O-carborane-based biphenyl and p-terphenyl derivatives

Article abstract of DOI:10.1002/asia.201400067

The synthesis and properties of biphenyl- and p-terphenyl-fused o-carboranes are described. Aryl rings in the biphenyl and p-terphenyl skeletons are highly coplanar because of the presence of the o-carborane unit. o-Carborane exhibits an electron-withdrawing character via the inductive effect, resulting in a decrease in both the HOMO and LUMO levels of oligophenyls without causing electronic perturbation. To be free from perturbations: The synthesis and properties of biphenyl- and p-terphenyl-fused o-carboranes are described. Aryl rings in the biphenyl and p-terphenyl skeletons are highly coplanar because of the presence of the o-carborane unit. o-Carborane exhibits an electron-withdrawing character via the inductive effect, resulting in a decrease in both the HOMO and LUMO levels of oligophenyls without causing electronic perturbation.

Full text of DOI:10.1002/asia.201400067

COMMUNICATION
DOI: 10.1002/asia.201400067
o-Carborane-based Biphenyl and p-Terphenyl Derivatives
Yasuhiro Morisaki,* Masato Tominaga, Takuya Ochiai, and Yoshiki Chujo*[a]
Abstract: The synthesis and properties of biphenyl- and p-
terphenyl-fused o-carboranes are described. Aryl rings in
the biphenyl and p-terphenyl skeletons are highly coplanar
because of the presence of the o-carborane unit. o-Carbor-
ane exhibits an electron-withdrawing character via the in-
ductive effect, resulting in a decrease in both the HOMO
and LUMO levels of oligophenyls without causing electron-
ic perturbation.
pected to be beneficial for applications in nonlinear optical
(NLO) materials.[6a] Marshall, Heeney, and co-workers re-
ported on the transistor performance of conjugated poly-
mers containing the bithiophene-fused o-carborane as an ac-
ceptor.[6b] A variety of electron-deficient and highly planar
oligoaryls can be obtained by the combination of o-carbor-
ane and oligoaryl systems; from this perspective, p-oligo-
phenyls were chosen for investigation in this study. Herein,
we report the synthesis and optoelectronic properties of p-
oligophenyl-fused o-carboranes; that is, compounds in which
biphenyl and p-terphenyl are attached to the two adjacent
carbons of o-carborane.
Recently, icosahedral carboranes (C2H12B10)[1] have been
used as building blocks for p-conjugated molecules[2] and
polymers.[3] In particular, o-carborane (o-C2H12B10), contain-
ing two adjacent carbon atoms, imparts aggregation-induced
emission (AIE)[4] properties to the molecules in which it is
incorporated.[2e–k,3e–h] Optoelectronic properties, including
AIE, are drastically altered by changes in orientation be-
tween the p-conjugated systems and the two adjacent
carbon atoms in the o-carborane unit.[2i,j] When the torsion
angle between the p-conjugated plane and the carbon–
carbon bond approaches 908, AIE is observed due to intra-
molecular charge transfer (CT) from the p-conjugated
system to the o-carborane cluster, as well as suppression of
molecular motion. On the other hand, at a torsion angle
close to 08, o-carborane withdraws electrons inductively
from the p-conjugated unit. Previously, we synthesized a bi-
thiophene-fused o-carborane,[5] in which bithiophene and
the o-carboraneꢀs carbon–carbon bond were fused; that is,
the bithiophene unit and the carbon–carbon bond were on
the same plane (dihedral angle of 08). As a result, the o-car-
borane moiety acted as a strong electron-withdrawing group
through an inductive effect rather than a conjugation effect.
