Isomer design unlocks rainbow phosphorescence

Achieving predictable color-tunable organic room-temperature phosphorescence (RTP) remains challenging due to limited understanding of triplet-state regulation in heteroaromatic systems. Carbazole and benzindole isomers provide an ideal platform to clarify how nitrogen positional isomerism governs triplet exciton behavior and emission energetics. Here, we establish a unified comparative framework to systematically investigate carbazole together with Bd[f], Bd[e], and Bd[g]. Nitrogen-site modulation within the fused tricyclic skeleton generates distinct red, yellow, green, and blue phosphorescence, while mechanochemical solvent-free synthesis enables scalable preparation of previously inaccessible benzindole isomers. Photophysical measurements combined with DFT/TD-DFT calculations, single-crystal analysis, and interaction region indicator theory reveal that positional isomerism controls exciton localization, triplet stabilization, and nonradiative decay independent of the host matrix. Here, we show that isomer-regulated triplet dynamics enable full-spectrum RTP, ultralong lifetimes up to 4.23 s, TSFRET behavior, and matrix-universal multifunctionality, establishing a general molecular design principle for rainbow-like organic phosphorescent materials.