Issue 22, 2024

Sonocrystallization induced thermally activated delayed fluorescence via distortion of molecular geometry

Abstract

In this work, we present three donor–acceptor thermally activated delayed fluorescence (TADF) molecules comprising a 2,3,5,6-tetrafluorobenzonitrile acceptor with various electron donor units: phenoxazine (Phx-BzN), phenothiazine (Pht-BzN), and carbazole (Cz-BzN). These molecules have been studied using steady-state and time-resolved photophysical techniques in solution, film in polymer host and in crystal state. While Cz-BzN displays TADF in solution and polymethyl methacrylate (PMMA) films, Phx-BzN and Pht-BzN are non-emissive in solution and somewhat emissive in polymer films. More interestingly, while Pht-BzN remains virtually non-emissive in all studied solvents, it exhibits strong photoluminescence and TADF in crystal state, attributed to Crystallization Induced Emission (CIE). Interestingly, by applying ultrasound to an amorphous Pht-BzN water suspension we were also able to induce a reorganization of the molecules obtaining a microcrystal suspension. This can represents a new strategy to develop luminescent organic microcrystalline materials. We demonstrate through computational studies that the CIE properties arise due to intermolecular interactions in the crystal structure that result in locking the ground state molecular geometry and blocking relaxation in the excited state. As a result, the oscillator strength in the crystal form is enhanced leading to a highly luminescent behaviour, while in solution it equals nearly zero due to the molecule adopting a perfectly orthogonal D–A orientation in the excited state.

Graphical abstract: Sonocrystallization induced thermally activated delayed fluorescence via distortion of molecular geometry

Supplementary files

Article information

Article type
Paper
Submitted
17 Jan 2024
Accepted
05 May 2024
First published
20 May 2024
This article is Open Access
Creative Commons BY-NC license

J. Mater. Chem. C, 2024,12, 7943-7955

Sonocrystallization induced thermally activated delayed fluorescence via distortion of molecular geometry

A. Maggiore, Y. Qu, P. Pander, F. B. Dias, G. Clavier, R. Guillot, D. Altamura, C. Giannini, V. Maiorano, P. Audebert and F. Miomandre, J. Mater. Chem. C, 2024, 12, 7943 DOI: 10.1039/D4TC00245H

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