New optical devices based on organic phase change materials: from fluorochromic inks and sensors to smart windows

Abstract

This thesis presents new strategies for the preparation of stimuli-responsive fluorescent and thermo-optical materials. As an alternative to current fabrication methodologies which suffer from severe drawbacks such as low scalability, difficult tunability and limited fatigue resistance, our approach makes use of simple and cost-effective organic phase change materials (PCMs) as externally responsive building blocks (e.g., paraffins). For stimuli-responsive fluorescent materials, we have demonstrated new emission modulation mechanisms that rely on how the solid-to-liquid transition of PCMs modifies the fluorescence properties of dispersed emitters by affecting their intra- and intermolecular interactions. In that way, a portfolio of fluorophore-PCM mixtures has been developed that reversibly vary their emission color or intensity upon PCM melting and solidification. Moreover, by combining these materials with near-infrared (NIR)-absorbing photothermal agents, PCM melting and, therefore, emission modulation have been photothermally accomplished. By this simple approach, the intrinsically temperature-responsive fluorophore-PCM mixtures have been converted into NIR-responsive systems, which makes them more interesting for biomedical and security applications. In addition, fluorophore-PCM mixtures have been miniaturized into micro- and nanostructures that mainly preserve the capacity to undergo emission modulation upon PCM phase change. These miniaturized materials have then been integrated into functional platforms such as inks and polymeric films that exhibit the same stimuli-responsive fluorescent properties. Finally, all these bulk and miniaturized PCM-based systems have been investigated for different applications such as security inks, biosensing, and data encoding. On the other hand, the large change in refractive index (n) of PCMs upon phase transition has been utilized for the preparation of a new generation of thermo-optical materials. For this, paraffinic particles have been embedded in polymeric films to obtain solid composite materials that show large variations in n at temperatures around the melting temperature of the dispersed PCM. Interestingly, depending on the size of the embedded paraffin particles, large or neglectable variations in transparency were observed along with the change in n. For the composites showing large changes in transparency, their application as energy-saving smart windows has been explored. In contrast, the use of the systems with neglectable transparency modulation but ample n variation has been investigated for reconfigurable planar optics.

Date of Award	27 Oct 2023
Original language	English
Supervisor	Claudio Roscini . (Director) & Jordi Hernando Campos (Director)