We for the first time report a novel class of organic units (N-1 and N-2) and their derivatives (PNNA-1 and PNNA-2) with excellent property of ultralong organic room temperature phosphorescence (UORTP). In this work, N-1, N-2 and their derivatives function as the guests while organic powders (PNCz, BBP, DBT) and polymethyl methacrylate (PMMA) serve as the host matrixes. Amazingly, the color of ultralong phosphorescence can be tuned in different states or by varying the host matrixes. At 77 K, all the four molecules show green afterglow in the monomer state but yellow afterglow in the aggregated state because strong intermolecular interactions exist in the self-aggregate and induce a redshift of the afterglow. In particular, PNNA-1 and PNNA-2 demonstrate distinctive photo-activated green UORTP in the PMMA film owing to the generation of their cation radicals. Whereas the PNNA-1@PNCz and PNNA-2@PNCz doping powders give out yellow UORTP, showing matrix-controlled color-tunable UORTP. In the matrix PNCz, the cation radicals of PNNA-1 and PNNA-2 can stay stably and form strong intermolecular interactions with PNCz because of their high molecular structure similarity, leading to a redshift of ultralong phosphorescence. Additionally, PNNA-1 and PNNA-2 show green UORTP in other matrixes DBT and BBP probably because they have a low molecular structure similarity with DBT and BBP. It is exciting that N-1, N-2 and their derivatives perform much better than Bd (H-benzo[f]indole) and its derivatives in UORTP. This study provides another example to support that cation radical might be a universal mechanism in organic phosphorescence. We believe that this work will expand the scope of organic phosphorescence.
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.
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