J. Phys. Chem. 1995,99, 17505-17508
17505
Conformation-Dependent Intramolecular Excimer Formation in Jet-Cooled 1,3=Diphenylpropane Tapas Chakraborty and Edward C. Lim*ft Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 Received: September 13, 1995; In Final Form: October 20, 1 9 9 9
Intramolecular excimer formation from photoexcited diarylalkanes in a supersonic beam has been studied, for the first time, using laser induced fluorescence. The results on 1,3-diphenylpropane demonstrate the presence of four major ground-state conformations which display differing efficiency for the excimer formation. The conformation dependence of the excimer formation allows tentative assignments of the species in terms of the four major rotational isomers expected for the molecule.
Introduction Intramolecular photoassociation of diarylalkanes in a supersonic beam, leading to the formation of an excimer or an exciplex, is currently a topic of considerableinterest. This stems from the fact that, unlike the condensed-phase experiments that start from a Boltzmann distribution of many reactant states, the photoassociation in a supersonic jet starts with selective excitation of a specific quantum state of the reactant under isolated molecule conditions. Thus, important questions pertaining to the intramoleculardynamics, such as vibrational level (mode and energy) dependence of the reaction rate, can be addressed by supersonic-jet laser spectroscopy. Equally important, supersonic expansion often produces reactant molecules in multiple geometrical isomers, thus allowing exploration of the relationship between structure and reactivity. Because of the large conformational difference between the excimer (or exciplex) and the molecule in the ground electronic state, the excimer or the exciplex cannot be directly prepared by photoexcitation of the ground-state species. Instead, the photoexcitation first prepares the locally excited (LE) state of the light-absorbing chromophore, which subsequently forms an excimer or an exciplex via conformational changes induced by intermoiety aromatic-aromatic interactions. The photoassociation in a supersonic beam therefore involves intramolecular vibrational energy redistribution (IVR) from the initially excited ring mode in the LE state to the reactive chain modes that bring the initial LE geometry to the excimer (or exciplex) geometry.' The efficiency of the excimer (or exciplex) formation depends on the conformation of the ground-state molecule relative to that of the excimer (or exciplex). Hence, the photoassociation dynamics provides a useful qualitative probe of the groundstate conformation of diarylalkanes. Although the conformationdependent exciplex formation in the jet-cooled diarylalkanes containing two chemically different aromatic moieties has been reported,* the number of observed conformations was far less than the number of possible chain conformations expected from these molecular systems. In this letter, we present the results of a supersonic-jet LIF (laser-induced fluorescence) study of intramolecular excimer formation in jet-cooled 1,3-diphenylpropane (DPP), which indicate the presence of all four major chain conformations of the molecule. The species-selective excimer formation allows tentative assignments of the four conformations. To our t Holder of the Goodyear Chair in Chemistry at The University of Akron. Abstract published in Advance ACS Absrrucfs, December 1, 1995.
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knowledge, no intramolecular excimer formation in a supersonic beam has been reported for any bridged diary1 compound.
Experimental Section The gas mixture of DPP vapor (from samples maintained at 70 "C) with He carrier gas (pressure 40 psi) was expanded through a pulsed nozzle (repetition rate 10 Hz, diameter 0.5 mm) from General Valve to produce an uncollimated beam. The jet-cooled DPP molecules were then excited with a frequency doubled Lambda Physik FL 3002E dye laser, pumped by a Lambda Physik LPX l00i XeCl excimer laser. The laser beam intersects the molecular beam perpendicularly about 1.5 cm downstream from the nozzle. The fluorescence excitation (FE) spectra were obtained by collecting the emission with a lens-filter combination and detected by a Hamamatsu R928 photomultiplier. The dispersed fluorescence (DF) spectra were measured by dispersing the emission with a 0.64 m Jovin Yvon THR-640 monochromator and detecting the fluorescence with the same photomultiplier. With 3600 grooves/mm grating and 200 pm slits, the DF spectra could be obtained with about 9 cm-' resolution. For both the FE and DF fluorescence measurements, the signal from the photomultiplier and the reference laser intensity were averaged, normalized, and processed, by a homemade gated detection system described el~ewhere.~ The highest purity (>98%) DPP from Lancaster was used without further purification.
