Synthesis of 5-(Aryl)amino-1,2,3-triazole-containing 2,1,3-Benzothiadiazoles via Azide–Nitrile Cycloaddition Followed by Buchwald–Hartwig Reaction

An efficient access to the novel 5-(aryl)amino-1,2,3-triazole-containing 2,1,3-benzothiadiazole derivatives has been developed. The method is based on 1,3-dipolar azide–nitrile cycloaddition followed by Buchwald–Hartwig cross-coupling to afford the corresponding N-aryl and N,N-diaryl substituted 5-amino-1,2,3-triazolyl 2,1,3-benzothiadiazoles under NHC-Pd catalysis. The one-pot diarylative Pd-catalyzed heterocyclization opens the straightforward route to triazole-linked carbazole-benzothiadiazole D-A systems. The optical and electrochemical properties of the compound obtained were investigated to estimate their potential application as emissive layers in OLED devises. The quantum yield of photoluminescence (PLQY) of the synthesized D-A derivatives depends to a large extent on electron-donating strengths of donor (D) component, reaching in some cases the values closed to 100%. Based on the most photoactive derivative and wide bandgap host material mCP, a light-emitting layer of OLED was made. The device showed a maximum brightness of 8000 cd/m2 at an applied voltage of 18 V. The maximum current efficiency of the device reaches a value of 3.29 cd/A.


Introduction
Polyheteroaromatic compounds containing donor-acceptor (D-A) units linked through π-conjugation have been drawing considerable attention due to their unique electrochemical and photochemical properties.They have found many important applications in organic electronics and luminescence materials [1][2][3].In the past decade, various electron-deficient (hetero)aromatics have been widely used as acceptor blocks in developing advanced functional materials.Among them, 2,1,3-benzothiadiazole (BTD) derivatives are recognized as privileged building blocks for assembling various optoelectronic devices [4], e.g., organic light-emitting diodes (OLEDs) [5,6], field effect transistors (OFETs) [7][8][9], solar cells (OSCs) [10][11][12][13][14], luminescent solar concentrators (LSCs) [15], as well as fluorescent tags for molecular and cellular imaging [16][17][18][19].The successes of these benzothiadiazole-based D-A molecules are mainly beneficial from their ability to provide for intramolecular chargetransfer effects.The nature of donor and acceptor components and the linker between them are particularly crucial factors that affect the properties of such compounds [20].The introduction of rigid chromophores with twisted molecular geometry or additional substituents into the linker can prevent unfavorable π-aggregates in the solid state, which often causes fluorescence quenching.Therefore, the development of new benzothiadiazole-based molecules with extended π-conjugation is currently of great interest.
The aromatic triazole ring is a versatile and readily installed linkage that has been used to extend the conjugation between different aromatic systems.Usually, it can be easily achieved via copper-catalyzed azide-alkyne cycloaddition, well known as "click" reaction.The beneficial effect of 1,2,3-triazole-linked BTD derivatives has been demonstrated for some dyes with a high photo and chemical stability in their excited states, which is a very desirable feature for both the technological and biological applications of new fluorophores [18,[21][22][23][24][25].
and the linker between them are particularly crucial factors that affect the properties of such compounds [20].The introduction of rigid chromophores with twisted molecular geometry or additional substituents into the linker can prevent unfavorable π-aggregates in the solid state, which often causes fluorescence quenching.Therefore, the development of new benzothiadiazole-based molecules with extended π-conjugation is currently of great interest.
The aromatic triazole ring is a versatile and readily installed linkage that has been used to extend the conjugation between different aromatic systems.Usually, it can be easily achieved via copper-catalyzed azide-alkyne cycloaddition, well known as "click" reaction.The beneficial effect of 1,2,3-triazole-linked BTD derivatives has been demonstrated for some dyes with a high photo and chemical stability in their excited states, which is a very desirable feature for both the technological and biological applications of new fluorophores [18,[21][22][23][24][25].

