Energy Transfer from a Fluorescent Hydrogel to a Hosted Fluorophore

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Langmuir 2006, 22, 2299-2303

2299

Energy Transfer from a Fluorescent Hydrogel to a Hosted Fluorophore Marco Montalti,*,† Luisa Stella Dolci,† Luca Prodi,† Nelsi Zaccheroni,† Marc C. A. Stuart,‡ Kjeld J. C. van Bommel,‡ and Arianna Friggeri*,‡ Dipartimento di Chimica “G. Ciamician”, UniVersita` degli Studi di Bologna, Via Selmi 2, 40126 Bologna, Italy, and Biomade Technology Foundation, Nijenborgh 4, 9747 AG Groningen, The Netherlands ReceiVed NoVember 9, 2005 The fluorescent properties of a new 1,3,5-cyclohexyltricarboxamide-based low-molecular-weight hydrogelator (1) derivatized with one hydrophobic fluorophore and two hydrophilic substituents have been investigated. Gels of 1 are composed of long, nonbranched fibers of uniform diameter, as shown by cryo-transmission electron microscopy (cryo-TEM). The aggregation of the naphthalene fluorophore moieties of the gelator molecules in the gel fibers favors the occurrence of a fast energy migration process that allows a very efficient sensitization of the fluorescence of a hosted fluorophore. Such processes have been investigated by the addition of propyldansylamide (PDNS), at two different concentrations, to gels of 1. Around 30% of the total PDNS added to the gels was found to be incorporated in the gel fibers, as confirmed by deconvolution of the fluorescence spectrum, excited-state lifetime measurements, and steady-state and time-resolved fluorescence anisotropy measurements. Moreover, anisotropy measurements show that the fluorophore that is incorporated within the gel fibers is almost completely immobilized, indicating that the interactions of PDNS with the gelator moieties are very strong. This particular configuration of donor (1) and acceptor (PDNS) molecules leads to a very efficient antenna effect, where 50% of the absorbed photons are funneled through to the dansyl derivative when one PDNS molecule is incorporated in the gel fibers for every 100 gelator molecules. A 5-fold higher concentration of PDNS increases the percentage of funneled photons to 75%.

Introduction A major challenge in the field of electronics based on organic molecules is the design of functional structures exhibiting longrange order.1 The use of relatively simple molecules capable of gelling water or organic solvents (low-molecular-weight gelators, LMWGs)2 can lead to well-ordered, 1D architectures via hydrogen bonding and/or other noncovalent intermolecular interactions. Therefore, the functionalization of gelator molecules with chromophores can result in novel materials, providing valuable information regarding light-harvesting systems, fluorescence sensors, and other photonic devices.3,4 Moreover, LMWGs are also of importance in the field of drug delivery with respect to the entrapment of drugs in gels and their subsequent controlled release.5 In this article, we report energy transfer from the fluorescent moieties of a LMWG gel toward a hosted fluorophore, demonstrating the incorporation of the fluorophore in the gel * To whom correspondence should be addressed. E-mail: [email protected] (M.M.), [email protected] (A.F.). Fax: (+31) 50-363 4429 (A.F.). † Universita ` degli Studi di Bologna. ‡ Biomade Technology Foundation. (1) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491-1546. (2) (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-3159. (b) Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237-1247. (c) van Bommel, K. J. C.; van der Pol, C.; Muizenbelt, I.; Friggeri, A.; Heeres, A.; Meetsma, A.; Feringa, B. L.; van Esch, J. Angew. Chem., Int. Ed. 2004, 43, 1663-1667. (d) Friggeri, A.; Gronwald, O.; van Bommel, K. J. C.; Shinkai, S.; Reinhoudt, D. N. J. Am. Chem. Soc. 2002, 124, 10754-10758. (3) (a) Mukhopadhyay, S.; Maitra, U.; Ira; Krishnamoorthy, G.; Schmidt, J.; Talmon, Y. J. Am. Chem. Soc. 2004, 126, 15905-15914. (b) Mukhopadhyay, S.; Ira; Krishnamoorthy, G.; Maitra, U. J. Phys. Chem. B 2003, 107, 2189-2192. (c) Tovar, J. D.; Claussen, R. C.; Stupp, S. I. J. Am. Chem. Soc. 2005, 127, 7337-7345. (d) Jover, A.; Meijide, F.; Rodrı´guez Nu´n˜ez, E.; Va´zquez Tato, J. Langmuir 1996, 12, 1789-1793. (e) Ihara, T. Yamada, H.; Nishihara, M.; Sakurai, T.; Takafuji, M.; Hachisako, H.; Sagawa T. J. Mol. Liq. 2004, 111, 73-76. (f) George, S. J.; Ajayaghosh, A. Chem. Eur. J. 2005, 11, 3217-3227.

