Synthesis of new polyconjugated molecules with biphenyl, dibenzothiophene, carbazole and phenanthrene units

The simple methods of synthesis of hardly accessible substituted biphenyl, dibenzothiophene, carbazole and phenanthrene derivatives were elaborated starting from dimethyl 4,4’-biphenyldicarboxylate. The series of new luminophores with extended π -conjugated chains based on combinations of biphenyl, carbazole, dibenzothiophene, phenanthrene fragments and alternating phenyl, vinyl or heterocyclic units were synthesized by the Wittig and the Knoevenagel reactions of corresponding aromatic dialdehydes and different CH-acids or phosphonium salts. Investigation of the effect of various substituents on the luminescent properties has been presented. The new luminophores could be used as emissive or charge transport layers in organic light emitting diodes (OLEDs).


Introduction
The biaryl axis is the central building block in a very large number of various molecules such as natural and pharmacologically active products, 1 chiral reagents, 2 as the source of chiral phases for chromatography, 3 as inflexible "spacers" between two parts of a molecule, 4,5 as the basis of liquid crystals 6 and fluorescent layers in OLEDs. 7Different substituted biphenyls very often are used as suitable synthetic intermediates for synthesis heteroaromatic and polycyclic aromatic compounds. 8The key step in such synthesis almost always is the coupling of two matched aromatic parts of the target molecule.Another way is the directed modification of easily accessible biphenyls such as, for example, 4, 4'-biphenyldicarboxylic acid.
Methyl ester of 4, 4'-biphenyldicarboxylic acid 1 is a very convenient and cheap starting material for synthesis of wide range of various biphenyl derivatives, especially because it can be separated from wastes of DMT manufacture. 9,10 wing to presence of two para-carboxylic groups in this molecule, the conjugated system of biphenyl can be easily involved in construction of extensive polyconjugated molecules like 2, which are potential luminophores and electroactive materials.[13] Similar heteroaromatic and polycyclic aromatic compounds 3-5 are of great interest as intermediates for synthesis of such kind polyconjugated systems.The analysis of literature shows that whereas of 2, 7-carbazolyl, 2,7-phenanthrenyl and 3,7-dibenzothiophenyl fragments have the lengthiest conjugation, they are not very often used as chromophores for preparation of dyes and luminophores.[16][17] Figure 1 The object of this work was elaboration of effective methods of synthesis of difficult-toaccess 4,4'-disubstituted biphenyl, 3,7-disubstituted dibenzothiophene, 2,7-disubstituted carbazole and phenanthrene derivatives followed by their application in synthesis of polyconjugated systems -potential electroluminescent and electroactive materials (Figure 1).

