Structure-Dependent Photochemical Dynamics of Modified <i>ortho</i>-Terphenyls Studied with One and Two-Photon Probes

We report on structure-dependent one- and two-photon induced photochemical dynamics of modified <i>ortho</i>-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 <i>trans</i>-4<i>a</i>,4<i>b</i>-dihydrotriphenylene (<i>trans</i>-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 <i>cis</i>-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.