Organic light-emitting diodes using a neutral π radical as emitter: The emission from a doublet

Abstract Triplet harvesting is a main challenge in organic light-emitting devices (OLEDs), because the radiative decay of the triplet is spin-forbidden. Here, we propose a new kind of OLED, in which an organic open-shell molecule, (4-N-carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl)methyl (TTM-1Cz) radical, is used as an emitter, to circumvent the transition problem of triplet. For TTM-1Cz, there is only one unpaired electron in the highest singly occupied molecular orbital (SOMO). When this electron is excited to the lowest singly unoccupied molecular orbital (SUMO), the SOMO is empty. Thus, transition back of the excited electron to the SOMO is totally spin-allowed. Spectral analysis showed that electroluminescence of the OLED originated from the electron transition between SUMO and SOMO. The magneto-electroluminescence measurements revealed that the spin configuration of the excited state of TTM-1Cz is a doublet. Our results pave a new way to obtain 100 % internal quantum efficiency of OLEDs. Organic electronics: An organic open-shell molecule (TTM-1Cz) was used as an emitter in organic light-emitting diodes (OLEDs). There is only one electron at the highest occupied molecular orbital (see picture), leading the excited state of TTM-1Cz to be a doublet. The key issue of how to harvest the triplet energy in an OLED is thus circumvented because the radiative decay of the doublet is totally spin-allowed.