Vibrational Control of Spin-Orbit Coupling Enhances Triplet-Triplet Annihilation Upconversion Efficiency

Singlet-triplet intersystem crossing (ISC) underlies key processes in photovoltaics, photocatalysis, and photochemistry and is conventionally attributed to spin-orbit coupling (SOC) treated as a purely electronic interaction. In reality, strong coupling among electronic, spin, and vibrational motions can allow nuclear dynamics to modulate SOC, usually called vibronic SOC. Despite the fact that a considerable number of synthesized materials have demonstrated that vibronic SOC plays a key role in enhancing ISC, experimental approaches for direct manipulating such vibrationally mediated SOC have been lacking. Here, we report a direct experimental approach that proved vibrational excitation can enhance SOC and accelerate singlet-triplet intersystem crossing. Using a BODIPY sensitizer in a triplet-triplet annihilation upconversion system, selective mid-infrared excitation of skeletal modes increases upconversion emission by 156%, driven by the acceleration of ISC. Time-resolved mid-infrared spectroscopy and density functional theory reveal that these gains arise from vibrationally strengthened SOC, establishing a clear mechanistic link between specific nuclear motions and spin dynamics.