Computational and experimental characterization of a fluorescent dye for detection of potassium ion concentration

The fluorescence of the SKC-513 ((E)-N-(9-(4-(1,4,7,10,13-pentaoxa-16-azacyclooctadecan-16-yl)phenyl)-6-(butyl(3-sulfopropyl)amino)-3H-xanthen-3-ylidene)-N-(3-sulfopropyl)butan-1-aminium) dye is shown experimentally to have high sensitivity to binding of the K(+) ion. Computations are used to explore the potential origins of this sensitivity and to make some suggestions regarding structural improvements. In the absence of K(+), excitation is to two nearly degenerate states, a neutral (N) excited state with a high oscillator strength, and a charge-transfer (CT) state with a lower oscillator strength. Binding of K(+) destabilizes the CT state, raising its energy far above the N state. The increase in fluorescence quantum yield upon binding of K(+) is attributed to the increased energy of the CT state suppressing a nonradiative pathway mediated by the CT state. The near degeneracy of the N and CT excited states can be understood by considering SKC-513 as a reduced symmetry version of a parent molecule with 3-fold symmetry. Computations show that acceptor-donor substituents can be used to alter the relative energies of the N and CT state, whereas a methylene spacer between the heterocycle and phenylene groups can be used to increase the coupling between these states. These modifications provide synthetic handles with which to optimize the dye for K(+) detection.