Synthesis, crystal structure and fluorescence behavior of 2,6-Di(thiophen-2-yl)-benzo[1,2-d:4,5-d']bisoxazole

Article abstract of DOI:10.5560/ZNB.2013-2301

An effective and clean new aerobic approach for the synthesis of 2,6-disubstituted benzobisoxazole by using a one-pot reaction of an organic aminoxyl radical as the catalyst is reported. 2,6- Di(thiophen-2-yl)-benzo[1,2-d : 4,5-d]bisoxazole was synthesized with catalysis by the free radical 4-methoxy-TEMPO and characterized by ¹H and ¹³C NMR spectroscopy, HRMS, as well as by elemental analysis, UV/Vis and emission spectroscopy. The crystal structure of the title compound has been determined by single-crystal X-ray diffraction. It crystallizes in the monoclinic space groupC2/c with a=12.531(1), b=3.8960(2), c = 28.733(2) A, β = 100:760(1), Z = 4. Through intermolecular weak C-H O hydrogen bonding and p-p stacking interactions, a supramolecular 3D structure is formed.

Full text of DOI:10.5560/ZNB.2013-2301

Synthesis, Crystal Structure and Fluorescence Behavior of 2,6-
Di(thiophen-2-yl)-benzo[1,2-d:4,5-d0]bisoxazole
Lan-Qin Chaia, Yu-Li Zhanga, Jun-Feng Tongb, and Gang Liua
a
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou,
Gansu 730070, P. R. China
Key Laboratory of Opto-Electronic Technology and Intelligent Control, Ministry of Education,
b
Lanzhou Jiaotong University, Gansu Province, Lanzhou, 730070, P. R. China
Reprint requests to Prof. Lan-Qin Chai. E-mail: chailq@mail.lzjtu.cn
Z. Naturforsch. 2013, 68b, 239 – 244 / DOI: 10.5560/ZNB.2013-2301
Received November 5, 2012
An effective and clean new aerobic approach for the synthesis of 2,6-disubstituted benzobisox-
azole by using a one-pot reaction of an organic aminoxyl radical as the catalyst is reported. 2,6-
Di(thiophen-2-yl)-benzo[1,2-d : 4,5-d0]bisoxazole was synthesized with catalysis by the free radical
4-methoxy-TEMPO and characterized by 1H and 13C NMR spectroscopy, HRMS, as well as by ele-
mental analysis, UV/Vis and emission spectroscopy. The crystal structure of the title compound has
been determined by single-crystal X-ray diffraction. It crystallizes in th◦e monoclinic space groupC2/c
˚
with a = 12.531(1), b = 3.8960(2), c = 28.733(2) A, β = 100.760(1) , Z = 4. Through intermolec-
ular weak C–H···O hydrogen bonding and π-π stacking interactions, a supramolecular 3D structure
is formed.
Key words: 4-Methoxy-TEMPO Free Radical, 2,6-Dithiophene Disubstituted Benzobisoxazole,
Synthesis, Crystal Structure, Fluorescence Behavior
Introduction
with excellent mechanical strength and thermal stabil-
ity [8 – 10].
Aerobic oxygenation/oxidation of hydrocarbons, al-
It has been reported that benzoxazole deriva-
cohols and amines catalyzed by aminoxyl radicals have tives were widely used in thin membrane field ef-
been extensively studied [1, 2]. However, its applica- fect transistors [11, 12], electrophosphorescent ma-
tion in the catalytic oxidative synthesis of heterocycles terials [13], light cells, molecular electrodes [14],
is rare. Recently, we reported an expeditious, one-pot, high-performance fibers and in other fields [15 – 17].
efficient method for the preparation of a series of asym- Compared with benzoxazoles, 2,6-disubstituted benzo-
metric semicarbazides with heteroarene core [3 – 5].
bisoxazoles have larger conjugated systems. Herein, an
The advantages of using organic materials to fab- effective and more eco-friendly new aerobic approach
ricate electroluminescent (EL) devices are their high for the synthesis of 2,6-di(thiophen-2-yl)-benzo[1,2-
brightness, high efficiency and potential color tun- d:4,5-d0]bisoxazole (3) by using a one-pot reaction
ing as well as their low cost of fabrication, which of an organic aminoxyl radical as the catalyst and 2-
shows great commercial potential [6]. 2-(Biphenyl-4- thiophenecarboxaldehyde is reported.
yl)-5-(tert-butylphenyl)-1,3,4-oxadiazole (PBD), was
the first example and one of the most efficient elec- Results and Discussion
tron transport materials in an organic EL device [7].
