J . Phys. Chem. 1991,95,3663-3670
3663
Picosecond Time Resolution of Vibrational Relaxation In Molecular van der Waals Complexes: Perylene with Naphthalene and Benzene Andrew J. Kaziska, Stacey A. Wittmeyer, and Michael R. Topp* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 (Received: October 8, 1990)
Fluorescence time profiles following excitation of naphthalene, benzene, and some alkane 1:l complexes with perylene under supersonic jet conditions have been recorded at =+ps resolution by using time-correlated single-photon counting techniques. Variations in rate constants over 2-3 orders of magnitude for the naphthalene case show that the vibrational coupling of in-plane modes is substantially weaker than for intermolecular or out-of-plane mod= at similar energies. Moreover, vibrational relaxation in perylenelnaphthalene is considerably slower than for some other molecular van der Waals complexes having fewer internal modes. For example, at 705-cm-I internal energy, the naphthalene complex shows >1 order of magnitude longer vibrational relaxation time (Le. 4 ns) than is the case for the corresponding benzene complex (120ps). At 353 cm-', the difference is even more pronounced, intramolecular vibrational relaxation (IVR) lifetimes of >15 ns (naphthalene) and 140 ps (benzene) being observed. On the other hand, the vibrational coupling in the benzene complex is shown to be more extensive than for the pentane complex at 353 cm-I. This behavior would not have been predicted by applying statistical theories based on available potential energy data. It is evident that the role of multipolar interactions in limiting largeamplitude relative motion of the components of such interaromatic complexes needs to be carefully examined with respect to its role in promoting vibrational coupling.
There is currently much interest in detailed comparisons of structure and dynamics in a wide variety of chemical systems ranging from isolated small molecules and clusters to large biological aggregates. Such studies examine the fate of energy deposited in vibrational or electronic chromophores in a molecule or molecular aggregate, its migration into other degrees of freedom, and the selectivity of such coupling. These kinds of measurements can be used to provide insight into energy transduction events ranging from single-molecule processes to photochemical change in the condensed phase. They may also be helpful in analyzing the motion of molecules adsorbed onto extended surfaces or included in mesophases such as membranes and micelles. Related experimentson molecular clusters isolated in collision-free environments, prepared initially in configurations determined by zero-point motion, provide valuable opportunities to examine molecular interactions, energy transfer, and relative motion under controlled conditions. Although no energy loss is possible in isolated clusters except by radiation or dissociation, there are some similaritieswith respect to vibrational relaxation phenomena in condensed media. For example, the persistence times of "resonance" emission in aromatic molecules such as anthracene and perylene and are not very different in different environments. Vibrational relaxation times for such molecules can exceed 10 ps in fluid solutions and 100 ps in rigid media at 4 K,depending on the available vibrational energy.'J Relaxation times of similar states in simple molecular van der Waals complexes, which lack the high phonon-mode density of an extended phase, can vary into the same time regime, depending on the nature of the complexing species. In both cases, the description of the "relaxing" levels involves mixing some zero-order promoting mode with a manifold of other states3" While condensed media are usually taken to represent a statistically limiting case, isolated molecules and their clusters span
