The excitation of lanthanide complexes plays a crucial role in their luminescence properties. Lanthanide ions Ln^3+ have unique electronic configurations with 4f orbitals that are shielded by the outer 5s and 5p orbitals. This shielding leads to sharp and well-defined emission bands, long luminescence lifetimes, and large Stokes shifts, which are the differences between the excitation and emission wavelengths. These properties make lanthanide complexes highly attractive for various practical applications, such as sensing and imaging.The luminescence of lanthanide complexes is mainly due to the electronic transitions within the 4f orbitals. However, direct excitation of the 4f orbitals is parity-forbidden, which means that the absorption of light by these orbitals is weak and inefficient. To overcome this limitation, lanthanide ions are often coordinated with organic ligands, which act as "antenna" molecules. These ligands, also known as sensitizers, can efficiently absorb light and transfer the excitation energy to the lanthanide ions through a process called the "antenna effect." This energy transfer enhances the luminescence properties of the lanthanide complexes.The excitation of lanthanide complexes can be tuned by modifying the ligands' structure, which affects the energy transfer efficiency and the overall luminescence properties. For example, the choice of ligand can influence the absorption and emission wavelengths, the luminescence intensity, and the lifetime of the excited state. By carefully designing the ligands, it is possible to create lanthanide complexes with specific luminescence properties tailored for various applications.Lanthanide complexes have been widely used in sensing and imaging applications due to their unique luminescence properties. Some examples include:1. Time-resolved fluorescence TRF assays: The long luminescence lifetimes of lanthanide complexes allow for time-gated detection, which reduces background interference and improves the sensitivity of the assay.2. Fluorescence resonance energy transfer FRET probes: Lanthanide complexes can act as donors or acceptors in FRET-based sensing systems, enabling the detection of various analytes, such as metal ions, small molecules, and biomolecules.3. Bioimaging: Lanthanide complexes have been used as luminescent markers in cell imaging and tissue staining, providing high-resolution and low-background images due to their sharp emission bands and long luminescence lifetimes.4. Upconversion nanoparticles UCNPs : Lanthanide-doped UCNPs can convert near-infrared NIR light into visible light, allowing for deep tissue imaging with minimal photodamage and autofluorescence.In summary, the excitation of lanthanide complexes significantly affects their luminescence properties, and by carefully designing the ligands, it is possible to create complexes with tailored properties for various sensing and imaging applications.