Principles

Photochemical reactions occur when substances interact with light energy, triggering chemical transformations. The core principle lies in the interaction between photons and molecules, which induces electronic transitions or bond cleavage. When the energy of a photon (determined by its wavelength) matches the electronic energy gap of a molecule, the molecule absorbs the light energy and transitions to an excited state (e.g., singlet or triplet state). Excited molecules exhibit heightened reactivity and can undergo transformations through the following pathways:

1. Direct Reaction: The excited molecule directly participates in bond recombination or cleavage (e.g., photolysis).

2. Energy Transfer: The excited molecule transfers its energy to another molecule, initiating subsequent reactions (e.g., photosensitized reactions).

3. Electron Transfer: The excited molecule undergoes electron transfer with another molecule, forming free radicals or ion pairs (e.g., photo-redox reactions).

Advantages

1. Mild Conditions: These reactions proceed under ambient temperature and pressure, eliminating the need for high temperatures or pressures and reducing energy consumption and equipment costs.

2. High Selectivity: By tuning the light wavelength, specific molecules can be precisely excited, minimizing side reactions and enhancing product purity.

3. Green and Sustainable: Light serves as the energy source, avoiding the use of toxic catalysts or strong oxidizing agents and reducing pollution.

4. Spatiotemporal Control: Reactions can be precisely controlled in time and space using light switches or photomasks (e.g., photolithography).

5. Unique Reaction Pathways: Light excitation enables reactions that are difficult to achieve thermally (e.g., C-H bond activation), expanding the boundaries of synthetic chemistry.

Photochemical reactions hold significant promise in fields such as energy conversion, pharmaceutical synthesis, and materials science.