Alkylated Indolo[3,2,1-Jk]carbazoles as New Building Blocks for Solution Processable Organic Electronics

A facile strategy for the introduction of tert-butyl and hexyl chains to the indolo[3,2,1-jk]carbazole scaffold is presented. With these building blocks six materials based on three different 4,4ʹ-bis(N-carbazolyl)-1,1ʹ-biphenyl derivatives with varying degree of planarization were prepared. Characterization of the materials showed that the alkyl chains have only minor effects on the photophysical properties of the compounds. In contrast, thermal robustness towards decomposition and electrochemical stability are increased by the introduced alkyl chains. Detailed investigation of the solubility in five different solvents revealed that the incorporation of the alkyl chains increases the solubility significantly. The increased solubility of the materials allowed the application as host materials in red, green and blue solution processed PhOLEDs. Hence, this work presents the first solution processed OLED devices based on the indolo[3,2,1-jk]carbazole scaffold. Abstract A facile strategy for the introduction of tert -butyl and hexyl chains to the indolo[3,2,1- jk ]carbazole scaffold is presented. With these building blocks six materials based on three different 4,4ʹ-bis( N -carbazolyl)-1,1ʹ-biphenyl derivatives with varying degree of planarization were prepared. Characterization of the materials showed that the alkyl chains have only minor effects on the photophysical properties of the compounds. In contrast, thermal robustness towards decomposition and electrochemical stability are increased by the introduced alkyl chains. Detailed investigation of the solubility in five different solvents revealed that the incorporation of the alkyl chains increases the solubility significantly. The increased solubility of the materials allowed the application as host materials in red, green and blue solution processed PhOLEDs. Hence, this work presents the first solution processed OLED devices based on the indolo[3,2,1- jk ]carbazole scaffold. constants are reported in Hertz; multiplicity of signals is indicated by using following abbreviations: s=singlet, d=doublet, t=triplet, q=quartet, quint=quintet and m=multiplet. High resolution mass spectra (HRMS) (m/z 50-1900) were obtained on a maXis UHR ESI-Qq-TOF mass spectrometer (Bruker Daltonics, Bremen, Germany) in the positive-ion mode by direct infusion. The sum formulas of the detected ions were determined using Bruker Compass DataAnalysis 4.1 based on the mass accuracy (Δm/z ≤ 5 ppm) and isotopic pattern matching (SmartFormula algorithm). experimental setup of the solubility study, cyclic voltammograms, DSC-/STA-curves, 1 H and 13 C NMR spectra of the materials, as well as external quantum efficiency – luminance curves and photoluminescence spectra of the devices can be found in the supporting information.


