- Email: [email protected]
Spectral characteristics of organic molecules embedded in solid silicate gels
Journal of Luminescence 94–95 (2001) 767–770
Spectral characteristics of organic molecules embedded in solid silicate gels S.M. Arabeia,*, J.-P. Galaupb, T.A. Pavicha, K.N. Solovyova a
Institute of Molecular and Atomic Physics of the NAS of Belarus, F. Skaryna Avenue 70, 220072 Minsk, Belarus b # 505, Universite! Paris-Sud, 91405 Orsay, France Laboratoire Aime! Cotton, Bat.
Abstract Data are presented concerning the influence of porous silicate matrices, obtained by the sol–gel method, on the spectral characteristics of embedded porphyrin- and hypericin-like molecules. The main cause of the spectral changes observed is the modification of the impurity molecular structure by means of hydrogen bonds formation involving the surface hydroxyl groups of the matrix pores. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Porphyrins; YbCl 1,2-naphthalocyanine; Hypericin; Sol–gel matrices
1. Introduction The influence of matrix on the electron–phonon interaction involved in vibronic transitions in impurity centres is determined by the chemical composition and structure of the matrix. Certain solid-state matrices (organic polymers, silicate gels, etc.) are prone to undergo temporal structural changes under the action of various factors which is reflected in the guest–host interaction. In this work, new results are presented on the influence of the nature of solid silicate gels and evolutional changes in their porous nano-dimensional structure on the spectral characteristics of embedded in them porphyrin-like and hypericin-like molecules, as well as the investigation of the effect of these factors on the excited states dynamics of the pigments. The paper generalizes briefly the results
*Corresponding author. Fax: +375-17-284-0030. E-mail address: [email protected] (S.M. Arabei).
published in Refs. [1–3] and contains new data on ytterbium complexes of phthalocyanines. 2. Experimental The molecules under study were incorporated in inorganic tetraethoxysilane (TEOS) or organic– inorganic vinyl-triethoxysilane (VTEOS) gel-matrices at the stage of condensation of sol–gel mixtures [1–4]. Hard sol–gel matrices coloured this way were obtained as solid monoliths. The methods of spectral investigations and conditions of the experiments are described in detail in Refs. [1–3]. 3. Results and discussion The choice of the organic molecules was based on the presence of atoms capable of formation of hydrogen bonds and located in different positions of the molecular system. For free-base porphin
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 5 9 - 3
768
S.M. Arabei et al. / Journal of Luminescence 94–95 (2001) 767–770
(H2P) and meso-tetraphenylporphin (H2TPP) these are the central nitrogen atoms, for YbCl phthalocyanine (YbPc) and YbCl 1,2-naphthalocyanine (YbNc), the bridge nitrogen atoms, for hypericin, the oxygen atoms of the carbonyl groups. Such structure should determine the specificity of their spectral changes on the incorporation into silicate gels. 3.1. Meso-tetraphenylporphin [1] The introduction of the H2TPP molecules into silicate gels leads, with the participation of the hydroxyl OH groups of the matrices, to the transformation of a fraction of neutral molecules into the protonated dicationic form (H4TPP2+) [1,4]: H2 TPP þ 2HOSi matr H4 TPP2þ y2O Si matr : In site-selected spectra of H2TPP in silicate matrices, a weak finely structured vibronic spectrum assigned to the normal neutral form of H2TPP appears superimposed on broad and intense fluorescence bands due to the H4TPP2+ form. Strong electron–phonon interaction between the dication and its environment and/or enhancement of non-planari are suspected to be responsible for the absence of the fine structure in the site-selected fluorescence spectra of the protonated form. The detailed vibrational analysis of the finestructure fluorescence and excitation spectra of H2TPP embedded in sol–gel matrices was given in Ref. [1]. The change in the activity of some low frequency vibrational modes observed in inorganic hosts is associated with a change in the orientation of the phenyl groups which intend to be coplanar with the macrocycle. 3.2. Free-base porphin [2] The main aim of this work was the investigation of the interaction of the H2P molecules with the silicate gel-matrices of different nature, by means of the analysis of the spectral-luminescent properties of such impurity systems. Purely inorganic TEOS and hybrid organo-inorganic VTEOS gel matrices were used as the subjects of inquiry [2].
