J . Phys. Chem. 1988, 92, 6537-6540
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Investigation of the Low-Temperature Multiple Fluorescence of 2-Acetylanthracene by Total Luminescence Spectroscopy+ Erich C. Meister, Georg W. Suter, and Urs P. Wild* Physical Chemistry Laboratory, Swiss Federal Institute of Technology, ETH-Zentrum, Uniuersitatstrasse 22, CH-8092 Zurich, Switzerland (Received: January 29, 1988)
The low-temperaturefluorescence of 2-acetylanthracenein rigid solutions was investigated using the method of total luminescence spectroscopy. In methylcyclohexane and ethanol at 77 K the spectra were found to be strongly dependent on the solvent and on the excitation wavelength. Seven different spectral components could be identified. In polycrystalline solutions of methylcyclohexane at 77 K two dominant sites are observed with an unusual large site splitting caused by the two crystal phases of methylcyclohexane. In addition, clear evidence is given for the formation of aggregates or microcrystals during a slow cooling process. The formation of H-bonded complexes of 2-acetylanthracenewith traces of water is strongly supported by measurements in ethanolic low-temperature glasses. Further results suggest that the anomalous low-temperature effect observed with 2-acetylanthraceneand reported by several authors is due to a solvent impurity capable of a strong proton-donor interaction with the solute. The method of total luminescence spectroscopy allows a concise characterization of multiply fluorescing samples.
Introduction The electronic spectra of 2-acetylanthracene (2AA) solutions have been the subject of several investigation^.^^^^^ They show a very pronounced sensitivity to solvent and temperature variations. In a very recent paper, Swayambunathan and Lim' found strongly different absorption and fluorescence spectra of 2AA in methylcyclohexane (MCH) solution when lowering the temperature from 300 to 77 K. Upon cooling of the solution, the room-temperature absorption spectrum disappears and a new spectrum shows up, which is shifted by about 1000 cm-I to the red and dominates the absorption in glassy M C H at 77 K. This behavior was explained by aggregation in the rigid glass, leading to the formation of stable ground-state dimers. The same experimental findings were reported earlier by Veselova et alS2in a MCH/isoprene 4:l mixture. However, they suggested different conformers, namely, s-cis-2AA and s-trans2AA, as the origin of the different spectra at ambient and at low temperatures. Since fluorescence phase-shift measurements gave multiple fluorescence only in a small temperature range around -100 OC and not at room temperature and a t 77 K, it was concluded that the conformational equilibrium is strongly influenced not only by the temperature but also by the low-temperature solvent structure and the free volumes in the liquids.2 Evidence for the observation of two conformers of 2AA is also given in ref 1 based on the results of a supersonic jet spectroscopy study. Excited-state s-cis/s-trans isomerization between two stable rotamers has been observed in the case of the nonpolar compound 2-vinylanthracene. Using low-temperature fluorescence spectroscopy, Cherkasov et ale3could describe the spectra taken at ambient temperature by a superposition of two subspectra, one of them being obtainable in pure form at low temperature. When applying picosecond time-resolved techniques as well as static variable-temperature fluorescence spectroscopy, Barbara and co-workers4 derived the spectra of both conformers directly from the observation of the equilibration process of 2-vinylanthracene and related alkenylanthracenes. In the case of the polar 2AA, however, the interpretations of the spectral effects are contradictory, as mentioned above. Since Tamaki, in a series of paper^,^ reported a strong dependence of the fluorescence of 2AA on the solvent, trivial phenomena such as H bonding to residual water traces or specific interaction of 2AA with a solvent impurity cannot rigorously be excluded. In this paper we present results from an investigation of the low-temperature effect on 2AA in glassy and polycrystalline MCH 'Parts of this work have been presented at the XIIIth International Conference on Photochemistry, Budapest, August 9-14, 1987 and at the 0 - E / LASE'88 Conference on Laser Spectroscopy, Los Angeles, January 10-17, 1988.
