Dibenzo[f,h]furazano[3,4-b]quinoxalines: Synthesis by Intramolecular Cyclization through Direct Transition Metal-Free C–H Functionalization and Electrochemical, Photophysical, and Charge Mobility Characterization

Abstract

Herein, we describe the synthesis of unsymmetrically substituted dibenzo[f,h]furazano[3,4-b]quinoxalines by intramolecular cyclization through direct transition metal-free C–H functionalization. The electrochemical and photophysical properties for several polycycles have been measured. In thin films of the dibenzo[f,h]furazano[3,4-b]quinoxalines, hole mobility is in the order of 10^–4 cm² V^–1 s^–1. The results show that the HOMO and LUMO energy levels are appropriate for using the compounds as hole-transport materials in thin-film devices, in particular, organic and perovskite solar cells.

Introduction

Polycyclic heteroaromatic compounds have received much attention as a fundamental framework in organic functional materials because their electrochemical and photochemical properties are useful in organic electronics and luminescent materials.¹ Among them, 1,4-diazatriphenylene (dibenzo[f,h]quinoxaline) derivatives (I) frequently appeared in a molecular unit in the preparation of extended π-conjugated systems with optical characteristic and electrochemical properties (Figure 1).² One of the rare azoannulated 1,4-diazatriphenylene derivatives is dibenzo[f,h]furazano[3,4-b]quinoxaline (II). Only two publications reported the synthesis of dibenzo[f,h]furazano[3,4-b]quinoxalines (II) by condensation of substituted 9,10-phenanthroquinone with 3,4-diaminofurazan under acidic reaction conditions.³ To the best of our knowledge, there are no synthetic procedures that give the asymmetrically substituted dibenzo[f,h]furazano[3,4-b]quinoxalines.

Figure 1.

Dibenzo[f,h]quinoxaline (I) and dibenzo[f,h]furazano[3,4-b]quinoxaline (II) derivatives.

We previously reported that 5-(hetero)aryl-[1,2,5]oxadiazolo[3,4-b]pyrazines (III) participated in a transition metal-free cross-coupling reactions with different nucleophiles.⁴ Therefore, our initial strategy toward dibenzo[f,h]furazano[3,4-b]quinoxalines assumed that acid-catalyzed intramolecular cyclization of aryl substituted 5-([1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazines to furnish 9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline intermediates, which could be further oxidized for the synthesis of targets 1,4-diazatriphenylenes (II) (Scheme 1).

Scheme 1. Interaction of 5-(Hetero)aryl-[1,2,5]oxadiazolo[3,4-b]pyrazines with Different Nucleophiles through a Transition Metal-Free Cross-Coupling Reactions.

In this paper, we report a new synthetic protocol for the preparation of unsymmetrically substituted dibenzo[f,h]furazano[3,4-b]quinoxalines and their photophysical and electrochemical properties; moreover, we fabricated hole-only devices to measure the hole mobility values in thin films.

Results and Discussion

Synthesis

At the first stage, the parent 5-(2-bromophenyl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (3) was obtained via the two-step procedure from readily available 2′-bromoacetophenone (1) in 79% overall yield (Scheme 2).

Scheme 2. Synthesis of 5-(2-Bromophenyl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (3).

Further, it was necessary to synthesize the series of 5-([1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine derivatives. 5-(2-Bromophenyl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (3) reacted with phenylboronic acid (4a) to give 5-([1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5a) under the Suzuki cross-coupling condition. Examination of the palladium catalysts showed that Pd(PPh₃)₄ in dry 1,4-dioxane gave the desired product 5a in the best yield (Table 1, entry 4).

Table 1. Optimization of the Suzuki Cross-Coupling Reaction Conditions.

entry	Pd catalyst/ligand (equiv)	base (2.5 equiv)	PhB(OH)2 content (equiv)	solvent	yield (%)
1	Pd(PPh3)4 (5 mol %)	K2CO3	2.0	1,4-dioxane/H2O (5:1)	16
2	Pd(PPh3)4 (10 mol %)	K2CO3	2.0	1,4-dioxane/H2O (5:1)	45
3	Pd(OAc)2 (5 mol %)/PPh3 (10 mol %)	K3PO4	2.0	1,4-dioxane	17
4	Pd(PPh3)4 (5 mol %)	K3PO4	1.2	1,4-dioxane	66

Based on the above results, the structural diversity of various arylboronic acids (4a–4h) was examined. As shown in Scheme 3, a wide range of mono-, di-, and trisubstituted phenylboronic acids, bearing both electron-donating and electron-withdrawing substituent groups, could react with 3 for the synthesis of the corresponding 5-bis(aryl)-substituted [1,2,5]oxadiazolo[3,4-b]pyrazines (5a–5h). As can be seen, the electronic character of the substituents of phenylboronic acids does not have an influence on their reaction activity. In all cases, the Suzuki cross-coupling occurred smoothly to give the desired products 5a–5h in good yields of 51–82% (Scheme 3).

Scheme 3. Synthesis of 5-([1,1′-Biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine Derivatives (5a–5h).

Next, we tried to intramolecular cyclize bis(aryl)-substituted [1,2,5]oxadiazolo[3,4-b]pyrazines (5a–5h) into corresponding 9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline by action of CF₃COOH as the best solvent, as shown earlier.⁴ The intermediate products 6 were precipitated as white powders in good yields of 56–95%. Notably, compounds 6b and 6d were not isolated in their pure forms due to high solubility in CF₃COOH. In these cases, the solvent was removed, and the resulting semisolid residue was oxidized to desired 1,4-diazatriphenylenes 7b and 7d without additional purification. Of note, it was found that 2,4-difluoro-9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6g) is not formed under the given reaction conditions. We suggest that it is due to the uncoordinated electron-withdrawing inductive effect of two fluorine atoms, which complicate the intramolecular electrophilic attack on the 1,1′-biphenyl substituents. The obtained 9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxalines (6a–6f and 6h) were oxidized with K₃Fe(CN)₆ in an alcohol solution (NaOH) for 24 h to afford target dibenzo[f,h]furazano[3,4-b]quinoxalines (7a–7f and 7h) in high yields of 69–96% (Scheme 4).

Scheme 4. Synthesis Dibenzo[f,h]furazano[3,4-b]quinoxaline (DBFQ) Derivatives (7a–7h).

