Design of Thermochromic Luminescent Dyes Based on the Bis(ortho-carborane)-Substituted Benzobithiophene Structure

Article abstract of DOI:10.1002/asia.201801529

To obtain solid-state emissive materials having stimuli-responsive luminescent chromic properties without phase transition, benzobithiophenes modified with two o-carborane units having various substituents in the adjacent phenyl ring in o-carborane were d

Full text of DOI:10.1002/asia.201801529

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Design of Thermochromic Luminescent Dyes Based on the Bis(o-
carborane)-Substituted Benzobithiophene Structure
Authors: Kenta Nishino, Yasuhiro Morisaki, Kazuo Tanaka, and Yoshiki
Chujo
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Chemistry - An Asian Journal
10.1002/asia.201801529
Design of Thermochromic Luminescent Dyes Based
on the Bis(o-carborane)-Substituted
Benzobithiophene Structure
1
1,2
1
1
Kenta Nishino , Yasuhiro Morisaki , Kazuo Tanaka * and Yoshiki Chujo
1Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
2Present address: Department of Applied Chemistry for Environment, School of Science
and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337,
Japan
E-mail: tanaka@poly.synchem.kyoto-u.ac.jp
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Chemistry - An Asian Journal
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Abstract
To obtain solid-state emissive materials having stimuli-responsive luminescent
chromic properties without phase transition, benzobithiophenes modified with two o-
carborane units having various substituents in the adjacent phenyl ring in o-carborane
were designed and synthesized. Their emission colors were strongly affected not only by
the substituents at the para-position of the phenyl ring but also by molecular distribution
in the solid state. In particular, the emission colors were changed by heating without
crystal phase transition. It was proposed that their thermochromic properties were
correlated not with isomerization but with the molecular motion at the distorted
benzobithiophene moiety.
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Introduction
Thermochromic materials are versatile for thermometers, wide-tunable micro laser
crystals and warning signals. There are many thermochromic dyes based on both organic
1
2
and inorganic compounds such as liquid crystals, stable radicals, polymorphic
3
4
5
I
clusters and inorganic oxides.7
6
compounds, conjugated polymers, leuco dyes, Cu
4
4
Proposed mechanisms of these compounds are classified into 1) the changes in the
structure by chemical reactions (i.e. neutral and biradical), 2) crystal phase transition (i.e.
liquid crystals and polymorphic compounds), 3) switching of the energy back transfer (i.e.
europium complexes), and 4) spin cross-over system (i.e. transition metal complexes).
Combination of “element-blocks”, which is defined as a minimum functional unit
composed of heteroatoms, with conjugated system is one of valid strategies for
8
fabricating advanced functions. From this view point, the aryl-substituted o-carborane
9
skeleton is a promising platform to obtain functional solid-state emissive materials. From
the first report on aggregation-induced emission (AIE) which can be detected only in the
aggregation and condensed state,10 the series of solid-state emissive materials were
11
discovered. In particular, stimuli-responsive luminescent chromic behaviors were often
1
2
found in these materials composed of the o-carborane “element-blocks”. Therefore, a
wide variety of luminescent chromism toward mechanical stresses, temperature changes
and vapor fuming were demonstrated with the o-carborane-containing conjugated
materials.13 In these materials, luminescent chromism was generally caused by
conformational or morphology changes in the solid materials. In order to obtain much
sensitive materials, our next goal is to realize luminescent chromic behaviors without
drastic structural changes as well as phase transition in the solid state.
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We have reported synthesis and optical properties of bis(o-carborane)-substituted
acenes.12,14 Intense solid-state emission bands from the intramolecular charge transfer
1
5
(
ICT) state in which the central aromatic system and the o-carborane unit respectively
worked as an electron donor and acceptor through the carbon in o-carborane can be
observed in the solid state of these compounds. These o-carborane units also played a
significant role in suppression of aggregation-caused quenching (ACQ) by disturbing
intermolecular interaction because of steric hindrances. Except for luminescent properties,
unique structural feature was found from the bis(o-carborane)-substituted aromatic ring.