Synthesis
Starting nitrile 3a was synthesized by the Pd-catalyzed cross-coupling reaction of readily available mono-Br-substituted BTD 1 [33,34] with boropinacolate derivative of 2phenylacetonitrile under standard Suzuki conditions.The reversed approach has proved to be more effective for the preparation of nitrile 3b.In this case, coupling between spe-

Synthesis
Starting nitrile 3a was synthesized by the Pd-catalyzed cross-coupling reaction of readily available mono-Br-substituted BTD 1 [33,34] with boropinacolate derivative of 2phenylacetonitrile under standard Suzuki conditions.The reversed approach has proved to be more effective for the preparation of nitrile 3b.In this case, coupling between specially prepared 4-BPin-substituted BTD 2 (see Section 3.2) and ortho-Br 2-phenylacetonitrile led to the formation of the desired BTD-containing nitrile 3b in an acceptable yield using the same catalytic system (Scheme 2).
cially prepared 4-BPin-substituted BTD 2 (see section 3.2) and ortho-Br 2-phenylacetonitrile led to the formation of the desired BTD-containing nitrile 3b in an acceptable yield using the same catalytic system (Scheme 2).Then, following our previous experience [31] and literature precedents [29], we performed the pre-optimization of the dipolar [3 + 2]-cycloaddition (DCR) for the reaction of BTD-nitrile 3a with benzyl azide by screening simple catalysts (K2CO3, Cs2CO3, KOtBu) and solvents (DMF, DMSO) under moderate heating (40-80 °C).As a result, we found that the best yield of the desired product 4a (94%) can be achieved using 3.0 equiv. of azide, 50 mol.% of KOtBu in DMSO solution at 70 °C for 3 h.With a lower catalyst loading of catalyst (20-30 mol.%) or azide (1-2 equiv.), the reaction became less efficient with respect to rate and yield.These conditions were further applied for the cycloaddition of nitriles 3a and 3b with different aliphatic and aromatic azides to afford the corresponding 5-amino-1,2,3-triazolelinked BTDs 4a-g in good to excellent yields (Scheme 3).Then, following our previous experience [31] and literature precedents [29], we performed the pre-optimization of the dipolar [3 + 2]-cycloaddition (DCR) for the reaction of BTD-nitrile 3a with benzyl azide by screening simple catalysts (K 2 CO 3 , Cs 2 CO 3 , KOtBu) and solvents (DMF, DMSO) under moderate heating (40-80 • C).As a result, we found that the best yield of the desired product 4a (94%) can be achieved using 3.0 equiv. of azide, 50 mol.% of KOtBu in DMSO solution at 70 • C for 3 h.With a lower catalyst loading of catalyst (20-30 mol.%) or azide (1-2 equiv.), the reaction became less efficient with respect to rate and yield.These conditions were further applied for the cycloaddition of nitriles 3a and 3b with different aliphatic and aromatic azides to afford the corresponding 5-amino-1,2,3-triazolelinked BTDs 4a-g in good to excellent yields (Scheme 3).The Buchwald-Hartwig amination (BHA) is one of the most efficient cross-coupling reactions to access a wide range of N-mono-and N,N-disubstituted aryl amines.Despite noticeable advances in this area [35][36][37][38][39], coupling heteroaromatic amines with arylhalides is still a challenging transformation that often requires time-consuming searches for optimal conditions and catalytic systems [40][41][42][43][44][45][46][47].In our previous study, we revealed that pal- The Buchwald-Hartwig amination (BHA) is one of the most efficient cross-coupling reactions to access a wide range of N-monoand N,N-disubstituted aryl amines.Despite noticeable advances in this area [35][36][37][38][39], coupling heteroaromatic amines with arylhalides is still a challenging transformation that often requires time-consuming searches for optimal conditions and catalytic systems [40][41][42][43][44][45][46][47].In our previous study, we revealed that palladium complex with expanded-ring NHC ligand (THP-Dipp)Pd(cinn)Cl (er-NHC-Pd) is the most competent catalyst in the BHA reaction for low-reactive 5-amino-1,2,3-triazoles [32].Now, we investigated its activity in the BHA of amino-triazoles 4 with different (het)aryl bromides.First, we checked the reaction of 4a with phenyl bromide using the same conditions, namely, equal amounts of the reagents were heated in 1,4-dioxane at 110 • C (oil bath temperature) in the presence of 2 mol.