Azaindolo[3,2,1‐jk]carbazoles: New Building Blocks for Functional Organic Materials

Abstract The preparation and characterization of 12 azaindolo[3,2,1‐jk]carbazoles is presented. Ring‐closing C−H activation allowed for the convenient preparation of six singly and six doubly nitrogen‐substituted indolo[3,2,1‐jk]carbazole derivatives in which ten of the materials have not been described in the literature before. The detailed photophysical and electrochemical characterization of the developed materials revealed a significant impact of the incorporation of pyridine‐like nitrogen into the fully planar indolo[3,2,1‐jk]carbazole backbone. Furthermore, the nitrogen position decisively impacted intermolecular hydrogen bonding and thus the solid‐state alignment. Ultimately, the versatility of the azaindolo[3,2,1‐jk]carbazoles scaffold makes this class of materials an attractive new building block for the design of functional organic materials.


Introduction
The development of p-conjugated small molecules with tailored molecular properties has been one of the major driving forces for the rapid development of the field of organic electronics over the last decades. [1,2] In particular, small to medium sized polycyclic (hetero)aromatic molecules with ad efined molecular structure are of tremendous importance. [2][3][4][5][6] Accordingly,m any materials based on these building blocks have been developed for applications in organic light-emitting diodes (OLEDs), [2,[7][8][9][10][11][12] organic field-effect transistors (OFETs), [1,2,6,7,[13][14][15] organic photovoltaics (OPVs), [2,[16][17][18][19] and sensingt echnology, [11,[20][21][22] to name af ew.A sac onsequence, there is an ongoing quest for novel fused aromatic moieties to fulfill the requirementsoft he respective technological application. [2,3,12] Recently,w ei ntroduced indolo[3,2,1-jk]carbazole (ICz) as new building block for optoelectronic materials. [23][24][25][26] ICz can be considered af ully planarized derivative of triphenylamine (TPA). Although arylamines are widely employeda se lectrondonating moieties, reports on the application of ICz have been scarce for al ong time, [27][28][29] owingt ot he elaborate preparation of the planarized scaffold (e.g.,v acuumf lash pyrolysis). [30][31] However, an ew synthetic strategy based on modern CÀHa ctivationr enders the wider application of this buildingb lock possible. [23,32] Compared with TPAa nd phenylcarbazole (PCz), ICz is aw eaker electron donor ( Figure 1). Due to the increasing planarization, the electron-donatingp ower of the triarylamines significantly decreasesf rom TPAt oP Cz and ICz. This gradual decreaseo fd onor strength is due to the contribution of the lone pair of the nitrogen to the aromaticity of the pyrroles formed by planarization. Therefore, the lone pair is more tightly bound to the arylamine core andl ess pronet ob ed elocalized in ad onor-acceptor molecule. [24] Effectively, the ICz moiety can exhibit light acceptorp roperties itself as evidenced in bipolar host materials. [23,33] Furthermore, the ICz building block has been utilized as aw eak electron acceptor in thermally activated delayedf luorescent (TADF) emitters. [34] Increasing the electron-acceptings trength by substitution of the ICz scaffold with electron-withdrawingc yano groups enabled the preparation of pure blue TADF emitters. [35] Based on these resultsa nd the growing interest regarding the application of the indolo[3,2,1-jk]carbazole as ab uilding block for optoelectronic materials, we set out to further expand the concept of planarization to intrinsically modulate the acceptor strengthb yi ncorporation of electron-withdrawing pyridine-liken itrogen atoms into the ICz moiety [ Figure 1, henceforth the atomsi nt he nitrogen-substituted ICzs (NICzs) are numbered according to the original numbering of ICz,i n which the numbers specify the positions of nitrogen atoms]. The concept of heteroatom doping is widelye mployedi ns ilicon-based semiconductor technology. Analogously,t he incorporationo fh eteroatomsi nto polycyclic aromatic scaffolds has been demonstrated to effectively modify the photophysical and electrochemical properties of the parent material. [36,37] In particular, nitrogen incorporation into acenes has been successfully realized to induce n-type charge-transport properties in materials forO FET applications. [38][39][40] Moreover,c arbolines, which contain an electron-deficient pyridine unit, were successfully employed as electron-transporting units in host materials for efficient phosphorescent OLEDs. [41,42] Notably,the exact positiono ft he nitrogena tom within the carboline scaffold significantly impacted the molecular properties of the materials and the device efficiency. [42,43] In additiont ot he electronic effects, the structure directing potentialo fn itrogen atoms by means of weak CÀH···N hydrogen bonds can also be employed to induce intra- [44,45] or intermolecular [46,47] interactions to control optical or electronic properties of the organic materials. [48] Accordingly,t he NICzs are an interesting class of materials. A reliable synthetic access to this scaffold would allow for the intriguing possibility to fine-tunet he molecular properties of optoelectronic materials by the subtle variation of the nitrogen positiona nd content of the NICz building block. Notably,t hree NICz derivatives have already been prepared, namely 5NICz, 7NICz,a nd 7,9NICz,w hich are accessible as single regioisomers by vacuum flash pyrolysis. [49] However,a ll other NICzs are unknown in the literature.
In this work, we presentacomprehensive synthetic approach towards all possible singly,a sw ell as six doubly substituted NICzs employing ac onvenient CÀHa ctivation reaction. This strategy allowed for the control of the substitution pattern of the NICz by choice of substrate as well as the regioselectivity of the ring-closing reactionb ye lectronic manipulation of the nitrogen atom by oxidation. Ultimately,o ur approach enabled the preparationo fafull set of NICz building blocks with tunable photophysical and electrochemical properties.

