Visualizing Excited-State Dynamics of a Diaryl Thiophene:Femtosecond Stimulated Raman Scattering as a Probe of ConjugatedMoleculesGiovanni Batignani,†,‡,∥ Emanuele Pontecorvo,†,∥ Carino Ferrante,† Massimiliano Aschi,‡

Christopher G. Elles,¶ and Tullio Scopigno*,†,§

†Dipartimento di Fisica, Universita di Roma “La Sapienza”, Roma I-00185, Italy‡Dipartimento di Scienze Fisiche e Chimiche, Universita degli Studi dell’Aquila, L’Aquila I-67100, Italy¶Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States§Center for Life Nano Science @Sapienza, Istituto Italiano di Tecnologia, Roma I-00161, Italy

*S Supporting Information

ABSTRACT: Conjugated organic polymers based on substitutedthiophene units are versatile building blocks of many photoactivematerials, such as photochromic molecular switches or solar energyconversion devices. Unraveling the different processes underlying theirphotochemistry, such as the evolution on different electronic states andmultidimensional structural relaxation, is a challenge critical to definingtheir function. Using femtosecond stimulated Raman scattering (FSRS)supported by quantum chemical calculations, we visualize the reactionpathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initialwavepacket dynamics of the reaction coordinates occurs within the first ≈1.5 ps, followed by a ≈10 ps thermalization.Subsequent slow opening of the thiophene ring through a cleavage of the carbon−sulfur bond triggers an intersystem crossing tothe triplet excited state. Our work demonstrates how a detailed mapping of the excited-state dynamics can be obtained,combining simultaneous structural sensitivity and ultrafast temporal resolution of FSRS with the chemical information providedby time-dependent density functional theory calculations.

Light control of conjugated organic polymers based onsubstituted thiophene units promises to play a central role

in the design of many photoactive devices, such as photo-synthetic systems1−4 or photochromic molecular switches.5,6

Accessing and manipulating the photochemical properties ofthese versatile materials requires a fundamental understandingof the reaction pathway of their building blocks upon anultrashort optical excitation. In fact, although the performanceof such materials ultimately depends on the fundamentalproperties of the constituent molecules, the dynamics of largepolymeric systems are often difficult to discern because ofstructural heterogeneities. Fortunately, detailed studies ofsmaller molecular building blocks provide useful insight onthe microscopic behavior ruling the underlying dynamics ofconjugated systems.7−11 A key question to be answered is therole of structural recombination following photoexcitation. Forexample, ultrafast geometrical rearrangement affects the initialstages of charge separation and recombination and alsoprovides valuable clues about the charge-transport materialproperties.2-Methyl-5-phenylthiophene (MPT), an asymmetric diaryl

molecule (the schematic of the ground-state geometry structureis reproduced in the inset of Figure 1a), is a model compoundwhich mimics the basic underlying structure of typicalconjugated molecular systems. MPT has been recently studied

by transient absorption (TA) measurements12 (Figure 1a),elucidating the formation of a triplet electronic state viaintersystem crossing (ISC) from the initially excited electronicsinglet S1 state. The spectrum of S1 includes a strong excited-state absorption (ESA) band centered near 480 nm and astimulated emission (SE) band centered near 360 nm, both ofwhich decay with the same single-exponential time-constant asthe appearance of a triplet−triplet absorption band near 370nm.12 The triplet band, which has a weak tail extending tolower energy, does not decay to any measurable extent over aperiod of 1 ns. Unfortunately, because of the lack of structuralsensitivity of TA spectroscopy, the rearrangement reactioncould not be probed, and the temporal sequencing of thephotoexcited MPT geometrical reconfigurations and therelaxation path in the triplet manifold of states are stilluncharted.Femtosecond stimulated Raman scattering (FSRS) is a

recently developed technique allowing for vibrational spectros-copy with subpicosecond time resolution.13−15 In FSRS, afemtosecond actinic pulse (AP) initiates the photochemistry of

