J . Phys. Chem. 1990, 94, 3931-3937
3931
Spectral Evidence for Different Phases Exhibiting the Shpol’skii Effect in Concentrated Solutions of Pyrene and Dibenzofuran in Frozen +Alkanes C. Amine, L. Nakhimovsky,t F. Morgan,t and M. Lamotte* Photophysique Photochimie MolPculaire, U A CNRS 348, Universite de Bordeaux I , F33405 Talence. France (Received: August 3, 1989)
Concentrated frozen solutions of dibenzofuran in n-heptane and pyrene in n-hexane were investigated by both fluorescence and absorption spectroscopy. At extremely high concentrations or relatively high concentrationsand diminished crystallization rates, the “normal” quasiline spectra are replaced by “new” spectra that exhibit quasiline character and that are not due to the formation of microcrystals of the solute. Intensity distribution changes among the vibronic lines involving non totally symmetric vibrational modes in the first electronic transition of the new spectrum have been interpreted as a result of weak interactions between oriented solute molecules. A model of the centers from which these spectra are originating is proposed. It is suggested that the model may provide an alternative interpretation for the occurrence, only at high solute concentration, of quasiline spectra, in the case of aromatics in “unsuitable” frozen n-alkanes.
1. Introduction It follows that the observation of QL spectra in Shpol’skii solutions leads to a dilemma. If we accept the idea that the solute The Shpol’skii effect (commonly referred to as the occurrence molecules are molecularly dispersed in the host matrix, we must of sharply structured absorption and emission spectra, also called find an explanation for a systematic violation of the isomorphism quasiline (QL) spectra), although not restricted to a certain class condition. Conversely, if we assume that, for the formation of of organic compounds,l is observed most often in frozen n-alkane molecularly dispersed solid solutions, the isomorphism condition solutions of polycyclic aromatic hydrocarbons (PAHs). must be strictly fulfilled, we must envisage that the solute molThe equilibrium solubility limit for PAHs in n-alkane solutions ecules giving rise to QL spectra are, in general, not fully dispersed. M.Z Neverat the solvent freezing point can be as low as In other words, we must question whether the observation of theless, the Shpol’skii effect is observed at concentrations well quasiline spectra is necessarily related to perfectly isolated solute above this limit, Le., in conditions for which solute precipitation is expected to occur to some extent during the crystallization process. Indeed, besides quasiline (QL) spectra, broad band spectra assigned to preaggregate~,~ to molecules trapped in in(1) (a) Shpol’skii, E. V. Sou. Phys.-Usp. (Engl. Transl.) 1962, 5 , 522. (b) Shpol’skii, E. V.; Bolotnikova, T. N. Pure Appl. Chem. 1974, 37, 183. tergrain site^,^,^ and to aggregates or to microcrystallites of the (2) Bolotnikova, T. N.; Gurov, F. I. Opt. Spectrosc. 1970, 28, 94. solute6-8 have been observed. That all these types of solute (3) Rima, J.; Lamotte, M.; Merle, A. M. Nouu. J . Chim. 1981, 5, 605. trapping modes can coexist in a Shpol’skii solution above a certain (4) Shpol’skii, E. V.; Klimova, L. A,; Nersesova, G. N.; Glyadkovskii, V. concentration is now well accepted. It is also widely accepted that I . Opt. Spectrosc. 1968, 26, 25. ( 5 ) Dekkers, J. J.; Hoornweg, G. Ph.; MacLean, C.; Velthorst, N. H. J . the centers responsible for the QL spectra are associated with the Mol. Spectrosc. 1977, 68, 56. formation of nonequilibrium solid solution^.^^^^^^ (6) Pfister, C. Chem. Phys. 1973, 2, 181. According to most authors, the molecules giving rise to QL (7) Nakhimovsky, L. A. Opt. Spectrosc. 1968, 24, 105. spectra are well dispersed and trapped essentially in substitutional (8) Glyadovskii, V. I.; Klimova, L. A,; Nersesova, G. N. O p f .Spectrosc. 1967, 23, 219. Glushkov, Yu. I.; Yavorskii, B. M.; Ganin, V. A. Ibid. 1971, sites in the n-alkane l a t t i ~ e . l - ~Evidence ,~~ for orientation of PAH 31, 43. Klimova, L. A.; Nersesov, G. N. Ibid. 1966, 21, 167. molecules in the host matrix derived from studies of polarized (9) Nakhimovsky, L. A. Bull. Acad. Sci. USSR, Phys. Ser. (Engl. Transl.) absorption and fluorescence spectra of monocrystallized solutions 1968,32, 1408. Ustyugova, L. N.; Nakhimovsky, L. A. J . Appl. Spectrosc. of PAHs in normal alkane^^^^^^^'^ have given strength to the idea (Engl. Transl.) 1968, 9, 1396. (IO) Gurov, F. I.; Nersesova, G. N. Bull. Acad. Sci. USSR, Phys. Ser. that the phases associated with QL spectra are indeed substitu(Engl. Transl.) 1970, 1 1 , 1135. tional solid solutions, but the state of dispersion of the aromatic (11) Pfister, C. Chem. Phys. 1973, 2, 171. component in these solutions is subject to c o n t r o v e r ~ y . ~ ~ (12) (a) Malikhina, N. N.; Shpak, M. T. Opt. Spectrosc. 