Paramagnetic Excitons in Molecular Crystals

The possibility of using electron spin resonance to study paramagnetic excited electronic states of aromatic crystals is explored. Two limiting cases are considered: On the one hand the excitation energy moves from molecule to molecule by a diffusion process; on the other the triplet excitation is described by a Frenkel exciton. In either case intermolecular propagation of the triplet excitation is assumed to proceed through virtual triplet ionization states. The theory of the spin resonance of triplet excitons is developed in some detail. It is shown that there is only a weak dependence of the spectra on the k vector of the exciton wave, except when k is near regions of band degeneracies. Band degeneracies arise from time-reversal symmetry in benzene, naphthalene, and coronene crystals and occur on boundary planes of the first Brillouin zone. A spin Hamiltonian is calculated for these three aromatic crystals for most k vectors which are far removed from regions of band degeneracy. Two-line spectra (with no hyperfine structure), representing an average over the molecular sites in the unit cell, are obtained. Scattering of the exciton wave by lattice vibrations can cause line broadening and also can serve as a mechanism for spin lattice relaxation.