% of er-NHC-Pd catalyst and 3.0 equiv. of sodium tert-butoxide for 24 h.As a result, the desired cross-coupling product 5a was isolated in 58% yield.The increase in the catalyst loading up to 5 mol.% and the amount of phenyl bromide up to 5 equiv.allowed reaching the full conversion of the starting amine 4a and, as a consequence, improving the yield of 5a to 91% (Scheme 4).With these conditions in hand, a series of N-aryl substituted 5-amino-1,2,3-triazole-linked BTDs 5a-f was synthesized in good to excellent yields (Scheme 4).It should be noted that N,N-diarylated products were not detected in the reaction mixtures despite using a large excess of amine component.This phenomenon can be rationalized by the unique feature of Pd-catalyst with bulky NHC ligand [48,49].Scheme 4. Buchwald-Hartwig cross-coupling synthesis of N-monosubstituted arylamino-1,2,3triazole-2,1,3-benzothiadiazoles 5a-f.Reaction conditions: 4 (0.2 mmol), (het)aryl-Br (5 equiv.),(THP-Dipp)Pd(cinn)Cl (5 mol.%),NaOtBu (3 equiv.),1,4-dioxane (1.0 mL), 110 • C, 24 h.It was previously shown that the introduction of auxiliary ligands to NHC-Pd complexes can provide new beneficial features in their catalytic activity [50][51][52][53].We recently established [54] that a combination of er-NHC-Pd with phosphine ligand can efficiently catalyze the arylation both of primary and secondary amines to produce the corresponding triaryl amines.This finding has prompted us to investigate the double Buchwald-Hartwig coupling of NH 2 -containing BTD compounds 4 with aryl bromides using a combination of the er-NHC-Pd complex with commercially available tert-butyl phosphine (protected as the BF 4 salt).Thus, we initially tested a catalytic activity of er-NHC-Pd/t-Bu 3 P•HBF 4 (5/10 mol.%) in the model reaction of 4a with phenyl bromide.Fortunately, we revealed that the reaction smoothly proceeds in 1,4-dioxane at 110 • C in the presence of 3.0 equiv. of sodium tert-butoxide and goes to completion for 24 h, furnishing the desired N,N-diphenyl derivative 6a in excellent yield and selectivity.These conditions were further applied for the preparation of a series of the corresponding N,N-diarylated 5-amino-1,2,3-triazol-linked BTDs 6a-l in high yields (Scheme 5).It is worth noting that this reaction demonstrates the first example of a double Buch wald-Hartwig cross-coupling reaction of 5-amino-1,2,3-triazoles with aryl halogenides.
Given the great value of electron-donor carbazole block in the development of ad vanced optoelectronic materials [55][56][57][58], as well as to be inspired by the results described above, we decided to study a possibility to perform one-pot diarylative cyclization of pri It is worth noting that this reaction demonstrates the first example of a double Buchwald-Hartwig cross-coupling reaction of 5-amino-1,2,3-triazoles with aryl halogenides.
Given the great value of electron-donor carbazole block in the development of advanced optoelectronic materials [55][56][57][58], as well as to be inspired by the results described above, we decided to study a possibility to perform one-pot diarylative cyclization of primary aminogroup of 4 with 2,2 ′ -dibromobiaryls under found catalytic conditions.
As it was expected, the same catalytic system er-NHC-Pd/t-Bu 3 P•HBF 4 (5/10 mol.%) has proved to be sufficiently active for the direct installation of a carbazole block into BTD derivatives 4a-d.The reactions have been accomplished under heating in dioxane solution in the presence of t-BuONa (3 equiv.),affording the desired products 7a-d in acceptable yields (Scheme 6).All synthesized compounds isolated in analytically pure form via flash chromatography were fully characterized by means of standard physicochemical methods (see Supplementary Materials).In addition, single crystals of good quality were obtained from 6a, 6g, and 7a for X-ray analysis (Figure 1).All synthesized compounds isolated in analytically pure form via flash chromatography were fully characterized by means of standard physicochemical methods (see Supplementary Materials).In addition, single crystals of good quality were obtained from 6a, 6g, and 7a for X-ray analysis (Figure 1).°C, 24 h.
All synthesized compounds isolated in analytically pure form via flash chromatography were fully characterized by means of standard physicochemical methods (see Supplementary Materials).In addition, single crystals of good quality were obtained from 6a, 6g, and 7a for X-ray analysis (Figure 1).

