Heat-Set Gel-like Networks of Lipophilic Co(II) Triazole Complexes in Organic Media and Their Thermochromic Structural Transitions

Article abstract of DOI:10.1021/ja037847q

A novel class of thermally responsive supramolecular assemblies is formed from the lipophilic cobalt(II) complexes of 4-alkylated 1,2,4-triazoles. When an ether linkage is introduced in the alkylchain moiety, a blue gel-like phase is formed in chloroform, even at very low concentration (ca. 0.01 wt %, at room temperature). The blue color is accompanied by a structured absorption around 580-730 nm, which is characteristic of cobalt (II) in the tetrahedral (T _d) coordination. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) of the gel-like phase confirms the formation of networks of fibrous nanoassemblies with widths of 5-30 nm. The observed widths are larger than a molecular length of the triazole ligand (ca. 2.2 nm) and they are consisted of aggregates of T_d coordination polymers. Very interestingly, the blue gel-like phase turned into a solution by cooling below 25 °C. A pale pink solution is obtained at 0 °C. indicating the formation of octahedral (O_h) complexes. The observed thermochromic transition is totally reversible. The formation of gel-like networks by heating is contrary to the conventional organogels, which dissolve upon heating. Temperature dependence of the storage and loss moduli (G′ and G″) shows minima around at 27 °C, at which temperature they gave comparable values. On the other hand, G′ exceeds G″ both in the gel-like phase (temperature above 27 °C) and in the solution phase (temperature below 25 °C). These observations indicate that T_d complexes are present as low-molecular weight species around at 25-27 °C. They are self-assembled to polymeric T_d complexes by heating and form gel-like networks. Upon cooling the solution below 25 °C, T_d complexes are converted to O_h complexes and they also self-assemble into oligomeric or polymeric species at lower temperatures. The observed unique thermochromic transition (pink solution → blue gel-like phase) is accompanied by an exothermic peak in differential scanning calorimetry (DSC), and is shown to be an enthalpy-driven process. The lipophilic modification of one-dimensional coordination systems provides unique solution properties and it would be widely applicable to the design of thermoresponsive, self-assembling molecular wires.

Full text of DOI:10.1021/ja037847q

Published on Web 01/28/2004
Heat-Set Gel-like Networks of Lipophilic Co(II) Triazole
Complexes in Organic Media and Their Thermochromic
Structural Transitions
Keita Kuroiwa,† Tomoko Shibata,† Akihiko Takada,‡ Norio Nemoto,‡ and
Nobuo Kimizuka*,†
Department of Chemistry and Biochemistry, Graduate School of Engineering, and the
Department of Molecular and Material Sciences, Graduate School of Engineering Sciences,
Kyushu UniVersity, Fukuoka 812-8581, Japan
Received August 10, 2003; E-mail: kimitcm@mbox.nc.kyushu-u.ac.jp
Abstract: A novel class of thermally responsive supramolecular assemblies is formed from the lipophilic
cobalt(II) complexes of 4-alkylated 1,2,4-triazoles. When an ether linkage is introduced in the alkylchain
moiety, a blue gel-like phase is formed in chloroform, even at very low concentration (ca. 0.01 wt %, at
room temperature). The blue color is accompanied by a structured absorption around 580-730 nm, which
is characteristic of cobalt (II) in the tetrahedral (Td) coordination. Atomic force microscopy (AFM) and
transmission electron microscopy (TEM) of the gel-like phase confirms the formation of networks of fibrous
nanoassemblies with widths of 5-30 nm. The observed widths are larger than a molecular length of the
triazole ligand (ca. 2.2 nm) and they are consisted of aggregates of Td coordination polymers. Very
interestingly, the blue gel-like phase turned into a solution by cooling below 25 °C. A pale pink solution is
obtained at 0 °C, indicating the formation of octahedral (Oh) complexes. The observed thermochromic
transition is totally reversible. The formation of gel-like networks by heating is contrary to the conventional
organogels, which dissolve upon heating. Temperature dependence of the storage and loss moduli (G′
and G′′) shows minima around at 27 °C, at which temperature they gave comparable values. On the other
hand, G′ exceeds G′′ both in the gel-like phase (temperature above 27 °C) and in the solution phase
(temperature below 25 °C). These observations indicate that Td complexes are present as low-molecular
weight species around at 25-27 °C. They are self-assembled to polymeric Td complexes by heating and
form gel-like networks. Upon cooling the solution below 25 °C, Td complexes are converted to Oh complexes
and they also self-assemble into oligomeric or polymeric species at lower temperatures. The observed
unique thermochromic transition (pink solution f blue gel-like phase) is accompanied by an exothermic
peak in differential scanning calorimetry (DSC), and is shown to be an enthalpy-driven process. The lipophilic
modification of one-dimensional coordination systems provides unique solution properties and it would be
widely applicable to the design of thermoresponsive, self-assembling molecular wires.
