The role of Zundel-like ions in the supramolecular self-organization of porphyrin assemblies

Aggregates mainly consisting of mono-protonated meso-tetraphenylporphine (TPP) dimers and water self-assembled from the dimers in 0.4 N aqueous HCl with 0.86 mol L−1 tetrahydrofuran have been investigated by means of infrared spectroscopy (IR), electronic absorption spectroscopy, and scanning electron microscopy (SEM). IR studies using deuterium labeling of small-size protonated water clusters located in the cage between TPP in the dimers allow for the assignment of a doublet at 2993, 3009 cm−1 to stretching vibrations of H⋯ODH and H⋯O+DH, respectively. In the absence of heavy water in the thin film a doublet at 982, 1000 cm−1 with the same energy gap of 16–18 cm−1 is identified with proton sharing in the OH+⋯O moiety of the cluster. The main features of the dimers, which provide their hydration, are the protonated N groups of pyrrole rings observed in the IR near 1486 cm−1 that are involved in the conjugated π-system of the porphyrin macrocycle. Stabilization of the electronic structure in the mono-protonated dimers occurs by H5O2+ ions present in the cage between TPP units and charge delocalization through hydrogen bonding. It is proposed that together with the cooperative structure of the cluster the H5O2+ ions are responsible for efficient self-assembling and self-organization of the highly ordered aggregates obtained. Based on surface studies a molecular mechanism for proton mobility is suggested to explain the origin of a slanting cross with the angle of 55° ± 3°, half the tetrahedral angle, on the surface of the aggregates in the solid state. Estimated proton hopping times of within 6.4–7.9 ps supports this molecular mechanism.