Thermal reversion of spirooxazine in ionic liquids containing the [NTf2]− anion

We have investigated the kinetic and thermodynamic parameters of thermal reversion of the spirooxazine 1,3,3-trimethyl-5′-(2-benzothiazolyl)-spiroindoline-2,3′-naphtho(2,1-b)(1,4) oxazine (SO) in molecular solvents and ionic liquids containing the [NTf2]− anion. ET(30) and Kamlet–Taft parameter studies were employed to interpret IL polarity and attempt to rationalise the kinetic–polarity relationship of MC→SO relaxation. The observed thermal relaxation of SO within ionic liquids was slower than that of molecular solvents with similar polarity, indicating a greater degree of interactions between the ionic liquid ions and the zwitterionic MC isomer, which led to increased lifetimes for the MC–ion complexes (19.6 s in acetonitrile and 90.9 s in [P6,6,6,14][NTf2]). Thermodynamic parameters of activation implied enhanced ordering of SO in ILs and its subsequent effect on thermal reversion of SO. Activation parameters found MC→SO relaxation was more temperature dependent in polar protic ILs such as [bmIm]+ than aprotic ILs such as [P6,6,6,14][NTf2] with ΔS‡ values of 40 J K mol−1 and −8.6 J K mol−1, respectively. Activation energies were generally higher in ILs (83.9–97 kJ mol−1) compared to molecular solvents (67.8–89.8 kJ mol−1), which was reflected in longer lifetimes of MC due to greater energy barriers of relaxation to SO. Pre-metathesis cleaning of precursor salts was found to be necessary in order to obtain spectroscopic grade ILs for physicochemical analysis using solvatochromic probe dyes. Inconsistencies in the polarity–kinetic relationship further reinforced the belief that measuring polarity of ILs may not be possible with solvatochromic probe dyes.