fibers. Recently, fluorescence-spectroscopy-based investigations of a few LMWG gels have shed light on the gel formation processes and on the nature of the environments (hydrophobic, hydrophilic) present within these gels.3 For the study presented here, a 1,3,5-cyclohexyltricarboxamide-based gelator (1) comprising two hydrophilic moieties and one hydrophobic substituent containing a naphthalenic group was designed and synthesized (Scheme 1). Dansyl was the fluorophore of choice for probing the gel environment because its fluorescence band is strongly solvent dependent and informative of the environment experienced by the fluorophore.6 Energy transfer in gels has been previously demonstrated to occur between two different hosted molecules7a or in a mixed gel.7b Ajayaghosh and co-workers7c have nicely reported how (4) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Horwood: Chichester, U.K., 1991. (b) Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Am. Chem. Soc. 2004, 126, 12268-12269. (c) Kodis, G.; Herrero, C.; Palacios, R.; Marino-Ochoa, E.; Gould, S.; De la Garza, L.; van Grondelle, R.; Gust, D.; Moore, T. A.; Moore, A. L.; Kennis, J. T. M. J. Phys. Chem. B 2004, 108, 414-425. (d) Bauer, R. E.; Grimsdale, A. C.; Muellen, K. Top. Curr. Chem. 2005, 245, 253-286. (e) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57-69. (f) Adronov, A.; Frechet, J. M. J. Chem. Commun. 2000, 1701-1710. (g) Montalti, M.; Prodi, L.; Zaccheroni, N. J. Mater. Chem. 2005, 15, 2810-2814. (5) (a) Friggeri, A.; Feringa, B. L.; van Esch, J. J. Controled Release 2004, 97, 241-248. (b) van Bommel, K. J. C.; Stuart, M. C. A.; Feringa, B. L.; van Esch, J. Org. Biomol. Chem. 2005, 3, 2971-2920. (6) The fluorescence band of dansyl derivatives is usually red-shifted on going from organic solvents to water: (a) Ghiggino, K. P.; Lee, A. G.; Meech, S. R.; O’Connor, D. V.; Phillips, D. Biochemistry 1981, 20, 5381. (b) Prodi, L.; Bolletta, F.; Montalti, M.; Zaccheroni, N. Eur. J. Inorg. Chem. 1999, 5, 455-460. (c) Pagliari, S.; Corradini, R.; Galaverna, G.; Sforza, S.; Dossena, A.; Montalti, M.; Prodi, L.; Zaccheroni, N.; Marchelli, R. Chem. Eur. J. 2004, 10, 2749. (d) Prodi, L.; Montalti, M.; Zaccheroni, N.; Dallavalle, F.; Folesani, G.; Lanfranchi, M.; Corradini, R.; Pagliari, S.; Marchelli, R. HelV. Chim. Acta 2001, 84, 690. (e) Summers, W. A.; Lee, J. Y.; Burr, J. G. J. Org. Chem. 1975, 40, 1559-1561. (7) (a) Nakashima, T.; Kimizuka, N. AdV. Mater. 2002, 14, 1113-1115. (b) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 12291233. (c) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem., Int. Ed. 2003, 42, 332-335.

