- Email: [email protected]
Amplified spontaneous emissions from (oxalato)(dibenzoylmethanato)boron
Optical Materials 27 (2005) 1815–1818 www.elsevier.com/locate/optmat
Amplified spontaneous emissions from (oxalato)(dibenzoylmethanato)boron Hockchuan Lim a, Seongshan Yap b, Teckyong Tou a
b,*
, Seikweng Ng
a
Institute for Postgraduate Studies and Research, University of Malaya, 50603 Kuala Lumpur, Malaysia b Faculty of Engineering, Multimedia University Cyberjaya, 63100 Selangor, Malaysia Received 22 July 2003; accepted 15 November 2004 Available online 13 January 2005
Abstract Amplified spontaneous emissions (ASE) from two boron-containing organic compounds, (oxalato)(dibenzoylmethanato)boron, were obtained for the first time using a nitrogen laser as the pump source. Compared to commercially available laser dyes, there were only small red shift and minimum broadening in the peak ASE wavelength. ASE conversion efficiencies obtained were 8–10% in chloroform solution and 5% when doped in thin films of poly(methyl methacrylate). 2004 Elsevier B.V. All rights reserved. PACS: 42.55.Mv; 42.70.Hj; 32.50+d Keywords: Amplified spontaneous emission; Laser dye; Boron; Organic compounds
1. Introduction Stimulated emission from organic dyes in solution was first reported almost forty years ago by Sorokin and Lankard [1] and Schafer and Schmidt [2], and thousands of organic compounds that can exhibit strong fluorescence have been evaluated for the laser action. Research in developing new laser dyes continues, especially on those with little triplet-state loss [3] such as the quasi-aromatic [4] and the intramolecular proton transfer [5] compounds. Recently, there was an interest in synthesizing novel dyes with large two-photon absorption cross-section, which can enable up-conversion lasing [6], and are applicable to optical power limiting [7], three dimensional lithographic microfabrication [8] and optical data storage [9]. For development of solid-state dye laser, the research interest started in 1960s was renewed by doping laser dyes in different types of host materials such as polymer [10], liquid *
Corresponding author. E-mail address: [email protected] (T. Tou).
0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.11.032
crystal [11], sol–gel [12], and organic–inorganic composite [13]. This article presents the measurements of amplified spontaneous emission (ASE) from two boroncontaining organic compounds that can be classified as b-diketoboronates. The general structure of b-diketoboro-nates, as shown in Fig. 1, represents a special type of chelated boron (atomic number 5) complexes. The tetrahedrally-coordinated boron compounds having two conjugated p-entities that bind to the boron atom through their pairs of oxygen ends display an unusual type of electron delocalisation over the boron atom that occurs without the intervention of conventional p-bond resonance. The two conjugated p-entities (five- and sixmembered chelates) are orthogonal to each other. Such a configuration shows a special type of electron delocalisation that has been called as spirointeraction, which was proposed on the basis of spectroscopic evidence and derived from their absorption and fluorescence spectra [14]. The preparation method for b-diketoboronate compounds from enolisable b-dicarbonyl compounds, a diol
1816
H. Lim et al. / Optical Materials 27 (2005) 1815–1818
R
R O B O
O O
Fig. 1. General structure of b-diketoboronates.
and boric acid, were documented [15,16]. The structures of the two boron-containing organic compounds, BOCI and BOC-II, used for this work are shown in Fig. 2. Both are structurally the same and known as (oxalato)(dibenzoylmethanato)boron except that BOC-II is deuteriated. In comparison with the conventional and commercially available laser dyes, these organic compounds have simpler structures and they can be readily synthesized by conventional organic reactions. They have also been proposed for laser action [17] but to the best of our literature search, this suggestion has yet been taken up.
