Synthesis and application of AIE active chromophores in supramolecular chemistry
Creators: name: Anuradha
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)
classificationCode: 340304
subject: Optical properties of materials
classificationCode: 340503
subject: Organic chemical synthesis

description: Conventional luminophores such as perylene diimides, naphthalene diimides and porphyrins, despite possessing near unity quantum yield in dilute solution, undergo Aggregation Caused Quenching (ACQ), where aggregation results in quenching of luminescence of these molecules. In 2001, Tang and co-workers from HKUST University, Hong Kong, discovered a contrary phenomenon –so called Aggregation Induced Emission (AIE), in which molecules show weak emission or no emission in dilute solution, however become active in the aggregated state as well as in the solid state. This phenomenon revolutionised the use of luminescent materials in organic light emitting diodes (OLEDs), and chemo and biosensors, as these applications involve the use of luminescent materials in a thin film form (solid state) or in aqueous surroundings. Amongst AIE-active luminophores, tetraphenylethene (TPE) derivatives have attracted much attention due to their capability of self-organisation, possibility of incorporation into larger multicomponent assemblies with ACQ fluorophores. The TPE luminogen is a propeller-shaped AIE-active molecule, bearing four phenyl rings around the central C=C double bond, and is non-emissive in solution, mainly due to lack of restriction of intramolecular rotation (RIR), but is highly fluorescent in the solid and aggregated states.<br><br>Chirality is prevalent in all life forms in nature. The scientific literature reveals few examples that demonstrate the formation of a twisted assembly either from homo-chiral molecules, or from a mixture of chiral and achiral molecules, where the molecular chirality is transferred to the handedness of the self-assembled helical structure. However, most of these molecules suffer from the problem of the ACQ effect and, and lack control of the handedness of chiral superstructures (i.e. chiral self-assemblies) lead to racemic mixture formation from achiral molecules.<br><br>The formation of chiral helical supramolecular structures with the induction and control (non-racemic) of supramolecular chirality from achiral functional organic molecules is a worthy challenge, due to potential applications in sensing, as template agent and catalysis. Furthermore, the induction of chirality into supramolecular assembled structures through hierarchical self-assembly of achiral compounds, and the control of their handedness, is closely related to the evolution of life and the chiral amplification found in nature.<br><br>In this thesis, we show that the combination of achiral TPE, an AIE-active luminophore bearing four alkyl chains with either an odd or even number of carbon atoms via an amide linkage in the molecular structure, allows the induction and control of supramolecular chirality into well-defined helical superstructures by controlling the solvent composition and polarity.<br><br>Chapter-2 describes the synthesis and self-assembly of TPE based achiral molecule (i.e. alkyl-TPE in which alkyl (decyl) chain is linked to central TPE core through amide linkage) into right handed helical structures through solvophobic control. The twisted structure of the central core of TPE, along with the positioning of the peripheral carbon atoms in the alkyl chain away from carbonyl group, maximises the van der Waal interactions in the alkyl chains. This results in the assembly of this molecule into three dimensional helical ribbons, which to our knowledge is the first report of the self-assembly with selective handedness from an AIE active achiral molecule.<br><br>These results further encouraged us to probe the effect of changing the number of carbon atoms of alkyl-TPE derivatives on the supramolecular behaviour. Thus, in chapter-3 three new analogues of alkyl-TPE were synthesised, bearing seven, eight and nine carbon atoms in their long alkyl chains. Self-assembly of alkyl-TPE bearing even number of carbon atoms (i.e. eight and ten) produced right handed helical structures whereas the alkyl-TPE derivative bearing an odd number of carbon atoms (i.e. seven and nine), resulted in left handed helical superstructures. The chiral helical structures were visualised by scanning electron and transmission electron microscopy and the mode of aggregation determined by circular dichromism. Thus, two chapters 2 and 3 clearly demonstrate induction and controlled chiral helical structures from achiral AIE active TPE luminophores with solvophobic control.<br><br>In chapter 4 the use of AIE an active TPE based molecule i.e. tetraamine-TPE (TA-TPE) for selective nitrite sensing is described. This molecule senses the nitrite ion in aqueous medium by a fluorescence turn-on mechanism. This method is environmentally friendly and offers the advantage of a low detection limit.<br><br>To my knowledge, all results described here in thesis are first reports in the scientific literature and are totally novel.