J . Phys. Chem. 1988, 92, 2422-2429
2422
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
Spectroscopy of the S2 State of Coronene TABLE I: Comparison of the So
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The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2423
S2 Excitation Spectrum of Coronene in an +Heptane Crystal and a Free-Jet Expansion (See Figures 1-3)
heptane crystal (a site) band
freq (abs)’
re1 vb
1 2 3 4 5’ 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
28 643 28 960 29 112 29313
13701 -687839 1040
29 501 29 806 29 937 29 978 30 158 30 284 30 337 30 408 30451 30 486 30 522 30 632 30 684 30 869 31 177 31 302 31 342 31 519
1228 1533 1664 1705 1885 2012 2064 2135 2178 2213 2249 2359 241 1 2596 2904 3029 3069 3246
supersonic expansion
tentative assigment
re1 vb
freq (abs)
nloc
nlo
29951 30 090 30 289 30 442 30 508/26d 30 776 30 902 30978196 31 126 31 252 31 312 31 379 nlo 31 454 31 530 31 604 31 653 31 877194 32 144 32271 32 347164 32 490
705 844 1043 1176 FR 1262/80 1530 1656 FR 1732150 1880 2006 2066 2133
n/o
2208 2254 2358 2407 F R 263112648 2898 3025 FR 3101/3118 3244
(e2g) 5+A f
6+A 2+c 7+A 5 2A 3 + c 5+Ef 9+A 4 + c 5 + c 6 + C 7 + c 5+A+C 9 + c
+
‘All values given in cm-I. bSolid-state values referenced to the matrix-induced 370-cm-I band; jet values estimated assuming that the average shift of the weak fundamental bands 3, 4, and 7 is zero between the crystal and supersonic jet experiments. ‘n/o = not observed. dThe secondary component shifts into a Fermi resonance (FR) from the crystal to the jet. ‘The alg modes observed in combinations are as follows: A, 475; B, 1020, (see footnote f); C, 1370 cm-l. fBand 15 is assigned as a combination involving a new alg mode, B, since it shifts identically with bands 5, 8, 18, and 21 from the gas phase to the crystal
large-molecule systems tend to favor the use of electronically excited-state rather than ground-state techniques, because of the higher sensitivity and since ultrashort pulse techniques can more readily be used. However, two potential limitations of photofragmentation experiments on the Si surface are that FranckCondon factors tend to be very small at vibrational energies >3000 cm-’ and that spectral congestion erases detailed structure above ==2000 cm-I so that the photoselection of different aggregates is impracticable. Because of favorable absorption cross sections, higher electronic states are potentially important as a means of injecting large amounts of energy, but it is well-known that the So S, and higher energy transitions for many species are broadened by intermediate-level structure. This was demonstrated in early solid-state ~ o r kand ~ ,has ~ more recently been observed on many occasions under supersonic jet conditions, where the absence of matrix perturbations clarifies the spectra. Furthermore, the extent of intermediate level structure varies considerably from one molecule to another, for example, being appreciably more highly congested for naphthalene6 than for pyrene? On the other hand, a number of statistical limit cases are also known, which present a particularly attractive prospect for higher energy jet studies, since the relatively modest broadening (generally 3200 cm-I above the So S2 origin. Combinations of the band cluster 5 with known alg modes (bands 8, 18, and 21) are readily identified. Another alg combination band (15) is identifiable, on the basis of both the form of the Fermi resonance cluster and the crystal to gas-phase shift (see Table I).13 The rest of the bands involve four other fundamentals (bands 2-4 and 7), two unassigned bands ( 6 and 9), and the two prominent alg modes, which appear in combinations. Calibration. Several factors complicate the assignment of the So S2 excitation spectrum of coronene as follows: (a) The electronic origin transition is not observed, even under solid-state conditions.13 (b) Evidence from solid-state work13 has indicated that the active vibrational modes in So SI do not correspond to those in So S,. The one identifiable feature ( v I ) common to the So S1 and So S2 solid-state spectra is not observed for the isolated molecule, as will be shown. (c) Unlike the solid-state case, no S2 So fluorescence transitions have yet been observed in the jet. In the absence of more direct information, the So S2 jet spectra have been calibrated against solid-state data. The multiplet structure of the So S1 transition of coronene in n-heptane at
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Figure 2. Comparison of the excitation spectra for SI So fluorescence of coronene in the 0-850-cm-' absorption region of So S1. Behavior for three molecular sites in the n-heptane: (upper left, a;lower left, y; upper right, 6) is compared with that for the jet-cooled molecule (lower right). No evidence for an origin transition is seen even for the noncentrosymmetric y site. The 370-cm-, feature, observed at varying relative intensities in the solid-state data, is absent from the jet spectrum.
low temperatures, both mono- and polycrystalline, has been well d ~ c u m e n t e d . l ~ There - ' ~ are four principal groupings, labeled a through 6, of which the principal resonances in fluorescence, due to 0 1 transitions in v3 (998 cm-' in So), lie at 443.4 (a), 444.5 (p), 444.8 (y), and 445.3 nm (S).16 The single-site So S2 excitation spectrum for SI So fluorescence of coronene, in the region of low vibrational energy, is shown for several sites in Figure 2. (Data for the @ site, which is present in small concentration, are not presented here.) These data show in part that the observations made on the SI and T, states by Pitts et a1.I6 are extendable to the S2level. EarlierI3 we reported that the So S2excitation of a-site resonances resulted in no 0; transition in the S l So emission spectrum. On the other hand, excitation of any noncentrosymmetric site ( p through 6) gave rise to a prominent origin, so a strong perturbation for these sites is not unexpected. Also, as shown in Figure 2, the y-site spectrum is doubled on all bands due to the existence of two distinct environmentsI6 while the other sites show single resonances. The notable exception is the large splitting of the 6-site 687-cm-' band, which is known16 to be present in the SI spectrum and has been
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Spectroscopy of the S2 State of Coronene
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2425
5
SI
I
I1
1
I u4
2
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Figure 4. Comparison of parts of the jet-cooled excitation spectra for SI So fluorescence of coronene in (top) the So SI and (bottom) the So
Figure 3. Comparison of the So
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S2excitation spectra for SI So fluorescence of wronene in (top) n-heptane at 12 K (a site) and (inverted) supersonic jet conditions. The spectra have been shifted and rescaled for direct comparison of vibrational frequencies. Details of the spectra and assignments are given in Table I (see also ref 13). attributed to crystal-field-induced perturbation of prominent eZg modes. The data of Figure 2 show clearly that the weak feature observed near 370 cm-' (band 1, u l ) in the solid state is absent from the jet spectrum. Thus, it is shown that the intensity ratio in the a and y sites, while the ratio increases (1370/1687) is 4% for the 6 site. Within the limits of validity of the asto ~ 2 0 % sumption of the frequency of q ,its presence provided the means for an accurate calibration of the So S2 solid-state spectra. However, the transition is shown to be completely matrix induced, since it is below the level of detectability (intensity ratio
