Room temperature phosphorescence vs. thermally activated delayed fluorescence in carbazole–pyrimidine cored compounds

Abstract

Utilization of “dark” triplet states is a key task for design of novel emissive organic compounds. Thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) are two promising pathways to employ triplet excitons and reach high internal quantum efficiencies. In this paper we present a series of carbazole–pyrimidine cored compounds, showing RTP and TADF. A specific energy level scheme was shown to promote intersystem crossing and RTP in carbazole–pyrimidine derivatives with phosphorescence quantum yield values of up to 0.07 in rigid Zeonex films. The modification of the acceptor core with 2-methylthio and dimethylamine fragments allowed us to reduce the singlet–triplet energy gap, enhance the rISC (reverse intersystem crossing) rate and obtain high intensity TADF with a fluorescence quantum yield of up to 0.32. Time resolved fluorescence analysis revealed the presence of conformational disorder of the donor–acceptor core governing TADF properties in solid films. A method to control the conformational disorder by enhancing the host rigidity was demonstrated. Sky-blue electroluminescence was shown for the dimethylamino-modified compound with the most intense TADF. Our results show that carbazole–pyrimidine cored materials are versatile donor–acceptor systems for achieving RTP and TADF via simple structural modifications.