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

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.