Computational approaches to design narrow TADF emitter Shawana Ahmad, Julien Eng, Thomas J. Penfold Newcastle University, UK Organic light-emitting diodes (OLEDs) with high resolution and energy efficiency require luminescent organic materials with narrowband emission, but this is a highly challenging task. Thermally activated delayed fluorescence (TADF) materials hold great potential and allow the design of a highly efficient narrow emitter 1 . Achieving colour purity and high efficiency at the same time is an important challenge in developing OLEDs. Herein, we explore two approaches to design efficient OLED molecules. In the first approach, the multiple resonance effect of atoms is used, which can induce significant localisation of the HOMO and LUMO on different atoms. This localisation reduces the singlet-triplet gap, promoting rISC and minimising their bonding/antibonding character. As a result, the vibronic coupling and vibrational relaxation in the material minimise giving a narrow emission 2 . The second approach offers a new way to improve donor-acceptor D-A molecular design, leading to highly efficient narrow emitters. Here, we are using the non-covalent interactions, i.e., van der Waals interactions, to control the Donor-Acceptor connecting bond of a molecule. These weak interactions allow the conformational control enabling to twist and hold the connecting bond between D-A units toward a more rigid TADF structure. In addition, we establish the key factors determining the emission full-width at half-maximum (FWHM) of 27 organic functional molecules exhibiting emitting states of different characteristics, including π–π*, charge transfer, and multiple-resonance 3 . We demonstrate that the emission FWHM can be interpreted within the displaced harmonic oscillator model (DHO). As a results, predictions can be made using ground state frequency and excited state gradient calculations only. This eliminates the need for time-consuming calculations of excited state geometries and Hessians. While formally only valid within the Condon approximation, the DHO model provides reasonable correlations for spectra exhibiting significant Herzberg–Teller effects and not only makes it possible to predict emission FWHM, but also informs on the normal modes responsible for emission band broadening. In addition, quantum chemistry and rate calculations of three multiple-resonance type thermally activated delayed fluorescence (TADF) emitters demonstrate how the importance of direct intersystem crossing (ISC) can be increased when sulphur is used within the B–N framework. Overall, this work offers new perspectives for incorporating considerations of emission FWHM into the rational molecular design and high-throughput screening procedures aiming to develop high-efficiency luminescent organic materials. References 1. Chem. Commun., 2018, 54, 3926-3935Nat. Photonics, 2019, 13, 678–682J. Mater. Chem. C, 2022,10, 4785-4794
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