The relationship between the chemical structure of a sunscreen agent and its ability to absorb UV radiation lies in the presence of specific functional groups and chromophores within the molecule. These functional groups and chromophores are responsible for absorbing the UV radiation and dissipating it as heat, thereby protecting the skin from harmful effects of the sun.There are two main classes of sunscreen agents commonly used in cosmetic products: organic chemical sunscreens and inorganic physical sunscreens.1. Organic sunscreens: These are carbon-based molecules that contain chromophores, which are groups of atoms that can absorb UV radiation. The most common chromophores in organic sunscreens are aromatic rings conjugated with carbonyl groups, such as benzophenones, cinnamates, and avobenzone. The photochemical properties of organic sunscreens are determined by the extent of conjugation and the presence of electron-donating or electron-withdrawing substituents on the aromatic rings. The more extended the conjugation, the more effective the sunscreen is at absorbing a broader range of UV radiation. Organic sunscreens are typically lightweight and easily absorbed by the skin, making them popular in cosmetic formulations.2. Inorganic sunscreens: These are mineral-based sunscreens that contain metal oxide particles, such as titanium dioxide TiO2 and zinc oxide ZnO . In contrast to organic sunscreens, inorganic sunscreens work by reflecting and scattering UV radiation, rather than absorbing it. The photochemical properties of inorganic sunscreens depend on the size and shape of the metal oxide particles. Smaller particles provide better UV protection, but they can also cause a white cast on the skin. To overcome this issue, manufacturers often use nanoparticles of metal oxides, which are transparent on the skin while still providing effective UV protection. Inorganic sunscreens are generally less irritating and more suitable for sensitive skin compared to organic sunscreens.In summary, the chemical structure of a sunscreen agent plays a crucial role in its ability to absorb or reflect UV radiation. Organic sunscreens rely on conjugated chromophores to absorb UV radiation, while inorganic sunscreens use metal oxide particles to reflect and scatter it. Both types of sunscreens have their advantages and disadvantages, and the choice between them depends on factors such as skin type, cosmetic preferences, and desired level of sun protection.