Shaping the Afterglow of Indolo[3, 2‐b]Carbazoles via Modulating Intersystem Crossing (ISC/rISC) and Phosphorescence Rates

ABSTRACT The development and modulation of dual‐mode organic afterglows, which integrate persistent thermally activated delayed fluorescence (pTADF) and room‐temperature phosphorescence (pRTP), remain challenging. This work presents afterglow modulation studies of indolo[3,2‐b]carbazole derivatives (X‐ICZ‐ p 1) via molecular engineering that regulates intersystem crossing (ISC), reverse ISC (rISC) and phosphorescence rates. As embedded in polymethyl methacrylate films and photoactivated, F‐ICZ‐ p 1 and Cl‐ICZ‐ p 1 exhibit color‐tunable afterglows, with a dual‐mode green one from pTADF plus pRTP emissions at 298 K and a pTADF‐type blue one at 320 K. Br‐ICZ‐ p 1 shows only a pRTP‐type green afterglow. Among these, F‐ICZ‐ p 1 achieves optimal performance, with an afterglow duration of ∼20 s and a pTADF‐pRTP lifetime > 2 s. Results reveal that nitrogen and halogen atoms jointly contribute to realizing obvious 1 (n,π*)→ 3 (π,π*) and 1 (π,π*)→ 3 (n,π*) transitions. The presence of a minimum‐energy crossing point between the S 1 and T 1 minima, along with small energy gaps, promotes efficient interconversion of T 1 and S 1 excitons. These factors collectively enhance spin‐orbit coupling effects and modulate the T 1 ‐S 1 energy splitting. Consequently, the rISC and phosphorescence rates are tuned to 10 − 1 –10 0 s − 1 for F/Cl‐ICZ‐ p 1 , but remain as fast as 10 1 s − 1 for Br‐ICZ‐ p 1 . Slower and comparable rates yield long‐lived hybrid pTADF‐pRTP afterglows, whereas faster and outcompeting rates yield short‐lived, single‐mode afterglows, shaping afterglow properties. Based on the photoactivatable afterglow behavior, potential application in optical information storage is explored.