Recently, the bithiophene-fused o-carborane has attracted
attention for optoelectronic materials.[6] Sun, Su, and co-
workers predicted that bithiophene-fused o-carborane deriv-
atives would exhibit large hyperpolarizability values by
virtue of the electron-withdrawing character, which is ex-
Synthetic routes to biphenyl- and p-terphenyl-fused o-car-
boranes (4 and 7) are shown in Scheme 1. Treatment of bis-
(2-bromophenyl)acetylene[7] (1) with decaborane (B10H14, 2)
in N,N-dimethylaniline afforded bis(2-bromophenyl)-o-car-
[a] Dr. Y. Morisaki, M. Tominaga, T. Ochiai, Prof. Dr. Y. Chujo
Department of Polymer Chemistry
Graduate School of Engineering
Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax : (+81)75-383-2607
E-mail: ymo@chujo.synchem.kyoto-u.ac.jp
chujo@chujo.synchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201400067.
Scheme 1. Synthesis of compounds 4 and 7.
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Yasuhiro Morisaki, Yoshiki Chujo et al.
borane (3) in 16% yield.[8] Subsequently, the reaction of 3
with nBuLi was carried out to obtain the dilithiated species,
and successive treatment with ZnBr2 and CuCl2 in situ af-
forded biphenyl-fused o-carborane 4 in 41% isolated yield.
Target compound 7 was prepared by the same method from
planar p-terphenyl skeleton with a torsion angle of 178.9(4)8
(C1-C2-C3-C7). From the structures of 4 and 7, it appears as
if the 2- and 2’-positions in the two adjacent phenyl rings
are rigidly held in the same plane by a clasp.
À
To obtain further insight into the aromaticity of the C1
C6 benzocarborane rings in 4 and 7, nucleus-independent
1,4-dibromo-2,5-bis(2-bromophenylethynyl)benzene
(5)[9]
(Scheme 1). Compounds 4 and 7 were obtained as air- and
moisture-stable white solids, and characterized by 1H, 13C,
and 11B NMR spectroscopy, high-resolution mass spectrome-
try (HRMS), and X-ray crystallography.
chemical shift (NICS) values were calculated at the B3LYP/
(d,p) level.[12] The results are sum-
6-31GACTHNUTRGNENG(U d,p)//B3LYP/6-31GHCATUNGTRENNUGN
marized in Figure 2, together with values for the related
compounds phenanthrene and 9,10-dihydrophenanthrene.
The NICS(1) value of central aromatic ring of phenanthrene
was À9.2 ppm, whereas that of the central 6-membered ring
of 9,10-dihydrophenanthrene was À0.1 ppm. In compound 4,
Compound 4 was soluble in common organic solvents
such as CHCl3, CH2Cl2, THF, toluene, benzene, and DMF.
Single crystals of 4 were readily obtained from CHCl3 and
MeOH, and the molecular structure, as determined by X-
ray crystallography, is shown in Figure 1.[10] The crystal
structure of 4 revealed a high planarity of the biphenyl skel-
eton with a torsion angle (C1-C2-C3-C7) of 179.2(2)8 be-
tween the two aromatic rings,[10a] which is almost same as
À
the NICS(1) values of the arene ring and C1 C6 benzocar-
borane ring were found to be À10.3 and À1.2 ppm, respec-
À
tively. The NICS(1) calculation of the C1 C6 benzocarbor-
ane ring of 7 was also carried out, and the value was esti-
mated to be À1.0 ppm (Figure 2). These results indicate that
that of phenanthrene (178.388).[11] The C5 C6 bond length
the C1 C6 benzocarborane rings of 4 and 7 are non-aromat-
À
À
(1.643(2) ꢂ) of the carborane cage in 4 is longer than those
in benzene (1.397 ꢂ), a general carbon–carbon double bond,
and a single bond. The solubility of 7 in organic solvents
was much lower than that of 4; the former could be dis-
solved only in hot CHCl3, benzene, and toluene. Single crys-
tals of 7 were obtained from hot benzene and MeOH, and
its molecular structure was confirmed by X-ray crystallogra-
phy (Figure 1B).[10b] The structure of 7 revealed the highly
ic rings.
Figure 2. NICS values (ppm) of 4, phenanthrene, 9,10-dihydrophenan-
threne, and
level.
