To give a better insight into the phosphorescent properties of excimers one may choose quasi-degenerate perturbation theory (QDPT) to calculate the mixing between singlet and triplet TDDFT states (SOC-TDDFT). The archetypal complex 1 and its excited state dimer serve as an example in this work. Optimisation of the T1 state for the purpose of this calculation is undertaken similarly as above and for anti excited state dimers, but using a larger basis set def2-TZVP. The SOC matrix and TDDFT excited states are calculated at the B3LYP/def2-TZVP level using ZORA-corrected def2-TZVP basis sets (see details in the ESI). Similar approach has previously been used to obtain accurate predictions of phosphorescence radiative decay rates in platinum(II) complexes. Summary of predicted properties for mono-and bimolecular systems is presented in Table , while associated frontier molecular orbitals are shown in the ESI (Fig. .22). One may note that the two systems do not display significant variations in the properties of the T1 despite obvious differences in orbital character and especially the shape of the d orbital of the metal centre involved in the S0→T1 transition. <T1|HSO|S1> and zero-field splitting (ZFS) in the dimer are roughly half of their respective values in the single molecule, in line with earlier findings by the author for analogous mono-and dinuclear platinum(II) complexes. The average radiative rate, kr,av remains virtually identical between the two systems. Note that the computational value of kr,av for 1 is comparable to that obtained experimentally. Experimentally estimating the radiative decay rate of the excimer is a complex task and thus the figure is rarely presented in the literature. Excimers or aggregates however generally display shorter luminescence lifetimes than their respective monomeric Pt(II) complexes, suggesting that the photoluminescence quantum yield of the bimolecular state is lower than that of the complex itself. The reduction in <T1|HSO|S1> and ZFS in the dimer is another example, beyond the previously discussed cases of dinuclear complexes, indicating the need for further studies to understand impressive luminescent properties of some dinuclear complexes of platinum(II) or iridium(III) .