Spirooxazine loading effects on the photochromism of polymer films

Spirooxazines are well-known photochromic dyes that, upon proper illumination, convert between uncolored/ coloured forms. In order to take advantage of their photoswitchable properties, one strategy is represented by their encapsulation in transparent host matrices, like polymers, which may improve the performance of the material, thanks to a major protection from oxidative degradation, or induce a variation of the optical properties, since the photochromic systems are affected by the surrounding environment. The fine-tuning and the strict control of the photochromic behaviour of the polymeric films is extremely appealing for several purposes, going from optical switches to sensors and light harvesting devices. One of the most elegant ways to exploit the photochromic characteristic of these materials is the controlled switch of the colour and the colouring/bleaching rate of a single photochromic system, since this versatility can open the possibility to use the same material for different applications. Therefore, a study of the modulation of the colourability and the colouring/bleaching rates of two spirooxazines in different polymeric matrices is herein reported. The influence of the photochromic concentration and the chemical nature of the polymeric matrix on the photophysical behaviour of the proposed systems is investigated to achieve a photochromic/polymeric dual system with tailored properties. The optical characterization of increasing concentrations of photochromic compound in solution was essential for the design of colour-defined polymeric films, highlighting, indeed, a correlation between photochromic concentration and colour switching from blue to pink, effect assigned to formation of aggregate species. By the proper choice of photochromic concentration and polymeric matrix, solid films showing great photoconversion efficiency and different colours were achieved under irradiation, together with the possibility to modulate the bleaching rate from few seconds to tens of minutes. Furthermore, polymeric films with different colours before irradiation could be produced as well. The obtained results open the possibility of a multi-purpose use of a photochromic system by simply varying its concentration within the polymeric film. Moreover, interesting colouring effect can be obtained by changing the polymeric matrix, therefore further widening their applicability.