Effects of Multi-Carborane Substitution on the Photophysical and Electron-Accepting Properties of o-Carboranylbenzene Compounds

Article abstract of DOI:10.1002/ejic.201700192

Multiple o-carborane substituted compounds, mono-, 1,3-bis-, and 1,3,5-tris[2-(4-butylphenyl)-o-carboran-1-yl]benzene (1–3), were prepared and characterized by multinuclear NMR spectroscopy and elemental analysis. The solid-state structures of 2 and 3 wer

Full text of DOI:10.1002/ejic.201700192

A Journal of
Accepted Article
Title: Effects of multi-carborane substitution on the photophysical
and electron accepting properties of o-carboranylbenzene
compounds
Authors: Dong Kyun You, Ji Hye Lee, Byung Hoon Choi, Hyonseok
Hwang, Min Hyung Lee, Kang Mun Lee, and Myung Hwan
Park
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10.1002/ejic.201700192
European Journal of Inorganic Chemistry
FULL PAPER
Effects of multi-carborane substitution on the photophysical and
electron accepting properties of o-carboranylbenzene
compounds
Dong Kyun You,[a]† Ji Hye Lee,[a]† Byung Hoon Choi,[a] Hyonseok Hwang,[a] Min Hyung Lee,*[b] Kang
Mun Lee,*[a] and Myung Hwan Park*[c]
Abstract: Multiple o-carborane substituted compounds, mono-, 1,3-
bis-, and 1,3,5-tris-(2-(4-butylphenyl)-o-carboran-1-yl)benzene (1−3),
were prepared and characterized by multinuclear NMR spectroscopy
and elemental analysis. The solid-state structures of 2 and 3 were
also confirmed by single crystal X-ray diffraction. While the mono-
carborane compound 1 was non-emissive in solution state at 298 K,
the photoluminescence (PL) spectra of 2 and 3 exhibited weak to
moderate emission (λem = 352 nm for 2 and 363 nm for 3 in THF).
dimensional (3D) analogues to aromatic derivatives.[1] Because
of these unique properties, carboranyl compounds have been
utilized as functional materials for a wide range of applications.[2]
In particular, o-carborane (closo-1,2-C2B10H12)-appended
compounds have recently attracted great interest for
optoelectronic applications due to their unique luminescent
properties and high thermal/chemical stabilities.[3-5] These
intriguing properties are attributed to the intrinsic nature of the o-
carborane units, such as strong electron-withdrawing effects
through the C atom on the cage and highly polarizable σ-
aromatic characters,[6] leading to the expansion of their
utilization as promising luminescent materials.[7] To date, in order
to induce fascinating photophysical properties, structural
modifications have been made through the incorporation of o-
carborane groups into transition-metal complexes[8-12] or organic
luminophores.[13] In particular, o-carborane-incorporated organic
luminophores often exhibit unusual multiple emission behaviour
derived from discrete intramolecular charge transfer (ICT) and
locally excited (LE) states.[13-25] These features have been
mainly observed in the donor-acceptor dyad systems in which
the o-carborane group acts as an acceptor. For example, Kang
and co-workers have reported donor-acceptor dyads (I in Figure
1) having carbazole units as a donor, demonstrating unique
charge-separated excited states between the individual
moieties.[14,15] Fox et al. have also reported C-diazaboryl-o-
carborane dyads (II) exhibiting dual emissions from high- energy
LE and low-energy CT states depending on the substituents and
molecular geometry.[16,17] Furthermore, o-carborane derivatives
Compounds 2 and 3 showed intriguing dual emission bands (λem
=
361 and 537 nm for 2 and λem = 387 and 520 nm for 3) at 77 K and
in film, of which the low-energy band was dominant in the solid state.
TD-DFT calculations on the S1 optimized structures suggested that
the low-energy fluorescence of 2 and 3 was attributed to the π(4-
butylphenyl) → π*(phenylene-o-carborane) intramolecular charge
transfer (ICT) transition. The low-energy electronic transition of 2
and 3 was apparently associated with an aggregation-induced
emission (AIE), and enhanced emission intensity (λem = ca. 570 nm
for 2 and λem = ca. 550 nm for 3) was observed upon increasing the
water fraction (fw) in THF/water mixtures. Furthermore, the PL
experiments of poly(3-hexylthiophene-2,5-diyl) (P3HT) polymer films
doped with 3 revealed the excellent electron-accepting properties of
3.
Introduction
Icosahedral carboranes (C2B10H12) are well-known boron-cluster
compounds that can be often characterized as three-
[a]
Dr. J. H. Lee, D. K. You, B. H. Choi, Prof. Dr. H. Hwang, Prof. Dr. K.
