Transient ground-state bleaching (GSB), stimulated emission (SE), and excited-state absorbance (ESA) are measured for betaine-30 (B30) in the first excited singlet state (S1) on picosecond time scales. In the viscous solvents glycerol, ethylene glycol, and deep eutectic solvents with intermediate viscosities, SE is seen to the red of the ground-state absorption band and ESA at shorter wavelengths. By contrast, SE is not seen in the nonviscous solvents acetone and ethyl acetate, although both GSB and ESA are prominent. Fluorescence is detectable at room temperature in ethylene glycol and glycerol and increases as the temperature is lowered but is not seen in acetone or ethyl acetate. The absence of SE and fluorescence in nonviscous solvents is consistent with previous suggestions that S1 can relax to a conformation in which radiative transitions to the ground state (S0) are forbidden. Viscous solvents evidently suppress this relaxation. Time-dependent DFT calculations for B30 in various solvents show that the relaxed configuration is strongly twisted. Molecular orbitals, excitation energies, and oscillator strengths are calculated for structures on minimum-energy paths for twisting using six different density functionals, including spin-opposite, range-separated double-hybrid functionals optimized for excited states. As the molecule twists, the oscillator strength for S0-S1 transitions decreases while ESA shifts to shorter wavelengths. Excitation from S0 to S1 has substantial π−π* character in the ground-state conformation but becomes almost entirely a charge-transfer transition as S1 relaxes, accounting for the loss of oscillator strength. The need for solvent rearrangement explains the sensitivity of the relaxation dynamics to viscosity and temperature.
Parson et al. (Mon,) studied this question.