2. Experimental The boron-containing organic compounds, BOC-I and BOC-II, used in this work were prepared and given as gifts from Prof. H. Hartmann of Technische Hochschule Merseburg, Germany, whose method was already referred in the above section [15,16]. These two compounds could only be dissolved in chloroform and not in alcohol, which is a disadvantage against many commercial laser dyes. The fluorescence spectra from chloroform solution was collected using Shimadzu UV–Vis-NIR Scanning Spectrophotometer (Model UV3101 PC) and Perkin–Elmer LS 50B Luminescence Spectrometer, so as to detect the spectral narrowing for ‘‘laser action’’. However, the ‘‘laser action’’ obtained
in this work is better classified as the amplified spontaneous emission (ASE) without using a laser resonator. The ASE was obtained from chloroform solutions of BOC-I and BOC-II, contained in a 1 cm UV quartz cuvette, upon transverse pumping by a 1 mJ nitrogen laser. And the ASE energy and spectra were measured, respectively, by a pyroelectric energy meter (Ophir, model Nova PE-10) and an optical multichannel analyser (EG&G Inc., model 1461). The ASE from BOC-I and BOC-II were also obtained when they were doped in 20 lm thin films of poly(methyl methacrylate) (Mw > 900,000), such interest arises from a stable dye-doped polymer lasers in UV-blue region [12]. The dip-coating technique was used to prepare doped PMMA thin films (20–60 lm) on the microscope slides. After a brief dipping, films coated on both sides of the microscope slide were allowed to dry in draft-free area in order in order to obtain uniform film. The ASE beam emerged from both ends of the microscope slide acting as a planar waveguide [18,19].
3. Results and discussion The two compounds, BOC-I and BOC-II, exhibit similar fluorescence spectra but with a small difference, of about 5 nm, in their peak wavelengths. This may be attributed to deuterium in BOC-II molecular structure which causes the red shift. The full-width, half-maximum (FWHM) of their fluorescence spectra (Fig. 3) may provide a dye-laser tunable range of 30–40 nm that is comparable to most of the commercially available laser dyes. When doped in thin films of PMMA, there is an additional but relatively small red shift of (3–5 nm) in their peak fluorescence wavelengths, as compared to the many commercial dyes when doped in different types of solid hosts [10–13,19,20].
6 -3
2.61x10 M -3
2.14x10 M D
O
O
O
B O
O B
O
O
O
O
O BOC-I
Intensity (a.u.)
-3
H
1.82x10 M
4
-3
1.25x10 M 2
O
O BOC-II
0 400
410
420
430
440
450
460
470
480
490
Wavelength (nm) Fig. 2. Structures of boron-containing organic compound, BOC-I and BOC-II, of (oxalato)(dibenzoylmethanato)boron.
Fig. 3. Fluorescence spectra of BOC-I in chloroform.
500
H. Lim et al. / Optical Materials 27 (2005) 1815–1818
1817
12 10
3.00x10-3 M 10
Intensity
Intensity (a.u.)
8
5.00x10-3 M
8
1.50x10-3 M
6
1.07x10-3 M
4
4
2
2 0 400
6
0
410
420
430
440
450
460
470
480
1.0x10-3
2.0x10-3
Wavelength (nm)
3.0x10-3
4.0x10-3
5.0x10-3
6.0x10-3
Concentration (M)
Fig. 4. ASE spectra from chloroform solution of BOC-I.
Fig. 6. ASE conversion efficiency of BOC-I in chloroform.
The ASE spectra from chloroform solutions, as shown in Fig. 4, shows significant spectral narrowing to 5 nm (FWHM) which is not commonly achieved in commercial laser dyes. Not only the ASE broadening of BOC-I and BOC-II in thin films of PMMA by merely (2–3) nm (Fig. 5) is very small and the blue shift is extremely little as compared to commercially available Coumarin dyes [21]. This work thus suggests that strong and tunable laser actions are highly possible if BOC-I and BOC-II are to be tested with a laser resonator. The ASE conversion efficiencies, as defined by its output energy with respect to the nitrogen laser energy, were 10% for BOC-I and 8.3% for BOC-II at the same concentration of 3.0 · 10 3 M in chloroform (Fig. 6). These conversion efficiencies are comparable to that of the more efficient commercial dye, Coumarin-460 (10%) when tested under the same condition [22]. Although the difference in conversion efficiencies is obviously due to hydrogen and deuterium in their molecular, it may contradict with the general understanding [3] that replacing the hydrogen with deuterium in the chromophore of laser dye increases the fluorescence efficiency. This dis-
crepancy was not investigated because not only our nitrogen laser output was unstable (10% fluctuation), the amounts of BOC-I and II were insufficient for further work. The ASE conversion efficiencies for BOC-I and BOCII doped in thin films of PMMA were (4–5%), a 50% reduction from that for chloroform solutions. The concentration dependence of the ASE conversion efficiency for doped thin films of PMMA was transformed from a curve to a linear function (Fig. 7). This may be explained by the formation of dye–molecule aggregates, as a result of reduced free volume in a more rigid host such as the PMMA. While these aggregates reduced the BOC concentration and hence the ASE output, they were still photo-absorptive but non-radiative. The formation of dye–molecule aggregates was accelerated by addition of more BOC compounds in PMMA, and led to precipitation of very fine particulates that could be observed with naked eyes even before drying the PMMA thin film. Upon formation of fine particulates, the ASE
10
Intensity (a.u.)
