Modulation of the Excited-State Intramolecular Proton Transfer (ESIPT) process in 2-(2′-Hydroxybenzofuran)benzoxazole (HBBO) dimers

A series of four dimers based on the connection of two 2-(2′-Hydroxybenzofuran)benzoxazole (HBBO) subunits are described in this article. HBBO fluorophores are especially known for displaying sizable excited-state intramolecular proton transfer (ESIPT) process and a subsequent fluorescence emission stemming from a keto tautomer. Two HBBO monomers, connected through perfluoropropane, sulfonyl spacers or directly connected were synthesized and characterized, in order to assess the influence of the bridge on the photophysical properties. All HBBO monomers comprise of a benzofuranol heterocycle functionalized with an electrodonating group, either p-methoxy (OMe) or p-N,N'-dibutylamino (NBu2). The studies of the optical properties in solution and in the solid-state reveal the presence of either a single keto (K∗) or a dual enol/keto (E∗/K∗) emission, depending on the electronic substitution on the benzofuranol ring but also on the electronegativity of the connecting bridge and the close environment (solvent or matrix). Structure/properties relationships confirmed the strong influence of the electronic nature of the spacer on the pKB of the aromatic amine of the benzoxazole units and consequently on the intensity of the proton transfer in the excited-state. The sulfonyl linker was found to be most electronegative spacer in the series, leading to the strongest ESIPT frustration and thus, the highest E∗/K∗ intensity ratio. All dimers containing a NBu2 moiety were responsive to protonation, leading to an enhanced acidity of the phenolic proton in each case.