Introduction
transfer absorption characteristics of these complexes are
controlled by tuning the chemical structure of lipid counter-
One-dimensional coordination compounds are attracting much
interest as scaffolds for designing materials with unique
electronic and magnetic properties.1,2 These low dimensional
structures are usually available only as the structural motifs of
bulk crystalline materials, and accordingly they have been a
subject of solid-state inorganic chemistry and physics. The
ability to isolate and maintain such linear complexes in solution
would provide a new family of functional molecular wires. We
have recently developed an ingenious approach to disperse
halogen bridged, mixed valence platinum complexes in organic
media as lipid-packaged nanowires.3,4 The intervalence charge-
anions. These nanowires show solvatochromism5 and supra-
molecular thermochromism,4,6 the latter occurs as a consequence
of reversible dissociation and polymerization of the conjugated
wires. Pillared honeycomb stereo-architectures have also been
constructed by the water microdroplet-templated self-assembly
of the nanowires at solid surfaces.7
In this paper, we describe development of a novel class of
gel-like networks which are self-assembled from lipophilic
Co(II)-1,2,4-triazole (Trz) complexes (Chart 1).8 1,2,4-Triazoles
are known as bridging ligands and their linear iron(II) complexes
have been rigorously studied, because of the spin crossover
† Department of Chemistry and Biochemistry.
‡ Department of Molecular and Material Sciences.
(1) (a) Kahn, O.; Martinez, C. J. Science 1998, 279, 44. (b) Kahn, O. Acc.
Chem. Res. 2000, 33, 647.
(4) (a) Kimizuka, N.; Oda, N.; Kunitake, T. Inorg. Chem. 2000, 39, 2684. (b)
Kimizuka, N.; Lee, S.-H.; Kunitake, T. Angew. Chem., Int. Ed. 2000, 39,
389.
(5) Lee, C.-S.; Kimizuka, N. Chem. Lett. 2002, 1252.
(6) Kimizuka, N. AdV. Mater. 2000, 12, 1461.
(7) Lee, C.-S.; Kimizuka, N. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4922.
(2) Okamoto, H.; Yamashita, M. Bull. Chem. Soc. Jpn. 1998, 71, 2023 and
references therein.
(3) Kimizuka, N.; Oda, N.; Kunitake, T. Chem. Lett. 1998, 695-696.
9
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10.1021/ja037847q CCC: $27.50 © 2004 American Chemical Society

Gel-like Networks of Co(II) Triazole Complexes
A R T I C L E S
Chart 1. Chemical Structures of Lipophilic Co(II) Triazole
Complexes
characteristics.1,8b,9 However to date, studies of the triazole
complexes have been limited to the bulk crystalline samples,
and there have been no reports on the maintenance of polymeric
structures in solution. To develop soluble triazole complexes,
we introduced a solvophilic, dodecyloxypropyl chain in the
ligand. When a cobalt(II) complex of the lipophilic triazole
ligand 1 (C12OC3Trz) is dissolved in chloroform, a blue gel-
like phase is formed at room temperature. Interestingly, it turns
into a pale pink solution upon cooling, and the gel-like phase
is regenerated by heating. This is a first example of reversible,
heat-set gel-like networks formed in organic media. The unusual
thermal transition characteristics of the chloroform solutions was
characterized by UV-vis spectra, AFM, TEM, scanning electron
microscopy (SEM), DSC, and rheology measurements.
