Evaluation and optimization of factors affecting light capacity of a high-intensity peroxyoxalate chemiluminescence system

Spectrochimica Acta Part A 54 (1998) 1067 – 1072

Evaluation and optimization of factors affecting light capacity of a high-intensity peroxyoxalate chemiluminescence system Zhengliang Zhi, Xujie Yang, Lude Lu, Xin Wang * Materials Chemistry Laboratory, Nanjing Uni6ersity of Science and Technology, Nanjing 210094, People’s Republic of China Received 29 September 1997; received in revised form 6 January 1998; accepted 13 January 1998

Abstract A high-light capacity chemiluminescence system produced by the oxidation of bis(2-butoxycarbonyl-3,4,6trichlorophenyl)oxalate by hydrogen peroxide in a suitable solvent in the presence of a fluorescent compound 9,10-diphenylanthracene was studied. The effects of stoichimetry and other variables affecting the chemiluminescent reaction and the amount of light output with respect to the light intensity and time of illumination were investigated and discussed. An optimum component formulation for high-light capacity illumination was present. Under these conditions, the chemiluminescence system allowed efficient illumination for ca. 6 h. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Peroxyoxalate chemiluminescence; Bis(2-butoxycarbonyl-3,4,6-trichlorophenyl)oxalate; 9,10-Diphenylanthracene

1. Introduction The chemiluminescence system using the reaction of an diaryl oxalate ester and a peroxide in a dilutent in the presence of a fluorescer is known to be capable of producing light emission of sufficient intensity and efficiency, which may be manipulated to meet the requirement for various practical illuminating purposes [1 – 5]. Many possible applications of the light system, however, depend on the availability of efficient fluorescent compounds and diaryl oxalate esters. Extensive research conducted by Paul [1–6], Rauhut [7 – 13] et al. have led to * Corresponding author.

enormous progress in this research field. Mechanisms of the reaction have also been studied extensively as reviewed recently by Orosz et al. [14] While the efficiency of the chemiluminescence is primarily dependent on the reactivity of the oxalates towards hydrogen peroxide, and the fluorescent quantum yield and chemical stability of the fluorescer, major improvement to light emission is also possible through the optimization in the stoichiometric and other related variables of the reaction system. In this study, a peroxyoxalate chemiluminescence system using the laboratory-synthesized bis(2-butoxycarbonyl-3,4,6-trichlorophenyl)oxalate (BBTPO) and an efficient purple emitter 9,10-diphenylanthracene was adopted as a

1386-1425/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S1386-1425(98)00021-3

1068

Z. Zhi et al. / Spectrochimica Acta Part A 54 (1998) 1067–1072

model system. The former was selected as the energy source because of its capability of producing high intensity light, the later was chosen as the fluorescent compound because of its stability under the reaction conditions and its excellent chemiluminescence efficiency. A number of variables affecting the light capacity were studied in a search for improved emission intensities and extended lifetime, and an optimum component formulation for high-light capacity was accordingly proposed.

2. Experimental

2.1. Materials and instrumentation Proton magnetic resonance spectra were recorded at 80 MHz using a Varian FI-80A spectrometer in DMSO-d6 with tetramethysilane (TMS) as internal reference. Infrared spectra were recorded with an IR 7400 instrument (Shanghai) with KBr pellets. A Waters 510 HPLC instrument furnished with a 3.0 mm ×15.0 cm Novo-Pak C18 column was used to assay the purity of the oxalate products. Light intensity measurements were performed with a 930-fluorometer (Shanghai) without external excitation and operated at an emission wavelength of 410 nm. The sensitivity of the instrument was set at minimum to reduce the intensity to a measurable level. Relative intensity readings were obtained in arbitrary units. 9,10-Diphenylanthracene was synthesized according to the published produces [1]. The product which was recrystallized from toluene as a yellow solid was obtained in a yield 80%. m.p. 245 – 246°C, (lit. 245 – 248°C). (Found: C, 91.80; H, 8.20, C26H28 requires: C, 91.77; H: 8.23). nmax (KBr)/cm − 1: 1388, 1444, 1505, 1610, 3100. 1H NMR (DMSO-d6) d/ppm: 7.2 – 7.5 (10H, m, Ar); 7.5 – 7.8 (8H, m, anthracene). Fluorescence lem = 417 nm, lex = 436 nm; chemiluminescence lmax = 437 nm. Bis(2-butoxycarbonyl-3,4,6-trichlorophenyl)oxalate was synthesized by the established procedures [2] using 3,5,6-trichlorosalicylic acid as the raw material, which was supplied by a local chemical plant in China. The crude product ob-

