Core-Twist Modulated Intersystem Crossing in a π-Fused Single-Molecule

Herein, we present direct evidence of intersystem crossing (ISC) in a core-twisted naphthalenemonoimide-fused perylenediimide (NP) at the single-molecule level. The ISC process was characterized by frequent quantum jumps (short off states) in the single-molecule fluorescence measurements. The singlet to triplet quantum jumps in NP was corroborated by ultrafast transient absorption measurements. NP exhibits intersystem crossing with a rate constant k_ISC = 6.5 × 10⁷ s^-1 and a triplet quantum yield ϕ_T = 25.8 ± 2.4%. Our theoretical investigations unveil the potential crossing and state mixing between first singlet excited state S₁ and third triplet excited state T₃, thus providing insight into the ISC mechanism during the relaxation of NP from the Franck-Condon region to its optimal geometry in the S₁ state. The current investigation provides critical insights into the twist-induced ISC process in a polyaromatic hydrocarbon at both the single-molecule and ensemble levels.