Luminescence of N-arylbenzamides in low-temperature glasses

The low-temperature luminescence of benzamide and six additional N-arylbenzamides has been investigated in a methyltetrahydrofuran glass. The luminescence of microcrystalline benzamide has also been studied. Assignments of the fluorescence and phosphorescence have been made on the basis of comparisons with the spectra of benzamide and the six aminoarenes and with the results of semiempirical ZINDO calculations for the amines and amides. This information was used to construct state-energy diagrams for the six amides. The 430 nm fluorescence of benzanilide and the amides derived from 4-aminobiphenyl and 4-aminodiphenyl-acetylene is assigned to a n,pi* state having a relaxed geometry and localized on the benzoyl group. The structured fluorescence of the amides derived from 4-aminostilbene and 2-aminoanthracene is attributed to a delocalized pi,pi* singlet slate, The amide derived from 2-aminophenanthrene displays dual fluorescence. The amides derived from 4-aminostilbene and 2-aminoanthracene are nonphosphorescent. The structured phosphorescence of the amides derived from the other aminoarenes is assigned to a pi,pi* triplet state similar to that for the aminoarene. These results serve to further elucidate the unusual luminescence properties of the benzamides in solution as well as in low-temperature glasses.