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

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.