A facile designed highly moderate craspedia flowerlike sulphated Bi₂O₃-fly ash catalyst: Green synthetic strategy for (6H-pyrido[3,2-b]carbazol-4-yl)aniline derivatives in water

2.1 Fourier transform infrared (FT-IR)

The Fourier transform infrared (FT-IR) spectra of fly ash (Class C) and sulfated Bi₂O₃-fly ash (12 wt%) are shown in Fig. 1a and b. Fig. 1a shows that the broad OH peak was appeared at 3476 cm⁻¹ denotes the OH stretching vibration. The powder nature of fly ash is associated with the hydrogen-bonded water molecules (Jain et al., 2012a,b). The broad peak appeared at 1166 cm⁻¹ is assigned as asymmetric stretching vibration of sulfate (Liu et al., 2011) which depicted in Fig. 1b. The FT-IR spectrum of Bi₂O₃ two peaks appeared at 473 cm⁻¹ and 525 cm⁻¹ is due to the stretching vibration of Bi–O bonded of α-Bi₂O₃ which is also in good agreement with earlier reports (Ai et al., 2011). The stretching vibration mode of Bi–O–Bi appears at 668 cm⁻¹ is good agreement with earlier reports (Elkholy et al., 1995).

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Figure 1 FT-IR spectra of (a) fly ash and (b) sulfated Bi₂O₃-fly ash.

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2.2 Confocal Raman

Confocal Raman spectra of fly ash and sulfated Bi₂O₃-fly ash are shown in Fig. 2a and b. The fly ash has no significant Raman shift observed, it suggests that because of the amorphous nature of fly ash, no shift was obtained as shown in Fig. 2a. Fig. 2b reveals that the sulfated Bi₂O₃-fly ash catalyst, the sulfate antisymmetric stretching vibration was observed at 1063 cm⁻¹ as good agreement with earlier report in the literature (Dong et al., 2007). The new peak was appeared at 1694 cm⁻¹ it corresponds to Bi–O bond stretching vibration (He et al., 2014). The results were seen that the facile designed sulfated Bi₂O₃-fly ash has good crystalline phase.

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Figure 2 Confocal Raman spectra of (a) fly ash and (b) sulfated Bi₂O₃-fly ash.

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2.3 Powder X-ray diffraction (PXRD)

The crystalline nature of fly ash and sulfated Bi₂O₃-fly ash (12 wt%) was examined, by powder X-ray diffraction (PXRD) patterns is shown in Fig. 3a and b. The fly ash (Class C) is a mixture of various inorganic oxides viz. silica, alumina, ferric oxide, calcium oxide and other metal oxides (Mn₂O₃ and TiO₂) along with mullite, quartz and calcite which denoted the letters M, Q and C respectively as shown in Fig. 3a and b. Fig. 3b shows the facile designed sulfated Bi₂O₃-fly ash in the diffraction patterns. The XRD patterns of Bi₂O₃ 2θ (degree) values are appeared at 11.86, 31.42, 33.02, 64.21 and 77.82, and the corresponding planes are (1 1 0), (2 2 2), (1 2 2), (4 4 4) and (0 2 8). The diffraction patterns were good agreement with the literature value for Bi₂O₃ (JCPDS card no-74-1375) (Sillen, 1938).

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Figure 3 Powder XRD pattern of (a) fly ash and (b) sulfated Bi₂O₃-fly ash.

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2.4 Field emission scanning electron microscopy (FE-SEM)

The morphology of sulfated Bi₂O₃-fly ash was examined by field emission scanning electron microscopy (FE-SEM) as shown in Fig. 4a–d. As can be seen in Fig. 4a two types of morphology were observed of 1 μm (indicated in green and red spherical in color) respectively. Fig. 4b shows the close-up view on morphology about 2 μm, it is seen that the flowers like morphology were observed channels in a large domain. In this observation, further reviewed individual morphology of dandelion (inserted) was clearly exhibited which is shown in Fig. 4c. Fig. 4d shows that the craspedia flower (inserted) like regular arrangement was observed. The regular arrangement of rod like petals is notable to see that some of relatively rod like particles appeared in Fig. 4d. It is believed that the flower like morphology has strongly bonded with atom which is highly stable in nature, moral active site sulfated Bi₂O₃-fly ash might have good crystalline nature (Wang et al., 2013).

