Perpendicular State of an Electronically Excited Stilbene: Observation

Letter pubs.acs.org/JPCL

Perpendicular State of an Electronically Excited Stilbene: Observation by Femtosecond-Stimulated Raman Spectroscopy Martin Quick,*,† Alexander L. Dobryakov,† Ilya N. Ioffe,‡ Alex A. Granovsky,§ Sergey A. Kovalenko,† and Nikolaus P. Ernsting*,† †

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, D-12489 Berlin, Germany Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia § Firefly Project, Moscow 117593, Russia ‡

S Supporting Information *

ABSTRACT: In the photoisomerization path of stilbene, a perpendicular state P on the S1 potential energy surface is expected just before internal conversion through a conical intersection S1/S0. For decades the observation of P was thwarted by a short lifetime τP in combination with slow population flow over a barrier. But these limitations can be overcome by ethylenic substitution. Following optical excitation of trans-1,1′-dicyanostilbene, P is populated significantly (τP = 27 ps in n-hexane) and monitored by an exited-state absorption band at 370 nm. Here we report stimulated Raman lines of P. The strongest, at 1558 cm−1, is attributed to stretching vibrations of the phenyl rings. Transient electronic states, resonance conditions, and corresponding Raman signals are discussed.

perpendicular state P of stilbene in the first excited singlet manifold S1 has been discussed for decades.1−5 Located close to a conical intersection S1/S0, a short lifetime τP in the subpicosecond range was considered to prevent its observation (hence the name “phantom state”). For example in transstilbene, a 1200 cm−1 torsional barrier in S1 must first be overcome to reach P. Its creation is therefore slow (∼100 ps) compared with fast decay to the electronic ground state, so that no population can be accumulated in P. But even if cis-stilbene is optically excited and then twists fast in a nearly barrierless photoreaction, τP is considered still too short for observation by transient absorption.6 By exploiting our photometric accuracy for transient spectra in the near-UV, we could recently show that P of stilbene has an electronic absorption band at 340 nm and that τP ≈ 0.3 ps in n-hexane.7 Theoretical descriptions (most recently refs 8 and 9) agree that P has peculiar properties, with sudden polarization10 and geometric distortions. But how predictive are the various quantum-chemical approaches quantitatively in this fundamental case? To answer this question, one would like to compare calculated vibrational modes of P with an experimental vibrational spectrum. A resonance Raman (RR) spectrum, with the P-band for resonance, should be recorded for this purpose. Unfortunately τP ≈ 0.3 ps (line width >50 cm−1) is too short for this experiment at present. A different kinetic situation is encountered when the central double bond is chemically modified. 1,1′-Dimethyl substitution, for example, eliminates the barrier between excited trans and P forms, while τP is increased to 19 ps (in n-hexane).11 As a consequence, significant population can now be accumulated at

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the perpendicular configuration even by excitation of the trans isomer. This opportunity is preserved in 1,1′-dicyano-stilbene where τP = 27 ps; in addition, the absorption band of P is shifted to longer wavelengths around 370 nm.12 Separation from competitive kinetics and an electronic resonance closer to the visible range are helpful for femtosecond-stimulated Raman (FSR) spectroscopy.13,14 For this reason, we study 1,1′-dicyano-stilbene, starting from the trans isomer in n-hexane. Transient absorption spectra12 are now described quantitatively. Quantum-chemical calculations provide initial structures and the likely direction of change after UV excitation. Both aspects are combined in a kinetic model of transient states with associated spectra. The model, in turn, allows us to formulate resonance conditions for transient Raman spectroscopy. Focusing on the late (10 ps) time scale, we report the first few Raman lines of the perpendicular state of an electronically excited stilbene. Transient absorption spectra of trans-1,1′-dicyanostilbene are shown in Figure 1. Positive induced absorbance ΔA > 0 is caused by excited-state absorption (ESA) S1 → Sn, and negative contributions indicate bleach and stimulated emission (SE) S1 → S0. Spectra have been measured repeatedly over several years; the result in Figure 1 reproduces the one of ref 12 but with improved accuracy. A global fit requires five exponential time functions, one oscillatory time function, and an offset. Received: August 25, 2016 Accepted: September 29, 2016

