A Water-Soluble Poly(phenylene ethynylene) with Pendant

Kai Sun , Haobin Chen , Lei Wang , Shengyan Yin , Haiyu Wang , Gaixia Xu , Danni ...... Yan-Qin Huang , Qu-Li Fan , Sheng-Biao Li , Xiao-Mei Lu , Fan ...
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Langmuir 2003, 19, 6523-6533

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A Water-Soluble Poly(phenylene ethynylene) with Pendant Phosphonate Groups. Synthesis, Photophysics, and Layer-by-Layer Self-Assembled Films† Mauricio R. Pinto, Boris M. Kristal, and Kirk S. Schanze* Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200 Received February 24, 2003. In Final Form: May 4, 2003 This paper reports the synthesis, optical, and thin-film-forming properties of a new water-soluble poly(phenylene ethynylene) that features phosphonate solubilizing groups. The new polymer, PPE-PO3-, is prepared by a synthetic route that involves initial preparation of a neutral analogue polymer in which the phosphonate groups are in the form of dibutyl esters. The neutral polymer is converted to the water-soluble form by cleavage of the phosphonate ester groups using (CH3)3SiBr. PPE-PO3- absorbs in the blue of the visible region and features a moderately intense fluorescence. The absorption and fluorescence of the polymer vary strongly with pH, an effect which is believed to arise because the polymer exists in a strongly aggregated form at low pH and in a relatively unaggregated state at high pH. Ultrathin multilayer films of the PPE-PO3- are deposited by the layer-by-layer (LbL) method using either the cationic polyelectrolyte poly(diallyldimethylamine) or Zr(IV) as deposition “partners”. Absorption spectroscopy indicates that the average bilayer thickness of the LbL films increases as the pH of the deposition solution decreases. This effect is believed to be due in part to the fact that the R-PO3- groups are partially neutralized at low pH; however, aggregation of the polymer in solution is also believed to contribute to the increase in the amount of polymer deposited at low pH. The fluorescence of PPE-PO3- is quenched very strongly in solution by cationic electron and energy acceptors, with Stern-Volmer quenching constants of ≈107 M-1. Finally, an electroluminescent device was fabricated using a 10 bilayer film consisting of PPE-PO3-/Zr(IV). The device turns on between 5 and 6 V and exhibits a yellow-orange emission.

Introduction Over the past decade a number of reports concerned with the synthesis and photophysical characterization of water-soluble conjugated polyelectrolytes (CPEs) have been published.1 Interest in CPEs is driven by the exceptional features they exhibit in aqueous solution, for example, high fluorescence quantum yields,2-4 unique solution behavior,5 the ability to interact electrostatically with other charged species,6,7 and an extraordinarily high sensitivity to fluorescence quenchers.8-10 This latter property has been exploited in sensing experiments in which a variety of organic and inorganic analytes and biomolecules can be detected at picomolar concentration levels.8-13 New efficient biosensors based on CPEs have also been used to detect DNA hybridization.11,13 Another useful characteristic of CPEs is their propensity to adsorb at interfaces.2,14,15 Specifically, thin and * To whom correspondence may be addressed. E-mail: [email protected]. † Part of the Langmuir special issue dedicated to David O’Brein. (1) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 9, 1293. (2) Kim, S.; Jackiw, J.; Robinson, E.; Schanze, K. S.; Reynolds, J. R.; Baur, J.; Rubner, M. F.; Boils, D. Macromolecules 1998, 31, 964. (3) Tan, C.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 446. (4) Liu, B.; Yu, W.-L.; Lai, Y.-H.; Huang, W. Chem. Commun. 2000, 551. (5) Schnablegger, H.; Antonietti, M.; Go¨ltner, C.; Hartmann, J.; Co¨lfen, H.; Samori, P.; Rabe, J. P.; Ha¨ger, H.; Heitz, W. J. Colloid. Interface Sci. 1999, 212, 24. (6) Thunemann, A. F. Adv. Mater. 1999, 11, 127-130. (7) Thunemann, A. F.; Ruppelt, D. Langmuir 2001, 17, 5098. (8) Chen, L.; McBranch, D. W.; Wang, H.-L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287. (9) Harrison, B. S.; Ramey, M. B.; Reynolds, J. R.; Schanze, K. S. J. Am. Chem. Soc. 2000, 122, 8561. (10) Wang, D. L.; Wang, J.; Moses, D.; Bazan, G. C.; Heeger, A. J. Langmuir 2001, 17, 1262.

uniform multilayer CPE films can be produced via layerby-layer (LbL) self-assembly.2,14,15 This feature is important from the standpoint of application of CPEs, since it facilitates the fabrication of thin-film-based electro-optical devices.16 In addition, adjustment of deposition conditions allows control of film morphology, i.e., layer thickness and interpenetration, and film porosity.17 Poly(phenylene ethynylene)s (PPEs) are a class of conjugated polymers that fulfill many of the requirements needed for CPE applications.18,19 Specifically, PPEs feature high fluorescence quantum yields, both in solution and as thin films.18-20 In addition, fundamental studies suggest that the singlet exciton in PPEs is highly delocalized and is able to diffuse very rapidly along a chain making the materials amenable to application in fluorescence sensing schemes where signal amplification is important.3,21,22 PPEs are relatively easy to synthesize in multigram quantities via Pd-catalyzed coupling of aryl acetylenes with aryl iodides (the Sonogashira reaction).18 This reaction is particularly useful for the synthesis of func(11) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 10954. (12) DiCesare, N.; Pinto, M. R.; Schanze, K. S.; Lakowicz, J. R. Langmuir 2002, 18, 7785. (13) Kushon, S. A.; Ley, K. D.; Bradford, K.; Jones, R. M.; McBranch, D.; Whitten, D. Langmuir 2002, 18, 7245. (14) Baur, J. W.; Rubner, M. F.; Reynolds, J. R.; Kim, S. Langmuir 1999, 15, 6460. (15) Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Rubner, M. F. Adv. Mater. 1998, 10, 1452. (16) Cutler, A. C.; Bouguettaya, M.; Reynolds, J. R. Adv. Mater. 2002, 14, 684. (17) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213. (18) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605. (19) Bunz, U. H. F. Acc. Chem. Res. 2001, 34, 998. (20) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864. (21) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593. (22) Swager, T. M. Acc. Chem. Res. 1998, 31, 201.

