JP2006013182A - Organic transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture an element through easy processes and to provide an organic thin film transistor using an organic semiconductor having high transistor characteristics. <P>SOLUTION: The transistor has the organic semiconductor layer. The organic semiconductor layer contains a gallium complex shown by a general formula (1), (L<SP>1</SP>)nGa(L<SP>2</SP>)m, where L<SP>1</SP>is a ligand expressed by a formula (2), and L<SP>2</SP>is a ligand shown by a halogen atom, -OR<SP>1</SP>(R<SP>1</SP>: hydrogen atom, etc.), or -O-Ga-(L<SP>1</SP>); and (n) is 2 or 3, but when (n) is 2, (m) is 1 or when (n) is 3, (m) is 0. In the formula (2), Rings A<SP>1</SP>and A<SP>2</SP>represent ring systems which may have substituents and are mutually condensed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an organic transistor, and more particularly to a field effect transistor using a thin film formed of an organic semiconductor capable of forming a thin film by a simple process.

  Field effect transistors are widely used as important switches and amplifying elements along with bipolar transistors. Until now, elements using silicon have been widely used. A field effect transistor has a structure in which a gate electrode is provided on a semiconductor material via a source and drain electrodes and an insulator layer. Basically, one of the p-type and n-type carriers transports a charge and is a typical monopolar device.

In particular, a material using silicon dioxide (SiO ₂ ) called a MOS (Metal-Oxide-Semiconductor) structure as an insulator is widely used for memory elements, gate elements, and the like. In addition, those using amorphous silicon as a semiconductor can be manufactured as thin film transistors by forming a semiconductor layer on a substrate such as glass, and this is widely used in active matrix liquid crystal displays that are used as switching elements in each cell. Has been.

  The operational characteristics of such a field effect transistor are as follows: carrier mobility μ, electrical conductivity σ of the semiconductor used, capacitance Ci of the insulating layer, element configuration (source-drain electrode distance L and width W, insulating layer) Of these, semiconductor materials having high mobility (μ) exhibit good characteristics.

  Among semiconductor materials, inorganic semiconductors typified by Si need to be processed at a high temperature of 300 ° C. or higher at the time of production, and it is difficult to use a plastic substrate or film for the substrate, and a lot of energy is required for production. . In addition, the device manufacturing process has to repeat a vacuum-type manufacturing process many times, and the apparatus cost and running cost are extremely enormous.

  On the other hand, in recent years, organic semiconductor materials have been energetically studied as organic compounds having high charge transport properties. These compounds are not only charge transport materials for organic EL devices, but also organic laser oscillation devices such as those discussed in “Science” (2000, 289, 599), It is expected to be applied to organic thin-film transistors that have been reported in many papers such as “Nature” (2000, 403, 521). Transistors using organic semiconductors have the potential to obtain semiconductors that can be dissolved by simplifying the manufacturing process by vacuum or low-pressure deposition at relatively low temperatures, and by appropriately improving the molecular structure. Conceivable. Furthermore, by using an organic semiconductor solution as an ink, manufacturing by a printing method including an ink jet method is also conceivable, and a large-area element can be manufactured at low cost. Manufacturing using these low-temperature processes has been considered impossible for conventional Si-based semiconductor materials, but there are possibilities for devices using organic semiconductors, and plastic substrates and films can be used as substrates. An element that is light and difficult to break can be produced. Furthermore, since there are a wide variety of materials, and it is possible to easily change the material properties easily by changing the molecular structure, functions and elements that are impossible with inorganic semiconductors by combining different functions Can also be realized.

Conventionally, gallium complexes have been actively studied for application as charge transport materials for organic EL devices. These compounds can be easily formed into thin films by vapor deposition or coating, and are known as good charge transport materials. For example, in Japanese Unexamined Patent Publication No. 2000-68062, a gallium complex is used as an electron transport material. However, it is unknown whether it has practical characteristics as an organic transistor.
Science (2000, 289, 599 pages) "Nature" (2000, 403, 521 pages) JP 2000-68062 A

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an organic thin film transistor using an organic semiconductor that can be manufactured by an easy process and that exhibits high transistor characteristics.