We are interested in developing benzobisoxazoles for Synthesis of compound 3
their use as building blocks for novel organic semicon-
ductors because conjugated small molecules and poly-
As shown in Scheme 1, the new compound 2,6-
mers based on benzobisoxazoles are well suited for use di(thiophen-2-yl)-benzo[1,2-d:4,5-d0]bisoxazole (3)
in organic semiconducting applications. These mate- was synthesized with 4-methoxy-2,2,6,6-tetramethyl-
rials combine efficient electron transport, photolumi- 1-piperidinyloxy (4-methoxy-TEMPO) as
a free
nescence, and third-order nonlinear optical properties radical catalyst starting from 1 and 2. The 4-methoxy-
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240
L.-Q. Chai et al. · 2,6-Di(thiophen-2-yl)-benzo[1,2-d : 4,5-d0]bisoxazole
Scheme 1. The synthetic route to 2,6-di(thiophen-2-yl)-benzo[1,2-d : 4,5-d0]bisoxazole (3).
Table 1. Crystal data and structure refinement for com-
pound 3.
Molecular formula
Molecular weight, g mol−1
Crystal color, habit
Crystal size, mm3
Crystal system
C16H8N2O2S2
324.36
colorless, needle-like
0.49 × 0.12 × 0.09
monoclinic
C2/c
Fig. 1. Molecular structure in the crystal with atom number-
Space group
ing of compound 3.
˚
a, A
12.5309(1)
3.8960(2)
˚
b, A
˚
c, A
28.733(2)
TEMPO radical is widely used for the selective β, deg
100.7600(1)
1378.10(2)
4
298(2)
1.56
3
˚
V, A
oxidation of primary or secondary alcohols [18 – 20].
4-Methoxy-TEMPOH can be slowly oxidized to the
4-methoxy-TEMPO radical in air at room tempera-
ture, so a strategy for the aerobic catalytic oxidative
synthesis of benzoxazole using 4-methoxy-TEMPO as
the catalyst was developed [21]. Moreover, the crystal
structure of the new compound was determined by
single-crystal X-ray diffraction.
Z
T, K
Dcalcd., mg m−3
µ(MoKα ), cm−1
F(000), e
3.9
664
hkl range
−14 : 10, ±4, −33 : 34
2.889–25.919
3183 / 1212 / 0.0477
1212 / 0 / 100
0.0533 / 0.1414
0.0697 / 0.1527
1.032
θ range for data collection, deg
Refl. collected / unique / Rint
Data / restraints / parameters
Final R1/wR2 [I > 2σ(I)]a
Final R1/wR2 (all data)b
Goodness-of-fit (F2)c
As shown in Fig. 1, the X-ray crystallographic anal-
ysis (Table 1) of the title compound 3 revealed that
the dihedral angles defined by the central benzo[1,2-
d:4,5-d0]bisoxazole ring plane and the side thiophene
planes is only 2.12(2)◦ , the planes defined by two side
thiophene rings are parallel, and their distance is about
−3
˚
Largest diff. peak / hole, e A
0.263 / −0.406
a
R1 = Σ||Fo|−|F ||/Σ|Fo|; b wR2 = [Σw(F2 −F2)2/Σw(F2)2]1/2
,
c
w = [σ2(Fo2) + (AP)2 + BP]−1, where P =o(Max(Fo2,0) + 2F2)/3;
c
o
c
c
GoF = [Σw(Fo2 −F2)2/(nobs −nparam)]1/2
.
c
˚
0.242(1) A, showing that the title molecule has a good
coplanar property owing to the absence of steric hin-
drance of alkyl chains [22]. Selected bond lengths and ing interactions between neighboring thiophene rings,
bond angles as well as hydrogen bonds parameters are oxazole rings and benzene rings with distances be-
˚ ˚
tween the centroids of 3.896(2) A and 3.829(2) A (Ta-
listed in Tables 2 and 3, respectively [23].
As shown in Fig. 2, in the crystal structure of the ti- ble 4 [24], Fig. 3).
tle compound, the neighboring units are linked into an
infinite chain by intermolecular hydrogen bonding C4– UV/Vis and photoluminescence spectra
H4···O1 (Table 3) forming six-membered rings with
a graph motif R22(6). Furthermore, in Fig. 4 the pack-
The UV/Vis absorption and photoluminescence
ing diagram shows that the title compound 3 forms spectra of the title compound 3 were determined in
a herringbone arrangement with abundant π −π stack- 5 × 10−5 mol L−1 CH2Cl2 solution at room tempera-
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L.-Q. Chai et al. · 2,6-Di(thiophen-2-yl)-benzo[1,2-d : 4,5-d0]bisoxazole
241
Fig. 2. Part of the infinite
chain motif of compound 3
(hydrogen atoms, except those
forming hydrogen bonds, are
omitted for clarity).
