How does photoisomerization change a molecule without breaking its chemical bonds?

In photoisomerization, a molecule changes its shape without breaking any chemical bonds by undergoing a structural rearrangement triggered by the absorption of light.1

When a molecule absorbs light, it enters an excited state where the distribution of its electrons differs significantly from its ground state. This change in electron distribution alters the stability of the molecule's current geometry, often making a twisted conformation—such as a 90° rotation about a double bond—more energetically favorable.1

As the molecule rotates to match this new electronic configuration, it eventually returns to the ground state. Because the 90° twist is unstable in the ground state, the molecule continues to rotate to reach a more stable shape, which may result in the formation of a new isomer or a return to the original structure. This process is fundamental to biological functions, such as vision, where the photoisomerization of retinal within rhodopsin triggers a change in protein structure that initiates nerve impulses.12