On the Fine-Tuning of the Excited-State Intramolecular Proton Transfer (ESIPT) Process in 2-(2′-Hydroxybenzofuran)benzazole (HBBX) Dyes

Herein, a full investigation of the optical properties and first-principles calculations of a large series of original 2-(2'-hydroxybenzofuran) benzazole (HBBX) dyes is described. The electronic substitution on the p-conjugated core of the fluorophores and the nature of the heteroatom (O, S, N) was varied extensively to assess the necessary parameters to trigger a partial frustration of the excited-state intramolecular proton transfer (ESIPT) process, which results in the emission of both tautomers, that is, enol and keto (E* and K*). The optical properties, studied in solution and in the solid state, revealed the appearance of either an intense single K* or a dual E*/K* emission; a feature that is highly dependent on the electronic substitution (donating or accepting), the heteroelement, and the close environment. Subtle modifications of these parameters allowed the establishment of structureproperty relationships that were successfully rationalized by first- principles calculations. In particular, the E*/K* emission intensity ratio was shown to be directly related to the free energies of the two emissive tautomers in the excited state.