Halochromic isoquinoline attached mechanochromic triphenylamine, N-phenyl-N-(4-(quinolin-2-yl)phenyl)benzenamine (PQPBA) and tris(4-(quinolin-2-yl)phenyl)amine (TQPA), smart fluorescent materials exhibit thermo/mechanochromism and tunable solid state fluorescence and their unusual halochromic response in PMMA matrix have been used for fabricating rewritable and self-erasable fluorescent platforms. PQPBA and TQPA showed strong fluorescence in solution (Φ = 0.9290 (PQPBA) and 0.9160 (TQPA)) and moderate solid state fluorescence (Φ = 20 (PQPBA) and 17% (TQPA). Interestingly, they exhibited a rare temperature (0-100 °C) dependent positive fluorescence enhancement via activating radiative vibrational transition. The deaggregation of PQPBA and TQPA in PMMA polymer matrix lead to the enhancement of fluorescence intensity strongly and fabricated strong blue fluorescent thin films (Φ = 58% (PQPBA) and 54% (TQPA). The halochromic isoquinoline has been exploited for demonstrating reversible off-on fluorescence switching by acid (TFA (trifluoroacetic acid)/HCl) and base (NH) treatment in both solids as well as PMMA thin films. Importantly, rewritable and self-erasable fluorescent platform has been achieved by make use of unusual fluorescence responses of PQPBA/TQPA with TFA/HCl after exposing NH. Single crystal and powder X-ray diffraction (PXRD) studies provided the insight on the solid-state fluorescence and external stimuli-induced fluorescence changes.

2-(4-(diphenylamino)-2-methoxybenzylidene) malononitrile (DPAMBM), an organic aggregation enhanced emission material, showed external stimuli mediated reversible phase change (crystalline to amorphous) and fluorescence switching as well as multiple factor mediated fluorescence tuning in the solid state. Polymorphic crystals obtained from CH 3 CN (DPAMBM-1) and CH 3 OH ( DPAMBM-2) 10 exhibited fluorescence at 588 nm and 538 nm, respectively. Single crystal analysis showed slightly twisted molecular conformation between aminophenyl (donor) and malononitrile (acceptor) and the molecules are well separated in the crystal lattice. Whereas linear molecular conformation and strong intermolecular interaction with anti-parallel dipole arrangement was observed in DPAMBM-2. Slight breaking and strong grinding of DPAMBM-1 crystals showed blue shifting of fluorescence to 571 nm and 15 562 nm, respectively. Interestingly, annealing/solvent vapor exposure further blue shift the fluorescence to 545 nm and convert the grounded powder to DPAMBM-2. Strong grinding of DPAMBM-2 red shifted the fluorescence to 562 nm and annealing/vapor exposure switched back to 545 nm. DPAMBM-2 was converted to DPAMBM-1 by annealing at 120 °C that tuned the fluorescence from 538 nm to 572 nm and mechanical grinding and annealing further blue shift the fluorescence to 545 nm. Nanofabrication of 20 DPAMBM also showed fluorescence tuning from 562 nm to 536 nm due to morphological change. Thus DPAMBM showed a rare combination of gradual fluorescence tuning from 588 nm to 536 nm and external stimuli controlled phase change with fluorescence switching between 545 nm and 562 nm.

A triphenylamine-based fluorophore, 4-((4-methoxyphenyl)(phenyl)amino)benzaldehyde (1), exhibits externalstimuli-responsive self-reversible solid-state fluorescence switching, tunable fluorescence, and a rare phenomenon of temperaturedependent fluorescence. Mechanically grinding a crystalline powder of 1 converts the blue fluorescence (λ max = 457 nm) to green (λ max = 502 nm), but blue fluorescence robustly self-recovers within 8 min. X-ray analysis and theoretical studies suggest that the change from a highly twisted molecular conformation and crystalline form into an amorphous phase with more planar conformation is responsible for the fluorescence switching. Self-reversible fluorescence switching did not show a significant change in fluorescence for several cycles of measurement. Interestingly, 1 in toluene showed a rare phenomenon of fluorescence enhancement with increasing temperature via activating more vibrational bands that lead to stronger twisted intramolecular charge-transfer (TICT) emissions. Morphological-change-mediated fluorescence tuning has also been demonstrated by fabricating nanoparticles of 1. The conversion of highly polydispersed, featureless, different-shaped nanoparticles into nearly uniformly sized spherical nanoparticles (20−25 nm) converts green (λ max = 502 nm) to blue fluorescence (λ max = 478 nm). The self-reversible multi-stimuli-responsive fluorescence switching and polymorphism and nanofabrication-mediated fluorescence tuning suggest its potential application in sensors, particularly for fluorescent thermometers.

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 (Φf = 85% (TCAAD-1, 55% (TDEM)), (ii) higher dihedral angle (τ) between donor (aminophenyl) and acceptor lead to weak/non fluorescent material, (iii) substituent at the ortho position to acceptor increased the dihedral angle (τ = 26.49 (TCAAD-2), τ = 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.

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