Visualizing Excited-State Dynamics of a Diaryl Thiophene

Jul 18, 2016 - Subsequent slow opening of the thiophene ring through a cleavage of the carbon–sulfur bond triggers an intersystem crossing to the tr...
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Visualizing Excited-State Dynamics of a Diaryl Thiophene: Femtosecond Stimulated Raman Scattering as a Probe of Conjugated Molecules Giovanni Batignani,†,‡,∥ Emanuele Pontecorvo,†,∥ Carino Ferrante,† Massimiliano Aschi,‡ Christopher G. Elles,¶ and Tullio Scopigno*,†,§ †

Dipartimento di Fisica, Universitá di Roma “La Sapienza”, Roma I-00185, Italy Dipartimento di Scienze Fisiche e Chimiche, Universitá 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 substituted thiophene units are versatile building blocks of many photoactive materials, such as photochromic molecular switches or solar energy conversion devices. Unraveling the different processes underlying their photochemistry, such as the evolution on different electronic states and multidimensional structural relaxation, is a challenge critical to defining their function. Using femtosecond stimulated Raman scattering (FSRS) supported by quantum chemical calculations, we visualize the reaction pathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initial wavepacket 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 to the 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 provided by time-dependent density functional theory calculations.

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by transient absorption (TA) measurements12 (Figure 1a), elucidating the formation of a triplet electronic state via intersystem crossing (ISC) from the initially excited electronic singlet S1 state. The spectrum of S1 includes a strong excitedstate absorption (ESA) band centered near 480 nm and a stimulated emission (SE) band centered near 360 nm, both of which decay with the same single-exponential time-constant as the appearance of a triplet−triplet absorption band near 370 nm.12 The triplet band, which has a weak tail extending to lower energy, does not decay to any measurable extent over a period of 1 ns. Unfortunately, because of the lack of structural sensitivity of TA spectroscopy, the rearrangement reaction could not be probed, and the temporal sequencing of the photoexcited MPT geometrical reconfigurations and the relaxation path in the triplet manifold of states are still uncharted. Femtosecond stimulated Raman scattering (FSRS) is a recently developed technique allowing for vibrational spectroscopy with subpicosecond time resolution.13−15 In FSRS, a femtosecond actinic pulse (AP) initiates the photochemistry of

ight control of conjugated organic polymers based on substituted thiophene units promises to play a central role in the design of many photoactive devices, such as photosynthetic systems1−4 or photochromic molecular switches.5,6 Accessing and manipulating the photochemical properties of these versatile materials requires a fundamental understanding of the reaction pathway of their building blocks upon an ultrashort optical excitation. In fact, although the performance of such materials ultimately depends on the fundamental properties of the constituent molecules, the dynamics of large polymeric systems are often difficult to discern because of structural heterogeneities. Fortunately, detailed studies of smaller molecular building blocks provide useful insight on the microscopic behavior ruling the underlying dynamics of conjugated systems.7−11 A key question to be answered is the role of structural recombination following photoexcitation. For example, ultrafast geometrical rearrangement affects the initial stages of charge separation and recombination and also provides valuable clues about the charge-transport material properties. 2-Methyl-5-phenylthiophene (MPT), an asymmetric diaryl molecule (the schematic of the ground-state geometry structure is reproduced in the inset of Figure 1a), is a model compound which mimics the basic underlying structure of typical conjugated molecular systems. MPT has been recently studied © XXXX American Chemical Society

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

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

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

interest. The system is subsequently interrogated by a pair of overlapped pulses: the joint presence of a broadband ultrashort probe pulse (PP) and a narrowband picosecond Raman pulse (RP) induces vibrational coherences which are read out as heterodyne coherent Raman signals free of fluorescence background, with simultaneous high spectral and temporal resolution.16−18 During the last 10 years, the inherent sensitivity of FSRS to the ultrafast vibrational dynamics has been demonstrated in a number of photoactive systems.19−27

Of relevance for the present context, FSRS has been recently applied to quaterthiophenes,28 addressing the torsional relaxation 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 broad tunability of the Raman pulse which, by matching the electronic transitions corresponding to ESA and SE measured in the TA spectrum, allows us to selectively isolate dynamics from singlet 2982