The measurements of the absorption and fluorescence spectra at room and low temperatures led to the following main results: (1) at the sol stage, at a definite pH value, the spectra of porphin are analogous to its spectra in analogous acidified organic solutions; (2) the spectra of porphin give evidence that for sol–gel at pH 6–3, as the solid framework of the TEOS matrices is formed, the relative content of the cationic form prevails more and more which is not observed for the VTEOS matrices. Thereby, the visible four-band absorption spectrum of H2P is transformed into a singleband spectrum. Based on the totality of the experimental data obtained, a conclusion was drawn that the neutral H2P molecules interact with the surface of the TEOS pores by means of the formation of hydrogen bonds between the central nitrogen atoms and the hydrogen atoms of the surface silanol groups. As a result, the centre of the H2P molecules is protonated, i.e. the dicationic form appears H2 P þ 2HOSi matr -H4 P2þ y2O Si matr to which the transformed absorption spectrum belongs. Evidently, in the case of VTEOS gel matrices the surface vinyl groups are not capable of protonating the H2P molecules. Subsequent prolonged drying process of doped TEOS matrices leads to the reverse process of H4P2+ deprotonation, the result of which is the formation of the monocationic species H3P+ with the corresponding absorption spectrum. The fluorescence and absorption spectra of all protonated forms of H2P have been studied in detail at 300 and 77 K. At 4.2 K the fluorescence line-narrowing spectrum of H4P2+ in the TEOS gel matrix has been obtained. The causes of the changes in the frequencies and activity of the H4P2+ vibrations in the excited S1 state are discussed. A number of low-frequency outof-plane vibrations of the porphyrin macrocycle has been revealed in the fluorescence spectrum; essential contributions to these modes is brought by folding and tilting of pyrrole rings, as well as by the Cb H and Ca Cm out-of-plane displacements. It has been inferred that the structural rearrangement (increase in non-planarity) of the dication takes place in the S1 state.
S.M. Arabei et al. / Journal of Luminescence 94–95 (2001) 767–770
3.3. YbCl phthalocyanine and YbCl 1,2-naphthalocyanine It has been established that YbPc and YbNc are embedded into the VTEOS gel matrix without essential changes in their initial spectral-luminescent properties. On the incorporation of YbPc in the neutral TEOS sol–gel mixture, a rapid bleaching of the starting solution is observed, contrary to YbNc. The very unstable YbPc easily dissociates in the TEOS sol–gel medium under the action of proton-donating silanol HO–SiR groups and then undergoes hydrolytic splitting to colourless products. Under similar conditions YbNc is stable. However, a considerable bathochromic shift (B20 nm) is observed for the spectra of YbNc in passing from the liquid sol to the hard gel matrix (Fig. 1). This indicates that YbNc suffers protonation at the macrocycle periphery due to the formation of hydrogen bonds between the azabridge nitrogen atoms with the surface HO– groups and the appearance of the Hn (YbNc)n+ cation: YbNc þ nHO2Si matr Hn ðYbNcÞnþ ynO 2Si matr :
Absorption (a.u.)
0.5
680.7
1.0
699.0
The results of the temperature dependence investigations for the spectroscopic properties of the
1
769
YbNc in TEOS gel matrix show that the ytterbium complex is thermally decomposed at temperatures as high as 250–2701C, which opens a prospect for obtaining silicate gel-glasses coloured by pigments of this class. 3.4. Hypericin [3] It was established that the process of hydrolysis and polymerisation of the starting TEOS mixture containing hypericin molecules leads to almost complete bleaching of the resulting solid polymeric TEOS gel-matrix, broadening and displacement of the absorption bands of hypericin, as well as to the absence of fluorescence. The observed spectral changes in the hypericin–TEOS system were attributed to the presence of the hydroxyl groups on the surface of the gel-matrix pores (as distinct from the VTEOS gel-matrix where the pores are covered by the vinyl groups) which form intermolecular hydrogen bonds with the hypericin molecules destroying the intramolecular hydrogen bonds (C=OyH) of the pigment molecule itself which results in the alteration of the absorption spectrum. At 4.2 K, fine-structure fluorescence and fluorescence excitation spectra of hypericin in polymeric silicate gel-matrices were obtained. Changes in the activity of modes in the S1 state were attributed to the alteration of the strength of intramolecular hydrogen bonds of hypericin, and the differences in the line intensities of the fluorescence spectra in different gel-matricesFto different orientation of the molecules with respect to the surface of the pores of polymeric gel-matrices.
