9676
J . Phys. Chem. 1991, 95, 9676-9681
Thus the L absorption edge white lines may provide key complementary information in the analysis of unknown configurations around transition-metal centers with low d-orbital occupancies. EXAFS alone cannot provide accurate coordination numbers or configurations, but provided the L-edge spectra can be obtained, deduction of the configuration may still be possible. This simplistic approach may yield more useful chemical information than the branching ratio, electronic configuration, and white line intensity methods, since at the present time the theory of the latter is not fully developed.
However, for the usefulness of these techniques to be further tested, improvements in both the experimental resolution and the sampling methods are required.
Acknowledgment. We acknowledge Dr. J. T. Gauntlett and R . J. Perry for assistance in collecting the data and Dr. G . van der Laan for helpful discussions. We are grateful to the S.E.R.C. for financial support, a studentship (J.F.W.M.), and access to the facilities of the S.R.S., whose staff we thank for their help with our experiments.
A Suitable Environment for the Formation of a Naphthalene-I ,2,5-Trimethylpyrrole Exciplex in a Low-Temperature Alkane Matrix Sadao Matsuzawa,*.+Philippe Garrigues,f Michel Lamotte,i and Mitsuhisa Tamura' National Research Institute for Pollution and Resources, 16-3 Onogawa, Tsukuba, Ibaraki 305, Japan, and Photochimie et Photophysique MolPculaire URA 348 CNRS, UniversitP de Bordeaux I, F33405 Talence. France (Received: February 27, 1991; In Final Form: July 8, 1991)
The effect of crystallization conditions of aromatic-alkane (cyclohexane, n-pentane, n-hexane, n-heptane, and n-octane) systems on exciplex formation between naphthalene and 1,2,5-trimethylpyrrole (TMP) in a rigid matrix at low temperatures was studied using a high-resolution fluorescence spectrometer. From fluorescence and excitation spectra, obtained at low temperature, it is concluded that such exciplexes are formed in peculiar mixed-solute-molecule assemblies we call "mixed crystallites", which are constituted of solute molecules expelled from monocrystallized zones (single crystals) of the solvent. We found that they do not occur in metastable micropolycrystallites of the solvent, which are formed predominantly in fast cooling conditions and which exhibit highly resolved spectra (Shpol'skii effect) of the solute. Furthermore, this suitable environment for exciplex formation appears to be favored in cyclohexane due to the occurrence of a phase transition at 186 K. Thus, slow cooling and the choice of cyclohexane as a solvent greatly increased the probability of observing exciplex formation between naphthalene and 1,2,5-TMP in frozen solutions.
Introduction It is generally accepted that an exciplex between electron donor and electron acceptor molecules, as well as excimers.' has a face-to-face (sandwich) configuration.2 The formation of this type of structure follows an electronic excitation and usually requires some reorientation of the involved molecules. Rigid matrices are believed to be not suitable for exciplex formation because they impose limitations in the reorientation of the solute molecules. In general, a lowering of the temperature decreases the probability for observing exciplex fluorescence, as reported for the intramolecular exciplex of w-phenyl-cu-(N,N-dimethylamino)alkanes in i ~ o p e n t a n e . ~Similarly in a polymer matrix, exciplex stability decreases below the glass transition temperature T 4-7 However, several cases have been reported of the formation of'an exciplex in rigid matrices, such as low-temperature solv e n t ~ ~and -~.~ For example, the pyrene-dimethylaniline (DMA) solute pair shows exciplex emission in a rigid cyclohexane m a t r i ~ . In ~ , this ~ case, a preexisting loosely bound pair of donor and acceptor, having a folded structure in the low-temperature matrix, may be excited by light. Possible exciplex formation by charge transfer through the a-bond framework has also been reported.1° According to this mechanism, exciplex formation in a rigid matrix may be relatively easy due to the fact that it does not require a face-to-face configuration, and therefore reorientation is not necessary. Although not yet confirmed by other studies, an exciplex emission from a semiflexible donor/ acceptor/solvent system has recently k e n reported by Verhoeven." Whether exciplex emission can be observed in a rigid matrix is solvent dependent. For example, while an intramolecular exciplex of 2-anthryl-(CH2),-DMA is observable in 3: 1 softened 'National Research Institute for Pollution and Resources. 'UniversitE de Bordeaux I .
