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Cyclohexadiene Revisited – a Time Resolved Photoelectron Spectroscopy and Ab Initio Study Oliver Schalk, Ting Geng, Travis Thompson, Noel K Baluyot, Richard D. Thomas, Enrico Tapavicza, and Tony Hansson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10928 • Publication Date (Web): 28 Mar 2016 Downloaded from http://pubs.acs.org on March 28, 2016

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The Journal of Physical Chemistry

Cyclohexadiene Revisited – a Time Resolved Photoelectron Spectroscopy and Ab Initio Study Oliver Schalk,

1,*

1

Enrico Tapavicza, 1

2

2

Ting Geng, Travis Thompson, Noel Baluyot, Richard D. Thomas, 2,*

Tony Hansson,

1

1

Department of Chemical Physics, AlbaNova University Centre, Stockholm University, Roslagstullsbacken 21, 109 61 Stockholm, Sweden

2

Long Beach, Department of Chemistry and Biochemistry, California State University, Long Beach, CA, United States of America

* O. Schalk. Electronic address: [email protected]; E. Tapavicza. Electronic address: [email protected]

Abstract We have reinvestigated the excited state dynamics of cyclohexa-1,3-diene (CHD) with timeresolved photoelectron spectroscopy and fewest switches surface hopping molecular dynamics based on linear response time-dependent density functional theory after excitation to the lowest lying ππ* (1B)-state. The combination of both theory and experiment revealed several new results: First, the dynamics progress on one single excited state surface. After an incubation time of (35 ± 10) fs on the excited state, the dynamics proceed to the ground state in an additional (60 ± 10) fs, either via a conrotatory ring-opening to hexatriene or back to the CHD ground state. Moreover, ring-opening predominantly occurs when the wavepacket crosses the region of strong non-adiabatic coupling with a positive velocity in the bond alternation coordinate. After 100 fs, trajectories remaining in the excited state must return to the CHD ground state. This extra time-delay induces a revival of the photoelectron signal and is an experimental confirmation of the previously formulated model of two parallel reaction channels with distinct time constants. Finally, our simulations suggest that, after the initially formed cis-Z-cis HT rotamer, the trans-Z-trans isomer is formed, before the thermodynamical equilibrium of three possible rotamers is reached after 1 ps.

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I. INTRODUCTION Cyclohexa-1,3-diene (CHD) is one of the most thoroughly studied molecules in time resolved photoexcitation experiments1-16 and ab initio computations9, 15, 17-31 and is the subject of two recent reviews.32, 33 The large interest originally stems from CHD being the prototype for a pericyclic reaction used to explain the Woodward-Hoffmann rules.34 The earliest models of the photodynamics were extended later via the van der Lugt / Oosterhoff model35 and beyond.28 In addition, the photoinduced dynamics of many larger systems are based on the cyclohexadiene dynamophore36, fulgide,

38, 39

37

such as the photoswitches diarylethylene and indolyl-

and the ring-opening dynamics of provitamin D, which is an intermediate in the

synthesis of vitamin D.28-30 The general view of the photo-induced dynamics upon excitation to the lowest lying ππ*-state (1B-symmetry in the Franck-Condon region) is as follows: After excitation, the dynamics proceed to a first conical intersection (CoIn) with a state that is both dark to one photon absorption and has 2A-symmetry in the Franck-Condon region. In the gas phase, a time constant of (60 ± 15) fs for the initial excited state to reach this CoIn was extracted7, 10, 14, 40 (slightly varying between the experiments, the shortest value being 30 fs16 ). After switching to the 2A-surface, the seam with the ground state is reached within 60 - 80 fs after excitation7, 10, 14, 16, 40

where the molecules can either return to the ground state without bond breaking or

undergo a conrotatory ringopening reaction to give Z-hexatriene. Quantum yields for the ringopening process were calculated to 60 % 15 and 62 % 30 in the gas phase, while experimental studies on CHD solvated in pentane gave a value of 41 %.41 In the gas phase, early scattering experiments provided no evidence for return to the CHD ground state,8, 42 but later work using multiphoton probe provided a maximal hexatriene yield of 73%

