Substituent Tightrope: Balancing Energy Storage and Stability of Dihydroazaborinine Molecular Solar Thermal Energy Storage Candidates.

1,2-Dihydro-1,2-azaborinines (BN-benzenes) are promising candidates for molecular solar thermal energy storage (MOST) as substitution-dependent isomerization to either the Dewar BN-benzene or benzvalene isomers is possible. Since not only photoisomerization behavior but also key MOST parameters, such as the half-life of the photoisomers, the energy storage density, and the absorption wavelength, are strongly influenced by substitution, a systematic investigation of structure-property relationships is essential to accurately predict performance and design optimized storage systems. We report the synthesis of 1,2-disubstituted dihydroazaborinines varying steric demand at the 1- (nitrogen) and 2-position (boron) independently. Comprehensive investigations of their photoswitching behavior, half-life and stored energy of the metastable state demonstrated that a sterically demanding substituent at the boron atom is sufficient to stabilize the dihydroazaborinine itself under ambient conditions, while an additional bulky group at the nitrogen atom is required to suppress the spontaneous thermal back-reaction of the Dewar BN-benzene photoisomer at room temperature. Consequently, the steric demand of both substituents is crucial for achieving a promising MOST system.