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Article

[1,2,5]Oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline as a Versatile Scaffold for the Construction of Various Polycyclic Systems as Potential Organic Semiconductors

by
Elizaveta M. Krynina
1,
Yuriy A. Kvashnin
1,
Ekaterina F. Zhilina
1,
Denis A. Gazizov
1,
Pavel A. Slepukhin
1,2,
Gennady L. Rusinov
1,
Egor V. Verbitskiy
1,2,* and
Valery N. Charushin
1,2
1
I. Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, S. Kovalevskaya Str., 22, 620137 Ekaterinburg, Russia
2
Department of Organic and Biomolecular Chemistry, Ural Federal University, Mira St. 19, 620002 Ekaterinburg, Russia
*
Author to whom correspondence should be addressed.
Chemistry 2025, 7(5), 158; https://doi.org/10.3390/chemistry7050158
Submission received: 3 September 2025 / Revised: 15 September 2025 / Accepted: 29 September 2025 / Published: 1 October 2025
(This article belongs to the Special Issue Oxygen-Containing Heterocyclic Compounds: Recent Advances in Chemistry)
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Abstract

A straightforward synthetic method is advanced to produce hard-to-reach polycyclic compounds belonging to the [1,2,5]oxadiazolo[3,4-b]quinoxaline ring system. This approach draws on a combination of the nucleophilic aromatic substitution of hydrogen (SNH) and Scholl cross-coupling reactions, followed by reduction of the 1,2,5-oxadiazole fragment under mild reaction conditions. All compounds were obtained for the first time with moderate to excellent yields. Electrochemical and photophysical measurements show that the synthesized compounds may serve as narrow-band n-type organic semiconductors, with energy levels ranging from 2.00 to 2.28 eV, comparable to those of the best commercially available electronic semiconductors.

1. Introduction

Derivatives of polycyclic heteroaromatic conjugated systems are widely recognized as organic semiconductors with various applications, including light-emitting diodes, photovoltaic devices, and thin-film transistors [1,2,3,4,5,6]. Many of compounds in this family have demonstrated promising performance that is comparable to that of amorphous silicon [7]. One of the promising structural motifs in materials science is the ring system of [1,2,5]oxadiazolo[3,4-b]pyrazine, also known as furazanopyrazine [8,9]. Over the past decade, numerous reports have emerged, detailing the development of molecular magnets [10,11,12,13], chemosensors [14,15,16], and semiconductor materials [17] that utilize the furazanopyrazine scaffold.
In particular, we have recently demonstrated the strong application potential of polycyclic (I) and push-pull systems (II and III), bearing the furazanopyrazine core, as effective charge transport layers for organic light-emitting diodes and perovskite solar cells (Figure 1) [18,19].
Notably, two convenient synthetic methods for obtaining 1,4-diazatriphenylene derivatives fused with the 1,2,5-oxadiazole ring have been proposed (Scheme 1). The first pathway involves the intramolecular Scholl cyclization of 5,6-di(hetero)aryl substituted [1,2,5]oxadiazolo[3,4-b]-pyrazines, which have preliminarily been obtained through the SNH reaction [20,21]. The second pathway exploits intramolecular forces, catalyzed by Lewis acids SNH reactions of 5-bis(hetero)aryl substituted furazanopyrazines, preliminarily obtained using the Suzuki reaction [18,22].
In addition, substituted [1,2,5]oxadiazolo[3,4-b]pyrazines have considerable synthetic potential, as convenient synthons for preparing various fused pyrazines. This is due to the ease with which the furazan ring can be reduced to yield the corresponding 1,2-diaminopyrazine derivatives (Scheme 2) [16,23]. It should be noted that 1,2-diaminopyrazines can be used to obtain not only condensed imidazoles, but also a variety of 1,2,5-chalcogenadiazole derivatives. These derivatives are of significant interest as functional materials with promising optical and electrochemical properties [24,25,26,27,28].
Through the reduction–condensation sequence, this study demonstrates a novel multi-step synthetic approach for creating various polycyclic systems of the family of condensed 1,4-diazines, using 5-(hetero)aryl-substituted furazanopyrazines as the starting materials. We intend to provide a comprehensive examination of their photophysical properties, both in solution and in the solid state, alongside an analysis of their single-crystal X-ray diffraction and electrochemical data. Additionally, the potential use of these polycyclic compounds as organic semiconductors will be demonstrated.