At the cyclization of the compound 5e, the formation of two regioisomeric products, which differ at the C(1) or C(3) position of the fluorine atom, is possible (Scheme 5). To unambiguously confirm the structure of the polycyclic compound 7e or 7e′, two-dimensional NOESY spectrum was recorded for this compound (Figure 2). Particularly, for the proposed structure of 7e′ in the NOESY spectrum, cross-peaks due to spatial interactions of the protons H(2), H(3), and H(4) forming the common system should be expected. Besides, the interactions with neighboring protons H(2) and H(4) should be observed for proton H(3). However, no cross-peaks with any of the neighboring protons were detected for proton H(4) in the NOESY experiment. Furthermore, for the other protons of the aromatic system H(1) and H(2), the interactions only with each other have been observed. This evidence unambiguously supports the structure of 7e.

Scheme 5. Structure of Two Possible Regioisomers: 3-Fluorodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7e) or 1-Fluorodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7e′).

Figure 2.

2D NMR NOESY (400 MHz, CDCl₃) spectrum of 7e at 313 K.

Notably, the structure of 7h was established by X-ray diffraction analysis (Figure 3). According to the XRD data, compound 7h crystallized in the noncentrosymmetric space group. The polycyclic system is planar (Figure 3). The fluoroaryl fragment is disordered in the plane with an occupancy coefficient of 0.85/0.15. The mean bond distances and angles of the molecules are near to standard. In the crystal, the molecules form the twisted slanting stacks (Figure 4a–c), with interfacial distances of adjacent molecules being approximately 3.3 Å (Figure 4d), which is near the layer distance in graphite (3.35 Å).⁵ This is a characteristic for a pronounced π–π interaction between two aromatic cores. These results suggest that the other analogue polycycles 7a and 7c–7f without the bulky propping tert-butyl group also may hold strong intermolecular π–π stacking interactions. In fact, relatively poor solubility was observed for 7d, 7e, and 7h, which reflects the strong π–π interactions.⁶

Figure 3.

ORTEP of 7h with thermal ellipsoids at the 50% probability level.

Figure 4.

Molecular packing for 7h (a) in the plane [100], (b) along the plane [8 24 0], and (c) in the plane [001]. (d) Fragment of stack with interfacial distances in Å.

Optical and Photophysical Properties

The electrochemical properties of dibenzo[f,h]furazano[3,4-b]quinoxalines (7a–7f and 7h) were studied using cyclic voltammetry under conditions previously described for similar heteroacene systems.⁷ The cyclic voltammograms shown in Figures S1–S7 demonstrate the irreversible character of the reduction of compounds 7a–7f and 7h. Given that no anodic behavior of 1,4-diazatriphenylenes (7a–f and h) could be recorded by CV, their excited state oxidation potentials (corresponding to the HOMO energy levels) were calculated by subtracting the optical energy gap E_g^opt (estimated from the long-wavelength absorption edge of the absorption spectra⁸ recorded in CH₂Cl₂ solution shown in Figures S8–S14) from the LUMO energy values (Table 2). The compounds under study are considered to be used in the solid state, so the similar optical and electrochemical parameters, as well as hole mobility (Table 2), were measured in their thin solid films (see Experimental Section). Both HOMO and LUMO energy levels of the compounds in solution are downshifted compared with those in solid films because of the influence of the reorganization energy of the electrolyte on the dispersion of the energy level of the substance in solution.

Table 2. Optical, Electrochemical, and Hole Mobility Data for Dibenzo[f,h]furazano[3,4-b]quinoxalines.

compound	Eredonseta (V)	EHOMOb/EHOMOc (eV)	ELUMOb,d/ELUMOc,d (eV)	Egoptb/Egoptc (±0.03 eV)	Absorptionb λmax (nm)/ε (103 M–1 cm–1)	hole mobilityc (10–5 cm2 V–1 s–1) (CIe 95%)	dipole momentf (D)
7a	–0.81	–7.15/–5.61	–4.29/–3.01	2.86/2.60	387/12.95	12.7 ± 1.5	6.81
					305/21.35
					294/13.52
					267/19.20
7b	–0.83	–7.12/–5.23	–4.27/–2.68	2.85/2.55	389/14.21	2.5 ± 0.5	6.87
					308/21.46
					297/15.73
					275/25.02
7c	–0.93	–6.74/–5.24	–4.17/–3.04	2.57/2.20	412/13.80	5.1 ± 0.4	8.62
					311/15.98
					279/29.98
					253/49.20
7d	–0.75	–7.11/–5.23	–4.35/–2.99	2.76/2.24	395/12.43	16.0 ± 4.0	5.43
					304/20.16
					293/12.97
7e	–0.80	–7.16/–5.27	–4.30/–2.72	2.86/2.55	386/14.73	13.3 ± 1.6	5.57
					303/20.52
					292/13.24
					275/22.76
7f	–0.75	–7.25/–5.25	–4.35/–2.80	2.90/2.45	384/11.85	2.4 ± 0.4	6.77
					307/26.34
					296/16.82
					271/18.09
7h	–0.78	–7.18/–5.27	–4.32/–2.87	2.87/2.40	384/14.54	3.5 ± 1.6	6.57
					301/16.47
					290/11.49
					267/21.02

^aE_red^onset, onset of reduction potential (vs Ag/AgCl reference electrode).

^bParameters measured in solutions at room temperature at C = 1.0 × 10^–5 M (for optical) and C = 5.0 × 10^–3 M (for electrochemical).

^cParameters measured for solid films at room temperature.

^dE_LUMO = E_HOMO – E_g^opt.

^eConfidence interval calculated from 10 replicates.

^fDFT calculation data.

Computational Data

The DFT calculations of the HOMO levels and the energy gap, as well as the dipole moment of molecules in the neutral state, were done at the B3LYP/6-31G* level of theory in vacuum.⁹ The computational data are shown in Table 2. The results show that the HOMO and LUMO levels were uniformly overestimated in the DFT calculations, presumably because the calculations were performed in a vacuum. The calculated band gap energies E_g^calc are systematically higher than the experimental E_g^opt values.

Hole Mobility

Organic semiconductors that can potentially be used as a charge-transporting material must possess reasonable charge carrier mobility values. Thus, we prepared specimens of a diode structure ITO/SiO₂/ DBFQ/Au where the SiO₂ insulator layer blocked injection of charge carriers from the ITO electrode in CELIV measurements (see Experimental Section). The data on hole mobility in the solid films of dibenzo[f,h]furazano[3,4-b]quinoxalines are also summarized in Table 2. The hole mobility in the 7a, 7d, and 7e films is 2.5–6 times larger than that in the 7b, 7c, 7f, and 7h films.

