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Understanding Spin-Triplet Excited States in Carbene-Metal-Amides.

Accepted version
Peer-reviewed

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Authors

Reponen, Antti-Pekka M 
Londi, Giacomo 
Matthews, Campbell SB 
Olivier, Yoann 
Romanov, Alexander S 

Abstract

Carbene-metal-amides (CMAs) are emerging delayed fluorescence materials for organic light-emitting diode (OLED) applications. CMAs possess fast, efficient emission owing to rapid forward and reverse intersystem crossing (ISC) rates. The resulting dynamic equilibrium between singlet and triplet spin manifolds distinguishes CMAs from most purely organic thermally activated delayed fluorescence emitters. However, direct experimental triplet characterization in CMAs is underutilized, limiting our detailed understanding of the ISC mechanism. In this work, we combine time-resolved spectroscopy with tuning of state energies through environmental polarity and metal substitution, focusing on the interplay between charge-transfer (3CT) and local exciton (3LE) triplets. Unlike previous photophysical work, we investigate evaporated host:guest films of CMAs and small-molecule hosts for increased device relevance. Transient absorption reveals an evolution in the triplet excited-state absorption (ESA) consistent with a change in orbital character between hosts with differing dielectric constants. Using quantum chemical calculations, we simulate ESAs of the lowest triplet states, highlighting the contribution of only 3CT and donor-moiety 3LE states to spectral features, with no strong evidence for a low-lying acceptor-centered 3LE. Thus, our work provides a blueprint for understanding the role of triplet excited states in CMAs which will enable further intelligent optimization of this promising class of materials.

Description

Keywords

laser spectroscopy, density functional calculations, transition metals, time-resolved spectroscopy, luminescence

Journal Title

Angew Chem Int Ed Engl

Conference Name

Journal ISSN

1433-7851
1521-3773

Volume Title

Publisher

Wiley
Sponsorship
Isaac Newton Trust (22.08(i))
Leverhulme Trust (ECF-2022-445)