A comparative study of the solid-matrix phosphorescence of heterocyclic aromatic amines in glucose glasses as a function of temperature

Solid-matrix phosphorescence intensities and lifetimes were obtained for 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 2-amino-9H-pyrido[2,3-b]indole (AαC) as a function of temperature from room temperature to 93 K. The two solid matrices employed were glucose glasses prepared with crystalline glucose and a glucose melt, both with and without a heavy-atom salt, NaI. In general, the phosphorescence intensities and lifetimes of PhIP increased as the temperature was lowered. The intensity-to-lifetime ratios showed that the triplet state formation efficiency of PhIP changed as a function of temperature for glasses prepared without NaI. This illustrated that the phosphorescence intensity was a function of both triplet state formation efficiency and phosphorescence lifetime. For the glasses prepared with NaI, the triplet state formation efficiency of PhIP remained essentially constant with temperature. Thus, the phosphorescence intensity was only a function of phosphorescence lifetime. The phosphorescence intensity for AαC was essentially the same at room temperature and at 93 K in glasses prepared with 10% NaI. Thus, this heavy-atom salt/glucose system was able to achieve the maximum phosphorescence signal at room temperature. Using the phosphorescence lifetimes as a function of temperature, activation energies for the phosphors in the glasses were calculated. The phosphorescence lifetime data fit a biexponential equation and two activation energies were obtained. The lower activation energy was related to low-frequency vibrations in the glucose matrix, and the higher activation energy was related to β-relaxation processes that occur in glucose glasses.