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Low temperature emission spectra of trapped tetracene pairs from ditetracene
Volume 17 1, number 56
CHEMICAL PHYSICS LETTERS
17 August 1990
Low temperature emission spectra of trapped tetracene pairs from ditetracene Mark A. Iannone ’ and Gary W. Scott Department of Chemistry, University of California at Riverside, Riverside?CA 92521, USA Received 24 May 1990
Interacting pairs of tetracene molecules are prepared in a PMMA host matrix by photodecomposition of ditetracene at 15 K. Excitation and emission spectra of these pairs are red-shifted and broadened relative to those of isolated tetracene molecules under the same conditions. No differences are seen in the spectra of pairs formed fram synditetracene and from antiditetracene. The results suggest that these pairs of tetracene molecules are not in the parallel (sandwich) configuration and that a distribution of intermolecular distances and orientations is obtained. A large geometry change in the excited state, akin to excimer formation, is postulated to explain the fluorescence spectrum.
1. Intruductlon When a degassed, concentrated solution of tetracene is irradiated into the SltSo tetracene absorption band, dimerization may occur across the 6,l l5’,12’ or 6,l l-6/,11’ positions. This bridging produces one of two isomers of ditetracene, anti and syn respectively, each of which have two naphthalene and two benzene chromophores. The structures of the two isomers, anti and syn, are shown in fig. 1. Isolation of one of these dimers was first reported by Wei and ’ Present address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA.
0
Fig. I. (a) Synditetracene.
b (b) Antiditetracene.
Livingston [ 11, and the infrared spectrum of this product was later studied in detail [ 21. Lapouyade and co-workers [ 3 ] isolated both of the products of tetracene dimerization, and the previously studied insoluble product was identified as anti-ditetracene by X-ray crystallography [ 41. The second, more soluble product was assigned as syn-ditetracene. An earlier report of the respective dimerizations of anthracene, tetracene, and pentacene in dilute solutions [ 51 was incorrect, as the observed products were apparently due to decomposition of the molecules by the unfiltered light from a mercury lamp. In a solid matrix, photodecomposition of dianthracene (AZ) produces two anthracene molecules which are presumed held in a face-to-face “sandwich” configuration, resulting in a structureless, extimer-like fluorescence, red-shifted by x 4500 cm-’ with respect to monomer fluorescence [ 6 ] I The behavior of these sandwich pairs and their tendency to undergo photodimerization to reform dianthracene has been extensively investigated, especially by Ferguson and co-workers [ 6-81. In particular, in dianthracene crystals, dimer formation was found to have a thermal activation energy of 7 kJ/mol [7]. Upon cooling solutions of anthracene, spectral changes were observed that were attributed to formation of a “stable dime? in which the long axes of the two molecules are parallel, but the short axes form
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an angle of 55” [ 93. This “stable dime? absorption and its fluorescence are red-shifted from the monomer spectra. Similar spectral results were obtained when a matrix containing anthracene sandwich pairs was softened in a controlled manner. Liquid solutions of anthracene were observed at 77 K and higher temperatures, but no excimer emission [ 61, only dimer emission, was seen. For the anthracene pair, the 55 o dimer configuration has been reported to be the most stable configuration, both for the ground and excited states [ 6 1. Pairs or larger aggregates of tetracene have been reported to form upon cooling solutions of tetracene in various solvents [ 10,111. There are several reports of emission attributed to excimer formation in pure [ 12-141 and mixed [ 151 crystals of tetracene, but there is no report of tetracene excimer formation in solution. We know of no previous study of the properties of the tetracene pairs such as are formed by decomposition of Tz in a solid matrix. The present paper describes the emission spectra of such pairs.
