C O M M U N I C A T I O N S
in conjugated polymers has been effected with nonconjugated blocks
or pendant groups. We have discovered that phase separation as
well as the optical properties can be controlled by the heterocycle
in the polymer chain. This represents a distinct type of phase
separation that is driven by elemental composition and simulta-
neously offers a direct means for controlling the optical properties.
Because of this remarkable ability and resultant properties, poly(3-
hexylselenophene-block-3-hexylthiophene)s should find utility in
fundamental studies, such as testing the limits of phase separation,
as well as in optoelectronic applications.
Figure 2. (a) Absorbance spectrum, (b) AFM height image, and (c) dark-
field STEM image of poly(3-hexylselenophene-block-3-hexylthiophene)
films. In (c), electron-dense regions appear brighter.
Acknowledgment. This work was supported by The University
of Toronto and NSERC. J.H. is grateful for a graduate scholarship
(NSERC CGS-M). The authors thank Guillermo Bazan and Yanika
Schneider for assistance with GPC measurements, Gilbert Walker
and Shell Ip for assistance with AFM microscopy, and Mitchell
Winnik and Meng Zhang for assistance with fluorescence
measurements.
which the block copolymer contains intact polythiophene and
polyselenophene chromophores.
The solution fluorescence spectra are also indicative of structure.
For reference, P3HT and P3HS emit at 578 and 623 nm,
respectively (see the SI). The block copolymer emission spectrum
is dominated by emission from the thiophene block because of this
block’s greater fluorescence intensity. On the other hand, the
statistical copolymer emits weakly at 598 nm, a frequency
positioned between the P3HT and P3HS emissions. Taken together,
the 1H NMR and solution optical measurements are consistent with
the proposed structures of the polymers.
Supporting Information Available: Experimental procedures and
additional figures. This material is available free of charge via the
We next designed experiments to examine the solid-state
properties of the block copolymer. Films were prepared by spin-
casting followed by annealing (150 °C, 1 h), and the absorbance
properties were measured. Interestingly, the absorption profile of
the block copolymer possesses shoulders (marked by arrows in
Figure 2) that coincide with the π-stacking bands of both P3HT
and P3HS. These features indicate association and organization of
blocks with corresponding blocks in adjacent chains. This observa-
tion stands in contrast to that for the statistical copolymer, which
has a nearly featureless absorption profile. The film morphology
of the block copolymer was also investigated using atomic force
microscopy (AFM). Distinct domains are present in the film (Figure
2). This morphology is striking when compared with the statistical
copolymer film, which has much smaller domains with a smoother
morphology, or when compared with blends of the two pure
polymers, which appear unstructured at the nanoscale (see the SI).
Taken together, the absorption and AFM data demonstrate that the
block copolymer undergoes a significant degree of phase separation
in the solid state.
More detailed information was available from dark-field scanning
transmission electron microscopy (STEM) measurements. In these
unstained images (Figure 2c), bright cylindrical features similar in
size to those observed using AFM can be seen. These features
represent domains with a high electron scattering ability and are
most likely due to selenophene-rich phases. Topographic elemental
mapping lends support to this hypothesis, showing that the bright
regions are rich in selenium and deficient in sulfur. Conversely,
the darker regions are rich in sulfur and deficient in selenium (see
the SI), confirming that the features arise from blocks of distinct
heterocycles that preferentially associate in the film. This preference
is quite surprising in view of the fact that selenophene and thiophene
are so structurally similar.
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In conclusion, poly(3-hexylselenophene-block-3-hexylthiophene)s
are an important new class of copolymer because they have broad
optical absorption properties, with an onset that is red-shifted by
80 nm relative to that of P3HT, and the ability to undergo phase
separation in the solid state. Until the present work, phase separation
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