Synthesis of 1-phospha-2-boraacenaphthenes: Reductive 1,2-aryl migration of 1-diarylboryl-8-dichlorophosphinonaphthalenes

Article abstract of DOI:10.1002/anie.201104971

On the move: The title compounds 1, which are unique heterocyclic systems containing a P-B bond, have been synthesized by the reduction of 1-diarylboryl-8-dichlorophosphinonaphthalene derivatives (see scheme). Both experimental and theoretical results for

Full text of DOI:10.1002/anie.201104971

Communications
DOI: 10.1002/anie.201104971
Heterocyclic Compounds
Synthesis of 1-Phospha-2-boraacenaphthenes: Reductive 1,2-Aryl
Migration of 1-Diarylboryl-8-dichlorophosphinonaphthalenes**
Akihiro Tsurusaki, Takahiro Sasamori,* Atsushi Wakamiya, Shigehiro Yamaguchi,
Kazuhiko Nagura, Stephan Irle, and Norihiro Tokitoh*
The chemistry of organic p-electron conjugated systems has
attracted much attention in recent years from the viewpoint of
material science, such as organic light-emitting diodes, photo-
voltaic cells, and liquid crystals. It is well known that the
incorporation of main group elements into a p-electron
conjugated backbone can effectively modify the electronic
properties of a p-electron conjugated molecule.[1,2] In partic-
ular, main group elements from groups 13 and 15 are
employed in these systems because they can drastically
change the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) levels
of the p-conjugated systems as a result of the characteristic
interactions of their vacant p orbital (group 13) and lone pair
(group 15). With regard to this type of compound, azabor-
ine,[3] phosphaborine,[4] boryl-substituted azobenzene,[5]
diborin,[6] and ladder-type p-electron conjugated systems[7]
have been synthesized and revealed unique photophysical
compound, because phosphinoborane undergoes facile oligo-
merization in a head-to-tail manner by the formation of
[8a,10]
À
intermolecular P B dative bonds.
A series of kinetically
stabilized phosphinoboranes bearing mesityl, phenyl, trime-
thylsilyl, and 1-adamantyl groups have been reported by
Power and co-workers,[11] and phosphinoboranes ((LA)H2P-
BH2(LB); LA = Lewis acid, LB = Lewis base) that are
thermodynamically stabilized by the coordination of a
Lewis acid and base have been reported by Scheer and co-
workers.[12] We report herein the synthesis and properties of 1-
phospha-2-boraacenaphthene 1, which is a unique heterocy-
À
clic compound bearing a P B bond attached to a naphthyl
unit at the 1- and 8-positions.[13] Compound 1 was synthesized
by the reduction of 1-diarylboryl-8-dichlorophosphinonaph-
thalenes 2 by a unique intramolecular aryl migration from the
boron atom to the phosphorus atom (Scheme 1).
and electrochemical properties.
[8]
À
Phosphinoborane (R2P BR’2) is a heavier analogue of
À
aminoborane (R2N BR’2), which exhibits an isoelectronic
=
structure to an alkene (R2C CR’2). Theoretical calculations
À
showed that the ground state of H2P BH2 has a Cs symmetric
structure with a pyramidal phosphorus atom; this structure is
À
in contrast to the planar structure of the ground state of H2N
BH2, which has C2v symmetry.[9] Although H2P BH2 and
À
À
H2N BH2 have different conformations in their ground
=
states, their H2E BH2 (E = N, P) p-bond energies are
estimated to be almost the same (34 kcalmolÀ1 for E = N,
36 kcalmolÀ1 for E = P).[9d,f] Thus, a P B bond is a candidate
À
to be a p-electron unit in p-electron conjugated systems.
However, kinetic or thermodynamic stabilization would be
necessary to isolate monomeric phosphinoborane as a stable
Scheme 1. Synthesis of 1. THF=tetrahydrofuran.
