INTERNAL MIXING, PHENYL RING TORSION AND EXCITONIC INTERACTION IN DIPHENYLMETHANE

Author Institution: Department of Chemistry, Purdue University, West Lafayette, IN 47907-2084The close proximity of two identical ultraviolet chromophores render diphenylmethane~(\mbox{DPM}) an interesting case for the study of the dependence of excitonic coupling on the distortion along low-frequency large-amplitude vibrational coordinates, in particular the phenyl ring torsional coordinates present in~\mbox{DPM}. We have studied the fluorescence excitation spectrum and several single vibronic level fluorescence~(\mbox{SVLF}) spectra of the $\tilde{A}^{1}B(\mathrm{S}_{1})$\,$\leftarrow$\,$\tilde{X}^{1}A(\mathrm{S}_{0})$ and the $\tilde{B}^{1}A(\mathrm{S}_{2})$\,$\leftarrow$\,$\tilde{X}^{1}A(\mathrm{S}_{0})$ transition of~\mbox{DPM} cooled in a supersonic jet. The band in the excitation spectrum blue-shifted by~$123\,\mathrm{cm}^{-1}$ from the $\mathrm{S}_{1}$\,$\leftarrow$\,$\mathrm{S}_{0}$ origin was assigned to the $\mathrm{S}_{2}$\,$\leftarrow$\,$\mathrm{S}_{0}$ origin. Its \mbox{SVLF} spectrum shows two-region fluorescence reminiscent of that previously observed in the gas-phase and supersonic jet spectra of naphthalene$^{1}$ and~ovalene$^{2}$. The high-energy region of this $\mathrm{S}_{2}$~$0^{0}$~\mbox{SVLF} spectrum was tentatively assigned to transitions originating in vibrationally excited levels of the $\mathrm{S}_{1}$~state. This finding indicates the presence of efficient internal mixing of the $\mathrm{S}_{2}$~$0^{0}$ level with the sparse manifold of $\mathrm{S}_{1}$ vibronic background levels. The quantum number changes in the non-totally symmetric torsional mode~(see\,figure) upon internal mixing suggest that additional to the internal conversion transitions accounted for by the Jortner-Berry $\Delta v$\,$=$\,$\pm 1$ rule higher order vibronic mechanisms$^{3}$ have to be considered as well. \vspace*{0.2cm} (1) S.~M.~Beck, D.~E.~Powers, J.~B.~Hopkins and R.~E.~Smalley, \textit{J.~Chem. Phys.}, \textbf{1980}, \textit{73}, 2019.\\ (2) A.~Amirav, U.~Even and J.~Jortner, \textit{J.~Chem. Phys.}, \textbf{1981}, \textit{74}, 3745.\\ (3) B.~Scharf, \textit{Chem. Phys.}, \textbf{1975}, \textit{7}, 478