138
M. Nazim et al. / Chemical Physics Letters 663 (2016) 137–144
and constructing the optical and organic electronic materials [21].
On the other hand, the trifluoromethyl group (ACF3) shows a sig-
nificant structural motif for organic molecules with a wide range
of interesting applications in medicinal and agricultural chemistry
[22–24]. The fluorinated organic materials have shown the high
hydrophobicity and electro negativity which are responsible for
the strong polarization behavior and high bond energy (ca.
480 kJ molꢀ1) of the carbon–fluorine bond [25,26]. The fluorinated
organic compounds contain a strong tendency to have hydrogen
bonds Fꢂ ꢂ ꢂHAC interactions with much lower energy that might
play an important role in the solid state organization. Moreover,
the conversion of CAH bonds to CAF bonds might have several
potential advantages as, the CAF bond is a very effective promoter
for radiation-less decay which could reduce the rate of radiation-
less deactivation and enhance the photoluminescence efficiency
[27]. In addition, the morphology of film with CF3-based material
and PC61BM blend is fibrous because the presence of fluorine atom
in CF3 strengthens the intermolecular interactions between two
active materials. Thus, the fluorine-termination of organic mole-
cules could induce the morphology of blend film and increase
2.2. Synthesis of small molecule
The synthetic route of spirobifluorene-based organic small
molecule, RTh-Sp-CF3 is shown in Scheme 1. The monomeric pre-
cursors 2, 3, of the n-hexyl bithiophene boronic acid pinacole ester,
4 and other related intermediates were synthesized as previously
reported procedures [40–42]. The intermediate product, 6 (RTh-
Sp-Br) was obtained by the successive Suzuki cross-coupling reac-
tions between 2,7-dibromo-9,90-spirobifluorene, 5 and n-hexyl
bithiophene boronic-acid pinacole ester,
4 using Pd(PPh3)4
(2.5 mol%) as catalyst and potassium carbonate as a base in anhy-
drous toluene solvent and then it finally coupled again by Suzuki
coupling with the 3,5-bis(trifluoromethyl)phenyl boronic acid to
give the final product, 7 named as RTh-Sp-CF3. The synthesized
small molecule was purified by flash column chromatography
and repeated recrystallization in the mixed solvent of dichloro-
methane/methanol (2:1, v/v) as pale yellow solid (Yield: 69.5%).
2.2.1. 1-(5-(thiophen-2-yl)thiophen-2-yl)hexan-1-one (2)
the
p p interactions between both organic molecules [28].
ꢀ
In a solution of 2,20-bithiophene 1 (3.17 g, 19.1 mmol) in anhy-
drous benzene (20 mL), add hexanoyl chloride (4.07 mL,
20.0 mmol) was added at room temperature. The TiCl4 (2.25 mL,
20.5 mmol) was added slowly to the reaction mixture at 0 °C and
then stirred for 15 min at 0 °C. After completion of the reaction,
ice water was added to quench the reaction and the resulting mix-
ture was diluted with CH2Cl2 (50 mL), washed successively with
water (200 mL) and saturated aqueous solution of NaHCO3
(100 mL), then dried over MgSO4 and evaporated under reduced
pressure evaporator to afford 5.00 g (85%) of yellow solid expected
as the desired ketone intermediate which was used for next step
without purification.
Recently, the organic small molecules containing trifluoromethyl-
benzene as the end-group presented high PCE (ꢁ6.0%) in BHJ fab-
ricated solar cell devices [29]. It is realized that the subtle
changes in the end-groups like fluorinated organic unit in small
molecules could significantly influence the photovoltaic parame-
ters of SMOSCs [30].
However, the spiro-based organic molecules with inherent non-
linear and rigid structures have attracted great attentions as the
organic functional materials owing to their physical properties,
high glass transition temperatures, good solubility and amorphous
nature which make them very promising for optoelectronic mate-
rials [31,32]. The spirobifluorene-based derivatives have expressed
an excellent thermal and chemical stabilities with high quantum
efficiencies as well as non-dispersive ambipolar carrier transport-
ing properties [33,34]. Most of the spirobifluorene-based small
molecules are synthesized from the central spirobifluorene, but it
requires expensive tools and chemicals [35–37]. The introduction
of D and A groups in two biphenyl branches of the spirobifluorene
core affords a class of spiro compounds with an asymmetric 2,7-
substitution pattern, resulting a good candidate for the construc-
tion of highly transparent nonlinear organic materials due to the
spiro-conjugation effects between the two fluorene units [38,39].
