Transfer of Chiral Information from Silica Hosts to Achiral Luminescent Guests: a Simple Approach to Accessing Circularly Polarized Luminescent Systems

Article abstract of DOI:10.1002/cplu.201900615

Systems that show circularly polarized luminescence (CPL) are usually constructed in one of two possible ways: either covalently binding the chiral moieties (usually organic compounds) to luminophores (inorganic or organic compounds) or associating the luminophores as guests with chiral hosts (usually organic compounds). Herein, we propose inorganic-based CPL-active systems constructed by the “chiral host-luminescent guest” strategy, in which silica acts as a chiral host to endow various luminescent guests with CPL. The chiral silica was modified by silane coupling with amino or phenyl groups to allow interaction with luminescent guests, and then used in combination with acidic achiral dyes, lead-halide type perovskites, and aggregation-induced emission luminogens (AIEgens). Interestingly, when these achiral guests were noncovalently confined in surface-modified chiral silica, the guests showed chiroptical behavior in the circular dichroism (CD) spectra, and thus became CPL active, even though they are not inherently chiral. The surface functional groups on the silica play very important roles in transferring the chiral information from the silica to the guests. This work provides a new concept for constructing CPL-active systems using inorganic materials as a chiral source.

Full text of DOI:10.1002/cplu.201900615

Accepted Article
Title: Transfer of Chiral Information from Silica Host to Achiral
Luminescent Guests: A Simple Approach to Accessing Circularly
Polarized Luminescent Systems
Authors: Ren-Hua Jin, Seiji Tsunega, Takuya Nakashima, and
Tsuyoshi Kawai
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10.1002/cplu.201900615
Transfer of Chiral Information from Silica Host to Achiral Luminescent Guests:
A Simple Approach to Accessing Circularly Polarized Luminescent Systems
[
a]
*[a]
[b]
[b]
Seiji Tsunega, Ren-Hua Jin, Takuya Nakashima, Tsuyoshi Kawai
[
a] S. Tsunega, Prof. R.-H. Jin,
Department of Material and Life Chemistry,
Kanagawa University,
3
-2-7 Rokkakubashi, Yokohama 221-8686, Japan
E-mail: rhjin@kanagwa-u.ac.jp
b] Prof. T. Nakashima, Prof. T. Kawai
[
Division of Materials Science,
Nara Institute of Science and Technology,
Ikoma, Nara 630-0192, Japan
1
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10.1002/cplu.201900615
Abstract
In constructing CPL-active systems, two feasible ways often conducted: designing the
chiral moieties covalently bonded to luminophores or associating the guest of luminophores to
a certain chiral host. Whatever the luminescent compounds were alternatively organics or
inorganics, chiral sources in CPL materials were commonly from organics. Herein, we
propose tolerant inorganic-based CPL-active systems based on the “chiral host – luminescent
guest” strategy, in which the inorganic silica itself is a chiral host to endow various
luminescent guests with CPL-feature. The chiral silica were firstly modified by silane coupling
agencies with amino or phenyl groups to bear the interaction sites with luminescent guest and
then used in combinations with acidic achiral dyes, lead-halide typed perovskites, and
aggregation-induced emission luminogens (AIEgens). Interestingly, these achiral guests
confined in surface-modified chiral silica through the non-covalent interactions behaved
chiroptically in the circular dichroism (CD) spectra, and thus became CPL-active even though
they were not inherently chiral. It was found that the surface functional groups on the silica
play very important roles to transfer the chiral information from the silica to the guests. This
work provides a new concept for constructing CPL-active systems using inorganic materials
as a chiral source.
