EPR study of the trans and cis isomers of a ferrocenyl Schiff-based polychlorotriphenylmethyl radical

The isomeric trans-1 and cis-1 radicals have been studied by EPR spectrosco py. At 220 K, the EPR spectrum of the trans-1 isomer shows a hyperfine stru cture corresponding to the coupling of the unpaired electron with the diffe rent nuclei with non-zero magnetic moments. In contrast, the EPR spectrum o f the cis-1 isomer at similar temperatures shows the characteristic structu re of a triplet species. The differences in the EPR spectra of both isomers have been attributed to the fact that the cis-1 isomer aggregates in solut ion originating the thermodynamically stabilized hydrogen-bonded diradical species, in which strong antiferromagnetic interactions are developed. EPR frozen solution experiments, down to liquid helium temperature, showed that the magnetic ground state of the resulting dimeric species is the singlet state and the triplet should be associated with a thermally accessible exci ted state. (C) 2001 Elsevier Science Ltd. All rights reserved.