Reversal of the intramolecular charge transfer in p-dimethylaminobenzanilides by amido anilino substitution

p-Dimethylaminobenzanilides with a para or meta substituent at the amido anilino moiety were designed to generate a series of dual fluorescent molecules of variable electron acceptors. Ab initio calculations indicated that the anilino substitution did not lead to an obvious change in the ground-state structures of the fluorophores, and the H-1 NMR signal of the amido -NH proton was found to experience a linear downfield shift with increasing a of the substitutent, supporting the fact that the amido anilino moiety was indeed varied comparably by the substitution. The intramolecular charge transfer dual fluorescence was indeed observed in solvents over a large polarity range from the nonpolar cyclohexane (CHX) through diethyl ether (DEE) and tetrahydrofuran (THF) to highly polar acetonitrile (ACN). It was found that the CT emission shifted to the blue with increasing electron-withdrawing ability of the amido anilino substituent up to a Hammett constant sigma of ca. +0.39 and to the red at higher sigma. The solvent polarity variation did not change the sigma value at which the CT emission shift direction reverses. Similar variation profiles were also observed with the CT to LE emission intensity ratio and the total fluorescence quantum yield. It was concluded that the CT direction in p-dimethylaminobenzanilides was reversed by the amido,anilino substitution, and the CT occurs from amido anilino to benzoyl moiety at low sigma, whereas at high sigma, the CT switches to that from dimethylamino to the benzanilide moiety, which was also supported from the Hartree-Fock calculations. This finding provides an alternative method based on substitutent effect for identifying the charge-transfer direction in multiple charge-transfer systems. The results suggested that the anilino group could be a much stronger electron donor than an aliphatic amino group, which would be of use in designing electron donor substituted molecules. In both cases the dependences of the CT emission energy against sigma of the substituent at the amido aniline phenyl ring were found to be much stronger than that in the ester counterparts of p-dimethylaminobenzanilides. This interesting sigma dependence in the CT emission would be of significance in developing new fluorescent sensors based on electron donor/acceptor variations in p-dimethylaminobenzanilides.