2. Experimental 2.1. Sample preparation Ditetracene was synthesized by the method of ref. 3. A suspension of 500 mg of tetracene (Aldrich Chemical Co. ) in 80 ml of dry benzene was degassed and then irradiated for 36 h with the filtered light (A> 345 nm ) from a 200 W high pressure mercury lamp. The resulting reaction mixture was filtered to separate the precipitate. The recovered solid anti-T, (443 mg) was washed first with benzene, and then with methanol. The filtrate was irradiated in the presence of oxygen to oxidize any residual tetracene, then evaporated to dryness. The residue of syn-T, was purified by passing it through a column of silica gel, eluting with a 1: 1 mixture of benzene and petroleum ether. The products were identified as the indicated isomers of Tz based on previous reports [ 3,4] their absorption and fluorescence spectra, and their mass spectra. The insoluble product, pure anti-T2, had a O-O absorption maximum at 325.4 nm at 1.5 K in PMMA. The soluble product was at least 90% synT2, perhaps with an impurity of anti-T2, as deter570
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mined by UV spectroscopy (O-O absorption maximum at 326.8 nm at 1.5 Kin PMMA with a shoulder at 325 nm). For both products, the mass spectrum showed only one major peak at 228 amu, suggesting that under the conditions of desorption used (both electron impact and fast atom bombardment desorption were tried) the dimers quantitatively cleave to tetracene. Most spectroscopic experiments were carried out on x0.4 mm thick films of PMMA containing approximately 0.01 M of one of the ditetracenes. The purification of the PMMA has been previously described [ 16 1, The films of syn-T, in PMMA were case from CHzClz solution at room temperature. Due to the lower solubility of anti-T2, it was necessary to cast films containing anti-T, at 60°C from 1,2-dichloroethane to avoid its precipitation from the film. 2.2. Methods Fluorescence and phosphorescence spectra were obtained using a SPEX Fluorolog 212 spectrofluorimeter, equipped with a 450 W Xe lamp and double monochromators. The spectrofluorimeter was calibrated using a standard lamp in order to obtain intensity corrections at all wavelengths. For spectra at temperatures of 12 K and above, the film samples were sandwiched between sapphire disks in the sample holder of a Displex 202E closed-cycle refrigerator (Air Products). The film samples were degassed in vacuum at room temperature for 12 h before cooling. Spectra were also obtained at 4.2 K by immersing degassed samples in liquid helium. Photolysis of ditetracene to produce tetracene pairs prior to taking pair spectra was accomplished by irradiating the cold films with the output of a 450 W xenon lamp dispersed through the SPEX monochromator which was set at 3 10 nm with 8 mm slits ( z 15 nm bandwidth). A Schott WG-305 filter was placed in the excitation beam.
3. Results 3.1, Ditetracene fluorescence and phosphorescence The low-temperature emission spectra of both the ditetracenes exhibit fluorescence (from 325 to 360
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nm) and phosphorescence (from 470 to 600 nm). These spectra are shown in figs. 2a (syn-T2) and 2b (anti-T, ) . 3.2. Tetracene pair fluorescence When ditetracene was photolyzed at low temperature in PMMA, the fluorescence of the product was broadened and red-shifted with respect to the fluorescence of an isolated tetracene molecule. Fig. 3 shows the fluorescence spectra of photolyzed syn-T, (solid line) and photolyzed anti-T, (dashed line) following a 2 h photolysis of each sample at 15 K. These plotted spectra were obtained by subtracting
a
h 16000
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Fig. 3. Fluorescence resulting from 420 nm excitation of photolyzed syn-T, (solid line) and anti-T, (dashed line) samples after a 2 h photolysis at 3 10 nm. In each instance, the emission of a fresh sample of the ditetracene was subtracted from the emission of the photolyzed sample. Spectral bandwidths: excitation, 1.8 nm: emission 3.6 nm.
r x20
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Fig. 2. (a) Fluorescence and phosphorescence of syn-Tz at 12 K in PMMA. (b) Fluorescence and phosphorescence of anti-T* at 12 K in PMMA. In both spectra the phosphorescence regions are shown with the intensities multiplied by 20. Excitation was at 310 nm, while excitation and emission bandwidths were 1.8 nm.
the emission observed with 420 nm excitation before photolysis from that obtained after photolysis for each of the two ditetracenes. The solid lines in figs. 4a and 4b represent the excitation spectra of emission at 650 nm ( ~7 nm bandwidth) from T*T at 15 K derived by photolysis of anti-T, and syn-T,, respectively. (650 nm was chosen because of the large ratio of product to tetracene monomer emission at this wavelength. In this paper, we will use T.T to indicate a pair of tetracene molecules in proximity with no covalent bonds between them.) Excitation spectra of emission at 475 nm ( 1.8 nm bandwidth) were also obtained for both samples, but are not shown. After the above experiment, the photolyzed samples of syn- and anti-T2 were allowed to warm to room temperature overnight. Then, they were again cooled to 15 K and the excitation spectra of the emission at 5 10 nm were obtained. These excitation spectra are the dashed curves also shown in figs. 4a and 4b. These spectra are quite similar with respect to structure and peak positions to the excitation spectra of 475 nm emission obtained before cycling to room temperature. Emission spectra for 420 nm excitation of photolyzed syn- and anti-T, samples before and after cycling are given in figs. 5a and 5b.
