Tuning of luminescence color of π-conjugated liquid crystals through co-assembly with ionic liquids

Article abstract of DOI:10.1039/c7tc02767b

Luminescent oxadiazole-based columnar liquid crystals containing a diethanolamine moiety forming intramolecular donor-acceptor charge transfer states have been designed and synthesized. These molecules exhibit strong solvatochromism depending on the solvent polarity. The co-assembly of oxadiazole derivatives and nonvolatile 1-butyl-3-methylimidazolium-based ionic liquids through hydrogen bonding leads to the formation of thermally stable nanosegregated columnar structures with confined channels of ionic liquids. Modulation of the emission color from light-blue to blue-green in the liquid-crystalline states has been achieved by incorporating the ionic liquid. This red-shifted emission can be attributed to the stabilization of the excited intramolecular charge transfer state of the donor-acceptor luminophore by polar molecules. The presented tunable luminescent soft materials obtained through noncovalent supramolecular approaches will open a new avenue in the fields of optoelectronic organic materials.

Full text of DOI:10.1039/c7tc02767b

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Tuning of luminescence color of π-conjugated liquid crystals
through co-assembly with ionic liquids
Received 00th January 20xx,
Accepted 00th January 20xx
Masato Mitani, Masafumi Yoshio* and Takashi Kato*
Luminescent oxadiazole-based columnar liquid crystals containing a diethanolamine moiety forming intramolecular donor-
acceptor charge transfer states have been designed and synthesized. These molecules exhibit strong solvatochromism
depending on the solvent polarity. Co-assembly of oxadiazole derivatives and nonvolatile 1-butyl-3-methylimidazolium-
based ionic liquids through hydrogen bonds leads to the formation of thermally stable nanosegregated columnar
structures with confined channels of ionic liquids. Modulation of the emission color from light-blue to blue-green in the
liquid-crystalline states has been achieved by incorporating the ionic liquid. This red-shifted emission can be attributed to
the stabilization of the excited intramolecular charge transfer state of the donor-acceptor luminophore by polar
molecules. The presented tunable luminescent soft materials obtained through noncovalent supramolecular approaches
will open a new avenue in the fields of optoelectronic organic materials.
DOI: 10.1039/x0xx00000x
www.rsc.org/
limited8 and still in a preliminary stage. If these π-conjugated
donor-acceptor molecules with a dipole moment could form
dynamically ordered states, a new type of sensing materials
Introduction
Visible light emission of π-conjugated molecules in the
condensed states has attracted much attention owing to their
application in optoelectronic devices such as organic light-
emitting diodes (OLEDs).1 The tuning of the emission color is
commonly achieved by the chemical modification of the π-
conjugated structure and the blending of luminescent
molecules exhibiting different emission colors.2 However,
multicolor emissions from single-component luminophore in
the solid crystals3 and liquid-crystalline (LC) states4 are readily
available nowadays, which uses the change in the molecular
conformation and packing structures upon the application of
external stimuli. These stimuli-responsive luminescence
properties expand the applications of π-conjugated molecules
in the fields of sensors and memories.5
that vary the luminescence color in response to polar
molecules and ions would be developed. In general, the
dipole-dipole interactions induce a J-aggregate with a head-to-
tail slipped π-stacking structure and a H-aggregate with a face-
to-face stacked arrangement, which normally leads to strong
emission and emission quenching, respectively.9
In the previous study on the stimuli-responsive
luminescence based on
a donor-acceptor π-conjugated
molecule,8 an amphiphilic oligo(p-phenylenevinylene),
consisting of an electron-donating N,N-dialkyl moiety and an
electron-accepting ester moiety tethering an oligo(ethylene
glycol) chain, was designed to form metastable J-aggregates
that possessed the parallel aligned dipole moment in the
layered structures. The nanosegregation of hydrophobic and
hydrophilic side chains is a key to stabilize the unusual dipole
orientation, which could induce the stimuli-responsive
luminescent properties by applying a mechanical force and by
the addition of lithium ion.
On the other hand, intramolecular charge-transfer (ICT)6
compounds that consist of donor and acceptor moieties
connected through
a
π-conjugated bridge exhibit
luminescence sensitive to environment and solvent polarity.
