Synthesis and fluorescence of N-squaraine dianions derived from electron-deficient primary anilines. (November 2022)

• Record Type: Journal Article
• Authors: Vega, Manel; Gomila, Rosa M.; Pons, Jordi; Frontera, Antonio; Rotger, Carmen; Costa, Antonio
• Is Part Of: Dyes and pigments. Volume 207(2022)
• Publication Date: 2022-11
• Subjects: Squaraine dyes -- Dianion -- Fluorescence -- DFT calculations; Dyes and dyeing -- Periodicals; Pigments -- Periodicals; 667.2
• DOI: 10.1016/j.dyepig.2022.110746
• Languages: English
• ISSNs: 0143-7208

Abstract

Abstract: Squaraines are 1, 3-disubstituted squaric acid derivatives featuring intense fluorescence in the visible. Unlike their parent compounds, N-squaraines obtained by condensation reaction between squaric acid and primary anilines are non-fluorescent compounds. The incorporation of electron-withdrawing substituents in the para position of the phenyl rings turns these compounds acidic enough to form the corresponding dianions by acid-base reaction with alkaline hydroxide in mixed DMSO-H2 O solvent mixtures. This work reports the microwave-assisted synthesis of N-squaraines and the investigation of the optical characteristics of their dianions. The dianionic squaraines have a C4 N2 O2 2− squaryl core with absorption maxima at 450–748 nm (molar absorptions up to 7.6 × 10 4 M −1 cm −1 ). The N-squaraine dianions are highly fluorescent in the 507–644 nm range (quantum yields 0.18–0.72). Due to their negative charge, and contrary to the expectations, anionic N-squaraines are chemically stable for hours in 50% v/v H2 O–DMSO solvent mixtures. DFT calculations show the absorption bands are S0 → S1 excitations (HOMO → LUMO) and S1 → S0 transitions (LUMO → HOMO) for the emission. In both cases, the calculated wavelengths compare well with the experimental observations. Graphical abstract: Image 1 Highlights: Discovering the fluorescent behaviour of classical N-squaraines dianions derived from primary anilines. Aryl rings having electron-withdrawing substituents turn N-squaraines slightly acidic. N-squaraine dianions can be brightly fluorescent in alkaline dimethylsulfoxide-water solutions. Experimental and calculated absorption and emission data support a simple HOMO-LUMO excitation-emission pathway.