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Chemiluminescence characteristics of cumarin derivatives as blue fluorescers in peroxyoxalate–hydrogen peroxide system
Spectrochimica Acta Part A 59 (2003) 1145 /1150 www.elsevier.com/locate/saa
Chemiluminescence characteristics of cumarin derivatives as blue fluorescers in peroxyoxalate hydrogen peroxide system /
Mohammad Javad Chaichi a, Ali Reza Karami b, Abbas Shockravi a, Mojtaba Shamsipur c,* a b
Department of Chemistry, Tarbiat Moalem University, Tehran, Iran Department of Chemistry, Tarbiat Modarres University, Tehran, Iran c Department of Chemistry, Razi University, Kermanshah, Iran Received 12 December 2001; accepted 9 August 2002
Abstract The chemiluminescence characteristics of seven different cumarin derivatives were studied in detail. The fluorescence and chemiluminescence spectra were compared; all cumarins used were found to act as blue fluorescers. The intensity and kinetic parameters for the chemiluminescent systems were evaluated from computer fitting of the resulting intensity /time plots. Among different cumarin derivatives used, 7-amino-4-trifluoromethylcumarin revealed the most promising characteristics as an efficient blue fluorescent emitter. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cumarins; Chemiluminescence; Peroxyoxalate /H2O2; Blue fluorescer
1. Introduction Peroxyoxalate chemiluminescence (PO-CL) is well known as a powerful means of detecting various fluorophores [1 /3] and hydrogen peroxide [4,5]. This is based on the reaction of H2O2 with an activated oxalate which results in the formation of one or more energy-rich intermediates [5]. These intermediates can easily excite a large number of
* Corresponding author. Tel.: /98-831-422-3307; fax: /98831-422-8439. E-mail address: [email protected] (M. Shamsipur).
fluorophores [6 /10] through the chemically initiated electron exchange luminescence mechanism [9]. The choice of activated oxalate for PO-CL reactions has been the subject of numerous efforts [10 /14]. It has been clearly shown that there are at least two characteristics necessary in the design of useful reagents for the PO-CL. First, the existence of some electron-withdrawing groups around the central peroxyoxalate moiety to facilitate the generation of reactive intermediates responsible for the excitation energy transfer to a fluorophore compound [14], and second, sufficient solubility of the leaving group in the solvent used. Detailed discussion of the possible mechanisms of the PO-
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CL reaction are frequently reported in the literature [3,15,16]. Cumarin derivatives are widely distributed in the plant kingdom, some of them are physiologically active and many of them are of great practical interest [17]. They are widely used as laser dyes [18,19], optical brighteners [20,21] and fluorescent markers [22]. On the other hand, some of the cumarin derivatives possess antimicrobial properties [17,23] and also employed in fluorometric assay of proteolytic enzymes in biological fluids [24], in fluorescent immunoassays [25], in brain intracellular pH measurements [26] and as a powerful drug in skin diseases [27]. We have recently reported the PO-CL study [28] and quenching effect of triethylamine on the chemiluminescence of 7-amino-4-trifluoromethylcumarin [29]. In this paper we wish to report the chemiluminescence characteristics and kinetic parameters of seven different cumarin derivatives. The structures of cumarin derivatives used are shown in Fig. 1.
2. Experimental 2.1. Reagents All chemicals used were of the reagent-grade from Fluka chemical company and used as received. bis-(2,4,6-Trichlorophenyl)oxalate (TCPO) was prepared from the reaction of 2,4,6-trichlorophenol with oxalyl chloride in the presence of triethylamine, as described elsewhere [30]. Hydrogen peroxide (30%) was concentrated via freeze drying (using a model FD-1 Fyela freeze dryer) up to 60% mixed with dimethyl phthalate in a 1:1 v/v portions and shook well on an electrical shaker. After 10 h, the organic phase was separated, dried on anhydrous Na2SO4 and the H2O2 concentration was determined by a standard potassium permanganate solution. 2.2. Apparatus Chemiluminescence detection was carried out with a homemade apparatus equipped with a model BPY47 photocell (Leybold, Huerth, Germany). The apparatus was connected to a personal computer via a suitable interface (Micropars, Tehran, Iran). Experiments were carried out with magnetic stirring (500 rpm) in a light-tight flattened bottom glass cell of 15 mm diameter at room temperature. All fluorescence and chemiluminescence spectra were recorded on a Model LS-50B Perkin /Elmer instrument. 2.3. Procedures
Fig. 1. Structures of cumarin derivatives C1 /C7.