8
Conversion Efficiency (%)
2.61x10-3 M -3
2.14x10 M
6
1.58x10-3 M
4
1.25x10-3 M
2
0 400
4.5
4.0
3.5
1.0x10-3
410
420
430
440
450
460
470
480
1.5x10-3
2.0x10-3
2.5x10-3
3.0x10-3
Concentartion (M)
Wavelength (nm) Fig. 5. ASE spectra of BOC-I doped in PMMA thin films.
Fig. 7. ASE conversion efficiency of BOC-I doped in PMMA thin films.
1818
H. Lim et al. / Optical Materials 27 (2005) 1815–1818
output was terminated abruptly probably as a result of a combined effect of competition for pump source and optical scattering by fine particulates which inhibited ASE. 4. Conclusion In short, both the boron compounds have been tested positive for ASE in the simple setup without a laser resonator. The ASE spectra were narrower and their tunable range were comparable to commercial dyes. Conversion efficiencies, from nitrogen laser to ASE energies, were lower than the best commercial dye in the blue region, Coumarin 460, but are probably comparable or even higher than the others. The main disadvantage was that they have a limited solubility in alcohol.
Acknowledgments The authors acknowledge the Ministry of Science, Technology and the Environment, Malaysia for R&D grant 09-02-03-0662.
References [1] P.P. Sorokin, J.R. Lankard, IBM J. Res. Dev. 11 (1966) 162.
[2] F.P. Scha¨fer, W. Schmidt, J. Volze, Appl. Phys. Lett. 9 (1966) 306. [3] K.H. Drexhage, in: F.P. Sha¨fer (Ed.), Dye Lasers, Springer-Verlag Topics in Applied Physics, Springer-Verlag, New York, 1977. [4] T.G. Pavlopoulos, D.A. Lightner, J.O. Brower, Appl. Opt. 42 (2003) 3555. [5] M.L. Ferrer, A.U. Acuna, F. Amat-Guerri, A. Costela, J.M. Figuera, F. Florido, R. Sastre, Appl. Opt. 33 (1994) 2266. [6] G.S. He, J.D. Bhawalkar, C. Zhao, P.N. Prasad, IEEE J. Quantum Electron 32 (1996) 749. [7] G. Zhuo, Z. Wang, D. Wang, C. Wang, X. Zhao, Z. Shao, M. Jiang, Opt. Laser Technol. 33 (2001) 209. [8] M.J. Miller, S.H. Wei, I. Parker, M.D. Cahalan, Science 296 (2002) 1876. [9] S. Maruo, S. Nakamura, S. Kawata, Opt. Lett. 22 (1997) 132. [10] R. Sastre, A. Costela, Adv. Mater. 7 (1995) 198. [11] P.V. Shibaev, V.I. Kopp, A.Z. Genack, J. Phys. Chem. B 107 (2003) 6961. [12] D. Lo, J.E. Parris, J.L. Lawless, Appl. Phys. B 55 (1992) 365. [13] Y. Yang, M. Wang, G. Qian, Z. Wang, X. Fan, Opt. Mater. 24 (2004) 621. [14] Y.L. Chow, Y.H. Zhang, M.X. Zheng, A. Rassat, Chem. Phys. Lett. 272 (1997) 471. [15] H. Hartmann, J. Prat. Chem. 328 (1986) 755. [16] P. Rapta, K. Erentova´, A. Stasˇko, H. Hartmann, Electrochim. Acta 39 (1994) 2251. [17] W. Rettig, Angew. Chem., Int. Ed. Engl. 25 (1986) 971. [18] R. Reisfeld, D. Shamrakov, Chem. Phys. Lett. 213 (1993) 47. [19] S.S. Yap, W.O. Siew, T.Y. Tou, S.W. Ng, Appl. Opt. 41 (2002) 1725. [20] K.C. Yee, T.Y. Tou, S.W. Ng, Appl. Opt. 41 (1998) 6381. [21] J. Huang, V. Bekiari, P. Lianos, S. Couris, J. Lumin. 81 (1999) 285. [22] K.W. Yee, Master Thesis, University of Malaya, 1997.
