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The structural assembly of organic fluorescent molecules exerts a significant influence on solid-state fluorescence properties, including fluorescence tuning and switching. Herein, we have synthesized a Schiff base based organic fluorophore, 2-((E)-((E)-(4-hydroxybenzylidene)hydrazono)(phenyl)methyl)phenol (1), with intra- and intermolecular H-bonding hydroxyl functionalities, and studied its solid state structural arrangement and fluorescence properties. 1 exhibited aggregation induced enhanced emission (AIEE) in the solid state. The crystallization of 1 produced geometrical (E/Z) isomers (1a (E-isomer) and 1b (Z-isomer)) with completely different intra- and intermolecular H-bonding. 1a showed typical intramolecular H-bonding with an adjacent nitrogen and showed six membered planar rings while 1b revealed unusual intramolecular H-bonding with seven membered non-planar rings. The intermolecular H-bonding of the additional hydroxy group in 1a produced a hexagon arrangement with the inclusion of a water molecule whereas a 2D network structure is formed in 1b. However, both 1a and 1b isomers showed large Stokes shifted solid state fluorescence at 570 and 565 nm, respectively, due to the intramolecular H-bonding facilitated excited state intramolecular proton transfer (ESIPT) process. Interestingly, both 1a and 1b exhibited self-reversible fluorescence switching after crushing without showing any structural transformation. 1a exhibited a relatively slow fluorescence recovery compared to 1b. Computational studies showed that intramolecular H-bonding/conformational twisting controlled the electron density distribution in the HOMO. PXRD studies confirmed the structural stability and complete regeneration of the crystalline phase after mechanical crushing. Thus, the simple interplay of intra/intermolecular H-bonding in 1 led to the formation of geometrical isomers with distinct self-reversibility. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Stimuli-responsive organic solid state fluorescent materials are considered as potential candidates for optoelectronic application as well as in the biomedical field. Molecular design and supramolecular interaction controlled organization in the solid state played an important role in producing switchable and tunable fluorescent materials. Excited state intramolecular proton transfer (ESIPT) mechanism-based solid state fluorescent materials showed unique photophysical properties such as a large Stokes shift and local environment (pH, polarity, ions and viscosity) responsive fluorescence modulation. The unique photophysical properties of ESIPT molecules made them interesting for various fields including laser dyes, molecular probes, optoelectronics, white emissive materials and optical information storage materials. Systematic fluorophore structural engineering has been performed over the years to gain insight on the ESIPT mechanism in order to improve the quantum efficiency and introduce desirable material attributes for functional applications. The substitutional unit, molecular conformation and supramolecular interactions played a significant role in transforming planar ESIPT fluorophores to stimuli-induced fluorescence switching materials either between two states or off–on states. In this review article, we have presented the recent developments in ESIPT-based solid state fluorescent materials and external stimuli-induced fluorescence switching. [ABSTRACT FROM AUTHOR]