Optical and Electrochemical Investigation
We performed the initial estimation of photophysical properties of each type of the prepared compounds (4b, 5f, 6d, 6g, and 7a).Optical and electrochemical data are collected in Table 1.
Table 1.The long-wavelength absorption and emission bands, quantum yields, the frontier molecular orbitals (FMOs) energies a for compounds 4b, 5f, 6d, 6g, and 7a in DCM.All compounds exhibit intense absorption bands at the edge of the visible region (Figure 2).For all compounds except 6g, the absorption maxima are almost the same (scatter 404-407 nm).The compounds have strong emissions with maxima at 529-562 nm.Compounds 4b and 5f show significant quantum yields (86-89%), but the quantum yields of 6d and 7a are close to 100%, making them the most promising candidates for creating light-emitting OLED layers.Compound 6g has a reduced quantum yield compared to the rest of the series (although still substantial).The differences in the optical properties of 6g are clearly due to a change in the position of the triazole moiety in the benzene ring.
Voltammograms of the compounds are presented in ESI.All compounds show a reversible reduction in the region of −0.86 V relative to the selected reference silver chloride electrode.This similarity is due to the similar localization and energy of LUMO on the acceptor fragment of benzothiadiazole, common to all compounds.The irreversible oxidation of compounds is observed in the region of 1.2-1.5 V relative to the reference electrode.The differences are due to the different structures of donor substituents.
ter 404-407 nm).The compounds have strong emissions with maxima at 529-562 nm.Compounds 4b and 5f show significant quantum yields (86-89%), but the quantum yields of 6d and 7a are close to 100%, making them the most promising candidates for creating light-emitting OLED layers.Compound 6g has a reduced quantum yield compared to the rest of the series (although still substantial).The differences in the optical properties of 6g are clearly due to a change in the position of the triazole moiety in the benzene ring.Voltammograms of the compounds are presented in ESI.All compounds show a reversible reduction in the region of −0.86 V relative to the selected reference silver chloride electrode.This similarity is due to the similar localization and energy of LUMO on the acceptor fragment of benzothiadiazole, common to all compounds.The irreversible oxidation of compounds is observed in the region of 1.2-1.5 V relative to the reference electrode.The differences are due to the different structures of donor substituents.

Electroluminescent Properties of 6d
Most of the synthesized compounds have a high photoluminescence quantum yield (PLQY) of over 80% in dichloromethane solution (DCM).Since compound 6d exhibits

