Optical absorption and emission properties of rubrene: insight from a combined experimental and theoretical study

Tetracene (Tc) and rubrene (Rub) are two prototype fluorescent molecules. Both molecules exhibit the same 'fluorescent backbone', but due to the additional phenyl groups, the backbone of Rub is twisted, whereas it is planar for Tc. In agreement with earlier investigations, optical spectroscopy of the respective solutions reveals that the S0→S1 transition in Rub is red-shifted with respect to Tc by ∼2000 cm−1 and that Rub exhibits a considerably larger Stokes shift. In order to unravel the physical origin of these differences, we have performed a detailed normal coordinate analysis and frequency calculations using density functional theory (DFT) in conjunction with linear response time-dependent DFT (TD-DFT) energy scan calculations. The calculations yield dimensionless normal coordinate displacements of the excited-state origin that were employed for the calculation of the vibrational finestructure of the absorption and fluorescence spectra of Tc and Rub. The purely theoretical displacements were subsequently refined through fitting to the experimental spectra using the time-dependent theory of electronic spectroscopy. The analysis reveals that the ∼2000 cm−1 red shift of the 0–0 vibronic band of Rub relative to Tc is mainly caused by the inductive effect of the phenyl substituents that leads to destabilization of the donor molecular orbital (MO) (the highest occupied molecular orbital (HOMO)). The large Stokes shift of 820 cm−1 observed for Rub is found to originate mainly from unresolved vibrational progressions involving low-frequency modes that are characterized by appreciable displacements in the excited state. The analysis shows that the spectra of Rub are strongly subject to temperature induced broadening, whereas for Tc this is much less significant.