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

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