It should be remarked that the central anthracene moiety was critically distorted because
1
2
of the bis(o-carborane) substitution according to the X-ray single crystal analyses. From
this fact, we presumed that electronic properties of the central distorted aromatic ring
could be sensitive toward environmental factors, such as temperature and molecular
morphology. In the cooling state or crystalline packing, molecular motions would be
highly restricted. Thereby, the distorted center can work as an electron donor, and ICT
emission is observable, similarly to the commodity system. Meanwhile, under heating
condition or in the solution state, vigorous intramolecular motions should be induced
around the distorted center. Therefore, solid-state emission can be varied by external
stimuli through perturbation of the electronic state at the distorted center. Based on this
assumption, we designed the bis(o-carborane)-substituted molecule to demonstrate a
luminescent chromic material without structural changes and phase transition.
Herein, synthesis and optical properties of bis(o-carborane)-substituted
benzobithiophenes are reported. To clarify the structure-property relationship with
luminescent behaviors, various substituents were introduced into the para-position of the
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Chemistry - An Asian Journal
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adjacent phenyl ring in the o-carborane unit. From optical and structural analyses, it was
found that the synthesized compounds had solid-state emission with the ICT characters,
and especially, their emission colors were distinctly altered not only by the substituents
at the para-position of the phenyl ring but also by molecular distribution in the solid state.
Furthermore, thermochromic luminescent behaviors were obtained without crystal phase
transition as we expected. Electronic structures at the distorted aromatic core created by
the bis(o-carborane) substitution should be responsible for luminescent chromism.
Results and Discussion
Synthesis of DCB-OMe was performed by employing the previous method for the
preparation of DCB-R (R = H, Bu) as shown in Scheme 1.16 Benzo[1,2-b:4,5-
b']dithiophene-4,8-dione was added to the THF solution of the lithiated ethnylbenzene
derivative 1-OMe. Following reduction with the quinone form of benzodithiophene by
t
SnCl
coupling reaction. After the decaborane insertion at the ethynyl moiety, DCB-OMe was
obtained. DCB-OH was prepared by deprotection of the methoxy groups with BBr
2
in the HCl aq. solution, the diethynyl derivative 2-OMe was obtained via the
3
1
11
13
(
Scheme 2). All compounds were characterized by H, B and C NMR spectroscopies
(Charts S1−S9), elemental analyses and HRMS measurements. The products showed
good stability and solubility in common organic solvents such as CHCl
3
, CH
2
Cl ,
2
tetrahydrofuran (THF) and benzene. Thus, we concluded that the products should have
the designed structures and enough stability for performing the series of measurements.
Schemes 1 & 2
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The X-ray single crystal analysis was applicable for determination of the structure of
DCB-OH (Figure 1). The single crystal of DCB-OH was collected by recrystallization
from benzene. Although the crystal incorporated benzene as a crystal solvent, π‒π
interaction between benzene and aromatic rings was hardly observed. Therefore,
electronic perturbations to DCB-OH by benzene should be negligible. Similarly to the
1
2
crystal structure of DCB-H as reported in the previous study, DCB-OH possessed the
cis-type conformation in the crystal packing. In particular, it was found that DCB-OH
had intermolecular and intramolecular hydrogen bonds. The distance of intermolecular
and intramolecular hydrogen bonds were 1.907 and 1.938 Å, respectively. By the
assistance with these hydrogen bonds, DCB-OH could be facilitated to form the cis-
conformation and construct the dimeric structure.