Results and Discussion
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Figure 1 presents a low-energy segment ( 1500 cm-I leads to the appearance of a strongly red shifted, structureless fluorescence at about 325 nm, in addition to the structured LE emission in the shorter wavelengths. This is illustrated in Figure 3, which displays the DF spectra from the vibronic levels at 1571 cm-I and 1900 cm-I above the Oo level of the A isomer. The structureless fluorescence can be assigned to the emission from the intramolecular singlet excimer of DPP, based on its spectral similarity to the well-known excimer fluorescence of the compound in solution.' The blue shift of the excimer fluorescence at higher excitation energy (viz., E, = 1900 cm-l) is consistent with the increased vibrational excitation in the excimer, which is predicted8to lead to an increased transition energy of the excimer emission (the radiative transition from the right tuming point of the excimer potential is much more allowed than that from the left tuming point which terminates on the very steeply rising portion of the repulsive ground-state potential*). Figures 4 and 5 represent the S I SO FE spectra of total emission (LE plus excimer) and of excimer emission in the
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J. Phys. Chem., Vol. 99, No. 49, 1995 17507
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CHART 1 H H
H,
I
940
960
1000
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1020
1040
1060
1080
Wovenumber (cm' )
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Figure 4. SI SO fluorescence excitation spectra of the total fluorescence (top) and excimer fluorescence (bottom) of DPP in the lower energy region. The frequency displacement is with respect to the feature assigned to the origin band of the conformer A. Experimental conditions were the same for both emissions, except for the use of a sharp cutoff filter (which transmits longer than 310 nm) for the excimer fluorescence.
1 d o I Emission
r
c C
al
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C
Toluene
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1200
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Wavenumber ( c 6 '
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1600
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SO fluorescence excitation spectra of the total Figure 5. SI fluorescence and excimer fluorescence of DPP in the higher energy region (top panel). The frequency displacement is with respect to the origin band of the conformer A. Experimental conditions were the same for both emissions, except for the use of a sharp cutoff filter (which transmits longer than 310 nm) for the excimer fluorescence. The lower panel is the fluorescence excitation spectrum of toluene for the same energy range. The intensity scales are not the same for DPP and toluene. spectral region of E, = 920- 1100 cm-I and E, = 1100- 1600 cm-I, respectively. The vibronic bands most pertinent to our
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discussion are indicated in the spectra. Comparison of the intensity ratio of the two FE spectra, which is a qualitative measure of the efficiency of the excimer formation, leads to the following major conclusions: (1) The efficiency of the excimer formation varies substantially among the four conformational isomers. More specifically, the 12' and 18' levels of B and D isomers undergo excimer formation efficiedtly, whereas the corresponding levels of the A and C isomers show little, or no, tendency to form excimers (Figure 4). Isomer A, with the least tendency to form an excimer, exhibits mostly, if not exclusively, LE fluorescence even at E, as large as 1190 cm-' (7 (2) Isomer B, which forms excimer most efficiently, displays extremely weak fluorescence (LE as well as excimer emissions) at E, exceeding about 900 cm-I. Thus, no prominent spectral feature assignable to the isomer is evident in the FE spectra for E, 900 cm-' (Figures 4 and 5). (3) The conformation-dependent excimer formation is no longer apparent for E, greater than about 1500 cm-I, (as can be seen by comparing the spectral features due to the C and D isomers in Figure 5 ) . The vibrational energy flow (Le., IVR) from the optically excited ring mode into the reactive torsions leading to the excimer formation, competes with the energy flow into nonreactive modes. The ratio of the reactive trajectories (or the density of the reactive bath states) relative to the nonreactive trajectories therefore determines the efficiency of the excimer formation. For diarylalkanes, the number of possible intermediate conformations and nonreactive trajectories increases sharply with increasing number of the torsions participating in the reaction.'*9 Thus, the rate of the excimer formation is expected to increase with a decreasing number of the reactive torsions required to bring a given conformer into a parallel sandwichpair geometry favored by the singlet excimer. Using this as a guide, one can propose tentative assignments of the four DPP isomers in terms of chain conformations (Table 1). There are four major rotational isomers in DPP (C,-CI-C~-C~-C~): tt, tg+ (=tg-), g+g+ (=g-g-) and g+g(=g-g+), where t and g denote trans and gauche, respectively, and the signs and - refer to the relative signs (right- or lefthanded) of rotation about the CI-C2 and C r C 3 bonds (Chart l).9x'0Of the four major rotational isomers, the folded g+gconformer, which places the two phenyl groups in a near-parallel arrangement, is expected to be most effective in forming an intramolecular singlet excimer of sandwich-pair geometry. Conversely, the extended tt conformer is predicted to be the least effective in adopting the excimer geometry, since two torsions (about C I - C ~and c2-c3 bonds) are involved in the formation of the sandwich-pair geometry. On the basis of these considerations, we identify the A isomer with the tt conformer, and the B isomer with the g+g- conformer. These assignments
4).