2
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10.1002/cplu.201900615
Introduction
Recently, the development of circularly polarized luminescence (CPL)-active materials,
which can produce left- or right-handed circularly polarized light in their luminescent transition
process, has been drawing attention due to its potential application in display techniques,[1-3]
optical devices,[4-7] probing and sensor materials,[8-11] and even catalysts for asymmetric
synthesis.[12-13] In comparison with the physical method which uses a linear polarizer and
quarter-wave plates, circularly polarized light could be directly generated by chiral
luminescent materials without a mass of energy loss during the conversion process. In order
to achieve CPL-active materials, many efforts have concentrated on designing both chirality
and luminescent centres and can be classified into two strategies. The first way is designing a
chiral moiety covalently bonded to a luminophore. Thus, the CPL-property could be found in
the chiral organic dyes,[14,15] polymers,[16,17] metal complexes,[18,19] and supramolecular
systems.[20,21] However, such a case sometimes needs a tediously long synthesis process,
and the CPL-property of the obtained chiral luminophores is unpredictable. The second way is
the guest-host systems in which the luminescent centres (guests) often are achiral. However,
after encapsulation or in site formation of the guest in a certain chiral host, the guest can be
activated to be chiroptical due to induced effect and hence become CPL active. In comparison
to the first way, the second way is very handy to construct the CPL systems with a wide color
gamut and adjustable intensities as a choice from the diverse combinations of both
luminescent guests and chiral hosts. The recent works demonstrated that supramolecular
assemblies based on chiral organo-gelators could be promising chiral hosts to bestow the
CPL-feature to the achiral guest luminophores.[22-25] For instance, Liu and co-workers reported
that the glutamic acid-based lipids could form chiral nanotube in the presence of various
luminophores such as luminescent dyes,[26] aggregation-induced emission luminogens
(
AIEgens),[27] and semiconductors (quantum dots, lead halide perovskites).[28,29] Consequently,
3
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10.1002/cplu.201900615
the supramolecular chirality was transferred to the encapsulated guest luminophores, which
could enable them to be CPL-active. In addition to this, chiral capping agents[30-31] and
solvents,[32] protein nanocages,[33] and liquid crystals[34-36] have been investigated as a
candidate of chiral host to induce CPL-activity from guest luminophores. Although there are
lots of reports on organic and/or inorganic achiral luminophores with an outstanding
luminescent property, chiral host materials for the CPL-active system mostly come from chiral
organics. In the field of chiral materials, although there are great achievements in the
development of inorganic chiral materials,[37-40] its potential to a chiral source for the CPL
system has been rarely explored. Herein, we propose a new concept for the architecture of
solid-state CPL-active materials based on host-guest systems employing the inorganic chiral
silica as a chiral host, which can force diverse achiral luminophores (guests) to become
CPL-active. In this system, it is not necessary to utilize asymmetric assistances of chiral
organics, but tailoring the chiral silica surface by the chemically or physically modification is
implemented to encapsulate or attach guest luminophores into chiral silica.
In our previous work, we have reported a unique method to prepare the nanofiber-based
chiral silica and its potential as a chiral source to transfer its chirality to other organic and/or
inorganic materials.[41,42] It was demonstrated that linear polyethyleneimine (PEI), possessing
only secondary amine units (-CH
2
CH NH-) in its backbone, can be complexed with chiral
2
tartaric acids (tart) in water and then self-assembled into crystalline aggregates (PEI/tart) with
nano-fibrous structures. In these crystalline aggregates, the amine (-NH-) and carboxylic
+
-
(
O=C-OH) groups bind as a charged pair -HNH ... O-C=O to result in the chiral catalytic sites
which promote the hydrolytic polycondensation of alkoxysilanes. Thereby, the
polycondensation of silane was stereochemically controlled by the surface of chiral crystalline
complexes, and hence the asymmetrically centred Si would be selectively formed in the
framework of polysilicates.[42-44] By taking advantage of the peculiar chiral environment of the
4
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silica, recently, we have developed the chirality transfer systems from the silica to other
materials such as metallic NPs,[45] metal oxides,[46] and even cross-linked polymeric
materials[47,48] through the in-situ synthesis around the chiral silica. As one example, very
recently, we reported on the CPL-active lanthanide oxides encapsulated in the chiral silica.[46]
Luminescent sub-10 nm lanthanide oxides (Eu
2
O
3
2
or Tb O
3
) nanoparticles (guest) were in-situ
o
formed in the chiral SiO
chiral SiO
2
nanofibres (host) through high-temperature calcination (900 C) of
hybrid nanofibres trapping Eu3+ (or Tb ) ions. Interestingly, these lanthanide
3+
2
oxides not only displayed optical activity in the solid-state CD spectra but also showed CPL
signals around their emission bands. This is the first report on inorganic-based CPL-active
systems by host-guest strategy employing the silica as a chiral host. This motivated us to
expand the chiral silica-based CPL systems to not only lanthanoid oxides but also diverse
achiral luminophores. In such a case, the key is the surface modification of the silica by which
the surroundings of silica support binding pockets to enclose the guest luminophores. Based
Scheme 1. The preparation of the chiral silica and its application to chiral
host materials to endow achiral luminophores with CPL-feature.
on these perspectives, this work can be categorized into three sections to construct chiral
silica-based CPL-active systems (Scheme 1): In the first, carboxylic type porphyrin, pyrene,
5
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10.1002/cplu.201900615
and fluoresceine were complexed with chiral silica whose surface was physically or
chemically modified by amine residues; In the second, the luminescent lead halide perovskite
nano-crystal consisted of methylammonium and lead-bromide components was in-situ formed
in the chiral silica bearing the primary amine groups; In the third, AIEgens of
tetraphenylethylene (TPE) aggregates were formed in the hydrophobized silica covered with
phenyl groups. Surprisingly, all the luminescent guests embed in chiral silica exhibited CD
and CPL activities in the range of its absorption and emission bands, respectively, even
though they were not inherently chiral. These findings suggest that the chiral silica promoted
by Tart/PEI complexes is an effective chiral host to generate CPL activity from diverse achiral
luminescent guests.
Results and Discussion
Complexation of achiral dyes with chiral silica possessing amino residues
As shown in Scheme 1, we firstly prepared chiral silica according to our established
o
Figure 1. SEM images of calcined a) D-SiO
2
o
and b) D-1 P-SiO
2
. c) TGA
curves and d) DRCD spectra of D- and L-1 P-SiO . The solid-state CD
2
o
spectra of D- and L-1 P-SiO
L-form).