Figure 1
1
From H NMR spectroscopy, the conformations of the compounds were evaluated
(Figure S1). In the previous report, it was demonstrated that DCB-H can form two types
of structural isomers as the cis- and trans-conformations in the solution state although the
cis-conformation was dominant in the crystal packing, and these structures were readily
1
16
distinguishable by the signal patterns in the H NMR spectra. Based on this fact, next,
1
to speculate the conformations of DCB-OMe and DCB-CN, variable-temperature H
NMR spectra in CD
2
Cl were measured (Figure S1). It was known that the signal peak at
2
8
.5 ppm attributable to the cis-conformation was small even at low temperature. This
result suggests that both compounds should form the trans-conformations. The energy
differences between the cis- and trans-conformations of DCB-OMe were estimated by
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Chemistry - An Asian Journal
10.1002/asia.201801529
the calculation of thermodynamic parameters with the plot (Figure S2). As a result, it was
observed that the enthalpy difference between both conformations was 2.76 kJ/mol, and
the trans-conformation had more stable than the cis-one. Moreover, the peaks assigned to
cis-conformation of DCB-CN were too small to prepare the plot. These data indicate that
the DCB-OMe and DCB-CN should form the trans-conformation in the crystal state
similarly to DCB-tBu. It is likely that steric hindrances of the substituents contributed to
formation of the trans-conformation.
The UV‒vis absorption and photoluminescence (PL) spectra of DCB-R in THF (1.0 
‒
5
1
0 M) were measured (Figure S3, Table 1). All samples showed the identical absorption
spectra. The absorption bands in longer wavelength region had the peaks around 400 nm.
These data suggest that the substituent group at the para position of the phenyl ring should
slightly influence on the electronic structure in the ground state. In contrast, emission
wavelengths of DCB-R were varied by the substituents. In particular, introduction of
cyano groups induced red-shifted emission (Figure S3b). Electron-withdrawing ability of
the o-carborane units can be enhanced by cyano groups via the σ*−π* conjugation on the
C−C bonds in the o-carborane unit. Thereby, the emission band should be influenced. It
was presumed that emission should be originated from the ICT state according to the
previous reports on photophysical properties of the aryl-substituted o-carborane
9
derivatives. To confirm this mechanism, UV−vis absorption and PL spectra were
recorded in various solvents. Corresponding to the results from these previous reports,9
the peak positions of absorption bands were detected at the similar positions, while
bathochromic shifts were induced by increasing solvent polarity in the PL spectra (Figure
S4, Table 2). From the Lippert−Mataga plots in which the extent of slopes of the fitting
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Chemistry - An Asian Journal
10.1002/asia.201801529
line represents the degree of the ICT character in the emission, it was shown that DCB-
R showed the emission bands from the ICT state. Since the o-carborane units can work
as a strong electron-accepting unit toward the central benzodithiophene unit, formation
of the ICT state should proceed in the excited state.
Tables 1 and 2
Solid-state PL spectra of DCB-R were recorded at room temperature. Figure 2 shows
the PL spectra in the crystalline state, and the results are listed in Table 1. All compounds
showed blue-shifted emission compared to those in the THF solutions. It was implied that
re-orientation energy could decrease in the aggregation with o-carboranes, and
1
7
stabilization of the ICT state by solvation could be reduced. As a result, blue-shifted
emission bands were detected. The first impressive point is the peak positions of emission
bands. Compared to the THF solutions, DCB-tBu, DCB-OMe and DCB-CN which had
the trans-conformation presented emission bands in the shorter wavelength region than
DCB-H and DCB-OH. In the previous report on the crystal packing of DCB-tBu, it was
found that intramolecular twist was induced at the benzobithiophene moiety.16 This
conformational distortion should disturb electronic conjugation, followed by emission in
the shorter wavelength region. Similar intramolecular twists could occur in DCB-OMe
and DCB-CN. Another feature in the luminescent property was the dependency of
emission efficiency on the conformation. Because of larger emission efficiencies in the
crystalline state than those in the solution, it was clearly indicated that DCB-R had
crystallization-induced emission enhancement properties. Moreover, in the crystalline
state, the emission efficiencies of DCB-H and DCB-OH were relatively higher than those
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Chemistry - An Asian Journal
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of DCB-tBu, DCB-OMe and DCB-CN. Although the o-carborane units located at both
sides of the benzodithiophene moiety would suppress ACQ, lower emission properties
could be caused by the intramolecular twist in the trans conformation of DCB-tBu, DCB-
OMe and DCB-CN.
