<p>Two perfluorinated derivatives of benzoylacetone—pentafluorobenzoyl-trifluoroacetone (PFBA) and decafluorodibenzoylmethane (DFDM)—were synthesized and studied by electronic absorption spectroscopy in aqueous solution to evaluate the effect of σ-acceptor substituents in the benzene ring on spectral and thermodynamic properties. The acid–base properties were determined over the pH range 1–9 using protein buffers at an ionic strength of <i>I</i> = 0.5&#xa0;mol·L^–1 (NaCl). PFBA exists predominantly in the keto form in aqueous solution, whereas DFDM occurs in the enol form both in water and in organic solvents. Chelation processes with gadolinium(III), thulium(III), and scandium(III) ions were investigated and revealed a non-monotonic correlation between the formation constants of the monocomplexes and the number of fluorine atoms. Comparison with previously obtained data showed that fluorination of the aliphatic chain does not affect the spectral properties, whereas fluorination of the phenyl group leads to significant changes in both spectral and thermodynamic parameters. The spectra of the chelated and ionized forms of the fluorinated derivatives are blue-shifted depending on the degree of fluorination. Decafluorodibenzoylmethane exhibits the largest known red shift among diketonates. The difference in the absorption maxima between the spectra of 4<i>f</i>- and 5<i>f</i>-metal complexes with this ligand exceeds 20&#xa0;nm, making it a promising candidate for use as an effective chemosensor. DFT modeling indicates that delocalization energy is a major contributor to the energy of electronic π–π* transitions.</p>