Synthesis of superstructures of AIE active tetraphenylethylene through solvophobic controlled self-assembly

Aggregation induced emission (AIE) is an attribute associated with the novel type of macromolecules that emit light under UV when aggregated in the solid state as opposed to Aggregation Caused Quenching (ACQ)–type macromolecules that quench their luminescence upon aggregation. Tetraphenylethylene is the most famous AIE-gen and also a building block that can neutralize the notorious ACQ effect.

Several derivatives of TPE and mono- & di-acids with odd and even number of CH2 groups are synthesized to firstly study the effect of odd & even number of carbon chain on the self-assembly and also to investigate the mechanochromic features of the synthesized molecules.

Traditional luminophores, namely naphthalene diimides or porphyrins, succumb to Aggregation Caused Quenching (ACQ), in a way that aggregation ends up with quenching of luminescence of molecules. Ben Zhong Tang from HKUST University, Hong Kong, unearthed an antithetical phenomenon, named as Aggregation Induced Emission (AIE), in which molecules demonstrate weak or no emission in dissolved state, whereas, they become luminiscent in the aggregated state also in the solid state. This breakthrough metamorphosed the application of luminescent materials in organic light emitting diodes (OLEDs), and chemo- & biosensors, where the use of luminescent materials in a thin film form (solid state) or in aqueous surroundings is sought. The most distinguished attention-grabber AIE-active luminophores have been tetraphenylethene (TPE) derivatives owing to their capability of self-organisation, feasibility of embodiment into larger multicomponent assemblies with ACQ fluorophores. The TPE luminogen, a propeller-shaped AIE-active molecule, enjoys the presence of the four phenyl rings around the central C=C double bond that turn distinctly emissive in solid and aggregated state thanks to the restriction of the intramolecular rotation abbreviated as RIR.

In this thesis, I have shown that the combination of TPE, an AIE-active luminophore consisting of four alkyl chains with either an odd or even number of carbon atoms via two amide linkages in the molecular structure, that direct the superstructures morphology by controlling the solvent composition.

Chapter-2 describes the synthesis and self-assembly of two TPE-based derivatives complied of one and two amide linkages and indicates a flower-like self-assembly in specific solvent mixtures, which to our knowledge is the first report of the self-assembly with the flower-like 3D morphology under solvophobic control.

These results further encouraged us to probe the effect of changing the number of carbon atoms of alkyl-TPE derivatives on the supramolecular behaviour. Therefore, in chapter-3 three new analogues of alkyl-TPE were synthesised, including seven, eight and nine carbon atoms in their long alkyl chains. Self-assembly of alkyl-TPE containing an even number of carbon atoms (i.e. eight and ten) produced various structures, TPE with eight carbons formed micro-belts and the one with ten carbons aggregated into flower-like superstructure, whereas the alkyl-TPE derivative bearing an odd number of carbon atoms (i.e. seven and nine), resulted in nano-sphere superstructures.