J . Phys. Chem. 1994,98, 5597-5606
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FEATURE ARTICLE Determination of Pericyclic Photochemical Reaction Dynamics with Resonance Raman Spectroscopy Philip J. Reid,? Mary K. Lawless, Steven D. Wickham, and Richard A. Mathies’ Department of Chemistry, University of California, Berkeley, California 94720 Received: February 1 , 1994; In Final Form: March 25, 1994”
Resonance Raman intensity analysis and picosecond time-resolved resonance Raman spectroscopy are used to elucidate the reaction dynamics of the electrocyclic ring-openings of 1,3-~yclohexadiene(CHD) and 1,3,5cyclooctatriene (COT) as well as the hydrogen migration in 1,3,5-cycloheptatriene (CHT). The resonance Raman intensities of C H D demonstrate that evolution along the conrotatory reaction coordinate occurs immediately after photoexcitation, in agreement with the prediction of the Woodward-Hoffmann rules. The 900-cm-l optical T2 combined with the 2 X 10“ fluorescence quantum yield shows that the initially prepared excited state of C H D depopulates on the 10-fs time scale due to internal conversion to a lower energy, optically dark surface. The Raman intensities of C O T and C H T demonstrate that for these molecules, the initial excited-state dynamics consist principally of ring planarization with no evidence for motion along reactive coordinates. This suggests that the establishment of a planar excited-state geometry is a prerequisite for reactive pericyclic nuclear motion. Picosecond time-resolved resonance Raman Stokes and anti-Stokes spectra of the above reactions reveal that the ground-state photoproducts appear on the 10-ps time scale. Analysis of the time-resolved vibrational spectra also demonstrates that population of the ground state is followed by vibrational relaxation and single-bond isomerization of the ring-opened photoproducts on the 10-ps time scale. This work demonstrates that resonance Raman spectroscopy is a powerful methodology for elucidating condensedphase chemical reaction dynamics.
Introduction Pericyclic rearrangements are an important class of chemical reactions due to the precise stereo- and regioselectivity of these reactions and the paradigmatic theories that have been developed to predict the structure of the reaction pr~ducts.1-~These reactions are termed pericyclic because the transition can be described by a concerted bond rearrangement through a closed loop of interacting orbitals. The confluence of experimental and theoretical work on these rearrangements led to the award of the 1981 Nobel Prize in Chemistry to Hoffmann and Fukui.* In their development of pericyclic reaction theory, Woodward and Hoffmann stressed the concepts of orbital symmetry, while Fukui was concerned with the properties of the frontier orbitals-the highest occupied and lowest unoccupied molecular orbitals of the molecular groups involved in the reaction.Is2 Over the past 20 years, a variety of “traditional” physical-rganic experiments have been used to demonstrate the overwhelming applicability of these theories in predicting the structuraloutcome of both thermal and photochemical rearrangements. Since the photochemical reaction products are ultimately determined by electronic state surfaces rather than molecular orbitals, we might ask: What state or states are involved in these reactions? What are the excited-statestructural changes that occur in these photochemical rearrangements? How fast do pericyclic photochemical reactions occur? Finally, how do these factors conspire to produce a class of chemical reactions with such unique reactive properties?Despite the rapid advances of physical chemical techniques which have increased our understanding of photochemical reaction dynamics into the femtosecond time regime, these important questions have To whom correspondence should be addressed. Present address: Department of Chemistry, University of Minnesota, Minneapolis, MN 55455. Abstract published in Advance ACS Abstracts, May 1, 1994.
not been answered for pericyclic chemi~try.~-lsThis article will describe some of the first studies designed to answer the above questions about pericyclic photochemical rearrangements. We will focus on the use of resonance Raman intensity analysis and picosecond time-resolved resonance Raman spectroscopy in elucidating solution-phase pericyclic reaction dynamics. The results of these investigations reveal key aspects of pericyclic reactions that have advanced our understanding beyond the Woodward-Hoffmann rules. Although a wide variety of pericyclic photochemical reactions are known, this article will concentrate on the prototypical electrocyclic ring-openings and sigmatropic shift reactions presented in Figure 1. In the condensed phase, photoexcitation of 1,3-~yclohexadiene(CHD) leads to an efficient ring-opening that has been shown to occur in a conrotatory fashion (i.e,, with a twisting of the terminal methylene groups in the same direction).s76J6J7 The photochemistry of CHD is particularly interesting since it is the photoreactive moiety in the conversion of 7-dehydrocholesterol to pre-vitamin D.5918-20 In contrast, the photochemical ring-opening of 1,3,5-~yclooctatriene(COT) is predicted to occur with a disrotatory rotation of the terminal methylene groups.Z1-22Finally, sigmatropic shifts (the migration of a a-bond with the accompanying rearrangement of olefinic bonding) are well represented by 1,3,5-~ycloheptatrienein which photoexcitation initiates the suprafacial migration of a hydrogen atom to a carbon center adjacent to the methylene portion of the ring.23-27 The orbital symmetry theory developed by Woodward and Hoffmann has allowed chemists to predict the photochemical outcome of pericyclic rearrangements.’ The application of this theory to the photochemical ring opening of CHD is presented in Figure 2. Here, an orbital correlation diagram is constructed by linking reactant and product molecular orbitals of the same
0022-365419412098-5597$04.50/0 0 1994 American Chemical Societv
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The Journal of Physical Chemisiry, Vol. 98. No. 22, 1994
" 1 .3Scyclohspfafnme
ul~iaiadai1.7.ylm
Figure 1. Perieycliephotahemicaalrcarrangcmcnllof I,3cyelohsxsdime
(CHD),I,3.5-cyclooctatriene(COT),and 1,3,5cyclohcptatnenc(CHT). The frontier orbital description of these rearrangemenu is illustrated. CHD is predicted to undergo ring-opening with a conrotatory rotation of the terminal methylene groups while COT ring opens in a disrotatory fashion. ThesigmatropicshiftofCHTarurssuprafa~allywithmigration of the C-H band to the adjacent position of the ring.
