(: R)-Binaphthyl derivatives as chiral dopants: Substituent position controlled circularly polarized luminescence in liquid crystals

Article abstract of DOI:10.1039/c9cc02253h

A series of nonbridged axially chiral binaphthyl derivatives were synthesized and used as chiral dopants for chiral nematic liquid crystals (N?-LCs). It was found that binaphthyl derivatives substituted at 3,3′ positions or 2,2′ positions of binaphthyl rings could induce N?-LCs to show opposite circularly polarized luminescence (CPL) signals, despite having the same configuration. Additionally, a CPL switch could be constructed in these N?-LCs regulated by the applied electric field.

Full text of DOI:10.1039/c9cc02253h

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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DOI: 10.1039/C9CC02253H
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(
R)-Binaphthyl Derivatives as Chiral Dopants: Substituent Position
Controlled Circularly Polarized Luminescence in Liquid Crystals
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaohui Gao,ab Xujin Qin, Xuefeng Yang, Yuangang Li, and Pengfei Duan
b
b
*a
*b
DOI: 10.1039/x0xx00000x
www.rsc.org/
A series of nonbridged axially chiral binaphthyl derivatives were
So far, it has been elucidated that binaphthyl derivatives
synthesized and used as chiral dopants for chiral nematic liquid have a good miscibility towards N-LCs as well as large helical
crystals (N*-LCs). It was found that binaphthyl derivatives twisting power which can control the screw sense (helical
1
1
substituted at 3,3’ positions or 2,2’ positions of binaphthyl rings, sense) and strength of N*-LCs . On the other hand, binaphthyl
1
2
could induce N*-LCs to show opposite circularly polarized derivatives are ease of synthesis, easy to modify and low cost .
luminescence (CPL) signals, despite having the same configuration. Therefore, they are not only useful for asymmetric reaction
Additionally, CPL switch could be constructed in these N*-LCs solvents to produce chiral conducting polymers, but also are
1
3
regulated by applied electric field.
used as dopants of achiral liquid crystals . By modifying the
chemical structures of binaphthyl derivatives to control the
screw sense of N*-LCs have been reported before. For
example, the mono-and di-substituted binaphthyl derivatives
with the same configuration gave opposite screw senses in N*-
In recent decades, the chiroptical materials with circularly
polarized luminescence have attracted considerable attention
due to their promising applications in 3D imaging , high-
1
2
3
4
security systems , CPL laser ， biological probe , enantio-
8a
LCs, though they both had the bridged structures . Another
example is that nonbridged and bridged binaphthyl derivatives
added into N-LCs leading to opposite screw senses, though
they had the same configuration . However, by changing the
positions of substituents of binaphthyl derivatives with the
same configuration to control the CPL signals in liquid crystal
has never been involved. In addition, chirality switching
bestows much potential applications in CPL-active materials
and electro-optic displays.
5
6
selective CPL sensors or electro-optic display . Besides, CPL
provides a probe of configuration and conformation in the
emitting state of chiral luminescence systems. The magnitude
of circular polarization in the excited state is quantified by the
14
L R L R L R
dissymmetry factor glum=2(I -I )/(I +I ), where I and I denote
the intensities of the left and right circularly polarized light,
respectively. However, the level of CPL for most chiral organic
molecules are much smaller in solvents or in aggregation state
−5
−2 7
(
|glum| = 10 −10 ) . To develop CPL-active materials with high
Herein, according to rational molecule design, a kind of
aggregation-induced emission (AIE) chiral binaphthyl
derivatives were synthesized, where substituents were
introduced at 3,3’ positions or 2,2’ positions at binaphthyl
rings, abbreviated as R/S-1 and R/S-2, and used as chiral
dopants for N*-LCs. The commercially available liquid crystal
medium 5CB (4-Cyano-4'-n-pentylbiphenyl), shows a N-LC
phase within the range of 24-35 °C, which was used as host
liquid crystalline compound. The N*-LCs samples were
prepared by adding a small amount of chiral dopant into liquid
crystal. We took insight into the chiroptical properties of the
N*-LCs. It was found that, as chiral dopants, R-1 and R-2 could
induce N*-LCs with left-handed and right-handed CPL,
respectively, though having the same (R)-configuration (Fig. 1).
We inferred that the opposite CPL signals of N*-LCs should be
assigned to the different dihedral angle of binaphthyl
derivatives, which could be explained in terms of substituent
positions modifying the molecule structures. Additionally, the
g
lum, doping chiral chromophores into achiral nematic liquid
8
crystals is one of the most effective approaches . Akagi’s group
have reported one kind of chiral liquid crystal polyacetylene
devitatives, which exhibited CPL activity with high glum
Recently Cheng et al. have reported chiral liquid crystals, which
showed aggregation-induced circularly polarized luminescence
9
.
1
0
(AI-CPL) with an extremely large glum value up to 0.41 .
a. College of Chemistry and Chemical Engineering，Xi’an University of Science and
Technology ， No.58 ， YanTa Road ， Xi’an 710054 ， P. R. China. E-mail:
liyuangang@xust.edu.cn
b.