Table 2. Selected bond lengths
(A) and bond angles (deg) for
compound 3.
Bond
S(1)–C(8)
O(1)–C(2)
C(1)–C(5)
C(3)–C(4)
C(6)–C(7)
Bond
Lengths
Bond
S(1)–C(5)
N(1)–C(1)
C(2)–C(4)
C(4)–C(2)
C(7)–C(8)
Bond
Lengths
Bond
Lengths
˚
1.684(4)
1.380(3)
1.439(4)
1.385(4)
1.416(4)
Angles
1.722(3)
1.304(4)
1.383(4)
1.383(4)
1.367(5)
Angles
O(1)–C(1)
N(1)–C(3)
C(2)–C(3)
C(5)–C(6)
1.367(3)
1.404(4)
1.409(4)
1.386(4)
Bond
Angles
C(8)–S(1)–C(5)
N(1)–C(1)–O(1)
O(1)–C(2)–C(4)
C(4)–C(3)–N(1)
C(2)–C(4)–C(3)
C(1)–C(5)–S(1)
C(7)–C(8)–S(1)
91.83(16)
114.9(3)
128.5(2)
129.7(3)
113.4(2)
120.0(2)
112.6(3)
C(1)–O(1)–C(2)
N(1)–C(1)–C(5)
O(1)–C(2)–C(3)
C(4)–C(3)–C(2)
C(6)–C(5)–C(1)
C(5)–C(6)–C(7)
104.7(2)
126.0(3)
107.5(2)
122.6(3)
128.4(3)
111.1(3)
C(1)–N(1)–C(3)
O(1)–C(1)–C(5)
C(4)–C(2)–C(3)
N(1)–C(3)–C(2)
C(6)–C(5)–S(1)
C(8)–C(7)–C(6)
105.2(2)
119.1(3)
124.0(3)
107.7(2)
111.6(2)
112.9(3)
Table 3. Weak hydrogen bond parameters for compound 3a.
D–H···A
d(D–H)
d(H···A)
d(D···A)
∠DHA
160.44
C4–H4···O1i
0.930
2.565
3.455
a
Symmetry code: (i) −1/2+x, −1/2+y, z.
Conclusion
We have reported the synthesis and structural char-
acterization of 2,6-dithiophene disubstituted benzo-
bisoxazole. Future efforts will focus on the synthesis of
other useful heterocycles based on these materials, as
well as device construction and evaluation of the elec-
tronic and optical properties of oligomers and poly-
mers containing the benzobisoxazole moiety.
Fig. 3. Face-to-face π-π stacking diagram of compound 3.
ture and are shown in Figs. 5 and 6, respectively. The
absorption spectrum exhibits the maximum peak at
about 361 nm, a shoulder peak at 344 nm and another
peak at about 380 nm. Comparatively, the fluorescence
spectrum shows a maximum peak at 408 nm, a shoul-
der peak at 428 nm and another peak at 387 nm (Ta-
ble 5). The Stokes shift between the maximum wave-
length of the absorbance and fluorescence spectrum is
47 nm.
Experimental Section
Materials and physical measurements
2,5-Dimethoxycyclohexa-2,5-diene-1,4-dione (1) and
2,5-diaminobenzene-1,4-diol (2) were prepared ac-
cording to literature procedures [25]. Pyrocatechol and
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242
L.-Q. Chai et al. · 2,6-Di(thiophen-2-yl)-benzo[1,2-d : 4,5-d0]bisoxazole
Table 4. π-π Stacking interactions
˚
˚
˚
Ring 1
Ring 2
α
0
DCC (A)
CgI–perp (A) CgJ–perp (A)
Slippage
a
˚
(A, deg) for compound 3 .
Cg1
Cg2
Cg2
Cg3
Cg1
Cg2
Cg3
Cg3
3.896(2)
3.896(2)
3.829(2)
3.896(2)
−3.476(1)
−3.476(1)
3.4743(2)
−3.476(1)
−3.476(1)
−3.476(1)
−3.4689(2)
−3.476(1)
1.761
1.761
0
0
0.27(15)
0
1.761
a
DCC = distance between ring centroids; α = dihedral angle between planes I and J; CgI–
perp = perpendicular distance of Cg(I) from ring J; CgJ–perp = perpendicular distance of Cg(J)
from ring I; slippage = distance between Cg(I) and perpendicular projection of Cg(J) on ring I;
Cg1, Cg2 and Cg3 are the centroids of the thiophene ring S1-C5-C6-C7-C8, the oxazole ring
O1-C1-N1-C3-C2 and the benzene ring C2-C3-C4-C2(a)-C3(a)-C4(a), respectively.
Fig. 4. Packing diagram of
compound 3 viewed along the
a and b axis, respectively.