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Reid et al. How can the excited-state dynamics and overall time course of pericyclic reactions be revealed? In our laboratory, we have focusedon theuseofresonanceRaman spectroscopy. Specifically, we have employed resonance Raman intensity analysis and picosecond time-resolved resonance Raman spectroscopy to elucidate the reaction dynamics from the initial femtosecond nuclear evolution of the vertically excited molecule to the appearance and conformational relaxation of ground-state phctoproduct. Theconnection between resonance Raman intensities and excited-state nuclear dynamicscan be made through various formalisms, but the important concept is that resonance Raman intensities provide information on the change in geometry of the excited-electronic state relative to the ground state along the ground-state vibrational normal modes. Therefore, Raman intensitiesprovideadirect measureofthe excited-statestructural evolution occurring out of the Franck-Condon region. In the picosecond resonance Raman experiments, a laser pulse is used to initiate the photochemistry and the Raman scattering from a second, temporally delayed, resonant pulse probes photoproduct formation kinetics and structural evolution. By applying both resonance Raman intensity analysis and time-resolved resonance Raman spectroscopy, we have been able to monitor the photochemical reaction dynamics of pericyclic rearrangements from the initial excited-state nuclear evolution to the appearance and relaxationoftheground-state products. In the followingsections, we will outline the application of these techniques to problems in photochemical reaction dynamics and present the results of our studies on pericyclic photochemical rearrangements. Excited-State Dynamim of Pericyclic Reactions from Resolunce Ramnn Intemities
Ca"
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Ryrc 2. Orbital symmetry correlation diagram for the photochemical ring opening of CHD. The #-and --bonding molecular orbitals of the reactant and product areordered in cncrgyandcategorizcd assymmetric (S) or antisymmetric (A) with respect to C2 rotation of the ring. The singly excited state of CHD correlates barrierlasly to the corresponding
singly excited state of the product.
symmetry defined with respect to C2 rotation of the ring which is conserved in a conrotatory rearrangement. The correlation of theelectronconfigurationofthereactant with thatoftheproduct allows one to crudely predict the 'allowedness" of the reaction. For example, the figure illustrates that the lowest singly excited configuration of CHD correlates with the excited state of the product withoutan increasein energy,suggesting that thereshould be no symmetry-imposed barrier to ring opening along the conrotatory reaction coordinate. In contrast. the ground-state configuration of CHD would correlate to a doubly-excited configurationof the product, suggesting the presence of a barrier tothe thermalreactionalongthiscoordinate. Indeed, thethermal ring-opening is observed to occur in a disrotatory rather than a conrotatory fashion.[ Although this approach is of predictive importance, it iscritical to realize that the character of theexcited s f a m and the nuclear evolution which occurs on these surfaces is more complicated than the simple Hiickel molecular orbital description would lead us to believe. Furthermore, an accurate descriptionof theelectronicstatesofpolyenesismorecomplicated than we have depicted with configuration interaction playing an important role.28
Thedependenceof both resonance Raman intensities and the electronic absorption spectrum on the excited-state potential surface permits theelucidation oftheinitial femtosecond reactive dynamics of a photoexcited molecule.29-3l The correlation betweenrcsonancc Raman intensities and excited-statestructural dynamics can be made through the traditional stationary state or sum-over-states picture32or the more conceptual approach which is provided by the time-dependent wave-packet formalism presented in Figure 3.33-35 The equations presented in the figure illustrate that both the absorption and Raman cross sections depend on the Fourier transform of their respective correlation functions (ili(f)) and (fli(i)), where li(f)) represents the time evolution of the ground-state wave function propagating under the influence of the excited-state vibrational Hamiltonian. Pictorially, optical excitation creates a replica of the groundstate nuclear wave function on the excited-state surface, which undergoes evolution since the ground-state wave function is not an eigenstateofthissurface. Asli(1)) evolves, itaquiresmerlap withthe final stateina Ramantransition If). Therefore,excitedstate evolution gives rise to resonance Raman intensity. In this simple picture of fundamental Raman scattering, the intensity is often approximately proportional to the square of the displacement of the excited-state minimum relative to the ground state (A2). However, it shouldbenoted that inall theanalyses presented here exact calculations were performed to determine these displacements. The Raman intensity of each normal coordinate provides a mode-specific measurement of the extent of nuclear evolution following photoexcitation. Implicit in this description of resonance Raman scattering is that the excited-state geometry must be displaced relative to the ground state in order to observe intensity. By symmetry, displacements can only occur along totally symmetric coordinates (assuming that the excited and ground electronic states belong to the same point group). However, activity along non-totally symmetric coordinates may alsobeobservedin theresonanceRamanspectrum. Forexample, a change in excited-state frequency relative to the ground state would cause li(f)) to spread, leading to overlap with even
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The Journal of Physical Chemistry, Vol. 98, No. 22, 1994 5599
.-fn2. e
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Normal Coordinate
Absorption
oA(EL/= *4
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~ ( i l i ( t ) ) e l ~ E ~ + ED(t) , ) ~ hdt
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Figure 3. Schematic of the time-dependent picture of resonance Raman scattering. The expressions for the absorptionand Raman cross sections are given at the bottom. Me*is the electronic transition length, EL is the excitation energy, Es is the energy of the scattered &oton, 81 is the vibrational energy, and D(r) is the homogeneous broadening function.