1963, 14,442. There are many aromatic guest-alkane host couples for which (b) Personov, R. I.; Bykovskaya, L. A. Sou. Phys. Dokl. (Engl. Transl.) 1972, 16, 1972. the Shpol’skii effect is observed only above a certain solute con(13) Lamotte, M.; Merle, A. M.; Joussot-Dubien, J. Chem. Phys. Lett. ~ e n t r a t i o n . ~ J This ~ J ~ fact may indicate, among other possible 1975, 35, 410. Vo Dinh, T.; Wild, U. P.; Lamotte, M.; Merle, A. M. Ibid. interpretations, the requirement of a certain degree of solute 1976, 39, 118. Lamotte, M.; Risemberg, S.; Merle, A. M.; Joussot-Dubien, precipitation. On the basis of this hypothesis, Nakhimovsky et J. J . Chem. Phys. 1978,69, 3639. (14) Zalesskii, I . E.; Sevchenko, A. N.; Nizhnikov, V. V.; Gorbachev, S. al. have proposed a model that assumes that both the QL and the M. Opt. Spectrosc. 1979, 46, 268. Gorbachev, S. M.; Zalesskii, I. E.; Nibroad band spectra, associated with aggregates (pseudocrystals) zhnikov, V. V. Ibid. 1980, 49, 37. of the solute, arise from the same solute precipitation ~ o n e . ~ J ~ (15) Klimova, L. A,; Ogloblina, A. I.; Nersesova, G. N.; Glyadkovskii, V. Comparison of the molecular shapes and the molecular packing I . Opr. Spectrosc. 1973, 35, 488. (16) Red’kin, Yu. R.; Mikhailenko, V. I. Bull. Acad. Sci. USSR, Phys. in the neat crystals of PAHs and n-alkanes shows that the two Ser. (Engl. Transl.) 1970, 34, 1205. components are not fulfilling the necessary conditions established (17) Klimova, L. A.; Nersesova, G. N.; Naumova, T. M.; Ogloblina, A. by Grimm and W01ff’~and developed later by KitaigorodskiiZo I.; Glyadkovskii,V. I. Bull. Acad. Sci. USSR, Phys. Ser. (Engl. Transl.) 1968, for the formation of molecularly dispersed substitutional solid 32, 1471. Klimova, L. A,; Ogloblina, A. I.; Shpol’skii, E. V. Ibid. 1970, 34, 1210. solutions. Even when the Bolotnikov rulez1is satisfied (similar (18) Kleschchev, G. V.; Lyemaev, A. I.; Mishina, L. A.; Nakhimovsky, L. length for the aromatic solute and the n-alkane molecules), the A. Opr. Spectrosc. 1974, 36, 49. Nakhimovskaya, L. A.; Mishina, L. A.; Shpol’skii solutions cannot be considered as isomorphic component Kleshev, G. V . Bull. Acad. Sci. USSR, Phys. Ser. (Engl. Transl.) 1970, 34, systems, even in a broad sense.Ia 1370. Present address: Department of Biochemistry, University of Connecticut Health Center, Farmington, CT 06032. *Visitor from Physics Department, York University, Downsview, Ontario, Canada M3J 1P3.
0022-3654/90/2094-393 1$02.50/0
(19) Grimm, H. G.; Wolff, H. In Handbuch der Physik, Atombau und Chemie Sec III F: Atombau und Kristallchemie; Geiger, H., Scheel, K., Eds.; Springer: Berlin, 1933; Vol. 24, Part 2. (20) Kitaigorodskii, A. I. Mixed Crystals; Springer-Verlag: Berlin, 1984. (21) Bolotnikova, T. N. Opt. Specfrosk. 1959, 7, 44.
0 1990 American Chemical Society
3932 The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 molecules free from mutual interactions. In previous papers,22we reported anomalous intensity distributions in the 4 K QL absorption spectra of concentrated solutions of dibenzofuran in n-pentane and n-heptane and phenanthrene in n-hexane, which were tentatively interpreted as resulting from weak solute-solute interactions between oriented chromophores. Weak solute-solute interactions can induce small effects such as spectral shifts or changes in oscillator strengths. Due to the difficulty in attributing a specific cause to the first effect, the second one appears to be the most convenient for such investigation, and absorption spectroscopy is obviously the most appropriate method. Surprisingly, however, this technique has been much less employed than the luminescent technique for investigating Shpol'skii solutions even though it provides important complementary information since it allows the detection of a larger number of species by including those that do not fluoresce or phosphoresce. In this paper, we present further evidence for the manifestation of solute-solute interactions in the quasiline spectra of dibenzofuran and pyrene. Results are drawn from experiments carried out with a setup built for recording absorption spectra of strongly scattering polycrystallized samples.23 We discuss in more detail the spectral properties of the phase associated with the quasiline spectrum of dibenzofuran in n-heptane, which exhibits an anomalous intensity distribution. We show that a similar phase is formed in the pyrene-n-hexane binary system.24 The results lead us to conclude that, under certain conditions of concentration and cooling rate, nonequilibrium phases, responsible for anomalous vibronic features in quasiline absorption spectra and low fluorescence quantum yield revealing solutesolute interactions, can be formed in Shpol'skii solutions.
