Thermally Activated Delayed Fluorescence from Locally Excited State: Theoretical Prediction and Experimental Validation

<div>As is well known, the thermally activated delayed fluorescence (TADF) is always generated from charge-transfer (CT) excited states in electron-donor (D) – electron-acceptor (A) systems. Here, a novel design strategy is proposed for realizing TADF from a locally excited (LE) state through controlling the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes between the LE singlet and higher triplet CT states. Based on the strategy, a boron difluoride derivative is theoretically predicted to emit TADF from the LE state, whose radiative decay rate constant is much larger kr (S₁ →S₀ )=1.12 * 10⁸ s ^-1 , two orders of magnitude larger than those of common TADF systems. And its lifetimes of the prompt and delayed fluorescence are experimentally validated to be 0.44 ns and 0.7 μs, respectively. This work is a breakthrough in the understanding of TADF and opens a new avenue for extending the TADF materials.</div>

Thermally Activated Delayed Fluorescence from Locally Excited State: Theoretical Prediction and Experimental Validation

<div>As is well known, the thermally activated delayed fluorescence (TADF) is always generated from charge-transfer (CT) excited states in electron-donor (D) – electron-acceptor (A) systems. Here, a novel design strategy is proposed for realizing TADF from a locally excited (LE) state through controlling the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes between the LE singlet and higher triplet CT states. Based on the strategy, a boron difluoride derivative is theoretically predicted to emit TADF from the LE state, whose radiative decay rate constant is much larger kr (S₁ →S₀ )=1.12 * 10⁸ s ^-1 , two orders of magnitude larger than those of common TADF systems. And its lifetimes of the prompt and delayed fluorescence are experimentally validated to be 0.44 ns and 0.7 μs, respectively. This work is a breakthrough in the understanding of TADF and opens a new avenue for extending the TADF materials.</div>

Electron Transfer in a Naphthalene Diimide System Studied by Single-Molecule Delayed Fluorescence

Electron transfer (ET) is a key chemical reaction in nature and has been extensively studied in bulk systems, but remains challenging to investigate at the single-molecule level. A previously reported naphthalene diimide (NDI)-based system (Higginbotham et al., Chem. Commun. 2013, 49, 5061–5063) displays delayed fluorescence with good quantum yield (~0.5) and long-lived (nanoseconds) prompt and delayed fluorescence lifetimes, providing an opportunity to interrogate the underlying ET processes in single molecules. Time-resolved single-molecule fluorescence measurements enabled forward and reverse ET rate constants to be calculated for 45 individual molecules embedded in poly(methylmethacrylate) (PMMA) film. Interpretation of the results within the framework of Marcus–Hush theory for ET demonstrates that variation in both the electronic coupling and the driving force for ET is occurring from molecule to molecule within the PMMA film and over time for individual molecules.