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

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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. E(T)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 deltaS++ 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.