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Macromolecules 2000, 33, 9518-9521
Synthesis and Characterization of Nonfluorescent Poly(p-aryleneethynylene)s by Alkyne Metathesis Winfried Steffen and Uwe H. F. Bunz*,† Department of Chemistry and Biochemistry, The University of South Carolina, Columbia, South Carolina 29208 Received August 8, 2000; Revised Manuscript Received October 3, 2000
ABSTRACT: Alkyne metathesis of mixtures of 2,5-didodecyl-1,4-dipropynylbenzene and {1,3-bis[2′,5′didodecyl-4′-propynyl(phenylethynyl)]-2,4-bis(trimethylsilyl)cyclobutadiene}cyclopentadienylcobalt in the presence of Mo(CO)6 and 4-trifluoromethylphenol at 150 °C furnishes a series of organometallic PPE copolymers in which CpCo-ligated (cyclobutadienylene)ethynylene units are interspersed into the phenyleneethynylene chains. The copolymers were investigated by UV-vis and fluorescence spectroscopy. Both techniques showed a dramatic influence of the presence of even small amounts of organometallic modules on the photophysical properties of the polymers.
Introduction
Result and Discussions
Conjugated polymers1 are semiconductors and as such valuable as easily processable materials for electronic devices. In most of these applications, strong solid-state emission (i.e., maximum fluorescence) of thin films is desirable or necessary for device function. Such devices include light-emitting diodes,2 light-emitting electrochemical cells,3 and polymer-based “plastic” lasers.4 There is however a demand for nonfluorescent conjugated polymers, where solid-state emission is either unnecessary (antielectrostatic coatings),5 unwanted (thin film transistors),6 or even detrimental (polymer-based photovoltaic cells)7 toward the projected use. In particular, in photovoltaic cells, charge separation is desired while emission is not. In this paper, we describe the synthesis and spectroscopic examination of a family of organometallic, nonfluorescent poly(p-phenyleneethynylene) derivatives (PPE) (Scheme 1). We have recently shown that alkyne metathesis with “instant” catalysts is a superbly simple way to prepare PPE (2)8 and related alkyne-bridged polymers9 of high molecular weight and purity. On the other hand we have a long-standing interest in organometallic polymers such as 3, which show a remarkable phase behavior.10,11 Early on, we observed that samples of 3 were stable for years under atmospheric conditions and completely nonfluorescent. However, due to the presence of the cyclobutadiene complex, synthesis of larger amounts of polymers of type 3 is laborious.10 PPEs 2 on the other hand can be made regularly in our laboratory on a 5-10-g scale without any problems.8 As a consequence, we explored the possibility to “dope” PPE chains covalently with small amounts of fluorescence-quenching cyclobutadiene complexes, to confer their optical properties to the base polymer, PPE. For inorganic semiconductors this strategy has been extremely successful. In silicon, doping with either As or Ga is used to significantly alter electronic properties.12 In conjugated polymers this concept (i.e., covalent doping) has been used occasionally,13 but has by far not been exploited to its full potential.
In a series of experiments, mixtures of the monomers 58b,d and 611 were treated with Mo(CO)6 in the presence of trifluoromethylphenol in off-the-shelf 1,2-dichlorobenzene (Scheme 2). The reaction mixtures were kept at 150 °C for 14-22 h under a slight stream of pure nitrogen. Standard workup furnished the polymers 7a-e in good-to-excellent yields as yellow or yellow-tan, nonfluorescent powders soluble in chloroform and dichloromethane. The degree of polymerization (Pn) of 7a-e ranges from 40 to 250 repeating units. As a consequence (Table 1) each of the polymer chains carries at least one cyclobutadiene unit according to a simple statistics (the exception is 7a