Transition Metal‐Involved Photon Upconversion

Upconversion (UC) luminescence of lanthanide ions (Ln3+) 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 Ln3+, transition metal (TM) ions, e.g., Mn2+, usually possess a single broadband emission due to its 3d5 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 Ln3+ can be modulated by TM ions (specifically d-block element ions with unfilled d orbitals), which benefits from the specific metastable energy levels of Ln3+ owing to the well-shielded 4f electrons and tuneable energy levels of the TM ions. The electric versatility of d0 ion-containing hosts (d0 normally viewed as charged anion groups, such as MoO66- and TiO44-) may also have a strong influence on the electric dipole transition of Ln3+, 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 Ln3+ tuned by TM or d0 ions, and the UC of d0 ion-centred groups, as well as their potential applications in bioimaging, solar cells and multifunctional devices.