Langmuir 2003, 19, 7963-7969
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Enhanced Fo1 rster Resonance Energy Transfer in Electrostatically Self-Assembled Multilayer Films Made from New Fluorescently Labeled Polycations Jean-Franc¸ ois Baussard,†,‡ Jean-Louis Habib-Jiwan,*,† and Andre´ Laschewsky*,‡,§ Department of Chemistry, Universite´ Catholique de Louvain, Place L. Pasteur 1, B-1348 Louvain-la-Neuve, Belgium, Fraunhofer Institute fu¨ r Angewandte Polymerforschung FhG-IAP, Geiselbergstrasse 69, D-14476 Golm, Germany, and Universita¨ t Potsdam, Institut fu¨ r Chemie, P.O. Box 601553, D-14415 Potsdam, Germany Received April 23, 2003. In Final Form: June 26, 2003 The synthesis of two new complementary fluorescently labeled polycations, which are derived from 4-(vinylbenzyl chloride) and coumarin, and their use for layer-by-layer multilayers films applying electrostatic self-assembly (ESA) are reported. The alternating polyelectrolyte deposition cycles were followed by UVvis spectroscopy, ellipsometry, and X-ray reflectivity. Regular growth was observed. Fluorescence and UV-vis measurements showed the formation of fluorescent dye aggregates for one coumarin derivative in the ESA multilayers. The resulting shift in the spectra enhances the spectral overlap between the two fluorescently labeled polycations when used in mixed thin films, improving the efficiency of the Fo¨rster resonance energy transfer between the chromophores. The nonradiative nature of the energy transfer was confirmed by fluorescence decay time measurements.
Introduction The alternating adsorption of polyelectrolytes of opposite charge, the so-called electrostatical self-assembly (ESA) or layer-by-layer (LbL) method, has evolved in the past years as a versatile and powerful method to grow thin polymeric films on a variety of solid substrates.1,2 A number of studies on such layer-by-layer grown polyelectrolyte films have employed fluorescent species. In the majority of cases, their use was linked to the construction of light-emitting devices (LEDs). Typical fluorescent species used in this context have been poly(phenylene vinylene) and its derivatives,3-6 derivatives of the ruthenium(II) bipyridyl complex,7,8 and semiconductor nanoparticles, such as CdSe or CdTe.9-11 Alternatively, fluorophore-labeled polyelectrolytes have been * To whom correspondence should be addressed. E-mail: habib@ chim.ucl.ac.be (Jean-Louis Habib-Jiwan); laschews@ rz.uni-potsdam.de (Andre´ Laschewsky). † Universite ´ Catholique de Louvain. ‡ Fraunhofer Institute fu ¨ r Angewandte Polymerforschung FhG-IAP. § Universita ¨ t Potsdam. (1) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (2) Multilayer Thin Films. Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003. (3) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (4) Hong, H.; Davidov, D.; Tarabia, M.; Chayet, H.; Benjamin, I.; Faraggi, E. Z.; Avny, Y.; Neumann, R. Synth. Met. 1997, 85, 1265. (5) Ho, P. K. H.; Granstro¨m, M.; Friend, R. H.; Greenham, N. C. Adv. Mater. 1998, 10, 769. (6) Marletta, A.; Castro, F. A.; Borges, C. A. M.; Oliveira, O. N., Jr.; Faria, R. M.; Guimara˜es, F. E. G. Macromolecules 2002, 35, 9105. (7) Wu, A.; Yoo, D.; Lee, J. K.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 4883. (8) Jiang, X. P.; Clark, S. L.; Hammond, P. T. Adv. Mater. 2001, 13, 1669. (9) Gao, M.; Lesser, C.; Kirstein, S.; Mo¨hwald, H.; Rogach, A.; Weller, H. J. Appl. Phys. 2000, 87, 2297. (10) Gao, M. Y.; Sun, J. Q.; Dulkeith, E.; Gaponik, N.; Lemmer, U.; Feldmann, J. Langmuir 2002, 18, 4098. (11) Crisp, M. T.; Kotov, N. A. Nano Lett. 2003, 3, 173.