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Figure 4 FE-SEM images of sulfated Bi₂O₃-fly ash scale bars (a) 1 μm, (b) 2 μm, (c) 1 μm (Dandelion flower inserted), and (d) 1 μm (Craspedia flower inserted).

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2.5 FE-SEM elemental color mapping

FE-SEM elemental color mapping reveals that the uniform distribution of the chemical composition of sulfated Bi₂O₃-fly ash on the surface which is shown in Fig. 5a. The elemental composition of sulfated Bi₂O₃-fly ash has been examined by energy-dispersive X-ray spectroscopy (EDS) as the results are shown in Fig. 5b. The sulfated Bi₂O₃-fly ash elements of Ca, O, Fe, Al, Na, Mg, Si, S and Bi are displayed in various colors which are shown in Fig. 5c–k respectively. It is noteworthy that no other elements seen in the modified sulfated Bi₂O₃-fly ash catalyst reveal the designed catalyst pure in form.

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Figure 5 FE-SEM elemental color mapping image of sulfated Bi₂O₃-fly ash (a) mix color, (b) EDS, (c) Ca, (d) O, (e) Fe, (f) Al, (g) Na, (h) Mg, (i) Si, (j) S, and (k) Bi.

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2.6 Transmission electron microscopy (TEM)

The morphology of sulfated Bi₂O₃-fly ash is examined by Transmission electron microscopy (TEM) analysis as shown in Fig. 6a–d. Fig. 6a shows that the structure length of the microrod is 1.09 μm and width is 0.23 μm. The corresponding fast Fourier transform (FFT) image was inserted which is regular shape of microrod which is indicated by red arrow. In Fig. 6c the single microrod was observed and small particles are present in the surface of microrod that may be the formation of bismuth oxide particles. Fig. 6d shows that the selected area electron diffraction (SAED) pattern of sulfated Bi₂O₃-fly ash was recorded which seems that the regular, numeral of parallel axis diffraction spots array are preferentially microrod shape (red line). The bright consistent spots confirm the crystalline behavior of sulfated Bi₂O₃-fly ash good crystalline nature. From the SAED patterns inter planar spacing is 0.712, 0.301, 0.267, 0.145 and 0.122 nm. The corresponding diffraction planes are (1 1 0), (2 2 2), (1 2 2), (4 4 4) and (0 2 8) of Bi₂O₃ respectively.

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Figure 6 TEM images of sulfated Bi₂O₃-fly ash (a) 0.5 μm (FFT image inserted), (b) 0.2 μm (FFT image inserted), (c) 0.5 μm rod (FFT image inserted), and (d) SAED pattern 5 1/nm.

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2.7 N₂ adsorption–desorption isotherms

The pore structure of sulfated Bi₂O₃-fly ash catalyst was investigated by N₂ adsorption–desorption isotherms. The corresponding pore size distribution and specific surface area were examined by the Barrett–Joyner–Halenda (BJH) method and Brunauer–Emmett–Teller (BET) method respectively. The N₂ adsorption–desorption isotherms and BJH description pore size distribution curve are shown in Fig. 7. This isotherm could be categorized as a typical Type III isotherm according to the classification of IUPAC (Sing et al., 1985). Fig. 7 shows a pore size distribution of sulfated Bi₂O₃-fly ash which is obtained from the corresponding desorption branch of the N₂ isotherm, by the BJH method. The sulfated Bi₂O₃-fly ash possesses BET surface area of 5.87 m²/g. The absorption of the sulfated Bi₂O₃-fly ash results shows the nonporous structure and small surface area (Xie et al., 2013). Total pore volume for pores with radius less than 1053.06 Å at P/Po = 0.990813 is 2.15 cm³/g. The quantitative textural information of the as-designed sulfated Bi₂O₃-fly ash is BET surface areas, total pore volume 0.023 and pore radius 18.0.