0022-3654/9 1 /2095-9676$02.50/0
glasses of isopentane and methylcyclohexane at 77 K, exciplex emission from the pyrene-DMA and the pyrene-diethylaniline (DEA) systems is observable in the cyclohexane matrix at 77 K.8,9 This behavior may be due to the existence of favorable peculiar conditions in the crystallized solvents. Apart from the type of solvent itself, the solute concentrations as well as the physical characteristics of the frozen matrices and particularly the size and the crystallinity of the grains (metastable micropolycrystallites as in the Shpol'skii matrix,'*-I6 monocrystallites,~6~17 highly dis(1) (a) Chandross, E. A.; Dempster, c. J . J . Am. Chem. SOC.1970, 92, 3586. (b) Davidson, R. S.; Whelan, T. D. J . Chem. Soc., Chem. Commun. 1977, 361. (2) (a) Mataga, N. Electronic Structure and Dynamic Behavior of Some Exciplex Systems. In The Exciplex; Gordon, M., Ware, W. R., Eds.; Academic Press: New York, 1975; pp 113-144. (b) Ide, R.; Sakata, Y.; Misumi, S.; Okada, T.; Mataga, N. J . Chem. Soc., Chem. Commun. 1972, 1009. (c) Okada, T.;Fujita, T.; Kubota, M.; Sasaki, S.; Mataga, N. Chem. Phys. Left. 1972, 14, 563. (d) Davidson, R. S.;Trethewey, K. R. J . Chem. SOC.,Chem. Commun. 1976, 827. (3) Van der Auweraer, M.; Gilbert, A,; De Schryver, F. C. J . Am. Chem. SOC.1980, 102, 4007. (4) Mattes, S. L.; Farid, S . Science 1984, 226, 917. ( 5 ) Martic, P.A.; Daly, R. C.; Williams, J . L. R.; Farid, S. J . Polym. Sci., Polym. Lerr. Ed. 1977, 15, 295. (6) Martic, P. A.; Daly, R. C.; Williams, J . L. R.; Farid, S. J . Polym. Sci., Polym. k i t . Ed. 1979, 17, 305. (7) Saeva, F.; LUSS, H.; Martic, P. J . Chem. Soc., Chem. Commun. 1989,
1476. (8) Mataga, N.; Okada. T.; Oohari. H. Bull. Chem. SOC.Jpn. 1966, 39,
2563. (9) Okada, T.; Mataga, N . Bull. Chem. SOC.Jpn. 1976, 49, 2190. (IO) Pasman, P.;Rob, F.; Verhoeven, J. W. J . Am. Chem. SOC.1982, 104, 5127. ( I I ) Verhoeven, J. W. Pure Appl. Chem. 1990, 62, 1585. (12) Rima, J.; Nakhimovsky, L. A.; Lamotte, M.; Joussot-Dubien, J. J . Phys. Chem. 1984.88. 4302.
0 199 1 American Chemical Society
Formation of the Naphthalene-TMP Exciplex ordered monocrystallites, or plastic crystals'*J9 ) are expected to play major roles in the formation of a solute exciplex at low temperatures, but up to now these situations have not been studied in detail. This report mainly describes the effect of crystallization conditions of aromatic-alkane systems on exciplex formation between naphthalene and 1,2,5-trimethylpyrrole (TMP).20*21 Evidence for the crucial role of this parameter was obtained by comparing the bandwidths of the spectra of naphthalene solutions, measured by a high-resolution fluorescence spectrometer,22 for different crystallization conditions. Experimental Section
A computer-controlled high-resolution fluorescence spectrometer (Shpol'skii spectraZZare obtainable), assembled by us, was used to obtain fluorescence and excitation spectra for naphthalene and the exciplex. For measurement of fluorescence spectra, light from the excitation source (JASCO PS-X500; 500-W xenon lamp) was dispersed by a monochromator (JASCO CT-25C; focal distance 0.25 m, 1200 lines/" grating) and focused on a rigid solution. Fluorescence was observed at 90' through a high-resolution monochromator (JASCO CT-SOCS; focal distance 0.5 m, 1800 lines/" grating) and detected with a photomultiplier (Hamamatsu R923). For measurement of fluorescence excitation spectra, a 1200 lines/" grating in the CT-25C monochromator was replaced by a 3600 lines/" grating to increase resolution. Sample solutions, taken in fused-silica tubes, were contained in a gilded-copper sample holder and frozen at the cold head of a closed-cycle helium cryogenerator (Daikin, Cry0 Kelvin 202A SL). To study the effect of crystallization conditions on exciplex formation, two ways of cooling, rapid cooling by immersion of the sample holder into liquid nitrogen followed by freezing on the head of the cryogenerator and slow cooling by freezing from room temperature