7832
J. Phys. Chem. 1994, 98, 7832-7836
Naphthalene-Amines Exciplex Formation Promoted by Phase Transition in Crystallized Cyclohexane Sadao Matsuzawa,*J Michel Lamotte,* Philippe d;arrigues,* and Yukio Shimizut National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305, Japan, and URA 348 CNRS, Universitb de Bordeaux I, 33405 Talence Cedex, France Received: December 27, 1993; In Final Form: May 4, 1994”
The effect of the phase transition of cyclohexane at 186 K on the formation of exciplex in a rigid matrix was studied for the naphthalene (Np)-N,N-dimethylaniline(DMA), NpN,N-diethylaniline (DEA), N p triethylamine (TEA), and pyrene (Py)-DMA systems by using a high-resolution fluorescence spectrometer. All systems show appearance of an exciplex emission around 186 K. However, a drastic increase in the probability of exciplex formation with the effect of the phase transition of cyclohexane, which has been previously found by us for the Npl,2,5-trimethylpyrrole system, was observed only for the N p D M A and N p D E A systems. In both of these systems the maximum intensity of the exciplex emission is observed for low concentration of the electron donor molecule, and we have obtained spectral evidences for the formation of solute L(mixed microcrystallites” in the low-temperature cyclohexane matrix. The occurrence of these exciplex emissions can be explained by an exciton-assisted mechanism taking place in guest mixed crystallites. Proof for the formation of mixed crystallites was not obtained for the N p T E A and Py-DMA systems. TEA seems to have no mutual affinity with N p at low temperature and rather accelerates the creation of N p crystallites. Thus, exciplexes formed in rigid solvent matrix could be categorized into two types: crystalline type and encounter type.
Introduction In general, the formation of an excimer or an exciplex in rigid matrices such as low-temperature solvents and plastics is not expected to occur readily because these matrices impose limitation in the reorientation of the electron-donor and electron-acceptor molecules. However, in some cases, the formation of excitedstate complexes in rigid matrices has been Lowtemperature cyclohexane has been known as one of a few rigid matrices in which the formation of an excimer or an exciplex can be observed. In 1960s, Ferguson] and Mataga et al.z reported on the spectroscopic properties and structures of excimers of polycyclicaromatic hydrocarbons (anthracene, pyrene, perylene, etc.) formed in this matrix. On the basis of their results, Mataga et a1.2 concluded that the excimers have the symmetrical overlappingsandwich (face-to-face)configuration. Furthermore, in a subsequent paper,3 they described the spectroscopicproperty of the exciplex (they called it “heteropolar excimer”) between pyrene (Py) and dimethylaniline (DMA) formed in rigid cyclohexane matrix. This exciplex was later shown to be formed by the excitation of a weak ground-state c o m p l e ~ . ~However, .~ the type of the sites, in which the ground- or the excited-state complexes are located, and the mechanism for making the faceto-face configuration of the solute molecules have not yet been elucidated. Only the presence of favorable sites in the rigid matrices could have beeninferred from the facts that the formation of an excimer and an exciplex is solvent dependenP and is affected by the crystallization conditions of the s ~ l u t i o n . ~ ~ ~ ~ ~ In a previous paper,8 we reported on some spectral evidences for the existence of suitable conditions for promoting exciplex formation in a low temperature alkane matrix. For the naphthalene (Np)-1,2,5-trimethylpyrrole (TMP) system,8 it was concluded that exciplexes were formed in peculiar mixed-solutemolecule assemblies, “mixed cyrstallites”, which are constituted of solute molecules expelled from the monocrystallizedzone (single crystals) of the solvent. Furthermore, it was found that this suitable environment for the exciplex formation appeared to be National Institute for Resources and Environment. t Universite de Bordeaux I. @Abstractpublished in Aduunce ACS Abstrurrs, July 15, 1994.
favored in matrices formed upon slow cooling of the cyclohexane solution due to the Occurrence of the phase transition (disordered crystal tosinglecrystal transition) at 186K.9 Theaforementioned results prompted us to study whether a similar process, for the exciplex formation in rigid solvents at low temperature, could be observed for other pairs of compounds forming exciplexes. In this report, the effect of the phase transition of cyclohexane on the formation of exciplex at low temperature for the N p DMA, Np-diethylaniline (DEA), Py-DMA, and Nptriethylamine (TEA) systems is described. Furthermore, the reason for the efficient formation of exciplexes at low concentration of the electron donor molecule, as observed for the N p D M A and N p DEA systems, is briefly discussed in comparison with the results we obtained with the N p T M P system.