They have versatile applications such as fluorescent probes for
cell imaging.7 However, the development of stimuli-responsive
luminescent materials based on ICT interactions is highly
Our design strategy for new luminescent materials is to
organize
amphiphilic
π-conjugated
donor-acceptor
luminophores into columnar LC structures10 with confined
ionophilic channels (Fig. 1). These columns are expected to
possess oppositely oriented local dipoles in the cross section. If
with the installation of ions in the center of columns, the
dipolar structures of luminophores in the excited states are
stabilized by the ion-dipolar interactions, the luminescent
color change would be induced. This radial self-assembly of ICT
compounds in the columnar structures may have a beneficial
effect on the ion sensitivity compared to the layered
assemblies. This is deduced from the formation of closer
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Fig. 1 Schematic illustration of design strategy for amphiphilic liquid-crystalline π-conjugated donor-acceptor luminescent materials with confined ionophilic channels for the
tuning of the luminescence color through co-assembly with ionic liquids.
interactions between ions and dipolar luminophores in the
columns. We intend to highlight the use of ionic liquids. Ionic
liquids are nonvolatile liquids and their polarity are easily
tuned by the combination of organic cations and inorganic
anions.11
We previously reported the development of ion-conductive
LC materials based on the self-assembly of ionic liquids.12 Ionic
liquids are confined in the ionophilic 1D columnar, 2D smectic,
and 3D bicontinuous cubic channels formed by the amphiphilic
LC molecules having diol groups through nanosegregation and
intermolecular interactions.12 These LC assembled structures
and ion-conductive properties of the mixtures are modulated
by changing the mixing ratio. This basic strategy for the
formation of supramolecular LC structures by two-component
co-assembly should be applicable to the development of
luminescent liquid crystals incorporating ionic liquids.
Fig. 2 Molecular structures of compounds 1–3.
Here we report on ICT luminescent columnar liquid crystals
containing an electron-donating diethanolamine moiety and
–
–
anions (Br–, BF4 , CF3SO3 , and (CF3SO2)2N–)11 were chosen due
to their miscibility with amphiphilic molecules containing diol
groups.12c,d
an electron-accepting oxadiazole moiety, and the tuning of
luminescent color by co-organization of the liquid crystals with
ionic liquids (Fig. 2).
Luminescence properties in solution
Results and Discussion
Compound 1a shows obvious positive solvatochromism (Fig.
3a). As the polarity of solvent increases, the luminescence
color changed from deep blue to green. For example,
compound 1a in dichloromethane solution (1 × 10–5 M)
Molecular design
Luminescent amphiphilic compounds 1a
designed and synthesized (Fig. 2). The luminescent cores of
compounds 1a and contain the electron- and proton-
,b and 2 have been
exhibits light-blue luminescence centered at 464 nm (ΦPL
0.96, Table S1, ESI†). The absorption and emission spectra of
compounds 1a and in solution were obtained to study the
=
,
b
2
accepting oxadiazole moiety13 and the electron-donating
amino phenyl moiety, therefore they are expected to show the
ICT characters. In addition, compound 1a possesses the
proton-donating phenol group at the luminescent core with
the aim of the formation of intramolecular hydrogen bonds
with the oxadiazole moiety and the induction of excited state
intramolecular proton transfer (ESIPT).14 In contrast,
compound 1b that corresponds to the O-benzyl protected 1a
was chosen as a reference compound in comparison with the
self-assembling and luminescent properties. The diol moiety is
2
photophysical properties (Fig. 3b,c, Table S1, ESI†). Both of the
absorption and emission spectra of compound 1a show red-
shifts in the polar solvents. Luminescence of ESIPT-active
molecules often displays the dual emission due to the
existence of the tautomers in the excited states.14 Compound
1a can adopt the enol form stabilized by the intramolecular
hydrogen bond and the keto form generated by the
intramolecular proton transfer. However, compound 1a shows
a single emission band in solution (Fig. 3c). This result provides
two possible explanations: either no ESIPT undergoes, or the
tautomer has close excited state electronic energy to the enol
form in the excited state, therefore their luminescence spectra
attached to luminescent cores of compounds 1a,b and 2 as a
ionophilic part. Hydroxyl groups have been used to promote
nanosegregation,15 to induce LC phases,16 and to increase the
miscibility with ionic liquids.17 As the ionic liquids, the typical 1-
butyl-3-methylimidazolium ionic liquids 3(X) with various
overlap. Compound 2 without the proton donating phenol
2 | J. Name., 2012, 00, 1-3
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Fig. 4 Chemical structures of model compounds for 1a ((a) in the enol form; (b) in the
keto form) and 2 (c) and their HOMOs and LUMOs obtained by DFT calculations of the
optimized structure.
