Conducting Polymer Networks Cross‐Linked by “Isolated” Functional Dyes: Design, Synthesis, and Electrochemical Polymerization of Doubly Strapped Light‐Harvesting Porphyrin/Oligothiophene Monomers

Abstract

Dye-functionalized conducting polymers: A new concept in the design of monomers gives bithiophene-based monomers as "isolated" functional dyes, thus yielding electroactive films through anodic electrochemical polymerization (see picture).We have synthesized doubly strapped porphyrin derivatives with four bithiophene segments that diverge from the porphyrin core, namely, Por(BT)(4) and PorZn(BT)(4). These molecules are designed as electrochemically polymerizable monomers that will yield the highly cross-linked conducting polymeric networks poly[Por(BT)(4)] and poly[PorZn(BT)(4)], respectively, through an oxidative coupling reaction among the bithiophene moieties. Selective synthesis of the distal doubly strapped porphyrin derivatives was successful from the bis(formylphenyl) and bis(dipyrrolylmethylphenyl) straps under Lindsey conditions. Slow kinetics observed for the zinc insertion reaction toward the doubly strapped porphyrin derivative revealed that both faces of the porphyrin plane are overlaid with alkyl chain straps. To probe this "isolation" effect, photophysical properties of the monomers and their model components were investigated by UV/Vis and fluorescence spectroscopic analysis. An absorption spectral comparison between the diluted solution and the thin solid film of Por(BT)(4) demonstrated that the porphyrin molecule is shielded by the double strap and self-aggregation is prevented. It is noteworthy that the fluorescence of the spin-coated film of the doubly strapped porphyrin monomer was twice as strong as that of an unstrapped porphyrin, thus indicating that the double strap can suppress undesired deactivation processes from the photoexcited state. In addition, fluorescence spectral measurements revealed that quantitative energy transfer from the oligothiophene segments to the porphyrin molecule takes place, which demonstrates an effective electronic interaction between these two chromophores. Electrochemical polymerization of the monomers Por(BT)(4) and PorZn(BT)(4) gave robust films that showed stable electrochemistry. Absorption spectral measurements and electrochemical characterization of the obtained films showed that the doubly strapped porphyrins are incorporated into the conducting polymer networks without any decomposition and protonation. Given all these observations above, our new monomer design based on functional dyes shielded by the double strap will lead to new organic optoelectronic materials in which functional molecules are spatially incorporated and isolated and yet show an effective electronic interactions with the conducting polymer backbones.