The Journal of
Physical Chemistry
0 Copyright, 1992, by the American Chemical Society
VOLUME 96, NUMBER 11 MAY 28,1992
LETTERS Ultrafast Dynamics of Isomerization Reactions: Structural Effects in Stilbene@) L.B a b e s , +A. A. Heikal, and A. H. Zewail* Arthur Amos Noyes Laboratory of Chemical Physics,$ California Institute of Technology, Pasadena, California 91 I25 (Received: March 4,1992; In Final Form: April 2, 1992)
We report our study of the ultrafast dynamics of isomerization and IVR in a designed series of stilbenes. Striking structural effects on the dynamics are observed and related to the stabilization or destabilization of the planar and twisted states.
The dynamics of isomerization reactions, in particular stilbenes, have received a wealth of experimental and theoretical studies (for recent reviews, see refs 1 and 2). Of particular interest to this laboratory is the study, under collision-free molecular beam conditions, of the microcanonical rates k(E)3and intramolecular vibrational-energy redistribution (IVR)? following excitation to the first singlet potential energy surface (PES). Twisting in the trans form of stilbene along the reaction coordinate is shown to be subject to an internal barrier (at energy E = 3.3 i 0.2 kcal/mol), and IVR is found to be restricted among vibrational states at energies below the barrier. The rates3Jv6and yieldSaof isomerization above the barrier have provided an opportunity to test the statistical theories for IVR and k(E).3-" The spectroscopy3J2of both So and SIat the equilibrium configuration and force field c a l c ~ l a t i o nhave ~ ~ also been studied, and a great deal is now known about the vibrational modes of the molecule. In this letter we present our first results on the picosecond dynamics of isomerization and IVR in a designed series of stilbene derivatives (see Figure 1) in (isolated) supersonic beams. The key role of the torsional (twisting) motion about the C,-C, double and c p h refer bond and the motion about the Ce-Cph bond, (C, to ethylenic and phenyl carbon atoms, respectively) are examined Fulbright/Spanish Ministry of Education and Science Postdoctoral Fellow. *Contributionno. 8582.
PI
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Figure 1. Series of molecules studied in this work.
by "anchoring" the double bond to the phenyl moiety. The effects of symmetry, the number of atoms, and electron delocalization are studied by considering the influence of one OCH3 (methoxy) versus two OCH3 groups substitution on IVR and k ( E ) . From these studies of (1) the barrier height, (2) the microcanonical rate (1) Saltiel, J.; Sun, Y.-P. Phorochromism-Molecules and Systems; Dum, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 64. ( 2 ) Waldeck, D. H. Chem. Rev. 1991, 92, 415.
0022-3654/92/2096-4127$03.00/00 1992 American Chemical Society
4128 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Letters
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Excess Vibrational Energy (cm-’) Figure 2. The microcanonical rate constants k(E) measured for stilbenes isomerization in the first excited state23as a function of excess vibrational energy. Open triangles: data from ref 3. Note that the rates were obtained from the single-exponential decays observed at all energies. The biexponential decays in Figure 3 are reflection of IVR measurements which were made by frequency and time resolution (see ref 4 for more details).