by the cryogenerator, were used. Rapid cooling mainly gives metastable micropolycrystalline matrices which have a snowlike appearance and are generally called Shpol'skii matrices,I2-I6 while slow cooling gives partially monocrystalline matricesI6I9 constituted of larger grains with pronounced monocrystal character. In the case of slow cooling using cyclohexane as the solvent, the sample solution was maintained, for a specified period (0-40min), at temperatures a little below 186 K to promote phase transition from disordered crystals (plastic crystal) to monwrystallites. Partially formed micropolycrystallites were also converted to monocrystallites by this procedure (annealing). Quenching rate constants for naphthalene and the lifetime of the exciplex were determined by a steady-state fluorescence spectrometer (Hitachi 650-60) and a nanosecond time-resolved spectrometer (Horiba NAES- 1 loo), respectively. Absorption spectra were measured on a JASCO 670 UV/vis spectrometer equipped with a temperature controller (JASCO E H C 363). In this measurement, a thin cuvette cell was used to increase transparency.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9677 TABLE I: Ouenchinn of Naphthalene Fluorescence by Pyrroles kqT,,O k,O 104k quencher L mol-l L m o b s-I L mol-Ts-1 ~
2.5-DMP 1,2,5-TMP P
2.26 X 1O'O 1.92 X 10" 5.10 X IO9
2170
1840 490
0
I-MP
0
"In n-hexane at 20 O C . The fluorescence lifetime of naphthalene (96 ns) was used as 7, for the determination of k,. bThese values were cited from the l i t e r a t ~ r e . ~ ~
High-purity naphthalene for the scintillation spectrometer (Eastman Kodak Co.) was used as received. 1,2,5-TMP, 2,5dimethylpyrrole (DMP), 1-methylpyrrole (MP), and pyrrole (P) (all from Aldrich >97%) were distilled prior to use. Alkanes (cyclohexane, n-pentane, n-hexane, n-heptane, and n-wtane) used as solvents, were HPLC grade or analytical reagent grade (Wako and Tokyo Kasei). It should be noted that, upon fast cooling, polycrystallized solutions of n-pentane and cyclohexane give narrow band (quasi-lines) spectra for naphthalene. This results from the so-called Shpol'skii effect, which has been shown to characterize metastable frozen solutions of aromatics in n-alkanesI4 but which could also be observed in other frozen solvents such as THFZ3or cyclohexane. Results Emission Spectra at Room Temperature. Table I shows rate constants for the quenching of naphthalene fluorescence by pyrroles in n-hexane at 20 OC. For comparison, literature values for benzene, acetonitrile, and ethanol solutions are also included. The quenching rate constants (kqs), in n-hexane, for 2,5-DMP and 1,2,5-TMP were 1O'O L mol-l s-l, i.e., larger than that for P. I-MP did not quench naphthalene fluorescence a t all in nhexane. The two methyl groups on the pyrrolic ring thus largely contribute to the large kqs determined for 2,5-DMP and 1,2,5TMP. Suppression of quenching by an N-methyl group was apparent from the decrease in kqs for 1,2,5-TMP and 1-MP compared with that for 2,5-DMP and P, respectively. Quenching is also found to be greatly affected by solvent polarity as indicated by kg = 0 for 1-MP compared with the values found in other solvents.z4 In the case of the four pyrrolic compounds listed in Table I, exciplex emission was observed only for 1,2,5-TMP in cyclohexane and Cs-CBn-alkanes. The quenching of naphthalene fluorescence by 2,5-DMP and P apparently might involve a nonradiative transition pathway b or ion formation pathway c according to the following scheme: N
_._ _ _
Commun 1975 201 _. .
(21) Davidson, R. S.;Lewis, A,; Whelan, T. D. J . Chem. SOC.Perkin Trans. 2 1977, 1280. (22) (a) Garrigues, P.; Ewald, M. Inr. J . Enuiron. Anal. Chem. 1985,21, 185. (b) De Lima, C. G. Crii. Reu. A n d . Chem. 1985, 16, (3), 177. (c) Hofstraat, J. W.; Jansen, H. J. M.; Hoomweg, G. Ph.; Gooijer, C.; Velthorst, N. H. Inr. J . Enoiron. Anal. Chem. 1985, 21, 299. (d) Bykovskaya, L. A,; Personov, R. 1.; Romanovskii, Yu. V. Anal. Chim. Acta 1981, 125, 1.
