New core-pyrene π structure organophotocatalysts usable as highly efficient photoinitiators

Summary Eleven di- and trifunctional compounds based on a core-pyrene π structure (Co_Py) were synthesized and investigated for the formation of free radicals. The application of two- and three-component photoinitiating systems (different Co_Pys with the addition of iodonium or sulfonium salts, alkyl halide or amine) was investigated in detail for cationic and radical photopolymerization reactions under near-UV–vis light. The proposed compounds can behave as new photocatalysts. Successful results in terms of rates of polymerization and final conversions were obtained. The strong MO coupling between the six different cores and the pyrene moiety was studied by DFT calculations. The different chemical intermediates are characterized by ESR and laser flash photolysis experiments. The mechanisms involved in the initiation step are discussed, and relationships between the core structure, the Co_Py absorption property, and the polymerization ability are tentatively proposed.

The mixture was heated under reflux for 48 h under a nitrogen atmosphere. Then the mixture was poured into water, and the aqueous layer was extracted with CHCl 3 . The combined organic phases were washed with brine and dried (MgSO 4 ). The residue was purified by column chromatography (SiO 2 ) by using DCM/acetone 1:1 as the eluent. During evaporation, a light brown solid formed, which was filtered off, washed with pentane, and dried under vacuum. 1.90 g of the photoinitiator was obtained in 88% yield. 1  Synthesis of 1,4-di(pyren-1-yl)benzene (Py_4).
First procedure: To a solution of 1,4-benzenediboronic acid (1 g, 6.03 mmol,) and 1bromopyrene (5.1 g, 18.1 mmol, 3 equiv) in THF (80 mL), was added Pd(PPh 3 ) 4 (60 mg) and aqueous KOH (5 mL, 2 M). The mixture was heated under reflux for five days under a nitrogen atmosphere. Then the mixture was poured into water, and the aqueous layer was extracted with CHCl 3 . The combined organic phases were washed with brine and dried (MgSO 4 ). During evaporation, a light yellow solid formed, which was filtered off, washed several times with pentane, and dried under vacuum. 2.65 g of the title compound were obtained in 92% yield.
Then the mixture was poured into water, and the aqueous layer was extracted with CHCl 3 .
The combined organic phases were washed with brine and dried (MgSO 4 ). During evaporation, a light yellow solid formed, which was filtered off, washed several times with pentane, and dried under vacuum. 1.18 g of the title compound was obtained in 89% yield. 1
The solution was cooled to −78 °C and 2.86 mL (7.15 mmol, 2.5 M in hexanes, 1.1 equiv) of n-BuLi was slowly added. The reaction was stirred for 30 min and 1.20 g (6.50 mmol) of 4bromobenzaldehyde was added. The reaction was allowed to warm to room temperature and stirring was maintained overnight. The solution was quenched with aqueous HCl (6 N) and stirring was maintained for 10 min. Water was added and the solution was extracted with chloroform 3 times, the organic phases were combined, dried over magnesium sulfate and the solvent removed under reduced pressure. The residue was purified by column chromatography (SiO 2 ) using DCM/acetone 1:1 as the eluent. During evaporation, a precipitate formed, which was filtered off, washed with pentane, and dried under vacuum.
To a solution of 616 mg (1.59 mmol) of alcohol 1 in 10 mL of CH 2 Cl 2 was added 515 mg (2.39 mmol) of PCC. The mixture was stirred at room temperature for 5 h. The solution was directly subjected to column chromatography (SiO 2 ) using DCM as the eluent. The ketone was eluted as the first band. The title compound was isolated as a light yellow solid (410 mg, 67% yield). 1