500
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Energy I cm" vibrational quanta of the ground state. In this case, intensity in even overtones of a non-totally symmetric coordinate is an indication of substantial excited-state geometric e v ~ l u t i o n . ~ ~ . ~ ~ A quantitative measurement of the nuclear displacements and optical dephasing can be achieved through an absolute resonance Raman intensity analysis. The absolute Raman cross sections ca? be determined by comparison to a scattering standard, such as a particular line of the solvent, after correction for the concentration and polarization difference in scattering between the two lines. In the experiments presented here, the ringbreathing and C-H stretching modes of cyclohexane provided convenient standards by which to measure the cross section of the strongest mode in the Raman spectrum. The importance of absolute scattering standards has been recognized by several g r o ~ p s . 3 8 Once ~ the cross sections for all the modes are 400 800 1200 1600 determined, an excitation profile for at least one mode must also Energy I cm" be obtained by measuring the Raman cross section a s a function of excitation wavelength. With these two studies and the Figure4. Resonance Ramanspectraof (A) 1,3-cyclohexadiene(CHD)?' (B) 1,3,5-cyclooctatriene and (C) 1,3,5-cycloheptatriene absorption spectrum, a determination of the absolute excited(CHT)63obtained with 257-nm excitation. Asterisks denote solvent lines. state displacements as well as the homogeneous and inhomogeneous line widths can be performed. The integrated absorption photochemical rearrangements of 1,3-~yclohexadiene(CHD), cross section is insensitive to the homogeneous broadening 1,3,5-cyclooctatriene (COT), and 1,3,5-cycloheptatriene function, D(t),however theRamancrosssectionis not. Therefore, (CHT).-3 Representative resonance Raman spectra of these the magnitude of the scattering cross section determined by the compounds are presented in Figure 4. A glance at these spectra excitation profile provides a constraint on the homogeneous line reveals the similarity of the Raman scattering to that observed width. If more broadening is required to reproduce the diffuseness for linear polyenes where the C-C and C-C stretching modes of the absorption spectrum, inhomogeneous broadening cordominate the observed spectrumnZ8However, closer examination responding to solvent-induced differences in molecular environreveals key intensity patterns that demonstrate the nature of the ments can be included as a Gaussian distribution of the electronic initial geometric relaxation after excitation and important origin.30 A b s o l ~ t e ~ ~ ~ 3 1and ~ ~ relative5' ~ - 5 3 56 resonance Raman differences between the excited-state dynamics of these molecules. intensity analysis has been applied to a variety of photochemically 1,3-CyclohexadieneExcited-StateDynamics. In the resonance reactive systems by our group as well as others. Finally, the Raman spectrum of CHD, the highlighted modes at 507,948, methodology and subtleties of this technique have been explored and 1321 cm-1 are of particular interest.61 The 507-cm-1 line in greater detail elsewhere and the interested reader is directed corresponds to an olefinic torsional mode. Intensity in this mode to these manuscripts.29-31,57-59 To illustrate the wealth of demonstrates that excitation causes the C=C-C=C portion of information available from an absolute resonance Raman intensity this slightly distorted ring to p l a n a r i ~ e . ~ lMore > 6 ~ dramatic is the analysis, we present a summary of our results on the pericyclic intensity of the CH2-CH2 single-bond stretch and the CHI
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Energy/" Figure 5. Fluorescenceemission spectrumof 1,3-~yclohexadieneobtained with 282.4-nm excitation. The absorption and emission spectra are indicated. The sharp features superimposed on the emission band are due to Raman scattering. The integrated intensity of the broad fluorescence band establishesa fluorescence quantum yield of 2 X 10-6. symmetric rocking modes at 948 and 1321 cm-I, respectively. The large intensity of these modes indicates that the aliphatic C-C bond is lengthening and that the molecule is distorting along the symmetric rocking mode with the atomic motion mainly on the axial C-H positions. Both of these geometric changes project directly on the conrotatory reaction coordinate. The absolute magnitude of theexcited-state displacements of the Raman active modes and the electronic homogeneous line width were also determined. The 900-cm-l optical T2 determined from this analysis corresponds to a 10-fs dephasing time. The origin of this fast dephasing was revealed through a measurement of the fluorescence quantum yield (Figure 5). With excitation in the lowest electronic absorption band, the total emission is nearly obscured by the Raman scattering (the sharp features in Figure 5) on top of the broad fluorescence emission. A comparison of the integrated fluorescence intensity to the ethylenic Raman line provides an upper limit on the fluorescence quantum yield of 1 2 X 10-6. This value combined with a Strickler-Berg analysis indicates that the fast dephasing is dominated by population decay from the initially prepared excited ~ t a t e . ~ s . ~ ~ The information provided by the resonance Raman intensity analysis can be used to develop a detailed picture of the excitedstate dynamics of CHD as illustrated in Figure 6. The arrows represent the cumulative evolution along the 507,948,1321, and 1578 cm-l normal coordinates 10fs after excitati0n.6~ An increase in C-C bond length and the symmetric twisting of the methylene groups is evident. Although this motion is consistent with the predicted conrotatory reaction coordinate, the brevity of evolution on this surface allows for a structural change which is only 10% of that necessary for the formation of photoproduct.61 Therefore, the initially prepared excited state accelerates the molecule along the conrotatory reaction coordinate. However, the majority of the structural evolution must occur on another surface which will be discussed in detail below. 1,3,5-Cyclooctatriene Excited-State Dynamics. Photoexcitation of 1,3,5-cyclooctatriene (COT) is predicted to result in the production of ring-opened photoproduct consistent with a disrotatory motion of the methylene gr0ups.l The resonance Raman spectrum of COT (Figure 4) is noticeably more complicated than CHD. Intensity is observed in 19 modes demonstrating that the excited-state evolution occurs along many morevibrational degrees of freedomS62The majority of the resonance Raman intensity is observed in the in- and out-of-phase C=C modes at 1610 and 1640 cm-l, the CHz-CHz stretch at 962 cm-l, and ring planarization and ethylenic torsion modes at 140, 339, and 404 cm-I. Like CHD, excitation of COT leads to a more planar excited-state geometry and lenghtening of the bond between the two methylene groups. However, evolution corresponding to the disrotatory rotation of the methylene groups is not observed.