2
(Red line for D-form while blue line for
6
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10.1002/cplu.201900615
method [41-44] using the crystalline complexes of PEI/tart as chiral catalytic template and the
o
resulted hybrids of PEI/Tart@SiO
the SiO
2
were sintered at 600 C to obtain chiral silica. Intriguingly,
2
after calcined showed the CD and VCD (vibrational circular dichroism) activities on
its absorption band which indicates that the chirality is involved in Si-O bond.[43, 45] That is, a
tetrahedron-based asymmetrically-centred Si atoms would be selectively formed in the
framework of polysilicates. [42 ] Subsequently, the primary amine was covalently introduced
into the sintered SiO
2
via the reaction between the silica and 3-aminopropyltrimethoxysilane in
o
toluene, which resulted in 1 P-SiO
2
covered by the basic surface. It can be seen from SEM
templated by D-PEI/tart aggregates is
image (Figure 1a) that the morphology of sintered SiO
2
based on sub-micro scale bundles consisted of a lot of nanofibers. After reacted with
o
3
-aminopropyltrimethoxysilane, the resulting D-1 P-SiO
2
also showed the same morphology
but the nanofibers became thicker due to the introduction of 3-aminopropylsilyl groups on the
silica surface (see Figure 1b). To determine the introduction amount of 3-aminopropyl on the
o
silica, we conducted the D- and L-1 P-SiO
2
to TGA measurement. As shown in Figure 1c, the
o
o
weight-loss ranging from 200 - 800 C is about 9.3 wt% for both D- and L-1 P-SiO
2
corresponding to 1.6 mmol of -NH
2
group in per gram of silica. Although the silica itself has no
o
absorption in the wavelengths over 200 nm, the 1 P-SiO
2
bearing aminopropyl-group showed
apparent absorption in the wavelength ranged 200 ~ 300 nm. Interestingly, in this absorption
o
region, the D- and L-1 P-SiO
2
exhibited remarkable CD activity with opposite spectral lines
with mirror-relationship though the aminopropyl-group is achiral. This means that the CD
activity due to aminopropyl-group is practically induced by the chirality of silica (see Figure 1d).
o
Since the 1 P-SiO
2
covered by amino group is able effectively to capture acidic organic
compounds, we selected three acidic fluorophores of TCPP, PyC and FR and prepared the
o
o
o
corresponding D- and L-type TCPP@1 P-SiO
2
, PyC@1 P-SiO
2
and FR@1 P-SiO by binding
2
o
physically the fluorophores onto 1 P-SiO
2
in methanol. To prove the chirality information
7
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transfer from the silica to the fluorophores, the three pairs were subjected to solid-state CD
spectroscopy. In Figure 2a-c, UV-Vis and the corresponding CD spectra were recorded. It is
o
o
o
clear that TCPP@1 P-SiO
2
, PyC@1 P-SiO
2
and FL@1 P-SiO
2
displayed the characteristic
o
Figure 2. Solid-state UV-vis and CD spectra of a) TCPP@1 P-SiO
2
@, b)
o
o
PyC@1 P-SiO
TCPP@1 P-SiO
2
, and c) FR@1 P-SiO
2
. Solid-state CPL spectra of d)
o
o
o
2 2
, e) PyC@1 P-SiO , and f) FR@1 P-SiO .
2
light absorption at UV and/or visible light regions contributed from each residue of TCPP
(
Soret and Q-bands), PyC (below 400 nm) and FR (above 400 nm). Interestingly, as
o
responses to these absorption bands, CD spectra for each pair of TCPP@1 P-SiO
2
,
o
o
PyC@1 P-SiO
2
and FR@1 P-SiO exhibited very well mirror-relationships in which L-type
2
showed positive cotton effect while D-type did negative one (see Figure 2a-c). These results
indicate that the chiral information of silica could transfer to the guests of the physically
adsorbed luminophores, and consequently the chiral silica is capable of directing the guests
of luminophores to play as chiral component even if the guests of luminophores themselves
have not chirality in their chemical structure. We think that the power of silica to transfer the
chiral information to the guest is very strong as long as the guests are trapped on silica within
quasi-intermolecular interaction regime. Here, just the interactions between amino group in
silica side and carboxyl group in luminophores side tie the two sides together to form an
acid-base couple. When the chiral silica has not amino group, the silica could form only a
8
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mixture with TCPP without special interactions, thus the mixture did not show meaningful CD
activity in the Soret band of TCPP (see Figure S1). In order to imply the role of amino group in
chirality transfer from silica to luminophores, we also used the CD active chiral silica
chemically binding secondary/tertiary amine groups or physically binding PEI residues (see
Figure S2) in the preparation of a series of acid-base (TCPP and basic silica) coupled
o
complexes such as D- and L-TCPP@PEI/SiO
2
, D- and L-TCPP@2 P-SiO
2
, D- and
o
L-TCPP@3 P-SiO
2
. Their UV-Vis and CD spectra were shown in Figure S3. As expected, all
the three pairs of D- and L- form exhibited chiroptical activity with opposite CD signs to form
mirror relationship on the Soret and Q-bands of porphyrin residue. These results suggest that
all the chiral silica with basic sites could capture acidic TCPP and endow the TCPP with
chirality.
