Excited state intramolecular proton transfer in 1,8-Dihydroxy-9,10-anthraquinone dye: Revealing microstructures in [CnmIm][NTf2] and [CnmIm][BF4] series of ionic liquid solvents

[Display omitted] •ESIPT process of 18DHAQ dye in [CnmIm][NTf2] and [CnmIm][BF4] IL solvents (n = 2, 4, 6, 8 and 10) display noteworthy modulations in relative emission intensities for normal and tautomeric forms.•Observed results reveal the microstructure formations in the [CnmIm][NTf2] and [CnmIm][BF4] series of the IL solvents.•More extensive microstructures are inferred as the alkyl chain length of the [CnmIm]+ cations of IL solvents is gradually increased.•The small differences between the results in [CnmIm][NTf2] and [CnmIm][BF4] series of ILs are understandably due to the dissimilar size, shape and basicity of the [NTf2]− and [BF4]− anions of the two series of IL solvents.•To the best of our knowledge, this is the only study involving ESIPT process to realize microstructure formations in neat IL solvents based on the [CnmIm]+ cations. Excited state intramolecular proton transfer (ESIPT) in 18-dihydroxy-9,10-anthraquinone (18DHAQ) dye has been investigated in two series of 1-alkyl-3-methylimidazolium ([CnmIm]+) based ionic liquid (IL) solvents, [CnmIm][NTf2] and [CnmIm][BF4], with n=2, 4, 6, 8 and 10, using steady-state (SS) and time-resolved (TR) fluorescence studies. In both the IL series, fluorescence intensity for tautomer (T*) form gradually decreases relative to normal (N*) form with increasing n value for [CnmIm]+ cations. Observed results suggest microstructure formation and its consequent effect on the ESIPT process of the dye in these IL solvents. Since more extensive microstructures are likely with larger n values of [CnmIm]+ cations, the dye is expectedly solubilized more in the polar microdomains of the solvent microstructures, resulting better stabilization for the more dipolar N* state and hence a higher emission intensity from this state. This proposition is clearly supported by ultrafast (sub-picosecond) fluorescence kinetics for both N* and T* states, though sub-nanosecond TR results indicate very similar fluoresce decays for both N* and T*, suggesting an eventual kinetic equilibrium between two states subsequent to the initial ultrafast and fast forward and backward ESIPT processes. Small differences between the results in the [CnmIm][NTf2] and [CnmIm][BF4] series of ILs are attributed to dissimilar size, shape and basicity of [NTf2]− and [BF4]− anions, responsible for some characteristic changes in the microstructures formed in the respective solvent series. To the best of our knowledge, present study is the only report demonstrat