Dissipative dynamics of spin-dependent electron–hole capture in conjugated polymers

Abstract

Spin-dependent electron–hole (e–h) recombination in poly(p-phenylenevinylene) chains is modeled by the dissipative dynamics of the multilevel electronic system coupled to the phonon bath. The underlying Hamiltonian incorporates the Coulomb and exchange interactions of spin-singlet and spin-triplet monoexcitations in Wannier-orbital basis and their coupling to the prominent Franck–Condon active modes. In agreement with experiment, we obtain that the ratio of singlet versus triplet exciton formation rates is strongly conjugation-length dependent and increasing on going from the model dimer to the extended chain. The result is rationalized in terms of a cascade interconversion mechanism across the electronic levels. In parallel to the direct formation of spin-dependent excitons, e–h capture is found to generate long-lived charge-transfer states, whose further phonon-mediated relaxation to the bottom of the density of states is hindered by the near e–h symmetry of conjugated hydrocarbons. Being nearly spin independent, such states most likely form an intersystem crossing pre-equilibrium, from which the singlet e–h binding channel is about ten times faster than the triplet one.