13‐cis‐β,β‐Carotene and 15‐cis‐β,β‐carotene

13‐cis‐β,β‐Carotene, C₄₀H₅₆, crystallizes with a complete molecule in the asymmetric unit, whereas 15‐cis‐β,β‐carotene, also C₄₀H₅₆, has twofold symmetry about an axis through the central bond of the polyene chain. The polyene methyl groups are arranged on one side of the polyene chains for each molecule and the 6‐s‐cisβ end groups, with the cyclohexene rings in half‐chair conformations, are twisted out of the planes of the polyene chains by angles ranging from 41.37 (17) to 52.2 (4)°. The molecules in each structure pack so that the arms of one occupy the cleft of the next, and there is significant π–π stacking of the almost‐parallel polyene chains of the 15‐cis isomer, which approach at distances of 3.319 (1)–3.591 (1) Å.     

Sb‐Triggered β‐to‐α Transition: Solvothermal Synthesis of Metastable α‐Cu2Se

Control over phase stabilities during synthesis processes is of great importance for both fundamental studies and practical applications. We describe herein a facile strategy for the synthesis of Cu₂Se with phase selectivity through a simple solvothermal method. In the presence and absence of SbCl₃, monoclinic α‐Cu₂Se and cubic β‐Cu₂Se can be synthesized, respectively. The formation of α‐Cu₂Se requires optimization of the Cu/Se molar ratio in the starting reagents, the reaction temperature, as well as the timing for the addition of SbCl₃. Differential scanning calorimetry of the synthesized α‐Cu₂Se has shown that a part of it undergoes a phase transition to β‐Cu₂Se at 135 °C, and that this phase transition is irreversible on cooling to ambient temperature. Kinetic studies have revealed that in the presence of Sb species the kinetically favored β‐Cu₂Se transforms to the thermodynamically favored α‐Cu₂Se. In this β‐to‐α phase transition process, the distribution of Cu ions in β‐Cu₂Se, as determined by the Cu/Se ratio and temperature, is likely to play a crucial role.