10.1021/la053015p CCC: $33.50 © 2006 American Chemical Society Published on Web 01/13/2006

2300 Langmuir, Vol. 22, No. 5, 2006 Scheme 1

energy transfer from an oligo(p-phenylenevinylene)- (OPV-) based organogel to an organic dye can be controlled by thermally disassembling the gel structure. In that case, however, a 5-fold excess of dye compared to gelator was required to achieve a significant quenching of OPV fluorescence, a situation not ideal for a light-harvesting system where the energy adsorbed by several units is usually expected to be funneled to a small number of “active” sites.4 This condition is nicely satisfied in the system described in this article, where the inclusion of 1% of acceptor fluorophore causes a 50% quenching of the fluorescence of the gel via an energy-transfer process. This feature makes the lightharvesting efficiency of this self-assembling water-compatible architecture comparable to that of multibranched supramolecular systems such as dendrimers.4 Experimental Section Materials. All chemicals were purchased from Aldrich, Fluka, or Bachem and used without further purification. HOBt is 1-hydroxybenzotriazole; CDI is 1,1′-carbonyldiimidazole. DMSO UVASOL grade from Merck and water ultra-purified by a Millipore Direct-Q system were used. Instrumentation. NMR experiments were performed using a Varian Gemini NMR spectrometer operating at 200 MHz, a Varian VXR NMR spectrometer operating at 300 MHz, or a Varian AS spectrometer operating at 400 MHz. All spectra were recorded in DMSO-d6. MS spectra were measured on a JEOL JMS-600H or a Science API 3000 mass spectrometer. Synthesis. Propyldansylamide (PDNS) was synthesized according to ref 6e. CHex(AmPhe-AmβNA)(COOH)2 (2). To a solution of cis,cis1,3,5-cyclohexanetricarboxylic acid (11.18 g, 51.71 mmol) and HOBt (2.55 g, 18.87 mmol) in DMSO (200 mL) was added CDI (2.80 g, 17.27 mmol). After the mixture had been stirred for 2 h at room temperature, H-Phe-βNA (5.00 g, 17.22 mmol) was added, and stirring was continued overnight, after which the solution was poured into H2O (600 mL), resulting in the formation of a gelly precipitate. The precipitate was filtered off and washed repeatedly with H2O (4 × 100 mL) and once with cold MeOH (50 mL). The remaining gelly solid was dissolved in hot acetone (ca. 400 mL) and filtered, after which the solvent was removed in vacuo. The remaining solid was further purified by dissolving it in a mixture of hot MeOH (400 mL) and 2 N NaOH (aqueous) (200 mL). After filtration of the solution, it was poured onto a mixture of ice (ca. 150 mL) and concentrated HCl (aqueous) (50 mL). The resulting precipitate was filtered off, washed with H2O (2 × 150 mL), dissolved in acetone (ca. 200 mL), and filtered over a double paper filter. The filtrate was concentrated in vacuo to give pure 2 as a white solid. Yield: 5.20 g (10.64 mmol ) 61.8%). 1H NMR: δ ) 1.24 (m, 3H, CHex), 1.77/1.94/2.09 (3 × m, 3 × 1H, CHex), 2.20-2.45 (m, 3H, CHex), 3.03 (m, 2H, CH2Ar), 4.72 (m, 1H, CH), 7.05-7.55 (m, 7H, Ar + H-βNA +

