Letter pubs.acs.org/JPCL
Direct Observation of Ground-State Product Formation in a 1,3Cyclohexadiene Ring-Opening Reaction Shunsuke Adachi,†,‡ Motoki Sato,† and Toshinori Suzuki*,†,‡ †
Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan RIKEN Center for Advanced Photonics, RIKEN, Wako, 351-0198, Japan
‡
S Supporting Information *
ABSTRACT: Ultrafast photoelectron imaging using a 90 nm vacuum-UV probe pulse is applied to the ring-opening reaction of 1,3-cyclohexadiene (CHD) in the gas phase, and formation of 1,3,5-hexatriene (HT) and CHD in their electronic ground states is observed in real time. The analysis of the transient photoelectron kinetic energy spectra reveals the branching ratio into HT and CHD as 3:7 upon 270 nm photoexcitation. The ratio is in reasonable agreement with the experimental values reported for the liquid phase and theoretical values for the gas phase, resolving the discrepancy.
photoelectrons. The bandwidth of the pump pulse is ∼300 cm−1, and the influence of impulsive Raman scattering13 is safely neglected. The CHD excitation probability by the deepUV pump pulse is kept low enough (8.3 eV at τ = 30 fs corresponds to ionization from the 1B state to a cation ground state, while the broad peak in the eKE region of >7.8 eV at τ = 90 fs is attributed to ionization from the 2A state to the cation ground state. In this higher (>6 eV) eKE region, there are no contributions from ground-state HT/CHD, as will become apparent in Figure 2. In Figure 1b, photoelectron signal intensities at eKEs of 8.6 (black circles) and 8.3 eV (red squares), representing 1B and 2A state populations, are shown. These signals are expected to exhibit respectively an exponential decay with a 1B/2A internal conversion time constant and an exponential rise and decay with 1B/2A and 2A/1A internal conversion time constants. We
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he photochemical electrocyclic ring-opening reactions are essential in synthetic organic chemistry and biochemistry (e.g., biosynthesis of previtamin D by solar UV).1 A prototypical ring-opening reaction of 1,3-cyclohexadiene (CHD) to 1,3,5-hexatriene (HT) has been the focus of many experimental and theoretical studies over the years,1−12 which provided deeper understanding of the mechanism. Photoexcitation places a nuclear wavepacket on a steeply repulsive part of the 1B potential energy surface, inducing ultrafast internal conversion through a conical intersection to the 2A state and then to the 1A ground state via the second conical intersection.1 Kuthirummal et al. estimated the time constants for the 1B/2A and 2A/1A transitions to be 55 and 84 fs, respectively,2 both of which are in reasonable agreement with the results from photoionization mass spectrometry3,4 and theoretical simulations.5−7 While the reaction pathway seems well understood, the reaction branching ratio remains a controversy.1,8 As the system passes the 2A/1A conical intersection, a part of the wavepacket may return to the reactant CHD structure rather than proceeding to a ring-opening reaction to form HT. Previous studies in the gas phase suggested unity quantum yield (QY) of the ring-opening reaction,9,10 which is contrasted with ∼0.4 reported for the reaction in solution.11,12 On the other hand, computational studies on the gas-phase reaction predicted a QY of ∼0.5, which is in agreement with the experimental values for solution.5−7 In this Letter, we shed light on this problem by ultrafast photoelectron spectroscopy using 90 nm vacuum-UV pulses. In this experiment, a deep-UV pump pulse (270 nm, 40 fs) initiates the CHD ring-opening reaction, and a time-delayed probe pulse (91 nm; 13.6 eV) ionizes the HT product to detect © 2015 American Chemical Society
Received: November 25, 2014 Accepted: January 5, 2015 Published: January 5, 2015 343
DOI: 10.1021/jz502487r J. Phys. Chem. Lett. 2015, 6, 343−346
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The Journal of Physical Chemistry Letters
Figure 1. Excited-state dynamics. (a) eKE spectra at τ = 30 (red) and 90 fs (black). (b) Photoelectron signal intensities at the eKEs of 8.6 (black circles) and 8.3 eV (red squares), representing 1B and 2A state populations, respectively. Black and red dashed curves are fits with an exponential decay and an exponential rise and decay using a 80 fs instrumental response function, respectively.
