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-p1) via molecular engineering that regulates intersystem crossing (ISC), reverse ISC (rISC) and phosphorescence rates. As embedded in polymethyl methacrylate films and photoactivated, F-ICZ-p1 and Cl-ICZ-p1 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-p1 shows only a pRTP-type green afterglow. Among these, F-ICZ-p1 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 S1 and T1 minima, along with small energy gaps, promotes efficient interconversion of T1 and S1 excitons. These factors collectively enhance spin-orbit coupling effects and modulate the T1-S1 energy splitting. Consequently, the rISC and phosphorescence rates are tuned to 10−1-100 s−1 for F/Cl-ICZ-p1, but remain as fast as 101 s−1 for Br-ICZ-p1. 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.
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