10.1021/la034324n CCC: $25.00 © 2003 American Chemical Society Published on Web 06/24/2003

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tionalized polymers because it can be carried out under mild conditions and in a number of different solvents, including water.3,23 The reaction is also tolerant of a wide variety of functional groups, including highly polar and ionic moieties.18 We recently reported the synthesis and photophysical characterization of a PPE that features sulfonate side groups (PPE-SO3-).3 This polymer is highly fluorescent and is soluble in water and low-molecular-weight alcohols. Fluorescence sensors for biologically relevant analytes have been developed which rely on PPE-SO3- as the active material.12,13 The results obtained on PPE-SO3- encouraged us to develop additional PPE-type CPEs which feature high specificity and sensory response in solution and in the solid state.

In this paper we report the synthesis and properties of a new water-soluble PPE-type polymer that features phosphonate side groups (PPE-PO3-). This polymer consists of a poly(phenylene ethynylene) backbone in which the repeat unit contains two phenylene rings, one of which is substituted with two R-PO32- substituents. Because of this substitution pattern, the polymer features (4-) charge per repeat unit (two phenylene rings), or (2-) charge per phenylene ring. It is known that phosphonate-substituted small molecules and polymers can be deposited in conjunction with Zr(IV) to form stable, structurally welldefined LbL self-assembled films.24,25 We anticipated that by incorporating the phosphonate side groups, it would be possible to fabricate Zr(IV)-templated LbL films using PPE-PO3-. It was further hoped that these films could be used as platforms for solid-state fluorescence sensor and electroluminescent device applications. In the present study we evaluate the ability of PPE-PO3- to form thin films by means of LbL self-assembly with Zr(IV) and with poly(dimethyldiallylammonium chloride) PDDA. In addition, we demonstrate the application of the PPE-PO3-/ Zr(IV) films as the active material in an electroluminescent device. Experimental Section Materials and Structural Characterization Methods. Carbon tetrabromide, tributyl phosphite, triphenylphosphine, bis(trifluoracetoxy)iodobenzene, rhodamine 6G (Aldrich), copper(I) iodide, 1,4-bis(hydroxyethyloxy)benzene, boron tribromide, ethidium bromide (ACROS), and tetrakis(triphenylphosphine)palladium(0) (Strem) were used without further purification. (23) McQuade, D. T.; Hegedus, A. H.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 12389. (24) Guang, C.; Hong, H. G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 5, 420. (25) Kohli, P.; Blanchard, G. J. Langmuir 1999, 15, 1418.

Pinto et al. N,N-Dimethylacetamide (DMAc) was distilled under reduced pressure, 2,6-lutidine and diisopropylamine were distilled over CaH2, and acetonitrile and dichloromethane were treated with a small amount of P2O5 and then distilled over activated basic aluminum oxide. 1,4-Diethynylbenzene26 and 1,1′-bis(2-phosphonoethyl)-4,4′-bipyridinium dibromide27 were synthesized according to procedures described elsewhere. NMR spectra were recorded on a Varian VXR-300 FT-NMR, operating at 300 MHz for 1H NMR and at 75.4 MHz for 13C NMR. Gel permeation chromatography (GPC) analyses were carried out on a system comprised of a Rainin Dynamax SD-200 pump and a Beckman Instruments Spectroflow 757 absorbance detector, using THF as eluant and two PLgel 5 µm MIXED-D columns (Polymer Laboratories) calibrated using polystyrene standards. FTIR spectra were taken on a Perkin-Elmer 1600 spectrometer. Synthesis. 1,4-Bis(2-bromoethoxy)benzene (1). Carbon tetrabromide (39.8 g, 120.0 mmol) was slowly added in small portions to a solution of 1,4-bis(2-hydroxyethoxy)benzene (10.0 g, 50.4 mmol) and triphenylphosphine (31.5 g, 120.0 mmol) in 300 mL of dry acetonitrile at 0 °C with stirring. The reaction mixture was allowed to warm to room temperature, and the resulting clear solution was stirred for another 4 h under N2. Then 200 mL of cold water was added to the reaction, whereupon product 1 precipitated as a white solid. The product was collected by vacuum filtration, thoroughly washed with methanol/water 60:40, and then recrystallized from methanol. The white flakelike crystals were dried under high vacuum, yield 13.8 g (85%), mp 112-114 °C (lit. 111-113 °C28). 1H NMR (CDCl3; δppm from TMS): 3.61 (t, 4H), 4.24 (t, 4H), 6.86 (s, 2H). 13C NMR (CDCl3; δppm from TMS): 29.95, 69.28, 116.64, 153.38. FTIR (νmax, KBr pellet): 525, 574, 730, 823, 870, 967, 1017, 1074, 1111, 1122, 11217, 1231, 1276, 1414, 1455, 1506, 2864, 2968, 3017 cm-1. 2,5-Diiodo-1,4-bis(2-bromoethoxy)benzene (2). A mixture of 13.0 g of 1 (40.0 mmol), 19.4 g of bis(trifluoracetoxy)iodobenzene (45.0 mmol), and 5.33 g of iodine (42.0 mmol) in 200 mL of dichloromethane was stirred at room temperature for 6 h. The reaction mixture was then diluted with 100 mL of pentane and cooled in an ice/water bath, which promoted the crystallization of product 2. Light pink crystals of the product were obtained after vacuum filtration. The product was washed with cold diethyl ether and crystallized twice from acetone/water 90:10, which furnished pure 2 as white needlelike crystals, yield 14.5 g (63%), mp 140-142 °C. 1H NMR (CDCl3; δppm from TMS): 3.66 (t, 4H), 4.27 (t, 4H), 7.21 (s, 2H). 13C NMR (CDCl3; δppm from TMS): 28.16, 70.41, 86.58, 124.09, 152.82. FTIR (νmax, KBr pellet): 468, 580, 755, 858, 880, 972, 1017, 1056, 1080, 1220, 1266, 1289, 1351, 1390, 1416, 1447, 1486, 2861, 2915, 3086 cm-1. 2,5-Diiodo-1,4-bis(2-dibutoxyphosphonoethoxy)benzene (3). A mixture of 4.0 g of 2 (6.94 mmol) and 17.5 g of tributyl phosphite (70.0 mmol) was refluxed in 50 mL of xylene for 30 h. The solvent, excess triibutyl phosphate, and any dibutyl butylphosphonate formed were removed by vacuum distillation. The remaining light yellow oil was diluted with 100 mL of a diethyl ether/dichloromethane 90:10 mixture, and the resulting clear solution was cooled in a refrigerator overnight. The product was obtained as fine white crystals by vacuum filtration. It was further purified by recrystallization two times from diethyl ether, affording pure 3 as white, fiberlike crystals, yield 4.46 g (80%), mp 66-68 °C. 1H NMR (CDCl3; δppm from TMS): 0.94 (t, 12H), 1.42 (m, 8H), 1.67 (m, 8H), 2.35 (m, 4H), 4.07 (t, 8H), 4.17 (t, 4H), 7.21 (s, 2H). 13C NMR (CDCl3; δppm from TMS): 13.84, 18.95, 25.76, 32.68, 64.91, 65.92, 85.55, 123.60, 152.82. HRMS (FAB): 89.036, 108.998, 221.111, 325.021, 451.874, 803.083 (M + H+) (calcd: 803.083). FTIR (νmax, KBr pellet): 704, 754, 789, 803, 848, 903, 975, 994, 1021, 1059, 1208, 1257, 1354, 1391, 1410, 1453, 1465, 1488, 2872, 2909, 2956 cm-1. 2,5-Diiodo-1,4-bis(2-phosphonoethoxy)benzene, Sodium Salt (4). A solution of 15.3 g of bromotrimethylsilane (13.2 mL, (26) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 627. (27) Felderhoff, M.; Heinen, S.; Molisho, N.; Webersinn, S.; Walder, L. Helv. Chim. Acta 2000, 83, 181. (28) Ashton, P. R.; Boyd, S. E.; Claessens, C. G.; Gillard, R. E.; Menzer, S. Chem. Eur. J. 1997, 3, 788.

