Transition Metal‐Involved Photon Upconversion

Upconversion (UC) luminescence of lanthanide ions (Ln 3+ ) has been extensively investigated for several decades and is a constant research hotspot owing to its fundamental significance and widespread applications. In contrast to the multiple and fixed UC emissions of Ln 3+ , transition metal (TM) ions, e.g., Mn 2+ , usually possess a single broadband emission due to its 3 d 5 electronic configuration. Wavelength‐tuneable single UC emission can be achieved in some TM ion‐activated systems ascribed to the susceptibility of d electrons to the chemical environment, which is appealing in molecular sensing and lighting. Moreover, the UC emissions of Ln 3+ can be modulated by TM ions (specifically d ‐block element ions with unfilled d orbitals), which benefits from the specific metastable energy levels of Ln 3+ owing to the well‐shielded 4 f electrons and tuneable energy levels of the TM ions. The electric versatility of d 0 ion‐containing hosts ( d 0 normally viewed as charged anion groups, such as MoO 6 6‐ and TiO 4 4‐ ) may also have a strong influence on the electric dipole transition of Ln 3+ , resulting in multifunctional properties of modulated UC emission and electrical behaviour, such as ferroelectricity and oxide‐ion conductivity. This review focuses on recent advances in the room temperature (RT) UC of TM ions, the UC of Ln 3+ tuned by TM or d 0 ions, and the UC of d 0 ion‐centred groups, as well as their potential applications in bioimaging, solar cells and multifunctional devices.