Monitoring the evolution of relative product populations at early times during a photochemical reaction

TL;DR

This study combines ultrafast electron diffraction and ab initio calculations to track and quantify the evolving populations of reaction products in a photochemical process, revealing rapid isomer interconversion within picoseconds.

Contribution

It introduces a novel combined experimental and computational approach to determine time-resolved populations of isomeric products in ultrafast photochemistry.

Findings

High (~50%) yield of episulfide isomer within 1 ps

Rapid interconversion between vibrationally excited photoproducts

Validation of theoretical predictions with experimental data

Abstract

Identifying multiple rival reaction products and transient species formed during ultrafast photochemical reactions and determining their time-evolving relative populations are key steps towards understanding and predicting photochemical outcomes. Yet, most contemporary ultrafast studies struggle with clearly identifying and quantifying competing molecular structures/species amongst the emerging reaction products. Here, we show that mega-electronvolt ultrafast electron diffraction in combination with ab initio molecular dynamics calculations offer a powerful route to determining time-resolved populations of the various isomeric products formed after UV (266 nm) excitation of the five-membered heterocyclic molecule 2(5H)-thiophenone. This strategy provides experimental validation of the predicted high (~50%) yield of an episulfide isomer containing a strained 3-membered ring within ~1 ps of photoexcitation and highlights the rapidity of interconversion between the rival highly vibrationally excited photoproducts in their ground electronic state.