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Photophysics of the porphyrins; unusual fluorescence of europium porphyrin complex entrapped in sol–gel silica matrix
Journal of Alloys and Compounds 380 (2004) 380–388
Photophysics of the porphyrins; unusual fluorescence of europium porphyrin complex entrapped in sol–gel silica matrix J. Dargiewicz-Nowicka a , M. Makarska a , M.A. Villegas b , J. Legendziewicz c , St. Radzki a,∗ a b
Faculty of Chemistry, M. Curie-Sklodowska University (UMCS), M. Curie-Sklodowska Sq. 2, 20-031 Lublin, Poland Centro Nacional de Investigaciones Metalurgicas (CENIM), CSIC, Avda. Gregorio del Amo 8, 28040 Madrid, Spain c Faculty of Chemistry, Wroclaw University, 50-383 Wroclaw, Poland
Abstract In this paper, the study on encapsulation of water-soluble cationic porphyrins: methyl-pyridyl porphyrin (H2 TMePyP) and its Eu(III) complex in the monolith gels prepared by sol–gel method are reported. The samples doped with the porphyrins were prepared by tetraethoxysilane (TEOS) hydrolysis and condensation. Their absorption and emission spectroscopic properties in comparison with the spectra of the same compounds in various solvents are investigated. The spectra of metal complex were compared with those of free-base porphyrin. The strong fluorescence of europium porphyrin in the silica matrix is observed under excitation in Q-band (530 nm), while at the same time emission neither free-base porphyrin nor europium chloride does not occur. It can be explained by the strong interaction of the Eu(III)TMePyP(acac) with the silica. © 2004 Elsevier B.V. All rights reserved. Keywords: Water-soluble europium porphyrin; Sol–gel; Optical properties; UV-Vis spectroscopy; Fluorescence spectroscopy
1. Introduction The possibility of mixing organic and inorganic compounds in a new unique hybrid material is realized by sol–gel method. The entrapment of organic reagents into sol–gel matrices and coatings has been the objective of much research since Avenir and co-workers pointed out the role of such systems for sensing purpose [1]. Sol–gel monoliths and sol–gel thin films are very useful to encapsulate various guests such as inorganic clusters, lanthanide complexes, laser dyes and bioactive molecules [2–4]. Porphyrins are known to play a significant role in several biological systems. For example, the presence of such complexes is essential for the activation and storage of oxygen (haemoglobin and myoglobin), electron transfer (cytochrome c, cytochrome oxidase) and solar energy transfer (chlorophyll). Porphyrins are versatile molecules whose physicochemical properties can be readily adjusted by modifications of the electronic distribution on the aromatic ring via peripheral substitution. Following both the porphyrins properties mentioned above and variety of chemical reac∗
Corresponding author. E-mail address: [email protected] (St. Radzki). URL: http://hermes.umcs.lublin.pl/users/radzki. 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.089
tions they are involved in, new kinds of macrocyclic compounds have been synthesized in order to their future potential applications. The research conducted for many years proved versatility of porphyrin applications, including often the different areas of life. These extraordinary interesting compounds can act for example as catalysts of many chemical reactions, they also play the role of pigments and dyes, photoconductors and semiconductors, analytical reagents, as well as sensitizes in photodynamic therapy (PDT). In the future, they will be probably used as active elements of biosensors, molecular switches, elements of selective electrodes, non-linear optical materials, parts of electro-chromic displays or special equipment cumulating solar energy [5,6]. The sol–gel immobilization of porphyrins in suitable matrices has been reported in literature due to the importance of these systems from many points of interest, such as: chemical and biochemical sensing [7–11]; pH sensing [12], and optical limiting [13]. The spectroscopic properties of such materials has been intensively investigated [14–18] by different methods, mostly by absorption and fluorescence spectroscopy, inasmuch sol–gel materials are characterized by good transparency in the UV-Vis region. In this paper, we report study on the immobilization and spectroscopic properties of water-soluble free-base 5,10, 15,20-tetrakis(1-methyl-4-pirydyl) porphine (H2 TMePyP)
J. Dargiewicz-Nowicka et al. / Journal of Alloys and Compounds 380 (2004) 380–388
and its complex with Eu(III) in monolithic transparent silica gels prepared by the sol–gel method. These porphyrins were chosen because of their good solubility in aqueous solutions. They are also a great interest in a variety of applications including phototherapy and interactions with molecules of biological importance (proteins, DNA and RNA).
