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N-heterocyclic carbene (NHC) iridium (III) complexes are considered aspromising candidates for blue emitters in organic light-emitting diodes. Theycan play the roles of the emitter as well as of electron and hole transportersin the same emission layer. We investigate optical transitions in suchcomplexes with account of geometry and electronic structure changes uponexcitation or charging and exciton transfer between the complexes from firstprinciples. It is shown that excitation of NHC iridium complexes is accompaniedby a large reorganization energy $\sim$0.7 eV and a significant loss in theoscillator strength, which should lead to low exciton diffusion. Calculationswith account of spin-orbit coupling reveal a small singlet-triplet splitting$\sim$0.1 eV, whereas the oscillator strength for triplet excitations is foundto be an order of magnitude smaller than for the singlet ones. Thecontributions of the F\"orster and Dexter mechanisms are analyzed via theexplicit integration of transition densities. It is shown that for typicaldistances between emitter complexes in the emission layer, the contribution ofthe Dexter mechanism should be negligible compared to the F\"orster mechanism.On the other hand, the ideal dipole approximation, although gives the correctorder of the exciton coupling, fails to reproduce the result taking intoaccount spatial distribution of the transition density. For charged NHCcomplexes, we find a number of optical transitions close to the emission peakof the blue emitter with high exciton transfer rates that can be responsiblefor exciton-polaron quenching. The nature of these transitions is analyzed.

Optical properties and exciton transfer between N-heterocyclic carbene  iridium (III) complexes for blue light-emitting diode applications from first  principles