‘Excimer’ fluorescence - X. Spectral studies of 9-methyl and 9, 10-dimethyl anthracene

The monomer and excimer fluorescence spectra and quantum yields of solutions of 9-methyl anthracene and 9-10-dimethyl anthracene in several solvents were observed as a function of concentration and temperature (T). The monomer and excimer fluorescence quantum efficiencies, the molar excimer/monomer fluorescence yield K, and the Stern-Volmer concentration quenching parameter $K'$, were determined at room temperature. K was observed as a function of T, and the excimer binding energy B evaluated. At high T, K for a given solute is independent of the solvent, indicating that B, $\Delta S$, the entropy change on excimer formation, and (k$_{fD}$)$_0$/(k$_{fM}$)$_0$, the ratio of the excimer and monomer radiative transition probabilities in vacuo, are solvent-independent molecular properties. It is proposed that the excimer structure of any compound is similar to that of two adjacent molecules in the crystal lattice, with a reduced intermolecular spacing, and that the excimer fluorescence polarization is that of the $^1$L$_a$-$^1$A monomer fluorescence. This model accounts for the different types of photodimerization behaviour in the meso-substituted anthracenes. It is proposed that all concentration quenching in aromatic hydrocarbons is due to the formation of excimers and/or stable photodimers.