Ultrafast Structure and Dynamics in the Thermally Activated Delayed Fluorescence of a Carbene-Metal-Amide.

Thermally activated delayed fluorescence has enormous potential for the development of efficient light emitting diodes. A recently discovered class of molecules (the carbene-metal-amides, CMAs) are exceptionally promising as they combine the small singlet-triplet energy gap required for thermal activation with a large transition moment for emission. Calculations suggest excited state structural dynamics modulate the critical coupling between singlet and triplet, but they disagree on the nature of those dynamics. Here we report ultrafast time-resolved transient absorption and Raman studies of CMA photodynamics. The measurements reveal complex structural evolution following intersystem crossing on the tens to hundreds of picoseconds time scale, and a change in the low-frequency vibrational spectrum between singlet and triplet states. The latter is assigned to changes in Raman active modes localized on the metal center.