Efficient Intersystem Crossing and Long‐lived Charge‐Separated State Induced by Through‐Space Intramolecular Charge Transfer in a Parallel Geometry Carbazole‐Bodipy Dyad

Abstract The design of efficient heavy atom‐free triplet photosensitizers (PSs) based on through bond charge transfer (TBCT) features is a formidable challenge due to the criteria of orthogonal donor‐acceptor geometry. Herein, we propose using parallel (face‐to‐face) conformation carbazole‐bodipy donor‐acceptor dyads (BCZ‐1 and BCZ‐2) featuring through space intramolecular charge transfer (TSCT) process as efficient triplet PS. Efficient intersystem crossing (Φ Δ =61 %) and long‐lived triplet excited state (τ T =186 μs) were observed in the TSCT dyad BCZ‐1 compared to BCZ‐3 (Φ Δ =0.4 %), the dyad involving TBCT, demonstrating the superiority of the TSCT approach over conventional donor‐acceptor system. Moreover, the transient absorption study revealed that TSCT dyads have a faster charge separation and slower intersystem crossing process induced by charge recombination compared to TBCT dyad. A long‐lived charge‐separated state (CSS) was observed in the BCZ‐1 (τ CSS =24 ns). For the first time, the TSCT dyad was explored for the triplet‐triplet annihilation upconversion, and a high upconversion quantum yield of 11 % was observed. Our results demonstrate a new avenue for designing efficient PSs and open up exciting opportunities for future research in this field.