Figure 1. Pictures of Co(1)3Cl2 in chloroform: (a) a blue gel-like phase
at 25 °C; (b) a pale pink solution at 0 °C.
complex left on the filter is consisted of higher molecular weight
polymers.
Atomic force microscopy (AFM) was conducted to investigate
their nanostructures. Figure 2a shows an AFM image of
Co(1)3Cl2 transferred on highly oriented pyrolytic graphite
(HOPG).12 Networks of fibrous nanoassemblies with widths of
5-30 nm are abundantly seen. As the observed widths are larger
than a molecular length of 1 (ca. 22 Å, estimated by CPK
model), they must be aggregates of the linear complexes.
Developed networks of fibrous assemblies were also observed
in transmission electron microscopy (TEM, Figure 2b). In
contrast to Co(1)3Cl2, chloroform solution of Co(2)3Cl2 gives
crystalline nano-aggregates with lengths of ca. 500-1500 nm
and widths of 30-50 nm (Figure 2c). Apparently, developed
networks are not formed from Co(2)3Cl2, and this is consistent
with the observed lower ability to form gel-like phase. Scanning
electron microscopy (SEM) of the freeze-dried gel of Co(1)3Cl2
afforded thicker entwined fibers (Figure 2d, width, ca. 100 nm),
and these thicker structures must be formed during the drying
process.12 Wide-angle X-ray diffraction measurement (WAXD)
of the powdered blue xerogel of Co(1)3Cl2 showed an intense
(001) peak and weak (002), (003) peaks at 2θ ) 2.5°, 5.2°, and
7.1° respectively. These diffractions indicate that the xerogel
fibers are formed from lamella structures with a long spacing
of 35.4 Å, which corresponds to a tilt bilayer structure of
Co(1)3Cl2.
Results and Discussion
The lipophilic triazole ligands 1 (C12OC3Trz) and 2 (C16Trz)
were synthesized by modifying the literature method.9 A flexible
ether linkage is introduced in 1, since it enhances the solubility
in organic media and improves the packing of alkyl chains in
the supramolecular assemblies.10a,b The complexes of Co(1)3Cl2
and Co(2)3Cl2 were obtained as blue powders by mixing each
ligands with CoCl2 in methanol.
Very interestingly, when Co(1)3Cl2 is dissolved in spectral
grade chloroform, a blue gel-like phase is formed at room
temperature (Figure 1a; concentration, 5 unit mM).11 A mini-
mum concentration required for the gelation was very low, (ca.
0.1 unit mM, 0.007 wt %) and much harder gel-like phase is
obtained at higher concentrations (above ca. 20 unit mM). On
the other hand, Co(2)3Cl2 requires considerably higher concen-
tration (ca. 10 wt %) to gelatinize chloroform. These gel-like
structures are stably maintained over a period of one month.
Gel permeation chromatography (GPC) was conducted for the
freshly prepared chloroform solution of Co(1)3Cl2 (concentra-
tion, 1 unit mM) after passing the solution through a membrane
filter (Advantec Toyo Co. Ltd. I3JP050AN, pore diameter, 0.5
µm). It was found that the most of blue complex is remained
on the membrane filter, and a chromatogram of the filtrate
showed presence of the oligomers with an average molecular
weight of ca. 9000 (data not shown). It seems that the blue
UV-vis absorption spectra of Co(1)3Cl2 and Co(2)3Cl2 in
chloroform are shown in Figure 3. Structured absorptions are
observed at 25 °C (peaks at 639, 673 and 698 nm), which are
4
characteristic of the tetrahedral cobalt(II) complex (Td, A2 f
4T1(P)).13 As the lipophilic ligand 1 without cobalt(II) is freely
dissolved in chloroform, van der Waals forces alone cannot
provide the nano-fibrous networks in Figure 2. The developed
nanofibers observed in Figure 2 must rely on the additional inter-
molecular interactions which will be provided by the polymeric
coordination. A possible model for the tetrahedral polymers is
shown in Figure 4a. The presence of polymeric structures is
further supported by rheological studies described later.
