An explanation of the very low fluorescence and phosphorescence in pyridine: a CASSCF/CASMP2 study

In this work, we applied the multiconfigurational complete active space self-consistent field method and the multiconfigurational second-order perturbation theory CASMP2 to study the fundamental excited states of pyridine and its possible photophysical and photochemical transformations. Our calculations, which are in agreement with the experimental results corresponding to excitations around the 0-0 transition, showed that the very low experimentally observed fluorescence of pyridine is due to the presence of two almost isoenergetic crossings, one of triple character, S-1/T-1/S-0 and the other of S-1/S-0 character. Both crossings are below the minimum of S-1(n pi*) and have a common transition state (S-1(TS)) with a very low energy barrier (1.85 kcal/mol or 0.08 eV at the CASMP2 level of theory) separating them. A third triple crossing of the type S-1/T-1/S-0 lying lower with respect to the other two elucidates the observed T-1 -> S-0 radiationless transition. This explains not only pyridine's very low fluorescence and phosphorescence but also its almost negligible photochemistry, showing that photo-physics is the prevalent process in this molecule. [GRAPHICS] .