tained was recrystallized from hexane/acetone with charcoal, giving a ca. 65% yield. Physical properties are listed below. m.p. 119–122°C (lit. 120–123°C), (Found: C, 44.00; H, 3.10; Cl, 32.95. C24H20Cl6O8 requires C, 43.86; H, 3.11; Cl, 33.13). nmax (KBr)/cm − 1: 1735, 1783 (lit. 1735, 1780), 2900–3100, 1440, 1105, 870, 600–700. 1H NMR (DMSO-d6) d/ppm: 0.90 (6H, t, CH3), 1.36– 1.95, [8H, m, (CH2)2], 4.30–4.50 (4H, s, OCH2), 7.63 (2H, m, Ar). Above-listed data confirmed the expected structures. The purity of the product was monitored by HPLC to be over 98%. Concentrated H2O2 was prepared from a commercial reagent (30%) by placing an appropriate volume of the 30% H2O2 in a desiccator over concentrated sulfuric acid under vacuum for 3–5 days, the concentration of the product was standardized by titration of a diluted solution with 0.006 mol l − 1 KMnO4 [15]. Other chemicals used in this study were of commercially available products and purchased from a local chemical reagent company. All these chemicals were of reagent grade and used as supplied without any further purification.

2.2. Chemiluminescence measurements The light capacity (L) of the chemiluminescence system is related to the intensity and lifetime by L=

&

T= 8

I dT/V

T=0

where I is the intensity, T the time, V the volume of the reaction solution. It is clear from this equation that light capacity can be evaluated in terms of I and T. All chemiluminescent intensity-time decay curves were determined at 25°C (298 K). Chemiluminescence intensity data measured in this study were not calibrated to the absolute units. Thus, the data obtained in this study cannot be compared to those reported by other laboratories, nevertheless, it may well provide a parameter for evaluation and comparison of the effect of various variables, and therefore optimize the reaction system.

Z. Zhi et al. / Spectrochimica Acta Part A 54 (1998) 1067–1072

1069

Table 1 Variables affecting the performance of the peroxyoxalate system Type of variable

Variable

Range studied

Adopted value

Stoichiometric

BBTPO concentration Fluorescent concentration H2O2 concentration Solvent for H2O2 Solvent for oxalate Catalyst Inhibitor Other additive

0.033–0.084 g ml−1

0.05 g ml−1

0.2–0.8 mg ml−1

0.5 mg ml−1

0.5–9%

4.5%

Dimethyl phthalate, tertiary butanol

80% Dimethyl phthalate and 20% tertiary butanol Dibutyl phthalate

Other variable

Dimethyl phthalate, dibutyl phthalate, di(2-ethylhexyl)-phthalate Potassium salicylate Oxalic acid PS

Quantitative chemiluminescence experiments were carried out in a 3-cm deep, 3.5 ml tip-fitted quartz cuvette, positioned vertically at the entrance of a fluorometer. For practical and convenience reason, a 2-component system in which an appropriate mass of the oxalate and fluorescer in a solvent (dibutyl phthalate) were combined as an ‘oxalate’ component; the H2O2, catalyst, and other additives in a solvent (80% dimethyl phthalate + 20% tertiary butanol) as a ‘peroxide’ component was adopted. The chemiluminescent reaction was effected by combining a 2-ml aliquot of oxalate component and 1-ml aliquot of the peroxide component, and shaking the tip-stopped cuvette to provide rapid mixing. The variation of intensity with time was measured at 5 min intervals during the next 6 h.