Precursor synthesis
Compared with plain ICz, the preparation of the substrates for CÀHa ctivation towards NICz provedt ob em ore challenging, owing to altered reactivity,s tability,a nd availability of the required substituted pyridines. Furthermore, the issue of regioselectivity during the ring-closing step arises as ar esult of the desymmetrization of the ICz scaffold by nitrogen incorporation. Therefore, we opted to investigate different approaches for the synthesis of these precursors( Scheme 1). These different strategies not only allow for the convenient synthesis of the required precursors buta lso pave the way for the future preparation of substituted NICzs as well as for the incorporation of the NICz moiety into larger p-conjugated systems. The pursued approaches are divided into two groups and the resultso ft he respective reactions towards NICz precursors are summarized in Ta ble 1.
The first strategy focusedo nt he establishment of ac arbazole unit attached to ah alogen-substituted pyridine. Notably, the outcome of the cyclization reactionf or these precursors was solely determined by the substitution pattern of the pyridine. Accordingly,t his strategy allowed for the preparation of NICzs with the nitrogen atom incorporated in the peripheral Scheme1.Synthetic approaches towards precursors for CÀH-activation;carbazole and carboline strategy.Xand Yare either Br and Cl or Cl and F, respectively. benzene unit of the ICz scaffold (Table1, 4PCz-7PCz,p osition 4-7). The most straightforward preparation of the respective precursors wasanucleophilic aromatic substitution of a appropriately substituted dihalogenated pyridine (Scheme1). Following this approach, the brominated precursors bromo-5PCz and bromo-7PCz were obtainedi n6 9a nd 37 %y ield, respectively,byheatingthe corresponding bromochloropyridines and carbazole in the presenceo fabase (see Table 1f or reaction conditions).T he two remaining carbazole precursors, bromo-4PCz and bromo-6PCz,h owever, could not be prepared using this method. Instead, nucleophilic substitution oc-curred in the more activated 2and 4positions of the pyridines, giving chloro-7PCz and chloro-5PCz in low yields. Therefore, we used brominated aminopyridinesa sa lternative substrates (Scheme1); bromo-4PCz was obtained by condensation of 3amino-2-bromopyridine and three equivalents of 2,5-dimethoxytetrahydrofuran [50] in 52 %y ield. In the case of bromo-6PCz, the carbazole moietyw as formedb yaPd-catalyzed Nozakitype Buchwald-Hartwig amination. [51] Notably,t he condensation reaction did not yield bromo-6PCz and the Nozaki approach failed for the preparation of bromo-4PCz.T of ully explore the accessibility of the carbazole precursors, we also employed theset wo methods for the preparation of bromo-5PCz, which was obtainedi ng ood yields from both reactions, as well as bromo-7PCz,w hich could not be successfully prepared following these approaches.
The second strategy employs the four different carboline derivatives as startingm aterials (Scheme 1, right). The corresponding four carboline-basedN ICz precursors (PCbs) were prepared by nucleophilic aromatic substitution of 1-bromo-2fluorobenzene and were obtained in excellent yields (82-92%) with the exception of bromo-7PCb (34 %; Ta ble 1). In contrast to the PCz precursors, the unsymmetrical nature of carbolines 4PCb, 5PCb,a nd 6PCb leads to two possible sites at which ring closure may occur.Although reactions at the benzene unit of the carbolines would yield the same products, which are availablet hrough the carbazole strategy,r ing closure at the pyridine unit would give the two remaining NICz derivatives. Notably,t he site selectivity and ratio of product is determined by the electronic demand of the ring-closing reaction and thus, may be significantly influenced by the nitrogen position.
Furthermore, we aimed to explore the substrate scope of the CÀHa ctivation. Accordingly,t he chloro-PCz precursors were prepareds tarting from carbazole and the corresponding chlorofluoropyridines. In contrast to the bromo-precursors all chloro-derivatives were obtained by nucleophilic aromatic substitutionina cceptable to excellent yield (57-95%;T able 1).