Results and Discussion
There are at least two reasons to consider 4, 4'-biphenyldicarboxylic acid as a versatile starting material for synthesis of the above mentioned system.First, two carboxylic groups in this molecule are the very convenient functions to develop and to lengthen side chains.Second, the cross-effect of the para-carboxylic group and the second ring gives the only answer about the direction of electrophilic substitution in this molecule and allows the carrying out a necessary functionalization of the ring system.
The double sulphurization of diester 1 with chlorosulphonic acid gives 5, 5dioxodibenzothiophen-3, 7-dicarboxylic acid 3 in quantitative yield (Scheme 1).This compound itself is the suitable chromophoric building block for synthesis of the rod-shaped molecules with a lengthy π-conjugated system like 10, which are potential electroluminescent materials.The dioxazole derivative 10 was prepared in one step by refluxing of acid 3 with o-aminophenol in the presence of boric acid 10 in diphenyl ether.
The molecule of dibenzothiophene 3 contains three electron-withdrawing groups and we anticipated that the introduction of an additional electron-donating function in this molecule could dramatically affect its properties as a chromophor.Amino and hydroxy derivatives 7, 9 are the most interesting from the viewpoint of their further use in synthesis of luminescent materials.The key intermediate in the synthesis of both amino and hydroxy derivatives is the nitro dibenzothiophene 6.The nitration of diacid 3 with nitric acid in concentrated H 2 SO 4 proceeded with the highest regioselectivity giving 1-nitro-5,5-dioxodibenzothiophen 6 in 80% yield.The minor 2-nitro derivative (5-7%) was easily separated with thrice-repeated crystallization from isopropyl alcohol.The presence of two carboxylic and sulfo electron-withdrawing groups in the molecule makes possible the direct nucleophilic substitution of the nitro group in relatively mild conditions, and, therefore, 1-hydroxy derivative 9 was obtained by treatment of nitro compound 6 with NaOMe in DMSO at 100°С followed by fusion with pyridine hydrobromide.In contrast to the starting compound 3 1-hydroxy-5,5-dioxodibenzothiophen-3,7-dicarboxylic acid 9 intensively fluoresces in solutions and displays the strong red solvatochromic shift in emission spectra (λ fluor.= 395 (toluene), λ fluor.= 525 (ethanol)).
We used these chromophores for the synthesis of new polyconjugated systems, which comprised dibenzothiophene and stilbene structural elements.The simplest model compounds Scheme 2 17-19 with phenylenevinylene fragments in main chain were prepared by the Wittig reaction of aldehydes 14-16 with benzyltriphenylphosphonium bromide as depicted in the Scheme 2.
The aldehydes were synthesized in two steps by the reduction of esters 11-13 followed by oxidation of corresponding alcohols with PCC.The ylide was generated under treatment of the phosphnium salt with t-BuOK in THF and the only trans-trans isomers of 17-19 were isolated in 45-75% yields.In comparison with the starting diesters, the highest effect was observed in the emission spectra of 2-alkoxy derivative 17.The red-shift of luminescence maximum was 50 nm (λ fluor.= 397 (13), λ fluor.= 448 (17)) because of expanding of π-conjugated backbone.In the case of amines 18, 19 only as much as three times the increase of emission intensity was observed without any changes of maximums positions in PL spectra.
Generally, luminophores from non-substituted biphenyls have low solubility and high crystallization tendency that impedes their application as EL materials, 18,19 but owing to presence of hydroxy functions in compounds 28-30, one can correct their solubility and the glass transition temperature by introduction of long alkoxy groups.
2-Hydroxy-4, 4'-biphenyldicarboxylic acid 20 was obtained in high yield by simply fusing of sulphone 3 with NaOH at 250-280°С. 10The same procedure with 2-methoxy sulphone 8 gave the mixture of 2-and 2, 2'-dihydroxy derivatives in a ratio 1:2 and 80% total yield, and their dibutyl esters 21 a-b can be easy separated by extraction of monoester with petroleum ether in the Soxhlet extractor.The strong influence of the substituent at C2 position on luminescence was demonstrated by the example of bis-benzoxazolyl derivative 22, which was synthesized by condensation of diacid 20 with 4-methyl-2-aminophenole in mixture of diphenyl ether/pyridine and boric acid as catalyst.This compound shows intensive blue photoluminescence in DMSO solution (λ max.= 420 nm) and, in the same time, luminescence of its salt form solution has maximum at 570 nm with the anomalous big Stokes shift (~240 nm), when pair electrons of phenolate can be drawn into conjugated system. 11he synthesis of the rod-shaped molecules with ethenyl linkers between the biphenyl moiety and electron-withdrawing 1,3-oxazole group was fulfilled by the Knoevenagel reaction of 2substituted-4,4'-biphenyldicarbaldehydes 26 and 27 with 2-methyl-1,3-benzoxazole.The benzoxazole 28, due to its electron-rich and good thermal stability, was used as emissive layer to enhance organic light-emitting diodes (OLEDs). 13 series of new soluble compounds based on a combination of biphenyl and stilbene structural elements were synthesized by the Wittig reaction of the aldehyde 26 with benzyl (triphenyl)phosphonium salts or diphosphonium bromide 25 with various aromatic aldehydes.The dimethyl ester of 20 was reduced with LiAlH 4 in THF and the resulting alcohol 23 was used for preparation both dialdehyde 26 and diphosphonium salt 25 as key intermediates in the synthesis of the EL dyes.
