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Chemie
Accepted Article
Title: Planar Chiral Organoboranes with Thermoresponsive Emission
and Circularly Polarized Luminescence: Integration of
Pillar[5]arenes with Boron Chemistry
Authors: Pangkuan Chen, Jin-Fa Chen, Xiaodong Yin, Bowen Wang,
Kai Zhang, Guoyun Meng, Songhe Zhang, Yafei Shi, Nan
Wang, and Suning Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001145
Angew. Chem. 10.1002/ange.202001145
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http://dx.doi.org/10.1002/ange.202001145
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Planar Chiral Organoboranes with Thermoresponsive Emission
and Circularly Polarized Luminescence: Integration of
Pillar[5]arenes with Boron Chemistry
Jin-Fa Chen,
[a]
Xiaodong Yin,
[a]
Bowen Wang,
[a]
Kai Zhang,
[a]
Guoyun Meng,
[a]
Songhe Zhang,
[a]
Yafei
Shi,
[a]
Nan Wang,
[a]
Suning Wang,
[b]
and Pangkuan Chen*
[a]
Abstract: Enantiopure molecules based on macrocyclic architecture
are unique for applications in enantioselective host-guest recognition,
chiral sensing and asymmetric catalysis. Taking advantage of chiral
transfer from the intrinsically planar chirality of pillar[5]arenes, we
herein present an efficient and straightforward approach to achieve
early examples of highly luminescent chiral systems (P5NN and
P5BN). The optical resolution of their enantiomers has been carried
out via preparative chiral HPLC, which was ascribed to the molecular
functionalization of pillar[5]arenes with π-conjugated, sterically bulky
triarylamine (Ar
3
N) as an electron donor and triarylborane (Ar
3
B) an
acceptor. This crucial design enabled investigations of the chiroptical
properties, including circular dichroism (CD) and circularly polarized
luminescence (CPL) in the solid state. Furthermore, the
intramolecular charge transfer (ICT) nature in P5BN afforded an
interesting thermochromic shift of the emission over a wide range of
temperature.
Chirality represents one of the most important phenomenon in
nature due to a high relevance to biological mechanism, and a
better understanding of this can also significantly advance wide
applications in life, medicine and material science.
[1]
As a recently
burgeoning investigation of chiroptical materials, the circularly
polarized luminescence (CPL) is of particular interest in many
areas such as photonics, information technology, smart sensing
and 3D (bio)imaging.
[2]
The CPL technique is a measure of
differential emission of the excited states in chiral systems upon
the circularly polarized radiation, and it depends both on the
remarkable chirality and fluorescent property of materials.
[3]
Each
requirement in principle can be fulfilled via supramolecular
assembly (e.g. supramolecular chiral induction, chiral liquid
crystals, and aggregation-induced emission),
[4-6]
but sometimes
these systems likely suffer from low efficiency and strong
dependence on external stimuli. As a consequence, the design
and construction of CPL-persistent, structurally well-defined
organic dyes are essentially vital to the development of next-
generation functional materials.
The common approach to achieve robust CPL-active materials
is to covalently functionalize a chiral skeleton using highly
luminescent fluorophores. Several types of organic CPL materials
have been reported over years, such as axially chiral binaphthyls,
helicenes, and mechanically planar chiral cyclophanes.
[7-10]
Pillar[n]arenes that show size-dependent cavity structures are
recognized as a class of unique macrocycles with intrinsic planar
chirality due to the specific orientation of phenyl rings.
[11]
A variety
of advances have been derived from considerable efforts in
supramolecular chemistry and host-guest recognition of
pillararene since its first discovery by Ogoshi in 2008.
[12-16]
In
contrast, the remarkable promise in chiral scaffolds remains rarely
explored because of the dynamic rotation of phenyl subunits
against methylene bridges (-CH
2
-), leading to fast inversion of the
enantiomers (pS and pR) (Scheme 1). To tackle this challenge,
dramatically enhanced steric effects should be sought by
replacement of the alkoxyl groups on parent pillar[n]arenes with
sufficiently bulky substituents.
[17]
Scheme 1. Proof-of-concept design of planar chirality via the integration of
conjugated boron chemistry and macrocyclic pillar[5]arenes for circularly
polarized luminescence (CPL).
In view of the weakly emissive nature of non-substituted
pillar[n]arenes, a judicious choice of appropriate fluorophores can
greatly impact their CPL characters. To address this issue, some
highly emissive π-conjugated substitutions would preferably be
considered to incorporate electron donors and acceptors.
[18]
As a
typical electron-accepting functionality, triarylborane (Ar
3
B) has
been widely employed in luminescent materials and many other
applications.
[10,19,20]
Organoboranes are enabled to show unusual
tunable emissions and fully reversible optical responses once
they are electronically coupled with donors via π-conjugation.
