Background and Originality Content
A variety of chiral motifs encompassed in all the biosphere on Earth has been long intriguing subjects among scientific communities; i.e. homochiral proteins made of achiral glycine and L-amino acids and, helical nucleic acids composed of achiral nucleobase and D-pentose, cellulose as a chainlike sequence of D-hexose, and handed spiral vines and conch.1-4 Among a range of chiral motifs at molecular and macromolecular levels, stimuli-responsive, non-rigid chiral supramolecular architectures are possible to noncovalently build-up from combination of achiral and chiral, chiral and chiral, and achiral and achiral building blocks.
The stimuli-responsivity on chiral supramolecules relies on several intermolecular interactions, involving conventional strong and non-conventional weak hydrogen bonds, strong but long-range electrostatic, weak and short-range π-π, and weak but short-range van der Waals interactions in addition to non-rigid molecular and macromolecular frameworks made of covalent bonds, allowing rotational and pyramidal inversion freedom associated with lower barrier heights.5
The stimuli-responsive chiral supramolecules conceptionally differ from inherently rigid chiral molecules and helical polymers consisting of enantiomerically pure stereogenic centers and sterically overcrowded stereogenic bonds.6-9 In this regard, exploring the underlying natural laws of chirality induction mechanisms of chiral-and-achiral building blocks, and realizing the designed on-demand smart chiral functions to supramolecular materials play important clues in evolution of the homochiral biosphere, followed by an elegant bottom-up process of practical chiral materials.10
So far, among rigid and non-rigid polymers, cholesteric liquid crystalline (Ch*LC) polymers capable of solid films and µm-size aggregate/droplet as suspension are classified to chirality-related soft matters.11-13 Generally, the chirality of Ch*LC polymers (Ch*LCP) arises from stereogenic centers of Ch*LCP itself. However, doping a small fraction of chiral molecules to achiral nematic LC molecules and polymeric materials is the most common approach to induce Ch*LC phases and to achiral nematic LC is the most common approach to generate supramolecular chirality.14-16
Such chiral amino acids, sugars, synthetic molecules and polymers are often added to LC polymers, enabling co-assemblages to induce helical motifs of multiple mesogens, leading to a macroscopically asymmetric superstructures like Ch*LC phase and chiroptical generation (so-called chirogenesis).17 Actually, the co-assemblages with the chiral dopants lead to several unique chiroptical properties revealed by circular polarization as selective reflection bands as one-dimensional photonic bandgap engineering and induced circular dichroism (ICD) booted resonantly in optofluidic medium in UV-Vis-near infrared (NIR) region.18 The fatal drawback in the chiral molecular and polymeric dopants in most Ch*LCPs, however, is a difficulty to design multiple switching, rewritable memory (RW), and write-once-read-many (WORM) modulated by condensational physical stimuli,19 like UV-vis light, thermal energy, and electric and magnetic fields.
Regarding the chirality induction to achiral systems, Li et al. reported that the chiral dopants determine the sign and magnitude of the pitches in Ch*LC phase due to its high fluidity.20 Our group demonstrated that chiral molecules designed newly work as effective dopants in Ch*LCP, allowed for the supramolecular chiroptical induction and long-term memory.21 However, the construction of Ch*LC polymers often requires the use of expensive and/or environmentally unfavorable dopants. In this regard, naturally-occurring chiral bio-resources and semi-synthetic derivatives are promising to afford inexpensive, renewable, and environmentally friendly, and greener chirality inducible scaffolds.22
Representative bio-resources are L-/D-amino acids, L-/D-saccharides, terpenes, and alkaloids. Particularly, cellulose and amylose as polysaccharides consisting of D-glucose repeating units withβ -(1-4) and α -(1-4) linkages, respectively, are the most abundant carbohydrates in most plants due to main components of cell walls and roots.23 The contents of cellulose in plant as dry matter attains as high as 30–50%.24
Thus far, a few experiments of cellulose-driven intermolecular chirality transfer to circular dichroism (CD)-silent and circularly polarized luminescence (CPL)-silent chromophoric and luminophoric molecules and polymers suggested three possible mechanisms, as follows.
