Synthesis of Linearly Fused Benzodipyrrole Based Organic Materials

The objective of this review is to give an overview of the synthetic methods to prepare different indolo[3,2-b]carbazoles and similar systems with a potential use in electro-optical devices such as OLEDs (organic light emitting diode), OPVs (organic photovoltaic) and OFETs (organic field effect transistor). Some further modifications to the core units and their implications for specific applications are also discussed.


Introduction
Polycyclic compounds containing two pyrrole rings have been widely studied because they possess many interesting properties. One of them is the good charge transfer properties these type of products possess [1,2], a second one is the feasibility to tune the electronic levels of these compounds for different applications. This causes these compounds to be excellent candidates for applications such as OPVs (organic photovoltaics) [3,4], DSSCs (dye-sensitized solar cell) [5], OLEDs (organic light emitting diodes) [6,7], and OFETs (organic field effect transistor, including thin film transistors) [8][9][10]. Advantages of organic materials for these applications are potentially low cost [11], lightweight and flexibility.
The main focus of this review will be the indolo [3,2-b]carbazoles, but also smaller benzodipyrrole systems like pyrrolo [2,3-f]indoles and pyrrolo [3,2-b]carbazoles (Figure 1), larger systems, and heterocyclic analogs of indolo [3,2-b]carbazole will be discussed. The smaller systems can be considered as indolo [3,2-b]carbazoles with one or two of the outer benzo-rings missing. The larger systems have one or more extra rings compared to indolo [3,2-b] carbazole. We will only focus on the linear indolo [3,2-b]carbazole isomer, and its smaller and larger analogs in this review due to the more interesting spectroscopic properties [12]. The smaller systems can be considered as indolo [3,2-b]carbazoles with one or two of the outer benzo-rings missing. The larger systems have one or more extra rings compared to indolo [3,2-b]carbazole. We will only focus on the linear indolo [3,2-b]carbazole isomer, and its smaller and larger analogs in this review due to the more interesting spectroscopic properties [12].
In this review, we will discuss the synthesis of the parent indolo [3,2-b]carbazole scaffold and further functionalization and polymerization of this compound for applications such as OPVs, OLEDs and OFETs. However, the focus of the review is synthetic and we will not go in the details of the applications.

Scheme 2. Pd-catalyzed quadruple N-arylation.
Chang et al. developed a general oxidative method starting from N-substituted amidobiphenyls to prepare carbazoles. In order to obtain a high yield, electron withdrawing groups such as acetyl or phenylsulfonyl should be placed on the amines. Alkyl substituted analogs seem to be disfavorable for the reaction.
PhI(OAc)2 was shown to be the stoichiometric oxidant with the best results for the carbazole synthesis. Copper triflate was used as a catalyst and this improved the yield of the reaction going from 75% up to 93%. The optimized reaction conditions (for carbazole) were used on 2,2″-bis(sulfonamide)p-terphenyl 10 to afford indolo [3,2-b]carbazole 11 in 40% yield after a double cyclization (Scheme 3) [23]. Chang et al. developed a general oxidative method starting from N-substituted amidobiphenyls to prepare carbazoles. In order to obtain a high yield, electron withdrawing groups such as acetyl or phenylsulfonyl should be placed on the amines. Alkyl substituted analogs seem to be disfavorable for the reaction.
PhI(OAc) 2 was shown to be the stoichiometric oxidant with the best results for the carbazole synthesis. Copper triflate was used as a catalyst and this improved the yield of the reaction going from 75% up to 93%. The optimized reaction conditions (for carbazole) were used on 2,2"-bis(sulfonamide)-p-terphenyl 10 to afford indolo [3,2-b]carbazole 11 in 40% yield after a double cyclization (Scheme 3) [23].
to prepare carbazoles. In order to obtain a high yield, electron withdrawing groups such as acetyl or phenylsulfonyl should be placed on the amines. Alkyl substituted analogs seem to be disfavorable for the reaction.
PhI(OAc)2 was shown to be the stoichiometric oxidant with the best results for the carbazole synthesis. Copper triflate was used as a catalyst and this improved the yield of the reaction going from 75% up to 93%. The optimized reaction conditions (for carbazole) were used on 2,2″-bis(sulfonamide)p-terphenyl 10 to afford indolo [3,2-b]carbazole 11 in 40% yield after a double cyclization (Scheme 3) [23].

