Dopant Deuteration Enables Long‐lived Room‐temperature Phosphorescence

ABSTRACT Organic host–guest systems exhibiting room‐temperature phosphorescence (RTP) hold great promise for sensing, encryption, and bioimaging. However, achieving both long lifetimes and high efficiency remains challenging, as enhanced spin–orbit coupling (SOC) often competes with efficient intersystem crossing (ISC). We report an isotope‐engineering strategy that overcomes the lifetime–efficiency trade‐off in classical carbazole (Cz) host–guest systems. Doping just 0.5 wt% of deuterated 1H‐benzo[f]indole (BdD8) into Cz extends the RTP lifetime from 0.485 to 1.771 s, a 3.65‐fold enhancement without compromising the phosphorescence quantum yield. Replacing the N–H group in BdD8 with a CD3 moiety (BdD8CD3) and using a methylated host (CzCH3) further extends the RTP lifetime to 1.870 s. This improvement arises from isotope‐induced suppression of non‐radiative decay and enhancement of ISC, as evidenced by a reduction in the non‐radiative rate from 2.01 to 0.51 s −1 and an increase in the ISC rate from 4.91 × 10⁷ s −1 to 6.21 × 10⁷ s −1 . Building on this success, we applied the strategy to benzo[b]carbazole (BCz) derivatives, which similarly exhibited enhanced RTP performance. Finally, we demonstrate time‐resolved multi‐information encoding enabled by this ultralong afterglow system.