6322
J . Phys. Chem. 1989, 93, 6322-6329
Activated Barrier Crossing in van der Waals Complexes of Perylene with Alkyl Halides Andrea L. Motyka, Stacey A. Wittmeyer, R. Jefferson Babbitt,? and Michael R. Topp* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 (Received: January 1 1 , 1989; In Final Form: April 4, 1989)
van der Waals complexes of perylene with 1-chlorobutane and 1-chloropentane, cooled in a supersonic jet, each exhibit at least three conformational isomers in the electronic ground state, which appear in the fluorescenceexcitation spectra as distinct band combinations. In emission from the zero-point levels in S1, the spectra of the different conformers are very similar to each other and to that of free perylene but are displaced according to the positions of the excitation resonances. For comparison, the corresponding n-pentane and n-hexane complexes exhibit simple spectra and single isomers. At 355 cm-I internal energy, the emission spectra of the pentane and hexane complexes, measured with - 2 5 cm-' resolution, each show a symmetric broadening consistent with extensive vibrational coupling, while some conformers of the alkyl halide complexes give rise to shifted spectra. The spectral shifts indicate that activated barrier crossing between different conformers is taking place. A semiquantitative model is presented for differences in the relative well depths for different sites.
1. Introduction There is currently much interest in determining the properties of aromatic van der Waals complexes, involving species from rare gas atoms to large molecules. Levels of aggregation can be determined by mass-resolved techniques involving two-photon ionization, while electronic spectral shift data permit some assignments of structural types. For example, the sign of the shift or the amount of Franck-Condon activity in "van der Waals" modes may be used to distinguish between different attachment sites.' In a small number of cases, rotational band contour analysis has been used for structural estimation of aromatic van der Waals complexes,2and to probe. single vibronic levels of large molec~les,~ where individual rotational sublevels are not resolved. Supersonic jet techniques have also allowed detailed studies of potential energy surfaces governing large-amplitude motion. For example, toluene and derivatives4 exhibit vibronic progressions associated with rotation (torsion) of the methyl group. The Franck-Condon activity of such modes is a consequence of electronic-state-dependent equilibrium geometries, and comparison of absorption and emission data has allowed detailed fitting of the spectra and assignment of the local potential energy surfaces. Similar experiments have been carried out for more strongly hindered rotors, such as b i p h e n ~ lbinaphthyl: ,~ and 9-phen~lanthracene.~Barrier crossing has also been studied in flexible, covalently bonded systems under supersonic jet conditions, such as the vibrationally activated electron transfer in the species A*-(CH2)3-B (A* SI-excited 9-anthry1, B = p-N,N-diethylanilino).8 The picosecond study of vibrational mode mixing (IVR) in different conformers of alkylanilines, involving energy coupling from a chromophore to a flexible alkyl chain, is of particular interest from the point of view of the present study.9 Numerous instances have been reported where van der Waals complexes are present in more than one ground-state conformation or exhibit barriers to the interchange of indistinguishable conformers. Examples are found over a large range of sizes, including A P - H C ~ , ~the ~ H20I1and s-tetrazine12dimers, and the acetylene complex of s - t e t r a ~ i n e . ' ~The latter has been shown to exist in two distinct forms since it is still small enough for the structure to have been determined on the basis of rotational structure in the vibronic spectrum. On the other hand, the spectrum of the fluorene dimer is highly complex, and this species has been shown to undergo barrier-free conversion to an excimer form in the excited state.I4 In general, techniques for analysis of the structures of van der Waals complexes involving large aromatic molecules are not well refined. For aromatic molecular complexes involving atoms and small molecules, two general types of conformational variant are encountered: Present address: Union Camp Corporation, Research and Development Division, P.O. Box 3301, Princeton NJ 08543.