4. Conclusion
2
0.0 600 700 Wavelength (nm) Fig. 1. Absorption spectra of YbNc in TEOS liquid sol-mixture (1) and in TEOS hard gel-matrix (2) at room temperature.
Unique properties and vast diversity of porphyrin- and hypericin-like molecules enable their wide practical use in sol–gel materials. The stability of these molecules towards destruction in sol–gel matrix environment or under various external attacks is one of the most important conditions of their specific applications. The results obtained may be used in the development of silicate gels, having certain spectral and photochemical properties, which may find promising
770
S.M. Arabei et al. / Journal of Luminescence 94–95 (2001) 767–770
applications in the creation of materials for optics, spectroscopy, and laser techniques. Acknowledgements This work was performed with financial support from the Belarusian Republican Foundation for Fundamental Research (Grant No. F99-207).
References [1] S.M. Arabei, S.G. Kulikov, A.V. Veret-Lemarinier, J.P. Galaup, Chem. Phys. 216 (1997) 163. [2] S.M. Arabei, T.A. Pavich, K.N. Solovyov, Zh. Prikl. Spektrosk. 68 (2001) 51. [3] S.M. Arabei, T.A. Pavich, J.-P. Galaup, P. Jardon, Chem. Phys. Lett. 306 (1999) 303. [4] S.G. Kulikov, A.V. Veret-Lemarinier, J.P. Galaup, F. Chaput, J.P. Boilot, Chem. Phys. 216 (1997) 147.
Spectral characteristics of organic molecules embedded in solid silicate gels S.M. Arabeia,*, J.-P. Galaupb, T.A. Pavicha, K.N. Solovyova a
Institute of Molecular and Atomic Physics of the NAS of Belarus, F. Skaryna Avenue 70, 220072 Minsk, Belarus b # 505, Universite! Paris-Sud, 91405 Orsay, France Laboratoire Aime! Cotton, Bat.
Abstract Data are presented concerning the influence of porous silicate matrices, obtained by the sol–gel method, on the spectral characteristics of embedded porphyrin- and hypericin-like molecules. The main cause of the spectral changes observed is the modification of the impurity molecular structure by means of hydrogen bonds formation involving the surface hydroxyl groups of the matrix pores. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Porphyrins; YbCl 1,2-naphthalocyanine; Hypericin; Sol–gel matrices
1. Introduction The influence of matrix on the electron–phonon interaction involved in vibronic transitions in impurity centres is determined by the chemical composition and structure of the matrix. Certain solid-state matrices (organic polymers, silicate gels, etc.) are prone to undergo temporal structural changes under the action of various factors which is reflected in the guest–host interaction. In this work, new results are presented on the influence of the nature of solid silicate gels and evolutional changes in their porous nano-dimensional structure on the spectral characteristics of embedded in them porphyrin-like and hypericin-like molecules, as well as the investigation of the effect of these factors on the excited states dynamics of the pigments. The paper generalizes briefly the results
*Corresponding author. Fax: +375-17-284-0030. E-mail address: [email protected] (S.M. Arabei).