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Fig. 4. (a) Excitation spectra of a photolyzed anti-T* in PMMA sample at I5 K. (b ) Excitation spectra of a photolyzed syn-T, in PMMA sample at I5 K. Solid lines: excitation of emission at 650 nm. Spectral bandwidths: excitation, 1.8 nm; emission 7.2 nm. Dashed lines: excitation of emission at 5 10 nm after cycling to room temperature. Spectral bandwidths: excitation, 1.8 nm; emission,1.8 nm. 4. Discussion
When ditetracene is photolyzed in solid solution at low temperature, the product tetracene molecules cannot move arbitrarily far apart since they are trapped by the matrix. The interaction between the two molecules affects the eigenstates of the system, giving rise to a broadened, red-shifted absorption and emission. The transition dipoles of the states resulting from two interacting molecules are given by the vector sum and difference of the two transition moments of the individual molecules. The magnitude of the interaction between the two molecules has often 572
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Wavelength/nm Fig. 5. (a) Comparison of the emission from a photolyzed sample of anti-T2 in PMMA at I5 K before and after cycling to room temperature. (b) Comparison of the emission from a photolyzed sample of syn-T1 in PMMA at I5 K before and after cycling to room temperature. Solid lines: freshly photolyzed sample. Dashed lines: same sample after cycling to room temperature. Excitation was at 420 nm. Spectral bandwidths: excitation, 1.8 nm: emis sion, 3.6 nm.
been approximated by a point-dipole-point-dipole model [ 8,9]. In this model, the interaction energy for a pair is of the order of 1p 1*/r3, in which p is the molecular transition moment and r is the distance between the chromophores. In the case of tetracene, the lowest-energy allowed . . . transition IS ‘BZUc‘A, (short-axis polarized), and has an oscillator strength of 0.44 [ 17 1, The red-shift observed in the T.T fluorescence excitation spectrum with respect to that of the monomer indicates that the short axes of the molecules in the pair are
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not parallel, since, for that case, a blue-shift of absorption would be expected. Transitions to the lower energy, out-of-phase dimer state in that case would not be allowed. In the case of chloroanthracene [ 81, a red-shift was observed for the presumed sandwich (parallel) dimer. This observation was attributed to an overall red-shift due to a dispersion interaction stronger than the pair splitting. However, this possibility is unlikely in the case of T-T since there is evidence that the intermolecular distance is larger in this case (see below). The observed red-shift of 250 cm-’ for the fluorescence excitation spectrum of T-T compared to the monomer (fig. 4 ) suggests that a splitting of around 500 cm-’ occurs in this system. A splitting of this magnitude would be consistent with a pair separation of x 8 A assuming a dipole-dipole interaction with the oscillator strength given above. No splitting is apparent in the excitation spectra, figs. 4a and 4b. These spectra are highly broadened due to the inhomogeneous environment. In particular, the very long tail at the red edge of the excitation spectra suggests a wide range of T*T distances in the pairs, e.g. from about 4 to 8 A, with the larger distances being most probable. In addition, the high-energy component would be difficult to observe if it has a smaller transition dipole than the lower-energy component. This would occur for an angle of around 60” between the short axes of the molecules as observed in the case of anthracene pairs under certain conditions I&91. Before photolysis of samples of either the ditetracenes, fluorescence was obtained upon excitation with 420 nm light. This emission is essentially identical with reported tetracene emission spectra [ 181 in solution and is therefore assigned to free tetracene present as an impurity in the samples, Additional emission observed after photolysis may be ascribed to a T.T trapped pair (fig. 3 ), whereas after cycling to room temperature, the T-T emission disappears and is replaced by emission nearly identical to that of isolated tetracene (fig. 5). It may be concluded that mobility of the tetracene solute molecules in PMMA at room temperature allows the T-T pairs to separate sufficiently to greatly weaken the interaction between them. The Stokes shift of the O-Opeak of fluorescence of T*T compared to the O-O of the excitation is z 3000
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cm-i. This appears to be inconsistent with a picture of rigidly trapped molecules. Relaxation to another, lower-energy state is unlikely since the fluorescence is strong and thus arises from an allowed state. The large Stokes shift suggests that the molecules move to an intermolecular distance of x 4 A in the excited state, lowering the energy of the pair. This might seem to justify the use of the term excimer in this case. However, this red-shift is smaller than that usually associated with excimers in solution. Furthermore, the fluorescence spectrum retains some vibronic structure. Both these observations are likely due to the broad distribution of orientations and distances occuring in the pair, due to the influence of the inhomogeneous matrix. If the PMMA host were sufficiently rigid and compact to prevent excessive movement of the two tetracene molecules trapped in a T.T pair, then the two tetracenes in pairs resulting from cleavage of each of the two different isomers of ditetracene should interact differently. If that were the case, one might expect a weaker interaction between tetracene molecules in a pair derived from anti-T, than in one derived from syn-T,. Such interaction differences should be reflected in the fluorescence spectra of T.T pairs from the two different isomers. Fig. 3 shows that there is no evidence for any difference in the fluorescence from T.T pairs formed from each of the two isomers in PMMA at 15 K inasmuch as there is no observed shift between the two spectra. (The apparent increase in emission for the pairs from synT2 in the region 475 to 550 nm is within the experimental uncertainty.) Likewise, the red-shift of the excitation spectrum of T.T emission with respect to that of tetracene emission is, within the experimental error in determining peak positions, G 5 nm for T.T from both isomers. There is a slight difference for the two isomers in the apparent Franck-Condon pattern of the monomeric product tetracene fluorescence that is produced after cycling to room temperature ( see fig. 5). This result is likely due to differences in the tetracene concentrations in the two samples resulting in differences in self-absorption of the fluorescence origin band. Emission very similar to that shown in fig. 3 was observed previously in cold solutions of tetracene [ 10 1. That emission was attributed to tetracene pair 573
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formation, based on the concentration dependence of the relative intensities of dimer and monomer fluorescence. The red-shift of the peak of that emission relative to that of the monomer O-O was so small ( z53000 cm-’ ) as to rule out excimer formation as the source. Thus such emission was attributed to a stable dimer of tetracene, analogous to the stable dimer of anthracene [9]. That assignment was later questioned [ 111, and the previously reported emission was attributed to microcrystals of tetracene. In the same report a shoulder in the fluorescence at 18550 cm-’ (539 nm) was assigned to the “true dimer” of tetracene. The present results from T2 decomposition seem, however, to support the original assignment, since under the conditions of this experiment, pairs of tetracene molecules are trapped by a solid matrix, and further aggregation is clearly impossible.
