Sensitivity of the photophysical properties of organometallic complexes to small chemical changes

We investigate an effective model Hamiltonian for organometallic complexes that are widely used in optoelectronic devices. The two most important parameters in the model are J, the effective exchange interaction between the pi and pi* orbitals of the ligands, and epsilon*, the renormalized energy gap between the highest occupied orbitals on the metal and on the ligand. We find that the degree of metal-to-ligand charge transfer character of the lowest triplet state is strongly dependent on the ratio epsilon*/J. epsilon* is purely a property of the complex and can be changed significantly by even small variations in the complex's chemistry, such as replacing substituents on the ligands. We find that small changes in epsilon*/J can cause large changes in the properties of the complex, including the lifetime of the triplet state and the probability of injected charges (electrons and holes) forming triplet excitations. These results give some insight into the observed large changes in the photophysical properties of organometallic complexes caused by small changes in the ligands. (C) 2010 American Institute of Physics. [doi:10.1063/1.3480981]