A stable free radical as donor: A layer-structure organic pressure sensor

A novel charge transfer salt was prepared by the addition of TCNQ to the stable free radical, 1,3-diphenyl-1,2,4-benzotriazin-4-yl (R•), resulting in (TCNQ)5(R•)2. Analysis by x-ray revealed the crystal structure to be triclinic, P1, with a = 7.696(4) Å, b=16.428(3) Å, c=16.706(2) Å, α=91.96(1)°, β=103.16(2)°, and γ=97.36(3)°, V=2035(1) Å3, Z=2, Dx=1.297 gcm−3. Both variable temperature ESR and magnetic susceptibility measurements indicated the material behaved as a Curie paramagnet down to 20 K, where a transition to an antiferromagnetically ordered state occurred. The pressed pellet room temperature conductivity was 10−4S/cm. Single crystal conductivity measurements of the current parallel to the TCNQ layer indicated a strong pressure dependence of the conductivity; where ‘sigma’ increased two orders of magnitude at 2 kbar.