7 calculated at the B3LYP/6-31GAHCTUNGTRENN(GUN d,p)//B3LYP/6-31ACHTUNGTRENNUNG(d,p)
Figure 3 shows the molecular orbitals of 4, 9,10-dihydro-
phenanthrene, biphenyl, and phenanthrene obtained by den-
sity functional theory (DFT) calculations. In compound 4,
no overlap of orbitals between the biphenyl moiety and car-
borane skeleton was observed. Thus, the molecular orbital
of 4 was identical to those of 9,10-dihydrophenanthrene and
biphenyl. The energy band gaps of 4 (4.90 eV) and 9,10-di-
hydrophenanthrene (4.99 eV) were narrower than that of bi-
Figure 1. ORTEP drawings of compounds 4 (A) and 7 (B). Thermal ellip-
soids are drawn at the 30% probability level. Selected bond lengths (ꢂ):
À
À
À
À
4 C1 C2 1.410(3); C2 C3 1.483(2); C3 C4 1.407(3); C4 C5 1.502(3);
À
À
À
À
C5 C6 1.643(2); C6 C1 1.492(3); 7 C1 C2 1.402(5); C2 C3 1.477(5);
À
À
À
À
C3 C4 1.407(5); C4 C5 1.510(5); C5 C6 1.633(5); C6 C1 1.497(5).
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Yasuhiro Morisaki, Yoshiki Chujo et al.
spectra of biphenyl and p-ter-
phenyl. Biphenyl exhibited
a broad absorption peak (Fig-
ure 4A), whereas 4 exhibited
a peak with a vibronic struc-
ture. In addition, the absorption
peak edge of
4 was batho-
chromically shifted compared
with that of biphenyl. As shown
in Figure 4B, the spectrum of 7
also exhibited a vibronic struc-
ture, which was bathochromi-
cally shifted in comparison with
that of p-terphenyl. These re-
sults are consistent with the
suppression of the rotary
motion of the phenylene rings
by the o-carborane skeleton. p-
Electrons are more effectively
delocalized through the highly
planar oligophenyl moieties
fixed by o-carborane than the
corresponding oligophenyls.
Photoluminescence
(PL)
Figure 3. The HOMO/LUMO orbitals and their energy levels calculated at the B3LYP/6-31GACHTNUTRGNE(NUG d,p) level of
theory for compound 4, 9,10-dihydrophenanthrene, biphenyl, and phenanthrene.
spectra of 7 and p-terphenyl are
shown in Figure S18 in the Sup-
porting Information (in CHCl3;
1.0ꢃ10À5 m). The PL spectrum
phenyl (5.37 eV) since the phenyl rings of biphenyl are
twisted due to the repulsion of hydrogen atoms. The highly
coplanar biphenyl moiety of 4 narrows the energy band gap
in comparison with 9,10-dihydrophenanthrene. The highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) levels of 4 were estimated to be
À6.55 and À1.65 eV, respectively; these values were lower
than those of the other compounds examined. Since a contri-
bution of the electrical perturbation of the o-carborane skel-
eton to the biphenyl moiety was not observed, the deep
HOMO and LUMO levels arose from the electron-with-
drawing feature of o-carborane via the inductive effect. The
LUMO energy level of 4 was estimated from the cyclic vol-
tammogram (CV) peak onset potentials measured in the
DMF solution, as shown in Figure S15 in the Supporting In-
formation. A reduction signal with a peak onset at À1.2 V
(vs. ferrocene/ferrocenium) appeared, whereas no peak was
observed in biphenyl.[13]
of 7 exhibited a clear vibrational structure, and 7 emitted
bright purple fluorescence with a PL quantum efficiency
(FPL) of 0.52.[14] The Stokes shift was considerably small; the
absorption peak at 336 nm (Figure 4B) and the PL peak at
337 nm (Figure S18, Supporting Information) indicated a 0–
0 transition band. On the other hand, the PL peak top of p-
terphenyl appeared at 340 nm with a higher FPL of 0.93. A
broad vibrational PL spectrum of p-terphenyl was observed,
thus suggesting that similar structures of p-terphenyl moiet-
ies in both 7 and p-terphenyl are formed in the excited
state. To understand the decrease in FPL of 7 (=0.52), PL
measurements in a frozen medium were carried out (in 2-
MeTHF; 1.0ꢃ10À5 m at 77 K), as shown in Figure S19 in the
Supporting Information. The PL intensity of 7 increased ap-
parently at 77 K; therefore, the main non-radiative deactiva-
tion process is caused by the molecular motion of the o-car-
borane moiety rather than by an intramolecular CT process.
