Role of intermolecular charge transfer towards fluorometric detection of fluoride ions with anthrapyrazolone derivatives†

Anion detection using small molecules as chemosensors holds significant importance in the biological and environmental fields, offering several advantages over conventional methods. In this report, we have developed three anthrapyrazolone derivatives, namely 2,7-dihydrobenzo[1,2,3-cd:4,5,6-c′d′]bis(indazole) (DHBBI), 2-(benzo[1,2,3-cd:4,5,6-c′]bis(indazole)-2(7H)-yl)ethanol (DHBBI-OH), and 2,2′-(benzo[1,2,3-cd:4,5,6-c′d′]bis(indazole)-2,7-diyl)diethanol (DHBBI-2-OH), utilizing straightforward chemical reactions. These compounds were investigated for the fluorometric detection of a series of anions viz. F⁻, Cl⁻, Br⁻, I⁻, OH⁻, CN⁻, ClO₄⁻, SO₄²⁻, NO₃⁻ and SCN⁻ in the form of tetrabutylammonium salts. Detailed photophysical and mechanistic investigations were conducted to understand the interactions of three fluorophores with different anions. Fluorescence analysis showed considerable changes in the emission intensity of the three fluorophores in the presence of a series of anions as mentioned earlier. Among the synthesized molecules, DHBBI (K_sv = 11.6 × 10⁴ M⁻¹) exhibited the highest selectivity with ∼1.30 and 1.80 fold higher association rate constants and sensitivity with a limit of detection of ∼10.3 mM towards F⁻ ions compared to DHBBI-OH and DHBBI-2-OH. The selectivity and sensitivity towards F⁻ ions were demonstrated in light of hydrogen-bonding interactions between DHBBI and F⁻ ions. Additionally, DFT and TDDFT studies were employed for DHBBI and its derivatives to investigate their structural insights and electronic properties comprehensively.