Electrochromic system

Details Electrochromic system Electrochromic system

1999-01-12

2001-06-05

Wu, Shean C. (Department: 1756)

Compositions

Light transmission modifying compositions

Modification caused by energy other than light

C345S049000, C359S265000, C359S267000, C359S272000, C359S276000

Reexamination Certificate

active

06241916

ABSTRACT:

The present invention relates to an electrochromic system, an electrochromic liquid compriesing this electrochromic system, an electrochromic device and novel electrochromic substances and processes for their preparation.
Electrochromic devices which comprise an electrochromic system are already known.
Such devices usually comprise, as the electrochromic system, pairs of redox substances dissolved in an inert solvent. They can additionally comprise conductive salts, light stabilizers and substances which influence the viscosity.
A reducible substance and an oxidizable substance are used as the pair of redox. substances. Both are colorless or only slightly colored. Under the influence of an electrical voltage, one of the substances is reduced and the other is oxidized, at least one becoming colored. When the voltage is switched off, the two original redox substances are formed again, decoloration or a lightening in color occurring.
It is known from U.S. Pat. No. 4,902,108 that those pairs of redox substances in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least two chemically reversible oxidation waves are suitable.
Electrochromic devices can be used in various ways. First, for example, they can be constructed as automobile rearview mirrors which can be darkened by applying a voltage during nighttime driving, so that blinding by headlamps of other vehicles is prevented (cf, for example, U.S. Pat. No. 3,280,701, U.S. Pat. No. 4,902,108, EP-A-0.435.689). Such devices can furthermore also be employed in window panes or automobile sunroofs, where they dim sunlight when a voltage is applied. Finally, a matrix display for graphic illustration of information such as letters, numbers and symbols can also be built up with such devices.
Electrochromic devices usually comprise a pair of panes of glass or plastic, one of which is metallized in the case of an automobile mirror. One side of these panes is coated with a transparent, electrically conductive layer, for example indium tin oxide (ITO). A cell is now built up from these panes by joining them, preferably gluing them, by means of an annular or rectangular sealing ring with their sides with the electrically conductive coating facing one another. The sealing ring produces a uniform distance between the panes, for example 0.1 to 0.5 mm. This cell is now filled with an electrochromic solution via an opening, and the cell is closed tight. The two panes can be contacted separately via the ITO layer.
The electrochromic systems known from the prior art comprise those pairs of redox substances which, after reduction or oxidation, form colored free radicals, cationic radicals or ionic radicals which are chemically reactive. As is known, for example, from Topics in Current Chemistry, Volume 92, pages 1-44 (1980), such free radicals (radical ions) may be sensitive to electrophiles or nucleophiles or else free radicals. To achieve a high stability of an electrochromic device which comprises such an electrochromic system and which should withstand several thousand switching cycles, it must therefore be ensured that the solvent used is absolutely free from electrophiles, for example protons, nucleophiles and oxygen. It must furthermore be ensured that such reactive species do not form due to electrochemical processes at the electrodes during operation of the electrochromic device.
The reverse reaction formulated according to the above equation to give RED
1
and OX
2
also takes place continuously away from the electrodes within the solution volume during operation of the electrochromic device. Because of the risks of degradation reactions of the free radicals (radical ions) due to electrophiles, nucleophiles or free radicals, it is important for the long-term stability of the display that the reverse reaction according to the above equation can take place as quickly as possible and without side reactions.
It has been found that, by coupling RED
1
and OX
2
via a covalent chemical bond, electron transfer is facilitated and the reverse reaction according to the above equation can thus be accelerated and side reactions avoided.
The present invention accordingly relates to an electrochromic system comprising at least one oxidizable substance RED
1
and at least one reducible substance OX
2
which, by electron donation of the former at an anode and by electron acceptance of the latter at a cathode, in each case with an increase in the extinction in the visible range of the spectrum, are converted from a slightly colored or colorless form into a colored form OX
1
and RED
2
respectively, in each case the slightly colored or colorless form being reformed after charge compensation, wherein at least one of the substances RED
1
and OX
2
contained therein are linked together covalently via a bridge.
The reduction and oxidation processes in the electrochemical system according to the invention are in general effected by electron acceptance or donation at a cathode or anode, a potential difference of 0.3 to 3 V preferably prevailing between the electrodes. When the electrical potential is switched off, a charge compensation generally takes place spontaneously between the substances RED
2
and OX
1
, decoloration or a lightening in color occurring. Such a charge compensation also already takes place during the flow of current inside the electrolyte volume.
The electrochromic system according to the invention preferably comprises at least one electrochromic substance of the formula (I)
in which
Y and Z independently of one another represent a radical OX
2
or RED
1
, but in which at least one Y represents OX
2
and at least one Z represents RED
1
,
in which
0
X
2
represents the radical of a reversibly electrochemically reducible redox system and
RED
2
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, and preferably represent an integer from 0 to 3.
The electrochromic system preferably comprises at least one electrochromic substance of the formula (I) in which
Y represents OX
2
and Z represents RED
1
and Y and Z alternate in their sequence.
The electrochromic system according to the invention particularly preferably comprises at least one electrochromic substance of the formulae
in which
OX
2
, RED
1
and B have the abovementioned meaning and
d represents an integer from 1 to 5.
The electrochromic system according to the invention especially preferably comprises at least one electrochromic substance of the formulae (Ia)-(Id)
in which
OX
2
represents the radical of a cathodically reducible substance which, in the cyclic voltammogram recorded in an inert solvent at room temperature, shows at least two chemically reversible reduction waves, the first of these reduction waves leading to an increase in the extinction at not less than one wavelength in the visible range of the electromagnetic spectrum,
RED
1
represents the radical of the anodically reversibly oxidizable substance which, in the cyclic voltammogram recorded in an inert solvent at room temperature, shows at least two chemically reversible oxidation waves, the first of these oxidation waves leading to an increase in the extinction at not less than one wavelength in the visible range of the electromagnetic spectrum and
B represents a bridge member.
An electrochromic system according to the invention which is particularly preferred is one which comprises at least one substance of the formula (Ia)-(Id), in which
OX
2
represents a radical of the formulae
in which
R
2
to R
5
, R
8
, R
9
and R
16
to R
19
independently of one another denote C
1
- to C
18
-alkyl, C
2
- to C
12
-alkenyl, C
3
- to C
7
-cycloalkyl, C
7
- to C
15
-aralkyl or C
6
- to C
10
-aryl or
R
4
and R
5
or R
8
and R
9
together form a —(CH
2
)
2
— or —(CH
3
)— bridge,
R
6
, R
7
and R
22
to R
25
independently of one another denote hy

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