EPR fine structure and spin densities in TCNQ salts

✍ Scribed by S. Flandrois; J. Boissonade

• Publisher: Elsevier Science
• Year: 1978
• Tongue: English
• Weight: 492 KB
• Volume: 58
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(78)80027-x

✦ Synopsis

The spin density distributions in TCN@anion+adicak are estimated from experimental fme s3rudure parameters and uystalIographic data of five TCNQ salts of l-1 stoichiometIy_ 1. rntroduclioIl Many TCNQ anion-radical s&s are known to exhibit EPR spectra characteristic of mobile excited tripIet states (triplet excitons), which are thought to arise from excited states of electronically coupled TCNQ entities [ 1 ] _ At low excitation densities a line splitting (tie structurz) is observed, resulting from dipoIe-dipole interactions between the two spins involved in the triplet, and represented by 2 OS: f E(S$ -St) term in the spin hamiltonian [2]

. The fine structure parameters D and E are determined from singIe crystal measurements of the anguIar dependence of the doublet splitting. They can aIso be computed tiom structural data and molecular spin distributions if they are available-Such computations have ahzady been carried out [3,4], based upon spin densities given by approximate quantum chemical methods-ActuaIIy a severs disc~pancy is observed between measured and calc~&ted values, the caIculated ones aiways being much larger-In order to explain this disagreement Hibma [S,S] proposed an extended triplet state with some spin density on non-nearest neighbour TCNQ-sites. Such a delocaiization is indeed expected for s&s where TCNQ dimers Iine up in the usual face-to-face chain-I&e fashion (Rb-TCNQ for exampIe i6]), but is Iess obvious when the dimew are isoIated as in trimet.hyIbenzimidazoIium TMB-TCNQ

[7] _ An alternative explanation was given by Soos [4] in the case of Rb-TCNQ. This salt consists of TCNQ stacks in which the plane-to-plane separations of anion-