Y. Jiang et al. / Tetrahedron Letters 56 (2015) 2324–2328
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1.0
0.5
General procedures
In the dark
1H NMR and 13C NMR spectra were performed on a Bruker
AVANCE III 300 (1H, 300 MHz and 13C, 75 MHz) or Bruker
AVANCE DRX 500 (1H, 500 MHz and 13C, 125 MHz). High resolution
mass spectra (HRMS) and matrix assisted laser desorption/ioniza-
tion time-of-flight mass spectrometry (MALDI-TOF) were
performed on Bruker Biflex III. Thermogravimetric analyses (TGA)
and differential Scanning Calorimetry (DSC) were carried out under
dry nitrogen gas flow with a 10 °C minꢀ1 heating rate on TGA Q500
V20.10 Build 36 and a DSC Q20 V24.4 Build 116, respectively.
4,8-Bis[5-(2-ethylhexyl)-2-thienyl]benzodithiophene (5): 2-ethyl-
hexyl-thiophene (1.21 g, 6.17 mmol) in 20 mL of dry THF was
cooled to 0 °C under argon atmosphere and n-butyllithium
(4.24 mL, 6.79 mmol) was added dropwise; the color of the mix-
ture turned from colorless to brown and orange. The mixture
was heated to 50 °C for 1 h, compound 6 (500 mg, 2.28 mmol)
was added in one portion and the mixture was stirred 2 h at room
temp. After the addition of SnCl2ꢃ2H2O + 10% HCl (8.46 g + 15 mL)
the mixture was left to hydrolyze overnight. The crude product
was washed with water, extracted with diethyl ether, and dried
over MgSO4. After solvent removal the crude compound was chro-
matographed on silica gel eluting with petroleum ether to give
(570 mg, 63%) of a yellow oil. 1H NMR (CDCl3, 300 MHz): d
(ppm) = 7.68, (d, J = 5.695Hz, 2H), 7.49, (d, J = 5.690 Hz, 2H), 7.33,
(d, J = 3.470 Hz, 2H), 6.92, (d, J = 3.444 Hz, 2H), 2.89, (d,
J = 6.758 Hz , 2H), 1.72, (m, 4H), 1.39, (m, 16H), 0.98 (m, 12H).
13C NMR (CDCl3, 75 MHz): d (ppm) = 145.7, 139.0, 137.2, 136.5,
127.7, 127.5, 125.4, 124.1, 123.4, 41.5, 34.3, 32.5, 28.9, 25.7, 23.1,
14.2, 10.9. MS MALDI-TOF calcd for C34H42S4 578, found 578.
4,8-Bis[5-(2-ethylhexyl)-2-thienyl]benzodithiophene-2,6-carbald-
ehyde (3): A solution of compound 5 (1.05 g, 1.82 mmol) in 30 mL of
dry THF was cooled to ꢀ78 °C under argon. n-butyllithium (2.2 mL,
5.5 mmol) was added dropwise and the color of the mixture turned
from yellow to fluorescent green. After 30 min stirring at room temp
the mixture was cooled to ꢀ78 °C, DMF (0.6 mL, 7.26 mmol) was
added dropwise, the solution was stirred for 30 min at ꢀ78 °C then
overnightat room temp. After quenching by ice water and extraction
with dichloromethane, the organic phase was dried over MgSO4.
Solvent removal and chromatography on silica gel (eluent 2:1 petro-
leum ether dichloromethane) gave 650 mg (57%) of an orange solid.
Mp 96 °C. 1H NMR (CDCl3, 300 MHz): d (ppm) = 10.14,(s, 2H), 8.40,
(s, 2H), 7.38, (d, J = 3.493 Hz, 2H), 6.99, (d, J = 3.487 Hz, 2H), 2.92,
(d, J = 6.649 Hz, 4H), 1.73, (m,2H), 1.40, (m, 16H), 0.99, (m, 12H).
13C NMR (CDCl3, 125 MHz): d (ppm) = 184.6, 147.3, 145.0, 141.1,
137.7, 134.7, 133.9, 128.5, 127.5, 125.7, 41.3, 34.0, 32.2, 28.7, 25.5,
22.8, 14.0, 10.7. HRMS calcd for C36H42O2S4 634.2068 found
634.2073.
