Theoretical Studyof Spectroscopy and Photodynamicsof Decatetraene as a Representative Dimethylated Polyene

The photodynamics and UV spectroscopy of decatetraene following excitation to the bright 1B_u state are studied theoretically, based on ab initio computations of the underlying potential energy (PE) surfaces. Both photophysical and photochemical aspects are investigated. The former involves smaller amplitude displacements, and – in addition to determining multidimensional PE surfaces – also a quantal treatment of the ensuing nuclear dynamics. The inclusion of the 1B_u–2A_g vibronic interaction allows to compute the vibrational structure of the 1A_g–1B_u UV spectral band and the femtosecond 1B_u–2A_g internal conversion (population transfer). The results are compared with analogous features of octatetraene and octatriene. The photochemical aspects involving larger-amplitude displacements are investigated from a quantum-chemical point of view, focusing on the stationary points and the seams of conical intersections that are involved. A comparison of decatetraene with octatetraene reveals the contrasting features of their 2A_g and 1B_u state minima, where the latter is more stable in the dimethylated system. The small barrier connecting these two states lies between 0.06 and 0.11 eV. The nonradiative decay channels originating from these minima are characterized by comparatively higher barriers in decatetraene and influence the outcome of the radiative processes in a different manner in comparison to that of octatetraene.