localized at the 1-(1,3,4-oxadiazole)-3,4,5-trialcoxylphenyl
moiety (Fig. 4a,b center and right panels). Similarly, the HOMO
and LUMO electron clouds for the model compound
spatially separated (Fig. 4c). These results support the ICT
characteristics of compounds 1a and . Therefore, compounds
1a and have potential to change their luminescent properties
2 are also
2
2
in response to the ionic environmental changes in the
condensed state.
Liquid-crystalline properties
The thermal properties of oxadiazole derivatives are
summarized in Table 1, which were determined by differential
scanning calorimetry and polarizing optical microscopic
observation. Compounds 1a and 1b exhibit thermotropic liquid
crystallinity, whereas no LC phases are formed for
2.
Fig.
3 (a) Luminescence images of compound 1a in solution (λex = 365 nm). (b)
Compound 1a shows an enantiotropic hexagonal columnar
(Colh) phase from 122 to 141 °C on heating and from 140 to 95
°C on cooling. Under polarizing optical microscopic
observation, a fan-shaped texture is seen for 1a at 130 °C on
cooling from the isotropic liquid (Fig. 5a). The small-angle X-ray
scattering (SAXS) pattern of 1a at 130 °C shows two peaks at
46.6 (100) and 26.8 Å (110) (Fig. 5b) with the reciprocal d-
spacing ratio of 1:√3, which is indicative of the hexagonal
arrangement of the columns. The number (n) of molecules in a
slice of column stratum was calculated to be 7 molecules,
Absorption and (c) emission spectra of compound 1a in diluted solution (λex = 365 nm).
(d) Lippert-Mataga correlations for compounds 1a and
parameter Δf. r: Correlation coefficient.
2 on the solvent polarity
group also exhibits the positive solvatochromic behavior (Table
S1, ESI†). The Lippert-Mataga plots for compounds 1a and
2
show that stokes shifts are directly proportional to the
orientation polarizability (Δf) of solvents (Fig. 3d and Table S1,
ESI†). These features are characteristic of an ICT in the excited
states.6 Moreover, the stokes shifts are also replotted against
the orientation polarizability (Δf’’) (Table S1, ESI†) to evaluate
the formation of a twisted ICT (TICT) in the excited state.18,19
The fitting of stokes shifts as a function of Δf’’ gives the higher
correlation coefficients (r) compared to those obtained for
Lippert-Mataga plots using Δf. These results suggest that
using the following equation: n = √3NAa2c
ρ /(2M), where NA is
Avogadro constant (6.02 x 1023 mol–1), a is the intercolumnar
distance (54 Å), c refers to the height of column stratum (4.5
Å),
ρ
is the density (1 g cm–3), and M represents the molecular
weight (970.41 g mol–1). As for compound 1b, a monotropic
Colh phase is formed at temperature from 106 to 91 °C on
cooling, in spite of the bulkiness of benzyl group (Fig. S2, ESI†).
Furthermore, interestingly, no LC phase is induced for diol-
compounds 1a and
In order to further confirm the ICT character of compounds
1a and the frontier molecular orbitals of triethoxy
derivatives as model compounds for 1a and were calculated
2 show the TICT emission.
2,
functionalized compound
2 without a phenol group, whereas
2
corresponding phenolic 1a can form the LC phase. It is
considered that compound 1a may prefer more planar π-
conjugated conformation via the intramolecular hydrogen
bond between phenol and oxadiazole moieties (See
supplementary information, ESI†). Hence, the stronger π-
by using the density functional theory (DFT) with the 6-31G
basis set. For the model compound of 1a, structures of both of
the enol form (Fig. 4a, left panel) and the keto form (Fig. 4b,
left panel) were optimized. The results of DFT calculation show
that the electron clouds in the HOMOs for the model
compound of 1a in the enol and keto forms are mainly
stacking of 1a compared to 2 could lead to the induction of the
LC phase. As a result, we have found that both of the planarity
of the π-conjugated moiety and the intermolecular hydrogen
localized
at
the
N,N-diethoxyl-4-aminophenyl-3’-
hydroxylphenyl moiety, whereas the LUMO electron clouds are
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Table 1 Thermal properties of compounds 1a,b and 2.
