Excited-state dynamics of ethyl 4-(N,N-dimethylamino)benzoate and ethyl 4-(N,N-diethylamino)benzoate in apolar and polar solvents

Abstract

An explanation is given for the red shift of the fluorescence band of ethyl 4-(N,N-diethylamino)benzoate (DEAEB) at ambient temperature (band F_X), relative to the normal fluorescence band (F_N) of ethyl 4-(N,N-dimethylamino)benzoate (DMAEB) in the same alkane as solvent, which is not based on the previously suggested twisted intramolecular charge-transfer nature of the fluorescent state, but on the proximity of low-lying nπ* and ππ* electronic states, having an energy separation depending on the electron donor strength of the substituted amino group. The magnitudes of the electric dipole moment of the fluorescent state of DMAEB (11.1 D) and DEAEB (12.2 D), in cyclohexane at ambient temperature, do not differ enough to allow a characterization of this fluorescent state of DEAEB as a state resulting from intramolecular electron transfer. The presence of small amounts of polar molecules in the alkane solution of DEAEB and DMAEB leads to the formation of fluorescing solute–solvent exciplexes (band F_A). The time dependence of the fluorescence of DEAEB and DMAEB in a viscous polar solvent like butan-1-ol reveals the presence of three types of species, leading to fluorescence of type F_N, F_X and F_A. The species emitting fluorescence of type F_N are converted into species yielding fluorescence band FX and the latter react with the solvent to give fluorescing solute–solvent exciplexes.