Electroluminescent Properties of 6d
Most of the synthesized compounds have a high photoluminescence quantum yield (PLQY) of over 80% in dichloromethane solution (DCM).Since compound 6d exhibits photoluminescence with the high PLQY of 95%, its electroluminescent properties were investigated in an OLED device with the following structure ITO/TAPC (53 nm)/TCTA (9 nm)/6d (15 wt.%):mCP (27 nm)/TPYMB (30 nm)/LiF (1 nm)/Al (80) nm.It contained a double-hole transport layer (HTL) consisting of 1,1-bis[(di-4-tolylamino) phenyl] cyclohexane (TAPC) and 4,4,4-tris(N-carbazolyl)triphenylamine (TCTA).The insertion of the thin layer of TCTA improved the hole-injection ability because of the step-wise hole injection from TAPC and TCTA to mCP (Figure 3), thereby ensuring a balance of charge carriers in the light-emitting layer (EML).In addition, it prevents the formation of exciplexes between the TAPC and the EML compounds [59].Tris-(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl) borane (3TPYMB) serves here as an electron transport layer (ETL), ITO is the anode and LiF/Al is the cathode.The light-emitting layer (EML) consists of a wide bandgap host material N,N-dicarbazolyl-3,5-benzene (mCP) and additive of dye 6d.EMLs with a dye content in the mCP matrix from 10 to 18 wt.%were investigated and optimal characteristics were obtained for OLED with a dye content of 15 wt.% (Table 2).All layers included in the OLED structures were formed using the method of thermal vacuum evaporation (TVE).OLED with EML based on pure 6d compound has also been produced.It demonstrated maximum brightness and a current efficiency two times lower than that of structures with co-deposited EML layers.Apparently, this is due to the concentration quenching of excited states.Tris-(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl) borane (3TPYMB) serves here as an electron transport layer (ETL), ITO is the anode and LiF/Al is the cathode.The light-emitting layer (EML) consists of a wide bandgap host material N,N-dicarbazolyl-3,5-benzene (mCP) and additive of dye 6d.EMLs with a dye content in the mCP matrix from 10 to 18 wt.%were investigated and optimal characteristics were obtained for OLED with a dye content of 15 wt.% (Table 2).All layers included in the OLED structures were formed using the method of thermal vacuum evaporation (TVE).OLED with EML based on pure 6d compound has also been produced.It demonstrated maximum brightness and a current ef-ficiency two times lower than that of structures with co-deposited EML layers.Apparently, this is due to the concentration quenching of excited states.The EL band of the device with a co-deposited 6d/mCP EML layer has a maximum at 555 nm (Figure 4a) and is shifted by 9 nm to longer wavelengths relative to the PL spectrum in a DCM solution (Table 1).The OLED emission color is green-white unsaturated and the CIE chromaticity coordinates do not lie on the locus boundary (Figure 4b) since the FWHM of the EL band is 79 nm.The broad PL and EL spectra arise as a result of charge transfer (CT) between the donor and acceptor fragments of dye 6d.As can be clearly seen from Figure 3, the HOMO and LUMO energy levels of compound 6d measured by CVA are well matched to the levels of mCP, which provides a balanced injection of electrons and holes into the EML and the onset voltage does not exceed 4 V. Since the HOMO levels of 6d and mCP are very close to each other, the 6d molecules are shallow traps and do not interfere with the transport of holes in the lightemitting layer.The maximum brightness of an OLED with the EML layer of 6d/mCP reaches 8000 cd/m 2 at an applied voltage of 18 V (Figure 5).The maximum current efficiency of the device reaches a value of 3.29 cd/A (Table 2).As can be clearly seen from Figure 3, the HOMO and LUMO energy levels of compound 6d measured by CVA are well matched to the levels of mCP, which provides a balanced injection of electrons and holes into the EML and the onset voltage does not exceed 4 V. Since the HOMO levels of 6d and mCP are very close to each other, the 6d molecules are shallow traps and do not interfere with the transport of holes in the lightemitting layer.The maximum brightness of an OLED with the EML layer of 6d/mCP reaches 8000 cd/m 2 at an applied voltage of 18 V (Figure 5).The maximum current efficiency of the device reaches a value of 3.29 cd/A (Table 2).
balanced injection of electrons and holes into the EML and the onset voltage does not exceed 4 V. Since the HOMO levels of 6d and mCP are very close to each other, the 6d molecules are shallow traps and do not interfere with the transport of holes in the lightemitting layer.The maximum brightness of an OLED with the EML layer of 6d/mCP reaches 8000 cd/m 2 at an applied voltage of 18 V (Figure 5).The maximum current efficiency of the device reaches a value of 3.29 cd/A (Table 2).

