Frenkel and Wannier Spin Excitons in Organic Free-Radical Crystals

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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.