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 (in) 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-(ZnP,), 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 similar to 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.