The Solvent-Dependent Photophysics of Diphenyloctatetraene

Despite many decades of study, the excited state photophysics of polyenes remains controversial. In diphenylpolyenes with conjugated backbones that contain between 2 and 4 double carbon-carbon bonds, the first two excited electronic states are nearly degenerate but of entirely different character, and their energy splitting is strongly dependent on solvent polarizability. To examine the interplay between these different states, steady-state and time-resolved fluorescence spectroscopies were used to undertake a comprehensive investigation of diphenylocatetraene's (DPO) excited state dynamics in 10 solvents of different polarizabilities and polarities, ranging from weakly interacting alkanes to polar hydrogen-bonding alcohols. These data revealed that photopreparation of the optically bright 1B_u state resulted in fast (<170 ps) internal conversion to the lower-lying optically dark 2A_g state. The 2A_g state is responsible for almost all the observed DPO fluorescence and gains oscillator strength via vibronic intensity stealing with the near-degenerate 1B_u state. The fluorescence lifetime associated with the 2A_g state decayed monoexponentially (4.2-7.2 ns) in contrast to prior biexponential decay kinetics reported for similar polyenes, diphenylbutadiene and diphenylhexatriene. An analysis combining the measured fluorescence lifetimes and fluorescence quantum yields (the latter varying between 7 and 21%) allowed for a 190 cm^-1 Herzberg-Teller vibronic coupling constant between the 1B_u and 2A_g states to be determined. The analysis also revealed that the ordering of electronic states remains constant in all the solvents studied, with the 2A_g state minimum always lower in energy than that of the 1B_u state, thus making it a relatively simple polyene compared to structurally similar diphenylhexatriene.