M. Lee
Department of Chemistry
Institute for Molecular Science and Fusion Technology
Kangwon National University, Chuncheon, Gangwon 24341
Republic of Korea
E-mail: kangmunlee@kangwon.ac.kr
Prof. Dr. M. H. Lee
Department of Chemistry and EHSRC
University of Ulsan, Ulsan 44610
Republic of Korea
[b]
[c]
E-mail: lmh74@ulsan.ac.kr
Prof. Dr. M. H. Park
Department of Chemistry Education
Chungbuk National University, Cheongju, Chungbuk 28644
Republic of Korea
E-mail: mhpark98@chungbuk.ac.kr
† These authors contributed equally to this work
Figure 1. o-Carborane derivatives exhibiting multiple emission behaviour.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201700192
European Journal of Inorganic Chemistry
FULL PAPER
(III) with 2-pyridyl or diphenylphosphino groups displayed multi-
emission features via different excited states.[25] Recently, we
have reported that a biphenylene-bridged dimeric o-carboranyl
triarylborane compound (IV) can show dual emissions resulting
from a triarylborane-based LE and an ICT-based aggregation-
induced emission (AIE) in a rigid state.[13a] We also found that
“through-space” ICT between the appended aryl group and the
central phenylene ring can lead to emission color tuning of para-
di-o-carboranylbenzene compounds (V) depending on the type
of substituent on the appended aryl group.[18]
r
A
C
C
C
C
r
r
A
A
,
,
r
A
a
2
2
33%
55
%
r
r
r
r
B
B
i
iii
)
( )
(
r
A
v
i
)
ii
( )
r
B
(
C
C
r
r
A
r
A
A
B
B
C
C
C
C
r
r
A
A
,
3 4
,
Although these emission behaviors in π-substituted o-
carborane derivatives have been extensively examined, detailed
investigation on the photophysical properties caused by the
alteration of the number of o-carborane units has not been
a
6%
3
2
6 %
-
=
n
u
r
b tyl
A
Scheme 1. Synthetic routes for
Pd(PPh3)2Cl2 , CuI, triethylamine, 4-butylphenylacetylene. (ii) toluene, 110 °C,
24 h. (iii) B10H14, Et2S. (iv) toluene, 110 °C, 72 h.
2 and 3. Reagents and conditions: (i)
reported. The introduction of multi-carborane units into
a
molecule could not only give rise to systematic variation of
optical properties, but also enhance electron-accepting ability.
The latter property could also be promising for optoelectronic
applications such as organic field-effect transistors (OFETs) and
organic photovoltaics (OPVs).[26] In an effort to investigate the
electronic effects of o-carborane and the photophysical
properties of the resulting aryl-o-carborane compounds, we
herein report mono-, bis-, and tris-o-carboranylbenzene (1−3)
compounds and their luminescent/electron-accepting properties
along with computational details.
Results and Discussion
Synthesis and characterization. The bis-acetylene (2a) and
tris-acetylene (3a) precursors were readily obtained from the
palladium-catalyzed Sonogashira coupling reactions of 1,3-
dibromobenzene
and
1,3,5-tribromobenzene
with
4-
butylphenylacetylene, respectively (Scheme 1). The final bis-
and tris-carborane compounds (2 and 3) were prepared from the
well-known boron cage forming reaction of decaborane (B10H14)
with the acetylene precursors (2a and 3a) in the presence of
excess Et2S (yield = 33% for 2 and 46% for 3). Note that the n-
Bu group was introduced to the 2-phenyl substituent of o-
carborane to endow high solubility in common organic solvents.
In addition, mono-carborane compound 1 was also similarly
prepared to compare the effects of multi-o-carborane
substitution on the luminescent properties.
Figure 2. X-ray crystal structures of 2 (left) and 3 (right) (40% thermal
ellipsoids). The H atoms are omitted for clarity.
X-ray diffraction analyses were obtained from a CH2Cl2/n-
hexane solution at −20 °C (Figure 2). While bis-carborane
compound 2 has syn conformation, tris-carborane compound 3
shows C1 symmetry and anti-stereochemistry, in which one 2-Ar
group is oriented to the opposite side of the others, probably due
to steric congestion between the 2-Ar groups. The anti-
conformation of 3 is in contrast to the previously reported crystal
structure of 1,3,5-tris(2-Ph-o-carboran-1-yl)benzene compound
without n-Bu group in the phenyl ring, which possessed C3
symmetry and syn stereochemistry of three phenyl groups.[28]
The substantial torsion angles of Ccage−Ccage−CPh−CPh in 2 (ψ =
51.3°) and 3 (ψ = 66.7°) can be mainly ascribed to the steric
hindrance between the 2-Ar and central phenylene rings.