Water-Soluble Polymers with Phosphonate Groups 0.1 mol) in 50 mL of CH2Cl2 was added dropwise to an ice-cold solution of 8.0 g of 3 (10.0 mmol) in 80 mL of dry CH2Cl2. After the addition, the resulting solution was warmed to room temperature and stirred for 24 h. The reaction mixture was then poured slowly into a large volume of cold water in order to quench excess bromotrimethylsilane. A thick slurry formed at once, which was collected by vacuum filtration. The solid product was dissolved in a solution of 2.0 g of sodium hydroxide in 40 mL of water. The resulting clear solution was filtered and added to a large volume of acetone, which effected precipitation of 4. The fine white solid obtained was collected by filtration and dried in a vacuum oven at 90-100 °C for 4 h. Salt 4 was very hygroscopic and therefore was stored under high vacuum, yield 6.1 g (92%). 1H NMR (D O; δ 13 2 ppm): 1.80 (m, 4H), 4.00 (t, 4H), 7.31 (s, 2H). C NMR (D2O; δppm): 30.83 and 32.47 (31P coupling), 70.55, 88.36, 126.07, 154.21. FTIR (νmax, KBr pellet): 726, 746, 834, 981, 1074, 1212, 1348, 1461, 1485, 1654, 2951 cm-1. 2,5-Bis-(phenyethynyl)-1,4-bis(2-phosphonoethoxy)benzene, Sodium Salt (PE-PO3-). A solution of 3.3 g of 4 (5.0 mmol), 2.0 g of phenylacetylene (16.6 mmol), 231 mg of tetrakis(triphenylphosphine) palladium(0) (20.0 µmol), 38 mg of copper(I) iodide (20.0 µmol), and 10 mL of diisopropylamine in 50 mL of a methanol/water mixture (60:40) was refluxed for 12 h under argon with stirring. The reaction mixture was then filtered through a 1.0 µm glass-fiber filter, the solvents were distilled off under reduced pressure, and the remaining solid was crystallized three time from methanol/water 20:80, yield 2.3 g (75%). 1H NMR (CD3OD; δppm): 2.01 (m, 4H), 4.06 (m, 4H), 6.82 (s, 2H), 7.16 (m, 6H), 7.29 (m, 4H). 13C NMR (CD3OD; δppm): 29.50 and 31.17 (31P coupling), 67.42, 96.49, 114.59, 118.32, 123.14, 129.52, 132.24, 153.53. FTIR (νmax, KBr pellet): 687, 757, 900, 1023, 1060, 1150, 1220, 1379, 1411, 1443, 1486, 1512, 1592, 1640, 2355, 2958 cm-1. PPE-PO3Bu2. A degassed solution of 139 mg of tetrakis(triphenylphosphine)palladium(0) (12.0 µmol) and 23 mg of copper(I) iodide (12.0 µmol) in 10 mL of a mixture of dimethylacetamide/diisopropylamine (50:50) was added, via cannula, to a degassed solution of 3.210 g of 3 (4.0 mmol) and 504.6 mg of 1,4-diethynylbenzene (4.0 mmol) in 50 mL of dimethylacetamide contained in a Schlenk flask under argon. The resulting deep yellow and blue fluorescent solution was stirred at 50 °C for 6 h. The final viscous solution was poured into a large amount of methanol, which induced the polymer to precipitate. The light yellow polymer fibers that were obtained from the methanol precipitation were redissolved in tetrahydrofuran and precipitated again by addition of methanol. This reprecipitation procedure was repeated five times, whereupon low molecular weight impurities could no longer be observed by GPC analysis. Polymer yield was 1.32 g (49%). GPC (THF, PS standards): Mw ) 109 000, Mn ) 55 000, Mw/Mn ) 1.98. 1H NMR (CDCl3 + 5% CD3OD; δppm from TMS): 0.87 (t, 12H), 1.33 (m, 8H), 1.59 (m, 8H), 2.34 (m, 4H), 4.03 (broad, 8H), 4.26 (broad, 4H), 7.02 (broad, 4H), 7.50 (broad, 2H). 13C NMR (CDCl3 + 5% CD3OD; δppm from TMS): 13.66, 18.83, 25.58, 32.63, 63.83, 66.01, 92.25, 108.20, 117.55, 126.69, 131.67, 153.11. FTIR (νmax, cast film): 545, 739, 800, 837, 898, 1023, 1219, 1275, 1385, 1413, 1493, 1519, 1667, 2716, 2766, 2874, 2962 cm-1. (Spectral and GPC data for PPEPO3Bu2 are available as Supporting Information.) PPE-PO3-. A solution of 2.0 mL of bromotrimethylsilane (15.6 mmol) in 10 mL of dry 1,2-dichlorobenzene was added to a solution consisting of 3.0 mL of dry 2,6-lutidine (21.6 mmol) and 1.00 g of PPE-PO3Bu2 (1.48 mmol PRU) in 150 mL of 1,2-dichlorobenzene. The resulting solution was stirred at room temperature for 6 h and then quenched by the addition of a mixture of 10 mL of methanol and 5.0 mL of 5 M aqueous NaOH. A precipitate formed, and it was collected by filtration, washed with methanol, and dissolved in 100 mL of water at 50-60 °C. An insoluble polymer fraction formed during this step was removed by filtration, and the filtrate was added to a large volume of methanol/acetone/ ether (10:30:60), which induced the polymer to precipitate. The polymer was purified by carrying out the reprecipitation procedure three more times. Finally, the polymer was dissolved in 150 mL of water and the resulting solution was dialyzed against Nanopure water (Millipore Simplicity water system) using a 6-8 kD MWCO cellulose membrane (Fisher Scientific). After dialysis the polymer solution was filtered through a 0.22 µm nylon membrane, and the concentration was adjusted to 1.0 mg mL-1.

