阅读说明：本技术 主体材料及包含其的有机电致发光器件 (Host material and organic electroluminescent device comprising same ) 是由 高春吉 赵晓宇 于 2021-09-15 设计创作，主要内容包括：本发明提供了主体材料和包含其的有机电致发光器件,该主体材料包含至少一种第一主体化合物和至少一种第二主体化合物,其中该第一主体化合物由下式1表示并且该第二主体化合物由下式2表示：本发明的有机电致发光器件具有优异的发光效率和寿命。(The present invention provides a host material and an organic electroluminescent device comprising the same, the host material comprising at least one first host compound and at least one second host compound, wherein the first host compound is represented by formula 1 below and the second host compound is represented by formula 2 below:)

1. A host material comprising at least one first host compound and at least one second host compound, wherein the first host compound is represented by formula 1 below and the second host compound is represented by formula 2 below:

wherein Ar is₁、Ar₂、Ar₃And Ar₄Each independently selected from substituted or unsubstituted C6-C30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

X₁、X₂、X₃and X₄Each independently selected from carbon or nitrogen;

Y₁and Y₂Each independently selected from NR₁Or oxygen, wherein R₁Is substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C6-30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

L₁and L₂Each independently selected from a single bond, a substituted or unsubstituted C6-C30 arylene group, or a substituted or unsubstituted C3-C30 heteroarylene group.

R₂To R₉Each independently represents hydrogen, deuterium, halogen, cyano, carboxyl, nitro, hydroxyl, substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C3-C30 cycloalkyl, substituted or unsubstituted C3-C30 cycloalkenyl, substituted or unsubstituted C3-C7 heterocycloalkyl, substituted or unsubstituted C6-C30 aryl, substituted or unsubstituted C3-C30 heteroaryl; or R₂To R₉May be connected to each other to form one or more rings, with the proviso that R is₂And R₃、R₃And R₄、R₄And R₅、R₅And R₆、R₆And R₇、R₇And R₈And R₈And R₉May be connected to each other to form one or more rings, wherein the ring has 1 to 5 monocyclic rings.

2. The host material of claim 1, wherein the first host compound is represented by any one of the following formulas 1-1 to 1-20:

wherein Ar is₁、Ar₂And Ar₃Each independently selected from substituted or unsubstituted C6-C30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

X₁is carbon or nitrogen;

R₁is substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C6-30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

L₁is a single bond, substituted or unsubstituted C6-C30 arylene or substituted or unsubstituted C3-C30 heteroarylene.

3. The host material of claim 2, wherein the first host compound is any one of:

4. the host material of claim 1, wherein the second host compound is represented by any one of the following formulas 2-1 to 2-5:

wherein Ar is₄Is substituted or unsubstituted C6-C30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

X₁is carbon or nitrogen;

R₁₀is substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C6-30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

L₂is a single bond, substituted or unsubstituted C6-C30 arylene or substituted or unsubstituted C3-C30 heteroarylene.

5. Host material according to claim 4, wherein the second host compound is any one of:

6. an organic electroluminescent device comprising an anode, a cathode and an organic functional layer, wherein the organic functional layer is at least one of a hole injection layer, a hole transport layer, a light emitting layer, a hole blocking layer, an electron transport layer and an electron injection layer, wherein the organic functional layer comprises the host material of any one of claims 1-5.

7. The organic electroluminescent device according to claim 6, wherein the organic functional layer is a light-emitting layer further comprising a dopant therein.

8. The organic electroluminescent device according to claim 7, wherein the mass ratio of the host material to the dopant is 10:1 to 100: 1.

9. The organic electroluminescent device according to claim 8, wherein the mass ratio of the first host compound to the second host compound in the host material is 1:9 to 9: 1.

Technical Field

The present invention relates to a host material, and more particularly, to a host material and an organic electroluminescent device including the same.

