Evidence for Interfacial Halogen Bonding

A homologous series of donor–π–acceptor dyes was synthesized, differing only in the identity of the halogen substituents about the triphenylamine (TPA; donor) portion of each molecule. Each Dye‐X (X=F, Cl, Br, and I) was immobilized on a TiO2 surface to investigate how the halogen substituents affect the reaction between the light‐induced charge‐separated state, TiO2(e−)/Dye‐X+, with iodide in solution. Transient absorption spectroscopy showed progressively faster reactivity towards nucleophilic iodide with more polarizable halogen substituents: Dye‐F < Dye‐Cl < Dye‐Br < Dye‐I. Given that all other structural and electronic properties for the series are held at parity, with the exception of an increasingly larger electropositive σ‐hole on the heavier halogens, the differences in dye regeneration kinetics for Dye‐Cl, Dye‐Br, and Dye‐I are ascribed to the extent of halogen bonding with the nucleophilic solution species. Scratching the surface of halogen bonding: A homologous series of donor–π–acceptor dyes bearing different halogen substituents, adsorbed onto a semiconductor surface, showed differences in reactivity towards nucleophiles that track with the extent of halogen bonding. Transient spectroscopic methods were used to show that this intermolecular interaction is most significant for the most polarizable halogen substituent.