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Can. J. Chem. Vol. 89, 2011
Fig. 1. Structure of head-to-tail regioregular P3HT.
The dinitro derivative, on reduction with stannous chloride in
HCl, gave the corresponding diamino derivative (V), which
on diazotization followed by displacement by sodium azide
yielded the crosslinker (VI).
2D NMR studies of 2,2′,3,3′,6,6′-hexamethoxy-5,5′-
dinitrobiphenyl (IV)
Previously we reported an aromatic nitro ipso substitution
leading to a molecule belonging to a rare space group, P42.30
Synthesis of compound IV was taken up to determine
whether it too would belong to the rare space group P42. 2D
NMR studies were then undertaken to establish its structure.
The nuclear Overhauser effect (NOE) spectrum (see Fig. S1
in the Supplementary data) of the compound shows that the
1
assignment from H-3 at d 7.59 corresponds to a peak at d
3.94 (CH3-10), indicating this methyl is ortho to C-3. In this
NOE spectrum, cross peaks are observed between CH3-8 at d
3.77 and both of the other methyl groups, i.e., CH3-10 at d
3.94 and CH3-14 at d 3.62. 13C NMR assignments from het-
eronuclear multiple bond coherence (HMBC) spectra (Fig. 3)
were based on the magnitude of the observed coupling con-
stants for C-4 (5.93 Hz) and C-6 (none resolved). However, it
cannot be ruled out that these assignments could be reversed.
The signal for C-4 appears at 137.7 ppm. From this, C-6 can
be calculated and placed as being extremely shielded at
SENTECH SE 500 ellipsometer. SEM investigations were
carried out on a LEO 1430 instrument using films coated
over indium tin oxide (ITO) coated glass.
88.9 ppm. The observance of a four-bond coupling (4JHC
)
Device fabrication
only for CH3-8 may be the result of this being the only
methyl group with the OCH3 aligned in the plane with the
aromatic ring. The other two methoxy groups exhibit an an-
gle of ∼125° from the plane of the ring. Complete NMR as-
signments for the compound are given in Table 1.
Devices were prepared on glass substrates coated with a
thin layer of ITO, which were patterned by an etching proc-
ess using a mixture of HNO3–HCl–H2O in a 1:12:12 ratio.
The coated and etched substrates were cleaned by a standard
wet process with detergent, followed by boiling acetone, iso-
propanol, and deionized water, with sonication at each step.
Solutions of regioregular P3HT in chlorobenzene (2 wt %)
were spin-coated at 2000 rpm, resulting in films with a thick-
ness of about 175 nm. The solutions were prepared with dif-
ferent concentrations of the crosslinker and were filtered
through a 0.2 µm pore size PTFE membrane syringe filter
before spin coating. The solvent was removed in vacuo and
metallic electrode contacts were formed by thermal evapora-
tion of the metal, aluminum (∼200 nm), in a vacuum coating
unit using a base pressure of 2 × 10–6 mbar (1 bar = 100 kPa)
in the evaporation chamber and a deposition rate of >1 Å s–1
over the film through a shadow mask. To avoid any doping
by oxygen, the devices were kept in vacuum till measure-
ments were done. Current–voltage (I–V) characteristics were
recorded under normal class 10 000 clean room environmen-
tal conditions (temperature 25 °C and relative humidity
∼45% to 50%) using a Keithley 2410 SourceMeter.
Crosslinking with poly(3-hexylthiophene)
In regioregular P3HT, inter-chain interactions are enhanced
by p-stacking of the polymer chains, and the formation of a
lamellar stacking of the polymer chains with good p-orbital
overlap between neighboring chains is promoted by the regu-
lar arrangement of the alkyl side chain in P3HT.18 With bet-
ter control of the structural order, the efficiency of charge
transport is expected to improve with stronger p–p interac-
tions. This p-overlap can be further improved by crosslinking
the polymer chains, which in turn will improve the electronic
communication between them. For this purpose a biaryl-
based bisazide was synthesized and crosslinked with the
polymer. Crosslinking was achieved in a Rayonet photoreac-
tor fitted with eight UV tubes (l = 254 nm) of 12 W each.
To avoid the effect of heat, the fan of the instrument was kept
on during the experiment. Photolysis of the azide generates
highly reactive nitrenes that can insert into a single bond
(C–H) or double bond (thiophene ring) of the polymer. IR
spectroscopy was used as a tool to determine the progress of
crosslinking in the polymer film. The disappearance of the
characteristic peak (20 min) for the azide group at
∼2100 cm–1 in the IR spectrum indicated the completion of
crosslinking (Fig. 4).
Results and discussion
Figure 2 shows the reaction sequence used to synthesize
the crosslinker. 1,2,4-Trimethoxybenzene (I) was synthesized
by Thiele’s acetylation of p-benzoquinone followed by meth-
ylation with dimethyl sulfate in aqueous alkali. Lithiation of
1,2,4-trimethoxybenzene using n-butyllithium in diethyl ether
gave the lithium salt, which on subsequent iodination gave
compound II. The iodo-derivative, on Ullmann coupling in
the presence of activated copper, yielded III, which on fur-
ther nitration gave the corresponding dinitro compound (IV).
To determine the effect of crosslinking on the hole mobi-
lity of P3HT, devices with a glass/ITO/polymer/Al structure
were prepared. Pristine P3HT films were prepared from a
2 wt % solution of polymer in chlorobenzene after stirring
for 24 h in the dark. For the crosslinking experiments, the
crosslinker was mixed in an appropriate ratio, namely 10 or
Published by NRC Research Press