Reentrant structural and optical properties of organic–inorganic hybrid metal cluster compound ((n-C4H9)4N)2[Mo6Bri8Bra6]

Abstract

We report the reentrant phase transition of the organic–inorganic hybrid metal cluster (MC) compound (TBA)₂[Mo₆Brⁱ₈Br^a₆] (TBA = ((n-C₄H₉)₄N)). Structural studies revealed that (TBA)₂[Mo₆Brⁱ₈Br^a₆] was crystallized in the conventional monoclinic phase with space group P2₁/n. When the crystal was cooled down to 143 K, we observed a structural phase transition from the conventional phase to an unknown phase with packing structure similar to the conventional one; however, the unit cell was extended to a double size. A reentrant transition to the conventional structure was induced when the crystal was further cooled to 113 K. These reentrant phase transitions occurred without any symmetry changes caused by slight motions of the MCs and TBA⁺ ions. Luminescence studies revealed that the phase transition poorly affected the emission spectra; however, the newly found phase exhibited 1.7 times longer emission lifetime than the conventional phase. These results indicate that MC-based compounds are promising and exhibit phase transitions and crystallographic polymorphisms.