singlettotriplet

Singlet to triplet transitions, often described as singlet-to-triplet intersystem crossing, refer to processes in which a molecule changes its electronic spin multiplicity from a singlet state (S) to a triplet state (T). In a single excitation, the molecule moves from a state with total spin S=0 to states with S=1; the singlet has multiplicity 1, while the triplet has multiplicity 3. Because electric-dipole transitions require ΔS=0, these transitions are formally spin-forbidden and thus relatively slow, but they can occur through spin-orbit coupling or vibronic interactions, enabling nonradiative conversion and, in some cases, phosphorescence or delayed fluorescence.

Two main manifestations are recognized. Intersystem crossing (ISC) is a nonradiative transition between electronic states of different spin multiplicities, typically S1 → T1 or higher triplet states. The rate of ISC is enhanced by heavy atoms or close-lying singlet and triplet levels that allow efficient mixing of spin characters. Reverse intersystem crossing (RISC) enables T states to repopulate S states and underpins thermally activated delayed fluorescence (TADF) in some organic emitters.

Another important phenomenon is singlet fission, in which one photoexcited singlet state splits into two triplet excitons (S1 → 2 T1). This process can, in principle, double the number of excitons available for charge generation, but requires specific energetic and electronic conditions (E(S1) ≥ 2 E(T1)).

Practically, singlet-to-triplet transitions are central to photophysics and applications such as phosphorescent OLEDs, organic photovoltaic devices, photodynamic therapy, and sensors. They are exploited or mitigated depending on whether triplet formation improves efficiency or causes quenching.

See also: phosphorescence, intersystem crossing, reverse intersystem crossing, thermally activated delayed fluorescence, singlet fission, spin-orbit coupling.