Preferential chiral solvation induced supramolecular chirality in optically inactive star Azo polymers: photocontrollability, chiral amplification and topological effects

Abstract

A series of star-shaped azobenzene (Azo)-containing side chain polymers with 3, 4 and 6 arms were synthesized by the atom transfer radical polymerization technique initiated by multifunctional initiators. The intense bisignated circular dichroism (CD) signals in the UV-vis region were observed when these star polymers aggregated in a dichloroethane/limonene mixed solvent, indicating the successful chirality transfer from limonene molecules to these star Azo polymers. The chirality originates from the preferential supramolecular chirality of well-organized optically inactive side-chain Azo units. The supramolecular chirality is closely related to the volume fraction of cosolvents, the enantiopurity (ee) of limonene, molecular weight and arm numbers of star Azo polymers. Interestingly, the maximum values of CD and g CD of the polymer aggregates increase with molecular weight in the case of 3-armed Azo polymers, and increase simultaneously with arm numbers of the polymers having similar repeating units under each optimized experimental condition. Surprisingly, the first chiral amplification behaviours were observed in the 4-and 6-armed Azo polymers, but absent in linear and 3-armed counterparts. The supramolecular chirality can be destroyed by trans-cis photoisomerization and recovered by thermal cis-trans isomerization of Azo units. The current research will provide further insight into the mechanism study and production of effective chiroptical materials in chiral solvation induced supramolecular chirality of the achiral polymers. † Electronic supplementary information (ESI) available: The GPC curves, 1 H NMR, UV-vis and CD spectra of star PAzoMAs. See

Chirality Construction from Preferred π-π Stacks of Achiral Azobenzene Units in Polymer: Chiral Induction, Transfer and Memory

“…However, this induction and assembly process of main chain Azo-containing polymers requires a complex system consisting of the good solvent, a chiral solvent, and a weak solvent. Recently, we further simplified the assembled components of a side-chain Azo-containing polymer by using limonene as the weak and chiral solvent simultaneously [27,29,30]. On the one hand, although much effort has been paid to simplifying the assembled components and studying the inducing mechanism, we still have a problem remaining to be solved of whether the construction of supramolecular chirality in the achiral side-chain Azo-containing polymer can be achieved by pure limonene under the controlled temperature.…”

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

“…In this system, chiroptical switching was achieved through the reversible modulation of the supramolecular helicity of Azo building blocks. Subsequently, Zhang and coworkers systematically investigated the effects of spacer lengths, topological structures and push-pull electronic substituents in Azo-polymer 20, on the chiral limonene induced supramolecular chirality [ 133 , 134 , 201 ]. The results of CD and UV-vis spectra suggested that the internal structures and external stimuli have distinctive influences on the induction and regulation of supramolecular chirality of achiral Azo-polymers.…”

Chiral Solvation Induced Supramolecular Chiral Assembly of Achiral Polymers

“…Meanwhile, many diverse factors, such as tersolvent composition, solvent polarity, polymer molecular weight, alkyl chain length, limonene enantiopurity, solution temperature, clockwise and counter-clockwise stirring, and aggregate size are confirmed to influence the magnitude of the induced CD and/or CPL amplitude. Through theoretical calculation, they assume that the inherent twisting ability (H-H repulsion) between the near- Zhang et al demonstrated that solvent chirality can be transferred to the aggregates of many optically inactive π-conjugated polymers with different backbone structures, such as main chain azo-containing polyfluorene (F8AZO), poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PF8), poly(9,9-di-noctylsila-fluorenyl-2,7-diyl) (PSi8), poly(9-(1-octylnonyl)-9H-carbazole-2,7-diyl) (PCz8), P(F8-alt-Si8), P(F8-alt-Cz8), and P(Si8-alt-Cz8), hyperbranched PF8s, side chain azo-containing polymers PAzoMA, P(AzoMA-rans-MMA), and star side-chain Azo polymers (star PAzoMAs) as shown in Figure 6 [69,[72][73][74][75][76]. Optically active F8AZO aggregates were successfully generated by the chirality transfer from (S)-and (R)-limonene, demonstrated by the intense ICD signals corresponding to F8AZO in the visible region [69].…”

“…However, a more recent study demonstrated that the signs depend essentially only on the nature of the polymer host supramolecular chirality, but not on the R or S solvent chirality, proven by the VCD spectra of the δ-phase s-PS film used [78]. To investigate the induction of supramolecular chirality for achiral side-chain polymers, Zhang et al designed serials of linear and star-shaped achiral side chain azobenzene-containing polymer (PAzoMA, P(AzoMA-rans-MMA), and star PAzoMAs) [75,76]. Chirality of nonracemic solvents were successfully transferred to these polymers, and well-assembled supramolecular transazobenzene aggregates are prepared in DCE/(R)-or (S)-limonene mixsolvents.…”