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

and are accompanied by the appearance of new vibrational bands. The ground-state Raman spectrum, calculated using density functional theory with B3LYP functional30,31 and the 6311++G** basis set, is reproduced in the bottom panel (green line), showing a good agreement with the SRS spectrum. The (scaled) cyclohexane solvent spectra are reported for both wavelengths as cyan shaded peaks. The FSRS signal for 366 nm Raman pulse wavelength clearly indicates different time scales for the vibrational spectra of the singlet and triplet excited states, with the most notable features appearing in the region of the CC 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 the appearance of a dominant vibrational Raman band at 1505 cm−1, attributed to a phenyl ring stretching-deformation, E1u, and a C−C inter-rings stretching, accompanied by weaker modes at 1550 cm−1 (in-plane thiophene ring deformation, A1), 890 cm−1 (in-plane thiophene ring deformation, A1) and 995 cm−1 (phenyl ring deformation, B1u). The common time scale of such processes (135 ps) is in excellent agreement with the ISC suggested by the TA evolution. Hence, we assign the decaying 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 intersystem crossing kinetics suggested by the TA excited-state absorption decay at 480 nm. Remarkably, a closer inspection of the same figure reveals that both 1505 and 1550 cm−1 peaks follow the TA at 366 nm only for time delays longer than 30 ps. Such mismatch emphasizes at the same time one of the main limitations of TA, the difficulty in disentangling overlapping contributions, and the main strength of FSRS, the ability to reveal structural modifications occurring on the excited singlet state. The origin of this conformational rearrangement can be investigated by considering FSRS spectra measured with the RP at 480 nm (Figure 1c and top panel of Figure 2), which are in

and triplet transient species. Experimental results are rationalized based on the insight of time-dependent (TD) density functional theory (DFT) calculations,29 performed using the Gaussian 09 software package. Specifically, we evaluated the potential energy surfaces (PESs), the corresponding normal modes, and the diabatic transition probabilities for the ISC between singlet and triplet excited states to dissect from FSRS spectra the different processes contributing to the electronic 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 to S1; the photoinduced dynamics is then monitored from 200 fs to 490 ps after the optical excitation, acquiring stimulated Raman spectra (SRS) induced by a third-order susceptibility and resulting as a modification of the PP spectral profile. The Raman gain is defined as the ratio

|E P|2 |E P(0)|2

, where EP and E(0) P

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 wavelength is, indeed, initially resonant with the ESA from S1 (populated for Δt < 200 ps), although the TA is dominated by the S1−S0 stimulated emission band. In fact, the Raman response for processes in resonance with SE induces negative contributions21 to the FSRS spectrum. The evidence of positive FSRS data indicates that the SE contributions are marginal. At later times, instead, it corresponds to the ESA transition (Tm−T1) of the triplet state. In striking contrast, the RP at 480 nm is solely resonant with the singlet excited-state absorption Sn−S1. Hence, such configuration allows for selective tracking of the structural rearrangement on the S1 surface (Figure 1c). FSRS spectra for the two different excitation wavelengths are compared in Figure 2 (with baseline and solvent subtracted as described in ref 27). Particularly the FSRS spectra at both RP wavelengths show a strong resonant enhancement with respect to the (off resonance) ground-state stimulated Raman spectrum measured at λRP = 366 nm (reproduced in the bottom panel of Figure 2) 2983

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The Journal of Physical Chemistry Letters resonance with the S1 ESA peak only, i.e., sensitive to the evolution of the molecule during its dynamics in the S1 potential energy surface. A closer look to the 1350−1650 cm−1 region, reported in Figure 1c, reveals indeed a rich behavior which has to be contrasted with the monotonic decay of the transient absorption. Specifically, the two modes located around 1505 and 1550 cm−1 develop within the first picosecond and then display a blue-shift of ∼10 cm−1. A similar behavior is observed for the Raman bands at 680 cm−1, ascribed to a phenyl ring-deformation (E2g) and a thiophene ring-deformation (A1), and at 1170 cm−1, assigned to a phenyl C−H bend (E1u), with an analogous initial increase of the Raman gain and a ∼5 cm−1 blue-shift. Furthermore, the complete decay (for time delays longer than 100 ps) of the FSRS bands measured at 480 nm confirms that the singlet excited state is efficiently converted to the triplet one. To determine the time scales involved in this dynamics we performed a global analysis over the vibrational features of the entire Raman spectra. The kinetics of different Raman modes, at both RP wavelengths, are reported in Figure 4 (square and