  As a result of intensive studies to achieve the above object, the present inventors have found that the object can be achieved by using an organic semiconductor compound of a specific metal complex, and have reached the present invention.

  That is, the present invention relates to an organic transistor having an organic semiconductor layer, wherein the organic semiconductor layer contains a gallium complex represented by the following general formula (1).

General formula [1]
(L ¹ ) nGa (L ² ) m

(In the formula, each L ¹ independently represents a ligand represented by the following general formula [2],
L ² is a halogen atom, —OR ¹ (R ¹ is a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group; Or -O-Ga- (L ¹ ) n, n represents 2 or 3, m is 1 when n is 2, m is 0 when n is 3. To express. )

General formula [2]

[Wherein, rings A ¹ and A ² each represents a ring structure condensed with each other which may have a substituent. ]
The present invention also relates to the above organic transistor, wherein L ¹ is represented by the following general formula [3].

General formula [3]

[Wherein R ^{2 to} R ⁷ are each independently a hydrogen atom, a halogen atom, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted amino group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted group, A cycloalkyl group, an aryl group which may have a substituted or unsubstituted nitrogen atom, an aryloxy group which may have a substituted or unsubstituted nitrogen atom, and adjacent R ^{2 to} R ⁷ substituents; May combine with each other to form an aromatic ring or a heteroaromatic ring. ]

  By using the organic semiconductor of the present invention, an organic thin film transistor element having both excellent ON / OFF ratio and high mobility could be produced. In addition, it was possible to provide an element having excellent characteristics not only by the vacuum deposition method but also by using the solution coating method.

(Organic semiconductor)
The present invention is characterized in that an organic semiconductor layer (active layer) which is a part of a layer structure of a transistor contains a metal complex compound represented by the following general formula [1].

General formula [1]
(L ¹ ) nGa (L ² ) m

L ¹ independently represents a ligand represented by the general formula [2], L ² represents a halogen atom, —OR ¹ (R ¹ is a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or A substituted cycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group.) Or a ligand represented by —O—Ga— (L ¹ ) n.
n represents 2 or 3, but when there are a plurality of L ^{1 s} , they may be the same or different. When L ² contains L ^1, they may be the same or different.
m represents 1 when n is 2 and 0 when n is 3.

General formula [2]

Rings A ¹ and A ² represented by the general formula [2] are substituted or unsubstituted aryl rings or heterocyclic structures bonded to each other. Examples of the aryl ring or the heterocyclic ring are the same as those described in the description of the substituent described later.

