Photophysical Behavior of Terpyridine−Lanthanide Ion Complexes

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Photophysical Behavior of Terpyridine-Lanthanide Ion Complexes Incorporated in a Poly(N,N-dimethylacrylamide) Hydrogel Vlasoula Bekiari and Panagiotis Lianos* UniVersity of Patras, Engineering Science Department, 26500 Patras, Greece ReceiVed May 18, 2006. In Final Form: June 29, 2006 Poly(N,N-dimethylacrylamide) hydrogel forms complexes with terpyridine and various trivalent ions, like Eu3+, Tb3+, Gd3+, and In3+. The hydrogel can be obtained in three different phases: swollen with water, lyophilized (i.e., dried by freeze-drying), where it loses the solvent but preserves the swollen configuration, and dried in the air where it shrinks. The three hydrogel phases affect the type of complex formed between terpyridine and the metal ion. Thus, in the swollen and lyophilized phases, metal-centered emission can be obtained by energy transfer from the excited ligand. In the shrunk phase, an intense green fluorescence is emitted, which is ligand-centered and is independent of the complexed ion. In the absence of any ion, the ligand emits blue luminescence, independently of the hydrogel phase. In the presence of europium(III) ions, blue, green, or red emission can be thus produced at appropriate compositions and hydrogel phases. Analysis of the photophysical behavior of the polymer-ligand-metal ion complex is related with the photophysical behavior of the ligand and its complexes in various pure solvents.

Introduction Hydrogels are 3D polymer networks that swell but do not dissolve in water. Shrinking or swelling of these gels can be activated by several external stimuli, such as solvent, heat, pH, and electric stimuli,1 and this makes them interesting materials for several useful applications: drug delivery,2-5 electrical applications,6 muscle mimetic actuators,2,7 hosts of nanoparticles and semiconductors,1,6,8-12 templates,13,14 etc.15 Even though studies on hydrogels already cover a broad domain in materials science, small attention has been paid to hydrogels as hosts of luminescent species. For example, studies of ligand-lanthanide luminescent complexes are proliferating in connection with other host matrixes, but similar studies associated with hydrogels are rare, if any. To fill the void, the present work introduces a complex formed between terpyridine (2,2′;6,2′′-terpyridine, abbreviated Tpy, see Figure 1 for the chemical structure), Eu3+ (or other ions), and poly(N,N-dimethylacrylamide) (PDMAM) hydrogel and studies the variations of the luminescence behavior of this complex system with respect to the phase transformations of the hydrogel during shrinking or swelling. The diversity of luminescence emissions that this system can offer may be useful for several photophysical applications, both in monitoring phase * To whom correspondence [email protected].

should

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addressed.

E-mail:

(1) Pardo-Yissar, V.; Bourenko, T.; Wasserman, J.; Willner, I. AdV. Mater. 2002, 14, 670. (2) Langer, R.; Tirell, D. A. Nature 2004, 428, 487. (3) Lee, K. Y.; Peters, M. C.; Mooney, D. J. AdV. Mater. 2001, 13, 837. (4) Han, J. H.; Krochta, J. M.; Kurth, M. J.; Hsieh, Y.-L J. Agric. Food Chem. 2000, 48, 5278. (5) Jeong, B.; Bae, Y. H.; Lee, D. S.; Kim, S. W. Nature 1997, 388, 860. (6) Pardo-Yissar, V.; Gabai, R.; Shipway, A. N.; Bourenko, T.; Willner, I. AdV. Mater. 2001, 13, 1320. (7) Liu, Z.; Calvert, P. AdV. Mater. 2000, 12, 288. (8) Wang, C.; Flynn, N. T.; Langer, R. AdV. Mater. 2004, 16, 1074. (9) Hu, Z.; Xia, X. AdV. Mater. 2004, 16, 305. (10) Yamashita, K.; Nishimura, T.; Nango, M. Polym. AdV. Technol. 2003, 14, 189. (11) Gattas-Asfura, K. M.; Zheng, Y.; Micic, M.; Snedaker, M. J.; Ji, X.; Sui, G.; Orbulescu, J.; Andreopoulos, F. M.; Pham, S. M.; Wang, C.; Leblanc, R. M. J. Phys. Chem. B 2003, 107, 10464. (12) Bekiari, V.; Pagonis, K.; Bokias, G.; Lianos, P. Langmuir 2004, 20, 7972. (13) Takeoka, Y.; Watanabe, M. AdV. Mater. 2003, 15, 199. (14) Xu, S.; Zhang, J.; Paquet, C.; Lin, Y.; Kumacheva, E. AdV. Funct. Mater. 2003, 13, 468. (15) Haraguchi, K.; Takehisa, T. AdV. Mater. 2002, 14, 1120.

