Understanding Ir(III) Photocatalyst Structure–Activity Relationships: A Highly Parallelized Study of Light-Driven Metal Reduction Processes

Abstract

High-throughput synthesis and screening methods were used to measure the photochemical activity of 1440 distinct heteroleptic [Ir(C^N) 2 (N^N)] + complexes for the photoreduction of Sn(II) and Zn(II) cations to their corresponding neutral metals. Kinetic data collection was carried out using home-built photoreactors and measured initial rates, obtained through an automated fitting algorithm, spanned between 0−120 μM/s for Sn(0) deposition and 0−90 μM/s for Zn(0) deposition. Photochemical reactivity was compared to photophysical properties previously measured such as deaerated excited state lifetime and emission spectral data for these same complexes; however, no clear correlations among these features were observed. A formal photochemical rate law was then developed to help elucidate the observed reactivity. Initial rates were found to be directly correlated to the product of incident photon flux with three reaction elementary efficiencies: (1) the fraction of light absorbed by the photocatalyst, (2) the fraction of excited state species that are quenched by the electron donor, and (3) the cage escape efficiency. The most active catalysts exhibit high efficiencies for all three steps, and catalyst engineering requirements to maximize these elementary efficiencies were postulated. The kinetic treatment provided the mechanistic information needed to decipher the observed structure/function trends in the high-throughput work.

Femtosecond extreme ultraviolet spectroscopy of an iridium photocatalyst reveals oxidation state and ligand field specific dynamics

“…Ir(III)(ppy) 3 serves as an excellent initial target for transient XUV spectroscopy due to its well-studied photochemistry, 34,35,39,40,42,46,[51][52][53][54][55][56][57][58][59][60][61] applicability as a dopant for OLEDs, 33,35,62 and common use as a photosensitizer and catalyst in organic transformations. [24][25][26]29 Examining the UV-Vis spectrum of Ir(III)(ppy) 3 (Fig. 2), three main bands are present: one below than 300 nm that corresponds to singlet π-π* transitions from the 2-phenylpyridine ligand, a broad band centered at 375 nm that corresponds to a 1 MLCT (d-π*) transition, and lower intensity triplet MCLT ( 3 MLCT) bands clustered between 430-500 nm.…”

“…Cyclometalated iridium(III) complexes are ubiquitous photocatalysts and sensitizers, with wide use in photoredox chemistry. [24][25][26][27][28][29][30][31][32] They are also common dopants in organic light-emitting diodes (OLEDs). [33][34][35] These complexes generally have strong molar absorptivity (~10,000 M -1 cm -1 ) in the UV-Vis spectral region, allowing for excitation at accessible single wavelengths or ambient light sources.…”

“…26,33,34,[36][37][38][39][40][41][42] Common ligand scaffolds, such as bipyridine (bpy) and 2-phenylpyridine (ppy), are modifiable through ligand exchange or substitutions at select positions, allowing for easy tuning of the charge transfer bands and dynamics. 29,30,37,41 Their excited state redox potentials are well-suited for transferring electrons in a variety of transformations. 28,41,[43][44][45] Finally, these triplet states are generally long-lived (ns-μs) due to the forbidden transition to return to the singlet ground state, allowing for sufficient time to transfer the energy into the desired process.…”

Ruthenium(ii) complexes with phosphonate-substituted phenanthroline ligands: synthesis, characterization and use in organic photocatalysis

“…21–23 However, despite the increasing amount of data on the photophysical and redox properties of such compounds, the development of reliable guidelines for designing complexes, which meet specific performance criteria remains a challenge. 24…”

“…[21][22][23] However, despite the increasing amount of data on the photophysical and redox properties of such compounds, the development of reliable guidelines for designing complexes, which meet specific performance criteria remains a challenge. 24 Our work is focused on the optimization of the Ru(II) photocatalyst efficiency. Although only a single polypyridyl ligand is required to acquire MLCT transitions in these complexes, 14 the replacement of one bpy ligand in the coordination shell of the ruthenium atom by another specific chelator can have a detrimental effect on the photophysical properties and reactivity.…”