Initial experiments on 1mmol scale were performed using carbazole precursors 4PCz-7PCz (Table 3). To our delight, the conversion of the bromo substrates smoothly delivered all four NICz derivatives (4NICz-7NICz)i ne xcellent yields of 80-96 % after reasonable reaction times (4-8 h). Inspired by this initial success, we investigated the reactivity of the respective chloro precursors. Surprisingly,t he results obtained for the chloro substrates in these quantitative experimentse xceeded those of the bromo precursors. This finding contrasts previousresults that suggested al ower reactivity of chloro substrates in the CÀ Ha ctivation stept owards ICz. [23] Nevertheless, 4NICz-7NICz were obtained from the corresponding chloro-PCzs in excellent yields between 92 and 98 %a fter as hort reactiont ime of 4h (Table 3). In the next step, we investigated the reactivityo f the carboline precursors (PCbs,T able 4). In contrast to the carbazole strategy,t wo possible products can be formed in the CÀHa ctivations tep from the PCbs. Although the formation of ap roduct mixture is ac leard isadvantage, 1NICz and 2NICz are only accessible throught his strategy.P recursors 4PCb-6PCb werea ll converted to ring-closed NICzs. In contrast, hardly any conversion of 7PCb was observed and starting material, alongw ith small amountso fd ehalogenated byproduct,  was recovered even after prolonged reaction times of 96 h. We assume that the ortho-nitrogen of 7PCb stabilizes the initially formed Pd complex and thus prevents productive ring closure. Analyzing the products formed in the CÀHa ctivation,w ed etermined that ar egioselective ring closure occurred at the 4 (4PCb)a nd 3( 5PCb)p ositions of the pyridine ring. Accordingly, 1NICz and 4NICz were obtained from 4PCb in a3 :1 ratio after separation, whereas the reactiono f5PCb yielded 2NICz and 5NICz in ar atio of 6.9:1. Notably,t he separation of the regioisomersw as achievedb ys imple column chromatography. Thus, the two missing NICz regioisomers were indeed prepared using the carboline strategy.I nt he case of 6PCb,r ing closure on the benzene ring of the carboline was preferred, presumably owing to the highly electron-poor alternative 2-position of the pyridine. Therefore, 1NICz and 6NICz were formed in a ratio of 1:3.9. Inspired by these results, we wondered if we could control the regioselectivity of the CÀHa ctivationb ym anipulation of the electron density of the pyridine unit. Thus, we oxidized the nitrogen atom of the pyridinet oi ncrease the electron density of the aromatic ring;t he reactionw ith meta-chloroperbenzoic acid (mCPBA) yieldedt he corresponding N-oxides in good to excellent yields (61-90 %). Employing these N-oxides, we indeed observed an increased tendency for the ring closure on the now more electron-rich pyridine rings (Table 4), whereas high yields (89-92 %) were retained. Reduction of the Noxides was smoothly accomplished using Fe powder in acetic acid (AcOH) in 76-97 %y ields. In the case of 4PCb,t he ratio of isolated 1NICz to 4NICz was significantly shifted towards 1NICz when starting form 4PCb-Ox (3:1 vs. 19.3:1). The ratio of the products remained roughly the same for 5PCb and 5PCb-Ox.M ost strikingly,h owever,t he regioselectivity could be inverted for 6PCb.A lthough 6NICz is the favored product startingf rom 6PCb, 1NCz is formed predominately from 6PCb-  Ox (1NICz:6NICz = 4.4:1). This remarkable result underlines the impact of the electronic layout of the carboline systems on the outcomeo ft he ring-closing step. This oxidation-reductions equencew as established as ap owerful tool to control the regioselectivity of the CÀHa ctivation reaction, which is of particular interestf or the synthesis of more complex and/or extended annulated systems( see sectiono nd iazaindolo[3,2,1-jk]carbazoles).

Diazaindolo[3,2,1-jk]carbazoles
Inspired by the successful preparation of all mono-substituted NICz regioisomers, we explored the potential to furtheri ncrease the electron deficiency of the ICz scaffold by double pyridine incorporation.G iven the numerous possibilities of where to positiont wo nitrogen atoms in the annulated system,w el imited these investigations to substrates that would yield symmetrically substituted products 4,12NICz, 5,11NICz,a nd 6,10NICz ( Table 5). The respective precursors 4,12PyCb, 5,11PyCb,a nd 6,10PyCb were preparedb yn ucleophilic substitution of suitably decorated dihalogenated pyridines with the respective carbolines. To our delight, these precursors also underwent smooth ring closure under the optimized conditions,g iving the doubly substituted NICzs in high overall yields. In analogy to the mono substituted compounds, ring closure occurred on the pyridine ring in the case of 4,12PyCb and 5,11PyCB,p referentially yielding unsymmetrical 1,4NICz and 2,5NICz,r espectively ( Table 5). Separation of the regioisomers wasa ccomplished by HPLC on the preparative scale with an acceptable product loss. In contrast, the reaction of 6,10PyCb yieldedsymmetric 6,10NICz as the major product. Compared with the mono-substituted precursor 6PCb,t he electron-deficient 2-position of the pyridine ring was disfavored to ag reater extent in 6,10PyCb because symmetric 6,10NICz and unsymmetrical 1,10NICz were obtained in a ratio of 15.2:1. To control the regioselectivity of the CÀHa ctivation, we again employed the oxidation-reduction strategy. Indeed, the selectivity was inverted and 1,10NICz was the preferred product starting from 6,10PyCb-Ox (6,10NICz:1,10-NICz = 1:12.7). The complete inversion of the regioselectivity clearly underlines the potentialo ft he N-oxidem ethodology to control the outcomeofthe ring-closing step.

Characterization
After the isolation of all six possible mono-substituted NICzs and three symmetrical doubly substituted NICzs as well as their respective unsymmetrical congeners, we investigated the molecular properties of the new building blocks.

Photophysical properties
To investigate the effects of the nitrogen incorporation on the photophysical properties of the synthesized buildingb locks, UV/Vis absorption and fluorescences pectra in dichloromethane, as well as low temperature phosphorescence spectra were recorded (Figures 3, 4, and Ta ble 6). Molar attenuation co-efficients are given in Table S1 (Supporting Information). Plain ICz shows adistinct absorption peak at 285 nm that is attributed to a p-p*t ransition of the conjugated molecular scaffold. [25,26] This signald isappeareda lmostc ompletely with the incorporation of nitrogen in the doubly annulated centralb enzene ring in 1NICz and 2NICz.T he lowest-energy transition of ICz can be observeda saclear signal at 363 nm with as houlder at shorter wavelength. In contrast, 1NICz and 2NICz show a ratherb road and weak absorption at longer wavelengths. Although ICz and 2NICz have nearly the same absorption onset at approximately 375 nm, the absorption onset of 1NICz is clearlyr edshifted to 400 nm. The isomersw ith an itrogen in one peripheral benzene ring show distinctp eaks between 280-300 nm. Analogous to ICz, 5NICz and 6NICz exhibit as harp absorption peak at about [a] Overall yield of ring-closed products (ratio determined by 1 HNMR) and yields of isolatedm aterial in brackets. For reactionss tarting from Noxides, yields over two steps (CÀHa ctivationa nd reduction) are given.
