Ultrafast PumpâRepumpâProbe Photochemical Hole Burning as a

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 5847−5854

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Ultrafast Pump−Repump−Probe Photochemical Hole Burning as a Probe of Excited-State Reaction Pathway Branching Joshua A. Snyder† and Arthur E. Bragg*

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Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States 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 S0 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 S1 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. fficient and robust molecular photoresponses require that photoinduced structural evolution proceeds as directly as possible toward desired photochemical outcomes and avoids undesired (i.e., unproductive) deactivation pathways. In this regard, numerous efforts have focused on elucidating key structure-dynamics relationships that impact the effectiveness and potential photophysical losses associated with photoisomerization reactions for chemical, biological, and material applications.1−10 On one hand, the presence of multiple stable molecular conformations can contribute significantly to photochemical losses. This has been demonstrated with the excited-state deactivation of cyclizable diarylethene photoswitches,11,12 for which the so-called photoexcited antiparallel and parallel ring conformations present at equilibrium deactivate by photocyclization and nonreactively, respectively;9 conformational dependence in excited-state relaxation pathways also explains complex spectral dynamics associated with E/Z thienyl-ethene photoswitches.8,13 On the other hand, branching between excited-state deactivation pathways can result in very different photochemical results for a molecular ensemble characterized roughly by a single conformation. In a classic example, S1 cis-stilbene relaxes predominantly via isomerization to trans-stilbene,14,15 but photocyclization to 4a,4b-dihydrophenanthrene (DHP) accounts for a nonnegligible (10%) quantum yield of excited-state relaxation.16,17 Photophysical investigations17−25 and dynamics simulations26−28 point to branching between cyclization and isomerization as a result of bifurcation of the nuclear wavepacket launched onto the excited-state potential energy surface with structural evolution that results in these different photochemical products. Pathway branching and the conformation dependence of excited-state deactivation in these systems give rise to pathway-

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specific spectral dynamics that may be observable with ultrafast time-resolved spectroscopies,1,9,19,20,25,29 such as transient absorption spectroscopy (TAS). However, when spectral features overlap significantly, as is often the case in the condensed phase, deciphering what changes are associated with specific photochemical channels necessarily falls within the realm of kinetic modeling rather than direct experimental observation.30 Thus spectroscopic probes that more deeply interrogate the connection between spectral dynamics and the formation of specific photochemical products are necessary to characterize simultaneously activated photophysical dynamics more directly. Here we demonstrate pump−repump−probe (PRP) photochemical hole burning as a means for deconvoluting complex excited-state dynamics to discriminate between reactive and nonreactive photochemical dynamics, applying it specifically to the interrogation of photoinduced dynamics of ortho-terphenyl (OTP) in solutions of tetrahydrofuran (THF), depicted in Figure 1a. We have previously reported on the ultrafast photochemistry of OTP and related compounds, as probed by TAS.31−34 The steady-state absorption of OTP, the photoinduced absorption spectrum of OTP*, and the spectrum of S0 4a,4b-dihydrotriphenylene (DHT) are presented in Figure 1b. Excitation of the lowest energy transition of OTP (250−300 nm) results in the immediate appearance of two transient absorption bands peaking at 375 and 600 nm attributed to S1 OTP.33,34 Decay of OTP* is bimodal over the course of picoseconds,31,32 with DHT formed as a result of excited-state deactivation. DHT exhibits two characteristic absorption Received: August 13, 2018 Accepted: September 18, 2018 Published: September 18, 2018 5847

DOI: 10.1021/acs.jpclett.8b02489 J. Phys. Chem. Lett. 2018, 9, 5847−5854

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The Journal of Physical Chemistry Letters

