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Preparation and characterization of pyrazoline nanoparticles
Materials Science and Engineering C 24 (2004) 131 – 134 www.elsevier.com/locate/msec
Preparation and characterization of pyrazoline nanoparticles Sun Wha Oh a, Dong Ri Zhang b, Young Soo Kang b,* a
b
Basic Science Research Institute, Pukyong National University, Busan 608-737, South Korea Department of Chemistry, Pukyong National University, 599-1 Daeyon 3-dong, Nam-gu, Busan 608-737, South Korea
Abstract Nanoparticles of 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline ranging from tens to hundreds of nanometers were prepared by the reprecipitation method. Their excitonic transitions responsible for absorption and emission, as compared with those of dilute solution, have been investigated as a function of nanoparticle size. We found that pyrazoline nanoparticles possess a special size dependence on their optical properties. As the nanoparticle size decreased, the molecular absorption peak of pyrazoline nanoparticles was observed to shift to the high-energy side due to size effect. D 2003 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Reprecipitation method; Optical properties; Size effect
1. Introduction
2. Experimental
Nanoparticles of semiconductors and metals have been an extremely active area of research because of their interesting quantum confinement effect on optical and electronic properties [1]. However, studies of organic nanoparticles have been paid little attention. But it is a tendency to extend the research on nanoparticles into organic materials [2,3], especially into general organic molecules due to their diversity. Pyrazolines have been widely used as optical brightening agents for textiles, paper and fabrics and as a hole-conveying medium in photoconductive materials [4 –8]. Specially, 2-pyrazolines containing electron donors and acceptors at 1and 3-positions were a typical heterocyclic ‘transition’ molecular crystal and have intrinsically large molecular hyperpolarizability which suggests the photorefractivity of two-dimensional array constructed with its nanoparticles in applied optical fields [9– 13]. In this study, nanoparticles of 1-phenyl-3-naphthyl-5((dimethylamino)phenyl)-2-pyrazoline ranging from tens to hundreds of nanometers by the reprecipitation method have been successfully prepared, and their optical sizedependent properties have been studied.
2.1. Materials
* Corresponding author. Tel.: +82-51-620-6379; fax: +82-51-6288147. E-mail address: [email protected] (Y.S. Kang). 0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2003.09.056
4-(Dimethylamino)aldehyde, sodium ethoxide, 2V-acetonaphthone and phenylhydrazine were purchased from Aldrich Chemical and used without any further purification. All solvents were obtained from Junsei Chemical and used without further purification. House-distilled water was passed through a four-cartridge Barnstead Nanopure II purification system consisting of macropure pretreatment, organic-free) for removing trace organics, two-ion exchangers and 0.2 mm hollow-fiber final filter for removing particles. Its resistivity was 18.3 MV and used in all the experiments. 2.2. Preparation of 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline Synthetic route of 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline is summarized in Fig. 1. To a solution of 4-(dimethylamino)aldehyde (14.9 g, 0.10 mol) in 300 ml of ethanol was added sodium ethoxide (21 wt.% solution in ethanol, 37.3 ml) and stirred for 1 h at 25 jC. To a solution was added 1V-acetonaphthone (17.0 g, 0.10 mol) and stirred for 24 h at 25 jC. The precipitate was filtered, washed with water and recrystallized with benzene to yield 24.1 g (80%) of chalone 1. 1H-NMR (CDCl3) 3.05 (s, 6H, N(CH3)2), 6.71 (d, 2H, J = 8.9 Hz, CH = CH), 7.47 – 7.62 (m, 6H, aromatic), 7.84 –8.00 (m, 4H, aromatic), 8.10 (dd,
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Fig. 1. Synthetic route of 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline.
