Buchwald–Hartwig Amination of Aryl Halides with Heterocyclic Amines in the Synthesis of Highly Fluorescent Benzodifuran-Based Star-Shaped Organic Semiconductors

The study of palladium-catalyzed amination of bromobenzene with aromatic and heterocyclic amines, widely used in the synthesis of organic semiconductors, was performed. The best conditions for the coupling of aryl bromides with carbazole, diphenylamine, phenoxazine, phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, and their derivatives have been developed. Based on the results, nine new star-shaped organic semiconductors, exhibiting up to 100% fluorescent quantum yield in the 400–550 nm range, have been synthesized in good yields. The TDDFT calculations of the absorption spectra revealed a good correlation with experimental results and slight solvatochromic effects with a change in the polarity of the solvent.


■ INTRODUCTION
Benzodifurans (BDFs), due to their p-type organic semiconductor properties, excellent light absorption and emission capability, and high hole mobility, are a group of compounds with a great potential application as luminescent and electroluminescent materials. 1−4 In addition, they are much less studied compared to benzodithiophenes widely used in optoelectronics. The appropriate molecular design of the BDFbased semiconductors allows for their application in many fields, including molecular switches and electrical regulators, 5−7 high-affinity fluorescent probes, 8 potential therapeutic agents, 9 dye-sensitized solar cell sensitizers, 10−13 polymer materials in polymer solar cells, 14−22 organic solar cells, 23,24 organic thin-film transistor materials, 25,26 and different layers in organic light-emitting diodes (LEDs). 27−34 A huge number of organic semiconductors used in optoelectronics as electron transport layers (ETL) and electron injection layers (EIL), 35,36 hole transport layers (HTL) and hole injection layers (HIL), 37,38 hosts for phosphorescent and thermally activated delayed fluorescent (TADF) materials, 39−43 and TADF materials themselves 44−51 contain aromatic or heterocyclic amines such as carbazole (Cz), diphenylamine (DPA), phenoxazine (PXZ), phenothia-zine (PTZ), and 9,9-dimethyl-9,10-dihydroacridine (DMAC). Although in the literature one can find numerous examples of coupling of the above-mentioned amines and their derivatives, 52−54 the comprehensive study of their palladiumcatalyzed coupling with aryl halides has not yet been performed.
Herein, the study of the Buchwald−Hartwig amination of aryl bromides with the amines mentioned above, leading to novel star-shaped BDF derivatives, along with their density functional theory (DFT) and spectral characteristics, is described.

■ RESULTS AND DISCUSSION
The benzodifuran core (TBBDF), containing four parabromophenylene groups and long alkyl chains in positions 4 and 8 of the BDF core, to improve the solubility of the desired   98  93  THF  83  67  83  84  72  DMF  28  3  60  19  35  DMSO  2  11  19  12  6 t-BuONa seems to be the most universal, although t-BuOLi and Cs 2 CO 3 also gave satisfactory results in individual cases (Table 3). Good results were also obtained using methylmagnesium chloride as a base. Weaker inorganic bases (K 2 CO 3 , K 3 PO 4 , and KOH) gave good results only for coupling with PXZ, but the amination with this amine is relatively easy. General Conditions. It was found that the Buchwald− Hartwig amination of bromobenzene with secondary amines is most preferably carried out in an environment of relatively non-polar solvents, although the base choice seems to be more complex; however, one can conclude that strong organic bases and Cs 2 CO 3 will work well in this reaction. The best reaction systems for palladium-catalyzed coupling of bromobenzene with Cz proved to be TrixiePhos/t-BuOLi/toluene, with DPA, PXZ, and PTXXPhos/t-BuONa/toluene, and with DMACt-BuXPhos/t-BuONa/toluene.
Star-Shaped BDF Synthesis. The developed conditions were applied to the synthesis of the fluorescent BDF starshaped compounds. Amine 1b was commercially available (Table 4), while the more complex amines (1a, 1c, and 1d) were synthesized according to the procedures presented in Scheme 2.
For 1a synthesis, the Sonogashira reaction of DICz and 1decyne, followed by hydrogen−Pd/C reduction, was performed. Amine 1c was obtained by the Buchwald−Hartwig reaction between DITosCz and Cz, followed by basic hydrolysis of the tosyl group. Although the Ullmann synthesis of 1c is described in the literature, our method allowed us to obtain the product in 68% yield using milder conditions (100°C , 24 h vs 166°C, and 48 h). 56 It was found that due to the better reactivity of aryl iodides compared to bromides, the same product yield was obtained after catalyst loading reduction to 0.5 mol % of [Pd(allyl)Cl] 2 and 2 mol % of t-BuXPhos per one iodine atom.