2. Experimental

The specifications of the used instruments, materials, synthesized chemicals, and characterization methods are provided in the Supporting Information.
The starting compound, 5-(thiophen-3-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (1), was synthesized using the procedure described earlier [20].
Synthesis of 5-(5-ethylthiophen-2-yl)-6-(thiophen-3-yl)-[1,2,5]oxadiazolo[3,4-b]-pyrazine (3). To a stirred mixture of 5-(thiophen-3-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (1) (204 mg, 1 mmol) and 2-ethylthiophene (227 μL, 2 mmol) in MeCN (5 mL), we added BF3·Et2O (124 μL, 1 mmol). The reaction mixture was stirred at room temperature for 48 h. The solvent was removed in vacuo, and the residual semisolid was washed with aqueous Na2CO3 and air dried. The resulting residue was purified via flash chromatography on silica gel using a hexanes/CHCl3 (1:1, v/v) solvent system to obtain the desired compound 3. It yielded 242 mg (77%), yellow solid, mp 130–132 °C. 1H NMR (400 MHz, DMSO-d6) δ 8.08 (dd, J = 3.0, 1.3 Hz, 1H), 7.74 (dd, J = 5.0, 3.0 Hz, 1H), 7.35 (dd, J = 5.0, 1.3 Hz, 1H), 6.86 (dt, J = 4.0, 1.0 Hz, 1H), 6.73 (d, J = 4.0 Hz, 1H), 2.88 (qd, J = 7.5, 1.0 Hz, 2H), 1.27 (t, J = 7.5 Hz, 3H). 13C NMR (126 MHz, DMSO-d6) δ 158.8, 157.0, 155.7, 151.2, 150.9, 138.5, 138.2, 134.6, 130.2, 128.5, 127.1, 125.8, 23.2, 15.3. Calcd. for C14H10N4OS2 (314.38): C, 53.49; H, 3.21; N, 17.82. Found: C, 53.34; H, 3.24; N, 17.79. HRMS (ESI): m/z calcd. for C14H11N4OS2: 315.0369 [M + H]+; found: 315.0373 and m/z calcd for C14H10N4NaOS2: 337.0188 [M + Na]+; found: 337.0190.
Synthesis of 2-ethyl-[1,2,5]oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline (4). In brief, 5-(5-Ethylthiophen-2-yl)-6-(thiophen-3-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (3) (157 mg, 0.5 mmol) was dissolved in dry CHCl3 (5 mL). Then, CF3COOH (37 μL, 0.5 mmol) and iron(III) chloride (325 mg, 2 mmol) were added to this solution. The reaction mixture was stirred at room temperature for 24 h. The solvent was distilled off under reduced pressure, and the residue was washed with water and air-dried. Purification via column chromatography (CHCl3) led to the desired polycycle 4. Yield: 287 mg (92%), dark green solid, mp 240–242 °C. 1H NMR (400 MHz, CDCl3) δ 8.24 (d, J = 5.3 Hz, 1H), 7.53 (d, J = 5.3 Hz, 1H), 7.26 (s, 1H), 3.08 (qd, J = 7.5, 1.1 Hz, 2H), 1.49 (t, J = 7.5 Hz, 3H). 13C NMR (126 MHz, CDCl3) δ 160.4, 151.0, 150.7, 148.0, 147.3, 141.3, 138.9, 134.5, 131.1, 126.5, 125.8, 120.5, 24.6, 15.4. Calcd. for C14H8N4OS2 (312.37): C, 53.83; H, 2.58; N, 17.94. Found: C, 53.97; H, 2.74; N, 17.96. HRMS (ESI): m/z calcd for C14H9N4OS2: 313.0212 [M + H]+; found: 313.0211 and m/z calcd for C14H8N4NaOS2: 335.0032 [M + Na]+; found: 335.0028. IR spectrum, ν, cm−1: 3106, 2974, 2922, 2853, 2703, 1799, 1741, 1715, 1677, 1631, 1569, 1532, 1508, 1487, 1470, 1440, 1426, 1375, 1357, 1342, 1327, 1313, 1300, 1268, 1258, 1222, 1190, 1153, 1140, 1109, 1093, 1061, 1051, 1030, 1017, 986, 952, 902, 888, 857, 862, 837, 798, 785, 770, 730, 686, 673, 640, 629, 596, 559, 518, 488, 470, 447, 430.
Synthesis of 6-ethyldithieno[2,3-f:2′,3′-h]quinoxaline-2,3-diamine (5). In brief, 2-Ethyl-[1,2,5]oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline (4) (200 mg, 0.64 mmol) and Fe (358 mg, 6.4 mmol, 10 equiv.) were mixed in glacial acetic acid (5 mL). The mixture was refluxed and stirred for 5 h. After that, the mixture was cooled down, and the solvent evaporated. The resulting residue was purified via flash chromatography on silica gel using ethyl acetate as an eluent to obtain the desired compound 5. Yield: 151 mg (79%), pale brown solid, mp 243–245 °C. 1H NMR (500 MHz, DMSO-d6) δ 7.85 (d, J = 5.3 Hz, 1H), 7.66 (d, J = 5.3 Hz, 1H), 7.34–7.31 (m, 1H), 6.74 (s, 2H), 6.69 (s, 2H), 2.97 (qd, J = 7.5, 1.2 Hz, 2H), 1.36 (t, J = 7.5 Hz, 3H). 13C NMR (126 MHz, DMSO-d6) δ 147.5, 145.0, 144.0, 133.7, 131.5, 129.5, 129.4, 128.9, 128.3, 123.9, 122.4, 118.3, 23.5, 15.6. Calcd. for C14H12N4S2 (300.40): C, 55.98; H, 4.03; N, 18.65. Found: C, 55.83; H, 4.08; N, 18.77. HRMS (ESI): m/z calcd. for C14H13N4S2: 301.0576 [M + H]+; found: 301.0576.
