Triplet Exciton EPR and Crystal Structure of [TMPD+]2[Ni(mnt)2]−2

We report in this paper the results of EPR and x ray diffraction studies on the ionic‐radical salt [TMPD⁺]₂[Ni(mnt)₂]⁻² composed of an organic free radical cation and a diamagnetic transition metal complex ion. This crystal exhibits the triplet exciton EPR spectrum characteristic of an alternating chain of spins. Exciton creation energy is $\text{Δ E=0.240 ± 0.005\hspace{0.17em}}\mathrm{eV};$ triplet spin Hamiltonian parameters are $\text{|\hspace{0.17em}D\hspace{0.17em}|\hspace{0.17em}=215 ± 1}$ G and $\text{|\hspace{0.17em}E\hspace{0.17em}|\hspace{0.17em}=28 ± 1\hspace{0.17em}}\mathrm{G}{\mathrm{;\hspace{0.17em}g}}_{\mathrm{zz}}{\text{=2.0029,\hspace{0.17em}g}}_{\mathrm{yy}}{\text{=2.0033,\hspace{0.17em}g}}_{\mathrm{xx}}\text{=2.0035}$ , all $\text{± 0.0005}$ . The complex crystallizes in $C_i^1\text{‐ P1̄}$ of the triclinic system, with cell dimensions of $\text{a=9.145\hspace{0.17em}(9)\hspace{0.17em}Å,\hspace{0.17em}b=11.502\hspace{0.17em}(9)\hspace{0.17em}Å,\hspace{0.17em}c=7.762\hspace{0.17em}(8)\hspace{0.17em}Å,\hspace{0.17em}α=98.24\hspace{0.17em}(5)°,\hspace{0.17em}β=105.69\hspace{0.17em}(2)°}$ and $\text{γ=92.64\hspace{0.17em}(4)°}$ . The organic free radicals form very strongly alternating linear chains and we examine the extent to which EPR results for a triplet exciton system may, through knowledge of the crystal structure of the system, be related in a detailed fashion to the zeroth order picture of the spin excitation of an isolated free radical pair.