Activationless nonradiative decay in rhodamines: Role of NH and lower frequency vibrations in solvent kinetic isotope effects

The solvent kinetic isotope effect in the nonactivated rate constants of Rhodamine 123 and Rhodamine B is discussed in water-H2, relatively to water-D2. In their mixtures the rate constants of the radiationless process in Rhodamine 123 decrease with the deuterium atomic content in the mixture. In water-H2 the process shows activation energy of the order of thermal fluctuations while in water-D2 it is activationless. Batochromic spectral shifts of the electronic absorption and emission are observed in water-H2, relatively to water-D2, and are of the order of the activation energy of nonradiative decay rate constants. The changes cannot be accounted for only in terms of dielectric contributions from solvent mode coupling to the Franck-Condon transitions. The variations are correlated with the molecular structural pattern of rigidified rhodamines and compared considering the energy release efficiency of vibrational donor modes to external acceptor modes. NH stretching vibrations are an order of magnitude more efficient than CH towards the dissipation of the electronic excitation energy of the first excited singlet-state of rhodamines and lower frequency vibrations also contribute.