Electrochromic system

Details Electrochromic system Electrochromic system

1999-05-25

2001-02-06

Lorin, Francis J. (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

Intermediate article

C359S265000, C252S301350

Reexamination Certificate

active

06183878

ABSTRACT:

The present invention relates to an electrochromic system, an electrochromic fluid comprising this electrochromic system, and an electrochromic device.
Electrochromic devices which contain an electrochromic system are already known.
Such devices customarily contain, as the electrochromic system, redox couples which are dissolved in an inert solvent. Additionally, conducting salts, light stabilizers and substances which affect the viscosity may be included.
Used as a redox couple is one reducible and one oxidizable substance each. Both are colorless or only weakly colored. Under the influence of an electric voltage, one of the substances is reduced and the other is oxidized, at least one becoming colored in the process. After the voltage is switched off, the two original redox substances are formed once more, accompanied by bleaching or fading of the color.
U.S. Pat. No. 4,902,108 discloses that those redox couples are suitable in which the reducible substance exhibits at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly exhibits at least two chemically reversible oxidation waves.
Electrochromic devices can be used in many different ways. For example, they may take the form of an automobile rearview mirror which during a journey at night can be darkened by a voltage being applied and thus prevents dazzling by the headlights of other vehicles (compare e.g. U.S. Pat. No. 3,280,701, U.S. Pat. No. 4,902,108, EP-A-0,435,689). Such devices may further also be employed in window panes or automobile canopies where, after a voltage has been applied, they black out the sunlight. Finally, such devices may also be used to build up a matrix display for graphic representation of information such as letters, numbers and symbols.
Electrochromic devices normally comprise a pair of glass or plastic panes, one of which being mirrored in the case of an automobile mirror. One side of these panes is coated with a transparent, electroconductive layer, e.g. indium tin oxide (ITO). These panes are then used to construct a cell by their electroconductively coated sides which face one another being bonded, preferably cemented, to an annular or rectangular gasket. The gasket establishes a uniform spacing between the panes, for example from 0.1 to 0.5 mm. This cell is then, via a port, filled with an electrochromic solution and is tightly sealed. Via the ITO layer contact can be made separately with the two panes.
The prior art electrochromic systems comprise redox couples which, after reduction and oxidation, form colored free radicals, cationic free radicals or anionic free radicals, which are chemically reactive. As disclosed, for example, by Topics in Current Chemistry, Vol. 92, pp. 1-44 (1980), such (ionic) free radicals may be sensitive with respect to electrophiles or nucleophiles or even free radicals. To achieve a high stability of an electrochromic device containing such an electrochromic system which is to survive several thousands of operating cycles it is therefore necessary to ensure that the solvent used is absolutely free from electrophiles, e.g. protons, nucleophiles and oxygen. It is also necessary to ensure that such reactive species are not formed by electrochemical processes on the electrodes during operation of the electrochromic device.
The reverse reaction to RED
1
and OX
2
formulated according to the above reaction equation also takes place continuously away from the electrodes within the volume of the solution while the electrochromic device is operated. Because of the described hazards of degradation reactions of the free radicals or (ionic) free radicals by electrophiles, nucleophiles or free radicals it is important for the long-term stability of the display that the reverse reaction in accordance with the above reaction equation can take place as rapidly as possible and without side reactions.
We have found that in an electrochromic system which comprises at least one Red-Ox couple linked covalently with one another via a bridge and further at least one electrochromic substance which after reduction or oxidation is not converted into an (ionic) free radical species but has a closed electron shell in the reduced or oxidized form, the electron transfer is facilitated and side reactions can be avoided.
The present invention therefore relates to an electrochromic system comprising oxidizable substances RED
1
which, by releasing electrons at an anode, are converted from a weakly colored or colorless form into a colored form OX
1
, and reducible substances OX
2
which, by accepting electrons at a cathode, are converted from a weakly colored or colorless form into a colored form RED
2
, the absorbance in the visible region of the spectrum increasing in each of these cases, and the weakly colored or colorless form being recovered in each case after charge equalization, wherein the substances RED
1
and OX
2
are a mixture comprising at least one couple RED
1
′/OX
2
′ which is linked covalently to one another via a bridge and at least one substance RED
1
″ or OX
2
″ in which the reciprocal conversion of oxidized and reducible form takes place by a &sgr; bond being broken or formed.
The reduction and oxidation processes in the electrochromic system according to the invention generally take place by electrons being accepted or released at a cathode or anode, respectively, a potential difference of from 0.3 to 3 V preferably obtaining between the electrodes. After the electric potential is switched off, charge equalization generally takes place spontaneously between the substances RED
2
′ and OX
1
′, RED
2
″ and OX
1
′ or RED
2
′ and OX
1
″, respectively, accompanied by bleaching or fading of the color. Such a charge equalization also takes place even while the current is flowing in the interior of the electrolyte volume.
The novel electrochromic system preferably comprises
a) as the couple RED
1
′/OX
2
′ at least one electrochromic substance according to the formula (I)
Y&Brketopenst;&Parenopenst;B-Z&Parenclosest;
a
&Parenopenst;B—Y&Parenclosest;
b
&Brketclosest;
c
B-Z  (I),
in which
Y and Z independently of one another represent a radical OX
2
′ or RED
1
,′but where at least one Y represents OX
2
′ and at least one Z represents RED
1
′,
where
OX
2
′ represents the radical of a reversibly electrochemically reducible redox system, and
RED
1
′ represents the radical of a reversibly electrochemically oxidizable redox system,
B represents a bridge member,
c represents an integer from 0 to 5, and
a and b independently of one another represent an integer from 0 to 5, preferably an integer from 0 to 3 and
b) at least one reducible substance OX
2
″ which represents a cyclic organic compound which, after accepting 1 or 2 electrons, is converted, one of the &sgr; bonds of the ring being broken, into an open-ring compound and which, by releasing 1 or 2 electrons, is converted once more into the cyclic starting compound, in each case precisely two electrons being transferred overall.
The ring opening may take place, for example, by two electrons being accepted directly, resulting in breaking of the bond. Alternatively it is possible for one electron to be accepted initially, whereupon ring opening takes place which may then be followed by further reactions such as a further electron and/or a proton being accepted and/or an anionic leaving group being eliminated, precisely 2 electrons being accepted overall, however. This ring opening results in an altered &pgr; electron system which has a corresponding higher-wavelength and/or higher-absorbance light absorption. Ring closure may correspondingly take place by simultaneous release of two electrons or successively by release of first one electron and possibly a further electron, and/or the proton which may have been bound is eliminated again and/or the eliminated anionic leaving group is bound once more, precisely 2 electrons being released overall, however

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