Ultrafast Pump–Repump–Probe Photochemical Hole Burning as a Probe of Excited-State Reaction Pathway Branching

Abstract

We demonstrate pump-repump-probe (PRP) transient hole burning as a spectroscopic tool for differentiating reactive from nonreactive deactivation of excited photochemical reactants observed by transient absorption spectroscopy (TAS). This method utilizes a time-delayed, wavelength-tunable ultrafast pulse to alter the excited reactant population, with the impact of "repumping" quantified through depletions in photoproduct absorption. We apply this approach to characterize dynamics affecting the nonadiabatic photocyclization efficiency to form S dihydrotriphenylene (DHT) following 266 nm excitation of ortho-terphenyl (OTP). TAS studies revealed bimodal deactivation of OTP*, but neither relaxation time scale (700 fs and 3.0 ps) could be assigned unambiguously to DHT formation due to overlap of excited-state and product spectra. PRP studies reveal that S OTP only cyclizes on the slower of these time scales, with the faster process attributable to nonreactive deactivation. We demonstrate that this method offers greater photochemical insights without assuming models to globally fit spectral transients collected by TAS.

Multiphoton Control of 6π Photocyclization via State-Dependent Reactant–Product Correlations

“…Figure a presents SR-PRP spectra collected for OTP in tetrahydrofuran (THF). This data exhibits a bleach band centered near 570 nm that is associated with the photoproduct depletion that we reported previously . Additionally, we observe a repump-induced absorption above 650 nm that signals a new photochemical product obtained via pump + repump (i.e., 1 + 1′) excitation.…”

“…As before, we find that only one lifetime (3.2 ps) is associated with the time dependence of the photoproduct depletion with variation in pump–repump delay. In contrast, the repump-induced absorption evolves biexponentially (360 fs and 3.2 ps with a 40:60 ratio of exponential prefactors), which is similar to what was observed in pump–probe TA data . These transients of ΔΔOD versus time (and their fits) are presented as logarithmic–linear plots in Figures S1 and S2 to highlight the single versus biexponential signal decay of the bleach and absorption features, respectively.…”

“…Our understanding of OTP photochemistry derives from analysis of ultrafast pump–probe transient absorption (TA) data. − We have previously observed that S 2 OTP* (which is populated on a time scale <100 fs) decays biexponentially with lifetimes of 700 fs and 3.0 ps in tetrahydrofuran (THF) solution. ,, However, we could not determine from TA which of these time scales corresponds with cyclization because the OTP* absorption spectrum completely spans that of S 0 trans -DHT, such that the kinetics of the photoproduct appearance cannot be determined unambiguously through global modeling of TA spectral dynamics . To resolve this issue, pump–repump–probe spectroscopy (PRP) was applied as a pathway-selective probe of dynamics .…”

“…Our understanding of OTP photochemistry derives from analysis of ultrafast pump–probe transient absorption (TA) data. − We have previously observed that S 2 OTP* (which is populated on a time scale <100 fs) decays biexponentially with lifetimes of 700 fs and 3.0 ps in tetrahydrofuran (THF) solution. ,, However, we could not determine from TA which of these time scales corresponds with cyclization because the OTP* absorption spectrum completely spans that of S 0 trans -DHT, such that the kinetics of the photoproduct appearance cannot be determined unambiguously through global modeling of TA spectral dynamics . To resolve this issue, pump–repump–probe spectroscopy (PRP) was applied as a pathway-selective probe of dynamics . PRP is a three-pulse experiment in which the species or state selectivity of the repump pulse and the relative timing of light pulses determine what spectroscopic or dynamical information is obtained. ,,, Scanning-repump pump–repump–probe (SR-PRP) spectroscopy (and pump–dump–probe spectroscopy − ), in particular, can be applied as a photochemical hole-burning experiment to identify what excited-state relaxation dynamics correspond with generation of a specific photoproduct: the repump pulse is timed to deplete the population in an excited electronic state that was prepared with the pump pulse; this depletion ultimately propagates to a drop in the photoproduct yield that would have been generated by the excitation pulse (measured at a time long after application of the pump–repump pulse pair), with the delay between the pump and repump pulses determining how strongly the excited-state population is depleted.…”

“…To resolve this issue, pump–repump–probe spectroscopy (PRP) was applied as a pathway-selective probe of dynamics . PRP is a three-pulse experiment in which the species or state selectivity of the repump pulse and the relative timing of light pulses determine what spectroscopic or dynamical information is obtained. ,,, Scanning-repump pump–repump–probe (SR-PRP) spectroscopy (and pump–dump–probe spectroscopy − ), in particular, can be applied as a photochemical hole-burning experiment to identify what excited-state relaxation dynamics correspond with generation of a specific photoproduct: the repump pulse is timed to deplete the population in an excited electronic state that was prepared with the pump pulse; this depletion ultimately propagates to a drop in the photoproduct yield that would have been generated by the excitation pulse (measured at a time long after application of the pump–repump pulse pair), with the delay between the pump and repump pulses determining how strongly the excited-state population is depleted.…”