Franck–Condon simulation for unraveling vibronic origin in solvent enhanced absorption and fluorescence spectra of rubrene

Abstract

Quantum chemistry calculations at the level of (TD)-DFT plus PCM solvent models are employed for analyzing potential energy surfaces and as a result two local minima with D₂, two local minima with C_2H, and one second-order transition state with D_2H group symmetry are found in both ground S₀ and excited-state S₁ potential energy surfaces. Simulated vibronic coupling distributions indicate that only second-order transition states with D_2H group symmetry are responsible for observed absorption and fluorescence spectra of rubrene and vibrational normal-motions related with atoms on the aromatic backbone are active for vibronic spectra. The Stokes shift 1120 cm⁻¹ (820 cm⁻¹) and vibronic-band peak positions in both absorption and fluorescence spectra in non-polar benzene (polar cyclohexane) solvent are well reproduced within the conventional Franck–Condon simulation. By adding damped oscillator correction to Franck–Condon simulation, solvent enhanced vibronic-band intensities and shapes are well reproduced. Four (three) normal modes with vibration frequency around 1550 cm⁻¹ (1350 cm⁻¹) related to ring wagging plus CC stretching and CH bend motions on the backbone are actually interpreted for solvent enhanced absorption (fluorescence) spectra of rubrene in benzene and cyclohexane solutions.