43

while a recent study with

90 nm probe pulses showed the build-up and recovery traces of hexatriene and CHD, respectively,14 from which the authors extracted a hexatriene quantum yield as low as 30 %. On the ground state surface, the molecules rearrange from cis-Z-cis-hexatriene (cZc) to cis-Ztrans-hexatriene (cZt) and finally to trans-Z-trans-hexatriene (tZt) which was studied in the liquid phase and takes place within a few hundred of femtoseconds.4, 5, 6 In the gas phase, a recent study suggests that several rotamers can be formed immediately; however, only the first 200 fs of the dynamics were investigated.15 Despite the apparently well studied reaction path, there are still unresolved questions concerning the photoinduced dynamics of CHD. In a recent theoretical treatment, Tapavicza et al. found no evidence for two excited states in the dynamics of CHD.30, 44 Instead, they


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report that the character of the excited state changed along the reaction coordinate, resulting in a decrease of the oscillator strength.44 The absence of a presumably present ‘dark’ valence state in the reaction path was also earlier discussed with respect to the structurally similar cyclopenta-1,3-diene (CPD) molecule. Early experiments on CPD seemed to confirm the presence of two excited states, i.e. analogous to CHD.45, 46 However, later calculations and further experiments revealed that the dynamics take place not only on one potential energy surface but that rearrangements first take place in the plane of the C5-ring before out-of-plane motions break the C2-symmetry of the system and drive the molecules toward a CoIn with the ground state.47,

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This step-wise mechanism thus was misinterpreted as dynamics on two

different states. A similar symmetry break was also observed for CHD28, and although the dynamics localize at different dynamophors (the CPD-CoIn is similar to ethylene while CHD shows delocalized dynamics), the similarities between the molecules might be large enough to rationalize the absence of the ‘dark’ 2A-state from the reaction coordinate. Further questions related to CHD concern the branching ratio between an unreactive return to the ground state and the ring opening reaction. As described earlier, gas phase experiments found no evidence for an unreactive return to the ground state8, 42 although theory predicts large quantum yields.30 Several mechanisms were proposed. The CoIn with the ground state was found to be near a ridge where the wavepacket could split, to proceed to either the ground state or to hexatriene.17 Later, several passes over the CoIn were discussed such that momentum toward the ground state structure is gained.17,

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Alternatively, calculations

suggested that the wavepacket split much earlier in the reaction process, which then could lead to the two different products.28 Finally, there used to be the issue of the absence of a prompt response in photoelectron pumpprobe experiments where only a delayed signal via ionization through Rydberg states was observed after excitation to the bright ππ*-state and a subsequent probe at 400 nm.10 In a very recent TRPES-study, Pemberton et al. resolved this issue and found a prompt response from CHD which they assigned to the 1B-state.16 However, in that study, the authors claim a static photoelectron band for the 1B-state on one hand, while, on the other, they analyze the shift of the photoelectron band along the 2A-state. It is evident that more research along that line is required. In the present study, we used time-resolved photoelectron spectroscopy (TRPES) in combination with the linear response time-dependent density functional theory fewest switching surface hopping (TDDFT-SH) method30,

44, 49

and tried to answer the above



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mentioned questions. In the following sections, we will first describe our experimental and theoretical methods (section II) before we present and analyze our results (section III). Thereafter, we discuss the impact of our findings and compare them to previous results (section IV) before we summarize and give an outlook to future experiments (section V).