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4 CHT
\
"1
Figure 6. Depiction of the excited-state dynamics determined from the absolute resonance Raman intensity analyses. The excited-stateevolution of 1,3-cyclohexadiene(CHD) is consistent with the predictedconrotatory ringopening. Incontrast,onlytheCH&H2bondisobserved toelongate in 1,3,5-cyclooctatrienewith the majority of the evolution consisting of ring planarization. The excited-statedynamicsof 1,3,5-cycloheptatriene (CHT) consist of planarizationwith no motion along the methylene C-H coordinates.
Disrotatory rotation corresponds to the non-totally symmetric methylene rocking mode at 1356 cm-I. Therefore, fundamental intensity is forbidden by symmetry consistent with the dearth of scattering between 1228 and 1400cm-1. Significant structural evolution along this coordinate would result in a decrease in the excited-state frequency relative to the ground state which would be manifested as intensity in even overtone tran~itions.33,3~.~~,3~ Careful analysis of the overtone region demonstrates that alrthe observed intensity can be assigned to combination bands involving the ethylenic modes and other coordinates. No intensity is observed in the 0-2 overtone transition for the disrotatory twist at 2712 cm-I (marked by arrow in Figure 4B). Calculations indicate that intensity corresponding to an excited-state frequency change greater than 200 cm-I would result in observable intensity. Therefore, significant structural evolution along this component of the predicted disrotatory reaction coordinate in the FranckCondon region does not occur.62 In contrast, the majority of the excited-state evolution corresponds to ring-planarization (Figure 6). The ground-state geometry of COT is nonplanar.68-69 Since an electrocyclic ring-opening also involves the formation of C=C bonds between the terminal methylene groups and the adjacent carbons with a corresponding rearrangement of the existing olefinic bonding, we might expect a planar geometry to enhance orbital overlap and facilitate the formation of these new bonds. These results provide strong evidence that the establishment of a planar or near-planar geometry is required before the predicted pericyclic rearrangement can take place. The absolute resonance Raman intensity analysis combined with a determination of the fluorescence quantum yield (-2 X 10-6)demonstrates that COT leaves the initially prepared excited state in -30 fs.62 This rapid internal conversion is similar to the behavior observed for C H D suggesting that a lower-lying excited state may be participating the photochemistry of COT as well. It has been established that for cis, cis-octatetraene, an electronic state of 2A1 character is the lowest energy excited ~tate.28.~0 1,3,5-Cycloheptatriene Excited-State Dynamics. Photoexcitation of 1,3,5-~ycloheptatriene(CHT) leads to migration of one
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The Journal of Physical Chemistry, Vol. 98, No. 22, I994
of the methylene C-H bonds to the adjacent position of the CHT ring with the corresponding rearrangement of the olefinic bond~.2~>2~-26 The resonance Raman spectrum of C H T (Figure 4) reveals that excited-state evolution occurs along numerous normal coordinates.63 The majority of the scattering intensity is observed in the in- and out-of-phase ethylenic modes at 1536 and 1610 cm-1 as well as the ring-planarization modes at 225, 354, and 419 cm-1.71972 Interestingly, intensity is not observed in the symmetric methylene C-H stretch coordinate a t 2900 cm-l. Therefore, excited-state evolution is not occurring along the coordinate which is predicted to be a major component of the sigmatropic shift. Instead, C H T evolves toward a more planar geometry with the largest displacement occurring along the "boatto-boat" coordinate at 225 cm-'. The predominant ground state conformationof C H T is a boatlike structure; therefore, excitation results in a flattening of the ground-state geometry as depicted in Figure 6.73974 Once again, the establishment of a planar or near-planar excited-state geometry precedes the reactive structural evolution. The absolute resonance Raman intensity analysis and the fluorescence quantum yield (1.4 X 1od) establish that the initially prepared surface undergoes an ultrafast, 20-fs depopulation consistent with the idea that a second electronic state is participating in the photochemistry.63 This rapid internal conversion is in agreement with gas-phase ionization studies.75 Also, magnetic-circular dichroism studies have indicated that a lower lying electronic state is present in CHT.76
Reaction Kinetics and Structural Evolution from Picosecond Time-Resolved Resonance Raman Spectroscopy Time-resolved resonance Raman spectroscopy provides an elegant complement to Raman intensity analysis by providing structural information about product formation kinetics and structural evolution on a longer time scale.77 The temporal resolution of this technique is dictated by the frequency resolution necessary to measure the vibrational spectrum. For example, frequency resolution of 20 cm-I limits the temporal resolution to 0.5 ps (assuming a transform limited, hyperbolic-secant pulse shape). Time-resolved Raman spectroscopy has been utilized to investigate many interesting chemical and biological To investigate pericyclic rearrangements, a source of tunable, picosecond, UV pulses with energies sufficient to initiate the photochemistry was developed. This laser system has been presented in detail elsewhere; therefore, only a brief description of the apparatus is presented here.8Q9 A synchronously pumped dye laser was used as the input