  N-heterocyclic carbene (NHC) iridium (III) complexes are considered as
promising candidates for blue emitters in organic light-emitting diodes. They
can play the roles of the emitter as well as of electron and hole transporters
in the same emission layer. We investigate optical transitions in such
complexes with account of geometry and electronic structure changes upon
excitation or charging and exciton transfer between the complexes from first
principles. It is shown that excitation of NHC iridium complexes is accompanied
by a large reorganization energy $\sim$0.7 eV and a significant loss in the
oscillator strength, which should lead to low exciton diffusion. Calculations
with account of spin-orbit coupling reveal a small singlet-triplet splitting
$\sim$0.1 eV, whereas the oscillator strength for triplet excitations is found
to be an order of magnitude smaller than for the singlet ones. The
contributions of the F\"orster and Dexter mechanisms are analyzed via the
explicit integration of transition densities. It is shown that for typical
distances between emitter complexes in the emission layer, the contribution of
the Dexter mechanism should be negligible compared to the F\"orster mechanism.
On the other hand, the ideal dipole approximation, although gives the correct
order of the exciton coupling, fails to reproduce the result taking into
account spatial distribution of the transition density. For charged NHC
complexes, we find a number of optical transitions close to the emission peak
of the blue emitter with high exciton transfer rates that can be responsible
for exciton-polaron quenching. The nature of these transitions is analyzed.
