Charge Resonance Interaction in Aromatic Trimer Radical Cations Revealed by IR Spectroscopy: The Case of Pyrrole Homo- and Heterotrimers.

Intermolecular interactions of aromatic molecules are crucial for many biological and functional materials. One of the strongest intermolecular forces is the charge resonance in an aromatic dimer radical cation (CR, ∼100 kJ mol^-1), in which the positive charge is delocalized over both molecules. It is of interest how a third unit influences the CR and binding motif of a dimer cation, including the degree of charge delocalization. Herein, we investigate the binding motifs, energies, charge distribution, and origin of the intermolecular forces of the pyrrole trimer cation (Py₃ ⁺). The NH stretch frequencies (ν_NH) recorded by infrared spectroscopy provide insight into the preferred binding motifs and interaction strengths by comparison to density functional theory calculations. We extend the study to Py₂ ⁺X by substituting one Py in Py₃ ⁺ with toluene or benzene. In both homo- and heterotrimers, the third unit solvates the CR-stabilized Py₂ ⁺ by an NH…π ionic hydrogen bond (H-bond) rather than by CR or π-stacking. This NH…π H-bond has only a minor effect on the CR of the Py₂ ⁺ core, because the positive charge is retained in the dimer core. Minor charge transfer to the third unit causes an uneven charge distribution in the Py₂ ⁺ core, thereby reducing its symmetry.