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.