CAS Center for Excellence in Nanoscience，CAS Key Laboratory of Nanosystem
and Hierarchical Fabrication ，Division of Nanophotonics ，National Center for
Nanoscience and Technology (NCNST)， No. 11 ZhongGuanCun BeiYiTiao，
Beijing 100190，P. R. China. E-mail: duanpf@nanoctr.cn; Tel: +86-10-82545510
†
Electronic Supplementary Information (ESI) available: Synthesis of dopants，
photophysical measurements and characterizations of N*-LCs. See
DOI: 10.1039/x0xx00000x
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effect of direct current (DC) electric field on the N*-LCs was CPL spectrum of R-1 and gave Cotton effects wViietwhAortipclpe oOsnilitnee
also investigated. Finally, CPL switch could be achieved in the sign to that of S-2. The opposite CPDLOsI:ig10n.a10ls39/imC9pClCie0d225th3He
induced N*-LCs by applying with a DC electric field.
different screw direction of N*-LCs. The chirality was also
confirmed via circular dichroism (CD) spectra, which measures
the differential absorption of left-handed and right-handed
circularly polarized light . Moreover, the signal of the CD
spectra is related to the dihedral angle (θ) of binaphthyl rings.
The R-1 and R-2 displayed negative Cotton effects either
1
7
dissolved in CH
the same conformation in isotropic solvent or at the film state
Fig.S4, ESI†). Unfortunately, CPL signals of the thin films
2 2
Cl or formed thin films, indicating they had
(
couldn't be detected yet, which might be due to the
amorphous phases. As shown in Fig. 2e and 2f, N*-LC doped
with R-1 showed a distinct Cotton effect with a positive
Fig. 1 Illustration of the helical structures of N*-LCs including
each chiral dopant with the same (R)-configuration. The
molecular structures of binaphthyl derivatives used as dopants
and the used liquid crystal.
The synthesis procedures of R/S-binaphthyl derivatives
where the substituents were introduced at 3,3’ positions or 2,2’
positions of binaphthyl rings, were synthesized via the Suzuki-
Miyaura cross-coupling and Knoevenagel reactions according
1
5
to the reported literatures . The chemical structures, detailed
synthesis processes and characterizations were described in
the electronic supplementary information (Fig.S1, ESI†). First,
the photo-physical properties of binaphthyl derivatives were
investigated. The fluorescent emission spectra of R-1 and R-2
2
were carried out in THF/H O mixed solution, which showed
typical AIE phenomena (Fig.S2, ESI†). It is well known that
Tang’s group demonstrated that AIE molecules are weakly
fluorescent or non-fluorescent in the solution state because of
the intramolecular rotation consuming energy from the
1
6
excited state . The CPL signals of R-1 and R-2 were silent in
the polar solvents. The chiral nematic liquid crystals consisting
of a host nematic liquid crystal (5CB) and each chiral dopant in
different concentrations were prepared. The fluorescence
intensities of N*-LCs samples showed enhancement as the
concentration increased. Meanwhile, the CPL level of N*-LCs
decreased slightly (Fig.S3, ESI†). These results indicated that
the AIE effect played a key role not only on the emission
intensity but also on the CPL degree of N*-LCs. The fluorescent
emission spectra and CPL spectra of N*-LC doped with 0.5 wt%
R-1 and N*-LC doped with 0.5 wt% R-2 in liquid crystal cells at
room temperature, were also measured (Fig. 2). The stop band
of N*-LCs induced by R-1 and R-2 couldn’t be detected in
visible light region, and N*-LCs were almost transparent
outside the stop band (inset pictures in Fig. 2). At the same
time, R-1 was found to induce N*-LC exhibited positive CPL
signal with blue emission (450 nm) (Fig. 2c). However, R-2
induced N*-LC showed negative CPL signal with yellow light
Fig. 2 (a) Fluorescence spectra of N*-LC doped 0.5 wt% R-1
excited at 340 nm; (b) Fluorescence spectra of N*-LC doped
0
0
.5 wt% R-2 excited at 360 nm; (c) CPL spectra of N*-LC doped
.5 wt% R/S-1 excited at 340 nm; (d) CPL spectra of N*-LC
doped 0.5 wt% R/S-2 excited at 360 nm. (e) CD spectra of the
N*-LC doped 0.5 wt% R/S-1; (f) CD spectra of N*-LC doped 0.5
wt% R/S-2.
signal. R-2 was found to induce N*-LC with a negative Cotton
effect. These results demonstrated that R-1 and R-2 had
different conformations in liquid crystal. To further investigate
the origins of opposite CPL signals, the screw senses of N*-LCs
films were investigated under polarized optical microscopy
(POM). As described in Fig. 3, N*-LC induced by 0.5 wt% R-1
and 0.5 wt% R-2 showed a clockwise screw direction and
counterclockwise screw direction in LC, respectively. The screw
senses of N*-LCs were coincided with the signals of CPL and CD.