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ground and excited states, subject to the constraints of the surrounding polymer. Evidence from the spectra indicates that there is considerable inhomogeneity in the pair structures. This is probably due to the inhomogeneity in the void size and shape in the polymer host.
Acknowledgement This research was supported by the Research Corporation with matching funds from the University of California, Riverside and from the Committee on Research at UCR. One of us (MAI) acknowledges stipend support of NASA Training Grant NGI50156.
References [I ] KS. Wei and R. Livingston, Photochem. Photobiol. 6
6. Conclusions Based on the present results, it is clear that the species formed upon photolysis of T2 in PMMA at 12 K is not an excimer-forming sandwich dimer. Evidence was presented which suggeststhat there is likely considerable relative movement of the tetracene molecules after cleavage of the Ta. The PMMA matrix is not compact or rigid enough to constrain the tetracene pair to remain in a repulsive sandwich configuration at low temperature, and the molecules can separate enough to greatly weaken their interaction at temperatures still well below the glass transition temperature of PMMA. Such a conclusion suggests that the T,-T-T system could be used to probe the rigidity and packing characteristics of different glassy hosts. The emission of T-T is red-shifted by w 3000 cm-’ with respect to free tetracene emission, while the excitation spectrum is red-shifted by only about 250 cm- ‘. The emission is not the characteristically structureless emission of an excimer, but the red-shift with respect to the absorption indicates a possible geometry change in the excited system. Presumably the components of the tetracene pairs are partially free to move and adopt a configuration which minimizes the pair state energy both in the
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(1967) 229. [2] S. Singh and C. Sandorfy, Can. J. Chem. 47 ( 1969) 257. [3] R. Lapouyade, A. Nourmande and H. Bouas-Laurent, Tetrahedron 36 ( 1980) 2311. [4] J. Gaultier, C. Hauw, J.-P. Desvergne and L. Lapouyade, Cryst. Struct 4 (1975) 497. [5] J.B. Birks, J.H. Appleyard and R. Pope, Photochem. Photobiol. 2 ( 1963) 493. [6] E.A. Chandross, J. Ferguson and E.G. McRae, J. Chem. Phys. 45 (1966) 3546. [ 71 J. Ferguson and A.W.-H. Mau, Mol. Phys. 27 (1974) 377. [ 81J. Ferguson, A.W.-H. Mau and J.M. Morris, Australian J. Chem. 26 (1973) 91. [ 91 T.T. Nakashima and H.W. Offen, J. Chem. Phys. 48 (1968) 4817. [ IO] J. Ferguson, A.W.-H. Mau and J.M. Morris, Australian J. Chem. 26 (1973) 103. [11]J.A.KatulandA.B.Zah1an,J.Chem.Phys.47(1967)1012. [ 121G. Foumie, F. Dupuy, M. Martinaud, G. Nouchi and J.M. Turlet, Chem. Phys. Letters 16 ( 1972) 332. [ 131M.V. Kurik and Yu.P. Piryantinskii, J. Luminescence 31/ 32 (1984) 619. [ 141H. Miiller, H. Biissler and G. Vaubel, Chem. Phys. Letters 29 (1974) 102. [ IS] P.F. Jones, J. Chem. Phys. 48 ( 1968) 5448. [ 161M. Iannone, G.W. Scott, D. Brinza and D.R. Coulter, J. Chem. Phys. 85 ( 1986) 4863. [ 171J.B. Birks, ed., Organic molecular photophysics, Vol. 1 (Wiley, New York, 1973). [ 161LB. Berlman, Handbook of fluorescence spectra of aromatic molecules (Academic Press, New York, 1971).
