Azaboradibenzo[6]helicene: Carrier inversion induced by helical homochirality

Article abstract of DOI:10.1021/ja310372f

Azaboradibenzo[6]helicene, a new semiconductor material possessing helical chirality, has been synthesized via a tandem bora-Friedel-Crafts-type reaction. Unprecedented carrier inversion between the racemate (displaying p-type semiconductivity) and the si

Full text of DOI:10.1021/ja310372f

Communication
pubs.acs.org/JACS
Azaboradibenzo[6]helicene: Carrier Inversion Induced by Helical
Homochirality
Takuji Hatakeyama,*,†,‡ Sigma Hashimoto,† Tsuyoshi Oba,† and Masaharu Nakamura*,†
†International Research Center for Elements Science (IRCELS), Institute for Chemical Research (ICR), Kyoto University, Uji, Kyoto
611-0011, Japan
‡PRESTO, Japan Science and Technology Agency (JST), Kawaguchi-shi, Saitama 332-0012, Japan
S
* Supporting Information
Scheme 1. Synthesis of Azaboradibenzo[6]helicene (A)
ABSTRACT: Azaboradibenzo[6]helicene, a new semi-
conductor material possessing helical chirality, has been
synthesized via a tandem bora-Friedel−Crafts-type reac-
tion. Unprecedented carrier inversion between the race-
mate (displaying p-type semiconductivity) and the single
enantiomer (displaying n-type semiconductivity) was
observed and can be explained by changes in the molecular
packing induced by helical homochirality.
elicenes, nonplanar screw-shaped polycyclic aromatic
compounds consisting of ortho-fused aromatic rings,
H
have attracted considerable attention because of their inherent
chirality.1 Recently, the self-assembly of helicenes via unique
π−π stacking interactions in solution as well as in crystals has
been studied extensively, since these aggregates display
intriguing properties such as liquid crystallinity,2 nonlinear
optical susceptibility,3 and circularly polarized luminescence.4
To date, however, the electrical properties of these aggregates
(e.g., charge mobility) have not been investigated well,5 even
though the π−π stacking interactions are expected to facilitate
charge transport. Herein we report the synthesis of
azaboradibenzo[6]helicene (A) via a tandem bora-Friedel−
Crafts-type reaction.6 Charge mobility measurements based on
the time-of-flight (TOF) method suggested that the racemate
and single enantiomer of A are p- and n-type semiconductors,
respectively. This unprecedented carrier inversion can be
explained by changes in the packing structure of the respective
hetero- and homochiral crystals of A, as revealed by calculation
of the electronic coupling.7
bonds are 1.5527(19) and 1.425(2), respectively, indicating
that they are single bonds. These observations, together with
the highly distorted BNC4 ring structure [B−N−C4(C4′)−
C3(C3′) dihedral angle = 21.66°] reveal the low aromaticity of
the BNC4 rings, which is consistent with the relatively small
nucleus-independent chemical shift [NICS(1)] value of −1.8
(Figure 1b).10,11 In contrast, the surrounding C6 rings are
nearly planar and show large NICS(1) values. Notably,
molecular orbital (MO) calculations10 indicated that the π
conjugation is spread over the entire molecule despite the large
distortion induced by the BNC4 rings (Figure 1c).
The unique packing structure of the heterochiral crystal is
shown in Figure 1d,e. The molecules are stacked in a head-to-
tail array with CH−π distances of 2.9−3.3 Å. Each array is
formed from a single enantiomer, resulting in alternating right-
handed (P-helical, shown in blue) and left-handed (M-helical,
shown in pink) configurations that are arranged in a face-to-face
fashion (π−π distance = 3.4−3.6 Å), while the local dipole
moments of the B−N bonds cancel each other.
Optical resolution of rac-A into enantiopure (P)-A and (M)-
A was carried out by chiral HPLC on a DAICEL CHIRALPAK
IA-3 column (eluent: n-hexane/CH2Cl2).12 The absolute
configuration of enantiopure A in each fraction was determined
by X-ray crystallographic analysis of the dibromo derivative of
(P)-A,13 while the structure of enantiopure (P)-A was
determined by X-ray crystallography (Figure 2). The molecular
Scheme 1 summarizes the synthesis of A. 1-Bromo-2-
phenylnaphthalene (1), prepared in two steps from commer-
cially available 1-bromonaphthalen-2-ol,8 was coupled with
lithium amide in the presence of tris(dibenzylideneacetone)-
dipalladium(0) [Pd2(dba)3] and 2-dicyclohexylphosphino-2′,6′-
dimethoxybiphenyl (SPhos) to give diarylamine 2 in 74% yield.
