Role of electronic localization in the phosphorescence of iridium sensitizing dyes

In this talk we present a recent systematic study\footnote{B. Himmetoglu, A. Marchenko, I. Dabo and Matteo Cococcioni, J. Chem. Phys. {\bf 137}, 154309 (2012)} of three representative iridium dyes, namely, Ir(ppy)$_3$, FIrpic and PQIr, which are commonly used as sensitizers in organic optoelectronic devices. We show that electronic correlations play a crucial role in determining the excited state energies in these systems, due to localization of electrons on Ir $d$ orbitals in the ground state. Electronic localization is treated by employing hybrid functionals within time-dependent density functional theory (TDDFT) and with Hubbard model based corrections within the $\Delta$-SCF approach. The performance of both methods are studied in a comparative fashion and shown to be in good agreement with experiments (within a few tenths of an electron-volt in predicting singlet-triplet splittings and optical resonances). The Hubbard corrected functionals provide further insights on the charge-transfer character of the excited states. The gained insight allows us to comment on envisioned functionalization strategies to improve the performance of these systems.