Compound
Phase transition behavior[a]
1a
Cr1 112 (46) Cr2 122 (33) Colh 141 (0.9) Iso
Iso 140 (0.9) Colh 95 (30) Cr2 75 (36) Cr1
1b
2
Cr1 41 (16) Cr2 75 (1) Cr3 95 (0.1) Cr4 112 (14) Cr5 117 (40) Iso
Iso 106 (0.9) Col 91 (43) Cr2 36 (18) Cr1
Cr1 23 (17) Cr2 64 (16) Cr3 87 (0.9) Cr4 124 (43) Iso
Iso 114 (41) Cr4 67 (7) Cr3 57 (14) Cr2 8 (16) Cr1
[a] Transition temperatures (°C) taken at the maximum of the transition peaks
and transition enthalpies (kJ mol–1, given in parentheses) determined by DSC on
2nd heating and 1st cooling at a scanning rate of 10 K min–1. Cr: crystalline; Colh:
hexagonal columnar; Col: unidentified columnar, Iso: isotropic liquid.
Fig. 6 Microscopic images of the contact region between (a) 1a and 3(BF4) at 150 °C, (b)
1a and 3((CF3SO2)2N) at 150 °C, (c) 1b and 3(BF4) at 140 °C, and (d) 2 and 3(BF4) at 140
°C. The optical textures of samples shown in (a), (c), and (d) are observed between the
crossed polarizers. The image of (b) is taken without polarizers to visualize the interface
between two immiscible 1a and 3((CF3SO2)2N) in the isotropic liquid states.
–
and CF3SO3 (See supplementary information, ESI†). In
contrast, 3((CF3SO2)2N) is immiscible with 1a and the liquid-
liquid phase separation is observed under a microscope
without polarizers (Fig. 6b). The increased ionophobic nature
of (CF3SO2)2N anion20 and its steric bulkiness may hinder
intermolecular
interactions
with
diethanolamine-
functionalized compound 1a. Compound 1b is also compatible
with 3(BF4) (Fig. 6c). The mixture in the contact region exhibits
the enantiotropic LC behavior, while compound 1b itself shows
the monotropic LC behavior as described above. It should be
noted that mixing of
2
and 3(BF4) induces a Colh phase (Fig. 6d
alone does not exhibit
and Fig. S4, ESI†) though compound
2
LC behavior. These results of contact test clearly show that the
introduction of proper ionic liquids into the amphiphilic
compounds 1a, 1b, and 2 leads to the stabilization and
induction of LC phases.
Compounds 1a and 3(BF4) were mixed with different molar
ratios to examine the LC behavior of the mixtures. Compounds
1a and 3(BF4) are miscible up to the mole fraction of 0.3 for
Fig. 5 (a) A polarizing optical microscopic image and (b) a SAXS pattern of compound 1a
in the Colh phase at 130 °C. Arrows in panel (a) indicate the directions of polarizer and
analyzer axes.
3(BF4) in the mixture. Mixtures of 1a and 3(BF4) (1a/3(BF4))
show enantiotropic Colh phases in wider temperature ranges
compared to 1a alone (Fig. S5, ESI†). For example, the mixture
of 1a and 3(BF4) in the 8:2 molar ratio shows the Colh phase
below the decomposition temperature and above around 90
°C on cooling. Moreover, the SAXS pattern of the mixture of 1a
and 3(BF4) in a 8:2 molar ratio at 130 °C (Fig. S6a, ESI†) shows
three peaks at 50.1 (100), 29.1 (110), and 25.1 Å (200) with the
reciprocal d-spacing ratio of 1:√3:2. The intercolumnar
distance (a) of the mixtures of 1a and 3(BF4) gradually
increased with the increase of mole fraction of 3(BF4) (Fig. S6b,
ESI†). The a value of the mixture of 1a and 3(BF4) containing
0.3 mole fraction of 3(BF4) is 58 Å at 130 °C, while a value of 1a
alone is 54 Å at the same temperature. This widening of
intercolumnar spacing suggests the incorporation of 3(BF4)
into the central part of columns.
bond of diol moieties play important roles in the formation of
the nanosegregated columnar LC phase.
