Charge Transfer and Solvation of Betaine-30 in Polar SolventsA Femtosecond Broadband Transient Absorption Study

Betaine-30 in polar solvents was studied with the pump−supercontinuum probe technique after ca. 50 fs excitation at 532 and 634 nm. By monitoring the spectral evolution of stimulated emission, excited-state absorption, and bleaching, photoinduced solvation and ultrafast intramolecular rearrangement in the excited state are resolved for the first time. This rearrangement is related to intramolecular electron transfer (iET). In acetonitrile, the rate coefficient k of iET satisfies the condition of solvent control, k = 1/τ 0 (±10%), where τ 0 is a characteristic solvent relaxation time deduced from the solvation correlation function C ( t ). Back ET or internal conversion proceeds from a molecular conformation which differs from that in the ground state. It develops on a picosecond time scale and is followed by cooling of hot betaine molecules by the solvent. By comparing transient Stokes shift data with C ( t ), we extract solvation and intramolecular reorganization energies associated with high and low frequency optically active modes. The solvatochromism of betaine-30 contains substantial solvent-dependent intramolecular contributions and, therefore, cannot be understood without accounting for internal degrees of freedom.