Compound 1d was synthesized analogously to 1c, but the use of the tert-butylcarboxycarbonate (Boc) protecting group was necessary since the reaction of DITosCz and standard [Pd(allyl)Cl] 2 /XPhos or Pd 2 (dba) 3 /[t-Bu 3 PH]BF 4 catalytic systems led to the deiodination of DITosCz, resulting in a complex mixture of byproducts. The synthesis of 1d from DBBocCz was also described in the literature; 57 however, we found that temperature reduction from the literature 220 to 100°C did not affect the yield of the reaction, which in both cases was 79%, but eliminates difficult to remove byproducts formed in the higher temperature.
Based on the developed amination conditions, it was found that TBBDF coupling with Cz, DPA, PXZ, and PTZ (Table 4, entries 1−4) undergoes smoothly, yielding desired products in good yields (61−92%), and only for DMAC, the yield was significantly lower (19%, entry 5). Since TBBDF contains four bromophenylene moieties, the total catalyst loading was 4 mol % of [Pd(allyl)Cl] 2 and 16 mol % of phosphine ligand. Larger loadings (8 and 32 mol %, respectively) were necessary to achieve satisfactory yields of 2f−2i (entries 6−9). Also, the ligand of choice for the more sterically demanding amines  proved to be t-BuXPhos and t-BuONa as a base, and only for 2h (entry 8), t-BuOLi gave better results. Additionally, the increase of amine and base load to 1.5 equiv per one bromophenylene moiety allowed us to obtain slightly better results. Even in these conditions, the low yield was obtained for DMAC coupling due to the significant amounts of partially substituted and debrominated byproducts, and in this particular case, other catalytic systems should be considered. Another approach that worked well for coupling 1d with TBBDF was using a pressure vessel and temperature above the boiling point of toluene (entry 9). Thus, it was possible to increase the yield of 2i synthesis from 29 to 43%.
Suzuki Reaction. To estimate the effect of the elongation of the conjugated system of phenylene rings on the spectral properties of star-shaped BDF, the 2i, an analogue of 2b, was prepared using the Suzuki reaction (Scheme 3).
Photophysical Properties. The star-shaped BDF derivatives were analyzed using ultraviolet−visible (UV−vis) (Supporting Information) and photoluminescence spectroscopies. Most of the obtained compounds revealed strong fluorescence both in solution and in solid states. Compounds 2b and 2f−2h revealed the highest, ∼100% quantum yield (QY) in toluene and chloroform. In addition, all these compounds have almost identical emission spectra profiles Scheme 2. Synthesis of Cz Derivatives Substituted in Positions 3 and 6 a a and maxima in a very narrow 418.4−419.8 nm range. In toluene, 2a, 2d, 2e, and 2i ( Figure 1) had broad emission with one maximum, while 2b, 2c, 2f−2h, and 2j had two or three maxima. The latter compounds show structured emission spectra and exhibit no or marginal solvatochromic effect (chloroform vs toluene). Moreover, Stoke's shift values for these emitters are rather small, 36−43 nm. These observations are typical of molecules with no or very little change in geometry after excitation. On the other hand, compounds 2a, 2d−e, and 2i show structureless emission spectra and exhibit substantial solvatochromism; the bathochromic effect was observed by changing the solvent from toluene to chloroform (Table 5). Stoke's shifts are much larger (80, 118, 61, and 76 nm for 2a, 2d, 2e, and 2i, respectively), which are typical for molecules whose excited-state geometry differ substantially from the geometry of the ground state. The elongation of the conjugated phenylene ring system (2j vs 2b) slightly shifted the emission maximum toward longer wavelengths and decreased QY by 15% but did not affect the emission spectrum profile. On analyzing the UV−vis spectra of 2a−2j in toluene and chloroform, it can be seen that they are rather insensitive to solvent change ( Figure 2). We found it interesting, bearing in mind some bathochromic shifts in emission spectra, so we decided to support these observations with computational chemistry methods.
