What is the main characteristic of photochemical interactions?

Photochemical interactions are fundamentally linked to the absorption of light, but their defining characteristic is the induction of a chemical reaction as a direct result of photon energy. When light photons are absorbed by molecules, they can provide the necessary energy to break bonds or stimulate new bond formations, leading to various chemical transformations. This process is critical in many natural and artificial systems, including photosynthesis and photopolymerization.

While light absorption is necessary for photochemical processes, it is the subsequent chemical reaction that defines these interactions, distinguishing them from purely thermal or physical effects. Other processes mentioned, such as heat generation or thermal expansion, are related to the consequences of energy absorption but do not capture the essence of what makes an interaction photochemical. In essence, photochemical interactions are characterized by the transformation of chemical species as a result of light energy absorption, emphasizing the profound relationship between light and matter.