UV-protected electrochromic solution

Details UV-protected electrochromic solution UV-protected electrochromic solution

2002-01-03

2003-04-08

Schwartz, Jordan M. (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S242000, C359S245000, C359S267000, C359S275000

Reexamination Certificate

active

06545793

ABSTRACT:

The present invention relates to a UV-protected electrochromic solution, to its use in an electrochromic device and to an electrochromic device comprising this solution.
Electrochromic devices comprising an electrochromic system are already known.
The electrochromic system of such devices customarily includes pairs of redox substances—redox couples—dissolved in an inert solvent. Additionally, conductive salts, light stabilizers and substances which influence the viscosity may be present
The redox couple used comprises one reducible and one oxidizable substance each. Both are colourless or have only a weak coloration. Under the influence of an electrical voltage the one substance is reduced and the other oxidized, with at least one becoming coloured in the process. After the voltage is switched off, the two original redox substances are formed once more, which is accompanied by the disappearance or fading of the colour.
RED
1
+OX
2
{right arrow over (←)}OX
1
+RED
2
(colourless) (coloured)
(low-energy couple) (high-energy couple)
U.S. Pat. No. 4,902,108 discloses that suitable such redox couples are those in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substances, correspondingly, has at least two chemically reversible oxidation waves.
Electrochromic devices can find multifarious applications. For example, they may take the form of a car rearview mirror which when travelling at night can be darkened by applying a voltage, thus preventing the driver being dazzled by headlights of other vehicles (cf. e.g. U.S. Pat. No. 3,280,701, U.S. Pat. No. 4,902,108, EP-A-0 435 689). Such devices may also be employed in window panes or car sun-roofs where, following the application of a voltage, they provide shade from the sunlight. Finally, it is possible to use such devices to construct a display device for the graphic representation of information in the form of letters, numbers and symbols.
Electrochromic devices normally consist of a pair of glass or plastic plates, one being mirrored in the case of a car mirror. One side of these plates is coated with a transparent, electroconductive layer, e.g. indium tin oxide (ITO). These plates are then used to construct a cell; to this end their facing, electroconductively coated side is attached, preferably by means of adhesive bonding, to an annular or rectangular sealing ring. The sealing ring establishes a uniform distance between the plates of, for example, from 0.1 to 0.5 mm. This cell is then filled, via an aperture, with an electrochromic solution and then tightly sealed. By way of the ITO layer it is possible to contact the two plates separately.
The electrochromic systems known from the prior art comprise redox couples which following the reduction and oxidation, respectively, form coloured free radicals, cationic free radicals or anionic free radicals that are chemically reactive. As known, for example, from Topics in Current Chemistry, Vol. 92, pp. 1-44 (1980) such (ionic) free radicals may be sensitive to electrophiles or nucleophiles or else to free radicals. In order, therefore, to achieve a high level of stability in an electrochromic device comprising an electrochromic system of this kind—a system which is intended to withstand several thousand switching cycles—it is necessary to ensure that the solvent used is absolutely free from electrophiles, e.g. protons, nucleophiles and oxygen. It must also be ensured that such reactive species are not formed by electrochemical processes taking place at the electrodes during operation of the electrochromic device.
The back-reaction to RED
1
and OX
2
that is formulated in the above equation also takes place continuously away from the electrodes within the volume of the solution while the electrochromic device is in operation. Owing to the above-described hazards of degradation reactions of the (ionic) free radicals by electrophiles, nucleophiles or free radicals it is important, for the long-term stability of the display, that the back-reaction in accordance with the above equation is able to take place as rapidly as possible and without side reactions.
Electrochromic devices of this kind generally exhibit sensitivity to light, especially UV light. Consequently, electrochromic devices have been disclosed which comprise UV stabilizers, for example in U.S. Pat. No. 5,280,380.
It has now been found that by coupling RED
1
and OX
2
by way of a covalent chemical bond the electron transfer is facilitated and thus the back-reaction indicated in the above equation can be accelerated and side reactions avoided.
It has likewise been found that electrochromic solutions comprising such RED
1
and OX
2
bridged via a covalent chemical bond can be effectively protected by particular UV absorbers against destruction by UV light.
The present invention accordingly relates to an electrochromic solution comprising at least one oxidizable substance RED
1
which releases electrons at an anode, and at least one reducible substance OX
2
which accepts electrons at a cathode and in so doing undergo transition from a weakly coloured or colourless form into a coloured form OX
1
and RED
2
, respectively, accompanied by an increase in the absorbance in the visible region of the spectrum, the weakly coloured or colourless form being restored after charge equalization, characterized in that at least one of the substances RED
1
and OX
2
that are present are linked covalently to one another via a bridge and in that there is at least one UV absorber selected from the classes of the unsubstituted and substituted cinnamic esters and of the unsubstituted and substituted 2-hydroxybenzophenones. At least one of the oxidation- or reduction-induced transitions RED
1
{right arrow over (←)}OX
1
or OX
2
{right arrow over (←)}RED
2
, respectively, is associated with an increase in the absorbance in the visible region of the spectrum.
The reduction and oxidation processes in the electrochromic system of 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 electrical potential has been switched off, charge equalization takes place—in general spontaneously—between the substances RED
2
and OX
1
, accompanied by disappearance or fading of the colour. Such charge equalization also takes place even while the current is flowing in the interior of the electrolyte volume.
The electrochromic solution of the invention preferably comprises at least one electrochromic substance of 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
, subject to the proviso that 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
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 at least one UV absorber selected from the classes of the unsubstituted and substituted cinnamic esters and of the unsubstituted and substituted 2-hydroxybenzophenones.
The electrochromic solution 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
With particular preference, the electrochromic system of the invention comprises at least one electrochromic substance of the formula
OX
2
—B—RED
1
  (Ia),
OX
2
—B—RED
1
—B—OX
2
  (Ib),
RED
1
—B—OX
2
—B—RED
1
  (Ic), or
OX
2
—(B—RED
1
—B—OX
2
)
d
—B—RED
1
  (Id),
in which
OX
2
, RED
1
and B have the meaning indicated above and
d represent

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