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High-level quantum chemical calculations, presented in this Letter, show that the unusual luminescence properties of the high-performance thermally activated delayed fluorescence emitter CMA1, observed recently by Di et al. Science 2017, 356, 159-163 , can be explained without resorting to the rotationally assisted spin-state inversion mechanism proposed by these authors. Multiconfiguration and relativistic effects lead to fast and efficient thermal equilibration of the excited singlet and triplet populations of this linear gold complex even for coplanar orientations of the ligands. The calculations predict S1 ⇝ T1 intersystem crossing outcompetes the submicrosecond fluorescence by 2 orders of magnitude, thus quenching prompt fluorescence. The significant time- and environment-dependent shifts of the CMA1 luminescence, observed in experiment, are attributed to effects of (hindered) solvent reorganization.
Föller et al. (Tue,) studied this question.