Intramolecular charge transfer and proton transfer fluorescence of salicylanilides

The intramolecular charge transfer (CT) reaction known to occur in benzanilide(BA) derivatives was coupled to salicylic acid (SA) for understanding the substituent effect on the proton transfer fluorescence in comparison with that on the CT fluorescence. A series of salicylanilides 1, in which the SA structural framework was incorporated in the electron acceptor moiety of the BA-like CT fluorophore, were designed and their absorption and fluorescence spectra were investigated in aprotic solvents CHX, DEE, and ACN and a protic solvent MeOH. It was found that 1 exhibited a long-wavelength emission at 470 mn in CHX that scarcely shifted when solvent changed from DEE to ACN of increasing polarity, whereas it drastically shifted to blue at ca 430 nm in protic solvent MeOH. It was therefore made clear that the emission of 1 was different from that of benzanilide, the latter being highly sensitive to solvent polarity. With 2, the o-OH methylated derivatives of 1, long-wavelength fluorescence emission with similar solvent dependence to that of benzanilides was observed. These facts suggested that the emission from 1 originated from the SA-like PT state while that of 2 from the BA-like CT state. The substituent effect on the emission of 1 and 2 with p- or m-substituent at the anilino phenyl ring was examined. Results showed that, whereas the emission of 1 scarcely varied, that of 2 dramatically shifted to the blue with increasing electron-withdrawing ability of the substituent. The emission energies of 2 in CHX were found to be correlated linearly to the Hammett constants of the substituents with a slope of + 0.378 eV, close to + 0.361 eV observed for benzanilides. It was concluded that the emission of 1 originated from the PT state that showed practically no dependence on solvent polarity and substituent at the proton acceptor moiety.