Imaging proton transfer dynamics in o-nitrophenol by Ultrafast Electron Diffraction*

Many important biological and chemical reactions are initiated by the transfer of a proton from a donor to an acceptor group within a molecule. These intramolecular proton transfer (IPT) reactions mediate fundamental biological processes, such as the oxidation of water during plant photosynthesis, the light-driven proton pump bacteriorhodopsin and light emission from green fluorescent protein. Moreover, IPT reactions have found wide industrial applicability as: fluorescence-based sensors, laser dyes, organic light-emitting diodes, solar collectors and molecular logic gates. Despite its biological and industrial importance, the structural rearrangements mediating IPT reactions had, until now, never been imaged on the atomic length and time scales.

In this talk we discuss the direct imaging of photo-induced intramolecular proton transfer in o-nitrophenol with high spatiotemporal resolution. We use a combination of mega-electron volt gas-phase ultrafast electron diffraction (MeV-UED) to capture the nuclear dynamics, and a genetic structural fitting algorithm (GSFA) to retrieve time-dependent three-dimensional structures directly from the diffraction data. Our results, which are supported by ab initio multiple spawning simulations, capture the moment of proton transfer, as well as ensuing nuclear wave packet dynamics leading to the relaxation of o-nitrophenol back to the ground-state through a conical intersection. These observations provide unprecedented mechanistic insights into light-induced proton transfer reactions. Furthermore, the novel analysis methodology (combination UED and GSFA) discussed in this talk enabled the retrieval of structural rearrangements thought to be too subtle for UED. In principle, this methodology could be leveraged to the study of other types of reactions.