Amplified spontaneous emissions from (oxalato)(dibenzoylmethanato)boron Hockchuan Lim a, Seongshan Yap b, Teckyong Tou a
b,*
, Seikweng Ng
a
Institute for Postgraduate Studies and Research, University of Malaya, 50603 Kuala Lumpur, Malaysia b Faculty of Engineering, Multimedia University Cyberjaya, 63100 Selangor, Malaysia Received 22 July 2003; accepted 15 November 2004 Available online 13 January 2005
Abstract Amplified spontaneous emissions (ASE) from two boron-containing organic compounds, (oxalato)(dibenzoylmethanato)boron, were obtained for the first time using a nitrogen laser as the pump source. Compared to commercially available laser dyes, there were only small red shift and minimum broadening in the peak ASE wavelength. ASE conversion efficiencies obtained were 8–10% in chloroform solution and 5% when doped in thin films of poly(methyl methacrylate). 2004 Elsevier B.V. All rights reserved. PACS: 42.55.Mv; 42.70.Hj; 32.50+d Keywords: Amplified spontaneous emission; Laser dye; Boron; Organic compounds
1. Introduction Stimulated emission from organic dyes in solution was first reported almost forty years ago by Sorokin and Lankard [1] and Schafer and Schmidt [2], and thousands of organic compounds that can exhibit strong fluorescence have been evaluated for the laser action. Research in developing new laser dyes continues, especially on those with little triplet-state loss [3] such as the quasi-aromatic [4] and the intramolecular proton transfer [5] compounds. Recently, there was an interest in synthesizing novel dyes with large two-photon absorption cross-section, which can enable up-conversion lasing [6], and are applicable to optical power limiting [7], three dimensional lithographic microfabrication [8] and optical data storage [9]. For development of solid-state dye laser, the research interest started in 1960s was renewed by doping laser dyes in different types of host materials such as polymer [10], liquid *
Corresponding author. E-mail address: [email protected] (T. Tou).
0925-3467/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.11.032
crystal [11], sol–gel [12], and organic–inorganic composite [13]. This article presents the measurements of amplified spontaneous emission (ASE) from two boroncontaining organic compounds that can be classified as b-diketoboronates. The general structure of b-diketoboro-nates, as shown in Fig. 1, represents a special type of chelated boron (atomic number 5) complexes. The tetrahedrally-coordinated boron compounds having two conjugated p-entities that bind to the boron atom through their pairs of oxygen ends display an unusual type of electron delocalisation over the boron atom that occurs without the intervention of conventional p-bond resonance. The two conjugated p-entities (five- and sixmembered chelates) are orthogonal to each other. Such a configuration shows a special type of electron delocalisation that has been called as spirointeraction, which was proposed on the basis of spectroscopic evidence and derived from their absorption and fluorescence spectra [14]. The preparation method for b-diketoboronate compounds from enolisable b-dicarbonyl compounds, a diol
1816
H. Lim et al. / Optical Materials 27 (2005) 1815–1818
R
R O B O
O O
Fig. 1. General structure of b-diketoboronates.
and boric acid, were documented [15,16]. The structures of the two boron-containing organic compounds, BOCI and BOC-II, used for this work are shown in Fig. 2. Both are structurally the same and known as (oxalato)(dibenzoylmethanato)boron except that BOC-II is deuteriated. In comparison with the conventional and commercially available laser dyes, these organic compounds have simpler structures and they can be readily synthesized by conventional organic reactions. They have also been proposed for laser action [17] but to the best of our literature search, this suggestion has yet been taken up.
