Optically active covalent organic frameworks and hyperbranched polymers with chirality induced by circularly polarized light

Article abstract of DOI:10.1039/d1cc02671b

Axial chirality was induced by circularly polarized light to covalent organic frameworks as well as hyperbranched polymers composed of bezene-1,3,5-triyl core units and oligo(benzene-1,4-diyl) as linker units where variation in induction efficiency was ra

Full text of DOI:10.1039/d1cc02671b

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Optically active covalent organic frameworks and
hyperbranched polymers with chirality induced
by circularly polarized light†
Cite this: Chem. Commun., 2021,
57, 7681
Received 21st May 2021,
Accepted 16th June 2021
c
Yuting Wang,a Koji Yazawa,b Qingyu Wang,a Takunori Harada,
Shuhei Shimoda,d
Zhiyi Song,a Masayoshi Bando,a Naofumi Nagae and Tamaki Nakano
*
af
DOI: 10.1039/d1cc02671b
rsc.li/chemcomm
Axial chirality was induced by circularly polarized light to covalent
Chirality induction by CPL has been realized for linear
organic frameworks as well as hyperbranched polymers composed of polyfluorene derivatives and other small molecules having
bezene-1,3,5-triyl core units and oligo(benzene-1,4-diyl) as linker aromatic–aromatic (Ar–Ar) single bonds through enantiomer-
units where variation in induction efficiency was rationally interpreted selective excitation of axially chiral Ar–Ar units.7–11 Biphenyl as
in terms of internal rotation dynamics studied through CPMAS the simplest Ar–Ar compound has a twisted conformation in
13C NMR experiments including CODEX measurements.
the ground state and is transformed to a coplanar conforma-
tion in excited states (twisted-coplanar transition),12,13 and if
Chiral polymers are an important class of materials because of one of the enantiomeric twists is preferentially excited by
their wide scope of applications.1,2 While most studies have CPL, the population of the un-excited twist increases, which leads
targeted linear polymers with a single-handed helical confor- to an optically active product (twist-sense-selective excita-
mation controlled by chiral monomeric units or by catalysis or tion).7–11,13–15 In this work, this methodology was applied for the
by external stimuli, chiral dendrimers and hyperbranched COFs and HBPs composed of benzene–benzene junctions. CPL-
polymers (HBPs) composed of chiral units have also been based chirality induction has been reported also for other types of
reported.3 In addition, as newer chiral polymers with branch- polymers.16,17
ing, covalent organic frameworks (COFs) have been drawing
Suzuki–Miyaura cross coupling reactions of the core mono-
attention.4,5 The chiroptical properties of COFs arise mostly mer (C) with a linker monomer (L), namely, the reactions of 1,
from chiral constituent units except in a case where an optically 3,5-tribromobenzene with 1,4-bis(dihydroxyboranyl)benzene, 1,3,
active small molecule induced chirality.6 With such a back- 5-tribromobenzene with 4,4,0-bis(dihydoxyboranyl)-1,10-biphenyl,
ground, we herein report the preparation of optically active and 1,3,5-tris(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzene
COFs and HBPs composed of benzene-1,3,5-triyl core and with 4,400-dibromo-1,10:40,100-terphenyl led to poly(benzene-1,3,5-
oligo(benzene-1,4-diyl) linker units having preferred-handed triyl-alt-benzene-1,4-diyl) [poly(Bz135-alt-Bz14)], poly(benzene-1,3,5-
axial chirality (twist) around the benzene–benzene junctions triyl-alt-1,10-biphenyl-4,40-diyl) [poly(Bz135-alt-BP440)], and poly
induced by circularly polarized light (CPL). Preparation of (benzene-1,3,5-triyl-alt- 1,10:40,100-terphenyl-4,400-diyl) [poly(Bz135-
optically active COFs and HBPs based on chirality of light has alt-TP4400)], respectively, at different [L]/[C] ratios (Scheme 1).18–20
been unprecedented.