(8) (a) Parts of this study were presented at the 49th SPSJ Annual Meeting:
Kimizuka, N.; Shibata, T. Polymer Preprints Jpn. 2000, 49, 3774. (b) Cast
films of related lipophilic Fe(II)-1,2,4-triazole complexes display reversible
spin transition characteristics: Shibata, T.; Kimizuka, N.; Kunitake, T. The
76th CSJ National Meeting; 4F137, Yokohama, 1999.
(9) Armand, F.; Badoux, C.; Bonville, P.; Teixier, A. R.; Kahn, O. Langmuir
1995, 11, 3467.
(10) (a) Kimizuka, N.; Kawasaki, T.; Hirata, K.; Kunitake, T. J. Am. Chem.
Soc. 1995, 117, 6360. (b) Kimizuka, N.; Kawasaki, T.; Hirata, K.; Kunitake,
T. J. Am. Chem. Soc. 1998, 120, 4094. (c) Kawasaki, T.; Tokuhiro, M.;
Kimizuka, N.; Kunitake, T. J. Am. Chem. Soc. 2001, 123, 6792.
(11) Samples of this kind are usually referred as “organogels”, but we avoid it
since the storage and loss moduli measured were dependent on angular
frequency ω. See Supporting Information.
(12) Kimizuka, N.; Shimizu, M.; Fujikawa, S.; Fujimura, K.; Sano, M.; Kunitake,
T. Chem. Lett. 1998, 10, 967.
(13) (a) Cotton, F.; Wilkinson, G. AdVanced Inorganic Chemistry, 4th ed.;
Wiley: New York, 1980. (b) Lever, A. B. P. Inorganic Electronic
4
4
Spectroscopy; Elsevier: Amsterdam, 1968; p 318. The T1g(F) f T1g(P)
transition of Oh complexes in the visible region near 500 nm is in admixture
with spin forbidden transitions (ꢀ, 5-40 M-1 cm-1). This absorption is
considerably weaker than 4A2 f 4T1(P) transitions of Td complexes, which
acquire some intensity by means of spin-orbit coupling.
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A R T I C L E S
Kuroiwa et al.
Figure 2. (a) AFM image of Co(1)3Cl2 (1 unit mM) transferred on HOPG. (b) TEM image of Co(1)3Cl2 (1 unit mM) transferred on a carbon-coated TEM
grid. Samples are not stained. (c) TEM image of Co(2)3Cl2 (1 unit mM). (d) SEM micrograph of a freeze-dried gel Co(1)3Cl2.
To our surprise, when the gel-like phase is cooled below 25
°C, absorption intensity of the Td complex is weakened and it
became a solution. A pale pink solution is obtained at 0 °C,
which color indicates the formation of octahedral (Oh) com-
plexes.13 Figure 5a displays the absorption intensity at 698 nm
(Td complex) measured in a heating-and-cooling cycle. The
absorption intensity of Co(1)3Cl2 changes almost monotonically
with the temperature changes (range, 0-23 °C). The 698 nm-
absorption intensity reaches a maximum above 25 °C and slight
increase in scattering is observed upon further heating. As the
Td complexes coexist with the Oh complexes in the solution
phase (below 25 °C), the dissolution of the gels by cooling
seems to occur as a consequence of decreased junctions (contacts
and interactions) between the bundled tetrahedral polymers. This
may be caused by the fragmentation process discussed below.
The temperature at which the heat-set sol-to-gel transition occurs
was almost invariable even for the higher concentration samples
(ca. 100 mM). In addition, the sol-to-gel transition is totally
reversible with subsequent temperature cycles.
tion by heating. Mixtures of Pd(II) ion and multidentate ligands
were reported to gelatinize dimethyl sulfoxide,16 however
formation of such 3D gel networks is irreversible and they are
insoluble in common organic solvents.