where the concentration of cyclobutadiene units is very low).14 In dilute chloroform solution the UV-vis spectra of the polymers 7a-c are almost superimposable onto each other and resemble that obtained for pure PPE. Aggregation15 is induced by addition of methanol (a non solvent) to chloroform solutions of 7. Figure 1, parts a-c, show the development of the UV/vis spectra of 7a-e in dependence of the methanol content. The classic aggregation behavior of PPEs is visible in 7a and 7b, where the content of organometallic quencher is 0.5 and 4% respectively. Once 25% of the units are organometallic (as in 7c-e) the planarization peak15-17 is not detected. The spectra of 7c-e are almost unaffected by the addition of methanol, similar to those of the pure organometallic polymers 3 and 4. As a consequence we can conclude that in poly(p-aryleneethynylene)s, classic PPE-type aggregation is diminished and disappears in 7d,e, which have a high content of organometallic modules. Fluorescence spectroscopy is a sensitive probe for energy transfer16 toward “defect sites”, in this case the cyclobutadiene(cyclopentadienyl)cobalt units. This intramolecular energy transfer occurs according to Swager’s molecular wire model16 and should be quite effective. PPEs have been determined to produce fluorescence quantum yields of unity in dilute solution. We prepared a series of solutions of didodecyl-PPE (2a) and 7a-d of equal concentration (6.0 mg/L), which allowed us to determine the quantum yields of the corresponding doped PPEs 7 by simple comparison of the intensities
†
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10.1021/ma001394h CCC: $19.00 © 2000 American Chemical Society Published on Web 12/01/2000
Macromolecules, Vol. 33, No. 26, 2000
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Figure 1. Absorption spectra of polymers 7a-e in (a) CHCl3, (b) 80% CHCl3/20% MeOH, (c) 60% CHCl3/40% MeOH, and (d) 80% MeOH/20% CHCl3. Scheme 1. Different Types of Aryleneethynylene Polymers
Scheme 2. Polymerization Reaction
of the emitted light to that of the standard (PPE). In Figure 2 the emission spectra of 7a-c are shown. The mechanism by which this quenching occurs is the
accessibility of the cobalt-centered charge-transfer MLCT states18,19 conferred onto the polymer by the presence of the cyclobutadiene complexes. As a consequence
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Macromolecules, Vol. 33, No. 26, 2000 Table 2. Relative Quantum Yields in Solution of Polymer Blend of Polymers 2 and 3 concn of polymer blend [mg L-1] 3 6 12
Figure 2. Emission spectra of polymers 7a-c (6 mg L-1) in CHCl3.
fraction of polymer 3 (R ) hexyl) in blend [%] 50 25 5 φ ) 0.53 φ ) 0.55 φ ) 0.58
φ ) 0.77 φ ) 0.79 φ ) 0.86
φ ) 0.94 φ ) 0.94 φ ) 0.95
their fluorescence at different concentrations (3, 6, and 12 mg L-1). Figure 3 and Table 2 show the results of the experiments. (1) The quantum yield is independent of the concentration of the added organometallic in the dilute regimen. (2) The decrease of the quantum yields seen in Figure 3 is merely a physical effect of dilution of the solution of the PPE by the dilute solution of the polymer 3. In consequence, there is no intermolecular quenching in these systems at low concentrations. However as discussed above, we do see dramatic quenching in our copolymers, strongly supporting the notion that (a) the cyclobutadiene complexes are in the polymer chain and (b) that the cyclobutadiene complex is likely to be distributed in a statistical fashion, suggested by the extremely close resemblance of the utilized organometallic monomer 6 to 5. Conclusion
Figure 3. Emission spectra of polymer blend of polymers 2 and 3 (sum of 2 + 3 ) 6 mg L-1) in CHCl3.