INTRODUCTION
Since the introduction of organic electroluminescent thin films, [1] followed by applications in first organic light emitting diodes (OLED) based on small molecules [2] and polymers, [3] functional organic materials for electroluminescent devices have been improved continuously. Especially the introduction of new light harvesting mechanisms like the use of transition metal emitters in phosphorescent OLEDs (PhOLEDs) [4,5] and organic emitters using thermally activated delayed fluorescence (TADF) [6,7] resulted in improved efficiencies. However, careful material design and device engineering is necessary to further improve device performance.
Recently, we investigated bipolar host materials using triarylamines with different degree of planarization, as donor subunits. Modification of the planarization allowed to control the intramolecular conjugation, as increasing planarization resulted in a decreased donor strength. Hence, the use of fully planarized indolo[3,2,1-jk]carbazole (ICz, molecular structure see chart 1, top, left) reduced intramolecular charge transfer and resulted in high triplet energies, as well as good thermal stability. [8] Besides its weakened donor strength, the versatility of the building block can be seen as itself has bipolar character [8][9][10] and can also be used solely as acceptor building block. [11,12] Furthermore, the electronic properties can be tuned by incorporation of sulfur atoms to increase the donor strength, [13] as well as nitrogen incorporation [14] or substitution with cyano groups [15] to increase the acceptor strength.
Although ICz is known to literature for a long time, [16] it took quite long until first studies on conducting thin-films were executed. [17,18] However, the development of more feasible synthetic approaches based on palladium catalyzed methodologies, [8,19,20] paved the way for the increased use of the ICz scaffold in organic electronics. In recent years the ICz scaffold became especially important as building block in dye sensitized [21][22][23] and perovskite solar cells, [24] and OLEDs. Particularly successful was the application of ICz as integral functional unit in host materials for PhOLEDs [8][9][10][25][26][27] and TADF-OLEDs, [28] but also as building block for fluorescent [12,27,29,30] and TADF emitters. [11,12,15] Additionally, the planar ICz motif was implemented in larger conjugated systems in electron donating molecules or materials employed as emitters. [30][31][32][33][34] So far, all ICz based materials for OLED devices were exclusively processed by vacuum deposition.
Besides the fact that small molecules are usually processed by vacuum deposition, [35] the planar and rigid backbone of ICz based materials causes low solubility which is usually not sufficiently high for solution processing techniques. With this work we aimed to further broaden the scope of the ICz building block by modifying the scaffold to increase its solubility and make it applicable for solution processing.
Despite some challenges, like the fabrication of multi-layered devices, there is an increasing interest in the fabrication of small molecule devices from solution, due to several advantages as milder conditions, faster processing and less material loss. [35][36][37] However, beside several other factors adequate solubility in certain solvents remains as crucial factor for small molecules to be processed from solution. Our goal was to introduce alkyl chains to increase the solubility of the ICz building block and demonstrate the applicability of ICz based materials for solution processed OLEDs for the first time. In order to compare the effect of different alkyl chains we have chosen tert-butyl and n-hexyl as target groups that should be incorporated into the scaffold.
While tert-butyl mono-substitution in position 2, double-substitution in positions 2 and 5 as well as trifold-substitution (2,5,11) are known in literature, [19] our goal was to achieve substitution in positions 5 and 11 to yield a symmetric building block (see chart 1). To get better insight on the effect of the alkyl chains on solubility and material properties we have chosen target materials based on the 4,4ʹ-bis(N-carbazolyl)-1,1ʹ-biphenyl (CBP) scaffold, that were already implemented as host materials for PhOLEDs (chart 1, bottom line). [9] Due to the different degree of planarization these materials have very diverse solubilities which allows for better comparison of the effect of alkylation. As further proof of concept, the new materials were used as host materials for solution processed PhOLED devices.

Synthesis
The synthetic strategy towards the alkylation of the ICz building block is depicted in scheme 1 (left side, a). The tert-butyl substituted building block 2a was synthesized in one step starting from the brominated ICz precursor 1 by Friedel Crafts alkylation using 2-chloro-2-methylpropane and ZnCl2 as lewis acid in nitromethane. As substitution occurs exclusively in the desired positions para to the central nitrogen atom an excess of the alkylation reagent could be used yielding 2a almost quantitatively. Introduction of the hexyl chains was achieved in a similar approach employing first a Friedel-Crafts acylation with hexanoyl chloride and AlCl3 as lewis acid. Subsequently, the carbonyl groups were reduced with LiAlH4 giving the hexyl building block 2c with an excellent yield of 80% over both steps. Finally, the halogenated precursors were converted to the corresponding boronic acid esters by Miyaura borylation, to prepare the precursors required for Suzuki cross coupling. Using standard conditions with PdCl2(dppf) and KOAc as base, 3a and 3b were obtained in good yields of 70% and 71%, respectively. Furthermore, for the synthesis of the CBP derivatives the alkylated carbazole and phenylcarbazole species were required. Alkylation of 9H-carbazole was achieved according to literature procedures. [38,39] While 5b is not known to peer-reviewed literature it can be prepared in the same way as 5a by a nucleophilic substitution of 1-bromo-4-fluorobenzene with the alkylated carbazole using Cs2CO3 as base. [40] Analogously to the ICz building blocks, 6a and 6b were converted to the boronic acid ester species by Miyaura borylation. [41] Scheme 1. Synthetic approach towards alkylation of 1 and synthesis of target materials RICzCz, RICzPCz and RICzICz (R = tert-butyl, hexyl). Reaction conditions: i: 2-chloro-2-methylpropane, ZnCl2, CH3NO2, 0 °C -rt; ii: hexanoyl chloride, AlCl3, CH2Cl2, 0 °C -rt; iii: LiAlH4, AlCl3, Et2O, CH2Cl2, 0 °C -rt; iv: bis(pinacolato)diboron, KOAc, PdCl2(dppf)*CH2Cl2, DMF, 100 °C; v: 1-bromo-4-fluorobenzene, Cs2CO3, DMF, 150 °C; vi: K2CO3, CuSO4*5H2O, 230 °C; vii: K2CO3 (2M in H2O), Pd(PPh3)4, THF, reflux.
With all the alkylated building blocks in hand the target materials RICzCz, RICzPCz and RICzICz (R= tertbutyl or hexyl) could be synthesized (scheme 1 right, b). C-N bond formation of the carbazole based materials was accomplished by Ullmann condensation of the halogenated ICz building block with the corresponding carbazole. A solvent free reaction protocol using CuSO4*5H2O and K2CO3 at 230 °C yielded tBuICzCz and HexICzCz with 77% and 78% yield, respectively. The remaining materials were synthesized by Suzuki coupling using Pd(PPh3)4 with aqueous K2CO3 as base in THF with yields ranging from 35% to 74%. All intermediates and target materials were characterized by 1 H and 13 C NMR spectroscopy as well as high-resolution mass spectrometry.

