Two-dimensional electronic spectroscopy of Betaine-30.

Betaine-30 is well-established as a standard dye for solvatochromism and has long been studied by ultrafast spectroscopy. Electronic excitation leads to rapid intramolecular electron transfer, while the decay of the resulting state corresponds to back electron transfer to the electronic ground state. Thus, Betaine-30's photophysics offers a route to probing the role that vibrational excitation and solvent dynamics play in electron transfer reaction rates. Here, we probe the excited state dynamics of Betaine-30 in two solvents (ethanol and acetonitrile) by means of two-dimensional electronic spectroscopy. Population dynamics in ethanol are measured at two pump wavelengths, and global analysis reveals a wavelength dependence of the electron transfer rate. This is assigned to excitation of distinct ground state conformers, which is confirmed by quantum chemical calculations. "Beatmaps" of coherently excited vibrations are recovered and analyzed in terms of the contribution of Raman active modes in ground and excited states. The contribution of modes in the excited state is a strong function of the rate of the electron transfer reaction.