Organic dopant cyclization and significantly improved RTP properties†

Abstract

The internal rotation of triplet-generating molecules is detrimental to room temperature phosphorescence (RTP) radiation, and this rotation is usually mitigated by doping into rigid microenvironments. The chemical locking of internal rotation units in advance should be an effective strategy but is rarely studied in comparison. Herein, a triplet-generating molecule with two rotatable phenyls (DIA) was designed, synthesized, and then cyclized using two types of bonding bridges. We found that DIA/PMMA film shows little observable RTP afterglow despite a 148 ms lifetime, whereas carbon bridge cyclized DIA (CDIA) and oxygen bridge cyclized DIA (ODIA) emitted green and blue ultralong RTP in PMMA film, with lifetimes of 2146 ms and 2656 ms, respectively, demonstrating the potent role of pre-locking of internal rotation units in promoting RTP. Benefiting from the good spectral overlap between the RTP emissions of dopants and the absorption of perylene red (PR) in PMMA film, the almost complete triplet-to-singlet Förster resonance energy transfer was achieved under trace doping (0.1%), providing red room temperature afterglow materials with lifetimes of 1567–1800 ms. Preliminary applications of blue, green, and red afterglow materials in optical encryption and anti-counterfeiting are demonstrated. This work not only develops new triplet-generating and -radiating molecules but also introduces an effective molecular strategy for achieving ultralong RTP polymers.