[*] Dr. A. Tsurusaki, Prof. Dr. T. Sasamori, Prof. Dr. N. Tokitoh
Institute for Chemical Research, Kyoto University
Gakasho, Uji, Kyoto 611-0011 (Japan)
1-Diarylboryl-8-dichlorophosphinonaphthalenes 2 were
synthesized from 1,8-dibromonaphthalene in two steps
(Scheme 1). The reaction of 1,8-dibromonaphthalene with
nBuLi and Ar2BX (X = F or OiPr; Ar= mesityl (a), 2,6-
dimethylphenyl (b), and 3,5-di-tert-butylphenyl (c)) in THF
E-mail: sasamori@boc.kuicr.kyoto-u.ac.jp
tokitoh@boc.kuicr.kyoto-u.ac.jp
Homepage: http://boc.kuicr.kyoto-u.ac.jp/www/index-e.html
Prof. Dr. A. Wakamiya, Prof. Dr. S. Yamaguchi, K. Nagura,
Prof. Dr. S. Irle
Department of Chemistry, Graduate School of Science
Nagoya University, Furo, Chikusa, Nagoya 464-8602 (Japan)
gave 1-bromo-8-diarylborylnaphthalenes
3 in moderate
yields. Then, treatment of 3 with nBuLi at low temperature
(À408C for 3a and 3b, and À788C for 3c) followed by the
addition of PCl3 gave 2 in relatively high yields.[14] It should be
noted that the 11B NMR spectra of 2a and 2b (d = 63.9 ppm
for 2a, d = 64.3 ppm for 2b), and 2c (d = 10.1 ppm) revealed
the tri- (2a and 2b) and tetra-coordination states (2c) of the
boron atom. The steric repulsion resulting from the methyl
[**] This work was partially supported by Grants-in-Aid for Creative
Scientific Research (B) (No. 22350017), the Global COE Program
B09, and the MEXT Project of Integrated Research on Chemical
Synthesis from the Ministry of Education, Culture, Sports, Science
and Technology (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104971.
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Angew. Chem. Int. Ed. 2011, 50, 10940 –10943

groups on the aryl group of 2a and 2b is believed to prevent
room temperature on exposure to air. The addition of
degassed water into the C6D6 solution of 1a afforded 1-
hydroxy(mesityl)boryl-8-mesitylphosphinonaphthalene (5a)
in 93% yield.[14]
À
the formation of an intramolecular P B dative bond.
The reduction of 2c with magnesium metal in THF at
room temperature gave a mixture of two isomers, (E)-4c and
(Z)-4c, which can be considered as dimers of the correspond-
ing 1-phospha-2-boraacenaphthene 1c, in 35 and 53% yields,
respectively, upon isolation (Scheme 1).[15,16] Therefore, the
introduction of 3,5-tBu2C6H3 groups would not be suitable for
the synthesis and isolation of a 1-phospha-2-boraacenaph-
thene in the monomeric form. On the other hand, the
reduction of 2a and 2b with magnesium metal afforded 1a
and 1b as orange crystals in 91 and 67% yields, respectively,
upon isolation.[14] These results suggest it is necessary to have
a substituent at the ortho positions of the aryl groups on the
boron atom to isolate 1 as a stable compound. The monomeric
molecular structure of 1a was unambiguously determined by
X-ray crystallographic analysis (Figure 1).[17] The deviation of
A plausible mechanism for the formation of 1a, 1b, and
À
4c is shown in Scheme 2. At first, the P Cl bond of
dichlorophosphine 2 could be reduced with magnesium
Scheme 2. Plausible mechanism for the formation of 1 and 4 from 2
by aryl migration from the boron atom to the phosphorus atom.
metal to give borate 8. Then, one of the two aryl groups on
the boron atom in 8 migrates to the phosphorus atom to
afford 1-phospha-2-boraacenaphthenes 1, together with the
elimination of MgCl2. Compounds 1a and 1b bearing mesityl
or 2,6-dimethylphenyl were isolated as monomers, while 1c
bearing 3,5-di-tert-butylphenyl groups is believed to undergo
ready head-to-tail dimerization, thus leading to the formation
of (E)-4c or (Z)-4c. When a 1:1 mixture of 2a and 2b was
reduced under the same reaction conditions as used for the
synthesis of 1, compounds 1a and 1b were obtained in a 1:1
ratio without the formation of any crossover product, as
1
determined by the H and 31P NMR spectra, thus suggesting
Figure 1. Molecular structure of 1a with thermal ellipsoids set at 50%
probability. Hydrogen atoms are omitted for clarity. Selected bond
lengths [ꢀ], angles [deg], and dihedral angles [deg]: P(1)–B(1)
1.889(3), P(1)–C(1) 1.817(3), B(1)–C(9) 1.582(4); P(1)-C(1)-C(10)
108.59(17), B(1)-C(9)-C(10) 113.6(2), P(1)-C(1)···C(9)-B(1) 10.24(14).
