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

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 ((1)ππ*) 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 ((1)πσ*) state, which is dissociative with respect to the non-hydrogen bonded 'free' O-H bond (labelled O(1)-H). This process occurs via tunnelling under an S1/S2 conical intersection (CI) on a timeframe of 5-11 ps, resulting in O(1)-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.

Bridging the Gap between the Gas Phase and Solution Phase: Solvent Specific Photochemistry in 4-tert-Butylcatechol

“…Studies on substituted phenols have also shown the quite dramatic effect substituents have on the topography of either the S1 or S2 (or both) potential energy surface (PES), resulting in noticeable effects on the S1/S2 tunneling probability and hence S1 lifetime. [17][18]21,45,46 Similar effects on the local topography of the S1 or S2 PESs from a weakly perturbing solvent will undoubtedly have a similar effect on the S1 lifetime as observed here. Whilst the gas-phase studies compare favorably with those in the non-polar cyclohexane, as expected, this is not the case in acetonitrile (r = 37.5).…”

“…Cursory inspection of the 4-TBC + parent transient relative to the corresponding H + transient reveals that the S1 state decays on a similar timescale to the appearance of H-atoms associated with O-H dissociation. Indeed, fitting this transient with a bi-exponential decay function (purple line) returns time constants of g τIVR(H) = 0.4 ± 0.1 ps, associated with rapid intramolecular vibrational energy redistribution (IVR) in the S1 state as seen in our previous investigation, 21 and g τP + = 4.9 ± 0.6 ps, which is assigned to excited state population flux from the S1 state, the P + signifying decay of the non-deuterated isotopologue parent ion signal (and therefore S1 state population). Importantly, this shows a clear correlation to the appearance time of H + .…”

“…+ (and thus H-atom) signal, g τH + = 5.9 ± 0.3 ps, which we attribute to quantum tunneling beneath an S1/S2 conical intersection (CI, vide infra). 21 In addition to the H + transient, Figure 2b also shows a complementary TR-IY trace of the 4-TBC + parent ion signal, as a function of t (purple diamonds), which provides a measure of the dynamics occurring on the photoprepared S1 state following excitation at 267 nm. Cursory inspection of the 4-TBC + parent transient relative to the corresponding H + transient reveals that the S1 state decays on a similar timescale to the appearance of H-atoms associated with O-H dissociation.…”

“…[23][24] However, gaining a complete understanding of its photodynamics is ongoing, with much progress to be made, [25][26][27][28] evidenced by recent work implicating the long-term (after many hours of UVA exposure) phototoxicity of eumelanins. 29 Previous gas-phase studies of catechol 21 have shown that after excitation to the first excited 1 ππ* state (S1), H-atom elimination is observed from dissociation of the non-intramolecular hydrogen bonded 'free' O-H bond, leading to the formation of the catechoxyl radical in approximately 10 ps. Immediately, this suggests that catechol is not photostable following UV exposure, and yet it is a subunit of a photoprotective biomolecule.…”

“…Many other phenol analogues have also been studied in the gas-phase, [17][18][19][20][21] one of which is catechol (1,2-dihydroxybenzene) -a UV chromophore in a range of biomolecules, one of which is eumelanin. As one of three types of melanin, 22 eumelanin is largely responsible for skin's frontline defense to UV exposure.…”