A novel mechanistic interpretation for the Aggregation-Induced Emission (AIE) phenomena is reported from the photophysical and computational studies of a newly synthesized luminogen, 2-amino-3-((E)-4-(diphenylamino)benzylidene)amino)maleonitile (DPA). DPA was characterized using fluorescence, UV-Visible, FTIR, XRD and NMR spectroscopy and quantum yields were observed. For computational studies, Electric Field Induced Second Harmonic Generation (EFISHG) and energy values were calculated (for monomer and dimer) using the B3LYP hybrid functional with 6-31G and 6-311G basis sets. Previously reported AIE molecule, 2-amino-3-((E)-(4-(diethylamino)benzylidene)amino)maleonitrile (A3MN) and an Aggregation-Caused Quenching molecule (fluorescein) were used as benchmarks to propose a new mechanistic interpretation for the AIE effect. Same calculations were performed for the comparison between the AIE and ACQ molecule pair.
DPA shows a phase change from crystalline to amorphous state upon grinding and solvatochromism with different solvents. Quantum yield values showed an enhancement when increasing the water percentage indicating the AIE effect of DPA upon aggregation. The gamma values yielded from EFISHG were used for the comparisons between dimers and monomers of the same molecule and dimer/monomer comparisons between different molecules. Fluorescein Dimer showed an enhanced gamma value compared to its monomer whereas A3MN showed the opposite effect. DPA Monomer showed a greater gamma value compared to A3MN Monomer while the pattern was reversed in their dimers. The gamma value of DPA monomer has a higher EFISHG value compared to its dimer. This indicates that the aggregation in AIE phenomenon effectively suppresses the energy transfer from the singlet excited state to a dark state resulting in enhanced emission.

Synthesis and characterization of a novel aggregation-induced emission (AIE) chromophore analogue: The revelation of a new photophysical interpretation for aggregation-induced emission phenomena

ACS Fall 2021

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