- Email: [email protected]
Photoacoustic and fluorescence studies of silica gels doped with rare earth salicylic acid complexes
Journal of Non-Crystalline Solids 278 (2000) 223±227
Letter to the Editor
www.elsevier.com/locate/jnoncrysol
Photoacoustic and ¯uorescence studies of silica gels doped with rare earth salicylic acid complexes Ronghu Wu, Huazhang Zhao, Qingde Su * Department of Chemistry, University of Science and Technology of China, Hefei Anhui 230026, People's Republic of China Received 13 December 1999; received in revised form 23 May 2000
Abstract Photoacoustic spectroscopy (PAS) is useful in the study of non-crystalline solids. By using PA amplitude and phase spectra, together with IR and ¯uorescence spectra, the variation of the energy levels and luminescence eciencies and the intramolecular relaxation processes of silica gels doped with rare earth complexes with salicylic acid have been studied in comparison with their corresponding pure complexes. After the complexes are doped into silica gels, the PA and emission peaks of the ligands show a shift, but the peaks of rare earth ions do not apparently shift. The complex molecules are dispersed in the silica gel, this reduces concentration quenching. In addition, the relatively rigid host structure of silica gel limits the vibrations of the ligands, which decreases the non-radiative transition and increases the lifetime of Tb3 . Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Photoacoustic spectroscopy (PAS) is a calorimetric technique that detects the heat generated through the non-radiative transitions in the sample after absorbing light. PAS is suitable for any type of solid, whether it be crystalline, powder, or gel. It has been widely used to investigate the chemical and physical properties of many kinds of samples [1,2]. It is a complement to absorption and photoluminescence spectroscopic techniques [3]. Recently, we have studied the absorption properties and relaxation processes of rare earth complexes [4,5]. PAS has been found to be very suitable for investigating the solid rare earth complexes.
*
Corresponding author. Tel.: +86-551 360 1624; fax: +86-551 363 1760. E-mail address: [email protected] (Q. Su).
Silica glass has been used extensively as the host matrix of optical and laser materials [6,7]. Developments in the low-temperature sol±gel technique have allowed doping organic compounds into silica glass. Rare earth complexes with organic ligands are potentially suitable optical materials [8]. So it is interesting to study the optical properties of silica gels doped with rare earth complexes with organic ligands. In the paper, silica gels doped with rare earth salicylate acid complexes have been studied by using the PA amplitude and phase spectra, together with IR and ¯uorescence spectra.
2. Experimental The alcohol solution of salicylic acid (the mole ratio of salicylic acid and Ln3 was 3:1) was mixed with the alcohol solution of LnCl3 (Ln La, Nd or Tb) under stirring. The pH was adjusted to about 5
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 3 0 9 - 4
224
R. Wu et al. / Journal of Non-Crystalline Solids 278 (2000) 223±227
with diluted aqueous ammonia. A white crystalline powder appeared. After ®ltering, washing and drying Ln(sal)3 H2 O was placed into a desiccator. Silica gels doped with rare earth complexes were synthesized using the sol±gel technique. The sol± gel solution consisted of tetraethyloxysilane (TEOS), ethanol and distilled water (the ratio of mole is 1:4:4). Ln3 (10% of the mole number of TEOS) was introduced by adding LnCl3 to the solution. Salicylic acid was added in appropriate proportions to Ln3 . The pH was controlled at about 3. After mixing for 1 h at room temperature under vigorous stirring, the solution was placed into an oven whose temperature was adjusted to 40°C. The temperature increased to 150°C step by step within 24 h. Then the sample was maintained at 150°C for 12 h. In the same way, silica gels doped with salicylic acid or Nd3 were synthesized. The photoacoustic spectra were recorded in the region 300±700 nm. The incident light was modulated by a mechanical chopper at a frequency of 12 Hz. The acoustic signal was detected with the sample placed in a photoacoustic cell ®tted with a microphone and then was fed to a lock-in-ampli®er to which a reference signal was input from the chopper. The amplitude signals were normalized for changes in lamp intensity using the reference signal of carbon black. The ¯uorescence spectra were measured with a spectro¯uorometer. The IR spectra of the samples ground into KBr pellets were recorded with an IR spectrometer. The IR spectra of silica gel doped with salicylic acid and Tb3 show the carboxyl stretching frequencies at 1629 and 1400 cmÿ1 , which suggest complex formation of Tb3 with salicylic acid. Compared with the IR spectra of Tb(sal)3 H2 O, some peaks of silica gel doped with salicylic acid and Tb3 disappear and the intensities of the other peaks decrease. These are because the concentration of complexes decreases and the vibrations of the ligands were restricted by the surrounding gel matrix [9]. 3. Results The PA amplitude spectra of the samples are shown in Fig. 1. The broad band from 300 to 400
Fig. 1. PA amplitude spectra of Nd(sal)3 H2 O (a); silica gels doped with salicylic acid (b); doped with Nd3 (c); doped with Ln(III) (Ln La (d); Nd (e); Tb (f)) salicylate complexes.
