How are photons used in photochemical reactions?

In a photochemical reaction, photons act as the primary source of energy that initiates a chemical reaction by being absorbed by molecules.1

When a molecule absorbs a photon, one of its electrons is promoted to a previously unoccupied orbital, creating a transient excited state. These excited states possess chemical and physical properties that differ significantly from the original ground-state molecules; specifically, they often become stronger acids and stronger reductants.21

Once in this high-energy state, the molecule may undergo several transformations:
* Structural changes: The molecule may fall apart, rearrange its structure, or isomerize (change shape).1
* Chemical interaction: It may combine with other molecules or transfer electrons, hydrogen atoms, protons, or its excitation energy to other species.1

This process is fundamental to life, such as in photosynthesis, where light energy is converted into stored chemical energy by driving electron transfers across membranes.13