The formation of 1−3 was characterized by multinuclear NMR
spectroscopy (Figure S15−S23 in the Supporting Information),
1
high resolution mass spectroscopy, and elemental analysis. H
and 13C NMR spectra of 1−3 exhibited the expected resonances
corresponding to the 2-aryl-o-carboranyl (aryl = 4-n-BuPh) and
central phenylene moieties. The 11B NMR signals of 1‒3
detected in the region of δ −1 to −11 ppm confirmed the
presence of o-carboranyl boron atoms. Moreover, the 11B NMR
spectra of 1‒3 distinctly exhibited two coupled peak sets in an
integral ratio of 3:7, although less resolved splitting patterns for 2
and 3 even at 60 °C (Figures S17, S20, and S23 in the
Supporting Information). The B−H coupling constants (1JB-H) of
ca. 126−166 Hz are also similar to the range observed for
carborane compounds.[27] Single crystals of 2 and 3 suitable for
Thermal and electrochemical stabilities. The thermal
stabilities of the bis- and tris-carborane compounds (2 and 3)
were examined by thermogravimetric analysis (TGA) (Figure 3a
and Table 1). Compounds 2 and 3 exhibited Td5 values of 361
and 376 °C, respectively, indicating high thermal stability.
Inherited from high thermal stability of o-carborane cage, the Td5
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10.1002/ejic.201700192
European Journal of Inorganic Chemistry
FULL PAPER
Table 1. Photophysical and electrochemical data of 1‒3
λem.max [nm]
DMSO[b]
298 K
[h]
[j]
λabs[a] /nm
(log ε)
Ered
/V
LUMO
/eV
Td5
fw
=
Cyhex[b]
298 K
THF[b]
298 K
film[c,d]
Solid[d]
Φem
[i]
90 %[d,e]
/°C
77 K
THF[b,f]
Solid[g]
233 (3.94),
273 (3.78)
[k]
[k]
[k]
[k]
[k]
1
‒
‒
‒
‒
‒
−1.99
−2.81
2
3
213 (4.67)
229 (4.81)
327
337
352
363
361, 537
387, 520
363
376
380, 538
382, 520
539
521
573
546
0.040
0.147
0.097
0.102
−1.49
−1.27
−3.31
−3.53
361
376
[a]c = 5.0 × 10−5 M in THF. [b]c = 5.0 × 10−5 M, determined at 298 K (Excitation, λex = 290 nm for 1, 296 nm for 2, and 309 nm for 3). [c]PMMA film doped with 20
[d]
wt% of compound.
λ
= 340 nm. [e]c = 5.0 × 10−4 M. [f]Quinine sulfate (Φ = 0.55) used as a standard. [g]Absolute photoluminescence quantum yield. [h]The
ex
reduction onset potential in DMSO (c = 5.0 × 10−4 M, scan rate = 100 mVs−1) with reference to the ferrocene/ferrocenium (Fc/Fc+) redox couple. [i]Calculated from
the Ered
[j]Determined by TGA at 5% weight loss. [k]Not observed.
.
value of tris-substituted 3 was also higher than that of bis-
substituted 2.
The electrochemical properties of 2 and 3 were investigated
by cyclic voltammetry (CV) measurements (Figure 3b and Table
1). Both 2 and 3 showed carborane-centred, quasi-reversible
reduction behaviour (Ered = −1.49 and −1.27 V, respectively),
which is similar to the typical characteristic of 1,2-di-aryl-o-
caborane compounds.[29,30] To examine the effect of increment in
the number of o-carborane groups, the reduction potential (Ered
=
−2.01 V) of a mono-o-carboranylbenzene compound (1) was
further measured (Figure S1a in the Supporting Information).
According to the reduction potentials of 1−3, which show gradual
anodic shifts with the number of o-carborane groups, the LUMO
energy levels for 1−3 were estimated to be −2.79,
−3.31, and −3.53 eV, respectively. These results clearly indicate
the increased stabilization of LUMO level with the increasing
number of 2-Ar-o-carborane groups. This could be attributed to
not only the enhanced inductive electron-withdrawing effects by
the o-carborane groups, but also the conjugation effects of o-
carboranes.[4b,8,10] Furthermore, multiple CV cycles revealed the
high electrochemical stability of 1−3 (Figure S1 in the Supporting
Information).
Figure 3. (a) TGA curves and (b) cyclic voltammograms (0.5 mM in DMSO,
scan rate = 100 mV s−1) of 2 and 3.
Optical properties. The optical properties of 1−3 were
examined by UV/Vis absorption and photoluminescence (PL)
measurements. The absorption spectra of 2 (λabs = 213 nm) and
3 (λabs = 229 nm) in THF exhibited intense absorption in the
range of 210−250 nm mainly assignable to ππ* transition in the
central phenylene and the appended 2-Ar rings (Figure 4 and
Table 1). In addition, 2 and 3 showed weak low-energy
absorption from ca. 275 nm to ca. 350 nm with the more red-
shifted feature in 3. These low- energy tails could be tentatively
assigned to π(2-Ar) → π*(central phenylene) charge transfer (CT)
transitions in nature (see TD-DFT results below). Similar weak
low-energy absorption (λabs = 250−325 nm, ε < 6000 M−1) was
also observed for 1 (Figure S2 in the Supporting Information).
These absorption features are similar to those of the para-
substituted di-o-carborane compounds.[18]
Figure 4. UV/Vis absorption in THF and PL spectra for (a) 2 and (b) 3.
