A new water-soluble near-neutral ratiometric fluorescent pH indicator

Donor-pi-acceptor fluorene derivative 1c is a near-neutral pH indicator whose pK(a) of similar to 7.0 was determined by both absorption and fluorescence methods. 1c satisfies important criteria for a sensitive ratiomeric fluorescent pH indicator with a distinctive isoemissive point, good dispersion in cell cytosol, and low cytotoxicity. Furthermore, its 2PA cross section of 100 GM in its neutral form suggests its. potential in two-photon fluorescence imaging applications.