Synthesis and photoluminescence properties of BF2 complexes with 1,3-diketone ligands

Article abstract of DOI:10.1016/j.tet.2007.07.004

BF₂ complexes with 1,3-diketone ligands were synthesized, and their optical and electrochemical properties were studied. The colors of the complexes varied depending on the structures of the 1,3-diketone ligands. The absorption and emission max

Full text of DOI:10.1016/j.tet.2007.07.004

Tetrahedron 63 (2007) 9354–9358
Synthesis and photoluminescence properties
of BF2 complexes with 1,3-diketone ligands
a
a
a
a
Katsuhiko Ono,a, Kyohei Yoshikawa, Yujiro Tsuji, Hiroyuki Yamaguchi, Rie Uozumi,
*
Masaaki Tomura,b Keijiro Tagaa and Katsuhiro Saitoa
aDepartment of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya 466-8555, Japan
bInstitute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan
Received 26 May 2007; revised 30 June 2007; accepted 2 July 2007
Available online 10 July 2007
Abstract—BF2 complexes with 1,3-diketone ligands were synthesized, and their optical and electrochemical properties were studied. The
colors of the complexes varied depending on the structures of the 1,3-diketone ligands. The absorption and emission maxima of the complexes
with 1,3-diaryl-1,3-diketone ligands were considerably red shifted as compared to those of the complexes with 1-aryl-3-trifluoromethyl-1,3-
diketone ligands, suggesting an extended p-conjugation of the 1,3-diaryl-1,3-diketone moieties. The molar absorption coefficients and quantum
yields of the complexes with 1,3-diaryl-1,3-diketone ligands were larger than those of the complexes with 1-aryl-3-trifluoromethyl-1,3-diketone
ligands. Cyclic voltammetry measurements revealed that the reduction potentials of the BF2 complexes were higher than those of the free
ligands. These complexes exhibited various emission colors in the solid states due to the intermolecular interactions.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
attracted attention as fluorophores due to their unique optical
properties.13 With regard to this, we synthesized BF2 com-
plexes with various 1,3-diketone ligands and studied their
optical and electrochemical properties (Fig. 1). Their emis-
sion maxima and quantum yields depended on the structures
of the 1,3-diketone ligands. The electron affinities of the
BF2-chelating moieties were investigated. In this paper, we
report the synthesis of boron complexes 1 and 2 and their
optical and electrochemical properties.
Currently, organic electroluminescent (EL) devices are an
active area of research because of their potential applications
in flat-panel displays.1 There has been considerable interest
in new light-emitting materials for the research and develop-
ment of organic EL devices. Synthetic studies have been
conducted on various phosphorescent materials with heavy
metals such as iridium2 and platinum3 and on fluorescent bo-
ron complexes.4–13 These boron complexes have attracted
considerable attention as fluorescent EL emitters. The boron
complexes with 1,6-bis(2-hydroxyphenyl)pyridine ligands
were synthesized and their EL properties were investigated.9
Recently, the introduction of electron-rich triphenylamine
into a boron complex led to the formation of a bipolar mate-
rial, which was used as a yellow light emitter to fabricate an
efficient single-layer EL device.10 Furthermore, organo-
boron quinolates with various functional groups were syn-
thesized,11 and their photoluminescence properties were
investigated in comparison to those of tris-8-hydroxyquino-
line aluminum (Alq3) analogues,1,14 which are well known
as EL emitters. The organoboron quinolate moiety was inte-
grated into polymers to produce light-emitting polymer ma-
terials.12 The BF2 complexes with 1,3-diketones have also
F
F
1a R1 = R2
1b R1 = R2
=
=
2a R1
2b R1
=
=
, R2 = CF3
, R2 = CF3
B
O
O
O
S
O
S
R1
R2
1c R1 = R2
=
2c R1
=
, R2 = CF3
Figure 1. Structure of boron complexes 1 and 2.
2. Results and discussion
Boron complexes 1 and 2 were obtained in yields of 50–85%
by the reaction between boron trifluoride–diethyl etherate
and the corresponding 1,3-diketones (Scheme 1). These
complexes were purified by sublimation under reduced pres-
sure and their structures were determined using spectral data
and elemental analysis. The melting points and colors of
Keywords: Boron complex; 1,3-Diketone; Photoluminescence; Electron
affinity.
