Photosensitized reactions are the reactions in which molecules absorbing light do not themselves react but induce other molecules to react. This process involves the transfer of energy from a photosensitizer molecule to another molecule, which then undergoes a chemical reaction. The photosensitizer molecule acts as a catalyst in the reaction.

Photosensitization involves three main steps: excitation, energy transfer, and reaction initiation.

1. Excitation:
- In photosensitized reactions, a photosensitizer molecule absorbs light energy and gets excited to a higher energy state.
- The photosensitizer molecule can absorb light of a specific wavelength due to its electronic structure and energy levels.
- This excitation of the photosensitizer molecule is a non-reactive process, and the molecule itself does not undergo any chemical changes.

2. Energy Transfer:
- The excited photosensitizer molecule then transfers its energy to another molecule, known as the substrate molecule.
- This energy transfer can occur through various mechanisms such as electron transfer, energy transfer via Förster resonance energy transfer (FRET), or energy transfer through singlet oxygen generation.
- The substrate molecule receives the energy from the photosensitizer and gets excited to a higher energy state.

3. Reaction Initiation:
- The excited substrate molecule can now undergo a chemical reaction that it would not have undergone without the energy input from the photosensitizer.
- This reaction can involve various processes, such as oxidation, reduction, or initiation of a specific chemical pathway.
- The photosensitizer molecule does not directly participate in the reaction but acts as a catalyst by providing the necessary energy to initiate the reaction.

Photosensitized reactions are widely used in various applications, including photodynamic therapy, organic synthesis, and environmental remediation. In photodynamic therapy, for example, photosensitizers are used to generate reactive oxygen species that can selectively destroy cancer cells. These reactions rely on the ability of the photosensitizer to transfer energy to molecular oxygen, leading to the production of highly reactive singlet oxygen species.

In summary, photosensitized reactions involve the absorption of light by a photosensitizer molecule, followed by energy transfer to a substrate molecule, which then undergoes a chemical reaction. The photosensitizer molecule acts as a catalyst in these reactions, inducing the reaction without itself being consumed or permanently altered.