Fluorescence imaging allows visualization of the processes on molecular level using fluorescent imaging reporters. Fluorescence imaging has been gaining its popularity due to its noninvasiveness, cost-efficiency, and absence of radioactive agents. The era of optical imaging with fluorescence probes started with conventional “always-on” probes that suffer from poor signal-to-background ratios (SBRs). Due to the limitations of the “always-on” probes use, there is an ongoing search for better fluorophores possessing “turn-on” or ratiometric properties that will alleviate background noise and low signal-to-noise ratio problems. Available ratiometric probes provide a limited color change in the overlapping range (red/yellow) and are high molecular weight fluorophores that are usually associated with high cytotoxicity. Within the broad range of fluorescent structures, smaller fluorophores appear to be advantageous for live cell analysis as they possess better cell permeability, high photostability, and lower cytotoxicity. We aimed to advance the development of small ratiometric probes based on coumarins. Color-changing property in the small coumarin fluorophores is advantageous comparing to such properties of high molecular weight fluorophores as coumarins stand apart due to their low cytotoxicity related to their low molecular weight which leads to their wide use in the live cell imaging. The approach is based on utilizing the fluorescence change upon hydrolysis of C4-substituted coumarin esters to achieve the transition of a red emissive coumarin ester to respective blue emissive coumarin acid. The work covers the advances towards establishing the efficient synthesis of coumarin aryl esters with the goal of obtaining large separation between two fluorescence emissions. Tuning Chan−Lam−Evans type coupling conditions allowed to overcome the challenges associated with synthesis of aryl esters and generate a library of aryl coumarin esters for the first time. Products of the modified copper-catalyzed Chan-Lam-Evans coupling reactions between the coumarin acid and different<br/>boronic acids will be presented. The effect of substitutions on aryl boronic acids on the emission properties of the final esters will be presented as well as the potential applications for cell imaging techniques.

Chan−Lam−Evans coupling offers an approach to coumarin aryl esters as ratiometric fluorophores

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