To a fluorometric cell containing 1 ml TCPO (0.01 M in ethyl acetate), 0.2 ml sodium salicylate (0.1 M in methanol) and 1 ml cumarin solution (0.01 M in methanol) was added 0.2 ml of 30% H2O2 in water and after vigorous stirring either the fluorescence or the chemiluminescence spectra were recorded. The CL intensity/time curves were obtained as follows. To a luminometer cell containing 250 ml cumarin (0.02 M in MeOH) and 500 ml TCPO (0.01 M in ethyl acetate) was introduced 100 ml H2O2 (1.5 M in 80:20 dimethyl phthalate: tert butanol containing 0.005 M sodium salicylate) and
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the CL intensity was recorded with elapse of time, while the solution was magnetically stirred at 500 rpm.
3. Results and discussion The PO-CL reactions (Scheme 1) are well known to produce light emissions of sufficient intensity for area illumination and different reading purposes [1,6,11]. The intensity, duration and color of emission of the PO-CL systems are of great importance [1]. Thus, selection of appropriate chemiluminescence fluorescers has significant consequences. Extensive research has been conducted to develop new fluorescent compounds to cover the yellow to green color range in PO-CL [1,3,31 /33]. In preliminary experiments, it was found that the addition of hydrogen peroxide to a 1:1 (v:v) methanol /ethylacetate solution containing TCPO and cumarin derivatives results in nice blue light, the intensity of which being strongly dependent on the structure of the fluorophore. It should be noted that, in the absence of sodium salicylate as a base, the emission of light was relatively long lived but the time taken to reach maximum emission and its duration was not reproducible. However, in the presence of the base, reproducible emission intensity decay curves were obtained. In Figs. 2 and 3, the CL spectra for the TCPO / H2O2 reaction in the presence of some of the cumarin derivatives used are compared with the corresponding fluorescence spectra, obtained under comparable conditions. The resulting lmax of the CL and fluorescence spectra and the maximum intensity of CL emission (Imax) are listed in Table
Scheme 1.
Fig. 2. Fluorescence ( */) and chemiluminescence (. . .) spectra of cumarins C1, C3, C4 and C5.
1. As is obvious from Fig. 2, there is a good correspondence for the CL and fluorescence spectral distributions of the fluoresceres, indicating that the singlet excited state of the fluorescent additives is formed in the reaction and is the emitting species [34,35]. The data given in Table 1 cleanly revealed that both the wavelength (lmax) and intensity at maximum CL (ICL max) are largely dependent on the structure of cumarin derivatives used. As is obvious, while the hydroxycumarin derivatives (i.e. C1 /C3) possess a lmax at about 400 nm, the cumarins containing an /NH2 group (C4 and C5) or a /COOH group (C6) as well as 8-methoxyporalen (C7) show a red shift of about 10 /55 nm to longer wavelengths. The observed shift is presumably due to the formation of a more facilitated conjugated system [31 /33], under the experimental conditions used. On the other hand, the ICL max would also decrease drastically in the order C5 /C4 /C3 /C1 /C2 /C6 /C7. It is well known that the increased conjugation in the homologue series not only results in a bathochro-
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Fig. 3. Computer fit of the CL intensity /time plot for C5: ( /) experimental point, (k) calculated point (/) experimental and calculated points are the same within the resolution of the plot. Table 1 Wavelengths of maximum intensity for the fluorescence (lFL max) and chemiluminescence (lCL max) and maximum CL intensity (ICL max) for cumarins C1 /C7 Cumarin