Specific examples of the substituents of the rings A ¹ and A ² that form the ligand of the general formula [2] include chlorine, bromine, iodine, halogen atoms of fluorine, methyl group, ethyl group, propyl group, Substituted or unsubstituted alkyl group such as butyl group, sec-butyl group, tert-butyl group, pentyl group, hexyl group, heptyl group, octyl group, stearyl group, trichloromethyl group, phenyl group, naphthyl group, biphenyl group, Anthranyl group, phenanthryl group, fluorenyl group, pyrenyl group, 3-methylphenyl group, 3-methoxyphenyl group, 3-fluorophenyl group, 3-trichloromethylphenyl group, 3-trifluoromethylphenyl group, 3-nitrophenyl group Substituted or unsubstituted aryl group such as methoxy group, n-butoxy group, tert-butoxy group, trichloro Methoxy group, trifluoroethoxy group, pentafluoropropoxy group, 2,2,3,3-tetrafluoropropoxy group, 1,1,1,3,3,3-hexafluoro-2-propoxy group, 6- (par Substituted or unsubstituted alkoxy groups such as fluoroethyl) hexyloxy group, phenoxy group, p-nitrophenoxy group, p-tert-butylphenoxy group, 3-fluorophenoxy group, pentafluorophenyl group, 3-trifluoromethylphenoxy Substituted or unsubstituted aryloxy group such as a group, methylthio group, ethylthio group, tert-butylthio group, hexylthio group, octylthio group, trifluoromethylthio group and the like substituted or unsubstituted alkylthio group, phenylthio group, p-nitrophenyl Thio group, p-tert-butylphenylthio Group, 3-fluorophenylthio group, pentafluorophenylthio group, substituted or unsubstituted arylthio group such as 3-trifluoromethylphenylthio group, cyano group, nitro group, amino group, methylamino group, diethylamino group, ethyl Mono- or di-substituted amino groups such as amino group, diethylamino group, dipropylamino group, dibutylamino group, diphenylamino group, bis (acetoxymethyl) amino group, bis (acetoxyethyl) amino group, bisacetoxypropyl) amino group, Acylamino group such as bis (acetoxybutyl) amino group, hydroxyl group, siloxy group, acyl group, methylcarbamoyl group, dimethylcarbamoyl group, ethylcarbamoyl group, diethylcarbamoyl group, propylcarbamoyl group, butylcarbamoyl group, phenylcarbamoyl group A carbamoyl group such as a group, a carboxylic acid group, a sulfonic acid group, an imide group, a cyclopentane group, a cyclohexyl group, a pyridinyl group, a pyrazinyl group, a pyrimidinyl group, a pyridazinyl group, a triazinyl group, an indolinyl group, a quinolinyl group, Acridinyl group, pyrrolidinyl group, dioxanyl group, piperidinyl group, morpholidinyl group, piperazinyl group, triatinyl group, carbazolyl group, furanyl group, thiophenyl group, oxazolyl group, oxadiazolyl group, benzoxazolyl group, thiazolyl group, thiadiazolyl group, There are heterocyclic groups such as a benzothiazolyl group, a triazolyl group, an imidazolyl group, a benzoimidazolyl group, and a pranyl group.
Moreover, the above substituents may combine to form a further 6-membered aryl ring or heterocyclic ring.

The ligand of L ¹ is preferably a quinolinolate ligand represented by the following general formula [3].

General formula [3]

R ^{2 to} R ⁷ are each independently a hydrogen atom, a halogen atom, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted amino group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, Represents an aryl group which may have a substituted or unsubstituted nitrogen atom, an aryloxy group which may have a substituted or unsubstituted nitrogen atom, and the adjacent substituents of R ^{2 to} R ⁷ are bonded to each other; An aromatic ring that may contain a nitrogen atom may be formed.
Specific examples of the substituents which are R ^{2 to} R ^{7 have} already been described as the substituents of the rings A ¹ and A ² .

  Representative examples of the residue of the general formula [3] include, but are not limited to, quinolinolato residues such as 8-hydroxyquinolinolato and 2-methyl-8-hydroxyquinolates.

The ligand L ² forming the metal complex compound of the general formula [1] is —OR ¹ (R ¹ is a hydrogen atom, an alkyl group, a cycloalkyl group, an aryl group, or a heterocyclic group), —O—. It is a ligand represented by Ga- (L ¹ ) n. Specific examples of the group or substituent represented by L ² or R ^{1 to} R ⁷ are the same as the specific examples of the substituent shown at the positions of the rings A ¹ and A ² . Further it can be mentioned -O-Ga- (L ¹⁾ a ligand or residues such as represented by n.

The compound represented by the general formula [1] of the present invention can be synthesized by the method described in JP-A-2000-68062. That is, it can be synthesized by using a gallium metal or a gallium compound and a compound having a ligand residue of L ¹ or L ² as a raw material in order to form a complex ligand. Examples of gallium compounds include gallium alkoxides such as alkyl gallium, gallium halide, gallium acetate, gallium nitrate, gallium sulfate, gallium hydroxide, gallium oxide, trimethoxy gallium, triethoxy gallium, triisopropoxy gallium, diethyl gallium chloride, or It may be a gallium compound partially substituted with acetylacetonate. The synthesis is preferably gallium alkoxide in terms of reactivity and safety, but is not limited thereto. Examples of the ligand residue represented by L ¹ include quinolinolato residues such as 8-hydroxyquinolinolato and 2-methyl-8-hydroxyquinolinolato.

