Hydrogen-Bonding Motifs and Proton-Transfer Dynamics in Electronically Excited 6-Hydroxy-2-formylfulvene.

To elucidate low-barrier hydrogen-bonding (LBHBing) motifs and their ramifications for hydron-migration dynamics, the Ã1B2-X̃1A1 (π* ← π) absorption system of 6-hydroxy-2-formylfulvene (HFF) and its monodeuterated isotopolog (HFF-d) has been probed under free-jet expansion conditions through synergistic application of fluorescence-based laser spectroscopy and quantum-chemical calculations. Neither the donor-acceptor distance nor the proton-transfer barrier is predicted to change markedly between the X̃1A1 and Ã1B2 manifolds, yet a radical alteration in the nature of the reaction coordinate, whereby the planar (C2v) transition-state configuration of the former is supplanted by a notably aplanar (C2) form in the latter, is suggested to take place following π* ← π electron promotion (owing, in part, to attendant rearrangements of π-electron conjugation about the molecular framework). In contrast to the strongly perturbed vibrational landscape (commensurate with LBHBing) reported for the X̃1A1 potential surface, the present measurements have revealed surprisingly regular patterns of Ã1B2 vibronic structure which are devoid of obvious band shifts/splittings that would be indicative of efficient proton-transfer processes. Detailed analyses enabled a total of 41 (6) and 28 (5) excited-state vibrational levels (fundamentals) to be assigned for HFF and HFF-d, with extensive activity found for modes involving displacement of the seven-membered chelate ring that harbors the O-H···O reaction center. Quantitative simulations of partially resolved rotational contours for the HFF origin band showed the transition dipole moment to possess hybrid type-a/b character, thereby allowing the tunneling-induced bifurcation of the vibrationless Ã1B2 level to be extracted, Δ0à = 0.119(11) cm-1. This represents an enormous (>1000-fold) decrease over the analogous ground-state metric and implies a pronounced quenching of excited-state hydron migration, in keeping with the kinematic penalties that would be exacted by requisite heavy-atom motion along a putatively aplanar reaction coordinate.