Flower-like superstructures of AIE-active tetraphenylethylene through solvophobic controlled self-assembly

Abstract

The development of well-organized structures with high luminescent properties in the solid and aggregated states is of both scientific and technological interest due to their applications in nanotechnology. In this paper we described the synthesis of amphiphilic and dumbbell shaped AIE-active tetraphenylethylene (TPE) derivatives and studied their self-assembly with solvophobic control. Interestingly, both TPE derivatives form a 3D flower-shape supramolecular structure from THF/water solutions at varying water fractions. SEM microscopy was used to visualise step-wise growth of flower-shape assembly. TPE derivatives also show good mechanochromic properties which can be observed in the process of grinding, fuming and heating. These TPE derivative self-assemblies are formed due to two main important properties: (i) the TPE-core along with alkyl chains, optimizing the dispersive interactions within a construct, and (ii) amide-linkage through molecular recognition. We believe such arrangements prevent crystallization and favour the directional growth of flower-shape nanostructures in a 3D fashion.

Figure 1. The chemical structure: dumbbell shaped TPE-1 and its amphiphilic analogue TPE-2 molecules used in this study.

Figure 2. Luminescence properties: photographic images of TPE-1 and TPE-2 solutions in various solvent mixtures irradiated by UV light (λ_ex = 365 nm).

Figure 3. Solution based self-assembly.

(A) The UV-vis absorption spectra of TPE-1 and TPE-2 (10 μM), (B) the fluorescence spectra of TPE-1 (10 μM) in THF/water at various water fractions, (C and D) florescence intensity changes of TPE-2 and TPE-1 at 475 nm as a function of water fraction in THF solvent mixture and irradiation at 365 nm, respectively.

Figure 4. Density functional theory (DFT) calculations: Electron density distribution of HOMO and LUMO orbitals of TPE-1 and TPE-2.

Figure 5. Visualisation of various self-assemblies by SEM analysis.

SEM micrographs of microstructures of TPE-1 from solutions in (A) f_w = 70%, (B) f_w = 80%, (C) f_w = 85%, (D) f_w = 95%, and TPE-2 deposited from solutions in (E) f_w = 70%, (F) f_w = 80%, (G) f_w = 85%, (H) f_w = 95% of water-THF solvent mixtures.

Figure 6. Flower like assembly formation process.

SEM micrographs of TPE-2 deposited by solvent evaporation of water-THF (f_w = 85%) solutions, showing a step-by-step growth of the flower-like 3D fractal microstructure (The scale bar indicates 10 μm).

Figure 7. The mechanochromic properties.

TPE-1 (A–D) and TPE-2 (E–H) showing the luminescence of these two compounds after grinding, fuming and heating. The florescence spectra of (I) TPE-1, and (J) TPE-2 at the powder crystalline, ground, fumed and heated states, respectively.

Figure 8

(A) The fluorescence spectra of TPE-1 in CHCl₃/hexane solvent mixtures, and (B) SEM micrograph of the fractal microstructure of TPE-1 deposited from CHCl₃/hexane at f_h = 95% v on a silicon wafer substrate, the inset shows a magnified area of the image where nanospheres can be seen.

Figure 9. Schematic diagram illustrating flower-like assembly of TPE-1 in CHCl₃/hexane (f_h = 95%) and TPE-2 in THF/water (f_w = 85%).

False coloured SEM images used to visualise the nanospheres and flower-shape assembly.