Structures and excited states of extended and folded mono-, di-, and tri-N-arylbenzamides

The relationship between molecular structure and electronic properties of 18 N-arylbenzamides has been investigated. The secondary amides adopt extended trans-amide conformations, whereas the tertiary amides adopt folded cis-amide conformations in which the N-aryl group is approximately perpendicular to the benzoyl group. The N-aryl groups of the tertiary di- and triamides occupy the same face of the benzoyl group and display edge-to-face aryl-aryl interactions. The long-wavelength absorption maxima of the secondary amides are red-shifted as the number of N-aryl groups increases, whereas the tertiary amides display are blue-shifted. ZINDO calculations aid in the assignment of the absorption and luminescence spectra. In all cases the lowest energy singlet state is assigned to a forbidden n,B* (carbonyl lone-pair to benzoyl) transition. The structureless fluorescence of the secondary and tertiary amides is assigned to planar and twisted n.B* singlets, respectively. The allowed absorption bands are assigned to A,B* (arene-to-benzoyl) or A,A* (arene-to-arene) transitions which are localized on a single arene, and the structured phosphorescence is assigned to A,A* triplet states.