Synthesis and properties of supramolecular ionic networks

Article abstract of DOI:10.1021/ja803248q

The synthesis of a supramolecular ionic network, its physical properties, and the use of this network property to form macroscopic porphyrin fibers are described. These ionic networks are compared to ionic liquids. Current ionic liquid compositions have a charge and molar ratio of 1:1 where an anionic species is matched with a cationic species; however, alteration of this molar ratio while maintaining the charge ratio of 1:1 by using multivalent cationic/anionic molecular pairs affords new ionically cross-linked networks with interesting properties. Copyright

Full text of DOI:10.1021/ja803248q

Published on Web 07/02/2008
Synthesis and Properties of Supramolecular Ionic Networks
Michel Wathier and Mark W. Grinstaff*
Departments of Biomedical Engineering and Chemistry, Metcalf Center for Science and Engineering,
Boston UniVersity, Boston, Massachusetts 02215
Received May 1, 2008; E-mail: mgrin@bu.edu
A hallmark of supramolecular chemistry is the use of well-defined
molecules or macromolecules and intermolecular forces to create
larger, more complex chemical systems with new and unique
properties.1–4 This noncovalent synthetic strategy affords a variety
of structures ranging from host-guest complexes and lipid organ-
izations to linear metallopolymers through the use of, for example,
hydrogen bonding, van der Waals forces, or metal-ligand bonds.5–9
Of the various supramolecular assemblies known, networks are
particularly interesting as their macroscopic properties can be
significantly different than the properties of the individual building
blocks.3 The creation of a supramolecular network requires two
different molecular structures whereby one structure possesses at
least two and the other three or more complementary molecular
recognition groups. Such a strategy has been applied to prepare
supramolecular polymer networks using hydrogen and metal-ligand
bonding.10,11 Herein, we report the discovery of “supramolecular
Figure 1. Chemical structures under investigation.
ionic networks” created using multicationic and multianionic
molecules. These fluidic materials, held together through non-
covalent electrostatic interactions, are easily prepared from a diverse
set of readily available starting materials, facilitating the design of
new ionic materials with a high level of molecular control.
As these materials are ionic networks that flow at ambient
temperatures, we chose to compare their properties to ionic liquids.
Ionic liquids are “salt-like” materials held together through electrostatic
interactions that have melting points at relatively low temperatures
(<100 °C). An example of a common ionic liquid is tetradecyl(tri-
hexyl)phosphonium chloride which has a melting point below 20 °C.12
Ionic liquids have found use in a wide range of applications, among
them industrial solvents, separation media, fuel cells, protein crystal-
lization matrices, liquid crystals, batteries, and thermal fluids.13–16
Current ionic liquid compositions have both a charge and molar ratio
of 1:1 (e.g., tetradecyl(trihexyl)phosphonium chloride) where a mono-
cationic species is stochiometrically paired with a monoanionic species.
Alteration of the molar ratio while maintaining the charge ratio of 1:1
by using multivalent cationic/anionic molecular pairs affords new
ionically cross-linked supramolecular networks.
Figure 2. Viscosity data for the various ionic liquids and network ionic
liquids at 25 °C (N ) 3; mean ( SD; *p < 0.001).
in the presence of para-toluene sulfonic acid. Addition of the silver
salt of EDTA4- to the P2+:2Cl- afforded ionic liquid 2P2+
:
EDTA4- and AgCl(s). A similar procedure was used to prepare
the ionic liquids and networks described herein.
Specifically, we chose to prepare an ionic network using a
phosphonium dication, P2+, and a tetraanion, ethylenediaminetetra-
acetate (EDTA4-). We selected a geminal dication since these
symmetric compounds possess two positive charges and have
recently been of interest for a variety of applications.17 The specific
chemical structures under investigation are shown in Figure 1. These
structures enabled us to evaluate the effect of varying the mole
and charge ratio for these ionic materials held together by
Coulombic interactions. The dichloride salt of the dicationic
phosphonium (P2+:2Cl-) was prepared by reacting 2 equiv of
trihexylphosphine with 1 equiv of 1,10-dichlorodecane at 140 °C
for 24 h. This ionic liquid readily flows and is a colorless liquid.
The silver salts of dodecanedioic acid, DDA2-, and ethylenedi-
aminetetraacetic acid, EDTA4-, were prepared as described in the
