Sensitivity of the photo-physical properties of organometallic complexes to small chemical changes

TL;DR

This study models how small chemical modifications in organometallic complexes significantly influence their photophysical properties, especially the triplet state lifetime and charge transfer characteristics, relevant for optoelectronic applications.

Contribution

It introduces an effective Hamiltonian model highlighting the impact of small chemical changes on the electronic and photophysical properties of organometallic complexes.

Findings

Small variations in $ ext{ε}^*/J$ cause large changes in triplet state properties.

The triplet state's MLCT character depends strongly on the ratio $ ext{ε}^*/J$.

Minor ligand modifications can significantly alter complex behavior.

Abstract

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 (MLCT) 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 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.