equivalent antiparallel ring geometries are energetically accessible in the ground state.35−37 Hence, the dynamics observed should be associated with a relatively structurally uniform ensemble of reactants. Therefore, do the two time scales observed reflect the initial relaxation of the excited reactant, S1* → S1, followed by electrocyclization, spectral dynamics associated with an intrinsically biexponential/ nonexponential process,6,7 or rather branching between distinct relaxation pathways, including a competing funnel to the ground state or the formation of other photoproducts (e.g., Figure 1a)? Motivated in part by recent successful applications of threepulse “pump−repump−probe” (PRP) and “pump−dump− probe” spectroscopies to interrogate and manipulate photoinduced dynamics in light-activated materials,10,38−41 biomolecules,6,42−44 and charge-transfer events,45−47 we have developed PRP transient photochemical hole burning, generalized schematically in Figure 1c, to unravel photochemical pathway branching for systems like OTP. Much like other PRP and pump−dump−probe experiments, our experiment utilizes a pulse pair to prepare and then manipulate the populations of excited states: Here the photochemical reactant is first excited to its lowest-lying optically accessible excited state with a 266 nm pulse, with a repump pulse subsequently applied at a controllable time delay after excitation to deplete photochemically the reactant excited-state population. The time-dependent propagation of this depletion to the nascent photoproduct population is probed via the impact on broadband absorption of the photoproduct, allowing us to identify which of the temporal responses of excited-state deactivation is associated with photoproduct formation and which must rather be associated with other (branched) processes. We note that whereas PRP and pump−dump− probe spectroscopies have been previously used to characterize spectral components associated with different photophysical pathways after branching has occurred,42,43 the experiment we present here is unique in that it interrogates the dynamics underlying the multiresponsive nature of a common, spectroscopically distinct excited state for improved insight into its photochemical reactivity. Experimentally, the PRP experiment involves three time delays between pump−repump, pump−probe, and repump− probe pulse pairs (t1,2, t1,3, and t2,3, respectively, as defined in Figure 1c). In practice, only two time delays must be defined, as the third is constrained by the other two. The relative timing of these pulses and the wavelengths of the repump pulse allow us to interrogate various aspects of excited-state and product photochemistry; the relative selectivity for excited-state and photoproduct excitation is illustrated in Figure 1b and is described below in the context of each of our measurements. As we seek to characterize the impact of repumping transient species on the net photochemical outcome, our PRP measurements are reported in terms of fractional depletion (FD) of pump−probe TAS signals (see Experimental Methods); notably, self-referencing via FD also improves signal-to-noise in the PRP measurement by effectively normalizing for fluctuations in the pump power. To apply this method, it is critical first to understand the impact of reexcitation on both the photoproduct (DHT) and excited reactant (OTP*). We expect that the repump pulse will deplete the photoproduct population; this is because photoinduced ring reopening of DHT (like photocyclization of OTP) is consistent with the Woodward−Hoffmann rules for

Figure 1. (a) Potential energy schematic for the photocyclization of ortho-terphenyl (OTP) to form 4a,4b-dihydrotriphenylene (DHT). (b) Absorption spectra of ground-state OTP (black), OTP* populated by 266 nm excitation of OTP (pink), and ground-state DHT (mauve) in solutions of tetrahydrofuran (THF). Spectra of pump and repump pulses employed in the present work (340, 400/ 380, and 580 nm) are overlaid. (c) Schematic depiction of photochemical hole burning via ultrafast pump−repump−probe spectroscopy as a tool for correlating excited-state relaxation with product formation channels.

features centered at 340 and 580 nm and is short-lived (45 ns) due to thermally activated ring reopening over a low energetic barrier (0.25 eV).34 Although this prior work reveals that electrocyclization must be a significant photochemical pathway, the dynamics between excited-state deactivation and photoproduct formation is inherently difficult to characterize because the absorption spectra of OTP* and S0 DHT overlap considerably across the visible and near-UV. Hence, our understanding of deactivation mechanisms is based on global kinetic modeling of transient spectral evolution subject to an assumed kinetic model, many of which could apply and therefore photochemical insights are limited. For example, global analysis of transients subject to a two-step sequential kinetic model (A → B → C) resulted in a reasonable fit to the data, revealing that S1 OTP deactivates on time scales of 700 fs and 3 ps.31,32,34 Unlike diarylethene compounds, OTP has little conformational flexibility due to steric crowding of the pendant phenyl rings, such that only two 5848


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between ground-state DHT and OTP (Figure 2a). We note that this time scale closely matches the DHT vibrational relaxation time scale measured following direct 266 nm excitation of OTP (also in THF solution).31 This would imply that deactivation of DHT* is rate-limited by vibrational relaxation or coincidentally occurs on a similar time scale as vibrational cooling. Alternatively, these data may reflect ultrafast (100 ps) only arises from excitation of the nascent photoproduct, we can use a scaled subtraction of these traces to isolate the time-dependent contribution attributable to reexcitation of S1 OTP (the “OTP* photochemical action”). The result of this is shown in 5850