11. Experimental Section Low-temperature absorption spectra were recorded with a homemade computerized double-beam ~pectrophotometer.~~ Two monochromators (Jobin-Yvon H R S f/7, dispersion 1.2 nm/mm) situated before and after the sample were used in order to minimize the stray light level and to avoid interference of the weak transmitted light with the sample fluorescence (or phosphorescence). The monochromators were set at the same wavelength and were scanned synchronously. Two out-of-phase rotating mirrors (25 Hz) provided alternate measurements of the intensity of the sample and the reference beams for absorbance calculations. The whole experimental process was controlled by a microcomputer. Fluorescence spectra were recorded with the same instrument after having rotated the sample cell and inserted a mirror in the reference beam for a front-face excitation and a 90' analysis of the emission, as in a regular fluorometer. The cells were circular (0.d. = 2.2 cm) all fused silica calibrated cells from Hellma. In all experiments, the sample cell, containing the solution, once attached to the sample holder was first immersed into liquid nitrogen for fast cooling and then rapidly introduced into the liquid helium cryostat (Meric). Pyrene (Fluka purum) was chromatographied over silica gel and sublimated under vacuum. Dibenzofuran (NDB, 99.99% purity) was used as received. n-Hexane and n-heptane (SDS Spectrosol) were dried and kept on molecular sieves (Merck, 5-10
A). 111. Results I . Absorption and Fluorescence Spectra of Concentrated Frozen Solutions of Dibenzofuran in +Heptane. Figure 1 displays (22) Rima, J.; Nakhimovsky, L. A.; Lamotte, M.; Joussot-Dubien, J. J . Phys. Chem. 1984,88, 4302. Mishina, L. A,; Nakhimovskaya, L. A,; Sviridova, K. A. Bull. Acad. Sei. USSR, Phys. Ser. (Engl. Transl.) 1975, 39, 134. (23) Soulignac, J. C.; Lamotte, M. Appl. Speclrosc. 1987, 41, 1220. (24) Contrary to dibenzofuran in any n-alkanes, the latter is a system in which steric conditions for the formation of molecularly dispersed solid solutions can be fulfilled if one molecule of pyrene substitutes two n-hexane molecules.
Amine et al. DIBENZOFU R A N in n - H e p t a n e
w U
Z Q m E
0
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240
260
280
300
WAVELENGTH (nm) Figure 1. Near-UV absorption spectra of a polycrystallized solution of dibenzofuran in n-heptane obtained with cells of different wall thicknesses. Conditions: C = 1.2 X M; T = 4.2 K; light path length = 0.01 cm. Key: (A) thin-wall cell ( = l mm), normal QL spectrum; (B) thick-wall cell (-2 mm), new spectrum.
two QL absorption spectra of dibenzofuran in n-heptane at 4.2 K. Both spectra have been recorded under the same experimental conditions of concentration (C = 1.2 X IOT2 M), sample thickness and cooling procedure. The only difference is in the cell wall thicknesses, which was about 2 times larger (=2 mm) for the sample corresponding to spectrum 1B than for that of spectrum 1A (51 mm). At the concentration employed and at temperatures close to the crystallization temperature, the sample solution is highly supersaturated and is therefore unstable. Hence, a small change in the crystallization rate (in this case induced by the difference in cell wall thickness) leads to changes in the structure of the solid solution reflected by the differences in the two QL spectra . The spectrum obtained with the thin-wall cell (spectrum A) is similar to the one recorded for more dilute solutions (C Ilo4 M) and can be considered as a "normal" (common) quasiline spectrum of dibenzofuran in n-heptane. It appears essentially as a one-site spectrum. The weak lines emerging from the broad phonon wings may correspond to minor sites or to localized phonon bands.25 Non totally symmetric (b2) vibrational modes are very active in the first A' symmetry26-28absorption transition of the normal QL spectrum of dibenzofuran so that many vibronic quasilines have a much higher intensity than the pure electronic quasiline found at 303.0 f 1 nm in agreement with others meas u r e m e n t ~ . ~ *A- ~similar ~ intensity distribution is reported for (25) Rebane, K. K.; Khisniakov, U. V. Opt. Spectrosc. 1963, 14, 362. (26) Popov, K. R.; Smirnov, L. V.; Gregneva, L. V.; Nakhimovsky, L. A. Opt. Spectrosc. 1971, 31, 363. Grebneva, V. L.; Nakhimovsky, L. A,; Nurmukhametov, R. N.; Popov, K. R.; Smirnov, L. V. Opt. Spectrosc. 1972, 33, 29. (27) Siegel, S.;Judeikis, H. S. J . Phys. Chem. 1966, 70, 2205. (28) Bree, A.; Vilkos, V. V. B.; Zwarich, R. J . Mol. Spectrosc. 1973, 48, 135.
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 3933
The Shpol'skii Effect in Pyrene and Dibenzofuran
II
1
3
DIBENZOFURAN in n- Heptane
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1 2 3 4 5 6 7 9 10
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TABLE I: Frequencies, Symmetries, and Assignment of tbe Most Important Vibronic Lines, Respectively, in tbe Fluorescence and in the First Absorption Transition of Dibenzofuran in n-Heptane at 4.2 K" Aii, cm-I line As, cm-I no. fluorescence IR/Ramanb symbd absorption assgnte
300
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23 1 430 557 620
218 425 556 616
669 77 1 864 1008 1010 1123 1207 1250 1297
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Figure 2. First transition absorption spectrum of dibenzofuran in frozen n-heptane, obtained with cells of different wall thicknesses. Conditions: C = 1.2 X IO-' M; 7' = 4.2 K; light path length = 0.01 cm. Key: (A) thin-wall cell ( = I mm), normal QL spectrum; (B) thick-wall cell (==2 mm), new spectrum. The lines marked with an asterisk are assigned to vibronically induced lines involving b2 vibrational modes (see Table I).
absorption. bBree, A.; Vilkos, V. V. B.; Zwarich, R. J . Mol. Spectrosc. 1973, 48, 124. (Taliani, C.; Bree, A. J. Phys. Chem. 1984, 88, 2351. dAuty, A. R.; Jones, A. C.; Phillips, D. Chem. Phys. Lett. 1984, 112, 529. 'The mode designation is arbitrary. The assignment for fluorescence conforms to literature data (see text).