Experimental Section For measurements of fluorescenceand fluorescenceexcitation spectra at low temperature, a high-resolution fluorescence spectrometer assembled by us enabled Shpol’skii spectralOJ1 to be recorded. Details for this instrument have been previously reported.8 Cyclohexanesolutionsof electron donor and acceptor molecules, contained in fused silica tubes (100 pL), were introduced in a gilded-copper sample holder and attached to the cold head of a closed-cycle helium cryogenerator (Daikin, Cry0 Kelvin 202A SL). Fluorescence was observed at 90° through a high-resolution monochromator and detected with a photomultiplier. To study the effect of the phase transition on the exciplex formation, fluorescencespectra were measured either under rapid or slow cooling conditions. Rapid cooling involves initial immersion of the sample holder into liquid nitrogen followed by a cooling to the working temperature at the cold head of the cryogenerator. Slow cooling is performed, starting from room temperature, with the sample holder already attached to the head of the cryogenerator. Primarily, sample solutions were cooled at the rate of 2 “C/min and then kept for 40 min without evacuation (the temperatureof the head of a cryogeneratorapproached about 160 K at this stage). The resulted rigid solutions were further frozen in vacuo to the lower temperature. Spectra and emission intensities were measured at 15 K except for those otherwise mentioned. For comparison, similar experiments were carried
0022-3654/94/2098-7832%04.50/0 0 1994 American Chemical Society
Naphthalene-Amines Exciplex Formation
The Journal of Physical Chemistry, Vol. 98, No. 32, 1994 7833
h
l
o
o PTT
1
I
I I
V
.-mc
CI
C
&I
n
B (b) 172-169K
-
0 3
0
250 Temperature/ K
315
365
415
465
Wavelength / n m Figure 1. Temperature variation, upon slow cooling,of the fluorescence spectra of the N p D M A system in cyclohexane: hx= 288 nm; concentration of Np and DMA, 1 X 10-3 M; solvent, cyclohexane.
F g w 2. Variation of the intensity of the exciplex emission, upon slow cooling, with the temperature of the solution: 0, N p D M A system; 0 , N p D E A system; A, Py-DMA system; A, N p T E A system; solvent, cyclohexane. The temperatureof the phase transition (186 K)is indicated by a vertical dotted line with an abbreviated symbol PTT.
I?\
Np-DMA
out with methylcyclohexane, cycloheptene, and cyclooctane solutions of exciplex-forming solute pairs. High-purity Np for the scintillation spectrometer (Eastman Kodak Co.) and analytical reagent grade Py (Wako >99%) were used as the electron-acceptormolecules without further purifications. DMA, DEA, and TEA, used as the electron-donor molecules, were all analytical reagent grade (>99%),which were purchased from Wako, Aldrich, and Tokyo Kasei, respectively. HPLC-grade cyclohexane (Wako), spectrometric-grade methylcyclohexane (Dohnin), reagent-grade cycloheptene, and cyclooctane were used as the solvents.
