Water-soluble Nile Blue derivatives: syntheses and photophysical properties

Four water-soluble 2-hydroxy-Nile Blue derivatives, 1 a, 1 b, 2 a, and 2 b, were prepared by condensation reactions performed under relatively mild conditions (90 degrees C, N,N-dimethylformamide with no added acid). These fluorescent probes had more favorable fluorescence characteristics than two known water-soluble Nile Blue derivatives. Specifically, they were superior to the known dyes with respect to their quantum yields in aqueous media and the sharpness of their fluorescence emissions. Concentration-dependant UV absorption and fluorescence emission studies indicated that the dyes did not aggregate in aqueous solution at concentrations of less than 1-4 microM. The new water-soluble materials 1 a, 1 b, 2 a, and 2 b emit in a desirable region of the fluorescence spectrum (lambda=670-675 nm). Overall they are potentially interesting for labeling biomolecules in aqueous environments.