2. Experimental 2.1. Reagents The water-soluble porphyrin H2 TMePyP [5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine] (Scheme 1a) in the form of the tetra-4-tosylate salt, and in the form of tetra iodide were purchased in Aldrich and Strem Chemicals, respectively. All others reagents and solvents were obtained from commercial sources and used without any additional purification. 2.2. Synthesis of Eu(III)TMePyP(acac) The complex with europium Eu(III)TMePyP(acac) (Scheme 1b) was prepared by the method described earlier
in the literature for the synthesis of analogical complexes with Sm(III) and Gd(III) [19,20]. A mixture of hydrated Eu(acac)3 (300 mg) and free base porphyrin in the form of tetra iodide (100 mg) in 1,2,4-trichlorobenzene was heated at reflux under nitrogen atmosphere for 2 h. After completion of the reaction, the 1,2,4-trichlorobenzene was removed by evaporation under reduced pressure. The solid residue was vacuum dried overnight, dissolved in MeOH/CH2 Cl2 , and applied to the top of an neutral Al2 O3 column. The no reacted free-base porphyrine was eluted first with pyridine and than with mixture of toluene and MeOH (98:2, v/v). The pure Eu(III)TMePyP(acac) was eluted with DMSO. The solvent was removed by vacuum evaporation and the solid residue was washed with acetone. The final product, after vacuum drying overnight, was obtained as an amorphous powder. Its purity was checked by TLC chromatography and UV-Vis spectroscopy. 2.3. Synthesis of silica alcogels The alcogels with various amounts of H2 TMePyP or Eu(III)TMePyP(acac) were synthesized by the sol–gel polymerization of tetraethyl orthosilicate [Si(OC2 H5 )4 –tetraethoxysilane (TEOS)] according to the methods well described in the literature [21,22]. HCl was used as a
CH3
(a)
N
CH3
381
+
N
NH
+
+ N
N N
CH3
HN
N
H3C
-
SO3
4
+
CH3
(b)
CH3
CH3 O
O
Eu
H3C
+ N N
N
CH3 +
N N
N
CH3
+ N N + CH3
Scheme 1. Structural formulas of: (a) 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine, tetra-p-tosylate salt (H2 TMePyp); (b) europium(III)actylacetonate, 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphine tetra iodide [Eu(III)TMePyP(acac)].
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hydrolysis catalyst and NH3 aqueous as a condensation catalyst to synthesize TEOS alcogels. We mixed 10 cm3 of TEOS with 1 cm3 of formamide and added distilled water and HCl. Formamide was added in order to obtain crack-free monoliths. After hydrolysis, NH3 aqueous to the TEOS mixture was added. Mixture was doped with water solutions of H2 TMePyP or Eu(III)TMePyP(acac) to achieve appropriate concentration of porphyrin in alcogel. Then, the sol was gelled in disposal polyacrylic cells sealed with parafilm to measure light absorption after gelation. Later parafilm was perforated to allow evaporate pore solvent during monolith drying. The following mole ratio was used: [TEOS]:[H2 O]:[formamide] = 2:11:1. Gelation was finished within 1 h after 120 min of hydrolysis.
2.4. Preparation of silica aerogels The alcogels synthesized according to the above procedure have the structure filled with water. Wet gels were dried in the desiccator first over water, later over concentrated H2 SO4 at temperature of 40 ◦ C for 3–4 weeks up to the moment when dimensions of the shrinking monoliths were constant. 2.5. Measurements UV-Vis measurements were performed on a Carl ZeissJena M42 spectrophotometer using 1 cm Hellma quartz cells. Porphyrin solutions were freshly prepared in the
Fig. 1. Absorption spectra of the 10−5 M/dm3 solutions of the H2 TMePyP and Eu(III)TMePyP(acac) in various solvents.
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spectral purity solvents at the concentration range about 10−5 M/dm3 . The same range of porphyrin concentrations was kept in gel samples. Absorption spectra were recorded in the 200–900 nm region at a temperature of 21 ± 1 ◦ C. Spectra were stored on disk under control of Carl Zeiss program Winaspect. Fluorescence measure-
383
ments were carried out on Fluoromax-2 spectrofluorometer (Jobin Yvon-Spex, Horiba Group), with the detector oriented at 90◦ relative to the light source and using 1 cm quartz cell. All the excitation and fluorescence spectra were recorded at temperature of 21 ± 1 ◦ C in the range 200–900 nm.
Fig. 2. Excitation and emission spectra of the 10−5 M/dm3 H2 TMePyP solutions in various solvents. The strong lines at 300 and 600 nm are the excitation and the second-order ones.
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Absorption and emission spectra were recorded digitally and the data base program Sigma Plot (Jandel Corp.) was used for manipulation and plotting of the spectra.