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The Journal of Physical Chemistry Letters Figure 4b alongside TAS probed at 400 nm that has been fit with a triexponential signal decay (600 fs, 2.8 ps, and 22 ps). The fastest decay component of the TAS signal is conspicuously absent from the action trace, highlighting that the repump pulse only actively depletes the photoproduct yield on one of the two time scales observed in the spectral dynamics of OTP* probed with TAS. Figure 4b shows that the slower components of the TAS signal decay closely match the OTP* action trace. On the basis of these results, we conclude that the faster S1 signal decay observed by TAS does not correspond to photoproduct formation and rather must correspond to an alternate, branched electronic deactivation pathway, as schematized in Figure 1a. Notably, this decay component accounts for ∼40% of the excited-state signal and therefore has a significant impact on the efficiency of photochemical bond formation. Although only weakly fluorescent, OTP emits between 300 and 450 nm.6 Therefore, a relevant consideration is whether the 400 nm pulse induces a “repump” (absorption) rather than a “dump” (stimulated emission) transition.43 Importantly, the photoproduct bleach presented in Figure 4 is largest at a perpendicular relative pump−repump polarization, indicating that the repump transition must be largely perpendicular to the OTP S0−S1 transition. In contrast, bleaching induced by dumping from the lowest excited state should be largest with parallel pulse polarizations because both pulses induce transitions between the same electronic states. On the basis of this polarization specificity, and the sizable signal recovery observed in Figure 3b on an ultrafast time scale (∼100 fs) that can only occur from a higher lying state, we conclude that the S1 bleaching observed must result from photophysics or photochemistry associated with repumping and not dumping. Realistically, either a repump or dump transition would enable the discrimination between reactive and nonreactive pathways via three-pulse transient hole burning. From the action traces collected at 380 and 340 nm, we can also assess the relative efficiency of depleting the DHT photoproduct through OTP* versus S0 DHT reexcitation. The relative absorption cross sections for OTP* and S0 DHT at the two wavelengths were determined from TAS measurements. Here we have assumed that the photoproduct depletion induced with short pump−repump delays arises from reexcitation of S1 OTP. The ratio of the fractional depletion measured at early versus late delay was calculated for each trace and then scaled according to the relative absorption cross sections for the two states at corresponding repump wavelengths. On the basis of this analysis, we find that repumping S1 OTP depletes DHT with ∼1/3 the efficiency of direct excitation of DHT at both repump wavelengths. Because the relative absorption cross sections for the two states differ considerably at the two repump wavelengths, the consistency in relative efficiency for depletion supports our assumption that no significant population of the cyclized product is formed within the first several hundred femtoseconds, that cyclization occurs on a ∼3 ps time scale, and that the second pulse applied repumps (rather than dumps) the S1 population. The fact that nonadiabatic electrocyclization only accounts for some of the excited-state decay observed by TAS is somewhat surprising: Unlike cis-stilbene, the structure of OTP is expected to constrain nonadiabatic dynamics associated with large-scale structural changes in the excited state (i.e., cis−trans isomerization);23,24,33 as increased inter-ring bond order in S1 (vs S0) should favor a planar molecular structure, one might

anticipate that structural dynamics would strongly favor cyclization. What, then, is the nature of the pathway branching in the excited state? We note that multiple low-energy (S1−S0) conical intersections have been located in aromatic cycles (e.g., benzene) that are associated with out-of-plane ring deformations.54 Similar crossing could be expected for OTP and would lead to alternate, nonreactive pathways for excited-state deactivation. It is also surprising that the cyclization pathway in OTP occurs over a relatively long time scale when compared with related photochemical systems, indicating either greater energetic barriers or an indirect approach of the excited-state wavepacket toward state-crossings for cyclization. We note that OTP is comparable in size to tetraphenylethene derivatives; excited-state dynamics of the latter have been explored recently using nonadiabatic surface-hopping trajectories.5,55 Similar studies with OTP would be highly informative about the nature and possible generality of “lossy” photophysical deactivation pathways in aromatic photochemical materials. In conclusion, we have demonstrated the use of three-pulse pump−repump−probe photochemical hole burning to differentiate between branched reactive and nonreactive excitedstate relaxation channels with distinct time scales that are otherwise spectroscopically indistinguishable with conventional optical pump−probe experiments. This approach takes advantage of differences in photochemical responses of excited versus product states to excitation and is valuable when the spectra of transient species overlap to a degree that limits insights from transient absorption measurements to assumptions of global kinetic modeling. This method holds great promise for deconvoluting complex photoinduced dynamics of molecular excited states attributable to pathway branching or the presence of reactive and nonreactive molecular conformers.

