The synthesis and single and two-photon excited fluorescence of a new quasi-quadrupolar organoborane compound

Article abstract of DOI:10.1016/j.molstruc.2010.02.007

A new acceptor-π-acceptor quadrupolar compound with a dimesitylboryl as acceptor and 2,7-dithienylfluorene as the conjugated bridge has been synthesized using the Suzuki-Miyaura coupling reaction. Its single and two-photon related photo-physical properties were experimentally examined. The combination of a large two-photon cross-section (δ = 1150 GM at 730 nm in hexane), high emission quantum yield (F{cyrillic} = 0.81 in hexane) and a strong binding constant with fluoride anions (K₁ = 3.0 × 10⁵ mol^-1 L) make this compound attractive for application as a two-photon excited fluorescent chemosensor for fluoride anions.

Full text of DOI:10.1016/j.molstruc.2010.02.007

Journal of Molecular Structure 969 (2010) 182–186
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
The synthesis and single and two-photon excited fluorescence of a new
quasi-quadrupolar organoborane compound
a
a
a,
Ying Chen a, Duxia Cao b, , Shasha Wang , Changqiao Zhang , Zhiqiang Liu
*
*
a State Key Laboratory of Crystal Materials, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China
b School of Material Science and Engineering, University of Jinan, Jinan 250022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new acceptor-p-acceptor quadrupolar compound with a dimesitylboryl as acceptor and 2,7-dithienyl-
Received 21 November 2009
Received in revised form 30 January 2010
Accepted 1 February 2010
Available online 6 February 2010
fluorene as the conjugated bridge has been synthesized using the Suzuki–Miyaura coupling reaction. Its
single and two-photon related photo-physical properties were experimentally examined. The combina-
tion of a large two-photon cross-section (d = 1150 GM at 730 nm in hexane), high emission quantum
yield (U = 0.81 in hexane) and a strong binding constant with fluoride anions (K1 = 3.0 ꢀ 105 molꢁ1 L)
make this compound attractive for application as a two-photon excited fluorescent chemosensor for fluo-
ride anions.
Keywords:
Organic borane
Two-photon excited fluorescence
Chemosensor
Ó 2010 Elsevier B.V. All rights reserved.
Fluoride anion
1. Introduction
the D–p–D system containing diarylamino moieties, some mole-
cules containing B(Mes)2 made by Marder et al. [13–15] and our
group [16,17] during the past few years have exhibited large TPA
cross-sections with remarkable high quantum yields, which are
Organic boron compounds play important roles in modern opto-
electronic materials. Generally, these compounds can be classified
into two types. The organic boronic acid/boronate ester act as key
intermediates for the further building of conjugated systems [1]
and triarylboranes act as functional materials [2,3]. For the latter,
physical stability is necessary. Trivalent boron is inherently elec-
tron-deficient and isoelectronic with a carbocation. When con-
comparable with the D–p–D analogues.
However, more and more research indicated that the photo-
chemical stability are also very important factor for the application
of TPA dyes. Although vinyl connected aromatic systems are one of
the best option for TPA materials, their relative poor stability is an
important drawback. For example, the one- and two-photon pho-
tochemical stability of two vinyl-containing fluorene derivatives
had been investigated using both absorption and fluorescence
methods by Belfield and coworkers [18]. Photochemical reactions
involve the olefinic C@C double bonds, especially cyclo-addition
reaction of oxygen with olefinic C@C double bonds were believed
to be one main photo-degradation process [18]. We have reported
compound 1 (see Scheme 1) possesses strong TPEF and large TPA
cross-section up to 1340 GM [16] and can be used as a TPEF active
chemosensor for fluoride anions with high sensitivity and
selectivity [17]. However, the intrinsic cis–trans isomerization of
the vinyl moiety and the possible photobleaching processes as
mentioned above certainly would limit its usefulness. To overcome
this shortcoming, many researchers would like use other rigid and
stable groups to build conjugated system in their design of TPA
materials [13–15]. Herein, we present details of the synthesis
and single and two-photon excited fluorescence properties of
nected with a
p
system, trivalent boron will behave as a strong
p
acceptor through p-
p* conjugation. However, the p orbital is read-
ily attacked by nucleophiles such as water, which results in poor
stability of normal trivalent organoboranes. It was found that the
addition of two bulky groups (normally mesityl, mesityl = 2,4,6-tri-
methylphenyl) offered enough steric hindrance to protect the
boron center from nucleophilic attack [2–4]. Lots of new
boron-containing materials have exhibited interesting photolumi-
nescence and electroluminescence properties in the past
decades [2–5].
