Aggregation-induced emission of triphenylamine substituted cyanostyrene derivatives

Article abstract of DOI:10.1039/c3nj01343j

Cyanostyrene-based D-π-A type conjugated compounds bearing triphenylamine units (G1, G1-N and G2) have been synthesized. By tuning the conjugated skeleton and the electron withdrawing ability of the acceptor, green, orange and red light emitters were achi

Full text of DOI:10.1039/c3nj01343j

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Aggregation-induced emission of triphenylamine
substituted cyanostyrene derivatives†
Xin Zhao,a Pengchong Xue,a Kai Wang,a Peng Chen,b Peng Zhanga and Ran Lu*a
Cite this: New J. Chem., 2014,
38, 1045
Cyanostyrene-based D–p–A type conjugated compounds bearing triphenylamine units (G1, G1-N and
G2) have been synthesized. By tuning the conjugated skeleton and the electron withdrawing ability of
the acceptor, green, orange and red light emitters were achieved. It is interesting that although their
emissions in solutions were weak, we observed strong emission in solid states. For example, the FF of
the dendritic molecule G2 in powder reached 0.67, which was more than 20 times of that in THF. The
single crystal X-ray diffraction data revealed that the intermolecular H-bonds of C–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀN and
C–Hꢀ ꢀ ꢀp led to the increased rigidity of the molecular skeleton, which would restrict the intramolecular
rotation, yielding high FF in solid states. These AIE compounds may become candidates for emitting
materials.
Received (in Montpellier, France)
30th October 2013,
Accepted 11th December 2013
DOI: 10.1039/c3nj01343j
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in OLEDs, laser devices, and so on, various conjugated organic
molecules emitting different colors have been synthesized.13 For
Introduction
Organic p-conjugated molecules have been intensively studied example, the oligofluorenes were linked to different cores, such
due to their potential applications in opto-electronic materials.1–5 as porphyrin and tetraphenylsilica, to give red and blue emission
Most of the organic luminogens are highly emissive in dilute in OLEDs, respectively.14,15 The trimer of tetraphenylethene
solutions but exhibit weak or quenched fluorescence upon (TPE) gave a fluorescence peak at 494 nm, which shifted to
increasing the concentrations of the solutions or in the solid 575 nm when fumaronitrile was used as a center linked to TPE
states because of strong p–p interactions and the non-radiative units. In addition, an emission at 713 nm was achieved in the
decay process. This phenomenon, which is known as aggregation- fumaronitrile-centered TPE bearing terminal N,N-diethylamino
caused quenching (ACQ),6,7 has greatly limited the practical group.16 Tian and coworkers designed a series of 2,20-biindene-
applications. Numerous efforts have been made to tackle this based fluorophores with strong solid-state emissions ranging
problem,8,9 and the most important development in this field was from deep blue to red, which were tuned by the variation of the
the introduction of the term aggregation-induced emission (AIE) substituents on the 2,20-biindenyl unit or by adjustment of the
by Tang and co-workers in 2001.10 They found that 1-methyl- aggregation state.17 Moreover, the luminogens emitting blue,
1,2,3,4,5-pentaphenylsilole gave very weak emission in solution, green, and red colors constructed from tetraphenylethene,
but the obtained nanoparticles could emit strong fluorescence benzo-2,1,3-thiadiazole and thiophene were prepared.18,19 It is
because of restriction of intramolecular rotation (RIR). Additionally, known that triphenylamine (TPA) has been widely used as the
Park reported that 1-cyano-trans-1,2-bis-(40-methylbiphenyl)- electron donor (D) in opto-electronic materials on account of its
ethylene (CN-MBE) was non-luminescent in solution but highly good hole transport mobility20–22 and the cyanostyrene is a typical
emissive in the aggregated states.11,12
electron acceptor (A) group with AIE activity.23–25 Recently, Tian
Because the achievement of the emitting materials with found that the multi-branched D–p–A–p–D compounds with the
strong emission in solid states is important for their applications triphenylamine (or carbazole) group as an electron-donor and the
cyanostyrene (or triazine) group as an electron-acceptor showed
excellent two-photon absorption activity and AIE properties.26
a State Key Laboratory of Supramolecular Structure and Materials,
Herein, in order to prepare new AIE materials emitting
College of Chemistry, Jilin University, Changchun 130012, PR China.
