J. Phys. Chem. 1988, 92, 2419-2422 that intercalation of a layer compound does not modify slabs of the host, 23NaKnight shift measurementsz9indicate that the Tisz slabs are modified during the phase transition. It must be assumed that the Ti-S bond distance itself remains nearly unchanged. Evidently, the phase transition both flattens each Tisz layer and also shortens the interlayer separation. The elastic strain energy associated with the slab deformation is offset by changes in the electronic band structure. In this regard, the d-block bands, consisting of three bands derived from the tp level and two bands derived from the e8 level, are of primary Interest. Tight-binding band structure calculations have beem performed as a function of an angular variable (identical with the trigonal angle defined herein) which was used to parametrize the Tisz slab deformati01-1.~~A small change in the trigonal angle modifies the lowest lying d-band and is responsible for an abrupt change in the density of states near the Fermi energy and is directly responsible for the transition. When Fe2P2S6is intercalated with ,,75,the n-propylamine (C3H7NH2)to form Fe2P2S6(n-C3H7NHZ) 4A,, transition shifts to 1945 cm-'. If the values of Dq 4E, and Cp do not dramatically change from the pure Fe2P2S6to the n-propylamine intercalate, then the position of the I R electronic band can be used to estimate the trigonal angle in the intercalated lattice. The change in the trigonal splitting upon n-propylamine
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(29) Molinie, P.; Trichet, L.; Rouxel, J.; Berthier, C.; Chabre, Y.; Segransan, P. J. Phys. Chem. Solids 1984, 45, 105. (30) Whangbo, M. H.; Trichet, L.; Rouxel, J. Znorg. Chem. 1985,24,1824.
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intercalation is +60 cm-'. Using (4), the change in the trigonal angle is estimated to be -0.1 O . Variations of this magnitude in the trigonal angle will produce fluctuations in the M-M distances on the order of -0.005 A. This value is considerably smaller than found in EXAFS studies of MnzP2S6and its cobaltocenium intercalation compound.31 The root-mean-square deviations induced by intercalation lie in the range 0.05-0.07 A for Mn-S distances and 0.1-0.15 8,for the second- and third-shell Mn-Mn and Mn-P distances. It is reasonable to suppose that smaller distortions are indued by the propylamine intercalate than by the cobaltocenium ion. ESR measurements also indicate magnetic inequivalence of Mn sites after intercalation with pyridine.32 Moreover, the value of magnetic susceptibility has been shown to be strongly correlated with lattice disorder. The reciprocal magnetic susceptibility was reduced by a factor of about 60 upon i n t e r ~ a l a t i o n . ~ ~ Acknowledgment. Acknowledgment is due to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the National Science Foundation for funds used in the execution of this research. Registry No. Fe2P2S6,79235-97-5; Co2P2S6,20642-12-0; Cd2P2S6, 28099-03-8; Mn2P2S6,20642-09-5; n-propylamine, 107- 10-8. (31) Michalowicz, A.; Clement, R. Znorg. Chem. 1982, 21, 3872. (32) Lifshitz, E.; Gentry, A. E.; Francis, A. H. J . Phys. Chem. 1984, 88, 3038. (33) Michalowicz, A.; Clement, R. J. Inclusion Phenom. 1986,4,265-271.
Luminescence Processes in [TbCbpy.bpy.bpyl3' Cryptate: A Low-Temperature Solid-state Study G. Blasse,* G. J. Dirksen, Physical Laboratory, State University, POB 80.000, 3508 TA Utrecht, The Netherlands
N. Sabbatini, S. Perathoner, Dipartimento di Chimica "G. Ciamician", Via Selmi 2, 401 26 Bologna, Italy
J. M. Lehn, and B. Alpha Le Bel Institute, Louis Pasteur University, 6700 Strasbourg, France (Received: September 16, 1987)
The luminescence properties of solid [TbCbpybpybpy]", [LaCbpybpybpy13+,and [Nacbpybpybpy]' are reported for temperatures down to 4.2 K. The 2,2'-bipyridine phosphorescence in the case of the La3+ and Na+ cryptate is quenched by energy migration to killers. In the case of Tb3+,however, efficient energy transfer from the cryptand to Tb3+takes place up till 100 K. At that temperature migration competes with energy transfer to Tb3+,reducing the Tb3+luminescence output. The rates of the processes involved are determined as accurately as possible. A comparison is made with the same cryptate molecules in solution.