0022-3654/89/2093-6322$01.50/0
(a) Planar aromatic molecules such as anthracene, perylene, and coronene effectively present two equivalent faces to potential complexing species, and distributional isomers can exist where the occupation numbers of these two sites are different. Cases such as these have been structurally analyzed for single-ring aromatics by Levy and co-worker~.'~Examples relevant to the present work include the cis (or (2,O)) and trans (or (1,l)) isomers of Ar2/perylene,I6 for which the emission spectra indicate differences in the vibrational coupling dynamics.I7 The equilibrium between these two types of isomer is maintained in the early part of the expansion, requiring collisions with complexing species. Some empirical correlations of the relative amounts of cis and trans conformers have been established, for example, for a homologous series of alkanes adsorbed onto perylene'8*19and anthracene.20 However, a detailed thermodynamic analysis has yet to be carried out. (b) Further into the expansion, once the distributional isomer ratio has been determined, collisions with buffer gas continue to cool the complexes. In this region vibrational isomers of 1:l complexes, associated with local minima in the intermolecular potential surface, can be trapped out. There are important similarities here between the present case of van der Waals com(1) Hager, J.; Wallace, S. C. J. Phys. Chem. 1984, 88, 5513. (2) Philips, L. A,; Levy, D. H. J. Chem. Phys. 1986.85, 1327. Beck, S. M.; Liverman, M. G.;Monts, D. L.; Smalley, R. E. J. Chem. Phys. 1979, 70, 232. (3) Keelan, B. W.; Zewail, A. H. J. Chem. Phys. 1985,82,3011. Keelan, B. W.; Zewail, A. H. J. Phys. Chem. 1985,89, 4939. (4) Murakami, J.; Ito, M.; Kaya, K. Chem. Phys. Left.1981, 80, 203. Ito, M. J . Phys. Chem. 1987, 91, 517 and references cited. (5) Murakami, J.; Ito, M.; Kaya, K. J. Chem. Phys. 1981, 74, 6505. (6) Jonkman, H. T.; Wiersma, D. A. Chem. Phys. Lett. 1983, 97,261. J . Chem. Phys. 1984, 81, 1573. (7) Werst, D. W.; Gentry, W. R.; Barbara, P. F. J. Phys. Chem. 1985,89, 729. (8) Felker, P. M.; Syage, J. A,; Lambert, W. R.; Zewail, A. H. Chem. Phys. Lett. 1982, 92, 1. (9) Baskin, J. S.; Dantus, M.; Zewail, A. H. Chem. Phys. Lett. 1986, 130, 471 -.
(10) Robinson, R. L.; Ray, D.; Gwo, D.-H.; Saykally, R. J. J . Chem. Phys. 1987.87, 5149. ( 1 1) Huang, Z. S.; Miller, R. E. J. Chem. Phys. 1988, 88, 8088.
(12) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H. J. Chem. Phys. 1983, 79, 1581. Young, L.; Haynam, C. A,; Levy, D. H. J. Chem. Phys. 1983, 79, 1592. (13) Morter, C. L.; Koskelo, A,; Wu, Y. R.; Levy, D. H. J . Chem. Phys. 1988, 89, 1867. (14) Saigusa, H.; Itoh, M. J . Phys. Chem. 1985, 89, 5436. (15) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H. J. Chem. Phys. 1984, 80, 2256. (16) Leutwyler, S. Chem. Phys. Lett. 1984, 107, 284. Leutwyler, S.; Jortner, J. J. Phys. Chem. 1987, 91, 5588. (17) Doxtader, M. M.; Gulis, I. M.; Schwartz, S. A,; Topp, M. R. Chem. Phys. Lett. 1984, 112, 483. (18) Schwartz, S. A.; Topp, M. R. J. Phys. Chem. 1984, 88, 5673. (19) Babbitt, R. J. Ph.D. Dissertation, University of Pennsylvania, 1988. (20) Babbitt, R. J.; Topp, M. R. Chem. Phys. Lett. 1987, 135, 182.
0 1989 American Chemical Society
Complexes of Perylene with Alkyl Halides plexation and the localization of an individual “flexible” molecule such as tryptophan in different conformational traps.21 While in many cases symmetry considerations restrict the number of distinguishable forms, alkane complexes of 2,5-diphenylfuran (PPF)22have indicated the coexistence of symmetrically distinct conformers. These results have been successfully compared with semiempirical atom-atom pair-potential calculation^,^^ which indicate interconversion barriers in the range 50-200 cm-I. However, these calculations are currently unconfirmed by experiment. In view of the predicted low barriers to interconversion, it is reasonable to suppose that coupling of vibrational energy within a molecular aggregate could lead to conformational exchange. Where multiple potential minima are found, such as for alkyl halide complexes of perylene, quantitative determinations of such barriers will provide a means of improving computational models. In general, by studying the photoisomerization dynamics of large-molecule complexes, insights can be gained into the study of molecular migration on extended surfaces. In the present study, we demonstrate that van der Waals complexes of perylene with 1-chlorobutane and 1-chloropentane exist in at least three resolvable isomeric forms under supersonic jet conditions, while complexes with alkanes of similar size (Le., n-pentane and n-hexane) exhibit only a single isomer each. The main result of this paper is to demonstrate that injection of -350 cm-l of vibrational energy can bring about conformational mixing for both chlorobutane and chloropentane complexes.