Received: May 25, 2016Accepted: July 18, 2016

Letter

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interest. The system is subsequently interrogated by a pair ofoverlapped pulses: the joint presence of a broadband ultrashortprobe pulse (PP) and a narrowband picosecond Raman pulse(RP) induces vibrational coherences which are read out asheterodyne coherent Raman signals free of fluorescencebackground, with simultaneous high spectral and temporalresolution.16−18 During the last 10 years, the inherentsensitivity of FSRS to the ultrafast vibrational dynamics hasbeen demonstrated in a number of photoactive systems.19−27

Of relevance for the present context, FSRS has been recentlyapplied to quaterthiophenes,28 addressing the torsionalrelaxation in the electronic excited state.Here we reveal the full reaction pathway of photoexcited

MPT, a model compound for conjugated organic polymers.This is achieved by developing an FSRS setup with broadtunability of the Raman pulse which, by matching the electronictransitions corresponding to ESA and SE measured in the TAspectrum, allows us to selectively isolate dynamics from singlet

Figure 1. Transient absorption and femtosecond stimulated Raman spectra of methyl-phenylthiophene. (a) TA spectrum of MPT following anexcitation to the S1 potential energy surface. The arrows indicate the Raman pulse wavelengths used in the FSRS experiments (i.e., 366 nm in panel band 480 nm in panel c). The inset shows the structure of MPT in the ground singlet state (S0). Tuning the Raman pulse into electronic resonancewith the singlet (c) and triplet (b) excited states selectively isolates the contributions from the two transient species, unveiling the dynamicalevolution pathway.

Figure 2. Femtosecond stimulated Raman spectra of methyl-phenylthiophene. Contributions from the transient singlet and triplet excited electronicstates are measured tuning the Raman pulse at 480 nm (top panel) and 366 nm (middle panel), respectively, for selected time delays (indicated inpicoseconds in the legend). The calculated vibrational frequencies of the system in the S1 state, evaluated over the explored geometricalconfigurations (see the Supporting Information), are indicated by the blue spectral regions. Red vertical lines in the middle panel reproduce thenormal mode on the relaxed T1 geometry. The blue trace in the bottom panel reproduces the (off-resonance) stimulated Raman of theunphotoexcited MPT measured with RP at 366 nm, while the green line is the calculated ground-state off-resonance Raman spectrum. The spectrahave the solvent contribution and a baseline removed, while the cyan shaded peaks show the scaled solvent SRS spectra, obtained at the different RPwavelengths. The asterisk indicates an artifact from solvent subtraction.

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and triplet transient species. Experimental results arerationalized based on the insight of time-dependent (TD)density functional theory (DFT) calculations,29 performedusing the Gaussian 09 software package. Specifically, weevaluated the potential energy surfaces (PESs), the correspond-ing normal modes, and the diabatic transition probabilities forthe ISC between singlet and triplet excited states to dissectfrom FSRS spectra the different processes contributing to theelectronic deactivation mechanism of MPT.MPT photochemistry is initiated by a 266 nm, 100 fs actinic

pump which promotes the system from the S0 ground state toS1; the photoinduced dynamics is then monitored from 200 fsto 490 ps after the optical excitation, acquiring stimulatedRaman spectra (SRS) induced by a third-order susceptibilityand resulting as a modification of the PP spectral profile. The

Raman gain is defined as the ratio | || |

E

EP

2

P(0) 2

, where EP and EP(0)

indicate the PP with and without the presence of the RP,respectively.The intersystem crossing is monitored by tuning the Raman

pulse to 366 nm (blue arrow in Figure 1a). Such a wavelengthis, indeed, initially resonant with the ESA from S1 (populatedfor Δt < 200 ps), although the TA is dominated by the S1−S0stimulated emission band. In fact, the Raman response forprocesses in resonance with SE induces negative contribu-tions21 to the FSRS spectrum. The evidence of positive FSRSdata indicates that the SE contributions are marginal. At latertimes, instead, it corresponds to the ESA transition (Tm−T1) ofthe triplet state. In striking contrast, the RP at 480 nm is solelyresonant with the singlet excited-state absorption Sn−S1. Hence,such configuration allows for selective tracking of the structuralrearrangement on the S1 surface (Figure 1c). FSRS spectra forthe two different excitation wavelengths are compared in Figure2 (with baseline and solvent subtracted as described in ref 27).Particularly the FSRS spectra at both RP wavelengths show astrong resonant enhancement with respect to the (offresonance) ground-state stimulated Raman spectrum measuredat λRP = 366 nm (reproduced in the bottom panel of Figure 2)

