J. Phys. Chem. 1995, 99, 17748-17753
17748
Vibrational Population Relaxation of Tetracene in n-Alkanes. Evidence for Short-Range Molecular Alignment P. K. McCarthy and G. J. Blanchard” Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322 Received: July 11, 1995’
W e report on the vibrational population relaxation dynamics of three tetracene vibrational modes (1462, 1383, and 1293 cm-I) in eight n-alkane solvents. The 1462 and 1383 cm-I resonances in tetracene are Raman-active fundamental vibrational modes, and the 1293 cm-’ mode is a Raman-active combination mode (307 986 cm-I). The vibrational population relaxation times (TI)for these modes range from 1 1 0 to -40 ps, depending on the identity of the solvent and the vibrational mode. These data are indicative of local solvent organization around the tetracene molecule. Comparison of these data to those we reported previously for perylene in the same solvents indicates that n-alkane solvent-solute coupling is more efficient for tetracene, in general, than for perylene. W e interpret the comparatively efficient relaxation of tetracene in terms of short-range molecular alignment.
+
Introduction The exchange of energy between dissimilar molecules is a fundamental process that, in many cases, plays a central role in determining the macroscopic properties of materials and solutions. The exchange of vibrational and rotational energy occurs spontaneously in liquids due to thermally induced inelastic collisions between molecules. For systems where there exists an energy gradient over short (molecular) ranges, such as solutions where solute molecules have been excited selectively by an external electric field, the exchange of energy is predominantly from the solute molecule to the surrounding solvent molecules. It is this type of energy transfer that is the focus of this work. In particular, we are interested in vibrational energy transfer between dissimilar molecules. Our previous work on the probe molecules ~ e r y l e n e ’ -and ~ 1-meth~lperylene~ in n-alkanes has demonstrated the existence of short-range organization of the solvent surrounding the solute. In addition, depending on the solute vibrational resonance excited and the identity of the solvent acceptor mode, vibrational population relaxation can proceed over a range of length scales. We have reported before on the relaxation of the perylene 1375 cm-’ vibration in n-alkanes.2 The terminal methyl group rocking motion in n-alkanes occurs at 1378 cm-I and is virtually independent of chain length. This condition is ideal for studying resonant, noncollisional energy transfer: and because the solvent “acceptor” mode is isolated (to some extent) to the termini of the molecules, it is possible to infer the existence of local organization in the solvent medium surrounding the solute (donor). For perylene, a molecule possessing a center of inversion, the vibrational modes we accessed were Raman-active and therefore infrared-inactive. Because the solvent acceptor vibrational mode is infrared-active, the coupling between the perylene 1375 cm-’ mode and the n-alkane 1378 cm-I mode is dominated by quadrupole-dipole interactions. The energy of these interactions scales as r-’: and we have observed experimentally that the solvent organization sensed by relaxation of the perylene 1375 cm-I mode persists for significantly less than 10 A.* For 1-methylperylene, the methyl group serves to remove the center of inversion, and the 1370 cm-I vibrational
* To whom correspondence @
should be addressed. Abstract published in Aduunce ACS Absrructs, December 1, 1995
0022-3654/95/2099- 17748$09.00/0
resonance we accessed modulates both the dipole and quadrupole moments of the molecule. The dominant mechanism of n-alkane energy transfer we observe the 1-methylperylene is dipole-dipole coupling, where the interaction energy scales as r-6.6 For the 1-methylperylene 1370 cm-I mode, we have observed that vibrational population relaxation j s sensitive to the local environment of the solute on a -10 A length scale, based on the correlation between T I times and reorientation times.4 Thus, the choice of probe molecule and vibrational mode can affect the length scale over which vibrational energy transfer occurs. Our work on perylene also indicated that its vibrational relaxation behavior depends sensitively on both the particular vibrational mode examined and the identity of the solvent surrounding the molecule. The donor-acceptor frequency detuning is clearly one important factor in accounting for these observations, but there is also a significant contribution from specific intermolecular interactions; it is the purpose of this paper to further examine these interactions. To this end, we have investigated the vibrational relaxation behavior of three tetracene vibrational modes in a series of eight n-alkane solvents. Tetracene was chosen because its aspect ratio (lengtwwidth) is significanty different than that of perylene. Our data indicate that, for two of the vibrational modes, we can understand the T I relaxation behavior in the context of quadrupole-dipole coupling, and for the third tetracene resonance, there are several factors that make the interpretation of its T I response less clear. We discuss these data and the complications associated with their interpretation below.
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Background
We have discussed the details of our T I measurement scheme and cover only the main points here. We use a pump-probe experimental configuration and detect the stimulated gain on the probe beam that results from the action of the pump beam. We choose the pump and probe wavelengths so that the pump laser excites the spectroscopic origin of the solute molecule, and the probe laser wavelength is adjusted to be one vibrational resonance lower in energy than the pump laser. The probe laser can be tuned to access all Raman-active vibrational resonances between -300 and -4000 cm-’. The practical lower limit for these experiments is -300 cm-I due to absorptiodemission band overlap as well as thermal popula0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 50, 1995 17749
Vibrational Population Relaxation of Tetracene
JA>. . . k. .? . . . .