the region from "small-molecule" through statistical limit behavior. Vibrational coupling in clusters depends on the kind of vibrational level structure present, as well as on the available channels for energy transfer.',* Vibrational coupling in aromatic molecules occurs through mixing of an optical promoting mode with levels usually of the same symmetry and similar energy. The energy range of significantly coupled states is usually limited by vibrational coupling matrix elements to the order of a few wavenumbers or less. In such cases, the bandwidth of an exciting picosecond laser pulse effectively brackets all of the eigenstates having a significant contribution from the "promoting" mode of interest. Effects associated with the spectral band shape and longitudinal mode structure of a laser pulse can usually be neglected. The dynamics subsequent to picosecond-order laser excitation can usually be determined reliably, and time-domain approaches can make important contributions to understanding level-mixing phenomena in largemolecule systems. Timedomain approaches are restricted by time resolution and dynamic range, which limit the extent to which time-dependent intensity modulations can be determined, yet they offer the advantage that the effects of rotational level structure can in many cases be averaged out. This has been shown for aromatic systems by experiments revealing simple quantum beats due to as few as two vibrational levels, largely independent of rotational level s t r u c t ~ r e . ~ - ~ Work from this laboratory has been reported on the dispersed fluorescence spectra of several van der Waals complexes involving perylene. Examples include Ar,? different alkanes,I0benzene," and naphthalene.11J2 All spectra could be analyzed in terms of contributions from two types of emission. Unrelaxed emission has the characteristic Franck-Condon profile of the vibronic level excited. In the case of the perylene a, mode (A)" at 353 cm-l, this profile has the distinctive intensity pattern (approximately 2, 1, 2, 1, ...) of a vibrational oscillator emitting from the v'=
(1) Tamm, T.; Saari, P. Chem. Phys. 197!4,40,311. Freiberg, A.; Tamm, T.; Timpmann, K.Laser Chem. 1983,3, 249. (2) Boczar, B. P.; Topp, M. R. Chem. Phys. Lcrr. 1984, 108, 490. (3) Felker, P. M.; Zewail, A. H. Ado. Chem. Phys. 1988, 70, 265. (4) Demmer, D. R.; Hager. J. W.; Leach, 0 . W.; Wallace, S. C. Chem. Phys. Lcrr. 1987, 136, 329. (5) Kaufman, J. F.; CBtt, M. J.; Smith, P. G.; McDonald, J. D. J. Chem. Phys. 1989,90,2818. (6) Mukamel, S . J . Chem. Phys. 1985,82, 2867. Shan, K.;Yan, Y. J.; Mubmel, S . J . Chem. Phys. 1987, 87, 2021.
(7) Nimlos, M. R.; Young, M. A.; Bemstein, E. R.; Kclley, D.F.J . Chem. Phys. 1989. 91, 5268. ( 8 ) Weber, P. M.; Rice, S. A. J . Chem. Phys. 1988,88,6120. (9) Doxtader, M. M.; Gulis, I. M.; Schwartz, S. A.; Topp, .. M. R. Chem. Phys. Lorr. 1984, 112, 483. (IO) Doxtader, M. M.; Topp, M. R. J . Phys. Chem. 1985, 89, 4291. (1 1) Motvka, A. L.; Wittmeyer, S.A. Babbitt, R. J.; TODD, ._M. R. J. Chem. Phys. 1988,89,4586. (12) Babbitt, R. J.; Topp, M. R. Chem. Phys. Leu. 1986, 127, 111. (13) Fourmann, B.; Jouvet. C.: Tramer. A.: le Bars. J. M.: MilliC. Ph. Chem. Phys. 1985.92, 25.
1. Intrduction
0022-3654/91/2095-3663$02.50/0
0 1991 American Chemical Society
3664 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 ,
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Figure 1. Fluorescence spectra for jet-cooled perylene, excited into different levels of the A-mode (353 cm-I) sequence. 1 level to u " = 0, 1, 2, 3, ... on a displaced lower surface. The traces shown in Figure 1 illustrate the trend in the Franck-Condon profile for a sequence of A-mode (353 cm-I) overtones. It was shown in ref 11 that this sequence fits harmonic-oscillator behavior quite closely. The important result from those experiments was that the naphthalene complex of perylene also exhibits a strong unrelaxed fluorescence component not only at 353 cm-* but also at 705 cm-I for the overtone band. By comparison, the benzene complex showed no resonance emission above the instrumental background. In fact, only the Ar 1:l complex showed an amount of unrelaxed emission comparable to that found in the naphthalene case. This paper reports time-resolved fluorescence experiments on benzene and naphthalene complexes with perylene in the vibrational energy region 0-705 cm-'. Preliminary data on alkane complexes are included for comparison. Time resolution of 40 ps has allowed resolution of a number of fast transients in the emission spectra. The dynamics of vibrational relaxation for the benzene complexes is observed to be orders of magnitude faster than for comparable excitations of perylene/naphthalene. This result was contrary to initial expectations and indicates that, even in systems involving large molecules, vibrational coupling can be mode selective and is subject to the influence of large-amplitude intermolecular modes.