hv
N*
exciplex
~~
(13) Amine, C.;Nakhimovsky, L. A,; Morgan, F.; Lamotte, M. J . Phys. Chem. 1990, 94, 393 1. (14) Nakhimovsky, L. A.; Lamotte, M.; Joussot-Dubien, J. Hundbook of Low Temperature Electronic Spectra of Polycyclic Aromatic Hydrocarbons; Elsevier Physical Science Data 4 0 Elsevier: Amsterdam, 1989; pp 32-72. (15) Hofstraat, J. W.; Freriks, 1. L.; De Vreeze, M. E. J.; Gooijer, C.; Velthorst, N. H. J . Phys. Chem. 1989, 93, 184. (16) Lamotte, M.; Merle, A. M.; Joussot-Dubien, J.; Dupuy, F. Chem. Phys. Leu. 1975, 35, 410. (17) Lamotte, M.; Joussot-Dubien, J. J . Chem. Phys. 1974, 61, 1892. (18) Kahn, R.; Fourme, R.; Andr€, D.; Renaud, M. Acra Crystallogr. 1973,829. I3 I. (19) Andr€, D.; Szwarc. H. Acra Crystallogr. 1988, B44, 651. (20) Davidson, R. S.;Lewis, A.; Whelan, T. D. J . Chem. SOC.,Chem.
2.43 (ethanol) 3.26 (acetonitrile) 2.53 (benzene) 1.01 (ethanol) 1.65 (acetonitrile)
N*
N + hvf
+
Py
N,3N
e
N
+
(N-Py*)*
Py
+ hVCT
A
N
+
N-
+
Py*
PyS3N + Py
where N and Py denote naphthalene and a pyrrolic compound, respectively. A broad emission (hum) via pathway a, in t h e case of the naphthalene-l,2,5-TMP exciplex, is observed with a maximum in the spectral range 390-400 nm in nonpolar solvents at room temperature, in good agreement with literature data.21 The lifetime associated with this exciplex emission was found to be 85 ns. (23) Kirkbright, G. F.; De Lima, C. G. Chem. Phys. Lett. 1976,37, 165. (24) McCullough, J. J.; Wu, W. S.; Huang, C. W. J . Chem. Soc., Perkin Trans. 2 1972, 370.
9678 The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
Matsuzawa et al. 1
4
,
A
I
(X6.7)
I
1
I
I
1
A‘ (X6.7)
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(X6.7)
(X6.7)
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1
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I
I
I
I
365
415
465
320
330
340
350
Wavelength /nm Figure 1. Fluorescence spectra for the naphthalene and naphthaleneI ,2,5-TMP systems obtained upon rapid cooling of the solution (Aex = 288 nm): spectrum A, naphthalene ( 1 X IO-) M) in cyclohexane; spectrum B, naphthalene ( I X M) and 1,2,5-TMP ( 1 X M) in cyclohexane. Spectra were measured at 15 K. The multiplying factor for sensitivity, indicated in parentheses, is based on that of spectrum C in Figure 3. Emission Spectra at Low Temperature. T o study the effects of the crystalline states of alkanes on the formation of the naphthalene-I ,2,5-TMP exciplex, high-resolution fluorescence spectra were obtained under rapid- and slow-cooling conditions. (a) Rapid Cooling. Figures 1 and 2 show variations in the 15 K fluorescence spectra of rapidly frozen cyclohexane solutions of naphthalene upon addition of M 1,2,5-TMP to the solution. When 1,2,5-TMP was not present in the solution (spectrum A in Figure l ) , narrow bands (quasi-lines) with about 0.1-nm fwhm (full width at half-maximum) of naphthalene were mainly observed (Shpol’skii effect), due to the radiative deactivation of excited naphthalene molecules incorporated in micropolycrystallites of cyclohexane formed by rapid cooling of the solutions. For concentrations of naphthalene above 1 X M, semibroad bands of about 0.5-nm fwhm could also be seen in addition to the narrow bands. They are obscure in spectrum A, but they can be seen distinctly (asterisked bands) in the partially expanded spectrum A (spectrum A’ in Figure 2). Their origin is discussed later. Similarly n-pentane also gave both narrow and semibroad bands for naphthalene. Spectrum B in Figure I shows that the addition of 1.2.5-TMP to the solution of naphthalene gives rise to a structureless broad = 374 nm), implying the formation of an exciplex band (A,, between naphthalene and 1,2,5-TMP in the rigid cyclohexane matrix. Interestingly, examination of the partially expanded spectrum B (spectrum B’ in Figure 2) shows evidence that some kind of interaction occurs between naphthalene molecules responsible for the semibroad bands and 1,2,5-TMP molecules. This is deduced, in spectrum A’, from the disappearance of the semibroad bands upon addition of 1,2,5-TMP. Only one of the semibroad bands at 329.84 nm (doubly asterisked) appeared to remain, due to the probable overlapping with a narrow band. Naphthalene molecules incorporated in the micropolycrystallites of cyclohexane apparently did not participate in exciplex formation,
,
io
W a v e l e n g t h / n in Figure 2. Partially expanded fluorescence spectra for the naphthalene and naphthalene-l,2,5-TMP systems obtained upon rapid cooling of the solution. Spectra A‘ and B’ are partially (315-360 nm) expanded spectra of spectrum A and spectrum B in Figure 1. Numbered bands correspond to those in Figure I , and asterisked bands indicate the semibroad bands described in the text. TABLE 11: Fluorescence of Naphthalene Observed in Rigid Cyclohexane Matrices upon rapid cooling upon slow cooling band no. A, nm 3, cm-’ A, nm 8 , cm-’ I