  The solvent used for the synthesis is selected from methanol, ethanol, isopropyl alcohol, ethyl acetate, acetonitrile, 1,4-dioxane, tetrahydrofuran, benzene, toluene, xylene, n-hexane, dimethylformamide, quinoline, sulfolane, water, and the like. . The reaction temperature is determined by the metal complex formation rate of the ligand. It is preferably 0 ° C to 250 ° C, more preferably 20 ° C to 80 ° C. The reaction is carried out for 10 minutes to 24 hours. The synthesis conditions are determined by conditions such as a metal compound, a ligand, a solvent, and a catalyst, and are not limited to these.

  Although the typical example of the compound of general formula [1] of this invention is specifically illustrated in Table 1, it is not restricted to these.

(Transistor element)
By installing the organic semiconductor of the present invention in the active layer of the organic thin film transistor element, a transistor device that can be driven satisfactorily can be provided.

  The organic thin film transistor has a source electrode and a drain electrode connected by an organic semiconductor channel (active layer) on a support, a top gate type having a gate electrode on the support, and a gate electrode on the support. First, it is roughly classified into a bottom gate type having a gate electrode and having a source electrode and a drain electrode connected by an organic semiconductor channel through a gate insulating layer.

  In order to install the compound of the present invention in the active layer of the organic thin film transistor device, it can be installed on the substrate by vacuum deposition, but a solution prepared by dissolving in an appropriate solvent and adding additives as necessary is cast coated. It is preferably installed on the substrate by spin coating, printing, ink jet method, ablation method or the like. In this case, the solvent for dissolving the organic semiconductor of the present invention is not particularly limited as long as the organic semiconductor can be dissolved to prepare a solution having an appropriate concentration. Specifically, diethyl ether, diisopropyl ether, etc. Chain ether solvents, cyclic ether solvents such as tetrahydrofuran and dioxane, ketone solvents such as acetone and methyl ethyl ketone, alkyl halide solvents such as chloroform and 1,2-dichloroethane, toluene, o-dichlorobenzene, nitrobenzene, Aromatic solvents such as m-cresol, N-methylpyrrolidone, carbon disulfide and the like can be mentioned.

  In the present invention, the material for forming the source electrode, the drain electrode, and the gate electrode is not particularly limited as long as it is a conductive material. Platinum, gold, silver, nickel, chromium, copper, iron, tin, antimony lead, tantalum, Indium, palladium, tellurium, rhenium, iridium, aluminum, ruthenium, germanium, molybdenum, tungsten, tin oxide / antimony, indium tin oxide (ITO), fluorine doped zinc oxide, zinc, carbon, graphite, glassy carbon, silver paste and Carbon paste, lithium, beryllium, sodium, magnesium, potassium, calcium, scandium, titanium, manganese, zirconium, gallium, niobium, sodium, sodium-potassium alloy, magnesium, lithium, aluminum, magnesium / Copper mixture, magnesium / silver mixture, magnesium / aluminum mixture, magnesium / indium mixture, aluminum / aluminum oxide mixture, lithium / aluminum mixture, etc., especially platinum, gold, silver, copper, aluminum, indium, ITO And carbon are preferred. Alternatively, a known conductive polymer whose conductivity is improved by doping or the like, for example, conductive polyaniline, conductive polypyrrole, conductive polythiophene, a complex of polyethylenedioxythiophene and polystyrenesulfonic acid, or the like is also preferably used. Among them, those having low electrical resistance at the contact surface with the semiconductor layer are preferable.

  As a method for forming an electrode, a method for forming an electrode using a known photolithographic method or a lift-off method, using a conductive thin film formed by a method such as vapor deposition or sputtering using the above as a raw material, a metal foil such as aluminum or copper There is a method of etching using a resist by thermal transfer, ink jet or the like. Alternatively, a conductive polymer solution or dispersion, or a conductive fine particle dispersion may be directly patterned by ink jetting, or may be formed from a coating film by lithography or laser ablation. Furthermore, a method of patterning an ink containing a conductive polymer or conductive fine particles, a conductive paste, or the like by a printing method such as relief printing, intaglio printing, planographic printing, or screen printing can also be used.