Background

With the development of multimedia technology and the increase of information-oriented requirements, the requirements for the performance of panel displays are increasing. The OLED has a series of advantages of self-luminescence, low-voltage direct current driving, full curing, wide viewing angle, rich colors and the like, and is widely noticed due to potential application in display and lighting technologies, and has a very wide application prospect. The organic electroluminescent device is a spontaneous light emitting device, and the OLED light emitting mechanism is that under the action of an external electric field, electrons and holes are respectively injected from a positive electrode and a negative electrode and then migrate, recombine and attenuate in an organic material to generate light. A typical structure of an OLED comprises one or more functional layers of a cathode layer, an anode layer, an electron injection layer, an electron transport layer, a hole blocking layer, a hole transport layer, a hole injection layer and an organic light emitting layer.

Although the research on organic electroluminescence is rapidly progressing, there are still many problems to be solved, such as the improvement of External Quantum Efficiency (EQE), the design and synthesis of new materials with higher color purity, the design and synthesis of new materials with high efficiency electron transport/hole blocking, and the like. For the organic electroluminescent device, the luminous quantum efficiency of the device is the comprehensive reflection of various factors and is an important index for measuring the quality of the device.

Luminescence can be divided into fluorescence and phosphorescence. In fluorescence emission, an organic molecule in a singlet excited state transits to a ground state, thereby emitting light. On the other hand, in phosphorescence, organic molecules in a triplet excited state transition to a ground state, thereby emitting light.

At present, some organic electroluminescent materials have excellent performance and certain application value, but as a host material in an organic electroluminescent device, the host material has good hole transport performance except that the triplet state energy level is higher than that of a guest material, and energy reverse transfer for exciton transition release is prevented. Currently, materials having both a high triplet level and good hole mobility in the host material are still lacking. Therefore, how to design a new host material with better performance is a problem to be solved by those skilled in the art.

Disclosure of Invention

In order to overcome the disadvantages of the prior art, it is an object of the present invention to provide a host material and an organic electroluminescent device including the same, which has excellent luminous efficiency and lifetime.

In order to realize the purpose of the invention, the technical scheme of the invention is as follows:

the present invention provides a host material comprising at least one first host compound and at least one second host compound, wherein the first host compound is represented by formula 1 below and the second host compound is represented by formula 2 below:

in the above structural formula, Ar₁、Ar₂、Ar₃And Ar₄Each independently selected from substituted or unsubstituted C6-C30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

X₁、X₂、X₃and X₄Each independently selected from carbon or nitrogen;

Y₁and Y₂Each independently selected from NR₁Or oxygen (O), wherein R₁Is substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C6-30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

L₁and L₂Each independently selected from a single bond, a substituted or unsubstituted C6-C30 arylene group, or a substituted or unsubstituted C3-C30 heteroarylene group.

R₂To R₉Each independently represents hydrogen, deuterium, halogen, cyano, carboxyl, nitro, hydroxyl, substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C3-C30 cycloalkyl, substituted or unsubstituted C3-C30 cycloalkenyl, substituted or unsubstituted C3-C7 heterocycloalkyl, substituted or unsubstituted C6-C30 aryl, substituted or unsubstituted C3-C30 heteroaryl; or R₂To R₉May be connected to each other to form one or more rings, with the proviso that R is₂And R₃、R₃And R₄、R₄And R₅、R₅And R₆、R₆And R₇、R₇And R₈And R₈And R₉At least one pair of them may be linked to each other to form one or more rings, wherein the ring has 1 to 5 monocyclic rings.

Preferably, the first host compound is represented by any one of the following formulae 1-1 to 1-20:

wherein Ar is₁、Ar₂And Ar₃Each independently selected from substituted or unsubstituted C6-C30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

X₁is carbon or nitrogen;

R₁is substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C6-30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

L₁is a single bond, a substituted or unsubstituted C6-C30 arylene group or a substituted or unsubstituted C3-C30 heteroarylene group; more preferably, the first host compound is any one of, but not limited to:

preferably, the second host compound is represented by any one of the following formulae 2-1 to 2-5:

wherein Ar is₄Is substituted or unsubstituted C6-C30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

X₁is carbon or nitrogen;

R₁₀is substituted or unsubstituted C1-C20 alkyl, substituted or unsubstituted C6-30 aryl or substituted or unsubstituted C3-C30 heteroaryl;

L₂is a single bond, a substituted or unsubstituted C6-C30 arylene group or a substituted or unsubstituted C3-C30 heteroarylene group;

more preferably, the second host compound is any one of, but not limited to:

the invention also provides an organic electroluminescent device which comprises an anode, a cathode and an organic functional layer, wherein the organic functional layer is at least one of a hole injection layer, a hole transport layer, a light emitting layer, a hole blocking layer, an electron transport layer and an electron injection layer, and the organic functional layer contains the host material.