1H, J = 1.7 Hz and J = 8.5 Hz, aromatic), 13C-NMR (CDCl3) 40.5 (N(CH3)2), 111.8 and 116.8 (CH = CH), 122.6, 124.6, 126.5, 127.5, 127.7, 127.9, 128.2, 129.3, 129.4, 129.5, 130.4, 132.6, 135.2, 136.3, 145.8 and 151.9 (aromatic), 190.3 (C = O). A solution of 1 (5.0 g, 16.6 mmol) and phenylhydrazine (1.98 g, 18.3 mmol) in 50 ml of acetic acid was heated for 4 h at 100 jC and then cooled to room temperature. The precipitate was filtered, washed with ethanol and recrystallized with benzene to yield 4.4 g (68%) of pyrazoline 2. 1HNMR (CDCl3) 2.92 (s, 6H, N(CH3)2), 3.23 and 3.27 (two d, 1H, J = 7.1 Hz, NCH), 3.91 (dd, 1H, J = 12.3 Hz and J = 16.9 Hz, CH2CHN), 5.27 (dd, 1H, J = 7.0 Hz and J = 12.1 Hz, CH2CHN), 7.14– 7.22 (m, 7H, aromatic), 7.45 –7.48 (m, 3H, aromatic), 7.79– 7.85 (m, 5H, aromatic), 8.17 (dd, 1H, J = 1.4 Hz and J = 18.6 Hz, aromatic), 13C-NMR (CDCl3) 43.5 (N(CH 3 ) 2 ), 64.2 (NCHCH 2 ), 111.7 (NCHCH 2), 116.8 (N = C), 122.6, 124.6, 126.5, 127.7, 127.9, 128.3, 129.3, 129.4, 130.4, 132.6, 135.6, 136.3, 145.8, 151.9 (aromatic). 2.3. Preparations of pyrazoline nanoparticles 1-Phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline (2) nanoparticles were prepared as follows. Pyrazoline acetone solution (1.0 mM) was injected into 10 ml of water with vigorous stirring, using a 100-Al microsyringe. Pyrazoline molecules began to be aggregated at once and its nanoparticles dispersed in water were obtained. By controlling the quantity of pyrazoline acetone solution injected into water and temperature, the size of nanoparticles was controlled [10 –13]. For example, when 100 and 50 Al of pyrazoline acetone solution were injected at 25 jC, the nanoparticle sizes finally prepared were 65 and 40 nm, recpectively. 2.4. Methods The 1H-NMR and 13C-NMR spectra were recorded using 400 MHz spectrometers (Varian); Abbreviations used are s
(singlet), d (doublet), t (triplet), and m (multiplet). All compounds in purities of greater than 90% as judged by 1 H-NMR and 13C-NMR. The UV – Vis optical absorption spectra of nanoparticles dispersion in water were measured using a Varian Carry UV – Vis spectrophotometer. The size and shape of nanoparticles were observed by means of the transmission electron microscope (TEM: Hitachi S-2400, Japan). The size and distribution of nanoparticles dispersed in water were also evaluated by the dynamic light scattering (DLS) technique using a Zetaplus 1246 (Brookhaven Instruments USA).
3. Results and discussion We successfully prepared a series of pyrazoline (2) nanoparticles from tens to hundreds nanometers, which were stable even after several weeks. Some of their TEM photographs were in Fig. 2, in which the average nanoparticle sizes were 40 and 65 nm, respectively. In this experiment, the nanoparticles gave the more aggregated state as the particles size increased. The nanoparticles with homogeneously dispersed TEM image are shown in Fig. 2a. The nanoparticles showed aggregated state in Fig. 2b. TEM images also showed that the size distribution of pyrazoline nanoparticles tends to be uniform in the same period and as confirmed by DLS. Fig. 3 shows the DLS measurement of a nanoparticle size. The value agreed roughly with that determined by TEM. The average diameter of particles is 40 nm, and the size distribution of the nanoparticles is remarkably narrow. Fig. 4 displays the UV – Visible absorption spectra of pyrazoline (2) nanoparticles dispersed in water with different size and the spectrum of pyrazoline (2) acetone solution (1.0 10 2 mM). In Fig. 4, upon going from 120 to 40 nm, kmax of pyrazoline (2) nanoparticles was observed to be shifted to a shorter wavelength region. It is shown that the more electron delocalization on the main backbone gets kmax
S.W. Oh et al. / Materials Science and Engineering C 24 (2004) 131–134
133
Fig. 2. The TEM photographs of pyrazoline (2) nanoparticles with different sizes: (a) 40 nm; (b) 65 nm.