constants k(E) and IVR, and (3) the relevant spectra in these series of stilbenes, we report striking effects with the changes in molecular structure. The experimental apparatus has been described e1~ewhere.l~ Briefly, helium (50-70 psi) was passed over tram-stilbene, or the stilbene derivatives, heated gently in a Pyrex tube prior to beam
(3) (a) Syage, J. A.; Lambert, W. R.; Felker, P. M.; Zewail, A. H.; Hochstrasser, R. M. Chem. Phys. Lett. 1982,88,266. (b) Syage, J. A.; Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1984,81,4685. (c) Syage, J. A.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1984, 81, 4706. (d) Felker, P. M.; Zewail, A. H. J . Phys. Chem. 1985, 89, 5402. (4) (a) Felker, P.M.; Lambert, W. R.; Zewail, A. H. J . Chem. Phys. 1985, 70, 265. (b) Felker, P. M.; Zewail, A. H. Adv. Chem. Phys. 1988, 70, 265. (5) (a) Amirav, A,; Jortner, J. Chem. Phys. Lett. 1983, 95, 295. (b) Majors, T. J.; Even, U.; Jortner, J. J . Chem. Phys. 1984, 81, 2330. (c) Rademann, K.; Even, U.; Rozen, S.;Jortner, J. Chem. Phys. Lett. 1986,125, 5. (6) Troe, J. Work in progress (private communication). (7) Khundkar, L. R.; Marcus, R. A,; Zewail J. Phys. Chem. 1983, 87, 2473. (8) (a) Troe, J. Chem. Phys. Lett. 1985, 114, 241. (b) Trw, J.; Weitzel, K.M. J . Chem. Phys. 1988,88, 7030. (9) Courtney, S. H.; Balk, M. W.; Philips, L. A.; Webb, S. P.; Yang, D.; Levy, D.H.;Fleming, G. R. J. Chem. Phys. 1988, 89, 6697. (IO) Nordholm, S. Chem. Phys. 1989, 137, 109. (11) Negri, F.; Orlandi, G. J . Phys. Chem. 1991, 95, 748. (12) (a) Zwier, T. S.; Carrasquillo, E. M.; Levy, D. H. J. Chem. Phys. 1983, 78, 5493. (b) Suzuki, T.; Mikami, N.; Ito, M. J . Phys. Chem. 1986, 90,6431. (c) Ito, M. J . Phys. Chem. 1987, 91, 517. (d) Spangler, L. H.; van Zee, R.; Zwier, T. S.J . Phys. Chem. 1987, 91, 2782. (e) Spangler, L. H.; van Zee, R. D.; Blankespoor, S. C.; Zwier, T. S. J . Phys. Chem. 1987, 91, 6077. (0 Spangler, L. H.; Bosma, W. B.; van Zee, R. D.; Zwier, T. S . J . Chem. Phys. 1988,88,6768. (g) Urano, T.; Hamaguchi, H.; Tasumi, M.; Yamanouchi, K.; Tsuchiya, S. Chem. Phys. Lett. 1987.137, 559. (h) Urano, T.; Maegawa, M.; Yamanouchi, K.; Tsuchiya, S. J . Phys. Chem. 1989, 93, 3459. (i) Urano, T.; Hamaguchi, H.; Tasumi, M.; Yamanouchi, K.; Tsuchiya, S.;Gustafson, T. L. J. Phys. Chem. 1989, 91, 3884. (13) (a) Warshel, A. J . Chem. Phys. 1975, 62, 214. (b) Gruner, D.; Brumer, P.;Shapiro, M. J . Phys. Chem. 1992, 96, 281. (14) Khundkar, L. R.; Zewail, A. H. Ann. Rev.Phys. Chem. 1990,41, 15,
and references therein.
