Synthesis of mono-functionalized core-modified expanded porphyrin building blocks and covalently linked expanded porphyrin dyads

Abstract

A series of mono-functionalized core-modified expanded porphyrin building blocks such as thiasapphyrin, thiarubyrin, oxasmaragdyrin, and BF₂–oxasmaragdyrin have been synthesized by simple condensation of readily available precursors. The mono-functionalized core-modified expanded porphyrin building blocks were used to synthesize the first three examples of covalently linked diphenylethyne-bridged dyads containing two different expanded porphyrin macrocycles, namely thaisapphyrin–BF₂–oxasmaragdyrin, thiarubyrin–BF₂–oxasmaragdyrin, and thiasapphyrin–thiarubyrin, by coupling appropriate mono-functionalized expanded porphyrin building blocks under mild Pd⁰ coupling reaction conditions. The three dyads were freely soluble in common organic solvents and characterized by MS, NMR, absorption, electrochemical and fluorescence techniques. The NMR, absorption and electrochemical studies indicated that the two macrocycles in the dyads interact weakly with each other and maintain their independent characteristic features. The steady-state fluorescence studies of the dyads showed that the thiasapphyrin and thiarubyrin units are nonfluorescent but fluorescence was observed from the BF₂–oxasmaragdyrin unit. However, the quantum yield of the BF₂–oxasmaragdyrin unit in the dyads was less than that of monomeric BF₂–oxasmaragdyrin because of an enhancement of nonradiative decay channels operating in the dyads. The potential use of two of the three dyads containing the BF₂–smaragdyrin subunit as fluorescent sensors for anions was explored. The studies showed that the binding of the anion at the protonated sapphyrin and rubyrin sites in the respective dyads can be followed by the clear changes in the fluorescence band of the BF₂–oxasmaragdyrin unit, which indicates that these dyads can be used as fluorescent anion sensors.