Intramolecular charge transfer dual fluorescence of substituted-phenyl p-dimethylaminobenzoates with comparable electron acceptors

A series of substituted-phenyl p-dimethylaminobenzoates with para- or meta-substitution at the ester phenyl ring (p-OH, p-CH3, H, p-Br, m-Cl, and p-NO2) were synthesized and their fluorescence spectra were recorded. Intramolecular charge transfer (CT) dual fluorescence was observed in polar solvents. The CT emission in the same solvent was found to shift to lower energy with increasing electron-withdrawing ability of the substituent, whereas the locally excited (LE) emission hardly showed any change. A linear correlation was found between the CT emission energies of the four of the six esters and the Hammett constants of the substituents in diethyl ether, tetrahydrofuran and ethyl acetate and the linear slopes were −0.190, −0.169 and −0.138 eV, respectively, that decreased with increasing solvent polarity. The implications of the present findings were discussed in terms of structural relaxation during the excited-state CT reaction and of designing new fluorescence chemosensors.