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Low temperature emission spectra of trapped tetracene pairs from ditetracene Mark A. Iannone ’ and Gary W. Scott Department of Chemistry, University of California at Riverside, Riverside?CA 92521, USA Received 24 May 1990
Interacting pairs of tetracene molecules are prepared in a PMMA host matrix by photodecomposition of ditetracene at 15 K. Excitation and emission spectra of these pairs are red-shifted and broadened relative to those of isolated tetracene molecules under the same conditions. No differences are seen in the spectra of pairs formed fram synditetracene and from antiditetracene. The results suggest that these pairs of tetracene molecules are not in the parallel (sandwich) configuration and that a distribution of intermolecular distances and orientations is obtained. A large geometry change in the excited state, akin to excimer formation, is postulated to explain the fluorescence spectrum.
1. Intruductlon When a degassed, concentrated solution of tetracene is irradiated into the SltSo tetracene absorption band, dimerization may occur across the 6,l l5’,12’ or 6,l l-6/,11’ positions. This bridging produces one of two isomers of ditetracene, anti and syn respectively, each of which have two naphthalene and two benzene chromophores. The structures of the two isomers, anti and syn, are shown in fig. 1. Isolation of one of these dimers was first reported by Wei and ’ Present address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA.
0
Fig. I. (a) Synditetracene.
b (b) Antiditetracene.
Livingston [ 11, and the infrared spectrum of this product was later studied in detail [ 21. Lapouyade and co-workers [ 3 ] isolated both of the products of tetracene dimerization, and the previously studied insoluble product was identified as anti-ditetracene by X-ray crystallography [ 41. The second, more soluble product was assigned as syn-ditetracene. An earlier report of the respective dimerizations of anthracene, tetracene, and pentacene in dilute solutions [ 51 was incorrect, as the observed products were apparently due to decomposition of the molecules by the unfiltered light from a mercury lamp. In a solid matrix, photodecomposition of dianthracene (AZ) produces two anthracene molecules which are presumed held in a face-to-face “sandwich” configuration, resulting in a structureless, extimer-like fluorescence, red-shifted by x 4500 cm-’ with respect to monomer fluorescence [ 6 ] I The behavior of these sandwich pairs and their tendency to undergo photodimerization to reform dianthracene has been extensively investigated, especially by Ferguson and co-workers [ 6-81. In particular, in dianthracene crystals, dimer formation was found to have a thermal activation energy of 7 kJ/mol [7]. Upon cooling solutions of anthracene, spectral changes were observed that were attributed to formation of a “stable dime? in which the long axes of the two molecules are parallel, but the short axes form
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an angle of 55” [ 93. This “stable dime? absorption and its fluorescence are red-shifted from the monomer spectra. Similar spectral results were obtained when a matrix containing anthracene sandwich pairs was softened in a controlled manner. Liquid solutions of anthracene were observed at 77 K and higher temperatures, but no excimer emission [ 61, only dimer emission, was seen. For the anthracene pair, the 55 o dimer configuration has been reported to be the most stable configuration, both for the ground and excited states [ 6 1. Pairs or larger aggregates of tetracene have been reported to form upon cooling solutions of tetracene in various solvents [ 10,111. There are several reports of emission attributed to excimer formation in pure [ 12-141 and mixed [ 151 crystals of tetracene, but there is no report of tetracene excimer formation in solution. We know of no previous study of the properties of the tetracene pairs such as are formed by decomposition of Tz in a solid matrix. The present paper describes the emission spectra of such pairs.