Figure 1. Absorption spectra of (1) 10-6 M and (2) 10-3 M Tpy in cyclohexane. Inset: chemical structure of Tpy.

transformations of the hydrogel itself or of other macromolecules and in sensing or monitoring other environments or stimuli. Ligand-lanthanide ion complexes are well-known efficient luminescent agents.16-18 They emit a long-living, narrow band, of practically monochromatic radiation, by ligand-to-metal energy transfer. They are endowed with two major advantages: when excitation is made through the ligand,19 they solve the problem of small light absorption cross section by lanthanide ions, and by complexing lanthanide ions, they protect them from luminescence quenching processes. Typical complexes involve a combination of three β-diketone molecules20 and one molecule of a heterocyclic ligand.16-18 Complexes between a heterocyclic ligand alone with a lanthanide ion are also studied with a lot of interest. In this respect, complexes of Tpy with lanthanides as well as with other trivalent metals are a very interesting subject (16) Capecchi, S.; Renault, O.; Moon, D.-G.; Halim, M.; Etchells, M.; Dobson, P. J.; Salata, O. V.; Christou, V. AdV. Mater. 2000, 12, 1591. (17) Robinson, M. R.; Ostrowski, J. C.; Bazan, G. C.; McGehee, M. D. AdV. Mater. 2003, 15, 1547. (18) Lenaerts, P.; Driesen, K.; van Deun, R.; Binnemans, K. Chem. Mater. 2005, 17, 2148. (19) Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. ReV. 1993, 123, 201. (20) Binnemans, K. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Bunzli, J.-C. G., Percharsky, V. K., Eds.; Elsevier: Amsterdam, 2005; Vol. 35, p 107.

10.1021/la061406d CCC: $33.50 © 2006 American Chemical Society Published on Web 08/29/2006

