‘Excimer’ fluorescence VII. Spectral studies of naphthalene and its derivatives

The fluorescence spectra of solutions of naphthalene and fourteen of its methyl, ethyl, dimethyl and trimethyl derivatives were observed as a function of molar concentration c at 20 $^\circ$C and as a function of temperature from 20 to -70 $^\circ$C. All the compounds show similar excimer fluorescence characteristics with mean values of the excimer binding energy B = 0.28 $\pm$ 0.03 eV and of the excimer interaction energy $V'_m$ = 0.765 $\pm$ 0.03 eV, with the exception of 1,8-dimethyl naphthalene (B = 0.14 eV) in which excimer formation is sterically hindered. The monomer fluorescence quantum efficiencies and ionization potentials were also determined. The fluorescence spectra of liquid 1,6-dimethyl naphthalene and its solutions in three solvents were observed, and the rate parameters and the entropy change $\Delta S$ on excimer formation were determined, B and $\Delta S$ (=-19.1 cal mole$^{-1}$ deg$^{-1}$) have similar values in the pure liquid and in cis-decalin and dilute (c $\leqslant$ 1 M) cyclohexane solutions, but slight differences occur in n-heptane and concentrated (c > 1 M) cyclohexane solutions.