Modulation of red organic room-temperature phosphorescence in heavy atom-free phosphors

Red room temperature phosphorescence (RTP) was conventionally obtained by incorporating heavy metal ions or halogens. However, heavy metals are toxic and expensive while C-halogen bonds are usually prone to photobleaching. Meanwhile, heavy atoms often lead to much reduced RTP lifetimes, precluding their use in certain fields. Herein, a series of naphthalene diimides with various electron-donor substituents on the nitrogen atoms were used to achieve red RTP. It was found that a moderate intramolecular charge transfer (CT) state provides mediating intersystem crossing channels for promoting red RTP with quantum yields circa. 5%. However, much stronger donors can in fact inhibit all forms of luminescence emission including RTP via photo-induced electron transfer quenching at room temperature, in which case the red phosphorescence could be restored if the donor, such as an amine, is suppressed by protonation.