Spin–Orbit versus Hyperfine Coupling-Mediated Intersystem Crossing in a Radical Pair

Abstract

While spin–orbit coupling (SOC) is typically the dominant interaction that couples singlet and triplet states within individual chromophores, hyperfine coupling (HFC) becomes important in multichromophoric systems, particularly in relation to the radical pair mechanism. Here, we use TD-DFT to calculate the spin–orbit coupling and hyperfine coupling between the first singlet and triplet charge transfer states of the radical pair 2Pyrene– and 2 N,N-dimethylaniline+. We show that, as the intermolecular donor–acceptor distance grows, SOC decays to zero (as one would expect) because singlet and triplet states are characterized by identical orbitals in space, while the HFC remains comparatively constant. The switching region occurs around 4 Å, beyond which HFC dominates over SOC as far as defining the rate of intersystem crossing (ISC).

Molecularly Engineered Room-Temperature Phosphorescence for Biomedical Application: From the Visible toward Second Near-Infrared Window

“…197 As the field of room-temperature phosphorescence developed, fundamental photophysical principles of hyperfine-coupling-driven intersystem crossing have been systematically uncovered. 198 If spin−orbit coupling is relatively small, 199 intersystem crossing can proceed through hyperfine interaction, which especially becomes important in multiple-chromophore systems and brings a conceptual revolution into long-lived phosphorescence. For example, Yuasa's research group reported visible room-temperature phosphorescence of pure organic crystals with an afterglow, which was strong and long enough to visually inspect.…”

“…According to above working principle, preparing heavy-atom-free organic crystals with long lifetimes and superior conversion efficiency has proved difficult . As the field of room-temperature phosphorescence developed, fundamental photophysical principles of hyperfine-coupling-driven intersystem crossing have been systematically uncovered . If spin–orbit coupling is relatively small, intersystem crossing can proceed through hyperfine interaction, which especially becomes important in multiple-chromophore systems and brings a conceptual revolution into long-lived phosphorescence.…”