to be shifted to a longer wavelength. This indicates that the size-dependent property of pyrazoline (2) nanoparticles is completely different from isolated pyrazoline (2) molecules. In the previous study, 1,3-diphenyl-2-pyrazoline nanoparticles with different size were prepared by the reprecipitation method [12]. It was reported that 1,3-diphenyl2-pyrazoline nanoparticles did not possess the optical size-dependent properties. Therefore the difference between 1,3-diphenyl-2-pyrazoline nanoparticles and pyrazoline (2) nanoparticles possibly results from the different molecular structures. Pyrazoline (2) possessing the dimethylaminophenyl ring at the 5-position results in the further non-conjugated charge
Fig. 3. The DLS characterization of pyrazoline (2) nanoparticles (40 nm).
transfer from C-5 to N-1. This possibly leads to the changes of steric effect of the 5-position dimethylaminophenyl group in different cases. In the pyrazoline (2) monomer, the dipole-dipole interaction energies are small, leading to wide band gaps. For bulk crystal, the dipole– dipole interaction energies are larger, leading to narrower band gaps. The dipole– dipole interaction energies in nanoparticles gradually decrease upon going from pyrazoline (2) bulk crystals to single molecule, leading to gradual widening of band gaps. In the case of 1,3-diphenyl-2-pyrazoline, which does not have the phenyl at 5-position, there was no blue shift of kmax. So dipole – dipole interaction energy among 1,3-
Fig. 4. The UV – Vis absorption spectra of pyrazoline (2) nanoparticles with different sizes: (a) 40 nm; (b) 65 nm; (c) 120 nm. (f) The spectrum of pyrazoline (2) acetone solution (1.0 10 2 mM).
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diphenyl-2-pyrazoline molecules is presumably to be the same. This possibly results in no shift of kmax. On the other hand, 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2pyrazoline molecules have the blue shift of kmax due to dimethylaminophenyl group at 5-position.
4. Conclusions 1-Phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline (2) nanoparticles with different size were prepared by the reprecipitation method. We found that size effect plays a critical role in optical properties of organic nanoparticles. The optical size effect is useful for controlling the wavelength of organic photoconductors or electroluminescent materials.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2002-050-C00007).
References [1] E. Hanamura, Phys. Rev. B 37 (1988) 1273 – 1278. [2] R. Micheletto, H. Fukuda, M. Ohtsu, Langmuir 11 (1995) 3333 – 3336. [3] N.D. Denkov, O.D. Velev, P.A. Kralchevsky, I.B. Ivanov, Nature 136 (1993) 26. [4] J.O. Morley, V.J. Docherty, D.J. Pugh, J. Mol. Electron. 5 (1989) 117 – 121. [5] S.R. Sandler, S. Loshaek, E. Broderick, K.C. Tsou, J. Phys. Chem. 66 (1962) 404 – 408. [6] R.H. Wiley, C.H. Jarboe, F.N. Hayes, E. Hansbury, J.T. Nielsen, P.X. Callahan, M.C. Sellars, J. Org. Chem. 23 (1958) 732 – 738. [7] S.R. Sandler, K.C. Tsou, J. Chem. Phys. 39 (4) (1963) 1062 – 1067. [8] Y.R. Huang, J.A. Katzenellenbogen, Org. Lett. 2 (18) (2000) 2833 – 2836. [9] X.H. Zhang, W.Y. Lai, Z.Q. Gao, T.C. Wong, C.S. Lee, H.L. Kwong, S.T. Lee, S.K. Wu, Chem. Phys. Lett. 320 (2000) 77 – 80. [10] H. Fu, J. Yao, J. Am. Chem. Soc. 123 (2001) 1434 – 1439. [11] R. Xie, H. Fu, X. Ji, J. Yao, J. Photochem. Photobiol. A Chem. 147 (2002) 31 – 37. [12] H. Fu, X. Ji, X. Zhang, S. Wu, J. Yao, J. Colloid Interface Sci. 220 (1999) 177 – 180. [13] H. Fu, R. Xie, Y. Wang, J. Yao, Colloids Surf. A Physicochem. Eng. Asp. 174 (2000) 367 – 373.