Next, the PL spectra of 1−3 were measured in solution and
solid state (Figure 4 and Table 1). In contrast to the non-
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10.1002/ejic.201700192
European Journal of Inorganic Chemistry
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observed for 2 and 3 in solution could be attributed to the weak
conjugation between the central phenylene ring and the meta-
substituted carborane cages. As similarly observed in the
absorption spectra, the position of the emission band for tris-
carborane substituted 3 was red-shifted as compared to that for
bis-substituted 2. The narrower band gap for 3 than for 2 can be
related to the greater LUMO stabilization in the former by the
three carborane moieties, as shown in the reduction potential
measurements. Furthermore, the quantum efficiency of 3 (Φem
=
0.147) was much higher than that of 2 (Φem = 0.040) in THF.
Note that the mono-substituted 1 was non-emissive in THF. This
finding indicates that the increase in the number of o-carborane
units could enhance the quantum efficiency. This might be partly
ascribed to the increased steric congestion between the 2-Ar-o-
carborane moieties, restricting the free rotation of the carborane
moieties.[16,17] Additional PL experiments of 2 and 3 were
performed in solvents of different polarities (Table 1 and Figure
S3 in the Supporting Information). The emission spectra of 2 and
3 clearly showed bathochromic shifts with increasing solvent
polarity, indicating polarized excited states in 2 and 3. Along with
the broad featureless emission bands, these results indicate that
the emissive excited states of 2 and 3 in solution bear a CT
character.
Figure 5. PL spectra of (a) 2 and (b) 3 in THF−water mixtures (5.0 × 10−4 M,
λex = 340 nm). Inset shows the emission colour of 2 and 3 in 0% or 90% water
mixtures under a hand-held UV lamp (λex = 365 nm).
emissive nature of 1 in solution state, the emission spectra of 2
and 3 in THF at 298 K exhibited weak to moderate emission
bands (λem = 352 nm for 2 and 363 nm for 3). This feature is
different from that found for the para-substituted di-o-carborane
compounds,[18] as well as other o-carborane derivatives,[15,29a,31-
36]
which are almost non-emissive in solution. For the para-
substituted di-o-carboranes, it was suggested that the
substantial conjugation between two carborane cages and a
central phenylene ring, which is coupled with the C−C bond
variation of o-carborane, was responsible for the emission
quenching in solution.[18] Thus, the distinct emission bands
Most interestingly, 2 (ca. 370 nm and 538 nm) and 3 (ca.
380 nm and 510 nm) exhibited dual emission bands (Figure 4
and Table 1) in a rigid state, such as at 77 K and in film,
Figure 6. Frontier molecular orbitals for 2 and 3 at their ground state (S0) and first excited singlet state (S1) with their relative energies from DFT calculation. The
transition energy (in nm) was calculated using the TD-B3LYP method with 6-31G(d) basis sets.
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10.1002/ejic.201700192
European Journal of Inorganic Chemistry
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although the intensities of the high-energy and low-energy
bands are differently observed for 2 and 3 at 77 K. In addition, a
broad low-energy emission band was dominantly observed in
the solid state (λem = 539 nm for 2 and λem = 521 nm for 3). The
high-energy emission bands for 2 and 3 in the rigid state were
similar to those observed in solution, indicating the same origin
of emission. Thus, the low-energy emission observed for 2 and 3
in the rigid state and solid state could be involved with
aggregation-induced emission (AIE), as in the case of previously
reported o-carborane-appended luminophores.[7a,13a,16‒18,33] To
clarify the AIE phenomenon in 2 and 3, PL experiments were
performed in THF−water mixed solvents (Figure 5). The
emission intensity of 2 and 3 was remarkably enhanced upon
increasing the water fraction (fw) in THF−H2O mixtures. In
particular, when fw is as high as 90%, the intensity of the broad
low-energy emission (ca. 570 nm for 2 and 550 nm for 3) was
maximized. Consequently, while the compound 2 and 3 showed
blue emission in THF, aggregates of 2 and 3 in a THF−H2O (fw =
90%) mixture exhibited orange and yellow fluorescence,
respectively, similar to the low-energy emission bands in the
solid state above. Furthermore, the quantum efficiency of 2 (Φem
= 0.097) in solid was comparable to that of 3 (Φem = 0.102).
Table 2 The major low-energy electronic transition for 2 and 3 at their first
excited singlet state (S1) calculated using the TD-B3LYP method with 6-
31G(d) basis sets.[a]
λcalc/ nm
f
Assignment
2
3
484.61
399.88
0.0006
0.3882
HOMO−1 → LUMO (99.7%)
HOMO−4 → LUMO (72.4%)
HOMO−5 → LUMO (23.6%)
493.03
386.57
0.0030
0.4121
HOMO−1→ LUMO (99.7%)
HOMO−6 → LUMO (97.3%)
[a]Singlet energies for the vertical transition calculated at the optimized S1
geometries.
thus, the transition can be described as local transition centred
on the phenylene-carborane fragments. On the other hand, the
experimentally observed low-energy AIE in the rigid state of 2
and 3 could be mainly assigned to the HOMO−1 → LUMO
transition (λcalc = 484.61 nm for 2 and λcalc = 493.03 nm for 3).
