Addressing the Aggregation Phenomenon in a Model Aggregation Induced Enhancement of Emission‐Coupled Excited State Intramolecular Proton Transfer Active Probe: Correlating Aggregation States to their Photodynamics

Steady-state and ps-ns time-resolved emission spectroscopic behavior is observed for the benchmark excited-state intramolecular proton transfer-coupled AIEgen 4-dimethylamino-2′-hydroxychalcone (DMHC). In solid-state, DMHC shows slow phototautomerization from J-aggregates, whereas H-aggregates with faster spectrodynamics are dominant in water alongside solvated molecules of DMHC.

We report a comprehensive study on the spectrodynamic behavior 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. The studies reveal that the excited-state intramolecular proton transfer (ESIPT) process is slow in the crystalline state (τ _ESIPT = 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 (τ _{local-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 (τ _{local-emission} = 90 ps and τ _ESIPT= 35 ps and 1.4 ns). It is 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 spectroscopic dissection of the aggregation phenomenon of a model aggregation induced enhancement of emission (AIE)-coupled ESIPT active molecule and delineates how the spectroscopic behavior 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.

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