Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-05-03
2004-11-30
Dye, Rena (Department: 1774)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S917000, C313S503000, C313S504000, C313S506000, C252S301160, C558S411000, C564S305000
Reexamination Certificate
active
06824890
ABSTRACT:
BACKGROUND
The notion that &pgr;-conjugated organic materials should possess interesting and useful electronic properties analogous to conventional inorganic semiconductors such as silicon or gallium arsenide is now well established in the scientific literature.
1,2
Many of these expectations have been realized with the practical demonstration of electronic devices in which conjugated organic materials are responsible for charge transport and/or light generation. Examples include polymer or small molecule light-emitting diodes (LEDs)
3,4,5
, photovoltaic devices
6
and field-effect transistors.
7.8
The prospect of realizing electrically driven organic lasers is also under intense investigation.
9
Much of the motivation for studying organic materials stems from the potential to tailor desirable optoelectronic properties and processing characteristics by manipulation of the primary chemical structure. Strategies for raising or lowering the highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) levels include conjugation length control, as well as the introduction of electron donating or withdrawing groups to the parent chromophore.
10
Regulating the HOMO and LUMO energy levels permits fine tuning of charge injection properties. In emissive devices, the HOMO/LUMO energy difference directly controls emission frequency. Organic materials also offer the opportunity to adjust optical properties by taking advantage of processes unique to the excited state, i.e. excimer and exciplex formation.
11
Several classes of luminescent polymers have been disclosed in the art heretofore. These include, for example, poly[1,4-phenylene vinylene], PPV
12
, soluble derivatives of PPV, such as MEH-PPV
13
, Aryl-substituted-PPV
14
, and PPV copolymers
15
. Soluble derivatives of polythiophene are also known in the art, for example the poly(3-alkylthiophenes)
16
. The photoluminescent spectra of these polymers typically fall in the visible spectral region with colors ranging from green to red. Considerable progress has been made toward using these materials in light emitting displays with lifetimes sufficient for commercial products
17
.
Low molar mass organic molecules can also be used for electroluminescent (EL) applications
18
. Disadvantages of these materials include their propensity for crystallization and difficulties in obtaining films by solution processing.
It is generally appreciated that the morphology of organic films plays a fundamental role in defining the functional characteristics of the material. However, studies that clearly relate electroluminescence and charge transport properties with molecular morphology remain scarce. The tendency of many small molecules to spontaneously crystallize
19
presents a limitation for LED applications because crystal formation destroys film homogeneity and crystal boundaries raise the resistance of the sample, eventually leading to electric shorting.
20
The thermal stability of amorphous molecular solids as measured by the glass transition temperature has been shown to directly correlate with electroluminescence stability.
21
It has been argued that in some cases thermal cycling of an organic LED heterostructure device above the glass transition temperature causes degradation resulting from disruption of the organic-organic interface rather than crystallization.
22
It is proposed based on X-ray specular reflectivity data that large thermal expansion of one of the components associated with its glass transition causes catastrophic strain release at the hetero-interface between materials.
For transistor applications proper alignment of chromophores is desired because it enhances charge transport.
23
On the other hand, in the case of polymer LEDs, ordered regions result in strong interchain coupling and lower emission quantum yields.
24
Despite the obvious need to control the final arrangement of individual molecules in the bulk a priori, a detailed understanding of the relationship between chemical structure of a given organic material with the resulting morphology is still lacking.
Strategies to minimize interchain contacts in conjugated polymers have generally implemented the use of bulky sidegroups on the polymer backbone. These attachments improve solubility by limiting interchain contacts but are typically aliphatic in nature and therefore limit charge injection and migration across the solid sample.
25
In response to these limitations, considerable efforts have been dedicated to developing molecules of intermediate molecular weight that minimize the aliphatic content and at the same time resist crystallization. Molecular shape is an important parameter in these efforts. Some of the recent strategies rely on creating molecular shapes that, from an intuitive perspective, can be considered “awkward” to packing. Examples include “starburst”, dendritic, tetrahedral, and spiro shaped molecules, as shown below.
26
Solution processing methods can be employed with these materials to yield kinetically trapped, amorphous solids that resist crystallization. Furthermore, these materials show elevated glass transition temperatures despite their modest molecular dimensions. Molecules such as the cues above embody all of the beneficial properties of small molecules, namely purity and well-defined structure, combined with the ability to cast thermally robust films directly from solution, a property characteristic of polymeric materials.
The use of semiconducting (conjugated) polymers and oligomers as EL materials in light emitting displays offer a number of advantages, including high brightness at low operating voltage, low weight, thin profile and low power consumption over conventional display elements such as incandescent lamps and liquid crystal displays. The relatively simple processing enabled by the use of soluble semiconducting polymers provides a pathway to low cost, high volume fabrication.
Accordingly, there is intense interest in developing improved organic EL materials of intermediate dimensions with topological attributes that discourage crystallization. Few guidelines are available for this purpose and new general structures that combine a preference to form useful films with the electrooptical requirements for EL are needed for the fabrication of more efficient LEDs.
SUMMARY OF THE INVENTION
The present invention is directed to a novel class of tetrahedral compounds that satisfy the need for improved organic materials that are luminescent, electroluminescent, and emit visible light with high photoluminescence efficiency. Moreover these materials are thermally stable, resist crystallization, soluble in common organic solvents and can be cast into films for use in electroluminescent devices.
The soluble tetrahedral compounds of the present invention are of general formula (I), where R1, R2, R3 and R4 are optoelectronic arms, generally comprised of organic electrochromophores, which are similar or independent of each other, and TS is a tetrahedral junction unit.
The tetrahedral junction unit, TS, includes but is not limited to tetraphenylmethane, tetraphenylsilane, sp
3
hybridized C or Si atoms, tetraphenyladamantane, adamantane and cubane.
R1, R2, R3 and R4 can be different or the same and are optoelectronic arms corresponding to conjugated monomers, oligomers, polymers, copolymers or other organic electrochromophores that are used in EL applications.
Particular preference is given to R1, R2, R3 and R4 corresponding to general formulas (II) or (Ill):
wherein the symbols have the following meaning:
R=H;
R′=alkoxy, alkyl aryl, aryloxy, cyano, halide, amido or other analogous functionalities that influence morphology and electrooptical properties; and
n=1−100
Preferred versions of general formula (II) have n values greater than 2.
The present invention also includes a method of making a tetrahedral compound having one or more organic chromophore arms attached to a tetrahedral junction site. This method includes the steps of providing a tetrahedral junction molecule
Bazan Guillermo C.
Robinson Matthew R.
Wang Shujun
Dye Rena
Fulbright & Jaworski
The Regents of the University of California
Thompson Camie S
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