Figure 2
Stimuli-responsiveness of solid-state emission properties of DCB-R was evaluated by
adding various types of external stimuli to the crystalline samples (Figures 3, 4, 5 and S5).
Initially, thermochromic luminescent behaviors were investigated (Figure 3). The
emission bands showed bathochromic shifts as temperature increased, and consequently
luminescent color was gradually changed by heating (Figure S5). As a representative
result, DCB-OMe showed the emission band around 570 nm at room temperature, and
the bathochromic shift by 20 nm was observed by heating at 200 °C. In particular, it
should be emphasized that this luminescent chromism proceeded rapidly and repeatedly
(Figure 4). The degree of luminescent chromism seemed to be independent of the
conformations. Moreover, from DSC measurements, phase transitions were not observed
in the detection temperature range from 0 to 220 °C (Figure S6). It is assumed that
intramolecular motions might be induced by heating especially at the benzodithiophene
moiety in which distortion of the molecular plane and twists between the connections
1
6
with the o-carborane units were induced. Indeed, emission intensities of all compounds
decreased as temperature increased (Figure S7). This fact also supports that
intramolecular motions should be activated by heating, and luminescent properties were
critically influenced. The crystalline sample of DCB-OH showed slight luminescent
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Chemistry - An Asian Journal
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chromism. It is likely that intermolecular and intramolecular hydrogen bonds suppressed
molecular motions even at high temperature.
Figures 3 and 4
The stimuli-responsiveness of the crystalline sample of DCB-tBu toward another
external stimulus such as fuming with organic volatile compounds (VOCs) was able to
be examined owing to the largest value of emission efficiency in the crystalline state.
Interestingly, emission color was changed by exposing other solvents such as benzene,
CHCl
3
, THF, furan and 1,4-dioxane (Figure 5a). In contrast, emission bands were
1
insensitive to vapors of n-hexane, toluene, CH
2
Cl
2
, acetone and MeCN. From the H
NMR measurements, it was found that the former solvents can be incorporated into the
crystal packing (Figure 5b). Furthermore, from the powder X-ray diffraction analyses,
different patterns were obtained from that of DCB-tBu with these encapsulated solvents
(Figure 5c). Hence, it is suggested that vapochromic luminescent behaviors of DCB-tBu
should be originated from the crystal−crystal phase transition by insertion of fumed VOCs
into the crystal packing. Morphology changes could be reflected to luminescent chromism
via the alteration of electronic structures of DCB-tBu.
Figure 5
The similar tendencies of a peak position and emission efficiency were also observed
by the formation of amorphous states (Table 1). Aggregation was induced by the 100-
–
3
folds dilution of the THF solution containing 1.0  10 M DCB-R with deionized water.
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In the aggregation state, all compounds also demonstrated blue-shifted emission with
larger emission efficiency compared to those in the THF solutions. These data mean that
DCB-R should have the aggregation-induced emission enhancement (AIEE) property.
Similarly to the crystalline samples, the molecules having the trans-conformation
provided emission bands in the shorter wavelength region with larger emission efficiency.
Intramolecular distortion could be responsible for optical properties. Significant AIEE
was observed from DCB-OH. It is likely that intermolecular interaction could be
disturbed by the hydrogen bond formation.
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Conclusion
Stimuli-responsiveness in the solid-state emissive properties of DCB-R via the unique
process is demonstrated. Structural diversity was obtained from the derivatives with
various types of substituents. In particular, these molecules showed rapid and reversible
thermochromic luminescence in the crystalline state. From the series of analyses, the
plausible mechanism was suggested. By heating, molecular motions should be activated
in the crystalline packing. Then, the room to form thermally-stable conformations could
be generated. As a result, extension of conjugated systems could occur, resulting in the
bathochromic shifts of luminescence bands. Owing to small structural changes, rapid and
reversible responses can be obtained. This strategy to tune luminescent color and to
realize thermochromism could be useful for construction the chemical sensors, smart
windows, multi-color lasers and molecular thermometers.
