Influence of Heavy Atom Effect on the Photophysics of Coinage Metal Carbene‐Metal‐Amide Emitters

Jiale Feng,Antti‐Pekka M Reponen,Alexander S Romanov,Neil C Greenham,Mikko Linnolahti,Manfred Bochmann,Dan Credgington,Thomas Penfold

doi:10.1002/adfm.202005438

Abstract

AbstractThe effect of the heavy metal atom on the photophysics of carbene‐metal‐amide (CMA) photoemitters is explored, where the metal bridge is either Au, Ag, or Cu. Spectroscopic investigations reveal the coupling mechanism responsible for communication between the singlet and triplet manifolds. The photophysical properties do not reflect expected trends based upon the heavy atom effect, as both direct coupling between charge‐transfer states and spin‐vibronic coupling via a ligand‐centered triplet state are present. Direct coupling is weakest for CMA(Ag), increasing the importance of the spin‐vibronic pathway and rendering its properties more sensitive to inter‐state energy gaps than for the Au and Cu‐bridged analogues. The measured activation energy correlates with the expected exchange energy of the charge‐transfer state, which is also closely related to the length of the bonds joining the carbene and amide ligands, and decreases in the order CMA(Cu) > CMA(Au) > CMA(Ag). These findings reveal that reducing interference between charge‐transfer and ligand‐centers excited, and minimizing exchange energy, are required for developing efficient luminescent CMA complexes.

Highlights

We have previously shown that direct spin-orbit coupling (SOC) between singlet and triplet charge-transfer (CT) states is implicated in Carbene-metal-amides (CMAs) are a promising family of orga- gold-bridged coinage metal variants,[8] in contrast with many nometallic donor-bridge-acceptor photoemitters with potential prevailing models for SOC in organic thermally-activated delayed-fluorescence (TADF) archetypes.[9,10,11,12]
We find that the strength of direct SOC between 1CT-3CT reduces in the order CMA(Cu) > CMA(Au) > CMA(Ag) as a result of both decreased d-orbital contribution to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and the decreased heavy atom effect
We have previously shown that strong electrostatic interactions between the CMA emitters and polar host molecules are able to increase CT state energies relative to the triplets localized to the carbazole donor 3LE(Cz).[8]

Summary

Introduction

We have previously shown that direct SOC between singlet and triplet charge-transfer (CT) states is implicated in Carbene-metal-amides (CMAs) are a promising family of orga- gold-bridged coinage metal variants,[8] in contrast with many nometallic donor-bridge-acceptor photoemitters with potential prevailing models for SOC in organic thermally-activated delayed-fluorescence (TADF) archetypes.[9,10,11,12] this. Credgington Cavendish Laboratory Department of Physics might be expected to change as the mass of the heavy bridging atom is reduced. We explore the influence of the heavy atom effect on the emission kinetics of CMAs

Results

Discussion

Conclusion