1424
J. Phys. Chem. 1995,99, 1424-1431
Rotational Coherence Measurements of Perylene Complexes with Benzene and Naphthalene: Vibronic Excitation and Structural Relaxation John R. Stratton? Thomas Troxler, Brian A. Pryor, Philip G. Smith, and Michael R. Topp* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 Received: October 3, 1994; In Final Form: November 21, 1994@
Experiments involving rotational coherence spectroscopy have measured moments of inertia of the jet-cooled 1:l complex of perylene with naphthalene in the SIstate. This complex is found to have overlapped rings, resembling the 1:1 complex of perylene with benzene. The high rotational symmetry of the aggregate prevents unambiguous determination of the angle subtended by the molecular long axes. Two possible structures are therefore suggested, corresponding to parallel and perpendicular alignment of the molecular long axes. The effects of vibronic excitation for both the benzene and naphthalene 1:l complexes of perylene were also investigated. Electronic-origin excitation of either complex results in prominent rotational coherence transients. The signals persisted with the same amplitude for 353 cm-’ vibronic excitation of naphthalene and were still present, at reduced intensity, at 705 cm-I. The naphthalene complex shows little or no vibrational coupling for these excitation energies. On the other hand, vibronic excitation of the benzene complex at 2 3 5 3 cm-I or the naphthalene complex at 540 cm-I showed no rotational coherence signal, both cases signifying the occurrence of vibration-rotation coupling.
Introduction Spectroscopic studies under supersonic jet conditions are providing much information about the energetics, dynamics, and structure of molecular aggregates and single molecules capable of large-amplitude intemal motion. In many cases, the fluorescence excitation spectra suggest structures, especially where the intermolecular interactions are site-specific. However, direct structure measurements are needed, in view of the competition between different types of nonbonded interactions. Much of the interest in this area stems from the opportunities to study differences in the internal level structure and coupling resulting from subtle changes in molecular arrangement. Moreover, vibronic excitation can lead to photochemistry, photoisomerization, and predissociation, which can be monitored on an ultrashort time scale. Experiments to measure structures make it possible to study in detail the particular types of interactions involved and the trajectories along which systems evolve following excitation. Structural measurements also serve a valuable function, providing information to refine procedures for calculating inter- and intramolecular potential energy surfaces. In turn, these can lead to more reliable predictions of the behavior of larger clusters for which structural information may not be readily obtained. Semiempirical calculations of the structures and binding energies of clusters involving rare gases112and other nonpolar species3s4with large aromatic molecules are often in reasonable agreement with e~periment.~-’OHowever, structural predictions of clusters involving polar interactions of aromatic molecules’ are more divergent and have not yet been extensively tested by experiment. One case where structure calculation and experimental structure determinations have come close together is in the spectroscopic and computational work by Brenner et aLI6 on polar complexes involving 1-cyanonaphthalene. These calculations, which were corrected for polarization effects, yielded structures similar to that recently measured for the triethylamine complex of 1-cyanonaphthalene by Berden and + Present address: Atlantic Community College, Mays Landing, NJ 08330. Abstract published in Advance ACS Absfracts, January 15, 1995. @
0022-365419512099-1424$09.0010
Meerts,I7using high-resolution frequency-domain spectroscopy. Wider application of experimental techniques for structure determinations will allow refinement of computational approaches to achieve more uniform results and to allow more accurate predictions of nonbonded potential energy surfaces in large-molecule systems. Rotational coherence spectroscopy has become an important tool for the structural analysis of larger molecular aggregates. Published examples of its application to van der Waals dimers of aromatic molecules include the 1:l benzene complex of fluorene by Joireman et aL1* The measured structure involving displaced, nearly parallel rings was shown to be consistent with semiempirical calculations using a “Buckingham” 6-exp approach augmented by a distributed point-charge term to accommodate the electrostatic interaction (6-exp- l).12 Another important case was the benzene complex of perylene,I9where the measured structure involves overlapping rings within IO” of parallel-plane configuration and with the benzene molecule located near the perylene center of mass. This structure is also similar to a prediction based on spectroscopic measurements, supported by pair-potential calculation^.