Electronically Excited States of Protonated Aromatic Hydrocarbons: Phenanthrene and Pyrene

The first and second electronic excited states (S-1 and S-2) of protonated phenanthrene and protonated pyrene, having the pi pi* nature, are strongly red-shifted compared to corresponding electronic transitions in neutral homologues. The CC2 calculations identify an out-of-plane deformation as the most important photochemical reaction coordinate in protonated phenanthrene as well as protonated benzene. It was shown that the excited S-1 states of protonated phenanthrene and protonated benzene are unstable via a torsional motion, which provides a fast access to a S-1-S-0 conical intersection. From the conical intersection, a barrier-less reaction path directs the system back to the minimum of the S-0 potential-energy surface. In contrast to the most stable isomer of protonated phenanthrene, the most stable structure of protonated pyrene shows planar structure in both the S-1 and S-2 excited states, without considerable geometry deformations.