Intermolecular,pi-to-pi bonding between stacked aromatic dyads. Experimental and theoretical binding energies and near-IR optical transitions for phenalenyl radical/radical versus radical/cation dimerizations

Abstract

The high symmetry and stability of phenalenyl systems, both as the planar pi-radical (P-o) and as the pi-cation (P+), are desirable characteristics of prototypical aromatic donor/acceptor pairs that encourage their use as (binary) models for the study of intermolecular interactions extant in stacked molecular arrays. Thus, quantitative ESR spectroscopy of the paramagnetic P-o identifies its spontaneous self-association to the diamagnetic P-2, previously characterized as the stacked pi-dimer by X-ray crystallography. Likewise, the rapid cross-association of P-o with the closed-shell P+ leads to the stacked pi-dimer cation P-2(.+) with the "doubled" ESR spectrum diagnostic of complete (odd) electron delocalization. These 7-associations are confirmed by UV-vis studies that reveal diagnostic near-IR bands of both P-2 and P-2(.+)-strongly reminiscent of intermolecular charge-transfer absorptions in related aromatic (donor/acceptor) pi-associations. Ab initio molecular-orbital calculations for the pi-climer P-2 predict a binding energy of DeltaE(D) = -11 kcal mol(-1), which is in accord with the experimental enthalpy change of DeltaH(D) = -9.5 kcal mol(-1) in dichloromethane solution. Most importantly, the calculations reproduce the intermonomer spacings and reveal the delicate interplay of attractive covalent and dispersion forces, balanced against the repulsions between filled orbitals. For comparison, the binding energy in the structurally related cationic pi-pimer P-2(.+) is calculated to be significantly larger with DeltaE(P) approximate to -20 kcal mol(-1) (gas phase), owing to favorable electrostatic interactions not present in the neutral pi-dimer (which outweigh the partial loss of covalent interactions). As a result, our theoretical formulation can correctly account for the experimental enthalpy change in solution of DeltaH(P) = -6.5 kcal mol(-1) by the inclusion of differential ionic solvation in the formation of the pi-pimer.