Preferably, the organic functional layer is a light-emitting layer, and the light-emitting layer further contains a dopant.

More preferably, the mass ratio of the host material to the dopant is 10:1 to 100: 1.

More preferably, the mass ratio of the first host compound to the second host compound in the host material is 1:9 to 9: 1.

The invention also provides application of the main body material in an organic electroluminescent device.

Preferably, the organic electroluminescent device comprises an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer and a cathode deposited in sequence, and the organic compound is used as a host material of the light emitting layer.

Preferably, the organic light emitting device comprises an anode, a cathode and a plurality of organic functional layers located between the anode and the cathode, wherein the organic functional layers contain the one or more compounds.

Detailed Description

The organic electroluminescent device of the present invention preferably comprises an anode, a cathode and a plurality of organic functional layers located between the anode and the cathode. The term "organic functional layer" refers to all layers disposed between an anode and a cathode in an organic electroluminescent device, and the organic functional layer may be a layer having a hole property and a layer having an electron property, for example, the organic functional layer includes one or more of a hole injection layer, a hole transport layer, a hole injection and transport functional layer, an electron blocking layer, a light emitting layer, a hole blocking layer, an electron transport layer, an electron injection layer, and an electron transport and injection functional layer.

The hole injection layer, the hole transport layer, and the functional layer having both hole injection and hole transport properties of the present invention may include an electron-generating substance in addition to a conventional hole injection substance, a conventional hole transport substance, and a substance having both hole injection and hole transport properties. For example, the organic functional layer is an emissive layer, and the emissive layer includes one or more of phosphorescent hosts, fluorescent hosts, phosphorescent dopants, and fluorescent dopants. The compound for the organic electroluminescent device can be used as a fluorescent main body, can also be used as fluorescent doping, and can be used as the fluorescent main body and the fluorescent doping at the same time.

The light-emitting layer of the present invention may be a red, yellow or blue light-emitting layer. And when the luminescent layer is a red luminescent layer, the compound for the organic electroluminescent device is used as a red main body, so that the organic electroluminescent device with high efficiency, high resolution, high brightness and long service life can be obtained.

The organic electroluminescent device comprises an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, an electron injection layer and a cathode which are sequentially deposited, wherein the red phosphorescent compound is used as a host material of the light-emitting layer.

The dopant in the organic electroluminescent device of the present invention may be at least one phosphorescent or fluorescent dopant, preferably a phosphorescent dopant. The phosphorescent dopant material is not particularly limited, but may be preferably selected from a metallized complex compound of iridium (Ir), osmium (Os), copper (Cu), and platinum (Pt), more preferably an ortho-metallized complex compound selected from iridium (Ir), osmium (Os), copper (Cu), and platinum (Pt), and even more preferably an ortho-metallized iridium complex compound.

Specific examples of the dopant compound are as follows, but are not limited thereto.

In order to form each layer of the organic electroluminescent device of the present invention, a dry film forming method such as vacuum evaporation, sputtering, plasma and ion plating methods, or a wet film forming method such as inkjet printing, nozzle printing, slit coating, spin coating, dip coating, and flow coating methods may be used.

When a solvent is used in the wet film forming method, a thin film may be formed by dissolving or diffusing the material forming each layer into any suitable solvent (e.g., ethanol, chloroform, tetrahydrofuran, dioxane, etc.). The solvent may be any solvent in which a material forming each layer can be dissolved or diffused and in which there is no problem in film forming ability.

