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Switchable materials have tremendous potential for application in sensor development that could be applied to many fields. We are focusing on emerging area of wireless sensor networks due to the potential impact of this concept in society. Spiropyran-based sensors are probably the most studied type of photoswitchable sensing devices. They suffer from many issues but photofatigue, insufficient selectivity and lack of sensitivity are probably the most important characteristics that hinder their wider application. Here, we are address these issues and demonstrate that covalent attachment of modified spiropyran into a polymeric film significantly reduces photodegradation. The observed signal loss after 12 th cycle of switching between the spiropyran and merocyanine forms is only about 27% compared to the loss of 57% of the initial signal in an equivalent experiment based on nonimmobilized spiropyran. This has enabled us to demonstrate at least 5 reversible cycles of detection of an ion of interest (in our case H + ) with minimal signal loss. Furthermore, we demonstrate that the sensitivity can be increased by incorporation of additional binding groups in the parent spiropyran molecule. Using molecular modeling to calculate the relevant bond lengths as a measure of interaction between MC and H + , the calculated increase of H-bond strength is approximately an order of magnitude for a derivative containing a methoxy group incorporated in the o-position of the parent spiropyran in comparison to the equivalent unsubstituted phenol. This theoretical result was found to correspond very well with experimental observation. As a result, we have increased the sensitivity to H + by approximately one order of magnitude.3

Spiropyran derivatives have been immobilised on the surface of polystyrene microbeads using different immobilisation strategies. These functionalised polymeric beads can be reversibly switched between the colourless inactive spiropyran (SP) and highly coloured (purple) active merocyanine (MC) forms using low power light sources, such as light emitting diodes (LEDs). A UV-LED (375 nm) is used for the SP to MC conversion, and a white LED (430-760 nm) for the reverse MC to SP conversion. The photochromic behaviour of the coated beads has been characterised using different LEDs and reflection spectroscopy, employing optic fibres and an in-house designed holder. Investigations into the metal-ion binding behaviour of the spiropyran modified microbeads have shown that Cu 2+ ions cause an appreciable colour and spectral change when brought into contact with the beads in the MC form, suggesting that a significant interaction is occurring. However, the Cu 2+ ions can be completely expelled by photonicconversion of the beads into the inactive SP form using a white LED. This sequence has been successfully repeated six times suggesting that it is possible to cycle through activation of the functionalised beads from a non-binding to a binding form (SP to MC) using a UV-LED, allow binding with Cu 2+ ions to occur, and subsequently, expel the bound ions and regenerate the passive SP surface using a white-LED. Other metals, such as calcium, do not cause any appreciable colour or spectral change over the same concentration range and in the presence of the same anion (final concentration 7.1 x 10 -4 M nitrate salt in ethanol). The system is therefore self-indicating in terms of whether the active MC or inactive SP forms are present, and whether Cu 2+ ions are bound to the MC form. In principle, therefore, these functionalised beads could form the basis of a photoswitchable stationary phase for metal ion binding and detection: irradiation of the stationary phase with UV-LEDs causes retention of guest species due to the presence of the MC form, while subsequent exposure to white LEDs causes release of guest species into the mobile phase.

We investigate the photo, solvate and thermochromic properties of a novel photoswitchable spiropyran derivative; SP Im in imidazolium based ionic liquids (ILs). SP Im was prepared by alkylation of the photoswitchable compound to an imidazolium cation and is added to imidazolium based ILs with increasing chain length to examine the stability of funtionalised cations its merocyanine (MC) and spiropyran (SP) forms and compared to standard spiropyran; BSP. The rate of thermal relaxation of the new derivative is found to be about ten times faster than that of BSP as reflected in rates of 13.9 x10 -3 s -1 and 1.0 x10 -3 s -1 for SP Im and BSP respectively in [C 6 mIm][NTf 2 ]. Since ILs are believed to form nano-structured domains it is proposed that the covalent attachment of the imidazolium side group of SP Im fully integrates the photoswitchable moiety into the non-polar region through side-chain association. In contrast, unbound BSP is relatively free to migrate between both polar and non-polar regions and the MC form is more readily stablised by the IL charge via through space interactions and spontaneous movement to charged nano-domains leading to enhancement of the MC lifetime. At higher concentrations, rheological and transport properties were investigated to determine the impact of covalent attachment of the BSP fragment to an imidazolium cation had upon the ionic liquid structure. Ionic Conductivity was found to decrease by up to 23% for SP Im with effects increasing with cation side-chain length. Unlike BSP, the photoswitching of the SP Im did not affect conductivity or viscosity values. This may indicate that the mobility of the photoswitchable compound and the resulting disruption of such movement may be critical to the evolution of this physical property.2

Novel modified spirooxazines (SO) with additional chelating groups were synthesised and the crystal structure of one of these was determined. UV-vis spectroscopic characterization of the photoisomerization of the SO derivatives shows that the photochromic behaviour is altered with Zn 2+ coordination. In particular, addition of a carboxylic acid (5) to the indole section of the SO increases the lifetime of the merocyanine Zn 2+ complex by 20-fold compared to the methylated indole (6).

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