Environmental Control of Triplet Emission in Donor-Bridge-Acceptor Organometallics

TL;DR

This study demonstrates a universal method to tune triplet emission energy in carbene-metal-amides OLED emitters through environmental control, enabling significant spectral shifts without chemical modifications, supported by experimental and computational analysis.

Contribution

Introduces a novel environmental approach to tune charge-transfer emission in CMAs, revealing mechanisms and effects on triplet diffusion and emission properties.

Findings

Achieved up to 210 meV blue shift in emission

Identified electrostatic interactions as key to emission tuning

Monte-Carlo simulations accurately model triplet diffusion

Abstract

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.