Twisted, Two-Faced Porphyrins as Hosts for Bispyridyl Fullerenes: Construction and Photophysical Properties

A new class of multichromophoric host compounds capable of binding guest species through two-point coordinative interaction is reported. The hosts contain appended zinc porphyrin (ZnP) moieties connected covalently by acetylene linkages to a central oxoporphyrinogen (OxP) unit through its central macrocyclic nitrogen atoms. Orientation of the ZnP groups was controlled to some extent by variation of the substitution pattern at the OxP N-substituent. Up to four porphyrin appendages are accommodated, and two cofacial bis-porphyrin interaction sites can be created in orthogonal geometry within the same molecule. The multiporphyrin hosts, termed “twisted, two-faced porphyrins”, abbreviated as OxP−(MPx)n where M = 2H or Zn, n = 2 or 4, and x = meta (m) or para (p), and their complexes with a bis-4-pyridyl-substituted fullerene derivative were investigated using spectroscopic, electrochemical, and photochemical methods. Porphyrin fluorescence emission of the OxP−(ZnPx)n compounds is quenched substantially compared to that of pristine ZnP with this quenching being more significant for OxP−(ZnPx)2 than for the OxP−(ZnPx)4. The electrochemically determined highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO−LUMO) gap for the OxP−(ZnPx)n series is ∼1.65 eV. For the OxP−(ZnPx)n series, charge separation from the singlet excited ZnP to OxP occurs, while, for their complexes with the bis-pyridyl fullerene derivative, charge separation appears to occur predominantly from the singlet excited porphyrin to fullerene.