Quantum-Indeterminate Proton Positions in Ultrafast Excited-State Intramolecular Proton Transfer

Although molecular vibrations are fundamentally described within a quantum mechanical framework, nuclear motion in condensed-phase chemistry is typically treated semiclassically, because direct experimental signatures of quantum behavior are difficult to access. Excited-state intramolecular proton transfer (ESIPT), which occurs before significant intra- and intermolecular perturbations can influence the system, provides a unique opportunity to probe such regimes. Here we investigate the ESIPT dynamics of [2,2'-bipyridyl]-3,3'-diol (BP(OH)₂), a system involving the transfer of two protons, using coherent vibrational spectroscopy. In a semiclassical picture, proton transfer is expected to impulsively drive coherent vibrations of the molecular framework through momentum conservation. In contrast to this expectation, only one of the two proton transfer events exhibits such a proton-transfer-induced coherent vibration, while the other shows no evidence of a coherent response associated with proton transfer. This observation cannot be straightforwardly explained within a simple semiclassical description of sequential proton transfer. Our results demonstrate how coherent vibrational spectroscopy can reveal subtle quantum aspects of nuclear motion in ultrafast proton-transfer reactions.