Characterization of Cyclic [<i>n</i>]Spirobifluorenylene Compounds and Electron Delocalization in Their Radical Cation Species

Charge delocalization through spiro-conjugation provides insights for the rational design of next-generation charge transport materials. We have previously conducted studies using terminally alkyl-protected cyclic [3]spirobifluorenylene compound 1-[3], which features a cyclic structure composed of bifluorenyl units. ESR studies on this radical cation species have shown that its electron is delocalized over approximately three bifluorenyl units, meaning that it is delocalized across the entire molecule. In this study, we newly synthesized cyclic [n]spirobifluorenylene compounds 1-[4] and 1-[5] and investigated their spectroscopic and electrochemical properties, and chemical oxidation behavior. Additionally, DFT calculations were performed not only for the neutral species but also for the radical cation species. Furthermore, we examined the influence of the number of perpendicular π-conjugated units on electron delocalization in their radical cation species. As a result, ESR spectral analysis of the radical cation species 1-[4]^·+ and 1-[5]^·+ revealed that electrons were delocalized over 3.37 and 3.87 units, respectively, when considering bifluorenyl as the unit structure.