Remarkable Angular and Temperature Dependence of the ESR Linewidth in Organic Ion-Radical Salts: 3,3'-Diethyl-6,6'-Dimethyl-2,2'-Thiacyanine-[TCNQ]2 and 1-Ethyl-3'-Methyl-2,2'-Quinoselenacyanine-[TCNQ]2

In the wide temperature range of 77–360 K, the angular dependence of the ESR linewidth, Δ H , in the single crystals of organic ion-radical salts, 3,3'-diethyl- 6,6' - dimethyl - 2,2' - thiacyanine - [TCNQ] 2 and 1 - ethyl - 3' - methyl - 2,2' - quinoselenacyanine-[TCNQ] 2 , can be described well by an expression, Δ H (θ)=α(3cos 2 θ-1) 2 +βsin 4 θ, where α and β are temperature-dependent parameters and θ is the angle between the axis perpendicular to the crystal planes of the plate-like single crystals of the salts and the direction of applied magnetic field. However, the two salts differ in the temperature dependence of the ESR linewidth. The difference may result from the difference of the structural symmetry of the cations.