J O U R N A L O F C H E M I S T R Y Materials Azacrown-CH2-bipyridine receptors in silica xerogel. Optical and coordination properties† Andrzej M. K�onkowski,a Krzysztof Kledzik,a Tadeusz Ossowskia and Anna Jankowska-Frydelb aFaculty of Chemistry, University of Gdansk, ul. Sobieskiego 18, 80–952 Gdansk, Poland bInstitute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 7, 80–952 Gdansk, Poland Samples of silica xerogel doped with encapsulated series of three receptors of the type (aza-3n-crown-n)-CH2-(2,23-bipyridine), where n=4, 5 and 6, were prepared by the sol–gel process.Fluorescence excitation spectra of the encapsulated receptors diVer from the absorption spectra. Emission of the samples can be quenched specifically owing to the coordination reaction with transition metal ions.Emission decays can be described by a double exponential, the lifetime values being short at room temperature. EPR spectra of CuII ions complexed on the surface of the receptor doped xerogel have fairly well resolved hyperfine structure and show an elongated rhombic octahedral environment for the receptors with 2,23-bipyridine groups. Owing to the utilizing of a low temperature preparation of (with two amine nitrogen atoms as donors) and aza-3n-crownn, i.e. 3n-membered macrocycle (with n-1 ethereal oxygen oxide materials, the so-called sol–gel process,1 the previous problem related to the inability to incorporate organic mol- atoms and an amine nitrogen atom as donors). The fluorescent sensing is studied with respect to Cu2+ as a model transition ecules into an inorganic oxide glass is resolved.Using this technique, a three-dimensional network of metal-oxide bonds metal ion and other transition metal ions for comparison. The aim of this study is to prepare material which could is formed at room temperature by the polymerisation–condensation reaction of metal alkoxides, followed by low temperature possibly be proposed as an optical recognition phase for a chemical optical sensor.The structure of metal complexes dehydration. The porous xerogel matrix thus obtained can trap receptor molecules.2 formed after reaction with the studied receptors in silica xerogel matrix is also reported. Inorganic oxides are superior to organic polymer matrices due to the fact that the excited state of the trapped organic molecule is capable of undergoing photochemical reactions Experimental with the surrounding organic matrix.This results in low photostability of both dopants and carrier.3 Chemicals Silica xerogels can play the role of host matrices doped with Tetramethoxysilane, copper(II ) perchlorate Cu(ClO4)2 6H2O encapsulated sensing organic indicators. The gels have the (both from Fluka A.G., Switzerland), NiII, CoII, MnII and CrIII obvious advantage of chemical inertness, mechanical stability nitrates (from Aldrich Co.) as well as vanadyl sulfate and optical transparency.What is more important, they are VOSO4 5H2O (from Merck A.G. Germany) of analytical physically able to encapsulate the indicator molecules in pores grade were used without further purification.in the gels, so that these molecules cannot be leached out in The series of (aza-3n-crown-n)-CH2-(2,23-bipyridine) ligands solution. At the same time the host material is suYciently (receptors) were synthesized by a preparation method similar porous to enable transport of metal ions, solvent and other to that in ref. 7 from azacrowns aza-3n-crown-n (abbr. A3nCn, small molecules into the interior.4 where n=4, 5 and 6) and 2,23-bipyridine (bpy), both from Several recent observations revealed the feasibility of making Aldrich Chemical Co.optical recognition phases with receptor molecules based on Methanol and ammonia were of analytical grade purity. The silica gel for chemical sensors.5 The exciting outcome of these water used was triply distilled from glass.observations is the feasibility of preparing gels boasting