Influence of chemical substitution and electronic effects on the triplet state kinetics of xanthene dyes

Abstract

Intersystem crossing rate constants and triplet lifetimes of structurally related xanthene dyes in air-equilibrated ethylene glycol are presented. By using a new time-resolved laser-scanning-microscopy technique the results are obtained with high accuracy. The relationship between the triplet quantum yield and the molecular structure is investigated. Electron withdrawing substituents yield a decrease of the intersystem crossing rate constant. This result is obtained for all investigated amino-group substitution patterns of the xanthene dyes. The introduction of additional branching of the chromophoric system causes an enhancement of the intersystem crossing rate constant by more than a factor of 2. These results cannot be explained by the energy gap rule. The triplet lifetime is in the range of a few microseconds and increases with the relative molecular mass of the dye.