I
1 0-0transition
Figure 1. Schematic three-level system used in the interpretation of the data we present here. The terms k are the rate constants for the transitions and relaxation processes discussed in the text.
tion of low-energy vibrations. For molecules of comparatively low symmetry, this tunability allows access to both infraredand Raman-active vibrational resonances. For molecules possessing a center of inversion, like tetracene, only Raman-active vibrational resonances can be accessed’directly. A significant experimental advantage of our pump-probe measurement scheme is that we can access solute vibrational resonances that are degenerate with solvent acceptor modes. Other methods developed to measure TI relaxation that use IR lasers for direct vibrational e x c i t a t i ~ ncannot ~ , ~ excite solute modes that are in close energetic proximity to solvent vibrations because of spectral overlap. The time evolution of the stimulated gain we detect on the probe laser beam contains information on the population dynamics of both the ground state vibration accessed and the excited electronic “intermediate” state. We model these data in the framework of a strongly coupled three-level system (Figure 1). Assuming that the population of the excited electronic state is created by -&t) pump excitation, the relaxation rate constant of the vibrational state (k3) is much greater than the relaxation rate constant of the exctied electronic state (kl), and k4 < kl, the equation describing the time course of the stimulated gain signal is
+
;[
:?bo
S(r) = A ( f ) B(r) = - exp(-k,r) -
where A0 represents the initial excited state population and the rate constants ki are as indicated in Figure 1. We note that the rate constant kl is the sum of the spontaneous and stimulated relaxation rate constants. Because the probe molecules we examine are characterized by k3 >> kl = k2, the second term in S(t) produces a small buildup in intensity subsequent to excitation, followed by a slow decay corresponding to the contribution of the fiist exponential term in eq 1. The information of interest here is the dependence of the quantity TI = k3-I on the identity of the tetracene vibrational mode and the n-alkane solvent.
Experimental Section Ultrafast Stimulated Spectroscopy. The stimulated emission pump-probe spectrometer used for these measurements has been described in detail previo~sly,~ and we provide only an outline of the system here. The light source is a mode-locked CW Nd:YAG laser (Coherent Antares 7 6 4 ) that produces 30 W average power at 1064 nm with 100 ps pulses at 76 MHz repetition rate. The 1064 nm light is frequency-doubled to produce > 3 W average power at 532 nm. The residual IR is mixed with the green light to produce -1.1 W of 354.7 nm light with the same pulse repetition rate and duration as the fundamental light. The W light pumps two dye lasers
350
400
450
500
550
wavelength (tun)
600
Figure 2. Absorption and emission spectra of tetracene in n-nonane. and the boxed region The arrow denotes the 0-0 transition (,Ipump), indicates the spectral region containing the vibrational modes studied here (Apr0d.
(Coherent 701-3) synchronously. The pump dye laser is operated with Stilbene 420 dye (Exciton) in the range 468473 nm, depending on the solvent used. The probe laser is operated with Coumarin 490 dye (Exciton) between 498 and 508 nm. Both dye lasers are cavity dumped at -8 MHz, and the pulses from each yield second-order autocorrelation traces of -7 ps fwhm. The time resolution of this spectrometer is determined by the cross-correlation of the two laser pulses, which is typically 10 ps fwhm. Detection of the (small) transient gain signals generated with this spectrometer is accomplished using a radio- and audio-frequency triple modulation, shot noise limited signal encoding scheme.I0-l2 For all T I measurements, the probe laser polarization is set to 54.7” with respect to the pump laser polarization to ensure the absence of rotational diffusion contributions to our data. Steady State Spectroscopies. The linear absorption and emission spectra of tetracene in the n-alkanes were recorded with -1 nm resolution using Hitachi U-4001 and F-4500 spectrometers. We show the linear response of tetracene in n-nonane in Figure 2. The 0-0 transition for tetracene in each solvent was determined from these data. Tetracene is sensitive to oxidative degradation, and to ensure that the degradation did not occur to any significant extent due to laser excitation, absorption spectra of the samples were taken before and after the TI measurements. Chemicals and Sample Handling. Tetracene and the n-alkane solvents were purchased from Aldrich Chemical Co. in the highest purity grade available and were used as received. Tetracene solution concentrations were M and were prepared by saturating the solution and then removing excess solid tetracene by filtration prior to the TI measurements. The solubility of tetracene increases with increasing solvent aliphatic chain length, and thus the chromophore concentrations used in these experiments varied (by a factor of -4) over the range of solvents used. For all measurements, however, saturation was achieved at a concentration sufficiently low that no aggregation effects were observed. For the pump-probe measurements, thermal lensing contributions to the signal were minimized by flowing the sample through a 1 mm path length cell. For all measurements, the sample temperature was maintained at 300
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f 0.2K. Results and Discussion As mentioned in the Introduction, we have previously investigated the TI relaxation behavior of perylene and l-me-
17750 J. Phys. Chem., Vol. 99, No. 50, 1995
McCarthy and Blanchard a
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Figure 3. Schematic atomic displacements for the vibrational modes studied here. Frequencies and activities are indicated for each mode. The arrows are directional indicators only and are not meant to indicate the amplitudes of the displacements.
