Synthesis and fluorescence of N-squaraine dianions derived from electron-deficient primary anilines

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 acidbase reaction with alkaline hydroxide in mixed DMSO-H2O 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 C4N2O22- squaryl core with absorption maxima at 450-748 nm (molar absorptions up to 7.6 x 104 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 Nsquaraines are chemically stable for hours in 50% v/v H2O-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.