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Figure 7 N₂ adsorption–desorption isotherm of the sulfated Bi₂O₃-fly ash catalyst. Inset of the corresponding pore volume of sulfated Bi₂O₃-fly ash catalyst.

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2.8 Optimization of the catalyst

At first we have examined the reaction of for the preparation of 3a with (E)-1-(4-aminophenyl)-3-phenylprop-2-en-1-one (1 mmol), 3-amino-9-ethylcarbazole (1 mmol) and water (20 mL) in the absence of catalyst at 95 °C for 12 h. In this reaction condition, no product was obtained Table 1 entry 1. Next, we have examined the reaction in the presence of metal oxide catalyst CuO at 85 °C for 8 h. The yield was obtained 18% Table 1 entry 2. We have also examined the reaction in the presence of H₂SO₄ at 85 °C for 7 h. The desired product was isolated and the yield is (26%) Table 1 entry 3. From the results, we have preferred further, design of solid phase sulfated fly ash catalyst. The catalytic activity of sulfated fly ash was found for the reaction at 95 °C for 12 h. The furnished yield could reach 37% Table 1 entry 5. Our optimization studies have preferred metal oxides loaded fly ash catalyst such as Bi₂O₃-fly ash and CuO-fly ash. The significant product yield was observed in the Bi₂O₃-fly ash catalyst (50%) Table 1 entry 6. Considering the favorable reaction condition, concise of Bi₂O₃-fly ash and sulfated fly ash was shown in Table 1 entry 5 and entry 6. In this observation, the reaction was carried out using different weight percentage of the sulfated Bi₂O₃-fly ash catalyst (8 and 12 wt%) and was designed. The yield progresses are shown in Table 1 entry 8–9.

The scope of the optimal condition further entries was examined such as both electron withdrawing and electron donating substituent groups, the isolated product is shown in Table 2 entry (3a–3g). The electron withdrawing group of (E)-1-(4-aminophenyl)-3-phenylprop-2-en-1-one derivatives was smoothly furnished.

Table 2 Synthesis of (6H-pyrido[3,2-b]carbazol-4-yl)aniline derivatives.^a,b

a Reaction condition: (E)-1-(4-aminophenyl)-3-phenylprop-2-en-1-one derivatives (1 mmol), 3-amino-9-ethylcarbazole (1 mmol) and 100 mg of sulfated Bi₂O₃-fly ash in water (20 mL) were refluxed at 85 °C for 1–4 h.

b Isolated yield (%).

2.9 Hot filtration test

The significance of sulfated Bi₂O₃-fly ash catalyst was examined by hot filtration test for compound 3f. A mixture of (E)-1-(4-aminophenyl)-3-(4-(methylthio)phenyl)prop-2-en-1-one (1 mmol 269 mg), 3-amino-9-ethylcarbazole (1 mmol 210 mg), water (20 mL) and sulfated Bi₂O₃-fly ash (100 mg) was refluxed at 85 °C for 1 h. The reaction was allowed to proceed for first 15 min and then catalyst was filtered off from the reaction mixture. The isolated product yield (40%) was confirmed by ¹H NMR and LC–MS which is given in Figs. S1a and S4a (see Supporting Information). The reaction was continued for another 60 min and monitored, obviously no changes, in the product yield it was confirmed by LC–MS analysis which is given in Fig. S4. In this observation, we concise in the absence of catalyst, and no reaction is obtained which is shown in Fig. S2 (see Supporting Information). Without removal of the catalyst, the reaction was conducted for 60 min. The reaction was monitored, at different time pause in minutes 30 (85%), 45 (90%), and 60 (99%) and furnished yield is given in Figs. S1 and S4 (see Supporting Information). After 60 min without column chromatographic purification we have attained (99%) yield. The detailed ¹H NMR spectra are given in the Supporting information Fig. S3 (see Supporting Information).