3. Results and discussion The effects of stoichiometric and other variables such as solvent, catalyst, inhibitor, etc. on the light emission with respect to the intensity and the time of effect illumination were examined in a search for satisfactory intensity and duration of light emission. First, the stoichiometric variables were optimized under the condition of no additive (catalyst, inhibitor) addition. Moreover, as various concentrations of catalyst and inhibitor affect the light emission intensity and illuminating duration, it was

None 1 – 2 mg ml−1 0.3%

then necessary to check these variables before the final formulation could be obtained. The study of variables was performed by the univariate method and the relative intensity obtained were used as the evaluating parameter. Preliminary experiments showed that under the sensitivity of the fluorometer set in the present study, the most suitable intensity range for practical illumination purposes was between 100 and up to 3000 units. Intensities less than 10 were considered to be very weak and unsuitable for practical uses. Table 1 lists those variables having a marked influence on the performance of the lighting system with the values finally adopted.

3.1. Effect of BBTPO concentration on the light emission In Fig. 1, the chemiluminescence decay curves with time are shown for BBTPO concentrations from 0.033 to 0.084 g ml − 1 in dibutyl phthalate in the presence of 0.5 mg/ml fluorescent and 4.5% of hydrogen peroxide. The result shows that the light intensity maxima of the system increased with the increase of BBTPO concentration until 0.05 g ml − 1. A more concentrated solution yet resulted in the higher intensity maxima, but increased decay rate considerably, possibly because the phenolic reaction products acted as chemiluminescence inhibitor and retarded the reaction rate. Thus, the optimum BBTPO concentration was found to be 0.05 g ml − 1.

1070

Z. Zhi et al. / Spectrochimica Acta Part A 54 (1998) 1067–1072

Fig. 1. Effect of solvent on the emission decay curve. (A) dimethyl phthalate, (B) dibutyl phthalate, (C) di(2-methylhexyl) phthalate, (D) 50% dibutyl phthalate+50% di(2-methylhexyl) phthalate.

3.2. Effect of fluorescent compound concentration on the light emission The influence of the concentration of fluorescent was studied by changing the variable from 0.2 to 0.8 mg ml − 1 while the other variables remained constant. The result shows that the light intensity maxima increase by ca. 30% on increasing the fluorescent concentration from 0.2 to 0.5 mg ml − 1, higher concentrations, however, resulted in virtually constant or even slightly lower light intensity, possibly because of the effect of concentration quenching. A 0.5 mg ml − 1 of the fluorescent was chosen as optimal as a result.

3.3. Selection of the sol6ent for the ‘oxalate’ component A suitable solvent for the peroxyoxalate system not only requires to provide reasonably high solubility for the chemiluminescence system components (oxalate, fluorescent, concentrated peroxide, etc.), but also should have as high a boiling point as possible in order to reduce the inflammability. Moreover, it should provide high quantum yields at high oxalate concentrations for the chemiluminescent reaction. Thus, for the oxalate compo-

nent, phthalate diester solvents, i.e. dimethyl phthalate, dibutyl phthalate, and di(2-ethylhexyl)phthalate were tested. The variations in light emission from the use of the three solvents are compiled in Fig. 2, as can be seen, dibutylphthalate was capable of providing the highest light capacity. Hence, for following examination, only dibutyl phthalate was used as the ‘oxalate’ component solvent.

3.4. Effect of H2O2 concentration on the light emission Primarily experiments show that a solvent containing 80% dimethyl phthalate and 20% tertiary butanol is necessary for the concentrated H2O2 (90%) to be completely solved. Increase of the ratio of tertiary butanol in the mixed solvent was found to increase the maximum intensity, but reduced illumination duration considerably, thus it was not recommended. Further studies were then conducted to investigate the chemiluminescence decay curves with time for H2O2 concentration from 0.5 to 9%. It was found that the relative intensity increased as the concentration of H2O2 decreased, but the decay rate also increased with time, a concentration of 4.5% proved to be the

Z. Zhi et al. / Spectrochimica Acta Part A 54 (1998) 1067–1072

1071

Fig. 2. Effect of the inhibitor on the emission decay curve. (A) blank, (B) oxalic acid 1.2 mg ml − 1, (C) oxalic acid 2.4 mg ml − 1.

most suitable as a compromise between a high intensity and a long duration of illumination.