Since the 1990s, two-photon absorption (TPA) and emission or-
ganic materials have attracted much attention due to potential
applications, such as 3D optical data storage [6] and laser scanning
microscopy [7]. Experimental and theoretical research indicated
that quadrupolar and octupolar molecular structure are successful
strategies for increasing the TPA and two-photon excited fluores-
cence (TPEF) properties [8–12]. Contrary to works focusing on
new A–p–A compound (2) with fluorene instead of divinylbenzene
as the conjugating core, namely, 2,7-bis(20-dimesitylborylthienyl)-
9,9-diethylfluorene.
* Corresponding authors. Tel./fax: +86 531 8836 2782 (Z. Liu).
E-mail addresses: duxiacao@ujn.edu.cn (D. Cao), zqliu@sdu.edu.cn (Z. Liu).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.02.007

Y. Chen et al. / Journal of Molecular Structure 969 (2010) 182–186
183
Mes
Mes
Mes
Mes
Mes
Mes
Mes
Mes
S
S
S
S
B
B
B
B
1
2
I
I
Mes
Mes
Mes
Mes
O
O
S
S
S
(b)
S-2
(c)
(a)
B
Br
B
B
2
S-1
S-0
Scheme 1. Synthesis of compound 2. Reagents and conditions: (a) n-BuLi, FB(Mes)2, THF, ꢁ78 °C to rt; (b) B2pin2, [Ir(COD)Cl]2, dtbpy, hexane, 80 °C; (c) Pd(PPh3)4, Na2CO3,
toluene, 80 °C.
stirred at 80 °C for 16 h. The mixture passed through a short silica
pad to remove the Ir-containing catalyst, then concentrated and
recrystallized from hexane at ꢁ20 °C to get 3.87 g white crystalline
product with acceptable purity (86% yield). 1H NMR (CDCl3,
400 MHz), d = 7.61 (d, J = 3.6 Hz, 1H), 7.40 (d, J = 3.6 Hz, 1H), 6.73
(s, 4H), 2.21 (s, 6H), 2.00 (s, 12H), 1.25 (s, 12H). 13C{H} NMR (CDCl3,
100 MHz): d = 140.79, 140.29, 138.61, 138.16, 128.15, 84.28, 24.82,
23.41, 21.20. Elemental analysis calcd. (%) for C28H36B2O2S: C,
73.39; H, 7.92; found: C, 73.27; H, 7.90.
2. Experimental
2.1. Chemicals and instruments
Nuclear magnetic resonance spectra were recorded on a Bruker
Avance-400 spectrometer. Mass spectrum was recorded on an Ap-
plied Biosystems Voyager DE STR (MALDI-TOF) spectrometer. Ele-
mental analysis was carried out on a PE 2400 autoanalyzer. n-
Butyllithium in hexane solution was obtained from Acros Ltd. Dim-
esitylboron fluoride (BF(Mes)2) was purchased from Aldrich Ltd.
Other reagents, such as fluorene and 2-bromothiophene were pur-
chased from Shanghai Reagents and were used as received directly
without further purification. All solvents were freshly distilled be-
fore using.
2.2.3. 2,7-Bis(20-dimesitylthienyl)-9,9-diethylfluorene (2)
2-Dimesitylboryl-5-Bpin-thiophene (S-1, 0.46 g, 1.0 mmol) and
2,7-diiodo-9,9-diethylfluorene (S-2, 0.24 g, 0.5 mmol) were
placed in a 100 mL Schlenk tube, which was evacuated and
purged with nitrogen gas three times. Pd(PPh3)4 (12 mg,
0.01 mmol), toluene (ꢂ10 mL) and saturated aquaues solution
of Na2CO3 (ꢂ0.5 mL) were added under nitrogen. The tube was
sealed with a Teflon cap and stirred at 80 °C for 16 h before it
was quenched. This mixture was extracted with dichlorometh-
ane, dried over MgSO4, concentrated in vacuo. Purification by col-
umn chromatography (silica gel using chloroform–petroleum
ether (1:6) gave a bright yellow powder with yield 64%. 1H
NMR (CDCl3, 400 MHz), d = 7.67 (s, 4H), 7.60 (s, 2H), 7.53 (d,
J = 3.6 Hz, 2H), 7.45 (d, J = 3.6 Hz, 2H), 6.86 (s, 8H), 2.33 (s,
12H), 2.17 (s, 24H); 13C {H} NMR (CDCl3, 100 MHz), d = 157.70,
151.05, 141.89, 141.48, 141.16, 140.89, 138.52, 133.18, 128.20,
125.37, 120.49, 120.24, 56.56, 32.84, 23.55, 21.28, 8.54. MS
(MALDI-TOF) m/z calcd for C61H64B2S2: 883; found 883; elemen-
tal analysis calcd. (%) for C61H64B2S2: C, 82.98; H, 7.31; S, 7.26;
found: C, 82.88; H, 7.30; S, 7.33.
