Down-conversion-induced delayed fluorescence via an inverted singlet-triplet channel

Inverted singlet-triplet molecules possessing a down-conversion process from the lowest excited triplet state (T1) to the lowest excited singlet state (S1) have recently roused much attention on account of potential merits in organic optoelectronics and photocatalysis. Nevertheless, investigations on inverted singlet-triplet molecules predominantly depend on the sophisticated ab initio calculations, while experimental attempts are extremely rare. In this work, we experimentally demonstrate the inverted singlet-triplet character of a heptazine derivative (HAP-3MF) which exhibits negative S1-T1 energy gaps (ΔEST) of −0.22 eV in toluene and −0.19 eV in acetonitrile based on meticulous photophysical investigations. To the best of our knowledge, this is the first time to directly obtain negative ΔEST based on fluorescence and phosphorescence spectra. Moreover, to better evaluate and reveal the photophysical features of extremely weak delayed emission in HAP-3MF, a mixed solution system of HAP-3MF:1,3-di(9H-carbazol-9-yl)benzene (mCP) in toluene at various molar ratios was subtly designed, and the energy transition mechanism involving a down-converted triplet-to-singlet channel was carefully elucidated. [Display omitted] •The inverted singlet-triplet character of a heptazine derivative (HAP-3MF) was experimentally demonstrated.•Negative S1-T1 energy gaps (ΔEST) of −0.22 eV in toluene and −0.19 eV in acetonitrile for HAP-3MF were obtained.•Photophysical features of extremely weak delayed emission in HAP-3MF was evaluated and revealed.•The energy transition mechanism involving a down-converted triplet-to-singlet channel was elucidated.