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of research both for their rich chemistry and physics and for their numerous applications. Thus, new functional hybrid organicinorganic polymeric structures have been synthesized by exploiting Tpy-metal complexation.21-23 Studies on photoinduced charge separation in many cases involve chemical structures also based on Tpy-metal complexation.24,25 Lanthanide ions complexed with Tpy impart additional luminescent characteristics, which can be useful in labeling complex structures of biological importance.26 The present work exploits complex formation between Tpy and Eu3+ to probe the phase transformations of PDMAM hydrogel. Complexes between Tpy and Gd3+, Tb3+, or In3+ are also studied for comparison and in an effort to clarify the behavior of the Tpy-Eu3+ complex itself. Experimental Section All reagents were from Aldrich, unless otherwise indicated, and were used as received. Synthesis of PDMAM is given in a previous publication.12 Tpy-M3+ Complexes (M ) Metal). An amount of 6.7mM Tpy and 6.7mM Eu(NO3)3‚5H2O, Tb(NO3)3‚5H2O, Gd(NO3)3‚6H2O, or InCl3 were dissolved in ethanol by preparing two separate solutions. Then, the ligand solution was added to the metal ion solution dropwise under stirring. Stirring continued for about 1 h at ambient conditions, and then it was slightly heated to 35 °C and was left in the dark to rest for 2 days. Precipitates were formed after about 5 h. Finally, the mixture was filtered to separate the solid phase. After several times of washing with ethanol, the precipitate was dried, and thus it was ready for characterization. The precipitate was either used in its solid form or redissolved in THF. PDMAM-Tpy-M3+ Complexes. An amount of 0.1 g of dry PDMAM was added in a 0.005 M Tpy solution in EtOH and was left to stand in a capped container for 3 days. The polymer swelled and thus adsorbed and retained a substantial quantity of Tpy. Then, it was taken out of the solution and was left to dry and shrink in the air. The remaining ethanolic solution retained only traces of Tpy. Almost all ligand was adsorbed by the polymer. Then, the PDMAMTpy complex was immersed in an aqueous solution of 0.001 M Eu(NO3)3‚5H2O, Tb(NO3)3‚5H2O, Gd(NO3)3‚6H2O, or InCl3 and was left to swell for 3 days. Finally, it was taken out and left to dry in air for 1 week. In the case of freeze-drying, the sample was frozen to liquid N2 temperature and pumped with a vacuum pump. Measurements. Absorption measurements were made with a Cary 1E spectrophotometer and luminescence measurements with a Cary Eclipse spectrofluorometer. Europium and terbium luminescence decay times were also measured with the Cary Eclipse spectrofluorometer. Nanosecond decay profiles were recorded by the photoncounting technique using a homemade apparatus employing an IBH hydrogen flash and Ortec electronics.

Results and Discussion To better understand the photophysical behavior of Tpy complexes in connection with the PDMAM hydrogel, the photophysical behavior of pure Tpy and of Tpy-M3+ complexes in neat solvents has been first studied and discussed in the following three paragraphs. Photophysical Behavior of Pure Tpy in Neat Solvents. Tpy was dissolved in several solvents, both polar and nonpolar, both at low and relatively high concentrations. The obtained absorption and emission spectra are shown in Figures 1 and 2. Figure 1 (21) Andres, P. R.; Schubert, U. S. AdV. Mater. 2004, 16, 1043. (22) Tzanetos, N. P.; Andreopoulou, A. K.; Kallitsis, J. K. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4838. (23) Dobrawa, R.; Wurthner, F. Chem. Commun. 2002, 1878. (24) Collin, J.-P.; Dixon, I. M.; Sauvage, J.-P.; Williams, J. A. G.; Barigelletti, F.; Flamigni, L. J. Am. Chem. Soc. 1999, 121, 5009. (25) Prasanna de Silva, A.; Gunaratne, H. Q. N.; Rice, T. E.; Stewart, S. Chem. Commun. 1997, 1891. (26) Mukkala, V.-M.; Takalo, H.; Liiti, P.; Hemmila, I. J. Alloys Compd. 1995, 225, 507.

Figure 2. Excitation spectra of cyclohexane solutions of (1) 10-6 M Tpy (emission wavelength, 334 nm) and (3) 10-3 M Tpy (emission wavelength, 437 nm); fluorescence spectra of cyclohexane solutions of (2) 10-6 M Tpy (excitation wavelength, 280 nm) and (4) 10-3 M Tpy (excitation wavelength, 361 nm); (5) fluorescence spectrum of 10-6 M Tpy in water at pH ) 1 (excitation wavelength, 290 nm).

