An inverted singlet-triplet excited state in a pentaazaphenalene derivative (5AP-N(C12)₂)

The characteristic of inverted singlet-triplet excited states, in which the lowest singlet excited state (S 1 ) is lower than the lowest triplet state (T 1 ) in energy, was observed in a dialkylamine-substituted pentaazaphenalene derivative, 5AP-N(C12) 2 . The transient photoluminescence measurements showed that the reverse intersystem crossing has virtually zero activation energy, whereas the intersystem crossing is proceeded by a thermal activation process. T 1 was located energetically above S 1 with a negative energy gap between S 1 and T 1 (Δ E ST ) of −37 meV. Fluorescence and phosphorescence spectra also confirmed the negative Δ E ST of −46 to −32 meV.