2 3 4 5 6
319.84 324.32 327.88 334.64 339.52 350.76
31266 30834 30499 29883 29453 28510
321.84 327.36 330.08 336.92 342.92 353.40
31071 30547 30296 29681 29161 28297
as evidenced by the absence of 1,2,5-TMP effect on the intensity of the narrow bands in spectrum B (or B’). The behavior of 1,2,5-TMP alone in a rigid cyclohexane matrix may be the same as that for naphthalene, since this molecule has a molecular size similar to that of naphthalene. Exciplex emission was observed for all solvents studied. However, the disappearance of the semibroad bands was not so obvious in C6-C8 n-alkanes due to the effect of band broadening, which makes their identification difficult in the recorded spectra. (b) Slow Cooling. Figure 3 shows the variation in the fluorescence spectra, measured at 15 K for slow-cooling conditions, upon adding 1,2,5-TMP to a cyclohexane solution of naphthalene (C, = M). The intensity of naphthalene fluorescence in slow-cooled samples appears to be much higher than that observed in rapid-cooled ones (compare inserted spectrum C’ in Figure 3 with spectrum A in Figure 1 , which were recorded at the same sensitivity). Moreover, the recorded fluorescence bands are found to be broader (semibroad) and slightly red-shifted (2-4 nm) as compared with the narrow bands. An important result is that the broadened spectra obtained upon slow cooling correspond very precisely to the semibroad band spectrum observed upon rapid cooling (asterisked bands in spectrum A’). In Table 11, the wavelengths and wavenumbers for the intense bands emitted upon slow cooling (semibroad bands) are compared with those emitted
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9619
Formation of the Naphthalene-TMP Exciplex
C (x 1)
l k
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I I
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a I
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[TMPI x 104/moI.I-' Fgue 4. Comparison of the intensity of the exciplex emission upon rapid and slow cooling: 0,rapid cooling; 0 , slow cooling. The time at which the temperature was maintained a little below 186 K, upon slow cooling, is 40 min. The fluorescence intensity was measured at 370 nm.
. E 0
V
m W L
0
a
in micropdycrystallites (Shpol'skii matrix) Phase III
k
h
4 Q
v
h V
1
1
I
5
365 415 465 Wavelength /nm Figure 3. Fluoresence spectra for the naphthalene and naphthalene1,2,5-TMPsystems obtained upon slow cooling (kx= 288 nm): spectra C and C', naphthalene ( 1 X M) in cyclohexane; spectrum D, naphthalene (1 X M) and I,Z,S-TMP (1 X M) in cyclohexane. All spectra were measured at 15 K. The time for maintaining the temperature a little below 186 K is 5 min for spectra C' and D and 40 min for spectrum C. 1
upon rapid cooling (narrow bands). When the time for maintaining the sample solution slightly below 186 K was short (0-20 min), narrow bands could still be weakly detected, as shown in spectrum C'. This may be due to uncontrolled cooling, which yields a mixture of monocrystallites and micropolycrystallites.I8 After maintaining the temperature for 40 min (spectrum C), no narrow band was detected, owing obviously to a thermal transformation (annealing) of the cyclohexane micropolycrystallites. According to previous observations, depending on cooling conditions and temperature, crystallized cyclohexane exhibits three phases:I8 phase I, a disordered crystal phase stable above 186 K; phase 11, a single-crystal phase stable below 186 K; and phase 111, metastable micropolycrystallites formed predominantly upon rapid cooling. Accordingly, the solid phase favored by slow-cooling below 186 K is phase 11. The spectral bandwidth for naphthalene fluorescence in phase I1 (spectrum C in Figure 3) is considerably smaller than that in phase I (see spectrum a in Figure 7) and not so different from that in phase 111 (narrow bands). In addition, this spectrum was very similar to that for the naphthalene crystal measured at 15 K.25 Judging from these facts, we deduce that the semibroad bands originate from segregated naphthalene molecules, which have formed aggregates (possibly "pseudocrystallites" as mentioned in ref 12) or small crystallites, in the cyclohexane rigid matrix. In both cases, excitation of molecules may take place by exciton-assisted energy transfer. The intensity of the semibroad bands upon slow cooling suggests that many pseudocrystallites or small crystallites of naphthalene are formed by phase transition of cyclohexane at 186 K. On the other hand, the presence of semibroad bands in the spectrum upon rapid cooling implies the partial formation of the same type of aggregates (25) Measurement of the fluorescence spectrum for the naphthalene crystal at I5 K was carried out in our laboratory.