expansion. The molecules were expanded (continuously) through a 60-pm nozzle. The cooled molecules were then excited a t a nozzle-to-laser distance of about 30 times the nozzle diameter (X/D 30). A tunable UV picosecond laser system was used with temporal and frequency pulse widths (fwhm) of 15 ps and 5 cm-’, re~pective1y.I~ The laser-induced fluorescence (LIF) was focused on the slit of a monochromator and then timeresolved by single-photon-counting techniques. A nonlinear least-squares method was used to fit the experimental time-resolved data.14J5 Care was taken to measure the response function accurately and to maintain reproducibility during the measurement of the fluorescence decays. Measurements of IVR and k(E) were made following the methodology given in ref 14. tram-Stilbene (S, Aldrich, 96%), 4-methoxy-trans-stilbene(MS, Parish), and 4,4’-dimethoxy-trans-stilbene(DMS,Aldrich, 97%) were used with the stated purity. 2-Phenylindene (PI) was synthesized following the method described in the literature.16 Figure 2 presents the measured rate constants &(E)as a function of the excess vibrational energy E for the series of molecules studied. For tram-stilbene (S),the results show a threshold of 3.3 f 0.2 kcal/mol, in agreement with the previously published works by Felker et aL3 They are also consistent with the quantum yield and rates measurements by Amirav, Even, and .iortnerS and with the more recent timeresolved work by Troe and co-workers.6 The 0; transition of S is at 310.1 nm, and the excess vibrational energy is taken above this value.” In this work, we have extended
-
-
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(15) (a) Semmes, D. H.; Baskin, J. S.; Zewail, A. H. J . Am. Chem. SOC. 1987, 109,4104. (b) Semmes, D. H.; Baskin, J. S.; Zewail, A. H. J . Chem. Phys. 1990, 92, 3359. (16) (a) Greifenstein, L. G.; Lambert, J. B.; Nienhuis, R. J.; Fried, H. E.; Pagani, G. A. J . drg. Chem. 1981,46,5125. (b) Bors, D. A.; Kaufman, M. J.; Streitweiser Jr., A. J . Am. Chem. SOC.1985, 107, 6975. (17) The precise value of the threshold energy cannot be determined without theory. For stilbene, the experimental value reported in ref 3 is 1200 cm-I, and this corresponds to a value of the rate of 0.08 X IO9 s-I. Using this 8% criterion, we deduced the corresponding values for the other derivatives. We have also confirmed these values by a plot of log T vs E.
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Figure 3. RepresentativeIVR data for MS,DMS and PI in both restricted and dissipative regimes. For comparison with S,the data at the top (from ref 4) are shown.
the energy range over the E values of ref 3 and for stilbene at E = 3500 cm-I the lifetime reaches the 80-ps range. The 0; transition of PI,MS, and DMS were measured at 318.3,325.0, and 333.4 nm (red shifts of 827,1478, and 2248 cm-I with respect to S),respectively. For MS,slight changes from S,in both threshold (E,,,= 3.7 f 0.2 kcal/mol) and rate constants were observed, but these changes are dramatic for DMS (threshold at 8.6 f 0.2 kcal/mol) and PI (6.5 0.6 kcal/mol). With no excess vibrational energy, the lifetime of S was measured to be 2.63 ns, in agreement with previous measurement^."^ The lifetimes for MS, DMS, and PI were found to be 2.96, 2.96, and 3.20 ns, respectively. Three structural-dynamical issues are of interest here: the inhence of the structural changes on the barrier heights, the IVR process, and the microcanonical rate constants k(E). The origin
of the energy barrier for isomerization'* is believed to be due to the avoided (or partial) crossing'g between the excited IB,,surface and a doubly excited 'A8 surface. The former is believed to be 'locally" excited on the phenyl rings (with some delocalization and ionic character), and in the twisted configuration is biradial in nature. The 'A, surface, however, because of its antibonding character, correlates with a 'zwitterionic" twisted state.20 The changes in the barrier height can be understood by considering (18) Saltiel, J. J . Am. Chem. Soc. 1968, 90,6394. (19) (a) Orlandi, G.; Siebrand, W. Chem. Phys. Lett. 1975,30,352. (b) Birks, J. B. Chem. Phys. Lert. 1978, 54, 430. (c) Orlandi, G.; Palmieri, P.; Poggi, G. J . Am. Chem. Soc. 1979, 101, 3492. (d) Olbrich, G. Ber. Bunsen-Ges. Phys. Chem. 1982,86, 209. (e) Hohlneicher, G.; Dick, B. J . Phorochem. 1984, 27, 215.
