Active solid-state devices (e.g. – transistors – solid-state diode – Organic semiconductor material
Reexamination Certificate
1999-02-12
2001-02-06
McKane, Joseph K. (Department: 1626)
Active solid-state devices (e.g., transistors, solid-state diode
Organic semiconductor material
C428S419000, C549S060000, C549S059000
Reexamination Certificate
active
06184540
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of preparing a thiophene-containing or furan-containing conjugated compound, whereby a soluble precursor compound, comprising a precursor unit having thermally separable substituents, is heated such that said thermally separable substituents are eliminated from said precursor compound while converting said precursor compound into said thiophene-containing or furan-containing conjugated compound. The invention also relates to precursor compounds suitable for use in such a method. The invention further relates to a semiconductor device in which use is made of a conjugated compound obtainable by using such a method.
BACKGROUND OF THE INVENTION
Conjugated compounds, or more in particular, thiophene-containing or furan-containing conjugated compounds are used in various industrial applications. For example, they can be used as dyes or pigments, as (semi)conductors, (electro)luminescent material or in electr(on)ical, optical and electro-optical devices such as light emitting diodes, field-effect transistors, solar cells, polarizing optical elements and batteries. In the context of the invention a compound is considered to be conjugated if, upon electronic excitation, it absorbs ultraviolet light or radiation of lower frequencies.
However, due to the inherent rigidity of the conjugated system, many potentially interesting conjugated compounds are insoluble. For many industrial applications processability from solution and therefore solubility is an essential requirement if an economically viable process is to be obtained.
To enhance the solubility of a conjugated compound it has been proposed to include solubilizing substituents such as large alkyl or alkoxy groups. However, this has the undesired side-effect that properties other than the solubility, such as the charge-carrier mobility, are adversely affected as well.
In order to enhance the processability from solution, it has also been proposed to use a method of the type mentioned in the opening paragraph. In such a method, referred to as a precursor method for short, the processing, such as, for example, the formation of a layer by spincoating, is done using the precursor compound. As a final step, the precursor compound is (thermally) converted by separating the separable substituents from the remainder of the compound thus forming the conjugated compound. Only a few conjugated compounds have been prepared by a method which uses such precursor compounds. One example is poly(2,5-thienylenevinylene) as disclosed in a publication by Kwan-Yue Jen et al. in J. Chem. Soc., Chem. Comm., 1987, p309. Since the precursor method is a very attractive method of rendering conjugated compounds processable from solution, there still exists a need for methods of preparing a conjugated compound using precursor compounds.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a method of preparing a thiophene-containing or furan-containing conjugated compound from a precursor compound which is processable from solution and thermally convertible to said thiophene-containing or furan-containing conjugated compound.
The object of the invention is achieved by a method of the type mentioned in the opening paragraph which, according to the invention, is characterized in that the precursor unit used is a tetrahydrothiophene or tetrahydrofuran unit having thermally separable substituents —SR
1
and —SR
2
, wherein R
1
and R
2
are independently selected as an all or aryl group. In the method according to the invention use is made of thermally convertible precursor compounds having precursor units which, after elimination of the thermally separable substituents by heating to an adequate temperature of, typically 200° C., are converted into thiophene or furan heterocycles. The presence of the precursor units enhances the solubility of the precursor compound of which they are part relative to the conjugated compound which contains the corresponding thiophene or furan units. The precursor units can be suitably used in any method of preparing a thiophene or furan-containing compound but preferably they are used to prepare conjugated compounds. Precursor units which can be converted into thiophene or furan units were hitherto unavailable and allow conjugated compounds which are known to be rather intractable, such as unsubstituted polythiophene, to be processed as though they are soluble.
The improved processability is most advantageously exploited in conjunction with a substrate. A preferred embodiment of the method according the invention is therefore characterized in that, before the precursor compound is heated, a solution comprising the precursor compound is prepared and provided onto a substrate. Suitably, the substrate is made, for example, of glass, quartz, silicon or a synthetic resin. The conjugated compound can be processed in the form of a layer by any conventional method which uses a solvent, for example, by spincoating a solution of the precursor compound onto the substrate.
Heating is suitably performed by conventional means such as an oven or a hot plate. Though not essential, heating may be performed under reduced pressure or under an inert atmosphere.
The precursor compound used in the method according to the invention contains precursor units derived from the heterocycles tetrahydrothiophene or tetrahydrofuran. Since in most thiophene-containing or furan-containing compounds the thiophene or furan ring is substituted at both the 2 and 5 position, use is made in particular of precursor units which are substituted correspondingly by non-separable substituents.
In addition to said substituents, the precursor unit has thermally separable substituents. By heating or, depending on the type of separable substituent, any other suitable method such as irradiation, these substituents are separated from the precursor unit. The separation takes place by way of a &bgr; elimination reaction which involves two groups on adjacent atoms, one group of which is the thermally separable group. The reaction produces a double bond on the heterocycle, thus converting the tetrahydrothiophene or tetrahydrofuran into the corresponding dihydro heterocycle. Two thermally separable substituents suffice to convert a tetrahydrothiophene (tetrahydrofuran) into its thiophene (furan) analog. The first of said two thermally separable substituents can be located at the 2 position, but from a synthetic point of view the 3 position is preferred. Analogously, the second thermally separable substituent is located at the 4 position.
Examples of-thermally separable substituents are sulfone, sulfoxide, alkoxy, aryloxy, such as phenoxy, amino, —NR
3
+
, or —SR
2
+
groups. Suitable precursor units are tetrahydrothiophene or tetrahydrofuran units having thermally separable substituents —SR
1
and —SR
2
, wherein R
1
and R
2
are independently selected as an alkyl or aryl group.
Preferably, a precursor unit is used according to the formula (I)
wherein X is equal to O or S, and R
1
and R
2
independently selected as an alkyl or aryl group. By heating to a temperature of, typically, 200° C. or higher, the thermally separable groups —SR
1
and —SR
2
are eliminated thus forming the sulfides R
5
SR
1
and R
6
SR
2
and a conjugated unit in the form of a thiophene (X=S) or furan (X=O) unit having substituents R
3
and R
4
located at the positions 3 and 4 respectively. The substituents R
3
and R
4
can be varied so as to obtain a range of conjugated compounds which can be rendered processable from solution. Since the tetrahydro unit itself is not conjugated, which has a solubility enhancing effect, and the substituents R
1
, R
2
, R
5
, and R
6
enhance the solubility as well, the choice of substituents R
3
and R
4
is not limited by solubility considerations. Suitable choices are for example methyl, ethyl, (m)ethoxy, nitro or hydrogen.
The substituents R
1
, R
2
, R
5
, and R
6
are not part of the conjugated compound which is obtained after thermal conversion and can, for examp
Brown Adam R.
Chmii Knut H.
De Leeuw Dagobert M.
Havinga Edsko E.
Hoeve Wolter Ten
Bartlett Ernestine C.
McKane Joseph K.
Solola Taofiq A.
U.S. Philips Corporation
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