Investigating Photophysical and Photochemical Structure-Activity Relationships of Heteroleptic Ir(III) Transition Metal Complexes Using High-throughput and Automated Techniques

Photocatalysis centers on the transformation of photon energy to chemical energy. These reactions are frequently driven by a photocatalyst, which is solely responsible for absorbing a particular wavelength of light and transferring that energy to selected substrates. The capacity of a photocatalyst to transform a molar amount of photons (1 mole of photons = 1 einstein) to a molar amount of photoproduct is governed by the molecular structure of the photocatalyst, which in the case of transition metal photocatalysts is strongly dependent on ligand molecular structure. It is therefore of theoretical interest to quantitatively elucidate correlations between features of ligand molecular structure and the resultant photocatalyst activity. Herein, high-throughput synthetic and experimental procedures as well as automated data analysis algorithms are invoked to investigate structure-function trends among heteroleptic [Ir(C^N)2(N^N)]+ transition metal complexes (where C^N represents an ortho-metalated cyclometalating ligand and N^N is a diimine ancillary ligand). Datasets pertaining to the photophysical properties and photochemical activity of 1,440 screened Ir(III) complexes (generated from 60 C^N ligands and 24 N^N ligands) were obtained through measurement of the UV-visible absorption spectra, deaerated excited state lifetime and emission spectra, and online kinetic data collection for the photodeposition of Sn(0) and Zn(0) from their respective cationic salts in solution. Automated analysis of this raw data, as well as the derivation of a photokinetic rate law, elucidated the sought-after ligand-based structure-function trends. This was achievable through the use of high-throughput experimental techniques capable of measuring features pertinent to each step in a photochemical cycle (i.e., absorption, intersystem crossing/internal conversion, and all associated electron transfer events). The findings from this unprecedentedly large dataset provided judicious guidelines to uncover ligands capable of making heteroleptic Rh(III) complexes of the same ligand stoichiometry possessing the photophysical and photochemical properties required for successful photocatalytic activity in water reduction systems, representing the first report of mononuclear Rh(III) photocatalytic activity. Finally, the ligand based photophysical trends provided engineering requirements needed to synthesize a novel heteroleptic Ir(III) guest in light-emitting electrochemical cells (LEECs) coupled to a perovskite host. These materials exhibited the highest reported efficiencies in the literature for this class of light emitting devices.