The stereochemistry of Wittig reaction products 30a-e depends very much on the structure of aromatic aldehydes.In case of relatively small molecules of benzaldehyde, 4-cyano-and 4fluorbenzaldehyde, the coupling of phosphorane generated from 25 does not proceed stereoselectively and inseparable mixtures of all four isomers (trans-trans, cis-cis, trans-cis) are formed.The same result was obtained when the aldehyde 26 reacted with benzyltriphenylphosphonium bromide.In both cases t-BuOK was used as base to generate corresponding ylides.The substances 30a-c show strong blue photoluminescence in solutions and, in spite of the fact that they are mixture of isomers, could be used as electroactive materials in manufacturing of OLEDs.Stereoselectivity of the reaction increased when we used the bulky 4-(diphenylamino)benzcarbaldehyde 20 and the trans-trans isomer of 30d was separated in 75% yield.The rest, approx.15%, was a mixture of other isomers, which were completely converted in the more stable trans-trans isomer of 30d by the refluxing in toluene with catalytic amount of iodine under argon atmosphere.The diphosphonium bromide 25, where R' is the branched 2ethyl-1-hexenyl, reacts with 4-(diphenylamino)benzcarbaldehyde giving the only trans-trans isomer 30e in 95% yield.We anticipated that the luminophores like 30d,e with electron donating aromatic amines in the main chain of conjugation have to possess the balanced hole and electron injection abilities and could be promising EL materials.
The most commonly used the hole-transporting materials in organic light-emitting devices are triarylamines like 4, 4'-bis [N-(1-naphthyl-)-N-phenyl-amino]-biphenyl (NPB). 21N-Arylated carbazole can be regarded as structural analog of triarylamines but having flat and rigid structure.That allows expecting good charge transporting properties, high termo-and light stability of polyconjugated molecules on their basis. 22One of the most common methods for synthesis of carbazole derivatives is the Cadogan reaction of 2-nitrobiphenyls in the presence of organophosphorus reagents. 23A requisite dimethyl 2-nitro [1, 1-biphenyl]-4,4'-dicarboxylate 31 was synthesized in high yield by nitration of diester 1 under thoroughly controlled temperature conditions.Unfortunately, the reductive intramolecular cyclization of 31 in the presence of triphenylphosphine gave only 30% yield of desired dimethyl 9H-carbazole-2,7-dicarboxylate 32 (Scheme 4).Furthermore, the reaction mixture was contaminated by a number of byproducts that were diddicult to separate.Another way for generation of singlet nitrenes that easily react with introduction into aromatic C-H bond is thermo-or photocyclization of 2-azidobiphenyls.The chain of transformations involved the reduction of nitro derivative 31 in amine 36, formation diazonium salt, its substitution with NaN 3 and thermocyclyzation of the azide 37.These transformations gave the carbazole 32 in 80% total yield, starting from nitrobiphenyl 31.The arylation of the electron-deficient nitrogen atom in dimethyl carbazole-2,7-dicarboxylate 32 demands harsh reaction conditions, however, the refluxing of it with bromobenzene in the presence of K 2 CO 3 , CuI and dibenzo-18-crown-6 gave the desired N-aryl carbazole 33 in unexpectedly high yield (92%).The corresponding dialdehyde 34 was obtained in two steps as depicted in Scheme 4. The carbazole-2, 7-dicarbohydrazide 35 was synthesized in quantitative yield by refluxing with hydrazine hydrate in ethanol.
Palladium-catalyzed annulation of aryl iodides with alkynes has been proved to be a useful method of forming polycyclic aromatic compounds, in particular substituted phenanthrenes. 24,25 e crucial 2-iodobiphenyl 38 was obtained by nucleophilic substitution of the amino group in 36 via the corresponding diazonium salt.To synthesize 9, 10-diphenylphenanthrene 5 we used an annulation procedure described for monosubstituted 2-iodobiphenyls. 25oth the carbazoles 34, 35 and phenanthrene 5 are versatile building blocks for synthesis of lengthy π-conjugated systems with alternating electron donor and electron acceptor groups in the main chain of conjugation, which could show good balanced luminescence and charge transport properties.The electron withdrawing ability of the 1, 3, 4-oxadiazole and 1,3-oxazole groups are very strong and comparable to the nitrile one.The presence of double bonds in these heterocycles allows using them as chromophore elements and in the same time as linkers for extending of a conjugated system.The synthesis of the carbazole derivative 39 with benzoxazole units in the side chain was accomplished by the condensation of the dialdehyde 34 with 2-methyl-1,3-benzoxazole under the Knoevenagel reaction conditions (Scheme 5).The oligomer 40, in which carbazole moiety is connected with bromophenyl unit via 1,3,4-oxadiazole linkers, was synthesized in 42% total yield by reaction of the dihydrazide 35 with 4-bromobenzoyl chloride followed by cyclization of the corresponding dibenzoyl carbazole-2,7-dicarbohydrazide in boiling Scheme 5 POCl 3 .The dibromide 40 itself intensively luminesces in the blue region (λ max = 428.5 nm) and could be used in synthesis of polymeric EL materials by the Suzuki-Miyaura cross coupling with boric acids. 26The invert succession of transformations was realized for preparation of phenanthrene 41 because of poor solubility of the diester 5 and the corresponding dihydrazide.9,10-Diphenyl-2,7-phenanthrenedicarbonyl dichloride was synthesized in two steps, and without further purification was condensed with 4-(octyloxy)benzenecarbohydrazide to give the mixed dihydrazide, which was cyclized in compound 41 in high total yield.This dye has strong blue fluorescence and good solubility in common organic solvents.