4130 The Journal of Physical Chemistry, Vol. 96, No. 11, 195’2
the effect of the substituents on the relative energy of both the trans and the twisted (90O) “phantom” states. At the planar trans configuration we observe a red shift, and this gives the extent of the substituent effect. The twisted “zwitterionic” state, according to the biradicaloid charge-transfer model,” should stabilize with the OCH,substitution. Taking into account the stabilization of the planar and the twisted forms, the barrier height does not change much. The stabilization of the twisted state is due to change in molecular symmetry, which presumably stabilizes the ionic character or, in other words, allows the ionic configurations to make a net contribution. For DMS, the twisted state acquires back the symmetry on both phenyls, and this results in a relative shift to higher energy, Le., relative destabilization of the twisted state. The planar configuration is stabilized by the electron density donation of the OCH3groups. From the measured red shift one can then understand the large and unexpected barrier (8.6 f 0.2 kcal/mol). This actually may explain the various results obtained for substituted trans-stilbenes where no change in the barrier height was found when only one substituent was used.5 More studies are in progress to quantify the degree of polarity in these states in view of more recent calculations which describe the isomerization surface as adiabaticsEbAlso, a large extent of the biradical contribution to the twisted state has been proposed theoretically for polyenes.21 The anchoring of the double bond to a phenyl group (PI) “closes” the isomerization channel and the appearance of a “threshold” (see Figure 2) in k(E) for this molecule is likely related to the (torsional) motion about the Ce4&, single bond (6.5 f 0.6 kcal/mol). This bond is expected to have some double-bond character in the excited state, and it is interesting to observe a (20) See,for example: (a) Lippert, E.; Rettig, W.; Bonacic-Koutecky, V.; Heisel, F.; Miehe, J. A. Adv. Chem. Phys. 1987,68, 1 . (b) Rettig, W. Modern models of bonding and delocalization (Molecular Siructure and Energetics; Liebman, J., Greenberg, A., Eds.; VCH Publishers: New York, 1988; Vol. 6. (21) Nebot-Gil, I.; Malrieu, J. P. J . Am. Chem. SOC.1982, 104, 3320.
Letters large value for the barrier height, reflecting the planarity of the molecule.22 Recent quantum calculation considers both double-bond torsion and single-bond rotation, but it gives the latter at higher energies.8bThe role of these coordinates on isomerization in solution has recently been discussed (see ref 2), and these measurements in isolated molecules are separating the importance of intrinsic barriers. IVR changes significantly upon methoxy substitution, less quantum-beat-modulated decays and smaller rates, with our time resolution (see Figure 3). In the dissipative regime, where enough states are available for energy redistribution irreversibly, Figure 3 shows a “biexponential” IVR behaviofa in the series of molecules but with the rate decreasing with substitution. This is in contrast with the expected acceleration of the process considering the increase in density of states and, in M S case, the symmetry lowering. The methoxy groups may have divided the vibrational/ rotational phase space. The k(E) for DMS above the barrier are smaller than those of S,consistent with a density of states that is larger than in S. In future experiments, we plan to study the extent of nonstatistical (or statistical) behavior by increasing the time resolution for the IVR from the initial state. We should then be able to examine the theory for IVR and k(E) in this interesting series of stilbenes which exhibits these striking effects of structure on dynamics. Acknowledgment. We thank Dr. V. Alcazar for helping us in the synthesis of 2-phenylindene and NMR analysis and Prof. W. Rettig for his suggestions and discussions. (22) (a) Baskin, J. S.; Zewail, A. H. J . Phys. Chem. 1989,93, 5701. (b) Champagne, B. B.; Pfanstiel, J. F.; Plusquellic, D. F.; Pratt, D. W.; van Herpen, W. M.; Meerts, W. L. J . Phys. Chem. 1990, 94, 6 . (23) The quantum yield for stilbene has been measured (see text), and there is no nonradiative channels other than isomerization. For the other molecules in the series we adopt the same conclusion as the lifetime with no excess vibrational energy is only (slightly) chan ed, actually increased. The excess vibrational energy E is taken above the 0,f which has been determined from the dispersed fluorescence spectra of all compounds.
