Lanthanide complexes are coordination compounds containing lanthanide ions elements with atomic numbers 57-71 surrounded by ligands. They exhibit unique photochemical properties due to their electronic configuration, which includes 4f orbitals that are shielded from the ligand field by the filled 5s and 5p orbitals. This shielding effect results in weak ligand field splitting and distinct spectroscopic and photochemical properties compared to other transition metal complexes.Photochemical properties of lanthanide complexes:1. Electronic transitions: Lanthanide complexes exhibit f-f transitions, which involve the excitation of an electron from one 4f orbital to another. These transitions are parity forbidden, making them weak and less intense compared to d-d transitions in other transition metal complexes. However, the presence of suitable ligands can enhance the intensity of these transitions through the ligand-to-metal charge transfer LMCT mechanism.2. Luminescence: Lanthanide complexes are known for their strong luminescence properties, particularly in the visible and near-infrared regions. The luminescence arises from the relaxation of the excited state to the ground state, accompanied by the emission of a photon. The long-lived excited states, sharp emission bands, and large Stokes shifts make lanthanide complexes excellent candidates for various applications, such as in sensors, imaging, and lighting.3. Photoreactivity: Lanthanide complexes generally exhibit low photoreactivity due to the shielding of the 4f orbitals. However, some lanthanide complexes can undergo photoinduced electron transfer PET or energy transfer processes, leading to photoreactivity in specific cases.Differences from other transition metal complexes:1. Transition metal complexes typically exhibit d-d transitions, which are Laporte allowed and have higher molar absorptivities compared to the f-f transitions in lanthanide complexes.2. The luminescence properties of transition metal complexes are generally weaker and have shorter lifetimes compared to lanthanide complexes.3. Transition metal complexes often exhibit more diverse photoreactivity, including photoinduced ligand exchange, redox reactions, and photocatalytic processes.Examples of applications of lanthanide complexes in photochemistry:1. Luminescent probes and sensors: Lanthanide complexes, such as those containing Eu III and Tb III , are used as luminescent probes for detecting various analytes, including metal ions, anions, and biomolecules.2. Bioimaging: Lanthanide complexes, particularly those containing near-infrared emitting ions like Nd III and Yb III , are used as contrast agents in optical imaging techniques, such as fluorescence microscopy and in vivo imaging.3. Light-emitting diodes LEDs : Lanthanide complexes, such as those containing Eu III and Ce III , are used as phosphors in LEDs to convert UV or blue light into visible light, resulting in energy-efficient and long-lasting light sources.4. Photodynamic therapy PDT : Lanthanide complexes, such as those containing Yb III and Nd III , can be used as photosensitizers in PDT for cancer treatment. These complexes can generate reactive oxygen species upon light irradiation, leading to the destruction of cancer cells.