Chiral performance of the above acidic luminophores captured by chiral silica could be
used as CPL-active candidates because the luminophores has intricate luminescent property
under certain excitation. We subjected all the samples of TCPP-associated chiral silica to
fluorescence and CPL spectroscopies. As shown in Figure S4, photoluminescence (PL)
o
spectra of TCPP@1 P-SiO
2
showed two emission peaks appeared ranging from 670 nm to
o
720 nm, which were attributed to the S
1
→S
0
transitions. Similarly, PyC@1 P-SiO
2
and
o
FR@1 P-SiO
2
also showed their emission under irradiation at 282 nm (Figure S5).
o
Interestingly, as shown in Figure 2d, the enantiomeric pair of TCPP@1 P-SiO
2
featured
photoluminescence exhibited oppositely signed CPL lines around at 650 nm due to TCPP,
showing plus for L-form while minus for D-form, although the TCPP itself is achiral. The
-3
luminescence dissymmetry factor (glum) was 5.0×10 , which is calculated from glum = 2 ×
-I )/(I +I ), where I and I refer to the intensity of left- and right-handed CPL, respectively.
(
I
L
R
L
R
L
R
This indicates two issues here: one is the TCPP residue interacts strongly with the amine
residues bonded on the silica, and the other one is the chiral information in the silica is
9
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effectively transferred to the TCPP to cause the induced CD and CPL. It seems that in this
event, the amines groups bonded to the silica with covalent or non-covalent play an important
role to mediate the interactions between the host of silica and the guest of TCPP. Surprisingly,
pairs of antipodal CPL signs with plus for L-forms while minus for D-forms, could be detected
from each enantiomeric pair of all the samples of the TCPP associated silica,
o
o
TCPP@2 P-SiO
2
, TCPP@3 P-SiO
2
, and TCPP@PEI/SiO
2
with the average glum values were
@TCPP without amine
about 2.0×10-3 - 5.0×10-3 (Figure S6). From the D- and L-SiO
2
mediation, we could not find CPL feature. This result further indicates that the amine residues
are indispensable in pulling TCPP around the neighbor of the silica via acid-base interaction
and thus inducing the chirality. Similar to the TCPP system, the prepared chiral hybrids of
o
o
PyC@1 P-SiO
2
and FR@1 P-SiO , also exhibited remarkable mirror-imaged CPL activity in
2
-3
-3
their emission bands with glum values 5.0×10 and 2.0×10 , respectively, when excited under
UV diode laser (375 nm) irradiation, suggesting the chirality transfer from silica to guest
molecules through the interaction between the carboxy groups and amine residues (Figure 2e,
f).
Preparation of MAPbBr and chiral silica composites
3
As discussed above, molecular dyes are capable of exhibiting CPL feature when they are
associated as guests to the host of chiral silica. In order to confirm the power of the chiral
silica to cause CPL from the non-molecular state of photoluminescent materials, we further
tried a nanocrystal of lead-halide perovskite (LHP) nanocrystals (NCs) as photoluminescent
candidate. The LHP NCs including organic-inorganic CH
3
NH
3
PbX NCs (often denoted as
3
MAPbX , MA = methylammonium cation) have recently been receiving a lot of attention due to
3
their outstanding photoluminescent properties such as bright photoluminescence within a
wide color gamut.[49] Since the luminescent property of LHPs has a correlation with the
crystallinity, LHPs are typically fabricated by the colloidal synthetic way using capping ligand
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reagents in order to control its crystal growth process. The chiral capping ligands were
occasionally used as a chiral source to aim at endowing the chirality with LHPs, but the report
on chiral LHPs is still limited.[50, 51] As the same is true of whole emissive semiconductors,
capping chiral molecules do not always make them become CPL-active though they can
behave optically-active in the CD spectra. So, the selection of chiral capping ligands is vitally
important.[52] On the other hand, there is no report on the chirality induction LHPs by
employing the inorganic chiral materials as a chiral source.
o
The sample of 1 P-SiO
2
impregnated with highly concentrated solutions of the precursor
) was heated under reduced pressure to induce the formation of
NCs on the silica. Finally, the products obtained were washed with toluene to
salt (CH
3 3
NH Br and PbBr
2
MAPbBr
3
Figure 3. TEM images of a) individual MAPbBr
3
NPs from
o
MAPbBr3@1 P-SiO
energy dispersive X-ray (EDX) spectra of MAPbBr
2
and b) its corresponding FFT analyses. c) The
o
3
@1 P-SiO
@1 P-SiO
2
. d) XRD
o
patterns of before and after washed MAPbBr
3
2
.
o
remove the excess NCs which formed outside the silica, giving MAPbBr
3
@1 P-SiO . A
2
structural analysis of the product obtained was presented in Figure S7 and Figure 3.