The charge carrier transport in organic and polymer solids can be represented in terms of the redox process.¹⁰⁻¹³ The hole transport is considered as a chain of redox reactions, consisting of one electron transfer from the HOMO of the neutral molecules to the single occupied molecular orbital (SOMO) of the neighbor radical cations. When the neutral molecule is charged positively, its conformation is transformed to accommodate the minimum energy state of the radical cation. In turn, when the SOMO of the radical cation accepts the second electron, its conformation is also transformed. Evidently, such molecule reorganization influences the probability of the intermolecular electron transfer and, as a consequence, the mobility of charge carriers. Indeed, molecule 7a consists of a solid core backbone, whereas other molecules have substituents, which can cause molecule conformation transformations during charge transfer. The fluorine substituent in the 7d and 7e molecules seems to initiate negligible transformations between the charged and neutral states of the molecules, so the high hole mobility for their films obeys the suggested approach as well.

It is understood that the packing of the molecules in solid films influences the charge carrier transport. As was discussed above, 7a and 7c–7h molecules tend to form strong intermolecular π–π stacking interactions. In consequence, we can expect relatively high charge mobility in their films in accordance with the fact that charge transport along the axis of such stack is faster than in any other direction.¹³ Nevertheless, the hole mobility in the 7c, 7f, and 7h films is lower than that in the 7a, 7d, and 7e films. In this regard, the dipole nature of the bulk of the films may be taken into account as well.^14,15

In accordance with the well-developed correlated disorder model (CDM),¹⁵ considering the Gaussian distribution of density of states, the following relationship describes charge mobility (see eq 1)

1

where C₀ = 0.78, Γ = 2, μ₀ is the pre-exponential factor, R is the distance between charge transport sites, F is the applied electric field, e is the electron charge, k is the Boltzmann constant, T is the temperature, and σ is the root mean square width of the energetic disorder. In general, the latter is determined by a dipolar component, σ_d, a van der Waals component, σ_v, and a quadrupolar component, σ_q, so as σ² = σ_d² + σ_v² + σ_q². To a quite good approximation, the contribution of the van der Waals and quadrupolar components may be neglected in comparison with the dipolar component, which is given by σ_d = 2.35ep/(εR) where p is the dipole moment of the transport molecule and ε is the permanent dielectric constant of the medium. It is seen that the lower the molecule dipole moment, the lower the amplitude of disorder and hence the larger the charge carrier mobility in the solid film. This correlation is revealed for 7d and 7e molecules, possessing the lower dipole moment compared with the 7c, 7f, and 7h molecules (Table 2). Meanwhile, the correlation fails if 7a molecules are compared to the 7b, 7f, and 7h molecules, with the molecules possess a similar dipole moment. It means that the CDM does not apply to the 7a films. We may suppose that the degree of ordering (stacking) of the 7a molecules in the film formed on a glass/ITO/SiO₂ substrate is higher than that of the 7b, 7f, and 7h molecules.

Conclusions

In summary, we have developed a new approach for the synthesis of unsymmetrical dibenzo[f,h]furazano[3,4-b]quinoxalines via intramolecular oxidative cyclodehydrogenation of the corresponding 5-phenylaryl(hetaryl)-substituted [1,2,5]oxadiazolo[3,4-b]pyrazines. The present synthetic procedure provides a facile way to the family of nitrogen-containing polycyclic heteroaromatic compounds that can potentially be used in organic electronics and luminescent materials.

Namely, we observed that a compromise between molecule stiffness, molecule dipole moment, and intermolecular packaging defines the mobility of holes in the solid films of the dibenzo[f,h]furazano[3,4-b]quinoxalines. In the films, hole mobility in the order of 10^–4 cm² V^–1 s^–1 was experimentally measured, which makes the compounds promising hole-transporting materials for organic electronics. In particular, 7b–7h compounds are promising candidates for the hole-transport layer in perovskite solar cells. Under similar conditions, the CELIV mobility of the pristine staple-compound Spiro-MeOTAD,¹⁶ which is commonly used as the hole-transport layer, was 8.5 × 10^–7 cm^–2 V^–1 s^–1 (for comparison, the hole mobility obtained from space charge limited current and time-of-flight measurements reached 2 × 10^–4 cm^–2 V^–1 s^–1 at high field).¹⁷ Moreover, their HOMO levels match well with those of MAPbI₃ and MAPbBr₃ hybrid perovskites.

Experimental Section

All reagents and solvents were obtained from commercial sources and dried by using standard procedures before use.

¹H and ¹³C NMR spectra were recorded on AVANCE 500 and AVANCE II 400 MHz instruments using Me₄Si as an internal standard. Elemental analysis was carried on a Eurovector EA 3000 automated analyzer. Melting points were determined on Boetius combined heating stages and were not corrected.

Flash column chromatography was carried out using Alfa Aesar silica gel 0.040–0.063 mm (230–400 mesh), eluting with ethyl acetate/hexane. The progress of reactions and the purity of compounds were checked by TLC on Sorbfil plates (Russia) in which the spots were visualized with UV light (λ, 254 or 365 nm).

The XRD experiment was performed on an Xcalibur 3 diffractometer on the standard procedure (MoK irradiation, graphite monochromator, ω scans with 1° step at T = 295(2) K). Empirical absorption correction was applied. Using Olex2,¹⁸ the structure was solved with the Superflip¹⁹ structure solution program using charge flipping and refined with the olex2.refine refinement package using Gauss–Newton minimization. All nonhydrogen atoms were refined in anisotropic approximation; H atoms were refined isotropically in the “rider” model. The mean crystallographic data and results of the refinements are presented in Table S1. The X-ray crystallography data for the structure reported in this paper have been deposited with Cambridge Crystallography Data Centre as supplementary publications CCDC 1976356 for 7h. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Cyclic voltammetry was carried out on a Metrohm Autolab PGSTAT128N potentiostat with a standard three-electrode configuration. Typically, a three-electrode cell equipped with a platinum working electrode, a Ag/AgNO₃ (0.01 M) reference electrode, and a glass carbon rod counter electrode was employed. The measurements were done in dichloromethane solution, containing the present compound (5 × 10^–3 M) with tetrabutylammonium perchlorate (0.1 M) as the supporting electrolyte under an argon atmosphere at a scan rate of 100 mV s^–1. The potential of the reference electrode was calibrated by using the ferrocene/ferrocenium redox couple (Fc/Fc⁺), which has a known oxidation potential of +5.1 eV versus vacuum for ferrocene.²⁰ After calibration of the measurements against Fc/Fc⁺, the HOMO energy level was calculated according to the following equation (eq 2)

2

where E_1/2(Fc/Fc⁺) is the half-wave potential of the Fc/Fc⁺ couple (the oxidation potential of which is assumed at 5.1 eV) against the Ag/Ag⁺ electrode.