mostly used for visualization or tracing purposes, respectively. Fluorescein- or rhodamin-labeled poly(amine)s have been typically employed in such studies.12-14 Only few other polymer-bound fluorophore systems have been reported in ESA films, so far.15-19 In any case, the use of fluorescent labels in multilayer films is not trivial, because when present in high concentration, efficient or even complete fluorescence quenching is frequently observed.19-25 Fluorescent polyelectrolytes have been rarely used to gain structural information about films made by the ESA method up to now. Some structural changes during the growth of the first deposited polymer layers were demonstrated by qualitative changes in the fluorescence emission spectra.26 Also, in polyelectrolyte films employing (12) Donath, E.; Moya, S.; Neu, B.; Sukhorukov, G. B.; Georgieva, R.; Voigt, A.; Ba¨umler, H.; Kiesewetter, H.; Mo¨hwald, H. Chem.sEur. J. 2002, 8, 5481. (13) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531. (14) Shenoy, D. B.; Antipov, A. A.; Sukhorukov, G. B.; Mo¨hwald, H. Biomacromolecules 2003, 4, 265. (15) Lee, T. S.; Yang, C.; Park, W. H. Macromol. Rapid Commun. 2000, 21, 951. (16) Zotti, G.; Schiavon, G.; Zecchin, S.; Berlin, A.; Giro, G. Synth. Met. 2001, 121, 1381. (17) Constantine, C. A.; Mello, S. V.; Dupont, A.; Cao, X.; Santos, D., Jr.; Oliveira, O. N., Jr.; Strixino, F. T.; Pereira, E. C.; Cheng, T.-C.; Defrank, J. J.; Leblanc, R. M. J. Am. Chem. Soc. 2003, 125, 1805. (18) Rousseau, E.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 2000, 16, 8865. (19) Saremi, F.; Lange, G.; Tieke, B. Adv. Mater. 1996, 8, 923. (20) Araki, K.; Wagner, M. J.; Wrighton, M. S. Langmuir 1996, 12, 5393. (21) Lowack, K.; Helm, C. Macromolecules 1998, 31, 823. (22) Lee, T. S.; Kim, J.; Kumar, J.; Tripathy, S. Macromol. Chem. Phys. 1998, 199, 1445. (23) Sun, J.; Zou, Sh.; Wang, Zh.; Zhang, X.; Shen, J. Mater. Sci. Eng., C 1999, 10, 123. (24) Kometani, N.; Nakajima, H.; Asami, K.; Yonezawa, Y.; Kajimoto, O. J. Phys. Chem. B 2000, 104, 9630. (25) Lee, S. H.; Kumar, J.; Tripathy, S. K. Langmuir 2000, 16, 10482. (26) Hong, H.; Tarabia, M.; Chayet, H.; Davidov, D.; Faraggi, F. Z.; Avny, Y.; Neumann, R.; Kirstein, S. J. Appl. Phys. 1996, 79, 3082.
10.1021/la030172k CCC: $25.00 © 2003 American Chemical Society Published on Web 08/19/2003
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exfoliated inorganic layered crystals, the lack of fluorescence quenching of a polycation redox pair indicated the efficient separation of two following polycation layers by the rigid and rather thick inorganic sheets employed as polyanions.27 Such an efficient layer separation in certain organic-inorganic hybrid films differs crucially from the normal interdigitated structure of layer-by-layer deposited films made from flexible organic polyelectrolytes.13,28,29 This was confirmed by Fo¨rster resonance energy transfer (FRET)30-32 studies. In fact, FRET is a powerful method to analyze the distribution of chromophores within a matrix and to evaluate their average separation. However, for purely organic ESA films, FRET studies corroborated the close mixing of polyelectrolytes as well as of dye aggregates within ESA films.24,33,34 Also, swelling/deswelling processes could be followed by nonradiative energy transfer studies.35 Here, we report on the use of coumarin-derivatized polyelectrolytes in multilayered thin films and their use for FRET studies. Coumarin dyes are known as efficient fluorophores whose spectral characteristics can be finetuned by appropriate substitution.36,37 But they have been hardly employed in ESA multilayer films so far.32 In this study, the coumarin derivatives coumarin 4 and coumarin 343 were employed because their spectral characteristics make them suited for FRET and because they dispose of a functional group suitable for grafting them onto the reactive polymer chosen as precursor, namely, on