Borylation of 2 by treatment with BuLi and BCl3 and the
subsequent tandem bora-Friedel−Crafts-type reaction with
AlCl3 and 2,2,6,6-tetramethylpiperidine afforded the racemate
of A (rac-A) in 68% yield.
The helical structure of rac-A was determined to be C2-
symmetric by X-ray crystallography (Figure 1a). The B−N
bond length [1.448(3) Å] is similar to that in typical B−N
aromatics (1.45−1.47 Å), indicating its strong π interactions.9
Furthermore, the lengths of the B−C1(C1′) and N−C2(C2′)
Received: October 20, 2012
© XXXX American Chemical Society
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Communication
Figure 2. (a) ORTEP drawing and (b, c) packing structure of (P)-A.
Thermal ellipsoids are shown at 30% probability. H atoms have been
omitted for clarity.
were then evaluated at room temperature using electric fields of
5.0 × 105 and 5.2 × 105 V cm−1, respectively. It was found that
rac-A showed high hole mobility (μh = 4.6 × 10−4 cm2 V−1
s−1),16 but no transient photocurrent was detected in electron
mobility measurements (Table 1). Interestingly, the enantio-
Table 1. Electronic Properties of Amorphous Films of rac-A
and (P)-A
a
b
Hole mobility (cm2 V−1 s−1). Electron mobility (cm2 V−1 s−1).
Figure 1. (a) ORTEP drawing and (d, e) packing structure of rac-A
obtained by X-ray crystal analysis. Thermal ellipsoids are shown at
50% probability. H atoms have been omitted for clarity. The P
enantiomer is shown in blue and the M enantiomer in pink. (b)
NICS(1) values and (c) the Kohn−Sham highest-occupied MO
(HOMO) and lowest unoccupied MO (LUMO) of (P)-A.
c
d
e
Ionization potential (eV). Electron affinity (eV). Transient
photocurrent was not detected.
pure (P)-A did show higher electron mobility (μe = 4.5 × 10−3
cm2 V−1 s−1) than hole mobility (μh = 7.9 × 10−4 cm2 V−1
s−1).16 Since the corresponding physical properties of the
vacuum-deposited films, such as ionization potential (IP) and
electron affinity (EA),17 were almost identical, we speculated
that the carrier inversion could be caused by the distinct
orientation of (P)-A in the homochiral film.
To gain deeper insight into the carrier-transport properties,
the electronic couplings V between neighboring molecules in
different stacks were calculated18 from the X-ray crystal
structures of rac-A and (P)-A (Figure 3). In the heterochiral
crystal of rac-A (Figure 3a), the P and M enantiomers are
arranged in a tightly offset, face-to-face stacking array. The
maximum coupling between the HOMOs of neighboring
molecules (42.0 meV) was 6 times that of the corresponding
LUMOs (7.2 meV). In contrast, in the homochiral crystal of
(P)-A (Figure 3b), the maximum coupling of the HOMOs
(30.1 meV) was only two-fifths that of the LUMOs (73.2
meV). These results are in good agreement with the carrier
structure of (P)-A in the homochiral crystal (Figure 2a) is C2-
symmetric, and the bond lengths and angles are almost identical
to those found in the heterochiral crystal (Figure 1a). On the
other hand, the packing structure of the homochiral crystal
differs significantly from that of the heterochiral crystal, as a
one-dimensional columnar alignment along the c axis was
observed (Figure 2b).14 Interestingly, the molecules in
neighboring columns are arranged parallel to each other and
show a rotation of 120° with each layer (Figure 2c). As a result,
the local dipole moments, which run perpendicular to the c axis,
are offset every third layer.
Since the enantiopurity of (P)-A did not decrease upon
heating (not even at 275 °C),15 we prepared films of rac-A and
(P)-A with thicknesses of 8.2 and 6.7 μm, respectively, by
vacuum deposition at 5.0 × 10−3 Pa for use in TOF
measurements.8 The carrier-transport properties of the films
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Journal of the American Chemical Society
Communication
performed on beamline BL19B2 at SPring-8 with the approval
of JASRI (2009B1785, 2010A1721).
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(10) DFT calculations, including NICS analyses, were performed at
the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level as implemented
in the Gaussian 09 program. See the Supporting Information for
details.
(11) Reported NICS(1) values for the planar BNC4 rings in BN-
substituted aromatics are −4.6 and −6.3 ppm. See: Bosdet, M. J. D.;
Jaska, C. A.; Piers, W. E.; Sorensen, T. S.; Parvez, M. Org. Lett. 2007, 9,
1395−1398.