Experimental Section
General Procedures.All melting points were measured using a Boetius apparatus and are uncorrected.IR spectra were recorded using a UR-20 IR spectrometer. 1 H NMR (100 or 500, 13 MHz) and 13 C NMR (125,75 MHz) spectra were recorded as CDCl 3 and DMSO-d 6 solutions on a Tesla BS-567A and Bruker AVANCE-500 instruments respectively.Chemical shifts (δ) are given from TMS (0 ppm) as internal standard for 1 H-NMR, and 13 CDCl 3 (77.0ppm) for 13 C-NMR.TLC was performed on aluminum backed silica gel 60 F254 plates and visualized by UV and/or exposure to I 2 .Column chromatography was conducted with Merck Kieselgel 60: 70-230 mesh.Solvents were dried and freshly distilled according to common practice.

N, N-Dimethyl-1-amino-5,5-dioxo-5H-dibenzo[b,d]thiophen-3,7-dicarboxylic acid diethyl ether (11)
. 1-Amino-5,5-dioxodibenzothiophen-3,7-dicarboxylic acid 7 (3.3 g, 10 mmol) was dissolved in 60 ml of ethanol saturated with HCl , then the reaction mixture was heated at reflux and stirred for 12 h.After cooling, a precipitate was filtered off and successively washed with ethanol and water.Dried crystals were solved in 40 ml of DMSO, 2,5 g (18.7 mmol) K 2 CO 3 and 2,5 g (17.7 mmol) CH 3 I were added, and then the reaction mixture was heated at 60°C and stirred for 140 h.The reaction mixture was diluted with water and extracted with CH 3 Cl.Combined organic solution was dried over Na 2 SO 4 .Concentration of the solution in vacuum afforded a residue, which was purified by column chromatography on silica gel (toluene) to give 1.9 g (60%) of 11 as a green solid.

Dibutyl 2, 2'-dihydroxy [1, 1'-biphenyl]-4, 4'-dicarboxylate (21a).
A mixture of well ground NaOH (10 g, 0.25 mol) and the diacid 8 (1 g, 3 mmol) was heated at 210°С under intensive stirring for 20 min.Then the cooled fuse was dissolved in 100 ml of water and was acidified with 10% hydrochloric acid.Precipitated crystals were filtered off, washed with water and air-dried.The crude product was boiled in mixture of 30 ml of butanol and 0.5 ml of H 2 SO 4 with regular removing of nascent water until TLC indicated no starting material.The reaction mixture was diluted with CHCl 3 , was washed with 100 ml of 5% NaHCO 3 and brine.Concentration of the solution in vacuum afforded a brown solid, which was placed in the Soxhlet apparatus and was extracted with petroleum ether to remove 2-hydroxy derivative 21b.The solid residue was recrystallazed from toluene to yield 21a (0.6 g, 52%) as white crystals.The extract was concentrated in vacuum and the residue was purified by column chromatography on silica gel (toluene) to give 21b (0.3 g, 27%) as a white solid.