II. METHODS A. Experimental methods CHD was purchased from Sigma-Aldrich with a nominal purity of 97% and used without further purification. Absorption spectra were taken in a 1 cm quartz cuvette (Hellma) under saturated vapor pressure using a Cary 5e photospectrometer (Varian). Our time-resolved photoelectron spectrometer experiment was partly described before.50 Briefly, the magnetic bottle type spectrometer consists of a 5.5 m long flight tube which allows for an E/∆E resolution of better than 100 which means that the energy resolution in the low energy part of the spectrometer is largely defined by the time resolution of our experiment. Femtosecond laser pulses were obtained from a Ti:Sapphire regenerative amplifier (Coherent Legend USP-HE) with an output energy of 4 mJ/pulse at a repetition rate of 1 kHz. Pump and probe pulses at 267, and 400 nm were generated by frequency doubling and consecutive mixing with the fundamental laser beam in beta-barium-borate crystals. In our experiments, the laser beams were attenuated to 1 µJ (267 nm pump) and 20 µJ (400 nm probe) for pump and probe pulses, respectively, and focused weakly into the interaction region by a concave spherical aluminum mirror (f/150 for the pump and f/125 for the probe pulse, ensuring a smaller spot size of the probe pulse in the interaction region). The cross-correlation between pump and probe pulses was measured in xenon and was (125 ± 15) fs. The spectral bandwidth of each of the pump −1

and the probe pulses was around 200 cm . The time delay between the two pulses was controlled by a motorized linear translation stage. Perpendicular to the incoming laser pulses, the sample was inserted in the interaction region of the magnetic bottle spectrometer via a gas needle. At each time delay, the measured pump-probe signal was corrected by subtracting the background signals due to the pump and probe laser pulses alone.

B. Computational methods All calculations were performed with the density functional (DFT)51 and time-dependent density functional theory (TDDFT)52 modules of the TURBOMOLE quantum chemistry


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program.53 Non-adiabatic molecular dynamics (MD) were carried out with the TDDFT-SH implementation.30, 44 All calculations employed the def2-SVP54 basis set and the PBE055, 56 hybrid exchange correlation functional. For CHD, usage of the larger basis sets def2-TZVP54 and aug-cc-pVTZ57, 58 leads to excitation energies that are 0.14 and 0.22 eV higher than the def2-SVP results. This leads to a def2-SVP S1 ionization potential that is less than 0.2 eV lower than the ones obtained by def2-TZVP and aug-cc-pVTZ calculations (Table S1 in the supporting information). Since this error is below the accuracy expected for the DFT/TDDFT approach,

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we use the computationally more efficient def2-SVP basis set for our calculations.

Along the entire ring-opening pathway, differences between the calculations using these basis sets never exceed 0.22 eV. To minimize problems associated with singlet instabilities at conical intersections,60,61 we used the Tamm-Dancoff approximation62 (TDA) to TDDFT. The ground state Boltzmann ensemble of CHD at 300 K was generated by ab initio MD60 with a time step of 50 au (~1.1 fs). A Nosé-Hoover63,

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thermostate with characteristic

response time of 500 au was used. 200 structures were taken from the MD trajectory to calculate the electronic absorption spectrum and Gaussian broadening with a FWHM of 0.4 eV was applied to each individual spectrum as described in ref 63. The broadened spectra were averaged to obtain the spectrum of the Boltzmann ensemble at 300K. For TDDFT-SH we followed the procedure previously described in refs 30 and 44. 119 structures and velocities were taken from the ground state Boltzmann ensemble and used as starting structures and starting velocities for the excited state TDDFT-SH simulations. To simulate the time-resolved photoelectron ionization spectrum (TRPES), we calculated the total ground state energy of a singly positively charged molecule for each molecular structure from each of the 119 TDDFT-SH trajectories. If the molecule is in the ground state, the ionization potential EIP is calculated by the difference of the ground state energy of the neutral molecule E0(0) and the total energy of the positively charged molecule E0(+1): (1) In the case that the trajectory is currently in the first excited state, the ionization potential is calculated by the ground state ionization potential plus the excitation energyω1. (2) The photoelectron kinetic energy is then obtained by subtracting the energy of the two-photon



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probe pulse energy of 6.18 eV plus a correction of 1.19 eV from the calculated ionization potential. This correction accounts for the systematic underestimation of the ionization potential of our calculation compared to the experimental value (see Table S1). The energy was chosen to obtain the same vertical ionization potential as in the experiment. For each trajectory the electron kinetic energy was broadened by 0.1 eV in the energy dimension. Afterward, the ionization energies of all trajectories were averaged to obtain the macroscopic ionization potential. The obtained signal was then temporally broadened using a Gaussian line shape with a FWHM of 120 fs.