Results Effect of Temperature on the Appearance of an Exciplex Emission. In a previous paper,8 we have reported, in the case of N p T M P system, that an exciplex emisison in cyclohexanematrix appeared at a low temperatureclose to that of the phase transition (186 K). In this paper, we have studied the Occurrence of this phenomenon for other exciplex systems in order to verify whether this behavior can be extended to other electron donor-acceptor systems. Figure 1 shows the temperature variation, upon slow cooling, of the fluorescence spectra of the Np (1 X 10-3 M)-DMA (1 X 10-3 M) system in cyclohexane. The variation was very similar to that previously observed for the N p T M P system.8 The structured fluorescence bands of Np observable at higher temperature (Figure la), are drastically reduced in intensity just below the phase transition temperature of cyclohexane (Figure 1b). Simultaneously, a new structureless broad band assigned to an exciplex formation appears in the large-wavelength range. A similar behavior is observed in the case of the Np-DEA system and to a lesser extent in the case of the well-known Py-DMA system. In contrast, the N p T E A system shows no such drastic change in the fluorescencespectrum in the temperature range of the phase transition. On the contrary, in the latter case, the fluorescence of naphthalene molecule decreases only slightly in intensity due to very little formation of exciplexes during the phase transition. Figure 2 shows the variation of the intensity of the exciplex emission with the temperature of solutions for the systems studied. The temperature of the phase transition (186 K) is indicated by a vertical dotted linewith an abbreviated symbol PTT. The intensities of the exciplex emissions, plotted in Figure 2, for the N p D M A , N p D E A , N p T E A , and Py-DMA systems were measured at 360,365, 380, and 433 nm, respectively. As can be seen from Figure 2, the intensity of exciplex emissions for the N p D M A and N p D E A systems drastically increased at a temperatureslightly below the phase transition temperature. The
315
I
I
365
415
I
I
465
Wavele n g th/nm Figure 3. Fluorescencespectra for the N p D M A system in cyclohexane upon rapid and slow cooling: (a) rapid cooling; (b) slow cooling; M; solvent, cyclohexane. concentration of Np and DMA, 1 X lack of coincidence with the phase transition temperature is probably due to the delay in the sample cooling process. The Py-DMA system shows a similar behavior. But, in contrast to the N p D M A and N p D E A systems, the change in the intensity of the exciplex emission occurs at slightly higher temperature than that of the phase transition (see Figure 2).
Effect of Phase Transition on the Probability of Exciplex Formation. Figure 3 displays the fluorescencespectra for the Np (1 X le3M)-DMA (1 X le3 M)system in cyclohexane upon rapid (spectruma) and slow cooling (spectrum b). Narrow bands present in the short-wavelength region of each spectrum are attributed to Np molecules incorporated in micropolycrystallites of cyclohexane. These molecules are trapped in the lattice and cannot interact with any other molecules in the matrix. By comparison of the two spectra, it is noteworthy that the intensity of the exciplex emission (Amx is nearly 360 nm) is drastically increased upon slow cooling (spectrum b). The difference in the intensity of the exciplex emission between rapid and slow cooling is still large in the case of the N p D E A system. Figure 4 shows the variation of the exciplex emission intensity observed for increasing concentration of the electron-donor molecule in the case of the above two systems. The dependence of the intensities with concentration, upon rapid and slow cooling, is shown by dotted and solid lines, respectively. Two characteristic features in these relations are (1) a large difference in the intensity of the exciplex emission between rapid and slow cooling as noted above and (2) the observation of the maximum of the fluorescence
Matsuzawa et al.
1834 The Journal of Physical Chemistry, Vol. 98, No. 32, 1994
?I
l o 0 r - - l l
Figure 4. Variation of the exciplex emission intensity for the N p D M A and N p D E A systems with the concentration of the electron-donor molecule; 0,N p D M A system; 0 ,N p D E A system; - - -rapid cooling; -,slow cooling; solvent, cyclohexane. The intensitiesof exciplex emissions for the N p D M A and N p D E A systems were measured at 360 and 365 nm, respectively. (a)
-
.-
Np-TEA
4 8 12 16 [TEA] X 103/m01.1”
L
290
I
300
- DMA
I
310
1
0
W a v e l e n g t h /nm
Figure 6. Fluorescence excitation spectra of exciplexes for the N p D M A and N p D E A systems: concentration of Np and electron-donormolecule, 1 X lW3 M; solvent, cyclohexane.