lFL max (nm)
lCL max (nm)
ICL max
C1 C2 C3 C4 C5 C6 C7
410.4 400.7 403.7 425.7 445.4 411.1 414.8
399.3 398.5 399.1 415.5 455.7 411.1 413.6
17.3 3.3 25.7 200.2 661.4 0.9 0.2
two aminocumarins used (i.e. C4 and C5) the compound C5 containing a /CF3 group at 7position possesses a higher CL intensity than C4 with a /CH3 group at the same position. This is because the methyl group is known to act as a source of photo-degradation, in comparison with a trifluoromethyl group [20,42]. In order to investigate the kinetics of the chemiluminescence process of TCPO /H2O2 /cumarin systems from the corresponding CL intensity versus time profiles, a previously reported simplified model was employed [36,37]: kr
mic shift to longer wavelengths, but also causes an increased light intensity [39,40]. The intensive work of Zechmeister and his coworkers in the caotenoid field has shown that the intensity is also very dependent on the stereochemical configuration [41]. It should be noted that the absence of phenolic-OH or /NH2 groups in cumarins C6 and C7, and the consequent prohibited conjugation, results in a significant diminished CL intensity. On the other hand, as it was expected [31 /33], the aminocurmarin derivative C4 and C5 show an enhanced CL intensity over the hydroxy anthraquinones used (i.e. C1 /C3). Finally among the
kf
R0X0P
(1)
where R1, X and P represent pools of reactants, intermediates and products, respectively, and both reaction steps are irreversible first-order reactions. The chemiluminescence signal is proportional to the concentration of intermediate X and the integrated rate equation for CL intensity versus time is: Mkf It [exp(kr t)exp(kf t)] (2) kf kr where It is CL intensity at time t, M is a theoretical maximum level of intensity if the
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Table 2 The CL parameters evaluated for different TCPO /H2O2 /cumarin systems Cumarin
kr (min1)
kf (min1)
M
J
Jexp
Tmax (min)
Texp (min1)
Y
T3/4
C1 C2 C3 C4 C5
36.7 8.1 31.8 35.3 29.1
1.7 3.4 1.9 0.25 0.18
37.8 30.7 22.4 47.6 447.9
33 16 19 46 434
35 18 21 52 428
0.09 0.18 0.09 0.11 0.17
0.11 0.15 0.09 0.13 0.19
22.6 9.1 11.8 188 2463
0.21 0.35 0.14 0.60 1.80
reactions were entirely converted to a CL-generating material and kr and kf are the pseudo-firstorder rate constants for the rise and fall of the burst, respectively. In addition, this pooled-intermediate model permits an estimate of intensity at the maximum CL, J, time of maximum intensity, Tmax, and the total light yield, Y , as follows: [kf =(kfkr )] k J M f (3) kr ln[kf =kr ) (4) Tmax [kf kr )
Y
g
IE dt
M kf
(5)
The kr, kf and M values for cumarins C1 /C5 were evaluated by computer fitting of the corresponding CL intensity /time profiles to Eq. (2), using a non-linear least-squares curve fitting program KINFIT [38]. It should be noted that, in the case of cumarins C6 and C7, the CL intensity was very low and, consequently, the intensity at different time intervals measured by the homemade used was subject to a large amount of error. Thus, these intensity/time data were not fitted to Eq. (2). The other parameters J, Tmax and Y were then evaluated from Eqs. (3) /(5), using the kr, kf and M values. All the resulting parameters for cumarins C1 /C5 are summarized in Table 2. The experimental values of J (Jexp) and Tmax (Texp) as well as the time needed for the CL intensity to decay to 3/4 of its maximum value (T3/4) are also included to Table 2 for comparison. As it is obvious from Table 2, there is a good agreement between the theoretical and experimental values obtained for the maximum CL intensity (J) and
the time of maximum CL intensity (Tmax) of different cumarin derivatives.
