Abstract

Transparent solid-state platforms for molecular solar thermal storage systems (MOST) are highly attractive for overcoming the intermittency of solar energy while enabling efficient light-matter interactions. Herein, we report the development of transparent photoactive polymer nanofiber composites based on azobenzenefunctionalized polycaprolactone (PCL) processed via electrospinning and subsequently infiltrated with lowmolecular-weight chitosan. A design of experiments (DOE) approach enabled the fabrication of uniform, beadfree nanofibers with an average diameter of 133 +/- 23 nm, leading to a significant increase in surface area and wettability. Vacuum-assisted impregnation with a dilute chitosan solution effectively filled inter-fiber voids, reduced refractive index mismatches, and yielded highly transparent nanofiber composites. Importantly, the azobenzene photoactivity was preserved in the solid state, displaying reversible Trans-Cis-Trans photoisomerization under alternating UV and visible light irradiation and maintaining stable performance over more than 100 charge-discharge cycles. The incorporation of chitosan further modulated the thermal backisomerization kinetics, extending the lifetime of the metastable Cis state. These results demonstrate a robust and transparent polymer nanofiber platform for solid-state MOST energy storage, with potential relevance for energy-related coatings and optically active materials.