and are accompanied by the appearance of new vibrationalbands. The ground-state Raman spectrum, calculated usingdensity functional theory with B3LYP functional30,31 and the 6-311++G** basis set, is reproduced in the bottom panel (greenline), showing a good agreement with the SRS spectrum. The(scaled) cyclohexane solvent spectra are reported for bothwavelengths as cyan shaded peaks.The FSRS signal for 366 nm Raman pulse wavelength clearly

indicates different time scales for the vibrational spectra of thesinglet and triplet excited states, with the most notable featuresappearing in the region of the CC stretch vibrations (1400−1600 cm−1)32 emphasized in Figure 1b. The 1470 cm−1 mode,which can be assigned to a phenyl ring stretching-deformation,E1u, and a thiophene ring deformation, decays along with theappearance of a dominant vibrational Raman band at 1505cm−1, attributed to a phenyl ring stretching-deformation, E1u,and a C−C inter-rings stretching, accompanied by weakermodes at 1550 cm−1 (in-plane thiophene ring deformation, A1),890 cm−1 (in-plane thiophene ring deformation, A1) and 995cm−1 (phenyl ring deformation, B1u). The common time scaleof such processes (135 ps) is in excellent agreement with theISC suggested by the TA evolution. Hence, we assign thedecaying and the developing modes to the S1 and the T1 states,respectively.Accordingly, Figure 3 shows that the evolution of the 1470

cm−1 Raman band intensity perfectly reproduces the inter-system crossing kinetics suggested by the TA excited-stateabsorption decay at 480 nm. Remarkably, a closer inspection ofthe same figure reveals that both 1505 and 1550 cm−1 peaksfollow the TA at 366 nm only for time delays longer than 30 ps.Such mismatch emphasizes at the same time one of the mainlimitations of TA, the difficulty in disentangling overlappingcontributions, and the main strength of FSRS, the ability toreveal structural modifications occurring on the excited singletstate.The origin of this conformational rearrangement can be

investigated by considering FSRS spectra measured with the RPat 480 nm (Figure 1c and top panel of Figure 2), which are in

Figure 3. Comparison between transient absorption and FSRS kinetics. The continuous blue lines are the TA profiles at 366 nm (top panel) and 480nm (bottom panel), revealing an intersystem crossing from the excited singlet to the triplet state with a ∼135 ps exponential decay. The FSRSRaman gains, obtained as the maxima of pseudo-Voigt fit to the measured spectra (see the Supporting Information), for selected modes in the high-frequency region (square and circle markers are used for RP at 480 nm and for RP at 366 nm, respectively) show similar evolutions on long timescales, while on the picosecond regime they exhibit kinetics much richer than the TA spectra, indicating a structural rearrangement of the system.

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resonance with the S1 ESA peak only, i.e., sensitive to theevolution of the molecule during its dynamics in the S1potential energy surface. A closer look to the 1350−1650cm−1 region, reported in Figure 1c, reveals indeed a richbehavior which has to be contrasted with the monotonic decayof the transient absorption. Specifically, the two modes locatedaround 1505 and 1550 cm−1 develop within the firstpicosecond and then display a blue-shift of ∼10 cm−1. Asimilar behavior is observed for the Raman bands at 680 cm−1,ascribed to a phenyl ring-deformation (E2g) and a thiophenering-deformation (A1), and at 1170 cm−1, assigned to a phenylC−H bend (E1u), with an analogous initial increase of theRaman gain and a ∼5 cm−1 blue-shift. Furthermore, thecomplete decay (for time delays longer than 100 ps) of theFSRS bands measured at 480 nm confirms that the singletexcited state is efficiently converted to the triplet one. Todetermine the time scales involved in this dynamics weperformed a global analysis over the vibrational features of theentire Raman spectra. The kinetics of different Raman modes,at both RP wavelengths, are reported in Figure 4 (square and

