Internal heavy atom effect on the radiative and non-radiative rate constants in xanthene dyes

Abstract

Flourescence quantum yield (ø f), phosphorescence and fluorescence quantum yield ratio (ø p/ø f) and the observed triplet decay time (τ po) of fluorescein and its halogen derivatives have been measured. On halogen substitution an increase in ø p/ø f and a decrease in ø f and τ po is observed in the sequence of Cl, Br, and I. The increase in ø p/ø f is not in the same proportion as the decrease in τ po. Although ø p/ø f is higher for cations than for dianions, it increases more for dianions on halogen substitution. However, the increase in 1/τ po is about 10 3 times more for cations than for dianions. The radiative and non-radiative rate constants viz., k is, k p and k qp have been calculated. On halogen substitution k f remains almost constant, whereas an increase in k p, k is and k qp is observed. For dianions the increase occurs in the order k qp> k is> k p while for cations the order is k qp> k p> k is. The deuterium solvent effect on triplet decay time and intensity is also observed.