Abstract
Near-infrared (NIR) emissive metal complexes have shown potential applications in optical communication, chemosensors, bioimaging, and laser and organic light-emitting diodes (OLEDs) due to their structural tunability and luminescence stability. Among them, complexes with bridging ligands that exhibit unique emission behavior have attracted extensive interests in recent years. The target performance can be easily achieved by NIR light-emitting metal complexes with bridging ligands through molecular structure design. In this review, the luminescence mechanism and design strategies of NIR luminescent metal complexes with bridging ligands are described firstly, and then summarize the recent advance of NIR luminescent metal complexes with bridging ligands in the fields of electroluminescence and biosensing/bioimaging. Finally, the development trend of NIR luminescent metal complexes with bridging ligands are proposed, which shows an attractive prospect in the field of photophysical and photochemical materials.
Highlights
Coordination compounds, such as Prussian blue and bile alum as dyes and fungicides, respectively, have been used in living very early on
This review introduces the recent development of NIR luminescent metal complexes with bridging ligands in the fields of electroluminescence, biosensing/bioimaging, etc
The luminescence behavior of metal complexes is directly related to the relative energy levels of metal ions and bridging ligands in excited states
Summary
Coordination compounds, such as Prussian blue and bile alum as dyes and fungicides, respectively, have been used in living very early on. Ligand-to-metal charge transfer (LMCT) is another common approach of excited state charge transition in luminescent metal complexes (Figure 1, mode IV), which is generally found in complexes with luminescent transition metal, especially the lanthanides, as the coordination center. This is because the forbidden d-d or f-f transitions make low absorption cross-sections of transition metal elements, and they are difficult to directly excite. High absorption organic ligands are used to coordinate with these emissive metal ions, which realizes energy transfer from the excited state of light-harvesting organic ligands to the metal center and enhances the luminescence of metal elements under low power excitation. Electronexcited metal complexes can carry out electron transfer and/or energy transfer with other molecules, and in the microenvironment, subtle changes would significantly perturb their luminescence, especially the charge-transfer excited metal complexes, which are the reason for their many applications
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