Jamie D. Young

Direct Observation of Hydrogen Tunneling Dynamics in Photoexcited Phenol

The excited-state dynamics of phenol following ultraviolet (UV) irradiation have received considerable interest in recent years, most notably because they can provide a model for understanding the UV-induced dynamics of the aromatic amino acid tyrosine. Despite this, there has been some debate as to whether hydrogen tunneling dynamics play a significant role in phenols excited-state O-H bond fission when UV excitation occurs below the (1)ππ*/(1)πσ* conical intersection (CI). In this Letter, we present direct evidence that (1)πσ*-mediated O-H bond fission below the (1)ππ*/(1)πσ* CI proceeds exclusively through hydrogen tunneling dynamics. The observation of hydrogen tunneling may have some parallels with proton tunneling dynamics from tyrosine residues (along the O-H bond of the phenol moiety) in a wide range of natural enzymes, potentially adding further justification for utilizing phenols as model systems for investigating tyrosine-based dynamics.

Unraveling Ultrafast Dynamics in Photoexcited Aniline

A combination of ultrafast time-resolved velocity map imaging (TR-VMI) methods and complete active space self-consistent field (CASSCF) ab initio calculations are implemented to investigate the electronic excited-state dynamics in aniline (aminobenzene), with a perspective for modeling (1)πσ* mediated dynamics along the amino moiety in the purine derived DNA bases. This synergy between experiment and theory has enabled a comprehensive picture of the photochemical pathways/conical intersections (CIs), which govern the dynamics in aniline, to be established over a wide range of excitation wavelengths. TR-VMI studies following excitation to the lowest-lying (1)ππ* state (1(1)ππ*) with a broadband femtosecond laser pulse, centered at wavelengths longer than 250 nm (4.97 eV), do not generate any measurable signature for (1)πσ* driven N-H bond fission on the amino group. Between wavelengths of 250 and >240 nm (<5.17 eV), coupling from 1(1)ππ* onto the (1)πσ* state at a 1(1)ππ*/(1)πσ* CI facilitates ultrafast nonadiabatic N-H bond fission through a (1)πσ*/S(0) CI in <1 ps, a notion supported by CASSCF results. For excitation to the higher lying 2(1)ππ* state, calculations reveal a near barrierless pathway for CI coupling between the 2(1)ππ* and 1(1)ππ* states, enabling the excited-state population to evolve through a rapid sequential 2(1)ππ* → 1(1)ππ* → (1)πσ* → N-H fission mechanism, which we observe to take place in 155 ± 30 fs at 240 nm. We also postulate that an analogous cascade of CI couplings facilitates N-H bond scission along the (1)πσ* state in 170 ± 20 fs, following 200 nm (6.21 eV) excitation to the 3(1)ππ* surface. Particularly illuminating is the fact that a number of the CASSCF calculated CI geometries in aniline bear an exceptional resemblance with previously calculated CIs and potential energy profiles along the amino moiety in guanine, strongly suggesting that the results here may act as an excellent grounding for better understanding (1)πσ* driven dynamics in this ubiquitous genetic building block.

Probing ultrafast dynamics in photoexcited pyrrole: timescales for 1πσ* mediated H-atom elimination

The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250–200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 11πσ* (1A2) state; and (ii) initial photoexcitation to the higher energy 1ππ* (1B2) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 11πσ* (1A2) at 250 nm, we observe a single time-constant of 126 ± 28 fs for N–H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 11πσ* (1A2) surface in the vertical Franck–Condon region, followed by non-adiabatic coupling through a 11πσ*/S0 conical intersection to yield pyrrolyl radicals in their electronic ground state (C4H4N()) together with H-atoms. At 238 nm, direct excitation to, and N–H dissociation along, the 11πσ* (1A2) surface is observed to occur with a time-constant of 46 ± 22 fs. Upon initial population of the 1ππ* (1B2) state at 200 nm, a rapid 1ππ* (1B2) → 11πσ* (1A2) → N–H fission process takes place within 52 ± 12 fs. In addition to ultrafast N–H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S0 ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 ± 0.4 ns. Analogous measurements on pyrrole-d1 reveal that these statistical H-atoms are released only through C–H bond cleavage.

Ultrafast excited-state dynamics of 2,4-dimethylpyrrole.

The dynamics of photoexcited 2,4-dimethylpyrrole (DMP) were studied using time-resolved velocity map imaging spectroscopy over a range of photoexcitation wavelengths (276-238 nm). Two dominant H atom elimination channels were inferred from the time-resolved total kinetic energy release spectra, one which occurs with a time constant of ∼120 fs producing H atoms with high kinetic energies centered around 5000-7000 cm(-1) and a second channel with a time constant of ∼3.5 ps producing H atoms with low kinetic energies centered around 2500-3000 cm(-1). The first of these channels is attributed to direct excitation from the ground electronic state (S0) to the dissociative 1(1)πσ* state (S1) and subsequent N-H bond fission, moderated by a reaction barrier in the N-H stretch coordinate. In contrast to analogous measurements in pyrrole (Roberts et al. Faraday Discuss. 2013, 163, 95-116), the N-H dissociation times are invariant with photoexcitation wavelength, implying a relatively flatter potential in the vertical Franck-Condon region of the 1(1)πσ* state of DMP. The origins of the second channel are less clear-cut, but given the picosecond time constant for this process, we posit that this channel is indirect and is likely a consequence of populating higher-lying electronic states [e.g., 2(1)πσ* (S2)] which, following vibronic coupling into lower-lying intermediary states (namely, S1 or S0), leads to prompt N-H bond fission.

Relaxation dynamics of photoexcited resorcinol: internal conversion versus H atom tunnelling

The excited state dynamics of resorcinol (1,3-dihydroxybenzene) following UV excitation at a range of pump wavelengths, 278 ≥ λ ≥ 255 nm, have been investigated using a combination of time-resolved velocity map ion imaging and ultrafast time-resolved ion yield measurements coupled with complementary ab initio calculations. After excitation to the 1(1)ππ* state we extract a timescale, τ1, for excited state relaxation that decreases as a function of excitation energy from 2.70 ns to ~120 ps. This is assigned to competing relaxation mechanisms. Tunnelling beneath the 1(1)ππ*/(1)πσ* conical intersection, followed by coupling onto the dissociative (1)πσ* state, yields H atoms born with high kinetic energy (~5000 cm(-1)). This mechanism is in competition with an internal conversion process that is able to transfer population from the photoexcited 1(1)ππ* state back to a vibrationally excited ground state, S0*. When exciting between 264-260 nm a second decay component, τ2, is observed and we put forth several possible explanations as to the origins of τ2, including conformer specific dynamics. Excitation with 237 nm light (above the 1(1)ππ*/(1)πσ* conical intersection) yields high kinetic energy H atoms (~11,000 cm(-1)) produced in ~260 fs, in line with a mechanism involving ultrafast coupling between the 1(1)ππ* (or 2(1)ππ*) and (1)πσ* state followed by dissociation. The results presented highlight the profound effect the presence of additional functional groups, and more specifically the precise location of the functional groups, can have on the excited state dynamics of model heteroaromatic systems following UV excitation.