[21]
[a] J.-F. Chen, Dr. X. Yin, B. Wang, K. Zhang, G. Meng, S. Zhang, Y.
Shi, Dr. N. Wang, Prof. Dr. S. Wang, Prof. Dr. P. Chen
Beijing Key Laboratory of Photoelectronic/Electrophotonic
Conversion Materials, Key Laboratory of Cluster Science of the
Ministry of Education, School of Chemistry and Chemical
Engineering, Beijing Institute of Technology of China
Beijing, 102488, China
E-mail: pangkuan@bit.edu.cn
[b] Prof. Dr. S. Wang
Department of Chemistry
Queen’s University
Kingston, Ontario, K7L 3N6 Canada
Supporting information and the ORCID identification number(s) for
the author(s) of this article is given via a link at the end of the
document.
10.1002/anie.202001145
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COMMUNICATION
Even though a few examples were reported to show pillararene-
based planar chirality,
[17]
to the best of our knowledge, the
analogues with CPL activity have not been experimentally
navigated. As one of our continuous pursuits of macrocycles
functionalized with boron chemistry,
[22]
we herein propose a facile
methodology to the first chiral luminescent molecules (P5NN and
P5BN) based on the easily accessible chirality transfer from
pillar[5]arenes. An axial functionalization of pillar[5]arene by π-
conjugated bulky fluorophores (Ar
3
B and Ar
3
N) would allow the
enantiomeric resolution of these systems. Meanwhile, this design
strategy can also serve purposes for the temperature-dependent
emission and CPL production.
Scheme 2. Structures and synthetic approach of rac-P5NN and rac-P5BN.
Figure 1. Crystal structures of (a) rac-P5NN and (b) rac-P5BN in elemental
colors (C: grey, O: red. 50% thermal ellipsoids). Distances (Å) are measured for
the cavity size of pillar[5]arenes and extension of bulky substitutions. (c)
Molecular packing in rac-P5NN with π-π stacking (d
= 3.28 Å) between
enantiomers. (d) Stacks in rac-P5BN via C-H…π interactions. Enantiomers are
highlighted in different colors. All the hydrogen atoms and solvent molecules are
removed for clarity.
The pillar[5]arene-based bifunctional triflate 1 (-OTf) as the key
intermediate was synthesized by a previously reported protocol.
[23]
Synthetic details were shown in Scheme 2 and the Supporting
Information (SI). A Suzuki coupling reaction of 1 with triarylamine-
containing boronic acid (2.0 equiv) gave P5NN in 76% yield.
P5BN was obtained very similarly via two-step reactions with Ar
3
N
followed by the Ar
3
B precursor under refluxed conditions in an
overall yield of 20%. These molecules were fully characterized
through
1
H,
13
C and
11
B NMR and HRMS spectroscopy after
standard workup and analytical purification.
Single crystals of rac-P5NN and rac-P5BN were grown by slow
evaporation from acetone, and their solid state structures were
examined by X-ray diffraction analysis. On equatorial position,
they clearly display well-defined cavities of the pillar[5]arenes,
and they are axially extended through π-conjugation with N donor
and B acceptor end-capped (Figure 1a and 1b). They show similar
angles (~64) of the π-extension against the pillar[5]arene plane
defined by five -CH
2
groups. A more sufficient π-conjugation was
observed in P5BN, as indicated by smaller dihedral angles
(57and 59) than those of 70 and 64 for P5NN within the
terphenyl units. Molecular size determination revealed that the
distance from centroid of the functionalized phenyl ring to the Ar
3
N
periphery (d1 = 10.70 Å) is greater than the cavity diameter of
P5NN (d = 9.26 Å). The π-substituents in P5BN (d1 = 10.12 Å, d2
= 11.56 Å) were also much larger than its cavity size of d = 9.11
Å (Figure 1 and S11). Thus, these conjugated substitutions have
been demonstrated to be bulky enough to circumvent possible
racemization of both molecules. Equimolar enantiomers of pR-
P5NN and pS-P5NN are contained in a unit cell with a face-to-
face π-π stacking (d = 3.28 Å). C-H…π interactions lead to the
supramolecular packing of rac-P5BN. Stacks in both cases adopt
the same alignment of homo- and heterochiral molecular packing
oriented in two directions (Figure 1c, 1d).
