Tuning Steric Hindrance and Intermolecular Interactions in Tetrahydroindolocarbazoles for Dual Thermally Activated Delayed Fluorescence and Room‐Temperature Phosphorescence

Synthesizing organic luminescent materials exhibiting long‐lived afterglow at room temperature, through room‐temperature phosphorescence (RTP) and thermally activated delayed fluorescence (TADF), properties in pure organic derivatives remains a challenge, as it requires precise control over molecular design and solid‐state packing. Here, we investigate the influence of steric hindrance and intermolecular interactions on the emissive behavior of 6,12‐diphenyl‐5,6,11,12‐tetrahydroindolo[3,2‐ b ]carbazole (ICZ) derivatives. By strategically positioning alkyl groups (methyl or tert ‐butyl) on external phenyl rings and grafting polyether chains onto nitrogen atoms, we modulate intramolecular vibrations, molecular stacking, and electronic communication between adjacent molecules. Our results reveal that the position of methyl substituents ( meta vs. para ) dramatically affects phosphorescence lifetimes, with the meta ‐substituted derivative achieving ultralong organic phosphorescence (UOP) persisting for up to 1.3 s, while the para isomer shows negligible phosphorescence. The introduction of bulky tert ‐butyl groups further enhances luminescence persistence by restricting phenyl rotation and promoting rigid molecular packing. Additionally, polyether chains on nitrogen atoms induce dual TADF‐RTP emission, with delayed fluorescence in the blue and phosphorescence in the orange, and significantly extend emission lifetimes (up to 487 ms for phosphorescence and 20 ms for TADF). Through X‐ray crystallography and photophysical analyses, we demonstrate that these modifications create a rigid molecular environment, suppressing nonradiative decay pathways and stabilizing triplet excitons.