To further investigate the nature of thermal transition in
chloroform, temperature dependences of the storage and the loss
shear moduli, G′ and G′′ were measured for Co(1)3Cl2 (Figure
5b). The cross over point where G′ ) G′′ provides a measure
for the gel-like network formation, marking the transition from
predominantly viscous to elastic properties.17a It is to note that
G′ and G′′ show minima, which is a quite unusual temperature
dependence. G′ value becomes almost comparable to G′′ at the
temperature range of 25-27 °C, and it indicates that the
complexes are present as nonpolymerized species. Upon heating
above 27 °C, G′ value shows increase and it exceeds G′′ at
higher temperatures. This is consistent with the observed
formation of polymeric Td complexes and their physical cross-
linking. Fragmentation of the polymeric complexes occurs by
cooling the gel-like phase around at 27 °C, which is ac-
companied by the dissolution and decrease in the absorption
intensity (Figure 3a). The fragmentation is promoted by water
molecules slightly present in the solution (less than 0.015 wt
%), since they are retarded in anhydrous chloroform (see
Supporting Information). Upon cooling the solution below 27
°C, G′ value is started to increase again and becomes almost
temperature-independent at lower temperatures. It is noteworthy
In contrast to Co(1)3Cl2, chloroform solution of Co(2)3Cl2
(1 unit mM) showed smaller temperature dependence and
considerable fraction of Td complex (ca. 60%) is maintained
even at 0 °C (Figures 3b and 5a). Thus, introduction of the ether-
linkage in alkyl chains not only promotes the formation of gel-
like nanoassemblies, but also gives the higher thermal respon-
siveness.
The heat-set gel-like network formation observed for Co(1)3Cl2
is in marked contrast to the conventional low molecular mass
gelators14 and metal soap organogels,15 which undergo dissolu-
(15) (a) Pilpel, N. Chem. ReV. 1963, 63, 221. (b) Fukasawa, J.; Tsutsumi, H. J.
Colloid Interface Sci. 1991, 143, 69.
(16) Xing, B.; Choi, M.-F.; Xu, B. Chem. Eur. J. 2002, 8, 5028.
(17) (a) Nowak, A. P.; Breedveld, V.; Pakstis, L.; Ozbas, B.; Pine, D. J.; Pochans,
D.; Deming, T. J. Nature 2002, 417, 424. (b) Shikata, T.; Hirata, H.
Langmuir 1989, 5, 398.
(14) (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133. (b) Abdallah, D.
J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237.
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Gel-like Networks of Co(II) Triazole Complexes
A R T I C L E S
Figure 5. (a) Temperature dependences of absorption intensity at 698 nm
in a heating-and-cooling cycle. (b) Temperature dependence of the storage
modulus G′ and the loss modulus G′′ for Co(1)3Cl2 at angular frequency ω
) 10 rad s-1. Concentrations, 1 unit mM in chloroform.
We presume that the octahedral oligomers or polymers
observed in the solution phase possess a rodlike structure with
alkyl chains radially oriented from the main chains, as schemati-
cally shown in Figure 4b. This triple N1, N2 bridged coordina-
tion structure has been suggested for Fe(II)(trz)3 complexes1,18
and was also reported for Fe(II)(pz)3 and Co(II)(pz)3 complexes
(pz, pyrazolate).19 The rodlike structure seems to be highly
lipophilic and would be well dispersed in chloroform, since no
aggregate structures were observed in TEM. Though the Oh
complexes are possibly fragmented by water molecules and it
may cause the distribution in both the molecular weight and
ligand compositions, the reversible thermal transition (Oh
polymers or oligomers (below ca. 25 °C) T dissociated
complexes (at. 25-27 °C) T gel-like networks of Td polymers
(above 27 °C)) clearly indicates the unique self-assembling
properties of the molecular wires.
Figure 3. Temperature dependence of UV-vis spectra: (a) Co(1)3Cl2; (b)
Co(2)3Cl2 in chloroform (1 unit mM).