Computational Results. Due to the structural flexibility of investigated systems, the first step was identifying the most representative rotamers for which absorption spectra should be calculated. This task was performed with the help of the CREST software, 58,59 which provides an automated scheme for finding rotamers based on the semiempirical tight-binding GFN2-xTB method 60 coupled to meta-dynamics simulations. 59 At first, in the case of every investigated system, hydrocarbon chains were replaced with methyl groups. Then, CREST calculations were performed, and as a result, the sets of rotamers were obtained. The rotamers with the lowest energy were taken to further calculations: they were enlarged by missing hydrocarbon groups and optimized within the PBE0/6-31G(*) approach with and without Grimme's GD3 empirical dispersion correction 60 provided by the Gaussian 19 package. Then, for every investigated system, the UV absorption spectra were recorded using several solvents: chloroform, dichloromethane (DCM), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and toluene. Solvents were mimicked by the polarizable continuum model, and the gas-phase geometry of the system was used. All bands for all investigated systems were found to be the π−π*-type transitions. For all systems, the lowest unoccupied molecular orbital (LUMO) orbitals lie on the BDF chain (Figures 3 and S23−S32). However, the position of the highest occupied molecular orbital (HOMO) orbitals let us divide investigated molecules into two groups. The first one consists of 2a−b, 2d, 2f−g, and 2j, where the HOMO orbitals lie on the BDF core, and the second is 2c, 2e, and 2h−i, where the HOMO orbitals occupy the outer part of the molecule. One could expect that manifested charge transfer in the second group will exhibit a more significant impact of the solvent on absorption or emission spectra. In Table 6, we compare S0 → S1 excitation wavelengths for different solvents (with toluene as a reference). Indeed, for the second group of molecules, the changes are much more significant. Additionally, the Δδ index, 62 which is the overall difference of root-mean-square deviation of electron and hole distributions, was calculated in MultiWFn, 63 for S0 → S1 excitation of the given system in toluene. The Δδ parameter allowed us to quantify the CT for the analyzed star systems. For 2c, 2e, and 2h−i, the absolute value of Δδ (4.46, 4.75, and 6.00, respectively) is much larger than for other systems. It indicates that for the star-shaped benzodifurans and similar systems, Δδ can be a good index for the charge transfer estimation. To provide a simple interpretation of excitation, the natural transition orbitals 61 have been obtained for the S0 → S1 excitation (the corresponding data are presented in the Supporting Information).

■ CONCLUSIONS
In conclusion, the condition development for palladiumcatalyzed amination of bromobenzene with Cz, DPA, PXZ, PTX, and DMAC was performed. The best catalytic system for palladium-catalyzed coupling with Cz proved to be Trix-iePhos/t-BuOLi/toluene, with DPA, PXZ, and PTXXPhos/ t-BuONa/toluene, and with DMACt-BuXPhos/t-BuONa/ toluene. Based on these results, the Buchwald−Hartwig reaction between the benzodifuran core (TBBDF) and the  55 tert-butyl-3,6-dibromo-9H-carbazole-9-carboxylate (DBBocCz), 57 3,6-di-tert-butyl-9Hcarbazole, 64 3,6-diiodo-9-tosyl-9H-carbazole (DITosCz), 65 and 3,6diiodo-9H-carbazole (DICz) 65 were synthesized according to a literature procedure. Solvents were purchased from Avantor, VWR,    Spectroscopic Measurements. Toluene and chloroform (spectrometric grade from Merck) were employed as solvents for absorption and fluorescence measurements. UV−vis absorption spectra were recorded on a PerkinElmer UV−vis Lambda 25 spectrometer in a 1 cm quartz cell compared to solvent blank. Emission spectra were obtained on a JASCO FP-8500 spectrometer. The QY was determined using 9,10-diphenylanthracene in toluene (θ ref = 0.95) at λ ex = 366 nm. The concentration of 9,10diphenylanthracene and the analyzed substances were set so that the absorbance at 366 nm was low enough to avoid the inner filter effect. The fluorescence lifetime was determined using a timecorrelated single-photon counting setup with a Maestro spectrum analyzer (EG&G Ortec, Oak Ridge, USA) and a pulsed LED (376 nm, PicoQuant GmbH, Berlin, Germany) with a pulse width of fewer than 1.5 ns (full width at half-maximum). All fluorescence light above 406 nm was detected using a low-pass filter. The decay traces were analyzed assuming a single exponential decay function.
Amination Screening. In a pressure vial closed with a septum, an amine (1.62 mmol) and a base (1.62 mmol) were mixed in a dry solvent (4 mL) and stirred for 5 min under argon. In a separate vial, [Pd(allyl)Cl] 2 (0.016 mmol; 5.9 mg; 1 mol %) and phosphine ligand (0.065 mmol; 4 mol %) in a dry solvent (1.5 mL) were stirred under argon for 5 min. Bromobenzene (1.62 mmol) and the catalyst mixture were added to the pressure vial, the septum was replaced by a screw cap, the vial was immersed in an oil bath, and the mixture was stirred at 100°C for 24 h. After this time, the reaction mixture was cooled to room temperature and analyzed by GC−MS.