On the Correlations Between Constitution and Scintillation Properties in the p-Oligophenylene Series

Abstract

The paper deals with the problem: What properties are requested from an organic scintillator solute and what structural principles are given to realize, or at least to influence them especially by looking at the p-oligophenylene series? According to their importance and also in respect to more or less special purposes one can distinguish the following sequence: 1) An organic scintillator solute must have a sufficient solubility in all types of solvents used for this counting method. As a result of extensive systematic studies substitution of larger alkyl groups has been recognized as a very effective principle of solubilization. By use of branched, particularly long-chain branched groups, even extremely insoluble substances can be solubilized to any extent in homopolar solvents. While substitution of alkyl groups is only effective for nonpolar solvents, the use of oxa-alkyl groups (derived from ethylene oxide) affords solubility also in polar solvents. It seems that with these two types of substituents almost all solubility problems in low molecular organic chemistry can be solved. 2) In any case a high light output is of greatest importance. In the p-coligo-phenylene series the scintillation efficiency increases with the degree of condensation (number of rings). Thus, with the higher homologs light yields could be obtained which were unknown up until now. Substituents disturbing the coplanarity cause a decrease in light output. 3) The fluorescence wavelengths should be in a favorable range in order to match the response of the commonly used phototubes. In the p-oligophenylene series with increasing degree of condensations the absorption and therefore also the fluorescence bands are shifted into longer wavelengths. Because of the convergency phenomena in connection with the lower accessibility of the higher homologs it is difficult to reach the range of 400 mp. By introducing other chromophoric systems, such as pyrene or fluoranthene, very long fluorescence wavelengths can be established. 4) For many measurements, especially in high energy physics, a short decay time is of greatest interest. For the p-oligophenylene series the decay time decreases as the degree of condensation increases. This can be easily understood, according to Forster, since in the same direction the molar extinction coefficient increases. 5) A large mean free path between absorption and fluorescence band (Stokes shift) guarantees a high optical transparency. In the p-oligophenylene series there is almost no overlap in these two bands. Substitution of alkyl groups seems to be a generalizable provision to further enhance the Stokes shift. 6) Organic scintillator solutes should have a low sensitivity to all types of quenching processes. So far, it is only known how to eliminate the self quenching. The solubilization groups, namely, affording on the one hand the necessary solubility seduce, or even completely exclude, the self quenching on the other hand.