optical properties which change in the presence of target molecules. Sample preparation In particular, gels with receptors producing characteristic colour changes when exposed to the metal ions were prepared. The sols were prepared by a typical sol–gel procedure1 from It is also noted that many other organic molecules incorporated a starting mixture of: tetramethoxysilane (TMOS), methyl into the sol–gel optical materials exhibit luminescence through- alcohol as diluent, distilled water (TMOS5H2O=154), out the UV–VIS region.2,6 In this case metal ions penetrating NH3(aq) catalyst and receptor A3nCn-CH2-bpy.The mixture a porous gel matrix can be complexed by the organic molecules was vigorously mixed at room temperature.The sol was which results in quenching of the fluorescence. allowed to gel for 3 days and then dried. The xerogel obtained We are interested in investigating the spectroscopic properties and studying the behaviour of an optical material with supramolecular dopants which exhibits chemical sensing. The optical material is a silica xerogel prepared by the sol–gel method and the supramolecular dopants (Fig. 1) are receptors containing two coordinatively active subunits: 2,23-bipyridine † Presented in part at the 11th International Symposium on the Photochemistry and Photophysics of Coordination Compounds, Fig. 1 Molecular structure of A3nCn-CH2-bpy receptor, where n=4 Cracow, Poland, July 1995. J. Mater. Chem., 1998, 8(5), 1245–1249 1245was heated for 3 h at 120 °C to remove ammonia and methanol as well as some of the water from the pores.Concentration of the receptor in the xerogel was 2.5×10-5 mol g-1 SiO2. The dry xerogel was then ground in a mortar and passed through standard sieves. Particles 0.75–1.50 mm in size were immersed in aqueous solutions containing such metal ions as: Cu2+, Ni2+, Co 2 +, Mn 2 +, Cr 3 + or VO2+ 200 mg (±0.1 mg) of the doped material was agitated in 0.01 M aqueous solution of the appropiate metal salt, maintaining the molar ratio [metal ion]5[receptor]=1051.The immersed xerogel was filtered oV after 24 h, rinsed with distilled water and then dried. By this chemisorption method complexes with transition metal ions were formed. The rate of uptake of the metal ions on the surface of the receptor doped xerogel was measured by cyclic voltammetry on a hanging mercury drop electrode, shaking 200 mg of the material with the metal ion solution.Complexes of CuII with A3nCn-CH2-bpy ligands (in a molar ratio 151) were also produced in the sol just before gelation. After 3 days the complexes were encapsulated by the sol–gel process in wet silica gel which was then dried.The procedure in the case of the CuII complex with bpy diVered. This complex, which was used for comparison, was Fig. 2 Optical absorption spectra of A3nCn-CH2-bpy receptors in methanol solution: (a) n=4; (b) n=5; (c) n=6 formed (in molar ratio Cu5bpy=152) separately and was dissolved in the sol before gelation. The silica xerogel with the trapped complex was obtained after gelation and drying.Apparatus Optical absorption measurements in the UV–VIS region were recorded on a Beckman DU 650 spectrophotometer. Spectra of the crushed xerogel samples were obtained in a silicon oil mull and were collected between 250 and 900 nm. Fluorescence emission (lexc=315 nm) and excitation spectra (lem=360 nm) were measured with a Perkin-Elmer LS 50B spectrofluorometer with reflection spectra attachment.None of the excitation spectra were corrected for the lamp and photomultiplier response. Fluorescence decays were measured using an Edinburgh Analytical Instruments CD 900 fluorometer. EPR spectra were obtained on a SE/X spectrometer (Radiopan, Poznan). Sample holders were sealed quartz capillaries (1 mmdiameter).A magnetic field modulation of 100 kHz was applied. Standard deviations of the EPR spectra param- Fig. 3 Optical absorption spectra of (a) bpy, and