Triple fluorescence of substituted benzanilides in solution and in solid states

The competitive triple fluorescence of benzanilide is studied by steady-sta te fluorescence investigations in dependence on the solvent polarity and th e para-substitution of the aniline core as well as by comparison with the f luorescence behavior of 4-methoxy-N-methylbenzanilide. The normal fluoresce nce of benzanilide S-1(LE) --> S-0 appears at lambda(max) = 345 nm, whereas a superposition of proton transfer (PT) fluorescence S-1'(PT) --> S-0'(PT) and intramolecular charge transfer (TCT) fluorescence S-1"(ICT) --> S-0(FC ) is responsible for the long-wavelength fluorescence in the 500-nm region. Different possibilities for the formation of the PT and ICT states are dis cussed. Investigations of the fluorescence behavior of benzanilides both in solution and as crystals in dependence on the para-substitution of the ben zanilide moiety support the PT/ICT model.