Since the HOMO−1 for 2 and 3 is localized on the 2-Ar groups
(91%), this transition involves the π(2-Ar) → π*(phenylene and
carborane) ICT transition. Owing to the restricted rotation of the
carborane cage, the ICT state may undergo radiative decay in
the rigid state.[10,13a,16,18]
Theoretical calculation. To elucidate the absorption and
emission features of 2 and 3, TD-DFT calculations on the
ground state (S0) and first excited state (S1) optimized structures
were performed with the B3LYP functional and 6-31G(d) basis
sets (Figure 6 and Table 2). The conductor-like polarizable
continuum model (CPCM) was also used to include the solvent
effects of THF.[37] Based on the TD-DFT results, the low-energy
absorptions of 2 and 3 are mainly associated with the HOMO−1
→ LUMO transition (f > 0.1). While the HOMO−1 for 2 and 3 is
predominantly localized on the 2-Ar group of the o-carborane
(91%, Tables S5 and S9†), the LUMO is delocalized over the
central phenylene ring (52% for 2 and 57% for 3) and carborane
cage (38% for 2 and 34% for 3). This result indicates that the
HOMO−1 → LUMO transition in both 2 and 3 is mainly
assignable to the π(2-Ar) → π*(phenylene and carborane) ICT
transition. This feature could also be characterized by through-
space CT transition.[30] In addition, the computed low-energy
absorption wavelength for 3 (λabs = 279 nm) is red-shifted as
compared to that for 2 (λabs = 267 nm), as similarly found in the
experimental absorption.
Electron accepting properties of 3. To examine the electron-
accepting properties of 3, the PL spectra of poly(3-
hexylthiophene-2,5-diyl) (P3HT) polymer film doped with 3 were
investigated (Figure 7). P3HT is well known as a donor for
typical electron-accepting materials such as fullerene (C60)
derivatives in OPVs. Moreover, the LUMO level (−3.53 eV) of 3,
calculated from CV measurements, is similar to those of
fullerene-based acceptors (ca. −3.7 eV).[38] The P3HT film itself
displayed intense fluorescence in the region between 600 and
800 nm (black line) with excitation at 525 nm, while 3 showed
virtually no emission (pink line) at the same excitation (Figure 7).
However, as the doping ratios of 3 to P3HT increased, the
overall intensity of P3HT emission was gradually quenched. In
particular, the PL intensity of the P3HT/3 film sample with a
According to the TD-DFT calculations on the S1 optimized
structures of 2 and 3, the high-energy emission was observed at
around 400 nm in both 2 and 3. The major contribution (f > 0.3)
to the emission of 2 and 3 involves HOMO−4 → LUMO and
HOMO−6 → LUMO transition, respectively. While both the
HOMO−4 (for 2) and HOMO−6 (for 3) are mainly located on the
central phenylene ring (72−75%) with an appreciable
contribution from the carborane cage (23−25%), the LUMOs (for
2 and 3) are predominantly localized on the carborane cage for
both compounds (ca. 85%, Table S7 and S11†). As a result, the
high-energy emissions can be assigned to partial CT transition
from the central phenylene to carborane. 2-Ar groups on the o-
carboranes have negligible contributions to both transitions, and
Figure 7. Photoluminescence spectra of P3HT films doped with 3 at 298 K (λex
= 525 nm).
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10.1002/ejic.201700192
European Journal of Inorganic Chemistry
FULL PAPER
experiments were performed using a Metrohm Autolab PGSTAT12
system.
1:1.5 weight ratio decreased over 97%. In contrast, the
absorption bands of the corresponding film samples remained
unchanged with the increase in the doping amount of 3 (Figure
S8 in the Supporting Information). These results strongly support
the fact that photoinduced CT from P3HT (as donor) to 3 (as
acceptor) occurs in the P3HT/3 films, which quenches the
P3HT-based fluorescence. This in turn indicates that the excited
electrons in the LUMO of P3HT (ca. −3.3 to −3.5 eV) can easily
move to the LUMOs of 3. Accordingly, it is expected that the tris-
carborane compound 3 could be utilized as a potential electron-
accepting or absorbing material in optoelectronic applications.
