Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible (400–750 nm), or infrared radiation (750–2500 nm).

In nature, photochemistry is of immense importance as it is the basis of photosynthesis, vision, and the formation of vitamin D with sunlight. It is also responsible for the appearance of DNA mutations leading to skin cancers.

Photochemical reactions proceed differently than temperature-driven reactions. Photochemical paths access high-energy intermediates that cannot be generated thermally, thereby overcoming large activation barriers in a short period of time, and allowing reactions otherwise inaccessible by thermal processes. Photochemistry can also be destructive, as illustrated by the photodegradation of plastics.

Photoexcitation is the first step in a photochemical process: the reactant is elevated to a state of higher energy, an excited state.

The first law of photochemistry, known as the Grotthuss–Draper law (for chemists Theodor Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place. According to the second law of photochemistry, known as the Stark–Einstein law (for physicists Johannes Stark and Albert Einstein), for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield.

When a substance in its ground state (S₀) absorbs light, one electron is excited. This electron maintains its spin. according to the spin selection rule; other transitions would violate the law of conservation of angular momentum. The excitation to a higher singlet state can be from HOMO to LUMO or to a higher orbital, so that singlet excitation states S₁, S₂, S₃... at different energies are possible.

Kasha's rule stipulates that higher singlet states quickly relax by radiationless decay or internal conversion (IC) to S₁. Thus, S₁ is usually, but not always, the only relevant singlet excited state. This excited state S₁ can further relax to S₀ by IC, but also by an allowed radiative transition from S₁ to S₀ that emits a photon; this process is called fluorescence.

Alternatively, it is possible for the excited state S₁ to undergo spin inversion and to generate a triplet excited state T₁ having two unpaired electrons with the same spin. This violation of the spin selection rule is possible by intersystem crossing (ISC) of the vibrational and electronic levels of S₁ and T₁. According to Hund's rule of maximum multiplicity, this T₁ state would be somewhat more stable than S₁.