poly(4-vinylbenzyl chloride). Experimental Section Materials. Substrates in the ESA process were either Suprasil quartz plates (no. 665-000 Hellma-Benelux) or one-side polished n-type 〈100〉 silicon wafers (ACM-France) that were cut into rectangles of 1 cm × 3 cm. The two substrates were cleaned by treating them for 30 min with hot “piranha acid” (1:1 v/v 98% H2SO4/28% H2O2; caution: piranha acid is extremely corrosive and reacts violently with organic compounds), followed by rinsing with ultrapure water and by drying with a stream of hot air. The so-prepared substrates were stored in Fluoroware individual boxes (Entregris) to avoid contamination and used within 24 h. This treatment produces silicon plates with a highly hydrophilic surface that is attributed to a dense layer of silanol groups left on the native layer of silicon oxide. The presence of acidic silanol groups on the surface results in a globally negative surface charge. Chemicals. 4-Vinylbenzyl chloride (VBC) (98% para-isomer, Aldrich, inhibited with tert-butylcatechol and nitroparaffin) was purified prior to polymerization by column filtration (aluminum oxide, activated, basic, 50-200 mesh, Acros Organics) to remove the inhibitors and stored at +4 °C in dark. The purity was checked by 1H NMR. Phenylmagnesium chloride (2 M) in THF, carbon disulfide, benzyl chloride, and N-methylmorpholine were used as received from Aldrich. Coumarin 4 (C4) laser grade (7-hydroxy4-methylcoumarin C10H8O3 M ) 176.17 g mol-1, Acros Organics) (27) Keller, S. W.; Johnson, S. A.; Brigham, E. S.; Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879. (28) Decher, G. Science 1997, 277, 1232. (29) Richert, L.; Lavalle, Ph.; Vautier, D.; Senger, B.; Stoltz, J.-F.; Schaaf, P.; Voegel, J.-C.; Picart, C. Biomacromolecules 2002, 3, 1170. (30) Kaschak, D. M.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 4222. (31) Kerimo, J.; Adams, D. M.; Barbara, P. F.; Kaschak, D. M.; Mallouk, T. E. J. Phys. Chem. 1998, B102, 9451. (32) Kaschak, D. M.; Lean, J. T.; Waraksa, C. C.; Saupe, G. B.; Usami, H.; Mallouk, T. E. J. Am. Chem. Soc. 1999, 121, 3435. (33) Dai, Z. F.; Da¨hne, L.; Donath, E.; Mo¨hwald, H. Langmuir 2002, 18, 4553. (34) Dai, Z.; Mo¨hwald, H.; Tiersch, B.; Da¨hne, L. Langmuir 2002, 18, 9533. (35) Ibarz, G.; Da¨hne, L.; Donath, E.; Mo¨hwald, H. Adv. Mater. 2001, 13, 1324. (36) Reynolds, G. A.; Drehxhage, K. H. Opt. Commun. 1975, 13, 222. (37) Drehxhage, K. H.; Erikson, G. R.; Hawks, G. H.; Reynolds, G. A. Opt. Commun. 1975, 13, 399.
Baussard et al. was crystallized from ethanol prior to use. Coumarin 343 (C343) laser grade (C16H15NO4, M ) 285.29 g mol-1), (benzotriazol-1yloxy)-tripyrrolidinophosphonium hexafluorophosphate (PyBOP, 99%), 3-(dimethyl-amino)propylamine, and N,N-diisopropylethylamine were used as received from Acros Organics. Poly(sodium 4-stryrenesulfonate) (PSS) (Mw ca. 70 000 g mol-1, Aldrich) was used as received. All solvents were analytical grade (Riedel-de Hae¨n). Ultrapure water (resistance, 18.2 MΩ cm-1) was purified by a Maxima ultrapure water system (Elga). Silicagel for column chromatography was purchased from Merck (Silicagel 60, 0.040-0.063 mm). Benzyl dithiobenzoate (BDTB) was synthesized according to a general procedure. BDTB was purified by flash chromatography on silicagel (eluent, diethyl ether), removing the eluent and some residual volatile impurities at 35 °C under reduced pressure (0.8 Torr). 