A Universal Framework for Ligand-Directed Artificial Photosynthesis by Hydrolyzed Titanium Complexes

Abstract

A mechanistic study of three hydrolyzed titanium–ligand complexes 2-phenylindole (PI)–TiCl₄, 2-phenylbenzoxazole (BZX)–TiCl₄, and
1,10-phenanthroline (Phen)–TiCl₄—reveals a unified pathway for Direct Air Capture (DAC) and photocatalytic conversion of atmospheric CO₂ under ambient conditions. The process exemplifies a radical-driven form of artificial photosynthesis powered by the intrinsic redox activity of hydrolyzed titanium species that continuously generate hydroxyl radicals (•OH), the true life-sustaining agents of the system.

To elucidate the underlying mechanism, the overall photocatalytic transformation can be resolved into three interconnected stages each governed by the evolving speciation, redox state, and ligand environment of titanium. Together, these stages trace the system’s progression from spontaneous hydrolysis to radical-driven CO₂ reduction and selective product formation.

Stage 1: Spontaneous hydrolysis and partial reduction of TiCl₄–ligand mixtures by atmospheric moisture and oxygen produce disordered yet functionally organized ensembles of hydrolyzed and reduced Ti species. This initial structural diversity provides a self-adjusting equilibrium of coordination environments that primes the system for autonomous catalysis. Each ligand introduces a distinct molecular template that shapes the redox potential landscape and directs the ensuing transformations.

Stage 2: Progressive hydrolysis establishes equilibria between hydrated Ti species and TiCl₂O units containing reduced Ti centers capable of capturing and activating atmospheric CO₂. Upon visible-light excitation, Ti–OH bonds act as radical precursors, releasing •OH radicals that concurrently regenerate reduced Ti sites and reduce carbonate intermediates to formaldehyde—the hallmark product of Reactive DAC.

Stage 3: This marks the divergence of ligand-controlled photochemistry. In the PI and BZX systems, hydroxyl-radical cascades extend formaldehyde into oxygenates reaching up to C₁₇, while in the Phen system, the same radical flux channels formaldehyde exclusively into polyoxymethylene (POM). These complementary outcomes expose the dual advantage of titanium chemistry: the ability to both generate and exploit hydroxyl radicals as reductants and molecular architects.

Collectively, the three systems define a self-sustaining, light-driven framework for artificial photosynthesis, in which hydroxyl radical dynamics, ligand templating, and environmental reactants (H₂O, O₂, CO₂) cooperate to achieve selective and extended CO₂ reduction under ambient conditions a level of autonomous photochemical integration not achieved by any other known photocatalytic system.