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understand the spectral information in Figure 1. We employed the density functional level of theory, using the Firefly package16 which is partly based on the GAMESS(US) source code.17 PBE0/Def2-TZVPP was used for S0 and TD-PBE0/ Def2-TZVPP with frozen chemical core for S1 at and near the FC geometry. The electronic ground state of the trans isomer is discussed first. Because of steric interaction of the cyano groups with the nearest phenyl hydrogens, the phenyl rings are rotated by ca. 40° with respect to the central ethylenic bond. This gives rise to two trans conformers that differ in the relative rotation directions of the phenyl rings: C2 and Ci symmetric. The roomtemperature C2/Ci ratio is calculated to be ∼3:1. The geometry of the cis-isomer is similar that of other cis-stilbenes. Neither of the trans-conformers was found to have local minima in the excited state. Nevertheless one can expect them to exhibit different dynamics. Similar to other stilbenes, the relaxation coordinate consists of torsion around the central ethylenic bond, which is weakened upon electronic excitation. In the ground state the torsional angle in the C2-conformer already differs from 180° (it is ca. 175°), so the excited molecule has a nonzero gradient along the relaxation path. This is in contrast with the Ci-conformer, where the central dihedral angle equals 180° by symmetry, and excitation initially creates a metastable saddle point. The saddle point is characterized by more moderately rotated phenyl rings (ca. 20°), vertical emission energy of 3.01 eV (compared with vertical absorption at 3.67 eV), and oscillator strength for emission increased from 0.49 to 0.57. The pyramidalized conformations are not accessible by conventional TDDFT. One can only hypothesize that there are two of them like in stiff-stilbene.18 Resonance conditions for transient FSR spectroscopy are examined so that changing Raman spectra and lineshapes can be rationalized. For this purpose, the transient optical spectra in Figure 1 are analyzed with the idea that the examined trans isomer in S0 in fact consists of two subpopulations, of Ci and C2 symmetry as calculated above, with a 1:3 ratio. Actinic excitation at λact = 350 nm transfers the subpopulations into the excited state. Let the corresponding Franck−Condon states in S1 be denoted F′ and F, respectively. Intriguingly, we observed experimentally that the P band rises in two tranches with 300 fs and 1 ps time constants. During the first tranche, the excited-state absorption by P coexists with a stimulated emission band, but during the second tranche the P-band stands alone. Both observations point to a heterogeneity in the kinetics and spectra. Taken together with the quantumchemical aspects above, we arrive at the kinetic scheme that is presented in Figure 2. The majority species, having C2 symmetry initially, is excited into FC state F. From there it relaxes fast along higher-frequency vibrational coordinates into a partly relaxed state R. Torsion around the central bond then creates the perpendicular state P with 300 fs time constant. The minority species, having Ci symmetry initially, is similarly excited into FC state F′ and relaxes equally fast to R′, but R′, unlike R, is a saddle point, which suggests slower torsional passage to the perpendicular state (labeled P′ for consistency). On this path a region X′ on the potential energy surface is crossed that causes the broad, red-shifted band for stimulated emission in Figure 1c. Note that TD-DFT results guided the formulation of the model only for the initial state, that is, at the FC geometry; the subsequent elements followed from observation. A quantitative decomposition of the spectral evolution is performed next. The state-associated spectra (SAS) of states

Figure 1. Transient absorption spectra upon ∼350 nm excitation of trans-1,1′-dicyanostilbene in n-hexane, recorded with magic-angle polarization. Time constants from global analysis are given in the corresponding time windows. For descriptions, see the text.