of A3nCn-CH2-bpy eters were estimated as follows: g||±0.003, g)±0.005 and receptors in silica xerogel: (b) n=4; (c) n=5; (d) n=6 A)±4×10-4 cm-1. Moreover, in this case, the higher wavelength wing shows comparativelyigher intensity.Results Silica gels prepared under basic conditions (pH>7) and high Coordination process water to silane ratios produce highly branched clusters which The rate of the coordination process (see Fig. 4) indicates that behave as discrete species. Gelation occurs by linking clusters complex formation on the xerogel surface is rather high together.1 This procedure makes the xerogels porous, these consequently being able to encapsulate and attach large supramolecular receptors of the type shown in Fig. 1. Probably owing to the hydrogen bond between oxygen atoms in the ether crown group and the silanol group, the receptors are practically non-leachable in aqueous solution. Absorption spectroscopy The receptors in methanol solution show strong absorption spectra with the characteristic band for free bpy at 290 nm (Fig. 2), whereas Fig. 3 shows room temperature absorption spectra of the receptors (and bpy for comparison) encapsulated in the xerogel. The bands for the receptors with n=4, 5 and 6 are centered at 301, 299 and 298 nm, respectively. The band for the first receptor shows the greatest intensity of UV absorption among the bands compared.Compared with the bpy derivatives, the absorption spectrum of bpy is much Fig. 4 Copper(II) uptake as a function of time for an A12C4-CH2-bpy receptor encapsulated in silica xerogel broader but has almost the same position of the main band. 1246 J. Mater. Chem., 1998, 8(5), 1245–1249(measured in seconds). However, later, due to the metal enrichment within pores controlled by diVusion, a slow complexation rate is observed. In any case the maximum duration time of the process is about 60 min.Luminescence spectroscopy Very weak emission was observed from the xerogel samples with the bpy derivatives on excitation with light of 290 nm (absorption lmax). The fluorescence excitation (lem=360 nm) and emission (lexc=315 nm) spectra of the encapsulated ligands are given in Fig. 5(A) and (B). The bands of A12C4-CH2-bpy exhibit much higher intensity than those of the ligands with larger crown groups (n=5 and 6), but the band positions are the same. The excitation spectra of the bpy derivatives encapsulated in the xerogel diVer from the absorption spectra [the excitation peaks are near zero at 310 nm where the species reach the maximum absorption, cf.Fig. 3 and 5(A)]. The Fig. 6 Characteristic quenching sequence for A18C6-CH2-bpy in silica emission originates from the higher wavelength wing of the xerogel, if the receptor is (a) uncomplexed; and complexed with absorption spectrum. (b) Cr3+; (c) VO2+; (d)Mn2+; (e) Co2+; (f ) Ni2+ and (g) Cu2+ A reasonable fit of the decay curves can be achieved using the fitting functions with more than one exponential component.The best results are achieved for two exponential functions. The adequacy of this exponential decay fitting was judged by an inspection of the plots of the standard deviation and by the statistical parameters x2 (Table 1). In general, the two components have very short lifetimes t. Participation of the longer lifetime is smaller and decreases with increasing n.The sequence of the fluorescence quenching of the receptor with n=6 complexed with transition metal ions such as CuII, NiII, CoII, MnII, CrIII and VOII is shown in Fig. 6. This sequence Fig. 7 Fluorescence quenching eVect due to the coordination of transition metal ions in a silica xerogel doped with A3nCn-CH2-bpy: (a) n= 4; (b) n=5; (c) n=6. Receptors uncomplexed (A), and complexed with (B) VO2+ and (C) Cu2+.is similar for the complexed species with n=4 and 5 and is characteristic of the transition metal ions used. The room temperature fluorescence spectra of the free