Diamagnetic to paramagnetic transition in trisdimethylaminocyclopropenium tetracyanoquinodimethanide (TDAC-TCNQ)

Abstract

Two crystalline forms of TDAC-TCNQ obtained under different crystallization conditions are both stable at room temperature. The diamagnetic yellow form belongs to the triclinic space group P1̅, with unit cell parameters a=7.030(2), b=8.791(2), c=17.306(6) Å, α=94.23(3)°, β=101.24(3)°, γ=90.95(2)°, V=1045.7(6) ų and Z=1 (R_w=0.073); it also has a long [sgrave]-bond between TCNQ^- in adjacent stacks, but shows no thermally activated triplets. The paramagnetic black form belongs to the monoclinic space group P2_l/c, with unit cell parameters a = 17.536(4), b = 7.272(2), c = 17.776(4) Å, β=113.36(2)°, V = 2081.2(9) ų and Z=4 (R_w = 0.058); it corresponds to a mixed stack of closed shell TDAC⁺ ions and TCNQ^- ion-radicals. Heating the yellow crystal above 79±3°C converts it to the black form. Subsequent cooling below -50±10°C regains the yellow form with paramagnetic TCNQ^- defects. The hysteresis is interpreted in terms of a high activation barrier for the large (>2Å) molecular motion of each TCNQ^-, in contrast to the unactivated, small amplitude (<0.2 Å) motions associated with spin-Peierls transitions.