Electronically excited states of protonated aromatic hydrocarbons: phenanthrene and pyrene.

The first and second electronic excited states (S1 and S2) of protonated phenanthrene and protonated pyrene, having the ππ* 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 S1 states of protonated phenanthrene and protonated benzene are unstable via a torsional motion, which provides a fast access to a S1-S0 conical intersection. From the conical intersection, a barrier-less reaction path directs the system back to the minimum of the S0 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 S1 and S2 excited states, without considerable geometry deformations.