e
.-
PI
4
I
V
W
In disordered crystals Phase I
E
.-* 0
(2)
I
* .V
x W
In single crystals
w&
0 V
e W
v1 V
0
L
0
/ I
v
k
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't
I
2 0
300 310 7 0 Wavelength / n m Figure 5. Fluorescence excitation spectra of naphthalene in the rigid cyclohexane matrices: ( I ) in micropolycrystallites (Shpol'skii matrix) formed upon rapid cooling ( X , = 334.64 nm), (2) in phase I (disordered crystals) formed at temperatures above 186 K upon slow cooling ( X ,= 335.90nm), (3) in phase I1 (mainly single crystals) formed at temperatures below 186 K upon slow cooling (Aob = 327.36 nm).
or crystallites in the cyclohexane matrix. Spectrum D in Figure 3 shows a drastic change in the fluorescence spectrum by the addition of 1,2,5-TMP. The intensity of the semibroad bands is considerably reduced (compare with spectrum C' measured under the same conditions) while an exciplex emission with an intensity much larger than that upon rapid cooling appears. Accordingly, the spectral change in Figure 3 supports the above idea that the naphthalene molecules which give rise to semibroad bands belong to the same type of centers as those involved in exciplex formation. In spectrum D, narrow bands are also observed due to the shorter temperature-maintaining time (5 min). The intensities of the exciplex emissions upon rapid and slow cooling are compared in Figure 4 . Upon slow cooling, the intensity at the maximum point was about 5 times as high as that upon rapid cooling. A suitable environment for the creation of
9680
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991
an exciplex between naphthalene and 1,2.5-TMP is thus obviously formed as a consequence of the phase transition of cyclohexane at 186 K . Excitation Spectra at L o w Temperature. To confirm that the naphthalene molecules which are responsible for semibroad fluorescence bands are correlated with those involved in exciplex formation, high-resolution fluorescence excitation spectra for solutions containing only naphthalene and the naphthalene1,2,5-TMP complex were recorded. Figure 5 shows the fluorescence excitation spectra of naphthalene for different types of the cyclohexane crystalline matrices: spectrum 1 corresponds to molecules trapped in micropolycrystallites (Shpol'skii matrix or phase III), spectrum 2 to those in phase I (disordered crystals) formed at temperatures above 186 K, and spectrum 3 to those in phase I1 (mainly single crystals) formed at temperatures below 186 K. The most interesting feature in the excitation spectra of naphthalene is the variation in the shape of the spectrum caused by the phase transition of cyclohexane at 186 K (phase I phase 11). While spectrum 2 in phase I appears simply as a broadened and red-shifted spectrum 1 and can be assigned to molecularly dispersed molecules in an inhomogeneous environment, spectrum 3 in phase IT may not be considered solely as a shifted spectrum of the former. Additional spectral bands particularly near 31 7.7, 3 16.2, 309.9. and 296.5 nm are clearly identified. A characteristic band at 316.2 nm appears particularly strong. A similar broad band in the long-wavelength side of the absorption spectrum, for naphthalene, 2-methylnaphthalene, phenanthrene, and diphenylene-type compounds, has been reported by Rima and Nakhimovsky at high concentration^.^^^"^^^^^^ They assigned these broad absorption bands to small crystallites of solute molecules in the rigid solvent matrix they called "pseudocrystallites". In the case of naphthalene, the first broad absorption bandI4 coincides with the strong band at 316.2 nm in spectrum 3. However, the spectrum of pseudocrystallites does not display a weak band at 317.7 nm. We noticed that both the weak band and the strong band at 3 16.2 nm have been observed in the absorption spectrum of naphthalene crystal.28 That is, the former band (indicated as A. in spectrum 3) and the latter one (Bo) correspond to the pure electronic transition on the