Synthesis and Characterization of Molecular Solar Thermal Energy Systems

Doctoral thesis, 2025

Molecular Solar Thermal (MOST) systems offer a promising route to sustainable solar energy storage by coupling light-driven isomerization with thermally activated back-conversion, addressing solar energy’s inherent intermittency. This study presents the targeted design, synthesis, and evaluation of norbornadiene/quadricyclane (NBD/QC) and azobenzene-based photoswitches, revealing critical structure–property relationships that govern photoswitching efficiency, thermal stability, solubility, and phase behavior. Substituent effects were systematically explored to optimize performance: alkyl chains enhanced solubility in nonpolar solvents, while aromatic groups improved compatibility with polarizable media such as the sustainable solvent anisole. In NBD systems, we show that both chain length and branching modulate activation parameters, with longer or branched chains increasing thermal half-lives by stabilizing the transition state. A pyrene-functionalized NBD achieved an isomerization quantum yield of 46% along with enhanced thermal stability, demonstrating the synergistic effect of π-extension and steric bulk. The most persistent QC isomers featuring ortho-substituted pyridyl groups exhibited half-lives up to 205 days, driven by reduced activation entropy. Parallel investigations into azobenzene-based MOST–phase change material (PCM) hybrids achieved total energy storage values (ΔHₜₒₜ) up to 307.42 J·g⁻¹. The top-performing compound combined high isomerization enthalpy, efficient crystallization, and a favorable thermal profile. We find that optimal MOST–PCM behavior arose from a balance between molecular rigidity and flexibility. Simple mono-substituted azobenzenes with medium-length alkyl chains outperformed bulkier analogues, which suffered from steric hindrance and diminished enthalpic output. Together, these findings provide design rules for tuning photoswitch performance across multiple metrics and establishing a blueprint for the development of efficient, tunable, and integrable MOST materials for solar thermal energy storage applications.