Spin-Vibronic Coupling in Indenoindenodibenzothiophene Diradicaloids.

Understanding how molecular structure governs photoinduced spin evolution in organic diradicals is essential for the rational design of spin-responsive molecular materials. Here, we investigate temperature-dependent excited-state relaxation dynamics of syn- and anti-indenoindenodibenzothiophene (IIDBT) diradicaloids using variable-temperature steady-state and transient absorption spectroscopies. Vibronic progressions in the high-energy, nonfrontier-orbital π-π* transition absorptions and low-energy frontier-orbital transition absorptions prompt strong couplings between electronic excitation, on one hand, and molecular vibrations, on the other hand. With the help of ultrafast spectroscopy, we identified multistep excited-state relaxation pathways including a "hot" singlet excited state, an intermediate state of mixed electronic and spin character, and a long-lived triplet excited state, following either low- or high-energy excitations. Notably, intersystem crossing (ISC) in IIDBT diradicaloids is enabled by vibrationally activated spin-orbit coupling, in which spin-vibronic coupling enhances singlet-triplet mixing along nuclear coordinates, leading to a fast and efficient interconversion between singlet and triplet excited states. Molecular control over both the vibronic coupling strength and spin evolution is also emphasized. This work establishes a clear experimental link between molecular vibration and spin-dependent excited-state dynamics in organic diradicals, providing a general framework for understanding and controlling ISC in open-shell organic systems.