Supramolecular Chirality in Azobenzene-Containing Polymer System: Traditional Postpolymerization Self-Assembly Versus In Situ Supramolecular Self-Assembly Strategy

Abstract

Recently, the design of novel supramolecular chiral materials has received a great deal of attention due to rapid developments in the fields of supramolecular chemistry and molecular self-assembly. Supramolecular chirality has been widely introduced to polymers containing photoresponsive azobenzene groups. On the one hand, supramolecular chiral structures of azobenzene-containing polymers (Azo-polymers) can be produced by nonsymmetric arrangement of Azo units through noncovalent interactions. On the other hand, the reversibility of the photoisomerization also allows for the control of the supramolecular organization of the Azo moieties within polymer structures. The construction of supramolecular chirality in Azo-polymeric self-assembled system is highly important for further developments in this field from both academic and practical points of view. The postpolymerization self-assembly strategy is one of the traditional strategies for mainly constructing supramolecular chirality in Azo-polymers. The in situ supramolecular self-assembly mediated by polymerization-induced self-assembly (PISA) is a facile one-pot approach for the construction of well-defined supramolecular chirality during polymerization process. In this review, we focus on a discussion of supramolecular chirality of Azo-polymer systems constructed by traditional postpolymerization self-assembly and PISA-mediated in situ supramolecular self-assembly. Furthermore, we will also summarize the basic concepts, seminal studies, recent trends, and perspectives in the constructions and applications of supramolecular chirality based on Azo-polymers with the hope to advance the development of supramolecular chirality in chemistry.

Keywords: Azo-containing polymers; in situ polymerization-induced self-assembly; postpolymerization self-assembly; supramolecular chirality.

Figure 1

Chiral architectures at various scales, from neutrino to enantiomeric molecules, proteins and DNA biomacromolecules, macroscopic living systems and galaxy.

Figure 2

Schematic representation of chiroptical switching between two enantiomers possessing (a) configurational chirality, (b) conformational chirality and (c) helical chirality.

Figure 3

(a) Illustration of chiral transfer from gelator to achiral polymer. (b) CD spectra of the supramolecular chiral co-gels. (c) SEM images of the helical fibers. (Reproduced from [79] with permission from The Royal Society of Chemistry).

Figure 4

Schematic representation of (a) traditional postpolymerization self-assembly, (b) previous supramolecular self-assembly in polymer systems and (c) in situ supramolecular self-assembly mediated by PISA.

Figure 5

The chemical structures of chiral Azo-polymers 1, 2, 3, 4, 5, P4S and P8S. (R, X, Y, and Z represent different groups).

Figure 6

The chemical structures of chiral Azo-polymers 6, 7, 8, 9, 10, 11, 12 and 13.

Figure 7

Idealized scheme of photoinduced and thermoinduced structural changes in the spin-coated film of the chiral Azo-polymer 11.

Figure 8

The chemical structures of achiral Azo-polymers 14, 15, 16, 17, 18, 19 and chiral additives.

Figure 9

The chemical structures of achiral Azo-polymers based on Azo-polymer 20 and chiral limonene. (x represents different spacer lengths; R represents different substituents).

Figure 10

Illustration of the supramolecular helical structures of achiral Azo-polymer 20 induced by chiral limonene and the construction of the chiroptical switching. (Reproduced from [135] with permission from The Royal Society of Chemistry).

Figure 11

The chemical structures of achiral Azo-polymers 21, 22, 23, 24, 25, 26, 27, 28, 29 and 30.

Figure 12

Sequence illustrating the formation of helical structure by CPL irradiation. (Reproduced from [206] with permission from The Royal Society of Chemistry).

Figure 13

Proposed scheme of twisted stack led by wavelength-dependent (a) l- and r-CPL sources. CD and UV-vis spectra of Azo-polymer 20 aggregates exposed to (b) 365 nm and (c) 313 nm l-CPL and r-CPL. (Reprinted with permission from [231]; Copyright (2017) American Chemical Society).

Figure 14

Schematic representation of Azo-polymer assemblies and morphological transition constructed by PISA strategy. (For (a–d), Reprinted with permission from [232,233,234,235]; Copyright (2018, 2019, 2017, 2018) American Chemical Society).

Figure 15

Scheme illustrating the construction of chiral or helical assemblies based on PISA strategy. (Red represents solvophobic segments, and blue represents solvophilic segments). ((a) Reproduced from [109] with permission from The Royal Society of Chemistry; (b) and (c), reprinted with permission from [238,239]; Copyright (2020, 2020) American Chemical Society).

Figure 16

(a) Synthetic route of PICSA strategy for constructing hierarchical supramolecular chiral Azo-polymer assemblies. Inset: representation of the dynamic reversibility. (b) AFM images and the representation of the helical fibers. (c) The maximum CD and g_CD values of the supramolecular chiral Azo-polymer assemblies with different morphologies and the representation of the supramolecular helical structure.

Figure 17

Six slightly twisted trans-Azo side chains and simulated CD and UV-vis spectra of the model. (Reprinted with permission from [231]; Copyright (2017) American Chemical Society).