Aggregation induced enhanced emission in Dimethyl-2,5-bis(4-methoxyphenylamino)terephthalate

Dimethyl-2,5-bis(4-methoxyphenylamino)terephthalate (1) exhibits very feeble emission in acetone solution, but is strongly emissive in the solid state. In order to develop an understanding of the processes responsible for such behavior, fluorescence studies have been performed on solutions of 1 in acetone-water mixtures. Increase in water content of these solutions is found to result in broadening of the absorption spectra along with red shift of the absorption maximum, suggesting the formation of aggregates. The process of aggregation is associated with a significant enhancement of the emission intensity. This is a manifestation of the aggregation induced enhanced emission (AIEE) phenomenon. Quantum chemical calculations reveal a propeller-like shape of the molecule, which is an essential criterion for AIEE-active molecules. The major nonradiative relaxation of the excited state of this molecule is likely to involve the rotation of the molecular segments with respect to each other. X-ray diffraction studies reveal a stacking pattern, which is likely to hinder such nonradiative decativation of the excited state, leading to the enhanced emission in solid state and aggregated state of the molecule. The trend of variation of emission intensity, as a function of water content of the mixture, is in line with the size of aggregates formed in these mixtures, as is brought out by light scattering studies. Electron microscopy reveals that the microstructure of the aggregates as well as the solid powder of 1 consists of nanorods. Fluorescence lifetimes indicate that the nature of the emissive species in the aggregates and the solid is the same. Thus, a systematic understanding of the structure and photoprocesses in the aggregates is developed.