shows the absorption spectrum of 10-6 M and 10-3 M Tpy dissolved in cyclohexane. At low concentration, Tpy has an absorption band stretching below 320 nm. The corresponding fluorescence spectrum (Figure 2) stretches between 300 and 400 nm with a maximum at 334 nm, while the corresponding excitation spectrum (Figure 2) lies in the same spectral region as the absorption spectrum. The assignment of Tpy electronic transitions has been made in an early work by Fink and Ohnesorge.27 These low-wavelength absorption and emission bands are due to π-π transitions. Tpy is most probably in a cis-cis conformation during these transitions.27 Both the absorption and fluorescence behavior of low-concentration Tpy remained practically the same in ethanol and other tested solvents. At high Tpy concentration, an additional absorption band was detected between 320 and 400 nm (Figure 1), while fluorescence was then registered above 400 nm (Figure 2). The low-wavelength emission was no longer detectable, apparently due to self-absorption by the concentrated solution. In this respect, the excitation spectrum of 10-3 M Tpy overlapped the fluorescence spectrum of 10-6 M Tpy (Figure 2). The fluorescence spectrum was substantially structured in cyclohexane but lost all its structure in polar solvents. This blue emission is not detected at low-concentration solutions because it is due to a weak n-π transition.27 The fluorescence decay time for the n-π transition was a single-exponential decay with a lifetime τ ) 9.3 ns in cyclohexane and 9.6 ns in ethanol solutions. Tpy is soluble in water only at low concentration. At neutral or basic pH, the spectral behavior of aqueous Tpy solutions is similar to that in all other tested solvents. However, Tpy becomes highly protonated in low-pH aqueous solutions.27 In that case, fluorescence is emitted in a drastically different spectral region, as seen by curve 5 of Figure 2. This emission is also a π-π transition,27 but it is most probably due to a cis-trans or transtrans conformation.27 This conclusion is drawn by the fact that in low-temperature rigid solutions the green emission is no longer detected, apparently because no conformation transition is then allowed. Photophysical Behavior of Tpy-M3+ Complexes in Neat Solvents. Figure 3 shows the fluorescence and excitation spectra of the Tpy-Eu3+ complex precipitated from ethanol and redissolved in THF. Luminescence is now characteristic of europium emission, both in terms of structure and of decay time (single-exponential decay: τ ) 0.58 ms). The corresponding excitation spectrum shows that light is absorbed by the ligand (27) Fink, D. W.; Ohnesorge, W. E. J. Phys. Chem. 1970, 74, 72.

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Figure 3. (1) Excitation spectrum (emission wavelength, 616 nm) and (2) luminescence spectrum (excitation wavelength, 336 nm) of the Tpy-Eu3+ complex dissolved in THF.

Figure 4. Luminescence spectrum of the Tpy-Gd3+ complex dissolved in THF and recorded at liquid nitrogen temperature.

Table 1. List of Eu3+ and Tb3+ Transitions, Peak Wavelengths, Peak Wavenumbers, and Relative Intensities, in Various Environments lanthanide ion Tpy-Eu3+ complex dissolved in THF

Tpy-Eu3+-PDMAM system Tpy-Tb3+-PDMAM system

transition 5D 1 5D 1 5D 0 5D 0 5D 0 5D 0 5D 0 5D 0 5D 4 5D 4 5D 4 5D 4

f 7F1 f 7F2 f 7F0,1 f 7F2 f 7F3 f 7F1 f 7F2 f 7F3 f 7F6 f 7F5 f 7F4 f 7F3

wavelength wavenumber relative (nm) (cm-1) intensitya 538 557 592 617 650 589 614 647 487 542 582 619

18587 17953 16892 16207 15385 16978 16287 15456 20534 18450 17182 16155

17 12 121 350 8 30 120 3 18 30 6 3

a The peak intensities in this table do not fit the values in Figures 7 and 8 because intensities are normalized for comparison reasons in the second case. In addition, no direct comparison of intensities can be made between solid samples and solutions.