Synthesis of 2-ethyl-9,10-diphenylpyrazino[2,3-b]dithieno[2,3-f:2′,3′-h]quinoxaline (7). In brief, 6-Ethyldithieno[2,3-f:2′,3′-h]quinoxaline-2,3-diamine (5) (50 mg, 0.17 mmol) and benzyl (6) (52 mg, 0.25 mmol) were mixed in glacial acetic acid (5 mL). The mixture was refluxed and stirred for 5 h. After that, the mixture was cooled down, and the solvent evaporated. The resulting residue was purified via flash chromatography on silica gel using CHCl3 as an eluent to obtain the desired compound 7. Yield: 71 mg (90%), bright red solid, mp 324–326 °C. 1H NMR (500 MHz, CDCl3) δ 8.60 (d, J = 5.3 Hz, 1H), 7.75–7.70 (m, 4H), 7.63 (d, J = 5.3 Hz, 1H), 7.49–7.41 (m, 3H), 7.39 (ddd, J = 8.5, 6.4, 1.5 Hz, 4H), 3.14 (qd, J = 7.5, 1.1 Hz, 2H), 1.52 (t, J = 7.6 Hz, 3H). 13C NMR (126 MHz, CDCl3) δ 157.8, 157.2, 156.8, 142.6, 142.50, 142.47, 142.4, 138.9, 138.2, 138.1, 137.1, 134.9, 132.1, 130.41, 130.37, 129.8, 129.7, 128.29, 128.27, 125.8, 125.0, 119.6, 24.6, 15.6. Calcd. for C28H18N4S2 (474.60): C, 70.86; H, 3.82; N, 11.81. Found: C, 70.92; H, 3.89; N, 11.93. HRMS (ESI): m/z calcd for C28H19N4S2: 475.1046 [M + H]+; found: 475.1042 and m/z calcd for C28H18N4NaS2: 497.0865 [M + Na]+; found: 497.0863. IR spectrum, ν, cm−1: 3114, 3074, 2958, 2927, 2870, 1996, 1944, 1882, 1797, 1684, 1579, 1568, 1532, 1501, 1481, 1456, 1444, 1421, 1372, 1337, 1321, 1289, 1274, 1233, 1178, 1153, 1081, 1062, 1026, 988, 977, 916, 883, 830, 806, 777, 769, 743, 725, 699, 691, 973, 643, 618, 601, 594, 557, 546, 504, 491, 471, 408.
Synthesis of 2-ethyl-[1,2,5]thiadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline (8). In brief, 6-Ethyldithieno[2,3-f:2′,3′-h]quinoxaline-2,3-diamine (5) (100 mg, 0.33 mmol) and pyridine (268 µL, 3.33 mmol) were mixed in CH2Cl2 (5 mL). The mixture was cooled down to −20 °C. Then, SOCl2 (100 µL, 1.37 mmol) was added to this solution. The reaction mixture was stirred at −20 °C for 30 min. After that, the mixture was refluxed and stirred for 12 h. Next, the mixture was cooled down, and the solvent evaporated. The resulting residue was purified via flash chromatography on silica gel using CHCl3 as an eluent to obtain the desired compound 8. Yield: 73 mg (67%), brown solid, mp 238–240 °C. 1H NMR (500 MHz, CDCl3) δ 8.43 (d, J = 5.3 Hz, 1H), 7.59 (d, J = 5.3 Hz, 1H), 7.37 (t, J = 1.1 Hz, 1H), 3.11 (qd, J = 7.5, 1.1 Hz, 2H), 1.50 (t, J = 7.5 Hz, 3H). 13C NMR (126 MHz, CDCl3) δ 157.9, 152.7, 152.5, 143.9, 143.8, 139.6, 137.6, 134.8, 131.7, 125.9, 125.2, 120.0, 24.6, 15.5. Calcd. for C14H8N4S3 (328.43): C, 51.20; H, 2.46; N, 17.06. Found: C, 51.18; H, 2.41; N, 17.11. HRMS (ESI): m/z calcd for C14H7N4S3: 326.9838 [M–H]+; found: 326.9848 and m/z calcd for C14H8N4S3: 327.9917 [M]+; found: 327.9905. IR spectrum, ν, cm−1: 3115, 3075, 2977, 2960, 2930, 2870, 1820, 1651, 1525, 1498, 1473, 1451, 1419, 1369, 1339, 1312, 1273, 1253, 1226, 1138, 1109, 1086, 1063, 1051, 993, 972, 909, 887, 862, 826, 812, 792, 783, 734, 672, 653, 639, 594, 544, 512, 452.
Synthesis of 2-ethyl-[1,2,5]selenadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline (9). In brief, 6-Ethyldithieno[2,3-f:2′,3′-h]quinoxaline-2,3-diamine (5) (100 mg, 0.33 mmol) and selenium dioxide (89 mg, 0.80 mmol) were mixed in CH3CN (5 mL). The mixture was refluxed under an argon atmosphere for 4 h. After that, the mixture was cooled down, and the solvent evaporated. The resulting residue was purified via flash chromatography on silica gel using CHCl3 as an eluent to obtain the desired compound 9. Yield: 98 mg (78%), dark purple solid, mp > 350 °C. 1H NMR (500 MHz, CDCl3) δ 8.38 (d, J = 5.2 Hz, 1H), 7.54 (d, J = 5.2 Hz, 1H), 7.32 (t, J = 1.1 Hz, 1H), 3.08 (qd, J = 7.5, 1.1 Hz, 2H), 1.49 (t, J = 7.6 Hz, 3H). 13C NMR (126 MHz, CDCl3) δ 158.3, 156.5, 156.3, 145.0, 144.7, 140.1, 138.0, 134.9, 131.7, 126.2, 125.2, 120.4, 24.6, 15.5. Calcd. for C14H8N4S2Se (375.34): C, 44.80; H, 2.15; N, 14.93. Found: C, 44.63; H, 2.31; N, 14.69. HRMS (ESI): m/z calcd for C14H9N4S2Se: 376.9428 [M + H]+; found: 376.9426 and m/z calcd for C14H8N4NaS2Se: 398.9248 [M + Na]+; found: 398.9246. IR spectrum, ν, cm−1: 3107, 2967, 2926, 2866, 1815, 1644, 1523, 1491, 1468, 1445, 1419, 1380, 1365, 1342, 1312, 1250, 1220, 1136, 1109, 1083, 1061, 993, 914, 891, 849, 826, 797, 781, 774, 747, 736, 672, 643, 618, 589, 557, 477, 448, 419.
Synthesis of 2-ethyl-10H-imidazo[4,5-b]dithieno[2,3-f:2′,3′-h]quinoxaline (10). Here, 6-Ethyldithieno[2,3-f:2′,3′-h]quinoxaline-2,3-diamine (5) (50 mg, 0.17 mmol) and triethyl orthoformate (52 mg, 0.25 mmol) were mixed in toluene (1 mL). The mixture was refluxed and stirred for 5 h. After that, the mixture was cooled down, and the precipitate was filtered. The resulting precipitate was washed with THF (2 × 2 mL) and dried at 100 °C for 1 h to obtain the desired compound 10. Yield 28 mg (55%), orange solid, mp > 350 °C. 1H NMR (500 MHz, DMSO-d6) δ 13.73 (s, 1H), 9.09 (s, 1H), 8.29 (s, 1H), 7.95 (d, J = 5.3 Hz, 1H), 7.60 (s, 1H), 3.09 (q, J = 7.5 Hz, 2H), 1.43 (t, J = 7.5 Hz, 3H). The 13C NMR spectra of 10 could not be obtained due to the poor solubility of this compound in deuterated solvents. Calcd. for C15H10N4S2 (310.39): C, 58.04; H, 3.25; N, 18.05. Found: C, 58.13; H, 3.33; N, 18.02. HRMS (ESI): m/z calcd. for C15H9N4S2: 309.0274 [M–H]+; found: 309.0276. IR spectrum, ν, cm−1: 3370, 3224, 3101, 2966, 2928, 1607, 1563, 1506, 1492, 1473, 1434, 1411, 1353, 1277, 1228, 1192, 1159, 1086, 981, 915, 873, 820, 780, 715, 670, 642, 614, 600, 513.