III. RESULTS A. UV-Vis absorption The UV-absorption spectrum of CHD is shown in Figure 1 and agrees with the spectra from literature.67, 68 The first absorption maximum lies at 251 nm (5.06 eV) and can be assigned to the lowest lying ππ*-state. This means that our pump laser pulse excites the sample on the red edge of the absorption band with little excess energy. The higher lying states around 200 nm were assigned to the π3px-Rydberg state and include a short vibrational progression.67

B. Time-resolved photoelectron spectroscopy The time resolved photoelectron spectrum of CHD excited at 267 nm and probed with 400 nm is shown in Figure 2. The highest lying peak of photoelectron kinetic energies lies at 2.48 eV while the onset of the band is located at 2.65 eV. The peak maximum is slightly below the energy cut-off of one pump (267 nm = 4.65 eV) and two probe photons (400 nm = 3.10 eV), which is 4.65 eV + 2 x 3.10 eV – 8.25 eV = 2.60 eV, where 8.25 eV is the vertical ionization potential (IP) of CHD.69 The differences of the two values will be discussed below. In Figure 3, we present the time zero spectra of CHD (open circles) which we fitted with a set of Gaussians (blue fits). As can be seen, the spectrum consists of a series of narrow banded peaks (FWHM = 30-100 meV) which are superimposed on a broad-banded background. These peaks originate from intermediate Rydberg states which are passed during the two photon probe process. Their assignment is challenging.10, 16 More information on this topic can be found in the supporting information. In addition to the Rydberg peaks, we observe a signal from a broad background. We tentatively fitted the spectrum with two Gaussians (green curves in Figure 3). This background must be assigned either to ionization via a valence state or to direct two photon


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ionization. More details on this band are found in the upcoming sections. The time-resolved photoelectron spectrum of CHD can be fitted with a monoexponential decay of 60 ± 10 fs with a tiny residual ( 100 fs), the BAC-velocity seems to decrease due to coupling with other modes (dephasing) and is not sufficient to overcome a possible barrier for breaking the C5-C6 bond so that the molecule is forced to return to the ring-closed ground state. This picture emphasizes that not only the momentum but also the geometry where the seam of a conical intersection is crossed plays an important role for product formation. The slow component found in the experimental TRPES, confirms the existence of a slow unreactive channel besides a fast parallel reactive channel.30, 33 For the hexatriene ground state, the simulations allowed us to follow the dynamics for 5 ps and we found that, after the Woodward-Hoffmann allowed conrotatory ring-opening, the tZt conformer is formed within 200 fs while the other conformers are formed afterwards, in slight disagreement with earlier work based on Ehrenfest dynamics simulations.15 Therefore, more work will be necessary to disentangle the ground state rearrangements on a 100 femtosecond time scale. A temperature dependent study of the reaction pathway might give further information.

In the near future, we plan further investigations on cyclic dienes before focusing more on the interplay of TRPES with TDDFT-SH trajectory calculations on larger molecular systems.

Acknowledgment We acknowledge the generous support and technical advice of Prof. Dr. Raimund Feifel and the outstanding technical help of Dr. John Alexander, especially with the development of our



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measuring software. This work was supported by the Swedish Research Council (VR), the Knut and Alice Wallenberg Foundation, the Lars Hiertas Minne Foundation, and the WennerGren Foundation. In addition, ET thanks the California State University, Long Beach for a start-up fund as well as the CSUPERB biotechnology fund.


