Ultrafast Dynamics of Highly Excited trans-Stilbene ... - ACS Publications

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Ultrafast Dynamics of Highly Excited trans-Stilbene: A Different Twist Jie Bao and Peter M. Weber* Department of Chemistry, Brown University, Providence, Rhode Island 02912

ABSTRACT Using femtosecond time-resolved resonant two-photon ionization coupled with photoelectron spectroscopy, we have explored the structural dynamics of a previously uncharted, highly excited state of trans-stilbene. The molecule responds to excitation to the S5 surface by twisting its phenyl groups about the carbon-carbon bonds. The motion of the coherent wave packet is reflected in the time-dependent spectrum, yielding an oscillatory frequency of 6.7 cm-1. This structural oscillation competes with a transition to a lower surface with a time constant of 280 ((30) fs. The dynamical motions are intriguingly different from those that have previously been observed in lower excited states, revealing a structural dynamics that strongly depends on the electronic surface. SECTION Dynamics, Clusters, Excited States

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TRPES is particularly suitable for monitoring coherent wave packet motions19 and has been proven to be an effective tool to study the dynamics of stilbene at energies above the barrier of the trans-cis interconversion.20 Trans-stilbene has three major absorption bands in the UV region.21 The first band, centered at 294 nm, accesses the S1 and S2 states. We utilized the second band, centered at 223 nm, which is the absorption to the S5 state of transstilbene. With a laser wavelength of 209 nm (5.93 eV), the excitation by the pump laser prepares the molecule in S5 with additional energy inserted into vibrations. The time evolution is probed with a 418 nm (2.97 eV) pulse that ionizes the molecules to the D0 (12Au) state, ejecting photoelectrons that are collected and analyzed. The photoelectron spectrum shown in Figure 1, which was taken with the pump and probe lasers overlapped at time zero, shows four dominant peaks labeled A-D. The spectral signature of the photoionization out of S5 persists for only a very short time, as shown in Figure 2. All peaks feature a fast decay time of 280 ((30) fs, as well as a slower component that decays on a picosecond time scale. This suggests that all peaks belong to ionization out of the same level but leading to different vibrational states of the ion. On the basis of a spacing of the peaks of 0.183 eV, and the ionization potential of trans-stilbene of 7.66 eV,20 we identify the peaks as a progression with vibrational quantum numbers from 3 to 6. The energy spacing of 0.183 eV, or 1480 cm-1, is close to the frequency of the central CdC double bond stretching mode, for which values of 1639 and 1551 cm-1 have been reported for the S0 and the S1 surface, respectively.1 The CdC stretch frequency is expected to drop as the electron

hile stilbene has been extensively studied experimentally and theoretically for more than 60 years,1 it retains its status as an attractive model system for photochemical dynamics.2-4 Its structural response to optical excitation is of broad interest to fundamental studies5-8 and of critical importance in many practical applications. For example, as the backbone of poly(paraphenylenevinylene), which is one of the most widely used polymers for low-cost electronic devices and solar cells, stilbene affects the polymer's charge carrier mobility because its structural distortions limit the π-conjugation.9-11 On the basis of previous research, it is well-known that upon photoexcitation from S0 (11Ag) to the S2 (21Bu) state, stilbene rapidly decays to the S1 (11Bu) state, on which the trans and cis forms meet at a conical intersection before relaxing back to the ground state.1,4 The vibrational motions and dynamics on these states have been elucidated in considerable detail using computational7,9,12,13 and experimental techniques, such as total fluorescence,7 dispersed fluorescence,14,15 multiphoton ionization, and femtosecond depletion spectroscopy.6 Even so, the exploration of stilbene's structural response to photoexcitation has been confined to its lowest singlet states, leaving a large range of the UV absorption region uncharted. In our work, we explored this high-energy region, uncovering dynamical processes in the S5 (31Bu) state that are distinctly different from the trans-cis interconversion motions observed on lower surfaces.1 These processes are described here. To explore the ultrafast dynamics of trans-stilbene, we applied femtosecond time-resolved resonant two-photon ionization coupled with photoelectron spectroscopy. Timeresolved photoelectron spectroscopy (TRPES) has been demonstrated to be a very informative differential detection technique,16-18 able to reveal fine details about molecular dynamics that are lost with integral detection techniques.

r 2009 American Chemical Society

Received Date: October 15, 2009 Accepted Date: November 13, 2009 Published on Web Date: November 20, 2009

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DOI: 10.1021/jz900147b |J. Phys. Chem. Lett. 2010, 1, 224–227

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Figure 1. Photoelectron spectrum upon ionizing trans-stilbene out of S5 at time zero. The ion energy (EIon = E4ω þ E2ω - EKE) is the net energy deposited by both pump (4ω) and probe (2ω) photons into the ion after ejection of the photoelectron. The vibrational energy scale (EVib = EIon - I.P.) refers to the energy deposited on the ion ground state (D0), which is higher than the molecular ground state by the ionization potential (I.P.). The peaks are labeled by their vibrational quantum number of the central CdC double bond stretching vibrational progression, as explained in the text.

