Hyperfine Interaction Promoted Intersystem Crossing

The mechanism of the intersystem crossing (ISC) in planar aromatic hydrocarbons is revised by considering hyperfine interaction promoted singlet-triplet transitions. The density matrix of the spin system of the metastable triplet state is derived. Extra terms including the electron-nuclear ordering, the ordering between the magnetic nuclei of the molecule, and the coherence between the nuclear spin sublevels are shown to be developed during the ISC. Several peculiarities of the spin system are predicted. The results are compared with the properties generated by the optical nuclear polarization. The proposed mechanism is examined by a qualitative analysis of the available experimental data on photoexcited pentacene in p-terphenyl.