*3.1. General Information*

− − – D3 version of Grimme's dispersion with Becke– – All reagents were purchased from commercial sources and used without further purification. Melting points were measured on a Stuart SMP3 melting point apparatus. The <sup>1</sup>H-NMR and <sup>13</sup>C-NMR spectra were recorded on an Agilent 400-NMR spectrometer in DMSO-*d*<sup>6</sup> solution using TMS as the internal standard. FT-IR spectra were recorded between 4000 and 650 cm−<sup>1</sup> using a FT-IR Nicolet 6700 apparatus with a Smart iTR accessory. UV-Vis spectra were recorded at room temperature in MeOH solutions (c = 4.0 × 10 <sup>−</sup><sup>5</sup> mol/L) on a Jasco V-660 spectrophotometer. High-resolution mass spectra were recorded on a Waters ACQUITY UPLC/Xevo G2QT instrument. Thin-layer chromatography was performed on silica gel 60 F<sup>254</sup> (Merck) TLC plates using CHCl3/EtOAc (5:1 *v*/*v*) as the mobile phase. Elemental analyses were performed with a VarioEL analyzer. The excited states of the obtained compounds were calculated using TD–DFT method. All calculations (including geometry optimization) were performed utilizing Gaussian09 rev. D.01 [36] with B3LYP functional and employing 6.31 g++(2d, 2p) basis set. The geometry of all compounds was optimized and during this calculation D3 version of Grimme's dispersion with Becke–Johnson damping [37] was used. In both, the optimization process and TD–DFT calculations, the polarizable continuum model [38] was used to include overall solvation effects (with methanol selected as a solvent). The assignment of the calculated excited states to the observed experimental maxima was based on the comparison of excitation energies and the oscillator strengths/intensities of the corresponding maxima. The analysis of the character of respective orbital excitations was based on orbital contour plots.

#### *3.2. Synthesis and Characterization*

#### – 3.2.1. General Procedure for the Synthesis of 2-Benzoylhydrazinecarbothioamide Derivatives (**2a**–**e**)

– – – – A carboxylic acid (**1a**–**e**, 0.10 mol) and thionyl chloride SOCl<sup>2</sup> (22 mL, 0.30 mol) were refluxed in dry toluene (10 mL) until the acid was fully consumed (TLC; 5–15 h). After cooling, the mixture was concentrated on a rotary evaporator, washed with additional dry toluene (10 mL) and concentrated again. The crude acid chloride was then dissolved in 50 mL of dry toluene and added dropwise to a mixture of thiosemicarbazide (9.11 g, 0.10 mol), NaHCO<sup>3</sup> (8.40 g, 0.10 mol) and H2O (150 mL). The whole solution was agitated at room temperature overnight. The precipitated solid was filtered off, dried in air and recrystallized from a mixture of EtOH–H2O to obtain pure 2-benzoylhydrazinecarbothioamide derivatives (**2a**–**e**).

*2-Benzoylhydrazinecarbothioamide* (**2a**). The product was obtained as a white solid (8.60 g, 44%); mp 197–198 ◦C (196–198 ◦C [39]).

*2-(4-Methoxybenzoylhydrazinecarbothioamide* (**2b**). The product was obtained as a white solid (15.32 g, 68%); mp 225–227 ◦C (226 ◦C [40]).

*2-(4-Nitrobenzoylhydrazinecarbothioamide* (**2c**). The product was obtained as a yellow solid (18.02 g, 75%); mp 212–214 ◦C (214 ◦C [41]).

*2-(4-Bromobenzoylhydrazinecarbothioamide* (**2d**). The product was obtained as a white solid (16.72 g, 75%); mp 216–218 ◦C. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 7.68–7.72 (m, 3H, Ar: H-3, H-5, NH), 7.82–7.88 (m, 3H, Ar: H-2, H-6, NH), 9.34 (s, 1H, NH), 10.45 (s, 1H, NH); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 125.5, 129.5, 129.9, 131.1, 131.7, 164.9 (C=O), 182.0 (C=S); Anal. Calcd for C8H8BrN3OS: C, 35.05; H, 2.94; N, 15.33. Found: C, 34.95; H, 2.89; N, 15.40.