The thermodynamic stability of the present coordination
polymers would be determined mainly by the following fac-
tors: (i) thermal stability of the coordination structures, (ii)
interactions of solvent molecules and alkyl chains (solvation
related interactions), (iii) interactions between the alkyl chains
in a molecular wire (van der Waals forces), (iv) interactions
between the aggregates, and (v) electrostatic interactions
between polymeric complexes and counteranions. The configu-
rational equilibrium between octahedral and tetrahedral struc-
tures of Co(II) ion is affected by several effects, including
crystal-field stabilization, properties of ligands (chemical struc-
Figure 4. Schematic illustrations of (a) polymeric Td complex and (b)
polymeric Oh complex. The polymeric Td complexes form fibrous nano-
assemblies that give gel-like networks. It is possible that coordination of
water molecules to the Oh complexes causes disruptions in the coordination
structure and in the degree of polymerization.
(18) Haasnoot, L. G. In Magnetism: A Supramolecular Function; Kahn, O.,
Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996; p
299.
(19) Masciocchi, N.; Ardizzoia, G. A.; Brenna, S.; LaMonica, G.; Maspero,
A.; Galli, S.; Sironi, A. Inorg. Chem. 2002, 41, 6080.
that G′ exceeds G′′ also in the liquid phase (below 25 °C). This
indicates that the fragmented octahedral complexes are re-
assembled into oligomers or polymers.17b
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Kuroiwa et al.
xerogels and in the crystalline samples (broken line), they are
compatible to the regularly oriented ligand molecules. The
observed negative enthalpy change is reasonably ascribed to
the ordering process of ligand alkylchains possibly to the
lamellar structures and interactions between the fibrous ag-
gregates. They would have balanced the free energy increase
accompanied by the reductions in translational and rotational
freedom of motion. Similar heat-induced mesophase to crystal
phase transition has been reported for liquid crystalline poly-
mers.22 Formation of organogels from crystalline fibrous ag-
gregates has been also widely observed.23
It is interesting to compare the present heat-set gel-like
network formation with heat-set hydrogels formed by globular
proteins,24 peptides,25 or synthetic polymers.26 Formation of
these heat-set hydrogels are known as entropy driven (positive
∆S) process which is in line with the hydrophobic inter-
actions.24-26 The heat-set hydrogels of proteins are associated
with denaturation which proceeds irreversibly. In organic liquids,
hydrophobic interactions are either not operative or of secondary
importance, and renders enthalpic stabilization responsible for
the heat-set phase transition.
Conclusion
Described herein is a first example of thermally reversible,
heat-set gel-like networks in organic media. It is developed by
combining the elements of the one-dimensional metal complexes
and mesoscopic supramolecular assemblies.3-8,10 There are
several appealing features in these self-assembling molecular
wires. The first is impartment of lipophilic nature to the one-
dimensional coordination complexes. The lipophilic modification
of bridging ligands gives an alternative strategy to the electro-
static binding of lipid molecules to the oppositely charged linear
complexes we reported previously.3-7 Introduction of ether
group in the 4-alkylated triazole Co(II) complex is important,
since it promotes the formation of developed gel-like networks
in chloroform. Second, fibrous nano-assemblies of tetrahedral
complexes in the gel-like state are thermally responsive and
they are transformed to the metastable, octahedral complexes
in solution by cooling. This self-assembly is reversible and the
heat-set transition (from the Oh complexes in solution to the Td
complexes in gel-like phase) is enthalpically driven. These
features give a clear distinction from the conventional organogels
that dissolve upon heating. In addition, it is also contrast to the
heat-set polymer hydrogels which formation is entropy driven.
The amphiphilic one-dimensional coordination systems thus
provide unique self-assembling properties which are not acces-
sible from the conventional inorganic or polymer chemistry.
We foresee that the heat-set organogels will find wide range of
applications as new thermoresponsive materials.