Synthesis Starting from Indoles
Ishii et al. investigated the oligomerization of indole in acidic conditions. One of the compounds the authors found in the mixture obtained by combining indole 12 with p-toluenesulfonic acid in Dowtherm A (mixture of biphenyl (26.5%) and diphenyl ether (73.5%)) was indolo[3,2-b]carbazole 13, but only 6% yield was obtained under these conditions (Scheme 4) [24]. The generality of this method was not investigated. Korolev et al. treated 3-formylindole with acid and they detected several indolocarbazoles [25].

Synthesis Starting from Indoles
Ishii et al. investigated the oligomerization of indole in acidic conditions. One of the compounds the authors found in the mixture obtained by combining indole 12 with p-toluenesulfonic acid in Dowtherm A (mixture of biphenyl (26.5%) and diphenyl ether (73.5%)) was indolo[3,2-b]carbazole 13, but only 6% yield was obtained under these conditions (Scheme 4) [24]. The generality of this method was not investigated. Korolev et al. treated 3-formylindole with acid and they detected several indolocarbazoles [25]. Cheng et al. started their synthesis from N-benzenesulfonylindole-2-carbaldehyde 14, which was subjected to a Horner-Wadsworth-Emmons reaction to obtain the corresponding cinnamate ester 15. This compound was then reacted with an excess of methylmagnesium iodide to obtain the tertiairy alcohol 16. The protecting benzenesulfonyl group was removed and then the obtained compound 17 was treated with a catalytic amount of acid, which generated a stabilized cation that dimerized headto-tail in acidic conditions. The resulting tetrahydro compound (structure not shown) was further oxidized with air oxygen to obtain the indolo [3,2- [27]. This synthesis is based on earlier work by Katritzky et al. in which benzotriazole was also used as a leaving group to perform a coupling reaction between indole and heteroaromatic Cheng et al. started their synthesis from N-benzenesulfonylindole-2-carbaldehyde 14, which was subjected to a Horner-Wadsworth-Emmons reaction to obtain the corresponding cinnamate ester 15. This compound was then reacted with an excess of methylmagnesium iodide to obtain the tertiairy alcohol 16. The protecting benzenesulfonyl group was removed and then the obtained compound 17 was treated with a catalytic amount of acid, which generated a stabilized cation that dimerized head-to-tail in acidic conditions. The resulting tetrahydro compound (structure not shown) was further oxidized with air oxygen to obtain the indolo[3,2-b]carbazole 18 (Scheme 5) [26].

