TRIPLET-TRIPLET ABSORPTION STUDIES ON AROMATIC AND HETEROCYCLIC MOLECULES AT $77^{\circ} K$.

Abstract

Triplet-triplet absorption spectroscopy has been extended both to numerous new molecules of current spectroscopic interest and to lower energy spectral regions (to $10,000 cm^{-1}$) of previously investigated molecules. Criteria for triplet-triplet absorption have been examined and listed and molecular mechanisms for increasing the probability of triplet-triplet absorption have been delineated. Measurements have been carried out by the method of steady cross-illumination using a modified Cary 14 spectrophotometer. Eight new T-T transitions have been found in the present research. New lower energy bands have been studied in the previously studied molecules triphenylene, phenanthrene and phenanthrene$-D_{10}$; in the latter two molecules, an evaluation was made of the absorption coefficient ($\epsilon = 675$), which has established the forbidden character of the observed bands. A tentative assignment as $^{3}L_{h} \leftarrow ^{4}L_{a}$ of the new triplet-triplet transition at $12,075 cm^{-1}$ in phenanthrene has been made from theoretical considerations. Some absorptions found previously in the literature for phenanthrene and fluorene have been questioned. Triplet-triplet absorption has been verified in diphenylamine by lifetime measurements, thus resolving an earlier uncertainty, and a similar band has been discovered in triphenylamine. Because of the relatively small attention that had been given previously to triplet-triplet absorption in heterocyclic molecules, many such molecules have been investigated and new transitions have been discovered in several, including quinoxaline, phenoxazine, phenothiazine, and carbazole. The related photochemistry of phenoxazine and phenothazine has also been studied briefly.