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

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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 S(1) 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 S(1) surface.