In addition, the compound represented by the formula I and the compound represented by the formula II may be subjected to film formation in the above-listed methods, usually by a co-evaporation method or a mixed evaporation method.

The present invention may provide a display system by including a plurality of host materials. In addition, it is possible to produce a display system or a lighting system by using the organic electroluminescent device of the present invention. In particular by using the various host materials of the invention it is possible to produce display systems, for example for smart phones, tablets, notebooks, PCs, TVs, or cars, or lighting systems, for example outdoor or indoor lighting systems.

The invention provides a host material, which comprises at least one first host compound and at least one second host compound, wherein an electron-rich heterocyclic chain structure in the host compound structure has great influence on the photoelectric properties of the whole compound molecule, and is beneficial to reducing unnecessary vibration energy loss, so that high-efficiency light emission is realized. By adjusting substituent groups, the complex has better thermal stability and chemical properties. In addition, the preparation method of the various main compounds is simple, the raw materials are easy to obtain, and the industrial requirements can be met.

The host compound disclosed by the invention is prepared into a device, and particularly used as a host material, the device has the advantages of low driving voltage, high luminous efficiency and the like, and is obviously superior to the conventional common OLED device.

Examples

The method for preparing the organic electroluminescent device of the present invention is not particularly limited, and other methods and materials for preparing the organic electroluminescent device known to those skilled in the art may be used in addition to the host materials represented by formula 1 and formula ii.

Example 1: synthesis of Compounds 1-10

1) Synthesis of intermediate 1-1-2

1-1-1(22.3g,81.3mmol), pinacol diboride (24.8g,97.5mmol), triphenylphosphine (1.6g, 6 mol%), bis (triphenylphosphine) palladium (II) dichloride (2.1g, 3 mol%), potassium phenoxide (16.1g,121.9mmol) and anhydrous toluene (300mL) were added to a three-necked flask under nitrogen protection. After the nitrogen substitution, the reaction was stirred at 50 ℃ for 5 hours, and then the system was cooled to room temperature and quenched by adding water. The reaction mixture was extracted with benzene solvent and saturated brine. The organic phase was dried over anhydrous magnesium sulfate. The dried mixture was filtered and concentrated under reduced pressure, and purification by column chromatography or distillation gave intermediate 1-1-2(20.4g, yield: 78%). LC-MS: M/Z321.15 (M +)

2) Synthesis of intermediates 1-1-3

After dissolving the compound 1-1-2(99.6g, 310mmol) in 1,4-dioxane (1,4-dioxane), methyl 2- (2-bromophenyl) acetate (73.3g, 320mmol), Pd (PPh) were added thereto₃)₄(18g, 15.5mmol) and K₂CO₃(128 g, 930mmol) and the resulting mass was stirred at 100 ℃ for 6 h. After the reaction was terminated, the resultant was cooled to room temperature and extracted with distilled water and ethyl acetate. The organic layer was MgSO₄Dried, and then filtered and concentrated. The concentrated residue was purified by column chromatography using ethyl acetate and hexane as developing agents to obtain the objective compound 1-1-3(68.3g, yield: 64%). LC-MS: M/Z344.12 (M +).

3) Synthesis of intermediates 1-1-4

Compound 1-1-3(17.2g, 50mmol) was added to a mixture of tetrahydrofuran and LTMP (tetramethyllithium piperidine) at 0 deg.C (the mixture was obtained by dissolving 27.2mL of LTMP in 250mL of tetrahydrofuran). After stirring for 8 hours, distilled water was added, and extraction was performed with ethyl acetate. Dried over magnesium sulfate and distilled under reduced pressure. Column chromatography gave 1-1-4(8.0g, yield: 51%) of the compound. LC-MS: M/Z312.09(M +).

4) Synthesis of intermediates 1-1-5

Under the protection of nitrogen, 1-1-4(15.6g, 50mmol), thionyl chloride (59.5g, 500mmol) and DMF (50g) are put into a three-necked flask, the temperature is slowly raised to 70 ℃, then the reaction is carried out for 3 hours, after confirming the follow-up reaction by TLC, the reaction product is decompressed, concentrated and most of the thionyl chloride is evaporated, and petroleum ether is added for pulping. The product was poured into 10L of ice water and filtered off with suction. The obtained solid was slurried with 5L of water and suction-filtered to obtain intermediate 1-1-5(9.8g, yield: 59%). LC-MS: M/Z330.06 (M +).

