through spin coating, densely and homogeneously distributed
nanospheres were formed. Sizes of the spheres decreased as the
solvent evaporation rate was increased. In contrast, when the
solvent evaporation rate is reduced, i.e. through drop casting,
spheres with size on the order of micrometres are formed (see
Supporting Information S3w).
Fig. 3 SEM images of C60–C12-XPPP 1 : 1 blend in toluene at
0.1 mg mLÀ1 (a), 0.2 mg mLÀ1 (b), 0.5 mg mLÀ1 (c).
To investigate the influence of concentration on morphol-
ogy, solutions of different concentrations, but with constant
ratio (1 : 1) between C60 and C12-XPPP, were prepared and
drop cast from toluene solutions. The SEM images revealed
that varying the solution concentration of the C60–C12-XPPP
blend did not hinder the formation of the spherical nanocrys-
tallites. As shown in Fig. 3, films of the blend consisting of
spherical structures were formed. Upon using a 0.1 mg mLÀ1
solution, crystallites with an average size of 200 nm were
obtained, while those prepared using the 0.2 mg mLÀ1 and
0.5 mg mLÀ1 solutions led to spherical particles with a size of
500 nm and 1200 nm, respectively. Moreover, with a lower
concentration, the spherical domains had a smaller size dis-
tribution and were more uniformly dispersed throughout the
film (see Supporting Information S4w).
Inter-chromophore order and homogenous dispersion of
the donor–acceptor system are essential for efficient perfor-
mance of materials in applications such as organic electro-
nics.15 It is further validated in the changes observed in
photophysical properties of the blend. With absorption in
the ultraviolet region, C60 has significant overlap with the
emission of the polymer (Fig. 4a). As a consequence, photo-
luminescence was quenched by photoinduced electron/energy
transfer with an intimate mixing of the donor–acceptor inter-
face. (UV absorbance of the polymer and blend is shown in
Supporting Information S5w). As shown in Fig. 4b, 31%
quenching was observed in the 1 : 2 C60–polymer blend and
64% in the 1 : 1 C60–polymer blends in toluene.
Fig. 4 Respective absorbance and fluorescence emission of individual
components (a), and quenching of emission (b) of C12XPPP with C60
blends.
Here we show a significant structural control of nanocrys-
tallites from a cross-conjugated polymer with C60 using a
conventional drop casting method. In line with previous
nanohybrid studies, where PPP backbones were engineered
with hydroxyl groups and single8 or double16 long alkoxy
chains on every repeat unit, significant inter-component inter-
actions were observed in the present system. Strong p–p
interactions between the polymer and C60s, coupled with the
weak interactions of the dodecyloxy chains, guided the self-
assembly to uniformly sized nanocrystallites. The aggregate
size and dispersions were tailored and studied. Extensive
photophysical and photocurrent studies of the hybrids are
underway.
crystallized from toluene solution gave non-uniform rods while
the polymer formed a featureless uniform film when cast from
the same solvent (see Supporting Information S2w). Addition of
C60 into the 0.1 wt% polymer solution in a 1 : 1 blend caused
substantial changes in morphology. Nanocrystallites of the
blend with uniform sized spheres were observed (Fig. 2). The
presence of aromatic groups on both the backbone and side
chains facilitated the dispersion of C60 and directed the p–p
interaction driven co-assembly (Fig. 1c–d).
In addition, there were no visible phase separated C60
aggregates in the nanocrystallites and well-defined lattice fringes
were observed in the high resolution TEM image (Fig. 2a). The
inter-lattice spacing measured here as well as in X-ray diffrac-
tion averages 0.3 nm, which is in agreement with the side chain
end-to-end distance within the polymer packing.13
The authors wish to thank the Agency for Science, Technology
and Research (ASTAR) for funding support and the National
University of Singapore for a scholarship to M. H. N.
Notes and references
As determined in previous studies, parameters such as
casting solvent, concentration and composition of the
polymer and C60 in the nanohybrid significantly influence
the morphology.14 The dynamics of solvent evaporation play
a vital role in crystallization behavior and morphology of the
nanohybrid. Under conditions of fast solvent evaporation, i.e.
1 V. Coropceanu, J. Cornil, D. A. da Silva Filho, Y. Olivier, R.
Silbey and J.-L. Bredas, Chem. Rev., 2007, 107, 926.
´
2 G. Li, V. Shrotriya, Y. Yao and Y. Yang, J. Appl. Phys., 2005, 98,
043704.
3 X. Yang, J. Loos, S. C. Veenstra, W. J. H. Verhees, M. M. Wienk,
J. M. Kroon, M. A. J. Michels and R. A. J. Janssen, Nano Lett.,
2005, 5, 579.
ꢀc
This journal is The Royal Society of Chemistry 2008
4946 | Chem. Commun., 2008, 4945–4947
Nurmawati, Muhammad Hanafiah
