Extended phenothiazines: synthesis, photophysical and redox properties, and efficient photocatalytic oxidative coupling of amines

Herein we successfully developed a ring-fusion approach to extend the conjugation length of phenothiazines and synthesized a series of novel extended phenothiazines 1–5. The intriguing π-conjugation length-dependent photophysical and redox properties of 1–5, and their photocatalytic performance towards visible-light-driven oxidative coupling reactions of amines were systematically investigated. The results indicated that this series of extended phenothiazines exhibited continuous red shifts of light absorption with increasing numbers of fused rings. As compared with the conventional phenothiazine (PTZ), all the extended phenothiazines displayed reversible redox behavior and maintained a strong excited-state reduction potential as well. Consequently, 3, 4 and 5 with longer effective conjugation lengths could efficiently catalyze the oxidative coupling of amines to imines under visible-light irradiation; by comparison, the shorter 1, 2 and PTZ could only catalyze such reactions in the presence of UV light. Moreover, 3 showed superior catalytic performance which can result in better yields within a shorter reaction time, and in a broad substrate scope. Finally, a direct and efficient conversion of amines to imines under sunlight in an air atmosphere was successfully realized. We believe that our study including the new phenothiazine modification methodology and the newly developed extended phenothiazine-based photocatalysts will open up a new way to develop novel phenothiazine-based materials for optoelectronic and catalytic applications.


Compound a-OTf
Compound a-OMe (1.00 g, 3.40 mmol) was dissolved in DCM (20 mL). BBr 3 (2.30 mL, 23.60 mmol) was added at 0 o C, and then the resulting mixture was stirred at room temperature for 1 h. The reaction mixture was quenched with MeOH (5 mL) dropwise at 0 o C. The resulting mixture was evaporated under reduced pressure to obtain crude product without purification. Then the crude product was dissolved in DCM (20 mL), pyridine (655 μL, 8.10 mmol) was added. Then triflic anhydride (1.70 mL, 10.1 mmol) was added at 0 o C, and the resulting mixture was stirred at room temperature for 2 h. After the reaction, the mixture was quenched with saturated NaHCO 3 aqueous solution and extracted with DCM (20 mL). The organic layer was dried over anhydrous Na 2 SO 4 , filtrated and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel (PE:DCM = 5:1) to give the compound a-OTf (1.39 g) as a white solid in 77% yield. 1 13

Compound b-OTf
Compound b-OMe (346 mg, 1.00 mmol) was dissolved in DCM (5 mL). BBr 3 (674 μL, 7.00 mmol) was added at 0 o C, and then the resulting mixture was stirred at room temperature for 1 h. The reaction mixture was quenched with MeOH (5 mL) dropwise at 0 o C. The resulting mixture was evaporated under reduced pressure to obtain crude product without purification. The crude product was dissolved in DCM (5 mL). Then pyridine (194 μL, 2.40 mmol) was added. Then triflic anhydride (505 μL, 3.00 mmol) was added at 0 o C. and the resulting mixture was stirred at room temperature for 2 h. After the reaction, the mixture was quenched with saturated NaHCO 3 aqueous solution and extracted with DCM (10 mL). The organic layer was dried over anhydrous Na 2 SO 4 , filtrated and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel (PE:DCM = 5:1) to give the compound b-OTf (483 mg) as a white solid with yield of 83%. 1

Compound 2-Br
9,10-dimesityl-2-methoxyanthracene was synthesized by previous reported procedures. [4] 9,10-Dimesityl-2-methoxyanthracene (889 mg, 2.00 mmol) was dissolved in THF 20 mL. Then ethyl n-BuLi (0.96 mL, 2.40 mmol) was added at -78 o C under N 2 protection and the resulting mixture was stirred at room temperature for 1 h. After that, 1,2-dibromoethane (190 μL, 2.20 mmol) was added to the mixture at -78 o C. The reaction mixture was stirred at room temperature overnight. After that, the mixture was quenched with saturated NH 4 Cl aqueous solution and extracted with AcOEt. The organic layer dried over anhydrous Na 2 SO 4 , filtrated and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel (PE) to obtain the compound 2-Br (846 mg) as a yellow solid in 81% yield. 1

Compound c-OTf
Compound c-OMe (582 mg, 1.00 mmol) was dissolved in DCM (10 mL). BBr 3 (674 μL, 7.00 mmol) was added at 0 o C, and the resulting mixture was stirred at room temperature for 1 h. The reaction mixture was quenched with MeOH (3 mL) dropwise at 0 o C. The resulting mixture was evaporated under reduced pressure to obtain crude product without purification. The crude product was dissolved in DCM (10 mL). Then pyridine (194 μL, 2.40 mmol) was added. Then triflic anhydride (505 μL, 3.00 mmol) was added at 0 o C. and the resulting mixture was stirred at room temperature for 2 h. After that, the mixture was quenched with saturated NaHCO 3 aqueous solution and extracted with DCM (10 mL). The organic layer was dried over anhydrous Na 2 SO 4 , filtrated and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel (PE:EA = 60:1) to give the compound c-OTf (622 mg) as a yellow solid with yield of 76%. 1