to a four-stage dye amplifier pumped by a Nd:YAG laser operating at 50 Hz. The amplified output consisted of 1.6-ps pulses with energies of 1 mJ. A fraction of the amplified output was removed with a beam-splitter and frequency doubled in KDP to produce a probe beam tunable from 284 to 298 nm. The remainder of the amplified output was used for continuum generation by focusing into a cell containing H20. A 10-nmportion of the continuum centered at 550 nm was isolated with a bandpass filter and amplified to -600 pJ/pulse in a second, two-stage amplifier. This beam was also frequency doubled to produce the actinic pulse centered at 275 nm. The different pump and probe frequencies allow for the acquisition of Raman scattering from the probe in the absence of background signal originating from the pump. The polarization of the pump was rotated to 55O relative to the probe to minimize the contribution of molecular rotations to the observed kinetics. The instrument response was determined by the two-photon dependent ionization of DABCO or triethylamine resulting in a 2-ps fwhm cross correlation.90 Spatial delay of the probe relative to the pump allows for temporal delays up to 10 ns. 1,3-CyclohexadienePicosecond Reaction Dynamics. Figure 7 presents Stokes and anti-Stokes difference spectra obtained for the photochemical ring opening of CHD.89,91 These difference spectra are produced by subtracting the probe-only spectrum
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Figure 7. Resonance Raman Stokes and anti-Stokes difference spectra of the photochemical ring opening of 1,3-~yclohexadiene.Anti-Stokes
spectra were obtained with 284-nm pump and probe wavelengths, while the two-color Stokes spectra were generated with a 284-nm probe and a 275-nm pump. The line at 801 cm-1 is due to the cyclohexane solvent. I
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Figure 8. Temporal evolution of the cis-hexatriene Stokes ethylenic intensity. Best fit by a single exponential resulted in a ground-state appearance time of 6 1 ps.
*
directly from the spectrum obtained in the presence of the actinic pulse. If we first look a t the Stokes spectra, the negative intensity a t 1578 and 1323 cm-1 in the 0-ps spectrum corresponds to the depletion of ground-state CHD created by the actinic pulse. Negative intensity is also observed in the cyclohexane solvent line a t 801 cm-l demonstrating that the optical absorbance of the sample has increased. At 4 ps, positive intensity at 1610, 1236, and 390 cm-1 is observed which is due to the ground-state cishexatriene (c-HT) p h o t o p r o d ~ c t . ~ 9 ~The ~ 2 - ~intensity ~ of the photoproduct ethylenic line increases for delays up to 100 ps with no further changes observed out to 10 ns. A determination of the ground-state c-HT appearance kinetics is provided by analyzing the temporal evolution of this intensity. These data were adequately modeled by single-exponential kinetics providing a ground-state appearance time of 6 i 1 ps (Figure 8). This measurement defines the time necessary for the completion of a condensed phase photochemical ring-opening reaction. Further information on the rate of production and vibrational
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relaxation of c-HT is provided by the anti-Stokes data (Figure 7).89 Measurement of the anti-Stokes intensity provides information on the extent and rate of excess vibrational energy dissipation.83.95-98 At 0 ps, minimal intensity is observed consistent with the absence of c-HT on the ground-state surface. The 4-ps spectrum exhibits intensity at 1614 and 1240 cm-1 in close agreement with the frequencies observed in the Stokes c-HT spectrum. Calculations of the anti-Stokes Raman cross sections for CHD and c-HT indicate that the scattering from the photoproduct should be the dominant source of intensity at this probe ~avelength.8~ This calculation combined with the agreement between the Stokes and anti-Stokes frequencies argues that we are observing anti-Stokes scattering from ground-state c-HT. The anti-Stokes intensity increases for delays up to 14 ps and then decays. The appearance and decay of the intensity at 1614 cm-l was adequately modeled assuming double-exponential kinetics with a best-fit appearance time of 8 f 2 and a decay time of 9 i 2 ps. The rate of photoproduct production determined from this analysis is in excellent agreement with that obtained from the Stokes data demonstrating that ground-state c-HT is produced with a -6-ps time constant and undergoes subsequent vibrational relaxation in -9 ps. Inherent in these time-resolved spectra is the ability to ascertain structural evolution by analyzing the changes in vibrational frequencies. The evolution in the c-HT ethylenic frequency from 1610 to 1625 cm-I (and the corresponding evolution in the antiStokes data) as well as the similar shift in the single-bond stretch frequency from 1236 to 1249 cm-I is consistent with single-bond i s o m e r i z a t i ~ n . ~ ~But , ~ which ~ - ~ ~ conformations ~ are present on the ground-state surface? Inspection of the 100-ps Stokes spectrum reveals a second ethyleniclineat 1572cm-1 whichcannot be assigned to the reactant since the ethylenic intensity of C H D due to ground-state depletion must be negative. Normal-mode calculations combined with symmetry arguments indicate that the observation of two ethylenic lines is only consistent with the lower symmetry of the s-cis,cis,s-trans-HTconformer. Evidence for its all-cis precursor is provided by the anti-Stokes spectra. The intensity at 828 cm-1 from 4 to 25 ps is assigned to the in-plane CHI rocking of the alf-cisconformer (the corresponding