Tandem cofacial stacks of porphyrin-phthalocyanine dyads through complementary coordination

A novel straightforward methodology to organize discrete heterogeneous stacks of porphyrin and phthalocyanine employed an imidazolyl-to-zinc complementary coordination protocol for a Zn(II) phthalocyanine that contains an imidazolyl terminal with an ethynylporphyrin as a coplanar spacer. Structural elucidation was performed by means of size-exclusion chromatography, spectral titration, and NMR spectroscopy. The association constants for the complementary coordination of the heterogeneous slipped-cofacial tetrads reached extremely high values, in the order of 10(14) M(-1). Close contact of the porphyrin and phthalocyanine planes led to a strong shielding of the cofacial protons, which were split due to the slipped-cofacial heterogeneous environment. In variable-temperature NMR spectroscopy, the split signals remained in the aromatic region, a result suggesting structural robustness. Addition of trifluoroacetic acid dissociated the coordination structure to unify the split signals. The stacked tetrads showed unique electronic structures, such as strong exciton coupling and charge-transfer properties between the porphyrin and phthalocyanine units, which were modulated by the peripheral substituents of the phthalocyanine subunit and by the solvent. Interconversion between the coordination tetrad and the corresponding dyad was observed upon addition of an axial ligand.