Photochemical reactions are chemical processes initiated when molecules absorb light energy. Unlike reactions driven by heat, these transformations depend directly on the specific wavelengths of light absorbed. This absorption provides the necessary energy to alter molecular structures or trigger new chemical bonds.
The Science of Light and Molecules
Light travels as discrete packets of energy called photons, each possessing a specific energy level determined by its wavelength. When a molecule encounters a photon, it can absorb this energy, provided the photon’s energy matches an energy difference within the molecule. This absorption elevates the molecule from its stable ground state to a higher-energy, unstable excited state.
Once in an excited state, the molecule becomes reactive and can undergo transformations. This absorbed energy can cause existing chemical bonds to break, leading to the formation of reactive intermediates like free radicals. Alternatively, the energy might induce a rearrangement of atoms within the molecule, forming a new isomer. The excited molecule can also transfer its energy to another nearby molecule, initiating a chain of reactions. Light energy acts as the activation energy, enabling reactions that would otherwise require higher temperatures or pressures.
Photochemistry in Living Systems
Light-driven chemical changes are fundamental to many biological processes. Photosynthesis, where plants, algae, and some bacteria convert light energy into chemical energy, is a prime example. Chlorophyll molecules absorb sunlight, initiating reactions that transform carbon dioxide and water into glucose and oxygen. This process involves light-dependent reactions where light energy drives the synthesis of ATP and NADPH, which then power sugar production.
Human vision relies on a photochemical reaction within the eye’s photoreceptor cells. When light strikes the retina, 11-cis-retinal, located within the rhodopsin protein, absorbs the photon. This absorption causes 11-cis-retinal to rapidly change its shape into all-trans-retinal. This conformational change triggers a cascade of biochemical signals, leading to electrical impulses sent to the brain, allowing us to perceive light.
Another biological photochemical reaction is the synthesis of Vitamin D in the skin. Exposure to ultraviolet B (UVB) radiation converts 7-dehydrocholesterol into pre-vitamin D3. This pre-vitamin D3 then undergoes a temperature-dependent rearrangement to form Vitamin D3.
Photochemistry in the Environment and Industry
Photochemical reactions play a significant role in environmental phenomena and industrial applications. The Earth’s protective ozone layer, located in the stratosphere, is formed through a photochemical process. Ultraviolet (UV) radiation from the sun splits oxygen molecules (O2) into individual oxygen atoms, which then react with other O2 molecules to form ozone (O3). Conversely, human-made pollutants like chlorofluorocarbons (CFCs) can be broken down by UV light in the stratosphere, releasing chlorine atoms that catalytically destroy ozone, leading to ozone depletion.
In urban environments, sunlight drives the formation of photochemical smog, a harmful type of air pollution. Nitrogen oxides and volatile organic compounds released from vehicle exhausts react in the presence of sunlight to produce ground-level ozone and other irritants. This complex series of reactions contributes to respiratory problems and environmental damage.
Industry utilizes photochemistry in various ways. In traditional photography, light striking silver halide crystals on film causes a chemical change that forms a latent image. Ultraviolet (UV) curing is another industrial application, where UV light rapidly polymerizes and hardens specialized inks, coatings, and adhesives. UV light is also employed in water purification systems to disinfect water by damaging the DNA and RNA of bacteria, viruses, and other microorganisms.