* Corresponding author. Tel./fax: +81 52 735 5407; e-mail: ono.
katsuhiko@nitech.ac.jp
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.07.004

K. Ono et al. / Tetrahedron 63 (2007) 9354–9358
9355
Table 2. Absorption and emission maxima and quantum yields of com-
plexes 1 and 2
compounds 1 and 2 are summarized in Table 1. The melting
point increases with an increase in the number of aryl groups
because the boron complex of hexafluoroacetylacetone was
obtained as an oil. The colors of these complexes varied de-
pending on the structures of the 1,3-diketone ligands. The
UV–vis spectra of 1 and 2 in dichloromethane are shown
in Figure 2. The longest absorption maxima of 1 are red
shifted as compared to those of 2 (Table 2) and the molar ab-
sorption coefficients of 1 are higher than those of 2. These
results suggested an effective p-conjugation between the
two aryl groups via the BF2-chelating moiety.
b
c
Compound labsa/nm
(log 3)
lema/nm lem,film/nm FPLðCH
FPL(toluene)
2Cl2
Þ
1a
1b
1c
2a
2b
2c
376 sh (4.61) 417
531
562
572
454
483
505
0.22
0.27
0.32
0.01
0.01
0.01
0.70
0.42
0.52
<0.01
<0.01
<0.01
411 (4.69) 454
419 (4.71) 459
329 (4.40) 397
361 (4.32) 410
361 (4.23) 417
a
In CH2Cl2.
Measured relative to 9,10-diphenylanthracene (FPL¼0.82), lex¼350 nm.
b
c
Measured relative to 9,10-diphenylanthracene (FPL¼0.82), lex¼375 nm.
F
F
O
O
B
BF3 • Et2O
dry CH2Cl2
O
O
are red shifted as compared to those in solution (Table 2).
The red shifts of 1a–c are larger than those of 2a–c. This re-
sult is because of the intermolecular interactions in the solid
states. Thus, compounds 1 and 2 exhibited various emission
colors in the solid states, as shown in Figure 3b.
R1
R2
R1
R2
Scheme 1. Synthesis of boron complexes with 1,3-diketone ligands.
Table 1. Melting points and colors of boron complexes 1 and 2
In order to investigate the redox properties of boron com-
plexes 1 and 2, cyclic voltammetry (CV) in acetonitrile
was performed. The voltammograms displayed reversible
(E1/2) and irreversible (Ep) reduction waves for 1 and 2,
respectively. No oxidation waves were observed up to
+1.9 V versus Fc/Fc+. The reduction potentials are listed
in Table 3. These electron affinities increased on BF2 chela-
tion as compared to those of the 1,3-diketone ligands (e.g.,
1,3-diphenyl-1,3-propanedione: Erped¼ꢀ1.96 V, 4,4,4-tri-
fluoro-1-phenyl-1,3-butanedione: Ep¼ꢀ1.58 V). The elec-
tron affinities of 2 are stronger than those of 1 due to the
electron-withdrawing trifluoromethyl groups of complexes
2. In addition, the 1,3-diaryl-1,3-diketone ligands increased
the stability of the anion radical states of complexes 1. The
electron-accepting units were determined to be the BF2-
chelating moieties because their electron affinities were
not affected by the kind of aryl groups.
Compound Mp/ꢁC Color
Compound Mp/ꢁC Color
1a
1b
1c
195–196 Light yellow 2a 127–128 Colorless
225–226 Orange
267–269 Orange
2b
2c
90–91 Pale yellow
150–151 Light yellow
These boron complexes exhibited photoluminescence (PL)
in solution and in the solid states. These PL data are listed
in Table 2. The emission maxima of 1 in dichloromethane
are red shifted as compared to those of 2, suggesting an ex-
tended p-conjugation of the ligands of 1. The quantum
yields of 1 and 2 in dichloromethane were measured relative
to 9,10-diphenylanthracene (FPL¼0.82).15 The quantum
yields of 1 are higher than those of 2 and these quantum
yields are not greatly affected by the kind of aryl groups.