UV–vis spectra were recorded for a 1 × 10^–5 M dichloromethane solution with a Shimadzu UV-2401PC spectrophotometer.

Since some of the dibenzo[f,h]furazano[3,4-b]quinoxaline derivatives under study were poor soluble, their thin solid films were prepared by thermal evaporation in vacuum. The morphology of the film was not studied. In thin layers of the dibenzo[f,h]furazano[3,4-b]quinoxaline derivatives, HOMO–LUMO energy levels were determined by cyclic voltammetry (CV). The CV experiment was carried out at a scan rate of 20 mV s^–1 in a three-electrode, three-compartment electrochemical cell in the glovebox with a dry argon atmosphere. Platinum sheets served as working and counter electrodes. A 20 nm solid layer of an examined material was preliminarily deposited onto the working electrode by thermal evaporation of the compound under 10^–6 mbar vacuum at a rate of 1 Å s^–1. A 0.2 M solution of tetrabutylammonium hexafluorophosphate (NBu₄PF₆, Fluka) in acetonitrile (MeCN (Aldrich, HPLC-grade)) was used as an electrolyte. The electrolyte solution was prepared as follows. To remove impurities and traces of water, MeCN was stirred with calcium hydride (10 g L^–1) for 2 days, fractionally distilled with P₂O₅ (5 g L^–1), then heated near the boiling point for 2 h with CaH₂ (5 g L^–1), and finally fractionally distilled again. NBu₄PF₆ was purified by recrystallization in ethanol and then dried and heated for 4 h at 80 °C in the inert dry argon atmosphere in the glovebox. The electrolyte solution was prepared in the glovebox also. A Ag wire immersed into the electrolyte solution with the addition of 0.1 M AgNO₃ was used as a pseudo reference electrode (Ag/Ag⁺). It was calibrated against ferrocene/ferrocenium couple (−0.039 V vs Ag/Ag⁺), and its potential was recalculated to the energy scale using a value of −4.988 eV for Fc/Fc⁺ in MeCN reported in the literature.²¹ Thus, the energy level of Ag/Ag⁺ (E_Ag/Ag+) in this case is −5.027 eV. Considering the accuracy of CVA experiment (±0.02 V), this value should be rounded to −5.03 eV.

A 20 nm solid film of an examined material was deposited onto the working electrode by thermal evaporation of the compound under 10^–6 mbar vacuum at a rate of 1 Å s^–1. The values of potentials corresponding to the HOMO levels were determined by applying a tangent to the onset of anodic and cathodic currents, followed by extrapolation of the tangent to zero current at the crossing with the axis of potentials. For all the substances, we failed to determine the LUMO level because of the rather wide optical band gap ranged around 2.4–2.6 eV. Therefore, considering the determined electrochemical response of HOMO, the response of LUMO should lay in the range of potentials near −2.4 V, which is close to the cathodic limit of the electrochemical stability window of MeCN.

Charge Carrier Mobility Measurements

In thin solid films, the charge mobility values were measured by using the technique of charge extraction by linearly increasing voltage (CELIV).²² Metal–insulator–semiconductor (MIS) diode structures, similar to those described in references,^23,24 were prepared.

Onto the ITO (indium tin oxide) electrode of the ITO/glass substrate, a SiO₂ insulator layer of 70 nm in thickness was deposited by conventional magnetron sputtering. Then, a 100 nm layer of the dibenzo[f,h]furazano[3,4-b]quinoxaline derivative and an 80 nm layer of the Au electrode were deposited successively by thermal evaporation of the material under 10^–6 mbar vacuum at a rate of 1 Å s^–1.

For CELIV measurements, an arbitrary waveform generator and oscilloscope (DLAnalog Discovery with WaveForms software) were used to generate the triangle voltage pulse with a proper ramp and offset. CELIV signals were recorded by a digital storage oscilloscope (Tektronix TDS3032B) as well. The charge carrier mobility μ was determined in accordance with eq 3(22)

3

where d is the film thickness, A is the applied voltage ramp, j(0) is the displacement current, and Δj is the maximum drift current at “t_max”. The latter three parameters were extracted from CELIV transient current curves.

Synthesis of 5-(2-Bromophenyl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (3)

Selenium dioxide (1.1 g, 10 mmol) was dissolved in 1,4-dioxane/H₂O (15:1, 16 mL), and the mixture was heated and refluxed for 5 min. 2′-Bromoacetophenone (1) (1.99 g, 10 mmol) was added, and heating was continued for the next 12 h. The reaction mixture was filtered, and the filtrate was concentrated under reduced pressure to give 2-bromophenylglyoxal as a light yellow oil. The solution of 3,4-diaminofurazan (2) (1.0 g, 10 mmol) and crude 2-bromophenylglyoxal in a mixture of EtOH (5 mL) and CH₃COOH (5 mL) was refluxed for 1 h. After that, the mixture was cooled down, and the precipitate was filtered off, washed with ethanol, and then air-dried. The desired product 3 was obtained as a crystalline yellow powder. Yield: 2.19 g, 79%. mp 135 °C. ¹H NMR (500 MHz, DMSO-d₆): δ 9.44 (s, 1H), 7.91 (dd, J = 8.0, 1.1 Hz, 1H), 7.79 (dd, J = 7.6, 1.7 Hz, 1H), 7.68 (td, J = 7.6, 1.1 Hz, 1H), 7.63–7.59 (m, 1H). ¹³C NMR (126 MHz, DMSO-d₆): δ 162.4, 157.1, 151.9, 151.1, 136.1, 133.4, 133.1, 132.4, 128.5, 121.0. Calcd for C₁₀H₅BrN₄O (277.08): C, 43.35; H, 1.87; N, 20.22. Found: C, 43.41; H, 1.84; N, 20.10.