3. Results and discussion Optical absorption was investigated for all the samples in the 200–900 nm range. However, due to the high absorbance of the silica gels in the UV, we could not determine any absorption shorter than 250 nm. The absorption spectra of porphyrins are extremely sensitive for such processes as metallation, protonation, ring oxidation or dimerization. The spectra shown in Fig. 1 illustrate the characteristic spectral changes that accompany porphyrin metallation. When we compare spectra of free base porphyrin with spectra of its Eu(III) complex we can observe only small shift of Soret band, while dramatically changes in the Q band could be noticed. The Q band of the free base porphyrin consists of four components: Qx (0, 0), Qx (1, 0), Qy (0, 0) and Qy (1, 0) which are associated with D2h (mmm) symmetry while in the spectra of Cu(II) porphyrins [symmetry D4h (4mmmm)] only one component Qy (0, 0) is observed. The appearance of additional Q bands on going from europium porphyrin to free base porphyrin is associated with an increase in the vibrational accessible modes. Also can be noticed that wavelength of the Soret and Q bands maximum alters with various solvent. The Soret band of H2 TMePyP is shifted about 3 nm, whereas this shift is stronger for Eu(III)TMePyP(acac) (10 nm) on going from H2 O to DMF. Less polar solvents were not examined, as these porphyrins are not soluble in such a solvents. The correlation between the change of the
porphyrin wavelength of the spectral band maxima and solvent properties (Reichardt’s solvent polarity parameter) was recently described [23]. Excitation and emission spectra of H2 TMePyP in various solvents are shown in Fig. 2. It can be seen that emission is quenching by the more polar solvents what is due to stronger dimerization and agglomeration of porphyrin in polar solvents [24]. H2 TMePyP shows red emission in the red region upon excitation in Soret band (429–440 nm), which correspond to the S1 → S0 transitions of the porphyrin. The excitation spectrum consists of few bands, from which the most intensive one centered at 440 nm can be ascribed as the Soret absorption band (S0 → S2 ) and four components of Q-band between 510 and 650 nm (S0 → S1 ). Note that the maximum of S0 → S1 band in absorption corresponds to the minimum in excitation spectrum, what is a result of surface quenching. Similar effect was reported earlier for other systems [25–27]. The europium complex Eu(III)TMePyP(acac) does not show any emission in solutions, what is in agreement with Gouterman’s theory [28] and additionally confirm purity of the europium complex synthesized in our laboratory. The luminescence properties of porphyrin complexes with rare earth metals have been reported, but strong emission was observed only for Sc, Y, Gd, Lu and Yb [29–32]. Moreover, the phenomenon of the ytterbium porphyrin quenching by the addition of the europium porphyrin was described by Sapunov [33]. Surprisingly for us, a very strong red emission of europium porphyrin in the sol–gel matrix can be observed. The lanthanide complexes with porphyrins are not very stable. Decomposition of lanthanide porphyrin may goes either in the acidic or in the basic environment according to the
Fig. 3. Absorption spectra of the H2 TMePyP (—), Eu(III)TMePyP(acac) (- - -) and partly decomposed Eu(III)TMePyP(acac) (· · · ) in monolithic aerogels.
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385
Fig. 4. Excitation spectra detected at 600 nm (upper figure) and 650 nm (lower) of the H2 TMePyP (—), Eu(III)TMePyP(acac) (- - -), partly decomposed Eu(III)TMePyP(acac) (· · · ) and EuCl3 (- · -) in monolithic aerogels. In the excitation spectra, strong lines at 300, 650 and, 300, 325 nm are the excitation and the second-order lines.
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Fig. 5. Emission spectra of the H2 TMePyP (—), Eu(III)TMePyP(acac) (- - -), partly decomposed Eu(III)TMePyP(acac) (· · · ) and EuCl3 (- · -) in monolithic aerogels upon excitation with 423 (Soret band), 443 and 530 nm (Q-band) wavelength light. The strong line at 530 nm is the excitation line which was not cut by filter.