In addition, the quantum yields of 1 in toluene are higher
than those in dichloromethane,13 and these values were af-
fected by the kind of aryl groups (Table 2). Figure 3a shows
the photograph of the PL emission of 1 and 2 in dichlorome-
thane, indicating that the blue emission of 2a–c is weaker
than that of 1a–c. These results are compatible with their
quantum yields. The fluorescence spectra in the solid states
were measured using cast films, which were prepared from
the solutions of 1 and 2 in dichloromethane. The PL maxima
The B3LYP/6-31G(d) calculations16 of complexes 1c and 2c
were performed in order to estimate the influence of the 1,3-
diketone ligands. The energies of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied mole-
cular orbital (LUMO) were higher in compound 1c than in
2c (HOMO/eV: 1c, ꢀ6.47; 2c, ꢀ7.27, LUMO/eV: 1c,
ꢀ2.80; 2c, ꢀ3.22). The TD calculations indicated that the
HOMO–LUMO gap of 1c is smaller than that of 2c (gap/
eV: 1c, 3.41; 2c, 3.86). These results are consistent with
those of the respective electron affinities and absorption
edges. The HOMOs and LUMOs are shown in Figure 4. In
the HOMO of 1c, the atomic orbital coefficients delocalize
on the whole of the 1,3-di(2-thienyl)-1,3-diketone ligand.
The HOMO of 2c has large coefficients on the 3-(2-thienyl)-
3-buten-2-one moiety. The LUMOs of 1c and 2c have large
coefficients on the BF2-chelating moieties.
The molecular structure of 1c was investigated by X-ray
crystallographic analysis. A single crystal of 1c was grown
from chloroform. The molecule is disordered in the region
of BF2-chelating moiety and thienyl groups. Therefore, the
structure was dissolved as a mixing model of stereoiso-
mers.17 The molecular structure is shown in Figure 5a.
The B–O bonds in the BF2-chelating moiety are equivalent
˚ ˚
and these bond lengths are 1.481 A and 1.482 A. The
Figure 2. UV–vis spectra of boron complexes 1 and 2.

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K. Ono et al. / Tetrahedron 63 (2007) 9354–9358
Figure 3. PL emission of 1 and 2 (a) in CH2Cl2 (1.0ꢂ10ꢀ4 M) and (b) in the solid states.
Table 3. Reduction potentials of boron complexes 1 and 2a
3. Conclusions
red
1/2
Compound
E
/V
Compound
Erped/V
In summary, we synthesized BF2 complexes with 1,3-di-
ketone ligands and investigated their optical and electro-
chemical properties as new light-emitting materials. The
colors of their complexes varied depending on the structures
of the 1,3-diketone moieties. The absorption and emission
maxima of 1 were considerably red shifted as compared to
those of 2, suggesting an extended p-conjugation of the
1,3-diketone moieties. The molar absorption coefficients
and quantum yields of 1 were larger than those of 2. In the
CV study of these boron complexes, the BF2-chelating
moieties behaved as electron-accepting units. The introduc-
tion of a trifluoromethyl group into the BF2-chelating moiety
increased the electron affinity. The anion radical states of
these complexes were stabilized by 1,3-diaryl-1,3-diketone
ligands. These complexes exhibited various emission colors
in the solid states due to their intermolecular interactions.
Because these are candidates for use as dopants in light-
emitting layers in a study of EL devices, further investigation
of their potential applications is in progress.
1a
1b
1c
ꢀ1.26
ꢀ1.24
ꢀ1.24
2a
2b
2c
ꢀ0.96
ꢀ0.96
ꢀ0.94
0.1 M n-Bu4NClO4 in MeCN, in Pt electrode, scan rate 100 mV sꢀ1
,
a
V versus Fc/Fc+.