(i) Multiple weak C-H/O-C, C-H/π, C-H/F-C, and strong N-H/O=C interactions between cellulose derivatives and achiral chromophores/luminophores are responsible for the induced chirality in the chiral-achiral hybridized supramolecular systems.
(ii) Switchable helicity in floppy frameworks, depending on natures of cellulose peralkyl esters and phenyl carbamoyl derivatives, permits a left-right preference in fluxional helical backbones, leading to optically active chromophores-and- luminophores.
(iii) Steric hindrance in certain cellulose derivatives restricts the rotational freedom of backbone, allowing formation of rigid chiral structures.25 Although mechanistic studies of cellulose-driven chirality induction in achiral polymers are on-going, semi-synthetic polysaccharide derivatives efficiently work as chirality inducible scaffoldings, affording excellent stereoselectivity and enantioselectivity by minimizing energy, time, chemicals, and cost.
Yet, several unanswered questions remain to us, as follow.
(i) Is natural cellulose made of only D-glucose repeating units essential for the chirality induction capability?
(ii) How do molecular weights (M n) and polydispersity index (Đ ) of polymers affect chirality and/or helical transfer capability?
(iii) Are the absolute magnitudes and sign of ICD spectral characteristics be tunable ?
To address these questions, here we chose cellulose triacetate (CTA) and cellulose acetate butyrate (CABu), that are readily soluble in common organic solvents. For comparison, we tested whether the respective D-/L-glucose permethyl esters (D-Glu and L-Glu) that are soluble in common organic solvents work as molecular chirality inducible scaffoldings.
Regarding chirality transfer capability endowed with cellulose alkyl esters, a helix sense between CTA and CABu is known to be opposite. CTA and CABu prefer right and left, respectively, although their constitutes have commonly the same D-glucose repeating units with β (1→4) linkage.26
In Scheme 1, we illustrate schematically how CTA and CAbu work as ambidextrous chirality inducing scaffoldings to form supramolecularly hybridized chiral structures with achiral azobenzene-pendant vinylpolymers (PMMAzo). The azobenzene moiety of molecules and polymers is established as a mesogen in nematic LC, producing Ch*LC supramolecular architectures. As expected, D-Glu and L-Glu induced the opposite ICD characteristics to the achiral polymers. Through our systematical study in this work, knowledge and understanding allow us to propose underlying relations and hidden laws between CTA, CABu, and D-/L-Glu and chiroptical characteristics of supramolecular azobenzene mesogens, proven by CD and UV-vis spectroscopy.
The present results should shed light on a new protocol for tailored chiroptical functionality of achiral polymers and switching helicity in CTA and CABu and switching in chirality of D- and L-Glu, leading to a new possibility for ambidextrous large-area applications, i.e. displays, sensors, and photonic devices. The chirality transfer with cellulose derivatives, inexpensive commodity polymer, to various achiral semi-commodity and purely synthetic polymers are thus very promising in future.
Results and Discussion
Results
In this work, achiral methacrylate monomer bearing 4,4’-dialkoxyazobenzene (mesogen) with a flexible hexamethylene linker, AzoMA (Scheme 1 and Figure S1) and the corresponding vinylpolymers, PMMAzo, were prepared by the reversible addition-fragmentation chain-transfer (RAFT) polymerization (Figure S2) in line with our previous works.16 Synthesis of AzoMA, PMMAzo, CTA, CABu, and D-/L-Glu were described in experimental section.27 All CTA, CABu, and D-/L-Glu were readily soluble in common organic solvents.
The values of M n and Đ of PMMAzo were controlled by molar ratios of AzoMA to 4-cyanopentanoic acid dithiobenzoate (CPADB) as chain transfer agent. The effects of thermal annealing time, ratios of the chirality/helicity transferable scaffoldings to PMMAzo, and the M n and Đvalues of PMMAzo affecting the magnitudes and sign in normalized CD-to-UV-vis signals at CD extremum known as Kuhn’s dissymmetry ratio,g CD at the first Cotton band around 350–400 nm, as chiroptical probes to evaluate the degree of ordered supramolecular mesogens were investigated, as summarized in Table 1.
Scheme 1 A schematic illustration of the preparation of chiral PMMAzo film by mixed with cellulose derivatives.