In conclusion, the synthesis, characterization, and proper-
ties of oligophenyl-fused o-carboranes have been described.
Owing to the effect of the o-carborane unit, high coplanarity
of the oligophenyl moieties was achieved. As a result, exten-
sion of p-conjugation, small Stokes shift, 0–0 transition
band, and clear vibronic structures in UV and PL spectra
were observed. In addition, o-carborane acted as a strong
electron-withdrawing group; in particular, o-carborane with-
drew electrons inductively. o-Carborane decreased both
HOMO and LUMO levels of oligophenyls without provid-
ing electronic perturbation. The energy levels of p-terphen-
yl-fused o-carboranes were lower than those of the corre-
In the case of the molecular orbitals of 7, a similar ten-
dency as for 4 was observed (see Figure S16 in the Support-
ing Information). The orbitals of 7 were identical to those of
5,6,12,13-dibenz
energy band gap (4.18 eV) was similar to that of 5,6,12,13-
dibenz
ACHTUNGTRENNUNG[a,h]anthracene and p-terphenyl, and the
[a,h]anthracene (4.26 eV). Deep HOMO (À6.53 eV)
and LUMO (À2.35 eV) energy levels were estimated, which
were derived from the inductive electron-withdrawal of o-
carborane.
UV/Vis absorption spectra of compounds 4 and 7 in
CHCl3 (1.0ꢃ10À5 m) are shown in Figure 4 together with the
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Yasuhiro Morisaki, Yoshiki Chujo et al.
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Acknowledgements
[8] Steric hindrance of bromides at 2,2’-positions resulted in low yields.
As shown in equation S2 in the Supporting Information, 2,2’-dichlor-
otolane yielded the carborane compound in 27% yield. 4,4’-Dibro-
motolane and tolane itself yielded the corresponding carboranes in
69% and 60% yields [Eqs. (S3) and (S4)], respectively.
This work was supported by Grant-in-Aid for Exploratory Research (No.
24655102) and Grant-in-Aid for Scientific Research on Innovative Areas
“Element-Blocks” (No. 24102013) from the Japan Society for the Promo-
tion of Science. M.T. appreciates Research Fellowships (No. 12J03404)
from the Japan Society for the Promotion of Science for Young Scien-
tists.
[9] G. Mao, A. Orita, D. Matsuo, T. Hirate, T. Iwanaga, S. Toyota, J.
Otera, Tetrahedron Lett. 2009, 50, 2860–2864.
[10] a) CCDC 978228 for 4; see Table S1 and Figure S13 in the Support-
ing Information; b) CCDC 978229 for 7; see Table S2 and Fig-
ure S14 in the Supporting Information. The crystallographic data for
this paper can be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Keywords: benzocarborane
·
biphenyl
·
carboranes
·
electron-deficient compounds · p-terphenyl
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Yasuhiro Morisaki, Yoshiki Chujo et al.
[11] V. Petricek, I. Cisarova, L. Hummel, J. Kroupa, B. Brezina, Acta
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[14] PL intensities of 4 and biphenyl were weak (FPL <0.01), and noisy
PL spectra were obtained (Figure S17, Supporting Information).
[12] Cartesian coordinates of the optimized structures are shown in Ta-
bles S3–S6 in the Supporting Information.
[13] Due to the low solubility of 7 in organic solvents, it was difficult to
compare the cyclic voltammograms.
Received: January 14, 2014
Published online: March 3, 2014
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