1,3-diketone skeleton is almost planar. The molecules are
stacked to form a columnar structure (Fig. 5b). In the col-
umn, two types of overlap mode were observed and their
face-to-face distances are 3.42 A and 3.48 A, as shown in
Figure 5c and d, respectively. These values are similar to
the sum of the van der Waals radii of carbon atoms
˚
˚
˚
(3.40 A). The molecules form a dimer by the overlap
mode shown in Figure 5c with an intermolecular F/H–
˚
C(2) distance of 2.82 A. Although the distance is slightly
˚
longer than the sum of their van der Waals radii (2.67 A),
the BF2-chelating moiety plays a crucial role as the building
block of the molecular assembly. The overlap mode in
Figure 5d reveals the interactions between the BF2-chelating
moieties and the thienyl groups. Because the BF2-chelating
moiety is electron deficient and the thienyl groups are
electron rich, the face-to-face stacking is attributed to an
intermolecular charge-transfer (CT) interaction. These inter-
molecular interactions cause the red shift of the PL emission
in the solid states more than in solution.
4. Experimental
4.1. General
Melting points were measured with a Yanaco micro melting
point apparatus and are uncorrected. IR, UV–vis, and PL
spectra were obtained with Jasco FT/IR-5300, Hitachi
U-3500, and Otsuka Electronics PTI-5100S spectrometers,
respectively. Mass spectra (EI) were determined with a
Hitachi M-2000S mass spectrometer operating at 70 eV by
a direct inlet system. Elemental analyses were performed
1
with a Perkin–Elmer 2400II analyzer. H and 13C NMR
spectra were recorded with Varian Gemini (300 MHz) and
Bruker Avance600 (150 MHz) spectrometers, respectively.
Tetramethylsilane was used for these spectra as an internal
standard.
4.2. Synthesis of boron complexes (1–2)
Boron trifluoride–diethyl etherate (0.38 mL, 2.94 mmol)
was added to a solution of 1,3-diphenyl-1,3-propanedione
Figure 4. HOMOs and LUMOs of (a) 1c and (b) 2c.

K. Ono et al. / Tetrahedron 63 (2007) 9354–9358
9357
Figure 5. Crystal structure of 1c: (a) molecular structure (ORTEP), (b) stacking, (c) overlap mode, (d) overlap mode.
(0.54 g, 2.42 mmol) in dry dichloromethane (10 mL) under
nitrogen. The reaction mixture was stirred for 1.5 h. After re-
moval of the solvent, the residue was filtered and washed
with hexane. The solid was purified by sublimation at
200 ꢁC under 10ꢀ3 Torr to provide boron complex 1a13 as
light yellow needles with a yield of 0.33 g (50% yield). IR
4.2.3. Compound 2a. Yield 79%. IR (KBr): 1611, 1352,
1
1215, 1055, 700 cmꢀ1. H NMR (CDCl3): d 7.04 (s, 1H),
7.65 (t, J¼7.8 Hz, 2H), 7.87 (t, J¼7.8 Hz, 1H), 8.21 (d,
J¼7.8 Hz, 2H). 13C NMR (CDCl3): d 94.4, 117.4 (q,
J¼277.4 Hz), 129.9, 130.3, 130.6, 138.5, 170.8 (q, J¼
38.9 Hz), 190.4. Mass: m/z (%) 264 (64) [M+], 245 (18),
195 (58), 105 (100). UV–vis (CH2Cl2): lmax (log 3) 329
(4.40) nm. Anal. Calcd for C10H6BF5O2: C, 45.50; H,
2.29. Found: C, 45.79; H, 2.14.
1
(KBr): 1543, 1489, 1375, 1047 cmꢀ1. H NMR (CDCl3):
d 7.20 (s, 1H), 7.54 (t, J¼7.5 Hz, 4H), 7.68 (t, J¼7.5 Hz,
2H), 8.13 (d, J¼7.5 Hz, 4H). 13C NMR (CDCl3): d 93.4,
128.9, 129.2, 131.9, 135.3, 183.2. Mass: m/z (%) 272 (96)
[M+], 271 (100), 253 (22), 195 (32), 105 (76). UV–vis
(CH2Cl2): lmax (log 3) 376 sh (4.61), 365 (4.66) nm. Anal.
Calcd for C15H11BF2O2: C, 66.22; H, 4.08. Found: C,
66.32; H, 4.11.
