Synthesis of a new class of highly fluorescent aryl-vinyl benzo[1,2-b:4,5-b′]difuran derivatives

Article abstract of DOI:10.1016/j.tetlet.2012.05.087

The synthesis of 2-(2-arylvinyl)- and 2,6-di(2-arylvinyl)dibenzo[1,2-b:4,5- b′]furan derivatives for application in optoelectronics is described. Wittig reaction of the triphenylphosphonium bromides derived from diethyl 2,6-dimethylbenzofuro[5,6-b]furan-3

Full text of DOI:10.1016/j.tetlet.2012.05.087

Tetrahedron Letters 53 (2012) 3923–3926
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Tetrahedron Letters
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Synthesis of a new class of highly fluorescent aryl-vinyl
benzo[1,2-b:4,5-b0]difuran derivatives
Mariusz J. Bosiak a, , Judyta A. Jakubowska , Krzysztof B. Aleksandrzak , Szymon Kaminski ,
⇑
a
a
a
´
Anna Kaczmarek-Ke˛dziera a, Marta Ziegler-Borowska a, Dariusz Ke˛dziera a, Jörg Adams b
a Department of Chemistry, Nicolaus Copernicus University, 7 Gagarin Street, 87-100 Torun´, Poland
b Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, 38678 Clausthal-Zellerfeld, Germany
a r t i c l e i n f o
a b s t r a c t
The synthesis of 2-(2-arylvinyl)- and 2,6-di(2-arylvinyl)dibenzo[1,2-b:4,5-b0]furan derivatives for appli-
cation in optoelectronics is described. Wittig reaction of the triphenylphosphonium bromides derived
from diethyl 2,6-dimethylbenzofuro[5,6-b]furan-3,7-dicarboxylate with aryl aldehydes gave the prod-
ucts in 70–99% yield. The corresponding products derived from furfural and cinnamic aldehyde were also
obtained. The prepared products reveal UV–Vis fluorescence with quantum yields, varying from 1% to
100%, and may be used as organic small molecule materials for organic light-emitting devices (OLED).
Ó 2012 Elsevier Ltd. All rights reserved.
Article history:
Received 3 April 2012
Revised 10 May 2012
Accepted 17 May 2012
Available online 26 May 2012
Keywords:
Benzodifuran
Fluorescence
OLED
Wittig reaction
Conjugated
p
-electronic systems are important building blocks
Ylides were generated from 4 and 5 for the Wittig reaction with
aromatic aldehydes. The best yields of products were obtained
using lithium hydroxide monohydrate in isopropanol as the sol-
vent, according to the method of Antonioletti14 (Tables 1 and 2).
Reactions, monitored by TLC, were continued until the starting
aldehyde was no longer apparent (0.5–2.5 h).
Pure E isomers of the products were obtained by heating E/Z
mixtures with a small amount of iodine in p-xylene for 2 h. Accord-
ing to this procedure, the products were obtained in good yields as
amorphous yellow to orange solids. The E/Z ratio assignment is
based on the 1H NMR coupling constants between hydrogen atoms
at the double bond.
for dye-sensitized solar cells (DSSC)1 and organic light-emitting
devices (OLED).2 Light-emitting polymers and organic small mole-
cule materials are used for the production of OLEDs.3 Among the
latter, the most efficient fluorescent emitters yet reported belong
to the distyrylarylenes4 and oligo-phenyl vinyl compounds.5 De-
spite numerous literature examples of the aforementioned conju-
gated
p-electronic systems, little attention has been paid to
heterocyclic compounds containing oxygen atoms. Benzodifuran
derivatives, due to their fluorescent properties, represent an
important group of compounds for OLED manufacturing. Recently,
the first examples of benzodifuran small molecule materials,6–10
and polymers,11,12 were reported. Herein, the synthesis and fluo-
rescence characterization of a novel group of substituted aryl-vi-
nyl, heteroaryl-vinyl, and 4-phenyl-1,3-butadienyl benzodifurans
designed for optoelectronic applications are described.
Furyl derivatives 13 and 23 decomposed under reflux in the I2/
p-xylene system, and crude E/Z mixtures were taken to fluores-
cence recording. All the products, except 13 and 23, were stable
in air and on exposure to direct sunlight.
Diethyl 2,6-dimethylbenzofuro[5,6-b]furan-3,7-dicarboxylate
(1) was easily prepared, in 25% yield, from p-benzoquinone and
ethyl acetoacetate using the Griniev method.13 Bromination of 1
with NBS (0.95 or 2.2 equiv) in the presence of a catalytic amount
of AIBN, gave monobromo and 2,6-(dibromomethyl) derivatives 2
and 3 in 63% and 67% yields, respectively. The reactions of 2 and
3 with triphenylphosphine in boiling toluene produced the corre-
sponding salts 4 and 5 (Scheme 1) in quantitative yields.
Among products 6–15, compound 11, containing a p-chloro-
phenyl group (Table 3), exhibited the highest fluorescence inten-
sity in dichloromethane, taken as a combination of the molar
extinction coefficient and fluorescence quantum yield. The
fluorescence for compounds 6 (phenyl), 7 (p-nitrophenyl), 10
(p-fluorophenyl), and 12 (p-bromophenyl) was lower, whereas
compounds 8 and 9, containing electron-donating groups, showed
poor fluorescence in dichloromethane. In the solid state, 6 and 11
proved to be the most fluorescent, and unexpectedly for
9
(p-methoxyphenyl), high fluorescence was also observed.
In the case of 2,6-disubstituted benzodifurans 16–25, similar
trends were observed in dichloromethane solutions. Compounds
⇑
Corresponding author. Tel.: +48 512038649; fax: +48 56 6542477.
E-mail address: bosiu@chem.umk.pl (M.J. Bosiak).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.087