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from Equilibrium. Proc. Natl. Acad. Sci. 2001, 98, 7117–7122. [43] Kotur, M.; Weinacht, T; Pearson, B. J.; Matsika, S. Closed-Loop Learning Control of Isomerization using Shaped Ultrafast Laser Pulses in the Deep Ultraviolet. J. Chem. Phys., 2009, 130, 134311. [44] Tapavicza, E.; Bellchambers, G. D.; Vincent, J. C.; Furche, F. Ab Initio Non-Adiabatic Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2013, 15, 18336–18348. [45] Fuß, W.; Schmid, W. E.; Trushin, S. A. Ultrafast Photochemistry of Cyclopentadiene: Competing Hydrogen Migration and Electrocyclic Ring Closure. Chem. Phys. 2005, 316, 225–234. [46] Schalk, O.; Boguslavskiy, A. E.; Stolow, A., Substituent Effects on Dynamics at Conical Intersections: Cyclopentadienes. J. Phys. Chem. A 2010, 114, 4058-4064. [47] Kuhlman, T. S.; Glover, W. J.; Mori, T.; Møller, K. B.; Martínez, T. J. Between Ethylene and Polyenes - the Non-Adiabatic Dynamics of cis-Dienes. Faraday Discuss. 2012, 157, 193-212. [48] Wolf, T. J. A.; Kuhlman, T. S.; Schalk, O.; Martinez, T. J.; Møller, K. B.; Stolow, A.; Unterreiner, A.-N. Hexamethylcyclopentadiene: A Test Case for the Combination of Time Resolved Photoelectron Spectroscopy and ab initio Multiple Spawning Simulations, Phys. Chem. Chem. Phys. 2014, 16, 11770-11779. [49] Tapavicza, E.; Tavernelli, I.; Rothlisberger, U. Trajectory Surface Hopping Within Linear Response Time-Dependent Density-Functional Theory. Phys. Rev. Lett. 2007, 98, 023001. [50] Kłoda, T.; Matsuda, A.; Karlsson, H. O.; Elshakre, M.; Linusson, P.; Eland, J. H. D.; Feifel, R.; Hansson, T. Strong-Field Photoionization of O2 at Intermediate Light Intensity. Phys. Rev. A 2010, 82, 033431. [51] Häser, M.; Ahlrichs, R. Improvements on the Direct SCF Method. J. Comput. Chem.

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Figures

Figure 1. Experimental (black) and calculated (red) normalized absorption spectra of cyclohexa-1,3-diene. The pump pulse of our experiment at 267 nm is indicated with a blue arrow.


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Figure 2. Time-resolved photoelectron spectrum of cyclohexa-1,3-diene excited at 267 nm and probed at 400 nm.



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Figure 3. Time zero spectrum of cyclohexa-1,3-diene (filled circles). The spectrum was fitted with a set of Gaussians (blue color for the Rydberg states and green color for the valence states). Superimposed on the spectrum is the time zero shift of the spectrum (open circles). See text for more details.


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Figure 4: a) Time evolution of the C5-C6 distance (density plot) upon excitation to the S1state of cyclohexa-1,3-diene. The red, solid line represents the population in the first excited state; the green curve represents the excited state population of the successful trajectories and the yellow curve the excited state population of the unsuccessful trajectories. b) Time evolution of the extended bond alternation coordinate BAC (BAC + d(C5-C6) - d(C2-C3), see section III.D for details). The red curve shows the absolute value of the non-adiabatic coupling between the S1- and S0-state in atomic units.



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Figure 5. Time evolution of the bond distance between C5 and C6 for all trajectories. The average values of the trajectories that lead to a certain rotational isomer are indicated by the colored lines.


