Molecular Engineering of Triphenylamine Based Aggregation Enhanced Emissive Fluorophore: Structure-Dependent Mechanochromism and Self-Reversible Fluorescence Switching

Triphenylamine (TPA), a propeller-shaped optoelectronic molecule, has been used to generate stimuli-responsive smart fluorescent organic materials and correlate the effect of subtle structural changes on the molecular packing and mechanochromic fluorescence (MCF). The substituent (OCH3) position in the TPA phenyl ring and acceptors (malononitrile, cyanoacetamide, cyanoacetic acid, ethyl cyanoacetate, and diethylmalonate) strongly influenced the solid state and mechanochromic fluorescence as well as the molecular packing. The structure-property studies revealed that (i) TPA derivatives without the OCH3 substituent exhibit strong fluorescence (Phi(f) = 85% (TCAAD-1, 55% (TDEM)), (ii) higher dihedral angle (tau) between donor (aminophenyl) and acceptor lead to weak/non fluorescent material, (iii) substituent at the ortho position to acceptor increased the dihedral angle (tau = 26.49 (TCAAD-2), tau = 27.14 (TDMM)), and (iv) the increase of alkyl groups produced self-reversible high contrast off-on fluorescence switching materials (TDEM). Powder X-ray diffraction studies indicate that stimuli induced reversible phase-transformation from crystalline to amorphous and vice versa was responsible for fluorescence switching. The computational studies also supported that OCH3 substitution at ortho to acceptor increased the dihedral angle and optical band gap. Thus, the present studies provide a structural insight for designing TPA based organic molecules for developing new smart organic materials.