The rate at which ISC — the non-radiative transition between two electronic states of different multiplicity — proceeds is decisive for the functional properties of many chromophores. Molecules with high triplet quantum yield and long triplet lifetime may serve as sensitizers of photochemical and photobiological transformations. Molecules with efficiently deactivated triplets may function as triplet quenchers and can thus protect other substances from photodamage. Until recently, it was believed that only El-Sayed allowed ISCs between a ππ* state and an nπ* state (direct spin–orbit coupling) are fast. However, in polar protic solvents or protein environments, nπ* states are often energetically not available for ISC. Vibronic spin–orbit coupling in out-of-plane normal modes can substantially accelerate transitions between two ππ* states (Penfold-2018). A prominent example is the xanthone family of dyes (xanthone, thioxanthone, acridone) where the ICS rate constants and triplet quantum yields are strongly solvent dependent. In molecules composed of light elements only, where the energy separation between the Born-Oppenheimer states typically is much larger than the spin–orbit interaction, we determine ISC rates according to Fermi's golden rule approximation. Electronic coupling matrix elements are computed using the SPOCK or SPOILER while the vibrational contributions to the rate are determined by the VIBES program. Detailed information on the underlying formalisms can be found in a recent review article (Penfold-2018).
You are welcome to join our team on this fascinating research topic which is funded in the framework of RTG 2482 (ModISC). Interested early-stage researchers should contact Prof. Christel Marian or Dr. Martin Kleinschmidt.