Preparation and characterization of pyrazoline nanoparticles Sun Wha Oh a, Dong Ri Zhang b, Young Soo Kang b,* a
b
Basic Science Research Institute, Pukyong National University, Busan 608-737, South Korea Department of Chemistry, Pukyong National University, 599-1 Daeyon 3-dong, Nam-gu, Busan 608-737, South Korea
Abstract Nanoparticles of 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline ranging from tens to hundreds of nanometers were prepared by the reprecipitation method. Their excitonic transitions responsible for absorption and emission, as compared with those of dilute solution, have been investigated as a function of nanoparticle size. We found that pyrazoline nanoparticles possess a special size dependence on their optical properties. As the nanoparticle size decreased, the molecular absorption peak of pyrazoline nanoparticles was observed to shift to the high-energy side due to size effect. D 2003 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Reprecipitation method; Optical properties; Size effect
1. Introduction
2. Experimental
Nanoparticles of semiconductors and metals have been an extremely active area of research because of their interesting quantum confinement effect on optical and electronic properties [1]. However, studies of organic nanoparticles have been paid little attention. But it is a tendency to extend the research on nanoparticles into organic materials [2,3], especially into general organic molecules due to their diversity. Pyrazolines have been widely used as optical brightening agents for textiles, paper and fabrics and as a hole-conveying medium in photoconductive materials [4 –8]. Specially, 2-pyrazolines containing electron donors and acceptors at 1and 3-positions were a typical heterocyclic ‘transition’ molecular crystal and have intrinsically large molecular hyperpolarizability which suggests the photorefractivity of two-dimensional array constructed with its nanoparticles in applied optical fields [9– 13]. In this study, nanoparticles of 1-phenyl-3-naphthyl-5((dimethylamino)phenyl)-2-pyrazoline ranging from tens to hundreds of nanometers by the reprecipitation method have been successfully prepared, and their optical sizedependent properties have been studied.
2.1. Materials
* Corresponding author. Tel.: +82-51-620-6379; fax: +82-51-6288147. E-mail address: [email protected] (Y.S. Kang). 0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2003.09.056
4-(Dimethylamino)aldehyde, sodium ethoxide, 2V-acetonaphthone and phenylhydrazine were purchased from Aldrich Chemical and used without any further purification. All solvents were obtained from Junsei Chemical and used without further purification. House-distilled water was passed through a four-cartridge Barnstead Nanopure II purification system consisting of macropure pretreatment, organic-free) for removing trace organics, two-ion exchangers and 0.2 mm hollow-fiber final filter for removing particles. Its resistivity was 18.3 MV and used in all the experiments. 2.2. Preparation of 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline Synthetic route of 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline is summarized in Fig. 1. To a solution of 4-(dimethylamino)aldehyde (14.9 g, 0.10 mol) in 300 ml of ethanol was added sodium ethoxide (21 wt.% solution in ethanol, 37.3 ml) and stirred for 1 h at 25 jC. To a solution was added 1V-acetonaphthone (17.0 g, 0.10 mol) and stirred for 24 h at 25 jC. The precipitate was filtered, washed with water and recrystallized with benzene to yield 24.1 g (80%) of chalone 1. 1H-NMR (CDCl3) 3.05 (s, 6H, N(CH3)2), 6.71 (d, 2H, J = 8.9 Hz, CH = CH), 7.47 – 7.62 (m, 6H, aromatic), 7.84 –8.00 (m, 4H, aromatic), 8.10 (dd,
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Fig. 1. Synthetic route of 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline.