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the fluorescence excitation spectrum of jet-cooled dibenzof~ran.~' A second absorption band system (IAl IB2) has been located at about 1800 cm-' (X(0,O) = 290 nm) from the first origin.28 This transition is thought to be responsible for the strong vibronic coupling explaining the very high activity of b2 symmetry vibrational modes in the first transition system. The origin line of the second transition cannot be clearly identified in our absorption spectrum in contrast with the third one, which begins with a strong (0,O) line at 250 nm and is also assigned to a 'Al IB2 transition (long axis polarized). The spectrum in Figure 1 B32obtained with the thick-wall cell ("new" spectrum in what follows) differs in several aspects from the normal QL spectrum. A striking feature, in the first transition, is the diminished intensity of most vibronic lines compared to the 0-0 line accompanied by a lowering of the intensity of the origin line of the third transition at 250 nm. Also to be noticed is the increase, in the high energy side, of the absorbance background, which could be assigned to the increase of the light scattering due to change in the sample texture and transparency. A spectrum similar to the "new" QL spectrum can also be obtained from samples with thin cell walls instead of thick ones but at concentrations close to 10-1 M. In these cases, however, the quasilines are slightly broadened probably because of the high concentration of the solute, which introduces large inhomogeneity. A detailed examination of the positions of the quasilines (Figure 2) shows that there is no energy shift between the normal and
-
(29) Danchinov, K. M.; Gastilovitch, E. A.; Rodionov, A. N.; Shigorin, D. N . Russ. J . Phys. Chem. (Engl. Transl.) 1982, 56, 1862. (30) Nurmukhametov, R. N.; Gobov, G. V. Opt. Spectrosc. 1%5,18, 126. (31) Auty, A. R.; Jones, A. C.; Phillips, D. Chem. Phys. Lett. 1984, 112, 529. (32) A spectrum similar to that in Figure 1B but recorded at 77 K was previously reported and discussed in ref 22.
WAVELENGTH InRl
Figure 3. Fluorescence spectrum of a polycrystallized solution of dibenzofuran in n-heptane. Conditions: C = 1.2 X IO-' M; T = 4.2 K. The sample is the same as that whose absorption spectra is shown in Figure 1 (spectrum A).
new QL spectra at the precision of our measurement. The large change in the relative intensities of the (0,O) line and the vibronic lines is particularly noticeable for line 3, which (along with other vibronic quasilines) has been interpreted28~29~33~34 resulting from vibronic coupling induced by a b2 symmetry mode. A similar relative intensity change affects all lines having the same symmetry in going from the normal Q L spectrum to the new one. This circumstance provides an opportunity to make a more precise assignment for the vibronic structure of the first absorption band of dibenzofuran in frozen n-heptane than was previously rep ~ r t e d . ~The ~ . ) results ~ are presented in Table I, where the main fundamental frequencies occurring in the fluorescence spectrum with their respective symmetry assignment drawn from IR/Raman data found in the l i t e r a t ~ r e ~are ~ "also ~ indicated. (33) Bree, A.; Lacey, A. R.; Ross, I. G.; Zwarich, R. Chem. Phys. Lett. 1974, 26, 329. (34) Grebneva, V. L.; Nakhimovsky, L. A.; Sukhina, G. V.; Popov, K. R.; Smirnov, L. V. Opt. Spectrosc. 1974, 37, 882.
3934 The Journal of Physical Chemistry, Vol. 94, No. 10, 1990
TABLE II: Frequencies, Symmetries, and Assignment of the Most Important Vibronic Lines, Respectively, Observed in the Fluorescence and in tbe First Absorption Transition of Pyrene in n-Hexane at 4.2 K"
PYRENE in n-Hexane
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line no. 1 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16 17 18 19 20 21
absorptionc
fluorescence IR/Raman,b 405 408 455 458 502 505 590 592 743 737 81 I 802 840 844 892 1068 1067 1105 1110 1142 1145 1174 1192, 1 I99 1237 1242 1329 1327 1377 1370 1410 1408
1510 1558 1597
1504 1553 1597
A
B
sym
390 450* 499: 574 721: 778 801 840* 1034
399 456' 494* 575 737* 785 814 (c) 1039 1114.
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"The asterisk indicates lines whose intensity is reduced with respect to the (0, 0) line on going from the normal QL spectrum to the new spectrum in absorption. bBree, A.; Kydd, R. A,; Misra, T. N.; Vilkos, V. V. B. Specrrochim. Acta 1971, 27A, 2315. e A , B refer respectively to the normal QL spectrum and to the new spectrum. dunobserved lines probably because of their weakness in the new spectrum B. 'The mode designation is arbitrary. The assignment for fluorescence conforms to literature data (see text).
