Customization of Cloud Temperature in Amphiphilic π-Systems by Photoisomerization and Supramolecular Co-assembly for Smart Window Application

Abstract

Controlling the phase transition temperature of amphiphilic polymers and small molecular systems exhibiting Lower Critical Solution Temperature (LCST) phenomenon is a crucial to regulate the transmittance of light, required for the construction of thermoresponsive smart windows. To address this problem, we have taken advantage of photoisomerization and supramolecular co-assembly of two photoresponsive amphiphilic molecules, an anthracene derived cyanostilbene (ANT) and a pyrene derived cyanostilbene (PYR), exhibiting different LCST phase transition at 27 and 37 °C, respectively. Initially, aqueous solutions (1 mM) of these molecules, when heated above their LCST under ambient light, exhibited a 90% reduction in solar light transmittance owing to the increase in particle size from 10-20 nm to 0.6-1.3 µm. Subsequently, the phase transition cloud temperatures (T_cloud) could be customized by photoisomerization and co-assembly of the two molecules at different mole ratios. This approach allowed control of the particles size between 675-1300 nm during the LCST phase change, enabling the fine tuning of T_cloud between 27-37 °C. Smart windows fabricated with ANT and PYR and their 1:1 combination exhibited solar and luminous transmittance reduction from 81% and 84% to 1.7% and 1.6% respectively, at 27 °C. Thermal IR transmittance was drastically reduced from 78% to 1.8%. This approach has been used to design several custom-made smart window prototypes with controlled transparency modulation suitable for tropical climate.