---A-
0
Np
3
20
[DMAl X103/m01.1”
Figure 5. Variation of the exciplex emission intensity for the N p T E A and Py-DMA systems with the concentration of electron-donor molecule: - - -, rapid cooling; -, slow cooling; solvent cyclohexane. The intensities of exciplex emissions for the N p T E A and Py-DMA systems were measured at 380 and 433 nm, respectively.
intensity in the low-concentration range in slow cooling condition. A comparable result has previously been obtained for the N p T M P system in a rigid cyclohexane matrix.* For the N p T M P system, the maximum change in intensity, upon slow cooling, was about 5 times higher than upon rapid cooling. The corresponding ratios for the N p D M A and N p D E A systems were found to be about 6 and 10, respectively (Figure 4). From these results, it is then clear that the Occurrence of exciplex emission is closely correlated with the transformations which take place in the cyclohexane solid solution during the phase transition and which are favored by slow cooling. In contrast, the N p T E A and Py-DMA systems show almost no or relatively small increase in the intensity of the exciplex emission upon slow cooling in comparison with fast cooling, although exciplex formations for these systems seems to be favored by the phase transition of cyclohexane (see Figure 2). Figure 5 shows the variation of the exciplex emission intensity for the N p T E A (Figure 5a) and Py-DMA (Figure 5b) systems with the concentration of the electron-donor molecule. The relations observed for these two systems were quite different from those for the N p D M A and N p D E A systems described above (Figure 4). In these cases, no maximum of the emission intensity over the low concentration range (below 2 X 10-3 M L-1) can be seen upon slow cooling. The intensity of exciplex emission gradually increased on increasing the concentration of the electron donor molecules (Le., TEA and DMA). It is interesting to note that the emission intensity upon slow cooling for the N p T E A system is lower than that upon rapid cooling. This peculiar behavior for the N p T E A system may be due to the incompatibility of TEA with naphthalene at low temperature as described later. The Py-DMA system, upon slow cooling, shows only about 2 times
of increase in the intensity of the exciplex emission with high concentration of DMA molecule. Fluorescence ExcitationSpectra of ExciplexesFormed in Rigid Cyclohexane Matrix. In previous experiments on the N p T M P system,* we found that the fluorescence excitation spectrum of the exciplex well coincided with the absorption spectrum of N p crystallites.I2 Hence, we inferred that the exciplex formation for the N p T M P system, in a rigid cyclohexane matrix, occurred especially in mixed crystallites containing electron-donor and -acceptor molecules. In these exciplexes, the excitation energy is transferred according to an exciton mechanism to centers where exciplexes are formed. Figure 6 shows the fluorescenceexcitation spectra of exciplexes for the N p D M A and N p D E A systems. Most of the bands, which appear in the spectra for these systems, are in good agreement with the excitation spectrum recorded in the case of N p T M P exciplex* and with the absorption spectrum of N p crystallites.I2 In the case of the N p T M P system, two exciton bands, BOand Ao, a t 316.2 and 317.7 nm (the position of the A. band is indicated by an arrow) were clearly observed. For the N p D M A and N p D E A systems, the Ao band was however not soclearly observed. This may be due to the weakness of the band which is obscured by the inherent noise. Judging from the appearance of strong Bo band, it seems that exciplexes for the N p D M A and N p D E A systems are formed by an excitation of naphthalene molecules in mixed crystallites as observed in the N p T M P system. In contrast, a proof for the formation of mixed crystallites was not obtained for the N p T E A system. In fact, two fluorescence spectra which imply no formation of mixed crystallites for this system were obtained. Figure 7 shows the high-resolution lowtemperature fluorescence spectra for the N p T E A system and N p in cyclohexane obtained upon rapid cooling. In the spectrum b for N p molecule, semi-broad bands (asterisked) arising from crystalline N p are detected. Despite the fact that these bands were found to disappear with the formation of N p T M P exciplex in rigid cyclohexane matrix: their intensities were found to increase when naphthalene was in the presence of TEA as shown in the spectrum a. From this result, it can be inferred that TEA has a tendency to promote the formation of N p crystallites rather than to form mixed Crystallites. Furthermore, evidences for no formation of mixed crystallites, for the N p T E A system, were obtained from the measurements of the low-temperature fluorescence spectra of a mixture of these two solute molecules. The low-temperature fluorescence spectrum of N p in TEA (molar
Naphthalene-Amines Exciplex Formation ? 3 d Y