and energy is transferred from the excited ligand to Eu3+ emissive states. The two most intense luminescence peaks at 592 and 616 nm correspond to transitions from the 5D0 state (17 250 cm-1) to the 7F1 and 7F2 Eu3+ states,28 respectively. In fact, the former overlaps the 5D0 f 7F0 transition28 at 583 nm. The weak transition at 650 nm is identified with the 5D0 f 7F3 transition.28 Two weak transitions at 538 and 557 nm are identified with the 5D1 f 7F1 and 5D1 f 7F2 transitions.28 The intensities and the positions of these peaks are all listed in Table 1. The 5D1 state lies28 at 19 000 cm-1. For ligand-to-metal energy transfer to occur to both the 5D (19 000 cm-1) and, more successfully, to the 5D (17 250 1 0 cm-1) states, it is necessary that the triplet state of Tpy, from where transfer is made, lies close and above 19 000 cm-1. The triplet state of Tpy was determined by a standard procedure,29,30 i.e., by studying the complex of the ligand with Gd3+. The presence of the heavy metal in the complex, i.e., in the close vicinity of the ligand, facilitates intersystem crossing and population of the ligand triplet state. However, the Gd3+ emitting level lies too high (>30 000 cm-1)31 to allow energy transfer from Tpy. Therefore, no other emission is expected but by transition from the ligand triplet state to its singlet ground state. This transition is, of course, forbidden, and for this reason it is very weak and is observed only at low temperature. Figure 4 shows the emission (28) Dejneka, M.; Snitzer, E.; Riman, R. E. J. Lumin. 1995, 65, 227. (29) Sager, W. F.; Filipescu, N.; Serafin, F. A. J. Phys. Chem. 1965, 69, 1092. (30) Weibel, N.; Charbonniere, L. J.; Guardigli, M.; Roda, A.; Ziessel, R. J. Am. Chem. Soc. 2004, 126, 4888. (31) Hemmila, I. J. Alloys Compd. 1995, 225, 480.

Figure 5. (1) Excitation spectrum of the Tpy-Eu3+-PEG-200 complex (emission wavelength, 509 nm); (2) luminescence spectrum of the Tpy-Eu3+-PEG-200 complex (excitation wavelength, 370 nm); (3) excitation spectrum (emission wavelength, 616 nm) of the Tpy-Eu3+ complex dissolved in THF.

of the Tpy-Gd3+ complex precipitated from ethanol and redissolved in THF. The spectrum was recorded with the help of a cryostat at liquid nitrogen temperature. This spectrum contains several peaks, but obviously the triplet emission is identified with the peak at 503 nm that corresponds to 19 880 cm-1. This level is convenient for energy transfer to both the 5D0 and 5D1 emissive states of Eu3+, as already discussed. Photophysical Behavior of the Tpy-Eu3+ Complex in a Nonconventional Solvent. Tpy-Eu3+ complex can be dispersed in polyether solvents such as poly(ethylene glycol) (PEG). PEG of an approximate molecular weight equal to 200 has been repeatedly used in our laboratory to disperse ligand-lanthanide ion complexes.32-36 Figure 5 shows the excitation and emission spectrum of Tpy-Eu3+ dissolved in PEG-200 together with the excitation spectrum of Figure 3, which corresponds to that of the precipitated from ethanol complex, for comparison. No metalcentered emission was observed in that case, and this is in accordance with previous findings.33,34 The broad green emission at 509 nm is a ligand-centered emission. This is concluded by the following findings: the decay time was only τ ) 2.8 ns; complexes formed with a multitude of other metals, like In3+, Tb3+, Gd3+, and several others, gave exactly the same green emission, independent of the metal used. Even though it lies close to the Tpy triplet emission at 503 nm (cf., Figure 4), it is not a triplet emission. This is concluded by the fact that its decay (32) Bekiari, (33) Bekiari, (34) Bekiari, (35) Bekiari, (36) Bekiari,

V.; V.; V.; V.; V.;

Lianos, P. J. Lumin. 2003, 101, 135. Lianos, P. Chem. Phys. Lett. 2004, 383, 59. Lianos, P. AdV. Mater. 2000, 12, 1603. Lianos, P. AdV. Mater. 1998, 10, 1455. Pistolis, G.; Lianos, P. Chem. Mater. 1999, 11, 3189.