3924
M. J. Bosiak et al. / Tetrahedron Letters 53 (2012) 3923–3926
NBS (2.2equiv.)
NBS(0.95equiv.)
AIBN, CCl4,
EtOOC
EtOOC
EtOOC
Me
AIBN, CCl4,
reflux, 20 h
O
Br
reflux, 20 h
O
O
Me
Me
Br
O
O
O
Br
COOEt
COOEt
COOEt
1
2
63%
3
67%
PPh3
PPh3
toluene
toluene
reflux, 2 h
reflux, 16 h
EtOOC
EtOOC
PPh3 Br
O
O
Me
O
Ph3P
O
Br Ph3 P
Br
COOEt
COOEt
4
99%
5
99%
1.LiOH•H2O,
i
-PrOH, rt, 10 min
2. Aldehyde, reflux, 0.5-2.5 h
3. I2, p-xylene, 2 h
EtOOC
EtOOC
Ar
O
O
Me
O
Ar
O
Ar
———
COOEt
COOEt
6-15
16-25
Scheme 1. Synthesis of aryl-vinyl benzodifuran derivatives.
Table 1
Table 2
Synthesis of monosubstituted aryl-vinyl benzodifurans by reaction of 4 with various
Synthesis of disubstituted aryl-vinyl benzodifurans by reaction of 5 with various
aldehydes
aldehydes
Aldehyde
Product
Yield (%)
Aldehyde
Product
Yield (%)
Benzaldehyde
6
7
8
9
10
11
12
13
14
15
99
95
73
73
82
70
77
94
94
79
Benzaldehyde
16
17
18
19
20
21
22
23
24
25
89
99
76
89
77
88
96
51
82
77
p-O2NC6H4CHO
p-CH3C6H4CHO
p-CH3OC6H4CHO
p-FC6H4CHO
p-ClC6H4CHO
p-BrC6H4CHO
Furfural
9-Anthracenecarboxaldehyde
C6H5CH@CHCHO
p-O2NC6H4CHO
p-CH3C6H4CHO
p-CH3OC6H4CHO
p-FC6H4CHO
p-ClC6H4CHO
p-BrC6H4CHO
Furfural
9-Anthracenecarboxaldehyde
C6H5CH@CHCHO
Table 3
Physical properties of benzodifurans 6–25
Compound
Dichloromethane solution
Solid state
(Mꢀ1 cmꢀ1
)
kem (nm)
hF
sF (ns)
HOMOc (eV)
LUMOc (eV)
Eg (eV)
kem (nm)
Fluorescence intensity
a
b
kabs (nm)
e
6
7
8
9
368
411
367
371
384
371
370
384
450
407
432
422
415
425
431
418
417
428
443
456
42480
32360
27940
29240
8080
>120000
32840
4620
56220
33480
37100
42120
40020
42600
37080
39840
37640
40060
27220
44200
451
553
456
471
486
455
458
496
558
484
478
560
483
498
480
484
484
500
557
517
0.34
1.00
0.07
0.01
0.27
0.45
0.58
0.05
0.04
0.06
1.00
0.76
0.66
0.18
0.98
1.00
1.00
0.25
0.01
0.06
1.0
2.6
0.2
0.1
1.5
1.2
1.3
1.0
0.7
0.2
1.4
1.8
1.2
0.1
1.5
1.4
1.5
1.0
1.6
0.2
ꢀ5.743
ꢀ5.944
ꢀ5.674
ꢀ5.543
ꢀ5.741
ꢀ5.783
ꢀ5.780
ꢀ5.591
ꢀ5.451
ꢀ5.576
ꢀ5.584
ꢀ5.888
ꢀ5.497
ꢀ5.347
ꢀ5.585
ꢀ5.647
ꢀ5.645
ꢀ5.406
ꢀ5.357
ꢀ5.393
ꢀ2.412
ꢀ3.134
ꢀ2.360
ꢀ2.284
ꢀ2.414
ꢀ2.501
ꢀ2.506
ꢀ2.394
ꢀ2.553
ꢀ2.554
ꢀ2.603
ꢀ3.293
ꢀ2.542
ꢀ2.453
ꢀ2.610
ꢀ2.706
ꢀ2.712
ꢀ2.583
ꢀ2.707
ꢀ2.732
3.331
2.811
3.315
3.259
3.327
3.282
3.274
3.198
2.898
3.021
2.981
2.595
2.955
2.894
2.975
2.940
2.933
2.823
2.651
2.661
494
590
520
519
550
509
524
—
570
530
566
601
567
570
560
560
568
602
565
560
9217
683
4201
7793
1367
8440
2313
—
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
282
1641
1855
386
9596
549
2408
428
924
153
26
653
a
Quantum yield measured against 9,10-diphenylanthracene in cyclohexane at kex = 366 nm. Absorbance of the dye solution between 0.066 and 0.145 at kabs = 366 nm.
Fluorescence lifetime determined by time-correlated single-photon-counting (TCSPC) with a PicoQuant (Germany) pulsed LED at kex = 376 nm as the light source. The
b
same solution as that described above was used.
c
Calculated using the B3LYP/6-311G(d,p) approach including the solvent (dichloromethane) effects via the PCM model.