[b] Not determined owingt oo verlapping signals from small amounts of dehalogenated byproduct. The absorption characteristics of unsymmetrically,d oubly nitrogen-substituted derivatives 1,4NICz and 1,10NICz as well as 2,5NICz clearly resemble those of 1NICz and 2NICz (Figure 3, Ta ble 6). This result is in line with the finding that compounds 4-7NICz more closely resemble plain ICz.T hus, nitrogen substitutioni nt he central benzene ring is the crucial factor determining the absorptionp roperties of the doubly substituted derivatives. Notably,t he distinct absorption band of 4NICz at   295 nm is also observed for 1,4NICz.T he onset of absorption of unsymmetrical 1,4NICz is identical to that of 1NICz at 400 nm, whereas the broad lowest-energy transition of 1,10NICz is shiftedt ol onger wavelengths with an absorption onset of 410 nm. In contrast, the absorption onset of unsymmetricali somer 2,5NICz at 353 nm is shifted towards higher energy compared with 2NICz (373 nm). Accordingly, the optical band gaps of the unsymmetrically,d oubly substituted derivatives span al arger range of 0.49 eV compared with the mono-substituted NICzs. Symmetricalb uildingb locks 4,12NICz, 5,11NICz,a nd 6,10NICz exhibit absorption properties similar to their respective mono-substituted congeners. The absorption onset of 4,12NICz and 5,11NICz are slightly blueshifted compared with 4NICz and 5NICz,w hereas the onseto f6,10NICz is shifted to as lightly longer wavelength than that of 6NICz.
Both 1NICz and 2NICz exhibit similar fluorescencec haracteristics to those of ICz.A nalogous to the absorption onset, the emission maximum of 2NICz is found at the same wavelength as that for ICz (375 nm), whereas the emission of 1NICz is distinctly redshifted to 407 nm. In contrast, all isomers with nitrogen substitution in the peripheral benzene ring exhibit a sharper emission peak in this area as well as ar edshifted shoulder. The nitrogen position influences the emission maxima, which follow the same order as the absorption onset from 5NICz (354 nm) to 7NICz (364 nm), 4NICz (375 nm), and 6NICz (384 nm).
The unsymmetrical substituted isomers 1,4NICz, 2,5NICz, and 1,10NICz each show one broad emission peak. Similart o 1NICz,t he emission maximao f1,4NICz and 1,10NICz are ratherr edshifted at 410 and 416 nm, respectively.I nc ontrast, 2,5NICz exhibits an emission maximuma t3 61 nm, which is blueshifted compared with that of 2NICz and closer to the emissiono fp eripherally substituted 5NICz.A nalogous to the absorption properties, 5,11NICz and 6,10NICz feature similar emission properties as their respective mono-substituted derivatives. Notably, 5,11NICz exhibits the highest-energy emission of the investigated materials with an emissionm aximum at 345 nm. Unlike the otherd erivatives, the emission characteristic of 4,12NICz clearly differs from that of 4NICz.A lthough the emission maximum of 4,12NICz is redshifted to 392 nm compared with 4NICz an additional high-energyb and at 372 nm emerges.
The triplet energy,c orresponding to the highest-energy phosphorescent transition, of buildingb locks for organic electronics is ad ecisive factor in particular for applicationsi n OLEDs. [10] Therefore, phosphorescence spectra of the developed buildingb locks werer ecorded. All compounds exhibit vibronically resolved phosphorescence. The phosphorescence spectra of the NICz isomers with the nitrogen in the peripheral ring are similar to the spectrum of ICz.A lthough the highest energy maximum of 5NICz at 436 nm is the same comparedt o ICz,t his transition is slightly redshifted for 7NICz (439 nm), 4NICz (443 nm), and 6NICz (446 nm). Notably,t his redshift occurs in the same order as for the fluorescence. In contrast, 1NICz and 2NICz show clearly different phosphorescence. The first maximum is blueshifted to 428 nm in the case of 2NICz, but redshifted to 445 nm for 1NICz.T he spectra of the unsymmetrically doublys ubstituted NICz isomers are clearly impacted by the nitrogen atoms in the central benzene ring because they show great similarity to the spectra of 1NICz and 2NICz. Although 1,4NICz and 1,10NICz have highest energy maxima at 456 and 455 nm, respectively, 2,5NICz is blueshifted to 429 nm. The spectra of the symmetrical doubly substituted isomers are similart ot he peripheral mono-substituted derivatives. Accordingly,t he highest-energy vibronic transitions of 5,11NICz and 4,12NICz are located at about the same energy as those of 5NICz and 4NICz at 432 and 442 nm, respectively. In contrast, this transition (436 nm) is slightly blueshifted for 6,10NICz compared with 6NICz.N otably,t he triplet energies of the NICz derivatives are located between 2.90 and 2.72 eV and are therefore suitable for the development of functional organic materials for applications in blue, and even deep blue, phosphorescent and TADF-based OLEDs. [57] In summary,c lear trends for the impacto ft he different nitrogens ubstitution positions weref ound. Substitution of one of the peripheral benzene rings (position 4-7, Figure 1) resulted in photophysical properties similart ot hose of plain ICz,i n which the exact energetic localization of absorption and emission (blueshift, redshift) depended on the substitution position. Derivatives with symmetrical doublesubstitution of the peripheral rings follow these tendencies, although the resulting shifts were generally more pronounced. In contrast, substitution of the central benzene ring (position 1o r2 )significantly altered the photophysical behavior,i nw hich nitrogen incorporation at position 1l ed to as hift towards lower energies and incorporation at position 2t owards highere nergies. In the case of the unsymmetrical doubly substituted derivativesw ith one pyridine-like nitrogen in the central ring and one in ap eripheral benzene ring, the effect of the nitrogen in the centralb enzene ring clearlyd ominates and those derivatives feature similar properties to 1NICz and 2NICz.

