title: Contrasting ring-opening propensities in UV-excited α-pyrone and coumarin.
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.
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK. daniel.murdock@bris.ac.uk.
authors.forename: Daniel
authors.initials: D
authors.lastname: Murdock
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK. daniel.murdock@bris.ac.uk.
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Ingle
authors.affiliation: Central Laser Facility, Research Complex at Harwell, Science and Technologies Facilities Council, Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, UK.
authors.forename: Igor V
authors.initials: IV
authors.lastname: Sazanovich
authors.affiliation: Central Laser Facility, Research Complex at Harwell, Science and Technologies Facilities Council, Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, UK.
authors.forename: Ian P
authors.initials: IP
authors.lastname: Clark
authors.affiliation: Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.
authors.forename: Yu
authors.initials: Y
authors.lastname: Harabuchi
authors.affiliation: Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.
authors.forename: Tetsuya
authors.initials: T
authors.lastname: Taketsugu
authors.affiliation: Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Maeda
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK. daniel.murdock@bris.ac.uk.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Orr-Ewing
authors.affiliation: School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK. daniel.murdock@bris.ac.uk.
authors.forename: Michael N R
authors.initials: MN
authors.lastname: Ashfold