the intramolecular migration of the aryl group in these
reactions.[14]
It is worth noting that compound 1a was found to be
orange in color, both in the crystalline form and in solution, in
contrast to the previously reported phosphinoboranes, which
were colorless or yellow crystals. Furthermore, 1a exhibited
weak but apparent orange emission in solution. Thus, we
measured the cyclic voltammetry, UV/Vis, and emission
spectra of 1a to clarify the influence of the rigid geometry
the phosphorus atom from the naphthalene plane is 0.36 ꢀ,
while the boron atom is located on the same plane as
naphthalene (Figure 1b). The sum of the bond angles around
the phosphorus and boron atoms are 328.4(3)8 and 359.5(6)8,
respectively, thus confirming the pyramidal (phosphorus) and
À
of the structure comprising of the P B bond and the naphthyl
moiety, to the electrochemical and photophysical properties
À
trigonal planar (boron) geometries. The P B bond length in
1a is 1.889(3) ꢀ, which is slightly longer than those in
Mes2PBMes2 (6; 1.839(8) ꢀ) and Ph2PBMes2 (7;
1.859(3) ꢀ).[11] The 11B NMR spectra of 1a and 1b in C6D6
showed broad signals at dB = 77.9 ppm (Dn1/2 = 1400 Hz) for
1a and dB = 77.8 ppm (Dn1/2 = 930 Hz) for 1b, thus suggesting
a trigonal planar geometry around the boron atom in solution.
These values are similar to those of 6 (dB = 82.4 ppm) and 7
(dB = 70.9 ppm).[11] On the other hand, the 31P NMR signals,
observed at dP = À28.2 ppm (1a) and dP = À27.7 ppm (1b),
were shifted relatively upfield compared to the signals of 6
(dP = 27.4 ppm) and 7 (dP = 26.7 ppm),[11] probably due to the
different geometries of the phosphorus atoms. That is, 1a
exhibits the pyramidal geometry, and 6 exhibits almost planar
geometry.[8a] 1-Phospha-2-boraacenaphthenes 1a and 1b were
stable under an inert atmosphere even on heating at 1008C
(C6D6 solution, sealed tube) for 24 h, but they underwent
decomposition within three hours in dilute C6D6 solution at
of 1a.[18]
While 1a showed an irreversible oxidation wave at Epa =+
0.65 V (versus Fc/Fc+; Fc = ferrocene; Figure S1 in the
Supporting Information), a reversible one-electron reduction
couple was observed at E1/2 = À2.22 V together with an
irreversible reduction wave at Epc = À3.28 V (versus Fc/Fc+;
Figure S2 in the Supporting Information).[14] The first reduc-
tion occurred at a less-negative region than that of dimesi-
tyl(1-naphthyl)borane (9; E1/2 = À2.51 V and Epc = À3.30 V)
under the same reaction conditions.[19] The less-negative
reduction potential of 1a compared to that of 9 is probably
due to the very effective pp–p* conjugation between the
naphthyl unit and the vacant p orbital of the boron atom; this
conjugation could be a result of the rigid conformation,
À
caused by the P B bond attached to the naphthyl unit.
The UV/Vis spectra of 1a in hexane, THF, and CH2Cl2
solutions were similar to each other (Figure 2). In all cases,
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www.angewandte.org 10941

Communications
Figure 2. UV/Vis (black) and fluorescence (gray) spectra of 1a in
hexanes.
three characteristic absorption maxima were observed at 452
(e, 0.23–0.25 ꢁ 104), 353 (e, 0.61–0.63 ꢁ 104), 302 (e, 1.4–1.5 ꢁ
104) together with 280 nm (sh, e, 1.2–1.3 ꢁ 104) as
a
shoulder.[14,20] The longest absorption of 1a is red-shifted in
comparision with that of acenaphthene (lmax = 320 nm).[21]
Compound 1a showed a broad fluorescence spectrum in the
range of ca. 510–750 nm.[22] It should be noted that a subtle
solvent effect [lmax = 595 nm (hexane), 609 nm (THF), and
612 nm (CH2Cl2)] with the large Stokes shift (D = 5300–
5700 cmÀ1) suggested a large structural change between
ground and excited states with the similar polarity. The
observed lifetimes were 5.2, 1.5, and 3.6 ns, and the quantum
yields were 0.032, 0.020, and 0.017 in hexane, THF, and
CH2Cl2, respectively. The calculated radiative (kr) and non-
radiative (knr) rate constants from the singlet excited state
were kr = 6.1 ꢁ 106, knr = 1.8 ꢁ 108 sÀ1 (hexane); kr = 1.3 ꢁ 107,
knr = 6.5 ꢁ 108 sÀ1 (THF); and kr = 4.7 ꢁ 106, knr = 2.7 ꢁ 108 sÀ1
(CH2Cl2).[23] The significantly small values of kr compared to
those of knr may retard the effective emission of 1a.
