Phosphorescent Soft Salts: Molecular Design, Synthesis, Characterization and Application Studies

“Soft salt” is a term used to describe ionic compounds that are composed of only metal complex components, without halide, alkali metal or other ions commonly present as counterions. While mononuclear iridium(III)-based soft salts are attractive alternatives to their neutral congeners for use in organic light-emitting devices, other application studies based on soft salts have not received much attention to date.

In contrast to traditional short-lived fluorescent probes, long-lived phosphorescent probes based on transition-metal complexes can effectively eliminate unwanted background interference by using time-resolved photoluminescence imaging technique. Hence, phosphorescent probes have become one of the most attractive candidates for investigating biological events in living systems. However, most of them are based on single emission intensity changes. Ratiometric probe based on soft salts comprising an analyte-sensitive cationic complex and an analyte-insensitive anionic complex allows accurate and quantitative measurements of the actual concentrations of intracellular molecules or the relative changes of concentrations in a living cell. It permits simultaneous recording of two separate wavelengths instead of measuring single emission intensity changes and thus offers built-in correction for environmental effects for imaging living cells and tissues. Phosphorescent soft salts, typically with long emission lifetime, evident Stokes shift and high photostability, are ideal materials for biological applications, especially for lifetime imaging. In this project, a series of rationally designed phosphorescent soft salts with different structures and advanced functionalities will be prepared from different constituent ion combinations. The structure of each metal complex ion can be easily altered to tailor for a specific property and function. Here, soft salt based phosphorescent probes will be developed for ratiometric and lifetime imaging of various analytes in real time. This research will offer a variety of phosphorescent ratiometric probes for high-tech applications in chemosensing and bioimaging technologies.

On the other hand, materials showing electrochromic luminescence are scarce. Here, new electrochromic luminescent materials derived mostly from soft salts will be developed which represent more sensitive candidates over the classical electrochromic materials based on absorption properties. Another advantage is that various electrochromic emission colors can be obtained easily by changing the ligands on each metal ion. We envision that these materials can be utilized in optical data recording, storage and security technologies based on the observed electrochromic luminescence due to their unique ionic characteristics. Depending on the associated electrochromic mechanism, data encrypton and decrypton can be realized for security protection via different advanced optical imaging or two-photon excitation techniques.