Photoswitchable Photochromic Chelating Spironaphthoxazines: Synthesis, Photophysical Properties, Quantum-Chemical Calculations, and Complexation Ability.

The stable and efficient photochromic and photoswitchable molecular systems designed from spirooxazines are of increasing scientific and practical interest because of their present and future applications in advanced technologies. Among these compounds, chelating spironaphthoxazines have received widespread attention due to their efficient optical response after complexation with some metal ions being of biomedical interest and environmental importance, as well as their good cycle performance and high reliability, especially by metal ion sensing. In this mini-review, we summarize our results in the design of novel photoswitchable chelating spironaphthoxazines with specific substituents in their naphthoxazine or indoline ring systems in view of recent progress in the development of such molecular systems and their applications as metal ion sensors. The design, synthesis methods, and photoresponse of such spirooxazine derivatives relevant to their applications, as well as quantum-chemical calculations for these compounds, are presented. Examples of various design concepts are discussed, such as sulfobutyl, hydroxyl, benzothiazolyl, or ester and carboxylic acid as substituents in the chelating spironaphthoxazine molecules. Further developments and improvements of this interesting and promising kind of molecular photoswitches are outlined.

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. 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.

Investigating nanostructuring within imidazolium ionic liquids: a thermodynamic study using photochromic molecular probes.

Molecular photoswitches have been used to investigate the possibility of nanostructured polar and nonpolar domains in ionic liquids (ILs). Two photochromic compounds, spiropyran (BSP) and spirooxazine (SO) were added to imidazolium based ionic liquids containing the anion [NTf2](-), and their photochromic behavior was monitored with increasing side chain length (C(2)-C(12)) of the imidazolium cation. Increasing side chain length was found to have only minor effects on the rate of thermal relaxation of the merocyanine form of spiropyran (MC(BSP)) and spirooxazine (MC(SO)) to BSP and SO, respectively. BSP was found to be a suitable optical probe, as linear correlations in parameters were observed for this compound. This is believed to be because BSP-IL interactions are based on hydrogen bonding between the MC(BSP) and the ionic liquid cations, compared to MC(SO), which is limited to electrostatic interactions. Hence, the sensitivity of MC(BSP) is enhanced in the charged polar regions of the IL. Increasing the side chain of the cation results in slight increases in MC(BSP) to BSP relaxation activation energy from 96.93 kJ x mol(-1) in [C(4)mIm][NTf(2)] to 105.27 kJ x mol(-1) in [C(12)mIm][NTf(2)]. MC(BSP) to BSP relaxation DeltaS(double dagger) and DeltaH(double dagger) values also increase with increasing side chain length. The ability for spirocyclic compounds to switch between polar and nonpolar forms appears to allow polar and nonpolar regions in ILs to be probed dynamically using a single probe dye. It appears that the value of the ground state equilibrium constant, K(e), is dominated by the nonpolar regions of the IL while the equilibrium constant of activation, K(double dagger), is dominated by the polar regions. A correlation of side chain length to equilibrium constant of activation is believed to be because polar regions are possibly expanding due to increasing influence of nonpolar side chain interactions and compound insertion upon the overall solvent structure. The result of such reordering and dispersion of polar regions reduces solvent-solute interactions which increases the rate of MC(BSP) to BSP relaxation.