nm is assigned to the p±p transition of salicylic acid. The PA peaks of the ligands in pure complexes are at 344 nm, but those in silica gels doped with complexes shift to 328 nm. The PA peaks from 400 to 700 nm are all assigned to the f±f transitions of Nd3 . The PA peaks of rare earth ion do not apparently shift. The emission spectra of La(sal)3 H2 O and silica gel doped with La(III) salicylate complex are shown in Fig. 2(a). The emission peak of the ligand in the La(sal)3 H2 O is at 408 nm, but that in the doped silica gel shifts to 427 nm. The emission spectra of Tb(sal)3 H2 O and silica gel doped with Tb(III) salicylate complex are shown in Fig. 2(b). The emission peaks at 485, 546, 589 and 621 nm are respectively assigned to 5 D4 ! 7 F6 , 5 D4 ! 7 F5 , 5 D4 ! 7 F4 and 5 D4 ! 7 F3 of Tb3 . The emission peaks of Tb3 in doped silica gel do not show shift apparently compared with those in Tb(sal)3 H2 O.
Fig. 2. Emission spectra of complexes and doped silica gels (excited at 344 and 328 nm, respectively): (a) La(sal)3 H2 O (solid line) and silica gel doped with La(III) salicylate complex (dashed line); (b) Tb(sal)3 H2 O (solid line) and silica gel doped with Tb(III) salicylate complex (dashed line).
R. Wu et al. / Journal of Non-Crystalline Solids 278 (2000) 223±227
From the PA and emission spectra, the PA and emission peaks of rare earth ions do not shift apparently, but the PA peaks of the ligands in doped silica gels show a blue shift and the emission peaks show a red shift. The PA signal is selectively sensitive only to the heat-producing deexcitation processes. For these samples, there are no other non-thermal deexcitation processes except the luminescence generally, The PA signal can be expressed as [10] P K1 Aabs 1 ÿ g;
1
where Aabs is the absorbance of the sample, K1 a coecient which is determined by the spectrometer, and g is the luminescence eciency. The ¯uorescence signal F can be expressed as F K2 Aabs g:
2
Combining Eqs. (1) and (2) 1 ÿ1 ; H c g
3
where H P =F , and c K1 =K2 . When the experimental condition is the same, we can take c as a constant. It can be inferred from Eq. (3) that as the luminescence eciency g of the sample increases, H exhibits a corresponding decrease. The relative PA and excitation intensities are listed in Table 1 with the values of H for comparison. From Table 1, it appears that the luminescence eciencies of La(III) and Tb(III) salicylate complexes in silica gel are higher than the corresponding pure complex. In Fig. 2(a), the luminescence intensity of La(III) complex in silica gel is higher, although the concentration in the doped silica gel is far lower than the pure complex. Photoacoustic phase is the time delay during the process from light absorption by the sample to
225
detection of the acoustic signal by microphone. The phase data is related to the optical and thermal properties of the sample as well as the relaxation processes, except for the instrumental elements [5] W tgÿ1 1 2=bls tgÿ1 xs;
4
where b is the absorption coecient of the sample, ls the thermal diusion length, x 2pf (f is the modulated frequency) and s is the relaxation time. When the absorption coecient is high enough, Eq. (4) is simpli®ed to W p=4 tgÿ1 xs:
5
The phase spectra of silica gels doped with complexes are shown in Fig. 3. The phase data of the samples at 300±340 nm remain constant, because the absorption coecient of the ligands is very high. The phase data of silica gel doped with La(III) salicylate complex are the highest. They are 3° higher than that of Tb(III) and are 14° higher than that of Nd(III) complex in the silica gel at 300±340 nm. In addition, the phase data of silica
Fig. 3. Phase spectra of silica gels doped with: (a) La(III); (b) Tb(III) or (c) Nd(III) salicylate complexes, and (d) Tb(sal)3 H2 O.