2.10 Reusability of sulfated Bi₂O₃-fly ash catalyst and its characterization

The catalyst reusability and the efficiency were examined under optimized condition for compound 3f. The reusability potential of sulfated Bi₂O₃-fly ash was examined up to five consecutive run. The results seen no appreciable changes in catalytic activity are shown in Fig. 8. The turnover number (TON) and turn over frequency (TOF) are examined and shown in Table 4. The catalyst was separated by filtration method and purified with DCM (30 mL) for unleashed organic substance. Then the catalyst was dried in hot air oven at 130 °C for 1 h. The catalytic activity was examined by green matrix method and it was presented in Table 3.

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Figure 8 Reusability characterization of fifth run sulfated Bi₂O₃-fly ash catalyst (a) powder XRD, (b) FE-SEM, (c) EDS and (d) TEM.

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Reused catalyst crystalline and a morphology studies were examined by powder XRD, FE-SEM and TEM. The powder XRD diffraction pattern shows similar than that catalyst which suggests that the there is no appreciable changes in crystallinity and it is shown in Fig. 8a. The surface morphology of fifth reaction run sulfated Bi₂O₃-fly ash catalyst was examined by FE-SEM. Fig. 8b result reveals that the no appreciable changes in morphology. The elemental composition was confirmed by EDS analysis which is shown in Fig. 8c. The TEM image shows that the rod like morphology observed of 0.5 μm, the fast Fourier transform (FFT) images inserted and it is also shown as regular shape of rod (indicated by red arrow).

3.1 Materials

All chemicals were procured from E-Merck and Sigma–Aldrich India. Fly ash (Class C) was collected from Thermal Power Plant-II, Neyveli Lignite Corporation (NLC), Neyveli, Tamil Nadu, India.

3.2 Characterization studies

Fourier Transform Infrared spectra (KBr, 4000–400 cm⁻¹) were recorded on a Nicolet-380 Fourier Transform spectrophotometer (FT-IR). Raman spectra were determined in confocal Raman (model witech alpha 300). The Powder X-ray diffraction (XRD) patterns were collected on a D8 Advance Bruker diffractometer operating at 230 V, 50 Hz, 6.5 KVA with Cu Kα source (λ = 1.5418 Å) at room temperature. The surface morphology, size and elemental composition of sulfated Bi₂O₃-fly ash were characterized by a Field-emission scanning electron microscopy (FE-SEM) recorded on Carl Zeiss (model Ultra 55) and Transmission electron microscopy (TEM) were recorded on (model Tecnai G²) techniques. The specific surface area of the samples was examined through nitrogen absorption at 77.3 K on the basis of BET equation using a Quantachrome QuadraWin Instruments version 5.02. Melting points of synthesized (6H-pyrido[3,2-b]carbazol-4-yl)aniline derivatives were determined in open glass capillaries on Mettler FP51 melting point apparatus and are uncorrected. The Nuclear magnetic resonance (NMR) spectra were recorded in Bruker AVIII 5000 instrument operating at 400 MHz (using solvent CDCl₃ tetramethylsilane as internal standard). Liquid chromatography–mass spectrometry (LC–MS) spectra were recorded on a Shimadzu LCMS-2010A model 230VCE.

3.3 Facile design of sulfated Bi₂O₃-fly ash catalyst

The sulfated Bi₂O₃-fly ash catalyst was prepared by hydrothermal method. Fly ash (1000 mg) was taken in clean conical flask and added to 20 mL of ethanol and water, followed by magnetic stirring (600 rpm) at room temperature for 15 min. After 15 min 1.2 mL of 0.05 M sulfuric acid was added to fly ash with vigorous stirring at room temperature for 2 h. Then sulfated fly ash (500 mg) was taken a clean conical flask and mixed with 10 mL of ethanol, stirred for 15 min. Ensuing 112 mg of Bi(NO₃)₃·5H₂O was dissolved in 20 mL of 0.005 M nitric acid to avoiding hydrolyzation of Bi³⁺ ions. The pH level 11 was added adjusted by using 180 mg of urea with constant stirring at 90 °C for 2 h. The resulting yellow color solid was obtained, and it is transferred into a Teflon-lined stainless-steel autoclave, sealed, and heated at 116 °C for 3 h. During the process, the pressure was maintained at 30 psi and then dried in an oven at 130 °C for 3 h and calcined at 450 °C for 4 h in a muffle furnace, finally product was obtained.