  In addition, the surface of the electrode is modified by using a self-assembled monolayer (SAM) to reduce the surface energy of the electrode surface, so that the crystal growth of the organic semiconductor material, the crystal arrangement, the organic semiconductor material, The paintability with the electrode can be improved. For example, when a gold electrode is used, it is desirable to modify the surface with alkanethiol or the like.

  Various insulating films can be used as the gate insulating layer, and an inorganic oxide film having a high relative dielectric constant is particularly preferable. Inorganic oxides include silicon oxide, aluminum oxide, tantalum oxide, titanium oxide, tin oxide, vanadium oxide, barium strontium titanate, barium zirconate titanate, lead zirconate titanate, lead lanthanum titanate, strontium titanate, Examples thereof include barium titanate, barium magnesium fluoride, bismuth titanate, strontium bismuth titanate, strontium bismuth tantalate, bismuth tantalate niobate, and yttrium trioxide. Of these, silicon oxide, aluminum oxide, tantalum oxide, and titanium oxide are preferable. Inorganic nitrides such as silicon nitride and aluminum nitride can also be suitably used.

  The hydrophilic gate insulating film can be changed from hydrophilic to hydrophobic by performing various chemical surface treatments. Thereby, the wettability of the gate insulating film and the hydrophobic organic semiconductor layer can be greatly improved. In addition, an effect of reducing leakage current can be obtained. Typical surface treatment materials include hexamethyldisilazane (HMDS), octyltrichlorosilane (OTS), 7-octenyltrichlorosilane (VTS), (tridecafluoro-1,1,2,2-tetrahydrooctyltri Silane-based materials such as chlorosilane (FTS) and benzyltrichlorosilane (BTS) are preferable, but are not limited thereto.

  Examples of the method for forming the film include a vacuum process, a molecular beam epitaxial growth method, an ion cluster beam method, a low energy ion beam method, an ion plating method, a CVD method, a sputtering method, an atmospheric pressure plasma method, and a spray process. Wet processes such as coating methods, spin coating methods, blade coating methods, dip coating methods, casting methods, roll coating methods, bar coating methods, die coating methods, and other wet processes such as printing and ink jet patterning methods, etc. Can be used depending on the material.

  The wet process is a method of applying and drying a liquid in which fine particles of inorganic oxide are dispersed in an arbitrary organic solvent or water using a dispersion aid such as a surfactant as required, or an oxide precursor, for example, A so-called sol-gel method in which a solution of an alkoxide body is applied and dried is used.

Among these, the atmospheric pressure plasma method and the sol-gel method are preferable.
The method for forming an insulating film by plasma film formation under atmospheric pressure is a process in which a reactive gas is discharged under atmospheric pressure or a pressure near atmospheric pressure to excite reactive gas to form a thin film on a substrate. The method is described in JP-A-11-61406, JP-A-11-133205, JP-A-2000-121804, JP-A-2000-147209, JP-A-2000-185362 (hereinafter referred to as atmospheric pressure). Also called plasma method). Accordingly, a highly functional thin film can be formed with high productivity.

  In addition, as the organic compound film, polyimide, polyamide, polyester, polyacrylate, photo radical polymerization type, photo cation polymerization type photo curable resin, or a copolymer containing an acrylonitrile component, polyvinyl phenol, polyvinyl alcohol, novolac resin, Also, cyanoethyl pullulan or the like can be used.

As the method for forming the organic compound film, the wet process is preferable.
An inorganic oxide film and an organic oxide film can be laminated and used together. The thickness of these insulating films is generally 50 nm to 3 μm, preferably 100 nm to 1 μm.

  Moreover, a support body is comprised with glass or a flexible resin-made sheet | seat, for example, a plastic film can be used as a sheet | seat. Examples of the plastic film include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfone (PES), polyetherimide, polyetheretherketone, polyphenylene sulfide, polyarylate, polyimide, polycarbonate (PC), Examples thereof include films made of cellulose triacetate (TAC), cellulose acetate propionate (CAP), and the like. Thus, by using a plastic film, the weight can be reduced as compared with the case of using a glass substrate, the portability can be improved, and the resistance to impact can be improved.