References [1] J.G. Burr, Chemi and Biolominescence, Marcell-Dekker, New York, 1985, pp. 245 /258. [2] K. Robards, P.J. Worsfold, Anal. Chim. Acta 266 (1992) 147. [3] A.G. Hadd, J.W. Birks, in: R.E. Sievers (Ed.), Selective Detectors, Chemical Analysis, vol. 131, Wiley, New York, 1995, p. 209. [4] P. Van Zoonen, D.A. Kamminga, C. Gooijer, N.H. Velthorst, R.W. Frei, G. Gu¨bitz, Anal. Chim. Acta 174 (1985) 151. [5] M. Stigbrand, A. Karlsson, K. Irgum, Anal. Chem. 68 (1996) 3945. [6] M.M. Rauhut, Acc. Chem. Res. 2 (1969) 80. [7] K. Honda, K. Miyaguchi, K. Imai, Anal. Chim. Acta 177 (1985) 111. [8] P.J.M. Kwakman, U.A.T. Brinkman, Anal. Chim. Acta 266 (1992) 175. [9] K. Katayama, H. Takeuchi, H. Taniguchi, Anal. Chim. Acta 281 (1993) 111. [10] A.R. Bowie, M.G. Sanders, P.J. Worsefold, J. Biolum. Chemilum. 11 (1996) 61. [11] M.M. Rauhut, L.J. Bollyky, B.G. Roberts, M. Loy, R.H. Whiteman, A.V. Iannotla, A.M. Semel, R.A. Clark, J. Am. Chem. Soc. 89 (1967) 6515. [12] S.-S. Tseng, A.G. Mohan, L.G. Haines, L.S. Vizcarra, M.M. Rauhut, J. Org. Chem. 44 (1979) 4413. [13] K. Nakashima, K. Maki, S. Akiyama, W.H. Wang, Y. Tsukamoto, K. Imai, Analyst 114 (1989) 1413. [14] C.V. Stevani, I.P. de Arruda Campos, W.J. Baader, J. Chem. Soc., Perkin Trans. 2 (1996) 1645. [15] H. Neuvonen, J. Chem. Soc., Perkin Trans. 2 (1994) 89. [16] G. Orosz, R.S. Givens, R.L. Schower, Crit. Rev. Anal. Chem. 26 (1996) 1. [17] P.M. Boisde, W.C. Meuly, Kirk-Othmer Encyclopedia of Chemical Technology, vol. 7, fourth ed., Wiley, New York, 1993, pp. 647 /658. [18] G.A. Reynolds, K.H. Drexhage, Optics Commun. 13 (1975) 222.
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[19] A.N. Fletcher, Appl. Phys. 14 (1977) 295. [20] H. Zollinger, Color Chemistry, VCH, New York, 1987. [21] B.M. Krasovitskii, B.M. Bolotin, Organic Luminescent Materials, VCH, New York, 1988. [22] E.R. Bissel, A.R. Mitchell, R.E. Smith, J. Org. Chem. 45 (1980) 2283. [23] J. Jurd, J. Corse, A.D. King, H. Bayne, K. Mihara, Photochem 10 (1971) 2971. [24] M. Zimmernan, Anal. Biochem. 70 (1976) 258. [25] S.G. Thomson, J.F. Burd, Antimicrob. Agents Chemother. 18 (1980) 264. [26] T.M. Syndt, R.E. Anderson, R.A. Van Dyke, J. Neurochem. 31 (1978) 627. [27] S.M. Sethna, N.M. Shah, Chem. Rev. 36 (1944) 1. [28] M. Shamsipur, M.J. Chaichi, Spect. Lett. 34 (2001) 459. [29] M. Shamsipur, M.J. Chaichi, Spectrochim. Acta 57A (2001) 2355. [30] A.G. Mohan, N.J. Turro, Chem. Educ. 51 (1974) 528. [31] P.J. Hanhela, D.B. Paul, Aust. J. Chem. 34 (1981) 1669.