different colors we designed D–p–A conjugated compounds
bearing TPA and cyanostyrene units G1, G1-N and G2
(Scheme 1). Although their emission in solutions was weak,
we observed strong emission in green, orange and red in solid
states, which were tuned by the conjugation length and the
electron withdrawing ability of the acceptor. For example,
the second generation of triphenylamine-based dendron was
E-mail: luran@mail.jlu.edu.cn; Fax: +86-431-88499179; Tel: +86-431-88499179
b Key Laboratory of Functional Inorganic Material Chemistry (MOE), School of
Chemistry and Materials Science, Heilongjiang University, Harbin 150080,
PR China
† Electronic supplementary information (ESI) available: Electrochemical properties
and 1H-, 13C-NMR and MALDI/TOF MS spectra. CCDC 968916 and 968917. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3nj01343j
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Scheme 1 Synthetic routes for G1, G1-N and G2.
involved in G227–29 leading to larger conjugation than that of and they are soluble in dichloromethane, chloroform and THF.
G1, so G2 emitted orange light and G1 emitted green light in From the FT-IR spectra of G2 we found that the vibration
solid states. When the nitro group was introduced into the absorption band appeared at ca. 960 cmꢁ1, meaning that the
cyanostyrene unit, red emitting compound G1-N was obtained.30 carbon–carbon double bonds adopted trans-form. In addition,
The single crystal X-ray diffraction data revealed that the inter- the 1H NMR spectra of G2 further confirmed that ethenyl
molecular H-bonds of C–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀp led to the groups adopted the trans-conformation on account of the
increasing rigidity of the molecular skeleton, which would absence of the signal at ca. 6.5 ppm assigned to the protons
restrict the intramolecular rotation, yielding high FF in solid in cis-double bonds.31
states. Therefore, green, orange and red emitting compounds
with AIE were obtained, which might have potential applications
UV-vis absorption and fluorescence emission spectra
in organic light emitting materials.
The UV-vis absorption and fluorescence emission spectra of
triphenylamine substituted cyanostyrene derivatives G1, G1-N
and G2 in THF (1.0 ꢂ 10ꢁ5 M) are shown in Fig. 1a and the
Results and discussion
Syntheses
corresponding photophysical data are presented in Table 1.
Two clear absorption peaks at ca. 300 nm and 400 nm were
The synthetic routes for triphenylamine substituted cyanostyrene detected for compounds G1 and G2. The former one can be
derivatives are described in Scheme 1. Firstly, the formyl-ended attributed to the p–p* transition of the triphenylamine group
triphenylamine derivatives 1 and 2 were prepared according to the and the latter one can be assigned to the intramolecular charge
methods reported by our group previously.31 The Knoevenagel transfer (ICT) band. In detail, the absorption bands of G1 were
condensation reaction between the formyl-ended triphenylamine located at 298 nm and 398 nm, which red-shifted to 303 nm
derivative (compound 1 or 2) and phenylacetonitrile catalyzed and 406 nm for G2 due to the enlarged conjugation of the
by sodium hydroxide afforded G132 and G2 in a yield of 85% increased dendritic structure. On account of the introduction of
and 79%, respectively. Similarly, the Knoevenagel condensation the nitro group the maximum absorption of G1-N red-shifted to
reaction between compound 1 and 2-(4-nitrophenyl)acetonitrile 439 nm, and its absorption intensity was quite lower than that
catalyzed by tetrabutylammonium hydroxide (TBAH) gave G1-N of G1 due to the spin-forbidden charge transfer.33 It is notable
in a yield of 90%. All the new compounds were characterized by that the intensity of the absorption peaks for G2 was higher
1
FT-IR, H NMR, 13C NMR and MALDI/TOF mass spectroscopy, than G1 and G1-N due to the increased number of vinyl
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Fig. 2 Photos of G1, G1-N and G2 in THF (a, b and c, respectively) and in
powders (d, e and f, respectively) under UV illumination (365 nm), and
fluorescent microscope (lex = 330–385 nm) images of G1 (g) and G1-N (h)
in crystals.