(12) UV−vis, fluorescence, and CD measurements were performed
for rac-A and enantiopure (P)-A and (M)-A. See the Supporting
Information for details.
(13) Dibromination of A took place selectively at the para position of
the aniline moiety in the presence of 2 equiv of N-bromosuccinimide.
See the Supporting Information for details.
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E.; McDonald, R.; Branda, N. R. Org. Lett. 2000, 2, 3169−3172.
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Angew. Chem., Int. Ed. 2012, 51, 695−699.
Figure 3. Electronic couplings V (in meV) between HOMOs of
neighboring molecules in the X-ray crystal structures of (a) rac-A (P in
blue, M in pink) and (b) (P)-A. The electronic couplings between
LUMOs are shown in parentheses.
mobilities determined by the TOF method,18 suggesting the
possibility that the molecular orientations in the amorphous
films might be similar to those in the crystals.19
In summary, we have synthesized azaboradibenzo[6]helicene
(A) via a tandem bora-Friedel−Crafts-type reaction. Charge
mobility measurements by the time-of-flight (TOF) method
suggested that the racemate and single enantiomer of A are
good p- and n-type semiconductors, respectively. This
unprecedented carrier inversion can be explained by differences
in the packing structures of the hetero- and homochiral crystals
of A, as revealed by electronic coupling calculations. The results
indicate the potential of these chiral organic semiconductors in
electronic applications such as bipolar junction transistors and
morphology-controlled bulk-heterojunction solar cells.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures; characterization, photophysical,
electrochemical, and crystallographic data and CIF files for
the products; and computational methods and data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
hatake@scl.kyoto-u.ac.jp; masaharu@scl.kyoto-u.ac.jp
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the PRESTO Program of JST
and a Grant-in-Aid for Young Scientists (23685020) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy (MEXT) of Japan. The study was also supported by the
NEXT Program of the Japan Society for the Promotion of
Science (JSPS) and the Asahi Glass Foundation. We thank Mr.
Toshiaki Ikuta and Dr. Jingping Ni (JNC Petrochemical
Corporation) for TOF and IP measurements and Professors
Mao Minoura (Kitasato University) and Hikaru Takaya and
Takahiro Sasamori (Kyoto University) for their guidance in the
X-ray crystallography experiments. For initial investigations,
synchrotron X-ray powder diffraction measurements were
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Journal of the American Chemical Society
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(15) The calculated racemization barrier [ΔH⧧ at the B3LYP/6-
31G(d) level] is 42.0 kcal mol−1, which is consistent with the
experimental results and considerably higher than that of the [6]
helicene (35.2 kcal mol−1). See: Grimme, S.; Peyerimhoff, S. D. Chem.
Phys. 1996, 204, 411−417.
(16) The charge mobilities are comparable to those in representative
organic semiconductors (measured by the TOF method), such as α-
naphthylphenylbiphenyldiamine (α-NPD, μh = 7 × 10−4 cm2 V−1 s−1),
1,3,5-tris(phenyl-2-benzimidazolyl)benzene (TPBI, μe = 9 × 10−5 cm2
V−1 s−1), and 4,4′-N,N′-dicarbazolebiphenyl (CBP, μh = 2 × 10−3 cm2
V−1 s−1, μe = 1 × 10−3 cm2 V−1 s−1). See: (a) Hung, W.-Y.; Ke, T.-H.;
Lin, Y.-T.; Wu, C.-C.; Hung, T.-H.; Chao, T.-C.; Wong, K.-T.; Wu, C.-
I. Appl. Phys. Lett. 2006, 88, No. 064102. (b) Li, C.; Duan, L.; Sun, Y.;
Li, H.; Qiu, Y. J. Phys. Chem. C 2012, 116, 19748−19754.
(17) IPs were measured by photoelectron spectroscopy in air (PESA)
using an AC-1 system (RIKEN KEIKI Co., Ltd.). EAs were estimated
from the IPs and UV−vis absorption-edge wavelengths.
(18) Electronic coupling (V) calculations were performed using the
PW91 hybrid functional with the DZP basis set as implemented in the
ADF2010 program. The electron-hopping rate (W) is proportional to
V2 on the basis of the Marcus−Hush theory. See ref 6 and citations
therein.
(19) Molecular orientation in vacuum-deposited amorphous films:
Yokoyama, D. J. Mater. Chem. 2011, 21, 19187−19202 and citations
therein.
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