Stokes intensity is difficult to assign due to interference from the solvent).89 Therefore, the increase in ethylenic frequency, the presence of two photoproduct ethylenic lines, and the observation of intensity assignable only to all-cis-HT demonstrate that c-HT initially appears on the ground state in the all-cis conformation and that it undergoes conformation relaxation to produce s-cis,cis,s-trans-HT. A more sophisticated kinetic analysis of these data suggests that this conversion occurs in 7 f 1 ps.89 Typical barriers to single-bond isomerization are -4 kcal/mol resulting in a -200 ps isomerization time constant assuming an Arrhenius preexponential of 5 X 10l2 s-1.103J04Therefore, we are observing an extremely rapid conformational relaxation. This enhanced rate is undoubtably due to steric interaction between the terminal methylene groups in the all-cis conformer as well as the elevated molecular temperature created after internal conversion from the excited state.Io5 1,3,5-Cyclooctatriene Picosecond Reaction Dynamics. To determine if the kinetics of photoproduct formation are dependent on thestereochemistry of the reaction, we examined thedisrotatory ring-opening of COT.106 The transient Stokes spectra obtained for this reaction are presented in Figure 9. Scattering assignable to the C=C and C-C modes of the &,cis-octatetraene (cis&OT) photoproduct is observed at 1590 and 1248 cm-'. These intensities increase as a function of probe delay up to 64 ps with no indication of further evolution observed for delays up to 12 nsec. The kinetics of cis,cis-OT production were determined by modeling the appearance of the photoproduct intensity a t 1590 cm-I by single-exponential kinetics giving a ground-state appearance time of 12 A 2 ps. This result demonstrates that the
Reid et al.
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.-cY) W
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Figure 9. Resonance Raman Stokes difference spectra of the photochemical ring openingof 1,3,5-cyclooctatriene. These data were obtained with pump and probe wavelengthsof 298 and 275 nm, respectively. Positive
intensity corresponds to the appearence of ground-state photoproduct. The intensity at 801 cm-I is due to the cyclohexane solvent.
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10-ps ground-state photoproduct appearance time is a general feature of electrocyclic ring-opening reactions regardless of the stereochemistry of the reaction. Also, the frequency evolution ofthe C-C a n d C = C modes observed in thesedata is remarkably similar to that observed for c-HT suggesting that ground-state conformational relaxation is a component of this reaction as well. 1,3,5CycloheptatrienePicosecond Reaction Dynamics. Figure 10presents the picosecond kinetics for the photochemical hydrogen migration of CHT.107 In the left panel, negative intensity at 1536, 1610, and 1758 cm-I in the 0-ps spectrum is assigned to depletion of the ground state. These lines decrease in intensity for delays up to 80 ps by which time the recovery of the groundstate is complete. Unlike our earlier analyses, the time evolution of the photoproduct ethylenic intensity was insufficient to determine the ground-state appearance time since the ethylenic frequency of the reactant and product are identical. The presence of intensity to the low-wavenumber side of the ground-state CHT ethylenic line at 1536 cm-1 (highlighted in the 8-25-ps spectra) indicates that other molecular species may be contributing to the observed ground-state repopulation dynamics. The right-hand side of Figure 10 presents the transient spectra obtained after the addition of vibrationally-relaxed CHT scattering to remove the negative intensity corresponding to ground-state depletion. These data demonstrate that two additional species are indeed present. At the earliest delays, positive scattering is observed at 1545 cm-l with the appearance and decay of this intensity following the instrument response. Furthermore, a transient increase in the optical absorbance of the sample is also observed which demonstrates the same kinetics.I'J7 The early-time scattering at 1545 cm-l is assigned to the initially prepared excited state of CHT.107-lm The second species corresponding to the intensity at 1529 cm-1 is consistent with the scattering from vibrationally hot ground-state CHT. The anharmonicity of this modecombined with the predicted Boltzmann distribution after internal conversion
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Energy (cm.')
Figure 10. Left: resonance Raman difference spectra of the (1,7)-
photochemical sigmatropicshiftof 1,3,5-~ycloheptatriene (CHT) obtained with pump-and-probe wavelengths of 284 nm. The negative intensity corresponds todepletionof ground-stateCHT. Right: differencespectra of the molecular intermediates in the ground-state appearance of CHT obtained by the addition of ground-state scattering to the difference spectra. These spectra illustratethe presenceof two intermediates, excitedstate CHT with an ethylenicat 1545 cm-1 and vibrationallyhot, groundstate CHT with an ethylenic at 1529 cm-*. is consistent with scattering as low as 1527 cm-1.107J10Kinetic analysis of this spectral region assuming scattering contributions from CHT, excited-state CHT, and hot ground-state C H T results in an appearance time of both vibrationally-relaxed and hot C H T of 26 f 4 ps and a vibrational cooling time of 15 f 5 ps.lO7 Although the structural rearrangement of C H T is dramatically different than the electrolytic ring-opening chemistry considered earlier, the rates of photoproduct production and ground-state cooling are remarkably similar.