1. Introduction
There is a growing interest in the luminescence properties of lanthanide ions in cryptates where the lanthanide ion is encapsulated into the ligand structure.'-5 Of special interest are those (1) Sabbatini, N.; Dellonte, S.; Ciano, M.; Bonazzi, A,; Balzani, V. Chem. Phys. Lett. 1984, 107, 212. (2) Blasse, G.; Buijs, M.; Sabbatini, N. Chem. Phys. Lett. 1986,124, 538. ( 3 ) Sabbatini, N.; Dellonte, S.; Blasse, G. Chem. Phys. Lett. 1986, 129, 541. .
(4) Sabbatini, N.; Perathoner, S . ; Lattanzi, G.; Dellonte, S.; Balzani, V. J . Phys. Chem. 1987, 91, 6136.
0022-3654/88/2092-2419$01.50/0
cryptates in which energy transfer between the cryptand and the rare-earth ion takes place. This has been observed for the macrobicyclic ligand containing three 2,2'-bipyridine group^.^^^ Up till now these studies were performed in aqueous solutions at 300 and 77 K. Here we present results of a study on solid cryptate powders down to 4.2 K. The complexes studied are ( 5 ) Alpha, B.; Balzani, V.; Lehn, J. M.; Perathoner, S . ; Sabbatini, N. Angew. Chem. 1987, 99, 1360. Sabbatini, N.; Perathoner, S.; Balzani, V.; Alpha, B.; Lehn, J. M. In Supramolecular Photochemistry; Balzani, V.,Ed.; Reidel: Dordrecht, Netherlands, 1987; p 187. (6) Alpha, B.; Lehn, J. M.; Mathis, G. Angew. Chem. 1987, 99, 259.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
Blasse et al. I.
c------j.
n
olr)(\
1
-b
/
EM
00
[LO1
300 K
Temperature dependence of luminescence intensities. The curve noted bpy relates to the phosphorescence of [LaCbpy.bpy.bpy13+ and [NaCbpy.bpy.bpy]+,that noted Tb to the Tb" luminescence of [TbCbpy.bpy.bpy13+,and that noted Tb(La) to the Tb3+impurity luminescence of [LaCbpy.bpy.bpy13+.All curves normalized to 10. Figure 2.
,--. 600
520
LLO
'-
380
300 nm
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Figure 1. Luminescence spectra at 4.2 K of solid [LaCbpy.bpy.bpyj3+. The emission spectrum shows a progresion in 1500 cm-I.
[TbCbpybpy.bpy13+, [LaCbpy~bpy.bpy13+,and [NaCbpy. bpybpy]'. The latter two species were studied in order to investigate the luminescence properties of the bpybpybpy cryptand. The La3+and the Na+ ions do not have energy levels in the spectral region of interest. The former cryptate is studied to investigate the energy transfer from the cryptand to the encapsulated Tb3+ ion. Actually, we observed a 100% quantum efficiency for the Tb3+emission upon cryptand excitation for temperatures below 100 K. The results are used to evaluate the energy transfer rates in these complexes as accurately as possible. Finally, we compare the present results on solids with those observed earlier for solut i o n ~ It . ~will ~ ~become clear that these results are compatible and that both types of investigations yield specific information which is mutually useful.
2. Experimental Section The cryptate complexes were prepared in Strasbourg as described elsewhere.6 The experimental techniques were described Since only small amounts of material were available, they were pressed in the cryostat on a bed of MgO. Corrections were made by measuring pure MgO. 3. Results and Discussion 3.1. [LaCbpy.bpy.bpy13+. The La cryptate shows below 100 K luminescence upon long-wavelength UV excitation corresponding to ligand-centered a b s o r p t i ~ n . ~Figure , ~ 1 presents the emission and excitation spectra at 4.2 K. The emission consists mainly of ligand phosphorescence.8-10 Three members of a progression are visible with mutual distance of 1500 cm-'. This value is in reasonable agreement with literature data.9 The zero-phonon transition is overlapped by a weak blue emission which might be Since its intensity is less than 5% of the total emission, it will be neglected below. The excitation spectrum must be considered with care. The cryptand absorption is extremely i n t e n ~ e .It~ is well-known that excitation and absorption spectra of strongly absorbing samples are strikingly different, because excitation is only effective in the tail of the absorption band. If excitation occurs in the band itself, the absorption occurs mainly in the surface layer, where usually the luminescence efficiency is low. This is what we observe. The excitation band of the solid has its maximum at 360 nm, whereas the excitation band and the absorption band of the cryptand in solution show their maximum at 300 nm.' The excitation spectrum of the solid shows even a minimum at that position. This is strong evidence that we excite into the l(t,a*) state of the bpy group. Harriman puts this level at -31.000 cm-' for 2,2'-bipyridine in water,8 which is in good
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(7) Blasse, G.; Dirksen, G. J.; Sabbatini, N.; Perathoner, S . Inorg. Chim. Acta 1987, 133, 167. (8) Harriman, A.; J . Phorochem. 1978, 8, 205. (9) Vincdgopal, K.; Leenstra, W. R. J . Phys. Chem. 1985, 89, 3824. (10) Suislau, A. P.; Kamyshnyi, A. L.; Zakharov, V. N.; Aslanov, L. A,; Avarmaa, R. A. Chem. Phys. Lett. 1987, 134, 617. (11) Henry, M. S.; Hoffman, M. 2.J . A m . Chem. SOC.1977, 99, 5211. (12) Kotlicka. J.; Grabowski, Z. R. J . Photochem. 1979, 1 1 , 413.