It is clear that the screw sense of N*-LCs is determined by the
(560 nm) (Fig. 2d). Moreover, S-1 exhibited a mirror image of
2
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stereospecific intermolecular interactions between the substituents were directly linked at 3,3’ positions at binaphthyl
View Article Online
DOI: 10.1039/C9CC02253H
binaphthyl moieties of chiral dopants and the surrounding N-
LC molecules. The former reports have demonstrated that the
helicity of binaphthyl derivatives depend on the dihedral angle
(
θ) of binaphthyl rings. (R)-binaphthyl derivatives have (R)-
cisoid conformation (0°< 90°) and (R)-transoid
conformation (90° < θ < 180°) corresponding to (M)-helicity
θ
<
1
8
and (P)-helicity, respectively . As the dopants, the R-1 and R-2
took an M-helicity and P-helicity in N-LCs, respectively, which
leading to oppositely screwed N*-LCs. Therefore, the reason of
the opposite CPL signals of the induced N*-LCs were caused by
1
9
the difference in dihedral angle .
rings leading to more rigid structure and R-2 had larger steric
Fig. 3 Polarized optical micrograph (POM) images of N*-LC
films including (a) 0.5 wt% R-1 ， (b) 0.5 wt% R-2 at room
temperature.
Fig.4 (a) and (b) CPL spectra of N*-LCs including 0.5 wt% R-1
and 0.5 wt% R-2 under the electric field “off” and “on”,
respectively; (c) and (d) intensity of the CPL glum value of N*-LC
including 0.5 wt% R-1 and 0.5 wt% R-2, respectively, against
the repeated electric field“off” and “on” cycles.
The helical pitch and the helical twisting power (β
w
) of N*-
LCs doped with each dopant were evaluated via the
2
0
Grandjean-Cano method and were summarized in table S1.
The Grandjean-Cano lines could be observed in the POM
images which indicated the formation of thermotropic chiral
N*-LCs phases (Fig.S5, ESI†). The helical twisting power of R-1
effect at 2,2’ positions leading a restriction in the internal
rotation of binaphthyl rings. Therefore, the opposite CPL
signals of N*-LCs arose from the difference of dihedral angle.
Recent research interests have focused on the
optoelectronics luminescent devices where the CPL is
-1
-1
and R-2 were calculated to be around 26.0 um and 15.6 um ,
respectively. The twisting power of R-1 was about 1.7 times
larger than that of R-2, which might be owing to the difference
in the conformation.
2
1
involved . To further take insight into the CPL-active materials
applications, a liquid crystal (LC) device was constructed on a
conductive ITO glass substrate. It was found that CPL signals of
the N*-LCs induced by this kind of AIE-active dye molecules
could be detected when a DC electric field was turned off,
which would represent the ‘‘on-state’’ of the system (Fig. 4a).
When a DC electric field (8 V) was applied to the LC cell, there
was no CPL signal, which would represent the ‘‘off-state’’ of
the system described in Fig. 4b. Interestingly, N*-LCs exhibited
almost the same degree of CPL when the DC electric field was
turned off again. The calculated value of the dissymmetry
factor (|glum|) of the N*-LCs doped with 0.5 wt% R-1 and 0.5
wt% R-2 were around 0.25 and 0.27, respectively. Meanwhile,
the glum values showed stable repeatability after several cycle
treatments (Fig. 4c and 4d). Therefore, we successfully
achieved an electric-field induced CPL switch that was
reversible and repeatable over multiple cycles. The POM
images of N*-LCs with electric field “ off” and “on” were
examined. As a result, the characteristic Grandjean texture of
N*-LC phases were observed without applied electric field.
Applying a DC electric field (8V), the characteristic of nematic
phases disappeared (Fig.S7, ESI†). This indicated that the
The finger-printed textures of N*-LCs and high glum values of
CPL were owing to the intermolecular interactions, including
the π-π interactions between binaphthyl derivatives and
surrounding liquid crystal molecules and dipolar interaction
from cyano groups. As demonstrated above, N*-LCs induced
by R-1 and R-2 showed opposite screw senses. The structural
difference between R-1 and R-2 was that the substituents
were linked at the 3,3’ positions and 2,2’ positions of
binaphthyl rings, respectively. In order to clarify the
relationship between the screw directions of N*-LCs and the
substituents positions of non-bridged binaphthyl derivatives,
we have particularly synthesized R/S-3 molecules which have
the same substituent position for comparison (Scheme S3).
The result of N*-LCs induced by R-3 was in accordance with
that of N*LCs induced by R-1 (Fig.S6, ESI†). This further
indicated that the screw direction of N*-LCs was dominated by
the substituent position of non-bridged binaphthyl derivatives.
It has been reported that the chirality of non-bridged
binaphthyl derivative depends on the change of the dihedral
angle between two binaphthyl rings. It is expected that the
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Conflicts of interest
The authors declare no conflict of interest.
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