A3nCn-CH2-bpy ligands in silica and complexed ones owing to coordination of the representative Cu2+ and VO2+ ions are shown in Fig. 7. Quenching of fluorescence for the complexed receptors is observed.The quenching eVect is especially great in the case of the copper(II ) complex with the A12C4-CH2-bpy ligand as compared with the fluorescence intensity of the respective free receptor immobilized in silica [cf. in Fig. 7(a)]. The receptors complexed with CuII cation in each case exhibit near zero emission intensity, whereas vanadyl cation in this Fig. 5 Excitation (A) and emission (B) spectra of A3nCn-CH2-bpy in situation possesses an intermediate position among the trans- silica xerogel: (a) n=4; (b) n=5; (c) n=6.The excitation spectra were ition ions studied. obtained by monitoring at 360 nm and the excitation wavelength for the emission was 315 nm. Recorded at 295 K. EPR spectroscopy Table 1 Photophysical properties of A3nCn-CH2-bpy receptors encap- EPR spectroscopy is a powerful tool with which to identify sulated in silica gel (measured at 295 K) changes in the coordination environment of CuII complexed with the supramolecular entities.EPR spectra of CuII comreceptor, n x2 t/ns contribution (%) plexed with the receptors and then encapsulated in silica xerogel are shown in Fig. 8. The CuII species give rise to typical 4 1.41 0.49 76.9 2.1 23.1 axial spectra. However, in the case of the complexes with bpy 5 1.08 0.11 99.2 and bpy derivatives with n=5 and 6, the spectra exhibit two 1.9 0.8 components. The hyperfine structure of the intense perpendicu- 6 1.40 0.097 99.9 lar signal on the high-field side is not resolved. 2.6 0.1 The EPR spectra of the samples with complexes formed by J.Mater. Chem., 1998, 8(5), 1245–1249 1247Table 2 EPR spectral parameters of CuII complexes with bpy and A3nCn-CH2-bpy trapped in silica xerogel but formed before gelation (spectra recorded at 295 K) ligand,n g||a A||b/10-4 cm-1 g) c bpy 2.289 154 2.055 4 2.265 154 2.041 5 2.259 158 2.033 6 2.258 156 2.027 a±0.003. b±4×10-4 cm-1. c±0.005. Table 3 EPR spectral parameters of CuII complexes formed with bpy and A3nCn-CH2-bpy ligands in silica gel by coordination from aqueous solution (spectra recorded at 295 K) ligand,n g||a A||b/10-4 cm-1 g) c bpy 2.280 154 2.044 4 2.257 155 2.035 5 2.258 157 2.031 6 2.258 156 2.027 a±0.003. b±4×10-4 cm-1.c±0.005. CuII ions on the surface and in the pores of the silica xerogel [see Fig. 8(b)] are similar to each other.Only in the spectrum of the sample with the n=6 receptor are two components clearly visible. In view of the fact that the CuII ion concentration in the gel samples is low (about 10-5 mol g-1 SiO2) we can neglect the possibility of interactions between copper(II) ions in the matrix. Fig. 8 X-Band EPR spectra of silica xerogel doped with: (A) CuII The lineshape of the EPR spectrum conforms satisfactorily complexed in the reaction mixture before gelation by (a) bpy; and by with the theoretical one which is obtained assuming the spin A3nCn-CH2-bpy ligands with (b) n=4; (c) n=5; (d) n=6.Recorded at 295 K Hamiltonian,8 provided that the current lineshape theory developed by Kneubuehl9 is corrected in respect of peak width. Sets of the g||, A|| and g) values for the encapsulated Cu2+ complexes and owing to coordination of CuII ions are presented in Tables 2 and 3.Owing to poor resolution of the spectra, it is impossible to determine the spectral parameters for the second component in the cases mentioned above. g|| and g) diVer distinctly in the CuII complexes with bpy and with the bpy derivatives on the other side, regardless of the preparation method.For all the CuII complexes, a g||>g)>ge= 2.0023 parameter sequence is observed. Discussion 2,2¾-Bipyridine (bpy) and its derivatives are renowned for their ability to form coordination compounds with metal ions. The description, based on MO theory, of the coordinative bonds