Addressing the aggregation phenomenon in a model AIE coupled ESIPT active probe: correlating aggregation states to their photodynamics.

In the current work, we report a comprehensive study on the spectrodynamic behaviour of 4-dimethylamino-2'-hydroxychalcone (DMHC)  in crystalline state and in aggregated state in water (pH 7.0) by the aid of steady-state and time-resolved spectroscopy. Our studies revealed that the ESIPT process is slow in the crystalline state (τESIPT=3.0 ns) with the emission dominated by ESIPT from 600 nm onwards, with little contribution (550-570 nm) from the local emission with lower lifetime of emission (τlocal-emission=140 ps). Spectrodynamic studies in the aqueous solution of DMHC shows the emission lifetime values are both lowered in the aqueous solution with emission from aggregates and the solvated DMHC molecules in aqueous solution (τlocal-emission =90 ps and τESIPT=35 ps and 1.4 ns). We concluded that DMHC shows ESIPT emission from the J-aggregated forms, whereas in aqueous solutions, the ESIPT emission in water occurs from the H-aggregates. The current report holds importance owing to the detailed spectrodynamic dissection of the aggregation phenomenon of a model AIE coupled ESIPT active molecule and delineates how the spectrodynamic behaviour of each aggregated form differs in terms of the nature of the aggregates formed (J- and H-aggregates), which shall aid in the understanding and construction of tailor-made AIE active molecules.