Synthesis Starting from Indoles
Ishii et al. investigated the oligomerization of indole in acidic conditions. One of the compounds the authors found in the mixture obtained by combining indole 12 with p-toluenesulfonic acid in Dowtherm A (mixture of biphenyl (26.5%) and diphenyl ether (73.5%)) was indolo[3,2-b]carbazole 13, but only 6% yield was obtained under these conditions (Scheme 4) [24]. The generality of this method was not investigated. Korolev et al. treated 3-formylindole with acid and they detected several indolocarbazoles [25]. Cheng et al. started their synthesis from N-benzenesulfonylindole-2-carbaldehyde 14, which was subjected to a Horner-Wadsworth-Emmons reaction to obtain the corresponding cinnamate ester 15. This compound was then reacted with an excess of methylmagnesium iodide to obtain the tertiairy alcohol 16. The protecting benzenesulfonyl group was removed and then the obtained compound 17 was treated with a catalytic amount of acid, which generated a stabilized cation that dimerized headto-tail in acidic conditions. The resulting tetrahydro compound (structure not shown) was further oxidized with air oxygen to obtain the indolo [3,2- [27]. This synthesis is based on earlier work by Katritzky et al. in which benzotriazole was also used as a leaving group to perform a coupling reaction between indole and heteroaromatic structures. The overall yield of these reactions is however lower (22% starting from the corresponding benzotriazole compound) [28]. Bergman et al. prepared 6-formyl-indolo[3,2-b]carbazole 25 starting from 2,3′-diindolylmethane 23, which was prepared in several steps. This compound was then condensed with dichloroacetylchloride in THF using pyridine as a base (84%) to form acylated compound 24. Acid catalyzed ring closure and hydrolysis of the dichloromethyl function at the meso position afforded the 6-formyl-indolo [3,2- Ivonin and coworkers used indole and phenylglyoxal to prepare 2-hydroxy-2-indol-3-ylacetophenone 26, which was then heated up to 200 °C. When the non-alkylated indole is used, only 10% of the dibenzoylated indolo[3,2-b]carbazole 27 is obtained. By using N-methylated indole for this reaction, the yield is increased up to 91% (Scheme 8) [30]. Bergman et al. prepared 6-formyl-indolo[3,2-b]carbazole 25 starting from 2,3 1 -diindolylmethane 23, which was prepared in several steps. This compound was then condensed with dichloroacetylchloride in THF using pyridine as a base (84%) to form acylated compound 24. Acid catalyzed ring closure and hydrolysis of the dichloromethyl function at the meso position afforded the 6-formyl-indolo[3,2-b]carbazole 25 (80%) (Scheme 7) [29]. Bergman et al. prepared 6-formyl-indolo[3,2-b]carbazole 25 starting from 2,3′-diindolylmethane 23, which was prepared in several steps. This compound was then condensed with dichloroacetylchloride in THF using pyridine as a base (84%) to form acylated compound 24. Acid catalyzed ring closure and hydrolysis of the dichloromethyl function at the meso position afforded the 6-formyl-indolo [3,2- Ivonin and coworkers used indole and phenylglyoxal to prepare 2-hydroxy-2-indol-3-ylacetophenone 26, which was then heated up to 200 °C. When the non-alkylated indole is used, only 10% of the dibenzoylated indolo[3,2-b]carbazole 27 is obtained. By using N-methylated indole for this reaction, the yield is increased up to 91% (Scheme 8) [30]. Ivonin and coworkers used indole and phenylglyoxal to prepare 2-hydroxy-2-indol-3-ylacetophenone 26, which was then heated up to 200˝C. When the non-alkylated indole is used, only 10% of the dibenzoylated indolo[3,2-b]carbazole 27 is obtained. By using N-methylated indole for this reaction, the yield is increased up to 91% (Scheme 8) [30]. Scheme 7. Synthesis of 6-formylindolo [3,2-b]carbazole from 2,3′-diindolylmethane.