Langmuir, Vol. 19, No. 16, 2003 6525 The polymer was stored in this format and diluted as appropriate for spectroscopic studies. Polymer yield was 52%. 1H NMR (DMSO-d6; δppm from TMS): 2.08 (broad, 4H), 4.36 (broad, 8H), 7.39 (broad, 4H), 7.66 (broad, 2H). FTIR (νmax, cast film): 496, 724, 971, 1063, 1216, 1283, 1370, 1414, 1519, 1648 cm-1. (Spectral data for PPE-PO3- are available as Supporting Information.) Absorption and Fluorescence Spectroscopy. UV-visible absorption spectra were obtained on a Varian Cary 100 dual beam spectrophotometer, with a scan rate of 300 nm/min. Corrected steady-state fluorescence spectra were recorded on a SPEX Fluorolog-2 fluorometer. A 450 W xenon lamp was used as excitation source, and emission was detected by a TE-cooled PMT (Hamamatsu R928) operated in photon-counting mode. A 1 cm × 1 cm quartz cuvette was used for solution spectra, and emission was collected at 90° relative to the excitation beam. Borosilicate glass slides were used as substrates for spectra of thin films. Light source/excitation/emission/detector slits were all set to 1.0 mm for all measurements. Fluorescence quantum yields are reported relative to coumarin 30 in acetonitrile (λexc ) 380 nm and φem ) 0.67).29 The optical density of solutions at the excitation wavelength was e0.1, and corrections were applied for differences in the refractive index of standard and sample solutions. All sample solutions were thoroughly purged with argon for 20 min prior to spectroscopic characterizations. Time-resolved fluorescence decays were obtained by time-correlated single photon counting on an instrument that was constructed in-house. A blue diode laser (IBH instruments, 405 nm) was used as excitation source and a photon-counting PMT (Hamamatsu R928) as detector. The fluorescence wavelengths were selected by using band-pass interference filters (fwhm ) 10 nm). Lifetimes were determined from the observed decays with the DAS6 deconvolution software (IBH instruments, Edinburgh). Layer-by-Layer Film Deposition and AFM Imaging. Borosilicate glass slides (0.8 × 3.0 cm) were soaked in piranha solution (H2SO4/H2O2 3:1) for 6 h, rinsed with water, sonicated with 5 N NaOH for 5 min, and rinsed again with water. The PPE-PO3-/PDDA multilayer films were obtained by sequentially soaking the glass substrate for 15 min in PDDA (1 mM PRU in water) and PPE-PO3- solutions (1 mM PRU, in 1:1 DMSO/water). Between each polymer deposition step the slides were rinsed in water and the films were kept wet during all dipping processes. Absorption spectra were obtained immediately after each deposition cycle, and the spectra were obtained with the slides contained in a quartz cuvette filled with water. For deposition of the PPE-PO3-/Zr(IV) films the borosilicate glass slides were cleaned as described above except that sonication with NaOH was omitted. The substrates were dried in an oven at 120 °C and then soaked in a boiling solution of (3-aminopropyl)trimethoxysilane in dry toluene (5 wt %) for 2 h. The amino groups on the silylated substrate surface were phosphorylated by soaking for 12 h in a solution of 10 mM POCl3 in dry 2,6lutidine followed by rinsing with toluene and acetone. Finally, the phosphorylated surfaces were zirconated by soaking the substrates in a 1:1 water/methanol solution of ZrOCl2 (c ) 10 mM) for 2 h followed by rinsing with water. The PPE-PO3-/Zr(IV) LbL films were obtained as described for the PDDA films, except that a 10 mM ZrOCl2 solution in water/methanol 1:1 was used in place of the PDDA solution. Topographic analysis of the LbL films was accomplished by means of atomic force microscopy (AFM) on a Digital Instruments NanoScope III system using a J-type scanner and silicon cantilevers in tapping mode. Scanning was carried out under ambient conditions (20-25 °C, 40-50% relative humidity). WSxM software (version 4.0, Scanning Probe Microscopy Software) was used for statistical analysis of the images. Fabrication and Testing of Polymer Light Emitting Diodes. One-inch square ITO coated glass plates (Delta Technologies, R ) 8-12 Ω-square-1) were used as the substrates. The slides were etched (∼3 mm on each side) by exposing the ITO-coated surface to the vapor of a solution of concentrated HNO3 and HCl in a 1:3 ratio. The ITO in the central area of the slides was protected by paraffin film. Prior to deposition of the conducting layers, the substrate was cleaned via RCA treatment. (29) Jones, G.; Jackson, W. R.; Choi, C.; Bergmark, W. R. J. Phys. Chem. 1985, 89, 294.

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Pinto et al. Scheme 1 a

a Key: (i) Br2/PPh3/CH3CN/room temperature/4 h, 85%. (ii) PhI(CF3CO2)2/I2/CH2Cl2/room temperature/6 h, 63%. (iii) P(OBu)3/ xylenes/reflux 30 h, 80%. (iv) (CH3)3SiBr/CH2Cl2/room temperature/24 h, 92%. (v) Phenyacetylene/Pd(PPh3)4/CuI/iso-Pro2NH/ H2O/MeOH/reflux/12 h, 75%.

Scheme 2 a

a Key: (i) 3/Pd(PPh ) /CuI/DMAc/iso-Pro NH/50 °C/6 h, 49%. (ii) (CH ) SiBr/2,6-lutidine/1,2-dichlorobenzene/room temperature/6 3 4 2 3 3 h, and then NaOH/H2O/MeOH, 52%.

