Two-photon fluorescence microscopy has become an indispensable
technique for cellular imaging. Whereas most two-photon fluorescent
probes rely on well-known fluorophores, here we report a new fluorophore
for bioimaging, namely azulene. A chemodosimeter, comprising a boronate
ester receptor motif conjugated to an appropriately substituted azulene,
is shown to be an effective two-photon fluorescent probe for reactive
oxygen species, showing good cell penetration, high selectivity for
peroxynitrite, no cytotoxicity, and excellent photostability.