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 (Delta E-ST) of -37 meV. Fluorescence and phosphorescence spectra also confirmed the negative Delta E-ST of -46 to -32 meV. (c) 2024 The Author(s). Published on behalf ofThe Japan Society of Applied Physics by IOP Publishing Ltd