Synthesis of N,n'-Dialkyl-4'bipyridinium Reagents

This application is based upon and claims the priority of International Patent Application No. PCT/US81/00949 filed July 14, 1981.

The field of this invention is chemical compounds and, particularly, chemical compounds suitable for use in electrochromic systems.

Recently there has been considerable interest in reduction-oxidation (redox) active compounds for use in electronic displays. Such electrochromic materials are suitable for use whenever a light absorbing, rather than a light emitting, display can be employed. Electochromic materials are often preferred because of their low power consumption and their ability to operate without a constantly applied power source.

A number of compounds which change color as a result of electrochemical reactions are known. For example, polyvinylferrocene, polynitrostyrene or polymers from hydrolytically unstable ferrocene monomers have been confined to the surface of electrode materials. All of these materials are electrochromic. Polynitrostyrene, a pale yellow solid, is claimed to give an electrode a red appearance upon reduction whereas ferrocene-centered polymers turn from yellow/orange to blue upon oxidation. These results are encouraging, but the ferrocenes are too weakly colored and the reduction of polynitrostyrene is not readily reversible in the presence of moisture.

Compounds formed from 4,4'-bipyridinium, known as viologens, also exhibit electrochromic behavior. For example, dialkyl-4,4'-bipyridinium di-cations and their associated anions (dichloride, dibromide or di-iodide) form contrasting colors. These viologen di-cations range from colorless, yellow, and to red, while mono-cations are a striking blue-purple. Two basic problems with viologens are their long term stability as coatings and their sluggish switching speeds (rates of color change) at typical coating or solution concentrations.

There exists a need for electrochromic materials which can be immobilized, and preferably covalently bonded, upon electrode surfaces and which can be repeatedly subjected to redox actions in display systems. Moreover there is a need for electrochromic materials that can respond rapidly to an applied potential and yield a strong color (i.e. high molar absorbivity).

We have discovered a novel class of dialkyl and dialkyl-aromatic viologens (4,4'-bipyridinium compounds) which may be polymerized and covalently bonded, or otherwise confined, to the surfaces of electrodes. These surface confined compounds exhibit the colors of viologen dications, colorless when oxidized, purple when reduced, while having the ability to avoid dissolution into the electrolyte in their oxidized or reduced, radical cationic, state. Further the surface confined compounds can be reduced and reoxidized rapidly and without degradation.

In one embodiment, our reagents include silicon groups which provide sites for attachment to surfaces bearing OH groups and, in the presence of H.sub.2 O, also promote polymerization. In particular, silicon alkoxides, Si(OR).sub.3, such as Si(OMe).sub.3 ; Si(OEt).sub.3 ; Si(OPr).sub.3 ; Si(OBu).sub.3 ; and silicon halogens such as SiCl.sub.3 may be used. When the silicon alkoxide reagents are applied to the OH bearing surface, the alkoxyls leave the compound and combine with hydrogen from the OH bearing surface to form alcohols while the silicon is thus free to bind to the surface oxide and also polymerize with neighboring silicon atoms. In a similar fashion silicon halogens may also be employed with the halogen being displaced to permit silicon oxide covalent bonding. The resulting coating is a polysiloxane polymeric structure with viologens as a functional group on each silicon atom. Although silicon groups are suitable binding agents other agents, such as boron dihalides and, in particular, BCl.sub.2, may also be used.

We have also found that a wide range of alkyl and aromatic groups can be used to link the viologen center of our compounds to the binding groups. Alkyl chains of one or more carbon atoms as well as alkyl-aromatic combinations may be employed. The only limitation appears to be that the alkyl-linking group must be short enough to support electron charge transfer.

Our compounds have been used in a number of electrochromic display systems. The mechanism for the electrochromic behavior is as follows: The compound is coated onto an electrode which is immersed in a electrolytic solution together with a second, physically remote, electrode. A voltage is applied between the electrodes. Our compounds are reduced in two distinct and reversible, one-electron, transfer steps, first to the highly colored radical mono-cation and then to the neutral molecule.

