Observing the photo-induced structural dynamics of proton transfer in o-nitrophenol by Ultrafast Electron Diffraction

Abstract

Many important biological and chemical processes are initiated by the transfer of a proton from a donor to an acceptor group within a molecule and are regulated by the interplay of nuclear and electronic dynamics. The electronic dynamics of proton transfer are well-studied, yet the nuclear dynamics have never been spatiotemporally resolved. Herein, we present a direct observation of the nuclear motions leading to and ensuing from photochemical proton transfer in o-nitrophenol using ultrafast electron diffraction. The proton transfer step is identified by the transient compression of the donor-acceptor distance and captured on the femtosecond timescale with sub-Angstrom resolution. Supported by ab initio multiple spawning simulations, our observations uncover the structural changes mediating excited-state relaxation. Our work provides an unprecedented mechanistic insight into the photochemical proton transfer mechanism and photo-relaxation dynamics of o-nitrophenol, unambiguously connecting an experimental observable to a process previously confined to theoretical models.

Imaging rehybridization dynamics into pericyclic minimum of an electrocyclic reaction in real-time

“…This is due to the redistribution of the absorbed photon energy during non-adiabatic dynamics lowering the molecular rigidity. 12,27 The corresponding signatures in a ΔPDF are negative peaks at the positions of the two cis and trans maxima (~3 Å and ~4 Å, respectively) of the third coordination sphere and a positive peak in the gap of the third coordination sphere (3.4 Å), overlapping with the observed ɣ signature. However, the early signature in the ɣ region as observed in the present study agrees only partially with this expectation: We observe a clear positive signature at 3.4 Å and a weak negative signature at 4 Å, which is close to the noise level in the experimental data, but clearly visible in the simulations (see Supplemental Figure S1), but a corresponding negative signature at smaller distances around 3 Å is missing.…”