3LS
350
355
360
365
370
WAVELENGTH (nm)
Figure 4. First transition absorption spectra of pyrene in frozen n-hexane obtained for two concentrations. Conditions: T = 4.2 K; light path length = 0.015 cm. Key: (A) C = 3 X M, normal QL spectrum: (B)C = 6 X M, new spectrum. Spectrum A is a one-site absorption spectrum reconstructed with the help of the site-selected fluorescence excitation spectrum (fluorescence intensity monitored at 384 nm (AX,,,, 0.1 nm). The lines marked with an asterisk are assigned to vibronically induced lines involving b,, vibrational modes (see Table 11). ii:
On the basis of frequency values, intensity behavior, and symmetries, a correspondance between the ground-state vibrational frequencies and the first-excited-state frequencies is also proposed. Interestingly, a consistent symmetry assignment is obtained for the most prominent vibronic lines, confirming that all the lines whose intensity is lowered in the new spectrum involve coupling modes having a b2 symmetry. The line at 287.2 nm (no. 23), which is the first line whose width increases noticeably in going to the high-energy range, can be tentatively identified with the origin 'Al), of the second electronic transition (IB2 A fluorescence spectrum of dibenzofuran obtained with the same concentration as for absorption is shown in Figure 3. The number on each line refers to the absorption line involving the same fundamental mode. This spectrum is similar to previously published The intensity of the resonant (0,O) line in our spectrum is, however, greatly reduced by reabsorption whereas in dilute solutions it dominates the vibronic components. On the low-energy side, it is accompanied by a sharp line that does not correspond to a secondary site but to a localized phonon line emerging from the phonon wing.25 Whatever is the dominant absorption spectrum, the normal or the new QL spectrum, the fluorescence spectrum is unchanged
-
(35) Bree, A.: Vilkos, V. V. B.: Zwarich, R. J . Mol. Spectrosc. 1973, 48, 124. (36) Danchinov, K. M.; Rodionov, A. N.; Shigorin, D. N. Russ. J . Phys. Chem. (Engl. Transl.) 1981, 55, 1442.
although its intensity seems to be lowered in the case where the new absorption spectra are observed (this point is discussed below with pyrene as solute). In both cases, the corresponding fluorescence excitation spectra are also similar and resemble closely the normal QL absorption spectrum. From these results, we are led to conclude that the solute molecules in centers associated with the new absorption spectrum are very weakly fluorescing or even nonfluorescing. The observed fluorescence may then be thought to arise from a small number of normal QL centers present in the frozen solutions that are not totally transformed into the new centers. 2. Absorption and Fluorescence Spectra of Concentrated Frozen Solutions of Pyrene in n-Hexane. Absorption Spectra. Like dibenzofuran in n-heptane, changing the pyrene concentration M to about 6 X in n-hexane from 3 X M or decreasing the crystallization rate when the concentration is below 3 X M leads to a new absorption spectrum with important changes in the intensity distribution between vibronic lines. However, contrary to dibenzofuran, there is a shift of this new spectrum by about 100 cm-' toward the high-energy side with respect to the normal QL spectrum. The normal QL spectrum of pyrene in n-hexane exhibits a complex environmental multiplet structure with at least three resolved component^.^' For an easier comparison with the new spectrum, which has one main component, we have reconstructed the absorption spectrum corresponding to the low-energy component ( A = 372.5 nm) with the help of a one-site fluorescence excitation spectrum (Aob = 384 nm). This reconstructed normal QL spectrum (A) is shown in Figure 4 where it is compared with the new spectrum (B). A one-to-one correspondence between their vibronic lines can be made, confirming that despite their different appearance they do belong to the same chemical species. The first absorption band of pyrene is weak but symmetry allowed with a short-axis polarization (IA, IB3"). Part of its intensity is derived from vibronic coupling with the strong second transition (IA, IB2J responsible for the occurrence of vibronic lines involving b,, modes. Irregularities in the activity of aBmodes have been interpreted by the contribution of a second coupling
-
-
(37) Klimova, L. A . Opt. Spectrosc. 1963, 15, 185: 1963. IS, 344
The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 3935
The Shpol'skii Effect in Pyrene and Dibenzofuran PYRENE
P Y RENE
in n - H e x a n e
6
0. A m 3 7 2 5nm
I 315
380
385
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InmI
Figure 5. Narrow band excited fluorescence spectra of pyrene in frozen n-hexane obtained from the sample whose absorption spectra is displayed in Figure 4 (spectrum B). Conditions: T = 4.2 K; C = 6 X IO-* M. Key: (a) A,, = 372.5 nm, Ah,,, = 0.1 nm, normal QL spectrum; (b) ,,A, = 37 1 .O nm, Ah,,, = 0. I nm, broad band spectrum.