The RCA protocol solution was prepared by mixing concentrated NH3 (70 wt %) and H2O2 (30 wt %) in distilled water in a ratio 1:4:20. The substrates were soaked in a warm RCA solution (60 °C) for 30 min and then rinsed twice in distilled water. To eliminate traces of water, the substrates were rinsed in 2-propanol (HPLC grade). All substrates were then dried in a flow of nitrogen. The cleaned ITO/glass substrates were spin-coated with a thin film (∼20 nm) of the hole-transporting polymer poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT-PSS, Bayer Baytron P VP 4083) by dropping 1 mL of a PEDOT-PSS aqueous dispersion (2 wt % in water, pH ) 2.0) onto the substrate and then spinning at 3000 rpm for 1 min. The PPE-PO3-/Zr(IV) films were then spin-assembled from water/methanol (1:1) polymer solution (1 mM, pH ) 7.0) at 3000 rpm. The spin-assembly process consisted of pipetting 0.2 mL of the PPE-PO3- solution onto the substrate followed by spinning for 30 s. Then the substrate was washed with 2-3 mL of a water/methanol (1:1) mixture and spun for 30 s more. Then 0.2 mL of 5.0 mM ZrOCl2 in water/ methanol (1:1) mixture (pH ) 3.5) was then pipetted onto the substrate followed by spinning for 30 s. The water/methanol rinse step was then carried out again. This sequence was then repeated as many times as needed to build up a multilayer film. The resulting films were dried under vacuum (1 × 10-6 Torr) for 12 h at ambient temperature. Calcium (5 nm) and then aluminum (200 nm) were deposited sequentially by thermal evaporation at 4 × 10-7 Torr. After metal deposition, the devices were encapsulated with epoxy (Loctite quick set epoxy) under an argon atmosphere. Electroluminescence spectra were recorded on a spectrometer consisting of an ISA-SPEX Triax 180 spectrograph equipped with a liquid N2 cooled CCD detector (EEV, back illuminated, 1024 × 128 array). Power for electroluminescence (EL) measurements was supplied using a Keithley 228 voltage/current source. A 100 W primary standard quartz halogen lamp was used to calibrate the Triax 180 spectrograph/CCD

detector system in irradiance units (µW cm-2 nm-1). Measurements were made normal to the surface of the devices, and in the computation of the EL quantum efficiencies it was assumed that the spatial distribution of the emission was Lambertian.30 External device quantum efficiencies were calculated as described in the literature.31

Results and Discussion Synthesis. In previous work we successfully prepared a sulfonate-substituted PPE polymer (PPE-SO3-) via direct polymerization of the ionic monomers in water-DMF solution.3 However, application of a similar strategy to directly prepare PPE-PO3- via Pd(0) catalyzed copolymerization of phosphonate monomer 4 with 1,4-diethynylbenzene did not afford a high molecular weight polymer, presumably because the ionic phosphonate groups coordinate with and deactivate the Pd(0) catalyst. Thus, a different strategy was developed which involved copolymerization of the dibutylphosphonate monomer 3 with 1,4-diethynylbenzene to afford the neutral polymer PPEPO3Bu2, followed by trimethylsilyl bromide (TMS-Br) promoted hydrolysis32 of the butylphosphonate ester side groups in PPE-PO3Bu2 to afford the desired ionic polymer PPE-PO3- (Scheme 2). An advantage of this polymer precursor method is that since the PPE-PO3Bu2 precursor polymer is neutral, it can be analyzed in organic solvents (30) Greenham, N. C.; Friend, R. H.; Bradley, D. D. C. Adv. Mater. 1994, 6, 491. (31) He, Y.; Hattori, R.; Kanicki, J. Rev. Sci. Instrum. 2000, 71, 2104. (32) Salomon, C. J.; Breuer, E. Tetrahedron Lett. 1995, 36, 6759.

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Figure 1. (left) Absorption spectra of PPE-PO3- in aqueous solution as a function of pH. (right) Fluorescence spectra of PPE-PO3in aqueous solution as a function of pH. [PPE-PO3-] ) 1 µM in phosphate buffer at 1 mM, pH range from 7.5 to 12.0 in 0.5 pH unit intervals.

by techniques such as GPC and high-resolution NMR spectroscopy, allowing full characterization of the molecular structure of the polymer. Precursor polymer PPE-PO3Bu2 was obtained by copolymerization of 1,4-diethynylbenzene and 3 in a dimethylacetamide/diisopropylamine solvent mixture. The polymerization afforded a relatively high molecular weight polymer. Hydrolysis of the butylphosphonate ester groups by TMS-Br followed by base extraction furnished the water-soluble PPE-PO3- along with a small amount of insoluble polymer. The amount of this insoluble material was reduced significantly when the hydrolysis was carried out in the presence of dry 2,6-lutidine. The insoluble material is believed to result from cross-linking reactions produced by HBr that is generated as a byproduct of the TMS-Br hydrolysis reaction. Photophysical Properties. Absorption and fluorescence spectra of PPE-PO3- were obtained in 5 mM phosphate buffer solution at pH values ranging from 7.5 to 12.0 (pH intervals of 0.5), and the spectra are illustrated in Figure 1. In general, the polymer features a strong absorption band in the blue region which arises from the long axis polarized π,π* transition along with a moderately strong blue (or green) fluorescence. Interestingly, as can be seen in Figure 1, the absorption and fluorescence are strongly dependent on pH. Specifically, the absorption spectrum undergoes a 35 nm red shift and becomes somewhat narrower with decreasing pH. In addition, an isosbestic point is clearly maintained throughout the entire pH range, suggesting that pH induces a change between two distinct chromophoric states of the polymer. The fluorescence undergoes even more pronounced changes with pH. At high pH, the fluorescence appears as a relatively narrow, structured band which is Stokes shifted very little from the absorption band. However, with decreasing pH, the fluorescence broadens significantly with a new “excimer-like” band appearing which is shifted >50 nm from the high pH fluorescence maximum. The changes observed in the absorption and fluorescence of PPE-PO3- as a function of pH strongly suggest that the polymer exists in an aggregated state at low pH and in a relatively less aggregated or monomeric state at high pH. This premise is supported by a number of facts. First, the absorption and fluorescence of PPE-PO3- at high pH closely resemble the spectra of neutral PPEs in good solvents such as THF or CHCl3.33,20 There is ample (33) Here the term “good solvent” is used in the formal sense to mean a solvent in which the polymer-solvent interactions are stronger than inter- and intrachain polymer-polymer interactions.

