Conformation of Alkyl to Tune Photochemical Topo-[2 + 2] Cycloaddition of Benzylidenethiazolidinedione for Construction of Photomechanical Crystals

The understanding of the influence of the alkyl chain conformation on the photochemical cycloaddition in crystals still remains a great challenge since the molecules may adopt thousands of different conformers in the crystallizing environment. In this work, the influence of alkyl chain conformation on solid-state photoinduced [2 + 2] cycloaddition of benzylidenethiazolidinedione derivatives (BTD and N-alkylated BTDn, n = 1–10) was investigated. It was found that BTD, BTD1, BTD2, BTD4, BTD5, and BTD8 underwent topo-[2 + 2] cycloaddition, but BTD3, BTD6, BTD7, BTD9, and BTD10 were photoinert. Single crystal structural analyses demonstrated that the alkyls in BTD3 and BTD8 adopted all-anti conformation, and the others exhibited C²–C³ gauche conformation. An independent gradient model based on Hirshfeld partition of molecular density (IGMH) and energy decomposition analyses (EDA) confirmed that the intermolecular interactions were stronger for the photochemical reactive systems than those for photoinert ones. Furthermore, the thermodynamic analysis of formation enthalpy and entropy suggested that an entropy change vs per increasing number of carbon atoms in alkyls could give information on how closely the molecules are arranged. Notably, the molecular crystals based on the photochemical reactive compounds exhibited significant photomechanical effects. This work demonstrated that the alkyl chain conformation is critical for tuning the solid-state photoinduced [2 + 2] cycloaddition, which is helpful for designing photomechanical crystalline materials.