Paramagnetic Excitons in Solid Free Radicals

It is suggested that the low-temperature paramagnetism of many aromatic free-radical solids may be due to triplet excitons, and that the formation of these exciton states rather than other low-temperature magnetic states is due to low-temperature crystal lattice structures that favor a singlet pairing of electrons on neighboring molecules. As an example it is shown that the paramagnetic resonance spectra of certain TCNQ charge-transfer complexes observed by Chesnut and Phillips may be interpreted in terms of triplet excitons.