Hydrogel-Terpyridine-Lanthanide Complexes

time is too short and that it is very intense, even though it is recorded at room temperature. Indeed, the green emission recorded with the Tpy-M3+/PEG-200 system has a fluorescence quantum yield that rises up to 0.63, while the fluorescence quantum yield of pure Tpy in PEG-200 was found to be only 0.13.34 Apparently, the green emission could be associated with the emission produced by a different Tpy conformer, as in the case of the highly protonated Tpy of Figure 2. We believe that, when the dispersion medium allows it, a different kind of Tpy-metal complex can be formed from that precipitated from standard solvents, like ethanol. The complexing power of the polyether groups in PEG200 apparently facilitates (or forces) the formation of such a modified complex between Tpy and the metal ion, and in that case a different ligand conformer takes part in the Tpy-M3+ complex. Tpy can form tridentate complexes with, for example, Eu3+ (cis-cis isomerization), bidentate complexes (cis-trans isomerization), or monodentate complexes (trans-trans isomerization),37 depending on the type of the metal ion and the dispersion environment. It is not surprising then that a different type of complex, based on a different conformer, can be obtained in the particular environment provided by PEG-200 or other complex environments, as in the present case of hydrogels, as will be seen in the next paragraph. A strong indication that a different Tpy conformer emits the luminescence of Figure 5 from that which emits the luminescence of Figure 3 is the fact that the excitation spectra are very distinct in the two cases. Indeed, as seen in Figure 5, the two spectra peak at different wavelengths, i.e., at 370 and 336 nm, respectively. Photophysical Behavior of the Tpy-Eu3+ Complex in PDMAM Hydrogels. PDMAM gives hydrogels, which swell in the presence of water or alcohol and shrink when they are dried. However, if freeze-drying is performed,12 that is, if the material is frozen at liquid nitrogen temperature and dried under vacuum, i.e., lyophilized, then the swollen polymer preserves its volume and its conformation even after all (or almost all) solvent is evaporated. The swelling hydrogel can adsorb a large percentage of substances dissolved in the solvent and retains them when it is dried. Thus, the polymer can be enriched with a great variety of chemical species both neutral and charged and both hydrophilic or hydrophobic.12 In the present work, a series of ligands have been dissolved in PDMAM and their luminescence has been recorded at all three polymer phases: swollen, lyophilized, and shrunk (ambient dried). No important variations of the emission properties of these ligands have been observed by going from one phase to the other. This holds true also for the blue-emitting Tpy, the fluorescence spectrum of which is shown in Figure 6 (curve 1). This fluorescence, together with the accompanying absorption profile, is identified with the high-concentration Tpy n-π fluorescence of Figure 2. However, when a metallic ionic species is codissolved with Tpy, then the emission spectrum changes in going from one polymer phase to the other. This phenomenon has been observed with a large variety of different metal ions. Figure 7 shows luminescence spectra by ligandcentered excitation (cf., absorption spectrum in Figure 6, curve 2) for the Tpy-Eu3+-PDMAM system. Water-swollen PDMAM emits metal-centered emission, characteristic of Eu3+ ions (cf., Figure 3 and Table 1). Exactly the same spectrum was recorded with a lyophilized sample. However, a shrunk sample (ambient dried) gave a completely different spectrum: a structureless green luminescence similar to that of Figure 5. This green emission of the Tpy-Eu3+-PDMAM system is very intense, stronger than the blue emission of the Tpy-PDMAM system (i.e., that of (37) Chapman, R. D.; Loda, R. T.; Riehl, J. P.; Schwarzt, R.W. Inorg. Chem. 1984, 23, 1652.

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Figure 6. (1) Fluorescence spectrum of PDMAM swollen in water retaining Tpy alone; (2) absorption spectrum of PDMAM swollen in water retaining Tpy alone; (3) absorption spectrum of shrunk PDMAM retaining both Tpy and Eu3+.

