Photochemical reactions in organic compounds involve the absorption of light energy by molecules, leading to changes in their electronic states and subsequent chemical transformations. The general mechanism of photochemical reactions can be divided into several steps:1. Absorption of light: When a molecule absorbs a photon of light, it gains energy and gets promoted from its ground electronic state S0 to an excited electronic state S1 or T1 . This process is called electronic excitation.2. Relaxation: Once the molecule is in an excited state, it can undergo various relaxation processes, such as internal conversion IC , intersystem crossing ISC , or fluorescence. IC and ISC involve the transfer of energy within the molecule, leading to the formation of different excited states or the return to the ground state. Fluorescence is the emission of light as the molecule returns to its ground state.3. Chemical reaction: If the molecule remains in an excited state, it can undergo a chemical reaction. The excited molecule can react with another molecule, or it can undergo a unimolecular reaction, such as bond cleavage or rearrangement. The excited state can also act as a catalyst, facilitating reactions that would not occur in the ground state.Examples of photochemical reactions in organic compounds include:a. Norrish Type I and Type II reactions: In these reactions, carbonyl compounds absorb light and undergo cleavage of the C-C bond Type I or the C-H bond Type II in the excited state.b. Patern-Büchi reaction: In this reaction, an excited carbonyl compound reacts with an alkene to form an oxetane ring.c. [2+2] Cycloaddition: In this reaction, two alkenes or an alkene and an alkyne absorb light and undergo a [2+2] cycloaddition to form a cyclobutane or cyclobutene ring.d. Photoisomerization: In this reaction, a molecule absorbs light and undergoes a change in its molecular geometry, such as cis-trans isomerization in alkenes.The energy from light is translated into chemical changes within the molecule by promoting the molecule to an excited state with a different electronic configuration. This change in electronic configuration can alter the molecule's reactivity, making it more susceptible to certain reactions or allowing it to participate in reactions that would not occur in the ground state. The energy from the excited state can also be used to overcome activation barriers for reactions, facilitating the formation of new chemical bonds or the cleavage of existing ones.