Intramolecular Electron Transfer in Multi‐Redox Systems Based on Cyclic [3]Spirobifluorenylene Compound**

Abstract

AbstractCyclic [3]spirobifluorenylene with bulky alkyl groups at the ends (1) was designed and synthesized to investigate the electron transfer phenomena in a π‐conjugated system including orthogonal π‐conjugated chains. The three bifluorenyl units in 1 are conjugated to each other via spiro‐conjugation, resulting in the splitting of the HOMO levels to a small extent. Therefore, the SOMO‐HOMO gap of the radical cation species is small, which is considered to allow the facile intramolecular electron transfer. The electronic properties of 1 and its partial structures were characterized by absorption and fluorescence measurements and electrochemical analysis. From the electrochemical oxidation, the interchain Coulombic repulsion was observed. In the TD‐DFT calculations for the radical cation species of 1, the geometry‐featured interchain electronic transitions were visualized by NTO calculations. The radical cation species of 1 generated by chemical oxidation with SbCl5 exhibited a broadened and lower‐energy NIR absorption band exceeding 2000 nm. Considering the results of the TD‐DFT calculations, the NIR band of the radical cation of 1 was attributed to the intramolecular electron transfer processes among the bifluorenyl units in the macrocycle. ESR experiments also indicated the delocalization of a spin of 1⋅+ in the whole molecule via hole hopping in the ESR time scale at room temperature. This work demonstrates the usefulness of spiro‐conjugation as a bridging unit in molecular wires to facilitate smooth electron transfer.