5) Synthesis of Compounds 1-10

Mixing compound 1-1-5(7.9g, 23.7mmol) and 2- ([1,1' -biphenyl)]-4-yl) -4-chloro-6- (4-phenylnaphthalen-1-yl) -1,3, 5-triazine (10.2g, 21.6mmol), tetrakis (triphenylphosphine) palladium (1.2g, 1.0mmol), potassium carbonate (7.5g, 59mmol), 90mL toluene, 30mL ethanol, and 30mL distilled water were added to a reaction vessel and the reaction was stirred at 120 ℃ for 4 hours. After completion of the reaction, methanol was added dropwise to the mixture, and the resulting solid was filtered. The obtained solid was purified by recrystallization through column chromatography to obtain compounds 1 to 10(8.3g, yield: 48%). LC-MS: M/Z729.25 (M)⁺)。

Example 2: synthesis of Compounds 1-39

Compounds 1 to 39 were synthesized by the method of reference example 1, and the synthesis of compounds 1 to 10 was referred to in all other steps to obtain compounds 1 to 39(8.8g, yield: 60%). LC-MS: M/Z617.19 (M +).

Example 3: synthesis of Compounds 2-35

Compounds 2 to 35 were synthesized by the method of reference example 1, and the other steps referred to the synthesis of compounds 1 to 10, to give compounds 2 to 35(10.4g, yield: 56%). LC-MS: M/Z779.27 (M +).

Example 4: synthesis of Compounds 3-145

Compounds 3 to 145 were synthesized by the method of reference example 1, and the other steps referred to the synthesis of compounds 1 to 10, to give compounds 3 to 145(10.5g, yield: 58%). LC-MS: M/Z766.28 (M +).

Organic electroluminescent device embodiment

1. First embodiment

The organic electroluminescent device was prepared as follows:

the ITO glass substrate was patterned to have a light-emitting area of 3mm × 3 mm. The patterned ITO glass substrate was then washed, and then placed in a vacuum chamber with a standard pressure set at 1X 10^-6And (4) supporting. Then, HI-1 is evaporated on the ITO substrate to form a film with a thickness ofAnd a first Hole Injection Layer (HIL) formed by evaporating HI-2 to a thickness ofAnd a second hole injection layer formed on the first hole injection layer by vapor deposition of HT-1 to a thickness ofThe Hole Transport Layer (HTL) of (2), and the EB-1 is continuously deposited on the hole transport layer to form a layer having a thickness ofThe Electron Blocking Layer (EBL) of (1), wherein the host material of the present invention (i.e., the compounds 4 to 28 and 1 to 10 of the present invention) and the guest compound (RD-1) are co-evaporated to a thickness ofWherein the compounds 4 to 28 and 1 to 10 of the present invention are evaporated at a rate of 2:3 and the mass ratio of the host material to RD-1 is 98:2, and finally HB-1 and ET-1 are sequentially used to form a layer having a thickness ofAnd a hole-blocking layer (HBl) having a thickness ofThen evaporating the cathode EI-1And AlThereby manufacturing an organic electroluminescent element.

2. Second embodiment

An organic light-emitting device of the second embodiment was fabricated by the same method as that of the first embodiment described above, except that the host material layer of the organic light-emitting device was replaced with compounds 1 to 39 instead of compounds 1 to 10 of the first embodiment.

3. Third embodiment

An organic light-emitting device of the third embodiment was fabricated by the same method as that of the first embodiment described above, except that the host material layer of the organic light-emitting device was replaced with compounds 2 to 35 instead of compounds 1 to 10 of the first embodiment.

4. Fourth embodiment

An organic light-emitting device of the fourth embodiment was fabricated by the same method as that of the first embodiment described above, except that the host material layer of the organic light-emitting device was replaced with compounds 3 to 145 instead of compounds 1 to 10 of the first embodiment.