Compound 3-methoxynaphthalene-2-thiol
2-Methoxynaphthalene (5.00 g, 31.6 mmol) was dissolved in THF (100 mL). Then ethyl n-BuLi (18.2 mL, 45.5 mmol) was added at -78 o C and the resulting mixture was stirred at room temperature for 1 h. After that, elemental sulfur (1.21 g, 37.9 mmol) was added to the mixture at -78 o C. Then the reaction mixture stirred at room temperature overnight. After the reaction, the mixture was quenched with saturated NH 4 Cl aqueous solution and extracted with AcOEt. The organic layer dried over anhydrous Na 2 SO 4 , filtrated and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel (PE) to give the compound 3methoxynaphthalene-2-thiol (2.90 g) as a white solid in 49% yield. 1 13

Compound d-OTf
Compound d-OMe (1.07 g, 1.70 mmol) was dissolved in DCM (20 mL). BBr 3 (1.16 mL, 12.0 mmol) was added at 0 o C, and then the resulting mixture was stirred at room temperature for 1 h. The reaction mixture was quenched with MeOH (10 mL) dropwise at 0 o C and evaporated under reduced pressure to obtain crude product without purification. The crude product was dissolved in DCM (20 mL). Then pyridine (330 μL, 4.08 mmol) was added. Triflic anhydride (858 μL, 5.10 mmol) was added at 0 o C, and the resulting mixture was stirred at room temperature for 2 h. After that, the mixture was quenched with saturated NaHCO 3 aqueous solution and extracted with DCM. The organic layer was dried over anhydrous Na 2 SO 4 , filtrated and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel (PE:EA = 100:1) to give the compound d-OTf (900 mg) as a yellow solid with yield of 61%. 1

Compound e-OTf
Compound e-OMe (1.45 g, 1.58 mmol) was dissolved in DCM (20 mL). BBr 3 (1.07 mL, 11.0 mmol) was added at 0 o C, and then the resulting mixture was stirred at room temperature for 1 h. The reaction mixture was quenched with MeOH (5 mL) dropwise at 0 o C and evaporated under reduced pressure to obtain crude product without purification. The crude product was dissolved in DCM (20 mL). Then pyridine (307 μL, 3.80 mmol) was added. Triflic anhydride (791 μL, 4.70 mmol) was added at 0 o C, and the resulting mixture was stirred at room temperature for 2 h. After that, the mixture was quenched with saturated NaHCO 3 aqueous solution and extracted with DCM. The organic layer was dried over anhydrous Na 2 SO 4 , filtrated and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel (PE:EA = 100:1) to give the compound d-OTf (1.28 g) as a yellow solid with yield of 70%. 1

Compound 5
A round-bottom flask was charged with e-OTf (1.00 g, 0.87 mmol), Pd 2 (dba) 3 (40.0 mg, 0.04 mmol), DPEPhos (48.5 mg, 0.09 mmol), Cs 2 CO 3 (847 mg, 2.60 mmol), and toluene (20 mL). 4-tert-butylaniline (152 μL, 0.95 mmol) was added to the mixture. The mixture was stirred at 130 o C for 24 h under N 2 protection. After reaction, the resulting mixture was diluted with H 2 O and extracted with AcOEt. The organic layer was dried over anhydrous Na 2 SO 4 , filtrated and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel (PE:DCM = 10:1) to give the compound 5 (210 mg) as a yellow solid with yield of 48%. 1 Table S1. Dihedral angle and selected bond length of DFT-optimized structure of PTZ, PTZ •+ , 1, 1 •+ , 2, 2 •+ , 3, 3 •+ , 4, 4 •+ , 5 and 5 •+ and dihedral angle and selected bond length of crystal structure of 1, 1 •+ , 2 and 2 •+ . S15 Figure S9. Calculated molecular orbitals of 3, 4, and 5 and their real space representations of hole and electron distributions for S0S1 excitation. Blue and green regions denote the hole and electron distributions, respectively (isovalue = 0.003). S0S1 was mainly attributed to electrons transition from HOMOLUMO (isovalue = 0.03).  Figure S11. Experimental and simulated EPR spectra of PTZ •+ in CH 2 Cl 2 (data processing (left) and screenshot (right) from the Bruker SpinFit software). The fitting parameters for the spectral simulation are: g = 2.00529, A N = 6.99874 G.                7 Photostability study Figure S24. Changes of absorbance of 3 (a), 4 (b) and 5 (c) (30 μM in toluene) in absorption spectra upon irradiation by 6 W white LED light every ten minutes. Details of photostability test: A solution of extended phenothiazine (30 μM in toluene) in quartz cell was subjected to 6 W white LED irradiation in the photoreactor, during which the UV-Vis spectra of the solution were measured every ten minutes over a period of one hour to record the absorbance changes. The photostability experiments were repeated 3 times under the same testing condition.           Figure S45.                        Table 3.