Notably, the FF values for these compounds in powders were
more than 20 times of those in solutions (Table 1), indicating
that they exhibited AIE properties. Moreover, the crystals of G1
and G1-N (the crystal of G2 was not obtained) also gave strong
emission, and their emission bands further red-shifted to
516 nm and 658 nm in the crystals. The FF values for G1 and
G1-N in crystals were 0.31 and 0.06, respectively, which were
similar to those in powders. In addition, the photos of G1, G1-N
and G2 in solutions and in solid states illuminated by UV light
as shown in Fig. 2 further illustrated that the emission was
enhanced significantly in powders as well as in crystals com-
pared with that in solutions.
Fig. 1 (a) UV-vis absorption and fluorescence emission (lex = 365 nm)
spectra of G1, G1-N and G2 in THF (1.0 ꢂ 10ꢁ5 M); (b) fluorescence
emission spectra of G1, G1-N and G2 in solid states (lex = 365 nm).
Table 1 UV-vis absorption and fluorescence data for triphenylamine
derivatives
Solution
Powder
Crystal
b
c
c
Molecule
l
abs/nm lema/nm FF
lema/nm FF
lema/nm FF
Electrochemical properties
G1
G1-N
G2
298, 398 494
0.02 506
0.01 624
0.03 596
0.33 516
0.31
0.06
—
d
The electrochemical behaviors of triphenylamine substituted
cyanostyrene derivatives were investigated by cyclic voltammetry
(CV) using a standard three-electrode cell and an electrochemical
workstation (CHI 604) under a N2 atmosphere. Fig. 3 shows the
cyclic voltammetry (CV) diagrams of G1, G1-N and G2 using
0.1 M tetrabutylammonium tetrafluoroborate (TBABF4) as a
439
—
0.09 658
d
d
303, 406 584
0.67
—
a
b
Excited at 365 nm. Quantum yields (FF) determined in THF using
c
an integrating sphere. Quantum yields (FF) determined using an
integrating sphere. Not obtained.
d
triphenylamine units in G2. From the fluorescence emission
spectra of G1, G1-N and G2 in THF we can find that G1-N was
almost non-emissive because the introduction of an electron
withdrawing nitro group into the cyanostyrene unit might lead
to the occurrence of photo-induced electron transfer from
triphenylamine to the acceptor. G1 gave a weak emission at
494 nm. Upon increasing the generation of the dendron based
on triphenylamine, a red-shift of the emission band located at
584 nm was observed for G2 due to the larger p-conjugation.
The fluorescent quantum yields (FF) of G1, G1-N and G2 in THF
were 0.02, 0.01 and 0.03 using an integrating sphere, meaning
that the emission was very weak.
The fluorescence emission spectra of G1, G1-N and G2 in
powders or in crystals are shown in Fig. 1b. It was found that
the emission of G1 and G2 in powders appeared at 506 nm and
596 nm, respectively, with a red-shift compared with those in
solutions due to the aggregation. Although G1-N was non-emissive
Fig. 3 Cyclic voltammograms of G1, G1-N and G2 in CH2Cl2 containing
in solution, its powder gave an obvious emission at 652 nm. 0.1 mol Lꢁ1 of tetrabutylammonium tetrafluoroborate (TBABF4).
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supporting electrolyte in dry CH2Cl2, glass carbon disc working I0 was that in pure THF, vs. the content of water in THF–water.
electrodes, a platinum-wire counter electrode, and an SCE As shown in the inset of Fig. 4b, when the volume content of water
reference electrode. The SCE reference electrode was calibrated was r50%, I/I0 values were quite low. Upon increasing the content
using the ferrocene/ferrocenium (Fc/Fc+) redox couple as an of water, I/I0 values increased significantly, suggesting that AIE
external standard. According to the redox potential of Fc/Fc+ happened in the aggregates of G2.
(with an absolute energy level of ꢁ4.8 eV relative to the vacuum
X-ray studies of the single crystals
level for calibration), the HOMO energy level was calculated to
be ꢁ5.52 eV for G1, ꢁ5.52 eV for G1-N and ꢁ4.91 eV for G2
In order to reveal the mechanism of the AIE phenomena, we
(Table S1, ESI†), respectively. The higher HOMO energy level of
G2 indicated its strong electron donating ability. The calculated
LUMO energy level of G1-N (ꢁ3.10 eV) was lower than that of G1
(ꢁ2.78 eV) and G1 (ꢁ2.35 eV) due to the strong accepting ability
of the nitro group.