3. Results and Discussion

3.1. Synthesis and Thermal Properties

For the synthesis of the target 2-ethyl-[1,2,5]oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]-quinoxaline (4), the premise was the sequential use of the nucleophilic aromatic substitution of hydrogen (the SNH-reaction) and the Scholl reaction, which we proposed earlier [21]. Namely, the reaction of 5-(thiophen-3-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (1) with 2-ethylthiophene (2) at room temperature in the presence of boron trifluoride etherate led to the SNH-product (3) in high yields. The Scholl reaction, using iron(III) chloride and trifluoroacetic acid, facilitated a coupling reaction between two thiophene substituents in 5-(5-ethylthiophen-2-yl)-6-(thiophen-3-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (3), yielding the desired polycycle (4) with a 92% yield (Scheme 3).
The reduction of the furazanopyrazine derivative (4) using iron powder in acetic acid resulted in the formation of the corresponding 6-ethyldithieno[2,3-f:2′,3′-h]quinoxaline-2,3-diamine (5). This 2,3-diamine-substituted derivative (5) served as a convenient building block for the development of new polycyclic systems 7–10.
Thus, 2-ethyl-9,10-diphenylpyrazino[2,3-b]dithieno[2,3-f:2′,3′-h]quinoxaline (7) was obtained with a high yield of 90% through the interaction of compound 5 with benzil (6) using the previously reported procedure [16].
According to the literature [29], the addition of excess thionyl chloride to 2,3-diamine (5) in the presence of pyridine at −20 °C enabled the preparation of 2-ethyl-[1,2,5]thiadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline (8) with a yield of 67%.
Additionally, 6-Ethyldithieno[2,3-f:2′,3′-h]quinoxaline-2,3-diamine (5) was treated with selenium dioxide in acetonitrile to give 2-ethyl-[1,2,5]selenadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline (9) in a 78% yield (Scheme 3) [30].
A general synthetic method for preparing imidazo[b]pyrazine derivatives involves reacting with triethyl orthoformate [31]. Consequently, 2-ethyl-10H-imidazo[4,5-b]dithieno[2,3-f:2′,3′-h]quinoxaline (10) was synthesized in a 55% yield by refluxing 2,3-diamine (5) with triethyl orthoformate in toluene.
The mechanism of the nucleophilic aromatic substitution of hydrogen (the SNH-reaction), which describes the interaction of compounds 1 and 2 to form product 3, is similar to the previously described processes [6]. Additionally, the Scholl cross-coupling reaction, which leads to the formation of polycycle 4, shares similarities with the mechanisms we discussed earlier [20].
All compounds 3–10 were systematically characterized using 1H and 13C NMR and HRMS data, which matched the anticipated structures (see Figures S1–S21 in the Supporting Information for corresponding spectra).
We observed an interesting feature in the 1H NMR spectra of most of the synthesized polycycles (compounds 4 and 7–9). The thiophene protons at position C(3), which would theoretically appear as a singlet, instead show a triplet at 7.26–7.37 ppm. This triplet is a result of spin–spin coupling with the CH2 protons of the ethyl group, exhibiting coupling constants of 1.0–1.2 Hz. Meanwhile, the CH2 group produces a quartet of doublets at 3.08–3.14 ppm, with splitting constants of 4JH–H =1.0–1.2 Hz and 3JH–H = 7.5 Hz (Figure 2). It is notable that this interaction between the CH2 group and the proton in the thiophene ring is also observed in compounds 3 and 5.