Solubility
The effect of the introduction of alkyl chains on the solubility of the materials at room temperature was determined in chloroform, isopropanol, n-hexane, tetrahydrofuran and toluene by absorption spectroscopy. The results of these experiments are summarized in table 1. A graphical comparison of the solubility in chloroform is depicted in figure 1.  all materials show a distinct absorption peak between 286 nm and 292 nm that can be attributed to the π -π* transitions of the materials. [9] In analogy to the absorption onset, these peaks are slightly redshifted in the case of alkylated RICzCz and RICzPCz. For ICzCz and ICzPCz a shoulder at higher wavelength is observed close to these peaks, which appears as distinct peak at 300 nm in the case of the alkylated species. In contrast this feature cannot be found for RICzICz derivatives.
Analogously, the emission maxima of the fluorescence spectra of the RICzCz and RICzPCz derivatives, are shifted to higher wavelengths. Notably, hexyl substituents induce a slightly more pronounced shift compared to tert-butyl. In contrast, the emission maxima of tBuICzICz and HexICzICz are not shifted compared to the non-alkylated material.  In order to determine the triplet energies of the materials, low temperature phosphorescence spectra were recorded at 77 K. Compared to the room temperature fluorescence all materials exhibit vibronically resolved spectra. Except of tBuICzCz and HexICzCz which show a more intense redshifted shoulder at the highest energy transition compared to ICzCz, the spectra of all other alkylated materials exhibit a very similar shape compared to their parent derivatives. In analogy to the fluorescence spectra, the phosphorescence spectra of the alkylated derivatives of RICzCz and RICzPCz are slightly redshifted compared to ICzCz and ICzPCz, albeit the shift is less pronounced compared to the fluorescence spectra. Again, such a redshift is absent in the RICzICz series. Moreover, all alkylated materials retain high triplet energies >2.81 eV, which makes them applicable as host materials in blue PhOLED devices.

Electrochemical properties
The HOMO energy levels of the materials were estimated from the onset of the oxidation peaks obtained during cyclic voltammetry measurements. LUMO energy levels were calculated from the HOMO levels and the optical gaps determined by UV/Vis absorption spectroscopy. The results are summarized in table 2. Furthermore, the alkylated materials show reversible oxidation (consecutive oxidation runs depicted in figure 3 and supporting information figures S2 and S3). For indolo[3,2,1-jk]carbazole, [17] as well as carbazole [42,43] based materials the high reactivity of the radical cationic species which is formed during oxidation, leads to formation of oligomeric species which are coupled via the para position of the nitrogen. This oligomerization can be observed by the formation of films on the electrodes. [17] Therefore, the incorporation of alkyl chains in these para positions increases the electrochemical stability of the materials making the oxidation reversible over several scans, without any electrochemical indication dimerization or polymerization and without the formation of thin films. This indicates that these don't solidify again after the first cycle.

