Abstract
The photoisomerisation dynamics following excitation to the S1 electronic state of two structurally related heterocyclic molecules, α-pyrone and coumarin, in acetonitrile solution have been probed by time-resolved vibrational absorption spectroscopy. Following irradiation at 310 nm, α-pyrone relaxes rapidly from its initially excited state, with a quantum yield for parent molecule reformation of 68%. Probing the antisymmetric ketene stretch region between 2100 cm(-1) and 2150 cm(-1) confirms the presence of at least two isomeric ring-opened photoproducts, which are formed highly vibrationally excited and relax on a picosecond timescale. Following vibrational cooling, a secondary, thermally driven, isomerisation is observed with a 1.8(1) ns time constant. In contrast, coumarin reforms the parent molecule with essentially 100% efficiency following excitation at 330 nm. The conical intersections driving the non-radiative relaxation of α-pyrone have been investigated using an automated search algorithm. The two lowest energy conical intersections possess remarkably similar structures to the two energetically accessible conical intersections reported previously for coumarin, suggesting that the differing photochemistry is the result of dynamical effects occurring after passage through these intersections.
Highlights
The photoisomerisation dynamics following excitation to the S1 electronic state of two structurally related heterocyclic molecules, a-pyrone and coumarin, in acetonitrile solution have been probed by time-resolved vibrational absorption spectroscopy
In order to better understand the relationship between molecular structure/conical intersection geometry and ring-opening propensity, we have studied the photoisomerisation dynamics of two structurally related heterocyclic a-carbonyl molecules, a-pyrone (2H-pyran-2-one; 1) and coumarin (1,2-benzopyrone; 2)
In order to explain the different near-UV photochemistry exhibited by a-pyrone and coumarin, the minimum energy conical intersection (MECI) structures linking the S1 and S0 potential energy surfaces in a-pyrone have been explored using an automated procedure[35] combining the seam model function approach[28] with a single component artificial force induced reaction methodology and compared with similar literature data for the coumarin molecule.[29]
Summary
The photoisomerisation dynamics following excitation to the S1 electronic state of two structurally related heterocyclic molecules, a-pyrone and coumarin, in acetonitrile solution have been probed by time-resolved vibrational absorption spectroscopy. Following irradiation at 310 nm, a-pyrone relaxes rapidly from its initially excited state, with a quantum yield for parent molecule reformation of 68%. Probing the antisymmetric ketene stretch region between 2100 cmÀ1 and 2150 cmÀ1 confirms the presence of at least two isomeric ring-opened photoproducts, which are formed highly vibrationally excited and relax on a picosecond timescale. A secondary, thermally driven, isomerisation is observed with a 1.8(1) ns time constant. Coumarin reforms the parent molecule with essentially 100% efficiency following excitation at 330 nm. The two lowest energy conical intersections possess remarkably similar structures to the two energetically accessible conical intersections reported previously for coumarin, suggesting that the differing photochemistry is the result of dynamical effects occurring after passage through these intersections
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