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350 Wavelength / n m Figure 7. Comparison of high-resolution low-temperaturefluorescence spectra for the NpTEA system and Np in cyclohexane: concentration of Np and TEA, 1 X 10-3 M. Stars in the spectra indicate semi-broad bands arising from Np crystallites. ratio 1:5), does not exhibit any broad emission band from an exciplex but only fluorescence bands of Np molecule. In contrast, solid solutions of Np in DMA and DEA exhibited exciplex emissions. These results imply that TEA has no mutual affinity for Np at low temperature and does not form mixed crystallites. The excitation spectrum for the Py-DMA system was the same as neither that of the Py monomer nor that of the Py crystal. This indicates that, as in the case of N p T E A system, the exciplex emission does not arise from mixed crystallites, The lowest energy band was located at about 345 nm, which is slightly red shifted (6.5 nm) as compared to that of the Py monomer. This spectrum may be attributed to the ground-state complex that had been suggested by Mataga et al.'v4
Discussion According to our results, it can be concluded that the phase transition occurring in cyclohexaneat 186 K givesdynamic forces to solute molecules in rigid matrix and is responsiblefor an increase in the probability of an exciplex formation (Figure 2). However, exciplexes formed during the phase transition should be categorized into two types. One type of the exciplexes arises within mixed crystallites of solute molecules formed during the solidification process (we label this the crystalline type). The other type results from the encounter of two solute molecules (pair encounter type) during cooling. From this point of view, the N p T M P , N p D M A , and N p D E A systems, which show a high probability of exciplex formation at low concentrations, belong to the crystalline type. On the other hand, the N p T E A and Py-DMA system, which show no evidence for "in crystalline formed exciplex" may be classified as belonging to the encounter type. The wavelengths of the exciplex emission maxima observed for the naphthalene-amine systems investigated here at low temperature are 2&30 nm shorter than those usually found for equivalent systems in fluid solutions. This may result from the rigidity of the host matrix (naphthalene microcrystals), which may be thought to impose some constraint on the guest molecules.'3J4 Decreasing temperature is supposed to stabilize the excited state, but this takes place only in soft matrices just below the freezing point.I4 The exciplexes formed within naphthalene microcrystals in cyclohexane may not therefore be fully stabilized.
For exciplexes stabilized by charge-transfer interactions, a correlation15J6between the energy of the fluorescencemaximum (hv) and the donor ionization potential I @ ) is expected. Such a correlation is not observed however in the case of Np-DEA and Np-TEA systems. Although DEA has an I @ ) value (6.99 eV) smaller than that of TEA (7.5 eV), the N p D E A system displays an exciplex emission at higher energy (a365 nm) than the N p TEA system (-380 nm). This anomaly may be relevant to the matrix-destabilizing effect invoked above or from very different charge-transfer contribution in the exciplex formation for these two donors.15J6 When comparing the fluorescence maxima of the exciplexes of naphthalene with TMP and DMA, better consistency with previous results was found. The fluorescence maxima are observed respectively at 374 nm for the Np-TMP system and at 360 nm for the N p D M A one. A similar order in the exciplex emission maxima was previously observed in the case of exciplexes of TMP and DMA with 2,4,5-triphenyloxazole in benzene which have been observed at 460 and 448 nm, respecti~e1y.l~However, we cannot comment further on the relationship between I @ ) and fluorescence maximum due to the lack of knowledge in the I @ ) value for TMP. The efficiency of the exciplex formation within mixed crystallites (crystalline type) seems to be very high, as shown by our experimental results for the N p D M A , N p D E A , and N p T M P systems. In these cases, the solute molecules may be envisaged to be expelled from single crystalsof cyclohexaneby a zone melting type phenomenon, and to make mixed crystallites in the microenvironment between single crystals of solvent (grain boundaries). The high intensity of the exciplex emission at low concentration may be explained by the efficient energy transfer to the existing pairs by exciton mechanism, within the mixed crystallites as shown in Scheme 1. Thecrystalof Np hasa monocliiicstructure with twomolecules per unit cell. The model illustrated above is a schematic illustrationof the moleculararrangement in the ab plane as viewed along the caxis. The Np (white ellipsoid) and the electron donor molecules (black ellipsoid) are considered to be ideally situated as shown in the illustration and nearly satisfy face-to-face orientation required for exciplex formation. As Np molecules in mixed crystallites can be continuously excited by fast excitonassisted energy-transfer,exciplex formation occursvery efficiently in comparison with encounter type. At a high concentration of the electron donor molecules, this possibility is reduced, most probably due to the decrease in the exciton migration. It should be noted that the model we propose for mixed crystallites, as described above, may be too much idealized. For instance, the distance between Np and the electron donor molecule along the a axis seems to be too large for each molecule to interact. But wemay postulate that parallel pairs (indicated by arrows) required
Matsuzawa et al.
