Coupling of two curved polyaromatic radical-anions: stabilization of dimers by counterions

In this study, a comprehensive theoretical investigation of both kinetic and thermodynamic stabilities was performed for dimeric dianionic systems (C₂₀H₁₀)₂^2- and (C₂₈H₁₄)₂^2-, neutralized by two alkali metal cations. The influence of the counterions was of primary interest. The impact of the additional/spectator ligand(s) was elucidated by considering adducts with four molecules of diglyme or two molecules of 18-crown-6 ether. Importantly, both types of systems - in the form of contact-ion pair (CIP) and solvent-separated ion pair (SSIP) - were considered. The SSIP set was augmented by the adduct, in which the dimeric dianionic species were neutralized with purely organic cations N(CH₃)₄⁺ and P(CH₃)₄⁺. Detailed analysis of the bonding revealed that the presence of the counterions made these systems thermodynamically stable. This finding is in sharp contrast with results obtained for isolated (PAH)₂^2- systems, which were previously found to be thermodynamically unstable, but kinetically persistent. The introduction of the alkali metal cations to the system significantly increases the ionic term (ΔE_elstat), whereas the repulsive ΔE_Pauli one was found to be substantially reduced. Considering that the orbital component (ΔE_orb) exhibited only a moderate decrease and the preparation energy (ΔE_prep) showed no changes, the above-mentioned changes in ΔE_elstat and ΔE_Pauli provided a clear explanation for the increase of the thermodynamic stability of the target species. Importantly, a clear correlation between the size of the alkali metal cation and stability of the target dimeric product was established. Thermodynamic stability of the system rises with a decrease in the size of M⁺ due to enlargement of the ΔE_orb. Evaluated energy barriers (as spin-crossing points between singlet and triplet energy surfaces) were found to be equal to +15.85 kcal mol^-1 and +18.5 kcal mol^-1 for [(Cs⁺)₂{(C₂₀H₁₀)₂^2-}] and [(Cs⁺)₂{(C₂₈H₁₄)₂^2-}], respectively, which is substantially higher than those calculated for isolated (PAH)₂^2- systems (+10.00 kcal mol^-1 for (C₂₀H₁₀)₂^2- and +12.35 kcal mol^-1 for (C₂₈H₁₄)₂^2-). Thus, this study identified the presence of counterions as the key factor, which have a dramatic influence on the thermodynamic and kinetic stabilities of the aimed dianionic dimeric systems, which are formed by two curved polyaromatic monoanion-radicals.