circle markers are used for λRP = 480 nm and λRP = 366 nm,respectively) together with the global fit traces (continuouslines) which have been obtained with the multiple exponentialfunction RGνi = Ai exp(−t/τ1) + Bi exp(−t/τ2)+Ci exp(−t/τ3),where RGνi indicates the Raman gain for the νi mode. Theslowest τ3 = 135 ps component can be clearly identified withthe intersystem crossing dynamics, in excellent agreement withTA measurements (see Figure 3). The assignment of the fastestτ1 = 1.5 ps and τ2 = 11 ps, however, is more critical. Such twotime scales cannot be attributed to resonance effects as the TAis monotonically decaying; here we propose that τ1 has to berelated to the structural rearrangement of the moleculeaccompanying the ultrafast internal vibrational relaxation ofthe wavepacket in S1. This is in line with recent observations onthe relaxation process along the torsional dihedral coordinate inquaterthiophenes.28 The blue-shift observed during the first 10ps for the S1 modes (Figure 1c and Figure 2, top panel)suggests vibrational cooling as a compelling likelihood for the τ2

assignment; furthermore, it rules out a reorientation of themolecule effect, which in principle may influence theexperimental Raman amplitudes, because of polarizationanisotropy, but which cannot explain frequency shifts.To validate such a scenario, and to elucidate its underlying

structural rearrangement pathway, we performed quantumchemical (QM) calculations to characterize the involved PESs.All the calculations have been performed using the Gaussian 09software package.33 Preliminary full optimizations performed atthe B3LYP30,31 TD-DFT level of theory, in conjunction withthe 6-31+G** basis set, have revealed that the relaxationprocess occurring in the conditions of our experiment isplausibly driven by two internal coordinates: the dihedral angle(ϕ) between phenyl and thiophene rings and the distance(dC−S) between the carbon (connected to phenyl) and thesulfur atoms in the thiophene. In oligothiophenes, for instance,the C−S distance is likely to play an important role in theelectronic relaxation and to be implicated in the rapidintersystem crossing between singlet and triplet states.34 As amatter of fact, in the ground electronic state (S0), MPT has anonplanar configuration with ϕ ≈ 30° and dC−S ≈ 1.76 Å(Figure 1a). On the other hand, and in agreement with recentQM calculations obtained in thiophene and bithiophenecompounds,35 the full minimum in the S1 electronic state hasa planar configuration with a sharply elongated C−S bond (ϕ =0°, dC−S ≈ 1.80 Å). Other internal coordinates such as ring-puckering, shown to be potentially active in higher-energychannels35 for unsubstituted thiophene, was revealed to beenergetically much higher for MPT and fully relaxed within thetemporal resolution of our experiment. Hence, we disregardedsuch modes for the topological definition of the PESs.Scanning the S1 PES along the selected coordinates, we

found that the structural relaxation starts from the initial excitedstate, S1* and involves a local minimum which eventuallyundergoes magnetic transition (intersystem crossing) to atriplet MPT (the experimentally observed τ3 = 135 ps process).In order to evaluate the kinetics of the nonradiative decay alongthe conformational S1 PES, we performed a semiclassicalsimulation from the Franck−Condon region, making use of theFokker−Planck equation36−39 (see the Supporting Informa-tion) for propagating the probability density onto the freeenergy surface. This approach is a convenient and computa-tionally less expensive alternative to ab initio and/or quantumdynamical methods. Remarkably, its applicability is subject tothe possibility of reducing the problem to a few semiclassicaldegrees of freedom, the low-frequency coordinates ϕ and dC−Sin our case. As shown in Figure 5b, the wavepacket evolvestoward the S1 PES local minimum within the first 2 ps,accompanied by a spread of the probability distribution alongthe dC−S coordinate (we hereafter concisely refer to theprobability density motion, along the free energy surface, as awavepacket evolution along the potential energy surface). Thisresult corroborates our assignment of the experimentallyobserved τ1 time scale to the wavepacket motion toward thelocal minimum of S1, away from the Franck−Condon region ofthe two low-frequency modes driving the reaction.In order to characterize the structural origin of ISC rate, we