The polymerization products were composed of a tetrahy-
drofuran-(THF)-soluble part having network structure
a
a Institute for Catalysis (ICAT) and Graduate School of Chemical Sciences and
Engineering, Hokkaido University, N21W10, Kita-ku, Sapporo 001-0021, Japan.
E-mail: tamaki.nakano@cat.hokudai.ac.jp
b JEOL RESONANCE Inc., 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan
c Department of Integrated Science and Technology, Faculty of Science and
Technology, Oita University, Dannoharu, 700, Oita 870-1192, Japan
d Technical Division, Institute for Catalysis, Hokkaido University, N21W10, Kita-ku,
Sapporo 001-0021, Japan
e Department of Applied Chemistry, College of Engineering, Shibaura Institute of
Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan
f Integrated Research Consortium on Chemical Sciences (IRCCS), ICAT,
Hokkaido University, Sapporo 001-0021, Japan
(COF) and THF-soluble part having no cross linking (HBP)
(Tables S1–S3 in the ESI†). The apparent averaged molar
masses of the HBPs were less than about 1 ꢀ 103 (SEC, vs.
polystyrene) while selected samples indicated mean diameters
of ca. 16–80 nm in dynamic light scattering measurements
(Fig. S14 in the ESI†).
The three COFs were irradiated with CPL in fine powder form
sandwiched between two quartz, and poly(Bz135-alt-TP4400) COF
exhibited intense and clear CD spectra. The spectra obtained on
L- and R-CPL irradiation were almost mirror images (Fig. 1A and
Fig. S17 in the ESI†), indicating that the preferred-handed
twist conformation was induced to benzene–benzene junctions
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cc02671b
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approximated with the power function indicated within
Fig. 1B. Furthermore, the maximum gCD of +2.5 ꢀ 10ꢁ4 at
326 nm after saturation of CD intensity upon L-CPL irradiation
was much higher than expected for aromatic compounds on
the basis solely of CPL chirality. Aromatic compounds at 100%
ee generally show gCD values of 10ꢁ3 to 10ꢁ4 order,13,23 and gCD
values attainable based only on CPL without amplification are
of 10ꢁ6 to 10ꢁ8 order because the maximum ee values (gPSS
)
attainable at the photo stationary state follow the equation, gPSS
= 1/2 gCD(100%ee) ꢀ 100%, which leads to maximum gCD values
decided by the equation, gCD = 1/100 ꢀ gPSS ꢀ gCD(100%ee) = 1/2
(gCD(100%ee)).2,13,24 These results suggest that efficient chirality
amplification took place through CPL irradiation of the COF.
On the other hand, poly(Bz135-alt-BP440) COF showed much
weaker CD spectra, and poly(Bz135-alt-Bz14) COF did not
respond to CPL irradiation (Fig. S17 in the ESI†). Regarding
the differences among the three COFs, the three COFs showed
rather similar transmission electron microscopy (TEM) images
without any particularly regular structures (Fig. 2A, Fig. S15 in
ESI†) while poly(Bz135-alt-Bz14) COF showed clear electron
diffraction (ED) spots and the other two COFs did not
(Fig. 2B, Fig. S15 in ESI†). On the other hand, none of the three
COFs showed any diffraction patterns indicating molecular
ordering in XRD measurements (Fig. S16 in the ESI†). These
results may mean that poly(Bz135-alt-Bz14) COF has a short-
distance ordered packing of benzene rings detected by ED but
not by XRD. CPL irradiation may not induce effective rotation
for poly(Bz135-alt-Bz14) COF due to a rigid structural order.
Furthermore, solid-state NMR analyses were conducted to
probe the twisting motion in the COFs. The 13C CPMAS spectra
of the COFs (Fig. 3A) showed two main signals of benzene rings
corresponding to ‘‘C4’’ having no hydrogens and to ‘‘C3’’
having hydrogens at around 140 ppm and 127 ppm, respec-
tively. The three spectra showed slightly different patterns
which may reflect the difference in chemical structure among
the three COFs.
