2. What is the non-radiative path from the emitting state? [i.e. what is the relevant vibrational co-ordinate?]
5. Is there a metal-centred (MC) excited state that is relevant to non-radiative decay?
[See also a Feature Article, Anthony Jacko, Ben Powell, and I wrote recently].
I just found a nice JACS paper, which comprises a combined theoretical and experimental study to help answer these questions. The abstract states:
The kinetic parameters for decay and results from density functional theory (DFT) calculations of the triplet state are consistent with a nonradiative process involving Ir−N (Ir−C for fac-Ir(pmb)3) bond rupture leading to a five-coordinate species that has triplet metal-centered (3MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and 3MC states.The paper has several helpful figures such as the one below.