have also evaluated the PESs for the ground T1 and first excitedT2 triplet states along the same dC−S and ϕ coordinates. Thesecalculations were first performed utilizing the S1 geometries(vertical calculations) to obtain the crossing seam from avibrationally relaxed MPT in the S1 state to a vibrationallyexcited MPT in the Ti state and then utilizing the relaxed triplet

Figure 4. Kinetics of the Raman gain at selected Raman modescompared with the global fit results (continuous lines). Filled squareand open circle markers are used for λRP = 480 nm and λRP = 366 nm,respectively. The three principal components identified suggest a fast(τ1 = 1.5 ps and τ2 = 11 ps) relaxation of the system on a localminimum of the S1 PES along the two different reaction coordinates,followed by a subsequent vibrational cooling. Then, a slower (τ3 = 135ps) time scale is required allowing the system to overcome the energybarrier (Figure 5a) to the intersystem crossing region.

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state coordinates to study the post ISC dynamics. The abovecalculations did not provide dramatically different PESs;therefore, in the rest of the discussion we will refer to thevertical calculations only. Remarkably, we found a crossingbetween S1 and T2 PESs, as shown in Figure 5a, for dC−S > 2.05Å. With the same analysis of the PES surfaces, we also identifieda possible conical intersection (CI) allowing the internalconversion (IC) between T2 and T1.The presence of efficient ISC conversion rate and the

subsequent IC between excited triplet states are in line withrecent work on gaseous oligothiophenes,34 showing, by high-level calculations, that the nonradiative decay from S1 isdominated by ISC involving different triplet states. In particularfor bithiophene, the species structurally most affine to MPTamong the investigated ones, the presence of a local minimumon the S1 surface close to the Franck−Condon (FC) region hasbeen reported, and it has been suggested the occurrence of adouble ISC between S1 and two different triplet surfaces,followed by triplet−triplet internal conversion, eventuallyleading to the deactivation.In addition to the local minimum near dC−S = 1.80 Å, the

calculated potential energy surfaces in Figure 5 reveal a second,lower-energy minimum on the S1 surface near dC−S = 2.6 Å.This lower-energy structure may be compatible with athiophene ring-opening reaction along the C−S bond, possiblyoccurring on the τ2 time scale, prior to ISC. Indeed, the 0.2 eV

barrier crossing from the local S1 minimum would lead to thisintermediate structure in the absence of efficient ISC. A ring-opening mechanism has been implicated in the efficientdeactivation of unsubstituted thiophene32,35,40 and could alsoplay a role in the aryl shift reaction that was observed previouslyfor aryl-substituted thiophenes.41,42 The aryl shift reactionrefers to a nonreversible translocation of the phenyl ring fromposition 2 (adjacent to S) to position 3 on the thiophene ringfor which a single, consistent mechanism has not yet beenidentified.12

However, the evaluation of calculated vibrational frequencieson the S1 state, shown in Figure S3 of the SupportingInformation, is in excellent agreement with the FSRS spectra(Figure 2, λRP = 480 nm) and allows us to shed light on theorigin of the τ2 process. Specifically, the predicted Raman shiftson their way to the open ring configuration show disparatetrends: the phenyl-ring deformation Raman mode (680 cm−1)undergoes a large blue shift (≈40 cm−1), while the phenyl ringstretching-deformation mode (1505 cm−1) gets slightly red-shifted. In addition, the frequency difference between theRaman doublet of the in-plane thiophene ring deformation(1550 cm−1 band) becomes increasingly larger, up to ≈50cm−1. In striking contrast, all the Raman bands observed byFSRS within the first 10 ps show a same behavior, i.e. a ≈5−10cm−1 blueshift, which can not be traced back to structuraleffects and clearly supports the identification of τ2 with the time