o
Transmission electron microscopy (TEM) images of MAPbBr
3
@1 P-SiO
2
showed entangled
11
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10.1002/cplu.201900615
silica nanofibers with about 20 nm diameter at low magnification-state (Figure S7, Figure 3a
and b), while about 5 nm of MAPbBr3 NCs distributed along to the fibrous structures were
observed at a high magnification-state. According to the elemental mapping by area-selective
energy-dispersive X-ray spectroscopy (EDXS), the main peaks of Pb and Br elements were
detected, suggesting the MAPbBr
3
NCs were successfully encapsulated in the chiral silica
(
Figure 3c). However, the exposure to the electron beam often caused the collapse of the
o
NCs. The X-ray diffraction (XRD) pattern of MAPbBr
3
@1 P-SiO is shown in Figure 3d. The
2
o
o
one broad halo peak at 2 = 10 - 25 was corresponding to its amorphous structure of the
silica. The sample encapsulating the perovskites showed the unique XRD pattern similar to a
cubic phase structure with a lattice constant = 5.9361 Å,which is well-agreement with a cubic
[53]
̅
organic-inorganic perovskite structure (space group = Pm3m, a = 5.9334 Å).
However,
the peaks are significantly broadened and split due to the finite grain size restricted by amine
residues binding chiral silica. Figure 3b showed the TEM image of the one nanoparticle and
its fast Fourier transform (FFT) pattern, indicating the interplanar distance of 2.97 Å
corresponding to the (200) crystals facet was well-consistent with XRD results. Both MAPbBr
NCs enclosed in D- and L-silica showed the PL emission in a narrow range around 525 nm
under excitation at 370 nm (Figure S8). Our interest here is whether the MAPbBr NCs
3
3
enclosed in chiral silica can show CPL-activity. The solid-state CD spectra of D- and
o
L-MAPbBr
3
@1 P-SiO
2
exhibited the mirror image CD signals in the wide range from 200 nm
to 400 nm, which were corresponding to the absorption of the amine residues and MAPbBr
NCs (Figure 4a). Surprisingly, the plus CPL signal for L-form was observed in the emission
wavelength of the MAPbBr NCs while the oppositely CPL sign in the case of D-form with glum
of 2.0×10 (Figure 4b). However, when the MAPbBr NCs formed on the sintered chiral SiO
composed of pure SiO ) in a similar way, it could show remarkable CD signs while no CPL
signs were detected. Therefore, the amine residue bonding chiral silica is crucial to the
3
3
-3
3
2
(
2
12
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CPL-induction of MAPbBr
3
NC. We are aware that the induction mechanism of the
CPL-activity in this system is not simple. We understood the optical activities from MAPbBr
3
NCs were not dependent on the shape chirality or chiral arrangement of the NCs on the silica,
which was supported by the SEM and TEM observation. During the crystal growing process, it
is needed to note that the primary amines bonding to chiral silica can strongly coordinate with
lead ions, which provide the scaffold of crystal seeds in the initial stage. Therefore, the
perovskites were epitaxially grown on the silica as the evaporation of the organic solvent while
they were strongly susceptible to the chirality of the silica. As a result, it might cause the chiral
distortion of their surface and/or interior crystalline structure with inducing the CPL-feature
Figure 4. Solid-state a) CD/UV-vis and b) CPL spectra of
o
MAPbBr
3
@1 P-SiO
2
, blue line for L-form while red-line for D-from.
from an inherently asymmetric crystalline structure. However, it is hard to reveal the atomic
arrangement of unstable MAPbBr NCs by TEM observation because the exposure to the
3
electron beam changed the initial crystalline structure of perovskites. Further experiments and
measurements are still necessary to clearly understand the CPL-induction mechanism.
Preparation of tetraphenylethylene (TPE) and chiral silica composites
Generally, most of -conjugated molecules emit intensively in the dilute solution but
become weak- or even non-emission in the aggregation or solid-state, due to the
aggregation-caused quenching (ACQ) effect. On the other hand, AIE-active luminogens
(
AIEgens), such as silole, tetraphenylethene (TPE), and their derivatives, show the reverse
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10.1002/cplu.201900615
phenomenon, that is, AIEgens are non-emissive in a good solvent, but become highly
luminescent upon the aggregation.[54] Herein, we tried to construct the CPL-active AIE system
in the combination with chiral silica, in which the chiral scaffold induces the CD and CPL from
an AIEgen of tetraphenylethene (TPE). The procedure for the preparation of chiral silica and
TPE hybrids can be accessed by two steps. In the first, the surface property of the sintered
silica needed to be modified with phenyl groups for providing the interaction site with the guest
of TPE. Then, the products obtained (Ph-SiO ) was immersed in TPE solution (dissolved in
2
THF), and then the impregnated samples were sandwiched with two sheets of filter paper to
remove the excess solution, which yielded the hybrids of chiral silica and TPE aggregate
(
abbreviated as TPE@Ph-SiO ).
2
Figure 5. Solid-state CD/UV-vis spectra of a) Ph-SiO
c) Photoluminescence spectra of TPE@Ph-SiO under excitation at 365 nm.
d) Solid-state CPL spectra of TPE@Ph-SiO
2
and b) TPE@Ph-SiO
2
.
