A research team led by Jye-Shane Yang from National Taiwan University has uncovered a molecular mechanism that explains how certain molecular crystals can bend, twist, or change shape when exposed to light without breaking. The study, published in the journal Angewandte Chemie International Edition, focuses on a group of related light-sensitive crystals made from anthracene, pentiptycene, and semifluorinated molecular segments. These crystals are able to undergo significant shape changes when light is applied, and the researchers have identified how their internal structure allows for this flexibility. The team discovered that the molecules in these crystals are arranged in a way that resembles a network of interconnected scissors. Neighboring molecules form joints that allow the structure to expand or contract in a coordinated manner. When a crystal is bent, the molecular "scissors" open on one side and close on the other, helping the crystal absorb the strain. This mechanism allows the crystal to undergo large deformations when exposed to light, rather than breaking apart. To understand these changes, the researchers used a variety of techniques, including X-ray measurements, computer simulations, single-crystal structural analysis, and solid-state nuclear magnetic resonance spectroscopy. These methods allowed them to observe the molecular changes from multiple angles. A key finding was the structural analysis of a crystal during its light-induced transformation, which provided direct evidence of the large-scale rearrangements associated with the scissor-like motion. The researchers tested several related crystals with slight variations in the length of the semifluorinated molecular segment. Surprisingly, even small changes in this segment led to very different behaviors. Some crystals were brittle, while others showed dramatic motion or expansion when exposed to light. The most flexible crystal had a balanced ability to both open and close its molecular scissors, allowing for greater deformation without breaking. Crystals that were too rigid or too loose in their structure were more likely to fracture under strain. "By understanding the coordinated movements at the molecular level, we can better explain how these crystals bend, expand, or break," says Jye-Shane Yang, co-corresponding author of the study. The findings may help guide future research into creating molecular crystals that can undergo large shape changes while maintaining mechanical strength.