Electrochemical properties
The exact energetic alignment of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of organic materialsi so ft remendous importance regarding charge transport in and charge injection into electronic devices. Therefore, cyclic voltammetry measurements were conducted to explore the effects of nitrogen incorporation andt he impact of the nitrogen position on the frontier molecular orbitals (Figures S67-S73, Supporting Information). All NICz derivatives exhibited irreversible oxidation, as typically observed for 9H-carbazole and ICz derivatives, owing to the instabilityo ft he formed oxidation products. [25,58] The determined energy values of the HOMOsa nd LUMOs of the developed NICzsa re summarized in Table 6a nd selected energyl evels are depicted in Figure 5 For the mono-substituted NICz variants, the biggest impact on the HOMO levels is observed for compounds 2NICz (À6.28 eV) and 5NICz (À6.32 eV) with the pyridine nitrogen para to the central nitrogen atom. Notably,t he effect on the LUMO levels of these two compounds is distinctly weakera nd thus 2NICz (À2.43 eV) and 5NICz (À2.42 eV) feature the highest LUMO levels of the mono-substituted series. In contrast, the overall effect of nitrogen incorporation is least pronounced for ortho-substituted 7NICz.T herefore, within this series, 7NICz exhibits the highest HOMO energy of À6.00 eV,c orresponding to a0 .22 eV decrease compared with ICz,a nd aL UMO level at À2.46 eV.C ompounds 1NICz, 4NICz,a nd 6NICz with a metasubstitution pattern display similar electrochemical behavior. The LUMO levels of these three compounds are impacted the most by nitrogen incorporation and decrease to À2.68, À2.57, and À2.62 eV,r espectively,w hereas their HOMO levelsa re located in an arrow range of À6.22 to À6.23 eV between those of the ortho-a nd para-substituted materials.
The influenceo ft he nitrogen positiono nt he electrochemical properties was rationalized by the exploration of the spatial distribution of the HOMO and the LUMO of the investigated derivatives (Figures 6a nd S74, S75, SupportingI nformation). HOMO and LUMO levels of the materials were calculated employing density functional theory.A sd epicted in Figure 6, significant electron density of the HOMO level of ICz is located on the central nitrogen,a sw ell as C2, C5, and C11, which are located para to the central nitrogen. Analogously, hardly any electron density is located on C1, C3, C6, and C10, which are all located meta to the central nitrogen atom. The distribution of the electron density explainsw hy para-nitrogens ubstitution influencest he energetic location of the HOMO levels the most. Indeed, the HOMO level of 2NICz is strongly distorted compared with ICz ( Figure 6). The concentrationo ft he electron density on the two benzene rings and the reduction of the spatiale xtension of the molecular orbitald ecrease the orbital energy of the HOMO of 2NICz.I nc ontrast, the shape of the HOMO of 1NICz is very similar to that of ICz and in the case of the meta-substituted NICz, the decreasedo rbitale nergy can be explained by ag eneral inductive effect of the pyridine nitrogen.T he electron density of the LUMO of ICz is located virtually exclusively on carbon atomst hat are meta to the central nitrogen and the annulation positions. Thisf inding explains the significantly decreased energy of the LUMOs of 1NICz, 4NICz,a nd 6NICz.A nalogous to the HOMO of 2NICz,asimilar distortion of the shape of the LUMO of 1NICz is observed. However,t he restriction of the LUMO is less pronounced, which is in agreement with the finding that the LUMO energy  levels are generally influenced to as lightly lower degree by the nitrogen incorporation. As expected, the energy levels of the HOMOsa nd LUMOs are further lowered by the introduction of as econd pyridine ring. Compared with ICz,t he HOMO levels of the doubly substituted NICzs are decreased by 0.50-0.70 eV and the LUMO energy levels are lowered by 0.27-0.58 eV.T he same trends as for the mono-substituted derivatives were observed. Accordingly, para-substituted 2,5NICz exhibits the lowest HOMO energy level among the compounds in this study at À6.48 eV whereas the HOMO level of 5,11NICz is located slightly higher at À6.42 eV.A nalogous to 2NICz and 5NICz,t he LUMO levels of the doubly substituted derivatives 2,5NICz and 5,11NICz are located at À2.58 and À2.54 eV and thus are significantly higher compared with the LUMOs of the meta-substituted compounds.T he meta-substituted derivatives can be divided into two groups;u nsymmetrical 1,4NICz and 1,10NICz feature as maller electrochemical band gap compared with symmetrical 4,12NICz and 6,10NICz.A ccordingly, 1,4NICz and 1,10NICz exhibit the lowest LUMO energy levels among the developed materials (À2.84 and À2.85 eV,r espectively).