2
2
.
The insertion of phenyl groups into chiral silica was confirmed by FT-IR and TGA
measurements shown in Figure S9 and Figure S10. In the FT-IR spectra of L-Ph-SiO , the
peaks around 2990 cm-1 related to the stretching vibration of (C-C)ring and deformation
2
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10.1002/cplu.201900615
vibration of (C-H)ring due to the presence of phenyl group (Figure S9). The weight loss from
o
TGA-curve in the temperature range from 200 to 800 C was assigned to the organic
components of phenyl groups (Figure S10). After the formation of TPE aggregates on the
SiO
2
, the ratio of organics toward inorganics calculated from TGA curves of L-Ph-SiO @TPE
2
got increased from 8.8 to 19.9 %, in which the TPE aggregates accounted for 12.2 % out of
total organics of 19.9 %. The optical properties of these samples were shown in Figure 5. The
solid-state CD-spectra of D- and L-Ph-SiO showed remarkable CD signs with mirror relation
2
within the UV absorption band (200 - 280 nm), which arose from the –* transition of phenyl
groups (Figure 5a). After the formation of TPE aggregates on the chiral silica, a new
absorbance appeared within a wide area from 200 – 400 nm wavelength corresponding to
TPE molecules, and the induced CD signals derived from TPE were also observed in the
same absorbance region (Figure 5b). TPE molecule itself is not chiral, but the non-covalent
interaction between TPEs and phenyl groups binding to chiral silica could induce the chirality
from TPE molecules and/or their aggregates. This motivated us to investigate whether the
CPL-feature can be detected from TPE confined in chiral silica. As shown in Figure 5c, both
enantiomers of TPE@Ph-SiO
2
displayed strong photoluminescence at emission wavelength
exhibited
around 460 nm owing to the AIE property of TPE. Amazingly, TPE@Ph-SiO
2
remarkable CPL signs at the emission maximum at 476 nm with different handedness,
positive sign for L-type while negative sign for D-type (Figure 5d). This is the very simple
method for endowing the CPL-feature with AIEgen, in which design of the AIE molecules with
covalently bonded chiral moieties is not necessary.[55]
Conclusion
In conclusion, nanofiber-based chiral silica, which has molecular scale chiral domains
through its siliceous frame, were employed as chiral host to construct the inorganics-based
CPL-active systems to endow various luminescent achiral guests with CPL-feature. It is worth
15
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10.1002/cplu.201900615
noting that the chiral source came from inorganics, which is different from the present
researches on CPL-active systems based on organics. More importantly, this CPL-active
system can be easily accessible without tedious synthesis and applied to the diverse guests
from molecular species to molecular packing entities including achiral dyes, perovskite NC,
and AIEgen. In this system, the key is the surface modification of the silica to fulfill the
non-covalent interactions between the host silica and luminescent guests. The robustness
and tolerance of the silica as a chiral host will provide diverse combinations with luminescent
guest materials regardless of organics and inorganics. We believe that this concept will
provide an idea for the future design and application of optical materials using chiral
inorganics such as sensors and solid-state optical devices.
Experimental section
Materials:
Poly 2-ethyl-2-oxazoline (average Mw ~50,000, Aldrich), ammonium solution (28 vol%,
Wako), hydrochloric acid (5 M, Wako), D-(-)-tartaric acid (D-Tart, > 99.0 %, TCI)、L-(+)-tartaric
acid (L-Tart, > 99.0 %, TCI), tetramethoxysilane (TMOS, > 99.0 %, TCI), toluene (> 99.5 %,
TCI), 3-aminopropyltrimethoxysilane (> 96.0 %, TCI), [3-(N,N-dimethylamino)-
propyl]trimethoxyslane (> 96.0 %, TCI), trimethoxy[3-(methylamino)propyl]silane (> 95.0 %,
TCI), N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (Gelest) methylamine solution (> 33
wt. % in absolute ethanol, Sigma-Aldrich), hydrobromic acid (> 47.0 wt. %, TCI), lead(II)
bromide (>99.999 %, Sigma-Aldrich), phenyltrimethoxysilane (> 98.0 %, TCI),
tetraphenylethylene (> 98.0 %, TCI), 5,10,15,20-tetrakis(p-carboxyphenyl)porphyrin (TCPP)
(
TCI), 1-pyrenecarboxyl acid (PyC), (TCI), and fluorescein (TCI) were used as purchased.
Synthesis of PEI:
16
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10.1002/cplu.201900615
The synthesis of PEI was performed by hydrolyzation of poly(2-ethyl-2-oxazoline)
according to our previous work.[56]
Preparation of PEI/tart and PEI/tart@SiO
2
In the typical procedure for preparing PEI/tart complexes, PEI (0.474 g, 6.0 mmol -NH-
groups; here one unit of (NHCH
2
CH
2
) possesses two water molecules) was dissolved in 100
o
o
mL of water at about 80 C. Then, 100 mL of an aqueous solution (ca. 80 C) containing 0.45
g of L- or D-tartaric acid (3.0 mmol carboxyl groups) was added to the hot PEI solution, stirred
for a few minutes and allowed to cool until it becomes room temperature. Subsequently, the
pH of the mixture solution was adjusted to 4 by using ammonia (NH , aq) and left it standing at
3
4 oC overnight. The formed crystalline complexes of PEI/tart were collected by centrifugation
and washed with water. Then, obtained products were re-dispersed in 40 mL of water, and to
this dispersion 6 mL of TMOS was added. The mixture was stirred for 2 h at room temperature.
