Photoisomerization is a process in which a molecule undergoes a structural change due to the absorption of light, leading to the formation of an isomer. This process can result in changes in the photophysical and photochemical properties of the molecule. Let's consider the example of azobenzene, a molecule that undergoes photoisomerization, to understand the mechanism and the resulting changes in its properties.Azobenzene is a molecule that consists of two phenyl rings connected by a N=N double bond azo group . It exists in two isomeric forms: trans-azobenzene and cis-azobenzene. The trans isomer is more stable and is the predominant form in the ground state.Mechanism of photoisomerization:1. Absorption of light: When azobenzene absorbs a photon of light, it gets excited from the ground state S0 to a higher-energy excited state S1 or S2 . This process is called electronic excitation.2. Isomerization: In the excited state, the molecule undergoes structural changes, leading to the rotation around the N=N double bond. This rotation causes the molecule to change its configuration from trans to cis or vice versa.3. Relaxation: After the isomerization, the molecule relaxes back to the ground state S0 by releasing the excess energy in the form of heat or light fluorescence or phosphorescence .Changes in photophysical and photochemical properties:The photoisomerization process leads to changes in the photophysical and photochemical properties of azobenzene, such as absorption and emission spectra, quantum yield, and photochemical reactivity.1. Absorption and emission spectra: The trans and cis isomers of azobenzene have different absorption spectra due to their distinct electronic structures. The trans isomer absorbs light in the UV region around 320 nm , while the cis isomer absorbs light in the visible region around 440 nm . This difference in absorption spectra can be used to monitor the photoisomerization process.2. Quantum yield: The quantum yield is the ratio of the number of molecules undergoing photoisomerization to the number of photons absorbed. The quantum yield of azobenzene depends on the wavelength of light used for excitation and the solvent used. Generally, the quantum yield for the trans-to-cis isomerization is higher than that for the cis-to-trans isomerization.3. Photochemical reactivity: The photoisomerization process can also affect the reactivity of azobenzene. For example, the cis isomer is more reactive towards nucleophilic addition reactions than the trans isomer due to its higher energy and strained geometry.In summary, the mechanism of photoisomerization involves the absorption of light, structural changes in the molecule, and relaxation back to the ground state. This process leads to changes in the photophysical and photochemical properties of the molecule, such as absorption and emission spectra, quantum yield, and reactivity.