Is There Symmetry Breaking in the First Excited Singlet State of 2-Pyridone Dimer?

We investigate the S-1 state potential energy surface of 2-pyridone dimer (2PY)(2) using time-dependent density functional and coupled cluster theory. Although the ground and S-2 excited states of (2PY)(2) have C-2h symmetry, the S-1 state shows symmetry breaking and localization of the excitation on one of the two monomers upon relaxation of the geometry. This localization is rationalized using a simple diabatic curve crossing model. As a consequence of the symmetry breaking, S-1 to S-0 transitions become optically allowed. We hypothesize that the band at 30 776 cm(-1) observed in the excitation spectrum of (2PY)(2) might be attributed to the S-1 state rather than the S-2 state; the S-2 state origin is predicted 3000-4000 cm(-1) above the S-1 state by hybrid density functional and coupled cluster methods. Asymmetric transfer of one hydrogen atom leads to a second Si state minimum that can rapidly decay to the ground state. This suggests that photoinduced tautomerization of (2PY)(2) occurs in a stepwise fashion, with only one hydrogen transfer taking place on the Si surface.