Table 1 Relative PA and excitation intensities of La(III) or Tb(III) complex-doped silica gel and pure complexes
PA relative intensity P Excitation relative intensity F H P =F
La(sal)3 H2 O
La(sal)3 in silica gel
Tb(sal)3 H2 O
Tb(sal)3 in silica gel
100 61 1.64
53 100 0.53
100 100 1.0
37 50 0.74
226
R. Wu et al. / Journal of Non-Crystalline Solids 278 (2000) 223±227
gel doped with Tb(III) complex are higher than that of the pure complex. According to Eq. (5), these re¯ect the variation of the relaxation time. At the location of the PA peaks of Nd3 , the phase is inversely proportional to the absorption coecient because the absorption coecient of f±f transition is small, which is consistent with Eq. (4).
4. Discussion The PA and emission peaks of rare earth ions do not apparently shift in doped silica gels, because the eect of environment on the bands 4f is very small. The locations of the energy levels of rare earth ions do not vary. As the ligand absorbs light, it is excited from the ground state to the Frank±Condon (F±C) excited state, then it relaxes to the equilibrium excited state. Finally, it returns to the ground state [11]. The energy levels of the ligand are aected by the polarization of the environment. The solid complexes Ln(sal)3 H2 O include crystalline and adsorbed water. The ligand is surrounded by H2 O that is polar. The ligand in doped silica gel is in the cage of SiO2 that has lower polarity than H2 O [12]. When the ligand transits to the F±C excited state after absorbing light, an induced dipole moment is formed. As the polarity of H2 O is higher than the walls of silica glass cage, it stabilizes the F±C excited state of the ligand. After the ligand relaxes to the equilibrium excited state the dipole moment disappears. Under this condition H2 O makes the ligand relatively unstable. In the doped silica gel, the energy of F±C excited state of the ligand increases and that of the equilibrium excited state decreases compared with the pure complex. So the PA peak of the ligand shows a blue shift and the emission peak shows a red shift. Both the PA relative intensities and the phase data of silica gels doped with Ln(III) (Ln La, Nd, Tb) salicylate complexes in the range of the ligands absorption were dierent in Figs. 1 and 3. They are directly related to their intramolecular energy relaxation processes. Energy level diagrams and intramolecular energy relaxation processes are shown in Fig. 4.
Fig. 4. Models of intramolecular energy relaxation processes of silica gels doped with Ln(III) (Ln La, Nd, Tb) salicylate complexes. (! radiative process; ÿ ÿ ÿ non-radiative process).
For La(III) salicylate complexes, La3 has no electron in 4f, energy cannot transfer from the lowest triplet state of sal to La3 . The ligand directly relaxes to the ground state radiatively or non-radiatively. The triplet state of the ligand is metastable [13], and its lifetime does not reduce, so the relaxation time is long. The phase data of silica gel doped with La(III) complex are the highest. The energy level of the triplet state of salicylic acid is 23 500 cmÿ1 [14]. It is higher than some energy level of Tb3 and Nd3 , so that in Tb(III) and Nd(III) salicylate complexes the energy may transfer from the triplet state of salicylic acid to the excited state of Nd3 and Tb3 . The lifetime of the triplet state becomes shorter. The numerous closely packed excited states of Nd3 provide paths for non-radiative transitions to the ground state so that the amount of heat given out from Nd(III) complexes is more than that from La(III) complexes. The lifetime of 4f excited state of Nd3 is short [15], so the relaxation time is short and the phase data is the smallest. However, Tb3 is a ¯uorescent ion, the energy that transfers from sal to Tb3 partly radiates so that the amount of heat given out from Tb(III) complexes is the least among these complexes. The longest lifetime of 4f excited state of Tb3 is the level of ms [15]. The lifetime of the triplet state of the ligand becomes short, but the relaxation time is still long. The phase data of the Tb(III) complex is higher than that of the Nd(III) complex in silica gel.