[32] P.J. Hanhela, D.B. Paul, Aust. J. Chem. 34 (1981) 1687. [33] P.J. Hanhela, D.B. Paul, Aust. J. Chem. 34 (1981) 1701. [34] R.M. Hochstrasser, G.B. Porter, Quat. Rev. (London) 14 (1960) 146. [35] M.M. Rauhut, B.G. Robert, A.M. Samsel, J. Am. Chem. Soc. 88 (1966) 3604. [36] R.S. Givens, R.L. Schower, in: J.W. Birsks (Ed.), Cheminescence and Rection Detection in Chromatography, VCH, New York, 1989. [37] A.G. Hadd, A. Seeber, J.W. Birks, J. Org. Chem. 65 (2000) 2675. [38] J.L. Dye, V.A. Nicely, J. Chem. Educ. 48 (1971) 443. [39] F. Sondreimer, D.A. Ben-Efraim, R. Wolovsky, J. Am. Chem. Soc. 83 (1961) 1675. [40] H.H. Jaffe’, M. Orchin, Theory and Applications of Ultraviolet Spectroscopy, Wiley, New York, 1962. [41] L. Zechmeister, Chem. Rev. 34 (1944) 267. [42] A.N. Fletcher, Appl. Phys. 14 (1977) 295.
Chemiluminescence characteristics of cumarin derivatives as blue fluorescers in peroxyoxalate hydrogen peroxide system /
Mohammad Javad Chaichi a, Ali Reza Karami b, Abbas Shockravi a, Mojtaba Shamsipur c,* a b
Department of Chemistry, Tarbiat Moalem University, Tehran, Iran Department of Chemistry, Tarbiat Modarres University, Tehran, Iran c Department of Chemistry, Razi University, Kermanshah, Iran Received 12 December 2001; accepted 9 August 2002
Abstract The chemiluminescence characteristics of seven different cumarin derivatives were studied in detail. The fluorescence and chemiluminescence spectra were compared; all cumarins used were found to act as blue fluorescers. The intensity and kinetic parameters for the chemiluminescent systems were evaluated from computer fitting of the resulting intensity /time plots. Among different cumarin derivatives used, 7-amino-4-trifluoromethylcumarin revealed the most promising characteristics as an efficient blue fluorescent emitter. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cumarins; Chemiluminescence; Peroxyoxalate /H2O2; Blue fluorescer
1. Introduction Peroxyoxalate chemiluminescence (PO-CL) is well known as a powerful means of detecting various fluorophores [1 /3] and hydrogen peroxide [4,5]. This is based on the reaction of H2O2 with an activated oxalate which results in the formation of one or more energy-rich intermediates [5]. These intermediates can easily excite a large number of
* Corresponding author. Tel.: /98-831-422-3307; fax: /98831-422-8439. E-mail address: [email protected] (M. Shamsipur).
fluorophores [6 /10] through the chemically initiated electron exchange luminescence mechanism [9]. The choice of activated oxalate for PO-CL reactions has been the subject of numerous efforts [10 /14]. It has been clearly shown that there are at least two characteristics necessary in the design of useful reagents for the PO-CL. First, the existence of some electron-withdrawing groups around the central peroxyoxalate moiety to facilitate the generation of reactive intermediates responsible for the excitation energy transfer to a fluorophore compound [14], and second, sufficient solubility of the leaving group in the solvent used. Detailed discussion of the possible mechanisms of the PO-
1386-1425/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 2 ) 0 0 3 1 1 - 6
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CL reaction are frequently reported in the literature [3,15,16]. Cumarin derivatives are widely distributed in the plant kingdom, some of them are physiologically active and many of them are of great practical interest [17]. They are widely used as laser dyes [18,19], optical brighteners [20,21] and fluorescent markers [22]. On the other hand, some of the cumarin derivatives possess antimicrobial properties [17,23] and also employed in fluorometric assay of proteolytic enzymes in biological fluids [24], in fluorescent immunoassays [25], in brain intracellular pH measurements [26] and as a powerful drug in skin diseases [27]. We have recently reported the PO-CL study [28] and quenching effect of triethylamine on the chemiluminescence of 7-amino-4-trifluoromethylcumarin [29]. In this paper we wish to report the chemiluminescence characteristics and kinetic parameters of seven different cumarin derivatives. The structures of cumarin derivatives used are shown in Fig. 1.