2.2. Synthesis
Compound 2 was prepared via a typical Suzuki–Miyaura cross-
coupling reaction (Scheme 1). The boronate ester S-1 was prepared
via Ir-catalysed borylation of 2-dimesitylthiophene [19]. Halo-flu-
orene S-2 was prepared via direct halogenations of fluorene at 2,
7 positions [20].
2.2.1. 2-Dimesitylborylthiophene (S-0)
To a Schlenk flask charged with 1-bromo-thiophene (2.45 g,
15 mmol) in 50 mL of dry Et2O and a stirrer bar, 1.6 M n-BuLi
in hexane (10 mL, 16 mmol) was added dropwise via a syringe
at ꢁ78 °C. The reaction was allowed to warm to room tempera-
ture for 1 h and was cooled to ꢁ78 °C again and a solution of
dimesitylboryl fluoride (4.0 g, 15 mmol) in 20 mL of Et2O was
added. The reaction was allowed to warm to room temperature
with stirring overnight, then quenched with 10 mL of 1 M HCl,
extracted into DCM, dried over Mg2SO4, and concentrated in va-
cuo. The residue was purified by recrystallization from hexane to
give 4.5 g white solid (90% yield). 1H NMR (CDCl3, 400 MHz),
d = 7.80 (q, 1H), 7.37 (q, 1H), 7.12 (m, 1H), 6.73 (s, 4H), 2.21 (s,
6H), 2.01 (s, 12H); 13C{H} NMR (CDCl3, 100 MHz): d = 140.85,
140.43, 138.55, 138.16, 129.15, 128.21, 23.45, 21.25. Elemental
analysis calcd. (%) for C22H25BS: C, 79.51; H, 7.58; found: C,
79.47; H, 7.60.
2.3. Measurements
Linear absorption spectra with C = 5.0 ꢀ 10ꢁ6 mol Lꢁ1 were re-
corded on a Shimadzu UV 2550 spectrometer. Single-photon ex-
cited fluorescence (SPEF) spectra with C = 5.0 ꢀ 10ꢁ6 mol Lꢁ1
were measured on an Edinburgh FLS920 fluorescence spectrome-
ter. The SPEF quantum yields
U for the compound were deter-
mined relative to coumarin 307 [21] using a standard method
[22]. TPEF experiments with C = 1.0 ꢀ 10ꢁ4 mol Lꢁ1 were per-
formed with a femtosecond Ti:sapphire laser (80 MHz, 200 fs pulse
width, Spectra Physics) as a pump source and a USB 2000 + Micro
Fiber Spectrometer (Ocean optics) as the recorder. The TPA cross-
sections of compound 2 in toluene and THF solutions were mea-
sured by using a TPEF measurement technique and coumarin 307
as Ref. [23].
