Ion-pairing π-electronic systems: ordered arrangement and noncovalent interactions of negatively charged porphyrins

In this study, charged π-electronic species are observed to develop stacking structures based on electrostatic and dispersion forces. π- π Interaction, defined herein, functions for the stacking structures consisting of charged π-electronic species and is in contrast to conventional π-π interaction, which mainly exhibits dispersion force, for electronically neutral π-electronic species. Establishing the concept of π- π interaction requires the evaluation of interionic interactions for π-electronic ion pairs. Free base (metal-free) and diamagnetic metal complexes of 5-hydroxy-10,15,20-tris(pentafluorophenyl)porphyrin were synthesized, producing π-electronic anions upon the deprotonation of the hydroxy unit. Coexisting cations in the ion pairs with porphyrin anions were introduced as the counter species of the hydroxy anion as a base for commercially available cations and as ion-exchanged species, via Na⁺ in the intermediate ion pairs, for synthesized π-electronic cations. Solid-state ion-pairing assemblies were constructed for the porphyrin anions in combination with aliphatic tetrabutylammonium (TBA⁺) and π-electronic 4,8,12-tripropyl-4,8,12-triazatriangulenium (TATA⁺) cations. The ordered arrangements of charged species, with the contributions of the charge-by-charge and charge-segregated modes, were observed according to the constituent charged building units. The energy decomposition analysis (EDA) of single-crystal packing structures revealed that electrostatic and dispersion forces are important factors in stabilizing the stacking of π-electronic ions. Furthermore, crystal-state absorption spectra of the ion pairs were correlated with the assembling modes. Transient absorption spectroscopy of the single crystals revealed the occurrence of photoinduced electron transfer from the π-electronic anion in the charge-segregated mode.