4/10 : G.Ulrich – Luminescent borate complexes and their emissive ligands 🗓 🗺
During the last decades, there has been a renewed interest in fluorescent organic dyes propelled by attractive applications in both the optoelectronic (OLED, OPV) and bioimaging (monitoring biological process with high resolution microscopy) fields. The organic fluorophores mostly used for these purposes, such as polyaromatics, xanthenes, coumarins, cyanines and BODIPY dyes  have their fluorescence mostly originating from a relaxation process of their lowest singlet excited state.
We and others introduced a decade ago several synthetic methodologies around fluorescent borate complexes such as BODIPY allowing to tune the color, the substituents on the boron, the solubility in various media including water.  More recently we investigated boron based complexes with stabilization of a cyanine backbone.  Bright fluorescent compounds with N^B^O or N^B^N chelation mode were synthesized, with 2-(2’-hydroxyphenyl)benzoxazole),  salicylaldimine,  or imino coumarin ligands.  These new dyes displayed competitive or complementary properties compared to BODIPY. The versatility and tunability of all these Borate complexes make them highly attractive fluorophores for wide areas of applications.
By investigating the ligand optical properties and chemically modifying the benzazole skeleton we could obtain very efficient ESIPT emitters. These molecules present an isomeric change in their excited state the Enol form is responsible of the absorption wavelength whereas the emission often comes from the Keto form; thus induces a large Stokes’shift. This last property makes ESIPT dyes very good candidates to obtain fluorophores not prone to reabsorption. ESIPT systems present several advantages including a high quantum yield of emission in the solid state. We have recently successfully engineered molecules exhibiting dual emission in more emissive 6-membered ring systems (benzazoles), by frustration of the proton transfer in excited state,  opening the way to white emissive materials and potential ratiometric sensors.
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