Compounds 1a
, 1b, and 2 were contacted with 1-butyl-3-
methylimidazolium ionic liquids 3(X) with various anions (Br–,
–
–
BF4 , CF3SO3 , and (CF3SO2)2N–) at temperatures above the
isotropization temperature of 1a 1b, and (150 °C for 1a, 140
°C for 1b and ) between sandwiched glass plates to examine
their miscibility. At the interface of 1a and 3(Br) 3(BF4), and
,
2
2
,
3(CF3SO3) (Fig. 6a and Fig. S3, ESI†), fan-shaped textures
showing the induction of the columnar phases are observed
under
a
polarizing optical microscope. This observation
3(BF4), and 3(CF3SO3) are miscible with 1a
indicates that 3(Br)
,
and the LC phases of mixtures are thermally stabilized
compared to 1a alone due to the formation of hydrogen bonds
–
between 1a and 3(X) containing
X
– = Br–, BF4 ,
4 | J. Name., 2012, 00, 1-3
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emission spectra of compound 1a and the mixtures of 1a and 3(BF4) in the Colh phase
at 130 °C.
Luminescence properties in the solid and LC states
We have found that the incorporation of ionic liquid 3(BF4)
Table
2 Photophysical properties of compounds 1a,b, and 2 and the mixtures
containing 3(BF4).
into diol-functionalized compounds 1a,b and
2 induces the
photoluminescent color change in the crystalline and LC states.
For example, compound 1a in the Colh phases at 130 °C
displays the light-blue luminescence (Fig. 7a), whereas the
mixture of 1a and 3(BF4) in the 8:2 molar ratio in the Colh
phase exhibits the blue-green luminescence (Fig. 7b).
Sample
λabs / nma
365
λem / nma
483
Phase
1a
Colh (130 °C)
Colh (130 °C)
Cr (100 °C)
Colh (100 °C)
Cr (100 °C)
Colh (120 °C)
1a/3(BF4)(0.2)b
368
498
The absorption and emission spectra of 1a alone and the
mixtures of 1a and 3(BF4) for the 9:1, 8:2, and 7:3 molar ratios
in the Colh phases at 130 °C are shown in Fig. 7(c) and (d),
respectively. Compound 1a and the mixtures of 1a and 3(BF4)
exhibit the absorption maxima in the region around 365-368
nm (Fig. 7c). In contrast, a noticeable red-shift of emission
peak (Fig. 7(d)) is observed upon self-assembly of the ionic
liquid and compound 1a (peak top: 483 nm for 1a, 498 nm for
the mixture of 1a and 3(BF4) in the 8:2 molar ratio, as
summarized in Table 2). These red-shifted emissions for the
mixtures of 1a and 3(BF4) can be ascribed to the stabilization
of the photoexcited states of 1a through ion-dipolar
interactions by the ionic liquids confined in the one-
1b
369
474
1b/3(BF4)(0.1)b
2
365
485
372
479
2/3(BF4)(0.2) b
367
500
[a] λabs: wavelength of absorption maxima; λem: wavelength of emission maxima.
(λex = 365 nm) [b] The mole fraction of 3(BF4) is shown in the parentheses.
luminescence color changes.
The photoluminescence quantum yields (ΦPL) of
compounds 1a, 1b, the 1a/3(BF4) mixture, and the 2/3(BF4)
mixture in the crystalline phases were measured. Compounds
1a and 1b exhibit the photoluminescence efficiency of 0.46
and 0.79 in the condensed states, respectively.2b,13a It is noted
dimensional channels of 1a
.
The photophysical properties of compounds 1b and
2
and
their mixtures with 3(BF4) were also examined to study the
effects of the proton donating phenolic group for 1a. The
1b/3(BF4) mixture and the 2/3(BF4) mixture in the Colh phases
also give the red-shifted emission spectra compared to 1b and
that the ΦPL values for the 1a/3(BF4) mixture and the
2/3(BF4)
mixture are larger than those for pristine compounds 1a and
2.
