Frenkel and Wannier Spin Excitons in Organic Free-Radical Crystals

Abstract

It is shown that organic molecular crystals containing linear chains of exchange‐coupled free radicals fall into two classes which may be characterized by their spin excitations. Two spins forming a Frenkel spin exciton remain adjacent in the crystal, while spins forming a Wannier spin exciton move through the crystal independently. This difference in the structure of the triplet excitations leads to a different temperature and pressure dependence for the width of the exchange‐narrowed line observed by spin resonance, to different exciton fine structure, and to different magnetic susceptibility‐vs‐temperature curves. It is suggested that paramagnetic excitations in organic charge‐transfer compounds containing chains of alternately positive and negative ion radicals are Wannier spin excitons.