evidence to support the notion that these neutral PPEs do not aggregate strongly in good solvents,20 and by analogy this suggests that at high pH PPE-PO3- is not strongly aggregated. Second, the broad fluorescence that is observed from PPE-PO3- at low pH signals that emission occurs exclusively from an “excimer-like” excited-state arising from interchain π-π stacking in the aggregate. Broad, structureless emission is typically observed from small-molecule excimers,34 and from conjugated polymers in the solid state,35 where interchain states dominate the photophysics.20 In addition, it is significant that we have observed very similar spectral changes (i.e., red shift and narrowing in absorption, and red shift and broadnening of the fluorescence) for the sulfonated polymer PPE-SO3when the solvent is changed from methanol to water. In this case the effects have also been attributed to strong aggregation in water.3 Aggregation of PPE-PO3- at low pH is likely induced by hydrophobic and π-π interactions between the polymer chains.36,37 Importantly, the second acid dissociation constant of organophosphonates (pKa2 corresponding to R-PO3H- T R-PO32-) is typically in the range of 7.58.2,38 and therefore the negative charge on the phosphonate side groups in PPE-PO3- is partially neutralized by protonation in the pH range between 8 and 9 (note that this is the pH range where the absorption and fluorescence spectral changes are most abrupt, see Figure 1). Thus, as the pH decreases, electrostatic repulsion between the PPEPO3- chains is decreased and interchain aggregates form. These aggregates may also be stabilized by hydrogen bonding between R-PO3H- residues on adjacent chains in the aggregate.39 Another interesting point concerns the red shift in the absorption that accompanies aggregate formation. The red shift signals an increase in conjugation along the poly(phenylene ethynylene) backbone. We infer from this observation that in the aggregate the chains align with their long axes parallel, and on average the phenylene rings in each PPE-PO3- chain are nearly coplanar (see Scheme 3). This aggregate conformation will optimize hydrophobic interactions between adjacent poly(34) Gordon, M., Ware, W. R., Eds. The Exciplex; Academic Press: New York, 1975. (35) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (36) Perahia, D.; Traiphol, R.; Bunz, U. H. F. J. Chem. Phys. 2002, 117, 1827. (37) Huang, W. Y.; Matsuoka, S.; Kwei, T. K.; Okamoto, Y. Macromolecules 2001, 34, 7166. (38) Crofts, P. C.; Kosolapoff, G. M. J. Am. Chem. Soc. 1953, 75, 3379. (39) Kim, B. S.; Chen, L.; Gong, J. P.; Osada, Y. Macromolecules 1999, 32, 3964.

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Table 1. Photophysical Properties of PPE-PO3Bu2 and PPE-PO3entry

pH

absorption λmax/nm

PPE-PO3Bu2a PPE-PO3PPE-PO3PPE-PO3

11.0b 9.0b 7.5b

424 412 420 444

fluorescence λmax/nm 443 437 437, 508 518

τem/ns at 450 nm (amplitude/%)

τem/ns at 550 nm (amplitude/%)

φfc

0.28 (100) 0.18 (100) 0.23 (85), 3.15 (15) 0.25 (43), 4.51 (57)

0.31 (100) 0.12d (100) 0.43 (35), 3.36 (65) 0.86 (35), 5.25 (65)

0.53 0.05 0.05 0.03

a Cyclohexanone solution. b Aqueous solution buffered with 5.0 mM Na PO and pH adjusted with 0.1 N HCl. c Fluorescence quantum 3 3 yields measured relative to coumarin-30 in acetonitrile (φf ) 0.67). d Upper limit, resolution limited by response of fluorescence lifetime instrument.

mer chains and also allows the polar phosphonate groups to extend into the solvent. More information regarding the effect of solution pH on the photophysics of PPE-PO3- comes from the study of the fluorescence quantum yields and lifetimes (φf and τem, Table 1). The neutral polymer precursor PPE-PO3Bu2 features a very high φf value (0.53) and a single-exponential lifetime of 280 ps. These fluorescence properties are similar to those observed for other neutral PPE-type polymers in

good solvents.20 At pH ) 11, the fluorescence decay of PPE-PO3- is single exponential; however, the lifetime and quantum yield are significantly lower compared to those of PPE-PO3Bu2. These observations support the hypothesis that PPE-PO3- is not strongly aggregated at high pH. However, some excited-state quenching is still evident, as reflected by the reduced lifetime and quantum efficiency, and this may arise from weak interchain interactions.

Water-Soluble Polymers with Phosphonate Groups

Evidence for strong aggregation of PPE-PO3- as the pH is decreased is also evident in the φf and τem data. Specifically, as the pH decreases, the emission decays become distinctly biexponential and wavelength dependent. At pH ) 9, the decay at 450 nm features a large amplitude, short-lived component and a low amplitude component with a lifetime of ≈3 ns. Interestingly, the amplitude of the 3 ns component is larger when the emission decay is monitored at 550 nm. Note that the amplitude of the long-lived component is even larger at pH ) 7.5. This evidence clearly indicates that the long lifetime emission decay component is due to emission from the “excimer-like” aggregate state. The longer lifetime for the excimer-like state is consistent with the fact that radiative (and nonradiative) decay from excited-state complexes (excimers and exciplexes) is forbidden.34 Layer-by-Layer Self-Assembled Films. One of our motivations for synthesizing PPE-PO3- was to produce a conjugated polyelectrolyte that could be used to fabricate self-assembled films via LbL methods. The resulting CPE films can be used in a variety of applications, including solid-state fluorescence sensors,20,40,41 electroluminescent devices,2,7,15 and photovoltaic devices.42,43 The phosphonate side groups in PPE-PO3- make it possible to deposit LbL films by two approaches. The first is based on electrostatic self-assembly of PPE-PO3- (a polyanion) with a polycation.44 A number of groups have used this method to fabricate ultrathin films using a variety of functional polyelectrolytes, including several examples that incorporate conjugated polyelectrolytes.2,14,15 The second approach is based on Zr(IV)-templated self-assembly of PPE-PO3- films. This method for LbL film fabrication is based on an extensive body of work that demonstrates the fabrication of nanostructured ultrathin films by alternate deposition of Zr(IV) and bis-phosphonates.24,25 In the succeeding section we describe the application of these two methods for film formation and compare the properties of the resulting films. Electrostatic self-assembled LbL films were fabricated by alternately dipping borosilicate glass substrates into a solution containing PPE-PO3- and one containing poly(diallyldimethylammonium chloride), PDDA (see structures), which is a cationic, semiflexible polyelectrolyte. The Zr(IV)-templated LbL films were deposited by alternately dipping substrates into a solution containing PPEPO3- and one containing ZrOCl2. In both approaches the solvent used to prepare the PPE-PO3- dipping solutions consisted of a 50-50 mixture of DMSO and water, with pH values adjusted between 4 and 8. The DMSO-water solvent mixture was used because it was found that addition of DMSO prevented PPE-PO3- from precipitating at low pH. Depositions were monitored by following the absorption spectra of the films as a function of deposition step. In addition, the surface morphology of five-bilayer films produced by deposition from solutions of differing pH was probed by ex situ AFM. Figure 2 illustrates the absorption spectroscopic data for the PPE-PO3-/PDDA films and the PPE-PO3-/Zr(IV) films, and Figure 3 illustrates typical AFM images. Table 2 lists parameters for the films computed from the absorption and AFM measurements. (40) Jones, R. M.; Bergstedt, T. S.; McBranch, D. W.; Whitten, D. G. J. Am. Chem. Soc. 2001, 123, 6726. (41) Liu, Y.; Mills, R. C.; Boncella, J. M.; Schanze, K. S. Langmuir 2001, 17, 7452. (42) Baur, J. W.; Durstock, M. F.; Taylor, B. E.; Spry, R. J.; Reulbach, S.; Chiang, L. Y. Synth. Met. 2001, 121, 1547. (43) Durstock, M. F.; Taylor, B.; Spry, R. J.; Chiang, L.; Reulbach, S.; Heitfeld, K.; Baur, J. W. Synth. Met. 2001, 116, 373. (44) Decher, G. Science 1997, 277, 1232.