Figure 7. Luminescence spectra of the Tpy-Eu3+-PDMAM system: (1) water-swollen or lyophilized (excitation wavelength, 336 nm) and (2) shrunk (excitation wavelength, 370 nm).

Figure 6, curve 1). Some contribution of blue luminescence adding to the form of curve 2 of Figure 7 obviously comes from isolated (noncomplexed) Tpy molecules. Also some weak Eu3+ luminescence at the long-wavelength tail of curve 2 of Figure 7 means that the green emission is produced at the broad expense of the metal-centered luminescence. When a nonemitting metal was used in the place of Eu3+, for example, Gd3+ or In3+, emission by energy transfer could not be produced. The swollen or the lyophilized phase did not offer any emission at all, while the shrunk phase again produced the intense green emission, as can be seen in Figure 7, curve 2. When another emitting ion was used in the place of Eu3+, i.e., Tb3+, the behavior was similar to that of the Tpy-Eu3+-PDMAM system. As seen in Figure 8, waterswollen PDMAM produced metal-centered emission characteristic of terbium ions (curve 1), while shrunk PDMAM produced the ligand-centered green emission observed with the other ions (curve 2). Metal-centered emission was obtained by ligandcentered excitation (excitation wavelength, 336 nm) and subsequent ligand-to-metal energy transfer. The characteristic Tb3+ peaks of Figure 8 (curve 1) correspond to transitions from the 5D (approximately 20 500 cm-1)38 state of Tb3+ to its 7F (487 4 6 nm), 7F5 (542 nm), 7F4 (581 nm), and 7F3 (619 nm) states, respectively38 (cf., Table 1). The emission of Tb3+ by energy transfer from Tpy is a surprise since the 5D4 energy level lies higher than the value of 19 880 cm-1 measured for the Tpy triplet state. Apparently, it is justified only as a result of perturbations since the two states lie close to each other. This (38) Joshi, B. C. J. Non-Cryst. Solids 1995, 180, 217.

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Figure 8. Luminescence spectra of the Tpy-Tb3+-PDMAM system: (1) water-swollen or lyophilized (excitation wavelength, 336 nm) and (2) shrunk (excitation wavelength, 370 nm).

is in accordance with the fact that the metal-centered luminescence of the Tpy-Tb3+-PDMAM system was much weaker than the corresponding luminescence of the Tpy-Eu3+-PDMAM system (Table 1). The above results show that two different types of complexes can be formed between Tpy and trivalent metal ions: (1) simple complexes of Tpy with Eu3+ or Tb3+, i.e., with emitting lanthanide ions, precipitated from common solvents such as alcohol, facilitate strong room-temperature metal-centered emission by ligandcentered excitation, population of the ligand excited triplet state, and subsequent energy transfer to the metal emitting states. Complexes of the same type but with nonemitting trivalent cations, like Gd3+ or In3+, do not lead to any luminescence at room temperature. However, some weak ligand triplet luminescence can be registered at low temperature. (2) Complexes coordinated in the presence of a third agent bearing polar chemical groups capable of binding metal ions, like polyether groups or acrylamide groups, produce ligand-centered intense green fluorescence. This fluorescence is independent of the nature of the metal ion, which seems to play only an intermediary role. The behavior of luminescence with respect to PDMAM hydrogel phase transformations is in line with the above two different types of complexes. In the case of water-swollen PDMAM, complexes of the first type apparently precipitate and are subsequently stabilized and retained by the polymer. Thus, metal-centered emission is observed in that case, red in the case of Eu3+ and green in the case of Tb3+. When the water-swollen hydrogel is lyophilized, i.e., freeze-dried, the polymer preserves its swollen conformation and retains the type 1 complexes without variation. When, on the contrary, the polymer is ambient dried and shrinks, the nature of the complex changes and becomes of the second type. Type 2 complexes are apparently coordination complexes between Tpy, metal ions, and the polymer polar groups themselves. Subsequently, no metal-centered emission but an intense ligand-centered green fluorescence independent of the nature of the complexed ion is in that case observed. The nature of the green fluorescence is not completely clear. As already discussed, most probably this fluorescence is generated by a different conformer than the normal cis-cis Tpy conformer, which makes tridentate complexes. In the presence of a third complexing agent, it is possible that isomerization is induced on Tpy so that bidentate (cis-trans conformers) or monodentate (trans-trans conformers)37 are formed and produce fluorescence similar to that of the highly protonated Tpy27 (cf., Figure 2) or of the Tpy-Eu3+-PEG-200 system (Figure 5). Most of the previous works studying Tpy-metal complexes deal with metal-