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Figure 6: Time-evolution of the dihedral angles C6-C1-C2-C3 (φ1) and C5-C4-C3-C2 (φ2) for different time intervals of the excited state dynamics. After ring opening, all successful trajectories (non-black) first form the trans-trans conformer (0.15-0.25 ps), characterized by the corners of the plot, i.e. φ1/φ2 combinations -180/-180, -180/180, 180/-180, and 180/180. At later times, during hot ground state dynamics, the trans-Z-trans (tZt, red) cis-Z-trans, trans-Z-cis (tZc or cZt, yellow), and cis-Z-cis (cZc, green) conformers are formed. In the last frame, the different reaction products correspond to the different C5C6 bond distances in Figure 5.



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Figure 7. Calculated time-resolved photoelectron spectrum of cyclohexa-1,3-diene upon excitation to the S1-state. The probe pulse was assumed to be 6.2 eV (two photons of 400 nm). A shift of 1.9 eV was applied to correct for the systematic overestimation of ionization potentials.


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Figure 8: Sketch of the potential energy surface and the reaction pathways of cyclohexa-1,3diene after excitation to the lowest lying excited state.



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Tables Assignment

3p

3d

4s

4p

4d

5s

5p

6s

Energetic position / eV

0.72 / 1 0.83

1.31 / 1 1.41

1.62

1.81

2.03

2.15

2.29

2.46

Time delay / fs

44

32

32

31

27

19

17

10

Table 1. Energetic positions and time delays for the intermediate Rydberg states observed in 1

the time-resolved photoelectron spectrum of cyclohexa-1,3-diene. Two peaks are assigned for ionization through the 3p- and 3d-Rydberg states.


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Figure for TOC



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Experimental (black) and calculated (red) normalized absorption spectra of cyclohexa-1,3-diene. The pump pulse of our experiment at 267 nm is indicated with a blue arrow.


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Time-resolved photoelectron spectrum of cyclohexa-1,3-diene excited at 267 nm and probed at 400 nm.



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Time zero spectrum of cyclohexa-1,3-diene (filled circles). The spectrum was fitted with a set of Gaussians (blue color for the Rydberg states and green color for the valence states). Superimposed on the spectrum is the time zero shift of the spectrum (open circles). See text for more details.


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a) Time evolution of the C5-C6 distance (density plot) upon excitation to the S1-state of cyclohexa-1,3diene. The red, solid line represents the population in the first excited state; the green curve represents the excited state population of the successful trajectories and the yellow curve the excited state population of the unsuccessful trajectories. b) Time evolution of the extended bond alternation coordinate BAC (BAC + d(C5-C6) + d(C2-C3)). The red curve shows the absolute value of the non-adiabatic coupling between the S1- and S0-state in atomic units. 202x172mm (96 x 96 DPI)



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Time evolution of the bond distance between C5 and C6 for all trajectories. The average values of the trajectories that lead to a certain rotational isomer are indicated by the colored lines.


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Time-evolution of the dihedral angles C6-C1-C2-C3 (φ1) and C5-C4-C3-C2 (φ2) for different time intervals of the excited state dynamics. After ring opening, all successful trajectories (non-black) first form the transtrans conformer (0.15-0.25 ps), characterized by the corners of the plot, i.e. φ1/φ2 combinations -180/180, -180/180, 180/-180, and 180/180. At later times, during hot ground state dynamics, the trans-Z-trans (tZt, red) cis-Z-trans, trans-Z-cis (tZc or cZt, yellow), and cis-Z-cis (cZc, green) conformers are formed. In the last frame, the different reaction products correspond to the different C5-C6 bond distances in Figure 5.



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Calculated time-resolved photoelectron spectrum of cyclohexa-1,3-diene upon excitation to the S1-state. The probe pulse was assumed to be 6.2 eV (two photons of 400 nm). A shift of 1.9 eV was applied to correct for the systematic overestimation of ionization potentials.


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Sketch of the potential energy surface and the reaction pathways of cyclohexa-1,3-diene after excitation to the lowest lying excited state.



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TOC Figure


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Cyclohexadiene Revisited: A Time-Resolved ... - ACS Publications

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