R. Wu et al. / Journal of Non-Crystalline Solids 278 (2000) 223±227
The luminescence eciencies of La(III) and Tb(III) salicylate complexes in silica gels are higher than the corresponding pure complex. The phase data of silica gel doped Tb(III) complex are also higher than that of Tb(sal)3 H2 O in the range of the ligand absorption. The excitation energy of the ligand can be lost by the vibration itself, and the excitation energy of Tb3 can also be lost by the vibration of its nearest ligands. The relatively rigid host structure of silica gel limits the vibrations of the ligands; this is consistent with the IR spectra, which decreases the non-radiative transitions caused by vibrations and increases the lifetime of Tb3 . In addition, the complex molecules are dispersed in the silica gel, which makes the concentration quenching lower greatly. All these make the luminescence eciency increase and the longest lifetime of the excited state of Tb3 to become longer.
5. Conclusions Compared with the pure complexes, the PA peaks of the ligands show a blue shift and the emission peaks show a red shift in silica gels doped with complexes, but the PA and emission peaks of rare earth ions do not apparently shift. The PA relative intensities and the phase data of silica gels doped with La(III), Tb(III) and Nd(III) salicylate complexes were dierent in the range of the ligand absorption, which shows the dierence of their intramolecular relaxation processes. The luminescence eciency of La(III) or Tb(III) salicylate complex in silica gel is higher than the corre-
227
sponding pure complex and the lifetime of Tb3 also increases in doped silica gel. Acknowledgements The authors gratefully acknowledge the National Nature Science Foundation of China for supporting this program (no. 29875026). References [1] C.S. Sunandana, Phys. Status Solidi A 105 (1988) 1. [2] A. Lachaine, R. Pottieer, D.A. Russell, Spectrochim. Acta Rev. 15 (1993) 125. [3] T. Ikarl, H. Yokoyama, S. Shigetomi, Jpn. J. Appl. Phys. 29 (1990) 887. [4] Y. Yang, Q. Su, G. Zhao, Spectrochim. Acta Part A 55 (1999) 1527. [5] Q. Mao, Q. Su, G. Zhao, Spectrochim. Acta Part A 52 (1996) 675. [6] E.J. Pope, J.D. Meddrano, J. Non-Cryst. Solids 106 (1988) 236. [7] I.M. Thomas, S.A. Payne, G.D. Wike, J. Non-Cryst. Solids 151 (1992) 183. [8] C.G.E. Buono, H. Li, Coord. Chem. Rev. 99 (1990) 55. [9] B. Yan, H.J. Zhang, S.B. Wang, J.Z. Ni, Mater. Chem. Phys. 51 (1997) 92. [10] W. Ronghu, S. Huiyu, S. Qingde, Spectrochim. Acta Part A. 56 (2000) 2073. [11] J.E. Bell, Spectroscopy in Biochemistry, vol. 1, CRC, Boca Raton, FL, 1981, p.158. [12] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956. [13] Z. Guiyun, Y. Jinghe, S. Zhikun, Chin. Rare Earth Soc. 7 (1989) 73. [14] Y.S. Yang, M.L. Gong, Y.Y. Li, H.Y. Lei, S.L. Wu, J. Alloys Compd. 207&208 (1994) 112. [15] W.T. Carnall, K.A. Gschneidner, L.R. Eyring, Handbook on the Physics and Chemistry of Rare Earth, vol. 3, NorthHolland, Amsterdam, 1979, p. 197, 199.
