Structural Constraint of Overcrowded Alkenes: Macrocyclization of 9,9′-Bifluorenylidene

Abstract

We report the successful synthesis of Cyclic BF and Cyclic Bis-BF, in which alkyl bridges installed at the 2-position serve as precise structural handles for constructing macrocyclic architectures with fine-tuned molecular strain. Enabled by a modified Barton–Kellogg reaction, this strategy overcomes the significant steric barriers typically associated with functionalizing 9,9′-bifluorenylidene. Comprehensive characterization, including NMR spectroscopy, single-crystal X-ray diffraction, DFT calculations, UV–Vis spectroscopy, and electrochemical measurements, reveals that strain-induced conformational locking governs the molecular structure–mechanical expansion–electronic property relationship. Notably, the macrocyclic frameworks exhibit electronic invariance despite increased molecular complexity, highlighting the dominant role of molecular strain induced by structural constraint. This work establishes a versatile strategy for designing strain-regulated macrocycles with predictable and tunable properties, providing new insights into the interplay between molecular architecture, geometry, molecular strain, and electronic properties.