Bekiari and Lianos

to-ligand charge-transfer (MLCT) luminescence,39-41 which, in most of the cases, is very weak and is observed at low temperature. The presently observed green luminescence is not of the MLCT type. This is concluded by the following data: MLCT luminescence has a rather long decay time, while τ was only 2.8 ns in the present case; MLCT luminescence is rather weak, while the fluorescence quantum yield rose as high as 0.63 in the present case.34 MLCT luminescence is usually observed by excitation in the visible, while excitation in the present case is obviously a ligand-centered n-π transition (cf., Figure 1 and the related discussion). Indeed the absorption spectrum of the complex (see curve 3 of Figure 6) largely extends into the visible. However, excitation in the visible did not produce any room-temperature luminescence at all. As already said, we also rule out the possibility that the green emission is phosphorescence. Phosphorescence is the ruling emission in the case of the Tpy-Ir complex.24,42 However, phosphorescence cannot be justified in the present case, neither in terms of intensity nor in terms of decay time. We then adopt the original explanation that the green emission is due to a different conformer obtained in the presence of a third complexing agent. If we take into account the fact that pure Tpy emits blue luminescence, we note that by combining PDMAM, Tpy, and Eu3+ at different proportions and physical states one can obtain all three basic colors, i.e., blue, green, and red.

Conclusion We have found that PDMAM hydrogels can solubilize and retain complexes of Tpy with various metal ions. The variation of the physicochemical state of the macromolecule affects the photoluminescence emission properties of the complex. Thus, the Tpy-Eu3+ complex solubilized in PDMAM emits red metalcentered emission when the polymer is in the solvent-swollen state or lyophilized swollen state but emits green ligand-centered luminescence when PDMAM is in its dried shrunk phase. Similar is the behavior of the Tpy-Tb3+ complex. If a nonemitting metal ion is used, then swollen PDMAM does not emit any luminescence under ambient conditions, but shrunk PDMAM emits green ligand-centered fluorescence. Comparison of these data with the behavior of pure Tpy or Tpy-metal complexes in various solvents leads to the conclusion that precipitated complexes give metalcentered luminescence but complexes formed in the presence of a third complexing agent emit green ligand-centered luminescence associated with a cis-trans or a trans-trans Tpy conformer. The behavior of the Tpy-M3+ complex in PDMAM can be used as a model for studying similar phenomena in other macromolecular phases, e.g., molecules of biological importance. Acknowledgment. We are thankful to Professor G. Bokias of the Chemistry Department of the University of Patras for providing PDMAM and for many helpful discussions concerning this work. We thank the European Social Fund (ESF), Operational Program for Educational and Vocational Training II (EPEAEK II), and particularly the Program PYTHAGORAS II, for funding the present work. LA061406D (39) McMillin, D. R.; Moore, J. J. Coord. Chem. ReV. 2002, 229, 113. (40) Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Thomson, A. M. W. C. Inorg. Chem. 1995, 34, 2759. (41) Goodall, W.; Williams, J. A. G. Chem. Commun. 2001, 2514. (42) Ayala, N. P.; Flynn, C. M.; Sacksteder, L. A.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. Soc. 1990, 112, 3837.