The absorption of light by organic compounds leads to the formation of excited states through a process called electronic excitation. When a molecule absorbs a photon of light, the energy of the photon is transferred to one of the molecule's electrons, causing it to jump from its ground state to a higher energy level, or excited state. This process is known as electronic excitation, and the molecule is said to be in an electronically excited state.The subsequent steps involved in the photochemical reactions of these compounds can be divided into four main categories:1. Internal conversion: After the molecule is excited, it can undergo internal conversion, a process in which the energy of the excited electron is transferred to the vibrational and rotational energy levels of the molecule. This causes the molecule to vibrate and rotate more rapidly, leading to the dissipation of the absorbed energy as heat. Internal conversion is a non-radiative process, meaning that no photons are emitted during the transition.2. Fluorescence: Alternatively, the excited molecule can undergo fluorescence, a process in which the excited electron returns to its ground state by emitting a photon of lower energy than the one initially absorbed. This results in the emission of light, typically at longer wavelengths than the absorbed light. Fluorescence is a radiative process and usually occurs on a timescale of nanoseconds to microseconds.3. Intersystem crossing: In some cases, the excited molecule can undergo intersystem crossing, a process in which the excited electron changes its spin multiplicity, transitioning from a singlet excited state to a triplet excited state. This process is spin-forbidden and typically slower than internal conversion or fluorescence. Triplet excited states are generally longer-lived than singlet excited states, lasting from microseconds to seconds.4. Photochemical reactions: Once the molecule is in an excited state, it can undergo various photochemical reactions, such as bond dissociation, electron transfer, or isomerization. These reactions involve the excited molecule interacting with other molecules or its environment, leading to the formation of new chemical species or changes in the molecular structure. The specific photochemical reaction depends on the nature of the excited molecule and its surroundings.In summary, the absorption of light by organic compounds leads to the formation of excited states through electronic excitation. Subsequent steps involved in the photochemical reactions of these compounds include internal conversion, fluorescence, intersystem crossing, and various photochemical reactions that can result in the formation of new chemical species or changes in molecular structure.