mechanism with the third t r a n ~ i t i o n . ~In~ Table 11, we have summarized the assignment of the main vibronic lines for the fluorescence and the absorption spectra. As with dibenzofuran, we observe that all the non totally symmetric lines, which involve bl coupling modes, have their intensity decreased compared to t i e totally symmetric ones in going from the normal spectrum to the new one. The effect appears, however, less pronounced for lines close to the (0,O) line (lines 2, 3, or 5) than for lines separated by more than 1000 cm-' from the origin (lines 10 and 21). This is probably due to a weaker coupling (because of a larger energy gap) between the second transition and the non totally symmetric vibronic transitions that are close in energy to the first pure electronic transition. It could be argued that an intensity distribution similar to that in the new QL spectra will be observed if there is texture in the sample, Le., preferential orientation of solvent crystallites induced by the cell walls or temperature gradient. If in a textured sample the solute molecules are so positioned that the transition moment of B, transitions for dibenzofuran in n-heptane and BzUtransitions for pyrene in n-hexane are oriented close to the direction of the propagation of light, the corresponding transition will not be excited with noticeable intensity. This would lead to anomalous intensity distribution in the absorption spectrum. To ascertain whether texture contributes to the intensity effects observed in the systems under study, the absorption spectrum of one of the samples exhibiting anomalous intensity distribution was measured in three mutually perpendicular directions. A sample of 10-2 M pyrene in n-hexane, 1.5 mm thick and 25 mm in diameter, was kept above liquid nitrogen in a vertical position at a temperature a few degrees below the melting point of the solvent until crystallization was completed. The sample was then gradually immersed into liquid nitrogen. The absorption spectrum of this sample, measured with light incident normal to the cell walls, was found to display an anomalous intensity distribution. Two rectangular ingots were then cut from the original sample so that the light propagation direction for absorption measurements was parallel to the cell walls of the original sample. For one of them, the measurement was performed in the direction of the temperature gradient and, in the other, in the direction perpendicular to it. The absorption spectra of the two rectangular ingots were found identical with the spectrum measured with light incident perpendicular to the cell walls. This experiment gives clear evidence t h a t the observed anomalies in the intensity distribution of the new spectrum are not due to texture in the sample. Fluorescence Spectra. Narrow band selective excitation in the origin line of the new spectrum (A,, = 371.0 nm) gives rise to a very weak fluorescence spectrum with a broad band structure as shown in Figure 5 (a). In the same figure is shown the QL fluorescence spectrum (b) obtained by selectively exciting the first (38) Bree, A.; Leyderman, A.; Salvi, P. R.; Taliani, C. Chem. Phys. 1986, 110, 211.
I
h
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310
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Figure 6. (a and b) Fluorescence excitation spectra obtained from the sample whose absorption spectrum is displayed in Figure 4 (spectrum B) and from which the two fluorescence spectra displayed in Figure 5 have been recorded: Conditions: T = 4.2 K C = 6 X lo-* M. Key: (a) A,b = 385.3 nm,AAh = 0.2 nm; (b) ,A = 455.0 nm, AAh = 1.0 nm. (c-e) Fluorescence spectrum obtained from the same sample by selective excitation in a vibronic line of the new spectrum a t 360.5 nm, AA = 0.15 nm. Key: (c) the broad band fluorescence spectrum in the origin range; (d and e) effect of the temperature on the excimer-like fluorescence band occurring in the long-wavelength side of the broad band fluorescence spectrum c, a t (d) T = 4.2 K and (e) T = 77 K.
multiplet component in the origin of the normal spectrum (A,, = 372.5 nm). The entire weak broad band fluorescence spectrum is red-shifted to about 70 cm-' with respect to the normal QL spectrum. This result is unexpected if we consider the lOO-cm-' blue shift observed for the new absorption spectrum. In Figure 6, the broad band fluorescence spectrum and its fluorescence excitation spectrum are reproduced (spectra c and a, respectively). Although the latter appears at the same place and with about the same vibronic structure as the new absorption spectrum, the lines look substantially broader. In fact, examination of the line profiles in spectrum B of Figure 4 makes it evident that the sharp lines of the new absorption spectrum are superimposed with broader lines. The corresponding excitation spectrum (Figure 6a) also seems to be a superposition of two spectra: the broader lines and extremely weak sharp lines. From this observation, it can be concluded that the sharpest absorption component that corresponds to the new phase of interest here should be assigned to very weakly fluorescing centers. The energy difference between the (0,O) lines in absorption and in emission suggests that the broad band features belong to associations in which the excitation energy is partly delocalized, the emission occurring from the lowest energy levels, as commonly observed in crystal^.^^^^^ The presence of collective centers is corroborated by the observation of a structureless emission (with a maximum at 455 nm) whose intensity increases upon increasing the temperature (Figure 6d,e), two features reminiscent of the excimer emission observed from the pyrene crystaL4' Two important differences, however, suggest that these associations have a different structure than that of a neat pyrene crystal. Firstly, = 371 nm) is shifted to the the (0,O) absorption band (A, high-energy side from the first absorption band in the pyrene crystal (A, = 375 nm41q4*). Secondly, the maximum of the structureless band is also blue-shifted with respect to that of the excimer (observed at around 470 nm in the pyrene crystaI4l4). The excitation fluorescence spectrum monitored at 455 nm, shown in Figure 6b, differs from the fluorescence excitation spectrum recorded by monitoring t h e broad band fluorescence intensity at (39) Prikhot'ko, A. F.; Fugol, I. Y. Opt. Spectrosc. 1958, 5 , 582. (40) Hochstrasser, R. M. Radiat. Res. 1963, 20, 107. (41) Ferguson, I. J . Chem. Phys. 1958, 28, 7 6 5 . (42) Chu, N. Y. C.; Kawaoka, K.; Kearns, D. R. J . Chem. Phys. 1971,55, 3059. (43) Klimova, L.A,; Nersesova, G.N.; Ogloblina, A. I.; Glyadkovskii, V. I. Bull. Acad. Sci. USSR, Phys. Ser. (Engl. Transl.) 1975, 39, 130. (44) Birks, J. B.; Kazzaz, A. A,; King, T. A. Proc. R . SOC.London 1966, A291, 556.
3936 The Journal of Physical Chemistry, Vol. 94, No. 10, 1990 PYRENE
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Figure 7. Variation with the concentration of the QL fluorescence intensity of pyrene in n-hexane at 77 K. The measurements were performed for two vibronic lines at 384 and 379 nm, respectively. The depletion of the fluorescence intensity in ranges B and C reflects the formation of the nonfluorescent phase I1 at the expense of phase I, which is the dominant phase in range A.