4.2.4. Compound 2b. Yield 85%. IR (KBr): 1618, 1472,
1
1343, 1273, 1204, 1051, 1022, 961, 677 cmꢀ1. H NMR
(CDCl3): d 6.90–6.91 (m, 2H), 7.92 (dd, J¼3.8, 0.6 Hz,
1H), 8.01 (d, J¼1.6, 0.6 Hz, 1H). 13C NMR (CDCl3):
d 94.1, 116.4, 117.6 (q, J¼276.7 Hz), 128.0, 147.7, 154.4,
169.2 (q, J¼37.4 Hz), 176.2. Mass: m/z (%) 254 (100)
[M+], 235 (94), 185 (100). UV–vis (CH2Cl2): lmax (log 3)
361 (4.32) nm. Anal. Calcd for C8H4BF5O3: C, 37.84; H,
1.59. Found: C, 37.96; H, 1.36.
Similar reaction conditions were applied to the synthesis of
boron complexes 1b–c and 2a–c.
4.2.1. Compound 1b. Yield 75%. IR (KBr): 1586, 1518,
1
1362, 1022 cmꢀ1. H NMR (CDCl3): d 6.73 (dd, J¼3.7,
1.7 Hz, 2H), 6.98 (s, 1H), 7.59 (d, J¼3.7 Hz, 2H), 7.79 (d,
J¼1.7 Hz, 2H). 13C NMR (CDCl3): d 91.8, 114.2, 121.5,
148.4, 149.6, 171.1. Mass: m/z (%) 252 (100) [M+], 233
(28), 187 (32). UV–vis (CH2Cl2): lmax (log 3) 411 (4.69),
395 (4.63), 313 (3.98) nm. Anal. Calcd for C11H7BF2O4:
C, 52.43; H, 2.80. Found: C, 52.30; H, 2.73.
4.2.5. Compound 2c. Yield 67%. IR (KBr): 1615, 1408,
1
1364, 1273, 1206, 1082, 1049 cmꢀ1. H NMR (CDCl3):
d 6.80 (s, 1H), 7.41 (dd, J¼4.9, 4.1 Hz, 1H), 8.22 (dd,
J¼4.9, 1.1 Hz, 1H), 8.26 (dd, J¼4.1, 1.1 Hz, 1H). 13C
NMR (CDCl3): d 94.4, 117.5 (q, J¼276.8 Hz), 130.9,
135.9, 139.5, 143.5, 168.8 (q, J¼38.9 Hz), 181.9. Mass:
m/z (%) 270 (64) [M+], 251 (20), 201 (67), 111
(100). UV–vis (CH2Cl2): lmax (log 3) 361 (4.23). Anal.
Calcd for C8H4BF5O2S: C, 35.59; H, 1.49. Found: C,
35.65; H, 1.40.
4.2.2. Compound 1c. Yield 64%. IR (KBr): 1549, 1319,
1
1049 cmꢀ1. H NMR (CDCl3): d 6.78 (s, 1H), 7.27 (dd,
J¼4.9, 3.9 Hz, 2H), 7.86 (dd, J¼4.9, 1.1 Hz, 2H), 8.06
(dd, J¼3.9, 1.1 Hz, 2H). 13C NMR (CDCl3): d 92.6, 129.4,
134.5, 136.8, 137.1, 175.6. Mass: m/z (%) 284 (100) [M+],
111 (67). UV–vis (CH2Cl2): lmax (log 3) 419 (4.71), 401
(4.63) nm. Anal. Calcd for C11H7BF2O2S2: C, 46.50; H,
2.48. Found: C, 46.23; H, 2.44.
4.3. Electrochemical measurements
CV was performed with a Toho Technical Research polari-
zation unit PS-07 potentiostat/galvanostat. The CV studies

9358
K. Ono et al. / Tetrahedron 63 (2007) 9354–9358
Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 74,
442; (c) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky,
S.; Thompson, M. E.; Kwong, R. C. Appl. Phys. Lett. 2001,
78, 1622; (d) Kavitha, J.; Chang, S.-Y.; Chi, Y.; Yu, J.-K.;
Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Tao, Y.-T.;
Chien, C.-H.; Carty, A. J. Adv. Funct. Mater. 2005, 15, 223;
(e) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Zhu, N.; Lee,
S.-T.; Che, C.-M. Chem. Commun. 2002, 206.
of compounds 1 and 2 were carried out in acetonitrile with
0.1 M n-Bu4NClO4 at a scanning rate of 100 mV sꢀ1 using
Pt and Ag/Ag+ electrodes. The values are expressed in
potentials versus Fc/Fc+.
