Indolenine-based derivatives as customizable two-photon fluorescent probes for pH bioimaging in living cells.

Novel pH probes based on 2-(6-methoxynaphthalen-2-yl)-3,3-dimethyl-3H-indole have been synthesized and charac-terized. These compounds display excellent "off-on" fluorescence responses to acidic pH especially under two-photon (TP) excitation conditions, as well as strong selectivity and sensitivity towards H+. These features are supported by fluo-rescence quantum yields over 35%, two-photon cross sections ca. 60 GM and good resistance to photodegradation un-der acidic conditions. The synthetic versatility of this model allows subcellular targets to be tuned through minor scaf-fold modifications without affecting their optical characteristics. The effectiveness of the probes' innate photophysical properties and the structural modifications for different pH-related applications is demonstrated in mouse embrionary fibroblast (MEF) cells.