General Procedure for the Synthesis of 5-([1,1′-Biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine Derivatives (5a–5h)

A mixture of 5-(2-bromophenyl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (1) (277 mg, 1.0 mmol), corresponding arylboronic acids 4a–4h (1.2 mmol), Pd(PPh₃)₄ (115 mg, 10 mol %), and K₃PO₃ (530 mg, 2.5 mmol) was dissolved in 1,4-dioxane (15 mL). The reaction mixture was degassed and refluxed for 15 h under an argon atmosphere. After completion of the reaction (monitored by TLC), 10 mL of water was added and extracted with ethyl acetate. The combined organic layer was washed with water and brine, dried over anhydrous Na₂SO₄, and concentrated in vacuo. Purification by silica gel column chromatography with ethyl CH₂Cl₂/hexane (1:2, v/v) as an eluent and recrystallization from EtOH were performed to afford the title compounds (5a–5h).

5-([1,1′-Biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5a)

Yield: 182 mg, 66%, pale yellow powder. mp 143 °C. ¹H NMR (400 MHz, DMSO-d₆): δ 8.25 (s, 1H), 7.93 (dd, J = 8.0, 1.2 Hz, 1H), 7.81–7.77 (m, 1H), 7.71–7.67 (m, 2H), 7.40–7.36 (m, 3H), 7.32–7.27 (m, 2H). ¹³C NMR (101 MHz, DMSO-d₆): δ 163.9, 156.6, 152.2, 150.7, 141.2, 139.0, 134.1, 132.0, 131.5, 130.5, 129.9, 129.1, 128.34, 128.32. Calcd for C₁₆H₁₀N₄O (274.28): C, 70.06; H, 3.68; N, 20.43. Found: C, 70.03; H, 3.74; N, 20.36.

5-(4′-tert-Butyl-[1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5b)

Yield: 270 mg, 82%, pale yellow powder. mp 142 °C. ¹H NMR (400 MHz, DMSO-d₆): δ 8.24 (s, 1H), 7.91 (d, J = 7.8 Hz, 1H), 7.80–7.75 (m, 1H), 7.70–7.64 (m, 2H), 7.41 (d, J = 7.9 Hz, 2H), 7.23 (d, J = 7.9 Hz, 2H), 1.28 (s, 9H). ¹³C NMR (101 MHz, DMSO-d₆): δ 164.0, 156.6, 152.3, 151.0, 150.7, 141.1, 136.1, 134.0, 132.0, 131.5, 130.5, 129.6, 128.2, 125.9, 34.3, 31.0. Calcd for C₂₀H₁₈N₄O (330.39): C, 72.71; H, 5.49; N, 16.96. Found: C, 72.51; H, 5.40; N, 17.18.

5-(3′,4′,5′-Trimethoxy-[1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5c)

Yield: 298 mg, 82%, orange powder. mp 154 °C. ¹H NMR (500 MHz, DMSO-d₆): δ 8.30 (s, 1H), 7.92 (d, J = 7.6 Hz, 1H), 7.80–7.72 (m, 2H), 7.70–7.66 (m, 1H), 6.52 (s, 2H), 3.67 (s, 3H), 3.58 (s, 6H). ¹³C NMR (126 MHz, DMSO-d₆): δ 164.1, 156.8, 153.2, 152.1, 150.5, 141.3, 137.7, 134.6, 134.0, 132.0, 131.3, 130.3, 128.3, 107.6, 60.1, 55.9. Calcd for C₁₉H₁₆N₄O₄ (364.36): C, 62.63; H, 4.43; N, 15.38. Found: C, 62.43; H, 3.53; N, 15.26.

5-(2′-Fluoro-[1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5d)

Yield: 210 mg, 72%, pale yellow powder. mp 151 °C. ¹H NMR (500 MHz, DMSO-d₆): δ 8.75 (s, 1H), 7.98 (dd, J = 7.7, 1.0 Hz, 1H), 7.80 (td, J = 7.6, 1.3 Hz, 1H), 7.73 (td, J = 7.6, 1.1 Hz, 1H), 7.65 (d, J = 7.6 Hz, 1H), 7.46–7.39 (m, 2H), 7.26 (td, J = 7.6, 0.8 Hz, 1H), 7.18–7.13 (m, 1H). ¹³C NMR (126 MHz, DMSO-d₆): δ 162.9, 158.4 (d, ¹J_C,F = 245.1 Hz), 156.5, 151.8, 150.7, 134.8 (d, ⁴J_C,F = 4.8 Hz), 132.2 (d, ⁴J_C,F = 2.8 Hz), 131.8, 131.5, 130.9, 130.8 (d, ³J_C,F = 8.4 Hz), 128.9, 126.6, 126.5, 125.2 (d, ⁴J_C,F = 3.5 Hz), 115.9 (d, ²J_C,F = 22.1 Hz). ¹⁹F NMR (471 MHz, DMSO-d₆): δ 45.87 (ddd, J = 10.6, 7.7, 5.4 Hz) ppm. Calcd for C₁₆H₉FN₄O (292.27): C, 65.75; H, 3.10; N, 19.17. Found: C, 65.70; H, 3.05; N, 19.24.

5-(3′-Fluoro-[1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5e)

Yield: 210 mg, 72%, pale yellow powder. mp 153 °C. ¹H NMR (500 MHz, DMSO-d₆): δ 8.42 (s, 1H), 7.94 (dd, J = 7.6, 0.9 Hz, 1H), 7.79 (td, J = 7.5, 1.3 Hz, 1H), 7.71 (ddd, J = 9.9, 5.6, 1.7 Hz, 2H), 7.35 (td, J = 7.9, 6.3 Hz, 1H), 7.32–7.28 (m, 1H), 7.24 (td, J = 8.4, 2.1 Hz, 1H), 6.99 (d, J = 7.8 Hz, 1H). ¹³C NMR (126 MHz, DMSO-d₆): δ 163.3, 162.3 (d, ¹J_C,F = 245.4 Hz), 156.7, 152.2, 150.7, 141.4 (d, ³J_C,F = 7.9 Hz), 139.9 (d, ⁴J_C,F = 2.1 Hz), 134.4, 132.0, 131.6, 130.9 (d, ³J_C,F = 8.6 Hz), 130.7, 128.8, 126.5 (d, ⁴J_C,F = 2.7 Hz), 116.4 (d, ²J_C,F = 22.0 Hz), 115.2 (d, ³J_C,F = 21.0 Hz). ¹⁹F NMR (471 MHz, DMSO-d₆): δ 50.83–50.78 (m). Calcd for C₁₆H₉FN₄O (292.27): C, 65.75; H, 3.10; N, 19.17. Found: C, 65.68; H, 3.17; N, 19.35.

