Tuning the photophysical and photo acidic properties of N-methyl-6-oxyquinolonium-based ionic liquid dyes: the role of solvent and substitution effects investigated by a TD-D3-DFT approach†

N-Methyl-6-hydroxyquinolinium-based IL dyes are a promising class of dyes with a wide range of potential applications. In this work, the photophysical and photo acidic properties of N-methyl-6-oxyquinolonium-based ionic liquid dyes [6MQc][Y1–6] (Y1–6 = CH₃CO₂⁻, CF₃CO₂⁻, NTf₂⁻, CF₃SO₃⁻, BF₄⁻, and PF₆⁻) were investigated in three solvent media at the TD-PBE0-D3/6-311++G(d,p) level of theory. The impact of solvent and anion on the absorption and emission spectra and the excited-state properties of these dyes were explored. The optimized structures revealed an increase in the O–H⋯X (X = O or F) and a decrease in the C–H⋯X hydrogen bond distances formed between the [6MQc]⁺ cation and anions due to S₀ to S₁ photoexcitation. The pairing of the [6MQc]⁺ cation with [CH₃COO]⁻ and [CF₃COO]⁻ anions leads to the formation of strong Brønsted acids, which exhibit the largest absorption wavelength and Stokes shift in the fluorescence emission (116–145 nm and 168–204 nm) and solvatochromic effect. The studied IL dyes have fluorescence emissions in the range of 430 to 565 nm. Our study contributes to the understanding of the unique properties of IL dyes and provides insights into the design and optimization of N-methyl-6-hydroxyquinolinium-based ionic liquid dyes for a range of applications.