^^^^^ The latter involved a Lennard-Jones-based 6- 12-1 expression. On the other hand, calculations by Price and Stone, which accommodated the electrostatic part of the interaction via a distributed multipole approach, favored structures in which the benzene molecule was displaced to the perylene periphery.I5 However, as Joireman et al. argued in their study of the benzene complex,I9 none of these semiempirical approaches are sufficiently refined that one would expect an accurate structure prediction. One weakness of these approximate methods is that they do not adequately allow for electronic interactions between the constituent molecules. This becomes a serious problem in multiple aromatic systems, because of the extensive overlap in energy between higher electronic states of different aromatic molecules. For example, upon formation of the 1:l and 2:1 complexes with naphthalene, the energy of the perylene SI state is lowered by 750 and 1440 cm-’.I4 Also, the radiative lifetime of perylene is significantly perturbed by aromatic complexation. Thus, while the fluorescencelifetimes of alkane and alkyl halide 0 1995 American Chemical Society
Perylene Complexes with Benzene and Naphthalene
J. Phys. Chem., Vol. 99, No. 5, 1995 1425
Figure 1. Fluorescence excitation spectrum of perylenehaphthalene 1:l complex (ref 14). The notation 0; marks the position of the perylene electronic origin band at 24 065 cm-'. The feature at -748 cm-' marks the origin of the complex, whereas those at -395, -208, and -43 cm-I mark vibronic bands of the complex at vibrational energies of 353, 540, and 705 cm-l.
complexes are 10-15% longer than for perylene itself, benzene and naphthalene complexes show increases of ~ 3 3 %and %50%.21 The fluorescence excitation spectrum of the benzene complex with perylene introduces little detail into the spectrum other than that of the perylene host. On the other hand, the excitation spectra of the benzene complexes with fluorene22 and anthraceneI3 are dominated by low-frequency progressions. The spacing, near 20 cm-', is characteristic of intermolecular "bending"-type motions, where a significant component of the displacement is parallel to the plane of the aromatic chromophore. Using results from the work of Joireman et al. on fluorene'* and peryleneIg complexes with benzene, one may conclude that the active low-frequency mode structure is associated with displaced-ring geometries. The fluorescence excitation spectrum of the naphthalene complex of perylene (see Figure 1) revealed, like perylene/ benzene, relatively weak low-frequency mode structure in the region C > 172 MHz. Finally, the data show no evidence of "hybrid" or K-type transients, leading us to conclude that the principal inertial axis is nearly parallel to the axis of the transition moment. To demonstrate the tolerance of the experiments, we plot in Figure 10 the variation in J-type recurrence time and K predicted for a simple rotation of the naphthalene molecule about the z-axis (symmetry axis) with a fixed interplanar distance (Az)
+
J. Phys. Chem., Vol. 99, No. 5, 1995 1429
Perylene Complexes with Benzene and Naphthalene I . ,
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Figure 10. Graph showing variation of simulated J recurrence time and asymmetry parameter ( K = (2B - A - C)/(A - C)) with rotational angle of the naphthalene molecule, for a fixed interplanar spacing of 3.35 A. The extreme values were as follows: 8 = 0", Az = 3.35 A, A = 262, B = 177, c = 175 MHZ; e = 900, AZ = 3.35 A, A = 233, B = 193, C = 175 MHz. 0
2
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Figure 12. Comparison of experimental data with simulations for three different structures. Structure T has the molecular planes perpendicular, with the long axes parallel, and Az = 4.9 A. The other two correspond to the structures shown in Figure 11.
Figure 11. Two structures for the perylenehaphthalene 1:1 complex consistent with the experimental data. These correspond to (upper) perpendicular (Az = 3.75 A) and (lower) parallel long axes (Az = 3.35 A).