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Figure 2. Absorption spectra as a function of the number of layers deposited for LbL self-assembled films of PPE-PO3-: (a) PPE-PO3-/PDDA film; (b) PPE-PO3-/Zr(IV) film. Insets illustrate plots of the absorbance at λmax vs layer number: (9) pH ) 8; (2) pH ) 6; ([) pH ) 4.

Figure 3. Tapping mode height AFM images of five-bilayer LbL films of PPE-PO3- deposited from DMSO/water solutions (1:1) of varying pH. Height scale is 0-200 Å. Set of three images at left is for PPE-PO3-/PDDA films (a) pH ) 8, (b) pH ) 6, and (c) pH ) 4. Set of three images at right is for PPE-PO3-/Zr(IV) films (d) pH ) 8, (e) pH ) 6, and (f) pH ) 4.

Several trends are apparent from inspection of the data. First, as shown by the insets in Figure 2, deposition of both PPE-PO3-/PDDA and PPE-PO3-/Zr(IV) films occurs very regularly. Specifically, almost all of the plots of film

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Pinto et al.

Table 2. Characterization of PPE-PO3- LbL Films deposition type PDDA Zr (IV)

pHa

abs inc/10-3 b

Γ / 10-10 mol cm-2 c

roughness/nmd

4.0 6.0 8.0 4.0 6.0 8.0

38.3 26.3 10.3 14.8 3.8 1.0

8.0 5.5 2.2 3.0 0.8 0.2

39.6 14.5 4.9 19.3 6.6 7.9

a PPE-PO - solution was buffered with 5.0 mM NaOAc and pH 3 adjusted with 10% acetic acid. b Absorption increment at λmax per bilayer adsorbed. c Average surface coverage per bilayer computed for five-bilayer thick films. d Surface roughness computed from AFM.

absorption vs layer number are linear. Second, the thickness of the polymer film deposited per bilayer increases significantly with decreasing pH. Third, the PPE-PO3-/PDDA bilayers, which are produced by electrostatic self-assembly, are thicker than the PPE-PO3-/ Zr(IV) bilayers, which are produced by templated selfassembly. Another subtle feature is that the absorption spectra of the PPE-PO3-/PDDA films are red shifted and spectrally narrower, while the spectra of the PPE-PO3-/ Zr(IV) films are blue shifted and broader. Note that the red-shifted absorption is associated with the polymer in a strongly aggregated form wherein the phenylene rings are coplanar (Scheme 3). The difference in the absorption spectra for the PPE-PO3-/PDDA and PPE-PO3-/Zr(IV) LbL films suggests that the polymer is present in a strongly aggregated form in films formed by electrostatic selfassembly and in a less ordered and less aggregated state in the films formed by templated self-assembly. Analysis of the absorption data under the assumption that the absorptivity (per polymer repeat unit, PRU) is  ) 4.8 × 107 cm2 mol-1 (value determined from PPE-PO3in solution, corresponds to 48 000 M-1 cm-1) allows estimation of the surface coverage (Γ) in PRU bilayer-1 cm-2 from the absorption spectra of the films (Γ ) ∆A/, where ∆A is the absorption increase per bilayer deposited). The surface coverage values are compiled in Table 2. As a point of reference, the approximate surface coverage of a monolayer of PPE-PO3- is ≈1.4 × 10-10 mol cm2 (this estimate is based on an area per PRU of 120 Å2, which would be consistent with layers in which the phenylenes are “face down”). This analysis suggests that for the PPEPO3-/Zr(IV) films deposited at pH g 6 less than a monolayer (on average) is deposited in each step. However, for pH ) 4, the Zr(IV) templated deposition affords films that contain slightly more than a monolayer per bilayer. By contrast, at all pH values PPE-PO3-/PDDA film deposition results in films that contain in excess of a single monolayer per deposition step. Indeed, at the lowest pH value, the absorption change suggests that the equivalent of five or more monolayers is deposited in each “layer”. To obtain additional information concerning the morphology of the PPE-PO3-/PDDA and PPE-PO3-/Zr(IV) LbL films, AFM imaging was carried out on a series of fivebilayer films produced at various pH values ranging from 4 to 8 (Figure 3). The AFM images reveal that both the PPE-PO3-/PDDA (Figure 3a-c) and PPE-PO3-/Zr(IV) (Figure 3d-f) LbL films are nonuniformly distributed on the surface of the substrate. Specifically, in both cases the polymer appears to be deposited onto the substrates as “clumps” or aggregates. For the PPE-PO3-/Zr(IV) films, as pH is decreased, the film structure appears to “fill-in”, resulting in a more uniformly distributed film. However, for the PPE-PO3-/PDDA films, at lower pH, the “clumps” appear to become even more pronounced, resulting in a