developed a strategy to prepare indolo[3,2-b]carbazoles starting from 3,3 1 -diindolylmethane 29. This diindolylmethane was obtained in situ by Bronsted-or Lewis acid-catalyzed condensation of indole 12 and an aliphatic aldehyde 28. The weak Lewis acid iodine was used in this case. In the second step, an orthoester and a strong Bronsted acid have been used to perform the ring closure (20%-50%). Previous to the ring closure, the 3,3 1 -connected diindolylmethane rearranged to a 2,3 1 -connected isomer to ultimately give indolo [3,2- Bhuyan et al. started their synthesis from isolated 3,3′-diindolylmethanes 29. The starting material was dimerized with iodine as a catalyst to achieve a symmetrical indolo[3,2-b]carbazole 34 in 50%-85% yield after 35 min. One equivalent of indole was left unreacted after elimination from the starting material. The final compound is symmetrical because the use of orthoformates is not required. The reaction however does not work with aliphatic and strong electron withdrawing aromatic R groups (Scheme 10) [33]. Another method by Bhuyan et al. is the three component reaction of indole with an aldehyde and N,N-dimethylbarbituric acid, which affords a 3-alkylindole that can dimerize to a symmetrical indolo [3,2-b]carbazole [34]. Another related method is the direct condensation of indole 12 with benzaldehyde 31 in the presence of hydrogen iodide to obtain 6,12-diphenyl-5,6,11,12-tetrahydroindolo[3,2-b]carbazole 32 in excellent yield in a single step. This compound then can further be functionalized and converted to the fully aromatic indolo[3,2-b]carbazole 33 (Scheme 9) [32].
Bhuyan et al. started their synthesis from isolated 3,3 1 -diindolylmethanes 29. The starting material was dimerized with iodine as a catalyst to achieve a symmetrical indolo[3,2-b]carbazole 34 in 50%-85% yield after 35 min. One equivalent of indole was left unreacted after elimination from the starting material. The final compound is symmetrical because the use of orthoformates is not required. The reaction however does not work with aliphatic and strong electron withdrawing aromatic R groups (Scheme 10) [33]. Another method by Bhuyan et al. is the three component reaction of indole with an aldehyde and N,N-dimethylbarbituric acid, which affords a 3-alkylindole that can dimerize to a symmetrical indolo [3,2-b]carbazole [34].
in 50%-85% yield after 35 min. One equivalent of indole was left unreacted after elimination from the starting material. The final compound is symmetrical because the use of orthoformates is not required. The reaction however does not work with aliphatic and strong electron withdrawing aromatic R groups (Scheme 10) [33]. Another method by Bhuyan et al. is the three component reaction of indole with an aldehyde and N,N-dimethylbarbituric acid, which affords a 3-alkylindole that can dimerize to a symmetrical indolo [3,2-b]carbazole [34].  Mohanakrishnan et al. developed a method where 2-methyl-indole-3-carboxaldehyde 35 has been used as a starting material. The aldehyde group was condensed with diethyl malonate and the 2-methyl group was brominated to obtain 36. This biselectrophilic compound can then be condensed with various electon rich (hetero)aromatic systems under the influence of a Lewis acid. N-alkyl-indole 37 has thus been used to obtain indolo[3,2-b]carbazole 38 in 55% yield after elimination of diethyl malonate [35].
Later, the aldehyde was converted to an acetal as an alternative to the condensation with diethylmalonate. Again, the methyl group was brominated. The obtained bis-electrophile 39 can be condensed with various aryl-and heteroaryl rings to get a polycyclic system. When an N-alkyl-indole 37 was used, in combination with ZnBr 2 as a Lewis acid catalyst, indolo[3,2-b]carbazole 38 was formed in 67%-69% yield. The bromide leaving group can also be replaced by an acetate [36]. The yield is however lower in this case (54%) (Scheme 11). Later, the aldehyde was converted to an acetal as an alternative to the condensation with diethylmalonate. Again, the methyl group was brominated. The obtained bis-electrophile 39 can be condensed with various aryl-and heteroaryl rings to get a polycyclic system. When an N-alkyl-indole 37 was used, in combination with ZnBr2 as a Lewis acid catalyst, indolo[3,2-b]carbazole 38 was formed in 67%-69% yield. The bromide leaving group can also be replaced by an acetate [36]. The yield is however lower in this case (54%) (Scheme 11). Reddy et al. prepared funtionalized indoles starting from N-Boc protected 2-aminobenzaldehyde 40. Nucleophilic attack of lithiated alkyne 41 and successive oxidation gave compound 42, which was converted by combination with 1-lithio-2-ethoxyethyne 43, acidic deprotection and cyclization to 3-alkynylindole-2-carboxaldehyde 44. This compound was then condensed with 1-methyl-indole 45 in oxidative conditions, using copper(II)triflate, to obtain indolo [3,2-b]carbazole 46 in 60% yield (Scheme 12) [37].

