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Rainbow Animation |
DNPO |
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Rainbow Animation |
DNPO |
The chemiluminescence produced by oxidation of diaryl ethanedioates [bis(aryl)oxalates], e.g., bis(2,4-dinitrophenyl)ethanedioate, in the presence of fluorescent dyes is far more impressive than that produced from luminol [1-3]. Peroxyoxalate chemiluminescence forms the basis of Cyalume "light sticks" (reference can be made to the previous experiment) used for emergency and underwater lighting, fishing lures and various novelty items such as glow necklaces. Since 1980 this reaction has become widely used as an analytical tool for trace analysis [4,5] and is currently the most sensitive and versatile chemiluminescence detection method for liquid chromatography [6]. The overall reaction may be represented by
ArO–C–C–OAr Fluorophor
|| || + H2O2 --> 2
ArOH + 2 CO2 + hv
O
O
(catalyst)
where Ar is an electronegative aryl group such as 2,4-dinitrophenyl or 2,4,6-trichlorophenyl.
Whereas most chemiluminescence reactions involve emission from a reaction intermediate derived from one of the reagents, the peroxyoxalate reaction transfers energy to a variety of fluorescent molecules, which in turn emit light during relaxation from the first singlet excited state. The general reaction scheme can be represented by:
oxalate ester + H2O2 --> intermediate (I) + products
I + fluorophor (F) --> F* + products
F* --> F + hv
The first substitution of H2O2 for a phenol (ArOH) in the oxalate ester is rate limiting, and as a result the chemiluminescence duration is indicative of the rate of reaction (26.2).
The reaction of an oxalate ester with hydrogen peroxide produces at least one, but possibly two or more, highly energetic intermediates capable of generatin the excited singlet state of fluorescent molecules. Radiative decay of the singlet-excited fluorescent molecule produces the observed light emission, and a variety of fluorophors may be used to produce a range of colours.
Preparation. Convenient esters are bis(2,4-dinitrophenyl)oxalate (DNPO) and bis(2,4,6-trichlorophenyl)oxalate (TCPO). Preparation [7,8]: dry a solution of 9.2 g of 2,4-dinitrophenol (or 9.94 g of 2,4,6-trichlorophenol) in 125 ml of benzene (or toluene) by azeotropic distillation of the water with 25 ml of the solvent. Cool the solution to 10 oC and add dropwise with stirring 5.05 g triethylamine followed by 3.5 g ethanedioyl dichloride (oxalyl chloride). Allow the mixture to warm up to ambient temperature and keep it at this temperature overnight. Distil off the solvent under reduced pressure (rotary evaporator if available). To the residue add 25 ml trichloromethane (chloroform), shake, filter off the solid product in a Buchner funnel, wash it with a little trichloromethane on the filter pad. Recrystallization of the product is no normally required but if desired DNPO can be recrystallised from nitrobenzene or ethyl acetate and TCPO - from benzene. Both, DNPO and TCPO can now be obtained commercially (e.g., from Aldrich, cat. Nos. 32,537-6 and 33,805-2, respectively) but they era relatively expensive to buy.
Demonstration. The experiment is better performed in a darkened room. Chemiluminescence is normally demonstrated in dimethyl phthalate - t-butanol system, but we are using a slightly modified procedure [6]. Stock solution of 150 mg of DNPO (or TCPO) is prepared in 90 ml of an ethyl acetate (80%) - acetonitrile (20%) solution. 10-15 ml portions of this solution are added into test tubes to which added a few mg of sensitizers. Reaction with TCPO also requires addition of ca. 0.2 g of sodium salicilate in each tube. Chemiluminescence is initiated by adding ca. 1-2 ml of 1% solution of hydrogen peroxide in acetonitrile (methyl cyanide), this solution is prepared by adding 1.5 ml of 30% H2O2 to a 50 ml volumetric flask and diluting to volume with acetonitrile. The following sensitizers can be used:
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Red light |
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Orange light |
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Yellow light |
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Green light |
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Blue light |
Amongst the many colours produced, the yellow emission from rubrene is the most easily observed for larger audiences or in rooms that are not completely dark.
Safety. The organic solvents are flammable, and care should be taken in their use and disposal. In particular, they should be stored in well-ventilated cabinets or hoods separate from acids and oxidising agents and isolated from sparks and flames. Organic solvents containing hydrogen peroxide should not be evaporated to dryness, as explosive organic peroxides may form. The fluorophors used in this demonstration are suspected carcinogens. The 30% hydrogen peroxide is caustic, and gloves must be worn when preparing the 1% solution.
References.
1. A.G. Mohan and N.J. Turro, “A facile and effective chemiluminescence demonstration”, J. Chem. Educ., 1974, 51, 528.
2. B.Z. Shakhashiri, L.G. Williams, G.E. Dirreen and A. Francis, “A cool-light chemiluminescence”, J. Chem. Educ., 1981, 58, 70.
3. D. Potrawa and A. Schleip, “Die Chemilumineszenz von Oxalestern - “Lightsticks”, MNU Mathematische und Naturwissenshaftliche Unterricht, 1983, 36, 284; Chem. Abstr., 1983, 99, 193921.
4. A.G. Hadd and J.W. Birks, in Selective Detectors: Environmental, Industrial, and Biomedical Applications, ed. R.E. Sievers, Wiley, New York, 1995, pp. 209-239.
5. P.J.M. Kwakman and U.A.T. Brinkman, Anal. Chim. Acta., 1992, 266, 175.
6. A.G.Hadd, D.W.Lehmpuhl, L.R.Kuck, J.W.Birks and G.P.Mell, “Chemiluminescence demonstration illustrating principles of ester hydrolysis reactions”, J. Chem. Educ., 1999, 76, 1237.
7. B.Iddon, The Magic of Chemistry, Poole, BDH, 1985 p. 37.
8. Tested Demonstrations in Chemistry, ed. L.Gilbert, et al., Denison University, Granville, OH, 1994, vol. 1, p. H-42.
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