Donor-Acceptor-Appended Triarylboron Lewis Acids: Ratiometric or Time-Resolved Turn-On Fluorescence Response toward Fluoride Binding

Abstract

Triarylboron Lewis acid compounds (CzmBoT (1c) and DPAmBoT (2c)), in which carbazole (Cz) or diphenylamine (DPA) donors are linked with a triazine acceptor in the ortho position of the phenylene ring are prepared and characterized. The treatment of 1c and 2c with the fluoride anion produces the corresponding fluoride adducts [1cF](-) and [2cF](-) as a tetraethylammonium salt. An X-ray diffraction study of [1cF](-) reveals a twisted conformation between the Cz and phenylene rings. The Cz-containing 1c shows a ratiometric fluorescence change upon fluoride binding, while the DPA-containing 2c exhibits a turn-on fluorescence response in tetrahydrofuran. In particular, both fluoride adducts exhibit thermally activated delayed fluorescence (TADF) properties with microsecond-range lifetimes. Electrochemical and theoretical analysis suggests that the intramolecular charge-transfer transition from the donor to a conjugated acceptor fragment is switched to the donor to a triazine transition after fluoride binding. Theoretical analysis further demonstrates the twisted structure, effective highest occupied molecular orbital-lowest unoccupied molecular orbital separation, and the small energy splitting between the excited singlet and triplet states for the fluoride adducts, with all supporting the observed TADF. The time-resolved fluorescence measurements of 2c in the presence of both fluoride and a competitive fluorescent dye (Coumarin 6) effectively eliminate the short-lived fluorescence of a dye, retaining long-lived fluorescence signals originating only from [2cF](-).