*2-(4-t-Butylbenzoylhydrazinecarbothioamide* (**2e**). The product was obtained as a beige solid (12.06 g, 48%); mp 210–211 ◦C. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 1.33 (s, 9H, C(CH3)3), 7.48 (d, 2H, *J* = 8.4 Hz, Ar: H-3, H-5), 7.52–7.60 (m, 2H, NH2), 7.84 (d, 2H, *J* = 8.4 Hz, Ar: H-2, H-6), 9.31 (s, 1H, NH), 10.39 (s, 1H, NH); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 30.8, 34.8, 124.9, 126.1, 128.3, 154.6, 165.6 (C=O), 182.0 (C=S); Anal. Calcd for C12H17N3OS: C, 57.34; H, 6.82; N, 16.72. Found: C, 57.21; H, 6.88; N, 16.63.

3.2.2. General Procedure for the Synthesis of 2-Amino-1,3,4-thiadiazole Derivatives (**3a**–**e**)

Derivatives of 2-benzoylhydrazinecarbothioamide **2a**–**e** (0.05 mol) were dissolved in 50 mL of concentrated H2SO<sup>4</sup> and agitated at room temperature overnight. Then, the mixture was alkalized with 25% ammonia and the precipitated product was filtered off, dried in air and recrystallized from a mixture of EtOH–H2O to yield the corresponding 2-amino-1,3,4-thiadiazoles **3a**–**e**.

*2-Amino-5-phenyl-1,3,4-thiadiazole* (**3a**). The product was obtained as a white solid (3.89 g, 45%); mp 232–233 ◦C (230 ◦C [42]).

*2-Amino-5-(4-methoxyphenyl)-1,3,4-thiadiazole* (**3b**). The product was obtained as a white solid (3.42 g, 33%); mp 184–185 ◦C (185–187 ◦C [43]).

*2-Amino-5-(4-nitrophenyl)-1,3,4-thiadiazole* (**3c**). The product was obtained as a yellow solid (8.33 g, 75%); mp 254–256 ◦C (250–252 ◦C [44]).

*2-Amino-5-(4-bromophenyl)-1,3,4-thiadiazole* (**3d**). The product was obtained as a white solid (8.96 g, 70%); mp 226 ◦C (222–224 ◦C [41]).

*2-Amino-5-(4-t-butylphenyl)-1,3,4-thiadiazole* (**3e**). The product was obtained as a white solid (4.78 g, 41%); mp 251–253 ◦C. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 1.29 (s, 9H, C(CH3)3), 7.35 (s, 2H, NH2), 7.48 (d, 2H, *J* = 8.4 Hz, Ar: H-3, H-5), 7.67 (d, 2H, *J* = 8.4 Hz, Ar: H-2, H-6); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 30.8, 34.5, 125.8, 126.0, 128.3, 152.2, 156.3, 168.1; Anal. Calcd for C12H15N3S: C, 61.77; H, 6.48; N, 18.01. Found: C, 61.70; H, 6.40; N, 18.12.

3.2.3. General Procedure for the Synthesis of 2-(4-Aminophenylazo)-5-phenyl-1,3,4-thiadiazole Derivatives (**4a**–**e**)

Sodium nitrite (1.31 g, 0.019 mol) was introduced portion wise into concentrated sulfuric acid (20 mL). The solution was heated to 50 ◦C in a water bath until complete dissolution and then rapidly cooled in an ice/salt bath to 0 ◦C. In the meantime, the solution of the appropriate 2-amino-5-aryl-1,3,4-thiadiazole **3a**–**e** (0.015 mol) in glacial acetic acid (30 mL) and propionic acid (15 mL) was prepared and added dropwise to an agitated solution of sodium nitrite in concentrated sulfuric acid at 0–5 ◦C. Then, the mixture was stirred for 24 h, and excess nitrous acid was decomposed by the addition of urea. The resulting diazonium salt solution was slowly introduced into the mixture of aniline (1.37 mL, 1.39 g, 0.015 mol) in 15 mL of water at 0–5 ◦C. The colored mixture was stirred at room temperature for the next 24 h and finally neutralized with saturated sodium carbonate solution (75 mL). The solid was filtered off, washed twice with hot water (2 × 25 mL) and dried in air. The crude product (**4a**–**e**) was purified by column chromatography on silica gel (CHCl3/EtOAc, 5:1 *v*/*v*).