assignment; furthermore, it rules out a reorientation of the molecule effect, which in principle may influence the experimental Raman amplitudes, because of polarization anisotropy, but which cannot explain frequency shifts. To validate such a scenario, and to elucidate its underlying structural rearrangement pathway, we performed quantum chemical (QM) calculations to characterize the involved PESs. All the calculations have been performed using the Gaussian 09 software package.33 Preliminary full optimizations performed at the B3LYP30,31 TD-DFT level of theory, in conjunction with the 6-31+G** basis set, have revealed that the relaxation process occurring in the conditions of our experiment is plausibly 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 the sulfur atoms in the thiophene. In oligothiophenes, for instance, the C−S distance is likely to play an important role in the electronic relaxation and to be implicated in the rapid intersystem crossing between singlet and triplet states.34 As a matter of fact, in the ground electronic state (S0), MPT has a nonplanar configuration with ϕ ≈ 30° and dC−S ≈ 1.76 Å (Figure 1a). On the other hand, and in agreement with recent QM calculations obtained in thiophene and bithiophene compounds,35 the full minimum in the S1 electronic state has a planar configuration with a sharply elongated C−S bond (ϕ = 0°, dC−S ≈ 1.80 Å). Other internal coordinates such as ringpuckering, shown to be potentially active in higher-energy channels35 for unsubstituted thiophene, was revealed to be energetically much higher for MPT and fully relaxed within the temporal resolution of our experiment. Hence, we disregarded such 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 excited state, S*1 and involves a local minimum which eventually undergoes magnetic transition (intersystem crossing) to a triplet MPT (the experimentally observed τ3 = 135 ps process). In order to evaluate the kinetics of the nonradiative decay along the conformational S1 PES, we performed a semiclassical simulation from the Franck−Condon region, making use of the Fokker−Planck equation36−39 (see the Supporting Information) for propagating the probability density onto the free energy surface. This approach is a convenient and computationally less expensive alternative to ab initio and/or quantum dynamical methods. Remarkably, its applicability is subject to the possibility of reducing the problem to a few semiclassical degrees of freedom, the low-frequency coordinates ϕ and dC−S in our case. As shown in Figure 5b, the wavepacket evolves toward the S1 PES local minimum within the first 2 ps, accompanied by a spread of the probability distribution along the dC−S coordinate (we hereafter concisely refer to the probability density motion, along the free energy surface, as a wavepacket evolution along the potential energy surface). This result corroborates our assignment of the experimentally observed τ1 time scale to the wavepacket motion toward the local minimum of S1, away from the Franck−Condon region of the 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 excited T2 triplet states along the same dC−S and ϕ coordinates. These calculations were first performed utilizing the S1 geometries (vertical calculations) to obtain the crossing seam from a vibrationally relaxed MPT in the S1 state to a vibrationally excited MPT in the Ti state and then utilizing the relaxed triplet

Figure 4. Kinetics of the Raman gain at selected Raman modes compared with the global fit results (continuous lines). Filled square and 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 local minimum of the S1 PES along the two different reaction coordinates, followed by a subsequent vibrational cooling. Then, a slower (τ3 = 135 ps) time scale is required allowing the system to overcome the energy barrier (Figure 5a) to the intersystem crossing region.

circle markers are used for λRP = 480 nm and λRP = 366 nm, respectively) together with the global fit traces (continuous lines) which have been obtained with the multiple exponential function RGνi = Ai exp(−t/τ1) + Bi exp(−t/τ2)+Ci exp(−t/τ3), where RGνi indicates the Raman gain for the νi mode. The slowest τ3 = 135 ps component can be clearly identified with the intersystem crossing dynamics, in excellent agreement with TA measurements (see Figure 3). The assignment of the fastest τ1 = 1.5 ps and τ2 = 11 ps, however, is more critical. Such two time scales cannot be attributed to resonance effects as the TA is monotonically decaying; here we propose that τ1 has to be related to the structural rearrangement of the molecule accompanying the ultrafast internal vibrational relaxation of the wavepacket in S1. This is in line with recent observations on the relaxation process along the torsional dihedral coordinate in quaterthiophenes.28 The blue-shift observed during the first 10 ps for the S1 modes (Figure 1c and Figure 2, top panel) suggests vibrational cooling as a compelling likelihood for the τ2 2984