Figure 3. a) HOMO of 1a(S0) and b) LUMO of 1a(S0) (isosurface at
0.04 a.u). c) Selected bond lengths of the optimized structures of
1a(S0) (normal font) and 1a(S1) (italic font). d) Schematic energy
diagram for the ground (S0) and excited (S1) states of 1a. Mes=me-
sityl.
À
nied by a weakening of the P B bond. Despite of the large
Theoretical calculations of the ground (S0) and excited
(S1) states of 1a (denoted as 1a(S0) and 1a(S1) hereafter)
were performed at the B3LYP/def2-TZVPP level of theory
with the TURBOMOLE program.[24] The optimized struc-
tural parameters of 1a(S0) are in good agreement with the
structural changes, the HOMO and LUMO of 1a(S1) are
similar to those of 1a(S0) (Figure S4 in the Supporting
Information). TDDFT calculations for 1a(S1) predicted the
wavelength and oscillation strength (f) of the HOMO–
LUMO transitions in the excited state are 703 nm and
0.0146, respectively, thus supporting the large Stokes shift
(calculated as D = 6800 cmÀ1) and slow radiative process of 1
(Figure 3d).[25] While the theoretical results are somewhat
red-shifted in comparison to the experimental transition
energies, a result that is typical for DFT calculations in the
case of charge-transfer (CT) excited states.
À
experimentally observed parameters (1a(S0): P B, 1.907 ꢀ;
À
À
P C(Naph), 1.822 ꢀ; B C(Naph), 1.556 ꢀ). The HOMO and
LUMO of 1a(S0) are shown in Figures 3a and b, respectively.
À
The HOMO of 1a(S0), which is similar to that seen in H2P
BH2 with Cs symmetric structure,[9a] exhibited a dominant
contribution from the lone pair n orbital of the phosphorus
atom (Figure 3a). On the other hand, the LUMO was the
result of the vacant p orbital of the boron atom delocalized
with the p* orbital of the naphthalene unit, thus indicating an
effective pp–p* conjugation (Figure 3b). TDDFT calcula-
tions for 1a(S0) (l = 475 nm (f = 0.0185)) indicate that the
longest absorption observed in the UV/Vis spectra (l =
452 nm) should be principally assignable to the intramolec-
ular charge transfer from the phosphorus atom to the boron
atom (HOMO–LUMO transition). It should be noted that the
In summary, 1,2-diaryl-1-phospha-2-boraacenaphthenes
1a and 1b have been synthesized by the reduction of 1-
diarylboryl-8-dichlorophosphinonaphthalenes 2a and 2b by a
unique intramolecular aryl migration from the boron atom to
the phosphorus atom. Attractive electrochemical and photo-
chemical properties of 1a are provided by not only the
À
intrinsic nature of the P B bond but also the rigid conforma-
À
tion of the structure owing to the P B bond being attached to
the naphthyl unit at the 1- and 8-positions. This work provides
À
À
À
À
P B, P C(Naph), and B C(Naph) bond lengths of 1a(S1)
insight into the intrinsic potential of a P B bond in p-electron
conjugated systems. Further investigations on the synthesis of
such molecules are currently underway.
were significantly different to those of 1a(S0) (Figure 3c).[14]
À
The P B bond length of 1a(S1) (2.040 ꢀ) is significantly
longer than that of 1a(S0) (1.907 ꢀ), while the P C(Naph)
(1.780 ꢀ) and B C(Naph) (1.482 ꢀ) bond lengths of 1a(S1)
are shorter than those of 1a(S0) (P C(Naph), 1.822 ꢀ; B
C(Naph) 1.556 ꢀ). Therefore, the phosphorus and boron
atoms strongly interact with the naphthalene unit accompa-
À
À
À
À
Experimental Section
Synthesis of 1a: In a glovebox filled with argon, a solution of 1-
dichlorophophino-8-dimesitylborylnaphthalene
(2a,
239 mg,
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Angew. Chem. Int. Ed. 2011, 50, 10940 –10943

0.501 mmol) and magnesium metal (12.3 mg, 0.506 mmol) in THF
(5 mL) was stirred at room temperature for 2 h. After the solvent was
removed under reduced pressure, n-hexane was added to the residue,
and then the mixture was filtered through Celite with hexanes. The
filtrate was concentrated under reduced pressure, and the residue was
washed with cooled hexanes to afford 1,2-dimesityl-1-phospha-2-
boraacenaphthene (1a, 185 mg, 0.455 mmol, 91%). 1a: orange
[9] a) T. L. Allen, A. C. Scheiner, H. F. Schaefer III, Inorg. Chem.