*2-(4-Aminophenylazo)-5-phenyl-1,3,4-thiadiazole* (**4a**). The product was obtained as a brown reddish solid (2.11 g, 51%); mp 236–238 ◦C; R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.18. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 6.75 (d, 2H, *J* = 8.8 Hz, 2-Ar: H-3, H-5), 7.13 (s, 2H, NH2), 7.57–7.59 (m, 3H, 5-Ar: H-3′ , H-4′ , H-5′ ), 7.77 (d, 2H, *J* = 8.8 Hz, 2-Ar: H-2, H-6), 8.04 (d, 2H, *J* = 8.4 Hz, 5-Ar: H-2′ , H-6′ ); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 114.1, 127.5, 127.8, 129.4, 130.1, 131.5, 142.3, 156.6, 165.7, 180.2. IR (ATR) ν: 3311, 1636,

1507 cm−<sup>1</sup> ; HRMS calcd for (C14H11N5S + H+): 282.0813; found: 282.0818; Anal. Calcd for C14H11N5S: C, 59.77; H, 3.94; N, 24.89. Found: C, 59.83; H, 3.97; N, 24.93.

*2-(4-Aminophenylazo)-5-(4-methoxyphenyl)-1,3,4-thiadiazole* (**4b**). The product was obtained as a brown reddish solid (2.47 g, 53%); mp 269–270 ◦C; R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.16. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 3.86 (s, 3H, OCH3), 6.74 (d, 2H, *J* = 8.8 Hz, 2-Ar: H-3, H-5), 7.06 (s, 2H, NH2), 7.11 (d, 2H, *J* = 8.8 Hz, 5-Ar: H-3′ , H-5′ ), 7.75 (d, 2H, *J* = 8.8 Hz, 2-Ar: H-2, H-6), 7.98 (d, 2H, *J* = 8.8 Hz, 5-Ar: H-2′ , H-6′ ); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 55.5, 114.1, 114.8, 122.6, 127.6, 129.2, 142.3, 156.3, 161.8, 165.6, 179.5. IR (ATR) ν: 3422, 3333, 1651, 1508 cm−<sup>1</sup> ; HRMS calcd for (C15H13N5SO + H+): 312.0919; found: 312.0912; Anal. Calcd for C15H13N5SO: C, 57.86; H, 4.21; N, 22.49. Found: C, 57.73; H, 4.24; N, 22.43.

*2-(4-Aminophenylazo)-5-(4-nitrophenyl)-1,3,4-thiadiazole* (**4c**). The product was obtained as a dark brown solid (3.42 g, 70%); mp 176–178 ◦C (178–179 ◦C [24]); R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.23. IR (ATR) ν: 3403, 1637, 1513 cm−<sup>1</sup> ; Anal. Calcd for C14H10N6O2S: C, 51.53; H, 3.09; N, 25.75. Found: C, 51.47; H, 3.05; N, 25.77.

*2-(4-Aminophenylazo)-5-(4-bromophenyl)-1,3,4-thiadiazole* (**4d**). The product was obtained as a dark violet solid (3.67 g, 68%); mp 261–263 ◦C; R<sup>f</sup> (CHCl3/EtOAc, 3:1 *v*/*v*) 0.23. <sup>1</sup>H-NMR (400 MHz DMSO-d6): δ 6.75 (d, 2H, *J* = 9.2 Hz, 2-Ar: H-3, H-5), 7.18 (s, 2H, NH2), 7.75–7.78 (m, 4H, 2-Ar, 5-Ar: H-2, H-6, H-3′ , H-5′ ), 7.97 (d, 2H, *J* = 8.8 Hz, 5-Ar: H-2′ , H-6′ ); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 114.2, 124.9, 129.2, 129.3, 130.4, 132.4, 142.3, 156.7, 164.6, 180.5. IR (ATR) ν: 3321, 1636, 1507 cm−<sup>1</sup> ; HRMS calcd for (C14H10N5SBr+H+): 359.9919; found: 359.9914; Anal. Calcd for C14H10N5SBr: C, 46.68; H, 2.80; N, 19.44. Found: C, 46.81; H, 2.79; N, 19.49.

