Decoupling Triplet Population from Intersystem Crossing in TADF Materials

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Abstract
The characterization of thermally activated delayed fluorescence (TADF) relies on detecting delayed emission following triplet formation. However, triplet population under optical excitation is fundamentally limited by the intrinsic intersystem crossing (ISC) efficiency of the emitter and in direct competition with often desirable singlet radiative decay. Here, we implement a general optical strategy to directly populate the triplet states of TADF chromophores in solution via diffusional triplet energy transfer, circumventing reliance on intrinsic ISC. This approach can enhance delayed-fluorescence detectability by up to 3 orders of magnitude across structurally diverse emitters spanning the blue to near-infrared and disparate emission time scales, converting near-threshold experiments into routine measurements. Through decoupling triplet population from ISC, we reveal previously hidden intrinsic TADF in two state-of-the-art chromophores, leading to a striking conclusion: unimolecular TADF remains highly relevant within multiple high-performing material families where delayed fluorescence─and the resulting high device performance─has previously been attributed solely to intermolecular solid-state effects. Accordingly, through providing an approach to directly interrogate the intrinsic triplet state photophysics of TADF materials, triplet sensitization can enable the identification and understanding of previously inaccessible photophysical behavior and property combinations, providing a foundation for future advances in TADF materials research.
Funding Information
  • Royal Society (URFR1241806)
  • University of Cambridge
  • Engineering and Physical Sciences Research Council (EPSRC) (EP/S022953/1)
  • Engineering and Physical Sciences Research Council (EPSRC) (EP/Z531303/1)
  • Swedish Research Council (VR) (2021-03744)

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