PHOTOCATALYSIS OF OLIGO(PARA-PHENYLENES) - PHOTOCHEMICAL REDUCTION OF CARBON-DIOXIDE WITH TRIETHYLAMINE

Photoreduction of CO2 to formic acid (HCO2-) and a small quantity of carbon monoxide (CO) can be achieved in nonaqueous polar solvent by using oligo(p-phenylenes) (OPP-n) as a photocatalyst and triethylamine (TEA) as a sacrificial electron donor under > 290-nm irradiation, where the photocatalysis in N,N-dimethylformamide leads to the most effective formation of HCO2- and CO. Among OPP-n, OPP-3 and OPP-4 show high photocatalytic activity for the formation of HCO2-, in which the apparent quantum yields of HCO2- formation for OPP-3 and OPP-4 are 0.072 and 0.084, respectively. Although photocatalyst OPP-3 itself concurrently undergoes photo-Birch reduction during the photocatalysis, the turnover number for the formation of HCO2- based on the reacted OPP-3 is calculated to be 4, implying a cyclic activity of the reduction system. The laser flash photolysis and pulse radiolysis studies reveal that the photocatalysis initially starts from the reductive quenching of the singlet state of OPP-n by TEA followed by the formation of the radical anion of OPP-n, OPP-n.-, resulting in direct electron transfer from OPP-n.- to CO2 molecules.