and in ethanol by the technique of total luminescence spectroscopy (TLS) at 77 K. This method has proven to be an excellent tool in the investigation of a wide field of excitation-dependent phenomena since it yields the full information of the luminescence intensity over the excitation-emission plane.6-7J4
Experimental Section Absorption spectra were recorded on a Perkin-Elmer Lambda-9 UV-vis-near-IR spectrometer that was equipped with an Oxford DN-704 liquid nitrogen cryostat. One- and two-dimensional luminescence measurements were made using a front-face geometry on a computer-controlled high-resolution spectrometer6v7 consisting of two Spex 1402 double-grating monochromators and an Osram XBO 2500-W xenon arc lamp as the excitation source. The dispersion of the monochromators was 5 A/mm. The detection system included a RCA C3 1034 photomultiplier in the photon-counting mode and digital data processing. Total luminescence spectra were obtained by sequentially recording emission spectra with different excitation frequencies. The spectra are corrected for the characteristics of the excitation subsystem. It should be noted that due to Kasha's rule, emissions from a single molecule that is vibrationally relaxed in its excited state appear in a rectangular grid pattern in the excitation-emission plane (Le., the TLS). Luminescence from a multicomponent system therefore is observed in TLS as a superposition of such patterns, and qualitative identification of the components is easily performed by simple visual pattern recognition. Fluorescence lifetime measurements were performed on a time-correlated single-photon-countingsystem*with a mode-locked Coherent Innova 1-15 argon ion laser and a synchronously pumped cavity-dumped frequency-doubled dye laser. The excitation wavelength was 300 nm. Emission from the sample was analyzed through a Spex 1400 double-grating monochromator that was modified for subtractive dispersion (10 A/mm). The apparatus (1) Swayambunathan, V.; Lim, E. C. Chem. Phys. Letr. 1987,134,255. (2) Veselova, T. V.; Stolbova, 0. V.; Cherkasov, A. S.Bull. Acad. Sci. USSR Phys. Ser. 1983,47, 1349. Veselova, T. V.; Stolbova, 0. V.; Cherkasov, A. S. Opt. Spectrosk. (USSR)1984, 57, 372. (3) Cherkasov, A. S.;Voldaikina, K. G. Bull. Acad. Sci. USSR Phys. Ser. 1963, 27, 630. (4) Brearley, A. M.; Strandjord, A. J. G.; Flom, S. R.; Barbara, P. F. Chem. Phys. Leu. 1985, 113, 43. Flom, S. R.; Nagarajan, V.; Barbara, P. F. J. Phys. Chem. 1986,90,2085. Brearley, A. M.; Flom, S.R.; Nagarajan, V.; Barbara, P. F. J. Phys. Chem. 1986, 90, 2092. ( 5 ) Tamaki, T.Bull. Chem. Soc. Jpn. 1978,51,2817; 1980,53,577; 1982, 55, 1756; 1982, 55, 1761; 1982, 55, 3321. (6) Suter, G. W.; Kallir, A. J.; Wild, U.P. Chimia 1983, 37, 413. (7) Suter, G. W. ETH Thesis No. 7064, Zurich, 1982. (8) Canonica, S.; Wild, U.P. Anal. Instrum. (N.Y.) 1985, 14, 331.
0022-3654/88/2092-6537$01.50/00 1988 American Chemical Society
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Figure 1. Fluorescence and excitation spectra of 2AA in solutions of different quality MCH at room temperature and at 77 K. Excitation was at 25 350 cm-I, emission was recorded at 22000 cm-*,concentration was M. (a) In a room-temperature solution of commercially available spectroscopic grade MCH. (b) In a glassy solution of commercially available spectroscopic grade MCH at 77 K. (c) In a room-temperature solution of purified MCH. (d) In a glassy solution of purified MCH at 77 K. Note that the fluorescence spectra in (a), (c), and (d) are independent of the excitation frequency, indicating a strongly emitting component A. The spectrum in (b) is strongly excitation dependent, showing two main components A and B.
