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bands of carboxyl at 1535 cm−1 for the antisymmetric stretch
and 1384 cm−1 for the symmetric stretch with Δvas–sym of
151 cm−1, indicating the bidentate complex18 between the car-
boxylic acid and Co(II) ions (Fig. S2 in the ESI†). However,
unreacted metalloporphyrins were identified from the dark pur-
ple solution that was obtained after the microsphere isolation.
Therefore, we have increased the reaction time to attempt
to complete the reaction during the particle growth. By
allowing longer reaction time, we would extend the assembly
time of particles in DMF to offer a potential to obtain larger
objects while maintaining the narrow particle size distribu-
tion.19 Therefore, we increased the reaction time by 45, 60, 75,
90, 120, and 200 min, respectively, and we obtained dark
brown suspensions as expected. SEM analysis revealed that
these suspensions were collections of uniform structures
showing systematically modulated morphologies and sizes
(Fig. 1, S3 and S4 in the ESI†). For example, when the reaction
mixture was vigorously stirred at 90 °C for 45 min, the apple-
like structures (Fig. 1b), i.e., spheres with a small opening at
the surface with approximately 650 60 nm average diameter
were obtained. This small opening grew bigger in size, and
eventually, the morphology of structure changed into donuts
(Fig. 1c) and stretched rubber bands (Fig. 1d) when the reac-
tion time was increased.
approximately 120
25 nm average diameter. When the
reaction is initiated in DMF at 90 °C, the seeds are immedi-
ately generated and quickly grow into large microspheres
until they are in a solution phase. As the reaction time is
extended, the surfaces of microspheres are redissolved and
a small opening is formed at the surface, thus producing
“apples”. The dissociated building blocks, (porphyrin)Mn(III)
biscarboxylic acid and Co(II) ions, from microspheres are
redeposited on the surface of particles, and thus, donuts
and stretched rubber band structures are formed. This pro-
cess continues until all the components are completely
dissolved in the hot DMF. After cooling, redissolved build-
ing components become less soluble, precipitate from the
DMF medium, and quickly generate nanospheres. As
expected, these materials exhibit similar powder X-ray dif-
fraction (PXRD) patterns with undetermined peaks, showing
typical amorphous characteristics despite their difference in
morphologies (Fig. S6 in the ESI†).
Remarkably, the dark brown suspension was again obtained
upon extending the reaction time to 450 min at 90 °C, and
narrowly dispersed microcubes, approximately 2.02
0.5 μm
in length, were confirmed by SEM analysis (Fig. 1f). PXRD
analysis revealed that these microcubes showed better crystal-
linity compared with other aggregates (Fig S6 in the ESI†).
Redissolved building components from amorphous spheres
are systematically assembled to produce stable crystalline
microcubes with a longer reaction time. However, when the
assemblies were carried out at 120 °C, morphologically
polydispersed particles were obtained (Fig. S5 in the ESI†).
Thermogravimetric analysis (TGA) of the samples obtained
at various reaction periods was performed to analyze the
thermal stability of all the particles. The TGA curves show an
initial weight loss due to solvent liberation and a high
thermal stability up to 300 °C. They show continuous weight
loss in the range from 300 °C to 600 °C due to the decomposi-
tion of frameworks (Fig. S7 in the ESI†). Furthermore, the
samples of nanospheres and the microcubes show relatively
large initial solvent-mediated weight losses, presumably due
to somewhat large surface areas.
Interestingly, the dark brown suspension became clear
with no indications of particle formation, when the reaction
time was extended to 300 min. However, a new dark brown
suspension was again observed after cooling it at ambient
temperature. SEM analysis revealed that the suspension
was a collection of uniform nanospheres (Fig. 1e) with
Therefore, the extreme cases of samples, i.e., micro-
spheres, and microcubes were selected to investigate their
surface area and gas sorption properties. Their volumetric
gas sorption measurements were carried out using N2 and H2
at 77 K and CO2 at 196 K. As illustrated in Fig. 2, the N2 and
H2 adsorption isotherms of the microsphere sample show
surface areas of 6.4 and 6.2 m2 g−1, respectively, while the
sorption of CO2 reveals an uptake value of 40.5 cm3 g−1. The
smaller surface area and gas uptake are attributed to the non-
porous nature of the microspheres. In the microcube sample,
the N2 and H2 adsorption isotherms reveal surface areas of
22.6 and 44.8 m2 g−1, respectively, while the sorption of
CO2 shows an uptake value of 61.8 cm3 g−1. The surface
area has been significantly increased due to the formation
of crystalline particles, although the particle size has also
been increased dramatically. However, the relatively small
values of crystalline microcubes are presumably due to the
Fig. 1 SEM images of (porphyrin)Mn(III) biscarboxylic acid- and Co(OAc)2-based
nano- and microsized CPAs obtained at various reaction periods: (a) 30 (micro-
sphere), (b) 45 (apple), (c) 60 (donut), (d) 90 (rubber band), (e) 300 (nanosphere),
and (f) ~450 min (microcube).
This journal is © The Royal Society of Chemistry 2013
CrystEngComm, 2013, 15, 9360–9363 | 9361