385.3 nm and appears to consist of only the boader lines. Aggregate formation is a common phenomenon observed in concentrated Shpol’skii solutions. Structured emissions arising from aggregates of pyrene formed in slowly crystallized solutions in n-pentane and n-decane have been reported by Klimova et The emissions were interpreted as arising from local states formed in the vicinity of defects. In both solvents, the origins (measured respectively at 377 and 383 nm) were found at a lower energy than the origin of the crystal absorption as expected for defectinduced emissions in a solid. Contrary to this, the new quasiline absorption spectrum is situated at higher energy than the crystal absorption, and as a consequence, it cannot be ascribed to the aggregates described by Klimova et al.43 in the case of pyrene. Only the weak broad band observed in the low-energy side of the spectrum in Figure 6b could be assigned to such aggregates. To summarize, the relatively weak broad component of the absorption spectrum of pyrene in n-hexane at concentrations above 3 X IO-* M (Figure 4B) gives rise to the weak broad band defect in aggregates type emission and to the structureless excimeric type emission, while the sharp features, which constitute the new QL spectrum (dominating in absorption), have an extremely weak fluorescent counterpart. Concentration Dependence of the Fluorescence Intensity. The variation of the pyrene Q L fluorescence intensity with concenM is illustrated in Figure 7. The meatration up to 5 X surements were performed at 77 K for two vibronic bands at 379 and 384 nm, respectively, using benzo[a]pyrene as an internal standard. Because of the much larger fluorescence quantum yield associated with the molecules responsible for the normal quasiline spectra, this experiment gives access to the variation, with the pyrene concentration, of the population of these centers. As expected, a linear dependence of the QL fluorescence intensity with concentration is found up to a concentration of about IO-* M (range A). Starting around this concentration, the slope of the intensity variation decreases drastically. The variation reaches a maximum at a concentration around 3 X M (range B). Interestingly, this corresponds exactly to the concentration range for which the QL absorption spectrum is replaced by the new QL absorption spectra. At higher concentration (range C), the fluorescence intensity does not increase anymore and even decreases slightly, due most probably to such a large value of the absorbance (Abs = 0.5 for c=5X M and 1 = 0.02 cm) that the nonlinearity domain is reached. The formation of the absorbing centers associated with the broader lines is not reflected distinctively in the concentration variation of the QL fluorescence intensity. Either they may be formed simultaneously with the new phase or they may be present
Figure 8. Postulated models for the arrangement of dibenzofuran and of pyrene molecules in centers fitted in the n-alkane lattice and respon-
sible for the new spectra. at concentration lower than M in an almost constant ratio relative to the centers responsible for the QL spectra. We have now some experimental evidence that even at a lower concentration not all the solute molecules are trapped in Q L centers and that some of them are included in precondensation zones giving rise to broad molecular ~ p e c t r a . ~These , ~ ~ centers, which we call preaggregates, may be responsible for the background noticed at the bottom of the quasilines in figure 4A. They may be responsible also for the broad band component seen in the fluorescence and in the excitation spectra in Figure 6 (spectra a and c). They can be ascribed to intermediate entities preceding the formation of aggregates or microcrystals of the solute.
IV. A Possible Model of Centers Associated with the “New” Quasiline Spectra A tentative description of the centers responsible for the new spectra can be made if we can envisage what type of molecular arrangement is consistent with the above observations. An important point to consider is the change in the relative intensity of vibronic lines (and electronic bands) having a symmetry different from that of the pure electronic band of the first transition. This intensity effect cannot be accounted for by a simple matrix effect but rather requires a more severe perturbation to occur. It may manifest itself each time identical chromophoric units are close enough to interact mutually. Indeed, it is commonly observed in ordered assemblies of molecules such as polymers4649 or crystal^.^^^^^ The spectra arising from localized excitation in pyrene aggregates were also found to exhibit this kind of feature.43 Accordingly, the intensity distribution in the new quasiline absorption spectra described above seems to be best interpreted by the existence of some solute-solute interactions within segregated centers whose structure however must be quite different from that of broad band aggregates or crystallites. Interaction between oriented chromophores could affect both the energy and the intensity of the electronic transitions. Hyperchromic or hypochromic effects were discussed in the first-order approximation of the perturbation theory by T i n o ~ c oand ~~ Rhodes!’ From their results, it is predicted that the first electronic transition of a given symmetry will be hypochromed if the interacting transition dipoles are parallel, while it will be hyper(45) Hofstraat, J. W.; Freriks, I. L.; de Vreeze, M. E. J.; Gooijer, C.; Velhorst, N. H. J . Phys. Chem. 1989, 93, 184. (46) Tinoco, I., Jr. J . Am. Chem. SOC.1960, 82, 4785. Tinoco, I., Jr. J . Chem. Phys. 1960, 33, 1332; 1961, 34, 1067. (47) Rhodes, W. J . Am. Chem. SOC.1961.83, 3609. (48) McRae, E. G.; Kasha, M. J . Chem. Phys. 1958, 28, 721. (49) Rich, A,; Kasha, M. J . Am. Chem. SOC.1960, 82, 6197. (50) Kasha, M. Radiat. Res. 1963, 20, 5 5 . ( 5 1 ) McClure, D. S. Solid State Phys.; Advances in Research and Applications; Academic Press: New York, 1958; Part I.
