Unveiling the Evolution of Afterglow in Diboraanthracene Scaffolds: From Thermally Activated Delayed Fluorescence to Room-Temperature Phosphorescence

Three-coordinate organoboranes have emerged as promising afterglow emitters by promoting intersystem crossing (ISC) through the introduction of boron centers. Among them, diboraanthracene (DBA) derivatives have recently shown great potential for achieving afterglow with second-scale durations. This study explores how the structural modulation of DBA scaffolds governs afterglow evolution. For the first time, ultralong thermally activated delayed fluorescence (TADF) was identified from 9,10-dimesityl-9,10-dihydro-9,10-diboraanthracene (MesDBA), which exhibits a delayed lifetime of 0.72 s and represents the longest pure TADF reported to date. Building on this insight, π-conjugation was extended to yield 6,13-dimesityl-6,13-dihydro-6,13-diborapentacene (MesDBP), which unlocks a new pathway for room-temperature phosphorescence (RTP) with a duration of 12 s and a lifetime of 1.41 s. An iptycene-derived DBA, mesityldiborapentiptycene (MesDBPI), was designed to modulate excited-state dynamics, affording a hybrid TADF-RTP afterglow lasting up to 40 s. Its deuterated analogue, MesDBPI-d₁₈, further extended the lifetimes to 4.00 s (TADF) and 4.22 s (RTP), establishing the longest values among organoboron emitters in an inert polymer. To rationalize these findings, a theoretical model grounded in Marcus theory was employed to predict the reverse intersystem crossing (RISC) rate constants, showing strong agreement with experimental measurements across all compounds. Furthermore, afterglow organic light-emitting diodes (OLEDs) based on MesDBPI achieved an external quantum efficiency (EQE) of up to 1.8%, demonstrating the validity of this molecular design strategy at the device level. In addition, color-tunable afterglows enable the demonstration of multilevel security applications. This controllable evolution from TADF to RTP underscores the versatility of DBA frameworks as a robust platform for next-generation optoelectronics and security technologies.