Discussion Resonance Raman Picture of Pericyclic Photochemical Reaction Dynamics. By combining resonance Raman intensity analysis and time-resolved resonance Raman spectroscopy, we have elucidated the reaction dynamics of pericyclic photochemical rearrangements from the initial excited-state nuclear evolution to the appearance and structural relaxation of the ground-state photoproducts. Our current understanding of these rearrangements is summarized in Figure 11. In the ring-opening of CHD, photoexcitation leads to immediate evolution along the predicted conrotatory reaction coordinate. However, the initially prepared excited state depopulates in only 10 fs. The magnitude of the displacements combined with the rapid internal conversion indicates that C H D proceeds only a fraction of the way along the conrotatory reaction coordinate on the 1B2 excited-state surface. The most reasonable explanation is that the rapid dephasing corresponds to internal conversion of the initially-prepared excited state to a lower-lying 2A1 surface. The excited state evolution begins on the strongly allowed 1B2 state, but the intersection of this surface with a weakly allowed Al state, analogous to the optically forbidden A, state of linear polyenes, leads to the observed rapid population decay.28Jll The picosecond appearance of ground-state photoproduct combined with the rapid decay of the
1A"I
CHT
Figure 11. Schematic reaction coordinates for the photochemical ringopening reactions of 1,3-cyclohexadiene, 1,3,5-cyclooctatriene,and the photochemical hydrogen migration of 1,3,5-~ycloheptatriene.
1Bz population is consistent with this picture of the reaction dynamics. Thisstateordering is in agreement with theobservation that the 2A1 state of cis-hexatriene is the lowest excited state in the gas phase.112J13 Also, recent resonance Raman experiments on cis-hexatriene and cyclopentadiene have demonstrated the presence of a lower energy, optically dark surface.1l4J1s Depopulation of the lower energy 2A1 surfaces should then lead to the appearance of ground-state cis-hexatriene in 6 ps. The 9-ps vibrational relaxation and 7-ps conformational relaxation of the photoproduct which complete the structural evolution are nearly synchronous with the ground-state appearance. This is the first measurement of the time necessary for the completion of a condensed-phase ring-opening reaction and provides a much more complete description of the reaction dynamics. Photoexcitation of COT and C H T also leads to rapid depopulation of the initially prepared excited state. In contrast to CHD, the nonplanar COT and CHT ground-state structures undergo evolution toward a planar structure before motion along the predicted reactive degrees of freedom occurs. This difference in excited-state dynamics is critical in defining the course of the pericyclic rearrangement. The near-planar ground-state geometry of C H D allows for concerted evolution along the predicted reaction coordinate immediately after excitation. However, thenonplanar
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The Journal of Physical Chemistry. Vol. 98. No. 22, 1994
ground-state geometries of COT and CHT result in a more sequential excited-statestructuralevolution as illustrated in Figure 1 I . The initial excited-state evolution of COT is along ringplanarizationcoordinates. This is followed by internalconversion tothe ZAsurfaceandpresumablyevolutionalongthedisrotatory twist coordinate on this surface. Since non-totally-symmetric rocking of the methylene groups is the major component of this coordinate, Figure I 1 depicts the 2A potential surface along the disrotatory-twistcoordinate as a saddle point. Similarly, Figure I 1 depicts the planarization of the ground-state boat structure in CHT which precedes any change in methylene C-H bonding. The large excited-state evolution along ring-planarization coordinates suggests that these modes serve to gate the pericyclic rearrangement leading to a multistep reaction mechanism. The times for photoproduct formation in CHD, COT, and CHT are remarkably similar. suggesting that the -IO-ps time scale is a general feature of pericyclic photochemistry. The 12ps appearance time of cis,&-octatetraene demonstrates that the time scale for photoproduct appearance is not dramatically affected by the stereochemistry of the reaction. The 26-ps groundstate recovery time of CHT is also similar. The small variation in time scalescould reflect differencesin coupling and/or energy differences between the lowest excited-stateand theground state resulting in different appearance times. Mechanismof Pericyclic PhotwhemicnlRearrangeolea** The structural evolution revealed by resonance Raman intensity analysis has important implications for the mechanism of pericyclic photochemical rearrangements. The excited-state dynamics are consistent with the formation of a planar structure before the pericyclic rearrangement can take place. Once this intermediate geometry is established, the rearrangement may occur in a concerted fashion as described by Woodward and Hoffmann.' However, it has recently been suggested that this single-step mechanism is not universally applicable?J'6 Dewar has proposed a "multiband" rule which states that since it is energetically costly to break all bonds before generating the product, a synchronous reaction should become more difficult with an increase in the number of bonds to be reformed. The excited-statedynamics of COT and CHT are consistent with this hypothesis in that evolution along only a subset of the predicted reactive coordinates, planarization, and alteration of olefinic bonding, occurs immediately after excitation. After a planar intermediate geometry is established providing good atomicorbital overlap between neighboring atoms, the pericyclic rearrangement becomes energeticallyfeasible. From this perspective, we would describe the chemistry as sequential rather than concerted. Although the initial nuclear evolution of these reactionsdiffers, internal conversion of the initially prepared excited state is extremely rapid for all of the compounds discussed here. This timescale for excited-state depopulation has both mechanistic and physical consequences on how we describe the reaction dynamics. First, the IO-fs decay of the initial site (denoted as 'E") dictates that motion along the reaction coordinate on this surface is extremely limited. However, the ground-state products do not appear until 3 orders of magnitude later in time. It is obvious that the majority of the excited-state nuclear evolution occurs on the optically dark state (denoted as "A") populated by internal conversion from theBstate. Theconcept oflower-energy excited-states participatingin pericyclic rearrangements has been proposed for the photochemical ringopeningofcyclobutene.'11J'8 Calculationsonthisreaction predictthat motionalongthereaction coordinate results in adecrease in energy of the A surface relative to the initially prepared excited-state resulting in an intersection of these surfaces. Therefore, evolution along the reaction coordinate results in internal conversion from the B to the A state, and the subsequent decay of the A state population leads to the appearance of ground-state photoproduct. The work
