Abstract
<jats:p>Photoactive molecules can be converted to a metastable high-energy isomer upon light irradiation. The stored energy is subsequently released as heat through on-demand back-isomerisation to the ground state. This reversible photochemical process forms the basis of molecular solar thermal (MOST) energy storage systems. However, the practical implementation of MOST systems remains limited by the difficulty of simultaneously achieving long storage lifetimes and high energy densities. To address this challenge, we recently described Micellar Solar Thermal Energy Storage (MIST) systems, in which photoactive molecules are incorporated into self-assembled micellar architectures. By restricting molecular motion, self-assembly substantially prolongs the lifetime of the metastable isomer while enabling highly concentrated, water-processable formulations. Here, we demonstrate that varying the counterion modulates the micellar structure, leading to pronounced differences in the thermal stability of the stored cis-isomer. In particular, the tetrabutylammonium salt exhibits an exceptional metastable half-life of up to 44 years. The same system also undergoes photo-induced phase separation, generating highly concentrated energy-storage states that release up to 18.2 °C of heat from 400 μL of a 2.38 mM solution upon phototriggered back-conversion. Together, these results demonstrate that self-assembly provides a powerful strategy for simultaneously tuning storage lifetime and energy density in MIST systems, offering a route to high-performance solar thermal energy storage without molecular redesign.</jats:p>