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited thereto.

Example 1
A thermal oxide film was formed on a Si wafer having a resistivity of 0.02 Ω · cm as a gate electrode to form a gate insulating layer, and then surface treatment was performed using octadecyltrichlorosilane (OTS). Exemplified compound 26 was deposited to a thickness of 70 nm by a vacuum deposition method (degree of vacuum: 2.2 × 10 −6 torr). Further, gold was deposited on the surface of the film using a mask to form a source electrode and a drain electrode. The film thickness of the source electrode and the drain electrode was 40 nm, and the organic thin film transistor element 1 having a channel width W = 5 mm and a channel length L = 25 μm was formed.

Using the exemplified compounds 19 and 20 in place of the exemplified compound 26, the organic transistor element 2 and the organic transistor element 3 were produced in the same manner as the organic transistor element 1.

Example 2
Using a substrate similar to that in Example 1, a chloroform solution of Exemplified Compound 26 was applied using a spin coater and naturally dried to form a semiconductor layer (thickness: 100 nm). A minute heat treatment was applied. Furthermore, gold was deposited on the surface of the semiconductor layer using a mask to form source and drain electrodes. An organic thin film transistor element 4 having a source and drain electrode thickness of 40 nm, a channel width W = 5 mm, and a channel length L = 25 μm was formed.

The following compound A and compound B were used in place of the exemplified compound 26, and an organic transistor element 5 and an organic transistor element 6 were produced in the same manner as the organic transistor element 1.

Compound A

Compound B

An attempt was made to produce the organic transistor element 7 and the organic transistor element 8 by the same method as the organic transistor element 4 using the following compound A and compound B instead of the exemplified compound 26, but the solubility of the material is very low. It was difficult to prepare a coating solution having a sufficient concentration, and a spin coat film could not be formed.

Using the organic transistor element produced as described above, transistor characteristics were measured under reduced pressure. The ratio of the maximum current value (ON current) to the minimum current value (OFF current) when a voltage of 30 V is applied between the source electrode and the drain electrode and the gate electrode is swept in the range of −30 V to 70 V Was the ON / OFF ratio. The results were shown as relative values when the value indicated for the organic transistor element 5 as a comparative example was 1, and the results were as follows. At the same time, the relative values of the mobility of other elements when the mobility of the element 5 in the saturation region is 1 are shown.

注)素子７および素子８においては、スピンコート膜を作製することが不可能であるため「ＯＮ／ＯＦＦ比」「移動度」を算出することは出来なかった。

Note) In the device 7 and the device 8, since it is impossible to produce a spin coat film, the “ON / OFF ratio” and “mobility” could not be calculated.

  From this result, it can be seen that the organic thin film transistor element manufactured using the semiconducting material of the present invention for the active layer exhibits an excellent ON / OFF ratio and mobility as compared with the conventional material.

Claims (2)

The transistor which has an organic-semiconductor layer, The said organic-semiconductor layer contains the gallium complex represented by following General formula (1), The organic transistor characterized by the above-mentioned.
General formula [1]
(L ¹ ) nGa (L ² ) m
(In the formula, each L ¹ independently represents a ligand represented by the following general formula [2],
L ² is a halogen atom, —OR ¹ (R ¹ is a hydrogen atom, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted heterocyclic group; Or a ligand represented by —O—Ga— (L ¹ ) n,
n represents 2 or 3, m represents 1 when n is 2, and m represents 0 when n is 3. )
General formula [2]

[Wherein, rings A ¹ and A ² each represents a ring structure condensed with each other which may have a substituent. ] The organic transistor according to claim 1, wherein L ¹ is represented by the following general formula [3].
General formula [3]

[Wherein R ^{2 to} R ⁷ are each independently a hydrogen atom, a halogen atom, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted amino group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted group, A cycloalkyl group, an aryl group which may have a substituted or unsubstituted nitrogen atom, an aryloxy group which may have a substituted or unsubstituted nitrogen atom, and adjacent R ^{2 to} R ⁷ substituents; May combine with each other to form an aromatic ring or a heteroaromatic ring. ]