, and pyridone-fused BOPHYs are comparatively studied. Fluorescence quantum yield was reduced in the presence of electric fields, which was attributed primarily to an enhancement of intramolecular nonradiative deactivation processes. Time-dependent density functional theory (TD-DFT) calculations and analyses of electron density redistribution induced by photoexcitation indicate that molecular symmetry determines the patterns of local charge redistribution and consequently, the magnitude of the net dipole moment change (|μ|). Among the three derivatives, pyridone-fused BOPHY exhibits the largest |μ| upon photoexcitation. These results indicate that the lowest excited state of pyridone-fused BOPHY has substantial intramolecular charge-transfer character. This study provides a basis for the functionalization of BOPHY for future photovoltaic and organic optoelectronic applications in addition to the conventional π-conjugation extension approach.