2.2.2. 2-Dimesitylboryl-5-Bpin-thiophene (S-1)
In a dry N2 filled Schlenk tube, the pre-catalyst [Ir(COD)Cl]2
(7 mg, 1% mol), ligand dtbpy (12 mg, 1% mol), and a little scale of
B2pin2 (ꢂ20 mg) were mixed in 2 mL hexane and stirred vigorously
until the solution turned brown. Then 2-dimesitylboryl-thiophene
(3.32 g, 10 mmol), B2pin2 (2.54 g, 10 mmol) and 15 mL hexane was
added to the tube. The tube was sealed with a Teflon cap and

184
Y. Chen et al. / Journal of Molecular Structure 969 (2010) 182–186
3. Results and discussion
3.1. Linear absorption and single-photon excited fluorescence
properties
Linear absorption and SPEF properties of 2 in various solvents
with different polarity along with compound 1 in toluene (Table
1) are reported. The absorption and emission spectra of 2 showed
clear vibronic structures (Fig. 1). Generally, compound 2 displays a
broad charge-transfer absorption band with maxima of 404–
412 nm and shoulders at higher wavelength. Compound 2 emits
strong fluorescence in the blue–green spectral region with a high
quantum yield over 70%. Upon increasing the polarity of the sol-
vents, the emission maximum, kmax(em), is gradually shifted to-
ward longer wavelengths by 20 nm. The weak solvatochromism
and small stokes-shift relate to the quasi-quadrupolar structural
motif of this compound and a rigid backbone. The dipole moment
of the molecule is small both in the ground and excited state due to
symmetric charge transfer, therefore the dipole–dipole interaction
between the solvent and solute should be quite weak, and only
small solvatochromism is observed.
Fig. 1. Linear absorption and single-photon excited fluorescence emission spectra
of 2 in various solvents (C = 5.0 ꢀ 10ꢁ6 mol Lꢁ1; kex = 400 nm).
Interestingly, compared with the spectral properties of vinyl-
phenylene-bridged compound 1, both the absorption and emission
of the fluorene-bridged compound 2 are significantly blue-shifted
by nearly 30 nm. In addition, the extinction coefficients, e, of 2
are also decreased by 20%, but the quantum yield of 2 is obviously
higher than that of 1. The differences between compounds 1 and 2
can be related to the differences in the structure of their cores. It
seems that the fluorine (in 2) does not provide as effective conju-
gation as the divinylbenzene (in 1), as evidenced by the spectral
blue-shift of 2. However, the higher rigidity of the fluorene com-
pared to the isomerizable divinylbenzene means less possibility
of a non-radiative transition, and should contribute to the higher
quantum yield.
3.2. Two-photon excited fluorescence properties
As shown in Fig. 1, there was not linear absorption over 550 nm
for compound 2. Therefore upon excitation with a laser in the
range of 700–900 nm, the frequency up-converted fluorescence
can be safely attributed to a multiphoton absorption process. In
our measurements, when the solution of 2 was excited with a fs la-
ser at 730 nm, it presented bright fluorescence emission. This
emission nearly overlaps the spectra of SPEF and its intensities
are linearly dependent on the square of input laser power. All these
results indicate a two-photon excitation mechanism for this
emission.
Further experiments reveal that the emission spectra of com-
pound 2 is independent of the excitation wavelengths (from 720
to 860 nm), but the intensity of emission is effected by altering
the excitation wavelength. By tuning the pump wavelengths incre-
mentally from 720 to 860 nm while keeping the input power con-
stant (100 mW) and then recording the emission intensity, the
two-photon excitation spectra of 2 was obtained. As shown in
Fig. 2. The normalized single and two-photon excitation and emission spectra of 2
in toluene. The wavelengths for the TPE spectrum have been divided by two.
Fig. 2, the peak position of single-photon excitation (SPE) spectrum
and two-photon excitation (TPE) spectrum of compound 2 is at 410
and 730 nm, respectively, which indicates that the energy level of
the TPE state is about 0.47 eV higher than that of the SPE state. This
is consistent with the prediction that the two-photon allowed
states for quadrupoles should be located at higher energy than
the single-photon allowed states [8–10,14], but fluorescence nor-
mally occurs from the same excited state.
The TPEF spectrum of compound 2 in Fig. 2 shows a red shift
relative to the SPEF spectrum. The shoulder at higher wavelength
is weaker in the TPEF spectrum compared to the SPEF spectrum.
This is due to a re-absorption effect, considering the partial overlap
of the SPE and SPEF spectra and the higher concentration of the
Table 1
Single and two-photon related photo-physical properties of compound 2.