Scheme 1 Synthesis of COFs and HBPs composed of benzene rings and
chirality induction of them by CPL.
through twist-sense-selective excitation. The chemical structure of
the COF was found to be intact through CPL irradiation by IR
spectra (Fig. S10 in the ESI†). In addition, the induced chirality
was stable over months at ambient temperature.
The evolution in gCD spectra was accompanied by a decrease
in absorbance spectral intensity (Fig. S19 and S20 in the ESI†).
This observation is ascribed to a change in the average dihedral
angle around single bonds which has been clarified for biphe-
nyl molecules by combined theoretical and experimental
studies.21
CD intensity is discussed in terms of the Kuhn’s anisotropy
factor [gCD = 2(eL ꢁ eR)/(eR + eL) where eL and eR are molar
absorptivities toward left-handed and right-handed CPL’s,
respectively]22 to eliminate the influence of absorbance inten-
sity. Fig. 1B shows the plots of gCD observed on L-CPL irradia-
tion against relative irradiation energy sum at 326 nm whose
definition is found in the ESI.† The gCD-vs.-energy sum plot
does not show a first-order type relation (an increasing form of
exponential decay), and the early-stage data can be well
To investigate the twisting motion, we first investigated the
13C longitudinal relaxation time in rotating flame (T1rC) mea-
surements which is sensitive to kHz order molecular motion.
Three COFs indicated clear dependence of T1rC on the struc-
ture (Fig. S6 in the ESI†). The decay profiles of the C3 signals
were well fitted with double exponential functions leading to
shorter and longer relaxation times (T1rCfast, T1rCslow) while
Fig. 1 CD-UV spectra of poly(Bz135-alt-TP4400) COF (run 3 in Table S3-2
in the ESI†) observed on L-CPL (a) and R-CPL (b) irradiation [A] and
g
CD-vs.-relative irradiation energy sum plots with curve fitting results (inset
plots and corresponding equation) [B]. Maximum gCD was +2.5 ꢀ 10ꢁ4 at
Fig. 2 TEM image (A) and electron diffraction profile (B) of poly(Bz135-
alt-Bz14) COF (run 4 in Table S1 in the ESI†).
326 nm.
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slow
former COF than the other two. T1rC
of C3 and T1rC of C4
may reflect motions of the relevant carbons other than rotation
including changes in their relative orientation within the COF
structures which could occur in association with the rotation.
More direct motional information was obtained by ‘‘center
band only detection exchange’’ (CODEX) NMR experiments.25
The CODEX experiment detects slow molecular dynamics
resulting from changes in the orientation dependent chemical
shift frequencies during the mixing time, tm. The C3 signals
exhibited a pure exchange signal at a sufficient mixing time
(100 ms) which was not confirmed for the C4 signals (Fig. 3B).
This is direct evidence of reorientation of phenylene flip,
namely the twisting motion. The correlation time (tc), which
reflects molecular motion during mixing time, for C3 was
estimated from the pure exchange intensity as a function of
tm (Fig. 3C).25,26 The obtained tc for poly(Bz135-alt-TP4400) COF
(11 ms) was much shorter than for the other two COFs showing
similar values (119 ms, 121 ms), indicating that the twisting
motion of benzene-1,4-diyl units occurs much smoother in
poly(Bz135-alt-TP4400) COF than in the other two COFs. It has
been reported that rotation of bezene-1,4-diyl units in linear
poly(benzene-1,4-diyl-alt-ethene-1,2-diyl) occurs in the millise-
cond time scale (tc = 10 ms).25 The twisting motion in
poly(Bz135-alt-TP4400) COF may occur at a similar rate.
