What is Photoisomerization and How Does It Work?

Photoisomerization is a process where a molecule absorbs light and undergoes a structural transformation without altering its underlying chemical formula. This means no atoms are added or removed, similar to folding a piece of paper into a new shape. The different structural forms of the molecule produced are known as isomers.

How Photoisomerization Works

The process begins when a molecule absorbs a photon of light, elevating it to a temporary, high-energy condition called an excited state. This energy allows rotation to occur around one of the molecule’s chemical bonds, specifically a rigid double bond that is not normally free to twist. The molecule cannot remain in this high-energy state for long.

As the molecule releases this excess energy, it settles into a stable, low-energy form. Because of the rotation that occurred, it now possesses a new three-dimensional shape. This new structure is an isomer of the original molecule. The event represents a direct conversion of light energy into molecular-scale mechanical motion.

A well-studied example is the cis-trans isomerization of a molecule called azobenzene. In its stable trans state, the molecule has a relatively straight, elongated shape. When it absorbs ultraviolet light, it switches to a U-shaped cis isomer. This geometric change is significant, as the distance between the ends of the molecule decreases substantially.

The Role in Biological Vision

This molecular light-switching is the basis for sight in humans and many other animals. Inside the retina, cells contain a protein called opsin, which holds a light-sensitive molecule named retinal. This complex, known as rhodopsin, functions through the photoisomerization of its retinal component. In its dark-adapted state, retinal exists in a bent configuration known as 11-cis-retinal.

When a single photon of light strikes this molecule, it provides the energy for an incredibly rapid transformation. The 11-cis-retinal isomer instantly straightens into a linear form called all-trans-retinal. This reaction is one of the fastest known in nature, completing in approximately 200 femtoseconds, or 200 quadrillionths of a second. This speed is possible because the process occurs with almost no energy barrier.

The efficiency of this conversion is high, with about two of every three absorbed photons successfully triggering the isomerization. This change in the shape of the retinal molecule, from bent to straight, forces a larger change in the opsin protein it is attached to. This protein movement is the trigger that generates a nerve impulse that travels to the brain to be interpreted as vision.

Harnessing Photoisomerization for Technology

Scientists are developing ways to use this molecular motion for applications like high-density data storage. Molecules that can be switched between two distinct shapes, such as cis and trans forms, can represent the 0s and 1s of binary code. By using techniques like two-photon absorption, a laser can be focused to flip molecules at a specific location within a three-dimensional medium. This could allow for 3D optical storage devices capable of holding vast amounts of information.

The principle also extends to the creation of smart materials. Photochromic lenses, which darken in sunlight and lighten indoors, are a common example, relying on molecules that reversibly change structure when exposed to UV light. Researchers are also exploring materials that change their physical shape or stiffness when illuminated for use as light-activated actuators or self-healing polymers.

In medicine, photopharmacology uses photoisomerization to create treatments with fewer side effects. The concept involves designing a drug molecule that is inactive in its initial form. Once the medication reaches a target area in the body, it can be activated by a focused beam of light. This switches the drug to its active form precisely where needed, improving effectiveness while minimizing impact on healthy tissues.

Controlling the Molecular Switch

For many of these applications to be practical, the molecular change must be reversible. A data storage medium that can only be written to once has limited utility, as does a smart material that cannot return to its original state. This reversibility is often achieved by using a second, different wavelength of light to flip the molecule back to its starting form.

For instance, with the azobenzene molecule, UV light is used to switch it from the trans to the cis isomer, while blue light can drive the reverse reaction. This provides direct control over the molecular switch in both directions. In other systems, the molecule may naturally revert to its more stable isomer over time or with heat, a process known as thermal relaxation. The rate of this relaxation can be tuned by making small chemical modifications to the molecule.