7836 The Journal of Physical Chemistry, Vol. 98, No. 12, 1994
temperature of the phase transitions. This could result from the fact that in these host molecules, contrary to cyclohexane, only limited changes in the molecular arrangement may occur during phase transition.
Conclusions
0
I
I
300
310
3 I
W a v e l e n g t h /nm Figure 8. Fluorescence excitation spectrum of N p in a rigid methylcyclohexane matrix formed upon slow cooling: concentration of Np, 1 X 10-3 M. Positions of the exciton and vibronic bands for N p crystallites are indicated by arrows.
to form an exciplex are more precisely located in defects of crystallites. Nevertheless, the possibility of exciplex formations for nonparallel pairs cannot be ignored. The determinant role played by change in the crystal structure of the host matrix, for the formation of mixed crystallites, was demonstrated by comparing the excitation spectrum of N p in rigid methylcyclohexane and cyclohexane matrices. Methylcyclohexane has been known to exhibit a phase transition at low temperature.l*J9 However, the change in the crystal structure for this solvent is not enough to induce a drastic change in the dispersion state of the solute molecules. This could be inferred from the fluorescence excitation spectrum of N p (1 X le3M) observed in rigid methylcyclohexane matrix, formed upon slow cooling (Figure 8). From the spectrum, it is evident that, in the experimental condition we used, N p can hardly crystallize in a rigid methylcyclohexane matrix. The exciton bands BO(3 16.2 nm) and A0 (317.7 nm) and some of the characteristic vibronic bands (peak positions are indicated by arrows) for crystallites, which appeared strongly in rigid cyclohexane matrix, were hardly observed. As a consequence, no exciplex emission was observed for all of the Np-containing systems investigated here. Similar results were obtained from a mixture of methylcyclohexane and isopentane as the solvent. It should be noted that observation of the excimer emission for perylene, reported by Ferguson’ on this solvent mixture, had involved an annealing process. The importance of a drastic change in crystal structure, for complex formations at low temperature, was further confirmed by an experiment with methylcyclohexane solution of the Py-DMA system. In this case, no exciplex emission was observed under the condition of slow cooling. It is interesting to note that the N p T E A system has to be classified as belonging to the encounter type. The reason may lie in the absence of affinity between TEA and N p molecules a t low temperature which leads to a separate segregation of these two species to form quasi-pure microcrystallites in cyclohexane matrix. In contrast, a phenyl-substituted molecule of TEA, Le., DEA, is found to form mixed crystallites with N p and to form exciplex. This fact means that the affinity for naphthalene is increased by the presence of the phenyl group in the molecule. Cycloheptene20-21 and cyclooctane22.23 also exhibit a phase transition at low temperature. However, any increase in the probability of exciplex formation is noticed in these solvents (no exciplex emission was observed for cyclooctane solutions) a t the
The effect of the phase transition of cyclohexane at 186 K on the formation of exciplex in rigid matrix was studied for the N p D M A , N p D E A , N p T E A , and Py-DMA systems. The formation of exciplex with the aid of the phase transition was observed for all systems investigated. Moreover, the results indicated the presence of two types of exciplexes, which were formed by different mechanisms. One is the crystalline type, which is formed in mixed crystallites of solute molecules, and the other is theencounter type formed by an incidental pair formation of solute molecules during the cooling process. The N p D M A and N p D E A (and N p T M P ) systems appeared to be involved in the former type and the N p T E A and Py-DMA systems in the latter. TEA seemed to be incompatible with N p at low temperature and appeared to promote the formation of microcrystallites of naphthalene.
