  Carbene-metal-amides (CMAs) are a promising family of donor-bridge-acceptor
molecular charge-transfer emitters for organic light-emitting diodes (OLEDs).
Here a universal approach is introduced to tune the energy of their
charge-transfer emission. A shift of up to 210 meV is achievable in the solid
state via dilution in a polar host matrix. The origin of this shift has two
components: constraint of thermally activated triplet diffusion, and
electrostatic interactions between the guest molecules and the polar host. This
allows the emission of mid-green CMA archetypes to be blue shifted without
chemical modifications. Monte-Carlo simulations based on a Marcus-type transfer
integral successfully reproduce the concentration- and temperature-dependent
triplet diffusion process, and reveal a substantial shift in the ensemble
density of states in polar hosts. In gold-bridged CMAs this substantial shift
does not lead to a significant change in luminescence lifetime, thermal
activation energy, reorganisation energy or intersystem crossing rate. These
discoveries thus offer new experimental and theoretical insight in to the
coupling between the singlet and triplet manifolds in these materials. Similar
emission tuning can be achieved in related materials where chemical
modification is used to modify the charge-transfer energy.
