Photo- and solvatochromic properties of nitrobenzospiropyran in ionic liquids containing the [NTf2]− anion

Abstract

The photo-, thermo- and solvatochromic properties of 2,3-dihydro-1′,3′,3′-trimethyl-6-nitrospiro-[1-benzopyran-2,2′-1H-indole] (BSP-NO₂) were studied in ILs containing the anion [NTf₂]⁻ by UV-Vis absorption spectroscopy, ab initio molecular orbital theory and density functional theory (DFT) calculations. It was found that the kinetics and thermodynamics of the BSP-NO₂ ↔ MC (merocyanine) equilibrium was sensitive to the nature of the cation. It was also observed that the imidazolium cation can form a through-space orbital interaction with the MC isomer, rather than a simple electrostatic interaction, thus preventing the MC conversion back to the BSP-NO₂ isomer. The BSP-NO₂ ↔ MC equilibrium thus serves as a model system for studying modes of interaction of the cations in ionic liquids.