5-(4′-Fluoro-[1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5f)

Yield: 232 mg, 80%, pale yellow powder. mp 151 °C. ¹H NMR (500 MHz, DMSO-d₆): δ 8.35 (s, 1H), 7.92 (dd, J = 7.6, 0.8 Hz, 1H), 7.78 (td, J = 7.6, 1.2 Hz, 1H), 7.70–7.65 (m, 2H), 7.36–7.32 (m, 2H), 7.23–7.17 (m, 2H). ¹³C NMR (126 MHz, DMSO-d₆): δ 163.6, 162.07 (d, ¹J_C,F = 246.3 Hz), 156.7, 152.2, 150.7, 140.2, 135.4 (d, ⁴J_C,F = 3.0 Hz), 134.1, 132.1, 132.0 (d, ⁴J_C,F = 3.8 Hz), 131.5, 130.6, 128.4, 116.0 (d, ²J_C,F = 21.6 Hz). ¹⁹F NMR (471 MHz, DMSO-d₆): δ 48.91 (dq, J = 8.9, 5.4 Hz). Calcd for C₁₆H₉FN₄O (292.27): C, 65.75; H, 3.10; N, 19.17. Found: C, 65.63; H, 3.04; N, 19.17.

5-(2′,4′-Difluoro-[1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5g)

Yield: 214 mg, 69%, yellow powder. mp 141 °C. ¹H NMR (500 MHz, DMSO-d₆): δ 8.86 (s, 1H), 7.99 (dd, J = 7.7, 1.2 Hz, 1H), 7.80 (td, J = 7.6, 1.4 Hz, 1H), 7.74 (td, J = 7.6, 1.3 Hz, 1H), 7.63 (d, J = 7.6 Hz, 1H), 7.47 (td, J = 8.7, 6.6 Hz, 1H), 7.22 (ddd, J = 10.6, 9.4, 2.5 Hz, 1H), 7.15 (td, J = 8.4, 2.3 Hz, 1H). ¹³C NMR (126 MHz, DMSO-d₆): δ 162.6, 162.2 (dd, ¹J_C,F = 248.2, ³J_C,F = 12.3 Hz), 158.5 (dd, ¹J_C,F = 247.7, ³J_C,F = 12.6 Hz), 156.5, 151.7, 150.7, 134.7, 134.0, 133.3 (dd, ³J_C,F = 9.8, ⁴J_C,F = 4.3 Hz), 131.8, 131.5, 130.9, 129.9 (d, ¹J_C,F = 240.1 Hz), 123.2 (dd, ²J_C,F = 15.0, ⁴J_C,F = 3.8 Hz), 112.3 (dd, ²J_C,F = 21.3, ⁴J_C,F = 3.6 Hz), 104.3 (t, ²J_C,F = 26.4 Hz). ¹⁹F NMR (471 MHz, DMSO-d₆): δ 53.22–53.15 (m, 1F), 50.70 (dd, J = 18.8, 8.9 Hz, 1F). Calcd for C₁₆H₈F₂N₄O (310.26): C, 61.94; H, 2.60; N, 18.06. Found: C, 61.91; H, 2.70; N, 18.26.

5-(3′,5′-Difluoro-[1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (5h)

Yield: 160 mg, 51%, yellow powder. mp 124 °C. ¹H NMR (400 MHz, DMSO-d₆): δ 8.62 (s, 1H), 7.99–7.96 (m, 1H), 7.82–7.71 (m, 2H), 7.70–7.67 (m, 1H), 7.33–7.25 (m, J = 9.3, 2.2 Hz, 1H), 7.11–7.04 (m, 2H). ¹³C NMR (101 MHz, DMSO-d₆): δ 162.6, 162.3 (dd, ¹J_C,F = 247.8, ³J_C,F = 13.5 Hz), 156.7 (t, ²J_C,F = 23.8 Hz), 152.1, 150.8, 142.9 (t, ³J_C,F = 9.8 Hz), 139.1, 134.0, 132.0 (d, ³J_C,F = 6.6 Hz), 131.6 (d, ²J_C,F = 18.5 Hz), 130.9, 129.1, 113.2 (dd, ²J_C,F = 25.8, ²J_C,F = 18.1 Hz), 103.6 (td, ²J_C,F = 25.7, ²J_C,F = 15.2 Hz). ¹⁹F NMR (376 MHz, DMSO): δ 54.04–53.9 (m). Calcd for C₁₆H₈F₂N₄O (310.26): C, 61.94; H, 2.60; N, 18.06. Found: C, 62.11; H, 2.62; N, 18.00.

General Procedure for the Synthesis of Dibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline Derivatives (7a–7h)

Corresponding 5-([1,1′-biphenyl]-2-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine derivatives 5a–5h (0.5 mmol) were dissolved in CF₃COOH (3 mL). The resulting mixture was stirred for 3 h at room temperature; the precipitate formed was filtered off, washed with MeCN (3 × 3 mL), and dried. The obtained 9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline derivatives 6a–6h were analyzed and further oxidized by the addition of the solution of NaOH (112 mg, 2.0 mmol, 4 equiv) and K₃Fe(CN)₆ (329 mg, 1.0 mmol, 2 equiv) in the mixture of EtOH/H₂O (1:5, v/v). The resulting mixture was stirred for 24 h at room temperature and diluted with H₂O (10 mL), and a precipitate was formed and filtered off. The residue was purified by flash column chromatography with CH₂Cl₂ as an eluent to afford the desired dibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline derivatives (7a–7h).

9,13-Dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6a)

Yield: 130 mg, 95%, white powder. mp 228 °C. ¹H NMR (400 MHz, DMSO-d₆): δ 9.95 (s, 2H), 8.77 (d, J = 8.2 Hz, 2H), 8.21 (d, J = 8.2 Hz, 2H), 7.68–7.64 (m, 2H), 7.60–7.57 (m, 2H). ¹³C NMR (101 MHz, DMSO-d₆): δ 148.0, 127.2, 126.6, 125.1, 123.2, 122.9, 120.5, 118.1. Calcd for C₁₆H₁₀N₄O (274.28): C, 70.06; H, 3.68; N, 20.43. Found: C, 70.15; H, 3.79; N, 20.44.

2-(tert-Butyl)-9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6b)

This substance could not be isolated in its pure form.

