Femtosecond Fluorescence Upconversion Study of a Naphthalimide–Bithiophene–Triphenylamine Push–Pull Dye in Solution

Abstract : There is a high interest in the development of new push-pull dyes for the use in dye sensitized solar cells. The pronounced charge transfer character of the directly photo-excited state is in principle favorable for a charge injection. Here, we report a time-resolved fluorescence study of a triphenylamine-bithiophene-naphthalimide dye in four solvents of varying polarity using fluorescence upconversion. The recording of femtosecond time-resolved fluorescence spectra corrected for the group velocity dispersion allows for a detailed analysis discriminating between spectral shifts and total intensity decays. After photoexcitation, the directly populated state (S1/FC), evolves towards a relaxed charge transfer state (S1/CT). This S1/CT state is characterized by a lower radiative transition moment and a higher non-radiative quenching. The fast dynamic shift of the fluorescence band is well described by solvation dynamics in polar solvents, but less so in non-polar solvents, hinting that the excited state relaxation process occurs on a free energy surface whose topology is strongly governed by the solvent polarity. Finally, the highly stabilized S1/CT state undergoes a fast non-radiative quenching. This study underlines the influence of the environment on the intramolecular charge transfer (ICT) process, and the necessity to analyze time-resolved data in detail when solvation and ICT occur simultaneously.