Fischer Indole Synthesis
Robinson was the first to prepare the indolo [3,2-b]carbazole scaffold by performing a double Fischer indolization. He started from bishydrazone 47 to obtain the indolo[3,2-b]carbazole 2 in 27% yield, using a mixture of sulfuric acid and acetic acid (Scheme 13) [38].

Fischer Indole Synthesis
Robinson was the first to prepare the indolo[3,2-b]carbazole scaffold by performing a double Fischer indolization. He started from bishydrazone 47 to obtain the indolo[3,2-b]carbazole 2 in 27% yield, using a mixture of sulfuric acid and acetic acid (Scheme 13) [38].

Oxidation of Indolo
This compound however is reactive towards nucleophiles and the reduction product of DDQ will do an addition on the oxidized indolo [3,2-b]carbazole to obtain the meso substituted compound 58 (Scheme 17) [44]. By putting t-butyl groups on the structure of 57, the oxidized molecule is stable and could be isolated and characterized by X-ray crystallography [45].
The same compound 59 can be obtained by reaction of anhydride 60 with metallated indole 12, followed by acid-catalyzed ring closure of the bisindole ketoacid 61 in a polar solvent and deprotection of 62 (30% overall yield) (Scheme 17) [46]. Substituted derivates of 59 were prepared by Youssef et al. by reacting substituted anilines and tetrabromo-p-benzoquinone in a three step reaction. Also similar ring expanded systems were prepared by this method [47]. 5,11-Dihydro-indolo[3,2-b]carbazole 2 can be oxidized to indolo[3,2-b]carbazole 57. This compound however is reactive towards nucleophiles and the reduction product of DDQ will do an addition on the oxidized indolo [3,2-b]carbazole to obtain the meso substituted compound 58 (Scheme 17) [44]. By putting t-butyl groups on the structure of 57, the oxidized molecule is stable and could be isolated and characterized by X-ray crystallography [45]. The same compound 59 can be obtained by reaction of anhydride 60 with metallated indole 12, followed by acid-catalyzed ring closure of the bisindole ketoacid 61 in a polar solvent and deprotection of 62 (30% overall yield) (Scheme 17) [46]. Substituted derivates of 59 were prepared by Youssef et al. by reacting substituted anilines and tetrabromo-p-benzoquinone in a three step reaction. Also similar ring expanded systems were prepared by this method [47].
A similar double N arylation was also performed by Hu et al. using 1-iodonaphtalene and substituted iodobenzenes, using even more drastic conditions [49,50].
The tetrahydroindolocarbazole 69 which was obtained when benzaldehyde and indole were used for the condensation, has phenyl groups as substituents at both meso position. The compound is however not yet fully aromatic. The tetrahydroindolo [3,2-b]carbazole is first alkylated twice in 50%-67% yield to the more soluble indolo[3,2-b]carbazole 70 and then brominated with an excess of NBS, which at the same time aromatizes the middle ring, to obtain dihydroindolocarbazole 71. These bromine atoms can be converted to aldehydes and further to alkynes (Scheme 20) [32]. A similar double N arylation was also performed by Hu et al. using 1-iodonaphtalene and substituted iodobenzenes, using even more drastic conditions [49,50].
The tetrahydroindolocarbazole 69 which was obtained when benzaldehyde and indole were used for the condensation, has phenyl groups as substituents at both meso position. The compound is however not yet fully aromatic. The tetrahydroindolo [3,2-b]carbazole is first alkylated twice in 50%-67% yield to the more soluble indolo[3,2-b]carbazole 70 and then brominated with an excess of NBS, which at the same time aromatizes the middle ring, to obtain dihydroindolocarbazole 71. These bromine atoms can be converted to aldehydes and further to alkynes (Scheme 20) [32]. The alkyl and hydrazon functionalities contain reactive groups like oxetanes or vinyl groups. These are introduced in the molecules to enable self-polymerization at high temperatures (up to 180 °C) to obtain a more stable morphology (Scheme 21) [53,54]. Also epoxides were used to crosslink indolo [3,2-b]carbazoles under influence of aromatic dithiols [55]. A similar polymerization reaction with vinyl end-capped indolo [3,2-b] The best results for the formylation reaction were obtained by using the "Rieche method", using SnCl4 and dichloromethylpentyl ether in excess. The di-formylated compound 77 was obtained in 80% yield [58].
Diacetylation of indolo[3,2-b]carbazole 76 was performed in 67%-90% yield, using BF 3 ‚OEt 2 to obtain indolo[3,2-b]carbazole 79 [59]. The prepared aldehydes and acetyl groups were further used to couple indolo [3,2- carbazole 82, which twice undergoes an intramolecular Buchwald-Hartwig amination. The second method begins with 5,11-bis(2-nitrophenyl)-5,11-dihydroindolo[3,2-b]carbazole 84, also obtained through Buchwald-Hartwig amination of the parent indolo [3,2-b]carbazole. Reduction, diazotation to 85 and insertion at the meso position gives the same ring closed product 83. The yield of the compound via this approach is however lower than for the previous method due to formation of other isomers (Scheme 23) [60,61]. Khodorkovsky and coworkers prepared a new fused indolocarbazole donor system 83, by two different approaches. The first is starting from 6,12-bis(2-chlorophenyl)-5,11-dihydroindolo[3,2-b] carbazole 82, which twice undergoes an intramolecular Buchwald-Hartwig amination. The second method begins with 5,11-bis(2-nitrophenyl)-5,11-dihydroindolo[3,2-b]carbazole 84, also obtained through Buchwald-Hartwig amination of the parent indolo [3,2-b]carbazole. Reduction, diazotation to 85 and insertion at the meso position gives the same ring closed product 83. The yield of the compound via this approach is however lower than for the previous method due to formation of other isomers (Scheme 23) [60,61].     (3,9-isomer). Photoluminescence was at 437 nm and 457 nm respectively (in THF) [74].  (3,9-isomer). Photoluminescence was at 437 nm and 457 nm respectively (in THF) [74].  (3,9-isomer). Photoluminescence was at 437 nm and 457 nm respectively (in THF) [74]. . The peaks of the absorption spectrum are located at 350 nm for the para-and the meso-polymer. The meta-polymer had a peak at 400 nm. We can again conclude that polymerization is best performed at the 3,9-positions and that the spacer used will cause a higher effective conjugation length [80]. are located at 350 nm for the para-and the meso-polymer. The meta-polymer had a peak at 400 nm. We can again conclude that polymerization is best performed at the 3,9-positions and that the spacer used will cause a higher effective conjugation length [80]. are located at 350 nm for the para-and the meso-polymer. The meta-polymer had a peak at 400 nm. We can again conclude that polymerization is best performed at the 3,9-positions and that the spacer used will cause a higher effective conjugation length [80].