2-thiophenecarboxaldehyde were purchased from Alfa were expressed in parts per million relative to the singlet
Aesar. All chemicals were of analytical reagent grade and (δ = 7.26) for CDCl3. Chemical shifts of 13C NMR were
were used without further purification.
expressed in parts per million relative to the central line of
1H and 13C NMR spectra were measured on Mercury the triplet (δ = 77.0 ppm) for CDCl3 and TMS as internal
Plus (400 MHz) spectrometers. Chemical shifts of 1H NMR standard. HRMS spectra were obtained with a Bruker APEX
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L.-Q. Chai et al. · 2,6-Di(thiophen-2-yl)-benzo[1,2-d : 4,5-d0]bisoxazole
243
Table 5. The absorption and fluorescence data for com-
pound 3.
Absorption
ε (L mol−1 m−1
Fluorescene
Relative intensity
λmax (nm)
)
λ (nm)
344
361
380
46 494
58 180
43 508
387
408
428
433.0
546.3
360.0
Fig. 6. Fluorescence spectrum of compound 3 in CH2Cl2 so-
lution (5.0 × 10−5 mol L−1, λex = 360 nm).
13C NMR spectroscopic analysis. – 1H NMR (400 MHz,
CDCl3, 25 ◦C, TMS): δ = 7.97 (q, 1J = 4.8 Hz, 2J = 3.2 Hz,
2H), 7.86 (s, 2H), 7.61 (q, 1J = 6.4 Hz, 2J = 1.6 Hz, 2H),
7.23 (q, 1J = 10.8 Hz, 2J = 3.6 Hz, 2H) ppm – 13C NMR
(400 MHz, CDCl3): δ = 160.22, 148.32, 140.28, 130.61,
130.17, 129.47, 128.40, 100.66 ppm. – MS (EI): m/z = 324
– Analysis for C16H8N2O2S2 (324.36): calcd. C 59.24, H
2.49, N 8.64; found C 59.36, H 2.63, N 8.76.
Fig. 5. UV/Vis absorption spectrum of compound 3 in
CH2Cl2 solution (5.0 × 10−5 mol L−1).
II FT-MS instrument using CD2Cl2 as solvent. UV/Vis spec-
tra were recorded on a Shimadzu UV-2550 spectrometer. The
fluorescence spectra were taken on a 970 CRT spectrofluo-
rometer (Spectro, Shanghai/China) with 370 nm excitation.
The single-crystal X-ray structure determination was carried
out on a Bruker Smart 1000 CCD area detector diffractome-
ter. Elemental analysis was performed on an Elementar Vario
EL Elemental Analysis instrument.
Crystal structure determination
The title compound was recrystallized from trichloro-
methane-acetonitrile. After several days, many colorless
needle-like single crystals suitable for X-ray diffraction were
obtained. A crystal of 3 was mounted on a Bruker Smart
1000 CCD area detector diffractometer. Intensity data were
collected using graphite-monochromatized MoKα radiation
˚
(λ = 0.71073 A) at 298(2) K. The structure was solved us-
ing the program SHELXS-97 [26, 27] and difference Fourier
techniques, and refined by full-matrix least-squares methods
on F2 using the program SHELXL-97 [28, 29]. All hydrogen
atoms were added in calculated positions. The crystal data
and parameters pertinent to data collection and structure re-
finement are given in Table 1.
CCDC 879998 contains the supplementary crystallo-
graphic 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.
Preparation of compound 3
2,5-Diaminobenzene-1,4-diol (2) (280.0 mg, 2.0 mmol)
and 2-thiophenecarboxaldehyde (0.5 mL, 4.5 mmol) were
reacted in o-xylene solution (25 mL) at 120 ◦C under ar-
gon atmosphere. The mixture was stirred for 5 h. After
cooling to room temperature, 4-methoxy-TEMPO (47.0 mg,
5.0 mmol) was added to the mixture, which was stirred at
120 ◦C for 15 h under oxygen atmosphere. After remov-
ing the solvent under reduced pressure, the obtained crude
product was extracted by chloroform (3 × 20 mL), washed
with distilled water (3 × 20 mL) and dried over anhydrous
Na2SO4. The residue was purified by chromatography us-
Acknowledgement
We are thankful for the financial support by the Natural
ing petroleum ether-ethyl acetate (3 : 2) to give a color- Science Foundation of Gansu Province (no. 1107RJZA165),
less crystalline solid (3) (535.9 mg). Yield: 87%. The iden- and the Innovation and Technology Fund of Lanzhou Jiao-
tidy and purity of the product was confirmed by 1H and tong University (no. DXS-2013035).
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L.-Q. Chai et al. · 2,6-Di(thiophen-2-yl)-benzo[1,2-d : 4,5-d0]bisoxazole
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