Optical and Electrochemical Investigation
UV-Vis spectra in dichloromethane solutions were recorded on an Agilent Cary 300 spectrometer (Santa Clara, CA, USA), for the fluorescence spectra, a Cary Eclipse spectrofluorometer has been used.All measured luminescence spectra were corrected for nonuniformity of detector spectral sensitivity.Rhodamine 6G (φ fl 0.95) in ethanol was used as a reference for the luminescence quantum yield measurements.The luminescence quantum yields were calculated using equation: where φ i and φ 0 are the luminescence quantum yields of the studied solution and the standard compound, respectively; A i and A 0 are the absorptions of the studied solution and the standard, respectively; S i and S 0 are the areas underneath the curves of the luminescence spectra of the studied solution and the standard, respectively; and n i and n 0 are the refractive indices of the solvents for the substance under study and the standard compound (n i 1.4242, DCM; n 0 1.361, EtOH).

Voltammetry Studies
Electrochemical measurements were carried out at 22 • C with a Metrohm Autolab B.V. potentiostat type: PGSTAT128N.Cyclic voltammetry (CV) experiments were performed in three-electrode cell equipped with a glassy carbon (GC) working electrode (disk d 2 mm), Ag/AgCl reference electrode, and platinum counter electrode.Compounds were dissolved in degassed dry DCM containing TBAHFP as the supporting electrolyte (0.1 M).Dry argon gas was bubbled through the solutions for 10 min before cyclic voltammetry experiments.The applied scan rate for CV was 200 mV s −1 .In ESI cyclic voltammograms are given relative to the Ag/AgCl reference electrode.Energies of the frontier molecular orbitals (FMOs) were calculated against ferrocene/ferrocenium (Fc/Fc+) as internal standard as follows: HOMO, eV = -Eox, V-4.8; LUMO, eV = -Ered, V-4.8, since the energy level of ferrocene/ferrocenium is 4.8 eV below the vacuum level [64].

OLED Fabrication and Characterization
OLED devices were fabricated on ITO-coated glass substrates that were pre-cleaned according to the standard procedure as described in [65].The hole transport, light-emitting, electron transport layers and LiF(1 nm)/Al(80) as cathode were deposited sequentially using thermal vacuum evaporation (TVE) at a residual pressure of 4 × 10 −6 mbar.The electroluminescence spectra of OLEDs were recorded using an Avantes 2048 fiber-optic spectrofluorimeter (Apeldoorn, The Netherlands).Voltage-current and voltage-brightness characteristics were recorded with a Keithley 2601 SourceMeter (Tektronix, Beaverton, OR, USA), a Keithley 485 picoampermeter, and a TKA-04/3 luxmeter-brightness meter (TKA Scientific Instruments, Saint Petersburg, Russia).Preparations of the OLED samples and the measurements of their spectral and optoelectronic characteristics were performed at room temperature in the argon atmosphere glovebox with maximum oxygen and moisture presence of 10 ppm.

Conclusions
In summary, we developed an efficient synthetic pathway to new series of 5-(aryl)amino-1,2,3-triazole-containing 2,1,3-benzothiadiazole derivatives.The method is based on 1,3dipolar azide-nitrile cycloaddition followed by Buchwald-Hartwig cross-coupling to afford the corresponding N-aryl and N,N-diaryl substituted 5-amino-1,2,3-triazolyl 2,1,3benzothiadiazoles under catalysis with (THP-Dipp)Pd(cinn)Cl complex thus opening the straightforward access to unknown before triazole-linked D-A systems.In addition, the one-pot diarylative Pd-catalyzed heterocyclization has been also performed to demonstrate a quick installation of carbazole donors into BTD structure.The quantum yields of photo-

Figure 3 .
Figure 3. Energy band diagram of the OLED structure with the co-deposited EML layer 6d/mCP.

Figure 3 .
Figure 3. Energy band diagram of the OLED structure with the co-deposited EML layer 6d/mCP.

Figure 5 .
Figure 5. Current density-voltage dependence and voltage-brightness characteristics of the OLED based on EML layer 6d/mCP.

Figure 5 .
Figure 5. Current density-voltage dependence and voltage-brightness characteristics of the OLED based on EML layer 6d/mCP.

Table 2 .
EL characteristics of the studied OLED with a dye 6d content in of 15 wt.%.
Max. Efficiency CIE λmax EL, nm CE, cd/A PE, lm/W x y

Table 2 .
EL characteristics of the studied OLED with a dye 6d content in of 15 wt.%.