2. Experimental Section Fluoresance lifetimes were measured by using time-correlated single-photon counting (TCSPC). The apparatus diagram is shown in Figure 2. Second-harmonic radiation (800-900 mW) from a continuously modelocked NdYAG laser (Quntronix 416) was used to pump a synchronous-cavitydye laser, cavity dumped at 3.8 MHz (Coherent 701-3CD using LDS-821, 30-40 mW). The output was frequency-doubled in LiI03, yielding secondharmonic power of 2-3 mW. The excitation bandwidth in these experiments was 552 cm-I, is virtually eliminated.
Vibrational Relaxation in van der Waals Complexes
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The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3669
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Time (ns)
Figure 13. Fluorescence-time profiles obtained following excitation of perylene/naphthalene at 550 and 540 c d . The 550-cm-l case still exhibits some intermediate-case behavior, while the 540-cm-l case resembles the case of benzene or alkane complexation,having T = 100 ps.
In view of this evidence, it is reasonable to expect that the dynamics of the C and N modes in naphthalene/perylene would be different from that of the A modes. Indeed, in ref 11, we reported that the dispersed emission spectra obtained by excitation into these two levels were qualitatively different, both from each other and from that of the A mode. The data reproduced in Figure 12 show that there is a residual “spike” in the emission spectrum for the 550-cm-’ band a t the position of the “Au = 0” transition, indicating a small contribution from unrelaxed fluorescence, while the data for the 540-cm-’ band show a broadened, effectively structureless spectrum. These spectra provide a dramatic contrast to the spectrum recorded for 705-cm-’ excitation of the same complex (Figure 8). Results from the time-domain study of the Cl, (55O-cm9 and Nb (540-cm-’) excitations are shown in Figure 13. For both cases, biphasic decays are found. The longer time components are similar, near 14 ns, indicating little singlet-triplet coupling. On the other hand, the shorter time components are substantially different, reflecting differences in the emission spectra of Figure 12. The “C” spectrum shows an evolution of the spectral bandwidth over 1.3 ns, while the “”-mode case gives a transient on the order of 120 ps. 4. Mscwsion
The data reported here have confirmed preliminary conclusions based on frequency-domain spectroscopy, which indicates that vibrational coupling in perylene/naphthalene is mode selective and considerably weaker than in the case of perylene/benzene. Superficially, this behavior would not have been expected on the basis of density-of-states considerations. The benzene complex involves only 44 atoms (26 carbon), compared with 50 (30 carbon) for the naphthalene case, and the benzene molecule itself has appreciably fewer low-frequency modes (Le., it is more rigid) than naphthalene. However, this difference may not be significant, since the vibrational mode density in the region 5705 cm-’ should be dominated by the large perylene molecule. The essential differences between the benzene and naphthalene cases must lie in the intermolecular modes generated by loss of three translations and three rotations on forming each complex. In zero order, three of these would transform into an intermolecular “stretch” and two ‘bends” corresponding to parallel motion of the two rings. The other three would approximately evolve into a weakly hindered rotation about a plane perpendicular to the perylene ring (Le. z-axis rotation) and two strongly hindered rotations appearing as internal “rocking” motions of the complex.
