A comparative photophysical study on the structurally related coumarin 102 and coumarin 153 dyes

Abstract

Photophysical properties of two 7-aminocoumarin dyes, namely, coumarin 102 (C102) and coumarin 153 (C153), have been investigated in different aprotic solvents to understand their polarity dependent behaviors. Both the dyes have the same closed-ring amine (julolidyl) substituent at 7-th position of their 1,2-benzopyrone moiety, but differ in regard to their 4-CH3 and 4-CF3 substituents, respectively. For both the dyes, absorption and fluorescence spectra undergo red shift, peak energies of these transitions decrease and the Stokes’ shifts follow linear correlations with the increasing solvent polarity parameter Δf, as defined by Lippert-Mataga. While fluorescence quantum yield (Φf) and lifetime values (τf) of C102 dye undergo linear changes with Δf, both these parameters for C153 display quite unusual deviations from linearity in the lower polarity solvents. For C102, the estimated fluorescence decay rates (kf) are significantly higher than C153 dye. The nonradiative decay rates (knr) are also substantially higher for C102 dye than C153 in most of the solvents. Interestingly, while kf values for both the dyes change linearly with Δf, the knr values show complete linearity with Δf for C102 dye only, but these values for C153 dye show quite substantial enhancements and an unusual positive deviation from linearity in lower polarity solvent. Quantum chemical studies indicate that for C153 dye in lower polarity solvents, especially, there is a possible involvement of intersystem crossing process, which might induce an additional nonradiative channel (kadd-nr) for the dye. Further, though we could not obtain any direct evidence in the present study, yet it is felt that some kind of fluorous effect, as might be arising due to large negative charge density on bulky perfluomethyl group of C153 dye, can also introduce additional kadd-nr, specifically in the nonpolar solvents, for which further investigations are continuing in our group.