Thermally Activated and Aggregation‐Regulated Excitonic Coupling Enable Emissive High‐Lying Triplet Excitons

Room‐temperature phosphorescence (RTP) originating from higher‐lying triplet excitons remains a rather rarely documented occurrence for purely organic molecular systems. Here, we report two naphthalene‐based RTP luminophores whose phosphorescence emission is enabled by radiative decay of high‐lying triplet excitons. In contrast, upon cooling the dominant phosphorescence originates from the lowest‐lying triplet excited state, which is manifested by a red‐shifted emission. Photophysical and theoretical studies reveal that the unusual RTP results from thermally activated excitonic coupling between different conformations of the compounds. Aggregation‐regulated excitonic coupling is observed when increasing the doping concentration of the emitters in poly(methylmethacrylate) (PMMA). Further, the RTP quantum efficiency improves more than 80‐fold in 1,3‐bis(N‐carbazolyl)benzene (mCP) compared to that in PMMA. This design principle offers important insight into triplet excited state dynamics and has been exploited in afterglow‐indicating temperature sensing.