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6323 I
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2. Experimental Section A pulsed supersonic free-jet system was used (He at 20-30 psig), as has been described e l s e ~ h e r e . ~Up ~ , to ~ ~three flow lines were employed: the main line carried pure helium, while other lines allowed the generation of binary and ternary complexes in the expansion, by bubbling helium at a controlled rate through a liquid sample. Alternatively, Ar and CH4 could be introduced by passing these gases through liquid paraffin. The excitation source was a nitrogen-pumped dye laser, having a bandwidth of 2 ps can provisionally be estimated since the complexation broadening of the "355p resonance is < 2 cm-I. A detailed examination of these dynamical considerations is currently planned, using picosecond time-resolved fluorescence measurements. From these data, we observe a symmetric broadening of the emission spectra, but no significant shift of the emission maximum from 419.8 nm for n-pentane and 420.5 nm for n-hexane. This indicates a single relevant ground-state conformation, and a small shift of the equilibrium conformation between SI and So. The broadening is a convolution of the excursions of ground versus SI modes, inhomogeneous broadening due to relative motion of the two species at equilibrium, possible contributions from rotational motion, and instrument factors. In the present context, it is significant that these spectra are straightforward, effectively unshifted, and symmetrically broadened. Data for the B' and (3' excitation bands of both the chlorobutane and chloropentane complexes show very similar behavior to the pentane and hexane cases. Thus, as Figure 9 shows, excitation of the B' resonance of perylene/l-chlorobutane (Le., the B conformer with 355 cm-' of excess vibrational energy (see Figure 2)) gives rise to a straightforward spectrum, strongly resembling that of the pentane complex, having the same symmetrical profile distributed about the position of the electronic origin of the B conformer. The spectral bandwidth is -80 cm-I, and the maximum is near 419.3 nm. Similarly, the chloropentane complex, excited via the /?' resonance, gives an emission band about 65 cm-I wide, centered at -420 nm, close to the position of vibrationally "cold" emission (see Figure 10). However, an important difference in behavior arises for both complexes when the A and C (or a and y) conformers are excited
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6327
Complexes of Perylene with Alkyl Halides I
0‘
A
P’
site C
JW’ !1-J.”L /I
6’
Figure 11. Potential energy surfaces schematically illustrating the ob-
served behavior for chlorobutane/perylene. The C site, having more intense excitation resonances, is presumed to be slightly lower in energy than A, by an amount AEAc. In the excited state the slight preference for A emission following excitation into A‘ or C’ indicates that this minimum may now be slightly lower. This is qualitatively consistent with the observed 80-cm-I red shift of the A resonances with respect to C.
I Y’
X (nm) Figure 10. Dispersed fluorescence spectra following excitation of dif5 vibraferent conformers of 1-chloropentane/perylenewith ~ 3 5 cm-’ tional energy in SI. As in Figure 9, two conformers (a’and y’) give
effectively the same emission spectrum while a third (6’) is distinct. The fourth conformer here (6’) is also shown to be distinct.
via the 355; resonance. For both complexes, the fluorescence spectra resulting from excitation into these pairs of resonances are strikingly similar. In the chlorobutane case, either A’ or C’ excitation gives a broad spectrum of -200 cm-’ fwhm. (The A’ emission spectrum is contaminated slightly by coincidentally excited bare perylene emission.) Analyzed in terms of A and C contributions, each modeled by a width of -80 cm-l, these spectra show approximately 60% A and 40% C. Excitation of the chloropentane complex via either a’or y’ also gives effectively the same emission spectrum. This time, excitation of either species at 355 cm-l gives rise to a emission, centered near 420.5 nm, having fwhm of 60-70 cm-’, indicating an effective interconversion of the y species to a under these conditions. The weak resonance 6’ of the chloropentane complex gives a fluorescence spectrum peaked near 4 19.1 nm, characteristic of the 6 conformer. It is significant to note that, although the 6’ resonance is 2 ps. Recognizing that we are, in fact, dealing with a multidimensional intermolecular potential energy surface, the effective barrier heights with respect to the reaction coordinate represented in Figures 11 and 12 will be a function of (i) the actual excess vibrational energy, Evib,injected through a zero-order promoting mode (e.g., 355'; see below); (ii) the overall size of the complex, which governs the number of modes into which energy can be coupled; (iii) the effective vibrational coupling, dependent on symmetry and relevant density of states; and (iv) the experimental conditions (Le., the irradiation and detection bandwidths, and the experimental time resolution). Thus, it is possible for Evibto exceed the energy barrier, but for some combination of conditions resulting from (ii)-(iv) above to restrict the site communication. It is useful to analyze such a barrier-crossing system in terms of current theories for vibrational coupling. In one approach commonly applied to systems of intermediate size, at zero time we excite a superposition of molecular eigenstates, represented by34
Here Is) represents the zero-order optical mode (e.g., Oo or 355') and Ill))is a relevant group of optically inaccessible states coupled nonradiatively to Is). Generally speaking, the { I I ) ) states have energy within -0.5 cm-I of Is), determined partially by the rovibrational coupling matrix element, V,135and the relevance of different (11))states in the vicinity of Is) is determined by symmetry through the specific form of Vsl. Operationally, at