2. Experimental 2.1. Reagents All chemicals used were of the reagent-grade from Fluka chemical company and used as received. bis-(2,4,6-Trichlorophenyl)oxalate (TCPO) was prepared from the reaction of 2,4,6-trichlorophenol with oxalyl chloride in the presence of triethylamine, as described elsewhere [30]. Hydrogen peroxide (30%) was concentrated via freeze drying (using a model FD-1 Fyela freeze dryer) up to 60% mixed with dimethyl phthalate in a 1:1 v/v portions and shook well on an electrical shaker. After 10 h, the organic phase was separated, dried on anhydrous Na2SO4 and the H2O2 concentration was determined by a standard potassium permanganate solution. 2.2. Apparatus Chemiluminescence detection was carried out with a homemade apparatus equipped with a model BPY47 photocell (Leybold, Huerth, Germany). The apparatus was connected to a personal computer via a suitable interface (Micropars, Tehran, Iran). Experiments were carried out with magnetic stirring (500 rpm) in a light-tight flattened bottom glass cell of 15 mm diameter at room temperature. All fluorescence and chemiluminescence spectra were recorded on a Model LS-50B Perkin /Elmer instrument. 2.3. Procedures
Fig. 1. Structures of cumarin derivatives C1 /C7.
To a fluorometric cell containing 1 ml TCPO (0.01 M in ethyl acetate), 0.2 ml sodium salicylate (0.1 M in methanol) and 1 ml cumarin solution (0.01 M in methanol) was added 0.2 ml of 30% H2O2 in water and after vigorous stirring either the fluorescence or the chemiluminescence spectra were recorded. The CL intensity/time curves were obtained as follows. To a luminometer cell containing 250 ml cumarin (0.02 M in MeOH) and 500 ml TCPO (0.01 M in ethyl acetate) was introduced 100 ml H2O2 (1.5 M in 80:20 dimethyl phthalate: tert butanol containing 0.005 M sodium salicylate) and
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the CL intensity was recorded with elapse of time, while the solution was magnetically stirred at 500 rpm.
3. Results and discussion The PO-CL reactions (Scheme 1) are well known to produce light emissions of sufficient intensity for area illumination and different reading purposes [1,6,11]. The intensity, duration and color of emission of the PO-CL systems are of great importance [1]. Thus, selection of appropriate chemiluminescence fluorescers has significant consequences. Extensive research has been conducted to develop new fluorescent compounds to cover the yellow to green color range in PO-CL [1,3,31 /33]. In preliminary experiments, it was found that the addition of hydrogen peroxide to a 1:1 (v:v) methanol /ethylacetate solution containing TCPO and cumarin derivatives results in nice blue light, the intensity of which being strongly dependent on the structure of the fluorophore. It should be noted that, in the absence of sodium salicylate as a base, the emission of light was relatively long lived but the time taken to reach maximum emission and its duration was not reproducible. However, in the presence of the base, reproducible emission intensity decay curves were obtained. In Figs. 2 and 3, the CL spectra for the TCPO / H2O2 reaction in the presence of some of the cumarin derivatives used are compared with the corresponding fluorescence spectra, obtained under comparable conditions. The resulting lmax of the CL and fluorescence spectra and the maximum intensity of CL emission (Imax) are listed in Table
Scheme 1.
Fig. 2. Fluorescence ( */) and chemiluminescence (. . .) spectra of cumarins C1, C3, C4 and C5.
