Vibrational energy redistribution in catechol during ultraviolet photolysis

This article reports the striking interplay between the molecular structure and the photodissociation dynamics of catechol (a key dihydroxybenzene), identified using a combination of electronic spectroscopy, hydrogen (Rydberg) atom photofragment translational spectroscopy, density functional theory and second order approximate coupled cluster methods. We describe how the non-planar (C1 symmetry) ← planar (Cs symmetry) geometry change during S1 (11ππ*) ← S0 excitation in catechol, as well as the presence of internal hydrogen bonding, can perturb the photodissociation dynamics relative to that of phenol (a monohydroxybenzene), particularly with respect to O–H bond fission via the lowest dissociative 1πσ* state. For λphot > 270 nm, O–H bond fission (of the non hydrogen bonded hydroxyl moiety) is deduced to proceed via H atom tunnelling from the photo-prepared 11ππ* state into the lowest 1πσ* state of the molecule. The vibrational energy distribution in the resulting catechoxyl product changes notably as λphot is tuned on resonance with either the v′ = 0, m2′ = 1+ or m2′ = 2+ torsional levels of the photo-prepared 11ππ* state: the product state distribution is highly sensitive to the degree of OH torsional excitation (m2) prepared during photo-excitation. It is deduced that such torsional excitation can be redistributed very efficiently into ring puckering (and likely also in-plane ring stretch) vibrations as the molecule tunnels to its repulsive 11πσ* state and dissociates. These observations can be rationalised by consideration of the photo-prepared nuclear wavefunctions. Analysis of the product vibrational energy distribution also reveals that the O–H bond strength of the non hydrogen bonded O–H moiety in catechol, D0(H–catechoxyl) ≤ 27 480 ± 50 cm−1, ∼2500 cm−1 lower than that of the sole O–H bond in bare phenol. As a consequence, the vertical excitation energy of the 11πσ* state in catechol is reduced relative to that in phenol, yielding a particularly broad distribution of product vibrations for λphot < 270 nm. This study highlights the interplay between molecular geometry and redistribution of vibrational energy during ultraviolet photolysis of phenols.