Scheme 35. Synthesis of pyrrolo[2,3-f]indole.
The angular isomer (pyrrolo[3,2-e]indole) was also formed during the reaction, the yield however was even lower for this compound. They showed an onset in the absorption spectrum at lower wavelengths, which makes the linear systems (pyrrolo[2,3-f]indole) more interesting for long wavelength absorption [12]. The angular isomer (pyrrolo[3,2-e]indole) was also formed during the reaction, the yield however was even lower for this compound. They showed an onset in the absorption spectrum at lower wavelengths, which makes the linear systems (pyrrolo[2,3-f ]indole) more interesting for long wavelength absorption [12].
By blocking the 4-position with a methyl group, both methods were suitable to prepare the linear isomer instead of the angular one that is normally formed. The yield of pyrrolo [2,3-b]carbazole 180 formation is 72% and 67%, respectively (Scheme 48). Nagarajan et al. used 3-amino-carbazoles 176, ethylene glycol 177 and a ruthenium catalyst to prepare pyrrolo [2,3-c]carbazoles 179 in good yield (73%), however this affords the angular isomer instead of the linear [98].
By blocking the 4-position with a methyl group, both methods were suitable to prepare the linear isomer instead of the angular one that is normally formed. The yield of pyrrolo [2,3-b]carbazole 180 formation is 72% and 67%, respectively (Scheme 48). Nagarajan et al. used 3-amino-carbazoles 176, ethylene glycol 177 and a ruthenium catalyst to prepare pyrrolo [2,3-c]carbazoles 179 in good yield (73%), however this affords the angular isomer instead of the linear [98].
By blocking the 4-position with a methyl group, both methods were suitable to prepare the linear isomer instead of the angular one that is normally formed. The yield of pyrrolo [2,3-b]carbazole 180 formation is 72% and 67%, respectively (Scheme 48).
By blocking the 4-position with a methyl group, both methods were suitable to prepare the linear isomer instead of the angular one that is normally formed. The yield of pyrrolo [2,3-b]carbazole 180 formation is 72% and 67%, respectively (Scheme 48).