tried to get single crystals for all compounds. The crystals of G1
and G1-N were obtained from the solutions in petroleum ether–
chloroform, but we could not get the crystal of G2. The single
crystal study revealed that the crystal structures of G1 and G1-N
belonged to the monoclinic space group P2(1)/n and mono-
clinic space group P21/c, respectively, and each cell contained
AIE properties
As discussed above, compounds G1, G1-N and G2 exhibited AIE
behaviors. In order to evaluate the AIE properties of these
compounds, G2 was selected as an example. The UV-vis absorption
and fluorescence emission spectral changes of G2 in THF–water
(G2 is soluble in THF and water is a poor solvent) with different
amounts of water are given in Fig. 4. It was found that when
the amount of water was r50% (volume content) the UV-vis
absorption spectra of G2 were similar to that in pure THF. Upon
further increasing the amount of water, the absorption band at
406 nm red-shifted and was broadened, which might be due to the
molecular aggregation.23 For example, the absorption red-shifted
to 420 nm and 422 nm when the contents of water were 70% and
90% in THF–water, respectively. In addition, the appearance of
tails in the visible region could be attributed to Mie scattering
caused by nanoparticles.11 From Fig. 4b, we could find that the
emission of G2 can hardly be detected when the volume content of
water was r50% in THF–water. However, upon further increasing
the amount of water in the system, the fluorescence emission of
G2 was enhanced significantly, accompanied by a blue-shift. The
emission appeared at 582 nm and 573 nm when the volume
contents of water were 70% and 90% in THF–water, respectively.
In addition, we presented the plot of the ratio of I/I0, in which I was
the integral of fluorescence emission in the mixed solvent and
Fig. 5 ORTEP drawings of G1 (a) and G1-N (b) at 50% probability level
ellipsoids.
Fig. 4 (a) UV-vis absorption and (b) fluorescence emission spectra (lex = 365 nm) of G2 (1.0 ꢂ 10ꢁ5 M) in THF–water with different amounts of water
(% volume). The inset of (b) shows a plot of PL integral versus water content of the solvent mixture for G2.
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four molecules (Z = 4). The molecular ORTEP drawings are and P4 planes. Each H-bond of C2–Hꢀ ꢀ ꢀO1 (2.60 Å) connected
displayed in Fig. 5. In the crystal of G1, the torsion angle one pair of molecules in adjacent layers, and the H-bond of
between phenyl rings P3 and P4 was 56.78(21)1, exhibiting a C5–Hꢀ ꢀ ꢀO2 (2.65 Å) provided a head-to-tail type connection.
more twisting structure of cyanostyrene than that in G1-N Meanwhile, the C–Hꢀꢀꢀp interactions of 2.61 and 2.93 Å happened
(8.38(6)1). The twisting structures between two phenyl rings in between parallel non-planar molecules. Accordingly, the RIR
G1 reduce the conjugation of molecules and prevent face-to- process caused by the intermolecular interactions, including
face p–p aggregations. Moreover, the intermolecular inter- C–Hꢀꢀꢀp, C–HꢀꢀꢀN and C–HꢀꢀꢀO, resulted in the increased rigidity
actions among molecules in G1 are shown in Fig. 6a. It was of the molecular skeleton to yield higher FF in crystals than those
found that the H-bond (2.44 Å) between N (in C–N triple bond) in solutions.
and the vinyl-H atom in two adjacent molecules was formed
along the short axis of the molecule. The C24–Hꢀ ꢀ ꢀp (P3) (2.87 Å)
and C4–Hꢀ ꢀ ꢀp (P3) (2.93 Å) provided two kinds of interactions for
Conclusions
each P3 ring. These C–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀp interactions resulted
in a rigid three-dimensional network. Thus, the twisting struc-
ture (torsion angle between P3 and P4) and the existence of
multiple C–Hꢀ ꢀ ꢀp H-bonds restricted the intramolecular rotation
and enabled the molecule to emit light intensively in the solid
states.34 In the crystal of G1-N (Fig. 6b and c), the rings of P3 and
P4 were almost coplanar (torsion angle 8.38(6)1) and four kinds
of H-bonds were formed. C18–Hꢀ ꢀ ꢀN2 (2.58 Å) and C24–Hꢀ ꢀ ꢀO1
(2.62 Å) linked the molecules in each layer together along the P3
In conclusion, cyanostyrene-based D–p–A conjugated com-
pounds bearing triphenylamine units (G1, G1-N and G2) have
been synthesized. By tuning the conjugated skeleton and the
electron withdrawing ability of the acceptor, green, orange and
red light emitters were achieved. It is interesting that although
their emissions in solutions were weak, we observed strong
emissions in solid states. For example, the FF of dendritic
molecule G2 in powder reached 0.67, which was more than
20 times of that in THF. The single crystal X-ray diffraction data
revealed that the intermolecular H-bonds of C–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀN
and C–Hꢀ ꢀ ꢀp led to the increased rigidity of the molecular
skeleton, which would restrict the intramolecular rotation,
yielding high FF in solid states. Therefore, the green, orange
and red emitting compounds with AIE properties were
obtained, which might have potential applications in organic
light emitting materials.