M. J. Bosiak et al. / Tetrahedron Letters 53 (2012) 3923–3926
3925
6
5
4
3
2
1
0
16 (phenyl), 17 (p-nitrophenyl), 18 (p-methylphenyl), 20 (p-fluoro-
phenyl), 21 (p-chlorophenyl), and 22 (p-bromophenyl) exhibited
the highest fluorescence intensities. Both 2-substituted and 2,6-
disubstituted benzodifurans containing furyl (13 and 23), 9-an-
thryl (14 and 24), and cinnamic (15 and 25) moieties revealed very
poor fluorescence.
The weak fluorescence observed for compounds 13 and 23 may
be due to the fact that crude E/Z mixtures of isomers were used for
the measurements, instead of the pure E isomer, as was the case for
other compounds. This was due to the instability of these two com-
pounds and may also explain the phenomenon of the long fluores-
cence lifetime observed for 13 despite the low fluorescence
quantum yield. Unexpectedly, a similar long fluorescence lifetime
was also observed for weakly fluorescent 24, containing two
anthracene moieties. The most intense emission spectra for the
2-substituted and 2,6-disubstituted benzodifurans are shown in
Figures. 1 and 2.
Generally, 2,6-disubstituted benzodifurans revealed higher
fluorescence, only for p-nitrophenyl, 9-anthryl, and p-cinnamic
derivatives was the trend opposite. Also, the absorption and emis-
sion wavelength maxima for all disubstituted benzodifurans, ex-
cept the 9-anthryl derivative, were shifted toward longer values.
Although the emission spectra of 2,6-disubstituted benzodifurans
(Fig. 2), revealed almost identical profiles and differed only in the
intensity, the spectra of 2-substituted benzodifurans revealed dif-
ferent profiles and wavelength maxima (Fig. 1).
An insight into the electronic structure of the investigated mol-
ecules was gained from quantum mechanical calculations within
the B3LYP/6-311G(d,p) approach including solvent (dichlorometh-
ane) effects via the PCM model. The calculations were performed
with the Gaussian0915 program package. Fig. 3 presents frontier
orbitals for compounds 6 and 16 as representatives of 2-disubsti-
tuted and 2,6-substituted benzodifurans, respectively. In the case
of the 2-substituted system, a significant charge transfer from
the central ring of the benzodifuran moiety to the substituent is
expected upon excitation. On the other hand, 2,6-disubstituted
benzodifurans possess zero dipole moment due to symmetry in
the ground state.
6, 7, 8, 10, 11, 12, 13
350
450
550
650
750
850
wavelength λ (nm)
Figure 1. Emission spectra of selected 2-substituted benzodifurans in dichloro-
methane. Excitation wavelength, kex = 366 nm. Concentration set to an absorbance
value between 0.09 and 0.11 to avoid the inner filter effect. The spectra are
corrected for the different absorbances to allow a quantitative comparison.
12
16, 18, 19, 20, 21, 22, 23
10
8
6
4
2
0
350
450
550
650
750
850
Figure 2. Emission spectra of selected 2,6-disubstituted benzodifurans in dichlo-
romethane. Excitation wavelength, kex = 366 nm. Concentration set to an absor-
bance value between 0.09 and 0.11 to avoid the inner filter effect. The spectra are
corrected for the different absorbances to allow a quantitative comparison.
Figure 3. Frontier orbitals for (a) 6 and (b) 16 calculated within the B3LYP/6-311G(d,p) approach including the solvent (dichloromethane) effects via the PCM model.

3926
M. J. Bosiak et al. / Tetrahedron Letters 53 (2012) 3923–3926
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Acknowledgements
Financial support from the Nicolaus Copernicus University, Tor-
un´ , Grant Rektorski 370-Ch, is acknowledged. The authors grate-
fully acknowledge Wroclaw Networking and Supercomputing
Center (WCSS) and Academic Computer Center Cyfronet AGH for
a generous allotment of computer time.
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Supplementary data
Supplementary data (complete experimental procedures, spec-
tral data characterization for compounds and HOMO/LUMO orbital
visualization) associated with this article can be found, in the on-
line version, at http://dx.doi.org/10.1016/j.tetlet.2012.05.087.
12. Yi, C.; Blum, C.; Lehmann, M.; Keller, S.; Liu, S.-X.; Frei, G.; Neels, A.; Hauser, J.;
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