Visible Light-activated 2-Phenyl Indole (PI)/TiCl\(_4\) Complexes for Atmospheric CO\(_2\) Capture and C\(_6\)-C\(_{17}\) Organic Synthesis via Hydroxyl Radical Synergism

Abstract

Preferred systems are catalysts that operate away from equilibrium, continuously supplied with energy and materials from inexhaustible sources such as sunlight, air, and water. This concept underpins artificial photosynthesis (AP). A novel self-organized chemical "living" system is presented that mimics natural photosynthesis by capturing CO₂ and H₂O from the atmosphere under ambient conditions. Using 2-phenyl indole (PI) and TiCl₄ complexes activated by visible light, this system autonomously reduces CO₂ with H₂O protons to produce long-chain oxygenated hydrocarbons up to C₁₇. The process begins with hydrolyzed PI/TiCl₄ complexes reacting with CO₂ to form organotitanium carbonates, with the 2:1 PI/TiCl₄ ratio proving most effective. Visible light reduces Ti⁺⁴ to Ti³⁺ and Ti²⁺, generating hydroxyl radicals, which play a critical role in the reduction of CO₂ to CO, H₂CO, and CH₃OH. These intermediate further couple to form C₆ to C₉ \(\alpha\)-carboxylic acid-\(\omega\)-aldehyde compounds, which serve as feedstocks for the subsequent synthesis of longer carbon chains (C₁₂ to C₁₇) through a radical mechanism. The system facilitates ligand exchange between PI and donor molecules, forming adducts involved in the photocatalytic process. Additionally, PI oligomerization contributes to the formation of functional oligomers. Over two dozen intermediates and products were identified using MALDI-TOF, ¹³C NMR, and IR spectroscopy, highlighting the complexity of the reaction network. This catalytic system uniquely integrates organotitanium chemistry and hydroxyl radical pathways, autonomously capturing CO₂ and humidity from the atmosphere while harnessing solar energy to produce valuable long-chain oxygenated hydrocarbons. This prototype offers a foundation for exploring broader applications and enhancements using other metals and ligands.