Figure 6. (a) DSC thermograms of Co(1)3Cl2. concentration, 1 unit mM
in chloroform. (b) Schematic temperature dependence of Gibbs free energy
for Co(1)3Cl2 in chloroform.
ture, polarizability, π-acceptor capacity) as well as crystal
packing.20 In the case of monomeric Co(II) complexes in
solution, octahedral coordination is favored at lower tempera-
tures.21 The equilibrium shifts to tetrahedral coordination at
higher temperatures, and this process is entropically driven.20,21
The observed temperature dependence, i.e., Oh complexes at 0
°C and Td complexes at room temperature, is consistent with
these observations.
On the other hand, the formation of gel-like networks as a
whole is enthalpically driven, as shown by the following
differential scanning calorimetry (DSC) data. Figure 6a shows
DSC thermograms obtained for Co(1)3Cl2 in chloroform. The
formation of gel-like networks is accompanied by an exothermic
peak between 5 °C and 40 °C and the peak was reproducibly
observed (∆H ) -33 kJ mol-1, ∆S ) 111 J K-1mol-1) though
the peak structure is slightly altered in the repeated scans.
The exothermic peak (negative ∆H) indicates an enthalpy
driven process that is in line with the transition from a metastable
solution state to a more stable gel-like state. A schematic plot
of the temperature dependence of the Gibbs free energy is shown
in Figure 6b. The exothermic transition upon heating arises
primarily from the greater thermodynamic stability of the nano-
fibrous networks (tetrahedral complexes) versus the octahedral
complexes in solution. The octahedral complexes in lower
temperature solutions are transformed to the tetrahedral com-
plexes that afford gel-like state (solid line, above 25 °C). As
the tetrahedral complexes are maintained also in the lamellar
(22) (a) Papadimitrakopulos, F.; Sawa, E.; MacKnight, W. J. Macromolecules
1992, 25, 4682. (b) Cheng, S. Z. D.; Yandrasits, M. A.; Percec, V. Polymer
1991, 32, 1284.
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(b) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Fages, F.; Pozzo, J.-L.
Langmuir 2003, 19, 2013.
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Langmuir 2001, 17, 3208. (b) Croguennoc, P.; Nicolai, T.; Kuil, M. E.;
Hollander, J. G. J. Phys. Chem. B 2001, 105, 5782.
(25) (a) Pochan, D. J.; Shneider, J. P.; Kretsinger, J.; Ozbas, B.; Rajagopal, K.;
Haines, L. J. Am. Chem. Soc. 2003, 125, 11802. (b) Wright, E. R.;
McMillan, A.; Cooer, A.; Apkarian, R. P.; Conticello, V. P. AdV. Funct.
Mater. 2002, 12, 149.
(20) King, H. C. A.; Ko¨ro¨s, E.; Nelson, S. M. J. Chem. Soc. 1963, 5449.
(21) (a) Sunamoto, J.; Hamada, T. Bull. Chem. Soc. Jpn. 1978, 51, 3130. (b)
Aizawa, S.; Iida, S.; Matsuda, K.; Funahashi, S. Inorg. Chem. 1996, 35,
1338. (c) Aizawa, S.; Funahashi, S. Inorg. Chem. 2002, 41, 4555.
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Okabe, S.; Sugihara, S.; Aoshima, S.; Shibayama, M. Macromolecules 2002,
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Gel-like Networks of Co(II) Triazole Complexes
A R T I C L E S
Acknowledgment. This work is supported by a Grant-in-Aid
for Scientific Research (B) (13440210) from Japan Society for
the Promotion of Science and by a Grant for 21st Century COE
Program from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan. We are grateful to Dr. S.
Sasaki and Mr. H. Imada (Department of Applied Chemistry,
Graduate School of Engineering, Kyushu University) for their
assistance in WAXD measurement. Fruitful discussions with
Professors H. Okawa and M. Ohba (Department of Chem-
istry, Graduate School of Science, Kyushu University) and
Professor A. Takahara (Institute for Materials Chemistry
and Engineering, Kyushu University) are also gratefully ac-
knowledged.
Supporting Information Available: Experimental section,
powder X-ray diffraction pattern of the xerogel, temperature
dependence of UV-vis absorption intensities measured in
anhydrous chloroform and in water-containing chloroform
(water content 0.015 wt % and 0.082 wt %), and rheological
data (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA037847Q
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