published in Refs. [1–3] and contains new data on ytterbium complexes of phthalocyanines. 2. Experimental The molecules under study were incorporated in inorganic tetraethoxysilane (TEOS) or organic– inorganic vinyl-triethoxysilane (VTEOS) gel-matrices at the stage of condensation of sol–gel mixtures [1–4]. Hard sol–gel matrices coloured this way were obtained as solid monoliths. The methods of spectral investigations and conditions of the experiments are described in detail in Refs. [1–3]. 3. Results and discussion The choice of the organic molecules was based on the presence of atoms capable of formation of hydrogen bonds and located in different positions of the molecular system. For free-base porphin
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 5 9 - 3
768
S.M. Arabei et al. / Journal of Luminescence 94–95 (2001) 767–770
(H2P) and meso-tetraphenylporphin (H2TPP) these are the central nitrogen atoms, for YbCl phthalocyanine (YbPc) and YbCl 1,2-naphthalocyanine (YbNc), the bridge nitrogen atoms, for hypericin, the oxygen atoms of the carbonyl groups. Such structure should determine the specificity of their spectral changes on the incorporation into silicate gels. 3.1. Meso-tetraphenylporphin [1] The introduction of the H2TPP molecules into silicate gels leads, with the participation of the hydroxyl OH groups of the matrices, to the transformation of a fraction of neutral molecules into the protonated dicationic form (H4TPP2+) [1,4]: H2 TPP þ 2HOSi matr H4 TPP2þ y2O Si matr : In site-selected spectra of H2TPP in silicate matrices, a weak finely structured vibronic spectrum assigned to the normal neutral form of H2TPP appears superimposed on broad and intense fluorescence bands due to the H4TPP2+ form. Strong electron–phonon interaction between the dication and its environment and/or enhancement of non-planari are suspected to be responsible for the absence of the fine structure in the site-selected fluorescence spectra of the protonated form. The detailed vibrational analysis of the finestructure fluorescence and excitation spectra of H2TPP embedded in sol–gel matrices was given in Ref. [1]. The change in the activity of some low frequency vibrational modes observed in inorganic hosts is associated with a change in the orientation of the phenyl groups which intend to be coplanar with the macrocycle. 3.2. Free-base porphin [2] The main aim of this work was the investigation of the interaction of the H2P molecules with the silicate gel-matrices of different nature, by means of the analysis of the spectral-luminescent properties of such impurity systems. Purely inorganic TEOS and hybrid organo-inorganic VTEOS gel matrices were used as the subjects of inquiry [2].
The measurements of the absorption and fluorescence spectra at room and low temperatures led to the following main results: (1) at the sol stage, at a definite pH value, the spectra of porphin are analogous to its spectra in analogous acidified organic solutions; (2) the spectra of porphin give evidence that for sol–gel at pH 6–3, as the solid framework of the TEOS matrices is formed, the relative content of the cationic form prevails more and more which is not observed for the VTEOS matrices. Thereby, the visible four-band absorption spectrum of H2P is transformed into a singleband spectrum. Based on the totality of the experimental data obtained, a conclusion was drawn that the neutral H2P molecules interact with the surface of the TEOS pores by means of the formation of hydrogen bonds between the central nitrogen atoms and the hydrogen atoms of the surface silanol groups. As a result, the centre of the H2P molecules is protonated, i.e. the dicationic form appears H2 P þ 2HOSi matr -H4 P2þ y2O Si matr to which the transformed absorption spectrum belongs. Evidently, in the case of VTEOS gel matrices the surface vinyl groups are not capable of protonating the H2P molecules. Subsequent prolonged drying process of doped TEOS matrices leads to the reverse process of H4P2+ deprotonation, the result of which is the formation of the monocationic species H3P+ with the corresponding absorption spectrum. The fluorescence and absorption spectra of all protonated forms of H2P have been studied in detail at 300 and 77 K. At 4.2 K the fluorescence line-narrowing spectrum of H4P2+ in the TEOS gel matrix has been obtained. The causes of the changes in the frequencies and activity of the H4P2+ vibrations in the excited S1 state are discussed. A number of low-frequency outof-plane vibrations of the porphyrin macrocycle has been revealed in the fluorescence spectrum; essential contributions to these modes is brought by folding and tilting of pyrrole rings, as well as by the Cb H and Ca Cm out-of-plane displacements. It has been inferred that the structural rearrangement (increase in non-planarity) of the dication takes place in the S1 state.
