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

Abstract

Abstract The characteristic of inverted singlet-triplet excited states (iST), in which the lowest singlet excited state (S1) is lower than the lowest triplet state (T1) in energy, was observed in a dialkylamine-substituted pentaazaphenalene derivative, 5AP-N(C12)2. The transient PL measurements showed that the reverse intersystem crossing has virtually zero activation energy, whereas the intersystem crossing proceeded by a thermal activation process. T1 was located energetically above S1 with the negative energy gap between S1 and T1 (ΔE ST) of −37 meV. Fluorescence and phosphorescence spectra also confirmed the negative ΔE ST of −46 – −32 meV.