The Contributions of Molecular Vibrations and Higher Triplet Levels to the Intersystem Crossing Mechanism in Metal-Free Organic Emitters

Huang, Rongjuan; Avó, João; Northey, Thomas; Chaning-Pearce, E.; dos Santos, Paloma L.; Ward, Jonathan S.; Data, Przemyslaw; Etherington, Marc K.; Fox, Mark A.; Penfold, Thomas J.; Berberan-Santos, Mário N.; Lima, João C.; Bryce, Martin R.; Dias, Fernando B.

Abstract

Intense, simultaneous, room temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF) is observed in a series of donor-acceptor-donor (D–A–D) molecules. This dual-luminescence is stronger in the “angular” isomers, compared to their “linear” regioisomers, which is consistent with an enhanced intersystem crossing (ISC) in the former. Herein, we demonstrate that the small energy gap between the triplet levels, T1-Tn, below the lowest singlet state, S1, in the “angular” regioisomers, enhances the coupling between S1 and T1 states and favors ISC and reverse ISC (rISC). This is consistent with a spin-vibronic mechanism. In the absence of this “triplet ladder”, due to the larger energy difference between T1 and Tn in the “linear” regioisomers, the ISC and rISC are not efficient. Remarkably the enhancement on the ISC rate in the “angular” regioisomers is accompanied by an increase on the rate of internal conversion (IC). These results highlight the contributions of higher triplet excited states and molecular vibronic coupling to harvest triplet states in organic compounds, and casts the TADF and RTP mechanisms into a common conceptual framework.

Citation

Huang, R., Avó, J., Northey, T., Chaning-Pearce, E., dos Santos, P. L., Ward, J. S., …Dias, F. B. (2017). The Contributions of Molecular Vibrations and Higher Triplet Levels to the Intersystem Crossing Mechanism in Metal-Free Organic Emitters. Journal of Materials Chemistry C Materials for optical and electronic devices, 5(25), 6269-6280. https://doi.org/10.1039/c7tc01958k

• Journal Article Type: Article
• Acceptance Date: May 23, 2017
• Online Publication Date: May 23, 2017
• Publication Date: Jul 7, 2017
• Deposit Date: May 8, 2017
• Publicly Available Date: May 30, 2017
• Journal: Journal of Materials Chemistry C Materials for optical and electronic devices
• Print ISSN: 2050-7526
• Electronic ISSN: 2050-7534
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 5
• Issue: 25
• Pages: 6269-6280
• DOI: https://doi.org/10.1039/c7tc01958k
• Public URL: https://durham-repository.worktribe.com/output/1388065