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Figure 5. Excited potential energy surfaces of methyl-phenylthiophene computed by time-dependent density functional theory with B3LYP functional 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, which are illustrated in panel c. The PESs for the triplet T2 and T1 states have been calculated at the optimized S1 geometry, with the same computational precision. 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 the probability distribution at different time delays, with steps of 800 fs.

barrier crossing from the local S1 minimum would lead to this intermediate structure in the absence of efficient ISC. A ringopening mechanism has been implicated in the efficient deactivation of unsubstituted thiophene32,35,40 and could also play a role in the aryl shift reaction that was observed previously for aryl-substituted thiophenes.41,42 The aryl shift reaction refers to a nonreversible translocation of the phenyl ring from position 2 (adjacent to S) to position 3 on the thiophene ring for which a single, consistent mechanism has not yet been identified.12 However, the evaluation of calculated vibrational frequencies on the S1 state, shown in Figure S3 of the Supporting Information, is in excellent agreement with the FSRS spectra (Figure 2, λRP = 480 nm) and allows us to shed light on the origin of the τ2 process. Specifically, the predicted Raman shifts on their way to the open ring configuration show disparate trends: the phenyl-ring deformation Raman mode (680 cm−1) undergoes a large blue shift (≈40 cm−1), while the phenyl ring stretching-deformation mode (1505 cm−1) gets slightly redshifted. In addition, the frequency difference between the Raman doublet of the in-plane thiophene ring deformation (1550 cm−1 band) becomes increasingly larger, up to ≈50 cm−1. In striking contrast, all the Raman bands observed by FSRS within the first 10 ps show a same behavior, i.e. a ≈5−10 cm−1 blueshift, which can not be traced back to structural effects and clearly supports the identification of τ2 with the time

state coordinates to study the post ISC dynamics. The above calculations did not provide dramatically different PESs; therefore, in the rest of the discussion we will refer to the vertical calculations only. Remarkably, we found a crossing between S1 and T2 PESs, as shown in Figure 5a, for dC−S > 2.05 Å. With the same analysis of the PES surfaces, we also identified a possible conical intersection (CI) allowing the internal conversion (IC) between T2 and T1. The presence of efficient ISC conversion rate and the subsequent IC between excited triplet states are in line with recent work on gaseous oligothiophenes,34 showing, by highlevel calculations, that the nonradiative decay from S1 is dominated by ISC involving different triplet states. In particular for bithiophene, the species structurally most affine to MPT among the investigated ones, the presence of a local minimum on the S1 surface close to the Franck−Condon (FC) region has been reported, and it has been suggested the occurrence of a double ISC between S1 and two different triplet surfaces, followed by triplet−triplet internal conversion, eventually leading 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 a thiophene ring-opening reaction along the C−S bond, possibly occurring on the τ2 time scale, prior to ISC. Indeed, the 0.2 eV 2985

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500 μm path length. The sample circulates through a flow-cell during the measurement to refresh the sample volume in the laser focus (∼70 μm fwhm) before every laser pulse. Experimental Setup. The femtosecond stimulated Raman scattering setup used for these experiments has been developed in the Femtoscopy Laboratory at the physics department of “La Sapienza” University (Rome). The details of the spectrometer have been described elsewhere;46,47 we briefly recall here the main 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 of the laser fundamental is frequency tripled via successive stages of second harmonic generation and sum frequency generation to give the 100 fs duration, 265 nm, vertically polarized actinic pulse (AP). The actinic pump energy is typically set at 2 μJ per pulse. The Raman pulses are synthesized from a two-stage OPA that produces tunable IR-visible pulses, followed by a spectral compression stage based on frequency doubling in a 25 mm Beta Barium Borate (BBO) crystal. This technique exploits the group velocity mismatch between the fundamental and second harmonic in the long BBO crystal to generate an output of few picosecond duration pulses with typical bandwidths of ≈15 cm−1 (refs 46, 48, and 49) without losing as much power as in a linear spectral filter in order to keep enough energy to stimulate the Raman effect even after the nonlinear tuning of the wavelength. Vertically polarized pulses with 10 cm −1 bandwidths and 600 nJ intensities are obtained. The femtosecond probe is a vertically polarized white-light continuum (WLC) generated by focusing the laser fundamental into a CaF2 crystal. The Raman features arise on top of the transmitted WLC, which is frequency dispersed by a spectrometer onto a charge-coupled device. A synchronized chopper is used to block alternating RP pulses to obtain the Raman gain using successive probe pulses; a second chopper blocks the actinic pump at 250 Hz to obtain Raman gain spectra with and without AP excitation. Quantum Chemical Calculations. The geometry optimization and normal mode calculations on the excited singlet S1 PES along the reaction coordinates ϕ and dC−S have been obtained by TD-DFT, with B3LYP functional and 6-31+G** basis set, using the Gaussian 09 software package. The energy calculation on the triplet T2 and T1 PESs has been performed on the S1 optimized geometry with the same functional and basis set. The MPT normal modes for both triplet T1 and singlet S0 states have been obtained after a complete geometry optimization using 6-311++G** basis set. The same calculations were also carried out with the 6-31+G** basis set to test the effect of the basis set size on the quality of the results. The negligible differences observed indicates that the (computationally less expensive) double-ζ basis set is suitable for the expensive calculations of the S1 PES scan. We also tested the performance of the effective core potential (Stevens/Basch/Krauss/Jasien/ Cundari, SBKJC)50,51 on the sulfur atom. Also in this case we did not observe appreciable differences in the definition of the valence excited states of MPT. Sketches of calculated MPT vibrational mode eigenvectors are provided in the Supporting Information.