response function of the detection system was 160-ps full width at half-maximum (fwhm) when a Hamamatsu R928 photomultiplier was used. Fluorescence decay deconvolution was performed in Fourier space with the method of Wild et al.' Chemicals. 2-Acetylanthracene (2AA) was synthesized from anthracene following the method described in ref 10 and purified by thin-layer chromatography. Methylcyclohexane (MCH; BDH, Spectrosol for fluorimetry) was either used as supplied or distilled and stored over sodium. Note that within this paper we refer to these solvents by the terms "commercially available" and "purified", respectively. 3-Methylpentane (3MP; Fluka, purum) was filtered through Celite/sulfuric acid and subsequently chromatographed on silica gel and basic aluminum oxide. Spectroscopic grade 96% ethanol (Fluka, UV grade) and absolute ethanol (Fluka, puriss. p,a., H 2 0 content (0.2%) were used without further purification. Glassy samples of M C H solutions were obtained by rapidly immersing 3-mm-i.d. quartz sample tubes into liquid nitrogen (unless otherwise specified). Polycrystalline samples were made by slowly freezing the solutions over about 10 min. Both methods were found to give easily reproducible spectra. Results Spectra in Glassy Methylcyclohexane (MCH). The excitation dependence of the emission of 2AA has been observed by several authors.',2 It is remarkable, however, that the effect has been reported only from experiments where MCH or a mixture containing M C H was used as a solvent. This led to the hypothesis that the effect originates from specific properties of MCH. In fact, the effect could be reproduced only in solutions containing MCH and was not found in 3MP. Therefore, further experiments with different qualities of M C H were carried out, applying different concentrations and sample-preparation procedures. The spectra obtained from 2AA in spectroscopic grade M C H taken without further purification and in purified M C H (cf. Experi( 9 ) Wild, U. P.; Holzwarth, A. R.; Good, H. P.Rev. Sci. Instrum. 1977, 48, 1621. (10) Etienne, A.; Arditti, G.; Chmelewski, A. Bull. SOC.Chim. Fr. 1965, 669.
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EMISSION ICM-11 Figure 2. Total luminescence spectra (TLS) of 2AA in commercially available spectroscopic grade glassy MCH at 77 K (a) and in purified glassy MCH at 77 K (b). Only one component A can be observed in (b), whereas a second component B dominates the emission in (a). Peaks belonging to the same component appear in a rectangular grid pattern in the TLS.
mental Section) are shown in Figures 1 and 2. Obviously, no spectral differences occur between the commercially available and the purified solvents at room temperature (see Figure la,c). Both exhibit anthracene-like structures in excitation (with maxima at 25 000,26 450 and 28 000 cm-') and in emission (with maxima at 24 770, 23 320 and 22 000 cm-l). Upon cooling the solution in commercially available MCH, the room-temperature spectra (labeled A) are reversibly replaced below about 160 K by a new transition B, appearing markedly red-shifted with 0-0 transitions at ijen = 24 100 cm-' and Be, = 23 650 cm-' (see Figure lb). This low-temperature effect is even more clearly shown in the total luminescence spectrum (TLS) in Figure 2a, where the new strong emission B dominates some remaining emission bands A, which themselves coincide with the transition frequencies of 2AA. The bands of the A pattern at 77 K (see Figure 2b) are only slightly red-shifted compared to their positions at room temperature and can be assigned to the emission from monomeric 2AA in a nonpolar environment. The appearance of the second emission B is in full agreement with the measurements of Swayambunathan and Lim' and Veselova et As reported in ref 2, the B absorption could also be observed to reversibly appear and disappear in a mixture of MCH and isopentane in the temperature interval from 130 to 150 K. From fluorescence measurements the intensity ratio of A to B was found to be independent of the solute concentration from t 6 X lo-' M. Therefore, ground-state dimerization between two 2AA molecules, as was proposed in ref 1, can be ruled out as the origin of the B spectrum. In carefully purified MCH (and in 3MP as well), however, 2AA reveals a much simpler spectral behavior (Figure Id) which is completely different from that in the commercially available solvent. The low-temperature induced B emission could no longer be observed in these solvents even at concentrations as high as lo4 M, and the total luminescence spectrum (Figure 2b) consists only of a single component that is identical with the A pattern in Figure 2a, provided that the solutions are frozen sufficiently fast to prevent aggregation (see the next subsection). However,
