X-ray-induced reduction of a surfactant/polyoxotungstate hybrid compound

© 2018 John Wiley & Sons, Ltd. We investigate the spontaneous reduction of a photochromic surfactant/polyoxotungstate hybrid during repeated X-ray photoelectron spectroscopy (XPS) scans and show how this effect may confound attempts to use soft X-rays to characterise materials of this nature. The W4f core-level spectra revealed a progressive increase of W5+ and W4+ species at the expense of W6+ as irradiation time increased. The samples developed a blue colour attributed to the presence of W4+ and/or W5+. The progressive photoreduction is also associated with a shift of the W6+ peak within the W4f spectrum to lower binding energies. This work highlights the need to consider inadvertent changes in oxidation state during X-ray photoelectron spectroscopy characterisation of samples containing photoreducible transition metals

Photomechanical photochromism in a cetyltrimethylammonium isopolytungstate

© 2018 The Royal Society of Chemistry. The photochromic properties of a hybrid compound comprised of the surfactant cation cetyltrimethylammonium [(C16H33)N(CH3)3]+ (CTA+) and the isopolytungstate anion [H2W12O40]6- is investigated. The compound, which has the nominal formula (CTA)7[H2W12O40]Cl·2H2O, changes from white to blue when exposed to UV radiation. The sample returns to the bleached state if stored in the dark-ambient. Application of XPS indicates that the coloring species are WV and WIV. The CTA+ component is found by XPS and FTIR to undergo progressive and irreversible oxidation during this cycle. Examination of FTIR suggests that the changes occur at multiple sites across the amphiphile. Surprisingly, the photochromic cycle is correlated with changes in the X-ray diffraction pattern, indicating partially reversible changes in the ordering of the Keggin ions and their spacing. In particular, application of the UV radiation causes the progressive accumulation of strain in the [001] direction. This is due to permanent oxidative changes in the CTA+ accumulating from cycle to cycle, resulting in an increase in interlamellar-distance due to less interdigitation of the chains. This provides a controllable photomechanical response