Direct versus Indirect H Atom Elimination from Photoexcited Phenol Molecules Azhar Iqbal, Lara-Jane Pegg, and Vasilios G. Stavros* Department of Chemistry, UniVersity of Warwick, Gibbet Hill Road, CoVentry CV4 7AL, U.K. ReceiVed: March 12, 2008; ReVised Manuscript ReceiVed: April 19, 2008 The active role of the optically dark πσ* state, following UV absorption, has been implicated in the photochemistry of a number of biomolecules. This work focuses on the role of the πσ* state in the photochemistry of phenol upon excitation at 200 nm. By probing the neutral hydrogen following UV excitation, we show that hydrogen elimination along the dissociative πσ* potential energy surface occurs within 103 ( 30 fs, indicating efficient coupling at the S 1 /S 2 and S 0 /S 2 conical intersections, with no identifiable role of statistical unimolecular decay of vibronically excited (S 0 ) phenol in the timeframe of our measurements. Introduction Aromatic amino acids like tyrosine, tryptophan, and pheny- lalanine have very large UV-absorption cross sections; however, the fluorescence quantum yields of these molecules are very small. This is an indication of efficient nonradiative processes, which effectively quench the fluorescence. 1–3 These nonradiative processes must be very fast in order to compete effectively with and overcome the fluorescence pathway. In such systems as those described above, it seems natural to study the chromophore of the biomolecule itself. Phenol, which is the chromophore of the amino acid tyrosine, shows similar low fluorescence quantum yields. 4 Recent ab initio calculations by Sobolewski, Domcke and co-workers 5–7 have shown that the low fluorescence quantum yield of phenol following excitation at the wavelength of interest here (200 nm) is primarily due to an excited singlet state of πσ* character, which is dissociative with respect to the stretching coordinate of O-H bond. This dissociative state (S 2 ) bisects both the optically bright ππ* state (S 1 ) and the ground state (S 0 ) through two successive conical intersections (CI, S 1 /S 2 and S 0 /S 2 , respectively), leading to elimination of neutral hydrogen (see Figure 1 in Nix et al. 8 ). The absorption of UV photons below 248 nm corresponds to the photoexcitation of phenol above the S 1 /S 2 CI. Generally, the optically dark S 2 state cannot be excited directly. Population from optically bright states such as S 1 can be transferred to the S 2 state through the S 1 /S 2 CI. Once on the S 2 state, the excited phenol evolves towards the S 0 /S 2 CI and can undergo two photochemical fates. The first is to continue its passage through the CI and dissociate directly. Alternatively, highly excited ground-state phenol may be formed which, following energy dissipation into the correct vibronic mode, i.e., O-H, can also lead to dissociation. The former and the latter pathways are referred to as direct dissociation and statistical unimolecular decay. 8 The dynamics of H atom and proton transfer have been heavily studied in phenol-ammonia clusters. 9–11 Pino et al. 12,13 first suggested that excited state hydrogen transfer could be used to explain the decrease in [PhOH-(NH 3 ) n ] + and the concurrent increase in the [NH 4 (NH 3 ) n-1 ] + signal. According to Pino et al. and later confirmed by Ishiuchi et al., 14 when using time- resolved ion-dip experiments, the initially excited optically bright ππ* (S 1 ) state couples to the optically dark πσ* (S 2 ) state through a CI. The decay of the phenol cluster can then be explained by tunnelling through a barrier along the O-H coordinate. Unlike in the bare phenol, the reorganization of the electronic levels due to clustering results in the S 2 state not intersecting the ground electronic state (S 0 ); that is, there is no S 0 /S 2 CI. The direct observation of H atom detachment driven through the S 2 state of phenol was first reported by Tseng et al., 15–17 and then by Nix et al., 8,18 by using multimass ion imaging and total kinetic energy release (TKER) measurement, respectively, although so far, no time-resolved measurements probing the absolute timescales of the direct and statistically unimolecular decay pathways of these two processes have been reported. This provides the driving forces for the work presented in this paper. In their work, Nix et al. 8 report how at the highest energies in their excitation, which corresponds to approximately the same region of excitation as that described in this work, they observed two primary peaks in the H atom kinetic energy release. They attributed the peak corresponding to the highest kinetic energy to direct dissociation. They proposed that the low-energy peak could be attributed to statistical unimolecular decay of highly excited phenol (S 0 ) molecules formed because of coupling at the S 0 /S 2 CI. A similar observation was also made by Tseng et al. 15–17 To the best of our knowledge, the only direct demonstration of time-resolved H atom elimination in such systems has been reported in pyrrole via the excited πσ* state by Lippert et al. 19 by using (2+1) resonance-enhanced multiphoton ionization (REMPI) by femtosecond laser pulses at 243.1 nm following photoexcitation at 250 nm in a pump-probe setup. Interestingly, Lippert et al. report two timescales for the dissociation. They attribute the first to direct dissociation, with a measured timescale of 0.11 ps. They measured the second pathway, due to indirect or statistical unimolecular decay, to occur in 1.1 ps. If indeed highly excited phenol molecules are undergoing a statistical unimolecular decay process, this should manifest itself in a similar two-step process in phenol. This article reports the results of a two-color pump-probe experiment with femtosec- ond laser pulses. The phenol molecules were excited at 200 nm, and the photoproducts were probed by (2+1) REMPI at 243.1 nm with TOF-MS detection. The results presented strongly implicate that a single pathway to dissociation is operative on the timescale of the experiment. The measured † Part of the “Stephen R. Leone Festschrift”. * Author to whom correspondence should be addressed. E-mail: v.stavros@warwick.ac.uk. J. Phys. Chem. A 2008, 112, 9531–9534 9531 10.1021/jp802155b CCC: $40.75  2008 American Chemical Society Published on Web 06/10/2008