Ultralong organic room-temperature phosphorescence modulated by balancing intramolecular charge transfer and localized excitation character of excited states

Long emission lifetime is the defining character of organic room-temperature phosphorescence (RTP) and afterglow materials. In the absence of heavy atom effect, organic localized excitation (LE) systems show small phosphorescence rates and exhibit potential to form long-lived phosphorescence materials. However, such LE systems usually have limited population of T1 states because of large singlet-triplet splitting energy (ΔEST, around 1 eV). Here we report ultralong RTP by balancing intramolecular charge transfer and localized excitation character of excited states. Aromatic functional groups of moderate electron-donating strength are selected to build difluoroboron β-diketonate (BF2bdk) systems with moderate intramolecular charge transfer (ICT) character in their S1 states. Such molecular design gives rise to ΔEST around 0.5 eV and is enough to populate BF2bdk's T1 states. Significantly, the resultant BF2bdk's T1 states exhibit 85–90% 3LE character and display long phosphorescence lifetimes up to 1.38 s upon doping under ambient conditions. This study shows that simple design of organic system can also lead to high-performance RTP materials and provide an insight into the fundamental photophysics of organic triplet system.