Femtosecond Dynamics of Pyridine in the Condensed Phase: Valence Isomerization by Conical Intersections

In this work, we combined femtosecond transient absorption (population and anisotropy) spectroscopy with ab initio electronic structure methods to study excited-state deactivation pathways of the pyridine molecule in liquid solutions. Studies of the effects of excitation energy, deuteration and substitution, solvent polarity and viscosity, and protonation of pyridine were performed. The experiments reveal the dynamics of S_1(nπ^*) and S_2(ππ^*) excited states of pyridine. The photoexcitation of the S_2(ππ^*) state leads to formation of the prefulvenic form of pyridine, a valence isomer, in ∼2.2 ps, while nonradiative deactivation of the S_1(nπ^*) state occurs in 9−23 ps and is to a large extent due to intersystem crossing. Using ab initio methods at the CASSCF and time-dependent DFT levels, we calculated the potential energy surfaces of the ground and S_2(ππ^*) states. A conical intersection was found responsible for the ultrafast deactivation of the pyridine molecule