in these complexes requires that the central ion and the ligands be able to form s- and p-bonds.It is no wonder, therefore, that bpy is a molecular block par excellence for a wide variety of types of supramolecular devices.10 Bpy, similarly to many of the optical active organic molecules tested, tends to dimerize and aggregate at moderate concentrations in aqueous solution.11 This tendency reduces the fluorescence quantum yield significantly.12 It is important to notice that dimerisation is greatly reduced by the trapping process in the silica xerogel, even though concentrations could be quite high (up to 10-2 M).The de-aggregation is a general phenomenon which indicates the lower polarity of the oxide cage11 than water and confirms the matrix isolation of the trapped molecules.13 The eVect is that maxima of absorption of organic, optically active molecules, are slightly red-shifted (ca. 5 nm) in silica xerogels, as compared with their aqueous solutions. These redshifts confirm the slightly less polar nature of the silica cage which is composed of SiMOH (silanol ) and SiMOMSi Fig. 9 X-Band EPR spectra of silica xerogel doped with (a) bpy, and with A3nCn-CH2-bpy: (b) n=4; (c) n=5; (d) n=6, complexed with groups.13 CuII in aqueous solution.Recorded at 295 K. It is known from previous studies14 that bpy in alcohol at a 1248 J. Mater. Chem., 1998, 8(5), 1245–1249concentration of 10-5 M does not aggregate, but the aggregates Conclusions exist at higher concentration (10-3–10-1 M) in water. In the The sol–gel process appears in this study to be a straightfor- latter case lmax of emission is at 430 nm.15 Thus, in the present ward and versatile fabrication method for the preparation of studies (ligand concentration 10-5 mol g-1 SiO2 and lmax of recognition phases for optical chemical sensors.emission at 345 nm) aggregation of the bpy derivatives cannot With the help of intensity quenching experiments we investi- be expected. It seems that the emission is probably from the gated three diVerent luminescent A3nCn-CH2-bpy receptors azacrown derivatives of bpy bonded to the silica network by (where n=4, 5 and 6) encapsulated in porous silica xerogel.hydrogen bonds between the silanol groups and oxygen atoms Among the samples the system with A12C4-CH2-bpy is the in the crown group. most promising as a component of the recognition phase in The considerable intensity of fluorescence emission for silica the optical chemical sensor for Cu2+ ions.xerogel doped with A12C4-CH2-bpy [Fig. 5(a)] is one of the The elongated rhombic octahedral environment of CuII ion promising features for the recognition phase based on this complexed with ligands of the A3nCn-CH2-bpy type (where receptor. It seems that the phase studied is suitable for selective n=4, 5 and 6) consists of two bpy ligand groups in the analysis of CuII ions in solutions owing to substantial quenchequatorial plane.ing of the emission by these ions [Fig. 7(a)]. The order of the emission intensity MnII>CoII>NiII>CuII is the reverse of the Irving–Williams order, i.e. order of the The financial support of this work by the Polish Scientific stabilities of corresponding complexes of the bivalent ions of Research Council (grant 7 T08A 028 10) is gratefully the first transition series, irrespective of the particular ligand acknowledged.involved.16 The bi-exponential decay observed means that two species take part in the excitation process. Probably one of the species References is the receptor supramolecule anchored with the xerogel network by hydrogen bonding and the second species is the non- 1 C.J. Brinker and G. W. Scherer, Sol–Gel Science. T he Physics and Chemistry of Sol–Gel Processing, Academic Press, London, 1990. bonded supramolecule. If the content of the non-bonded 2 D. Avnir, V. R. Kaufman and R. Reisfeld, J. Non-Cryst. Solids, species decreases with n then one of the decay components 1985, 74, 395.decreases (cf. in Table 1). 