Montalti et al. NH), 7.62 (d, 1H, J ) 8.8 Hz, H-βNA), 7.85 (m, 3H, H-βNA), 8.31 (m, 2H, H-βNA), 10.40 (s, 1H, NH-βNA), 12.19 (bs, 2H, COOH). 13C NMR: δ ) 30.1, 36.6, 37.2, 39.7, 41.0, 53.8, 114.4, 119.0, 123.6, 125.3, 126.2, 126.4, 126.9, 127.3, 128.2, 128.7, 132.3, 135.4, 136.7, 169.7, 173.4, 175.9. Elemental analysis for C28H28N2O6‚0.25 H2O: calcd C 68.21%, H 5.83%, N 5.68%; found C 68.14%, H 5.86% N 5.65%. EI-MS for C28H28N2O6: calcd 488.55, found 487.2 [M - H]-. CHex(AmPhe-AmβNA)(AmEtOEtOH)2 (1). A solution of compound 2 (1.50 g, 3.16 mmol), 2-(2-aminoethoxy)-1-ethanol (1.00 g, 9.49 mmol), and DMT-MM8 (1.92 g, 6.95 mmol) in MeOH (65 mL) and DMSO (20 mL) was stirred overnight at room temperature. The gelly precipitate that formed was filtered off, washed with H2O (4 × 50 mL), and dissolved in hot MeOH/H2O (20:1, 150 mL). The solution was filtered and subsequently evaporated to dryness by repeated azeotropic distillation with toluene to remove all of the water. Compound 1 was isolated as a white solid. Yield: 1.20 g (1.85 mmol ) 58.6%). 1H NMR: δ ) 1.2-1.75 (m, 6H, CHex), 2.05-2.40 (m, 3H, CHex), 2.95 (m, 2H, CH2Ar), 3.1-3.6 (m, partially covered by DMSO peak, EtOEt), 4.56 (m, 2H, OH), 4.71 (m, 1H, CH), 7.1-7.6 (m, 8H, Ar + H-βNA + NH), 7.80 (m, 5H, 3H-βNA + 2NHEt), 8.25 (m, 2H, H-βNA), 10.31 (s, 1H, NHβNA). 13C NMR: δ ) 30.4, 36.6, 37.2, 37.4, 39.7, 41.1, 41.5, 53.6, 59.1, 68.0, 71.0, 114.3, 119.0, 123.6, 125.4, 126.2, 126.4, 127.0, 127.3, 128.1, 128.7, 132.3, 135.4, 136.6, 169.6, 173.2, 173.3. Elemental analysis for C36H46N4O8‚H2O: calcd C 63.51%, H 7.11%, N 8.23%; found C 63.25%, H 6.73% N 8.17%. EI-MS for C36H46N4O8: calcd 662.79, found 663.3 [M + H]+, 685.4 [M + Na]+. Hydrogel Preparation. The hydrogel samples for photophysical characterization were prepared in a 50-µL Hellma cuvette by quickly mixing 5 µL of a 2.4 × 10-2 M DMSO solution of 1 with 45 µL of water. A slightly turbid hydrogel was immediately formed. Fluorescence measurements indicated a slow reorganization of the material during the first hours after the mixing. A stable gel structure was reached after about 3 h. For the preparation of PDNS-doped gels, 1 µL of solution of the dye (8 × 10-4 or 4 × 10-3 M) was mixed with 5 µL of the 2.4 × 10-2 M DMSO solution of the gelator, and then 45 µL of water was added. Also in this case, gel formation upon addition of water was very fast, and the fluorescence properties stabilized after about 3 h. Gel-to-Sol Transition Temperature Measurements. Gel-tosol transition temperature (Tgs) measurements were done using the “dropping ball” method, which consisted of carefully placing a steel ball (65 mg, 2.5 mm in diameter) on top of prepared gels in 2-mL glass vials and subsequently placing the closed vials in a heating block.5a The temperature of the heating block was increased by 5 °C/h, and the Tgs was defined as the temperature at which the steel ball reached the bottom of the vial, as observed by a CCD camera. All Tgs measurements were carried out in duplicate. The error in the Tgs values was (5 °C. Photophysical Measurements. Steady-state and time-resolved fluorescence measurements were performed with an Edinburgh FLS920 spectrofluorimeter equipped with a TCC900 card for TCSPC (time-correlated single-photon counting) data acquisition and GlanThompson polarizing prisms. For excitation in the TCSPC experiments, an LDH-P-C-405 pulsed diode laser and a D2 pulsed lamp were used. Cryo-Transmission Electron Microscopy (Cryo-TEM) Measurements. For cryo-TEM measurements, a few microliters of gel were deposited on a bare 700-mesh copper grid. After the excess liquid had been blotted away, the grids were plunged quickly into liquid ethane. Frozen-hydrated specimens were mounted in a cryoholder (Gatan, model 626) and observed in a Philips CM 120 electron microscope, operating at 120 kV. Micrographs were recorded under low-dose conditions on a slow-scan CCD camera (Gatan, model 794). (8) Kunishima, M.; Kawachi, C.; Morita, J.; Terao, K.; Iwasaki, F.; Tani, S. Tetrahedron 1999, 55, 13159-13170.