2. Experimental The boron-containing organic compounds, BOC-I and BOC-II, used in this work were prepared and given as gifts from Prof. H. Hartmann of Technische Hochschule Merseburg, Germany, whose method was already referred in the above section [15,16]. These two compounds could only be dissolved in chloroform and not in alcohol, which is a disadvantage against many commercial laser dyes. The fluorescence spectra from chloroform solution was collected using Shimadzu UV–Vis-NIR Scanning Spectrophotometer (Model UV3101 PC) and Perkin–Elmer LS 50B Luminescence Spectrometer, so as to detect the spectral narrowing for ‘‘laser action’’. However, the ‘‘laser action’’ obtained
in this work is better classified as the amplified spontaneous emission (ASE) without using a laser resonator. The ASE was obtained from chloroform solutions of BOC-I and BOC-II, contained in a 1 cm UV quartz cuvette, upon transverse pumping by a 1 mJ nitrogen laser. And the ASE energy and spectra were measured, respectively, by a pyroelectric energy meter (Ophir, model Nova PE-10) and an optical multichannel analyser (EG&G Inc., model 1461). The ASE from BOC-I and BOC-II were also obtained when they were doped in 20 lm thin films of poly(methyl methacrylate) (Mw > 900,000), such interest arises from a stable dye-doped polymer lasers in UV-blue region [12]. The dip-coating technique was used to prepare doped PMMA thin films (20–60 lm) on the microscope slides. After a brief dipping, films coated on both sides of the microscope slide were allowed to dry in draft-free area in order in order to obtain uniform film. The ASE beam emerged from both ends of the microscope slide acting as a planar waveguide [18,19].
3. Results and discussion The two compounds, BOC-I and BOC-II, exhibit similar fluorescence spectra but with a small difference, of about 5 nm, in their peak wavelengths. This may be attributed to deuterium in BOC-II molecular structure which causes the red shift. The full-width, half-maximum (FWHM) of their fluorescence spectra (Fig. 3) may provide a dye-laser tunable range of 30–40 nm that is comparable to most of the commercially available laser dyes. When doped in thin films of PMMA, there is an additional but relatively small red shift of (3–5 nm) in their peak fluorescence wavelengths, as compared to the many commercial dyes when doped in different types of solid hosts [10–13,19,20].
6 -3
2.61x10 M -3
2.14x10 M D
O
O
O
B O
O B
O
O
O
O
O BOC-I
Intensity (a.u.)
-3
H
1.82x10 M
4
-3
1.25x10 M 2
O
O BOC-II
0 400
410
420
430
440
450
460
470
480
490
Wavelength (nm) Fig. 2. Structures of boron-containing organic compound, BOC-I and BOC-II, of (oxalato)(dibenzoylmethanato)boron.
Fig. 3. Fluorescence spectra of BOC-I in chloroform.
500
H. Lim et al. / Optical Materials 27 (2005) 1815–1818
1817
12 10
3.00x10-3 M 10
Intensity
Intensity (a.u.)
8
5.00x10-3 M
8
1.50x10-3 M
6
1.07x10-3 M
4
4
2
2 0 400
6
0
410
420
430
440
450
460
470
480
1.0x10-3
2.0x10-3
Wavelength (nm)
3.0x10-3
4.0x10-3
5.0x10-3
6.0x10-3
Concentration (M)
Fig. 4. ASE spectra from chloroform solution of BOC-I.
Fig. 6. ASE conversion efficiency of BOC-I in chloroform.
The ASE spectra from chloroform solutions, as shown in Fig. 4, shows significant spectral narrowing to 5 nm (FWHM) which is not commonly achieved in commercial laser dyes. Not only the ASE broadening of BOC-I and BOC-II in thin films of PMMA by merely (2–3) nm (Fig. 5) is very small and the blue shift is extremely little as compared to commercially available Coumarin dyes [21]. This work thus suggests that strong and tunable laser actions are highly possible if BOC-I and BOC-II are to be tested with a laser resonator. The ASE conversion efficiencies, as defined by its output energy with respect to the nitrogen laser energy, were 10% for BOC-I and 8.3% for BOC-II at the same concentration of 3.0 · 10 3 M in chloroform (Fig. 6). These conversion efficiencies are comparable to that of the more efficient commercial dye, Coumarin-460 (10%) when tested under the same condition [22]. Although the difference in conversion efficiencies is obviously due to hydrogen and deuterium in their molecular, it may contradict with the general understanding [3] that replacing the hydrogen with deuterium in the chromophore of laser dye increases the fluorescence efficiency. This dis-
crepancy was not investigated because not only our nitrogen laser output was unstable (10% fluctuation), the amounts of BOC-I and II were insufficient for further work. The ASE conversion efficiencies for BOC-I and BOCII doped in thin films of PMMA were (4–5%), a 50% reduction from that for chloroform solutions. The concentration dependence of the ASE conversion efficiency for doped thin films of PMMA was transformed from a curve to a linear function (Fig. 7). This may be explained by the formation of dye–molecule aggregates, as a result of reduced free volume in a more rigid host such as the PMMA. While these aggregates reduced the BOC concentration and hence the ASE output, they were still photo-absorptive but non-radiative. The formation of dye–molecule aggregates was accelerated by addition of more BOC compounds in PMMA, and led to precipitation of very fine particulates that could be observed with naked eyes even before drying the PMMA thin film. Upon formation of fine particulates, the ASE
10
Intensity (a.u.)
