A novel approach to the design of time-resolved turn-on fluorescence sensors was proposed and utilized in hydrogen peroxide sensing.

These authors contributed equally to this work.We report the synthesis and thermally activated delayed fluorescent (TADF) properties of ortho-carbazole-appended triazine compounds. Two donor (D)-acceptor (A) compounds, CzMeoTRZ (1) and BuCzMeoTRZ (2), in which the carbazole (Cz and t-BuCz) donor and 4,6-dipheny-1,3,5-triazine (TRZ) acceptor moieties are linked in the ortho position of the 3-methylphenylene ring were prepared and characterized. Both compounds exhibit broad blue emission at 465 and 487 nm, respectively (photoluminescence quantum yield, Φ PL = 27% and 49% in toluene), which is typical of the donor to acceptor ICT transition. The transient PL decay measurements confirm that both compounds have TADF character with the microsecond-range delayed lifetimes (τ d = 1.09 μs for 1 and 1.51 μs for 2). Electrochemical analysis indicates that both compounds undergo carbazole-centered oxidation and triazine-centered reduction. Theoretical studies further show the twisted D-A structure, effective separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and small energy splitting between the excited singlet and triplet states for 1 and 2, all of which support the observed TADF.

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]−.