Time constants τ1−4 for the main relaxation processes are given as insets. (The additional process, with τ5 ≈ 7 ps, can be assigned to cooling of P. It contributes only marginally to the observed spectral evolution and is therefore treated in the Supporting Information (SI), where the oscillation is also discussed.) Next, the panels of the figure are commented on one-by-one. (a) Population is initially prepared in the Franck−Condon (FC) region of S1. Bleach of the absorption band is seen at 340 nm and SE at 400 nm. At time-zero, ESA S1(FC) → Sn is observed at 280 nm and broadly in the range 420−690 nm. As population leaves the FC region with τ1 = 0.16 ps, ESA between 440−690 nm rises to a peak at 560 nm. An initial red shift of SE is behind the apparent ESA decay around 440 nm. (b) Within half a picosecond the ESA band around 560 nm and beyond decays with τ2 = 0.3 ps; simultaneously the SE broadens and extends further to the red. With the same time constant a new absorption band rises with a peak at 374 nm. By comparison with various stilbenes,7,12 the 374 nm band is attributed to excited-state absorption from a perpendicular form P on the S1 potential energy surface (denoted ESA(P) in the following, or “P band” for short). (c) On a slower time scale τ3 = 1.0 ps the ESA(P) band continues to rise until 3 ps, while the red-shifted SE decays completely. (d) The ESA(P) band vanishes and the electronic ground state recovers with τ4 = 27 ps, but a small offset remains that is due to photoproducts after isomerization.15 Quantum-chemical calculations of 1,1′-dicyanostilbene (in vacuo) provide molecular structures and forces that help to 4048

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broad and strong emission band around 500 nm. For direct comparison with the measured spectra the corresponding SAS has been weighted by the relative population (25%; the spectrum of X′ in Figure 3a would appear four times stronger if all population went through the Ci path). The measured steady-state, spontaneous emission spectrum f(ν̃) (distribution of photons over wavenumber ν̃) is a valuable cross check for the consistency of the performed experiment and its data evaluation. The corresponding spectrum of cross sections for stimulated emission is given by σSE ∼ f/ν̃2; this curve is shown as a black noisy line in Figure 3b. Comparison with the state-associated spectra above shows that the steadystate emission comes mainly from the transient states R and X′. The two contributions are indicated by representative lognormal distributions in Figure 3b. Their optimized position and width are similar to those of the corresponding SAS, and their amplitude ratio reflects the different lifetimes of the emission (0.3 and 1.0 ps, respectively). The Raman wavelength λR in FSR spectroscopy was originally tuned to the (P, P′) band. Picosecond transformlimited pulses at 388 nm, an excellent position in the band, are directly available in our setup.19 Unfortunately we did not succeed in creating a femtosecond Stokes probe in that region with the required stability (see the SI). Therefore, we resorted to λR = 510 and 470 nm (vertical lines in Figure 3a) hoping to gain insight from a comparison of results. Raman-induced absorption spectra were recorded on the Stokes side (in the range indicated by red horizontal bars). Diagrams for nonlinear field-matter interactions in FSR spectroscopy are shown in Figure 4. The corresponding

Figure 2. Model for the photoreaction. Calculations predict two ground-state isomers of trans-1,1′-dicyanostilbene, of Ci/C2 symmetry, having a 1:3 population ratio. Upon excitation, they follow different paths on the S1 surface. This would explain the rise of excited-state absorption by P in two tranches (cf. Figure 1b,c). F,F′, Franck− Condon; R,R′, vibrationally relaxed but before CC torsion; P,P′, perpendicular states. Emissive X′ is encountered only by the minority species (which is marked with a prime).

F′,F are taken to be identical, and similarly of R′,R and P′,P. With this simple model the SAS in Figure 3a are obtained from the data in Figure 1. The emissive domain X′ is seen to have a

Figure 4. Energy level diagrams illustrating the generation of Raman signal via interaction with the Raman (blue) and probe (red) fields. A preceding “actinic” pulse (not shown) has placed initial population in S1. Diagram notation is taken from the supporting information of ref 21, where corresponding lineshapes are also shown. Vibrational modes in S1 are exposed by diagrams 6,7 and modes in the electronic ground state S0 by diagram 8b (green bars).

interaction sequences are assumed to be well-separated in time from the preceding actinic excitation S0 → S1. (For details, the reader is referred to ref 20 and the supporting information of refs 21 and 22.) In principle, they allow the prediction of vibrational lineshapes and their change when λR is varied provided that the vibronic structure of ESA and SE bands is well-known. In practice, that knowledge is not available, and we see no way how an observation of lineshapes, upon tuning λR, can be used to identify the underlying electronic transition. But two useful conclusions may be drawn regardless. First, if some lineshapes change while others remain, there must be different vibronic structures underneath. Second, by actinic excitation one induces the transient observation not only of vibrational