*2-(4-Aminophenylazo)-5-(4-t-butylphenyl)-1,3,4-thiadiazole*(**4e**). The product was obtained as a brown solid (2.43 g, 81%); mp 171–173 ◦C; R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.27. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 1.33 (s, 9H, C(CH3)3), 6.74 (d, 2H, *J* = 8.4 Hz, 2-Ar: H-3, H-5), 7.10 (s, 2H, NH2), 7.58 (d, 2H, *J* = 8.8 Hz, 5-Ar: H-3′ , H-5′ ), 7.76 (d, 2H, *J* = 8.4 Hz, 2-Ar: H-2, H-6), 7.96 (d, 2H, *J* = 8.8 Hz, 5-Ar: H-2′ , H-6′ ); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 30.8, 34.8, 114.1, 126.2, 127.3, 127.4, 130.2, 142.3, 154.5, 156.5, 165.7, 179.9. IR (ATR) ν: 3325, 1621, 1507 cm−<sup>1</sup> ; HRMS calcd for (C18H19N5S+H+): 338.1440; found: 338.1438; Anal. Calcd for C18H19N5S: C, 64.07; H, 5.68; N, 20.75. Found: C, 64.21; H, 5.70; N, 20.71.

## 3.2.4. General procedure for the synthesis of

2-[4-(*N,N*-dimethylamino)phenylazo]-5-phenyl-1,3,4-thiadiazole derivatives (**5a**–**e**)

Sodium nitrite (1.31 g, 0.019 mol) was introduced portion wise into concentrated sulfuric acid (20 mL). The solution was heated to 50 ◦C in a water bath until complete dissolution and then rapidly cooled in an ice/salt bath to 0 ◦C. In the meantime, a solution of the appropriate 2-amino-5-aryl-1,3,4-thiadiazole **3a**–**e** (0.015 mol) in glacial acetic acid (30 mL) and propionic acid (15 mL) was prepared and added dropwise to an agitated solution of sodium nitrite in concentrated sulfuric acid at 0–5 ◦C. Then, the mixture was stirred for 24 h and excess nitrous acid was decomposed by the addition of urea. The resulting diazonium salt solution was slowly introduced into the mixture of N,N-dimethylaniline (1.90 mL, 1.82 g, 0.015 mol) in 15 mL of water at 0–5 ◦C. The colored mixture was stirred at room temperature for the next 24 h and finally neutralized with saturated sodium carbonate solution (75 mL). The solid was filtered off, washed twice with hot water (2 × 25 mL) and dried in air. The crude product (**5a**–**e**) was purified by column chromatography on silica gel (CHCl3/EtOAc, 5:1 *v*/*v*).

*2-[4-(N,N-Dimethylamino)phenylazo]-5-phenyl-1,3,4-thiadiazole* (**5a**). The product was obtained as a brown solid (3.20 g, 69%); mp 194–195 ◦C (191 ◦C [26]); R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.32. IR (ATR) ν: 1603, 1525 cm−<sup>1</sup> ; Anal. Calcd for C16H15N5S: C, 62.11; H, 4.89; N, 22.64. Found: C, 62.08; H, 4.86; N, 22.67.

*2-[4-(N,N-Dimethylamino)phenylazo]-5-(4-methoxyphenyl)-1,3,4-thiadiazole* (**5b**). The product was obtained as a dark red solid (3.00 g, 59%); mp 219–221 ◦C; R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.27. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 3.17 (s, 6H, N(CH3)2), 3.86 (s, 3H, OCH3), 6.92 (d, 2H, *J* = 9.2 Hz, 2-Ar: H-3, H-5), 7.12 (d, 2H, *J* = 8.8 Hz, 5-Ar: H-3′ , H-5′ ), 7.85 (d, 2H, *J* = 9.2 Hz, 2-Ar: H-2, H-6), 7.99 (d, 2H, *J* = 8.8 Hz, 5-Ar: H-2′ , H-6′ ); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 40.2, 55.5, 112.2, 114.8, 122.5, 126.0, 129.2, 129.4, 142.0, 154.6, 164.8, 179.4. IR (ATR) ν: 1603, 1516 cm−<sup>1</sup> ; HRMS calcd for (C17H17N5SO+H+): 340.1232; found: 340.1233; Anal. Calcd for C17H17N5SO: C, 60.16; H, 5.05; N, 20.63. Found: C, 60.27; H, 5.03; N, 20.61.