film surface morphology that is very rough. The difference in film morphology derived from the two different deposition methods is also reflected by the average roughness values (Table 2, last column) which are computed from the AFM data obtained on the five-bilayer films. It is evident that on average the PPE-PO3-/PDDA films are “rougher” and the difference in surface roughness increases as the pH of the deposition solution is decreased. The general observation that the thickness of the PPE-PO3- LbL films deposited by either method increases with decreasing pH of the deposition solution is consistent with previous work by other groups on LbL deposition of weak polyelectrolytes such as poly(acrylic acid).17,45 This effect is believed to arise because of the influence of the deposition solution pH on the charge density of the adsorbing polymer (and on the previously adsorbed polymer layer).46 For a fixed charge density on the surface, a larger number of weak polyacid repeat units is required for charge compensation, leading to an increased effective layer thickness. The solution pH also affects the polyacid chain conformation, owing to the effect of pH on the extent of ionization of the weak acid side groups.17 Specifically, the polymer backbone of a weak polyacid assumes a coiled conformation at low pH due to the low level of ionization of the polyacid groups (which effectively decreases intrachain electrostatic repulsion). As a consequence, at low pH thicker films are produced by LbL deposition as a result of chain entanglements and the formation of loops. Similar effects are observed in LbL films fabricated with salt added to the deposition solutions; in this case the increase in the ionic strength of the deposition solution screens intrachain electrostatic repulsions inducing the polyelectrolyte chains to adopt coiled conformations in solution.47 Due to the fact that PPE-PO3- is a weak polyelectrolyte, the charge density on the polymer backbone is expected to decrease with decreasing pH due to protonation of the R-PO32- groups, which is expected to occur between pH ) 7 and 8. This factor alone is expected to increase the amount of material that deposits at low pH due to electrostatic effects. However, the situation with the PPEPO3- system is believed to be more complex than simple electrostatic effects. We believe that a second important feature which drives increased amount of PPE-PO3deposited with decreasing pH is aggregation of the polymer. Specifically, as shown above, optical absorption and photoluminescence studies provide clear evidence that the extent of aggregation of PPE-PO3- in solution increases with decreasing pH. Thus, it is likely that when LbL film deposition is carried out at low pH, the material adsorbs from solution in an aggregated state. This explains the AFM observations which show that the surface of the fivebilayer LbL films are comparatively rough and appear as though PPE-PO3- deposits onto the surface in “clumps”. There is another more subtle feature with respect to the LbL films which has implications with respect to the mechanism for the deposition. Specifically, close inspection of the AFM images reveals that, in general, Zr(IV)templated deposition produces films that consist of relatively small aggregate domains. By contrast, the electrostatic self-assembled films with PDDA feature much larger domains; indeed, the films deposited at pH ) 4 feature very large “mushroom-like” features on the surface. This difference likely arises because the Zr(IV)-templated deposition is likely irreversible, and the composition of (45) Mendelsohn, J. D.; Barret, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (46) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (47) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153.

Water-Soluble Polymers with Phosphonate Groups

the material that adsorbs directly reflects the structure of the aggregates that are present in solution. However, in LbL electrostatic deposition with PDDA, the deposition may be reversible such that rearrangement and aggregation of the polymer may continue to occur on the surface during the deposition period. This results in the larger aggregate domains and increases amount of material that ultimately deposits. Amplified Fluorescence Quenching in Solution. A number of recent studies have demonstrated that the fluorescence of conjugated polyelectrolytes is quenched with very high sensitivity by molecular quenchers that have opposite charge relative to the polymer.3,8-10,48 This “amplified quenching”21 effect is believed to arise due to two factors. First, the polymer and quencher interact by ion pairing, which effectively increases the local concentration of the quencher in the vicinity of the polymer. Second, the singlet excited state is significantly delocalized in the polymer and is also able to diffuse rapidly along a single chain and between chains due to interchain aggregation. These effects significantly increase the effective “quenching radius” of the quencher.10 One objective of the work on PPE-PO3- is to develop a material that can be used as a fluorescence sensor.8,41 This application requires that the polymer display the amplified quenching effect. Therefore, in the present investigation the quenching of PPE-PO3- fluorescence by charged electron and energy transfer quenchers was evaluated, with the objective of determining if the material exhibits amplified quenching. (The use of this polymer in sensor applications will be reported elsewhere.) Comparative experiments were performed using PPE-PO3- and the monomer model compound PE-PO3- along with the quenchers N,N′-dimethyl viologen (MV), ethidium bromide (EB), and rhodamine 6G (R6G). All three quenchers are cationic and, consequently, are anticipated to ion pair with the phosphonate side groups. MV is expected to quench by oxidative electron transfer,49 while EB and R6G are expected to quench via the singlet-singlet (Fo¨rster) energy transfer mechanism.50 Quenching experiments were carried out in unbuffered aqueous solutions, and the concentration of PPE-PO3and PE-PO3- was 1.0 µM ([PRU] for the polymer). No change was observed in the pH of the solutions during the titrations, and therefore the fluorescence response is not influenced by a pH change induced aggregation of PPEPO3-. Figure 4 illustrates Stern-Volmer fluorescence quenching plots (I°/I vs [Q]) for the polymer and monomer model compound. All plots exhibited nearly linear correlations within the quencher concentration range employed. The KSV values, calculated from the slopes of the linear plots, are collected in Table 3. First, for model compound PE-PO3- some quenching is observed even at sub-µM concentrations of the quencher, with KSV values in the range of 105 M-1 obtained. In view of the relatively short fluorescence lifetime of the model (τ ) 2.2 ns), the observation of such large KSV values demonstrates that static quenching dominates. Presumably, the cationic quenchers ion pair with PE-PO3-, and the KSV values reflect the stability constant for the complex.51 Interestingly, for all three quenchers the KSV values for quenching (48) Whitten, D.; Chen, L.; Jones, R.; Bergstedt, T.; Heeger, P.; McBranch, D. In Optical Sensors and Switches; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 2001; p 189. (49) Bock, C. R.; Meyer, T. J.; Whitten, D. G. J. Am. Chem. Soc. 1974, 96, 4710. (50) Fo¨rster, T. Discuss. Faraday Soc. 1959, 27, 7. (51) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plenum Publishers: Dordrecht, 1999.

Langmuir, Vol. 19, No. 16, 2003 6531

Figure 4. Stern-Volmer quenching plots for steady-state fluorescence quenching for (a) PPE-PO3-, x-axis scale at top, and (b) PE-PO3-, x-axis scale at bottom. Note difference in y axis scales in (a) and (b). Key: ([) rhodamine 6G; (2) methyl viologen; (1) ethidium bromide. Table 3. Fluorescence Quenching of PE-PO3- and PPE-PO3- a KSV/M-1

a

-

quencher

PPE-PO3

PE-PO3-

R6G MV EB

4.7 × 107 3.2 × 107 1.1 × 107

1.4 × 105 1.0 × 105 1.5 × 105

Experiments in unbuffered aqueous solution.

of PPE-PO3- are more than 100 times larger than those for the model compound. This observation clearly indicates that the quenching is amplified in the polymer. Although ion pairing between the polymer and the cationic quenchers is important, the increased quenching response for the PPE-PO3- must be due to intrachain singlet exciton delocalization and rapid intra- and interchain exciton transport. Interestingly, despite the difference in quenching mechanism for R6G and MV, the KSV values are comparable. This suggests that the amplified quenching effect does not require long-range Fo¨rster transfer from the polymer to a dye acceptor (since this quenching mechanism is not possible for MV). It is noteworthy that the sensitivity of PPE-PO3- to quenching by R6G and MV is so large that it is possible to detect these quenchers at concentrations