of 3.35 A. On a strict numerical basis, the data allow for an angle of up to 40" between the molecular long axes. However, if the angles between the long axes and the interplanar spacing are simultaneouslyvaried, the experimental data can be matched over the entire range from Az = 3.35 A, 8 = 0" to Az = 3.75 A, 8 = 90". The two extreme structures of this type are shown in Figure 11. The calculated rotational parameters are listed in Table 2, together with the rotational constants for the perylene model, matched to literature RCS A second degree of freedom is the tilt of the naphthalene plane with respect to that of perylene. We found that the RCS data would be insensitive to this degree of freedom for a fixed center-of-mass displacement. However, limits are imposed on such a motion by atomic van der Waals radii, giving allowable tilt angles of < 10" for case 8, = 0", Az = 3.35 A and %20° for 8, = 90", Az = 3.75 A. A third degree of freedom involved relative motion of the center of mass in the x-y plane, parallel to that of perylene. By observing in the simulations the excursions in J-type
recurrence times and K values, we determined that the allowable error in the x-y position for a long-axis parallel structure was only 0.5-1 A. A specific investigation of T-shaped structures was also performed. Recall that this type of structure could be favored since the quadrupolar interaction is in an attractive domain. These structures have in common that the aromatic ring planes are at 90". Most T-shaped structures do not fit the transient spacing, because the center-of-mass distances necessary to maintain allowable nonbonded atomic separations are too large. However, one T-shaped structure was seriously considered. This involves a naphthalene molecule directly over the center of the perylene plane, with the long axes parallel and a center-of-mass spacing of 4.9 A. For a ring dihedral angle of 90°, this would correspond to a distance between the closest hydrogen atoms of naphthalene and the perylene ring of %2.4 A. This structure differs from those shown in Figure 11, since the inertial axis frame has been rotated, so that the A axis is now perpendicular to the perylene plane and the B axis is parallel to the perylene long axis (i.e., the transition moment). Also, this structure is in the oblate top regime, having K %+0.4. The reason for considering this structure is that the principal J-type recurrences are now spaced by %(A B)-', which is close to the experimental result. Despite this coincidence, however, we have disregarded this structure for several reasons. First, the symmetry is low, so that the simulation of Figure 12 predicts that negative A-type asymmetry transients37should appear at ~4 ns and 4 0 ns, having comparable strength to the J-type transients. These were not observed. Also, the absolute magnitudes of the simulated J transients for the T-shaped form are inconsistent with the very strong J transients actually observed in the experiment. That is, the J transients observed were the strongest for any species we have so far investigated, which is only consistent with a near-perfect symmetric top. (This difference is not evident from the simulation in Figure 12, since the traces were scaled to the magnitude of the first J-type transient in each case, for clarity.) Second, the simulation predicts only weak negative J-type recurrences for this structure, whereas the experiment showed several prominent negative
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Stratton et al.
1430 J. Phys. Chem., Vol. 99, No. 5, 1995 transients. This is an important criterion since, as Figure 9 shows, these transients are quite sensitive to asymmetry. Third, since the transition moment is now b-type (Le., the perylene long axis is perpendicular to the A rotational axis), a K-type transient is also predicted near 4.7 ns. This also was not observed. The family of possible structures can be further refined with the aid of spectroscopic information. Recent experiments involving RCS measurements have shown that an asymmetric binding site in the cyclopropane/perylene 1:1 species leads to two symmetrically inequivalent (111) isomers at the 2:l aggregation level.9 This causes a splitting in the excitation spectra. However, the fluorescence excitation spectrum of the naphthalene/ perylene 2:l species showed no splitting in the 0; band, at a resolution of 2 cm-' l 4 This implies a symmetric 1:l binding site, supporting the structures close to 6, = 0" and 90". Also, we speculated above that the 6, = 90" structure would require a molecular tilt, in order to support the intermolecular centerof-mass spacing of 3.75A. This kind of tilt could arise from electrostatic interaction similar to that in fluorene/benzene.I6 However, like fluorene/benzene, the tilt should change significantly upon electronic excitation, generating active lowfrequency mode structure in the spectrum. In fact, the excitation spectrum of perylenehaphthalene is simple in the region ' 5 0 cm-'. On the other hand, the presence of several prominent vibronic bands in the range 70-95 cm-' is evidence for a change in interplanar displacement or an out-of-plane deformation of the perylene molecule. Since we do not know the extent to which the perylene molecule is deformed in this complex, both the 6 = 0" and 