Any polar solvent can serve as the solvent. The supporting electrolyte can be tetra alkyl ammonium halide [R.sub.4 N]X in nonaqueous solvents and alkaline halides such as LiCl NaCl or KBr can be used in aqueous solvents. In acetonitrile solvent we observe E.degree. values of -0.5.+-.0.05 and -0.9V.+-.0.05 (vs. a saturated caramel reference electrode) for the first and second reductions, respectively, for surface-confined polymer in Example I below. In other electrolytes such as propylene carbonate and methylene chloride, reversible charge transfer, spectral shifts for both reduction steps are observed. In water rapid reversible charge transfer, spectral shifts for the first reduction are observed.

In the preferred embodiment, our compounds are prepared as dication salts, with halides or other anions used to neutralize the compound. Two preferred salts are dichlorides and dibromides.

We have produced electrochromic coatings from our compounds up to 100 monomer layers thick while retaining the viologen's rapid electrochromic response (on the order of 10 milliseconds). Additionally the quality of the color exhibited has been steadfast; absorbion on the order of 90% has been observed in the visible region. These qualities have been maintained in samples tested over 20,000 redox cycles.

FIG. 1 is a graph of the cyclic voltammetry exhibited by one of our compounds in an electrolyte.

The invention is best described with reference to the following non-limiting examples:

The following reagent salt: ##STR1## was prepared by refluxing dry 4,4' bipyridine (Aldrich Chemical Co.) with 1-bromo-3-trimethoxysilylpropane (which was prepared by reacting HC(OMe).sub.3 with 1-bromo-3-trichlorosilylpropane purchased from Petrarch Chemical Co. in rigorously dry CH.sub.3 CN solution. Reagent I was isolated as a pale yellow solid di-bromide salt by crystallization from CH.sub.3 CN solution by adding Et.sub.2 0. UV-vis and .sup.1 H NMR spectroscopy and elemental analysis accords well with the structure shown.

The following reagent salt: ##STR2## was prepared in a similar fashion substituting 1-bromo-3-trichlorosilane for 1-bromo-3-trimethoxysilylpropane in Example 1.

The following reagent salt: ##STR3## was prepared in a similar fashion substituting 1-trimethoxy-2-(p,m-chloromethyl) phenylethane for 1-bromo-3-trimethoxysilylpropane in Example 1.

In FIG. 1 the electrochemical behavior of an electrode surface prepared from reaction with Reagent I of Example 1 is shown. The Reagent I was coated unto a platinum electrode by immersing the electrode in a .about.1 mM solution of the reagent salt in CH.sub.3 CN at 25.degree. C. for 3-8 hours. The measurements were taken in CH.sub.3 CN/0.1 M [n-Bu.sub.4 N]Br. The graph reveals clear, persistent and reversible cyclic voltammetric waves. The two waves observed correspond to the two, one-electron reductions of the reagent and are equal in area. The data were taken by starting at about 0 volts (right hand side of the graph) and sweeping negative. The first negative current peak corresponds to the 2+.fwdarw.1+ reduction (colorless to purple coloration) while the second peak corresponds to the second 1+.fwdarw.0 reduction (purple to yellow coloration). At about -1.2 volts the voltage sweep was reversed and the two, one-electron, oxidations steps induced. This electrode was cycled between oxidized and reduced forms thousands of times without deterioration.

Industrial Applications

Our invention may find application in many electronic display systems where low power consumption is required and a light absorbing rather than light emitting media can be employed. Coatings onto electrically conductive glasses are one such use.

Equivalents

It should be recognized that those skilled in the art may make various changes to our compounds without departing from the spirit and scope of our claims. For example, one may make various additions to the viologen center of our compounds, such as by adding methyl or other groups to the pyridine rings without affecting the usefulness or essential character of our compounds. Other modifications may include changes in the redox system itself.