2. Experimental 2.1. Sample preparation Ditetracene was synthesized by the method of ref. 3. A suspension of 500 mg of tetracene (Aldrich Chemical Co. ) in 80 ml of dry benzene was degassed and then irradiated for 36 h with the filtered light (A> 345 nm ) from a 200 W high pressure mercury lamp. The resulting reaction mixture was filtered to separate the precipitate. The recovered solid anti-T, (443 mg) was washed first with benzene, and then with methanol. The filtrate was irradiated in the presence of oxygen to oxidize any residual tetracene, then evaporated to dryness. The residue of syn-T, was purified by passing it through a column of silica gel, eluting with a 1: 1 mixture of benzene and petroleum ether. The products were identified as the indicated isomers of Tz based on previous reports [ 3,4] their absorption and fluorescence spectra, and their mass spectra. The insoluble product, pure anti-T2, had a O-O absorption maximum at 325.4 nm at 1.5 K in PMMA. The soluble product was at least 90% synT2, perhaps with an impurity of anti-T2, as deter570
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mined by UV spectroscopy (O-O absorption maximum at 326.8 nm at 1.5 Kin PMMA with a shoulder at 325 nm). For both products, the mass spectrum showed only one major peak at 228 amu, suggesting that under the conditions of desorption used (both electron impact and fast atom bombardment desorption were tried) the dimers quantitatively cleave to tetracene. Most spectroscopic experiments were carried out on x0.4 mm thick films of PMMA containing approximately 0.01 M of one of the ditetracenes. The purification of the PMMA has been previously described [ 16 1, The films of syn-T, in PMMA were case from CHzClz solution at room temperature. Due to the lower solubility of anti-T2, it was necessary to cast films containing anti-T, at 60°C from 1,2-dichloroethane to avoid its precipitation from the film. 2.2. Methods Fluorescence and phosphorescence spectra were obtained using a SPEX Fluorolog 212 spectrofluorimeter, equipped with a 450 W Xe lamp and double monochromators. The spectrofluorimeter was calibrated using a standard lamp in order to obtain intensity corrections at all wavelengths. For spectra at temperatures of 12 K and above, the film samples were sandwiched between sapphire disks in the sample holder of a Displex 202E closed-cycle refrigerator (Air Products). The film samples were degassed in vacuum at room temperature for 12 h before cooling. Spectra were also obtained at 4.2 K by immersing degassed samples in liquid helium. Photolysis of ditetracene to produce tetracene pairs prior to taking pair spectra was accomplished by irradiating the cold films with the output of a 450 W xenon lamp dispersed through the SPEX monochromator which was set at 3 10 nm with 8 mm slits ( z 15 nm bandwidth). A Schott WG-305 filter was placed in the excitation beam.
3. Results 3.1, Ditetracene fluorescence and phosphorescence The low-temperature emission spectra of both the ditetracenes exhibit fluorescence (from 325 to 360
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nm) and phosphorescence (from 470 to 600 nm). These spectra are shown in figs. 2a (syn-T2) and 2b (anti-T, ) . 3.2. Tetracene pair fluorescence When ditetracene was photolyzed at low temperature in PMMA, the fluorescence of the product was broadened and red-shifted with respect to the fluorescence of an isolated tetracene molecule. Fig. 3 shows the fluorescence spectra of photolyzed syn-T, (solid line) and photolyzed anti-T, (dashed line) following a 2 h photolysis of each sample at 15 K. These plotted spectra were obtained by subtracting
a
h 16000
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Wavenumber/cm-'
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Fig. 3. Fluorescence resulting from 420 nm excitation of photolyzed syn-T, (solid line) and anti-T, (dashed line) samples after a 2 h photolysis at 3 10 nm. In each instance, the emission of a fresh sample of the ditetracene was subtracted from the emission of the photolyzed sample. Spectral bandwidths: excitation, 1.8 nm: emission 3.6 nm.
r x20
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Fig. 2. (a) Fluorescence and phosphorescence of syn-Tz at 12 K in PMMA. (b) Fluorescence and phosphorescence of anti-T* at 12 K in PMMA. In both spectra the phosphorescence regions are shown with the intensities multiplied by 20. Excitation was at 310 nm, while excitation and emission bandwidths were 1.8 nm.
the emission observed with 420 nm excitation before photolysis from that obtained after photolysis for each of the two ditetracenes. The solid lines in figs. 4a and 4b represent the excitation spectra of emission at 650 nm ( ~7 nm bandwidth) from T*T at 15 K derived by photolysis of anti-T, and syn-T,, respectively. (650 nm was chosen because of the large ratio of product to tetracene monomer emission at this wavelength. In this paper, we will use T.T to indicate a pair of tetracene molecules in proximity with no covalent bonds between them.) Excitation spectra of emission at 475 nm ( 1.8 nm bandwidth) were also obtained for both samples, but are not shown. After the above experiment, the photolyzed samples of syn- and anti-T2 were allowed to warm to room temperature overnight. Then, they were again cooled to 15 K and the excitation spectra of the emission at 5 10 nm were obtained. These excitation spectra are the dashed curves also shown in figs. 4a and 4b. These spectra are quite similar with respect to structure and peak positions to the excitation spectra of 475 nm emission obtained before cycling to room temperature. Emission spectra for 420 nm excitation of photolyzed syn- and anti-T, samples before and after cycling are given in figs. 5a and 5b.