ACKNOWLEDGMENT
This work was partially supported by the Asahi Glass Foundation (for K.T.) and a
Grant-in-Aid for Scientific Research (B) (JP17H03067) and (A) (JP17H01220), for
Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-
Blocks (No.2401)” (JP24102013) and for Challenging Research (Pioneering)
(JP18H05356).
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Chemistry - An Asian Journal
10.1002/asia.201801529
Figures and Tables
t
Scheme 1. Synthesis of DCB-R (R = H, Bu, CN, OMe)
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Chemistry - An Asian Journal
10.1002/asia.201801529
Scheme 2. Synthesis of DCB-OH
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Chemistry - An Asian Journal
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Figure 1. (a, b) ORTEP diagrams, (c) packing structure and (d) hydrogen bonds and dimeric
structure of DCB-OH.
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Chemistry - An Asian Journal
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Figure 2. PL spectra in (a) the aggregation and (b) the crystalline state of DCB-R at room temperature.
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Chemistry - An Asian Journal
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Figure 3. Variable-temperature normalized PL spectra with solid DCB-R samples. All samples were
placed on the cover glass and heated by a hot plate.
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Chemistry - An Asian Journal
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Figure 4. Thermochromism of solid-state emission during stepwise heating and cooling cycles was
performed with DCB-H.
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Chemistry - An Asian Journal
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1
Figure 5. (a) PL spectra (b) H NMR spectra in CDCl
circles: solvent peaks of CHCl
DCB-tBu after exposing with various solvents.
3
for CHCl
3
-fumed sample and CD
2
Cl
2
for others (red
3
, dioxane, furan, THF and benzene, respectively) and (c) PXRD patterns of
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Chemistry - An Asian Journal
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Table 1. Wavelengths of maximum absorbance and emission and emission efficiencies of DCB-R under
various conditions
THF (nm)a
Φ
a,b
PL
λ
agg (nm)
638
Φ
PL
b
λ
cry (nm)
614
Φ
PL
b
R
H
λ
abs
λ
403
408
404
406
407
682
0.12
0.12
0.03
0.02
<0.01
0.23
0.72
0.43
0.25
0.17
0.90
0.94
0.67
0.64
0.32
OH
tBu
OMe
CN
670
627
611
698
615
586
686
628
563
727
648
645
a
b
−5
1
.0 × 10 M
Determined as an absolute value with the integration sphere method.
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Chemistry - An Asian Journal
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Table 2. Summary of changes in optical properties of DCB-R by altering solvent types
DCB-H
DCB-tBu
DCB-OMe
DCB-CN
DCB-OH
solvent
λ
ab
λ
PL
Δλ
λ
ab
λ
PL
Δλ
λ
ab
λ
PL
Δλ
λ
ab
λ
PL
Δλ
λ
ab
λ
PL
Δλ
(cm )
–1
–1
–1
–1
–1
(
nm) (nm)
(cm ) (nm) (nm)
(cm ) (nm)
(nm)
(cm ) (nm) (nm)
(cm ) (nm) (nm)
e
396
401
400
403
400
402
603
658
660
682
681
714
8668
9740
9848
10151
10315
10870
402
405
404
404
405
405
614
666
670
689
692
722
8588
9676
9827
10238
10240
10840
400
406
404
406
405
404
610
668
661
686
685
709
8606
9660
9623
10053
10092
10648
401
407
405
406
408
405
624
682
686
727
703
764
8912
9907
10114
10875
10285
11602
394
401
398
402
398
397
592
639
635
671
658
686
8488
9288
9377
9972
9928
10611
benzene
CHCl
THF
CH Cl
MeCN
3
2
2
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Chemistry - An Asian Journal
10.1002/asia.201801529
Graphical Abstract
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