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How does light change trans-stilbene into cis-stilbene?

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Underwater view of a glowing chain of tunicates with a scuba diver and boat visible near the water's surface in the background. — photochemical reaction Chain of fluorescent tunicates.

The transformation of trans-stilbene into cis-stilbene is a process known as photoisomerization, a type of photochemical reaction where a molecule changes shape without breaking chemical bonds.1

When a stilbene molecule absorbs light, it transitions from its ground state to an excited state. In this excited state, the electron distribution changes significantly, making the molecule most stable when the central double bond is twisted to a 90° angle—a position halfway between the trans and cis forms.1

As the molecule rotates to adopt this 90° conformation, it reaches a point where the energy levels of the ground and excited states are close, allowing the molecule to return to its ground state with excess vibrational energy. Because the 90° twist is unstable in the ground state, the molecule continues to rotate. This rotation can proceed in either direction, resulting in the formation of the cis isomer or returning to the original trans isomer.1