Enhancement of Fluorescence Emission Intensity in the Derivatives of Tri(biphenyl-4-yl)amine by Breaking of Aggregates via Solid-State Grinding with Various Metal Salts: Chemosensing and Light-Emitting Diode Applications

Abstract

In the present era, solid-state fluorescent materials are in huge demand due to their vast applications in LEDs, sensors, recognition, and imaging. However, molecular aggregations in the solid state limit their usage in practical applications. Typically, J-aggregates exhibit enhanced emission, while H-aggregates result in quenching the fluorescence emission in the solid state. Several techniques like making crystalline polymorphs, applying external stimuli in the solid state, cocrystallizing the fluorophores with other molecules, acid-protonated molecular crystals, supramolecular engineering, crystal engineering, and incorporating metal ions during solid-state grinding have been developed to manipulate the π-π and other supramolecular interactions in solid-state aggregates. Due to extended conjugations, triphenylamine-based small organic molecules and covalent organic cages show immense potential applications in developing optoelectronic devices and sensors. In the present thesis work, numerous tri(biphenyl-4-yl)amine (TBA) derivatives were synthesized and characterized using spectroscopic techniques. The synthesized tri(biphenyl-4-yl)amine derivatives showed J-aggregates and exhibited lower fluorescence emission intensity in the solid state compared to the solution form. These aggregations were disrupted through simple solid-state grinding combined with different inorganic metal salts, resulting in enhanced fluorescence emission. The DFT computational modeling revealed that dipole-ion interactions were the driving force for breaking the aggregations in TBA derivatives. Herein, the surfaces of the micro/nanometer-sized salt crystal particles serve as templates for the molecules of TBA derivatives, or in the broad sense, the inorganic metal salt crystal surfaces act as a “solid-state solvent.” The sulfonyl hydrazide derivative of the TBA (TBA-THZ) exhibited negligible emission in the solid state due to the aggregation- caused quenching phenomenon while grinding the TBA-THZ solid powder with the alkali and alkaline earth metal salts showed enhanced fluorescence emission through dipole-ion (S=O···Mn+) interactions. The fluorescent active salt ground matrices exhibited excellent acidochromism properties in the solid state. The TBA-based imine-linked covalent organic cage (COC1) molecular materials exhibited green emission in the solid state. The photophysical characteristics and crystal structure analysis revealed that the COC1 cage molecules form J-aggregates in the crystal lattice, which was disrupted by grinding the COC1 crystal with different inorganic metal salts, resulting in enhanced fluorescence emission through dipole-ion (C–H∙∙∙X−, X = oxygen or halogen from anion) interactions. The 2-hydrazinopyridine derivative of the TBA (TBA-2HP) also displayed green fluorescence emission in the solid state, and the photophysical properties, as well as DFT computational modeling, elucidated that the TBA-2HP molecules form J-aggregates in the solid state. Upon grinding the TBA-2HP solid powder with the different inorganic metal salts, it showed enhanced fluorescence emission through both dipole-ion (C–H∙∙∙X−, X = oxygen or halogen from anion) interactions and coordination bond formation (N→Zn2+). Interestingly, TBA-2HP, upon grinding with different salts of zinc, exhibited enhanced fluorescence emission intensity as well as a change in the maximum emission wavelength. Moreover, it was observed that the emission colour was tuned by varying the ratio of the TBA-2HP with the zinc acetate. Furthermore, the electroluminescence properties of the green emissive TBA derivatives were investigated. Green light emission occurred upon coating the green-emissive TBA derivatives (COC1 or TBA-2HP) doped PMMA polymer film over the surface of a near-ultraviolet LED. Upon coating, a thin layer of COC1 or TBA-2HP, along with a red phosphor-doped PMMA polymer on the surface of a blue LED, resulted in white light emission. Further investigation revealed that the COC1 cage material exhibited excellent acidochromic properties in the solid and solution states. Among different nitroaromatic compounds, picric acid is one of the most widely used chemicals in chemical laboratories and dye, pharmaceuticals, fireworks, and matchbox industries. Because of improper waste management, PA easily contaminates groundwater and soil, which causes severe health problems for human beings. Therefore, developing cost- effective sensory materials for detecting high-energy nitro-explosive materials has become a massive global demand due to the unremitting rise in health and environmental concerns. In the present thesis report, the TBA-based amine-linked fluorescent covalent organic cage (COC2) and TBA-2HP materials were utilized for highly sensitive and selective detection of picric acid at the nano to micromolar level. The COC2 cage material was also investigated for the acidochromism studies.