8
Conversion Efficiency (%)
2.61x10-3 M -3
2.14x10 M
6
1.58x10-3 M
4
1.25x10-3 M
2
0 400
4.5
4.0
3.5
1.0x10-3
410
420
430
440
450
460
470
480
1.5x10-3
2.0x10-3
2.5x10-3
3.0x10-3
Concentartion (M)
Wavelength (nm) Fig. 5. ASE spectra of BOC-I doped in PMMA thin films.
Fig. 7. ASE conversion efficiency of BOC-I doped in PMMA thin films.
1818
H. Lim et al. / Optical Materials 27 (2005) 1815–1818
output was terminated abruptly probably as a result of a combined effect of competition for pump source and optical scattering by fine particulates which inhibited ASE. 4. Conclusion In short, both the boron compounds have been tested positive for ASE in the simple setup without a laser resonator. The ASE spectra were narrower and their tunable range were comparable to commercial dyes. Conversion efficiencies, from nitrogen laser to ASE energies, were lower than the best commercial dye in the blue region, Coumarin 460, but are probably comparable or even higher than the others. The main disadvantage was that they have a limited solubility in alcohol.
Acknowledgments The authors acknowledge the Ministry of Science, Technology and the Environment, Malaysia for R&D grant 09-02-03-0662.
References [1] P.P. Sorokin, J.R. Lankard, IBM J. Res. Dev. 11 (1966) 162.
[2] F.P. Scha¨fer, W. Schmidt, J. Volze, Appl. Phys. Lett. 9 (1966) 306. [3] K.H. Drexhage, in: F.P. Sha¨fer (Ed.), Dye Lasers, Springer-Verlag Topics in Applied Physics, Springer-Verlag, New York, 1977. [4] T.G. Pavlopoulos, D.A. Lightner, J.O. Brower, Appl. Opt. 42 (2003) 3555. [5] M.L. Ferrer, A.U. Acuna, F. Amat-Guerri, A. Costela, J.M. Figuera, F. Florido, R. Sastre, Appl. Opt. 33 (1994) 2266. [6] G.S. He, J.D. Bhawalkar, C. Zhao, P.N. Prasad, IEEE J. Quantum Electron 32 (1996) 749. [7] G. Zhuo, Z. Wang, D. Wang, C. Wang, X. Zhao, Z. Shao, M. Jiang, Opt. Laser Technol. 33 (2001) 209. [8] M.J. Miller, S.H. Wei, I. Parker, M.D. Cahalan, Science 296 (2002) 1876. [9] S. Maruo, S. Nakamura, S. Kawata, Opt. Lett. 22 (1997) 132. [10] R. Sastre, A. Costela, Adv. Mater. 7 (1995) 198. [11] P.V. Shibaev, V.I. Kopp, A.Z. Genack, J. Phys. Chem. B 107 (2003) 6961. [12] D. Lo, J.E. Parris, J.L. Lawless, Appl. Phys. B 55 (1992) 365. [13] Y. Yang, M. Wang, G. Qian, Z. Wang, X. Fan, Opt. Mater. 24 (2004) 621. [14] Y.L. Chow, Y.H. Zhang, M.X. Zheng, A. Rassat, Chem. Phys. Lett. 272 (1997) 471. [15] H. Hartmann, J. Prat. Chem. 328 (1986) 755. [16] P. Rapta, K. Erentova´, A. Stasˇko, H. Hartmann, Electrochim. Acta 39 (1994) 2251. [17] W. Rettig, Angew. Chem., Int. Ed. Engl. 25 (1986) 971. [18] R. Reisfeld, D. Shamrakov, Chem. Phys. Lett. 213 (1993) 47. [19] S.S. Yap, W.O. Siew, T.Y. Tou, S.W. Ng, Appl. Opt. 41 (2002) 1725. [20] K.C. Yee, T.Y. Tou, S.W. Ng, Appl. Opt. 41 (1998) 6381. [21] J. Huang, V. Bekiari, P. Lianos, S. Couris, J. Lumin. 81 (1999) 285. [22] K.W. Yee, Master Thesis, University of Malaya, 1997.