Deciphering intermediate excited states in spin-flip transition in carbonyl-nitrogen multi-resonance molecule

Spin-flip process involving multiple excited states plays a key role for the function and application of many organic molecules. However, it remains extremely challenging to experimentally identify and investigate intermediate states in spin-flip transition due to its rapid inactivation via various electronic transitions. Herein, we report the deciphering of intermediate states involved in the spin-flip transition in a tailor-made carbonyl-nitrogen multi-resonance molecule (anti-DIQAO) designed with high symmetry. The transitions among different electronic states of anti-DIQAO are slowed by high molecular symmetry and distinguished using steady-state and transient spectroscopies. The second excited triplet (T₂) state is identified as the intermediate state in the spin-flip transition, whose transition to higher energy levels and phosphorescence radiation are recorded. It is demonstrated that, in reverse intersystem crossing, intramolecular vibration drives the rate-dominant reverse internal conversion from the lowest excited triplet (T₁) state to the T₂ state, and thermal activation triggers transition from mixed T₂ and T₁ states to the lowest excited singlet (S₁) state. Additionally, anti-DIQAO exhibits narrow-bandwidth electroluminescence with an outstanding external quantum efficiency of up to 32.6%. This research provides an effective molecular design to tune the populations of excited states, which is of high significance for exploring efficient luminescent materials.