Letter to the Editor
www.elsevier.com/locate/jnoncrysol
Photoacoustic and ¯uorescence studies of silica gels doped with rare earth salicylic acid complexes Ronghu Wu, Huazhang Zhao, Qingde Su * Department of Chemistry, University of Science and Technology of China, Hefei Anhui 230026, People's Republic of China Received 13 December 1999; received in revised form 23 May 2000
Abstract Photoacoustic spectroscopy (PAS) is useful in the study of non-crystalline solids. By using PA amplitude and phase spectra, together with IR and ¯uorescence spectra, the variation of the energy levels and luminescence eciencies and the intramolecular relaxation processes of silica gels doped with rare earth complexes with salicylic acid have been studied in comparison with their corresponding pure complexes. After the complexes are doped into silica gels, the PA and emission peaks of the ligands show a shift, but the peaks of rare earth ions do not apparently shift. The complex molecules are dispersed in the silica gel, this reduces concentration quenching. In addition, the relatively rigid host structure of silica gel limits the vibrations of the ligands, which decreases the non-radiative transition and increases the lifetime of Tb3 . Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Photoacoustic spectroscopy (PAS) is a calorimetric technique that detects the heat generated through the non-radiative transitions in the sample after absorbing light. PAS is suitable for any type of solid, whether it be crystalline, powder, or gel. It has been widely used to investigate the chemical and physical properties of many kinds of samples [1,2]. It is a complement to absorption and photoluminescence spectroscopic techniques [3]. Recently, we have studied the absorption properties and relaxation processes of rare earth complexes [4,5]. PAS has been found to be very suitable for investigating the solid rare earth complexes.
*
Corresponding author. Tel.: +86-551 360 1624; fax: +86-551 363 1760. E-mail address: [email protected] (Q. Su).
Silica glass has been used extensively as the host matrix of optical and laser materials [6,7]. Developments in the low-temperature sol±gel technique have allowed doping organic compounds into silica glass. Rare earth complexes with organic ligands are potentially suitable optical materials [8]. So it is interesting to study the optical properties of silica gels doped with rare earth complexes with organic ligands. In the paper, silica gels doped with rare earth salicylate acid complexes have been studied by using the PA amplitude and phase spectra, together with IR and ¯uorescence spectra.
2. Experimental The alcohol solution of salicylic acid (the mole ratio of salicylic acid and Ln3 was 3:1) was mixed with the alcohol solution of LnCl3 (Ln La, Nd or Tb) under stirring. The pH was adjusted to about 5
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 0 ) 0 0 3 0 9 - 4
224
R. Wu et al. / Journal of Non-Crystalline Solids 278 (2000) 223±227
with diluted aqueous ammonia. A white crystalline powder appeared. After ®ltering, washing and drying Ln(sal)3 H2 O was placed into a desiccator. Silica gels doped with rare earth complexes were synthesized using the sol±gel technique. The sol± gel solution consisted of tetraethyloxysilane (TEOS), ethanol and distilled water (the ratio of mole is 1:4:4). Ln3 (10% of the mole number of TEOS) was introduced by adding LnCl3 to the solution. Salicylic acid was added in appropriate proportions to Ln3 . The pH was controlled at about 3. After mixing for 1 h at room temperature under vigorous stirring, the solution was placed into an oven whose temperature was adjusted to 40°C. The temperature increased to 150°C step by step within 24 h. Then the sample was maintained at 150°C for 12 h. In the same way, silica gels doped with salicylic acid or Nd3 were synthesized. The photoacoustic spectra were recorded in the region 300±700 nm. The incident light was modulated by a mechanical chopper at a frequency of 12 Hz. The acoustic signal was detected with the sample placed in a photoacoustic cell ®tted with a microphone and then was fed to a lock-in-ampli®er to which a reference signal was input from the chopper. The amplitude signals were normalized for changes in lamp intensity using the reference signal of carbon black. The ¯uorescence spectra were measured with a spectro¯uorometer. The IR spectra of the samples ground into KBr pellets were recorded with an IR spectrometer. The IR spectra of silica gel doped with salicylic acid and Tb3 show the carboxyl stretching frequencies at 1629 and 1400 cmÿ1 , which suggest complex formation of Tb3 with salicylic acid. Compared with the IR spectra of Tb(sal)3 H2 O, some peaks of silica gel doped with salicylic acid and Tb3 disappear and the intensities of the other peaks decrease. These are because the concentration of complexes decreases and the vibrations of the ligands were restricted by the surrounding gel matrix [9]. 3. Results The PA amplitude spectra of the samples are shown in Fig. 1. The broad band from 300 to 400
Fig. 1. PA amplitude spectra of Nd(sal)3 H2 O (a); silica gels doped with salicylic acid (b); doped with Nd3 (c); doped with Ln(III) (Ln La (d); Nd (e); Tb (f)) salicylate complexes.