Heterocyclic Analogs
Wang et al. prepared several tetracyclic indolonaphthyridines 190 starting from methyl 2-iodobenzoate 186. The first step is a Sonogashira coupling (88%-99%), followed by a saponification of the ester to get the corresponding carboxylic acid (61%-94%). Then a Curtius rearrangement is performed (73%-78%) and the obtained isocyanate 187 is subjected to an aza-Wittig reaction with 188, immediately followed by thermal ring closure of the carbodiimide intermediate 189 to form the final product 190 (Scheme 50). When a pyridine analog of 188 is used, multiple isomers are possible [101].

Heterocyclic Analogs
Wang et al. prepared several tetracyclic indolonaphthyridines 190 starting from methyl 2-iodobenzoate 186. The first step is a Sonogashira coupling (88%-99%), followed by a saponification of the ester to get the corresponding carboxylic acid (61%-94%). Then a Curtius rearrangement is performed (73%-78%) and the obtained isocyanate 187 is subjected to an aza-Wittig reaction with Wang et al. prepared several tetracyclic indolonaphthyridines 190 starting from methyl 2-iodobenzoate 186. The first step is a Sonogashira coupling (88%-99%), followed by a saponification of the ester to get the corresponding carboxylic acid (61%-94%). Then a Curtius rearrangement is performed (73%-78%) and the obtained isocyanate 187 is subjected to an aza-Wittig reaction with 188, immediately followed by thermal ring closure of the carbodiimide intermediate 189 to form the final product 190 (Scheme 50). When a pyridine analog of 188 is used, multiple isomers are possible [101].  The reaction is also possible starting from 2,4-dibromo-nitrobenzene or 2,5-dibromo-nitrobenzene 198 (38%-61%) to obtain dibrominated compounds 199. These compounds are further functionalized by Suzuki, Stille and Yamamoto coupling. The non-functionalized compound shows an absorption onset at 385-390 nm. The compounds with functionalization at the 2-and 9-position (X 1 = Ar or CN) do not show different properties. The 3,8-functionalized isomer (X 2 = Ar or CN) show a slight red-shift in the absorption spectrum (30-50 nm).

Larger Systems
Earlier we mentioned a method to prepare indolonaphthyridines from methyl 2-iodo-benzoate The reaction is also possible starting from 2,4-dibromo-nitrobenzene or 2,5-dibromo-nitrobenzene 198 (38%-61%) to obtain dibrominated compounds 199. These compounds are further functionalized by Suzuki, Stille and Yamamoto coupling. The non-functionalized compound shows an absorption onset at 385-390 nm. The compounds with functionalization at the 2-and 9-position (X 1 = Ar or CN) do not show different properties. The 3,8-functionalized isomer (X 2 = Ar or CN) show a slight red-shift in the absorption spectrum (30-50 nm).

Larger Systems
Earlier we mentioned a method to prepare indolonaphthyridines from methyl 2-iodo-benzoate (Scheme 50) [67]. The authors started from diethyl 2,5-dihydroxyterephthalate 200 to quantitatively convert this compound to diethyl 2,5-dialkynylterephthalate 201 in two steps, which now was used as a substrate for a double cyclization reaction. After saponification to 202, Curtius rearrangement to 203, aza-Wittig reaction with 204 and heating for 15 h, a heptacyclic polyheteroaromatic system 205 was obtained. The final step of the reaction however only worked with phenyl substituted iminophosphoranes 204 and not with the pyridine analogs (Scheme 53) [101]. The reaction is also possible starting from 2,4-dibromo-nitrobenzene or 2,5-dibromo-nitrobenzene 198 (38%-61%) to obtain dibrominated compounds 199. These compounds are further functionalized by Suzuki, Stille and Yamamoto coupling. The non-functionalized compound shows an absorption onset at 385-390 nm. The compounds with functionalization at the 2-and 9-position (X 1 = Ar or CN) do not show different properties. The 3,8-functionalized isomer (X 2 = Ar or CN) show a slight red-shift in the absorption spectrum (30-50 nm).