Under illumination
0.0
-0.5
-1.0
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Voltage [V]
Figure 5. Current density versus voltage of a BHJ cell ITO/PEDOT:PSS/TCN+1/Al.
Dotted line: in the dark, solid line: under AM 1.5 simulated solar illumination at a
power light intensity of 90 mW cmꢀ2
.
efficiency (EQE) of the cell under monochromatic light shows a
maximum response of ꢂ8.0% around 420 nm followed by a net
shoulder around 520 nm (Fig. 4). The clear correlation of these
two maxima with the absorption maxima of the acceptor and
donor materials shows that despite the modest photo-conversion
efficiency, the acceptor compound 1 provides a substantial con-
tribution to the overall photo-current.
The current-density versus voltage curves obtained under AM
1.5 simulated solar illumination are shown in Figure 5. The devices
present a short-circuit current density (Jsc) of 0.84 mA cmꢀ2 and an
open-circuit voltage (Voc) of 1.02 V which combined with a low fill
factor (FF) of 0.23 gave a power conversion efficiency (PCE) of
0.24%. Although modest this value is comparable to the results
recently reported for BHJ cells based on other small size molecular
acceptors tested with poly(3-hexylthiophene),21 however it must
be underlined that in our case both the donor and acceptor materi-
als are small molecules. On the other hand, the high Voc, consistent
with the deep LUMO level of compound 1 ranks among the highest
reported so far for molecular BHJ cells.7–12
This result confirms that electron-acceptor materials with
appropriate energy levels can be designed using the BDT platform.
However, the low Jsc and FF values suggest insufficient charge-
mobility of the active materials and/or problems of charge-transfer
at the material/electrode interfaces. Although the insertion of
appropriate buffer layers at the material/electrode interfaces can
probably improve the performances of the devices, these first
results indicates that further molecular engineering is needed to
improve both the light-harvesting and charge-transport properties
of this kind of acceptor material.
2,6-Bis(tributyltin)-4,8-bis[5-(2-ethylhexyl)-2-thienyl]benzodithio-
phene (4): A solution of compound 5 (700 mg, 1.21 mmol) in 30 mL
dry THF was cooled to ꢀ78 °C under argon. n-butyllithium (1.3 mL,
3 mmol) was added dropwise and the mixture was stirred 30 min
at room temp. Trimethyltin chloride (863.6 mg, 4.33 mmol) was
added in one portion at ꢀ78 °C and the mixture was stirred over-
night at room temp. The organic phase was washed with ice water
and dried over MgSO4. Solvent removal gave 1 g, yield 91.2% of a
crude product which was used without further purification for
the next step. MS MALDI-TOF calcd for C40H58S4Sn2 906, found 904.
2,20-(4,8–Bis[5-(2-ethylhexyl)-2-thienyl]benzodithiophene-2,6-bis-
(methan-1-yl-1-ylidene))dimalononitrile (1): A mixture of compound
3 (47.88 mg, 0.075 mmol) and malononitrile (14.96 mg, 0.23 mmol)
and 2 drops of TBAH in 15 mL of chloroform was heated to 85 °C for
3 h, under argon. After cooling to room temp and solvent removal,
the crude product was chromatographed on silica gel (eluent 1:1
dichloromethane petroleum ether) to give 200 mg (87%) of a purple
Conclusion
Two examples of small molecular electron acceptors based on
the benzodithiophene platform have been synthesized. Optical
and electrochemical results show that the introduction of elec-
tron-withdrawing groups on the BDT core can lead to systems
combining interesting light-harvesting properties with a LUMO
level comparable to that of C60 derivatives. A preliminary evalua-
tion of the potential of one of these compounds as electron accep-
tor in ‘all-molecular’ BHJ cells shows that in spite of a high voltage
the devices present only a modest efficiency which suggests that
both the light-harvesting and the charge-mobility need to be
improved. Work aiming at the structural optimization of this class
of acceptors designed for ‘all-molecular’ OPV cells is now under-
way and will be reported in future publications.