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

Using 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 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.

Efficient Intra‐Supramolecular Electron Transfer in Zinc Porphyrin‐Coumarin Dyads

“…[23,24] To do this, a number of non-covalently supramolecular systems were created and synthesized using various binding mechanisms in order to mimic biological systems' operations and comprehend the fundamental ideas behind photoinduced energy-transfer and electron-transfer reactions in the photosynthetic systems' reaction centers. [25][26][27][28][29][30][31][32][33][34][35][36] Compared to covalent systems, such noncovalently connected donor-acceptor complexes have the benefit of being simpler to assemble.…”

“…The photo‐ and redox‐active elements in the bacterial photosynthetic reaction centers are grouped into a protein matrix in a well‐organized manner through non‐covalent interactions, as was previously shown [23,24] . To do this, a number of non‐covalently supramolecular systems were created and synthesized using various binding mechanisms in order to mimic biological systems’ operations and comprehend the fundamental ideas behind photoinduced energy‐transfer and electron‐transfer reactions in the photosynthetic systems’ reaction centers [25–36] . Compared to covalent systems, such non‐covalently connected donor‐acceptor complexes have the benefit of being simpler to assemble.…”