Larger Systems
Earlier we mentioned a method to prepare indolonaphthyridines from methyl 2-iodo-benzoate (Scheme 50) [67]. The authors started from diethyl 2,5-dihydroxyterephthalate 200 to quantitatively convert this compound to diethyl 2,5-dialkynylterephthalate 201 in two steps, which now was used as a substrate for a double cyclization reaction. After saponification to 202, Curtius rearrangement to 203, aza-Wittig reaction with 204 and heating for 15 h, a heptacyclic polyheteroaromatic system 205 was obtained. The final step of the reaction however only worked with phenyl substituted iminophosphoranes 204 and not with the pyridine analogs (Scheme 53) [101]. Yorimitsu et al. started from dibenzothiophene, which was oxidized to sulfone 210 by using aqueous hydrogen peroxide. In the next step, an aniline 211 is used to perform a nucleophilic aromatic substitution to obtain the corresponding carbazole 212 in 94% yield (Scheme 55).  Yorimitsu et al. started from dibenzothiophene, which was oxidized to sulfone 210 by using aqueous hydrogen peroxide. In the next step, an aniline 211 is used to perform a nucleophilic aromatic substitution to obtain the corresponding carbazole 212 in 94% yield (Scheme 55). Yorimitsu et al. started from dibenzothiophene, which was oxidized to sulfone 210 by using aqueous hydrogen peroxide. In the next step, an aniline 211 is used to perform a nucleophilic aromatic substitution to obtain the corresponding carbazole 212 in 94% yield (Scheme 55). On yet another substrate, benzothiophenesulfone, the authors first performed a Diels-Alder reaction with isobenzofuran 214 to afford the expanded benzonaphthothiophene sulfone, which can be converted to benzo[b]carbazole (not shown, 62%).
Hsu et al. prepared three heptacyclic carbazole derivatives 219, 222 and 223 by different annelation reactions to a carbazole precursor 216. The first one contains an sp 3 center between the carbazole moieties and the two thiophene rings. The two thiophene rings 217 were linked by a Suzuki reaction with carbazole 216, followed by Grignard addition of four aryl groups to the two ester groups to obtain 218. Acid catalyzed ring closure of the intermediate biscarbinol gave the final heptacyclic compound 219 (Scheme 56) [107].
For the two other analogs, the ester functionality in the thiophene starting material 220 was replaced by a bromine atom and additionally carbazole was dibrominated after protection of the α-positions of thiophene to get 221.
To obtain the bis(silacyclopentadiene) compound, the four bromine atoms are lithiated and the end product 222 is formed by addition of SiCl 2 Oct 2 (94%).
To obtain the bis(silacyclopentadiene) compound, the four bromine atoms are lithiated and the end product 222 is formed by addition of SiCl2Oct2 (94%).

Polymerization and Applications
The
These polymers (231 and 232 respectively) showed less red shifted absorption (up to 750 nm) in comparison to the two previous systems [102].
While the monomers show absorption up to 400 and 460 nm in toluene, the polymers show absorption up to 700 nm (carbon 234 and silicon 235 bridge) and 840 nm (nitrogen 236 bridge) [108].
These polymers (231 and 232 respectively) showed less red shifted absorption (up to 750 nm) in comparison to the two previous systems [102].
While the monomers show absorption up to 400 and 460 nm in toluene, the polymers show absorption up to 700 nm (carbon 234 and silicon 235 bridge) and 840 nm (nitrogen 236 bridge) [108].

Conclusions
The past two decades have seen a large activity in the domain of indolo [3,2-b]carbazoles and the related smaller and larger benzodipyrrole analogs. New synthetic methods were reported, leading to superior materials. Certainly, the full potential of this work has not been realized. It is expected that we will see further development in this area.

Author Contributions:
The two authors co-wrote this review.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript:  Triethylamine  TFAA  Trifluoroacetic anhydride  Tf  Triflate  THF  Tetrahydrofuran  THP  Tetrahydropyran  TIPS  Triisopropylsilyl  TMSCl  Trimethylsilylchloride  Ts Tosyl