Synthesis of 1-butyl-4-(phenylethynyl)benzene (1a). To the mixture of
bromobenzene (0.50 g, 3.18 mmol), copper iodide (80 mg), and
Pd(PPh3)2Cl2 (120 mg) in 10 mL of toluene was added triethylamine (90
mL) at room temperature. After stirring for 15 min, 4-
butylphenylacetylene (0.68 mL, 3.81 mmol) in toluene (10 mL) was
added to the resulting dark brown slurry. The reaction mixture was then
refluxed for 24 h. All the volatiles were removed at reduced pressure,
affording gray oily residue. Purification was carried out by column
chromatography (silica gel, CH2Cl2/hexane as an eluent) to afford the
desired product 1a as a pale yellow oil (0.32 g, 43%). 1H NMR (CDCl3): δ
7.50 (d, J = 8.0 Hz, 2H, C6H4), 7.42 (d, J = 8.0 Hz, 2H, C6H4), 7.31 (d, J =
7.6 Hz, 2H, Ph-H), 7.30 (t, J = 8.2 Hz, 1H, Ph-H), 7.14 (d, J = 8.0 Hz, 2H,
Ph-H), 2.60 (t, J = 15.6 Hz, 2H of n-butyl), 1.58 (m, 2H of n-butyl), 1.33
(m, 2H of n-butyl), 0.91 (t, J = 14.8 Hz, 3H of n-butyl). 13C NMR (CDCl3):
δ 143.41, 131.56, 131.52, 128.48, 128.32, 128.06, 123.53, 120.40, 89.62
(Acetylene-C), 88.73 (Acetylene-C), 35.62, 33.42, 22.33, 13.96.
Conclusions
We have prepared and characterized mono-, bis- and tris-o-
carborane-substituted benzene compounds (1−3). While the
mono-substituted compound 1 was non-emissive in solution
state at 298 K, 2 and 3 showed weak to moderate emission in
the high-energy region. Compounds 2 and 3 exhibited dual
emission bands at 77 K and in film, of which the broad low-
energy emission band was dominantly observed in the solid
state. Computational studies suggested that the additional low-
energy emission in 2 and 3 was involved with the ICT state. The
emission was also assigned to AIE in nature, showing enhanced
emission intensity upon increasing water fraction in THF/water
mixtures. Finally, PL experiments using P3HT polymer film
doped with 3 showed the excellent electron-accepting properties
of 3.
Synthesis of 1,3-Bis(2-(4-Butylphenylacetylenyl)benzene (2a). This
compound was prepared in a manner analogous to the synthesis of 1a
using 1,3-dibromobenzene. Purification was carried out by column
chromatography (silica gel, CH2Cl2/hexane as an eluent) to afford the
desired product as a pale gray solid (0.43 g, 55%). 1H NMR (CDCl3): δ
7.68 (s, 1H, C6H4), 7.45 (dd, J = 9.2 Hz, 2H, C6H4), 7.42 (d, J = 8.0 Hz,
4H, Ph-H), 7.30 (t, J = 7.6 Hz, 1H, C6H4), 7.15 (d, J = 8.0 Hz, 4H, Ph-H),
2.61 (t, J = 12.0 Hz, 4H of n-butyl), 1.59 (m, 4H of n-butyl), 1.33 (m, 4H of
n-butyl), 0.92 (t, J = 14.8 Hz, 6H of n-butyl). 13C NMR (CDCl3): δ 143.57,
134.47, 131.53, 131.01, 128.48, 128.36, 123.79, 120.13, 90.13
(Acetylene-C), 87.95 (Acetylene-C), 35.60, 33.38, 22.30, 13.92. Anal.
Calcd for C30H30: C, 92.26; H, 7.74. Found: C, 91.93; H, 7.50.
Synthesis of 1,3,5-Tris(2-(4-butylphenylacetylenyl)benzene (3a). This
compound was prepared in a manner analogous to the synthesis of 1a
using 1,3,5-tribromobenzene. Purification was carried out by column
chromatography (silica gel, hexane) to afford the desired product as a
colorless oil (0.48 g, 62%). 1H NMR (CDCl3): δ 7.62 (s, 3H, C6H3), 7.45 (d,
J = 8.0 Hz, 6H, Ph-H), 7.17 (d, J = 7.6 Hz, 6H, Ph-H), 2.63 (t, J = 15.6 Hz,
6H of n-butyl), 1.61 (m, 6H of n-butyl), 1.36 (m, 6H of n-butyl), 0.94 (t, J =
14.4 Hz, 9H of n-butyl). 13C NMR (CDCl3): δ 143.71, 133.72, 131.57,
128.49, 124.13, 119.93, 90.62 (Acetylene-C), 87.30 (Acetylene-C), 35.60,
33.35, 22.29, 13.91; Anal. Calcd for C42H38: C, 92.94; H, 7.06. Found: C,
92.51; H, 7.11.
Experimental Section
General considerations. All operations were performed under an inert
nitrogen atmosphere using standard Schlenk and glove box techniques.