The ΦPL value for the 1a/3(BF4) mixture and the 2/3(BF4)
mixture in a 8:2 molar ratio at 25 °C in the crystalline phase
reaches as high as 0.65 and 0.81, respectively. The reason for
the photoluminescence enhancement for the mixtures is not
clear, but introduction of ionic liquids may affect the molecular
packing, conformation, or excited states of compounds 1a,b
2
alone in the crystalline (Cr) phases (Table 2). Even
compounds 1b and 2 without a phenolic group necessary to
show the ESIPT emission have found to exhibit the
luminescent color change by the incorporation of the ionic
liquid. This fact suggests that the ion-dipolar interactions
and
2 in the crystalline phases as well as LC phases, which
between the ionic liquid and compounds 1b and
2 exhibiting
leads to suppression of the non-radiative deactivation
processes.
the ICT nature should play important roles to induce the
Proposed assembled structures of the mixture of 1a and 3(BF4)
The assembled structures of compound 1a and the mixture
with 3(BF4) are proposed in Fig. 8. Compound 1a and the
mixture of 1a and 3(BF4) exhibit the Colh phases. The results of
XRD measurements indicate that each disc of columns is
supposed to be composed of around 7 molecules of compound
1a (Fig. 8a). Due to nanosegregation, hydrophilic diol moieties
of compound 1a are organized at the center of the columns
and intermolecular hydrogen bonds between the hydroxyl
groups of the diol moieties are formed (Fig. 8b). As for the
mixture of 1a and 3(BF4), the ionic liquids are organized inside
the columns (Fig. 8c). Formation of hydrogen bonds between
the hydroxyl groups of compound 1a and BF4 anions was
confirmed by IR measurements (Fig. S7, ESI†). Furthermore,
the intercolumnar distance of the Colh phases for the mixtures
is longer than that for pure compound 1a. These results
support the proposed assembled structure shown in Fig. 8c.
The spatial proximity of the ionic liquids and the luminescent
cores may lead to the stabilization of compound 1a in the
Fig. 7 Luminescence images of the thin LC films of (a) 1a and (b) the mixture of 1a and
3(BF4) in the 8:2 molar ratio in the Colh phases at 130 °C under UV irradiation (λex = 365
nm). The LC samples are sandwiched between two quartz plates. (c) Absorption and (d)
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excited states, which results in the red-shifted emission of the Young Scientists and Program for Leading Graduate Schools
mixture.
(MERIT).
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Fig. 8 Schematic illustration of the Colh (a), and the proposed organization of molecules
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In summary, we have proposed a new approach to the tuning
of the luminescence color for supramolecular assemblies.
Donor-acceptor type luminescent amphiphilic molecules have
been designed and synthesized for this purpose. They possess
the ICT characters and show strong solvatochromism. In the
condensed states, they exhibit columnar LC structures through
nanosegregation to form ionophilic nanochannels surrounded
by the oxadiazole-based luminescent cores. We found that
incorporation of ionic liquids into the nanosegregated LC
structures lead to the confinement of ionic liquids in the
ionophilic channels through hydrogen bonds, resulting in the
change in the luminescence color of the molecular assemblies.
The spatial proximity of the ionic liquids and the luminescent
cores may allow the stabilization of the ICT excited state
through ion-dipole interactions, which causes the red-shifted
emission in the co-assembled structures. These results suggest
that this approach provide new molecular-based materials
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Acknowledgements
7
8
This study was partially supported by JST, CREST, JPMJCR1422
for T. K. This work was also partially supported by KAKENHI No.
22107003 for T. K. and KAKENHI No. 15H00921 for M. Y. from
the Ministry of Education, Culture, Sports, Science and
Technology (MEXT). M. M. thanks the Japan Society for the
Promotion of Science (JSPS) for a Research Fellowship for
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DOI: 10.1039/C7TC02767B
Tuning of luminescence color of πꢀconjugated liquid
crystals through coꢀassembly with ionic liquids
Masato Mitani, Masafumi Yoshio,* and Takashi Kato*
Excited intramolecular charge transfer states affect luminescence color of supramolecular assembly
formed by amphiphilic liquid crystals and ionic liquids.