Supporting Information. The ethyl ester analogue of ethylenedi-
aminetetraacetic acid, EE, was prepared by esterification in methanol
The viscosities of the ionic liquids were measured using a AR 1000
controlled strain rheometer (TA Instruments). Ionic liquid P2+:2Cl-
possesses a viscosity of 1000 Pa ·s. Substitution of the monoanion,
Cl-, for the tetraanion, EDTA4-, while maintaining the charge ratio
(1:1) with P2+, affords a significant increase in the viscosity (Figure
2). The resulting ionic liquid 2P2+:EDTA4- has a viscosity of
approximately 12 000 Pa ·s. The viscosity is dependent on the charge
ratio as evident by a significant decrease in the viscosity with ionic
liquid P2+:EDTA4- (see Supporting Information for graph). The
enhanced viscosity is a consequence of the network formed between
the dication and tetraanion. To confirm this ionic network formation,
several control experiments were performed. First, an ionic liquid was
prepared with the dianion, DDA2-, and P2+ which cannot form a
network due to the insufficient number of cooperative Coulombic
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9648 J. AM. CHEM. SOC. 2008, 130, 9648–9649
10.1021/ja803248q CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 4. (A) Scanning electron micrograph of a fiber composed of 2P2+
:
H2TPP4- (bar ) 100 µm). (B) Scanning electron micrograph of the fiber
at higher magnification (bar ) 50 µm). (C) Fluorescent micrograph of a
fiber. (D) Solid-state fluorescence spectrum of a fiber.
Figure 3. (A) Chemical structures of the geminal phosphonium dication
and porphyrins under investigation. (B) Photograph of an ionic network
fiber prepared from 2P2+:H2TPP4-. (C) Photograph of the P2+:2Cl- with
H2TPP.
cooperative Coulombic interactions. Incorporation of a porphyrin in
the network (2P2+:H2TPP4-) and the observation of fluorescence
suggest that ionic networks containing specific functional building
blocks continue to possess their original properties. For example, self-
assembled solid-state-based porphyrin materials are of interest for many
uses (e.g., sensors).19,20 The strategy demonstrated using multicationic
and multianionic molecules provides an opportunity to prepare new
supramolecular ionic networks with properties not attainable with
current ionic liquids. Moreover, the generality, ease of preparation,
diversity of starting materials available, and straightforwardness of the
approach will facilitate the design of new ionic materials for a range
of applications.
Acknowledgment. This work was supported by BU. The
authors like to thank A. Griset for the SEM experiment and P. Allen
for the fluorescence imaging.
Supporting Information Available: Syntheses and rheological
measurements. This material is available free of charge via the Internet
at http://pubs.acs.org.
interactions. Ionic liquid P2+:DDA2-, like P2+:2Cl-, has a viscosity
of ≈1000 Pa ·s. Next the esterfied EDTA, EE, which cannot form
ionic bonds, was mixed with P2+:2Cl-. The resulting mixture had a
low viscosity, further confirming the importance of carboxylate residues
in these networks. Moreover, substitution of the phosphonium dication,
P
2+, with a phosphonium monocation, P+, also gives low viscous fluids
because a network cannot be formed. Ionic liquids P+:Cl-, 2P+:
DDA2-, 4P+:EDTA4-, and P+:EE possess viscosities less than 1000
Pa ·s.
Importantly, the connectivity that is present in an ionic network
enables the preparation of macroscopic materials with specific sizes
and shapes. To illustrate this feature, we prepared ionic materials from
geminal phosphonium dication, P2+, with either para-tetracarboxy-
5,10,15,20-tetraphenyl-21H,23H-porphine, H2TPP4-, or as a control,
5,10,15,20-tetraphenyl-21H,23H-porphine, H2TPP (Figure 3). The
former is expected to form an ionic network as it possesses four
carboxylate residues available for ionic bonding. Ionic liquid 2P2+
:
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modulus were also determined and found to be ≈107 and 106 Pa,
respectively. The mixture of 2P2+ and H2TPP4- was heated to 160
°C, and a fiber was hand-pulled from the solution. The fiber was dark
purple in color, flexible, moldable into a coil, and was approximately
1 mm in diameter and 10 cm in length (Figure 3B). Under the same
conditions, if P2+ is mixed with H2TPP, which lacks the anionic
carboxylate groups for ionic bonding, a low viscous P2+:2Cl- ionic
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fluorescent micrograph at 40X magnification of the fiber is shown in
Figure 4C under 514 nm illumination. The solid-state fluorescent
spectrum possesses a λmax at 645 nm, consistent with the reported
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