J
I
560
I
I
I
l
I
180
I
I
I
1
LOO nm
The emission spectrum of the phosphorescence at 4.2 K of [NaCbpy.bpy.bpy1'.
Figure 3.
agreement with our results. We conclude also that the intersystem state is effective for >95%. crossing to the 3(r,n*) The quantum efficiency ( 4 ) of the phosphorescence is low. Its value is about 2%. An increase of temperature results in a drastic decrease (see Figure 2). Upon measuring the emission spectrum as a function of temperature, a peculiar phenomenon occurred. A certain amount of Tb3+emission appeared when the ligand phosphorescence intensity decreased. Obviously our sample contains a small amount of terbium, probably present as an impurity in the starting lanthanum compound. At 4.2 K the amount of Tb3+ emission is 0.1% of the total emission intensity. This amount increases one order of magnitude upon increasing the temperature (Figure 2). Below it will become clear that the bpybpyebpy cryptand transfers efficiently to Tb3+. The low quantum efficiency of the ligand phosphorescence, and the behavior of the Tb3+impurity emission, suggest strongly that the j(n,n*)excited state is not localized but migrates among the several bpy groups. In this way it reaches quenching centers, so that the total quantum efficiency is low. The Tb3+ions form a small part of these quenching centres. This concentration will be very low (IO-*% or less). The migration, which occurs already at 4.2 K, is also thermally activated, so that upon increasing the temperature the quenching increases, and simultaneously the Tb3+ intensity increases too. By plotting In 4-l vs T'for the ligand phosphorescence, we derived an activation energy of 100 cm-' for the migration. The rise of the Tb3" emission intensity with increasing temperature is exponential. The activation energy which can be derived in this way amounts also to 100 cm-' which confirms our model. This migration is hard to characterize further, because it seems obvious that the transfer rate between bpy groups of one and the same cryptate molecule will be different from that between bpy groups on different cryptate molecules. The literature gives usually a radiative rate of 1 S-I for the phosphorescence of the free 2,2'-bipyridine.'s9 This, together with our experimental quantum efficiency, yields for the total migration rate a value of 50 s-' at 4.2 K. This value increases rapidly with temperature. In this discussion we neglect the influence of nonradiative transitions within the bpy group itself, because for 2,2'-bipyridine molecules in solution efficient luminescence has been reported at 77 K and lower temperature^.^,^ 3.2. [NuCbpy.bpy.bpy]+. The luminescence properties of the sodium cryptate are similar to those of the lanthanum cryptate with some interesting differences. The emission is mainly phosphorescene (see Figure 3), but with a complicated vibrational structure. The quantum efficiency is about 2% and the quenching
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Luminescence of Lanthanide Ions in Cryptates
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2421
temperature about 100 K (Figure 2). The individual vibronic lines in the sodium cryptate are much narrower than in the lanthanum cryptate. We also observed that upon increasing the temperature above 25 K the vibrational pattern of the sodium cryptate changes. There is a considerable broadening and the zero-phonon transition disappears. We did not analyze the vibrational pattern of the sodium cryptate, because it seems clear that the energy migration among the bpy group is responsible for the complicated structure. Most probably the emissions observed, for sodium as well as for lanthanum cryptate, are due to trap emissions. If this is correct, the spectra suggest that the lanthanum sample contains a broad range of optical traps; Le., there is a wide range of defects in the solid phase. In the sodium sample there seems to be only a restricted amount of optical traps. The zero-phonon line mentioned above belongs then to a very shallow trap which is easily emptied. In the present samples these phenomena cannot be studied in more detail. Apart from the fact that they also point to energy migration, they are not of relevance for our aims. Therefore we refrain from further analysis and discussion. 