3 N. S. Allen and J. F. McKeller, Photochemistry of Dyed and The CuII ion can give numerous bis-bipyridine complexes Pigmented Polymers, Applied Science, Publishers, London, 1980. with the general formula Cu(bpy)2X2 nH2O. Complexes with 4 B. Dunn and J. I. Zink, J.Mater. Chem., 1991, 1, 903. anions which are generally reluctant to bond to the metal such 5 M.A. Arnold, Anal. Chem., 1992, 64, 1015. 6 Y. Kobayashi, Y. Imai and Y. Kurakawa, J.Mater. Sci. L ett., 1988, as X=ClO4- are of some interest. For these complexes the 7, 1148. axial CuMO bonds are considerably longer than the equatorial 7 J. C. Rodriguez-Ubis, B. Alpha, P. Plancherd and J.-M. Lehn, CuMN bonds. It is therefore uncertain whether, in the strict Helv.Chim. Acta, 1984, 67, 2264. sense, the oxygen atoms are bonded to the copper ion or not. 8 A. H. Maki and B. R. McGarvey, J. Chem. Phys., 1958, 29, 31. Hathaway et al.17–19 concluded that the compound should be 9 F. K. Kneubuehl, J. Chem. Phys., 1960, 33, 1074. formulated as [Cu(bpy)2](ClO4)2 and that the complex cation 10 F. Voegtle, Supramolecular Chemistry. An Introduction, Wiley, has an essentially planar structure, although the two bpy Chichester, 1991. 11 F. P. Schafer, T opics in Applied Physics, vol. 1: Dye L asers, 2nd ligands are twisted mutually by 10–30° towards a tetrahedral edn., Springer, Berlin, 1977. coordination. This is not entirely in agreement with results 12 K. Kemnitz, T. Murao, I. Yamazaki, N. Nakashima and obtained by Nakai20 regarding the aspect of the coordination K.Yoshihara, Chem. Phys. L ett., 1983, 101, 337. of the ClO4- ion. This author presented the conclusion that 13 D. Avnir, D. Levy and R. Reisfeld, J. Phys. Chem., 1984, 88, 5956. in the [Cu(bpy)2](ClO4)2 crystal the oxygen atoms complete 14 E. Castellucci, L. Angeloni, G. Marconi, E. Venuti and I. Baraldi, a distorted octahedron in the axial direction, though one of J.Phys. Chem., 1990, 94, 1740. 15 S. Dyanya and Bhattacharyya, J. Photochem. Photobiol. A: Chem., the CuMO distances is considerably longer than the other. 1992, 63, 179. Thus, the complex could be regarded as six-coordinate. 16 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Since the EPR spectral parameters obey the order Pergamon Press, Oxford, 1984, p. 1065. g||>g)>ge for all of the studied coordination species (see 17 J. M. Procter, B. J. Hathaway and P. Nicholls, J. Chem. Soc. A, Tables 2 and 3), it could be proposed that there exists a 1968, 1678. tetragonal coordination environment in the samples with CuII 18 D. S. Brown, J. D. Lee, B. G. A. Melson, B. J. Hathaway, complexes prepared by encapsulation and chemisorption. J. M. Procter and A. A. G. Tomlinson, Chem. Commun., 1967, 369. 19 B. J. Hathaway, J. M. Procter, R. C. Slade and A. A. G. Tomlinson, Assuming that complexes of the type [Cu(bpy)2]2+ are unable J. Chem. Soc. A, 1969, 2219. to adopt a square-planar configuration because of steric inter- 20 H. Nakai, Bull. Chem. Soc. Jpn., 1971, 44, 2412. action between hydrogen atoms to the nitrogen,21 the coordi- 21 P. J. Burke, D. R. McMillin and W. R. Robinson, Inorg. Chem., nation environment of CuII should be elongated rhombic 1980, 19, 1121. octahedral.22 22 J. Foley, D. Kennefick, D. Phelan, S. Tyagi and B. Hathaway, An increase of g|| indicates decreasing tetragonality of the J. Chem. Soc., Dalton T rans., 1983, 2333. 23 B. J. Hathaway, in Comprehensive Coordination Chemistry, ed. coordination sphere of CuII.23 It suggests that the tetragonal G. Wilkinson, Pergamon Press, Oxford, 1987, vol. 5, pp. 667 et distortion of the complexes increases as the ligands change seq., 730. from bpy to bpy derivatives. In addition, this eVect is greater for complexes created by the chemisorption method than in the reaction mixture before gelation. Paper 7/08131F; Received 11th November, 1997 J. Mater. Chem., 1998, 8(5), 1245–1249 1249