In organic chemistry, we use mechanisms to study the flow of electrons in the conversion of reactants to products. We also use mechanisms to study organic photochemical reactions, in which electrons start in a lower energy ground state and are raised into a higher energy excited state with the addition of energy. For some molecules, the excited state offers configurations of electrons not found in the ground state, and we see products that would not result from the ground state.
Photosynthesis is the process by which plants harness the energy from the sun and use it to synthesize glucose. The sun's energy is just the right wavelength to create photochemical reactions in the light-absorbing pigments (chlorophyll) in the plant's leaves. After chlorophyll reaches the excited state due to energy absorption, the energy is then passed along to another molecule, continuing the reactions involved in photosynthesis.
UV-VIS spectroscopy is one of the main applications of photochemistry. It allows us to determine the concentration of a molecule in a sample, and sometimes, it can aid in identifying an unknown molecule. The molecule being tested must absorb light in the ultraviolet (about 200 to 400nm) or the visible (about 400 to 700nm) range in order to be detected by this equipment. A light beam containing multiple wavelengths gets passed through a small container holding your sample, and the computer records which wavelength(s) were absorbed, and at which intensity.
Sometimes, the energy given off in a photochemical reaction is released in the form of light. These reactions are called chemiluminescent reactions. For example, fireflies light up when the enzyme luciferase contained in their body combines with oxygen. You might also be familiar with lightsticks, in which two light-producing chemicals get mixed together when the inner tube is snapped.
Photochemistry is pervasive in the world of industrial biology and chemistry. Synthetic photoreactions are important in creating polymers with interesting mechanical and electronic properties. Photochemical reactions are being explored that destroy organic waste, thereby controlling pollution. Drug precursors have been designed that are inactive until dosed with energy at the appropriate wavelength, which a physician can do once the precursor has reached the target tissue.