1990, 29, 1930; b) M. B. Coolidge, W. T. Borden, J. Am. Chem.
Soc. 1990, 112, 1704; c) T. L. Allen, W. H. Fink, Inorg. Chem.
1992, 31, 1703; d) T. L. Allen, W. H. Fink, Inorg. Chem. 1993, 32,
4230; e) H.-J. Himmel, Dalton Trans. 2003, 3639; f) D. J. Grant,
D. A. Dixon, J. Phys. Chem. A 2006, 110, 12955; g) T. M. Gilbert,
S. M. Bachrach, Organometallics 2007, 26, 2672.
[10] A. Staubitz, A. P. M. Robertson, M. E. Sloan, I. Manners, Chem.
Rev. 2010, 110, 4023.
[11] a) D. C. Pestana, P. P. Power, J. Am. Chem. Soc. 1991, 113, 8426;
b) X. Feng, M. M. Olmstead, P. P. Power, Inorg. Chem. 1986, 25,
4615.
1
crystals, m.p. 1278C (decomp.). H NMR (600 MHz, C6D6, RT): d =
2.02 (s, 3H), 2.18 (s, 3H), 2.280–2.282 (m, 6H), 2.30 (s, 6H), 6.67–6.68
(m, 2H), 6.798–6.800 (m, 2H), 7.28 (ddd, 3JHH = 8.2 Hz, 7.0 Hz, 4JHP
=
2.7 Hz, 1H), 7.30 (dd, 3JHH = 8.2, 6.8 Hz 1H), 7.47 (ddd, 3JHP = 7.3 Hz,
3
3JHH = 7.0 Hz, J = 1.0 Hz, 1H), 7.55 (d, JHH = 8.2 Hz, 1H), 7.75 (dd,
3JHH = 8.2 Hz, 4JHH = 1.0 Hz, 1H), 7.89 ppm (ddd, 3JHH = 6.8 Hz,
4JHH = 1.0 Hz, 4JHP = 1.0 Hz, 1H); 13C{1H} NMR (150 MHz, C6D6,
RT): d = 20.96 (CH3), 21.23 (CH3), 23.72 (d, 4JCP = 4.6 Hz, CH3), 24.51
(d, 3JCP = 14.2 Hz, CH3), 126.26 (CH), 126.74, 127.17 (CH), 127.37 (d,
3JCP = 7.7 Hz, CH), 128.00 (CH), 128.76 (d, 2JCP = 10.9 Hz, CH),
129.44 (d, 3JCP = 5.7 Hz, CH), 132.09 (d, 3JCP = 1.9 Hz), 133.81 (d,
4JCP = 1.8 Hz, CH), 134.87 (d, 3JCP = 8.6 Hz, CH), 137.52 (br), 137.57,
[12] a) U. Vogel, A. Y. Timoshkin, K.-C. Schwan, M. Bodensteiner,
M. Scheer, J. Organomet. Chem. 2006, 691, 4556; b) K.-C.
Schwan, A. Y. Timoskin, M. Zabel, M. Scheer, Chem. Eur. J.
2006, 12, 4900; c) U. Vogel, P. Hoemensch, K.-C. Schwan, A. Y.
Timoshkin, M. Scheer, Chem. Eur. J. 2003, 9, 515; d) A. Adolf, U.
Vogel, M. Zabel, A. Y. Timoshkin, M. Scheer, Eur. J. Inorg.
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[13] Although there are a few examples on the synthesis and
complexation of heteroatom-substituted acenaphthene deriva-
tives, their properties have not been studied. For examples, see:
a) R. Kçster, G. Benedikt, W. Fenzl, K. Reinert, Justus Liebigs
Ann. Chem. 1967, 702, 197; b) M. Pailer, H. Huemer, Monatsh.
Chem. 1964, 95, 373; c) B. J. Anderson, M. A. Guino-o, D. S.