3.3. [Tbcbpy.bpy.bpyl3+. The Tb3+cryptate shows at 4.2 K upon cryptand excitation a green emission of a very high intensity. The emission spectrum consists of the well-known 5D4 7FJ emission lines of the Tb3+ ion. We did not observe any other features in the emission spectrum. This holds also for higher temperatures. The excitation spectrum of this emission consists of a very weak 'F6 5D4 transition on the Tb3+ ion at about 490 nm and a dominating band which is similar to the ones describing above. The latter is due to cryptand excitation. Its presence points to efficient transfer from cryptand to Tb3+. The quantum efficiency is not easy to measure accurately. However, it will not be far from 100%. Figure 2 shows the temperature dependence of this emission. It is clear that quenching starts above 100 K. At room temperature q N 5% only. Note that the onset of the quenching coincides with the complete quenching of the cryptand luminescence. Since it is hard to imagine that the quenching of the Tb3+ emission which starts at 100 K is due to a process inside the Tb3+ ion itself, the conclusion must be that at 100 K the migration rate of the triplet excited state of the 2,2'-bipyridine group starts to compete with the cryptand Tb3+ transfer rate. Below we will try to make this qualitative statement more accurate. At this moment we conclude that [TbCbpybpybpy13+below 100 K shows complete energy transfer from cryptand to Tb3+. Due to the high absorption strength of the cryptand, this results in an enormous light output. 3.4. Transfer Processes in the Solid. The migration rate at 4.2 K was found to be 50 s-] (see section 3.1). Let us try to estimate a value for the transfer rate between two 2,2'-bipyridine groups on different cryptand species. It is well-known that such a transfer rate can be high.13 Since the zero-phonon line is relatively weak, the total number of jumps will probably not exceed 10000. However, intracryptand jumps will be more probable than intercryptand jumps by, let us say, a factor of 10. This brings the number of cryptand-to-cryptand jumps at 1000. This seems a reasonable value to explain a quantum efficiency of 2% at 4.2 K in an ill-defined solid. This rough estimate implies that the intercryptand transfer rate is about 5 X lo4 s-' at 4.2 K. This value will rapidly increase with temperature. From the temperature dependence of the phophorescence we derive, for example, for 50 and 100 K the following estimates of this rate, lo5 and 5 X lo5 s-l, respectively. Let us now turn to the transfer from the 2,2'-bipyridine group to the Tb3+ ion. If the transfer would occur by dipole-dipole interaction, this rate is estimated to be 40 s-l. For this purpose we use the Forster-Dexter formalismI4 with the experimental spectral overlap (2 eV-l) and the oscillator strength for Tb3+ given by Carnall (0.5 X lO6).l5 The low rate is mainly due to the-very
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(13) Wolf, H. C. Ado. A t . Mol. Phys. 1967, 3, 119. (14) Dexter, D. L. J . Chem. Phys. 1953, 21, 836.
log P I
I
\
'
0
1
I
5
io
15
-103
20 T
Figure 4. Semiquantitativepresentation of the temperature dependence of the radiative rate of the bpy phosphorescence ( l ) , the migration rate by the bpy excited state (2), the bpy-bpy triplet-transfer rate (3), and the rate of transfer from bpy to Tb3+ (4). See also text. Temperature scale is TI.Rates are denoted by P.