1,2,3-Trimethoxy-9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6c)

Yield: 170 mg, 93%, white powder. mp 221 °C. ¹H NMR (500 MHz, DMSO-d₆): δ 9.80 (s, 1H), 9.69 (s, 1H), 8.74 (d, J = 8.3 Hz, 1H), 8.12 (d, J = 8.3 Hz, 1H), 8.03 (s, 1H), 7.64–7.59 (m, 1H), 7.57–7.51 (m, J = 7.5 Hz, 1H), 4.04 (s, 3H), 3.97 (s, 3H), 3.91 (s, 3H). ¹³C NMR (126 MHz, DMSO-d₆): δ 151.5, 148.5, 147.9, 147.1, 142.2, 127.2, 125.6, 124.9, 124.4, 123.9, 122.6, 120.2, 118.2, 116.7, 111.2, 101.5, 62.5, 60.8, 56.1. Calcd for C₁₉H₁₆N₄O₄ (364.36): C, 62.63; H, 4.43; N, 15.38. Found: C, 62.59; H, 4.55; N, 15.38.

4-Fluoro-9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6d)

This substance could not be isolated in its pure form.

3-Fluoro-9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6e)

Yield: 130 mg, 89%, white powder. mp 245 °C. ¹H NMR (400 MHz, DMSO-d₆): δ 9.96 (s, 1H), 9.90 (s, 1H), 8.72 (d, J = 8.2 Hz, 1H), 8.54 (dd, J = 11.5, 2.4 Hz, 1H), 8.23 (dd, J = 9.2, 5.6 Hz, 1H), 8.18 (d, J = 8.3 Hz, 1H), 7.69–7.64 (m, J = 7.5 Hz, 1H), 7.59–7.50 (m, 2H). ¹³C NMR (101 MHz, DMSO-d₆): δ 156.0 (d, ¹J_C,F = 242.3 Hz), 148.0 (d, ⁴J_C,F = 3.5 Hz), 128.4 (d, ³J_C,F = 8.3 Hz), 127.8, 125.9 (d, ⁴J_C,F = 4.0 Hz), 125.0, 123.8, 123.4, 123.2, 123.1, 120.4 (d, ³J_C,F = 10.6 Hz), 119.8, 118.1, 117.7 (d, J = 2.3 Hz), 115.9 (dd, ²J_C,F = 23.7, ⁴J_C,F = 4.3 Hz), 108.40 (dd, ²J_C,F = 22.6, ⁴J_C,F = 3.2 Hz).¹⁹F NMR (376 MHz, DMSO-d₆): δ 47.05 (ddd, J = 11.5, 8.1, 5.7 Hz). Calcd for C₁₆H₉FN₄O (292.27): C, 65.75; H, 3.10; N, 19.17. Found: C, 65.63; H, 3.14; N, 19.19.

2-Fluoro-9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6f)

Yield: 84 mg, 56%, white powder. mp 267 °C. ¹H NMR (400 MHz, DMSO-d₆): δ 10.02 (s, 1H), 9.86 (s, 1H), 8.82 (dd, J = 9.2, 5.9 Hz, 1H), 8.72 (d, J = 8.0 Hz, 1H), 8.20 (d, J = 8.1 Hz, 1H), 8.01 (dd, J = 11.7, 2.4 Hz, 1H), 7.68–7.56 (m, 2H), 7.43 (td, J = 9.0, 2.4 Hz, 1H). ¹³C NMR (101 MHz, DMSO-d₆): δ 161.4 (d, ¹J_C,F = 243.5 Hz), 147.9 (d, ⁴J_C,F = 3.2 Hz), 127.0, 126.4, 126.2 (d, ³J_C,F = 9.3 Hz), 125.5, 124.4 (d, ³J_C,F = 9.3 Hz), 123.4, 123.4, 123.2, 122.4, 120.5, 119.5, 117.7 (d, ⁴J_C,F = 3.7 Hz), 113.5 (d, ²J_C,F = 23.6 Hz), 105.4 (d, ²J_C,F = 24.0 Hz). ¹⁹F NMR (471 MHz, DMSO-d₆): δ 49.61–49.55 (m). Calcd for C₁₆H₉FN₄O (292.27): C, 65.75; H, 3.10; N, 19.17. Found: C, 65.73; H, 3.21; N, 19.37.

2,4-Difluoro-9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6g)

This substance was not detected in the reaction mixture by ¹H NMR.

1,3-Difluoro-9,13-dihydrodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (6h)

Yield: 120 mg, 77%, white powder. mp 231 °C. ¹H NMR (400 MHz, DMSO-d₆): δ 9.96 (s, 1H), 9.07 (d, J = 10.9 Hz, 1H), 8.75 (d, J = 8.1 Hz, 1H), 8.50 (dd, J = 11.0, 1.4 Hz, 1H), 8.21 (d, J = 8.2 Hz, 1H), 7.72 (td, J = 8.0, 0.7 Hz, 1H), 7.64–7.53 (m, 1H). The ¹³C NMR spectrum cannot be correctly described in view of the extremely low solubility of 6h in common organic solvents. ¹⁹F NMR (376 MHz, DMSO): δ 53.76 (ddd, J = 14.3, 11.2, 7.4 Hz, 1F), 49.51–49.44 (m, 1F). Calcd for C₁₆H₈F₂N₄O (310.26): C, 61.94; H, 2.60; N, 18.06. Found: C, 61.83; H, 2.72; N, 18.12.

Dibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7a)

Yield: 170 mg, 73%, red powder. mp 281 °C. ¹H NMR (400 MHz, DMSO-d₆): δ 8.31 (d, J = 8.0 Hz, 2H), 8.03 (dd, J = 7.7, 1.2 Hz, 2H), 7.79 (td, J = 8.0, 1.2 Hz, 2H), 7.56–7.52 (m, 2H). ¹³C NMR (101 MHz, DMSO-d₆): δ 152.0, 151.6, 134.0, 133.4, 129.2, 128.7, 127.8, 124.2. Calcd for C₁₆H₈N₄O (272.27): C, 70.58; H, 2.96; N, 20.58. Found: C, 70.46; H, 2.91; N, 20.41.