Fluorescence of 2-Acetylanthracene
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988 6539
Figure 3. Effect of the cooling rate on the total luminescence spectra of 2AA in a 2.2 X IO4 M solution of purified MCH at 77 K. The spectra are normalized to equal intensity. (a) TLS after slow cooling. A simultaneous emission of three components can be. resolved from the polycrystalline sample, namely, two dominant sites A, and A2 of monomeric 2AA occupying the two crystal phases of MCH and a third fluorescence from aggregated or microcrystallinic 2AA (M). (b) TLS obtained after an intermediate cooling rate. The spectrum again shows aggregation M of 2AA within the glassy sample. Component A is characteristicof monomeric 2AA in a nonpolar glassy environment. (c) TLS after shock freezing. The glassy sample emits practically only from monomeric 2AA; aggregation could be avoided by rapid immobilization.
addition of a small amount of commercially available M C H to these solutions again initiated the B emission. It should be noted that we were not able to observe B in mixtures of purified M C H and isopentane, which gave more stable glasses than M C H alone and allowed the use of very different cooling rates. The fluorescence decay curves could be fitted well by two-exponential functions. On the basis of the spectral distribution of the two decay components, the fluorescence lifetimes of A and B are 17.9 f 0.2 and 12.9 f 0.3 ns, respectively. At 77 K no evidence for a reaction or a transition between the emitting states of A and B was found in the time-resolved measurements. From the above findings we conclude that B is the result of a specific low-temperature, ground-state interaction of 2AA with a solvent impurity that is common to M C H from different suppliers. Our attempts to identify this impurity by G C / M S have not been successful. The assignment of the B emission to Hbonded complexes between 2AA and traces of H 2 0 can be ruled out, since even saturating an M C H solution of 2AA with water resulted only in a small shoulder at the red edge of the longest wavelength excitation band at 77 K. On the other hand, addition of a small amount of phenol to a nonpolar solution of 2AA resulted in a red-shifted intense emission at 77 K nearly identical with B (see Conclusions). Spectra in Polycrystalline Methylcyclohexane (MCH). It has long been known that the highly resolved spectra of aromatic compounds in crystalline M C H matrices can be extraordinarily dependent on the rate of cooling" and the temperature of annealing.I2 This effect has been explained in terms of two crystal modifications a and 6 of M C H with different packing densities. The conditions for polycrystalline M C H lead also to an excitation-dependent fluorescence of 2AA at 77 K (Figure 3a), showing two sites A, and A2 a t 24 290 and 24 740 cm-', respectively, whose vibrational spectra were not resolved at 77 K. The bandwidths of the 0 4 transitions are about 210 cm-l. The relative intensities of the site origins were found to depend only slightly on the rate of cooling. However, they change rapidly as a function of temperature around the reversible phase transition in the 113-142 K temperature range.I2 The fluorescence lifetimes at 77 K are equal to 15.0 f 0.2 ns for A, and 15.4 f 0.2 ns for A2 and are significantly different from the value in glassy MCH. Again no evidence for an interaction between the origin levels of the emissions A, and A2 was found in the lifetime measurements. The large site splitting is probably due to a selective stabilization of the two coplanar molecular rotamers of 2AA by the a and p crystal phases of MCH, respectively. This explanation seems to be confirmed by a vibronic analysis of selectively excited highly resolved Shpol'skii fluorescence spectra of 2AA that showed striking differences in the low-frequency modes of A, and A2 sites at liquid-helium t e m p e r a t ~ r e . ' ~ ( 1 1) Khalil, 0. S . ; Goodman, L. J . Chem. Phys. 1976, 65, 4061. (12) Olszowski, A. Chem. Phys. Lerf. 1981, 78, 520.
