Intramolecular electron transfer in multi-redox systems based on cyclic [3]spirobifluorenylene compound

Cyclic [3]spirobifluorenylene with bulky alkyl groups at the ends (1) was designed and synthesized to investigate the electron transfer phenomena in a -conjugated system including orthogonal -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 shifted NIR absorption band compared to those of its partial structures. 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. This work demonstrates the usefulness of spiro-conjugation as a bridging unit in molecular wires to facilitate smooth electron transfer.