Symmetry breaking and spectra of diphenyloctatetraene in n-alkanes

Abstract

One- and two-photon excitation spectra, as well as absorption and emission spectra of diphenyloctatetraene (DPOT) in n-alkanes are investigated at low temperatures. For DPOT in n-octane we report on the measurements of one-photon excitation and emission spectra and for DPOT in n-tetradecane (TD) on the measurements of one- and two-photon excitation spectra and emission spectra. The spectra are governed by the transitions between the electronic ground (S0) and the two lowest electronic excited singlet states (S1,S2). The interpretation is based on allowed transitions and transitions induced by the S1–S2 coupling due to Herzberg–Teller promoting modes or due to static lattice-induced distortions of DPOT. A single noncentrosymmetric site is observed for DPOT in n-octane. For DPOT in TD a centrosymmetric and a noncentrosymmetric site are reported. The analysis indicates that there is a dynamical equilibrium in the population of these two sites. The experimental data are quantitatively studied by comparison with simulated spectra. The simulation calculations are based on the coupling between nonadiabatic S1 and S2 states, harmonic modes, and suitable transition moments and line-shape functions. For DPOT in TD the calculations reveal an interesting interference pattern in the vibronic progressions observed in two-photon excitation spectra.