Abstract
Halogen bonds are a highly directional class of intermolecular interactions widely employed in chemistry and chemical biology. This linear interaction is commonly viewed to be analogous to the hydrogen bond because hydrogen bonding models also intuitively describe the σ-symmetric component of halogen bonding. The possibility of π-covalency in a halogen bond is not contemplated in any known models. Here we present evidence of π-covalency being operative in halogen bonds formed between chloride and halogenated triphenylamine-based radical cations. We reach this conclusion through computational analysis of chlorine K-edge X-ray absorption spectra recorded on these halogen bonded pairs. In light of this result, we contend that halogen bonding is better described by analogy to metal coordination bonds rather than hydrogen bonds. Our revised description of the halogen bond suggests that these interactions could be employed to influence the electronic properties of conjugated molecules in unique ways.
Highlights
Background signalExcitation energyhalogen bonds[41,42]
The triphenylamine core stabilizes the radical cation sufficiently to allow for spectroscopic studies[43,44]
The Dye-X compounds were adsorbed to a solid mesoporous TiO2 thin film with a common molecular orientation (Supplementary Fig. 2), thereby facilitating the study of halogen bonding interactions with a solution-phase species while suppressing selfinteraction between Dye-X molecules[45]
Summary
Halogen bonds[41,42]. For the halogen bonded chloride adducts of Dye-I+ and Dye-Br+, we find that the well-resolved low-energy “pre-edge” spectral signature arises from π-orbital covalency.
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