UV/Vis absorption and emission spectra were recorded for
racemic P5NN and P5BN in THF, and the data of electronic
structures are summarized in Table 1. They exhibit a strong
absorption profile of π-π* transition at ~305 nm. The absorption
spectrum of P5BN also shows a shoulder peak red-shifted to 330
nm due to an intramolecular N/B charge transfer (ICT). The optical
energy gaps (E
g
) were estimated to be 3.49 and 3.41 eV for P5NN
and P5BN, respectively, and are close to the computational
results (Table S5). Both molecules show strong emissions in the
solid state as well as in solution (P5NN: Φ
L
= 0.61, Φ
S
= 0.19, τ =
1.2 ns; P5BN: Φ
L
= 0.99, Φ
S
= 0.57, τ = 6.4 ns), which is particularly
useful for potential applications in light-emitting devices (Figure
2a). These highly enhanced emissions are in sharp contrast to the
parent pillar[5]arenes with much lower fluorescence quantum
efficiency (Φ
fl
˂ 0.01), a result of the introduction of Ar
3
B and Ar
3
N
as key π-conjugated fluorophores. In addition, P5BN displayed a
considerably broad emission spectrum (λ
em
= 493 nm) and a larger
Stokes shift of approximately 12,700 cm
-1
in comparison to that of
7,200 cm
-1
for P5NN.
10.1002/anie.202001145
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COMMUNICATION
Figure 2. (a) UV−vis absorption and emission spectra of rac-P5NN (λ
ex
= 306
nm) and rac-P5BN (λ
ex
= 303 nm, THF, c = 1.0 × 10
−5
M). Inset: photographs
showing the emission colors of rac-P5NN and rac-P5BN in THF under 365 nm
UV irradiation. (b) Key electronic transitions contributing to vertical excitations
of pS-P5BN (TD-DFT, B3LYP/6-311G**) as an example with molecular orbital
plots (iso=0.02, DFT-B3LYP/6-31G*).
Table 1. The photophysical and computational data of P5NN and P5BN.
λ
abs
[a]
(nm)
λ
em
[a]
(nm)
Φ
L
[b]
(%)
Φ
S
[c]
(%)
τ
ave
[d]
(ns)
E
HOMO
[e]
(eV)
E
LUMO
[e]
(eV)
P5NN
306
393
61
19
1.2
‒4.88
‒0.64
P5BN
303
493
99
57
6.4
‒4.89
‒1.67
[a] Recorded in THF (1.0 × 10
−5
mol/L) at room temperature. [b] Fluorescence
quantum yield (Φ
L
) measured in THF. [c] Φ
S
measured in solid state. [d]
Average fluorescence lifetime in THF. [e] HOMO and LUMO energy levels
obtained by DFT calculations (B3LYP, 6-31G*).
The electronic π-conjugated characteristics calculated for
P5NN and P5BN using DFT (B3LYP/CAM-B3LYP, 6-31G*) and
TD-DFT (B3LYP, 6-311G**) are presented in Figure 2b and the
SI. According to TD-DFT studies, the absorption of P5NN is
mainly attributed to the π-π* transition to S
1
state (HOMO-
1→LUMO, f = 0.8991) of the axially delocalized π system. On the
other hand, P5BN indeed represents another distinct example of
π-extended ambipolar species with a macrocyclic decoration. All
of the lowest electronic transitions to S
1
, S
2
, S
3
and S
4
are derived
from vertical excitations to the B-centered LUMO orbital either
from the N-centered HOMO-2 or pillar[5]arene-localized HOMO,
HOMO-1 and HOMO-3 orbitals, featuring a typical charge transfer
character of transition. Cyclic voltammetry (CV) of P5BN showed
a reversible one-electron reduction band with half-wave potential
at E
1/2
= -2.49 V (vs Fc
+
/Fc, in THF) as well as multi-step oxidation
curves with the first half-wave potential at +0.59 V in CH
2
Cl
2
(Figure S13), leading to an electrochemical HOMO-LUMO gap
(3.08 eV) consistent well with the DFT calculation (3.22 eV). As
expected, the first electrochemical oxidation of N donor occurred
at a slightly lower potential of +0.52 V in P5NN.
Figure 3. Top: CD spectra of enantiomers for P5NN and P5BN after optical
resolution in THF (c = 1.0 × 10
−5
M); Bottom: solid-state CPL spectra of
enantiomers for P5NN (λ
ex
= 306 nm) and P5BN (λ
ex
= 303 nm) at r.t. Negative
(-) and positive (+) signs correspond to the first and second peak in HPLC traces,
respectively.
As indicated in the X-ray structures of rac-P5NN and rac-P5BN,
we envisaged that the configuration of both enantiomers could be
sufficiently maintained by bulky, rigid π-conjugated substituents.
Motivated by this fact, we have performed optical resolution of the
racemic mixtures for further investigation. The initial injection of
rac-P5NN into a chiral HPLC with a Daicel Chiralpak IA-3 column
(hexanes/isopropanol = 95 / 5, v/v) resulted in two separated
peaks with 1:1 area (Table S4). The single peak position was
reproduced in each collected fraction, indicative of a reasonably
good isolation of pR-P5NN and pS-P5NN with an enantiomeric
excess (>96% ee, Figure S17). In rac-P5BN, a similar procedure
gave rise to a pair of sufficiently well-separated enantiomers (>98%
ee) via preparative chiral HPLC with IG-3 column (Figure S18).