3.2. X-Ray Diffraction Analysis of the Polycyclic Compounds 4, 8, and 9

The structures of compounds 4, 8, and 9 were examined and confirmed using X-ray diffraction (XRD) data. The results are summarized in Tables S1–S21 in the Supporting Information. Figure 2, Figure 3, Figure 4 and Figure 5 illustrate the general view and numbering of the atoms in molecules 4, 8, and 9. The quality of the data for the observed small crystals at room temperature allows for a discussion only of the general geometry of the molecules, with limited accuracy.
According to the XRD data, the molecules of the compound are planar, except for the substituents. The bond distances and angles within the molecules are close to the expected values. Specifically, the bond distances for N–X in the diazole moieties increase from N–O at 1.38 Å (compound 4) to N–S at 1.61 Å (compound 8) and to N–Se at 1.78 Å (compound 9). The differences in the N–X bond distances are within the measurement error limits. The angles also reflect this trend: N–O–N measures 115.4(7)° (compound 4), N–S–N measures 101.92(18)° (compound 8), and N–Se–N measures 94.6(3)° (compound 9). These data indicate an increasing asymmetry in the diazole ring as the atomic weight of the substituent atom increases from lighter O to heavier Se.
The Et-substituent in compound 4 shows structural disordering at the C(2) positions within the plane of the heterocycle, with occupancy coefficients of 0.5. For clarity, the second part of the disordering is omitted in Figure 2. In contrast, this type of disordering is not observed in compounds 8 and 9.
The crystals of compound 4 are centrosymmetric, and their packing is arranged in stacks oriented along the 0a axis (Figure 6). The structure features a coplanar arrangement of the polycyclic moiety planes within the stacks. The distance between the planes of the molecules in the stack measures 3.512 Å, and the dihedral angle between the planes of the polycyclic moiety and the (001) plane is 62.5°. Neither significant C–H…π interactions nor π–π stacking is observed in the crystals. However, there are weak hydrogen bonds detected between the stacks, specifically between CAr–H(12)…N(3) [1 − x, 1 − y, −z] and CAr–H(13)…O(1) [x, y − 1, z], which involve the heteroatoms of the diazole moiety.
The compound 8 crystallizes in the centrosymmetric space group of the monoclinic system. Its crystal packing is layered, with the orientation of the layers in the (001) plane (Figure 7). The molecules within each layer are coplanar, with distances between the planes of adjacent molecules measuring 3.45 Å and 3.42 Å. The planes of molecules in adjacent layers form a dihedral angle of 34.6°. Notably, no significant C–H…π interactions or π–π stacking is observed in the crystals. The molecules in the layers form a centrosymmetric dimeric structure, characterized by shortened contacts between N(3) and S(2) at a distance of 3.22 Å, located at the coordinates [−x, 2 − y, −z]. Furthermore, structure 8 is distinguished by minimal equivalent isotropic displacement parameters compared to compounds 4, 8, and 9. This rigid molecular packing contributes to a reduced likelihood of thermal energy dissipation at high energy levels within the molecules.
Compound 9 crystallizes in a non-primitive, centrosymmetric space group within the monoclinic system. The molecules are situated in a special position on a plane. The crystal packing of compound 9 is layered, with the layers oriented in the (010) plane (Figure 8). The distance between the molecular planes is b/2 = 3.39 Å. However, no significant C–H…π interactions or π–π stacking was observed in the crystals, likely due to the high values of the interatomic distances.
Within the crystal layers, the molecules form a centrosymmetric dimeric structure characterized by particularly short contacts between N(4) and Se(1) [−x, y, −z], measuring 2.804 Å. The geometry of these contacts is similar to the N…S contacts observed in compound 8, possibly indicating a specific attractive interaction among the selenodiazole moieties of the azapolyheterocyclic system (Figure 9). Previously, extremely short N…Se contacts ranging from 2.8 to 2.9 Å have been reported for various selenodiazoles in the literature [32,33,34].

3.3. Photophysical Studies of the Polycyclic Compounds

The photophysical properties of compounds 4 and 7–9 were studied at room temperature using UV-Vis and photoluminescence spectroscopy in dichloromethane solutions and the solid state. (Table 1, Figure 10, Figure 11 and Figures S21–S24). Photophysical studies of 2-ethyl-10H-imidazo[4,5-b]dithieno[2,3-f:2′,3′-h]quinoxaline (10) were conducted solely in the solid state due to its extremely low solubility in most organic solvents.
The UV–vis spectra of compounds 4 and 7–9 exhibit maximum absorption bands between 220 and 380 nm, attributed to the allowed π–π* transition. Absorption in the longer wavelength range of 380 to 600 nm may be due to intramolecular charge transfer.
It is known that electronegativity has a noticeable effect on a compound’s absorption maximum, influencing the energy required for electronic transitions [35]. Generally, as electronegativity increases, the absorption maximum shifts to lower wavelengths (higher energy), and, as electronegativity decreases, the absorption maximum shifts to higher wavelengths (lower energy). This is because more electronegative atoms attract electrons more strongly, requiring more energy to promote them to higher energy levels, thus affecting the wavelength of light absorbed. The presence of atoms with varying electronegativities (χO = 3.44, χS = 2.58, χSe = 2.55) in the structures of compounds 4, 8, and 9 is indicated by a hypsochromic shift in the long-wave absorption band of compound 4 (423 nm) compared to those of compounds 8 (443 nm) and 9 (459 nm) (Figure 12). This shift may be attributed to an increase in transition energy due to the presence of oxygen as the most electronegative atom.
Compounds 4 and 7–9 in CH2Cl2 demonstrate a weak fluorescence (ΦF up to 0.02) in the red/infrared range. Varying the fused heterocyclic moiety in 4, 8, 9, especially to the near IR region for oxadiazole 4 and selenadiazole 9 derivatives, leads to a significant bathochromic shift of the emission spectrum compared to 7. The emission for compounds 4 and 7–9 was red-shifted compared to a solution (Table 1).
Compounds 4 and 7–10 exhibit a fluorescence in a broad range (550–850 nm) in the solid state with quantum yields up to 0.03 (Table 1, Figure 10). The higher absolute quantum yield of fluorescence in the solid state for compound 8, when compared to compounds 4 and 9, can be attributed to the molecular arrangements within the crystals. In the crystals of compounds 4 and 9, the molecules are stacked directly on top of each other, whereas in the crystals of compound 8, the molecular stacks are angled relative to one another. It is well known that excessive π–π stacking can quench emission in a solid-state [36].
The fluorescence decay of compounds 4 and 7–10 has been studied in both CH2Cl2 and the solid state. The excitation wavelength (λex) used was 450 nm, while the emission wavelength (λem) corresponded to the maxima of the fluorescence bands for these compounds at 293 K (Table 1). A bi-exponential function was employed to fit the fluorescence intensity decay for compounds 4 and 7–9. The fitted parameters are summarized in Table 1 and Table S22, and Figures S25–S33. The presence of two components in the fluorescence decay indicates that the fluorophores can exist in two conformational states. The excitation and emission spectra of these two fluorescent states for compounds 4 and 7–9 are highly overlapping and indistinguishable at room temperature. Consequently, the transition between these states can be examined using fluorescence lifetime measurements.

