The photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin related to the energy separation of the two lowest-lying singlet excited states

Abstract

The photophysical behavior of 3-chloro-7-methoxy-4-methylcoumarin (ClMMC) was studied as a function of the solvent and temperature. The radiative lifetime ${\tau }_F^0{\text{=1/k}}_F$ is essentially solvent independent and its value $\text{(≈4.6\hspace{0.17em}}\mathrm{ns})$ is totally commensurate with the fluorescence originating from a ${}^1{\mathrm{(\pi ,\pi }}^{*})$ state as the lowest excited state. From the fluorescence data obtained in 24 solvents plus nine solvent mixtures, and the triplet formation quantum yields for three representative solvents, it was found that the internal conversion rate constant ${\mathrm{(k}}_{\mathrm{IC}})$ dictates the photophysical behavior of ClMMC and changes of two orders of magnitude occur from nonpolar to polar solvents. From the temperature dependence (20 to $\text{−100\hspace{0.17em}°}\mathrm{C}$ ) of the fluorescence lifetimes in five solvents it was found that a change of the internal conversion rate constant of the same order of magnitude occured as above. The rate constants and the activation energies for the radiationless processes were determined. The results show that the reason for the dramatic variation of $k_{\mathrm{IC}}$ is the fact that when the $S_2{\mathrm{(n,\pi }}^{*})$ state is close lying to $S_1{\mathrm{(\pi ,\pi }}^{*}\mathrm{),}$ there is a decrease of the activation energy of the internal conversion process ${\mathrm{(S}}_1{\mathrm{\sim \sim \to S}}_0\mathrm{).}$ Increasing solvent polarity increases the energy gap between these states, and decreases the magnitude of the effect. Decreasing the temperature to sufficiently low values, disables the deactivation channel. The interpretation of the foregoing results can be satisfied by either a model involving a fast equilibrium between two close lying $S_2$ and $S_1$ states or in terms of the so-called “proximity effect.”