performed least-squares global fitting using the Levenberg− Marquardt method by considering a Gaussian instrumental response function (time constant of 80 fs). The resulting curves (dashed) well reproduce the experimental results, from which the 1B/2A and 2A/1A internal conversion time constants were obtained as 70 ± 10 and 60 ± 20 fs, respectively. They are consistent with previously reported values (∼60 and ∼80 fs) within the fitting errors,2−4 which are limited by the 80 fs instrumental response function. In order to discuss the QY from our experimental results, we consider here how the eKE spectrum reveals the ground-state dynamics. Figure 2a shows previously reported He(I) photo-
agreement with those in the calculated spectrum in Figure 2b. It is possible that the photoelectron spectra of highly vibrationally excited HT/CHD molecules deviate from those of molecules in thermal equilibrium in terms of spectral shape and intensity and, therefore, that the differential spectrum shown in Figure 2b is approximate. Pullen et al. considered this issue previously, which exhibited a drop in peak intensity and broadening as a result of vibrational excitation.12 However, they also demonstrated that the influence of vibrational temperature on the shape of a differential absorption spectrum is substantially less than that of CHD/HT isomerization12 (see the Supporting Information for more detail). The excellent agreement of the positive/negative peak positions between Figure 2b and d strongly suggests that the two peaks are attributed to hot HT and CHD produced by ultrafast internal conversion, respectively. Figure 3 shows transient differential signal intensities at eKE = 3.6 eV (Figure 3a, red squares) and eKE = 2.3 eV (Figure 3b, blue circles), representing the ground-state dynamics for HT production and CHD depletion, respectively. Note that only the data for τ > 200 fs are plotted in Figure 3. This is because the transient signals due to the ionization from the 1B/2A
Figure 2. Estimated and experimental differential spectra. (a) Previously reported He(I) photoelectron spectra for CHD14 and HT.15 (b) Estimated differential spectrum. (c) Experimental eKE spectra averaged for negative (black) and positive (red) time ranges. (d) Experimental differential eKE spectrum (solid, magnified by a factor of 200) and the negative-delay eKE spectrum (dashed).
electron spectra for CHD14 and HT15 (see the Supporting Information, Figure S1). Their first bands at the electron binding energy (eBE) of ∼8.5 eV are almost identical, while the second bands are different. Consequently, the differential spectrum between the HT and CHD He(I) photoelectron spectra (Figure 2b) exhibits positive and negative peaks at eBEs of 10.4 and 10.9 eV, respectively. Figure 2c shows experimental eKE spectra averaged for negative (−390 to −120 fs, black) and positive (+420 to +990 fs, red) time ranges. Because the excitation efficiency of CHD in our experiment is maintained low, the time-independent signal from unexcited CHD molecules obscures the time dependence of the spectrum. However, subtraction of the spectrum at a negative -delay from that at a positive delay successfully extracts the difference, as shown in Figure 2d (solid curve). The positive and negative peak positions in this differential spectrum are in excellent
Figure 3. Ground-state dynamics. Transient differential signal intensities at eKE = (a) 3.6 (red squares) and (b) 2.3 eV (blue circles), representing the dynamics for HT production and CHD depletion, respectively. Red and blue dotted curves are for eye guides. The black dashed curve shows the expected signal rise calculated from the obtained excited-state kinetic time constants. 344
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the QY for an unshaped pulse might be considerably smaller than unity, though the absolute QY has not been reported. Accordingly, our QY value (∼0.3) is more convincing, resolving the discrepancy between the two phases. In both gas and liquid phases, more than half of the wavepacket returns to the reactant CHD structure where the QYs are determined by potential landscapes in the vicinity of the conical intersections, which are not strongly affected by solvation. In summary, pump−probe photoelectron spectroscopy using 90 nm vacuum-UV pulses enabled the first spectroscopic observation of ground-state product formation in the CHD ring-opening reaction. We observed the partial recovery of the reactant CHD structure for the first time in gas-phase experiments. The estimated ring-opening QY of ∼0.3 resolves the discrepancy between the experiments in gas and liquid phases.