3.4. Electrochemical Properties of the Polycyclic Compounds

The electrochemical properties of compounds 4 and 7–9 were investigated using cyclic voltammetry in a CH2Cl2 solution. The measured oxidation onset (EOxonset) and reduction onset (ERedonset) potentials are summarized in Table 2, and the corresponding voltammograms are presented in Figure 11. All of the polycyclic compounds 4 and 7–9 exhibit irreversible oxidation, with potentials that range from 1.11 to 1.28 V relative to ferrocene. Additionally, reduction occurs reversibly within the potential range of −1.17 to −0.78 V (Table 2). The energies of the frontier molecular orbitals (HOMO and LUMO), as well as the electrochemical band gap for the compounds under consideration, were calculated based on the oxidation and reduction onset potentials.
The energies of the frontier molecular orbitals, known as EHOMO (the energy of the highest occupied molecular orbital) and ELUMO (the energy of the lowest unoccupied molecular orbital), along with the band gap (Egel), are critical factors that influence the electronic and conductive properties of materials. Research shows a direct correlation between the types of charge carriers in organic thin-film transistors and the experimentally measured levels of the frontier molecular orbitals [37].
Studies have shown that materials with a LUMO energy lower than −3.15 eV and a HOMO energy lower than −5.60 eV exhibit strict n-type semiconductor behavior. This behavior is attributed to their high barriers to hole injection. Based on the HOMO and LUMO energies obtained from electrochemical studies (Table 2), the synthesized polycyclic systems can be classified as narrow-band n-type organic semiconductors, with energy gaps ranging from 2.00 to 2.28 eV (Figure 13). The energies of their frontier orbitals are comparable to those of commercially available n-type semiconductors, such as [6]-phenyl-C61-butyric acid methyl ester (PCBM) [38]. PCBM is widely used as an active layer to enhance the performance of perovskite solar cells, improving energy conversion efficiency, stability, and reproducibility [39].

4. Conclusions

A convenient procedure has been proposed for synthesizing new heterocyclic systems, including [1,2,5]oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline, pyrazino[2,3-b]dithieno-[2,3-f:2′,3′-h]quinoxaline, [1,2,5]thiadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline, [1,2,5]selenadiazolo [3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline, and 10H-imidazo[4,5-b]dithieno[2,3-f:2′,3′-h]quinoxaline. This method combines nucleophilic aromatic substitution of hydrogen (the SNH reaction), the Scholl cross-coupling reactions, and mild reduction of the 1,2,5-oxadiazole fragment. All compounds described in this article (3–5 and 7–10) were obtained by us for the first time with moderate to excellent yields. The synthesized compounds show a considerable potential for use as new n-type electronic semiconductors in various organic electronic devices.
We propose a method that will significantly broaden the range of previously unknown 1,4-diazatriphenylene derivatives, serving as a notable enhancement to the established synthetic approaches for their preparation [40].