Finally, the white solid (PEI/tart@SiO
air-dried. Yield: 1.6 g.
2
) was collected, washed with water and acetone, and
Preparation of PEI/SiO
The PEI/tart@SiO
and tart components. Then, 0.1 g calcined SiO
2
o
2
powders were calcined at 600 C for 3 h under air to remove the PEI
2
was mixed with 0.05 g PEI in 4 mL methanol
by stirring for 12 h at room temperature. After that, PEI-adsorbed SiO
collected by centrifugation and dried at room temperature. Yield: 0.11 g.
2
(PEI/SiO ) was
2
o
Synthesis of surface modified chiral silica with basic residues (SCA-SiO
2
: 1 P-SiO
2
,
2oP-SiO
, 3 P-SiO
o
2
2
)
o
The silica hybrid (PEI/tart@SiO
components. Then, the calcined SiO
2
) was calcined at 600 C to remove the organic
2
(125 mg) and 0.7 mL of basic-type silane coupling
o
agencies (SCA) {3-(trimethoxysilyl)propylamine (1 P); N-methyl-3-(trimethoxysilyl)-
17
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10.1002/cplu.201900615
o
o
propylamine (2 P) and N,N-dimethyl-3-(trimethoxysilyl)-propylamine (3 P) were added into 20
o
mL of dehydrated toluene and the mixture was stirred at 85 C for 90 min. The products
obtained were collected by filtration and washed with methanol, water and acetone. We used
the three types of silane coupling agencies bearing primary, secondary and tertiary amino
groups for the modification of silica surface, and the corresponding products were abbreviated
o
o
o
o
in 1 P-SiO
2
, 2 P-SiO
2
, and 3 P-SiO
2
, respectively. Yield: 0.152 g of 1 P-SiO ; 0.151 g of
2
2oP-SiO
and 0.143 g of 3 P-SiO
o
2
2
.
Synthesis of hydrophobized chiral silica covered with phenyl groups (Ph-SiO )
2
o
The powders of PEI/tart@SiO
2
were calcined at 600 C to remove the organic residues of
PEI and tartaric acid. Then, the calcined SiO
2
(160 mg) and 2 mL of phenyltrimethoxysilane
o
(
Ph) were added into 8 mL of dehydrated toluene and the mixture was stirred at 90 C under
nitrogen atmosphere for 18 hours. The products obtained were collected by filtration and
washed with methanol and acetone and then dried at room temperature. Yield: 1.7 g.
Encapsulation of TCPP in chiral silica bearing amino residues
The preparation of complexes of 5,10,15,20-tetrakis(p-carboxyphenyl)porphyrin (TCPP)
and chiral silica is as follows: 80 mg of the chiral silica bearing amino residues (PEI/SiO ,
2
1oP-SiO
, 2 P-SiO
o
, or 3 P-SiO ) was added into methanol solution of TCPP (0.2 mM, 20 mL)
o
2
2
2
and the mixture was stirred at r.t. for 2 hours. The wine-red powders were collected by
centrifugation, washed with methanol and dried at r.t. The corresponding products were
o
o
o
abbreviated in TCPP@PEI/SiO
2
, TCPP@1 P-SiO
2
, TCPP@2 P-SiO
2
, and TCPP@3 P-SiO
2
,
o
respectively. Yield: 75 mg of TCPP@PEI/SiO
2
; 76 mg of TCPP@1 P-SiO
2
; 78 mg of
o
o
TCPP@2 P-SiO
2
; and 74 mg of TCPP@3 P-SiO
2
.
Encapsulation of 1-pyrenecarboxyl acid in chiral silica bearing amino residues
The preparation of complexes of 1-pyrenecarboxyl acid (PyC) and chiral silica is as
o
follows: 80 mg of 1 P-SiO
2
was added into ethanol solution of PyC (0.8 mM, 20 mL) and the
18
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ChemPlusChem
10.1002/cplu.201900615
mixture was stirred at r.t. for 2 hours. The white powders were collected by centrifugation,
washed with methanol and dried at r.t. The corresponding products were abbreviated in
o
PyC@1 P-SiO
2
. Yield: 78 mg.
Encapsulation of fluorescein in chiral silica bearing amino residues
The preparation of complexes of fluorescein (FR) and chiral silica is as follows: 80 mg of
1oP-SiO
was added into methanol solution of FL (0.8 mM, 20 mL) and the mixture was stirred
2
at r.t. for 2 hours. The orange powders were collected by centrifugation, washed with
o
methanol and dried at r.t. The corresponding products were abbreviated in FR@1 P-SiO
2
.