3.4 General procedure for synthesis of (6H-pyrido[3,2-b]carbazol-4-yl)aniline derivatives

To a mixture of (E)-1-(4-aminophenyl)-3-phenylprop-2-en-1-one derivatives (1 mmol), 3-amino-9-ethylcarbazole (1 mmol), and sulfated Bi₂O₃-fly ash (100 mg) in water (20 mL) at85 °C for 1–4 h, the completion of reaction was monitored by TLC. After completion the reaction mixture was cooled to room temperature, compound was separated with DCM. The organic layer was dried over on anhydrous Na₂SO₄, the excesses of solvent were removed. The crude products were purified by column chromatography, through silica gel (200 mesh) with (20%) 80:20 hexane/ethyl acetate as eluent afforded the desired pure products shown in Scheme 1. The purified products of 3a–3g were confirmed by FT-IR, NMR (¹H, ¹³C), and LC–MS techniques. The heterogeneous nature of sulfated Bi₂O₃-fly ash catalyst was purified with DCM (30 mL) and dried in hot air oven at 130 °C for 1 h. After purification sulfated Bi₂O₃-fly ash catalyst was reused for further reaction.

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Scheme 1 Synthesis of (pyrido [3,2-b] carbazol-4-yl) aniline derivatives.

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3.5 Characterization data of (6H-pyrido[3,2-b]carbazol-4-yl)aniline derivatives (Table 2 entry 3a–3g)

4-(6-Ethyl-2-phenyl-6H-pyrido[3,2-b]carbazol-4-yl)aniline (Table 2 entry 3a) Yield (339 mg 82%), brown solid, M.p. 258–259 °C; FT-IR (KBr, cm⁻¹) 3063, 2936, 1632, 1616; ¹H NMR (400 MHz, CDCl₃, δ ppm), 8.70 (s, 2H, NH₂), 8.18 (d, J = 7.68 Hz, 2H, ArH), 8.11 (d, J = 1.76 Hz, 2H, ArH), 8.04–8.01 (m, 3H, ArH), 7.90 (d, J = 7.24 Hz, 1H, ArH), 7.55–7.51 (m, 3H, ArH), 7.44–7.41 (m, 3H, ArH), 7.31 (t, J = 7.48 Hz, 2H, ArH), 4.38 (Q, J = 7.21 Hz, 2H, CH₂), 1.45 (t, J = 7.20 Hz, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃, δ ppm), 158.09, 143.82, 140.57, 138.87, 136.78, 130.94, 129.80, 128.83, 128.64, 125.96, 123.53, 123.13, 120.66, 120.06, 118.96, 112.65, 108.84, 108.75, 37.73, 13.94; LCMS m/z calculated for C₂₉H₂₃N₃ 413 found 413 [M].

4-(2-(3-Bromophenyl)-6-ethyl-6H-pyrido[3,2-b]carbazol-4-yl)aniline. (Table 2 entry 3b) Yield (463 mg 94%), brown solid, M.p. 292–293 °C; FT-IR (KBr, cm⁻¹) 3052, 2926, 1610; ¹H NMR (400 MHz, CDCl₃, δ ppm), 8.60 (s, 2H, NH₂), 8.17 (s, 1H, ArH), 8.13 (d, J = 7.81 Hz, 1H, ArH), 8.06 (s, 1H, ArH), 7.84 (d, J = 7.68 Hz, 2H, ArH), 7.60 (d, J = 7.92 Hz, 2H, ArH), 7.51 (t, J = 6.36 Hz, 3H, ArH), 7.42 (d, J = 8.52 Hz, 3H, ArH), 7.37 (t, J = 7.84 Hz, 1H, ArH), 7.27 (t, J = 6.82 Hz, 1H, ArH), 4.35 (Q, J = 7.21 Hz, 2H, CH₂), 1.45 (t, J = 7.16 Hz, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃, δ ppm), 155.85, 143.07, 140.55, 139.08, 138.76, 133.61, 131.03, 130.27, 127.36, 126.02, 123.51, 123.08, 120.62, 119.99, 119.03, 112.80, 108.85, 108.75, 37.75, 13.91; LCMS m/z calculated for C₂₉H₂₂BrN₃ 491 found 490 [M⁻¹].