These substitutions induce stability and electron transport or
ambipolar transport in organic materials by lowering the energy
levels (both the highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO). In this work, a
new and effective spirobifluorene-based D-D-A type organic small
molecule, RTh-Sp-CF3 with alkyl bithiophene donor and 3,5-bis
(trifluoromethyl) benzene as acceptor, has been synthesized and
utilized for SMOSCs. The terminal alkyl group in bithiophene units
of the small molecule have considerably improved its solubility in
common organic solvents and 3,5-bis (trifluoromethyl) benzene as
acceptor unit improves the generation of charge carriers.
Under inert atmosphere,
a suspension of LiAlH4 (4.6 g,
121 mmol) and AlCl3 (4.03 g, 30.3 mmol) in anhydrous Et2O
(100 mL) was added to the toluene (40 mL) solution of ketone
intermediate at 0 °C. Then the reaction was stirred for 1 h at the
room temperature. After completion, the reaction mixture was
cooled to 0 °C. Afterward, ethyl acetate (20 mL) and HCl (6 M) solu-
tion (50 mL) were added to the reaction mixture. The resulting
mixture was then extracted with diethyl ether (300 mL), washed
with brine solution and water (50 mL), dried over MgSO4 and evap-
orated in the vacuum oven. The obtained yellow residue was puri-
fied by flash column chromatography on silica gel (hexane) to give
a colourless oil (6.00 g, 93%). 1H NMR (400 MHz, CDCl3, d, ppm):
7.17 (dd, 1H), 7.13 (dd, 1H), 6.95 (dd, 1H), 6.97 (d, 1H), 6.66 (d,
1H), 2.74 (t, 2H), 1.67 (m, 2H), 1.37 (m, 14H), 0.92 (t, 3H); 13C
NMR (100 MHz, CDCl3, d, ppm): 146.5, 145.3, 138.1, 134.7, 127.4,
125.5, 124.4, 123.9, 123.7, 123.2, 32.1, 31.6, 30.3, 29.7, 29.5, 29.1,
22.7, 14.1.
2.2.2. 5-Bromo-50-decyl-2,20-bithiophene (3)
In solution of compound 2 (2.00 g, 6.53 mmol) in dimethylfor-
mamide (30 mL), N-bromo succinimide (NBS) (1.22 g, 6.86 mmol)
was added slowly and the obtained reaction mixture was stirred
for 30 min in dark. The reaction mixture was diluted with hexane
(50 mL), and washed with saturated aqueous solution of NH4Cl
(50 mL), dried over MgSO4 and evaporated under reduced pressure.
The obtained residue was purified by flash column chromatogra-
phy on silica gel (hexane) to give a white solid (2.36 g, 94%). mp
35–38 °C; 1H NMR (400 MHz, CDCl3, d, ppm): 6.96 (d, 1H), 6.91
(d, 1H), 6.87 (d, 1H), 6.69 (d, 1H), 2.75 (t, 2H), 1.65 (m, 2H), 1.37
(m, 14H), 0.93 (t, 3H); 13C NMR (100 MHz, CDCl3, d, ppm): 145.3,
139.7, 133.6, 130.1, 130.7, 124.5, 123.5, 123.3, 110.5, 32.3, 31.5,
30.6, 29.7, 29.5, 29.1, 22.7, 14.2.
2. Experimental
2.1. Materials and equipments
All the chemicals and reagents were purchased from the com-
mercial sources and used without further purification. The flash
Column chromatography was performed on a column packed with
silica gel (300–400 mesh). The thin layer chromatography (TLC)
plates of aluminum silica gel 60 F254 (Merck) were used to moni-
tor the reaction progress.
Nazim
