Chirality Remote Control in Nanoporous Materials by Circularly Polarized Light

Abstract

The ability to dynamically control chirality remains a grand challenge in chemistry. Although many molecules possess chiral isomers, lacking their isolation, for instance during photoisomerization, results in racemic mixtures with suppressed enantiospecific chiral properties. Here, we present a nanoporous solid in which chirality and enantioselective enrichment is induced by circularly polarized light (CPL). The material is based on photoswitchable fluorinated azobenzenes attached to the scaffold of a crystalline metal-organic framework (MOF). The azobenzene undergoes <i>trans</i>-to-<i>cis</i>-photoisomerization upon irradiation with green light and reverts back to <i>trans</i> upon violet light. While each moiety in <i>cis</i> conformation is chiral, we show the <i>trans</i> isomer also possesses a nonplanar, chiral conformation. During photoisomerization with unpolarized light, no enantiomeric enrichment is observed and both isomers, <i>R</i>- and <i>S</i>-<i>cis</i> as well as <i>R-</i> and <i>S</i>-<i>trans</i>, respectively, are formed in identical quantities. In contrast, CPL causes chiral photoresolution, resulting in an optically active material. Right-CPL selectively excites <i>R</i>-<i>cis</i> and <i>R</i>-<i>trans</i> enantiomers, producing a MOF with enriched <i>S</i>-enantiomers, and <i>vice versa</i>. The induction of optical activity is reversible and only depends on the light-handedness. As shown by first-principle DFT calculations, while both, <i>trans</i> and <i>cis</i>, are stabilized in nonplanar, chiral conformations in the MOF, the <i>trans</i> isomer adopts a planar, achiral form in solution, as verified experimentally. This shows that the chiral photoresolution is enabled by the linker reticulation in the MOF. Our study demonstrates the induction of chirality and optical activity in solid materials by CPL and opens new opportunities for chiral resolution and information storage with CPL.