1H, J = 1.7 Hz and J = 8.5 Hz, aromatic), 13C-NMR (CDCl3) 40.5 (N(CH3)2), 111.8 and 116.8 (CH = CH), 122.6, 124.6, 126.5, 127.5, 127.7, 127.9, 128.2, 129.3, 129.4, 129.5, 130.4, 132.6, 135.2, 136.3, 145.8 and 151.9 (aromatic), 190.3 (C = O). A solution of 1 (5.0 g, 16.6 mmol) and phenylhydrazine (1.98 g, 18.3 mmol) in 50 ml of acetic acid was heated for 4 h at 100 jC and then cooled to room temperature. The precipitate was filtered, washed with ethanol and recrystallized with benzene to yield 4.4 g (68%) of pyrazoline 2. 1HNMR (CDCl3) 2.92 (s, 6H, N(CH3)2), 3.23 and 3.27 (two d, 1H, J = 7.1 Hz, NCH), 3.91 (dd, 1H, J = 12.3 Hz and J = 16.9 Hz, CH2CHN), 5.27 (dd, 1H, J = 7.0 Hz and J = 12.1 Hz, CH2CHN), 7.14– 7.22 (m, 7H, aromatic), 7.45 –7.48 (m, 3H, aromatic), 7.79– 7.85 (m, 5H, aromatic), 8.17 (dd, 1H, J = 1.4 Hz and J = 18.6 Hz, aromatic), 13C-NMR (CDCl3) 43.5 (N(CH 3 ) 2 ), 64.2 (NCHCH 2 ), 111.7 (NCHCH 2), 116.8 (N = C), 122.6, 124.6, 126.5, 127.7, 127.9, 128.3, 129.3, 129.4, 130.4, 132.6, 135.6, 136.3, 145.8, 151.9 (aromatic). 2.3. Preparations of pyrazoline nanoparticles 1-Phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline (2) nanoparticles were prepared as follows. Pyrazoline acetone solution (1.0 mM) was injected into 10 ml of water with vigorous stirring, using a 100-Al microsyringe. Pyrazoline molecules began to be aggregated at once and its nanoparticles dispersed in water were obtained. By controlling the quantity of pyrazoline acetone solution injected into water and temperature, the size of nanoparticles was controlled [10 –13]. For example, when 100 and 50 Al of pyrazoline acetone solution were injected at 25 jC, the nanoparticle sizes finally prepared were 65 and 40 nm, recpectively. 2.4. Methods The 1H-NMR and 13C-NMR spectra were recorded using 400 MHz spectrometers (Varian); Abbreviations used are s
(singlet), d (doublet), t (triplet), and m (multiplet). All compounds in purities of greater than 90% as judged by 1 H-NMR and 13C-NMR. The UV – Vis optical absorption spectra of nanoparticles dispersion in water were measured using a Varian Carry UV – Vis spectrophotometer. The size and shape of nanoparticles were observed by means of the transmission electron microscope (TEM: Hitachi S-2400, Japan). The size and distribution of nanoparticles dispersed in water were also evaluated by the dynamic light scattering (DLS) technique using a Zetaplus 1246 (Brookhaven Instruments USA).
3. Results and discussion We successfully prepared a series of pyrazoline (2) nanoparticles from tens to hundreds nanometers, which were stable even after several weeks. Some of their TEM photographs were in Fig. 2, in which the average nanoparticle sizes were 40 and 65 nm, respectively. In this experiment, the nanoparticles gave the more aggregated state as the particles size increased. The nanoparticles with homogeneously dispersed TEM image are shown in Fig. 2a. The nanoparticles showed aggregated state in Fig. 2b. TEM images also showed that the size distribution of pyrazoline nanoparticles tends to be uniform in the same period and as confirmed by DLS. Fig. 3 shows the DLS measurement of a nanoparticle size. The value agreed roughly with that determined by TEM. The average diameter of particles is 40 nm, and the size distribution of the nanoparticles is remarkably narrow. Fig. 4 displays the UV – Visible absorption spectra of pyrazoline (2) nanoparticles dispersed in water with different size and the spectrum of pyrazoline (2) acetone solution (1.0 10 2 mM). In Fig. 4, upon going from 120 to 40 nm, kmax of pyrazoline (2) nanoparticles was observed to be shifted to a shorter wavelength region. It is shown that the more electron delocalization on the main backbone gets kmax
S.W. Oh et al. / Materials Science and Engineering C 24 (2004) 131–134
133
Fig. 2. The TEM photographs of pyrazoline (2) nanoparticles with different sizes: (a) 40 nm; (b) 65 nm.