(13) Meister, E. C.; Suter, G. W.; Wild, U. P., manuscript in preparation.
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EMISSION CCM-I1 Figure 4. TLS of 2AA in spectroscopic grade 96%ethanol at 77 K. The excitation-dependentsubspectrum C is due to complexes of 2AA with water traces in the solvent.
As can be expected from the low solubility in nonpolar solvents, 2AA should upon cooling spontaneously form aggregates or microcrystals from relatively concentrated solutions. Figure 3a shows this process in the TLS of a slowly cooled polycrystalline solution of 2AA in purified MCH (c = 2.2 X lo4 M): In addition to the two sites A, and A2 mentioned above, a third red-shifted huge emission M is observed that has a Stokes shift of 1600 cm-I and a fwhm of 900 cm-'. The same solution, when shock cooled to give a glass with its characteristic A emission, did not give rise to any M-type fluorescence (see Figure 3c). An intermediate case is shown in Figure 3b: Cooling was carried out fast enough to give a glassy sample (as indicated by an A-type emission) but significantly slower than in the experiment that led to the spectrum in Figure 3c. This difference in the cooling rate of the same solution is responsible for the appearance of the broad and unstructured emission M. Furthermore, in a slowly cooled polycrystalline sample of a very dilute (c = 6 X lo-' M) solution only A, and A2 but no M could be observed. In addition, we measured the fluorescence and excitation spectra of solid 2AA and found a close overlap with the bands characteristic of M. From these results we conclude that M is indicative of the formation of aggregates or microcrystals in low-temperature nonpolar solutions. Obviously, the aggregation process is inhibited by the immobilization of the monomeric entities through high freezing rates or at very low solute concentrations. Spectra in Ethanol. As was indicated above, the most simple and evident reason for the appearance of B in commercially available MCH would be the presence of residual water or some other protic component leading to H bonds or to keto-enol tautomerism in the aceto group of 2AA. It is therefore interesting to study the spectra of 2AA in more polar environments. The TLS spectrum of 2AA in 96% ethanol is shown in Figure 4. This spectrum again is strongly excitation dependent. In addition to the dominant emission P of 2AA in a polar solvent
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mmo zym ZlMD tam WAVENWERS KU-13 Figure 5. Influence of water and triethylamine (TEA) to the fluorescence excitation spectrum of 2AA in glassy ethanolic solution at 77 K. The monitoring wavenumber was 21 740 cm-I. The solvents are absolute ethanol (full line), ethanol with 1% H 2 0 (dash-dotted line), and ethanol with 1% TEA (dotted line). The shoulder C is due to the formation of H-bonded 2AA-H20 complexes. Note the opposite effects of H 2 0 and TEA on the spectrum. The excitation spectra are related to a corresponding cut through the T L S in Figure 4 at t,, = 21 740 cm-I. mum
cage, a second fluorescence C of substantial intensity appears upon red-edge excitation at I,, < 24 000 cm-I. This subspectrum C is clearly related to the presence of H 2 0 in the ethanol solution, since in absolute ethanol and in the presence of triethylamine (TEA) (TEA binds residual water; see, e.g., ref 14) C is mostly suppressed, whereas its relative intensity increases upon the addition of water to the solution. The influence of water traces to 2AA is also seen in Figure 5, where excitation spectra in ethanol with varying H 2 0 contents are plotted. As can be verified, the magnitude of the shoulder in the red edge of the longest wavelength excitation band is clearly related to the water concentration in the sample. TEA adds only minor effects to the spectrum in absolute ethanol (