Experimental section
General
Solvents were purified and dried using standard protocols. All
other chemical reagents were obtained commercially and were
1
used as received without further purification. H NMR spectra
were recorded on a Mercury plus 400 MHz using CDCl3 as
solvent in all cases. 13C NMR spectra were recorded on a
Mercury plus 100 MHz using CDCl3 as solvent. FT-IR spectra
were recorded using a Germany Bruker Vertex 80v FT-IR spectro-
meter by incorporating samples in KBr disks. Mass spectra were
recorded on an Agilent 1100 MS series and AXIMA CFR MALDI/
TOF (Matrix assisted laser desorption ionization/Time-of-fight)
MS (COMPACT). UV-vis absorption spectra were recorded on
a Shimadzu UV-1601PC spectrophotometer. Fluorescence emission
spectra were recorded on a Shimadzu RF-5301 luminescence
spectrometer. The emission spectra of solids were recorded
using a Maya2000 Pro CCD spectrometer. The absolute fluores-
cence quantum yields were measured on an Edinburgh FLS920
steady state fluorimeter. The fluorescence microscopy images
were obtained using an Olympus BX51 fluorescence micro-
scope. Cyclic voltammetry was carried out using a CHI 604B
electrochemical working station at room temperature at a scan
Fig. 6 Intermolecular H-bonds (C–Hꢀ ꢀ ꢀO, C–Hꢀ ꢀ ꢀN and C–Hꢀ ꢀ ꢀp) in the
crystals of G1 (a) and G1-N (b, c).
rate of 50 mV sꢁ1
.
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Table 2 Crystal data for G1 and G1-N
Synthetic procedures and characterization
(Z)-3-(4-(Diphenylamino)phenyl)-2-phenylacrylonitrile (G1).
A mixture of formyl-ended triphenylamine derivative 1 (0.16 g,
0.6 mmol), phenylacetonitrile (0.11 g, 0.9 mmol), and sodium
hydroxide (0.04 g, 0.9 mmol) in ethanol (50 mL) was stirred for
24 h at room temperature under an atmosphere of nitrogen.
Then, the precipitate was filtered off and washed with cold
Compound
G1
G2
Empirical formula
Formula wt
Temperature, K
Crystal system
Space group
a, Å
b, Å
c, Å
a/1
b/1
C27H20N2
372.45
293(2) K
Monoclinic
P2(1)/n
17.164(3)
6.6995(13)
19.809(4)
90.00
111.08(3)
90.00
2125.4(7)
4
1.164
0.068
3.09–27.48
19 408
4844
0.0598
0.982
0.0535
0.1219
0.1192
0.1451
C27H19N3O2
417.45
293(2) K
Monoclinic
P21/c
11.154(2)
7.5463(15)
25.707(5)
90.00
96.42(3)
90.00
2150.3(7)
4
1.289
0.083
3.14–27.48
19 989
4896
0.0507
1.031
0.0542
0.1212
0.1153
0.1423
1
ethanol. Yield: 85%. M.p. 148.0–151.0 1C. H NMR (400 MHz,
CDCl3) d 7.774 (d, J = 8 Hz, 2H), 7.642 (d, J = 8 Hz, 2H), 7.402–
7.440 (m, 3H), 7.294–7.365 (m, 5H), 7.100–7.168 (m, 6H), 7.048
(d, J = 8 Hz, 2H) (Fig. S1, ESI†). 13C NMR (100 MHz, CDCl3) d
149.98, 146.66, 141.73, 135.08, 130.69, 129.61, 129.02, 128.60,
126.50, 125.73, 124.41, 120.96, 118.79, 107.80 (Fig. S2, ESI†).
