Although long-range intramolecular electron transfer has been extensively demonstrated in covalently linked donor-acceptor conjugates, intramolecular singlet fission (ISF) over linker lengths significantly exceeding 15 Å has not yet been observed. Moreover, no studies have shown linker-length-dependent control of the singlet correlated triplet pair (TT) dynamics, where singlet TT undergoes spin conversion to quintet TT or dissociation into two individual triplet states (T1 + T1) via quintet TT. Here, we report pentacene dimers bridged by two different polyyne-based molecular wires, in which long-range ISF is successfully observed over linker lengths of up to ∼35 Å. As the linker length increases, we further demonstrate tunable singlet TT dynamics, achieving systematic control over pathways: spin conversion to quintet TT, and dissociation of quintet TT into T1 + T1. Transient absorption measurements reveal that the individual triplet yields increase with increasing linker length, while the nearly quantitative formation of the 1TT is maintained. The small attenuation factors for ISF and for recombination of singlet TT to the ground state indicate that the polyyne linker functions as an efficient molecular wire. Thermodynamic analysis and time-resolved electron paramagnetic resonance measurements clearly reveal that vibration-driven structural modulation of the polyyne-based molecular wire in the singlet TT directly controls the branching between (i) spin conversion from singlet TT to quintet TT and (ii) dissociation into T1 + T1 via quintet TT following spin conversion. These findings establish molecular wires as effective platforms for long-range ISF and provide a mechanistic foundation for engineering singlet TT dynamics across extended molecular distances.
Sakai et al. (Mon,) studied this question.