Solvents
Linear absorption
SPEF
TPEF
kmax (nm)
10ꢁ4
ꢃe
kmax (nm)
U
kmax (nm)
d (GM)
2
1
DMF
Acetone
THF
Toluene
Hexane
Toluene
412
407
409
410
404
435
7.8
7.5
8.0
7.5
8.4
9.3
459, 481
452, 475
451, 477
449, 477
440, 466
482, 515
0.70
0.78
0.81
0.78
0.87
0.61
462, 488
456, 488
453, 486
451, 477
450, 471
491, 514
780 (730 nm)
750 (730 nm)
1080 (730 nm)
990 (730 nm)
1150 (730 nm)
1340 (775 nm)

Y. Chen et al. / Journal of Molecular Structure 969 (2010) 182–186
185
samples in TPEF measurements. This re-absorption effect will in-
duce some experimental error in calculating the TPA cross-sections
from the generally used comparative TPEF measurement
technique.
Generally, the TPA cross-section of 2 is large (ꢂ1000 GM) and
dependent on solvent. For comparison, the value of 2 is less than
that of compound 1 (990 GM vs. 1340 GM in toluene), and the kTPE
of compound 2 is also blue-shifted by 45 nm (730 nm vs. 775 nm).
These results seem consistent with the single-photon related prop-
erties as discussed above. The relative short electron delocalization
range of compound 2 may decrease the quadrupolar charge trans-
fer and influence TPA.
ties in molecule 2, there were two clear stages of sensing the fluo-
ride ion in both absorption and emission spectra. In the first stage,
for F-addition from 0 to 2.0 equivalents, the main absorption be-
tween 390 and 450 nm of 2 greatly decreased and the absorption
edge was red-shifted from 460 to 500 nm. As a result, the whole
band degenerated to a broad single band centered at 410 nm.
The presence of two distinct isosbestic points at 362 and 443 nm
and molar ratio analysis indicate that the spectral change in this
concentration range can be ascribed to 1:1 complexation of 2 with
fluoride ion with a binding constant K1 = 3.0 ( 0.3) ꢀ 105 Mꢁ1 L.
Furthermore, the emission intensity also decreased in this process.
In the second stage, the further addition of a large excess of TBAF
caused subsequent spectral changes with a new absorption peak
at 390 nm, and the absorption edge greatly blue-shifted back to
around 440 nm. Correspondingly, the emission spectra are also
blue-shifted. These changes could be assumed to be complexation
with the second fluoride ion. The binding constant for this process
is estimated to be K2 = 1.8 ( 0.4) ꢀ 105 Mꢁ1 L.
3.3. Spectral response to fluoride complexation
Trivalent organoboron compounds have been studied as
chemosensors for fluoride anions with high selectivity and sensi-
tivity [24–30]. The recognition mechanism is attributed to the un-
ique steric structure of bulky dimesitylboryl group (offering
selectivity) and the Lewis acid–base interaction between the triva-
lent boron atom and fluoride anion (offering sensitivity). In the
presence of a fluoride anion, the strong B–F interaction is believed
4. Conclusions
A new A–p–A type compound with dimesityl protected triva-
lent borons as electron acceptors was synthesized. Its single- and
two-photon related photo-physical properties and complexation
with fluoride anions were examined. The high quantum yield and
strong two-photon absorption as well as a strong interaction with
fluoride anions make this compound a potential luminescent
material and chemosensors.
to interrupt the extended
change of the related photo-physical properties.
In our experiments, n-Bu4NF (TBAF) as the fluoride source was
gradually added to THF solutions of each borane. Fig. 3 shows
the spectral change of 2 in THF solution (6.5
p-conjugation and thereby causes a
lM) upon the addition
of TBAF. Perhaps due to the two symmetrical dimesityboryl moie-
Acknowledgments
Financial support from the National Natural Science Foundation
of China (20802026 and 50803033), Natural Science Foundation of
Shandong province (Q2008F02) and Shandong Encouraging Fund
for Excellent Scientists (BS2009CL013) are gratefully acknowl-
edged. Z. L. also thanks the Royal Society and BP for a China Incom-
ing Fellowship to support collaboration with Prof. T.B. Marder in
Durham University (U.K.).
References
[1] Dennis G. Hall (Ed.), Boronic Acids: Preparation and Applications in Organic
Synthesis and Medicine, Wiley-VCH, 2005.
[2] C.D. Entwistle, T.B. Marder, Chem. Mater. 16 (2004) 4574–4585;
C.D. Entwistle, T.B. Marder, Angew. Chem. Int. Ed. Engl. 41 (2002) 2927–2931.
[3] S. Yamaguchi, A. Wakamiya, Pure Appl. Chem. 78 (2006) 1413–1424.
