Thermally activated delayed fluorescence: A critical assessment of environmental effects on the singlet-triplet energy gap

The effective design of dyes optimized for thermally activated delayed fluorescence (TADF) requires the precise control of two tiny energies: the singlet-triplet gap, which has to be maintained within thermal energy, and the strength of spin-orbit coupling. A subtle interplay among low-energy excited states having dominant charge-transfer and local character then governs TADF efficiency, making models for environmental effects both crucial and challenging. The main message of this paper is a warning to the community of chemists, physicists, and material scientists working in the field: the adiabatic approximation implicitly imposed to the treatment of fast environmental degrees of freedom in quantum-classical and continuum solvation models leads to uncontrolled results. Several approximation schemes were proposed to mitigate the issue, but we underline that the adiabatic approximation to fast solvation is inadequate and cannot be improved; rather, it must be abandoned in favor of an antiadiabatic approach.

Automated summary

This paper warns chemists, physicists, and material scientists about the necessity of precise control over the singlet-triplet gap and strength of spin-orbit coupling for optimized dyes for TADF. It emphasizes the crucial role of low-energy excited states with charge-transfer and local character in TADF efficiency, and calls for the abandonment of the adiabatic approximation in favor of an antiadiabatic approach for fast solvation in models.