Surface Degradation of Spiropyran based films

Coatings with photoswitchable polarity can allow for control over the physical properties of surfaces. Such coatings can be prepared by combining molecular photoswitches with other molecular components that formation of amorphous films rather than crystallisation. Here, a thioctic acid unit was combined with a spiropyran photochromic switch to form spiropy-ran thioctic ester (SPTE), which can form amorphous films, melting at a relatively low temperature (70-90 °C). The photochromism of the films is reversible however, we show that the increase in surface polarity observed upon switching with UV light to the merocya-nine isomer is due in part to degradation not observed in the bulk of the film. Vibrational spectroscopy show the intial switching to the merocyanine state occurs but the prolonged exposure leads to degradation of the merocyanine formed. The dithiolane unit of the SPTE is shown to be innocent in regard to degradation by comparison with an non- substituted ni-trospiropyran (NSP). The films, subjected to extended irradiation, were monitored by DR-and ATR-FTIR, UV/vis absorption and Raman spectroscopy. The suitability of each of the techniques to report on the state of the surface of the film, of relevance to contact angle measurements, is determined and insight is gained into both the stability of the spiropyran photoswitch in and near the surface of the coating, as well as the pathways to degradation.