Chemistry in Action: Making Molecular Movies with Ultrafast Electron Diffraction and Data Science

A long-held thought experiment of science is the direct observation of the motions of atoms and molecules as they undergo chemical reactions and phase transitions. The advent of microscopy and diffraction beyond the visible spectrum of light ⁠— using X-rays, electrons, and neutrons ⁠— has since enabled such atomic-level elucidation of the structure of matter in the steady state. Recently, time-resolved techniques have finally enabled the making of the titular `molecular movie' on the time scale of chemical physics. In this thesis, I report on my works which further this quest by using ultrafast electron diffraction and data science to fully resolve the photoinduced structural dynamics of five large, low-symmetry, and weakly scattering molecular systems: two Bechgaard-Fabre salts of 4,5-ethylenedioxy-tetrathiafulvalene, a ring-closing diarylethene derivative, and two iron(II)-based spin-crossover complexes. Leveraging the ultrabright femtosecond electron source presently available, large overdetermining time-series maps of reciprocal space were measured and structurally refined to track the individual atomic motions that lead to each photoproduct: two transient metallic phases, a ring-closed isomer, and two expanded high-spin states. In all cases, a dramatic and heretofore unknown reduction in dimensionality was found, whereby the photoinduced structural dynamics is governed by key reaction modes that are much fewer in number than the degrees of freedom expected of the respective molecular systems. This insight — along with the experiments and methods presented herein — should find application in uncovering the physics of reduced dimensionality as a solution to the problem of how chemistry scales with complexity.This work lays the foundation to recast chemistry in terms of reduced reaction modes, unifying structure and dynamics as a new conceptual basis for chemistry.Ph.D