Acknhwledgment. The encouragements and supports of Dr. I. Tamori (chief of the atmospheric environmental department, NIRE) and Dr. R. Lesclaux (director of the Laboratoire de Photophysique et Photochimie Moleculaire, Universit6 de Bordeaux I) are gratefully acknowledged. Furthermore, we thank Miss Emma Davies for reading the manuscript. References and Notes (1) Ferguson, J. J . Chem. Phys. 1965,43, 306. (2) Mataga, N.; Torihashi, Y.; Ota, Y. Chem. Phys. Lett. 1967, I , 385. (3) Mataga, N.; Okada, T.; Oohashi, H.; Bull. Chem. SOC.Jpn. 1966, 39, 2563. (4) Okada, T.; Mataga, N. Bull. Chem. Soc. Jpn. 1976, 49, 2190. (5) (a) Mattes, S. L.; Farid, S. Science 1984, 226,917. (b) Saeva, F.; Luss, H.; Martic, P. J. Chem. SOC.Chem. Commun. 1989, 1476. (6) Yoshihara, K.; Futamura, K.; Nagakura, S. Chem. Lett. 1972, 1243. (7) Ferguson, J. J . Chem. Phys. 1966, 44, 2677. (8) Matsuzawa, S.; Garrigues, P.; Lamotte, M.; Tamura, M. J. Phys. Chem. 1991, 95,9676. (9) Kahn, R.; Fourme, R.; Andre, D.; Renaud, M. Acta Crystallogr. 1973, B29, 131. (10) (a) Garrigues, P.; Ewald, M. In?.J. Ewiron. Anal. Chem. 1985,21, 185. (b) De Lima, C. G. Crit. Rev. Anal. Chem. 1985, 16 (3), 177. (c) Hofstraat, J. W.; Jansen, H. J. M.; Hoornweg, G. P.; Gooijer, C.; Velthorst, N. H. In?. J . Ewiron. Anal. Chem. 1985, 21, 299. (d) Bykovskaya, L. A.; Personov, R. I.; Romanovskii, Yu. V. Anal. Chim. Acta 1981, 125, 1. (1 1) Nakhimovsky, L. A.; Lamotte, M.; Joussot-Dubien, J. Handbook of Low Temperature Electronic Spectra of Polycyclic Aromatic Hydrocarbons; Elsevier Physical Science Data 40; Elsevier: Amsterdam, 1989; pp 32-72. (12) Broude, V. L.; Rashba, E. I.; Sheka, E. F. Spectroscopy of Molecular Excitons; Speringer-Verlag: Berlin, 1985; pp 10, 207. (13) Ferguson. J.; Mau, A. W.-H.; Pun,M. Mol. Phys. 1974,28, 1457. (14) Ghoneim, N.; Rohner, Y.; Suppan, P. Furaduy Discuss. Chem. SOC. 1988,86, 295. (1 5 ) Gordon, M.; Ware, W. R. The Exciplex; Academic Press Inc.: New York, 1975. (16) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970. (17) Davidson, R. S.; Lewis, A.; Whelan, T. D. J . Chem. Soc., Perkin Trans. 2 1977, 1280. (18) Hankin, S. H.; Khalil, 0.S.; Goodman, L. Chem. Phys. Lett. 1979, 63, 11. (19) Olszowski, A. Chem. Phys. Lett. 1981, 78, 520. (20) Haines, J.; Gilson, D. F. R. J . Phys. Chem. 1990, 94, 3156. (21) Haines, J.; Gilson, D. F. R. Can. J . Chem. 1990, 68, 604. (22) Keller, R. C.; Coffey, M. S.; Lizak, M. J.; Conradi, M. S.; Bunnelle, W. J . Phys. Chem. 1989, 93, 3832. (23) Lizak, M. J.; Keller, R.C.; Coffey, M.S.; Conradi, M.S.; Bunnelle, W. J . Phys. Chem. 1990,94,992.