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Fig. 4. (a) Excitation spectra of a photolyzed anti-T* in PMMA sample at I5 K. (b ) Excitation spectra of a photolyzed syn-T, in PMMA sample at I5 K. Solid lines: excitation of emission at 650 nm. Spectral bandwidths: excitation, 1.8 nm; emission 7.2 nm. Dashed lines: excitation of emission at 5 10 nm after cycling to room temperature. Spectral bandwidths: excitation, 1.8 nm; emission,1.8 nm. 4. Discussion
When ditetracene is photolyzed in solid solution at low temperature, the product tetracene molecules cannot move arbitrarily far apart since they are trapped by the matrix. The interaction between the two molecules affects the eigenstates of the system, giving rise to a broadened, red-shifted absorption and emission. The transition dipoles of the states resulting from two interacting molecules are given by the vector sum and difference of the two transition moments of the individual molecules. The magnitude of the interaction between the two molecules has often 572
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Wavelength/nm Fig. 5. (a) Comparison of the emission from a photolyzed sample of anti-T2 in PMMA at I5 K before and after cycling to room temperature. (b) Comparison of the emission from a photolyzed sample of syn-T1 in PMMA at I5 K before and after cycling to room temperature. Solid lines: freshly photolyzed sample. Dashed lines: same sample after cycling to room temperature. Excitation was at 420 nm. Spectral bandwidths: excitation, 1.8 nm: emis sion, 3.6 nm.
been approximated by a point-dipole-point-dipole model [ 8,9]. In this model, the interaction energy for a pair is of the order of 1p 1*/r3, in which p is the molecular transition moment and r is the distance between the chromophores. In the case of tetracene, the lowest-energy allowed . . . transition IS ‘BZUc‘A, (short-axis polarized), and has an oscillator strength of 0.44 [ 17 1, The red-shift observed in the T.T fluorescence excitation spectrum with respect to that of the monomer indicates that the short axes of the molecules in the pair are
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not parallel, since, for that case, a blue-shift of absorption would be expected. Transitions to the lower energy, out-of-phase dimer state in that case would not be allowed. In the case of chloroanthracene [ 81, a red-shift was observed for the presumed sandwich (parallel) dimer. This observation was attributed to an overall red-shift due to a dispersion interaction stronger than the pair splitting. However, this possibility is unlikely in the case of T-T since there is evidence that the intermolecular distance is larger in this case (see below). The observed red-shift of 250 cm-’ for the fluorescence excitation spectrum of T-T compared to the monomer (fig. 4 ) suggests that a splitting of around 500 cm-’ occurs in this system. A splitting of this magnitude would be consistent with a pair separation of x 8 A assuming a dipole-dipole interaction with the oscillator strength given above. No splitting is apparent in the excitation spectra, figs. 4a and 4b. These spectra are highly broadened due to the inhomogeneous environment. In particular, the very long tail at the red edge of the excitation spectra suggests a wide range of T*T distances in the pairs, e.g. from about 4 to 8 A, with the larger distances being most probable. In addition, the high-energy component would be difficult to observe if it has a smaller transition dipole than the lower-energy component. This would occur for an angle of around 60” between the short axes of the molecules as observed in the case of anthracene pairs under certain conditions I&91. Before photolysis of samples of either the ditetracenes, fluorescence was obtained upon excitation with 420 nm light. This emission is essentially identical with reported tetracene emission spectra [ 181 in solution and is therefore assigned to free tetracene present as an impurity in the samples, Additional emission observed after photolysis may be ascribed to a T.T trapped pair (fig. 3 ), whereas after cycling to room temperature, the T-T emission disappears and is replaced by emission nearly identical to that of isolated tetracene (fig. 5). It may be concluded that mobility of the tetracene solute molecules in PMMA at room temperature allows the T-T pairs to separate sufficiently to greatly weaken the interaction between them. The Stokes shift of the O-Opeak of fluorescence of T*T compared to the O-O of the excitation is z 3000
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cm-i. This appears to be inconsistent with a picture of rigidly trapped molecules. Relaxation to another, lower-energy state is unlikely since the fluorescence is strong and thus arises from an allowed state. The large Stokes shift suggests that the molecules move to an intermolecular distance of x 4 A in the excited state, lowering the energy of the pair. This might seem to justify the use of the term excimer in this case. However, this red-shift is smaller than that usually associated with excimers in solution. Furthermore, the fluorescence spectrum retains some vibronic structure. Both these observations are likely due to the broad distribution of orientations and distances occuring in the pair, due to the influence of the inhomogeneous matrix. If the PMMA host were sufficiently rigid and compact to prevent excessive movement of the two tetracene molecules trapped in a T.T pair, then the two tetracenes in pairs resulting from cleavage of each of the two different isomers of ditetracene should interact differently. If that were the case, one might expect a weaker interaction between tetracene molecules in a pair derived from anti-T, than in one derived from syn-T,. Such interaction differences should be reflected in the fluorescence spectra of T.T pairs from the two different isomers. Fig. 3 shows that there is no evidence for any difference in the fluorescence from T.T pairs formed from each of the two isomers in PMMA at 15 K inasmuch as there is no observed shift between the two spectra. (The apparent increase in emission for the pairs from synT2 in the region 475 to 550 nm is within the experimental uncertainty.) Likewise, the red-shift of the excitation spectrum of T.T emission with respect to that of tetracene emission is, within the experimental error in determining peak positions, G 5 nm for T.T from both isomers. There is a slight difference for the two isomers in the apparent Franck-Condon pattern of the monomeric product tetracene fluorescence that is produced after cycling to room temperature ( see fig. 5). This result is likely due to differences in the tetracene concentrations in the two samples resulting in differences in self-absorption of the fluorescence origin band. Emission very similar to that shown in fig. 3 was observed previously in cold solutions of tetracene [ 10 1. That emission was attributed to tetracene pair 573
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formation, based on the concentration dependence of the relative intensities of dimer and monomer fluorescence. The red-shift of the peak of that emission relative to that of the monomer O-O was so small ( z53000 cm-’ ) as to rule out excimer formation as the source. Thus such emission was attributed to a stable dimer of tetracene, analogous to the stable dimer of anthracene [9]. That assignment was later questioned [ 111, and the previously reported emission was attributed to microcrystals of tetracene. In the same report a shoulder in the fluorescence at 18550 cm-’ (539 nm) was assigned to the “true dimer” of tetracene. The present results from T2 decomposition seem, however, to support the original assignment, since under the conditions of this experiment, pairs of tetracene molecules are trapped by a solid matrix, and further aggregation is clearly impossible.