scale of vibrational cooling, as opposed to a ring-opening scenario anticipating the ISC. On the basis of these results we can schematically describe the whole process as follows. The photoexcited MPT initially undergoes 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 = 11 ps by vibrational cooling, which manifest itself, via anharmonic coupling, in the (higher-frequency) modes tracked by FSRS.26,43,44 Then, the carbon−sulfur distance has to reach the ISC seam at dC−S > 2.05 Å (Figure S2). The long τ3 = 135 ps time scale is required to overcome a ∼0.2 eV potential energy barrier between dC−S = 1.80 Å and dC−S > 2.05 Å configurations. Such activation barrier on the ring-opening reaction may in principle give rise to a temperature and solvent effect on the τ3 crossing time scale, without changing the reaction mechanism, which may be interesting to address in future 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 between the 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 Supporting Information and has been already observed for bithiophene compounds.8 Although motion along the dC−S coordinate would lead to an open-ring structure on the S1 surface, we do not observe an experimental signature of that structure in the FSRS spectra, indicating that ISC is very efficient following the activated barrier crossing. Furthermore, the estimated position of T2−T1 CI, topologically close to the crossing seam, ensures a rapid transition from T2 to the ground triplet state after the ISC. Summing up, we reported FSRS spectra of photoexcited MPT, exploring Raman resonant enhancement45 both with the transient singlet state and with excited-state absorption of the triplet state. FSRS spectroscopy allows following the MPT transient states by recording Raman fingerprints from low- to high-frequency regions and selectively isolating singlet and triplet intermediate species with high temporal and spectral resolution. The experimental results are combined with quantum chemical calculations used to characterize the multidimensional potential energy surfaces and the vibrational properties of the system along the reaction coordinates. Taken together, our results elucidate the MPT photoreaction steps, identifying an initial rapid structural relaxation followed by 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 a cleavage of the carbon−sulfur bond (τ3 ≈ 135 ps) required to access the intersystem crossing curve. Our work indicates how FSRS spectroscopy, supported by DFT theoretical modeling, allows for an efficient tracking of complex atomic motions, including structural rearrangement, vibrational cooling, and intersystem crossing in conjugated molecular compounds.

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EXPERIMENTAL SECTION Sample Preparation. The 2-methyl-5-phenylthiophene was purchased from Sigma-Aldrich (96%) and dissolved in cyclohexane (Sigma-Aldrich, ≥99.9%) to a concentration that gives 10% transmission of the 266 nm actinic pulses through a

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01137. 2986

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

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ∥

G.B. and E.P. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.G.E. is grateful for the hospitality and support while at the Universitá di Roma “La Sapienza” as a Visiting Professor and for additional support from the donors of the American Chemical 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 received funding from the European Research Council under the European Community’s Seventh Framework Program (FP7/ 2007-2013)/ERC Grant Agreement No. 207916. The authors ackowledge CINECA-Italy for the project IsC34 MUPS (ISCRA - class C).

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