Study of halogen substitution effects on the structural and electronic properties of overcrowded alkene rotors

Abstract

Overcrowded alkenes are central building blocks in the design of molecular motors, whose conformational and electronic properties can be finely tuned through molecular design. 9,9′-bifluorenylidene (9,9′-BF) derivatives are intriguing scaffolds due to their remarkable conformational flexibility, with the angle between fluorene units varying dramatically from 31° to 52° depending on environmental conditions, crystal packing arrangements, and the nature of substituents present. The strategic placement of halogen atoms within the bifluorenylidene framework offers a powerful method for controlling intermolecular interactions and influencing how molecules arrange themselves in crystalline solids. Halogen bonding interactions exhibit remarkable geometric precision and considerable strength, making them valuable tools for creating materials with predictable structures. In this work, we systematically examine how halogen substitution affects the electronic structure and solid-state behaviour of π-conjugated 9,9′-BF derivatives. The complex interplay between halogen bonding and other weak interactions, including π–π stacking and C–H⋯π contacts, creates a rich landscape of possibilities that governs molecular self-assembly and ultimately determines the final crystal architecture. Single-crystal X-ray diffraction, spectroscopic methods, electrochemical analysis, and quantum chemical computations were used to understand how bromine atoms modify intramolecular twisting, supramolecular interactions, and crystallographic packing motifs. We also demonstrated that halogen substitution impacts thermal properties, phase transitions, and electrochemical features. Electrochemical studies show a progressive stabilisation of reduced states with increasing halogenation, attributed to the electron-withdrawing nature of bromine. The experimental findings are corroborated by theoretical calculations, highlighting how halogen substitution can manipulate the structure–property relationships in these π-conjugated systems, providing fundamental insights for their future application in sophisticated material design, including molecular machines.