Crystal packing
The arrangement and p-p interactions of individual molecules in the solid state can significantly impactthe macroscopic photophysical and electronic properties of thin films of organic materials. [59] In particular, CÀH···N hydrogen bonds can be used to induceo rc ontrol specific interactions between molecules and influence their alignment. [46,47] Hence, we were interested in the packing of the newly developed NICz molecules because the incorporation of the pyridine-like nitrogen atom into the molecular scaffold can lead to the potentiali nduction of CÀ H···N hydrogen bonding. Therefore, singlec rystals suitable for X-ray structure determinationw ere grown by recrystallization of 2NICz, 5NICz, 6NICz, 7NICz, 2,5NICz,a nd 6,10NICz.C rystals of 1NICz were obtained only as the acetonitrile solvate. Compounds 2NICz, 5NICz,a nd 2,5NICz featured temperature-dependentp olymorphism and twinning, which is beyond the scope of this contribution and will be discussed in detail elsewhere. [60] Atomsw ere labeled as outlined in Figure 1. In the case of more than one crystallographically unique molecule (Z' = 2), prime characters are added to the atom names.
The molecular packing in the solid state is generally determined by non-classical CÀH···N hydrogen bonding. [61,62] In these kind of non-classical hydrogen bonds, electrostatic interactions are more prominent than covalent bonding. [61] Their influence on the structure is not as pronounced as in the case of strong hydrogenb onds. Notably, some of the interactions reported here feature H···N distances longer than the sum of their van der Waals radii (2.75 ). If possible, the lone pairs of the Na toms connect to the hydrogen atoms at the 7a nd 9 positions (Table S2, Supporting Information). In af ew exceptions, the Ha tom in the 2p ositiona cts as donor.F rom at opological point of view,t he hydrogen contacts form one-dimensional chains, which are usually connectedb yp-p interactions to layers.The layers,inturn, are stacked to give the final crystal structure. Am ore detailed description of the molecular arrangements will be given first for the molecules with N-substitution para to the central N8 atom (2NICz, 5NICz, 2,5NICz), then meta (1NICz, 1,10NICz, 6NICz, 6,10NICz), and finally ortho (7NICz).
para-N-substitution:T he hydrogen bonding of 2NICz and 2,5NICz molecules forms straight chains as shown in Figure 7.
In both cases, the molecules are located on a( pseudo-)twofold rotationa xis;t his (pseudo)symmetry is the crucial feature of the phase transitions described elsewhere. [60] The mutual inclination of adjacent molecules with respectt ot his axis is determined by the crystal packing: In 2NICz,t he least-squares (LS) planes defined by the Ca nd Na toms of the individual aromatic planeso ft wo adjacent molecules are inclinedb y6 3.48.I n contrast, 2,5NICz exists as two polymorphs;the first is isostructural with 2NICz,w hereas in the second, adjacent molecules are perfectly coplanar (adjacent moleculesr elatedb yab + c lattice translation). In both 2,5NICz polymorphs, the molecule is 1:1d isordered with respect to the N5 atom, which is not involved in hydrogen bonding.
The chains are in all cases connected to layers by p-p interactions.A djacentc hains within these layers are relatedb ya na (2NICz, 2,5NICz first polymorph) or a b (2,5NICz,s econd polymorph)l attice translation.T he distances between the leastsquares planeso fi ndividual molecules of neighboringc hains are 3.45 in all cases. Neighboringc hains of two adjacent layers feature different propagation directions for the two structures ( Figure 8). In the crystalso f2NICz and the first polymorph of 2,5NICz,t he chains are parallel and extend in opposite directions. In contrast, in the second polymorph of 2,5NICz,n eighboring interlayer chains are inclinedb y4 2.718 but propagate in the same direction. No notablei nterlayer CÀ H···p interactions are observed in either of the structures.
For 5NICz,a nalogous chains form in the solid state (Figure 9a). Here, the hydrogen bond acceptor N5 is located off the moleculara xis (C2ÀN8). Accordingly,t he H7···N5 and H9···N5 contacts and the molecular axis form angles of approximately 908 andt he formed molecular chains adopt az ig-zag pattern. Adjacent molecules in these chains are related by a2 1 screw rotationa xis and are nearly coplanar (angle between LS planes: 3.558). As previously observed, the chains are connected by p-p interactions, in whicha djacent chains are related by inversion symmetry (Figure 9b). The distance between connected molecules is approximately 3.55 (a precisev alue cannotb eg iven because the connected molecules are not perfectly coplanar). The formedl ayers are stacked along [100]. In principle, nitrogen atoms at the 2a nd 5p ositions can form similar CÀH···N hydrogen bonds with the hydrogens at the 7 and 9p ositions. Yet, only N2 is involved in hydrogen bonding in 2,5NICz,indicating that this interaction is preferred. meta-N substitution:C oncerning the meta-substituted 1NICz,o nly acetonitrile solvate crystals were formed, in which the electron lone pair of N1 is directed towards the solvent filled voids of the structure ( Figure S76, Supporting Information). This is af irst indication that the formation of ah ydrogen-bond network is more difficult in thesec ases. The 1NICz molecules are connected through p-p interactions to rods extending along the [100] direction (Adjacentm olecules related by an a-lattice translation;d istance of LP planes:3 .41 ); these rods surround the solventfilled voids.