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ground and excited states, subject to the constraints of the surrounding polymer. Evidence from the spectra indicates that there is considerable inhomogeneity in the pair structures. This is probably due to the inhomogeneity in the void size and shape in the polymer host.
Acknowledgement This research was supported by the Research Corporation with matching funds from the University of California, Riverside and from the Committee on Research at UCR. One of us (MAI) acknowledges stipend support of NASA Training Grant NGI50156.
References [I ] KS. Wei and R. Livingston, Photochem. Photobiol. 6
6. Conclusions Based on the present results, it is clear that the species formed upon photolysis of T2 in PMMA at 12 K is not an excimer-forming sandwich dimer. Evidence was presented which suggeststhat there is likely considerable relative movement of the tetracene molecules after cleavage of the Ta. The PMMA matrix is not compact or rigid enough to constrain the tetracene pair to remain in a repulsive sandwich configuration at low temperature, and the molecules can separate enough to greatly weaken their interaction at temperatures still well below the glass transition temperature of PMMA. Such a conclusion suggests that the T,-T-T system could be used to probe the rigidity and packing characteristics of different glassy hosts. The emission of T-T is red-shifted by w 3000 cm-’ with respect to free tetracene emission, while the excitation spectrum is red-shifted by only about 250 cm- ‘. The emission is not the characteristically structureless emission of an excimer, but the red-shift with respect to the absorption indicates a possible geometry change in the excited system. Presumably the components of the tetracene pairs are partially free to move and adopt a configuration which minimizes the pair state energy both in the
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(1967) 229. [2] S. Singh and C. Sandorfy, Can. J. Chem. 47 ( 1969) 257. [3] R. Lapouyade, A. Nourmande and H. Bouas-Laurent, Tetrahedron 36 ( 1980) 2311. [4] J. Gaultier, C. Hauw, J.-P. Desvergne and L. Lapouyade, Cryst. Struct 4 (1975) 497. [5] J.B. Birks, J.H. Appleyard and R. Pope, Photochem. Photobiol. 2 ( 1963) 493. [6] E.A. Chandross, J. Ferguson and E.G. McRae, J. Chem. Phys. 45 (1966) 3546. [ 71 J. Ferguson and A.W.-H. Mau, Mol. Phys. 27 (1974) 377. [ 81J. Ferguson, A.W.-H. Mau and J.M. Morris, Australian J. Chem. 26 (1973) 91. [ 91 T.T. Nakashima and H.W. Offen, J. Chem. Phys. 48 (1968) 4817. [ IO] J. Ferguson, A.W.-H. Mau and J.M. Morris, Australian J. Chem. 26 (1973) 103. [11]J.A.KatulandA.B.Zah1an,J.Chem.Phys.47(1967)1012. [ 121G. Foumie, F. Dupuy, M. Martinaud, G. Nouchi and J.M. Turlet, Chem. Phys. Letters 16 ( 1972) 332. [ 131M.V. Kurik and Yu.P. Piryantinskii, J. Luminescence 31/ 32 (1984) 619. [ 141H. Miiller, H. Biissler and G. Vaubel, Chem. Phys. Letters 29 (1974) 102. [ IS] P.F. Jones, J. Chem. Phys. 48 ( 1968) 5448. [ 161M. Iannone, G.W. Scott, D. Brinza and D.R. Coulter, J. Chem. Phys. 85 ( 1986) 4863. [ 171J.B. Birks, ed., Organic molecular photophysics, Vol. 1 (Wiley, New York, 1973). [ 161LB. Berlman, Handbook of fluorescence spectra of aromatic molecules (Academic Press, New York, 1971).