Figure 5. Excited potential energy surfaces of methyl-phenylthiophene computed by time-dependent density functional theory with B3LYPfunctional and the 6-31+g** basis set (a). The energies of the excited S1 singlet state are obtained along the reaction coordinates ϕ and dC−S, whichare illustrated in panel c. The PESs for the triplet T2 and T1 states have been calculated at the optimized S1 geometry, with the same computationalprecision. In panel b, the wavepacket evolution from the Franck−Condon region (ϕ ≈ 30° and dC−S≈ 1.76 Å) is reproduced from Δt = 50 fs to Δt =3250 fs. The white line indicates the central position of the wavepacket, while the colored lines reproduce the half width at half-maximum of theprobability distribution at different time delays, with steps of 800 fs.

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scale of vibrational cooling, as opposed to a ring-openingscenario anticipating the ISC.On the basis of these results we can schematically describe

the whole process as follows. The photoexcited MPT initiallyundergoes a planarization process, evolving (τ1 = 1.5 ps)toward the local minimum on the S1 PES at dC−S = 1.80 Å.Subsequently, the broad distribution in both dC−S and ϕ(highlighted in Figure 5b) gets narrower within the next τ2 = 11ps by vibrational cooling, which manifest itself, via anharmoniccoupling, in the (higher-frequency) modes tracked byFSRS.26,43,44 Then, the carbon−sulfur distance has to reachthe ISC seam at dC−S > 2.05 Å (Figure S2). The long τ3 = 135ps time scale is required to overcome a ∼0.2 eV potentialenergy barrier between dC−S = 1.80 Å and dC−S > 2.05 Åconfigurations. Such activation barrier on the ring-openingreaction may in principle give rise to a temperature and solventeffect on the τ3 crossing time scale, without changing thereaction mechanism, which may be interesting to address infuture studies.A careful inspection of the ISC surface reveals that the

similarity of S1 and T2 PES derivatives along the crossing curve(CC), as well as a relatively high spin−orbit coupling betweenthe two electronic states (the order of magnitude of the spin−orbit coupling matrix elements along the CC is ∼5 × 10−3 eV),enables an efficient ISC rate, as we show in the SupportingInformation and has been already observed for bithiophenecompounds.8 Although motion along the dC−S coordinatewould lead to an open-ring structure on the S1 surface, we donot observe an experimental signature of that structure in theFSRS spectra, indicating that ISC is very efficient following theactivated barrier crossing. Furthermore, the estimated positionof T2−T1 CI, topologically close to the crossing seam, ensures arapid transition from T2 to the ground triplet state after theISC.Summing up, we reported FSRS spectra of photoexcited

MPT, exploring Raman resonant enhancement45 both with thetransient singlet state and with excited-state absorption of thetriplet state. FSRS spectroscopy allows following the MPTtransient states by recording Raman fingerprints from low- tohigh-frequency regions and selectively isolating singlet andtriplet intermediate species with high temporal and spectralresolution. The experimental results are combined withquantum chemical calculations used to characterize themultidimensional potential energy surfaces and the vibrationalproperties of the system along the reaction coordinates.Taken together, our results elucidate the MPT photoreaction

steps, identifying an initial rapid structural relaxation followedby thermalization on the way to the S1 PES local minimum.Also, we identified an intersystem crossing to the triplet state,which involves a slow opening of the thiophene ring through acleavage of the carbon−sulfur bond (τ3 ≈ 135 ps) required toaccess the intersystem crossing curve.Our work indicates how FSRS spectroscopy, supported by

DFT theoretical modeling, allows for an efficient tracking ofcomplex atomic motions, including structural rearrangement,vibrational cooling, and intersystem crossing in conjugatedmolecular compounds.

