Investigation of excited-state deactivation processes in benzofluorenones using time-resolved transient absorption spectroscopy

Three benzofluorenone derivatives, benzo[a]fluorenone (BFA), benzo[b]fluorenone (BFB), and benzo[c]fluorenone (BFC), were studied spectroscopically in comparison with 9-fluorenone (9-FL) in solvents of various polarities to better understand the solvatochromism and excited-state deactivation processes in these molecules. Time-resolved transient absorption, steady-state absorption, emission, and fluorescence lifetimes were utilized in this endeavor. Differences in the excited-state deactivation processes are observed which can be attributed to the varied characters of the singlet and triplet excited states in these molecules which arise from structural differences and solvatochromism. The main deactivation pathway in all of the studied molecules was internal conversion (IC). Ignoring IC, the competition between intersystem crossing (ISC) and fluorescence is highly dependent on the molecule-solvent system. BFA was found to decay mainly via a triplet state in cyclohexane but via fluorescence in ethyl acetate and acetonitrile. BFB was found to decay via both ISC to a triplet state and emission from the singlet state in cyclohexane but only via ISC in ethyl acetate and acetonitrile. Only emission from the singlet state was observed in BFC in all solvents.