Figure 3. Contour photoelectron spectrum of peak A and curve fit showing the time-dependent peak position. Left panel: Timeresolved photoelectron spectrum of peak A, showing the intensity as a function of the kinetic energy and the delay between the pump and the probe pulses. The intensity is on a logarithmic scale, with a red color representing a higher intensity. Right panel: The peak position of peak A at different delay times and the fit to a sine square function.

pulses. The time dependence of the peak center is plotted in the right panel of Figure 3. It is apparent that during its brief lifetime, the peak shifts from its initial value of 0.63 eV toward lower kinetic energies. After about 1 ps, it reaches a minimum kinetic energy of 0.59 eV and thereafter shifts back toward higher kinetic energies. The peak intensity decreases rapidly so that it is impossible to follow the peak position further. The kinetic energy of the ejected photoelectrons is known to depend sensitively on the molecular structure at the very moment of the ionization transition.16,22,23 The time dependence of the photoelectron peak center therefore reflects the structural change of trans-stilbene upon electronic excitation to S5. The oscillatory nature of the motion, of which we see only a part of an oscillation because of the rapid decay of the electronic level, results from the time evolution of a coherent wave packet with a frequency given by the period of the oscillation. The experimental data fit very well to an oscillatory function Ek ðtÞ ¼ Ek0 þ A 3 sin2 ðωt þ jÞ

Figure 2. Time dependence of the photoelectron peaks A, B, C, and D corresponding to the four peaks of Figure 1. Plotted with colored squares are the experimental data for each peak, while solid lines are fits using a doubly exponential function, convoluted by a Gaussian instrument function. All traces have a fast decay component of 280 ((30) fs and a slow decay component of about 5 ps.

where the time-dependent kinetic energy Ek oscillates with an angular frequency ω. Ek0 represents the electron kinetic energy corresponding to the molecular structure as it is projected from the ground electronic state. The phase shift arises because even at zero delay time, the molecular structure can evolve within the duration of the excitation and ionization pulses. The optimized parameters are Ek0 = 0.629 eV, A = -0.034 eV, ω = 1.27 1012 s-1, and j = 0.68. Consequently, the oscillation has a vibrational frequency of 6.7 cm-1. On the basis of previous studies,1,13 the only vibrational mode with a similar frequency is the antisymmetric torsional vibration of the phenyl groups about the C-C single bonds. Since this vibration is symmetric about the planar molecular geometry, displacement in one direction leads to the same spectral shift as the reverse displacement in

density between the central two carbon atoms decreases. On the D0 surface of the ion, the electron density of the central double bond is significantly reduced compared to that for the ground-state molecule. This accounts for the decrease of the vibrational frequency and makes 1480 cm-1 in good agreement with the values on other surfaces. Closer inspection of the vibrational peaks reveals that their center positions depend on the delay time between the pump and the probe pulses. Shown in the left panel of Figure 3 is a contour plot of the photoelectron spectrum of peak A (in Figure 1) of trans-stilbene. The logarithmic intensity of the peak, encoded by the color, is plotted against the kinetic energy and the delay time between the pump and probe


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approximately 200 μJ. The beam was upconverted to the second (2ω, 418 nm) and fourth (4ω, 209 nm) harmonics with BBO crystals. The laser beams intersected the molecular beam of stilbene, which was generated by entraining the sample in a stream of helium carrier gas at 130 °C and expanding through a 100 μm nozzle and a 150 μm skimmer. The molecular beam and the laser beams crossed at the laser focus, where the molecules were pumped by the 4ω beam (photon energy, 5.93 eV) and, after an adjustable delay time, were ionized by the 2ω beam (photon energy, 2.97 eV). The photoelectrons were recorded using a linear time-of-flight photoelectron spectrometer that provided us with the timedependent photoelectron spectra.

Scheme 1. trans-Stilbene Responding to Electronic Excitation with Structural Dynamics That Depends on the Excited Surfacea

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: [email protected]. a Excitation to the ion leads to an expansion of the CdC double bond; excitation to S1 and S2 launches the molecule on a path to isomerization; and, as shown here, excitation to S5 leads to oscillatory torsional motions of the phenyl groups before the molecule decays to a lower surface. (The electronic energies of S1 and S2 are from ref 27, and the experimental value of the ionization energy is from ref 20.)

ACKNOWLEDGMENT The authors acknowledge the assistance of Dr. Michael Minitti. This work is supported by the Army Research Office under the Contract W911NF-06-1-0463.

REFERENCES

the opposite direction. Therefore, the kinetic energy cycles twice while the molecule undergoes one vibrational period, as is modeled with the squared sine function. The vibrational frequency of 6.7 cm-1 that we find for the S5 state is comparable to the values reported for antisymmetric phenyl torsion about the C-C single bonds of 8 cm-1 for the S0 state and 34 cm-1 for the S1 state.1 In summary, trans-stilbene is found to exhibit a multitude of structural responses upon electronic excitation that depend on the initially prepared state. As summarized in Scheme 1, excitation to the S5 surface launches an antisymmetric torsional rotation of the phenyl groups. The frequency of oscillation of the coherent wave packet is found to be 6.7 cm-1. This structural response of the molecule is distinctly different from the responses in other electronic states. As revealed by the vibrational progression upon excitation to the ion, ionization from S5 is largely associated with the expansion of the carbon-carbon double bond. Furthermore, previous studies have shown that excitation to the lower surfaces of transstilbene leads toward isomerization to the cis structure. The newly found structural twist of the molecule upon electronic excitation in the far-UV range of the spectrum adds to the already impressive lineup of motions of trans-stilbene.

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Experimental Section The apparatus of this study has been described previously.24-26 Briefly, we used a regeneratively amplified laser system (Positive Light, Spitfire) with a near-IR tuning range between 760 and 840 nm, operating at 5 kHz, with pulse durations of about 100 fs. For this experiment, the system was optimized to operate at 836 nm with a pulse energy of


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Ultrafast Dynamics of Highly Excited trans-Stilbene ... - ACS Publications

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