The observations discussed so far suggest that the rotation
around the chirality axes was much more smoothly activated by
photo excitation in poly(Bz135-alt-TP4400) COF than in the other
two in which greater steric crowdedness inside the frameworks
can hamper such a motion. The density of branching points
(core units) thus sensitively affects the freedom of inner rota-
tion in the COFs.
Soluble HBPs were also subjected to CPL irradiation in
solution-cast film (Fig. 4). All three HBPs exhibited clear,
mirror-image CD spectra upon L- and R-CPL excitation where
the chemical structures were found intact by IR spectra
(Fig. S11 in the ESI†), indicating that CPL has introduced a
preferred-handed twisted conformation.
The maximum gCD values were much greater than those
expected based only on CPL chirality, and the early-stage data
in the gCD-vs.-relative irradiation energy sum plots were well
approximated with the power functions shown in Fig. 4,
suggestive of chirality amplification. The chirality induction
efficiency was rather similar among the three HBPs while the
maximum gCD value and the exponent of the power function
Fig. 3 13C CPMAS NMR spectra of poly(Bz135-alt-Bz14) COF (run 4 in
Table S1 in the ESI†) (a), poly(Bz135-alt-BP440) COF (run 3 in Table S2, ESI†)
(b), and poly(Bz135-alt-TP4400) COF (run 3 in Table S3-2 in the ESI†) (c) [A],
13C CODEX spectra of poly(Bz135-alt-TP4400) COF with mixing time of
1 ms (i) and 100 ms (ii) including reference spectra (top), CODEX exchange
spectra (middle), and pure exchange spectra (bottom) [B], and normalized
pure exchange CODEX intensity of the C3 signals as a function of mixing
time for poly(Bz135-alt-Bz14) COF (a), poly(Bz135-alt-BP440) COF (b), and
poly(Bz135-alt-TP4400) COF (c) [C]. The fit curve in C is a stretched
exponential a*(1 ꢁ exp(tm/tc))b.
those of the C4 signals were well fitted with single exponential were slightly greater for the poly(Bz135-alt-Bz14) HBP than the
functions leading to one relaxation time (T1rC) (Fig. S7–S9 in other two, which may mean that the higher branching density
the ESI†). Considering that the C3 signals arise from the in the poly(Bz135-alt-Bz14) HBP is advantageous in chirality
carbons at the 2- and 3-positions of the bezene-1,4-diyl ‘‘linker’’ induction, which was not the case for the COFs.
units and those at the 2-, 4-, and 6-positions of the benzene-
The sharp contrast in chirality induction between the COFs
fast
slow
1,3,5-triyl ‘‘core’’ units, T1rC
and T1rC
probably reflect and the HBPs having the same chemical structures may mean
molecular motions of the former and the latter, respectively. that the COFs have a much more rigid structure based on the
fast
The most plausible motion contributing to T1rC
is twisting extended network than HBPs, and the rotation motion is much
(flipping) of the bezene-1,4-diyl units around the chirality axes, more restricted in the COFs. This is supported by the fact that a
and it was shorter for poly(Bz135-alt-TP4400) COF (1.9 ms) than longer irradiation time was necessary for poly(Bz135-alt-TP4400)
for the other two COFs showing similar values (3.4 ms, 3.0 ms), (up to 45 min) than for the HBPs (up to 10 min) to complete
indicating that the small-angle twisting motion is faster for the chirality induction.
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dynamics may provide deeper insights into the molecular
mechanisms of chirality induction by light. Furthermore, the
facile preparation of chiral COFs and HBPs using CPL may lead
to development of novel optically active, solid-state materials
with branching showing various chiral functions.
This work was supported in part by the MEXT/JSPS KAKENHI
Grant Number JP 19H02759 and in part by the JST grant No.
JPMJTMl9E4. The Technical Division of Institute for Catalysis,
Hokkaido University is acknowledged for technical support.
Conflicts of interest
There are no conflicts to declare.
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Fig. 4 CD-UV spectra observed on L-CPL (a) and R-CPL (b) irradiation
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