Reversible fluorescence switching and topochemical conversion in an organic AEE material: polymorphism, defection and nanofabrication mediated fluorescence tuning

Abstract

2-(4-(Diphenylamino)-2-methoxybenzylidene) malononitrile (DPAMBM), an organic aggregation enhanced emission material, showed an external stimulus 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 CH₃CN (DPAMBM-1) and CH₃OH (DPAMBM-2) exhibited fluorescence at 588 nm and 538 nm, respectively. Single crystal analysis showed a 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 were observed in DPAMBM-2. Slight breaking and strong grinding of DPAMBM-1 crystals showed blue shifting of fluorescence to 571 nm and 562 nm, respectively. Interestingly, annealing/solvent vapor exposure further blue shifts the fluorescence to 545 nm and converts the ground 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 shifts the fluorescence to 545 nm. Nanofabrication of DPAMBM also showed fluorescence tuning from 562 nm to 536 nm due to morphological changes. Thus DPAMBM showed a rare combination of gradual fluorescence tuning from 588 nm to 536 nm and an external stimulus controlled phase change with fluorescence switching between 545 nm and 562 nm.