low transition probability of the phosphorescence transition. Note that the spectral overlap is favorable, the phosphorescence maximum being close to the position of the 7F6 5D4transition of Tb3+. It is clear that the value of 40 s-l is too low to explain the efficient transfer from ligand to Tb3+, since the intercryptate transfer rate is much higher. Note in passing that the spectral overlap is optimal at 4.2 K, so that the cryptand Tb3+transfer rate will be temperature independent in good approximation. We have to conclude that the transfer to Tb3+ occurs by exchange and is therefore hard to calculate. The spectral overlap Tb3+ transfer is found to be 20 times that for for cryptand intercryptand transfer. If the exchange integral would not differ too much, this would give a value of lo6 s-' for the transfer to Tb3+. This value is roughly equal to the intercryptand transfer rate at 100 K as required by the Tb3+ quenching above this temperature. For Gd3+ Tb3+transfer (also by exchange) similar values have been observed.16 In view of all this we conclude that our rough estimates are nevertheless of the order of magnitude to be expected. Finally we try to calculate from the quenching curve of the Tb3+ luminescence further values for the intercryptand migration. For this purpose we assume the quenching to be due to the migration (to killers), and the transfer to Tb3+to be. temperature independent. This gives for 200 and 300 K the migration rate lo6 and 2 X lo7 SKI,respectively. In Figure 4 we have plotted the deduced rates as a function of temperature. This figure illustrates our model. The phosphorescence radiative rate is too low to give efficient phosphorescence. At low temperatures the rate for transfer to Tb3+is larger than the migration rate, but at higher temperatures the reverse is true. Note a change in the temperature dependence of the migration rate above 100 K. This is not necessarily due to the migration itself. Nonradiative transitions on the 2,2'-bipyridine may become of influence. This will manifest itself in an apparently higher migration rate. Therefore we have not estimated transfer rates from the migration rates above 100 K. Actually, the phosphorescence of 2,2'-bipyridine is quenched in solutions at higher temperatures, although accurate data are lacking.8J0 Let us finally compare the situation in the solid with that in solution^.^^^ 3.5. Transfer Processes in Cryptates in Solution.5 The obvious difference between the solid and the solution is the fact that the migration process is vanishing in the latter. Figure 4 suggests therefore high quantum efficiencies for isolated [ T b c b p y b p y bpy13+ upon cryptand excitation at 300 K. This, however, has not been observed. One explanation is the occurrence of a nonradiative process on the 2,2'-bipyridine group at 300 K which is
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(15) Carnall, W. T. Handbook on the Physics and Chemistry Of Rare Earths; Gschneidner, K. A,, Jr., Eyring, L., Eds.; North-Holland: Amsterdam, Netherlands, 1979; Vol. 3, p 171. (16) Blasse, G. Prog. Solid State Chem. 1988, 18, 79.
J . Phys. Chem. 1988, 92, 2422-2429
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responsible for the greater part of the nonradiative loss. As discussed above, this term is comparable to the migration process in the solid, and they probably partly overlap. If this nonradiative process cannot be suppressed, this cryptate will never show high q at 300 K. This is still another process, viz. back transfer from Tb3+ to the 2,2'-bipyridine group.5 In view of the nonradiative losses on the latter, this back transfer implies a nonradiative loss. In ref 5 it has been shown that excitation into the SD4 level of Tb3+results in a considerable nonradiative loss due to back transfer. At 300 K the back transfer rate was found to be 2 X lo3 S - I . ~ Since every Tb3+is surrounded by three bpy groups, the rate per times the Tb-bpy pair is -0.6 X lo3 s-l. This is about transfer rate (to Tb3+). We will now consider this in more detail. There is no doubt that also for back transfer dipole-dipole interaction cannot be of any importance, so that the back transfer must be exchange mediated. Then the difference in the transfer rates is in the spectral overlap. For the transfer to Tb3+ this amounts to 2 eV-'; for the back transfer it is unknown, since the excitation or absorption spectrum of the ground state to 3(ir,ir*) state transition is unknown. However, extrapolation from the spectra shows that it will certainly not exceed 0.01 eV-I, in agreement with the derived values. Another approach is to relate the two rates as
transfer, Le., if we assume that the temperature dependence of the migration vanishes at higher temperatures, we can derive from an activation energy. This amounts to 1500 a plot of log P vs T1 cm-' which is rather close to the value which follows from the spectral data. Therefore, the back-transfer process seems to be confirmed by the present investigations.
4. Conclusions The quantitative results of the present investigations are rather inaccurate and can only be considered as rough estimates. Nevertheless, the processes in the solid bpy-bpysbpy cryptates become clear. (a) In case of Na+ or La3+ as the central ion, the phosphorescence intensity is considerably reduced down to 4.2 K due to migration of the excited state to quenching sites. The migration carries a thermal activation. (b) In case of Tb3+as the central ion, the cryptand transfers efficiently to Tb3+ up till 100 K. Above that temperature the migration becomes a competing process and the Tb3+ emission intensity starts to decrease. Back transfer from Tb3+ to the cryptand occurs also and quenches the Tb3+emisison even more. The results for solids and solutions are in line with each other in spite of the fact that the transfer processes are partly different. Note Added in Proof. Recently N.S. has found the phosphorescence lifetime of bpy in [LaCbpy.bpy.bpyl3' to be 0.40 s. This value does not change our calculations in view of the accuracy limits.