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Semiempirical calculation^^^-^^ have successfully predicted the binding energies of van der Waals complexes between aromatic molecules and one or two complexing species such as argon or methane. The agreement to within about 10%of experimental v a l ~ e s is~ reasonable, ~ ~ ~ * since the equations were parametrized according to thermodynamic properties of surfaceadsorption and some solid-state cases. Such techniques have also successfully computed some low-frequency vibrational modes of small van der Waals complexes involving aromatic hydrocarbon^.^^^^^ Some of these are listed in Table 111for systems of interest here. Similar numbers for this set of modes are obtained for the naphthalene and benzene cases, and even for the methane complex, which also shows substantial vibrational coupling at low energy (see Figures 5 and 8). Hindered rotational degrees of freedom are still not well understood. These motions rely more on subtleties in the shape of the intermolecular potential, and are especially sensitive to corrugation effects. As noted in ref 11, the only significant difference between benzene and naphthalene that could affect vibrational coupling a t low energies appears to be the weakly hindered rotational motion about the z axis. The distinction in this case rests on differences in the multipolar term, which predict that the naphthalene complex would be more rigid. Measurements of the binding energies of molecular complexes are needed to test the results of semiempirical calculations a t the simplest level, to gain insight into the importance of multipolar components of the intermolecular potential. At this point, however, only a few measurements have been made on small-molecule complexes, and we are currently aware of no study measuring the binding energy of complexes involving two or more aromatic molecules. Structures of key complexes also need to be determined precisely, in order to test subtleties of the balance between multipolar repulsion and dispersive attraction terms. Computations of large-amplitude motions in these kinds of aggregates depend critically on the solution of these problems. Qualitatively, the behavior of the n-alkane complexes, com red to that of benzene, is more straightforward. It is k n ~ w n ~ l - ~ c h a t the low-frequency modes of an alkyl chain provide a means for (23) Williams, D. E. J. Chem. Phys. 1967,47,4680. Acta Crystallogr. 1980, A36, 715. Hansen, F. Y.; Taub, H. Phys. Rev. 1979, 819, 6542. Ondrechen, M. J.; Berkovitch-Yellin,2.; Jortner, J. J . Am. Chem. Soc. 1981, 103, 6586. (24) Hutson, J. M.; Clary, D. C.; Beswick, J. A. J. Chem. Phys. P984,81, 4474. ( 2 5 ) Hall, D.; Williams, D. E. Acta Crystallogr. 1975, ,431,56. Williams, D. E. Acta Crystallogr. 1980, A36,715. Califano, S.;Righini, R.; Walmsley, S . H. Chem. Phys. Lett. 1979,64,491. Price, S.L.; Stone, A. J. J . Chem. Phys. 1987,86,2859. Stone, A. J.; Price,S. L. J. Phys. Chem. 1988,92,3325. (26) Law, K. S.; Bemstein, E. R. J. Chem. Phys. 1985,82,2856. Schauer, M.;Bernstein, E. R. J. Chem. Phys. 1985, 82, 3722. (27) Doxtader, M. M.; Gulis, I. M.;Schwartz, S.A,; Topp, M.R. Chem. Phys. Lett. 1984,112,483. Doxtader, M. M.;Topp, M. R. J. Phys. Chem. 1985, 89, 4291, (28) Wittmeyer,S.A.; Kaziska, A. J.; Motyka, A. L.; Topp, M. R. Chem. Phys. Lett. 1989, 154, 1. (29) Menapace. J. A.; Bernstein, E. R. J. Phys. Chem. 1987. 91, 2533, 2843. Nowak. R.; Menapace, J. A.; Bernstein, E.R. J. Chem. Phys. 1988, 89, 1309. (30) Leutwyler, S . Chem. Phys. Lett. 1984,107,284; J. Chem. Phys. 1984, 81, 5480. Bhiger, J.; Leutwyler, S. Chem. Phys. Lett. 1986, 126, 238. Leutwyler, S.;Jortner, J. J . Phys. Chcm. 1987, 91, 5558. (31) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1981, 74, 6986 and references therein. (32) Parmenter, C. S.;Stone, B. M. J. Chem. Phys. 1986,84,4710. Moas, D. B.; Parmenter, C. S.; Ewing, G . E. J. Chem. Phys. 1987, 86, 51. (33) Syagc, J. A.; Felker, P. M.;Semmes, D. M.;Zewail, A. H. J . Chcm. Phys. 1985,82,2896. Baskin, J. S.;Dantus, M.; Zewail, A. H. Chem. Phys. Lett. 1986, 130, 413.