nm is assigned to the p±p transition of salicylic acid. The PA peaks of the ligands in pure complexes are at 344 nm, but those in silica gels doped with complexes shift to 328 nm. The PA peaks from 400 to 700 nm are all assigned to the f±f transitions of Nd3 . The PA peaks of rare earth ion do not apparently shift. The emission spectra of La(sal)3 H2 O and silica gel doped with La(III) salicylate complex are shown in Fig. 2(a). The emission peak of the ligand in the La(sal)3 H2 O is at 408 nm, but that in the doped silica gel shifts to 427 nm. The emission spectra of Tb(sal)3 H2 O and silica gel doped with Tb(III) salicylate complex are shown in Fig. 2(b). The emission peaks at 485, 546, 589 and 621 nm are respectively assigned to 5 D4 ! 7 F6 , 5 D4 ! 7 F5 , 5 D4 ! 7 F4 and 5 D4 ! 7 F3 of Tb3 . The emission peaks of Tb3 in doped silica gel do not show shift apparently compared with those in Tb(sal)3 H2 O.
Fig. 2. Emission spectra of complexes and doped silica gels (excited at 344 and 328 nm, respectively): (a) La(sal)3 H2 O (solid line) and silica gel doped with La(III) salicylate complex (dashed line); (b) Tb(sal)3 H2 O (solid line) and silica gel doped with Tb(III) salicylate complex (dashed line).
R. Wu et al. / Journal of Non-Crystalline Solids 278 (2000) 223±227
From the PA and emission spectra, the PA and emission peaks of rare earth ions do not shift apparently, but the PA peaks of the ligands in doped silica gels show a blue shift and the emission peaks show a red shift. The PA signal is selectively sensitive only to the heat-producing deexcitation processes. For these samples, there are no other non-thermal deexcitation processes except the luminescence generally, The PA signal can be expressed as [10] P K1 Aabs 1 ÿ g;
1
where Aabs is the absorbance of the sample, K1 a coecient which is determined by the spectrometer, and g is the luminescence eciency. The ¯uorescence signal F can be expressed as F K2 Aabs g:
2
Combining Eqs. (1) and (2) 1 ÿ1 ; H c g
3
where H P =F , and c K1 =K2 . When the experimental condition is the same, we can take c as a constant. It can be inferred from Eq. (3) that as the luminescence eciency g of the sample increases, H exhibits a corresponding decrease. The relative PA and excitation intensities are listed in Table 1 with the values of H for comparison. From Table 1, it appears that the luminescence eciencies of La(III) and Tb(III) salicylate complexes in silica gel are higher than the corresponding pure complex. In Fig. 2(a), the luminescence intensity of La(III) complex in silica gel is higher, although the concentration in the doped silica gel is far lower than the pure complex. Photoacoustic phase is the time delay during the process from light absorption by the sample to
225
detection of the acoustic signal by microphone. The phase data is related to the optical and thermal properties of the sample as well as the relaxation processes, except for the instrumental elements [5] W tgÿ1 1 2=bls tgÿ1 xs;
4
where b is the absorption coecient of the sample, ls the thermal diusion length, x 2pf (f is the modulated frequency) and s is the relaxation time. When the absorption coecient is high enough, Eq. (4) is simpli®ed to W p=4 tgÿ1 xs:
5
The phase spectra of silica gels doped with complexes are shown in Fig. 3. The phase data of the samples at 300±340 nm remain constant, because the absorption coecient of the ligands is very high. The phase data of silica gel doped with La(III) salicylate complex are the highest. They are 3° higher than that of Tb(III) and are 14° higher than that of Nd(III) complex in the silica gel at 300±340 nm. In addition, the phase data of silica
Fig. 3. Phase spectra of silica gels doped with: (a) La(III); (b) Tb(III) or (c) Nd(III) salicylate complexes, and (d) Tb(sal)3 H2 O.