Energy Transfer from a Hydrogel to a Fluorophore

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Figure 3. Fluorescence spectrum (λexc ) 290 nm) of a hydogel of 1 (2.4 × 10-3 M in 9:1 H2O/DMSO (continuous line) and fluorescence spectra of the same hydrogel in the presence of PDNS at concentrations of 8 × 10-5 M (dotted line) and 4 × 10-4 M (dashed line). Figure 1. Cryo-TEM image of a gel of 1 (2.4 × 10-3 M) in 9:1 H2O/DMSO (scale bar is 100 nm).

Figure 2. Fluorescence spectrum (λexc ) 290 nm) of a DMSO solution of 1 (2.4 × 10-3 M, dashed line) and fluorescence spectrum (λexc ) 290 nm, continuous line) and fluorescence anisotropy spectrum (λexc ) 290 nm, solid circles) of a hydrogel of 1 (2.4 × 10-3 M in 9:1 H2O/DMSO).

Results and Discussion Fluorescent Properties of the Hydrogel. Gelator 1 is a 1,3,5cyclohexyltricarboxamide-based gelator comprising two hydrophilic moieties and one hydrophobic substituent containing a naphthalene fluorophore (Scheme 1). Cryo-transmission electron microscopy (cryo-TEM) images of hydrogels of 1 in pure water or water/DMSO (9:1) mixtures show that the gels consist of long, nonbranched fibers with very uniform diameters of ca. 3.9 nm (Figure 1), corresponding to approximately two gelator molecule lengths. In aqueous media, the gelator’s hydrophobic fluorophore is most likely situated in the interior of the gel fiber, and the hydrophilic moieties are pointing toward the aqueous surroundings. Compound 1 in DMSO solution shows an absorption band and a fluorescence band with maxima at 285 and 358 nm, respectively, typical of naphthalene derivatives (a fluorescence quantum yield of 0.12 was measured in aerated solution). The fluorescence spectrum of a hydrogel of 1 (2.4 × 10-3 M; Figure 2), is slightly blue-shifted with respect to the spectrum of 1 in DMSO solution (Figure 2), and its vibrational structure is distorted by the intermolecular interactions resulting from aggregation in the gel structure. The anisotropy spectrum