5. Fifth embodiment

An organic light-emitting device of the fifth embodiment was fabricated by the same method as that of the first embodiment described above, except that the host material layer of the organic light-emitting device was replaced with compounds 4-36 from compounds 4-28 of the first embodiment.

6. Sixth embodiment

An organic light-emitting device of comparative example was fabricated by the same method as in the first embodiment described above, except that the host material layer of the organic light-emitting device was replaced with the compound 4-36/1-39 instead of the compound 4-28/1-10 of the first embodiment.

7. Seventh embodiment

An organic light-emitting device of comparative example was fabricated by the same method as in the first embodiment described above, except that the host material layer of the organic light-emitting device was replaced with the compound 4-36/2-35 instead of the compound 4-28/1-10 of the first embodiment.

8. Comparative example 1

An organic light-emitting device of a comparative example was prepared in the same manner as in the first embodiment described above, except that the host material layer of the organic light-emitting device was replaced with the compound Ref-1 instead of the compounds 1 to 10 of the first embodiment.

9. Comparative example 2

An organic light-emitting device of a comparative example was prepared in the same manner as in the first embodiment described above, except that the host material layer of the organic light-emitting device was replaced with the compound 4-36/Ref-1 instead of the compound 4-28/1-10 of the first embodiment.

The fabricated organic light emitting device was tested for voltage, efficiency and lifetime under a current condition of 10mA/cm 2.

Table 1 shows the performance test results of the organic light emitting devices prepared in the examples of the present invention and the comparative examples.

TABLE 1

As shown in table 1, the organic electroluminescent device comprising the specific compound combination according to the present invention as a host material has higher luminous efficiency and longer life characteristics than the comparative organic electroluminescent device.

10. Eighth embodiment

Manufacturing of organic electroluminescent device:

the ITO glass substrate was patterned to have a light-emitting area of 3mm × 3 mm. The patterned ITO glass substrate was then washed, and then the substrate was mounted in a vacuum chamber with a standard pressure set at 1 x 10-6 torr. Then, HI-1 is evaporated on the ITO substrate to form a film with a thickness ofAnd a first Hole Injection Layer (HIL) formed by evaporating HI-2 to a thickness ofAnd a second hole injection layer formed on the first hole injection layer by vapor deposition of HT-1 to a thickness ofThe Hole Transport Layer (HTL) of (2), and the EB-1 is continuously deposited on the hole transport layer to form a layer having a thickness ofThe Electron Blocking Layer (EBL) of (1), wherein the host material of the present invention (i.e., the compounds 4 to 28 and 1 to 10 of the present invention) and the guest compound (RD-1) are co-evaporated to a thickness ofWherein the compounds 4 to 28 and 1 to 10 of the present invention are evaporated at a rate of 2:3 (mass ratio) and the mass ratio of the host material to RD-1 is 98:2, and finally 1 to 10 and ET-1 are sequentially formed to a thickness ofHas a Hole Blocking Layer (HBL) and a thickness ofThen evaporating the cathode EI-1And AlThereby manufacturing an organic electroluminescent device.

11. Ninth embodiment

An organic electroluminescent device of the ninth embodiment was fabricated in the same manner as the above-described eighth embodiment, except that the hole-blocking layer was replaced with compounds 2 to 35 from compounds 1 to 10 of the eighth embodiment.

12. Comparative example 3

An organic electroluminescent device of the ninth embodiment was fabricated in the same manner as the above-described eighth embodiment, except that the hole-blocking layer was replaced with the compound Ref-2 from the compounds 1 to 10 of the eighth embodiment.

Table 2 shows the performance test results of the organic electroluminescent devices prepared in the examples and comparative examples of the present invention.

TABLE 2

As shown in table 2, the organic electroluminescent device comprising the specific compound combination according to the present invention as a hole blocking layer had higher luminous efficiency and longer life characteristics than the comparative organic electroluminescent device.

The foregoing has described the general principles, principal features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are given by way of illustration of the principles of the present invention, and that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is defined by the appended claims and equivalents thereof.