Zinc Tetrapyrrole Coordinated to Imidazole Functionalized Tetracyanobutadiene or Cyclohexa‐2,5‐diene‐1,4‐diylidene‐expanded‐tetracyanobutadiene Conjugates: Dark vs. Light‐Induced Electron Transfer

Abstract

AbstractUsing the popular metal‐ligand axial coordination self‐assembly approach, donor‐acceptor conjugates have been constructed using zinc tetrapyrroles (porphyrin (ZnP), phthalocyanine (ZnPc), and naphthalocyanine (ZnNc)) as electron donors and imidazole functionalized tetracyanobutadiene (Im‐TCBD) and cyclohexa‐2,5‐diene‐1,4‐diylidene‐expanded‐tetracyanobutadiene (Im‐DCNQ) as electron acceptors. The newly formed donor‐acceptor conjugates were fully characterized by a suite of physicochemical methods, including absorption and emission, electrochemistry, and computational methods. The measured binding constants for the 1 : 1 complexes were in the order of 104–105 M−1 in o‐dichlorobenzene. Free‐energy calculations and the energy level diagrams revealed the high exergonicity for the excited state electron transfer reactions. However, in the case of the ZnNc:Im‐DCNQ complex, owing to the facile oxidation of ZnNc and facile reduction of Im‐DCNQ, slow electron transfer was witnessed in the dark without the aid of light. Systematic transient pump‐probe studies were performed to secure evidence of excited state charge separation and gather their kinetic parameters. The rate of charge separation was as high as 1011 s−1 suggesting efficient processes. These findings show that the present self‐assembly approach could be utilized to build donor‐acceptor constructs with powerful electron acceptors, TCBD and DCNQ, to witness ground and excited state charge transfer, fundamental events required in energy harvesting, and building optoelectronic devices.