Figure 3. (a) State-associated spectra SAS, when the transient absorption in Figure 1 is analyzed with the kinetic model of the previous figure. The SAS of X′ has been scaled by 1/4 for direct comparison with Figure 1b,c. Dashed vertical lines mark the Raman wavelengths λR used, and horizontal bars indicate the Stokes windows that were probed. (b) The stationary stimulated emission spectrum (black) is consistent with the SAS above (see text). 4049

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Figure 5. FSR spectra in the excited state with λact = 340 nm and λR = 470 and 510 nm, respectively. (a) Before t = 0.1 ps, spectral features in S1 (marked by dots or double-dots) are distinguished by their behavior upon changing λR (see text). (b) Both sets of lines decay with τ2 = 0.3 ps while new signals rise simultaneously. Some appear for both Raman excitations, but others show up only when λR = 470 nm. (c) The signals decay, similar to SE(X) in TA, with τ3 = 1 ps. (d) Very weak Raman features decay on the 30 ps time scale and are reproduced with both λR values. They are assigned to the perpendicular state P because of their lifetime (see Figure S3 for corresponding kinetics). For their search, time-independent Raman signal due to photoproducts has been subtracted. Also, the number of averages has been increased by a factor 5 compared with (a−c).

To understand this behavior at least qualitatively, consider the state-associated electronic spectra at t = 0.1 ps and just beyond. In this early time window, the excited population exists mostly in the vibrationally relaxed state (R, R′). Electronic resonances ESA(R, R′) and SE(R, R′) are engaged with both λR values but in different proportions, as can be seen from Figure 3a (green line). Correspondingly, diagrams 6, 7, and 8b (Figure 4) apply in different proportions, and modes in S1 as well as in S0 are expected to be seen transiently. This may explain the richness of the FSR spectra in Figure 5a and their sensitivity to λR. The fact that two sets of lines are observed (• and ••) could reflect the aforementioned heterogeneity, that is, the coexistence of states R and R′ corresponding to subpopulations of (approximately) C2 and Ci symmetry. (b) From 0.1 to 0.72 ps, all early Raman signals decay with 0.3 ps time constant, like ESA(R, R′) in transient absorption (τ2). On the same time scale, new Raman lines appear. The ones at 528 and 1546 cm−1 are observed with both Raman excitations, while the line at 456 cm−1 is visible with only λR =

modes in S1 (diagrams 6 and 7) but also of modes in S0 as seen from S1 (diagram 8b). These qualitative features can be recognized in the transient FSR spectra, which will be discussed next. Excited-state FSR spectra are shown in Figure 5. Constant contributions by the electronic ground state and solvent have been removed by subtracting the (averaged) signal from negative delays. As before, we discuss the panels of the figure one-by-one. (a) At early delay time, rich activity is seen for both Raman excitations. With λR = 510 nm, a negative sign for S1 Raman lines is concluded by inspection. When the Raman wavelength is decreased to λR = 470 nm, some S1 lines keep their position (marked •) while others appear to shift by +10 cm−1 (marked ••). Most likely the shift is actually a change of spectral shape. It follows that resonance conditions for the •• lines should be qualitatively different at the two Raman wavelengths, whereas for the • lines they are the same. 4050

