Temperature- and conformation-dependent luminescence of benzanilides

The luminescence of benzanilide, N-methylbenzanilide, and two cyclic analogues has been investigated as a function of temperature both in microcrystals and in the glass-forming solvent methyltetrahydrofuran. Potential energy surfaces for small-amplitude geometric relaxation of the excited states and large amplitude twisting about the amide bond in the ground and excited states have been explored using AM1 and RCIS calculations, respectively. ZINDO calculations have been used to probe the electronic configuration of the planar and twisted singlet and triplet states. Correlation of the spectroscopic and computational results indicates that at 77 K benzanilide phosphorescence occurs from a planar pi,pi* triplet state whereas fluorescence occurs from a singlet state with partially relaxed geometry. At higher temperatures, large-amplitude geometric relaxation results in the formation of an intramolecular charge-transfer (TICT) state that is twisted about the amide C-N bond. This TICT state is responsible for the weak Stokes-shifted fluorescence observed from benzanilide and N-methylbenzanilide in fluid solution. Inhibition of amide twisting in cyclic derivatives precludes observation of TICT fluorescence.