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reactions: LnP(acac) + 4H+ → H4 P2+ + Ln3+ + acac− LnP(acac) + 3H2 O → H2 P + Ln(OH)3 + Hacac. Since the HCl was used as a hydrolysis catalyst and NH3 aqueous as a condensation catalyst during alcogels preparation and additionally surface of silica aerogels is acidic, we suspected that europium complex in our samples was partly decomposed, and emission arises from free-base porphyrin or its dication. To exclude such a possibility, we decided to compare spectra of investigated gel samples with analogue gel samples doped with EuCl3 or partly decomposed Eu(III)TMePyP(acac) (by admixture of H2 SO4 ). Absorption spectra of the set aerogel samples are illustrated in Fig. 3. Comparing the aerogel porphyrins spectra to these in water solution, we can say that the overall characteristics are similar. However, some changes in peak intensity and wavelengths are observed. The Soret band of H2 TMePyP is red shifted (about 3 nm) and a broadening of this band occurred. The four Q-bands undergo similar changes. These result was interpreted as arising from the less polar silica matrix, which consisted of Si–O–Si and Si–O groups what was observed for the octaethyl porphyrin in the earlier report [34,35] and/or hydrogen bonding formation with protonated nitrogen of porphyrin ring leading to partial disorder. As a result, broadening of the bands occurs. For the metallated porphyrin, the changes are reversed. The Soret band and Q-bands are blue shifted (about 5 nm) what suggests different manner of Eu(III)TMePyP(acac) interaction with silica net of gel. It seems to be reasonable since the nitrogen atoms are deprotonated. In such a case, either silicate groups can create dimmer bridging two europium complexes engaging also a water or OH− groups and/or during drying significant deformation of the europium complex in cage of gel structure occurs presumably – electron of porphyrin ring interact also. Excitation spectra of the investigated aerogels detected at the 600 and 650 nm are shown in Fig. 4. We must underline, that absorption, excitation and emission spectra of gels presented here are recorded for the same samples. The untypical excitation spectrum of H2 TMePyP compared with the same spectrum in solutions can be explained by surface quenching. The molar coefficients of porphyrins in gels are higher than in solutions, and this phenomenon occurs despite the same porphyrin concentration in the solution and solid state. The presence of the strong band at the 529 nm observed for Eu(III)TMePyP(acac) suggests that this complex could be excited at the Q-band. Fig. 5 shows the emission spectra of the investigated porphyrins in aerogels upon excitation with 423, 443 and 530 nm wavelength of light. It can be noticed that upon this conditions of excitation emission of the europium from the EuCl3 is not observed. The strong emission of H2 TMePyP is observed upon all three excitation wavelengths (S1 →
387
S0 transitions of the porphyrin). The emission of the partly decomposed Eu(III)TMePyP(acac) sample is not observed under 530 nm excitation, while pure Eu(III)TMePyP(acac) shows very strong luminescence upon 443 nm excitation and even 100 times stronger upon 530 nm excitation. This emission was never earlier observed in solution. Its origin can be explained by porphyrin ring bonding interaction as it was mentioned above and by the interaction of methyl pyridyl porphyrin peripherals with Si–O–Si or Si–O groups located at the inner surface of gel pores.
4. Summary (1) Spectral characterization (absorption and emission) of cationic water-soluble porphyrin H2 TMePyP and its complex Eu(III)TMePyP(acac) in various solvents and incorporated into sol–gel monolithic material have been undertaken. (2) Absorption spectra of solutions and sol–gel matrices indicate only small differences. Insignificant changes in molar coefficients of extinction and wavelength are observed. (3) Unexpected intensive fluorescence of europium porphyrin is observed upon excitation in the Q-band wavelength light. It can be explained by the strong interaction of the Eu(III)TMePyP(acac) with the silica. (4) Fluorescent properties of the europium porphyrin in sol–gel matrices make this complex useful for some special applications as sensing of molecular oxygen or biomolecules.
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[14] S.G. Kulikov, A.V. Veret-Lemarinier, J.P. Galaup, F. Chaput, J.P. Boilot, Chem. Phys. 216 (1997) 147–161. [15] S.M. Arabei, S.G. Kulikov, A.V. Veret-Lemarinier, J.P. Galaup, Chem. Phys. 216 (1997) 163–167. [16] J.M. Kruger, H.D. Breuer, Ber. Bunsen-Ges. Phys. Chem. 102 (1998) 1554–1556. [17] D. Delmarre, A.V. Veret-Lemarinier, C. Bied-Charreton, J. Lumin. 82 (1999) 57–67. [18] M. Cervantes, A. Clark, V. Terpugov, F. Medano, J. Opt. Technol. 69 (2002) 61–63. [19] S. Radzki, P. Krausz, S. Gaspard, C. Giannotti, Inorg. Chim. Acta 138 (1987) 139–143. [20] S. Radzki, P. Krausz, Monatsh. Chem. 17 (1996) 51–61. [21] M.A. Villegas, Bpl. Soc. Esp. Ceram. Vidr. 29 (1990) 315–329. [22] H. Tamon, T. Kitamura, M. Okazaki, J. Colloid Interface Sci. 197 (1998) 353–359. [23] M. Makarska, St. Radzki, J. Legendziewicz, J. Alloys Compd. 341 (2002) 233–238. [24] K. Kano, M. Takei, S. Hashimoto, J. Phys. Chem. 94 (1990) 2181– 2187.