90"structures remain credible at the present level of experimental detail. Moreover, reference to the work of Felker and co-workers noted above shows that they obtained interplanar spacings for several double aromatic van der Waals complexes in the range 3.5-3.6 A. This falls between the values we find for the 6 = 0" and 90" forms. Finally, Table 2 shows one clear distinction between the two forms, in the value of the A rotational constant, to which our experiments are not sensitive. This issue could be resolved by pump-probe experiments exciting a higher-energy electronic transition of different polarization from that of perylene SO- 4 1 . This would then give K-type transients, generating a definitive value of the A rotational constant for the ground-state aggregate. Dynamics. The rotational coherence data obtained following vibronic excitation of perylenehaphthalene (Figure 7) parallel excitation gives the results presented in Figure 5. Thus, rise to strong recurrence transients that persist unattenuated throughout the observation window of e10 ns. This behavior is completely consistent with the dispersed fluorescence spectra since, if vibration-rotation coupling is sufficiently weak that resonance fluorescence persists for the fluorescence lifetime, the rotational coherence in this collision-free system must be preserved. Excitation to the higher energy level of 705 cm-' (i.e., the & transition, which is the overtone of the same inplane ag mode) still preserves the rotational coherence signal. Here, the signal is attenuated on a time scale comparable to the resonance fluorescence decay time of 4 ns. Vibronic excitation of jet-cooled trans-stilbene has been shown38 to produce rotational coherence modulations on the fluorescence time profile, even at internal energies where vibrational dephasing is extensive. This suggests that redistribution of the internal energy leads to little structural change, and the effects of small changes in the rotational constants are not significant under typical experimental conditions. However, coupling of vibrational energy into large-amplitude intermolecular modes of mdecular aggregates can dephase the initial
4
rotational coherence. For example, different isomers of alkyl halide complexes of perylene undergo different degrees of rotational coherence loss when excited via The loss in some of these cases can be related to an isomerization process reflected in the time-resolved fluorescence spectra. An important observation to be made here is that, although rotational dephasing results from vibrational energy redistribution, the time scale of the rotational coherence loss varies widely between different species. Typical behavior for alkane complexes is shown in Figure 8, where the rotational coherence excitation is the same during signal following either 0; or the first recurrence cycle (i.e., 24 ns). The two cases differ at longer times, where vibronic excitation leads to attenuated later recurrences. The benzene case provides a stark contrast. Thus, despite the strong signal following 0; excitation, perylene/ excitation, sugbenzene showed no recurrences following gesting that vibronic excitation induces large-amplitude internal motion on a time scale much shorter than the recurrence time. Excitation of the naphthalene complex via the 540 cm-' mode also resulted in no recurrences. Simulations indicate that this effect should not be due to a simple difference in orientation of the transition moment for this non-totally-symmetric mode; the high symmetry of the aggregate should permit the observation of K-type transients. Instead, the data suggest that this behavior mirrors the observation from Figure 5 that this mode is extensively vibrationally coupled. Time-resolved fluorescence measurements*I estimated the vibrational dephasing time to be ~ 1 8 ps. 0 This is evidence for mode-selective vibrational coupling, which in this case correlates with vibrational symmetry. Thus, complexation of perylene by successive atoms of argon or molecules of methane gradually changes the spacing and relative amplitudes of a pair of resonances in the range 535555 cm-1,39 The behavior of this doublet resembles a case of an avoided crossing, in which one of a coupled pair of levels is being tuned. On the basis of the hole-burning study, we attributed the mobile component to an out-of-plane mode, which is readily perturbed by complex formation. The coupling with the promoting "C" mode (548cm-I) of perylene was assumed to occur via a vibronic coupling interaction involving higherenergy o-z* electronic states of the perylene molecule. In this case, it is reasonable to expect that excitation of such a perturbed out-of-plane mode would lead to increased vibrational coupling, possibly along similar lines to the promotion of IVR by the methyl group in p - f l u o r o t ~ l u e n e . ~ ~ ~ ~ ~ Considering again the dependence of the RCS transients on vibrational energy, unpublished work in this laboratory suggested similar behavior for the PPF/benzene complex. In that case, excitation at 270 cm-' above 0; did not yield recurr e n c e ~ .Although ~~ there is some remaining question about the assignment of the low-frequency mode structure of this complex, the important result from that study was that, in contrast with the benzene case, PPF complexes with alkanes show retention of RCS signals at 256 cm-I. To attempt an explanation of the difference between the alkane and benzene complexes of perylene following 4 excitation, one should consider the types of degrees of freedom available for vibrational coupling. The available modes involve internal vibrations of either moiety or intermolecular modes. One possible explanation for the difference in behavior between the alkane and aromatic adduct species is in their internal mode structure. Thus, n-alkanes possess a high internal mode density as a result of torsional and low-frequency bending modes of the alkyl chain. On the other hand, a molecule such as benzene possesses few vibrational modes '353 cm-I, so that there is little internal mode density to participate in coupling with the
