Critical molecular design that can actively control intramolecular singlet fission by hydrostatic pressure

The active control of singlet fission (SF) by external stimuli presents a major challenge in current chemistry, although it is yet in the initial stages. In this study, we synthesized a series of SF-active chromophore pentacene dimers connected by the flexible linkers and investigated the ground- and excited-state properties, under hydrostatic pressure as an external stimulus. The flexibilities of the linkers were elucidated by the density functional theory. Hydrostatic pressure-UV/vis and hydrostatic pressure-circular dichroism spectroscopies showed the absence of a ground-state conformational change, indicating that the active switching of SF observed in this study comes from the excited-state dynamics. The hydrostatic pressure-fluorescence lifetime and hydrostatic pressure-nanosecond transient absorption measurements revealed the active inversion of SF process (acceleration vs. deceleration) and independent triplet (T₁) formation, in addition to the shortened T₁ lifetimes and non-decreasing T₁ quantum yields with elevating hydrostatic pressure. This study offers a valuable guideline through the critical molecular design revealed herein for further developing smart SF-controllable soft materials induced by hydrostatic pressure stimulations.