Extraordinary anisotropic thermal expansion in photosalient crystals

Abstract

Although a plethora of metal complexes have been characterized, those having multifunctional properties are very rare. This article reports three isotypical complexes, namely [Cu(benzoate)<i>L</i> ₂], where <i>L</i> = 4-styryl-pyridine (4spy) (<b>1</b>), 2'-fluoro-4-styryl-pyridine (2F-4spy) (<b>2</b>) and 3'-fluoro-4-styryl-pyridine (3F-4spy) (<b>3</b>), which show photosalient behavior (photoinduced crystal mobility) while they undergo [2+2] cyclo-addition. These crystals also exhibit anisotropic thermal expansion when heated from room temperature to 200°C. The overall thermal expansion of the crystals is impressive, with the largest volumetric thermal expansion coefficients for <b>1</b>, <b>2</b> and <b>3</b> of 241.8, 233.1 and 285.7 × 10^-6 K^-1, respectively, values that are comparable to only a handful of other reported materials known to undergo colossal thermal expansion. As a result of the expansion, their single crystals occasionally move by rolling. Altogether, these materials exhibit unusual and hitherto untapped solid-state properties.