[4] H.C. Brown, V.H. Dodson, J. Am. Chem. Soc. 79 (1957) 2302–2306.
[5] G. Zhou, Adv. Funct. Mater. 18 (2008) 499–511.
[6] D.A. Parthenopoulos, P.M. Rentzepis, Science 245 (1989) 843–845.
[7] W. Denk, J.H. Strickler, W.W. Webb, Science 248 (1990) 73–76.
[8] B. Strehmel, V. Strehmel, Adv. Photochem. 29 (2007) 111–354.
[9] G.S. He, L.-S. Tan, Q. Zheng, P.N. Prasad, Chem. Rev. 108 (2008) 1245–1330.
[10] M. Rumi, S. Barlow, J. Wang, J.W. Perry, S.R. Marder, Adv. Polym. Sci. 213
(2008) 1–95.
[11] F. Terenziani, C. Katan, M. Blanchard-Desce, E. Badaeva, S. Tretiak, Adv. Mater.
20 (2008) 4641–4678.
[12] H.M. Kim, B.R. Cho, Chem. Commun. (2009) 153–164.
[13] M. Charlot, L. Porres, C.D. Entwistle, A. Beeby, T.B. Marder, M. Blanchard-Desce,
Phys. Chem. Chem. Phys. 7 (2005) 600–606.
[14] J.C. Collings, S.-Y. Poon, Chem. Eur. J. 15 (2009) 198–208.
[15] C.D. Entwistle, J.C. Collings, A. Steffen, L.-O. Pålsson, A. Beeby, D. Albesa-Jové,
J.M. Burke, A.S. Batsanov, J.A.K. Howard, J.A. Mosely, S.-Y. Poon, W.-Y. Wong, F.
Ibersiene, S. Fathallah, A. Boucekkine, J.-F. Halet, T.B. Marder, J. Mater. Chem.
(2009) 7532–7544.
[16] Z.Q. Liu, Q. Fang, D.X. Cao, D. Wang, G.B. Xu, Org. Lett. 6 (2004) 2933–2936.
[17] Z.Q. Liu, M. Shi, F.Y. Li, Q. Fang, Z.H. Chen, T. Yi, C.H. Huang, Org. Lett. 7 (2005)
5481–5484.
[18] C.C. Corredor, K.D. Belfield, M.V. Bondar, O.V. Przhonska, J. Photochem.
Photobiol. A: Chem. 184 (2006) 105–112.
[19] I.A.I. Mkhalid, J.H. Barnard, T.B. Marder, J.M. Murphy, J.F. Hartwig, Chem. Rev.
110 (2010) 890–931.
[20] R. Kannan, G.S. He, L.X. Yuan, F. Xu, P.N. Prasad, A.G. Dombroskie, B.A.
Reinhardt, J.W. Baur, R.A. Vaia, L.-S. Tan, Chem. Mater. 13 (2001) 1896–1904.
Fig. 3. Absorption (a) and emission (b) spectra of 2 (6.5 lM; kex = 400 nm) in THF
upon the addition of fluoride anions.

186
Y. Chen et al. / Journal of Molecular Structure 969 (2010) 182–186
[21] G.A. Reynolds, K.H. Drexhage, Opt. Commun. 13 (1975) 222–225.
[22] J.N. Demas, G.A. Crosby, J. Phys. Chem. 75 (1971) 991–1024.
[23] M.A. Albota, C. Xu, W.W. Webb, Appl. Opt. 37 (1998) 7352–7356.
[24] Y. Kim, F.P. Gabbaï, J. Am. Chem. Soc. 131 (2009) 3363–3369.
[25] T.W. Hudnall, C.-W. Chiu, F.P. Gabbaï, Acc. Chem. Res. 42 (2009) 388–397.
[26] Z.M. Hudson, S. Wang, Acc. Chem. Res. 42 (2009) 1584–1596.
[27] T. Agou, J. Kobayashi, T. Kawashima, Chem. Eur. J. 13 (2007) 8051–8060.
[28] G. Zhou, M. Baumgarten, K. Müllen, J. Am. Chem. Soc. 130 (2008) 12477–
12484.
[29] H. Li, F. Jäkle, Angew. Chem. Int. Ed. 48 (2009) 2313–2316.
[30] T. Agou, M. Sekine, J. Kobayashi, T. Kawashima, Chem. Commun. (2009)
1894.