3670 The Journal of Physical Chemistry, Vol. 95, No.9, 1991 1000
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Figure 14. Fluorescence-time profiles for the pentane and benzene complexes of perylene, following excitation at 353 cm-' (A:). Although the benzene complex has a longer fluorescence decay time, the IVR rate is faster. (See Table 11.)
efficient coupling of vibrational energy deposited in a covalently attached chromphore. Other work has addressed the "insulation" of the two components by interposition of different kinds of linkages including heavy atoms.K38 In the present case, we are dealing with a nonbonded interaction, and by using a large aromatic substrate, we can carry out experiments with a smaller amount of vibrational energy (e.g. 353 cm-') than is the case in many infrared studies. Consequently, it is possible to observe differences in the behavior of the benzene and alkane complexes for different energies. The main ohservation from our preliminary study is that the benzene complex of perylene, at the low energy of 353 cm-', shows more extensive vibrational coupling (T = 140 ps) than does the n-pentane complex ( T = 240 ps; see Figure 14), but the coupling is about the same for the n-heptane complex ( T = 130 ps). Doubling the internal energy affected the vibrational (34) Kim, H. L.; Kulp, T. J.; McDonald, J. D. J. Chrm. Phys. 1987,87, 4376. (35) McIlroy, A.; Nesbitt, D. J. J. Chrm. Phys. 1989, 91, 104. (36) Merman, S. M.; Lbpez, V.; FairCn, F.; Voth, G. A,; Marcus, R. A. Chrm. Phys. 1989, 139, 163. (37) Uzer, T.; Hynw, J. T. Chrm. Phys. 1989, 139, 163. (38) Lehmann, K. K.;Pate, B. H.; Scoles, G . J . Chrm. Phys. 1990,93, 2152.
Kaziska et al. coupling rate of the benzene complex only slightly ( T = 120 ps), while both alkane complexes showed about a factor of 2 increase in the relaxation rate (T = 120 and 70 ps for pentane and heptane, respectively). Although a more detailed analysis is necessary, this implies that the vibrational coupling is becoming more efficient for the alkane complexes and less efficient for the benzene complex as the energy is raised from 353 to 705 cm-I. This indicates that the vibrational coupling of the benzene complex is obtained through lower frequency modes than is the case for alkane complexes. This is consistent with a picture where the benzene complex is very "loose" in regard to rocking and rotational modes, allowing energy coupling from the perylene substrate. On the other hand, longer chain alkanes do not rotate easily on the surface and, at low energies, do not have a great mode density. This is easily Seen from the result of increasing the chain length from pentane to heptane, where the 7 value drops appreciably. The naphthalene complex does not show strong vibrational coupling, unless energy is directly injected into intermolecular modes. Even in this case, the coupling is weaker than in the benzene case. This implies that the observed coupling is restricted because the low-frequency mode density is less than it is for the benzene complex or because the matrix element nesded to couple in-plane motions of perylene atoms to intermolecular motions of the complex is small. The mode density of the two complexes has to be at least comparable. Parallel to the case demonstrated by Parmenter for p-fluorotoluene?* it is possible that the results signify a rather rigid perylene/naphthalene complex. In this case, the absence of large-amplitude motion could explain a small coupling matrix element. This may indicate that rotation of the naphthalene moiety about all three possible axes relative to perylene is substantially hindered. By contrast, the benzene complex appears to exhibit significant large-amplitude motion, since the vibrational coupling is efficient at all levels studied. 5. Conclusion
These results for benzene and naphthalene complexes of perylene focus on the need to be able to compute intermolecular potential energy surfaces involving aromatic molecules with a precision capable of predicting hindered rotational motions. The evident dynamical differences between the naphthalene and benzene complexes of perylene originate in the different degree to which each complex can undergo internal large-amplitude motion. Such degrees of freedom are largely constrained by the multipolar component of the intermolecular potential. Calculations are needed that will allow such differences to be predicted more effectively. Acknowledgment. We acknowledge the support of this work by the Research Corp. and by the Research Foundation of the University of Pennsylvania. Some facilities were made available through the Laboratory for Research on the Structure of Matter at the University of Pennsylvania (NSF).