already in 7a, the fluorescence quantum yield is only 18% of that observed for pure PPE, even though the “quencher” substitutes less than 0.1% of the aryleneethynylene units. The fluorescence in solution has fully disappeared in 7d, (not shown in Figure 2) where every fifth unit is a cyclobutadiene complex. In the solid state even the polymers 7a and 7b are nonemissive, which strongly suggests that fluorescence quenching does not only occur intramolecularly but as well in an intermolecular fashion. The polymers 7a,b are only lightly substituted by cyclobutadiene complexes and thus show a similar solid-state structure as 2 does. As a consequence the order in these lamellar materials is high, which leads to efficient van der Waals promoted interchain contacts in these polymers. The closely packed nature of these organometallic-doped materials allows then efficient energy transfer from chain to chain, similar to that proposed by Swager for dye-doped PPE multilayers.16b An important question was if the energy transfer was purely intramolecular or if intermolecular energy transfer was likewise important in dilute solution. To investigate this hypothesis, we prepared mixtures of the pure polymers 2 and 3 where the content of the organometallic polymer was 5%, 25%, and 50% and measured
In conclusion, we have shown that incorporation of CpCo-stabilized cyclobutadiene complexes into PPEs even in small amounts leads to an efficient quenching of polyaryleneethynylene’s fluorescence in solution and in the solid state. Quenching occurs by inter- and intramolecular energy transfer from the excited state of the conjugated organic polymer to the covalently incorporated defect site, the organometallic unit, which has access to energetically favorable MLCT states. In future we will determine fluorescence lifetimes of the doped polymers and insert other organometallic units (including ferrocene and cymantrene) into the PPE backbone to compare quenching efficiencies and photophysics of poly(aryleneethynylene)s doped with different organometallic moieties. Experimental Section Molecular weight determinations were performed using a Waters Styragel HMW 6E (7.8 mm i.d. 300 mm) GPC column (20 µm particles/10 µm frits) eluted with CHCl3 at ambient temperature (flow rate of 1 mL/min). Solubility data for the polymers are provided with respect to chloroform at ambient temperature (25 °C), unless otherwise stated. Molecular weight results were based on 10 polystyrene standards (Mw) 3 900 000, 1 980 000, 996 000, 629 000, 210 000, 70 600, 28 600, 10 900, 3000, and 1300) purchased from Waters (type SM-105). The 1H and 13C NMR spectra were recorded with either a Bruker AM 300 MHz or a Varian Mercury 400 MHz spectrometer operating in the FT mode at 300 (1H) and 75.5 MHz (13C) and at 400 (1H) and 100.6 MHz (13C). The 1H chemical shifts are referenced to the residual proton peaks of CDCl3 at δ 7.24 (vs TMS) and C2Cl4D2 at δ 5.99 (vs TMS). The 13C resonances are
Table 1. Polymerization Results for Copolymer 7a
a
polymer
weight ratio 5:6 (g/g)
% organometallic
ratio of all/ organometallic units
Pn*
Mw/Mn
% yield
7a 7b 7c 7d 7e
2.00/0.030 1.00/0.100 0.080/0.120 0.080/0.120 0.00/0.100
0.07 1 21 22 33
1429 100 5 4.5 3
84 248 125 122 43
3.2 3.3 4.2 6.2 3.1
88 99 88 95 78
n ) [(x + y)m] in polymers 7.
Macromolecules, Vol. 33, No. 26, 2000 referenced to the central peak of C2Cl4D2 at δ 73.8 (vs TMS) and CDCl3 at δ 77.0 (vs TMS). Monomer 6 was synthesized as described.11 General procedure for the copolymerization of monomers 5 and 6: Monomers 5 and 6, Mo(CO)6 (5-20 mol %) and either 4-chlorophenol or 4-hydroxybenzotrifluoride (1 equiv with respect to the monomers 5 and 6; for 7c-e only 4-hydroxybenzotrifluoride works) were dissolved in 1,2-dichlorobenzene and stirred for 48 h at 150 °C, removing butyne by a slow stream of nitrogen. The solution was cooled and precipitated polymers 7a-e dissolved by the addition of CH2Cl2. The organic layer was washed with 20 mL each of H2O, 10% NaOH, and 25% HCl. Addition of methanol precipitates the polymers 7, which were filtered and vacuum-dried. Synthesis of 7a. 5 (2.00 g, 4.08 mmol), 6 (0.030 g, 0.023 mmol), 4-chlorophenol (0.527 g, 4.10 mmol), and Mo(CO)6 (0.054 g, 0.205 mmol, 5 mol %) in 40 mL of 1,2-dichlorobenzene were reacted according to the general procedure to give yellow, brittle polymer 7a (1.79 g, 88%). IR: ν 2922, 2850, 1772, 1693, 1601, 1503, 1467, 1377, 1260, 885, 721 cm-1. 