excited states to cation excited states and a coherent artifact obscure the analysis of the low eKE region at short time delays (see the Supporting Information, Figure S2). In Figure 3a, the signal rise at around τ = 200−500 fs (red squares) is assigned to the HT product formation in the ground electronic state. Additionally, the expected signal rise (black dashed curve) calculated from the obtained excited-state kinetic time constants is shown. There seems to be a certain delay (∼100 fs) between them, representing the time required for wavepacket motion from the 2A state to the 1A state. On the other hand, in Figure 3b, the depletion of ground-state CHD, which is denoted by a negative differential signal intensity, recovers on the similar time scale of HT production. The result clearly indicates that the bleach of the CHD population partially recovers by a back reaction from the conical intersection region between 2A and 1A. To evaluate the QY from our experiment, Figure 4 shows an averaged differential spectrum at τ = 210−240 fs (black solid
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EXPERIMENTAL METHODS The setup for 90 nm pulse generation has been described in detail elsewhere.17 The 90 nm pulse energy is 0.2 μJ at a 1 kHz laser repetition rate (i.e., 0.2 mW average power). Both the 270 nm pump pulse and time-delayed 90 nm probe pulse are focused onto the supersonic molecular beam of CHD (SigmaAldrich, 97%) formed by co-expanding with a helium carrier gas from a nozzle and a skimmer. The photoelectrons generated by the pump−probe photoionization are projected onto a 2D position-sensitive detector using a velocity map electrode, and 3D photoelectron velocity distributions are reconstructed from the images using the pBaseX method.18,19 In this manner, the eKE distribution and the angular distribution are reconstructed, but only the eKE distribution is discussed in this Letter.
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Figure 4. Averaged differential spectra at τ = 210−240 (black solid) and 510−990 fs (red solid). Four dashed curves represent estimated differential spectra after the reaction with the QYs of 0, 0.3, 0.6, and 1.0, from top to bottom.
ASSOCIATED CONTENT
S Supporting Information *
Further information/discussion on previously reported He(I) photoelectron spectra, ionization cross sections for hot HT/ CHD molecules, transient signals for τ < 200 fs in the lower eKE region, and thermal reactions on the ground states. This material is available free of charge via the Internet at http:// pubs.acs.org.
curve). For simplicity, we assume that the bleach recovery before τ = 210 fs is negligible and that the eKE spectra of hot HT/CHD are the same as the HT/CHD He(I) photoelectron spectra. In this spectral range, the more HT that is produced, the larger the differential signal intensity after the reaction (see the four dashed curves in Figure 4 representing estimated differential spectra after the reaction with the ring-opening QYs of 0, 0.3, 0.6, and 1.0.). The observed differential spectrum after the reaction (τ = 510−990 fs, red solid curve) provides the QY of 0.29 with the fitting error of ±0.03. The unit QYs reported in the previous experimental studies in the gas phases9,10 are in contrast with those in the solution phases (∼0.4)11,12 as well as theoretical studies (∼0.5).5−7 Ruan et al. proposed that the difference between the two phases might arise from solvation effect;9 however, there has been no evidence that the time scale for the ring-opening reaction is substantially different in the two phases. Alternatively, an interconversion between CHD and HT on their ground states seems to be a plausible explanation for this discrepancy because the internal energies of hot HT/CHD following the internal conversion from the excited state exceed the potential barrier between them. However, this possibility is unlikely as the interconversion time constant estimated using Rice−Ramsperger−Kassel−Marcus theory is a nanosecond or even longer12 (see the Supporting Information for more details). Meanwhile, a gas-phase optical control experiment reported a ∼37% increase in the QY for shaped excitation pulses,16 implying that
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partially supported by the Research Foundation for Opto-Science and Technology and by the Amada Foundation.
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REFERENCES
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