4.4. X-ray crystallographic analysis of 1c
A single crystal of 1c was obtained by recrystallization from
chloroform. Crystal data for 1c: 0.25ꢂ0.25ꢂ0.15 mm,
C11H7BF2O2S2, M¼284.10, orange prism, triclinic, space
4. Jaska, C. A.; Emslie, D. J. H.; Bosdet, M. J. D.; Piers, W. E.;
Sorensen, T. S.; Parvez, M. J. Am. Chem. Soc. 2006, 128, 10885.
5. Bosdet, M. J. D.; Jaska, C. A.; Piers, W. E.; Sorensen, T. S.;
Parvez, M. Org. Lett. 2007, 9, 1395.
6. Inokuma, Y.; Yoon, Z. S.; Kim, D.; Osuka, A. J. Am. Chem. Soc.
2007, 129, 4747.
˚
group P1, a¼7.786(4), b¼8.664(4), c¼9.388(4) A,
a¼91.780(10)ꢁ, b¼101.128(9)ꢁ, g¼110.790(8)ꢁ, V¼577.5
3
(5) A , Z¼2, Dc¼1.634 g cmꢀ3, m¼0.474 mmꢀ1, F(000)¼
˚
288. Reflection data were collected with a Rigaku MSC Mer-
cury CCD diffractometer using graphite-monochromated
7. Wakamiya, A.; Taniguchi, T.; Yamaguchi, S. Angew. Chem.,
Int. Ed. 2006, 45, 3170.
˚
Mo Ka radiation (l¼0.71070 A) at 173 K. No absorption
8. (a) Goze, C.; Ulrich, G.; Mallon, L. J.; Allen, B. D.; Harriman,
A.; Ziessel, R. J. Am. Chem. Soc. 2006, 128, 10231; (b) Goze,
C.; Ulrich, G.; Ziessel, R. J. Org. Chem. 2007, 72, 313.
9. Liu, Y.; Guo, J.; Zhang, H.; Wang, Y. Angew. Chem., Int. Ed.
2002, 41, 182.
10. Zhang, H.; Huo, C.; Zhang, J.; Zhang, P.; Tian, W.; Wang, Y.
Chem. Commun. 2006, 281.
€
11. Qin, Y.; Kiburu, I.; Shah, S.; Jakle, F. Org. Lett. 2006, 8, 5227.
12. Nagata, Y.; Chujo, Y. Macromolecules 2007, 40, 6.
correction was applied. The structure was solved as a disor-
dered model using the direct method (SHELXS-97).18 All
non-hydrogen atoms were refined anisotropically by the
full-matrix least-squares (SHELXL-97)19 and all hydrogen
atoms were placed in geometrically calculated positions
and refined by using a rigid model. The final values of
R1¼0.0550, GOF¼1.629, and max./min. residual electron
ꢀ3
˚
density 0.475/ꢀ0.230 e A were obtained for 2458 unique
reflections(I>2s(I)). CCDC-648258containsthesupplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
ꢀ
13. Cogne-Laage, E.; Allemand, J.-F.; Ruel, O.; Baudin, J.-B.;
Croquette, V.; Blanchard-Desce, M.; Jullien, L. Chem.—Eur.
J. 2004, 10, 1445.
14. (a) Montes, V. A.; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr.
Chem.—Eur. J. 2006, 12, 4523; (b) Montes, V. A.; Li, G.; Pohl,
R.; Shinar, J.; Anzenbacher, P., Jr. Adv. Mater. 2004, 16, 2001.
15. Morris, J. V.; Mahaney, M. A.; Huber, J. R. J. Phys. Chem.
1976, 80, 969.
Acknowledgements
This work was supported by a Grant-in-Aid (Nos. 17750037
and 19550034) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. We would like to
thank Research Center for Molecular-Scale Nanoscience,
Institute for Molecular Science, for the crystal analysis.
16. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.;
Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;
Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi,
M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.;
Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.;
Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski,
J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck,
A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui,
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