Isolation of the enantiomers was confirmed by the mirror images
in circular dichroism (CD) spectra with opposite Cotton effects in
THF (Figure 3). The CD dissymmetry factors (g
abs
, defined as Δɛ/ɛ
at 306 nm) were calculated to be 6.68 × 10
−6
and 1.32 × 10
−5
for
P5NN and P5BN, respectively. The first peak with shorter
retention time in HPLC traces and the second one were assigned
to be negative (-) and positive (+) sign, respectively (see the SI
and Table S4). Each pair of enantiomers exhibited an almost
mirror-symmetric CPL spectrum in the solid state, and the g
lum
values were determined to be -2.02 × 10
−4
for (-)-P5NN at 398 nm
10.1002/anie.202001145
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COMMUNICATION
and -8.78 × 10
−4
for (-)-P5BN at 455 nm. The CPL properties
demonstrated in these solid-state materials would be valuable for
future uses in chiral luminescence devices. In solution phase,
enantiopure P5BN showed CPL activity only in hexane at room
temperature (Figure S19b), and no CPL signal was observed for
P5NN in common organic solvents. These observations are
supposed to indicate that type of luminogens and solvent media
also take an important role in the production of CPL emission.
[9d]
Figure 4. (a) Temperature-dependent emission spectra of rac-P5BN recorded
in 2-methyltetrahydrofuran between 130 and 345 K (c = 5.0 × 10
−5
M, λ
ex
= 303
nm); (b) Photograph shown in an NMR tube and CIE color coordinates; (c) Data
fitting of the emission (λ
max
) dependence on T for rac-P5BN, and (d) reversible
emission modulation with cycling T changes for rac-P5BN.
To further explore the potential applications of P5BN as smart
materials in response to external stimuli, we first monitored the
photoluminescence at different temperatures. As shown in Figure
4, rac-P5BN experienced a bathochromic shift in the emission
spectra as temperature decreased between 345 and 130 K, and
a concomitant emission color change from blue to yellow occurred
in the low melting point solvent 2-methyltetrahydro-furan. The
emission wavelength was linearly enhanced with a high
correlation coefficient of 0.992. Furthermore, a fully reversible
emission response was corroborated by an excellent fatigue
resistance without emission degradation upon its exposure to 5
cycles of temperature change over a broad range of ~215 K
(Figure 4d). This thermochromic shift of the emission was likely
induced by the temperature-dependent stabilization of ICT
because the solvent properties show temperature dependence as
well.
[10a, 24]
Compared with similarly well-established fluorescent
thermometers,
[10a,19g,21b]
the emission responses of P5BN at low
temperature of ‒90 C are particularly distinct.
Additionally, P5BN
showed a gradual emission quenching in response to fluoride ion
titration (Figure S15 and S16), similar to that frequently observed
in other triarylboranes due to the loss of electron-deficiency of B
caused by anion binding with F
−
ion as a Lewis base. On account
of multiple characteristics such as macrocyclic host, ambipolar
donor-acceptor and tunable emission integrated in one molecule,
we anticipate that the enantiopure analogues of P5BN could be
applied to chiral separation of guest species and CPL sensing of
temperature, media and electron donors.
In summary, we have proposed a facile and highly efficient
strategy to achieve well-defined planar chiral luminescent
systems by taking advantage of pillararenes and boron chemistry.
Incorporation of the sterically bulky, rigid and π-conjugated
fluorophores (Ar
3
B and Ar
3
N) has enabled the optical resolution
of pR/pS isomers. Other than the chiral transfer from pillar[5]arene,
this approach also offered strong persistent luminescence in
P5NN and P5BN. The chiroptical properties of CD and solid-state
CPL emission were acquired, indicating a remarkable promise of
this modular design. Meanwhile, P5BN showed a unique
temperature dependence of the emission due to the ICT character
in the macrocyclic scaffold. These key findings provide an early
proof of principle for future applications in chiral luminescent
sensing, chiral supramolecular assembly (e.g. host-guest
recognition, 3D network gelation) and CPL-based electronic
devices. Systematic investigations of robust materials with large
g
lum
value will be pursed in our laboratory based on sophisticated
planar chiral organoboranes by structural tuning of other type of
functionalities.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC) (No. 21772012). We thank the
Analysis & Testing Centre at Beijing Institute of Technology for
advanced facilities.
Conflict of interest
The authors declare no conflict of interest
Keywords: pillar[5]arene • organoborane • circularly polarized
luminescence • planar chirality • thermochromic emission
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