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EXPERIMENTAL METHODS Sample Preparation. Solutions (5 mM) of ortho-terphenyl (Sigma) in tetrahydrofuran (Inert PureSolv SPS) were mixed by sonication. Solutions were transferred to a 0.5 mm path length fused silica flow cell (Spectrocell), circulated with a peristaltic pump (Cole-Palmer), and sparged with N2 for 15 min. Pump−Repump−Probe Spectroscopy. The laser system and experimental setup have been previously described;40 details relevant to PRP experiments are described here. The output of a Ti:sapphire amplifier (Coherent Legend Elite, 1 kHz, 4 mJ, 35 fs) was split to generate pump, repump, and probe light pulses: 266 nm pump/excitation pulses were generated with a SHG + SFG mixing cascade; ultraviolet (340 and 380 nm) and visible (580 nm) repump pulses were generated using an optical parametric amplifier (Light Conversion) or by second harmonic generation of the fundamental (400 nm). Supercontinuum probe pulses (400−800 nm) were generated by focusing the fundamental or its second harmonic into crystalline plates (United Crystals) to drive white-light generation: The visible supercontinuum was generated by focusing the fundamental into sapphire (2 mm), whereas the ultraviolet supercontinuum (280−400 nm, used to collect data shown in Figure 2) was generated with the second harmonic focused into CaF2 (2 mm). The polarization of the 266 nm pump was set relative to the repump or probe as required for each PRP configuration using a zero-order waveplate (Thorlabs) and was determined to have a ∼70 fs fwhm duration based on autocorrelation via two-photon absorption (2PA) in a 200 μm BBO crystal (United Crystals).56 Visible 5851


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The Journal of Physical Chemistry Letters Present Address

repump pulses (>400 nm) were determined to have duration of 40−60 fs fwhm based on autocorrelation via 2PA in a GaN LED57 or by SHG in a BBO crystal. The time-resolution of the UV-pump/visible continuum probe was determined to be 200−250 fs fwhm by cross-correlation in the BBO crystal or a BK7 coverslip. Consequently, our PRP action measurement has a somewhat better time resolution than our TAS measurement because the instrument response function for the former corresponds to the cross-correlation of the pump and repump pulses (cf. Figure 4b). The pump and repump were delayed independently with two motorized translation stages (Newport, ILS250). Both beams were focused with lenses at 15° relative to the probe and to ∼2 mm spot size with a fluence of ∼150 μJ/cm2. The polarizations of all of these beams were set immediately before the sample. The probe was focused at the sample using a parabolic mirror (Edmund), with its polarization set with the broadband waveplate immediately before the sample; transmitted probe light was collimated with a lens after the sample and filtered with a U-340 filter for the UV and a 400 nm longpass filter for the visible. Probe light was dispersed with a grating spectrograph (Princeton Acton) and detected with a CCD (Princeton Pixis 100). Data collection and computation of spectra were performed with an acquisition program written with Labview 2016. All beams were chopped using two optical choppers (Thorlabs). One chopper was equipped with a dual-phase blade allowing for the pump (Pu) and repump (Re) beams to be chopped in a phase-locked sequence. A second chopper alternately blocked and unblocked the probe (Pr) beam at a rate of 250 Hz. Chopping all three beams results in eight-phase data collection that enables simultaneous determination of fluorescence and scatter-corrected (FC) pump−probe (ΔODFC PuPr, eq 1), FC repump−probe (ΔODFC RePr, eq 2), FC pump−repump−probe (ΔODFC PuRePr, eq 3), and the differential pump−repump−probe signal (ΔΔODFC PuRePr, eq 4). i PuPr − Pu yz zz ΔODFCPuPr = −logjjj k Pr − BG {

i RePr − Re yz zz ΔODFCRePr = −logjjj k Pr − BG {

i PuRePr − PuRe yz zz ΔODFCPuRePr = −logjjj Pr − BG k {

†

J.A.S.: Department of Chemistry & Biochemistry, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the National Science Foundation (NSF), CHE-1455009 (A.B.). J.A.S. gratefully acknowledges support from the Langmuir-Cresap fellowship (JHU). We thank Dr. Timothy Magnanelli for assistance with developing data acquisition programs and Dr. Jamie Young for useful discussions, assistance with figure preparation, and critical reading of the manuscript.

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(1)

(2)

(3)

ΔΔODFCPuRePr = ΔODFCPuRePr − (ΔODFCPuPr + ΔODFCRePr ) (4)

Importantly, none of the repump wavelengths used in this experiment are resonant with the reactant’s absorption spectrum (Figure 1), such that repump−probe spectra (eq 2) had negligible contribution to PRP signals, greatly simplifying their analysis. Fractional depletion is determined from the ratio of eq 4 with eq 1.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Arthur E. Bragg: 0000-0002-3376-5494 5852


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