A versatile heterogeneous photocatalyst: nanoporous gold powder modified with a zinc(II) phthalocyanine derivative for singlet oxygen [4 + 2] cycloadditions

Abstract

Nanoporous gold was functionalized with a photosensitizer, a zinc(II) phthalocyanine derivative. Such systems are active for the generation of reactive singlet oxygen which can be used for photocatalytic oxidation reactions. This study aims to demonstrate the versatility of such an approach, in terms of substrates and the employed solvent, only possible for a truly heterogeneous catalytic system. The activity of the hybrid system was studied for [4 + 2] cycloadditions of three different types of dienes and a total of eight substrates in two organic solvents and once in water. The highest activity was measured for 1,3-diphenylisobenzofuran, which is also highest in terms of sensitivity for the reaction with 1O2. Trends in conversion could be anticipated based on reported values for the rate constant for the reaction of 1O2. In almost all cases, an amplification of the conversion by immobilization of the sensitizer onto nanoporous gold was observed. The limiting case was ergosterol, which was the largest of all substrates with a van-der-Waals radius of about 2.1 nm. Additional factors such as the limited lifetime of 1O2 in different solvents as well as the hampered diffusion of the substrates were identified. Graphic abstract

Sustainable Photooxidation using a Subpart-per-million Heavy-Metal-Free Red-Light Photocatalyst

“…Concerning the t -Bu-substituted ones, no substrate conversion was observed with NiPCy t Bu 4 or MgPCy t Bu 4 , while ZnPCy t Bu 4 provided slow conversion to reach only 19% in 90 min. This result was rather surprising as Zn-phthalocyanines are among the most widely used phthalocyanines for 1 O 2 generation. ,− In comparison, the metal-free H 2 PCy t Bu 4 gave a better conversion of 52% in a 90 min reaction time. We were delighted to find out that Si­(OH) 2 PCy t Bu 4 and Si­(Cl) 2 PCy t Bu 4 gave good conversions of 75 and 78%, respectively, and PtPCy t Bu 4 and PdPCy t Bu 4 were even more reactive with 88 and 100% conversions, respectively, within the same reaction time presumably as a result of a higher TOF in the latter cases due to heavy-atom enhanced singlet-to-triplet ISC.…”