Liquid Scintillators. The Stability of Organic Scintillator Solutes Toward Ultra-Violet Radiation. The Photooxidation Of 2, 5-Diphenyloxazole.

Author

Harold A. Mackay

Publication Date

6-3-1966

Abstract

An insight has been gained concerning the stability of organic scintillator solutes in toluene on exposure to ultraviolet light while in the presence of oxygen and water. Oxygen and water, from either solute or solvent, not only function as quenchers but may continue to react with the solute molecule via a photooxidation process. Removal of these materials from the 2, 5-diphenyloxazole toluene system resulted in a marked improvement in stability of the system on exposure to ultraviolet light. The choice of scintillator solute, in addition to water and oxygen, and the time of ultraviolet irradiation, has a pronounced effect on ultraviolet stability. In general, the larger molecules, of similar structure, with increased linear conjugation are more stable to ultraviolet irradiation than are the smaller molecules. For example, 2, 2'-p-phenylenebis­(5-phenyloxazole) (POPOP) is of greater stability than 2, 5-diphenyloxazole (PPO). The presence of a methyl group on the molecule, although enhancing solubility, reduces the stability of the solute thus 4-methyl-2, 5-diphenyloxazole (MPPO) is less stable than 2, 5-diphenyloxazole (PPO). Of all the primary solutes evaluated, p-terphenyl has the greatest stability to ultraviolet light by exhibiting essentially no reduction in pulse height after 1000 minutes of irradiation. PBD, 2-(4-biphenylyl)-5-phenyl-1, 3, 4-oxadiazole, is the most stable of the hetero molecules and approaches the stability of p-terphenyl. With respect to the secondary solutes employed as wavelength shifters, 2, 5-bis[5’-t-butylbenzoxazolyl (2 ')] -thiophene (BBOT) exhibits greater stability to ultraviolet light than 2, 2 '-p-phenyl­enebis(5-phenyloxazole) (POPOP). The trans-aryl ethylenes, some of which display good light output, are not as stable as their counterparts where the double bond is contained in a fused ring. For example, 3, 4-dihydro-2 -(4-biphenylyl)-6-phenylnaphthalene is of greater stability to ultraviolet light than trans-1, 2-di(4-biphenylyl)ethylene. The substituted naphthalenes and anthracenes, because of certain properties of their rings, exhibit less stability than p-terphenyl on ultraviolet irradiation. The reduction in pulse height upon exposure to ultraviolet light of a 2, 5-diphenyloxazole toluene solution, containing dissolved oxygen and water, is primarily due to photooxidation of the solute and the photooxidation products that result, which in turn act as quenchers. After prolonged periods of ultraviolet irradiation, a colorless crystalline product was formed above the meniscus of the 2, 5-diphenyloxazole toluene solution. This compound, m. p. 142-144° with decomposition, has an empirical formula of C₂₂H₁₇O₅N and was shown to be a tertiary amide tentatively identified as N-(phenylglyoxyl)dibenzamide hydrate. The formation of this compound is explained by 2, 5-diphenyloxazole forming a peroxide at the 2, 5 -ring position, which results in the opening of the oxazole ring followed by a rearrangement to a tertiary amide, C₁₅H₁₁O₃N. This tertiary amide reacts with a 4, 5-ring peroxidation product of 2, 5-diphenyl­oxazole via acylation affording the C₂₂H₁₇O₅N product, N-(phenylglyoxyl)­dibenzamide hydrate. Additional work remains to be done. The structure of the tertiaryamide C₂₂H₇O₅N requires positive confirmation, as well as the processes by which it is formed. This would appear to be a natural application for the tagged atom approach. Solute concentration effects on ultraviolet stability remain to be investigated particularly in the region of self quenching. Lastly, additional secondary solutes should be evaluated, since far too little is known concerning their stability to ultraviolet irradiation.

Project Sponsors

The Division of Biology and Medicine of the U.S. Atomic Energy Commision

Language

English

Document Type

Thesis

Degree Name

Chemistry

Level of Degree

Masters

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Guido Herman Daub

Second Committee Member

F. Newton Hayes

Third Committee Member

Milton Kahn

Fourth Committee Member

Raymond N. Castle

Recommended Citation

Mackay, Harold A.. "Liquid Scintillators. The Stability of Organic Scintillator Solutes Toward Ultra-Violet Radiation. The Photooxidation Of 2, 5-Diphenyloxazole.." (1966). https://digitalrepository.unm.edu/chem_etds/203