On the Fluorescence Properties and Nonradiative Transitions in Medium-Sized All-Trans Polyenes

The nonadiabatic photodynamics of all-trans linear polyenes with N = 4–8 conjugated double bonds is studied from an electronic structure perspective. Excitation energies and stationary points for the 1B u and 2A g singlet states have been computed by using the state-average complete active space (SA-CASSCF) method and its second-order perturbation theory variant (MS-CASPT2). The dependence of the two low-lying excited states on the “chain length” N has been elucidated. In addition, the 1B u –2A g crossing seam has been mapped out in a suitable two-dimensional coordinate space and its minimum within the subspace has been determined. This minimum is found to increase substantially and monotonously in energy with increasing N. This increase is discussed and interpreted in relation to the fluorescence properties of these systems. In particular, it allows to understand the crossover from S1­(2A g ) fluorescence for smaller N to S2­(1B u ) (or dual) fluorescence for larger N.