3.5. Effect of the catalyst and inhibitor The effect of potassium salicylate (catalyst) on chemiluminescence intensity are minor when used in concentration of 0.2 mg ml − 1 or less. A higher concentration of the catalyst may produce a dramatic increase in the maximum light intensity, but a considerable reduction in the efficient illuminating time, thus prohibiting its use in practical light systems. As a result, the addition of the catalyst was not further considered. On the other hand, as shown in Fig. 2, the addition of oxalic acid at concentration of 1.2 mg ml − 1 or higher significantly inhibited the chemiluminescent reaction rate. Consequently, in order to control the emission within the specified intensity for various practical illuminating purposes, the addition of a suitable amount (e.g. 1 mg ml − 1) of oxalic acid is recommended.

3.6. Further impro6ement of the chemiluminescent light capacity The effect of the other additives such as polymer on chemiluminescence intensity was further

tested. It was found that, by adding ca. 0.3% (added as a benzene solution) of polystyrene (PS), an increase of ca. 30% in the maximun chemiluminescence intensity was obtained while the variation of the life-time of illumination remains minor. A higher concentration of PS, however, had a negative effect on the light intensity. The former was believed to be the result of the modification of matrix which benefited an improvement of the yields of the chemiluminescent intermediate; the later, however, was believed to be caused by concentration quenching of the additive.

3.7. Figures of merit of the proposed chemiluminescence system According to the results above, the optimum component formulation was obtained and summarized in Table 1. Under these circumstances, the system can emit purple light of satisfactory intensity (intensity \ 10 units) for ca. 6 h. We have started investigations into the further improvement of the light capacity of the proposed peroxyoxalate chemiluminescence system, concentrating on other kinds of intensity-modifying additives such as polymers, surfactants, etc. The preliminary data of the experiment have shown the considerable improvement of the light emit-

Z. Zhi et al. / Spectrochimica Acta Part A 54 (1998) 1067–1072

1072

ting intensity and lifetime, the induction time of lighting was also shortened greatly, details of which will be reported later.

References [1] [2] [3] [4] [5] [6]

P.J. Hanhela, D.B. Paul, Aust. J. Chem. 34 (1981) 1687. P.J. Powd, D.B. Paul, Aust. J. Chem. 37 (1984) 73. P.J. Powd, D.B. Paul, Aust. J. Chem. 34 (1981) 1669. P.J. Powd, D.B. Paul, Aust. J. Chem. 37 (1984) 553. P.J. Hanhela, D.B. Paul, Aust. J. Chem. 34 (1981) 1701. D. Dowd, D.B. Paul, Report of the Material Research Laboratory, Australia, 1979, MRL-R-736, 16 pp.; CA 91:140490p.

.

[7] M.M. Rauhut, N.J. Bridgewoder, US Patent, 3740679 (1973). [8] Y. Bollyky, J. Laszlo, M.M. Rauhut, Ger. Offen. 2016582 (1970). [9] R.J. Manfre, A.G. Mohan, M.M. Rauhut, US Patent, 4308395 (1981), [10] M.M. Rauhut, L.J. Bollyky, B.G. Roberts, M. Loy, R.H. Whitman, A.V. Iannota, A.M. Semsel, R.A. Clarke, J. Am. Chem. Soc. 89 (1967) 6516. [11] M.M. Rauhut, A.M. Senkel, US Patent, 3974086 (1976). [12] D.R. Maulding, M.M. Rauhut, US Patent, 3994820 (1976). [13] D.R. Maulding, M.M. Rauhut, S. African Patent, 7302681 (1974). [14] G. Orosz, R.S. Givens, R.L. Schowen, Crit. Rev. Anal. Chem. 26 (1996) 1. [15] Z. L. Zhi, Postdoctoral research report, Nanjing University of Science and Technology, Nanjing, China,1997.