1H NMR (CDCl3): δ 7.36 (bs, 2H), 4.95 (bs, 0.04H), 2.82 (bm, 4H), 1.70 (bm, 2H), 1.24 (bm, 54H), 0.86 (bm, 12H), 0.38 (bs, 0.14H). 13C NMR (CDCl ): δ 141.80, 132.33, 122.74, 93.11, 34.33, 3 32.05, 30.87, 29.84, 29.80, 29.62, 29.50, 22.81, 14.24. UV/vis: (CHCl3) λmax () 383 (45 337); (20% MeOH) λmax () 383 (42 788), 439 (18 151); (40% MeOH) λmax () 397 (39 911), 439 (33 550); (80% MeOH) λmax () 401 (43 202), 414 (39 790), 439 (41 087), 437 (34 065). The polymers 7b-e were obtained by the same experimental protocol. Yields and spectroscopic data are listed. 7b (1.09 g, 99%). IR: ν 2922, 2851, 1503, 1467, 1378, 1258, 894, cm-1. 1H NMR (CDCl ): δ 7.36 (bs, 2H), 4.97 (bs, 0.16H), 2.82 (bm, 3 3H), 1.71 (bm, 2H), 1.24 (bm, 50H), 0.85 (bm, 11H), 0.38 (bs, 1H). 13C NMR (CDCl3): δ 141.80, 132.33, 122.74, 93.11, 34.3, 32.05, 30.87, 29.85, 29.80, 29.50, 22.82, 14.25, -0.07. UV/vis: (CHCl3) λmax () 391 (38 131); (20% MeOH) λmax () 405 (36 005), 441 (30 821); (40% MeOH) λmax () 404 (33 540), 416 (32 031), 441 (38 013); (80% MeOH) λmax () 405 (28 118), 416 (27 387), 442 (32 170). 7c (0.192 g, 76%). IR: ν 2923, 2852, 1499, 1466, 1379, 1260, 1246, 1095, 1021 cm-1. 1H NMR (CDCl3): δ 7.35 (bs, 2H), 7.28 (bs, 1H), 7.16 (bs, 1H), 4.97 (bs, 3H), 2.80 (bm, 6H), 2.70 (bm, 2H), 1.67 (bm, 10H), 1.23 (bm, 100H), 0.85 (bm, 18H), 0.37 (bs, 11H). 13C NMR (CDCl3): δ 141.90, 141.04, 132.43, 131.76, 123.61, 122.84, 122.03, 93.34, 89.90, 81.17, 75.59, 65.39, 34.13, 31.93, 30.77, 29.69, 29.39, 22.70, 14.11, -0.21. UV/vis: (CHCl3) λmax () 386 (55 678); (20% MeOH) λmax () 387 (58 738); (40% MeOH) λmax () 393 (42 036); (80% MeOH) λmax () 437 (34 065). 7d (0.177 g, 95%). IR: ν 2923, 2853, 1501, 1464, 1378, 1246, 1109, 1003, 894, 842, 810 cm-1. 1 H NMR (CDCl3): δ 7.35 (bs, 2H), 7.28 (bs, 1H), 7.16 (bs, 1H), 4.97 (bs, 3H), 2.80 (bm, 6H), 2.70 (bm, 2H), 1.67 (bm, 10H), 1.23 (bm, 120H), 0.85 (bm, 18H), 0.37 (bs, 12H). 13C NMR (CDCl3): δ 141.90, 141.04, 132.43, 131.76, 123.61, 122.84, 122.03, 93.34, 89.90, 81.17, 75.59, 65.39, 34.13, 31.93, 30.77, 29.69, 29.39, 22.70, 14.11, -0.21. UV/vis: (CHCl3) λmax () 384 (35 103); (20% MeOH) λmax () 383 (40 500); (40% MeOH) λmax () 388 (23 254); (80% MeOH) λmax () 393 (26 151). 7e (0.162 g, 82%). IR: ν 2955, 2919, 2872, 2186, 1503, 1449, 1415, 1379, 1264, 1090, 1025, 901 cm-1. 1H NMR (CDCl3): δ 7.21 (bs, 2H), 7.12 (bs, 2H), 4.96 (bs, 5H), 2.62 (bm, 8H), 1.60 (bm, 8H), 1.28 (bm, 72H), 0.89-0.84 (bm, 12H), 0.06 (bs, 18H). 13C NMR (CDCl3): δ 141.68, 140.89, 132.21, 131.66, 81.15, 77.32, 65.41, 34.21, 32.03, 30.73, 29.83, 29.76, 29.57, 29.46, 22.81, 14.24, -0.05. UV/vis: (CHCl3) λmax () 375 (27 048).
Acknowledgment. We wish to thank M. E. Vaughn for the measurement of several UV-vis spectra and Professors M. L. Myrick (USC) and M. D. Curtis (Ann Arbor) for a stimulating discussion. We thank the Commission of Higher Education of the State of South Carolina, the National Science Foundation (Grant CHE 9814118) and the Research Corporation for generous support for this work. U.H.F.B. is an NSF CAREER (CHE 9981765, 2000-2003) awardee and Camille Drey-
