The molecular structure and photophysical behavior of several secondary and tertiary N-(aminoalkyl)-phenanthrenecarboxamides have been investigated. Secondary (aminoalkyl)amides exist predominantly in the Z conformation, whereas tertiary amides exist as mixtures of Z and E conformers and semirigid piperazines as mixtures of chair conformers. Rate constants for endergonic intramolecular electron transfer are found to be highly dependent upon molecular structure. The aromatic and amide groups of the tertiary amides are essentially orthogonal, and thus, an E aminoalkyl group can adopt low-energy conformations in which there is spatial overlap between the aromatic and amine groups, whereas such overlap is not possible for either a Z aminoalkyl group or the piperazines. The observation of more rapid intramolecular electron transfer quenching of the phenanthrene singlet by an appended trialkylamine in the E vs Z conformation is attributed to this difference in overlap. An increase in the phenanthrene-amide dihedral angle is also found to result in a decrease in the rate constant for intramolecular electron transfer quenching by a Z aminoalkyl group. In the case of appended tertiary anilines, efficient electron transfer quenching occurs for both Z and E conformers. The Z conformers form fluorescent exciplexes, providing a new example of exciplex-type emission in the absence of direct π-π overlap. Exciplexes formed by the E conformers are nonfluorescent and apparently undergo rapid intersystem crossing. The strong exciplex emission observed at low temperatures both in solution and in frozen glasses is attributed to ground state dimers or aggregates.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry