Synthesis of covalently linked unsymmetrical porphyrin pentads containing three different porphyrin subunits

Abstract

The tetrafunctionalized AB₃-type porphyrin building blocks containing two different types of functional groups with N₄, N₃O, N₃S and N₂S₂ porphyrin cores were synthesized by following various synthetic routes. The AB₃-type tetrafunctionalized N₄ porphyrin building block was synthesized by a mixed condensation approach, the N₃S and N₃O porphyrin building blocks by a mono-ol method and N₂S₂ porphyrin building block by an unsymmetrical diol method. The tetrafunctionalized porphyrin building blocks were used to synthesize monofunctionalized porphyrin tetrads containing two different types of porphyrin subunits by coupling of 1 equiv of tetrafunctionalized N₄, N₃O, N₃S and N₂S₂ porphyrin building block with 3 equiv of monofunctionalized ZnN₄ porphyrin building block under mild copper-free Pd(0) coupling conditions. The monofunctionalized porphyrin tetrads were used further to synthesize unsymmetrical porphyrin pentads containing three different types of porphyrin subunits by coupling 1 equiv of monofunctionalized porphyrin tetrad with 1 equiv of monofunctionalized N₂S₂ porphyrin building blocks under the same mild Pd(0) coupling conditions. The NMR, absorption, and electrochemical studies on porphyrin tetrads and porphyrin pentads indicated that the monomeric porphyrin subunits in tetrads and pentads retain their individual characteristic features and exhibit weak interaction among the porphyrin subunits. The steady state and time-resolved fluorescence studies support an efficient energy transfer from donor porphyrin subunit to acceptor porphyrin subunit in unsymmetrical porphyrin tetrads and porphyrin pentads.