Structures and photoinduced electron transfer of protonated complexes of porphyrins and metallophthalocyanines

Porphyrins and phthalocyanines are planar two-dimensional π-compounds, which are normally difficult to protonate because of the low basicity. When many bulky substituents are introduced to porphyrins and phthalocyanines, however, the macrocyclic π-plane is distorted due to the steric repulsion of the bulky substituents. The π-plane distortion facilitates protonation to afford stable protonated porphyrins and phthalocyanines. Crystal structures of protonated porphyrins and phthalocyanines were determined to clarify the role of hydrogen bonding in the supramolecular assemblies. Protonated porphyrinoids can act as an electron acceptor rather than an electron donor in photoinduced electron-transfer reactions. The rate constants of photoinduced electron-transfer reactions of diprotonated porphyrin with different degrees of distortion were determined and they are evaluated in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer, which are affected by the distortion of the π-plane. A distortion of the macrocyclic ligands also affords higher Lewis acidity at a metal center to allow facile axial coordination of ligands, due to poor overlap of the lone pair orbitals with d _x2-y2 or p _x and p _y orbitals of the metal center. Thus, the distortion of the macrocyclic ligands enables one to construct various molecular and supramolecular complexes composed of porphyrins and phthalocyanines. The photodynamics of photoinduced electron-transfer reactions of various supramolecular complexes of distorted porphyrin and phthalocyanines are discussed in relation to structure and photofunction.