S.M. Arabei et al. / Journal of Luminescence 94–95 (2001) 767–770
3.3. YbCl phthalocyanine and YbCl 1,2-naphthalocyanine It has been established that YbPc and YbNc are embedded into the VTEOS gel matrix without essential changes in their initial spectral-luminescent properties. On the incorporation of YbPc in the neutral TEOS sol–gel mixture, a rapid bleaching of the starting solution is observed, contrary to YbNc. The very unstable YbPc easily dissociates in the TEOS sol–gel medium under the action of proton-donating silanol HO–SiR groups and then undergoes hydrolytic splitting to colourless products. Under similar conditions YbNc is stable. However, a considerable bathochromic shift (B20 nm) is observed for the spectra of YbNc in passing from the liquid sol to the hard gel matrix (Fig. 1). This indicates that YbNc suffers protonation at the macrocycle periphery due to the formation of hydrogen bonds between the azabridge nitrogen atoms with the surface HO– groups and the appearance of the Hn (YbNc)n+ cation: YbNc þ nHO2Si matr Hn ðYbNcÞnþ ynO 2Si matr :
Absorption (a.u.)
0.5
680.7
1.0
699.0
The results of the temperature dependence investigations for the spectroscopic properties of the
1
769
YbNc in TEOS gel matrix show that the ytterbium complex is thermally decomposed at temperatures as high as 250–2701C, which opens a prospect for obtaining silicate gel-glasses coloured by pigments of this class. 3.4. Hypericin [3] It was established that the process of hydrolysis and polymerisation of the starting TEOS mixture containing hypericin molecules leads to almost complete bleaching of the resulting solid polymeric TEOS gel-matrix, broadening and displacement of the absorption bands of hypericin, as well as to the absence of fluorescence. The observed spectral changes in the hypericin–TEOS system were attributed to the presence of the hydroxyl groups on the surface of the gel-matrix pores (as distinct from the VTEOS gel-matrix where the pores are covered by the vinyl groups) which form intermolecular hydrogen bonds with the hypericin molecules destroying the intramolecular hydrogen bonds (C=OyH) of the pigment molecule itself which results in the alteration of the absorption spectrum. At 4.2 K, fine-structure fluorescence and fluorescence excitation spectra of hypericin in polymeric silicate gel-matrices were obtained. Changes in the activity of modes in the S1 state were attributed to the alteration of the strength of intramolecular hydrogen bonds of hypericin, and the differences in the line intensities of the fluorescence spectra in different gel-matricesFto different orientation of the molecules with respect to the surface of the pores of polymeric gel-matrices.
4. Conclusion
2
0.0 600 700 Wavelength (nm) Fig. 1. Absorption spectra of YbNc in TEOS liquid sol-mixture (1) and in TEOS hard gel-matrix (2) at room temperature.
Unique properties and vast diversity of porphyrin- and hypericin-like molecules enable their wide practical use in sol–gel materials. The stability of these molecules towards destruction in sol–gel matrix environment or under various external attacks is one of the most important conditions of their specific applications. The results obtained may be used in the development of silicate gels, having certain spectral and photochemical properties, which may find promising
770
S.M. Arabei et al. / Journal of Luminescence 94–95 (2001) 767–770
applications in the creation of materials for optics, spectroscopy, and laser techniques. Acknowledgements This work was performed with financial support from the Belarusian Republican Foundation for Fundamental Research (Grant No. F99-207).
References [1] S.M. Arabei, S.G. Kulikov, A.V. Veret-Lemarinier, J.P. Galaup, Chem. Phys. 216 (1997) 163. [2] S.M. Arabei, T.A. Pavich, K.N. Solovyov, Zh. Prikl. Spektrosk. 68 (2001) 51. [3] S.M. Arabei, T.A. Pavich, J.-P. Galaup, P. Jardon, Chem. Phys. Lett. 306 (1999) 303. [4] S.G. Kulikov, A.V. Veret-Lemarinier, J.P. Galaup, F. Chaput, J.P. Boilot, Chem. Phys. 216 (1997) 147.