Glueck, J. A. Golen, A. G. DiPasquale, L. M. Liable-Sands,
A. L. Rheingold, Org. Lett. 2008, 10, 4425; d) T. Mizuta, T.
Nakazono, K. Miyoshi, Angew. Chem. 2002, 114, 4053; Angew.
Chem. Int. Ed. 2002, 41, 3897.
3
1
138.31 (d, JCP = 5.7 Hz), 139.01, 141.77 (d, JCP = 8.3 Hz), 142.66 (d,
2
2JCP = 17.6 Hz), 143.07 (br d, 2JCP = 12.0 Hz), 144.09 ppm (d, JCP
=
12.9 Hz); 11B NMR (95 MHz, C6D6, RT): d = 77.6 (Dn1/2 = 1390 Hz);
31P NMR (121 MHz, C6D6, RT): d = À28.2 ppm. HRMS (FAB): m/z:
calcd for C28H29BP ([M+H]+): 407.2100; found: 407.2098 ([M + H]+).
Elemental analysis calcd (%) for C28H28BP: C 82.77, H 6.95%; found:
C 82.80, H 6.96%. Assignment of the signals in 1H and 13C NMR
spectra is shown in the Supporting Information.
Received: July 15, 2011
Published online: September 21, 2011
[14] See the Supporting Information for details.
[15] The 1H NMR spectra of the crude mixture showed the signals of
(E)-4c and (Z)-4c in the ratio of 7:10.
Keywords: 1,2-aryl migration · acenaphthenes · conjugation ·
.
fluorescence · phosphorus heterocycles
[16] Bertrand and co-workers have reported P2B2 four-membered-
À
ring systems with B B bonds. For examples, see; a) D. Schesch-
kewitz, H. Amii, H. Gornitzka, W. W. Schoeller, D. Bourissou,
G. Bertrand, Science 2002, 295, 1880; b) D. Scheschkewitz, H.
Amii, H. Gornitzka, W. W. Schoeller, D. Bourissou, G. Bertrand,
Angew. Chem. 2004, 116, 595; Angew. Chem. Int. Ed. 2004, 43,
585.
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[17] CCDC 832939 (1a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[18] To the best of our knowledge, there has been no report on the
electrochemical and photophysical properties of phosphinobor-
anes.
[19] The reduction potentials of NaphBMes2 were previously
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À
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[20] The broad absorption was observed around 280–400 nm (lmax
=
334 nm; e, 1.4 ꢁ 103) in the UV/Vis spectrum of 2a in hexanes.
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[22] No emission was observed using the excitation wavelengths at
every 10 nm from 280 to 370 nm.
[23] Both kr and knr values of acenaphthene are calculated as 1.2 ꢁ
107 sÀ1 (quantum yield, 0.50; lifetime, 46 ns in cyclohexane), see:
M. Montalti, A. Credi, L. Prodi, M. T. Gandolfi in Handbook of
Photochemistry, 3rd. ed., CRC, Boca Raton, 2006, p. 86.
[24] a) R. Ahlrichs, M. Bar, M. Haser, H. Horn, C. Kolmel, Chem.
Phys. Lett. 1989, 162, 165; b) TURBOMOLE Userꢀs Manual,
Version 6.2; R. Ahlrichs, F. Furche, C. Hꢃttig, W. Klopper, M.
Sierka, F. Weigend, TURBOMOLE ver. 6.2, 2010.
[6] A. Wakamiya, K. Mori, T. Araki, S. Yamaguchi, J. Am. Chem.
Soc. 2009, 131, 10850.
[7] For a review, see: a) A. Fukazawa, S. Yamaguchi, Chem. Asian J.
2009, 4, 1386; for examples of phosphonium- and borate-
bridging zwitterionic stilbenes, see: b) A. Fukazawa, H.
Yamada, S. Yamaguchi, Angew. Chem. 2008, 120, 5664; Angew.
Chem. Int. Ed. 2008, 47, 5582; c) A. Fukazawa, H. Yamada, Y.
Sasaki, S. Akiyama, S. Yamaguchi, Chem. Asian J. 2010, 5, 466.
[8] For reviews, see: a) R. T. Paine, H. Nçth, Chem. Rev. 1995, 95,
343; b) P. P. Power, Chem. Rev. 1999, 99, 3463; c) R. C. Fischer,
P. P. Power, Chem. Rev. 2010, 110, 3877.
[25] The emission is most likely interpreted in terms of the intra-
molecular charge transfer (HOMO–LUMO) with
change of the polarity at the excited state.
a small
Angew. Chem. Int. Ed. 2011, 50, 10940 –10943
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