Multiporphyrin assemblies on cyclophosphazene scaffolds: synthesis and studies

Abstract

The stable and robust cyclotriphosphazene and cyclotetraphosphazene rings were used as scaffolds to prepare hexa- and octaporphyrin arrays by treating N₃P₃Cl₆ and N₄ P₄ Cl₈, respectively, with 5-(4-hydroxyphenyl)-10,15,20-tri(p-tolyl)porphyrin (N₄ core) or with its thiaporphyrin analogues (N₃S and N₂S₂ cores) in THF in the presence of Cs₂CO₃ under simple reaction conditions. Thiaporphyrins were examined in addition to the normal porphyrin to tune the electronic properties of the resultant arrays. Observation of the molecular ion peaks in the mass spectra confirmed the molecular structures of the arrays. 1D and 2D NMR techniques were employed to characterize the multiporphyrin arrays in detail. The ¹H NMR spectra of the multiporphyrin arrays each show a systematic set of signals, indicating that the porphyrin units are arranged in a symmetrical fashion around the cyclophosphazene rings. All signals in the ¹H NMR spectra were assigned with the aid of COSY and NOESY experiments. The protons of each porphyrin unit are subject to upfield and downfield shifts because of the ring-current effects of neighboring porphyrin units. Optical, electrochemical and fluorescence studies of the arrays indicated that the porphyrin units retain their independent ground- and excited-state characteristics. Cu^II and Ni^II derivatives of hexaporphyrin and octaporphyrin arrays containing N₄ porphyrin units and N₃ S porphyrin units were synthesized, and complete metalation of the arrays was confirmed by their mass spectra and by detailed NMR characterization of the Ni^II derivatives of hexa- and octaporphyrin arrays containing N₄ porphyrin units. Electrochemical studies indicated that Cu^II and Ni^II ions present in the thiaporphyrin units of the arrays can be stabilized in the +1 oxidation state, which is not possible with arrays containing normal porphyrin units.