Synthesis of ESIPT fluorophores with two intramolecular H-bonding functionalities: Reversible mechanofluorochromism and conformation controlled solid state fluorescence efficiency

We have synthesized two new ESIPT fluorophores (1-((E)-((E)-((2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl) naphthalen-2-ol (Naph-2-OH) and 5-(diphenylamino)-2-((E)-((E)-((2-hydroxyphenyl)(phenyl) methylene)hydrazono)methyl) phenol (TPA-2-OH)) with two unsymmetrical intramolecular H-bonding functionalities and investigated the solid-state structural assembly and fluorescence properties. The crystal structure analysis confirmed the formation of strong intramolecular H-bonding and twisted molecular conformation. The nonplanar twisted molecular conformation produced well separated molecules in the crystal lattice that lead to strongly enhanced emission ((λmax = 538 nm, Φf = 5.2% (Naph-2-OH), λmax = 562 nm, Φf = 13.4% (TPA-2-OH)). Density functional theoretical (DFT) calculation supported the fluorescence tuning and intramolecular charge transfer (ICT) from TPA donor to imine acceptor. Both Naph-2-OH and TPA-2-OH showed mechanical crushing and heating induced reversible fluorescence switching due to the reversible phase transformation between crystalline and amorphous state.