1H NMR (200 MHz, in CDCl3, δ in ppm): 4.60 (s, 2H, -S-CH2-); 7.20-7.60 (m, 8H, CH aryl); 8.00 (d, 2H, CH aryl o-position of Φ-CSS-). Elemental analysis (C14H12S2, Mr ) 244.38) calcd: %C ) 68.88, %H ) 4.95, %S ) 26.24. Found: %C ) 68.2, %H ) 4.7, %S ) 26.2. FT-IR selected bands: 3060-3030 cm-1 (aromatic C-H, ν), 2925 cm-1 (aliphatic C-H, ν), 1494 and 1444 cm-1 (aromatic ring, ν), 1225 and 1044 cm-1 (CdS, ν). Poly(4-vinylbenzyl chloride) (PVBC) was synthesized via free radical polymerization of VBC in bulk, using the reversible addition-fragmentation chain transfer (RAFT) method.38-40 Azoisobutyronitrile (AIBN) was recrystallized from ethanol just before use. VBC (4 mL), AIBN (4 mg, 2.4 × 10-5 mol), and the RAFT agent BDTB (40 mg, 1.6 × 10-4 mol) are placed in a Schlenk tube. After degassing by several freeze-thaw cycles, the tube is placed into an oil bath at 60 °C. The polymerization is stopped by cooling the mixture quickly to room temperature after 2 and 3 h for samples 1 and 2, respectively. Reaction mixtures were diluted by THF, and polymers were precipitated into methanol, isolated, and dried. The polymer characteristics were determined by size exclusion chromatography. The molar masses of sample 1 were determined as Mn ) 4135 g mol-1 (DPn ) 27), and Mw/Mn ) 1.22, whereas for sample 2, values of Mn ) 5929 g mol-1 (DPn ) 38), and Mw/Mn ) 1.14 were determined. For polymer PVBA-C4, 0.5 g (8.9 mmol) of powdered KOH in 10 mL of DMSO was stirred for 5 min, before adding 0.3 g (1.7 mmol) of coumarin 4 and 0.3 g (2.0 mmol) of sample 1 of poly(4-vinylbenzyl chloride). The mixture was heated at 60 °C for 12 h. By integration of the 1H NMR signals at 4.96 ppm (-O-CH2Aryl) and at 4.51 ppm (Cl-CH2-) of a sample taken at this time, a content of 10 mol % of chromophore in the polymer was estimated. Then, 0.4 g (4.0 mmol) of N-methylmorpholine was added, and the reaction was pursued at 60 °C for 72 h. The labeled polycation PVBA-C4 was dialyzed against pure water using a Spectra/Por membrane (nominal cutoff, 6000-8000). The dialyzed solution was lyophilized, and the polymer was stored in the dark under a nitrogen atmosphere; yield, 522 mg. 1H NMR (300 MHz, in D2O, δ in ppm): 0.7-2.2 (m broad, backbone); 2.53.6 (m broad, CH3-N+-CH2-, -CH2-O-); CH2-O-; 3.90 (s broad, Ar-CH2-N+); 4.45 (s broad, Ar-CH2-Cl); 6.60 and 7.20 (s broad, Ar-H). The molar ratio of fluorescently labeled repeat units p was determined via the absorbance band of PVBA-C4 in water at λmax ) 318 nm, assuming that the extinction coefficient of ) 12 000 L mol-1 cm-1 determined in CH3CN is not changed by the grafting. Though presumably less precise, the value of p is corroborated by integration of the 1H NMR signals (vide supra). The ratio of unmodified repeat units n and of quaternized repeat units m was determined by elemental analysis (%C ) 57.21, %H ) 7.48, %N ) 5.23, %Cl ) 9.05) from the chlorine and the nitrogen contents: m ) 0.73, n ) 0.21, and p ) 0.06. For polymer PVBA-C343, 200 mg (7.0 × 10-4 mol) of coumarin 343, 401 mg (7.7 × 10-4 mol) of PyBOP, 80 mg (7.7 × 10-4 mol) of 3-(dimethylamino)propylamine, and 317 mg (2.5 × 10-4 mol) of N,N-diisopropylethylamine in 10 mL of CH2Cl2/CH3CN (1:1 v/v) were stirred overnight at room temperature. The reaction (38) Le, T. P. T.; Moad, G.; Rizzardo, E.; Thang, S. International Patent Application PCT WO9801478 [CA, 1998: 115390]. (39) Rizzardo, E.; Chiefari, J.; Chong, B. Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Thang, S. H. Macromol. Symp. 1999, 143, 291. (40) Mayadunne, R. T. A.; Jeffery, J.; Moad, G.; Rizzardo, E. Macromolecules 2003, 36, 1505.
Enhanced Fo¨ rster Resonance Energy Transfer medium was concentrated and purified by column flash chromatography on silicagel (eluent, CHCl3/CH3OH 80:20 v/v). The second, colored fraction was collected, and the solvents were removed. The residue was dissolved in CHCl3/CH3OH 90:10 v/v. The modified coumarin 343, bearing now a tertiary amine moiety, was obtained by crystallization from this mixture by adding dropwise pentane; yield, 235 mg (81%). UV-vis: main absorbance bands at λ1 ) 438 nm and λ2 ) 269 nm. FT-IR selected bands: 1688 cm-1 st (amide I). Elemental analysis (C21H27N3O3, Mr ) 369.44 g.mol-1) calcd: %C ) 68.27, %H ) 7.36, %N ) 11.37. Found: %C ) 68.15, %H ) 7.72, %N ) 11.48. 1H NMR (300 MHz, in CD3OD, δ in ppm): 1.6-1.9 (m, 6H, N-C-CH2-C-N, Aryl-CH2-), 2.26 (s, 6H, -N(CH3)2), 2.43 (t, 2H, -CH2-N