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemistry7050158/s1, General Information; Table S1: Crystal data and structure refinement for compound 4; Table S2: Fractional Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for compound 4. Ueq is defined as 1/3 of of the trace of the orthogonalized UIJ tensor; Table S3: Anisotropic Displacement Parameters (Å2 × 103) for compound 4. The Anisotropic displacement factor exponent takes the form: −2π2[h2a × 2U11 +...+ 2hka × b × U12]; Table S4: Bond Lengths for compound 4; Table S5: Bond Angles for compound 4; Table S6: Torsion Angles for compound 4; Table S10: Anisotropic Displacement Parameters (Å2 × 103) for compound 8. The Anisotropic displacement factor exponent takes the form: −2π2[h2a × 2U11 + 2hka × b × U12 +…]; Table S11: Bond Lengths for compound 8; Table S12: Bond Angles for compound 8; Table S13: Torsion Angles for compound 8; Table S14: Hydrogen Atom Coordinates (Å × 104) and Isotropic Displacement Parameters (Å2 × 103) for compound 8; Table S15: Crystal data and structure refinement for compound 9; Table S16: Fractional Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for compound 9. Ueq is defined as 1/3 of the trace of the orthogonalized UIJ tensor; Table S17: Anisotropic Displacement Parameters (Å2 × 103) for compound 9. The Anisotropic displacement factor exponent takes the form: −2π2[h2a × 2U11 + 2hka × b × U12 +…]; Table S18: Bond Lengths for compound 9; Table S19: Bond Angles for compound 9; Table S20: Torsion Angles for compound 9; Table S21: Hydrogen Atom Coordinates (Å × 104) and Isotropic Displacement Parameters (Å2 × 103) for compound 9; Figure S1: 1H NMR (400 MHz, DMSO-d6) spectrum of 3; Figure S2: 13C NMR (126 MHz, DMSO-d6) spectrum of 3; Figure S3: 1H NMR (400 MHz, CDCl3) spectrum of 4; Figure S4: 13C NMR (126 MHz, CDCl3) spectrum of 4; Figure S5: 1H NMR (500 MHz, DMSO-d6) spectrum of 5; Figure S6: 13C NMR (126 MHz, DMSO-d6) spectrum of 5; Figure S7: 1H NMR (500 MHz, CDCl3) spectrum of 7; Figure S8: 13C NMR (126 MHz, CDCl3) spectrum of 7; Figure S9: 1H NMR (500 MHz, CDCl3) spectrum of 8; Figure S10: 13C NMR (126 MHz, CDCl3) spectrum of 8; Figure S11: 1H NMR (500 MHz, CDCl3) spectrum of 9; Figure S12: 13C NMR (126 MHz, CDCl3) spectrum of 9; Figure S13: 1H NMR (500 MHz, DMSO-d6) spectrum of 10; Figure S14: HRMS spectrum of 3; Figure S15: HRMS spectrum of 4; Figure S16: HRMS spectrum of 5; Figure S17: HRMS spectrum of 7; Figure S18: HRMS spectrum of 8; Figure S19: HRMS spectrum of 9; Figure S20: HRMS spectrum of 10; Figure S21: Excitation (–) and emission (–) spectra of 4 in CH2Cl2; Figure S22: Excitation (–) and emission (–) spectra of 7 in CH2Cl2; Figure S23: Excitation (–) and emission (–) spectra of 8 in CH2Cl2; Figure S24: Excitation (–) and emission (–) spectra of 9 in CH2Cl2; Table S22: Detailed data of the fluorescence lifetime measurements of in CH2Cl2: τ—lifetime, f—fractional contribution, τavg—average lifetime, χ2—chi-squared distribution; Figure S25: Time-resolved fluorescence lifetime decay profile of 4 (green) in CH2Cl2, instrumental response function (IRF, blue). λex = 450 nm, λem = 740 nm; Figure S26. Time-resolved fluorescence lifetime decay profile of solid 4 (green), instrumental response function (IRF, blue). λex = 450 nm, λem = 800 nm; Figure S27: Time-resolved fluorescence lifetime decay profile of 7 (green) in CH2Cl2, instrumental response function (IRF, blue). λex = 450 nm, λem = 695 nm; Figure S28: Time-resolved fluorescence lifetime decay profile of solid 7 (green), instrumental response function (IRF, blue). λex = 450 nm, λem = 732 nm; Figure S29: Time-resolved fluorescence lifetime decay profile of 8 (green) in CH2Cl2, instrumental response function (IRF, blue). λex = 450 nm, λem = 730 nm; Figure S30: Time-resolved fluorescence lifetime decay profile of solid 8 (green), instrumental response function (IRF, blue). λex = 450 nm, λem = 733 nm; Figure S31: Time-resolved fluorescence lifetime decay profile of 9 (green) in CH2Cl2, instrumental response function (IRF, blue). λex = 450 nm, λem = 730 nm; Figure S32: Time-resolved fluorescence lifetime decay profile of solid 9 (green), instrumental response function (IRF, blue). λex = 450 nm, λem = 733 nm; Figure S33: Time-resolved fluorescence lifetime decay profile of solid 10 (green), instrumental response function (IRF, blue). λex = 450 nm, λem = 650 nm; Figure S34: IR spectrum of 4; Figure S35: IR spectrum of 7; Figure S36: IR spectrum of 8; Figure S37: IR spectrum of 9; Figure S38: IR spectrum of 10.

Author Contributions

Conceptualization, E.V.V.; Methodology, E.M.K., Y.A.K., E.F.Z., D.A.G. and P.A.S.; Formal analysis, V.N.C.; Investigation, E.M.K., Y.A.K., E.F.Z., D.A.G. and P.A.S.; Resources, G.L.R.; Data curation, G.L.R. and E.V.V.; Writing—original draft, E.F.Z., D.A.G., P.A.S. and E.V.V.; Writing—review & editing, E.V.V. and V.N.C.; Project administration, V.N.C.; Funding acquisition, Y.A.K. All authors have read and agreed to the published version of the manuscript.

Funding

The synthetic part of the work was carried out with financial support from the Russian Science Foundation (project No. 24-23-00084).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