4.32
4
4
Perylene Complexes with Benzene and Naphthalene rest of the intemal modes of the cluster. This difference in behavior is reflected, for example, in the room-temperature heat capacities of the different gaseous species (e.g., C, (298 K) = 136 (benzene), 255 (octane), and 314 J/(mol/deg) (decane).42 At lower temperatures, the differences between benzene and the alkanes should be even more pronounced. The intemal energy of the perylene molecule in the benzene complex evidently couples readily to the large-amplitude intermolecular modes. On the other hand, in alkane complexes there is a competing channel for vibrational coupling due to the lowfrequency intemal modes of the alkane, which may retard the effective transfer of energy into the nanosecond domain. Although this simple analysis neglects the intemal modes of the aromatic chromophore, it is consistent with an observed trend for both the rigid perylene and the flexible PPF molecules. Our experiments do not directly measure the difference in rate of energy transfer from perylene to the intermolecularmodes of the cluster, because the observation window is limited to periodic transients in the polarized fluorescence time profile. However, for the aromatic cases we observe that, when vibrational relaxation occurs, the cluster structure changes on a time scale significantly faster than the time to the first negative recurrence. An upper limit on this change is 1 ns, whereas the lower limit is set by the vibrational relaxation time. On the other hand, for the alkane complexes, the structure changes on a much longer time scale, measurable by the attenuation of successive cycles of the rotational coherence transients. Although this has not yet been quantitatively measured, the data suggest that this process occurs on a 5-10 ns time scale for the octane and decane complexes of perylene at 353 cm-’.
Conclusions In conclusion, we have presented evidence that the structure of the 1:l complex of naphthalene with perylene involves superimposed rings, which are nearly parallel. Because of the limited information content of the experimental traces for this prolate symmetric top species, two structures could not be resolved. These involve cases in which the molecular long axes are parallel and perpendicular, involving intermolecular spacings of 3.35 and 3.75 A, respectively. Despite some spectroscopic evidence possibly favoring the more widely spaced (perpendicular long-axis) form, we cannot definitively eliminate either possibility. Structures involving intermediate angles are also allowed by the rotational coherence data but do not appear consistent with experimental data on higher aggregates. Two figures were presented to show that the unusually weak vibrational coupling of the naphthalene complex is reflected in the rotational coherence data. The RCS data also confirm much different behavior in the benzene complex of perylene, for which the vibrational redistribution rate is at least 100 times faster than for the naphthalene case at 353 cm-I. The reason for the difference in these rates is not yet understood but could begin to be modeled on the basis of the measured structures. We have found that the rotational coherence transients are highly sensitive to the occurrence of vibrational energy redistribution in the benzene and naphthalene complexes, whereas the alkane complexes are insensitive at 353 cm-’. For example, in the benzene complex at this energy, no recurrences were observed, whereas for the octane and decane cases, the recurrence amplitudes were undiminished after 5 ns. This observation also reflects an advantage of the TCSPC method for RCS over double-resonance techniques, since successive recurrences over a 10-20 ns time span can be more readily compared. The difference in behavior between the alkane and aromatic complexes of perylene, supported by preliminary data on PPF complexes, points to an important property of the rotational
J. Phys. Chem., Vol. 99, No. 5, 1995 1431
coherence technique beyond static structure measurements. Thus, it can be used to examine the consequences of the flow of energy into different kinds of intemal modes of a complex system.
Acknowledgment. The work reported here was supported by the National Science Foundation CHE 91-20767 and by the ACS-PRF (24816-AC6). Additionally, T.T. thanks the Swiss National Science Foundation for support. References and Notes (1) Ondrechen, M. J.; Berkovitch-Yellin, Z.; Jortner, J. J. Am. Chem.
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