Volume 250, 2024

Excited-state dynamics of C3-symmetric heptazine-based thermally activated delayed-fluorescence emitters

Abstract

Heptazine-based materials have recently emerged as a promising motif for thermally activated delayed fluorescence, as their near-zero or negative singlet–triplet energy gaps enable extremely fast reverse intersystem crossing (rISC) rates. Another method for achieving a high rate of rISC is through the use of highly symmetric emitters, which benefit from energy-level degeneracies and a high density of states. Here, we investigate the effect of combining these two design strategies on the excited-state dynamics of C3-symmetric emitters containing heptazine cores. We find that in two of the four emitters studied, the S1 state has a high degree of locally excited (LE) character with density on the heptazine moiety, preventing excited-state localization and a loss of symmetry in the energy-minimized S1 geometry. Surprisingly, these symmetric molecules still suffer from a loss of density of triplet states below the S1 state. Overall, we find that maintaining C3 symmetry will not necessarily maintain density of states, but that heptazine-based materials with LE S1 states still benefit from maximized rISC rates via increased spin–orbit coupling with low-lying charge-transfer triplet states and exhibit advantageous photophysical properties, such as near-unity photoluminescence quantum yields and high colour purity.

Graphical abstract: Excited-state dynamics of C3-symmetric heptazine-based thermally activated delayed-fluorescence emitters

Associated articles

Supplementary files

Article information

Article type
Paper
Submitted
13 juin 2023
Accepted
29 juin 2023
First published
17 nov. 2023

Faraday Discuss., 2024,250, 181-191

Excited-state dynamics of C3-symmetric heptazine-based thermally activated delayed-fluorescence emitters

K. Bergmann and Z. M. Hudson, Faraday Discuss., 2024, 250, 181 DOI: 10.1039/D3FD00121K

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