Table 1 Relative PA and excitation intensities of La(III) or Tb(III) complex-doped silica gel and pure complexes
PA relative intensity P Excitation relative intensity F H P =F
La(sal)3 H2 O
La(sal)3 in silica gel
Tb(sal)3 H2 O
Tb(sal)3 in silica gel
100 61 1.64
53 100 0.53
100 100 1.0
37 50 0.74
226
R. Wu et al. / Journal of Non-Crystalline Solids 278 (2000) 223±227
gel doped with Tb(III) complex are higher than that of the pure complex. According to Eq. (5), these re¯ect the variation of the relaxation time. At the location of the PA peaks of Nd3 , the phase is inversely proportional to the absorption coecient because the absorption coecient of f±f transition is small, which is consistent with Eq. (4).
4. Discussion The PA and emission peaks of rare earth ions do not apparently shift in doped silica gels, because the eect of environment on the bands 4f is very small. The locations of the energy levels of rare earth ions do not vary. As the ligand absorbs light, it is excited from the ground state to the Frank±Condon (F±C) excited state, then it relaxes to the equilibrium excited state. Finally, it returns to the ground state [11]. The energy levels of the ligand are aected by the polarization of the environment. The solid complexes Ln(sal)3 H2 O include crystalline and adsorbed water. The ligand is surrounded by H2 O that is polar. The ligand in doped silica gel is in the cage of SiO2 that has lower polarity than H2 O [12]. When the ligand transits to the F±C excited state after absorbing light, an induced dipole moment is formed. As the polarity of H2 O is higher than the walls of silica glass cage, it stabilizes the F±C excited state of the ligand. After the ligand relaxes to the equilibrium excited state the dipole moment disappears. Under this condition H2 O makes the ligand relatively unstable. In the doped silica gel, the energy of F±C excited state of the ligand increases and that of the equilibrium excited state decreases compared with the pure complex. So the PA peak of the ligand shows a blue shift and the emission peak shows a red shift. Both the PA relative intensities and the phase data of silica gels doped with Ln(III) (Ln La, Nd, Tb) salicylate complexes in the range of the ligands absorption were dierent in Figs. 1 and 3. They are directly related to their intramolecular energy relaxation processes. Energy level diagrams and intramolecular energy relaxation processes are shown in Fig. 4.
Fig. 4. Models of intramolecular energy relaxation processes of silica gels doped with Ln(III) (Ln La, Nd, Tb) salicylate complexes. (! radiative process; ÿ ÿ ÿ non-radiative process).
For La(III) salicylate complexes, La3 has no electron in 4f, energy cannot transfer from the lowest triplet state of sal to La3 . The ligand directly relaxes to the ground state radiatively or non-radiatively. The triplet state of the ligand is metastable [13], and its lifetime does not reduce, so the relaxation time is long. The phase data of silica gel doped with La(III) complex are the highest. The energy level of the triplet state of salicylic acid is 23 500 cmÿ1 [14]. It is higher than some energy level of Tb3 and Nd3 , so that in Tb(III) and Nd(III) salicylate complexes the energy may transfer from the triplet state of salicylic acid to the excited state of Nd3 and Tb3 . The lifetime of the triplet state becomes shorter. The numerous closely packed excited states of Nd3 provide paths for non-radiative transitions to the ground state so that the amount of heat given out from Nd(III) complexes is more than that from La(III) complexes. The lifetime of 4f excited state of Nd3 is short [15], so the relaxation time is short and the phase data is the smallest. However, Tb3 is a ¯uorescent ion, the energy that transfers from sal to Tb3 partly radiates so that the amount of heat given out from Tb(III) complexes is the least among these complexes. The longest lifetime of 4f excited state of Tb3 is the level of ms [15]. The lifetime of the triplet state of the ligand becomes short, but the relaxation time is still long. The phase data of the Tb(III) complex is higher than that of the Nd(III) complex in silica gel.