[25] A. Dossing, J. Sokolnicki, J.P. Riehl, J. Legendziewicz, J. Alloys Compd. 341 (2002) 150–155. [26] G. Blasse, G.S. Dirksen, D. Van Der Voort, N. Sabatini, S. Perathoner, J.-M. Lehn, B. Alpha, Chem. Phys. Lett. 146 (1988) 347. [27] G. Blasse, G.S. Dirksen, N. Sabatini, S. Perathoner, J.-M. Lehn, B. Alpha, J. Phys. Chem. 92 (1988) 2419. [28] M. Gouterman, in: D. Dolphin (Ed.), The Porphyrins, vol. 3, Academic Press, New York, 1978. [29] M. Gouterman, C.D. Schumaker, T.S. Srivastava, T. Yonetani, Chem. Phys. Lett. 40 (1976) 456–461. [30] L.A. Maratano, C.-P. Wong, W. De Horrocks, A.M.P. Goncales, J. Phys. Chem. 80 (1976) 2389–2393. [31] M.P. Tsvirko, K.N. Solovev, G.F. Stelmakh, V.E. Pyatosin, T.F. Kachura, Opt. Spectrosc. (USSR) 50 (1981) 300–303. [32] J.X. Meng, K.F. Li, J. Yuan, L.L. Zhang, W.K. Wong, K.W. Cheah, Chem. Phys. Lett. 332 (2000) 313–318. [33] W.W. Sapunov, Khim. Fiz. 12 (1993) 1455–1461. [34] D. Avenir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956. [35] Z. Grauer, D. Avenir, S. Yariv, Can. J. Chem. 62 (1984) 1889.
Photophysics of the porphyrins; unusual fluorescence of europium porphyrin complex entrapped in sol–gel silica matrix J. Dargiewicz-Nowicka a , M. Makarska a , M.A. Villegas b , J. Legendziewicz c , St. Radzki a,∗ a b
Faculty of Chemistry, M. Curie-Sklodowska University (UMCS), M. Curie-Sklodowska Sq. 2, 20-031 Lublin, Poland Centro Nacional de Investigaciones Metalurgicas (CENIM), CSIC, Avda. Gregorio del Amo 8, 28040 Madrid, Spain c Faculty of Chemistry, Wroclaw University, 50-383 Wroclaw, Poland
Abstract In this paper, the study on encapsulation of water-soluble cationic porphyrins: methyl-pyridyl porphyrin (H2 TMePyP) and its Eu(III) complex in the monolith gels prepared by sol–gel method are reported. The samples doped with the porphyrins were prepared by tetraethoxysilane (TEOS) hydrolysis and condensation. Their absorption and emission spectroscopic properties in comparison with the spectra of the same compounds in various solvents are investigated. The spectra of metal complex were compared with those of free-base porphyrin. The strong fluorescence of europium porphyrin in the silica matrix is observed under excitation in Q-band (530 nm), while at the same time emission neither free-base porphyrin nor europium chloride does not occur. It can be explained by the strong interaction of the Eu(III)TMePyP(acac) with the silica. © 2004 Elsevier B.V. All rights reserved. Keywords: Water-soluble europium porphyrin; Sol–gel; Optical properties; UV-Vis spectroscopy; Fluorescence spectroscopy
1. Introduction The possibility of mixing organic and inorganic compounds in a new unique hybrid material is realized by sol–gel method. The entrapment of organic reagents into sol–gel matrices and coatings has been the objective of much research since Avenir and co-workers pointed out the role of such systems for sensing purpose [1]. Sol–gel monoliths and sol–gel thin films are very useful to encapsulate various guests such as inorganic clusters, lanthanide complexes, laser dyes and bioactive molecules [2–4]. Porphyrins are known to play a significant role in several biological systems. For example, the presence of such complexes is essential for the activation and storage of oxygen (haemoglobin and myoglobin), electron transfer (cytochrome c, cytochrome oxidase) and solar energy transfer (chlorophyll). Porphyrins are versatile molecules whose physicochemical properties can be readily adjusted by modifications of the electronic distribution on the aromatic ring via peripheral substitution. Following both the porphyrins properties mentioned above and variety of chemical reac∗
Corresponding author. E-mail address: [email protected] (St. Radzki). URL: http://hermes.umcs.lublin.pl/users/radzki. 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.089
tions they are involved in, new kinds of macrocyclic compounds have been synthesized in order to their future potential applications. The research conducted for many years proved versatility of porphyrin applications, including often the different areas of life. These extraordinary interesting compounds can act for example as catalysts of many chemical reactions, they also play the role of pigments and dyes, photoconductors and semiconductors, analytical reagents, as well as sensitizes in photodynamic therapy (PDT). In the future, they will be probably used as active elements of biosensors, molecular switches, elements of selective electrodes, non-linear optical materials, parts of electro-chromic displays or special equipment cumulating solar energy [5,6]. The sol–gel immobilization of porphyrins in suitable matrices has been reported in literature due to the importance of these systems from many points of interest, such as: chemical and biochemical sensing [7–11]; pH sensing [12], and optical limiting [13]. The spectroscopic properties of such materials has been intensively investigated [14–18] by different methods, mostly by absorption and fluorescence spectroscopy, inasmuch sol–gel materials are characterized by good transparency in the UV-Vis region. In this paper, we report study on the immobilization and spectroscopic properties of water-soluble free-base 5,10, 15,20-tetrakis(1-methyl-4-pirydyl) porphine (H2 TMePyP)
J. Dargiewicz-Nowicka et al. / Journal of Alloys and Compounds 380 (2004) 380–388
and its complex with Eu(III) in monolithic transparent silica gels prepared by the sol–gel method. These porphyrins were chosen because of their good solubility in aqueous solutions. They are also a great interest in a variety of applications including phototherapy and interactions with molecules of biological importance (proteins, DNA and RNA).