(Figure 2) shows that the fluorescence is depolarized (anisotropy ≈ 0.07), suggesting either a high local mobility of the fluorophores or the occurrence of a fast depolarization via excitation energy migration. The first explanation can be ruled out because of the rigidity of the gel structure deriving from multiple noncovalent interactions between the gelator molecules and because of the short excited-state lifetime.9 Moreover, the blue shift observed upon gelation (Figure 2) suggests a parallel orientation of the naphthalene moieties to give H-type aggregates. Such an orientation together with the strong, short-distance intermolecular interactions supports the occurrence of a very fast energy migration process. Fluorescent Properties of the Hydrogel Doped with PDNS. Addition of PDNS to hydrogels of 1 does not lead to visible changes in the morphology of the gels as observed by cryo-TEM (see Supporting Information).10 However, upon excitation at 290 nm of the gel samples containing PDNS (8 × 10-5 M; Scheme 1b), a strong fluorescence band at 490 nm can be detected while the fluorescence of 1 (2.4 × 10-3 M) is reduced to around 50%; an even higher quenching (around 75%) is observed if the concentration of PDNS is increased to 4 × 10-4 M (Figure 3). The position of the fluorescence maximum (490 nm) in the gel samples suggests that the dansyl fluorophore is preferentially surrounded by a nonaqueous environment, despite the fact that the medium is 90% water. These observations indicate that the molecules of PDNS are incorporated in the gel fibers. Direct excitation of the dansyl (365 nm; Scheme 1c), on the other hand, results in a shift of the fluorescence band maximum to 535 nm (Figure 4). In this situation, both PDNS molecules in solution (λmax ) 560 nm) and incorporated in the gel fibers (λmax ) 490 nm) are excited, and the spectrum is the sum of these two different bands. Deconvolution of this fluorescence spectrum leads to an estimation of the fraction of dansyl molecules incorporated in the gel fibers of around 30% of the total PDNS present for both concentrations. To distinguish between fluorophores in solution and incorporated in the gel fibers, excited-state lifetime measurements (9) (a) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic/Plenum Publishers: New York, 1999. (b) Dutt, G. B.; Ameloot, M.; Bernik, D.; Negri, R. M.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 97519761. (10) Gel-to-sol transition temperature (Tgs) measurements of gels of 1 as such or containing PDNS also do not show differences in Tgs values measured. For a gel of 1 (2.4 × 10-3 M) in 9:1 H2O/DMSO, Tgs ) 111 °C. For a gel of 1 (2.4 × 10-3 M) and PDNS (8 × 10-5 M or 4 × 10-4 M) in 9:1 H2O/DMSO, Tgs ) 111 °C.

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Figure 4. Fluorescence (lines) and fluorescence anisotropy (squares) spectra (λexc ) 365 nm) of a hydrogel of 1 (2.4 × 10-3 M in 9:1 H2O/DMSO) containing PDNS at concentrations of 8 × 10-5 M (full line, solid squares) and 4 × 10-4 M (dashed line, open squares).

can also be used and were therefore employed to evaluate the distribution of PDNS in the gel. The observed excited-state decay upon direct dansyl excitation is biexponential, with a component having a longer lifetime of 20 ns attributable to the fluorophores trapped in the gel fibers and a component with a shorter lifetime, 4.0 ns, corresponding to the presence of PDNS in the aqueous environment between the gel fibers. Interestingly, the 30% fraction of trapped PDNS observed after deconvolution of the fluorescence spectrum was confirmed by the analysis of the preexponential terms of the excited-state decay. Further evidence that PDNS is immobilized in the gel fibers comes from the reduced mobility that the dansyl molecules experience in the gel, as attested by steady-state and time-resolved fluorescence anisotropy measurements (λexc ) 365 nm). The steady-state anisotropy of PDNS in the gel is quite high at low wavelengths (450-500 nm), whereas it falls to lower values at higher wavelengths (λ > 550 nm), as can be seen in Figure 4. Such behavior is again due to the presence of two different populations of fluorophores: one in solution and one incorporated in the gel fibers. The anisotropy spectrum recorded under steadystate conditions is, in fact, a weighted average of the anisotropy spectra of all of the fluorescent units. However, because of the above-mentioned solvent dependence of the dansyl luminescence, the weight of the fluorophores incorporated in the gel fibers prevails the blue region, whereas at longer wavelengths, it is the emission from the fluorophores in solution that dominates. From a quantitative point of view, it is possible to use the spectra resulting from the deconvolution of the fluorescence band to calculate the average fluorescence anisotropy of PDNS in solution and in the gel fibers. The deconvolution of the total anisotropy spectrum gave a value of 0.25 for the anisotropy of incorporated fluorophores and a value of around 0 for fluorophores in solution. This result is a further confirmation of the partition of PDNS between the gel fibers and the solvent in the hydrogel. Moreover, it is indicative of very strong interactions of the fluorophores with the gelator moieties, leading to almost immobilized dansyl moieties within the gel fibers. In turn, we observed a complete depolarization of the dansyl fluorescence upon excitation at 290 nm. This is additional evidence that energy transfer occurs from the naphthalenic units of the hydrogel to the PDNS moieties and is due to the random orientation of the dansyl moieties with respect to the chromophores of 1. Under these conditions, in fact, the energy-transfer process leads to the formation of randomly oriented dansyl excited moieties and then to nonpolarized emission.9