Yield: 76 mg.
Synthesis of methylammonium bromide (MABr)
Methylamine solution (24 mL of 33 wt%) in absolute ethanol were added to 100 mL of
ethanol (pre-cooled to 0 °C). Then 8.6 mL of hydrobromic acid were added and the resulting
mixture was stirred under Ar atmosphere for 1 hour. Then, water and ethanol were removed
by rotary evaporation at 40 °C. The product was dissolved in ethanol, recrystallized in diethyl
ether, and dried under vacuum at 40 °C for 24 hours.
o
Preparation of perovskite NC and chiral silica composites (MAPbBr
3
@1 P-SiO
2
)
o
The powders of 1 P-SiO
2
(40 mg) were mixed with 0.8 mL of 0.3 M solution of
) in DMF for 10
methylammonium lead-bromide (containing 0.24 mmol of MABr and PbBr
2
minutes. Then the mixture was conducted with suction filter. The retained powders placed in a
o
petri dish were heated at 100 C in a vacuum condition for 40 min. Afterwards, the powders
were allowed to cool to room temperature under vacuum condition. Finally, the dried sample
was washed with toluene and the yellow powders were recovered by suction filtration. Yield:
38 mg.
Preparation of tetraphenylethylene (TPE) and chiral silica composites (TPE@Ph-SiO
2
)
19
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10.1002/cplu.201900615
The powders of Ph-SiO
2
(30 mg) were mixed with 0.6 mL of 0.05 M solution of
tetraphenylethylene (TPE) in THF for 1 h. Then, the mixture was conducted with suction filter
and the retained powders were dried at r.t. Afterwards, the sample was washed with water
and the white powders were recovered by suction filtration. The product obtained was dried at
o
6
0 C. Yield: 25 mg
Characterizations
FTIR spectra were recorded on the NICOLET 380 Thermo ELECTRON FT-IR spectrometer,
with KBr as a reference. Elemental analysis was conducted by vario MICRO-cube instrument
(
(
Elementar Inc.). The TG-DTA analysis was conducted on an Exstar 6000 instrument
Elementar Japan K.K.). The SEM images were taken on a HITACHI SU8010 scanning
electromicroscope (SEM) equipped with energy dispersive spectrometer (EDS). The TEM
observation was performed on a JEOL JEM-2100 instrument with acceleration voltage of 200
kV equipped with energy dispersive spectrometer (EDS). The spectra of solid-state diffuse
reflectance circular dichroism (DRCD) and UV-Vis absorption of the samples of powders
dispersed in KCl were simultaneously recorded on a JASCO J-820 spectropolarimeter
equipped with a DRCD-466L unit. The X-ray diffraction (XRD) patterns were collected on a
Rigaku RINT Ultima III X-ray diffractometer with Cu Ka radiation ( = 0.1540 nm). Circular
polarized luminescence (CPL) spectra were measured using a home-made CPL
spectroscopy system, which is applicable to solid-state measurements of opaque samples
(
see in H. Tsumatori, T. Nakashima, T. Kawai, Org. Lett. 2010, 12, 2362-2365). A UV diode
laser (375 nm) was used for the excitation and the emission of the solid powder samples was
collected from the same side via an objective lens. The retardation of the emitted light was
controlled by a photo-elastic modulator (Hinds, PEM-90) that is modulated with the frequency
of 50 kHz. The circularly polarized component was converted to the linearly polarized light
and detected by a photomultiplier tube (PMT, Hamamatsu, H7732-10) after passing through
20
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10.1002/cplu.201900615
the linearly polarized cubic prism (200,000:1). The AC component of the PMT output with
frequency of 50 kHz was analyzed by a lock-in amplifier (EG&G, Model 7265), which can be
modulated by the reference frequency signal from the PEM. The emission intensity and the
CPL dissymmetry were evaluated by the DC component and ratio of AC and DC components,
respectively, as a function of wavelength.
Acknowledgment
The research was supported in part by the JSPS KAKENHI Grant Number JP16H06515
(
Coordination Asymmetry) and by the JSPS KAKEHI Grant-in-Aid for Scientific Research (B),
9H02767.
1
Conflict of interest
The authors declare no conflict of interest
Keywords: chirality · circular dichroism · circularly polarized luminescence · luminophores ·
chiral silica
21
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10.1002/cplu.201900615
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Entry for the Table of Contents
Full paper
CPL-active systems based on the
chiral host of silica-luminescent guest
of achiral dyes, lead-halide typed
perovskites and aggregation-induced
S. Tsunega, R.-H. Jin,* T. Nakashima,
T. Kawai
CPL
Page No. – Page No.
Chiral silica
Host)
(
Transfer of Chiral Information from
Achiral guests
emission
luminogens
All the
were
Silica
to
Photoluminescences:
Approach Accessing
Dyes
Emissive
Luminescent
perovskites
Nonemissive
constructed.
achiral
Aggregation
Simple
luminophores trapped on the silica
showed remarkable CPL-activity.
AIEgens
Circularly Polarized Luminescent
Systems
25
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