to be shifted to a longer wavelength. This indicates that the size-dependent property of pyrazoline (2) nanoparticles is completely different from isolated pyrazoline (2) molecules. In the previous study, 1,3-diphenyl-2-pyrazoline nanoparticles with different size were prepared by the reprecipitation method [12]. It was reported that 1,3-diphenyl2-pyrazoline nanoparticles did not possess the optical size-dependent properties. Therefore the difference between 1,3-diphenyl-2-pyrazoline nanoparticles and pyrazoline (2) nanoparticles possibly results from the different molecular structures. Pyrazoline (2) possessing the dimethylaminophenyl ring at the 5-position results in the further non-conjugated charge
Fig. 3. The DLS characterization of pyrazoline (2) nanoparticles (40 nm).
transfer from C-5 to N-1. This possibly leads to the changes of steric effect of the 5-position dimethylaminophenyl group in different cases. In the pyrazoline (2) monomer, the dipole-dipole interaction energies are small, leading to wide band gaps. For bulk crystal, the dipole– dipole interaction energies are larger, leading to narrower band gaps. The dipole– dipole interaction energies in nanoparticles gradually decrease upon going from pyrazoline (2) bulk crystals to single molecule, leading to gradual widening of band gaps. In the case of 1,3-diphenyl-2-pyrazoline, which does not have the phenyl at 5-position, there was no blue shift of kmax. So dipole – dipole interaction energy among 1,3-
Fig. 4. The UV – Vis absorption spectra of pyrazoline (2) nanoparticles with different sizes: (a) 40 nm; (b) 65 nm; (c) 120 nm. (f) The spectrum of pyrazoline (2) acetone solution (1.0 10 2 mM).
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diphenyl-2-pyrazoline molecules is presumably to be the same. This possibly results in no shift of kmax. On the other hand, 1-phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2pyrazoline molecules have the blue shift of kmax due to dimethylaminophenyl group at 5-position.
4. Conclusions 1-Phenyl-3-naphthyl-5-((dimethylamino)phenyl)-2-pyrazoline (2) nanoparticles with different size were prepared by the reprecipitation method. We found that size effect plays a critical role in optical properties of organic nanoparticles. The optical size effect is useful for controlling the wavelength of organic photoconductors or electroluminescent materials.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2002-050-C00007).
References [1] E. Hanamura, Phys. Rev. B 37 (1988) 1273 – 1278. [2] R. Micheletto, H. Fukuda, M. Ohtsu, Langmuir 11 (1995) 3333 – 3336. [3] N.D. Denkov, O.D. Velev, P.A. Kralchevsky, I.B. Ivanov, Nature 136 (1993) 26. [4] J.O. Morley, V.J. Docherty, D.J. Pugh, J. Mol. Electron. 5 (1989) 117 – 121. [5] S.R. Sandler, S. Loshaek, E. Broderick, K.C. Tsou, J. Phys. Chem. 66 (1962) 404 – 408. [6] R.H. Wiley, C.H. Jarboe, F.N. Hayes, E. Hansbury, J.T. Nielsen, P.X. Callahan, M.C. Sellars, J. Org. Chem. 23 (1958) 732 – 738. [7] S.R. Sandler, K.C. Tsou, J. Chem. Phys. 39 (4) (1963) 1062 – 1067. [8] Y.R. Huang, J.A. Katzenellenbogen, Org. Lett. 2 (18) (2000) 2833 – 2836. [9] X.H. Zhang, W.Y. Lai, Z.Q. Gao, T.C. Wong, C.S. Lee, H.L. Kwong, S.T. Lee, S.K. Wu, Chem. Phys. Lett. 320 (2000) 77 – 80. [10] H. Fu, J. Yao, J. Am. Chem. Soc. 123 (2001) 1434 – 1439. [11] R. Xie, H. Fu, X. Ji, J. Yao, J. Photochem. Photobiol. A Chem. 147 (2002) 31 – 37. [12] H. Fu, X. Ji, X. Zhang, S. Wu, J. Yao, J. Colloid Interface Sci. 220 (1999) 177 – 180. [13] H. Fu, R. Xie, Y. Wang, J. Yao, Colloids Surf. A Physicochem. Eng. Asp. 174 (2000) 367 – 373.