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The influence of molecular geometry on the efficiency of thermally activated delayed fluorescence

Lookup NU author(s): Jamie Gibson, Dr Beth Laidlaw, Professor Thomas Penfold

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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).

Abstract

In this work we successfully developed a strategy for positively influencing the conformation of thermally activated delayed fluorescence (TADF) molecules containing phenothiazine as the electron donor (D) unit, and dibenzothiophene-S,S-dioxide as the acceptor (A), linked in D–A and D–A–D structures. In this strategy the effect of restricted molecular geometry is explored to maximize TADF emission. The presence of bulky substituents in different positions on the donor unit forces the molecules to adopt an axial conformer where the singlet charge transfer state is shifted to higher energy, resulting in the oscillator strength and luminescence efficiency decreasing. With bulky substituents on the acceptor unit, the molecules adopt an equatorial geometry, where the donor and acceptor units are locked in relative near-orthogonal geometry. In this case the individual signatures of the donor and acceptor units are evident in the absorption spectra, demonstrating that the substituent in the acceptor uncouples the electronic linkage between the donor and acceptor more effectively than with donor substitution. In contrast with the axial conformers that show very weak TADF, even with a small singlet triplet gap, molecules with equatorial geometry show stronger oscillator strength and luminescence efficiency and are excellent TADF emitters. Acceptor-substituted molecules 6 and 7 in particular show extremely high TADF efficiency in solution and solid film, even with a singlet–triplet energy gap around 0.2 eV. This extensive study provides important criteria for the design of novel TADF and room temperature phosphorescence (RTP) emitters with optimized geometry.

Publication metadata

Author(s): Nobuyasu RS, Ward JS, Gibson J, Laidlaw BA, Ren Z, Data P, Batsanov AS, Penfold TJ, Bryce MA, Dias FB

Publication type: Article

Publication status: Published

Journal: Journal of Materials Chemistry C

Year: 2019

Volume: 7

Issue: 22

Pages: 6672-6684

Online publication date: 29/05/2019

Acceptance date: 03/05/2019

Date deposited: 30/05/2019

ISSN (print): 2050-7526

ISSN (electronic): 2050-7534

Publisher: Royal Society of Chemistry

URL: https://doi.org/10.1039/C9TC00720B

DOI: 10.1039/C9TC00720B

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