2-Hexyloxy-4, 4'-bis [hydroxymethyl]-1,1'-biphenyl (23).
To dry methanol (1l) saturated with HCl, 26.5 g (0.103 mol) of well-milled powder of 2-hydroxy [1,1'-biphenyl]-4,4'-dicarboxylic acid 20 was added.The resulting suspension was stirred under reflux for 12 h.After cooling, a precipitate was filtered off, washed with cold methanol.The crude product was dissolved in 180 ml of DMSO and 13.8 g (0.1 mol) of K 2 CO 3 was added.The reaction mixture was heated at 75°C, 16.5 g (0.1 mol) of 1-bromohexane was added and the resulting solution was stirred for 10 h at 75°C, then was poured into 0.5 l of 5% hydrochloric acid, and extracted with ethyl acetate.The organic layer was washed with water and dried over Na 2 SO 4 .After removal of solvent, the residue was recrystallized from ethanol.The obtained crystals were dissolved in 150 ml of THF and added dropwise to the suspension of 3.8 g (0.1 mol) LiAlH 4 in 100 ml of THF under stirring.The reaction mixture was refluxed for 2 h, excess of LiAlH 4 was quenched with addition of 15 ml of 15 % water solution of NaOH, and an inorganic solid was separated by filtration and washed with THF.Solvent was removed under reduced pressure, the residue was dissolved in ethyl acetate, washed with water and dried over Na 2 SO 4 .After removal of solvent, the residue was crystallized from toluene to afford 26.

2, 2'-Dihexyloxy-4, 4'-bis [hydroxymethyl]-1, 1'-biphenyl (24).
To a solution of dibutyl 2, 2'dihydroxy [1, 1'-biphenyl]-4, 4'-dicarboxylate 21a (0.6 g, 1.5 mmol) and 1-bromohexane (0.6 g, 3.6 mmol) in 40 ml of DMSO, anhydrous K 2 CO 3 (0.5 g, 3.6 mmol) was added and the resulting mixture was heated at 60°С and stirred for 12 h.Then the reaction mixture was diluted with water and extracted with chloroform.The combined organic solution was dried over Na 2 SO 4 , and then concentrated in vacuum.The residue brown oil was added as a solution in 10 ml of dry THF to a suspension of LiAlH 4 (0.37 g, 10 mmol) in 30 ml of THF and the reaction mixture was stirred at RT for 12 h.Excess of LiAlH 4 was quenched with 5% solution of NaOH in water and inorganic solids were filtered off and washed with hot THF.After concentration of the filtrate in vacuum the residue was purified by column chromatography on silica gel (toluene) to yield 24 (0.7 g, 80%) as colorless oil.Elemental Anal.Calcd for C 26 H 38 O 4 : C 75.32, H 9.24 Found: C

General procedure for the Wittig condensation
To the suspension of 1 g (1 mmol) 4,4'-bis[(triphenylphosphonio)methyl]-2-(hexyloxy)biphenyl dibromide (25) in 20 ml of dry THF, t-BuOK (0.28 g, 2.5 mmol) was added and the reaction mixture was stirred until complete dissolution of phosphonium salt happened with formation of the dark-red solution .Then a solution of 2 mmol corresponding aldehyde in 10 ml of THF was slowly added and the resulting mixture was stirred for an additional 2 h.After 2 h, the reaction mixture was poured into 100 ml of water and extracted with chloroform.The combined organic phase was washed with water and then dried over Na 2 SO 4. After removal of solvent, the residue was purified by column chromatography on silica gel (eluent: chloroform) and recrystallized from hexane.

2,7-Bis(hydroxymethyl)-9(N)-phenylcarbazole.
To a solution of 300 mg (0,8 mmol) of 9(N)phenylcarbazole-2,7-dicarboxylic acid dimethyl ester (33) in 50 ml of dry THF, 190 mg (5 mmol) of LiAlH 4 was added and the mixture was refluxed for 2 h under stirring.When TLC indicated the reaction was finished, the reaction mixture was treated with 10 ml of ethyl acetate followed by 50% aq.solution of potassium hydroxide until aluminum hydroxide precipitated, which was filtered off and washed several times with hot THF.The organic phase was combined and solvent removed on the rotary evaporator.Reaction mixture was heated at 40-50°С for 16 h under stirring, and then was diluted with brine and extracted with chloroform.Organic phases were combined and solvents removed in vacuum.The residue was purified by column chromatography on silica gel (chloroform) and dried under reduced pressure.Recrystallization from benzene gave 39 (138 mg, 78%), as bright green-yellow crystals.MP = 306-309°С (dec.);Elemental Anal.