Non-Markovian effects in the radiationless decay of rhodamine 3B+ in water : ethanol mixtures

Abstract

The intramolecular nonradiative decay of the first excited singlet of Rhodamine 3B cation, is studied in the full range of composition of water : ethanol mixtures with different polarities and viscosities. The correlation of experimental data of absorption, emission and rate constants, k_nr, with the dielectric and shear viscosity parameters of these mixtures shows that friction effects determine the reaction rates. These were modelled using the phenomenological dependence of radiationless decay rates which is expressed in terms of Kramers–Grote–Hynes theory. The reactant well wavenumber (ω₀/2πc = 30 cm⁻¹) obtained previously for the activated radiationless decay of Rhodamine 3B in a variety of solvents provides a means to estimate the reaction transmission coefficients (κ) in the mixtures. The trend of variation with the composition reflects the dependence on the viscoelastic response of water : ethanol mixtures. The clustering of ethanol molecules at low ethanol content disrupts the water structure and induces enhanced hydrophobicity around the chromophore which manifests in non-Markovian effects on the reaction rates, k_nr.