Crystalso f1,10NICz,i nc ontrast, consist of two crystallographically independentm olecules. These molecules are connected by H2···N10' and H9'···N1 contacts, with each molecule of the pair acting as donor and acceptor ( Figure 10 a). Adjacent molecules are related by ap seudo-b [001] glide reflection (the P2 1 structureh as pseudo-Pc2 1 b symmetry) forming chains extending in the [010] direction. The two crystallographicallyi ndependentm olecules are nearly coplanar (angle between LS planes:3 .658). Analogous to 2NICz and 2,5NICz,t he chains are connected to layers by p-p interactions (adjacent chainsr elated by an a-lattice translation;d istance of LS planes3 .47 in both cases). Notably,t he arrangement of the 1,10NICz chains closelyr esembles the structuref ormed by 2,5NICz (second polymorph). Chains in neighboring layers propagate in the same direction and the angle between the chains is 56.418 ( Figure 10 ba nd 10 c). The hydrogen-bonding networks in meta-substituted 6NICz is complex. Crystals of 6NICz contain two crystallographicallyi ndependent 6NICz molecules (Z' = 2). In contrast to 1,10NICz these molecules are not related by pseudo-symmetry.M olecules of the first kind are connected by CÀH···N hydrogen bonds, in which each molecule connects to two others, acting as hydrogen-bond donor and acceptor,r espectively.B ecause the donor( H7, H9) and acceptor (N6) atoms are in close vicinity,astrong twisting of the two connectingm olecules is required (angle betweenL Splanes: 77.88; Figure 11 a). The two molecules connected as donor and acceptor to one twisted molecule are coplanar (related by a blattice translation) and their LS planes are spaced by only 3.26 ,i ndicating as trong interaction of the p systems ( Figure 11 b). These fragments form chains extendinga long the [010] direction. Notably, p-p stacking does not occur between adjacent chains but between two molecules of the same chain, which are connected by at hird 6NICz molecule that    acts as ah ydrogen donor and acceptor for the stacked molecules, respectively (Figure 11 a). Therefore, in contrast to the previousc ases, the p-p interaction does not connect adjacent chains to two-dimensionall ayers but forms one-dimensional rods extending along [010] by intra-chain p-p interactions. For the second kind of 6NICz molecule in the unit cell, ar ather short CÀH···N contact to aH 2a tom is observed. The observed CÀH···N angle of 1358 indicates ar ather weaki nteraction that is probablyn ot structure-directing. These weak interactions form chains extending in the [001] direction (Figure 11 c). Analogous to 2NICz,t hese chains are connected tol ayers by p-p interactions through a b-lattice translation with ad istance between the LS planes of 3.31 .T hus, in the case of 6NICz,t he p-p interaction between the planar molecules results in different packing features for the two crystallographically independent molecules. Although intrachains tacking yields one-dimensional rods, interchain stacking leads to two-dimensional layers.
Finally,t he entire 6NICz crystal is built by stacking two alternating layers,t he first of which corresponds to the layer formed by the p-p stackingo fc hains of molecule 2a nd the second is made of neighboring rods of molecule 1t hat are packed by van der Waals interactions ( Figure S77, Supporting Information).
In 6,10NICz,o nly one of the Na toms is involved in hydrogen bonding. Similar to 6NICz,t he connected molecules( related by a2 1 screw rotation) are distinctly non-coplanar( angle between LS planes:5 3.08). In the resulting chains ( Figure S78, Supporting Information), pairs of moleculesa re again connected by intrachain p-p interactions (distance between LS planes: 3.41 )a nd form rods along [100].T hus, even thought he chains are relatedt opologically to one of the 6NICz chains (considering hydrogen bondinga nd p-p interactions), the actual geometry of the chains is influenced distinctly by packing effects. The chains are packed in ac heckerboardp attern connected only by van der Waals interactions ( Figure 12). ortho-N substitution:F inally,i n7NICz,n on otable intermolecular hydrogen-bonding interactions are observed, owing to steric shieldingo ft he nitrogen atom in the 7p osition.I nstead, molecules are connected by p-p interactions to rods (adjacent molecules related by an a-lattice translation;d istance of LS planes:3 .30 ); the rods are packed by van der Waals interactions ( Figure S79, Supporting Information).
In summary,t he investigated NICzs exhibited ar ich crystallization behavior,i nw hich p-p stacking and non-classical CÀ H···N hydrogen bonds proved to be structure-determining factors. Although all compounds featured p-p stacking of the planar aromatic scaffolds, intermolecular hydrogen bonds were not formed in crystalso f1NICz and 7NICz.N otably, the sole pyridine-like nitrogen of these two compounds is located next to the annulation positiona nd thus is not effectively available for hydrogen bonding due to steric reasons. Although the interactions between isolated molecules of the developed compoundsa re similar, the position of the nitrogen has ad ecisive impact on the exact packing of the crystals. Thus, depending on the nitrogen positiono ne-or two-dimensional supramolecular arrangements aref ormed. Compoundsl ackingh ydrogen bondsa re organized in rods by p-p stacking;t he rods are packed by van der Waals interactions (1NICz, 7NICz). When hydrogen bondingi sp ossible, chains are formed and connect to neighboring chains by p-p stacking to ultimately form layers (2NICz, 5NICz, 2,5NICz, 1,10NICz). Notably,the propagation direction of chains in adjacent layers depends on the nitrogen position. If the geometrical requirementso ft he hydrogen bonds preventlinear propagationoft he chains in the direction of the molecular axis (C2ÀN8), twisted chainsf orm and p-p stacking occurs between molecules of the same chain (6NICz, 6,10NICz). In such aw ay,t he chains are arranged to one-dimensional rods.

Conclusion
We have described the convenient synthesis of all six possible mono-substituted azaindolo[3,2,1-jk]carbazoles as well as three symmetric doubly substituted derivatives and their unsymmetrical regioisomers. Notably,t he presented CÀHa ctivation approach not only allows for the preparation of the azaindolo[3,2,1-jk]carbazole congeners described in this work, but the developedo xidation-reduction strategy also enables one to controlt he regioselectivityo ft he ring-closing CÀHa ctivation step. Therefore, many other doubly substituted regioisomers are now accessible employing this method and additional nitrogen incorporation can be achieved. We investigated the photophysical and electrochemical properties as well as the solid-state interactions of the developed materialsa nd revealed the impact of nitrogen incorporation in different positions of the indolo[3,2,1-jk]carbazole scaffold. Ultimately,w e have provided synthetic chemists with at oolbox full of NICz buildingb locks. The established structure-property relationships and the possibility to tune their molecular properties and controlt he supramolecular arrangement of individual molecules predestines the class of azaindolo[3,2,1-jk]carbazoles as a novel molecular platform and could guidem aterial scientists in the design of functional organic materials with tailored properties. Figure 12. Packing of 6,10NICz.E ach 6,10NICz moleculeconnects as hydrogen-bond donor and hydrogen-bond acceptor to two different adjacent 6,10NICz molecules.Color codes as in Figure 7.

