On the Stability and Properties of Organic Metals and Their Isomeric Charge-Transfer Complexes

A methodology for conceptualizing the aggregation isomers of D-A adducts is presented. It is concluded that the mixed-stacks isomer …D-A-D-A… leads to stabilization of the ground no-bond configuration. On the other hand, the segregated-stacks isomer …DD∥AA… leads to stabilization of excited charge-transfer configurations which form the delocalized state …D^+ρD^+ρ∥A^−ρA^−ρ… with a partial degree of electron transfer (ρ). It is argued from first principles that this state is not always truly delocalized. In such cases, there will be a barrier for electron hopping (e.g., A⁻A → AA⁻). A mechanism of curve-band crossing is responsible for stabilizing …D^+ρD^+ρ∥A^−ρA^−ρ… below the no-bond state. The value of ρ in this isomer is determined by a combination of two factors: I_D-A_A and the reorganization energy obtained by relaxing D⁺ and A⁻ from the geometry of their neutrals. The recognition that the mixed-stacks isomer originates from a ground no-bond configuration, while the (often conducting) segregated-stacks isomer arises from crossing of the excited charge-transfer pack of configurations, indicates that the latter isomer could in principle be the less stable one. Experimental data are examined in light of the model.