A and B axes for the crystal lattice of naphthalene, respectively. Other bands in spectrum 3 also coincide with those in the absorption spectrum of the naphthalene crystal. Accordingly, both semibroad fluorescence bands and broad excitation bands, observed in phase TI, can be assigned to small crystallites of naphthalene molecules. This is supported by the coincidence of the fluorescence spectra in phase I1 with that of naphthalene crystal. The semibroad fluorescence bands may then be assigned to molecules in low-energy defects of crystallite. populated by exciton-assisted energy transfer. The presence of I ,2,5-TMP molecules allows the formation of exciplexes, which traps the excitation energy at the expense of the naphthalene molecules responsible for the semibroad bands, thus explaining their disappearance. Our observation of emission from the charge-transfer exciplex, at high concentrations of naphthalene, can then be correlated with the presence of solute crystallites, which appears as a necessary conditions for energytransfer or charge-transfer interactions in rigid solvent mat rice^.^^,^^ This is further demonstrated by the excitation spectra of the naphthalene-I .2,5-TMP exciplex (Figure 6). formed upon rapid and slo* cooling. Excitation spectrum 5 for the exciplex formed upon slow cooling is quite in agreement with that observed for naphthalene in cyclohexane phase I1 (spectrum 3 in Figure 5 ) . The excitation spectrum of the weak exciplex emission observed from rapidly cooled solutions (spectrum 4)also resembles spectrum 3. These results indicate that the naphthalene-1.2.5-TMP exciplex
-
(26) Nakhimovsky, L. A . Bull. Acad. Sci. USSR, Phys. Ser. (Engl. Trans/.). 1968, 32, 1408. (27) Kleschchev, G. V.; Lyemaev, A. 1.; Mishina. L. A,: Nakhimovsky, L. A . Opt. Spectrosc. (Engl. Trans/.) 1974, 36, 49. (28) Broude, V . L.: Rashba. E. 1.: Sheka. E. F. Spectroscopy ofMolecular Excirons; Springer-Verlag: Berlin, 1985: pp IO and 207. (29) Rima. J.: Lamotte. M.: Joussot-Dubien. J. Chem. Phys 1986, 101.
439.
Matsuzawa et al. I
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300 310 320 Watelength /nm Figure 6 Fluorescence excitation spectra of the naphthalene-I ,2,5-TMP exciplex i n rigid cyclohexane matrices (hobs= 374 00 nm) rapid cooling. ( 5 ) upon slow cooling 290
~
( a ) 190 K-180 K
(b) 170 K-165 K CI
fi
+A-
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Below 165K
315
365 415 465 Wavele n g t h / n m Figure 7. Variation with temperature in the fluorescence spectrum of the naphthalene-] ,2,5-TMP system (hex = 288 nm). Concentration: naphthalene, 1 X M: 1,2,5-TMP,3 X M. Solvent: cyclohexane. is formed predominantly in phase 11, but some crystallites are already present in phase 111. Exciplex formation is induced by the phase transition of cyclohexane at I86 K and is related to the formation of mixed crystallites containing the two solute molecules. It was also confirmed that the fluorescence excitation spectra for the exciplex formed in other rigid n-alkane (C,-C8) matrices are almost the same as those formed in the cyclohexane matrix (phase II), indicating possibly similar processes and requirements for exciplex formation in all alkanes. Low-Temperature Dependence on Exciplex Formation. Figure 7 shows the variation with decreasing temperature in the fluorescence spectrum of the naphthalene-I ,2,5-TMP system in cyclohexane. The fluorescence spectrum a, from 190 to 180 K, indicates no exciplex formation between naphthalene and I .2,5-
Formation of the Naphthalene-TMP Exciplex Rapid Cooling
S l o w Cooling
Figure 8. Models for the formation of the naphthalene-] ,2,5-TMP exciplex in rigid solvent matrices. The meaning of each denotation is indicated in the text. This figure illustrates the formation of a precursor of mixed crystallites, "mixed pseudocrystallites", in the vicinity of a single-crystal phase of the alkane. Mixed crystallites may be formed by combination of some mixed pseudocrystallites.
