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

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.