The influence of internal charge transfer on nonradiative decay in substituted terthiophenes

Photophysical data for a series of end substitued 3',4'-dibutyl-2,2':5',2''-terthiophenes are reported. Static absorption and fluorescence, quantum yields, time-resolved fluorescence, and time- and frequency-resolved pump-probe spectra are applied to investigate excited state relaxation in bromo, nitro, and tricyanovinyl substituted species in a variety of solvents. The effect of solvent polarizability and end-group substitution is discussed in the context of charge transfer in the excited state and its impact on nonradiative decay rates. In solution at room temperature, both symmetric and asymmetric addition of electron withdrawing end groups generate an excited state with substantial charge transfer character. Solvent polarizability has a significant influence on the excited state dynamics in the charge transfer compounds. Examples include a 20-fold reduction in the intersystem crossing rate going from hexane to toluene and an order of magnitude increase in the internal conversion rate between toluene and acetone. The results demonstrate that the impact of the substituents on intramolecular charge transfer, and the resulting amplification of the interactions between the excited state(s) and the local molecular environment, can dramatically change the excited state relaxation dynamics in substituted terthiophenes.