2-(tert-Butyl)dibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7b)

Yield: 157 mg, 96%, red powder. mp 248 °C. ¹H NMR (400 MHz, CDCl₃): δ 9.15 (s, 1H), 9.07 (d, J = 8.1 Hz, 1H), 8.29 (d, J = 8.1 Hz, 1H), 8.24 (d, J = 8.4 Hz, 1H), 7.86 (dd, J = 8.5, 2.0 Hz, 1H), 7.81–7.76 (m, J = 7.6 Hz, 1H), 7.65–7.59 (m, J = 7.6 Hz, 1H), 1.52 (s, 9H). ¹³C NMR (101 MHz, CDCl₃): δ 152.6, 151.8, 151.7, 151.4, 151.4, 133.8, 133.7, 131.5, 131.2, 128.7, 128.6, 128.6, 128.5, 125.0, 123.3, 123.2, 35.2, 31.2. Calcd for C₂₀H₁₆N₄O (328.28): C, 73.15; H, 4.91; N, 17.06. Found: C, 73.18; H, 4.90; N, 17.16.

1,2,3-Trimethoxydibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7c)

Yield: 124 mg, 69%, red powder. mp 259 °C. ¹H NMR (400 MHz, CDCl₃): δ 9.13 (dd, J = 8.1, 1.1 Hz, 1H), 8.23 (d, J = 8.2 Hz, 1H), 7.83–7.77 (m, 1H), 7.68–7.62 (m, 2H), 4.17 (s, 6H), 4.06 (s, 3H). ¹³C NMR (101 MHz, CDCl₃): δ 157.7, 156.9, 151.5, 151.1, 150.9, 150.1, 145.0, 133.7, 133.4, 131.4, 129.2, 128.73, 128.65, 123.3, 117.5, 102.6, 61.3, 61.1, 56.2. Calcd for C₁₉H₁₄N₄O₄ (362.35): C, 62.98; H, 3.89; N, 15.46. Found: C, 62.99; H, 4.14; N, 15.44.

4-Fluorodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7d)

Yield: 122 mg, 84%, orange powder. mp 252 °C. ¹H NMR (400 MHz, CDCl₃): δ 9.25 (d, J = 7.7 Hz, 1H), 9.09 (d, J = 6.8 Hz, 1H), 8.86 (d, J = 5.0 Hz, 1H), 7.89–7.84 (m, 1H), 7.76–7.70 (m, 1H), 7.69–7.56 (m, 2H). The ¹³C NMR spectrum cannot be correctly described in view of the extremely low solubility of 7d in common organic solvents.¹⁹F NMR (471 MHz, CDCl₃): δ 53.53 (dt, J = 13.4, 4.2 Hz). Calcd for C₁₆H₇FN₄O (290.26): C, 66.21; H, 2.43; N, 19.30. Found: C, 66.50; H, 2.34; N, 19.43.

3-Fluorodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7e)

Yield: 135 mg, 93%, orange powder. mp 282 °C. ¹H NMR (400 MHz, CDCl₃): δ 9.24–9.19 (m, 2H), 8.30 (d, J = 8.2 Hz, 1H), 8.04 (dd, J = 10.2, 2.4 Hz, 1H), 7.91–7.85 (m, 1H), 7.77–7.73 (m, 1H), 7.41 (ddd, J = 9.0, 7.7, 2.5 Hz, 1H). ¹³C NMR (101 MHz, CDCl₃): cannot be correctly described in view of the extremely low solubility of 7e in common organic solvents. ¹⁹F NMR (376 MHz, CDCl₃): δ 58.73 (ddd, J = 10.3, 7.5, 6.2 Hz). Calcd for C₁₆H₇FN₄O (290.26): C, 66.21; H, 2.43; N, 19.30. Found: C, 66.23; H, 2.55; N, 19.34.

2-Fluorodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7f)

Yield: 114 mg, 78%, orange powder. mp 256 °C. ¹H NMR (500 MHz, CDCl₃): δ 9.18 (dd, J = 8.1, 0.8 Hz, 1H), 8.83 (dd, J = 9.5, 2.8 Hz, 1H), 8.39 (dd, J = 8.9, 5.1 Hz, 1H), 8.32 (d, J = 8.1 Hz, 1H), 7.87–7.83 (m, 1H), 7.72–7.68 (m, 1H), 7.57 (ddd, J = 9.0, 7.5, 2.8 Hz, 1H). ¹³C NMR (101 MHz, CDCl₃): δ 163.0 (d, ¹J_C,F = 250.9 Hz), 151.6, 151.5, 151.3, 150.6 (d, ⁴J_C,F = 3.1 Hz), 133.9, 133.1, 131.2 (d, ³J_C,F = 8.1 Hz), 130.3 (d, ⁴J_C,F = 3.0 Hz), 129.0, 128.7, 128.6, 125.8 (d, ³J_C,F = 8.1 Hz), 123.4, 121.6 (d, ²J_C,F = 22.9 Hz), 114.3 (d, ²J_C,F = 23.7 Hz). ¹⁹F NMR (471 MHz, CDCl₃): δ 51.23 (ddd, J = 9.5, 7.5, 5.1 Hz). Calcd for C₁₆H₇FN₄O (290.26): C, 66.21; H, 2.43; N, 19.30. Found: C, 66.29; H, 2.47; N, 19.24.

2,4-Difluorodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7g)

This substance was not detected in the reaction mixture by ¹H NMR.

1,3-Difluorodibenzo[f,h][1,2,5]oxadiazolo[3,4-b]quinoxaline (7h)

Yield: 118 mg, 77%, orange powder. mp 269 °C. ¹H NMR (400 MHz, CDCl₃): δ 9.24 (dd, J = 8.1, 1.2 Hz, 1H), 8.32 (d, J = 8.2 Hz, 1H), 8.01–7.96 (m, 1H), 7.94–7.89 (m, 1H), 7.80 (td, J = 7.7, 1.0 Hz, 1H), 7.23 (ddd, J = 11.5, 8.2, 2.5 Hz, 1H). The ¹³C NMR spectrum cannot be correctly described in view of the extremely low solubility in common organic solvents. ¹⁹F NMR (471 MHz, CDCl₃): δ 66.81 (dd, J = 14.9, 11.8 Hz, 1F), 62.03 (ddd, J = 15.7, 9.8, 8.3 Hz, 1F). Calcd for C₁₆H₆F₂N₂O (308.25): C, 62.34; H, 1.96; N, 18.18. Found: C, 62.47; H, 1.98; N, 18.16.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00479.

• Compound characterization checklist (XLS)

• Crystallographic data for 7h (CIF)

• Cyclic voltammograms of compounds; UV–vis absorption spectra of compounds 7a–7f and 7h; and ¹H, ¹³C, and ¹⁹F NMR spectra for all new compounds (PDF)

The authors declare no competing financial interest.