2. Experimental 2.1. Reagents The water-soluble porphyrin H2 TMePyP [5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine] (Scheme 1a) in the form of the tetra-4-tosylate salt, and in the form of tetra iodide were purchased in Aldrich and Strem Chemicals, respectively. All others reagents and solvents were obtained from commercial sources and used without any additional purification. 2.2. Synthesis of Eu(III)TMePyP(acac) The complex with europium Eu(III)TMePyP(acac) (Scheme 1b) was prepared by the method described earlier
in the literature for the synthesis of analogical complexes with Sm(III) and Gd(III) [19,20]. A mixture of hydrated Eu(acac)3 (300 mg) and free base porphyrin in the form of tetra iodide (100 mg) in 1,2,4-trichlorobenzene was heated at reflux under nitrogen atmosphere for 2 h. After completion of the reaction, the 1,2,4-trichlorobenzene was removed by evaporation under reduced pressure. The solid residue was vacuum dried overnight, dissolved in MeOH/CH2 Cl2 , and applied to the top of an neutral Al2 O3 column. The no reacted free-base porphyrine was eluted first with pyridine and than with mixture of toluene and MeOH (98:2, v/v). The pure Eu(III)TMePyP(acac) was eluted with DMSO. The solvent was removed by vacuum evaporation and the solid residue was washed with acetone. The final product, after vacuum drying overnight, was obtained as an amorphous powder. Its purity was checked by TLC chromatography and UV-Vis spectroscopy. 2.3. Synthesis of silica alcogels The alcogels with various amounts of H2 TMePyP or Eu(III)TMePyP(acac) were synthesized by the sol–gel polymerization of tetraethyl orthosilicate [Si(OC2 H5 )4 –tetraethoxysilane (TEOS)] according to the methods well described in the literature [21,22]. HCl was used as a
CH3
(a)
N
CH3
381
+
N
NH
+
+ N
N N
CH3
HN
N
H3C
-
SO3
4
+
CH3
(b)
CH3
CH3 O
O
Eu
H3C
+ N N
N
CH3 +
N N
N
CH3
+ N N + CH3
Scheme 1. Structural formulas of: (a) 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21H,23H-porphine, tetra-p-tosylate salt (H2 TMePyp); (b) europium(III)actylacetonate, 5,10,15,20-tetrakis(1-methyl-4-pyridyl) porphine tetra iodide [Eu(III)TMePyP(acac)].
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hydrolysis catalyst and NH3 aqueous as a condensation catalyst to synthesize TEOS alcogels. We mixed 10 cm3 of TEOS with 1 cm3 of formamide and added distilled water and HCl. Formamide was added in order to obtain crack-free monoliths. After hydrolysis, NH3 aqueous to the TEOS mixture was added. Mixture was doped with water solutions of H2 TMePyP or Eu(III)TMePyP(acac) to achieve appropriate concentration of porphyrin in alcogel. Then, the sol was gelled in disposal polyacrylic cells sealed with parafilm to measure light absorption after gelation. Later parafilm was perforated to allow evaporate pore solvent during monolith drying. The following mole ratio was used: [TEOS]:[H2 O]:[formamide] = 2:11:1. Gelation was finished within 1 h after 120 min of hydrolysis.