FT-IR (KBr, cmꢁ1): 2216, 1583, 1506, 1489, 1448, 1329, 1298,
1275, 1192, 1178, 831, 760, 696. MS, m/z: cal. 372.16, found:
372.9 (Fig. S3, ESI†).
(Z)-3-(4-(Diphenylamino)phenyl)-2-(4-nitrophenyl)acrylonitrile
(G1-N). A mixture of 4-(diphenylamino)benzaldehyde (0.16 g,
0.6 mmol), 2-(4-nitrophenyl)acetonitrile (0.15 g, 0.9 mmol), and
tetrabutylammonium hydroxide (TBAH) (0.13 g, 0.9 mmol) in
ethanol (50 mL) was refluxed for 12 h under an atmosphere of
nitrogen. After cooling down to room temperature, the precipitate
was filtered off and washed with cold ethanol to give a red solid.
Yield: 90%. M.p. 160.0–162.0 1C. 1H NMR (400 MHz, CDCl3) d 8.272
(d, J = 8 Hz, 2H), 7.784–7.832 (m, 4H), 7.548 (s, 1H), 7.324–7.361
(m, 4H), 7.147–7.190 (m, 6H), 7.038 (d, J = 8 Hz, 2H) (Fig. S4, ESI†).
13C NMR (100 MHz, CDCl3) d 151.08, 147.24, 146.14, 144.76, 141.45,
131.50, 129.74, 126.15, 126.10, 125.13, 125.04, 124.31, 120.00,
118.11, 104.60 (Fig. S5, ESI†). FT-IR (KBr, cmꢁ1): 2218, 1574,
1514, 1485, 1340, 1284, 1198, 1180, 849, 752. MS, m/z: cal. 417.15,
found: 417.2 (Fig. S6, ESI†).
g/1
Volume, Å3
Z
Density, Mg mꢁ3
M (Mo Ka), mmꢁ1
Y range, deg
No. of reflcns collected
No. of unique reflcns
R(int)
GOF
R1 [I > 2s(I)]
wR2 [I > 2s(I)]
R1 (all data)
wR2 (all data)
Preparation of aggregates for AIE measurements
The solution of G2 in THF was firstly prepared (1.0 ꢂ 10ꢁ5 M).
Then, 1 mL of the above solution was transferred into a
volumetric flask (10 mL). After adding an appropriate amount
of THF, water was added dropwise under vigorous stirring to
furnish a 10 mL solution in THF–water with a specific water
fraction. The water content was varied in the range of 0–90 vol%.
The UV-vis absorption and fluorescence emission spectra of the
resulting samples were recorded immediately after preparation.
(Z)-3-(4-(Bis(4-(4-(diphenylamino)styryl)phenyl)amino)phenyl)-
2-phenylacrylonitrile (G2). G2 was prepared from formyl-ended
triphenylamine derivative 2 and phenylacetonitrile by using a
similar procedure to G1. The crude product was purified by
column chromatography (silica gel) using dichloromethane as
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (51073068, 21374041), the 973 Program
(2009CB939701), and the Open Project of the State Key Laboratory
of Supramolecular Structure and Materials (SKLSSM201203).
1
an eluent. Yield: 85%. M.p. 112.0–116.0 1C. H NMR (400 MHz,
CDCl3) d 7.835 (d, J = 8 Hz, 2H), 7.686 (m, J = 8 Hz, 2H), 7.370–
7.476 (m, 13H), 7.275–7.312 (m, 5H), 7.137–7.176 (m, 14H), 6.976–
7.092 (m, 14H) (Fig. S7, ESI†). 13C NMR (100 MHz, CDCl3) d
149.43, 147.57, 147.32, 145.51, 141.58, 135.03, 133.79, 131.59,
130.72, 129.31, 129.02, 128.66, 127.65, 127.45, 127.31, 127.04, Notes and references
126.23, 125.76, 125.47, 124.52, 123.62, 123.06, 121.71, 118.72,
108.12, 77.37, 77.06, 76.74 (Fig. S8, ESI†). FT-IR (KBr, cmꢁ1): 2204,
1589, 1508, 1490, 1325, 1281, 1178, 960, 829, 754. MS, m/z: cal.
910.4, found: 912.4 (Fig. S9, ESI†).
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Diffraction data of single crystals were collected on a Rigaku
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