Anhydrous grade solvents (Sigma Aldrich) were dried by passing through
an activated alumina column and stored over activated molecular sieves
(5 Å). Spectrophotometric-grade tetrahydrofuran (THF) was used as
received from Sigma Aldrich. The following commercial reagents were
used without any further purification after purchasing from Sigma Aldrich:
bis(triphenylphosphine)palladium(II) dichloride, copper(I) iodide, n-
butyllithium hexane solution (2.5M), diethyl sulfide, triethylamine, 4-
butylphenylacetylene, bromobenzene, 1,3-dibromobenzene, 1,3,5-
Synthesis of 2-(4-Butylphenyl)-o-carboran-1-yl)benzene (1). To a
toluene solution (100 mL) of decaborane (B10H14, 0.13 g, 1.06 mmol) and
1a (0.21 g, 0.89 mmol) was added an excess amount of Et2S (5 equiv) at
room temperature. After heating to reflux, the reaction mixture was
further stirred for 3 days. The solvent was removed under vacuum. The
solution was purified by column chromatography on alumina using
toluene/n-hexane as an eluent, affording 1 as a pale yellow oil (0.17 g,
54%). 1H NMR (CDCl3): δ 7.39 (d, J = 8.4 Hz, 2H), 7.28 (d, J = 8.4 Hz,
2H), 7.21 (t, J = 14.8, 1H), 7.11 (t, J = 15.2 Hz, 2H), 6.90 (d, J = 8.4 Hz,
2H), 3.30-1.50 (br, 10H, CB-H), 2.45 (t, J = 15.6 Hz, 2H of n-butyl), 1.45
(m, 2H of n-butyl), 1.20 (m, 2H of n-butyl), 0.83 (t, J = 14.4 Hz, 3H of n-
butyl). 13C NMR (CDCl3): δ 145.29, 130.76, 130.62, 130.49, 130.01,
128.24, 128.17, 127.96, 85.56 (CB-C), 85.20 (CB-C), 34.96, 32.97, 22.14,
13.87. 11B NMR (acetone-d6, 25 °C): δ –2.4 (br d, 1JB-H = 153.5 Hz, 3B), –
tribromobenzene, poly(3-hexylthiophene-2,5-diyl). Decaborane (B10H14
)
was purchased from KatChem and used without further purification.
CDCl3 (Cambridge Isotope Laboratories) was used after drying over
activated molecular sieves (5 Å). NMR spectra were recorded on a
Bruker Avance 400 spectrometer (400.13 MHz for 1H, 100.62 MHz for
13C, and 128.38 MHz for 11B) at ambient temperature unless otherwise
stated. Chemical shifts are given in ppm, and are referenced against
external Me4Si (1H, 13C) and BF3·Et2O (11B). HRMS (EI) measurement
(JEOL JMS700) was carried out at Korea Basic Science Institute.
Elemental analyses were performed on an EA3000 (Eurovector) in the
Central Laboratory of Kangwon National University. UV/Vis absorption
and PL spectra were recorded on a Jasco V-530 and a Spex Fluorog-3
luminescence spectrophotometer, respectively. Thermogravimetric
analysis (TGA) was obtained using a TA Q500 under a flow of dry
nitrogen with a heating rate of 10 °C/min. Td5 values of 2 and 3 were
determined by TGA at 5% weight loss. Cyclic voltammetry (CV)
1
9.9 (overlapping dd, JB-H = 166.4 Hz, 7B). HRMS (EI): m/z Calcd for
C18H28B10: 354.3122. Found: 354.3123.
Synthesis of 1,3-Bis(2-(4-butylphenyl)-o-carboran-1-yl)benzene (2).
This compound was prepared in a manner analogous to the synthesis of
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700192
European Journal of Inorganic Chemistry
FULL PAPER
1 using 2a. After heating to reflux, the reaction mixture was further stirred
for 3 days. The solvent was removed under vacuum and MeOH (50 mL)
was added to the residue. The resulting yellow slurry was filtered and the
remaining solid was re-dissolved in toluene. The solution was purified by
column chromatography on alumina using toluene as an eluent.
Recrystallization from a mixture of CH2Cl2/n-hexane afforded 2 as a white
solid (0.48 g, 33%). Single crystals suitable for X-ray diffraction studies
were grown from cooling of a CH2Cl2/n-hexane solution of 2. 1H NMR
(CDCl3): δ 7.28 (s, 1H, C6H4), 7.25 (dd, J = 6.4 Hz, 2H, C6H4), 7.17 (d, J
= 8.4 Hz, 4H, CB-Ph-H), 6.93 (d, J = 8.4 Hz, 4H, CB-Ph-H), 6.92 (t, J =
11.2 Hz, 1H, C6H4), 3.30-1.50 (br, 20H, CB-H), 2.48 (t, J = 15.6 Hz, 4H of
n-butyl), 1.48 (m, 4H of n-butyl), 1.24 (m, 4H of n-butyl), 0.86 (t, J =
14.8 Hz, 6H of n-butyl). 13C NMR (CDCl3): δ 145.73, 132.21, 131.94,
130.82, 130.37, 128.49, 128.18, 127.49, 85.27 (CB-C), 83.20 (CB-C),
35.00, 32.97, 22.25, 13.83. 11B NMR (acetone-d6, 60 °C): δ –1.8 − –3.5
hexafluorophosphate was used as the supporting electrolyte. All solvents
were sufficiently degassed during
1 h before use. The reduction
potentials were recorded at a scan rate of 100 mV/s and reported with
reference to the ferrocene/ferrocenium (Fc/Fc+) redox couple.