Montalti et al.

Figure 5. TCSPC decay (full line) and fluorescence anisotropy decay (dashed line) of a hydrogel of 1 (2.4 × 10-3 M in 9:1 H2O/ DMSO) containing 8 × 10-5 M PDNS.

The time-resolved anisotropy recorded for the dansyl-doped gel is reported in Figure 5 and exhibits an initial increase followed by a plateau where the residual anisotropy is relatively high. This behavior is again due to the presence of free and trapped dansyl units. The free units present the shorter excited-state lifetime (τ ) 4 ns) and contribute mostly to the first part of the curve: because these molecules have high mobility and no residual anisotropy, the average anisotropy in this part of the curve is low. At longer times after the pulse, the contribution of the population of bound fluorophores (τ ) 20 ns) with hindered motion and high residual anisotropy increases, causing a parallel increase of the overall anisotropy. The anisotropy profile fitting9b gives a residual anisotropy value r∞ ) 0 for PDNS in solution as expected for a free-rotation condition and r∞ ) 0.26 for the dansyl derivative in the gel fibers.11 A value of 0.35 was reported as r0 for dansyl,6a and because this parameter represents an upper limit of the observable anisotropy, the observed residual anisotropy value of r∞ ) 0.26 confirms that the motion of the fluorophore trapped in the hydrogel is, during the deactivation time, strongly hindered.9 All of these data clearly indicate that 30% of the added PDNS molecules are immobilized in the gel fibers containing the naphthalene chromophores. It is worth noting here that, when only one molecule of PDNS is inserted in the gel structure per every 100 naphthalene derivatives, 50% of the photons absorbed by the gelator are funneled to the dansyl derivative, and this percentage is increased to 75% for a 5-fold higher PDNS concentration. This unprecedented result is of particular interest for all applications in which an efficient antenna effect is highly desired, such as sensor technology and solar energy conversion,4 especially where water-compatible systems are required.

Conclusions In summary, these results clearly show that PDNS molecules are incorporated in the gel fibers formed by gelator 1 and that energy transfer from the gel fibers to the fluorophores occurs with unprecedented efficiency. Furthermore, the rotation of incorporated PDNS molecules is strongly hindered, suggesting a compact organization of the gelator molecules. This system, based on active molecules incorporated in a scaffolding structure made of packed fluorescent units assembled via noncovalent interactions is, in our opinion, an important water-compatible (11) In the interpolation of the anisotropy decay, the fast anisotropy decay processes were neglected, and only the residual anisotropy (r∞) of the two populations was considered (see ref 9).

Energy Transfer from a Hydrogel to a Fluorophore

model for analogous systems that could find applications in fields related to sensors and energy conversion. Acknowledgment. Financial support from MIUR (SAIA and LATEMAR projects) is gratefully acknowledged.

Langmuir, Vol. 22, No. 5, 2006 2303

Supporting Information Available: Cryo-TEM images of 1 in the presence of propyldansylamide. This material is available free of charge via the Internet at http://pubs.acs.org LA053015P