Structure–property relationships in normal and mixed dithienylethenes – polyoxometalates supramolecular assemblies with fast solid-state photochromic properties

Abstract

Five new highly photochromic hybrid organic–inorganic materials were successfully prepared by supramolecular assembly of normal (1⁺) and mixed (2⁺) cationic dithienylethenes (DTEs) and polyoxometalates (POMs) units. Single-crystal X-ray diffraction studies reveal that (1)₃[PM₁₂O₄₀]·5ACN (M = W, Mo) (1-PW₁₂ and 1-PMo₁₂) and (2)₃[PM₁₂O₄₀]·5ACN (M = W, Mo) (2-PW₁₂ and 2-PMo₁₂) are isostructural, while (2)₄[Mo₈O₂₆]·4DMF (2-Mo₈) is an isotype of (1)₄[Mo₈O₂₆]·4DMF (1-Mo₈) i.e., the first POM–DTE ionic assembly, recently reported. The solid-state photochromic properties of these materials at room temperature are highly tunable with the nature of the DTEs and POMs used, which has been interpreted taking into account steric and electronic factors. Especially, while 1-PMo₁₂ and 2-PMo₁₂ exhibit negligible photoresponses which have not been considered herein, thorough investigations of the photocoloration and fading kinetics in ambient conditions have highlighted the precise impact of both organic and inorganic components on the photoswitching abilities of 1-Mo₈, 2-Mo₈, 1-PW₁₂ and 2-PW₁₂. Upon UV-light irradiation, the absorption of the normal and mixed DTEs in their closed-ring form exhibits a hypsochromic shift which is more pronounced in hybrid systems integrating β-[Mo₈O₂₆]⁴⁻ rather than α-[PW₁₂O₄₀]³⁻, due to the higher negative charge density of the octamolybdate unit. The photocyclization rates of DTEs are also systematically increased when combined with the β-[Mo₈O₂₆]⁴⁻ unit, while in contrast, their back cycloreversion rates are not affected whatever the nature of the POMs used. In addition, within isostructural series, hybrids integrating mixed DTEs exhibit faster fading processes than their normal DTE counterparts when exposed to visible light. Finally, among this new series, 2-Mo₈ shows the best photochromic performances with a fast photocoloration rate associated with a high coloration contrast, a fast and complete fading process and a high cyclability.