-
Cam*xl(Clmrar.,#on,
Orbital symmetry correlationdiagram for the photochemical ringopeningof 1.3-eyclohexadiene (CHD).This figuredepicua doubly excited electronic configuration of CHD which correlates to the groundstate configuration of the product. This configuration is expected to contribute significantly to the lowcr-lying excited state upon which the majority of the photochemical rearrangement occurs. F l y r e 1%
presented here provides experimental evidence in support of this hypothesis. The limited excited-state evolution on the B surface dictates that the A state is of mechanistic importance. Identification and elucidation of the extent of nuclear evolution on the lower-energy surface are the current frontiers in mechanistic studies of pericyclic photochemistry. Ground-State Reaction Dynamics. The temporal evolution in the photoproduct ethylenic intensities shows that all of these reactions are complete on the IO-ps time scale regardless of the stereochemistry and chemical nature of the rearrangement. The 9- and IS-ps vibrational relaxation times for cis-hexatriene and CHT, respectively, are in agreement with the cooling rates observed for other polyatomic molecules in the condensed phase.1'e'22 However, this is not to imply that the mechanism of ground-state vibrational relaxation is well understood. For example, ground-state frons-stilbene is observed to relax on a much slower time scale and this rate is solvent dependent.78 Furthermore, evidence exists for a nonstatistical distribution of energy throughout the modes of frons-stilbene as well as in rhodopsin and bacteriorhodopsin,challenging theassumption that intramolecular vibrational relaxation is complete on the subpicosecond time~cale.'~.'~l-'~~ In contrast, our analysisof the c-HT anti-Stokes scattering demonstrated that the Observed relative intensities are consistent with complete vibrational energy rand0mization.8~Theabilityto investigateandmonitor molecular temperatureusing transient Raman intensities should prove useful in furtherinvestigationsofvibrationalrelaxation dynamics. Also, theobserved frequencyevolutionin thespectraofthe ring-opened photoproducts is consistent with single-bond conformational relaxation. Other mechanisms for frequency evolution including coupling ofhigh frequencyvibrational modes toanharmonic lowfrequency vibrations (i.e., "exchange interaction") have been proposed for other reactions.82J2'Jz8 Determining the operative mechanism for frequency evolution in pericyclic reactions will involve evaluation of both Stokes and anti-Stokes intensities as well as consideration of the molecular details of the chemistry. Clearly, time-resolved vibrational spectroscopic techniques are naturally suited to these studies. The Right Answer from the Wrong Excited State! The Woodward-Hoffmann rules are based on consideration of the electronic description of the iniriarly prepared, one-electron excited state.' However, the results presented here demonstrate thatthemajorityoftherearrangementoccursonthelowerenergy A state which is not considered in orbital-symmetry conservation
Feature Article arguments. Why are the Woodward-Hoffmann rules so successful given that they consider an excited state on which the overall nuclear evolution is very limited? Figure 12 suggests a uotential answer to this question from the Woodward-Hoffmann molecular-orbital perspdctive. The doubly-excited configuration of CHD depicted in the figure correlates smoothly to the photoproduct ground state. This transition has no barrier; indeed, it is a downhill process. To the extent that the A state is described by this configuration, we would predict that evolution along the conrotatory reaction coordinate on this surface should occur readily without a symmetry imposed energy barrier. Indeed, calculations of the electronic structure of linear polyenes demonstrate that this is the dominant configuration of the lowest energy excited state.**." It is fortunate for the development of physical-organic chemistry that the primary photochemically active state in pericyclic reactions involves double excitation of the same orbitals considered in the earlier one-electron treatments! Conclusion. We have presented the results of our resonance Raman investigations of pericyclic reaction dynamics. The resonance Raman intensities observed for the electrocyclic ring openings of CHD and COT as well as the sigmatropic shift of C H T demonstrate the multidimensional nature of the initial excited-state nuclear evolution. Furthermore, a comparison of these intensities indicates that evolution along planarization coordinates serves to gate subsequent pericyclic rearrangement. Indeed, recent resonance Raman experiments on planar cyclobutene show that reactive disrotatory motion occurs immediately after photoexcitation.130 Time-resolved resonance Raman experiments on these reactions have demonstrated that the 10-ps photoproduct formation time is a general feature of pericyclic rearrangements. By combining these two techniques, we have been able to investigate chemical reaction dynamics from the initial excited-state nuclear evolution to the appearance of ground-state product and reveal some fundamental new concepts about the mechanism of these reactions. This approach should prove useful in elucidating the mechanisms of a variety of condensed-phase photochemical reactions.
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