Effect of Solvent Perturbation on the S3 → S1 Internal Conversion Efficiency of Benzene, Toluene, and p-Xylene

The $S_3{\mathrm{\hspace{0.17em}\to \hspace{0.17em}S}}_1$ internal conversion efficiency of benzene is found to depend critically on the effectiveness of environmental perturbation in mixing the $S_3$ and $S_1$ states. Evidence for this derives from the experimental correlation of the conversion efficiency with the intensities of the 0–0 absorptive and emissive transitions in solutions with the solvents benzene, methanol, isopropyl alcohol, tetrahydrofuran, ethyl ether, acetonitrile, hexane, cyclohexane, methylcyclohexane, decalin, isooctane, and perfluorinated hexane. Additionally, the importance of $S_3{\mathrm{\hspace{0.17em}-\hspace{0.17em}S}}_1$ mixing is shown to explain the marked increase in the efficiency of internal conversion observed upon methyl substitution of benzene.