Tuning the Spectroscopic Properties of Ratiometric Fluorescent Metal Indicators: Experimental and Computational Studies on Mag-fura-2 and Analogues

In this joint theoretical and experimental work, we investigate the properties of Mag-fura-2 and seven structurally related fluorescent sensors designed for the ratiometric detection of Mg²⁺ cations. The synthesis of three new compounds is described, and the absorption and emission spectra of all of the sensors in both their free and metal-bound forms are reported. A time-dependent density functional theory approach accounting for hydration effects using a hybrid implicit/explicit model is employed to calculate the absorption and fluorescence emission wavelengths, study the origins of the hypsochromic shift caused by metal binding for all of the sensors in this family, and investigate the auxochromic effects of various modifications of the "fura" core. The metal-free forms of the sensors are shown to undergo a strong intramolecular charge transfer upon light absorption, which is largely suppressed by metal complexation, resulting in predominantly locally excited states upon excitation of the metal complexes. Our computational protocol might aid in the design of new generations of fluorescent sensors with low-energy excitation and enhanced properties for ratiometric imaging of metal cations in biological samples.