Emissive tags based on three new electron acceptors and phenoxathiin containing compounds with enhancement of room-temperature phosphorescence

Herein we present three new electron-accepting moieties for organic triplet emitters. The synthesized electron accepting compounds show green-orange room-temperature phosphorescence (RTP), with quantum efficiency reaching 3.2 %. The attachment of two phenoxathiin moieties results in the increase of the quantum efficiencies of RTP up to 17.8 %. With the help of different experimental and theoretical approaches, we demonstrate that the observed RTP enhancement can be attributed to intersystem crossing via upper singlet-triplet levels. The study by to time-resolved emission spectroscopy shows, that upper intersystem crossing competes with the upper reverse intersystem crossing. This results in intensive RTP and weak thermally activated delayed fluorescence. The highest efficiency of RTP is observed for the compounds with the widest singlet-triplet energy gap of 0.66 eV and the most efficient upper intersystem crossing. One of the six synthesised compounds shows very high oxygen sensitivity. This compound shows the highest ratio of RTP-to-fluorescence intensities of 3.6. This property mainly determines its application for the emissive tag. The fabrication approach of orange emissive tags without strong UV excitation is proposed using the molecular dispersion of the RTP emitter in a rigid polymer matrix.