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

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

Phosphorescent carbene-gold-arylacetylide materials as emitters for near UV-OLEDs

A series of carbene-gold-acetylide complexes [(BiCAAC)AuCC]nC6H5-n (n = 1, Au1; n=2, Au2; n=3, Au3; BiCAAC = bicyclic(alkyl)(amino)carbene) have been synthesized in high yields. All complexes show excellent thermal stability up to 342℃. Compounds Au1–Au3 exhibit deep-blue to blue-green phosphorescence with good quantum yields up to 43% in all media. An increase of the (BiCAAC)Au moieties in gold complexes Au1–Au3 increases the extinction coefficients in the UV-vis spectra and stronger oscillator strength coefficients supported by theoretical calculations. The luminescence radiative rates decrease with an increase of the (BiCAAC)Au moieties. The time-dependent density functional theory (TD-DFT) study supports a charge-transfer nature of the phosphorescence due to the large (0.5–0.6 eV) energy gap between singlet excited (S1) and triplet excited (T1) states. Transient luminescence study reveals the presence of both non-structured UV prompt-fluorescence and vibronically resolved long-lived phosphorescence 428 nm. Organic light-emitting diodes (OLED) have been fabricated by physical vapour deposition with 2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) as a host material with complex Au1. The near-UV electroluminescence is observed at 405 nm with device efficiency of 1% while demonstrating OLED device lifetime LT50 up to 20 min at practical brightness of 10 nits, indicating a highly promising class of materials to develop stable UV-OLEDs

Research data supporting "Influence of Heavy Atom Effect on the Photophysics of Coinage Metal Carbene-Metal-Amide Emitters"

This data supports the research work "Influence of Heavy Atom Effect on the Photophysics of Coinage Metal Carbene-Metal-Amide Emitters". The data was collected by using steady-state and transient optical spectroscopy techniques combined with quantum-chemistry calculations

Understanding Spin‐Triplet Excited States in Carbene‐Metal‐Amides

Publication status: PublishedCarbene‐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.</jats:p

Understanding Spin‐Triplet Excited States in Carbene‐Metal‐Amides

Publication status: PublishedCarbene‐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

Deep-Blue and Fast Delayed Fluorescence from Carbene-Metal-Amides for Highly Efficient and Stable Organic Light-Emitting Diodes.

Publication status: PublishedLinear gold complexes of the "carbene-metal-amide" (CMA) type are prepared with a rigid benzoguanidine amide donor and various carbene ligands. These complexes emit in the deep-blue range at 424 and 466 nm with 100% quantum yields in all media. The deep-blue thermally activated delayed fluorescence (TADF) originates from a charge transfer state (CT) with an excited state lifetime as low as 213 ns, resulting in fast radiative rates of 4.7 × 106 s-1. The high thermal and photo-stability of these CMA materials enabled us to fabricate highly energy efficient organic light-emitting diodes (OLED) in host-guest architectures. We report deep-blue OLED devices with electroluminescence at 416 nm and 457 nm with practical external quantum efficiencies of up to 23% at 100 cd m-2 with excellent colour coordinates CIE (x; y) = 0.16; 0.07 and 0.17; 0.18. The operating stability of these OLEDs is the longest reported to date (LT50 = 1 hour) for deep-blue CMA emitters, indicating a high promise for further development of blue OLED devices. Our findings inform the molecular design strategy and correlation between delayed luminescence with high radiative rates and CMA OLED device operating stability. This article is protected by copyright. All rights reserved

Excited-State Lifetime Modulation by Twisted and Tilted Molecular Design in Carbene-Metal-Amide Photoemitters.

Funder: Samsung Display Corp.Carbene-metal-amides (CMAs) are an emerging class of photoemitters based on a linear donor-linker-acceptor arrangement. They exhibit high flexibility about the carbene-metal and metal-amide bonds, leading to a conformational freedom which has a strong influence on their photophysical properties. Herein we report CMA complexes with (1) nearly coplanar, (2) twisted, (3) tilted, and (4) tilt-twisted orientations between donor and acceptor ligands and illustrate the influence of preferred ground-state conformations on both the luminescence quantum yields and excited-state lifetimes. The performance is found to be optimum for structures with partially twisted and/or tilted conformations, resulting in radiative rates exceeding 1 × 106 s-1. Although the metal atoms make only small contributions to HOMOs and LUMOs, they provide sufficient spin-orbit coupling between the low-lying excited states to reduce the excited-state lifetimes down to 500 ns. At the same time, high photoluminescence quantum yields are maintained for a strongly tilted emitter in a host matrix. Proof-of-concept organic light-emitting diodes (OLEDs) based on these new emitter designs were fabricated, with a maximum external quantum efficiency (EQE) of 19.1% with low device roll-off efficiency. Transient electroluminescence studies indicate that molecular design concepts for new CMA emitters can be successfully translated into the OLED device