Harnessing VisibleLight: A Polydiacetylene–RhComplex 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]⁺.