Experimental Section
All reagents and solvents were obtained commercially and used without further purification. Anhydrous solvents were prepared by filtration through drying columns. The water content of purchased DMA was determined by Karl Fischer titration using aM itsubishi CA-21 Moisture Meter and corrected to 3000 ppm for CÀHa ctivation screening reactions. Given that further experiments indicated no significant impact of the water content on the reaction outcome, large scale experiments were conducted using unmodified DMA with roughly 100 ppm H 2 O. Screening reactions were performed in glass vials under argon atmosphere in ac ontrolled heating block using argon-saturated substrate solutions including 1methylnaphthalene as internal standard as well as ac atalyst solution. All screening experiments were performed three times independently.T he depicted results represent the mean of these experiments. Yields of the screening reactions were determined by GC using aT hermo Scientific TRACE 1310 gas chromatograph with dual configuration consisting of two AS 1310 autosamplers, Thermo Scientific TR-5MS columns and FID detectors. Absorption and photoluminescence measurements were conducted using a PerkinElmer Lambda 750 spectrometer and aP erkinElmer LS 55 fluorescence spectrometer,r espectively.C H 2 Cl 2 solutions (5 mm) were employed for solution measurements whereas phosphorescence spectra were recorded at 77 Ku sing solid solutions of the materials in toluene/EtOH (9/1;0 .5 mg mL À1 )w ith ad elay of 1ms. Cyclic voltammetry was measured using at hree-electrode configuration consisting of aP tw orking electrode and counter electrode, and an Ag/AgCl reference electrode and aP GSTAT128N potentiostat provided by Metrohm Autolab B.V.T he measurements were carried out in a0.5 mm solution in HPLC-grade acetonitrile employing nBu 4 NBF 4 (0.1 m)a ss upporting electrolyte. Prior to the measurements, the solutions were purged with nitrogen for approximately 15 minutes. The HOMO and LUMO energy levels were calculated from the onset of the oxidation and reduction peaks, respectively.T he onset potential was determined by the intersection of two tangents drawn at the background and the rising of the oxidation and reduction peaks. Synthetic details are described in the Supporting Information.
General procedure for CÀHa ctivation:Aglass vial was charged with the corresponding halogenated precursor (1 equiv.), K 2 CO 3 (2 equiv.), and (NHC)Pd(allyl)Cl (5 mol %) and flushed with argon. After addition of 10 mL mmol À1 degassed DMA, the reaction was stirred under argon atmosphere at 130 8Cu ntil full conversion was reached (4-8 h). After cooling, the reaction mixture was poured into water and repeatedly extracted with CH 2 Cl 2 .T he organic phases were dried over anhydrous Na 2 SO 4 and concentrated under reduced pressure. The crude product was purified by column chromatography.
Computational details:D FT calculations were performed using the Gaussian 09 package [63] applying the Becke three-parameter hybrid functional with Lee-Yang-Perdew correlation (B3LYP) [64][65] in combination with Pople basis sets 6-311G(d,p). [66] Geometry optimizations were performed in the gas phase and without symmetry constraints. Orbital plots were generated using GaussView. [67] Single-crystal diffraction:X -ray diffraction data of 1NICz·xCH 3 CN, 2NICz, 5NICz, 6NICz, 7NICz, 1,10NICz, 2,5NICz,a nd 6,10NICz were collected at T = 100-270 Ki nadry stream of nitrogen on aB ruker Kappa APEX II diffractometer system using graphite-monochromatized MoK a radiation (l = 0.71073 )a nd fine sliced f-a nd w-scans. Data was reduced to intensity values with SAINT and an absorption correction was applied with the multi-scan approach implemented in SADABS or TWINABS. [68] The structures were solved by the dual-space approach implemented in SHELXT [69] and refined against F 2 with JANA2006. [70] Non-hydrogen atoms were refined anisotropically.T he Ha toms connected to Ca toms were placed in calculated positions and thereafter refined as riding on the parent atoms. Contributions of disordered solvent molecules to the intensity data were removed for 1NICz using the SQUEEZE routine of the PLATON [71] software suite. Compounds 2NICz, 5NICz,a nd 1,10NICz crystallize as twins by pseudo-merohedry.C ompound 7NICz forms twins with non-overlapping reflections and was refined against HKLF5 data with information on reflection overlap. The absolute structure of non-centrosymmetric crystals (1NICz·xCH 3 CN, 1,10NICz, 2,5NICz,a nd 6,10NICz)w as not determined owing to a lack of resonant scatterers. Molecular graphics were generated with the program MERCURY. [72] CCDC 1864321 (1,10NICz·xCH 3 CN), 1864322 (1NICz·xCH 3 CN), 1864323 (2,5NICz), 1864324 (2NICz), 1864325 (5NICz), 1864326 (6,10NICz), 1864327 (6NICz)a nd 1864328 (7NICz)c ontain the supplementary crystallographic data. These data can be obtained free of charge by The Cambridge Crystallographic Data Centre.