4-(2-(3-Chlorophenyl)-6-ethyl-6H-pyrido[3,2-b]carbazol-4-yl)aniline (Table 2 entry 3c) Yield (418 mg 93%), brown solid, M.p 273–274 °C; FT-IR (KBr, cm⁻¹) 3063, 2926, 1610; ¹H NMR (400 MHz, CDCl₃, δ ppm), 8.62 (s, 2H, NH₂), 8.13 (d, J = 7.72 Hz, 2H, ArH), 8.05 (d, J = 1.62 Hz, 1H, ArH), 7.82–7.76 (m, 1H, ArH), 8.01 (s, 2H, ArH), 7.52–7.48 (m, 4H, ArH), 7.43 (t, J = 1.56 Hz, 3H, ArH), 7.27 (t, J = 4.88 Hz, 2H, ArH), 4.36 (Q, J = 7.24 Hz, 2H, CH₂), 1.45 (t, J = 1.56 Hz, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃, δ ppm), 156.00, 143.11, 140.55, 139.07, 138.53, 134.96, 130.71, 129.99, 128.08, 126.89, 126.01, 123.51, 123.05, 120.61, 119.99, 119.02, 108.84, 108.74, 37.75, 13.90; LCMS m/z calculated for C₂₉H₂₂ClN₃ 448 found 448 [M].

4-(6-Ethyl-2-(4-(trifluoromethyl)phenyl)-6H-pyrido[3,2-b]carbazol-4-yl)aniline (Table 2 entry 3d) Yield (471 mg 97%), brown solid, M.p 285–286 °C; FT-IR (KBr, cm⁻¹) 3046, 2926, 1600; ¹H NMR (400 MHz, CDCl₃, δ ppm), 8.08 (d, J = 7.36 Hz, 1H, ArH), 8.05 (t, J = 8.36 Hz, 1H, ArH), 7.97 (d, J = 7.92 Hz, 1H, ArH), 7.79 (t, J = 8.20 Hz, 1H, ArH), 7.64 (s, 2H, ArH), 7.57 (s, 1H, ArH), 7.49 (d, J = 7.32 Hz, 1H, ArH), 7.43 (s, 1H, ArH),7.38 (d, J = 8.00 Hz, 2H, ArH), 7.23 (d, J = 9.12 Hz, 2H, ArH), 6.93 (d, J = 8.28 Hz, 1H, ArH), 6.61 (d, J = 7.80 Hz, 1H, ArH), 4.25 (Q, J = 6.28 Hz, 2H, CH₂), 3.56 (s, 2H, NH₂), 1.38 (t, J = 6.92 Hz, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃, δ ppm), 155.80, 151.97, 142.94, 141.02, 140.50, 139.13, 138.79, 134.60, 131.32, 128.76, 128.40, 125.77, 125.59, 124.36, 123.72, 122.56, 120.54, 118.15, 115.78, 113.96, 109.16, 108.54, 106.45, 37.51, 13.36; LCMS m/z calculated for C₃₀H₂₂F₃N₃ 481 found 482 [M⁺¹].

4-(6-Ethyl-2-(4-fluorophenyl)-6H-pyrido[3,2-b]carbazol-4-yl)aniline (Table 2 entry 3e) Yield (403 mg 93%), brown solid, M.p 280–281 °C; FT-IR (KBr, cm⁻¹) 3052, 2931, 1605; ¹H NMR (400 MHz, CDCl₃, δ ppm), 8.63 (s, 2H, NH₂), 8.12 (d, J = 7.68 Hz, 1H, ArH), 8.03 (s, 2H, ArH), 7.97 (t, J = 6.56 Hz, 2H, ArH), 7.48 (d, J = 8.16 Hz, 3H, ArH), 7.42 (d, J = 8.36 Hz, 2H, ArH), 7.24 (t, J = 5.84 Hz, 3H, ArH), 7.17 (t, J = 8.36 Hz, 2H, ArH), 4.37 (Q, J = 7.20 Hz, 2H, CH₂), 1.45 (t, J = 7.28 Hz, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃, δ ppm), 165.68, 156.49, 143.59, 140.55, 138.85, 133.10, 130.50, 130.41, 125.93, 125.59, 123.51, 123.06, 120.58, 120.42, 119.94, 118.92, 118.73, 116.00, 115.78, 114.67, 112.47, 108.79, 108.79, 108.70, 108.42, 37.73, 13.88; LCMS m/z calculated for C₂₉H₂₂FN₃ 431 found 432 [M⁺¹].

