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Issue 18, 2013
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Manipulating dynamics with chemical structure: probing vibrationally-enhanced tunnelling in photoexcited catechol

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Abstract

Ultrafast time-resolved velocity map ion imaging (TR-VMI) and time-resolved ion-yield (TR-IY) methods are utilised to reveal a comprehensive picture of the electronic state relaxation dynamics in photoexcited catechol (1,2-dihydroxybenzene). After excitation to the S1 (1ππ*) state between 280.5 (the S1 origin band, S1(v = 0)) to 243 nm, the population in this state is observed to decay through coupling onto the S2 (1πσ*) state, which is dissociative with respect to the non-hydrogen bonded ‘free’ O–H bond (labelled O1–H). This process occurs via tunnelling under an S1/S2 conical intersection (CI) on a timeframe of 5–11 ps, resulting in O1–H bond fission along S2. Concomitant formation of ground state catechoxyl radicals (C6H5O2(X)), in coincidence with translationally excited H-atoms, occurs over the same timescale as the S1 state population decays. Between 254–237 nm, direct excitation to the S2 state is also observed, manifesting in the ultrafast (∼100 fs) formation of H-atoms with high kinetic energy release. From these measurements we determine that the S1/S2 CI lies ∼3700–5500 cm−1 above the S1(v = 0) level, indicating that the barrier height to tunnelling from S1(v = 0) → S2 is comparable to that observed in the related ‘benchmark’ species phenol (hydroxybenzene). We discuss how a highly ‘vibrationally-enhanced’ tunnelling mechanism is responsible for the two orders of magnitude enhancement to the tunnelling rate in catechol, relative to that previously determined in phenol (>1.2 ns), despite similar barrier heights. This phenomenon is a direct consequence of the non-planar S1 excited state minimum structure (C1 symmetry) in catechol, which in turn yields relaxed symmetry constraints for vibronic coupling from S1(v = 0) → S2 – a scenario which does not exist for phenol. These findings offer an elegant example of how even simple chemical modifications (ortho-hydroxy substitution) to a fundamental, biologically relevant, UV chromophore, such as phenol, can have profound effects on the ensuing excited state dynamics.

Graphical abstract: Manipulating dynamics with chemical structure: probing vibrationally-enhanced tunnelling in photoexcited catechol

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Publication details

The article was received on 05 Feb 2013, accepted on 18 Mar 2013 and first published on 22 Mar 2013


Article type: Paper
DOI: 10.1039/C3CP51108A
Citation: Phys. Chem. Chem. Phys., 2013,15, 6879-6892
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    Manipulating dynamics with chemical structure: probing vibrationally-enhanced tunnelling in photoexcited catechol

    A. S. Chatterley, J. D. Young, D. Townsend, J. M. Żurek, M. J. Paterson, G. M. Roberts and V. G. Stavros, Phys. Chem. Chem. Phys., 2013, 15, 6879
    DOI: 10.1039/C3CP51108A

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