J . Phys. Chem. 1990, 94, 3931-3944
3937
in the host l a t t i ~ e . ~ Increasing - ~ , ~ ~ the concentration is thought to increase the probability of incorporating the solute molecules in the lattice, thus favoring the Occurrence of the Q L spectra.15J6 As an alternative, we propose that the so-called incorporated molecules belong to centers whose structure is close to the model described above. At low concentration, we suggest that the incorporated molecules that give rise to broad molecular band spectra are trapped in precondensation zones or in intergrain sites that have a weak crystalline character and in which the mean solute-solute distance increases gradually with increasing concentration. The centers at low concentrations may correspond to the preaggregates described in refs 3 and 45. According to our model, in the case of an inappropriate matrix (r-type system) the formation of one-dimensional aggregates as described above is expected to be favored at the expense of trapped isolated solute molecules. In such centers, the QL absorption spectra may manifest some hyper- or hypochromic effect. In the spectrum of dibenzofuran in n-octane reported in ref 54 (Figure 2.14c), the large intensity of the 0-0 line as compared to the vibronic lines is most probably relevant of this case. Unfortunately, up to now, only a few accurate absorption experiments have been performed on these system^,*^*^^ and further investigations are required. An important argument against the formation of ordered segregate centers giving rise to QL spectra is the absence of energy transfer from the QL centers when an acceptor is added to the solution.53 In effect, with dibenzofuran, in the conditions for observation of the new spectra, we did not obtain any evidence for energy transfer to anthracene or phenanthrene when one of these acceptors was added at very low concentration as well as M. This result may be attributed at concentration up to tentatively to a high selectivity of the process of formation of these centers, which leads exclusively to the grouping of molecules having strictly the same shape. Registry No. Dibenzofuran, 132-64-9;pyrene, 129-00-0.
chromed if they are oriented head-to-tail. Applying these results and taking into account the known polarization of the pure electronic transition (short-axis polarized) and that of the lines induced by vibronic coupling (long-axis polarized), the intensity distribution in the new spectra is readily interpreted by the arrangements proposed in Figure 8 for both pyrene and dibenzofuran. According to this mode1,7J8-22the solute molecules are arranged side by side in one-dimensional assemblies that are fitted in the n-alkane lattice by substituting a finite number of the host molecules for a minimum disturbance of the lattice. This type of arrangement has been envisaged in the so-called organic alloys involving the syncrystallization of two similar types of molecules such as aliphatic fatty acids of different lengths.52 The model has the merit of offering both weak oriented interactions between solute molecules and a structure different from that of the neat crystal. The hypochromism, which affects the non totally symmetric vibronic lines, in the first transition may result from the decrease in the oscillator strength of higher electronic transition having the same polarizations as the vibronic lines. Thus, in the case of dibenzofran in n-heptane or of pyrene in n-hexane, we have observed two different nonequilibrium phases that give rise to quasiline spectra. Phase I is associated with the common QL spectra, while phase I1 gives rise to new spectra that exhibit an intensity distribution change characteristic of segregated structures. The fact that the respective spectra are observed in the same wavelength range leads to the assumption that the surroundings of the solute molecules in phase I and in phase I1 do not differ substantially. This may imply that, (as it has previously been s ~ g g e s t e d in ~ ~phase ~ ~ ) I, some segregation of the solute molecules may have already taken place (even at low concentration). The model proposed here could be of interest for interpreting the occurrence of QL spectra in the puzzling case of inappropriate matrices. In those systems, which do not satisfy Bolotnikova's rule,2' broad band spectra are observed at low concentration while QL spectra occur only at high concentration. It has been proposed that at low concentration the solute molecules are not incorporated
(53) Klimova, L. A.; Nersesova, G. N.; Naumova, T. M.; Ogloblina, A. I.; Glyadkovskii, V. I. Bull. Acad. Sci. USSR,Phys. Ser. (Engl. Trans.) 1968, 32, 1361. (54) Nakhimovsky, L. A. Proceedings of the 8th International Symposium on Polynuclear Aromatic Hydrocarbons; Battelle Press: Columbus, OH, 1985; p 937.
(52) Chanh, N . Ba.; Haget-Bouillaud, Y.; Bedoin, J. Bull. SOC. Fr. MinOral. Cristallogr. 1912, 95, 281.
Dynamical Stereochemistry of Elementary Reactions in Solution I. Benjamin,*?+Antonio Liu, Kent R. Wilson,* Department of Chemistry 8 - 0 3 9 , University of California, San Diego, La Jolla, California 92093-0339
and R. D. Levine* The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, Jerusalem 91 904, Israel (Received: August 14, 1989)
+
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Orientational steric effects in four A BC AB + C reactions in the gas phase and in solution are studied by molecular dynamics simulations. We investigate the role of the torque due to the reactant-reactant potential and that due to solvent-reactant interactions. The time scales for the development of the rotational energy and of the orientation needed for reaction are computed, the latter in terms of a BC angular correlation function. The results vary considerably from one type of reaction to another. We find that the gas and solution results are surprisingly similar and that analyses and models familiar from the gas phase, such as the cone of acceptance, can also be useful in solution.
I. Introduction There has been considerable progress in our understanding of the steric requirements of elementary chemical reactions in the gas phase.',2 The purpose of our present computational study
'
Permanent address: Department of Chemistry, University of California, Santa Cruz, CA 95064.
0022-3654/90/2094-3937$02.50/0
is to see whether some similar enlightenment is possible with respect to the molecular dynamics Of stereose~ectivityfor reactions (1) Dynamical Stereochemistry Issue.
5365-5516.
J . Phys. Chem. 1987, 91,
( 2 ) Orientation and Polarization Effects in Reactive Collisions. J . Chem. SOC.,Faraday Trans., in press.
0 1990 American Chemical Society
