Spectroscopic properties of cyclometallated iridium complexes by TDDFT

We provide a unified view of our recent work on TDDFT calculations on the class of Iridium(III) cyclometallated complexes, of relevance for their extensive use in OLED and LEEC devices. Our results, obtained for vastly different systems, allow us to trace some general conclusions concerning the modeling of these transition metal complexes. The effect of relativistic effects on both geometrical structures and emission properties is analyzed considering scalar relativistic corrections and quadratic response theory. Absorption spectra for these compounds can be satisfactorily reproduced by TDDFT calculations neglecting spin- orbit coupling, because of the strong intensity of singlet-singlet transitions which hinders the weaker singlet-triplet transitions. We also underline the importance of being able to simulate the emission spectra line-shapes, thus allowing for a direct comparison of calculated and experimental quantities and to estimate the emission color perceived by the human eye. Approximate treatments of spin-orbit coupling based on excited state analysis are in some case useful to rationalize experimental trends, but more rigorous and quantitative approaches are required for future applications. Finally, we stress the absolute relevance of solvation effects in the description of the excited states of this class of systems, with very large differences in the excited state properties calculated in vacuo and as a function of different solvents. These differences have profound consequences for the comprehension of the photophysical properties of this important class of systems and should always be considered in their effective modeling. On overall, our results indicate that electronic structure-driven tuning of the excited state properties of Iridium( III) cyclometallated complexes is possible, thus opening the way to a theoretical and computational strategy for the design of new phosphorescent compounds with specific target characteristics.