Calixarene Linked Bis-Porphyrin Hosts for Fullerenes and the Assembly of Supramolecular Ferrocene-Porphyrin-Fullerene Triads

Abstract

Porphyrins and fullerenes are excellent substituents for the construction of photoactive assemblies due to their unique redox and photoelectronic properties. Upon irradiation these assemblies can undergo photoinduced electron transfer between the porphyrin donor and the fullerene acceptor, however these lifetimes are short lived. This thesis describes the synthesis of several calixarene linked bis-porphyrin hosts for the binding of fullerenes. Also described in this thesis, is the functionalization of the calixarene scaffold with ferrocene for the preparation of supramolecular triads. The presence of a secondary donor is expected to increase the lifetime of the photoinduced charge separated state through a second electron transfer from the ferrocene to the porphyrin. Chapter Two describes new methods for the synthesis and modification of porphyrins bearing different substituents at the meso-positions. Suzuki coupling has been employed with various boronic acids to form A2BC substituted porphyrins with aminophenyl at the 5-position, 3,5 ditert- butyl phenyls at the 10 and 20-positions and various substituents at the 15-position. These porphyrins have been coupled to calixarenes to construct bis-porphyrin hosts. UV-visible complexation studies and co-crystallates of porphyrins with fullerene have revealed that the 15-position of the porphyrin exhibits a greater influence of the fullerene affinity than initially believed, with fullerene association increasing with a higher number of CH-π interactions from porphyrin to the fullerene. Nickel catalyzed reactions with nitro and acetamidophenols have been employed to give phenyl ether porphyrins. These porphyrins have also been coupled to calixarenes to form bis-porphyrins, however these host displayed no affinity for fullerenes. Chapter Three describes modification to the calixarene scaffold to increase the flexibility of the bis-porphyrin. This increased flexibility has been achieved though extension of the methylene linkers and via tetra-alkylation of narrow rim to remove the hydrogen bonding motif. The extended linker bis-porphyrin host was observed to have no affinity for fullerenes and X-ray structures of the bis-porphyrin reveal that through the addition of an extra methylene linker the porphyrin planes orientate to an incorrect geometry for the accommodation of fullerenes. The tetra alkylation of the calixarenes with methyl and butyl groups bis-porphyrin hosts displayed lower binding affinities for fullerene by an order of magnitude lower than their dialkylated counter parts. Computational modeling has indicated that the removal of the hydrogen bonding motif inverts the pinched cone conformation of the calixarene scaffold, which translates to an unfavorable changes in the geometry of the binding site between the porphyrin planes. Chapter Four describes the functionalization of the wide rim of the calixarene scaffold with ferrocene via palladium catalyzed coupling reactions. Two palladium coupling methods were employed. The first method was Sonogashira coupling which has been employed to couple ferrocene moieties with different linker lengths. The second was Suzuki coupling, which as has been used to couple ferrocene phenyl boronic acid to two isomers of a tetra-alkylated calixarene. These isomers varied by the order of alkylation and in turn affect with phenyl ring the ferrocene is appended to, either para to the porphyrin amide of the n-butyl chain. Chapter Five describes the functionalization on the wide rim of a tetra alkylated diaminocalixarene scaffold with ferrocene benzoic acid via an amide coupling reaction. Two different isomers of the calixarene were prepared in an analogous method to the tetra-alkylated calixarenes in Chapter Four. The ferrocene functionalized hosts display similar association constants as the non-ferrocene functionalized host. The functionalization of ferrocene to the wide rim of the calixarene bisporphyrin resulting in minor fluorescence quenching of the porphyrin moieties suggesting the ferrocenes are at a remote distance from the porphyrins promoting the lifetime of the charge separated state through a second electron transfer.