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 3d(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 4f 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 MoO6(6-) and TiO4(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.