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

## TL;DR

This paper investigates the use of iridium(III) complexes in solar cells to drive chemical reactions, finding that fluorinated versions perform better in some cases.

## Contribution

The work explores the underutilized potential of cyclometalated iridium(III) complexes in photoelectrochemical systems for organic synthesis.

## Key findings

- Fluorinated iridium(III) complexes showed higher reactivity in Diels–Alder reactions but lower efficiency in alcohol oxidation.
- Photoelectrochemical performance varied significantly depending on the presence of redox mediators and fluorination.
- Molecular design of sensitizers must align with reaction requirements for optimal performance.

## Abstract

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.