α- to β-(dmes)BiI₅ (dmes = Dimethyl(2-ethylammonium)sulfonium Dication): Umbrella Reversal of Sulfonium in the Solid State and Short I···I Interchain ContactsCrystal Structures, Optical Properties, and Theoretical Investigations of 1D Iodobismuthates

Syntheses, X-ray structural characterization, optical properties, and electronic structures of 1D metal(III) iodide hybrids, namely, α-((CH₃)₂S(CH₂)₂NH₃)BiI₅ (1a), β-((CH₃)₂S(CH₂)₂NH₃)BiI₅ (1b), ((CH₃)₂S(CH₂)₂NH₃)SbI₅ (2), and (HO₂C(C₆H₄)CH₂NH₃)BiI₄ (3), are reported. According to the results of single-crystal X-ray diffraction analyses, the 1D inorganic chains are constructed by corner-shared M(III)I₆ octahedra in 1a, 1b, and 2 and by edge-shared ones in 3. In polymorphs 1a, 1b, and 2, the polymeric BiI₅²⁻ anionic chains are charge-balanced by the dimethyl(2-ethylammonium)sulfonium (dmes) dications. Complex 1a crystallizes in the polar space group of P2₁cn. A spectacular umbrella reversal of half sulfonium parts together with the conformational change of half polymeric anions in the crystal structure of 1a occurs at moderate temperature (73 °C), leading to the β-phase 1b, through a reversible single-crystal-to-single-crystal process. Complex 1b, as well as the isotype structure of 2, crystallize in the nonpolar acentric space group of P2₁2₁2₁. Because of their acentric structural characteristic, second harmonic generation (SHG) optical properties are observed in the polycrystalline powder samples of 1a, 1b, and 2. It is notable that the SHG signal of 1a is much stronger than that of 1b and 2 owing to the polarity of 1a. Remarkably, the peculiar dissymmetrical dication of dmes is able to modify the bonding features of the inorganic frameworks through shortening I···I distances between adjacent chains (d(I···I) < 4Å). The structure of 3, which crystallizes in the triclinic space group P1̅, features a polymeric anionic chain constructed from edge-shared BiI₆ octahedra. The charge is balanced by the pairs of trans-4-(ammoniummethyl)-cyclohexane-carboxylic acid, which are linked together via the H bonding between the carboxylic groups to form a pseudodication. The results of DFT calculations based on the structures of 1a and 3 indicate that the narrower band gap in 1 appears to be associated on the one hand with a σ* I-p/Bi-s interaction that moves the Fermi level to higher energy and on the other hand with the interchain I···I contacts.