R. Wu et al. / Journal of Non-Crystalline Solids 278 (2000) 223±227
The luminescence eciencies of La(III) and Tb(III) salicylate complexes in silica gels are higher than the corresponding pure complex. The phase data of silica gel doped Tb(III) complex are also higher than that of Tb(sal)3 H2 O in the range of the ligand absorption. The excitation energy of the ligand can be lost by the vibration itself, and the excitation energy of Tb3 can also be lost by the vibration of its nearest ligands. The relatively rigid host structure of silica gel limits the vibrations of the ligands; this is consistent with the IR spectra, which decreases the non-radiative transitions caused by vibrations and increases the lifetime of Tb3 . In addition, the complex molecules are dispersed in the silica gel, which makes the concentration quenching lower greatly. All these make the luminescence eciency increase and the longest lifetime of the excited state of Tb3 to become longer.
5. Conclusions Compared with the pure complexes, the PA peaks of the ligands show a blue shift and the emission peaks show a red shift in silica gels doped with complexes, but the PA and emission peaks of rare earth ions do not apparently shift. The PA relative intensities and the phase data of silica gels doped with La(III), Tb(III) and Nd(III) salicylate complexes were dierent in the range of the ligand absorption, which shows the dierence of their intramolecular relaxation processes. The luminescence eciency of La(III) or Tb(III) salicylate complex in silica gel is higher than the corre-
227
sponding pure complex and the lifetime of Tb3 also increases in doped silica gel. Acknowledgements The authors gratefully acknowledge the National Nature Science Foundation of China for supporting this program (no. 29875026). References [1] C.S. Sunandana, Phys. Status Solidi A 105 (1988) 1. [2] A. Lachaine, R. Pottieer, D.A. Russell, Spectrochim. Acta Rev. 15 (1993) 125. [3] T. Ikarl, H. Yokoyama, S. Shigetomi, Jpn. J. Appl. Phys. 29 (1990) 887. [4] Y. Yang, Q. Su, G. Zhao, Spectrochim. Acta Part A 55 (1999) 1527. [5] Q. Mao, Q. Su, G. Zhao, Spectrochim. Acta Part A 52 (1996) 675. [6] E.J. Pope, J.D. Meddrano, J. Non-Cryst. Solids 106 (1988) 236. [7] I.M. Thomas, S.A. Payne, G.D. Wike, J. Non-Cryst. Solids 151 (1992) 183. [8] C.G.E. Buono, H. Li, Coord. Chem. Rev. 99 (1990) 55. [9] B. Yan, H.J. Zhang, S.B. Wang, J.Z. Ni, Mater. Chem. Phys. 51 (1997) 92. [10] W. Ronghu, S. Huiyu, S. Qingde, Spectrochim. Acta Part A. 56 (2000) 2073. [11] J.E. Bell, Spectroscopy in Biochemistry, vol. 1, CRC, Boca Raton, FL, 1981, p.158. [12] D. Avnir, D. Levy, R. Reisfeld, J. Phys. Chem. 88 (1984) 5956. [13] Z. Guiyun, Y. Jinghe, S. Zhikun, Chin. Rare Earth Soc. 7 (1989) 73. [14] Y.S. Yang, M.L. Gong, Y.Y. Li, H.Y. Lei, S.L. Wu, J. Alloys Compd. 207&208 (1994) 112. [15] W.T. Carnall, K.A. Gschneidner, L.R. Eyring, Handbook on the Physics and Chemistry of Rare Earth, vol. 3, NorthHolland, Amsterdam, 1979, p. 197, 199.