T M P occurs. In this temperature range, taking into account the delay of sample cooling, cyclohexane may still exist as phase I (disordered crystals). Spectrum b observed below the temperature of the phase transition ( 1 86 K) clearly indicates the occurrence of exciplex formation in phase 11. The observance of a structureless broad band assigned to exciplex formation is accompanied by a reduction in the intensity of the naphthalene fluorescence. This is also apparent in spectrum c a t a lower temperature. It is noteworthy that the fluorescence yield from the aromaticalkane systems extremely decreases just before the Occurrence of exciplex emission. In addition to the fluorescence spectra, we obtained absorption spectra at 5 and -1 0 O C for this system and confirmed that any ground-state charge-transfer complex, which could exhibit a characteristic absorption bands, is detected. Discussion From the experimental results, we may speculate how the naphthalene-I ,2,5-TMP exciplex is formed in rigid alkane matrices. The exciplex in this system seems to be formed in a different way from that for the previously reported pyrene-DMA system.*g Namely, the formation of the naphthalene-l,2,5-TMP exciplex does not result from the excitation of a loosely bound chargetransfer complex, which would be already in a suitable configuration in the ground state. Instead, it seems to require some displacement of the solute and solvent molecules, which appears to be only possible during a phase transition of the solvent. This was clearly shown by experiments in cyclohexane solutions. Although the precise configuration of the site in which exciplex formation occurs may be questioned, it is clear from the results that small crytallites of naphthalene, which are produced or altered by the phase transition, are responsible for the exciplex formation between naphthalene and 1,2,5-TMP. The probability of exciplex formation in a single crystal of solvent is very low because impurities (naphthalene and 1,2,5-TMP in this case) are expelled by zone melting. Hence, we assume the formation of suitable centers, in which naphthalene and 1,2,5-TMP molecules can be present, adjacent to small single crystalline zones of the solvent.
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 9681
On the basis of this idea, a structural model for the formation of the naphthalene-l,2,5-TMP exciplex in a rigid solvent matrix is illustrated in Figure 8. White ellipsoids, black ellipsoids, and zigzag lines represent naphthalene, 1,2,5-TMP, and solvent molecules, respectively. It should be noted that each denotation for naphthalene and 1,2,5-TMP in the models need not necessarily correspond to pairs of molecules but may correspond to a group of solute molecules. Three types of rectangles represent the different micro environments in the crystallized solvent matrix: small ones, micropolycrystallites of solvent; large ones delineated by a solid line, single crystals of solvent; and large ones delineated by a dotted line, the proposed micro environments in which both naphthalene and 1,2,5-TMP molecules are incorporated and the formation of mixed pseudocrystallites or mixed crystallites may occur. In Figure 8, the formation of mixed pseud~crystallites~~ is illustrated because we assume that pseudocrystallites are the precursors of crystallites. Upon rapid cooling (upper model in Figure 8), interactions between naphthalene and 1,2,5-TMP are mostly prevented by individual incorporation of molecules into micropolycrystallites of the solvent. The observed weak emission from the exciplex (e.g., in spectrum B) may be due to partial formation of mixed crystallites of the solutes, derived from mixed pseudocrystallites as shown in the center of the model, in the vicinity of single-crystal phases of the alkane (cyclohexane in the case of spectrum B). Solute molecules can be forced to migrate by zone-melting processes into spaces (proposed microenvironment which corresponds to the grain boundaries) between single crystals of the solvent, producing mixed pseudocrystallites. Furthermore, these mixed pseudocrystallites thus separated from the single-crystal phase of the solvent may be combined with another one to form mixed crystallites from which an exciplex can be formed. The suitable face-to-face structure needed for exciplex formation between naphthalene and 1,2,5-TMP may naturally be favored by the phase transition as well as the formation of naphthalene crystallites. Upon slow cooling, the higher probability for the formation of solvent single crystals by the phase trasnition at 186 K and by annealing of micropolycrystallites, which contain solute molecules in them, induces an increase in the formation of mixed pseudocrystallites, as shown in the lower model in Figure 8. Thus, the model well explains the increase in the intensity of the exciplex emission observed upon slow cooling. The formation of several kinds of exciplexes may be possible depending on the type of segregates14 which could exist in micropolycrystallites. Conclusions The effects of crystallization conditions on the exciplex formation in a rigid matrix were studied for the naphthalene1,2,5-TMP system using a high-resolution fluorescence spectrometer. The experimental results showed that the formation of the naphthalene-] ,2,5-TMP exciplex occurs in peculiar microenvironments, involving both mixed crystallites of the solute molecules and small single crystals of the solvent. It does not Occur in micropolycrystallites which prevail upon fast cooling, as in Shpol'skii matrices. This appears to be common to all alkane matrices studied. Moreover, the suitable microenvironment for exciplex formation appears to be largely favored by the phase transition of solid cyclohexane at 186 K. For this reason, the rate of cooling must be slow to promote a suitable crystallized structure, and this explains why cyclohexane appears to be a good solvent for the observation of exciplex emissions in low-temperature matrices. We are continuing this study to clarify whether similar processes for exciplex formation in rigid solvents at low temperatures can be also observed for other solute species.
Acknowledgment. We thank Dr.A. Wakisaka for his valuable comments. Registry No. TMP, 930-87-0; DMP, 625-84-3; MP, 96-54-8; P, 109-97-7; naphthalene, 91-20-3.