■ EXPERIMENTAL SECTIONSample Preparation. The 2-methyl-5-phenylthiophene waspurchased from Sigma-Aldrich (96%) and dissolved incyclohexane (Sigma-Aldrich, ≥99.9%) to a concentration thatgives 10% transmission of the 266 nm actinic pulses through a

500 μm path length. The sample circulates through a flow-cellduring the measurement to refresh the sample volume in thelaser focus (∼70 μm fwhm) before every laser pulse.Experimental Setup. The femtosecond stimulated Raman

scattering setup used for these experiments has been developedin the Femtoscopy Laboratory at the physics department of “LaSapienza” University (Rome). The details of the spectrometerhave been described elsewhere;46,47 we briefly recall here themain features of the setup. A Ti:sapphire laser generates 3.6 mJ,35 fs pulses at 800 nm and 1 kHz repetition rate. A portion ofthe laser fundamental is frequency tripled via successive stagesof second harmonic generation and sum frequency generationto give the 100 fs duration, 265 nm, vertically polarized actinicpulse (AP). The actinic pump energy is typically set at 2 μJ perpulse. The Raman pulses are synthesized from a two-stage OPAthat produces tunable IR-visible pulses, followed by a spectralcompression stage based on frequency doubling in a 25 mmBeta Barium Borate (BBO) crystal. This technique exploits thegroup velocity mismatch between the fundamental and secondharmonic in the long BBO crystal to generate an output of fewpicosecond duration pulses with typical bandwidths of ≈15cm−1 (refs 46, 48, and 49) without losing as much power as in alinear spectral filter in order to keep enough energy to stimulatethe Raman effect even after the nonlinear tuning of thewavelength. Vertically polarized pulses with 10 cm−1

bandwidths and 600 nJ intensities are obtained. The femto-second probe is a vertically polarized white-light continuum(WLC) generated by focusing the laser fundamental into aCaF2 crystal. The Raman features arise on top of thetransmitted WLC, which is frequency dispersed by aspectrometer onto a charge-coupled device. A synchronizedchopper is used to block alternating RP pulses to obtain theRaman gain using successive probe pulses; a second chopperblocks the actinic pump at 250 Hz to obtain Raman gainspectra with and without AP excitation.Quantum Chemical Calculations. The geometry optimization

and normal mode calculations on the excited singlet S1 PESalong the reaction coordinates ϕ and dC−S have been obtainedby TD-DFT, with B3LYP functional and 6-31+G** basis set,using the Gaussian 09 software package. The energy calculationon the triplet T2 and T1 PESs has been performed on the S1optimized geometry with the same functional and basis set. TheMPT normal modes for both triplet T1 and singlet S0 stateshave been obtained after a complete geometry optimizationusing 6-311++G** basis set. The same calculations were alsocarried out with the 6-31+G** basis set to test the effect of thebasis set size on the quality of the results. The negligibledifferences observed indicates that the (computationally lessexpensive) double-ζ basis set is suitable for the expensivecalculations of the S1 PES scan. We also tested the performanceof the effective core potential (Stevens/Basch/Krauss/Jasien/Cundari, SBKJC)50,51 on the sulfur atom. Also in this case wedid not observe appreciable differences in the definition of thevalence excited states of MPT. Sketches of calculated MPTvibrational mode eigenvectors are provided in the SupportingInformation.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpclett.6b01137.

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Additional details on wavepacket simulation during theFranck−Condon dynamics, probability of ISC evalua-tion, procedure to estimate the peak positions andintensities, and list of computed normal modes (PDF)Video of Franck−Condon dynamics (AVI)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: tullio.scopigno@phys.uniroma1.it.

Author Contributions∥G.B. and E.P. contributed equally to this work

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSC.G.E. is grateful for the hospitality and support while at theUniversita di Roma “La Sapienza” as a Visiting Professor andfor additional support from the donors of the AmericanChemical Society Petroleum Research Fund (53045-DNI6)and from a National Science Foundation CAREER Award(CHE-1151555). G.B., C.F., E.P., and T.S. have receivedfunding from the European Research Council under theEuropean Community’s Seventh Framework Program (FP7/2007-2013)/ERC Grant Agreement No. 207916. The authorsackowledge CINECA-Italy for the project IsC34 MUPS(ISCRA - class C).

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