Harnessing Visible Light: A Polydiacetylene–Rh Complex for NADH Photoregeneration and CO₂ Reduction

In the quest for artificial photosynthesis, this study introduces an approach involving the design and synthesis of key components for direct solar fuel production from CO2. We developed a conjugated polymer chromophore, specifically poly(diacetylene) (PDA), and covalently bonded it to a rhodium(III) catalyst. This polymer acts as both a visible-light harvestor and a structural scaffold for catalyst immobilization. UV irradiation polymerized the phenanthroline-doped diacetylene monomer, yielding (Cp*)Rh(phen-)-PDA (PDA-Rh), where Cp* is pentamethylcyclopentadienyl and phen is a 1,10-phenanthroline derivative. PDA-Rh proved capable of chemically regenerating NADH in the presence of sodium formate, albeit at a slower rate than [Rh(Cp*)(phen)Cl]+, attributed to PDA-Rh’s lower diffusion coefficient. Notably, PDA-Rh facilitated a 40% NADH regeneration within 24 h under visible light, significantly outperforming the [Rh(Cp*)(phen)Cl]+ and PDA mixture under the same conditions. Further investigations into the photophysical and electrochemical behaviors of PDA and PDA-Rh, both in solution and at the TiO2 interface, revealed electron transfer from the photoexcited PDA to [(Cp*)RhCl(phen-)], initiating the reduction of Rh(III) to active intermediates. Subsequent CO2 reduction experiments, leveraging the photogenerated NADH by PDA-Rh and NADH-dependent formate dehydrogenase, demonstrated that PDA-Rh significantly enhances CO2 conversion, surpassing the control system of PDA and [Rh(Cp*)(phen)Cl]+.