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fus Teacher Scholar (2000-2004). References and Notes (1) (a) Handbook of Conducting Polymers; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J., Eds.; Marcel Dekker: New York, 1997. (b) Zhang, X.; Shetty, A. S.; Jenekhe, S. A. Acta Polym. 1998, 49, 52. (c) For a comprehensive treaty on polythiophenes, see: Reddinger, J. L.; Reynolds, J. R. Adv. Polym. Sci. 1999, 145, 58. (2) (a) Neher, D. Adv. Mater. 1995, 7, 691. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. 1998, 37, 402. (3) Yu, G.; Cao, Y.; Andersson, M.; Gao, J.; Heeger, A. J. Adv. Mater. 1998, 10, 385. Gao, J.; Yu, G.; Heeger, A. J. Appl. Phys. Lett. 1997, 71, 1293. (4) Hide, F.; Diaz-Garcia, M. A.; Schwartz, B. J.; Heeger, A. J. Acc. Chem. Res. 1997, 30, 430. Kallinger, C.; Hilmer, M.; Haugeneder, A.; Perner, M.; Spirkl, W.; Lemmer, U.; Feldmann, J.; Scherf, U. Mu¨llen, K.; Gombert, A.; Wittwer, V. Adv. Mater. 1998, 10, 920. (5) Brendel, U.; Munstedt, H. Kunststoffe-Plast. Eur. 1996, 86, A-73. (6) (a) Nanos, J. I.; Kampf, J. W.; Curtis, M. D. Chem. Mater. 1995, 7, 2232. Koren, A. B.; Curtis, M. D.; Kampf, J. W. Chem. Mater. 2000, 12, 1519. (b) Yang, Y.; Heeger, A. J. Nature 1994, 372, 344. Garnier, F.; Horowitz, G.; Peng, X. H.; Fichou, D. Adv. Mater. 1990, 2, 592. Lovinger, A. J.; Rothberg, L. J. J. Mater. Res. 1996, 11, 1581. (7) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. Halls, J. J. M.; Arias, A. C.; MacKenzie, J. D.; Wu, W. S.; Inbasekaran, M.; Woo, E. P.; Friend, R. H. Adv. Mater. 2000, 12, 498. Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (8) (a) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605. (b) Kloppenburg, L.; Jones, D.; Bunz, U. H. F. Macromolecules. 1999, 32, 4194. (c) Bunz, U. H. F.; Kloppenburg, L. Angew. Chem. 1999, 38, 478. (d) Kloppenburg, L.; Song, D.; Bunz, U. H. F. J. Am. Chem. Soc. 1998, 120, 7973. (e) Kloppenburg, L.; Bunz, U. H. F. J. Organomet. Chem. 2000, 606, 13. (9) (a) Pschirer, N. G.; Vaughn, M. E.; Dong, Y. B.; zur Loye, H.-C.; Bunz, U. H. F. J. Chem. Soc., Chem. Commun. 2000, 85. (b) Pschirer, N. G.; Bunz, U. H. F. Macromolecules 2000, 33, 3961. (10) Altmann, M.; Bunz, U. H. F. Angew. Chem. 1995, 34, 569. (11) Steffen, W.; Ko¨hler, B.; Altmann, M.; Scherf, U.; Stitzer, K.; zur Loye, H.-C.; Bunz, U. H. F. Chem.sEur. J., in press. (12) Ball, P. Made to Measure; Princeton University Press: Princeton, NJ, 1997. (13) Tasch, S.; List, E. J. W.; Hochfilzer, C.; Leising, G.; Schlichting, P.; Rohr, U.; Geerts, Y.; Scherf, U.; Mu¨llen, K. Phys. Rev. B 1997, 56, 4479. (14) From the knowledge of the ratio (R) of the two monomers R ) [number of moles of 5]/[number of moles of 6] and the degree of polymerization Pn a good statistical estimate (probability PN) of how many chains are without a cyclobutadiene complex is available: PN ) RPn. From this number the probability P that a given chain contains at least one cyclobutadiene(cyclopentadienyl)cobalt fragment is P ) (1 PN). (15) Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; StuderMartinez, S. L.; Bunz, U. H. F. Macromolecules 1998, 31, 8655. Fiesel, R.; Halkyard, C. E.; Rampey, M. E.; Kloppenburg, L.; Studer, S. L.; Bunz, U. H. F. Macromol. Rapid Commun. 1999, 20, 105. (16) (a) Swager, T. M. Acc. Chem. Res. 1998, 31, 201. (b) Kim, J. S.; McHugh, S. K.; Swager, T. M. Macromolecules 1999, 32, 1500. (c) McQuade, D. T.; Kim, J. S.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 5885. (17) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F. Macromolecules 2000, 33, 652. (18) Rengel, H.; Altmann, M.; Neher, D.; Harrison, B. C.; Myrick, M. L.; Bunz, U. H. F. J. Phys. Chem. B 1999, 103, 10335. Harrison, B. C.; Seminario, J. M.; Bunz, U. H. F.; Myrick, M. L. J. Phys. Chem. A 200, 104, 5937. (19) For very interesting work of organometallic PPEs, see as well: Ley, K. D.; Li, Y. T.; Johnson, J. V.; Powell, D. H.; Schanze, K. S. J. Chem. Soc., Chem. Commun. 1999, 1749. Walters, K. A.; Ley, K. D.; Schanze, K. S. Langmuir 1999, 15, 5676. Ley, K. D.; Schanze, K. S. Coord. Chem. Rev. 1998, 171, 287. Ley, K. D.; Whittle, C. E.; Bartberger, M. D.; Schanze, K. S. J. Am. Chem. Soc. 1997, 119, 3423.
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