Phase Behavior of Light-Responsive Lyotropic Liquid Crystals for Molecular Solar Thermal Energy Storage

Molecular solar thermal energy storage (MOST) materials are a promising method for renewable energy storage that captures solar energy and releases it on demand as heat. Azobenzene is attractive for MOST applications due to its photoreversible <i>E-Z</i> isomerization. Recently, phase-change materials have been formed using azobenzene to increase their energy-storage capacity; however, these condensed phases often lower the isomerization degree, which is only recovered on dissolution. In this work, sparing solvent addition is used to drive the self-assembly of azobenzene photosurfactants (AzoPS) into lyotropic liquid crystal (LLC) phases, which are explored for MOST applications for the first time. Using small-angle X-ray scattering (SAXS), polarized optical microscopy, and differential scanning calorimetry (DSC), we show that the structure-isomerization behavior, and energy-storage properties of these light-responsive LLCs can be systematically tuned by adjusting the photosurfactant structure, solvent, and concentration. Furthermore, by developing a method that combines SAXS with <i>in situ</i> DSC, we directly correlate the isomerization-induced LLC phase transitions to their energy-storage contributions. The formation of LLC phases through solvent addition both enhances the degree of isomerization (by up to 20%) and amplifies the structural disordering on isomerization, resulting in energy-storage densities of up to 123 J g^-1. The ability to tune both the structure and isomerization properties in LLC materials suggests significant promise for MOST applications. In addition, the combination of advanced characterization methods used to establish the structure-isomerization-enthalpy (LLC-photoswitch-phase change) relationships provides unique insight into these multicomponent systems and accelerates the design pathways to future iterations for competitive solar energy storage devices.