Influence of electronic vs. steric factors on the solid-state photochromic performances of new polyoxometalate/spirooxazine and spiropyran hybrid materials

Abstract

Two new photochromic hybrid organic–inorganic supramolecular materials (SN)₂[W₆O₁₉] (SN₂W₆) and (SP)₂[W₆O₁₉]·0.5CH₃CN (SP₂W₆) have been elaborated by combining for the very first time an isopolyoxotungstate [W₆O₁₉]²⁻ with both photoswitchable cationic spirooxazine (SN) and spiropyran (SP), via electrostatic interactions. (SN)₂[Mo₆O₁₉] (SN₂Mo₆), a third new photochromic compound which contains the [Mo₆O₁₉]²⁻ unit, has been also designed. Their structure was solved by single-crystal X-ray diffraction analysis, and their solid-state photochromic properties in ambient conditions were thoroughly investigated by UV-visible diffuse reflectance spectroscopy. SN₂W₆ and SN₂Mo₆ have been proved to be isostructural while SP₂W₆ is isostructural with its already reported counterpart SP₂Mo₆. In the SN₂M₆ and SP₂M₆ series, the SN and SP molecules are in very close contact with the POM units, which a priori should dramatically limit their photoisomerization in the solid state. However, this strongly varies with the nature of the metal in the [M₆O₁₉]²⁻ unit. Indeed SN₂W₆ and SP₂W₆ exhibit remarkable photochromism under low-power UV excitation (λ_ex = 365 nm) while SN₂Mo₆ and SP₂Mo₆ develop very limited color-change effects under similar UV exposure. These results have unambiguously highlighted that the efficiency of the SN and SP photoisomerizations in these hybrid systems is much more impacted by an electronic factor resulting from the intrinsic optical absorption properties of both SP and POM components than by a steric factor induced by crystal packing.