2.4. Preparation of silica aerogels The alcogels synthesized according to the above procedure have the structure filled with water. Wet gels were dried in the desiccator first over water, later over concentrated H2 SO4 at temperature of 40 ◦ C for 3–4 weeks up to the moment when dimensions of the shrinking monoliths were constant. 2.5. Measurements UV-Vis measurements were performed on a Carl ZeissJena M42 spectrophotometer using 1 cm Hellma quartz cells. Porphyrin solutions were freshly prepared in the
Fig. 1. Absorption spectra of the 10−5 M/dm3 solutions of the H2 TMePyP and Eu(III)TMePyP(acac) in various solvents.
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spectral purity solvents at the concentration range about 10−5 M/dm3 . The same range of porphyrin concentrations was kept in gel samples. Absorption spectra were recorded in the 200–900 nm region at a temperature of 21 ± 1 ◦ C. Spectra were stored on disk under control of Carl Zeiss program Winaspect. Fluorescence measure-
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ments were carried out on Fluoromax-2 spectrofluorometer (Jobin Yvon-Spex, Horiba Group), with the detector oriented at 90◦ relative to the light source and using 1 cm quartz cell. All the excitation and fluorescence spectra were recorded at temperature of 21 ± 1 ◦ C in the range 200–900 nm.
Fig. 2. Excitation and emission spectra of the 10−5 M/dm3 H2 TMePyP solutions in various solvents. The strong lines at 300 and 600 nm are the excitation and the second-order ones.
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Absorption and emission spectra were recorded digitally and the data base program Sigma Plot (Jandel Corp.) was used for manipulation and plotting of the spectra.
3. Results and discussion Optical absorption was investigated for all the samples in the 200–900 nm range. However, due to the high absorbance of the silica gels in the UV, we could not determine any absorption shorter than 250 nm. The absorption spectra of porphyrins are extremely sensitive for such processes as metallation, protonation, ring oxidation or dimerization. The spectra shown in Fig. 1 illustrate the characteristic spectral changes that accompany porphyrin metallation. When we compare spectra of free base porphyrin with spectra of its Eu(III) complex we can observe only small shift of Soret band, while dramatically changes in the Q band could be noticed. The Q band of the free base porphyrin consists of four components: Qx (0, 0), Qx (1, 0), Qy (0, 0) and Qy (1, 0) which are associated with D2h (mmm) symmetry while in the spectra of Cu(II) porphyrins [symmetry D4h (4mmmm)] only one component Qy (0, 0) is observed. The appearance of additional Q bands on going from europium porphyrin to free base porphyrin is associated with an increase in the vibrational accessible modes. Also can be noticed that wavelength of the Soret and Q bands maximum alters with various solvent. The Soret band of H2 TMePyP is shifted about 3 nm, whereas this shift is stronger for Eu(III)TMePyP(acac) (10 nm) on going from H2 O to DMF. Less polar solvents were not examined, as these porphyrins are not soluble in such a solvents. The correlation between the change of the
porphyrin wavelength of the spectral band maxima and solvent properties (Reichardt’s solvent polarity parameter) was recently described [23]. Excitation and emission spectra of H2 TMePyP in various solvents are shown in Fig. 2. It can be seen that emission is quenching by the more polar solvents what is due to stronger dimerization and agglomeration of porphyrin in polar solvents [24]. H2 TMePyP shows red emission in the red region upon excitation in Soret band (429–440 nm), which correspond to the S1 → S0 transitions of the porphyrin. The excitation spectrum consists of few bands, from which the most intensive one centered at 440 nm can be ascribed as the Soret absorption band (S0 → S2 ) and four components of Q-band between 510 and 650 nm (S0 → S1 ). Note that the maximum of S0 → S1 band in absorption corresponds to the minimum in excitation spectrum, what is a result of surface quenching. Similar effect was reported earlier for other systems [25–27]. The europium complex Eu(III)TMePyP(acac) does not show any emission in solutions, what is in agreement with Gouterman’s theory [28] and additionally confirm purity of the europium complex synthesized in our laboratory. The luminescence properties of porphyrin complexes with rare earth metals have been reported, but strong emission was observed only for Sc, Y, Gd, Lu and Yb [29–32]. Moreover, the phenomenon of the ytterbium porphyrin quenching by the addition of the europium porphyrin was described by Sapunov [33]. Surprisingly for us, a very strong red emission of europium porphyrin in the sol–gel matrix can be observed. The lanthanide complexes with porphyrins are not very stable. Decomposition of lanthanide porphyrin may goes either in the acidic or in the basic environment according to the
Fig. 3. Absorption spectra of the H2 TMePyP (—), Eu(III)TMePyP(acac) (- - -) and partly decomposed Eu(III)TMePyP(acac) (· · · ) in monolithic aerogels.