*2-[4-(N,N-Dimethylamino)phenylazo]-5-(4-nitrophenyl)-1,3,4-thiadiazole* (**5c**). The product was obtained as a brown solid (3.19 g, 60%); mp 169–171 ◦C; R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.38. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 3.18 (s, 6H, N(CH3)2), 7.04 (d, 2H, *J* = 8.8 Hz, 2-Ar: H-3, H-5), 7.96 (d, 2H, *J* = 9.2 Hz, 5-Ar: H-3′ , H-5′ ), 8.37–8.40 (m, 4H, 2-Ar, 5-Ar: H-2, H-6, H-2′ , H-6′ ); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 40.0, 112.3, 124.4, 124.5, 126.9, 129.1, 130.1, 142.1, 155.5, 164.8, 179.8. IR (ATR) ν: 1597, 1516 cm−<sup>1</sup> ; HRMS calcd for (C16H14N6SO<sup>2</sup> + H+): 355.0977; found: 355.0979; Anal. Calcd for C16H14N6SO2: C, 54.23; H, 3.98; N, 23.71. Found: C, 54.40; H, 3.99; N, 23.76.

*5-(4-Bromophenyl)-2-[4-(N,N-dimethylamino)phenylazo]-1,3,4-thiadiazole* (**5d**). The product was obtained as a dark violet solid (3.49 g, 60%); mp 201–203 ◦C; R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.40. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 3.18 (s, 6H, N(CH3)2), 6.91 (d, 2H, *J* = 9.2 Hz, 2-Ar: H-3, H-5), 7.77 (d, 2H, *J* = 8.4 Hz, 5-Ar: H-3′ , H-5′ ), 7.84 (d, 2H, *J* = 9.2 Hz, 2-Ar: H-2, H-6), 7.97 (d, 2H, *J* = 8.4 Hz, 5-Ar: H-2′ , H-6′ ); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 40.0, 112.3, 124.9, 128.8, 129.2, 129.3, 132.4, 142.1, 154.8, 164.7, 180.5. IR (ATR) ν: 1597, 1522 cm−<sup>1</sup> ; HRMS calcd for (C16H14N5SBr+H+): 388.0232; found: 388.0238; Anal. Calcd for C16H14N5SBr: C, 49.49; H, 3.63; N, 18.04. Found: C, 49.37; H, 3.66; N, 18.09.

*5-(4-t-Butylphenyl)-2-[4-(N,N-dimethylamino)phenylazo]-1,3,4-thiadiazole* (**5e**). The product was obtained as a dark violet solid (3.07 g, 56%); mp 225–227 ◦C; R<sup>f</sup> (CHCl3/EtOAc, 5:1 *v*/*v*) 0.39. <sup>1</sup>H-NMR (400 MHz, DMSO-d6): δ 1.33 (s, 9H, C(CH3)3), 3.18 (s, 6H, N(CH3)2), 6.93 (d, 2H, *J* = 9.2 Hz, 2-Ar: H-3, H-5), 7.59 (d, 2H, *J* = 8.4 Hz, 5-Ar: H-3′ , H-5′ ), 7.86 (d, 2H, *J* = 9.2 Hz, 2-Ar: H-2, H-6), 7.97 (d, 2H, *J* = 8.4 Hz, 5-Ar: H-2′ , H-6′ ); <sup>13</sup>C-NMR (100 MHz, DMSO-d6): δ 30.8, 34.8, 40.1, 112.3, 126.2, 127.4, 127.6, 142.1, 153.4, 154.5, 154.7, 165.8, 179.8. IR (ATR) ν: 1604, 1524 cm−<sup>1</sup> ; Anal. Calcd for C20H23N5S: HRMS calcd for (C20H23N5S+H+): 366.1752; found: 366.1754; Anal. Calcd for C20H23N5S: C, 65.72; H, 6.34; N, 19.16. Found: C, 65.80; H, 6.33; N, 19.18.
