Related Experiment Video
Updated: Sep 19, 2025

Proton Transfer and Protein Conformation Dynamics in Photosensitive Proteins by Time-resolved Step-scan Fourier-transform Infrared Spectroscopy
Published on: June 27, 2014
Structure-Dependent Photochemical Dynamics of Modified ortho-Terphenyls Studied with One and Two-Photon Probes
Ryan P Brady1, Luke R O'Connor1, Rachel A Dziatko1
1Department of Chemistry, Johns Hopkins University, 3400 N. Charles St. Baltimore, Maryland 21218, United States.
Abstract:
We report on structure-dependent one- and two-photon induced photochemical dynamics of modified ortho-terphenyls (OTPs), model 6π electrocyclizing photoreactants that exhibit distinct branching between reactive and nonreactive nonradiative excited-state deactivation channels. Using both pump-probe transient absorption spectroscopy (TAS) and pump-repump-probe (PRP) TAS, we find an enhanced efficiency for cyclization over nonreactive deactivation with the addition of 4,4″ alkyl substitution on pendant rings. The nonreactive deactivation rate is largely unaffected by structural modifications, whereas the cyclization rate to form trans-4a,4b-dihydrotriphenylene (trans-DHT) decreases with increasing substituent size. Based on these observations, we conclude that the reactive and nonreactive deactivation channels involve distinctly different structural dynamics within low-lying electronic excited states to reach critical geometries (i.e., conical intersections). We show that the cis-DHT photoproduct yield obtained by resonant 1 + 1' excitation is sensitive to dynamics in the low-lying excited-states of OTP that are in turn sensitive to structural modifications. Within the sub-ps time regime, PRP and TAS signals evolve on significantly different timescales, highlighting that these methods have different contrasts for probing dynamics on low-lying potential energy surfaces.
Related Concept Videos
Photochemical Electrocyclic Reactions: Stereochemistry
Selection Rules: Photochemical Activation
Cycloaddition Reactions: MO Requirements for Photochemical Activation
Thermal and Photochemical Electrocyclic Reactions: Overview
ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3
ortho–para-Directing Deactivators: Halogens
Directing Effect of Substituents: ortho–para-Directing Groups

