Exploring the Boundaries of Cyclometalated Iridium(III) Sensitizers in Photoelectrochemical Organic Transformations

Dye-sensitized photoelectrochemical cells (DSPECs) are currently at the forefront of solar-to-chemical energy conversion technologies. Although water oxidation to dioxygen has long been the preferred reaction at the photoanodic compartment, recent research has increasingly focused on oxidation processes for the synthesis of value-added organic compounds. Quite surprisingly, within this framework, cyclometalated iridium(III) complexes have received negligible attention as photoactive components in DSPEC photoanodes, in spite of their intriguing photophysical and electrochemical properties. With the aim of filling this gap, this work explores the application of two iridium(III) complexes (Ir1 and Ir2), differing in the presence of fluorinated substituents, as light-harvesting sensitizers anchored onto mesoporous TiO₂ photoelectrodes. These systems were employed to drive two relevant oxidation processes: the TEMPO-mediated oxidation of benzyl alcohol (BzOH) to benzaldehyde and the radical cation Diels-Alder reaction between trans-anethole (TA) and isoprene (ISO). In the oxidation of BzOH to benzaldehyde, maximum photocurrent densities on the order of 0.5-0.7 mA·cm^-2 were recorded, but the photoelectrodes proved substantially inefficient (APCE between 2.2% and 2.4%). Under operative conditions, low Faradaic efficiencies (FEs) for benzaldehyde formation were also registered (42% and 32% for Ir1 and Ir2, respectively), associated with a rapid decrease in photocurrent densities, particularly in the case of the fluorinated complex. In contrast, the DSPEC system operating without a redox mediator exhibits markedly improved performances (photocurrent densities on the order of 0.7 mA·cm^-2, APCE up to 19%), with quantitative conversion of the TA substrate under bulk electrolysis conditions. Interestingly, for this latter reaction, the enhanced oxidative power of the fluorinated sensitizer contributes to the increased reactivity. A combination of photoelectrochemical and transient absorption spectroscopy studies has been performed to rationalize the observed behavior. The results highlight how the molecular design and electronic properties of the dye component in DSPECs should be rationally engineered to align with the thermodynamic and kinetic requirements of the targeted chemical transformation.