A Readily Obtained Alternative of 1H-benzo[f]indole towards Room Temperature Ultralong Organic Phosphorescence

Abstract

1H-benzo[f]indole (Bd) is a significant unit in the field of room temperature ultralong organic phosphorescence (RTUOP). However, the synthesis of Bd is rather hard and of low yield, which greatly limits the wide applications of RTUOP. Therefore, exploring readily obtained alternatives of Bd is of great significance and demands to be addressed though it is full of challenges. Herein, we report a new unit named 5H-benzo[b]carbazole (BCz) which can function similarly with 1H-benzo[f]indole (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 of Bd in RTUOP and shows some advantages in comparison with Bd. Firstly, BCz and its derivatives exhibit distinctive red-shifted red ultralong phosphorescence at 77 K while Bd does not. Secondly, BCz demonstrates remarkable photo-activated yellow ultralong phosphorescence at room temperature while the phosphorescence of Bd is difficult to be activated at room temperature. Thirdly, BCz derivatives (CNPyBCz and CNBrBCz) display similar photo-activated yellow ultralong phosphorescence with Bd derivatives at room temperature but the phosphorescent lifetimes are longer. Fourthly, it is shown that BCz and its derivatives emit yellow RTUOP in the powder matrixes besides their carbazole counterparts. 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 phosphorescence in the aggregated state arises from the enhanced π-π interactions among BCz units. To our best knowledge, this study paves a simple way for the future applications of RTUOP. Moreover, this work indicates that cation radical-involved mechanism may be universal in the field of RTUOP.