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17508 J. Phys. Chem., Vol. 99, No. 49, 1995 are consistent with the fact that the observed band spacing (47 cm-I) between the A and B isomers agrees with that (49 cm-I) between the trans and the gauche forms of n-propylben~ene.~ The fact that the intensity of the A band (including that due to the torsional progression) is by far the most intense is also consistent with the expectation that the tt is the most stable of the four rotational isomer^.^ The anomalously strong intensity of the band B, assigned to the least stable g+g- rotational isomer, might be an indication of the conformational deformation along soft degrees of freedom (such as the torsional angles)" which leads to an increased stability. The identification of the C and D isomers is also not difficult. On the basis of the expectation that the more stable conformer should be present in larger concentration, we assign the stronger D band to the more stable gt conformationg and the weaker C band to the less stable gg conf~rmation.~ The observation that the C isomer has much smaller tendency to form excimer than the D isomer is in accord with the expectation that the transformation to the face-to-face conformation of the excimer is substantially more difficult for gg than for gtS9 The lack of the spectral features clearly assignable to the B isomer in the higher energy region (Ex > 900 cm-I) of the FE spectrum can be rationalized by assuming that the vibrationally highly excited excimer (formed from the high-energy photoexcitation) decays largely by nonradiative processes (e.g., internal conversion). This will lead to a strong quenching of the LE fluorescence without the compensating increase in the intensity of the excimer fluorescence. A strong decrease in the intensity of total fluorescence accompanying the excimer formation is evident when a higher energy segment of the FE spectrum of DPP is compared with that of toluene (Figure 5 ) . The band intensities, measured relatiave to those of the 7 and the lower energy bands, are clearly much smaller in DPP than in toluene for E, > 1200 cm-I, where all four major conformers of DPP exhibit excimer formation. The high vibrational excitation of the intramolecular singlet excimer is expected, under the collision-free conditions of a supersonic jet, when the IVR from the optically prepared ring mode Is) into the reactive modes { Ir)} (leading to the excimer formation) can compete with the IVR into the nonreactive modes (In)} that yields the vibrationally relaxed LE fluorescence. At low excitation energies, where the { Ir)} and {In)} manifolds do not fully communicate through IVR, the branching ratio between the two IVR processes is expected to depend on the conformation of DPP as well as the nature of the initially excited vibrational level. For the DPP conformations that differ considerably from the face-to-face arrangement of the two phenyl moieties (favored by the singlet excimer), the Is) {In)} IVR is expected to be dominant over the Is) { Ir)} IVR due to the much greater number of nonreactive trajectories relative to the reactive trajectories. These conformations will therefore lead to the relaxed (redistributed) LE fluorescence (at the exclusion of the excimer fluorescence) at low excess vibrational energies. On the other hand, for the near-parallel g+g- (or B)
4
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conformation, an effective excimer formation is expected to compete with the nonreactive IVR. Under collision-free conditions of a supersonic beam, the energy conservation requires that theexcimer formed from the higher energy Is) state to be highly excited in the low-frequency intermoiety vibrations. This will lead to a rapid decay of the excimer by radiationless transitions. Since the efficiency of the radiationless transitions is expected to sharply increase with increasing vibrational excitation, the lack of the clear spectral features assignable to the B isomer in the higher energy region (Ex > 900 cm-I) of the FE spectrum can be rationalized. The low yield of the LE fluorescence from the photoexcited van der Waals dimer of benzene in a supersonic jet can also be rationalized by assuming the formation of vibrationally highly excited excimer which undergoes very efficient nonradiative decay.'* At very high excitation energies, interconversion between various conformers will lead to thermal equilibration and hence to the loss of the conformation dependence of excimer formation. In this energy range, the mode selectivity in excimer formation will also be lost due to very efficient IVR, leading to the vibrational energy randomization. The results of this study indicates that the loss of the conformation dependence occurs at E, greater than about 1500 cm-' for DPP. In conclusion, we have presented the first study of intramolecular excimer formation in jet-cooled diarylalkanes for the prototype molecule DPP. The results, based on the LIF measurements, demonstrate the presence of four ground-state conformations that can be associated with the four major rotational isomers expected for the molecule. The energy threshold for the excimer formation is different for each isomer, which allowed tentative assignments of the four isomers in terms of the four expected chain conformations.
Acknowledgment. We are grateful to the Office of Basic Energy Sciences of the Department of Energy for financial support and to Lou Allinger, Wayne Mattice, Hiroyuki Saigusa and Marek Zgierski for helpful discussions. References and Notes (1) Syage, J. A.; Felker, P. M.; &wail, A. H. J. Chem. Phys. 1984, 81, 2233.
(2) Kurono, M.; Takasu, R.; Itoh, M. J. Phys. Chem. 1995, 99, 9668. (3) Chakraborty, T.; Lim, E. C. Chem. Phys. Lett. 1993, 207, 99. (4) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J. Chem. Phys. 1980, 72, 5039. (5) Chakraborty, T.; Lim, E. C., unpublished results. (6) Hopkins, J. B.; Powers, D. E.; Mukamel, S.;Smalley, R. E. J. Chem. Phys. 1980, 72, 5049. (7) Hirayama, F. J. Chem. Phys. 1963, 42, 3363. (8) Sadygov, R.; Lim, E. C., manuscript in preparation. (9) Mattice, W. L.; Suter, U. W. Conformation Theory of Large Molecules; John Wiley & Sons: New York, 1994. (10) Florv. P. J. Statistical Mechanics of Chain Molecules: Hanser: New York, '1989.' (11) Mendicuti, F.: Mattice. W. Comuut. Polvm. Sci. 1993. 3. 131. (12) Saigusa, H.: Lim, E. C. J. Phys.'Chem. 1995, 99, 15738. JP952679C