1. As is obvious from Fig. 2, there is a good correspondence for the CL and fluorescence spectral distributions of the fluoresceres, indicating that the singlet excited state of the fluorescent additives is formed in the reaction and is the emitting species [34,35]. The data given in Table 1 cleanly revealed that both the wavelength (lmax) and intensity at maximum CL (ICL max) are largely dependent on the structure of cumarin derivatives used. As is obvious, while the hydroxycumarin derivatives (i.e. C1 /C3) possess a lmax at about 400 nm, the cumarins containing an /NH2 group (C4 and C5) or a /COOH group (C6) as well as 8-methoxyporalen (C7) show a red shift of about 10 /55 nm to longer wavelengths. The observed shift is presumably due to the formation of a more facilitated conjugated system [31 /33], under the experimental conditions used. On the other hand, the ICL max would also decrease drastically in the order C5 /C4 /C3 /C1 /C2 /C6 /C7. It is well known that the increased conjugation in the homologue series not only results in a bathochro-
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Fig. 3. Computer fit of the CL intensity /time plot for C5: ( /) experimental point, (k) calculated point (/) experimental and calculated points are the same within the resolution of the plot. Table 1 Wavelengths of maximum intensity for the fluorescence (lFL max) and chemiluminescence (lCL max) and maximum CL intensity (ICL max) for cumarins C1 /C7 Cumarin
lFL max (nm)
lCL max (nm)
ICL max
C1 C2 C3 C4 C5 C6 C7
410.4 400.7 403.7 425.7 445.4 411.1 414.8
399.3 398.5 399.1 415.5 455.7 411.1 413.6
17.3 3.3 25.7 200.2 661.4 0.9 0.2
two aminocumarins used (i.e. C4 and C5) the compound C5 containing a /CF3 group at 7position possesses a higher CL intensity than C4 with a /CH3 group at the same position. This is because the methyl group is known to act as a source of photo-degradation, in comparison with a trifluoromethyl group [20,42]. In order to investigate the kinetics of the chemiluminescence process of TCPO /H2O2 /cumarin systems from the corresponding CL intensity versus time profiles, a previously reported simplified model was employed [36,37]: kr
mic shift to longer wavelengths, but also causes an increased light intensity [39,40]. The intensive work of Zechmeister and his coworkers in the caotenoid field has shown that the intensity is also very dependent on the stereochemical configuration [41]. It should be noted that the absence of phenolic-OH or /NH2 groups in cumarins C6 and C7, and the consequent prohibited conjugation, results in a significant diminished CL intensity. On the other hand, as it was expected [31 /33], the aminocurmarin derivative C4 and C5 show an enhanced CL intensity over the hydroxy anthraquinones used (i.e. C1 /C3). Finally among the
kf
R0X0P
(1)
where R1, X and P represent pools of reactants, intermediates and products, respectively, and both reaction steps are irreversible first-order reactions. The chemiluminescence signal is proportional to the concentration of intermediate X and the integrated rate equation for CL intensity versus time is: Mkf It [exp(kr t)exp(kf t)] (2) kf kr where It is CL intensity at time t, M is a theoretical maximum level of intensity if the
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Table 2 The CL parameters evaluated for different TCPO /H2O2 /cumarin systems Cumarin
kr (min1)
kf (min1)
M
J
Jexp
Tmax (min)
Texp (min1)
Y
T3/4
C1 C2 C3 C4 C5
36.7 8.1 31.8 35.3 29.1
1.7 3.4 1.9 0.25 0.18
37.8 30.7 22.4 47.6 447.9
33 16 19 46 434
35 18 21 52 428
0.09 0.18 0.09 0.11 0.17
0.11 0.15 0.09 0.13 0.19
22.6 9.1 11.8 188 2463
0.21 0.35 0.14 0.60 1.80
reactions were entirely converted to a CL-generating material and kr and kf are the pseudo-firstorder rate constants for the rise and fall of the burst, respectively. In addition, this pooled-intermediate model permits an estimate of intensity at the maximum CL, J, time of maximum intensity, Tmax, and the total light yield, Y , as follows: [kf =(kfkr )] k J M f (3) kr ln[kf =kr ) (4) Tmax [kf kr )
Y
g
IE dt
M kf
(5)
The kr, kf and M values for cumarins C1 /C5 were evaluated by computer fitting of the corresponding CL intensity /time profiles to Eq. (2), using a non-linear least-squares curve fitting program KINFIT [38]. It should be noted that, in the case of cumarins C6 and C7, the CL intensity was very low and, consequently, the intensity at different time intervals measured by the homemade used was subject to a large amount of error. Thus, these intensity/time data were not fitted to Eq. (2). The other parameters J, Tmax and Y were then evaluated from Eqs. (3) /(5), using the kr, kf and M values. All the resulting parameters for cumarins C1 /C5 are summarized in Table 2. The experimental values of J (Jexp) and Tmax (Texp) as well as the time needed for the CL intensity to decay to 3/4 of its maximum value (T3/4) are also included to Table 2 for comparison. As it is obvious from Table 2, there is a good agreement between the theoretical and experimental values obtained for the maximum CL intensity (J) and
the time of maximum CL intensity (Tmax) of different cumarin derivatives.
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