Stiffening Organic Crystals through Polymerization Using Visible Light

Soft organic crystals that combine high strength and toughness are essential for flexible electronics and bioinspired devices, but they often compromise one property for the other. Here, we demonstrate a visible-light-driven, single-crystal-to-single-crystal photopolymerization of 1,1'-dioxo-1<i>H</i>,1'<i>H</i>-[2,2'-biindene]-3,3'-diyl-bis(decanoate) (<b>B10</b>) into a polymeric crystal (<b>PB10</b>) that simultaneously with polymerization enhances its mechanical strength and toughness. Under white-light irradiation (2.5 W cm^-2), centimeter-long <b>B10</b> needles exhibit splitting, coiling, and straightening, accompanied by a color change from red to colorless. This transformation is accompanied by a molecular reorganization, where the weak (π···π stacking) interactions are replaced by stronger (C-C) bonds, resulting in a drastic change in mechanical properties. As a result, upon photopolymerization, the <b>PB10</b> crystals transition from purely elastic to elastic/plastic, with a nearly 228-fold increase in toughness. This polymerization is also accompanied by increases in tensile modulus and a nearly 81-fold increase in tensile toughness. Remarkably, the <b>PB10</b> crystals exhibit a load-bearing capacity exceeding 1 × 10⁵ times their own mass, additionally reflecting the dramatic enhancement in mechanical strength.