OLED devices
The developed materials were evaluated as universal host materials for RGB PhOLED devices, to investigate the applicability of the alkylated ICz scaffold for solution processing. tBuICzCz (I),  In summary, employing the hexyl substituted host materials resulted in higher efficiency of the devices, compared to the tert-butyl substituted materials. Notably, employing HexICzPCz in RIV, GIV and BIV, yielded the best performance within each series. However, in the case of red and blue devices the hexyl substitution resulted in lower brightness and high efficiency roll-off, which might be attributed to varying film formation of the different alkyl chains.

CONCLUSIONS
We developed a reliable, easy and high yielding synthetic approach for the introduction of tert-butyl and hexyl chains into the ICz building block that can be easily upscaled to multi-gram synthesis. To

EXPERIMENTAL SECTION
All solvents and reagents were obtained commercially and used without further purification. Zinc chloride was dried by melting under vacuum prior to use. Anhydrous solvents were prepared by filtration through drying columns. Column chromatography was performed on silica 60 (Merck, 40-63 μm). Prior to device fabrication, all materials were purified by subsequent refluxing and filtration after cooling in tert-butanol, isopropanol and acetonitrile.
Absorption and photoluminescence measurements were conducted using a Thermo Scientific NanoDrop One C UV-Vis spectrophotometer and a PerkinElmer LS 55 fluorescence spectrometer, respectively. CH2Cl2 solutions (5 µM) were employed for solution measurements while phosphorescence spectra were recorded at 77 K using solid solutions of the materials in toluene/EtOH and isotopic pattern matching (SmartFormula algorithm).

Device fabrication
The devices were fabricated on clean glass substrates precoated with an indium tin oxide (ITO) layer

9-(4-Bromophenyl)-3,6-dihexyl-9H-carbazole (5b
flushed with argon. After addition of degassed DMF (2.5 ml/mmol) the reaction mixture was stirred at 100 °C until full conversion. The reaction was cooled to room temperature, poured into water and repeatedly extracted with CH2Cl2. The combined organic phases were dried over anhydrous Na2SO4 and concentrated under reduced pressure.

General procedure for the solvent free Ullmann reaction (GP1)
Alkylated 2-bromoindolocarbazole (1.00 eq.), alkylated carbazole (1.50 eq.), K2CO3 (2.00 eq.) and CuSO4*5H2O (0.10 eq.) were added to a glass vial stirred and at 230 °C in a heating block until full conversion according to TLC. After cooling to room temperature, the residue was dissolved in water and CH2Cl2. The phases were separated, and the aqueous phase extracted with CH2Cl2. The combined organic phases were dried over anhydrous Na2SO4 and concentrated under reduced pressure.

General procedure for the Suzuki coupling (GP2)
Alkylated 2-bromoindolocarbazole (1.00 eq.) and the corresponding boronic acid ester (1.25 eq.) were added to a three-neck flask and flushed with argon. Under argon counterflow Pd(PPh3)4 (5 mol%) was added. After addition of argon degassed anhydrous THF (20 ml/mmol) and aqueous K2CO3 (2.50 eq., 2M solution) the reaction was refluxed until full conversion according to TLC. After cooling to room temperature, the mixture was poured into water and extracted repeatedly with CH2Cl2. The combined organic phases were dried over anhydrous Na2SO4 and concentrated under reduced pressure.

1) Solubility study
The solubility of the materials was determined by absorption spectroscopy. Saturated solutions (each ~200 µl) of the materials in different solvents were prepared and stirred at 22 °C for one hour. The mixture was filtered through a syringe filter (Acrodisc 13 mm with 0.2 µm PTFE). The solvent of an exact aliquot (50 µl -100 µl) was removed under reduced pressure. Afterwards, the residue was dissolved in CH2Cl2 and diluted accordingly to reach the linear absorption range (0.1 -1.0 a.u.). All experiments were repeated three times. The calibration curves for all derivatives are shown in figure   S1. Figure S1. Calibration curves of the concentration dependent absorption measurements of all materials. Absorption wavelength used for the corresponding material noted in brackets.   Figure S4. TGA curves of tert-butyl (left) and hexyl (right) substituted materials.