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.

Triplet state formation and quenching dynamics of 2-mercaptobenzothiazole in solution

“…The T2 band is located to the lower frequency side of the T1 band, and its decay timescale (~6 ps) is comparable to typical timescales for vibrational relaxation in solution. 2,45 It could therefore be interpreted as deriving from vibrationally hot T1 molecules produced by direct ISC from the S1 state (with  = 0.4 ps). However, the band position at 1364 cm -1 and its shape do not change for at least the first 2 ps, as is demonstrated in the inset to Figure 3(c).…”

Molecular Photofragmentation Dynamics in the Gas and Condensed Phases

“…PESs upon extending one C-O bond, 132 yet TA and TRIR absorption studies following UV excitation in this case reveal essentially 100% parent bleach recovery within 100 ps and no ring-opened products 129,133 -reminding us that the product branching is likely to depend not just on the geometry of, but also the nuclear momenta on passage through, the regions of conical intersection between PESs.…”

“…129 Measurements at longer pump-probe delay times (out to 100 s) identify two further (thermally driven) isomerisations attributable to, first, thermalisation of the initially produced E-isomer population and, second, E- Z-isomerisation (involving net rotation about the C=C double bond) resulting in almost total eventual recovery of the ring-closed parent population. 130,131 The early time nuclear dynamics involve *-state mediated, asymmetric ring-expansion towards regions of conical intersection with the S 0 PES characterised by one much extended R C-O bond length and a non-zero torsional angle between the carbonyl and ketene moieties.…”

“…130,131 The early time nuclear dynamics involve *-state mediated, asymmetric ring-expansion towards regions of conical intersection with the S 0 PES characterised by one much extended R C-O bond length and a non-zero torsional angle between the carbonyl and ketene moieties. 129 Electronic structure calculations for coumarin (benzopyron-2-one) identify what, structurally, appears to be a very similar conical intersection between the * and S 0…”

Probing Photochemically and Thermally Induced Isomerization Reactions in α-Pyrone

“…19 Recent work from our group has used time-resolved infrared (TRIR) spectroscopy to investigate UV-induced ring-opening of several S-and O-containing heterocycles, 20,21 and has attempted to elucidate how the potential energy landscape influences such reactions and resultant quantum yields for ring-opened products. In particular, a recent study on the dynamics of α-pyrone following femtosecond pulsed laser irradiation at 310 nm, indicated that a maximum of 32% of the initially excited molecules were reacting to form photoproducts.…”