Molecular Preorganization Unlocks Reversible Solid‐State Photoconversion in Dianthrylnaphthalene‐Based Molecular Solar Thermal Systems

ABSTRACT Molecular solar thermal (MOST) systems capable of harnessing sunlight, storing it as chemical energy in the form of a photoisomer, and releasing it as thermal energy on demand represent a promising strategy for renewable solar energy utilization both in liquid‐based devices and the solid state. However, the implementation of MOST materials in the solid state is limited by restricted molecular motion in the condensed phase, which hampers the geometrical requirements for photoisomerization, as well as by inner filter effects which can prevent quantitative photoconversion or necessitate long irradiation times. To address these challenges, three dianthrylnaphthalenes were designed to pre‐organize two anthracene moieties in a face‐to‐face geometry favorable for intramolecular photodimerization. This molecular design increased the forward photoisomerization quantum yield compared to ortho ‐dianthrylbenzenes, reaching 32%, while enabling solid‐state switching. In particular, the acenaphthene‐linked derivative underwent quantitative conversion to its higher‐energy photoisomer in both thin films and bulk crystalline powders. Electron diffraction analysis revealed elongated newly formed C─C bonds in the photoisomer, with refined lengths of 1.81(2) and 1.63(2) Å, exceeding the generally observed length of a C sp3 ─C sp3 bond. These results demonstrate how molecular preorganization can overcome limitations of anthracene‐based MOST systems and expand their potential for solid‐state solar energy storage applications.