kb,tr= ktre-L\E/kT where k,, is the transfer rate to Tb3+,kb,trthe back-transfer rate, and AE the energy difference between the phosphorescence level and the 5D4level. The former is estimated from the progression in the phosphorescene spectrum of [LaCbpy.bpy.bpy13+ to be at level. This value of 22 160 cm-', Le., 1750 cm-I above the 5D4 AE yields for kb,,,/kt,at 300 K a value of lo4 which is in acceptable agreement with the ratio derived above. If we assume that the rapid increase above 100 K in Figure 4 of what is called there the migration rate is mainly due to back
Acknowledgment. G.B. and N.S. acknowledge NATO contract No. RG 937/86. This research originates from the NATO Workshop on Supramolecular Photochemistry (April 1987, Capri) directed by Prof. V. Balzani. G.B., N.S., and J.M.L. are grateful to Prof. Balzani for his stimulating interest in this cooperation. Registry No. [TbCbpybpybpy]'+, 107539-34-4; [LaCbpybpy bpy13+,113597-85-6;[Nacbpybpybpy]', 113597-86-7.
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Fluorescence Excitation Spectroscopy of the S2 State of Coronene: Solid State, Isolated Molecule, and van der Waals Clusters R. Jefferson Babbitt, Co-Jen Ho, and Michael R. Topp* Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 (Received: October 2, 1987)
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The So S2excitation spectrum of coronene under different environmental conditions is reported, including several individual sites in a Shpol'skii matrix and both isolated molecule and van der Waals clusters under supersonic jet conditions. It is confirmed that the vibronic structure of the isolated molecule So S2spectrum is distinct from that of the So SI transition, consistent with solid-state data for coronene, and with the case of benzene, also a D6* species. The jet spectra are calibrated by comparison with solid-state data, which are in turn calibrated through the observation of a weak matrix-induced vibronic band common to the SI and T I spectra. van der Waals clusters involving up to a dozen argon atoms are reported, and the data are compared to earlier results obtained for perylene. The line widths of the coronene So S2excitation transitions are found to be similar, regardless of the molecular environment. The absence of substantial line broadening in the clusters is attributed to the presence of ordered structures.
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Introduction There is a growing interest in the study of the dynamics, including photofragmentation, of molecular clusters isolated under supersonic jet conditions. A number of different approaches are possible, including direct infrared excitation' and stimulated (1) Morales, D. A.; Ewing, G. E. Chem. Phys. 1980, 53, 141. Pendley, R. D.; Ewing, G.E. J . Chem. Phys. 1983, 78, 3531. Liu, W.-L.; Kohlenbrander, K.; Lisy, J. M. Chem. Phys. Lett. 1984,112,585. Janda, K. C. Ado. Chem. Phys. 1985, 60, 201. Brady, B. R.; Spector, G.B.; Flynn, G.W. J . Phys. Chem. 1986, 90, 83. Miller, R. E. J . Phys. Chem. 1986, 90, 3301. Gough, T. E.; Knight, D. G.; Rowntree, P. A.; Scoles, G. J . Phys. Chem. 1986, 90, 4026.
0022-365418812092-2422$0 1.5010
emission pumping with visible and ultraviolet radiation* for electronic ground-state studies and direct electronic excitation for studies of the first excited singlet and triplet state^.^ Studies of (2) Kittrell, C.; Abramson, E.; Kinsey, J. L.; McDonald, S. A.; Reisner, D. E.; Field, R. W.; Katayama, D. H. J . Chem. Phys. 1981, 75, 2056. Lawrance, W. D.; Knight, A. E. W. J . Chem. Phys. 1982, 76, 5637. J . Phys. Chem. 1983, 87, 389. Abramson, E.; Field, R. W.; Imre, D.; Innes, K. K.; Kinsey, J. L. J . Chem. Phys. 1984, 80, 2298. Moll, D. J.; Parker, G.R.; Kupperman, A. J . Chem. Phys. 1984, 80, 4800. Suzuki, T.; Mikami, N.; Ito. M. Chem. Phys. Lett. 1985, 120, 333. Pique, J. P.; Chen, Y . ;Field, R. W . ; Kinsey, J. L. Phys. Rea. Lett. 1987, 58, 475.
0 1988 American Chemical Society