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470 nm. We associate the first two with emissive X′. The 456 cm−1 line presents a bit of a puzzle; it could belong to P, which has just been created and therefore is hot. Both signal types appear with τ2 = 0.3 ps and decay with τ3 = 1 ps. (c) By 3 ps the previous excited-state signals have decayed, and the corresponding spectrum at this delay time contains Raman signals from the P,P′-state and S0-signals (negative, that is, Raman gain ΔA < 0) of the photoproduct(s). We show this frame mainly so that the reader can form an impression of the size of the expected lines due to the perpendicular state, relative to early signal and to noise. The nonresonant FSR spectrum of the electronic ground state is shown in the SI. The main result of this Letter is shown in Figure 5d, where the long-time (picosecond) evolution of the FSR spectra is presented. Raman-induced signal that is constant during this time interval is attributed to the S0 spectrum of the photoproduct(s); it has been subtracted for the figure. Remaining spectral activity has amplitudes of a few μOD only. To recognize systematic evolution of specific signals, each spectrum shown in Figure 5d has been sampled five times more compared with Figures 5a−c. A few vibrational shapes are seen with both Raman excitations, and they decay with time constants between 20 and 30 ps. They are identified as Raman signals from the perpendicular state because their decay matches the electronic lifetime of the P state (see Figure S3 for corresponding kinetics). Three Raman lines can be ascribed with certainty to P at 320, 430, and 1558 cm−1. A conclusive vibrational assignment is beyond the scope of this work. Here we simply compare the strongest line of P, at 1558 cm−1, to modes observed in this region for unsubstituted stilbene14 and to previous computational findings for pristine and stiff-stilbene.9,18 In S0 of stilbene, the strongest (and highest-frequency) line is due to the central CC stretching motion where the bond order is highest. Not far from it one finds quadrant vibrations of the phenyl rings, still rather intense. In pretorsional S1, the central bond weakens while the 2−3 and 5−6 bonds in the phenyl rings show slight contraction; as a result, quadrant vibrations (with some contribution from the central bond) become both most intense and of highest frequency. In P of stilbene and stiff-stilbene the central bond is predicted to weaken further, while the 2−3 and 5−6 phenyl bonds remain of highest bond order. Coming back to 1,1′dicyanostilbene, it follows by analogy that the most intense line P should correspond to quadrant phenyl vibrations. For an outlook, we list the tasks, concerning trans-1,1′dicyanostilbene, which must be left to others. (i) Set up quantum-chemical calculations on a high level to predict the vibrational modes of the P state. (ii) Exploit electronic resonance at ∼400 nm to improve the signal of reported lines and then measure the entire Raman spectrum of P. (iii) Use ethylenic 12C/13C and phenyl H/D replacement14 to consolidate assignments.

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Letter

AUTHOR INFORMATION

Corresponding Authors

*M.Q.: E-mail: [email protected]. *N.P.E.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (ER 154/10-4). We are grateful to Th. Lenzer and K. Oum for a critical reading of the manuscript. I.I. and A.G. thank the Supercomputing Center of the Lomonosov Moscow State University for computational support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01923. Time traces for transient absorption and FSR signal, S0 Raman spectra of 1,1′-dicyanostilbene and of photoproduct(s), and considerations of signal/noise. (PDF) 4051

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The Journal of Physical Chemistry Letters and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347−1363. (18) Quick, M.; Berndt, F.; Dobryakov, A. L.; Ioffe, I. N.; Granovsky, A. A.; Knie, C.; Mahrwald, R.; Lenoir, D.; Ernsting, N. P.; Kovalenko, S. A. Photoisomerization Dynamics of Stiff-Stilbene in Solution. J. Phys. Chem. B 2014, 118, 1389−1402. (19) Kovalenko, S. A.; Dobryakov, A. L.; Ernsting, N. P. An Efficient Setup for Femtosecond Stimulated Raman Spectroscopy. Rev. Sci. Instrum. 2011, 82, 063102. (20) Weigel, A.; Dobryakov, A.; Klaumünzer, B.; Sajadi, M.; Saalfrank, P.; Ernsting, N. P. Femtosecond Stimulated Raman Spectroscopy of Flavin after Optical Excitation. J. Phys. Chem. B 2011, 115, 3656−3680. (21) Dobryakov, A. L.; Quick, M.; Ioffe, I. N.; Granovsky, A. A.; Ernsting, N. P.; Kovalenko, S. A. Excited-state Raman Spectroscopy with and without Actinic Excitation: S1 Raman Spectra of transazobenzene. J. Chem. Phys. 2014, 140, 184310. (22) Quick, M.; Dobryakov, A. L.; Kovalenko, S. A.; Ernsting, N. P. Resonance Femtosecond-Stimulated Raman Spectroscopy without Actinic Excitation Showing Low-Frequency Vibrational Activity in the S2 State of All-Trans beta-Carotene. J. Phys. Chem. Lett. 2015, 6, 1216−1220.

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