Analytical studies were performed using equipment of the Center for Joint Use “Spectroscopy and Analysis of Organic Compounds” at the Postovsky Institute of Organic Synthesis of the Ural Branch of the Russian Academy of Sciences.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Selected examples of furazanopyrazine derivatives that exhibit semiconductor properties.
Figure 1. Selected examples of furazanopyrazine derivatives that exhibit semiconductor properties.
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Scheme 1. Synthetic approaches to dibenzo[f,h]furazano[3,4-b]quinoxaline derivatives.
Scheme 1. Synthetic approaches to dibenzo[f,h]furazano[3,4-b]quinoxaline derivatives.
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Scheme 2. Transformation of furazanopyrazines into imidazo[4,5-b]pyrazine derivatives.
Scheme 2. Transformation of furazanopyrazines into imidazo[4,5-b]pyrazine derivatives.
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Scheme 3. Syntheses of various polycyclic systems 4, 7–10 starting from 5-(thiophen-3-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (1).
Scheme 3. Syntheses of various polycyclic systems 4, 7–10 starting from 5-(thiophen-3-yl)-[1,2,5]oxadiazolo[3,4-b]pyrazine (1).
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Figure 2. Molecular structure of compound 4, illustrated with 50% probability thermal ellipsoids.
Figure 2. Molecular structure of compound 4, illustrated with 50% probability thermal ellipsoids.
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Figure 3. Multiplets of protons at the C(3) position and the CH2 group in compounds 4 and 7–9 (on top). An example of these multiplets for polycycle 8 (on the bottom).
Figure 3. Multiplets of protons at the C(3) position and the CH2 group in compounds 4 and 7–9 (on top). An example of these multiplets for polycycle 8 (on the bottom).
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Figure 4. Molecular structure of compound 8, illustrated with 50% probability thermal ellipsoids.
Figure 4. Molecular structure of compound 8, illustrated with 50% probability thermal ellipsoids.
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Figure 5. Molecular structure of compound 9, illustrated with 50% probability thermal ellipsoids.
Figure 5. Molecular structure of compound 9, illustrated with 50% probability thermal ellipsoids.
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Chemistry 07 00158 g006
Figure 6. The crystal packing of compound 4 according to XRD data.
Figure 6. The crystal packing of compound 4 according to XRD data.
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Chemistry 07 00158 g007
Figure 7. The crystal packing of compound 8 according to XRD data.
Figure 7. The crystal packing of compound 8 according to XRD data.
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Chemistry 07 00158 g008
Figure 8. The crystal packing of compound 9 according to XRD data.
Figure 8. The crystal packing of compound 9 according to XRD data.
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Chemistry 07 00158 g009
Figure 9. The shortened Se…N contacts in compound 9 according to XRD data.
Figure 9. The shortened Se…N contacts in compound 9 according to XRD data.
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Chemistry 07 00158 g010
Figure 10. Emission spectra of 4 and 7–10 in solid state.
Figure 10. Emission spectra of 4 and 7–10 in solid state.
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Chemistry 07 00158 g011
Figure 11. Cyclic voltammogram curve (vs. Fc/Fc+) for polycycles 4 and 7–9 in CH2Cl2 solution.
Figure 11. Cyclic voltammogram curve (vs. Fc/Fc+) for polycycles 4 and 7–9 in CH2Cl2 solution.
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Chemistry 07 00158 g012
Figure 12. UV-Vis spectra of 4 and 7–9 in CH2Cl2.
Figure 12. UV-Vis spectra of 4 and 7–9 in CH2Cl2.
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Chemistry 07 00158 g013
Figure 13. Energy level diagrams (HOMO and LUMO) of polycyclic systems 4 and 7–9, as well as the well-known n-type organic semiconductor material PCBM, are provided for comparison.
Figure 13. Energy level diagrams (HOMO and LUMO) of polycyclic systems 4 and 7–9, as well as the well-known n-type organic semiconductor material PCBM, are provided for comparison.
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Table 1. Photophysical properties of 4, 9–11.
Table 1. Photophysical properties of 4, 9–11.
SampleAbsorptionFluorescence
Solution in CH2Cl2Solid
λabsmax (nm)/
ε (M−1·cm−1)		λex (nm)λem (nm)τavg, [ns]/χ2ΦFλem (nm)τavg, [ns]/χ2ΦF
4423/12,300
281/23,600
255/2300
230/27,100		288, 433, 5407400.57/1.267<0.018000.71/1.228<0.01
7456/34,400
312/40,900
248/50,600		313, 4586112.04/1.1510.026402.11/1.0510.02
8443/22,400
368/6100
295/24,900
286/26,500
245/37,100		305, 4646951.68/1.051<0.017322.76/1.0330.03
9459/26,300
443/25,200
286/20,000
249/31,800		4607300.79/1.188<0.017330.36/1.327<0.01
10-----6501.31/1.180<0.01
ΦF—the absolute quantum yield values were determined using the SC-30 integrating sphere of the FS5 Edinburgh Instruments spectrofluorometer.
Table 2. Electrochemical properties and energies of frontier molecular orbitals and the band gap of polycycles 4 and 7–9.
Table 2. Electrochemical properties and energies of frontier molecular orbitals and the band gap of polycycles 4 and 7–9.
CompoundEOxonset, VERedonset, VEHOMO, eVELUMO, eVEgel, eV
41.28−0.78−6.38−4.322.06
71.11−1.17−6.21−3.932.28
81.17−0.97−6.27−4.132.14
91.12−0.88−6.22−4.222.00
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Krynina, E.M.; Kvashnin, Y.A.; Zhilina, E.F.; Gazizov, D.A.; Slepukhin, P.A.; Rusinov, G.L.; Verbitskiy, E.V.; Charushin, V.N. [1,2,5]Oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline as a Versatile Scaffold for the Construction of Various Polycyclic Systems as Potential Organic Semiconductors. Chemistry 2025, 7, 158. https://doi.org/10.3390/chemistry7050158

AMA Style

Krynina EM, Kvashnin YA, Zhilina EF, Gazizov DA, Slepukhin PA, Rusinov GL, Verbitskiy EV, Charushin VN. [1,2,5]Oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline as a Versatile Scaffold for the Construction of Various Polycyclic Systems as Potential Organic Semiconductors. Chemistry. 2025; 7(5):158. https://doi.org/10.3390/chemistry7050158

Chicago/Turabian Style

Krynina, Elizaveta M., Yuriy A. Kvashnin, Ekaterina F. Zhilina, Denis A. Gazizov, Pavel A. Slepukhin, Gennady L. Rusinov, Egor V. Verbitskiy, and Valery N. Charushin. 2025. "[1,2,5]Oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline as a Versatile Scaffold for the Construction of Various Polycyclic Systems as Potential Organic Semiconductors" Chemistry 7, no. 5: 158. https://doi.org/10.3390/chemistry7050158

APA Style

Krynina, E. M., Kvashnin, Y. A., Zhilina, E. F., Gazizov, D. A., Slepukhin, P. A., Rusinov, G. L., Verbitskiy, E. V., & Charushin, V. N. (2025). [1,2,5]Oxadiazolo[3,4-b]dithieno[2,3-f:2′,3′-h]quinoxaline as a Versatile Scaffold for the Construction of Various Polycyclic Systems as Potential Organic Semiconductors. Chemistry, 7(5), 158. https://doi.org/10.3390/chemistry7050158

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