A Readily Obtained Alternative to 1H-Benzo[f]indole toward Room-Temperature Ultralong Organic Phosphorescence

1H-Benzo[f]indole (Bd) is a significant phosphorescence unit in the field of room-temperature ultralong organic phosphorescence (RTUOP). However, the synthesis of Bd is rather hard and has a low yield, which greatly limits the wide applications of RTUOP. Therefore, exploring readily obtained alternatives to Bd is of great significance and demands to be addressed although it is full of challenges. Herein, we report a new phosphorescence unit named 5H-benzo[b]carbazole (BCz), which can function similarly as Bd in RTUOP. BCz can be obtained facilely via two steps of reactions, while the synthesis of Bd requires seven steps of tedious reactions. Excitingly, readily obtained BCz is an excellent alternative to Bd in RTUOP in the following aspects and shows some advantages in comparison with Bd: First, BCz and its derivatives can exhibit distinctive redshifted red ultralong phosphorescence in the self-aggregated state at 77 K while Bd and its derivatives cannot. Second, BCz demonstrates remarkable photo-activated yellow ultralong phosphorescence at room temperature while the intrinsic phosphorescence of Bd is difficult to be activated at room temperature. Third, BCz derivatives (CNPyBCz and CNBrBCz) display similar photo-activated yellow ultralong phosphorescence as Bd derivatives at room temperature but their phosphorescent lifetimes are longer. Fourth, it is shown that BCz and its derivatives emit yellow RTUOP in powder matrixes as their carbazole counterparts do. It is revealed that BCz and Bd share the same cation-radical-involved phosphorescence mechanism featuring charge separation and charge recombination and the redshift of ultralong phosphorescence in the self-aggregated state arises from enhanced pi-pi interactions among BCz units. To the best of our knowledge, this study paves a simple way for future applications of RTUOP. Moreover, this work indicates that the cation-radical-involved mechanism may be universal in the field of RTUOP.