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Fig. 4. Excitation spectra detected at 600 nm (upper figure) and 650 nm (lower) of the H2 TMePyP (—), Eu(III)TMePyP(acac) (- - -), partly decomposed Eu(III)TMePyP(acac) (· · · ) and EuCl3 (- · -) in monolithic aerogels. In the excitation spectra, strong lines at 300, 650 and, 300, 325 nm are the excitation and the second-order lines.
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Fig. 5. Emission spectra of the H2 TMePyP (—), Eu(III)TMePyP(acac) (- - -), partly decomposed Eu(III)TMePyP(acac) (· · · ) and EuCl3 (- · -) in monolithic aerogels upon excitation with 423 (Soret band), 443 and 530 nm (Q-band) wavelength light. The strong line at 530 nm is the excitation line which was not cut by filter.
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reactions: LnP(acac) + 4H+ → H4 P2+ + Ln3+ + acac− LnP(acac) + 3H2 O → H2 P + Ln(OH)3 + Hacac. Since the HCl was used as a hydrolysis catalyst and NH3 aqueous as a condensation catalyst during alcogels preparation and additionally surface of silica aerogels is acidic, we suspected that europium complex in our samples was partly decomposed, and emission arises from free-base porphyrin or its dication. To exclude such a possibility, we decided to compare spectra of investigated gel samples with analogue gel samples doped with EuCl3 or partly decomposed Eu(III)TMePyP(acac) (by admixture of H2 SO4 ). Absorption spectra of the set aerogel samples are illustrated in Fig. 3. Comparing the aerogel porphyrins spectra to these in water solution, we can say that the overall characteristics are similar. However, some changes in peak intensity and wavelengths are observed. The Soret band of H2 TMePyP is red shifted (about 3 nm) and a broadening of this band occurred. The four Q-bands undergo similar changes. These result was interpreted as arising from the less polar silica matrix, which consisted of Si–O–Si and Si–O groups what was observed for the octaethyl porphyrin in the earlier report [34,35] and/or hydrogen bonding formation with protonated nitrogen of porphyrin ring leading to partial disorder. As a result, broadening of the bands occurs. For the metallated porphyrin, the changes are reversed. The Soret band and Q-bands are blue shifted (about 5 nm) what suggests different manner of Eu(III)TMePyP(acac) interaction with silica net of gel. It seems to be reasonable since the nitrogen atoms are deprotonated. In such a case, either silicate groups can create dimmer bridging two europium complexes engaging also a water or OH− groups and/or during drying significant deformation of the europium complex in cage of gel structure occurs presumably – electron of porphyrin ring interact also. Excitation spectra of the investigated aerogels detected at the 600 and 650 nm are shown in Fig. 4. We must underline, that absorption, excitation and emission spectra of gels presented here are recorded for the same samples. The untypical excitation spectrum of H2 TMePyP compared with the same spectrum in solutions can be explained by surface quenching. The molar coefficients of porphyrins in gels are higher than in solutions, and this phenomenon occurs despite the same porphyrin concentration in the solution and solid state. The presence of the strong band at the 529 nm observed for Eu(III)TMePyP(acac) suggests that this complex could be excited at the Q-band. Fig. 5 shows the emission spectra of the investigated porphyrins in aerogels upon excitation with 423, 443 and 530 nm wavelength of light. It can be noticed that upon this conditions of excitation emission of the europium from the EuCl3 is not observed. The strong emission of H2 TMePyP is observed upon all three excitation wavelengths (S1 →
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S0 transitions of the porphyrin). The emission of the partly decomposed Eu(III)TMePyP(acac) sample is not observed under 530 nm excitation, while pure Eu(III)TMePyP(acac) shows very strong luminescence upon 443 nm excitation and even 100 times stronger upon 530 nm excitation. This emission was never earlier observed in solution. Its origin can be explained by porphyrin ring bonding interaction as it was mentioned above and by the interaction of methyl pyridyl porphyrin peripherals with Si–O–Si or Si–O groups located at the inner surface of gel pores.
4. Summary (1) Spectral characterization (absorption and emission) of cationic water-soluble porphyrin H2 TMePyP and its complex Eu(III)TMePyP(acac) in various solvents and incorporated into sol–gel monolithic material have been undertaken. (2) Absorption spectra of solutions and sol–gel matrices indicate only small differences. Insignificant changes in molar coefficients of extinction and wavelength are observed. (3) Unexpected intensive fluorescence of europium porphyrin is observed upon excitation in the Q-band wavelength light. It can be explained by the strong interaction of the Eu(III)TMePyP(acac) with the silica. (4) Fluorescent properties of the europium porphyrin in sol–gel matrices make this complex useful for some special applications as sensing of molecular oxygen or biomolecules.
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