thylperylene in the n-alkanes under conditions where the donoracceptor frequency detuning is Av = 0. These experiments have indicated that the order of the polar donor-acceptor coupling is important in determining the length scale over which the relaxation event operates and also that solvent local organization exists around the solute molecule. The purpose of the present investigation is to achieve an understanding of how the shape of the solute moleucle alters the transfer of vibrational energy between molecules in solution. To achieve this understanding, we chose to examine three vibrational modes of tetracene in the n-alkanes. We chose tetracene because its shape is significantly different than perylene. In addition, the linear optical response of this molecule is well u n d e r ~ t o o d , ' ~ and -'~ there is no spectroscopic evidence of significant vibronic coupling effects between singlet electronic manifolds." We have examined three vibrational resonances in tetracene: two Raman-active fundamental modes (1462, 1383 cm-I) that are degenerate with IR-active solvent acceptor modes and a third, Raman-active combination vibrational mode (1293 cm-I) that is degenerate with an IR-active resonance in the tetracene molecule (1294 cm-I). We have calculated the atomic displacements for these modes and show them in Figure 3. We expected that the investigation of the two Raman-active fundamentals would yield information on the different environments probed by the two normal coordinates and that the combination mode could potentially provide insight into intramolecular energy transfer processes. The absorption and emission spectra of tetracene in all of the n-alkane solvents are virtually identical to those shown in Figure 2, with only minor spectral shifts arising from solutesolvent interactions. For all of the T I measurements, the pump laser wavelength is set to the spectroscopic origin to avoid contributions to the data from excited state vibrational population r e l a ~ a t i o n .The ~ probe laser wavelength was varied across the boxed region shown in Figure 2 to access the 1462, 1383, and 1293 cm-' modes in tetracene. The T I times we report here are the averages of at least two and usually four determinations. Each determination is, itself, the average of at least 10 (usually 20) individual time scans. As we had found for the TI relaxation
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number of carbons in n-alkane solvent Figure 4. T I times as a function of solvent aliphatic chain length: (a) 1383 cm-' mode, (b) 1293 cm-l mode, and (c) 1462 cm-I mode. For this mode, the relaxation behavior in n-hexadecane is anomalous and is not included in the figure. See text for a discussion of this point.
of perylene in the n-alkanes,2 the experimental data exhibit both a solvent and vibrational mode dependence. For this reason, we consider the behavior of the modes individually before comparing the results of one mode to those of another. The tetracene 1383 cm-I vibration should, in principle, provide a direct comparison to the perylene data because we are probing the same degenerate donor-acceptor relaxation effect in both experiments. We find that there is a measurable solvent dependence to the experimental data. Specifically, T I is fast in n-pentane, n-hexane, and n-heptane, and there is a significant increase in T I for this mode in n-octane. For the longer alkanes, TI gradually decreases from its high of 38 i 8 ps in n-octane to '10 ps in n-hexadecane (Figure 4a). The observation of a discontinuous T I dependence on the solvent aliphatic chain length is reminiscent of the behavior of perylene, but the details of the solvent dependence are significantly different. For the perylene 1375 cm-' mode, the fastest T I times were reported for n-octane and n-nonane,2 and this is exactly the opposite of the data we report here on tetracene. An obvious implication of these data is that the solvent environment formed around the tetracene molecule is significantly different than that formed around the perylene molecule. Because T I does not vary regularly with solvent aliphatic chain length, however, these data indicate the presence of solvent local organization about the tetracene molecule. It is also apparent that there is not enough information provided from these measurements to reveal the details of organization of the solvent molecules around tetracene. We offer a speculative explanation only and do not claim to achieve a detailed understanding of solvent local organization. The fact that we observe the increase in TI times at n-octane for this mode suggests that the shorter alkanes cannot span the length of the tetracene molecule, and therefore both terminal methyl groups, on which the solvent acceptor mode is significantly localized, are in close proximity to the vibrational coordinate. For the longer alkanes, where it is less likely that both methyl groups are in close proximity to the tetracene molecule, we observe longer T I times. In order to be consistent with this hypothesis, the longer alkanes would have to "wrap
J. Phys. Chem., Vol. 99, No. 50, 1995 17751
Vibrational Population Relaxation of Tetracene
TABLE 1: TILifetimes of Tetracene Dissolved in n-Alkanes solvent n-pentane n-hexane n-heptane n-octane n-nonane n-decane
n-hexadecane
vibrational mode (cm-')
1462 1383 1293 1462 1383 1293 1462 1383 1293 1462 1383 1293 1462 1383 1293 1462 1383 1293 1383 1293 1462 1383 1293
TI
* 10
(PS)
19 f 6 18f6 '10 31 f 16 '10 15f6 '10 '10 15f7 < 10 38 f 8
m
[
v
I
< 10
44fl 21 f 5 33 f 9 '10
0.4 O'
