ESIPT geometrical isomers with distinct mechanofluorochromism and intra/intermolecular H-bonding controlled tunable fluorescence

Abstract

Integrating an intramolecular H-bonding functionality in an organic fluorophore produces an excited state intramolecular proton transfer (ESIPT) mechanism facilitating large Stokes shifted fluorescent materials. The tunable and environmental sensitive fluorescence of ESIPT molecules makes them potential candidates for white light emitting, chemo-sensing and bioimaging applications. Herein, we designed organic fluorophores (2-((E)-((E)-(2-hydroxybenzylidene)hydrazono)(phenyl)methyl)phenol (1), 2-((E)-(((E)-(2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl)-4-methoxyphenol (2), 2-((E)-((E)-((2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl)-5-methoxyphenol (3) and 5-(diethylamino)-2-((E)-((E)-((2-hydroxy phenyl)(phenyl)methylene)hydrazono)methyl)phenol (4)) with two intramolecular H-bonding functionalities and unsymmetrical structures (salicylaldehyde-imine and benzophenone-imine) and explored their substituent-dependent structural assembly and solid-state fluorescence. 1–4 showed tunable solid-state fluorescence between 536 and 645 nm dependent on the inter/intramolecular H-bonding. 3 produced yellow (3-Y) and red (3-R) fluorescent E/Z geometrical isomers. Solid-state structural studies revealed the formation of intramolecular H-bonding at both functionalities in 1, 2a and 3-Y, whereas 2b, 3-R and 4 displayed one intra- and another intermolecular H-bonding. The formation of intra- and intermolecular H-bonding significantly influenced the solid-state fluorescence. Mechanofluorochromic (MFC) studies showed self-reversible fluorescence switching for 1–3 and stimuli-induced reversible fluorescence switching for 4. Importantly, the geometrical isomers 3-Y and 3-R exhibited distinct fluorescence switching without showing any structural transformation upon crushing/heating. Computational studies suggested that the 3-R isomer is more stable by 9.1 kcal mol⁻¹ compared to 3-Y. Powder X-ray diffraction studies further confirmed its reversible phase transition and structural stability. Overall, the present work explored the structural versatility of ESIPT molecules and its impact on fluorescence tuning and switching.