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]⁺.