4-(6-Ethyl-2-(4-(methylthio)phenyl)-6H-pyrido[3,2-b]carbazol-4-yl)aniline (Table 2 entry 3f) Yield (454 mg 99%), brown solid, M.p. 287–288 °C; FT-IR (KBr, cm⁻¹) 3353, 2931, 1589; ¹H NMR (400 MHz, CDCl₃, δ ppm), 8.04 (d, J = 7.28 Hz, 1H, ArH), 7.98 (d, J = 7.72 Hz, 1H, ArH), 7. 82 (s, 1H, ArH), 7.48–7.54 (m, 3H, ArH), 7.43 (s, 1H, ArH), 7.38 (d, J = 7.68 Hz, 2H, ArH), 7.24 (s, 4H, ArH), 6.93 (d, J = 7.08 Hz, 1H, ArH), 6.61 (d, J = 7.76 Hz, 1H, ArH), 4.25 (Q, J = 6.80 Hz, 2H, CH₂), 3.58 (s, 2H, NH₂), 2.48 (s, 3H, SCH₃), 1.38 (t, J = 6.36 Hz, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃, δ ppm), 151.61, 142.62, 141.74, 140.49, 139.17, 134.58, 131.89, 131.15, 130.86, 129.03, 128.79, 128.29, 127.02, 126.02, 125.86, 125.57, 123.72, 122.57, 121.11, 120.56, 118.14, 115.77, 113.97, 109.16, 108.53, 106.44, 37.54, 15.17, 13.92; LCMS m/z calculated for C₃₀H₂₅N₃S 459 found 458 [M⁻¹].

4-(6-Ethyl-2-(3-nitrophenyl)-6H-pyrido[3,2-b]carbazol-4-yl)aniline (Table 2 entry 3 g) Yield (419 mg 91%), brown solid, M.p. 284–285 °C; FT-IR (KBr, cm⁻¹) 3052, 2932, 1616; ¹H NMR (400 MHz, CDCl₃, δ ppm), 8.80 (s, 2H, NH_2, ArH), 8.76 (s, 1H, ArH), 8.32–8.30 (dd, J₁ = 1.76 Hz, J₂ = 1.76 Hz, 1H, ArH), 8.13 (d, J = 7.76 Hz, 1H, ArH), 8.10 (d, J = 1.96 Hz, 2H, ArH), 7.68 (t, J = 7.96 Hz, 1H, ArH), 7.55 (d, J = 2.04 Hz, 1H, ArH), 7.53 (d, J = 2.04 Hz, 1H, ArH), 7.51 (d, J = 1.04 Hz, 1H, ArH), 7.49 (d, J = 1.08 Hz, 1H, ArH), 7.46–7.43 (m, 2H, ArH), 7.29–7.25 (dd J₁ = 14.81 Hz, J₂ = 14.72 Hz, 3H, ArH), 4.37 (Q, J = 7.20 Hz, 2H, CH₂), 1.45 (t, J = 7.21 Hz, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃, δ ppm), 154.20, 148.74, 142.41, 140.59, 139.37, 138.49, 134.62, 133.78, 129.73, 128.62, 126.13, 124.99, 123.55, 123.23, 123.02, 120.62, 120.03, 119.15, 113.05, 108.92, 108.81, 37.78, 13.90; LCMS m/z calculated for C₂₉H₂₂N₄O₂ 458 found 457 [M⁻¹].