High-Throughput Screening and Automated Data-Driven Analysis of the Triplet Photophysical Properties of Structurally Diverse, Heteroleptic Iridium(III) Complexes

Abstract

Steady state emission spectra and excited state lifetimes were measured for 1440 distinct heteroleptic [Ir(C^N)₂(N^N)]⁺ complexes prepared via combinatorial parallelized synthesis; 72% of the complexes were found to be luminescent, and the emission maxima of the library spanned the visible spectrum (652–459 nm). Spectral profiles ranged from broad structureless bands to narrow emissions exhibiting vibrational substructure. Measured excited state lifetimes ranged between ∼0.1–14 μs. Automated emission spectral fitting with successive Gaussian functions revealed four distinct measured classes of excited states; in addition to well understood metal–ligand to ligand-charge transfer (³MLLCT) and ligand-centered (³LC) excited states, our classification also identified photophysical characteristics of less explored mixed ³MLLCT/³LC states. Electronic structure features obtained from DFT calculations performed on a large subset of these Ir(III) chromophores offered clear insights into the excited state properties and allowed the prediction of structure/luminescence relationships in this class of commonly used photocatalysts. Models with high prediction accuracy (R² = 0.89) for emission color were developed on the basis of experimental data. Furthermore, different degrees of nuclear reorganization in the excited state were shown to significantly impact emission energy and excited state lifetimes.