UV/Vis absorption and photoluminescence (PL) experiments. UV/Vis
absorption and PL measurements in solution (5
×
10−5 M) were
performed with a 1-cm quartz cuvette. The solution quantum efficiencies
were measured with reference to that of quinine sulfate (0.5 M H2SO4, Φ
= 0.55).[44] The absolute photoluminescence quantum yields (PLQYs) of
2 and 3 in the solid state were obtained using an absolute PL quantum
yield spectrophotometer (Quantaurus-QY C11347-11, Hamamatsu
Photonics) equipped with a 3.3 inch integrating sphere at 298 K. The thin
film samples in PMMA were obtained from spin-coating of a THF solution
(1 mL) of PMMA (50 mg) containing o-carborane compounds (20 wt%
versus PMMA) on 15 × 15 mm quartz plate (thickness = 1 mm). P3HT
films doped with 3 were obtained from spin-coating of CHCl3 solutions
(1 mL) containing P3HT (10 mg) and 0 ‒ 1.5 wt% of 3.
1
(br m, 6B), –9.9 (overlapping dd, JB-H = 147.7 Hz, 14B). Anal. Calcd for
C30H50B20: C, 57.47; H, 8.04. Found: C, 57.10; H, 7.89. HRMS (EI): m/z
Calcd for C30H50B20: 630.5774. Found: 630.5778.
Synthesis of 1,3,5-Tris(2-(4-butylphenyl)-o-carboran-1-yl)benzene (3).
This compound was prepared in a manner analogous to the synthesis of
2 using 3a. Recrystallization from a solution of CH2Cl2/n-hexane afforded
3 as a white solid (0.23 g, 46%). 1H NMR (CDCl3): δ 7.15 (s, 3H, C6H3),
7.03 (d, J = 8.4 Hz, 6H, CB-Ph-H), 6.94 (d, J = 8.4 Hz, 6H, CB-Ph-H),
3.30-1.50 (br, 30H, CB-H), 2.50 (t, J = 16.0 Hz, 6H of n-butyl), 1.50 (m,
6H of n-butyl), 1.28 (m, 6H of n-butyl), 0.88 (t, J = 14.8 Hz, 9H of n-butyl).
13C NMR (CDCl3): δ 146.10, 133.37, 131.11, 130.26, 128.66, 127.15,
84.93 (CB-C), 81.51 (CB-C), 35.10, 33.01, 22.45, 13.84. 11B NMR
(acetone-d6, 60 °C): δ –2.6 (overlapping dd, 1JB-H = 126.1 Hz, 9B), –10.3
(br d, 1JB-H = 146.7 Hz, 21B). Anal. Calcd for C42H72B30: C, 55.97; H, 8.05.
Found: C, 55.85; H, 8.17. HRMS (EI): m/z Calcd for C42H72B30: 906.8426.
Found: 906.8431.
Acknowledgements
This research was supported by the Nano·Material Technology
Development Program (NRF-2016M3A7B4909246 for K. M. Lee)
and
Basic
Science
Research
Program
(NRF-
2016R1C1B1008452 for M. H. Park) through the National
Research Foundation of Korea (NRF) funded by the Ministry of
Science, ICT & Future Planning. M. H. Lee is thankful for the
Priority Research Centers program (2009-0093818) through the
NRF funded by the Ministry of Education. K. M. Lee thanks the
support from the 2015 Research Grant from Kangwon National
University.
X-ray crystallography. Single crystals of 2 and 3 were coated with
Paratone oil and mounted onto a glass capillary. The crystallographic
measurements were carried out using a Bruker SMART Apex II CCD
area detector diffractometer with a graphite-monochromated Mo-Kα
radiation (λ = 0.71073 Å). The structures were solved by direct methods
and all non-hydrogen atoms were subjected to anisotropic refinement by
a full-matrix least-squares method on F2 by using the SHELXTL/PC
package, resulting in X-ray crystallographic data of 2 and 3 in CIF format
(CCDC 1463120 and 1032713). Hydrogen atoms were placed at their
geometrically calculated positions and refined riding on the
corresponding carbon atoms with isotropic thermal parameters. The
detailed crystallographic data, bond lengths and angles are given in
Table S1 and S2†.
Keywords: Carboranes • Charge transfer • Cyclic voltammetry •
Luminescence • Photophysics
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10.1002/ejic.201700192
European Journal of Inorganic Chemistry
FULL PAPER
FULL PAPER
Multiple o-carborane substituted
compounds, mono-, 1,3-bis-, and
1,3,5-tris-(2-(4-butylphenyl)-o-
carboran-1-yl)benzene (1−3), were
prepared and characterized. Various
photophysical properties of these
series were investigated.
multi-carborane
Dong Kyun You, Ji Hye Lee, Byung
Hoon Choi, Hyonseok Hwang, Min
Hyung Lee*, Kang Mun Lee* and Myung
Hwan Park*
Page No. – Page No.
Effects of multi-carborane
substitution on the photophysical
and electron accepting properties of
o-carboranylbenzene compounds
This article is protected by copyright. All rights reserved.