Coumarin- and Rhodamine-Fused Deep Red Fluorescent Dyes: Synthesis, Photophysical Properties, and Bioimaging in Vitro

A series of deep red fluorescent
dyes (CR1 to CR3) was developed via introduction
of a coumarin moiety into
the rhodamine molecular skeleton. The novel dyes possessed the individual
advantages of coumarin and rhodamine derivatives, and the emission
wavelength was extended to the deep red region (>650 nm) due to
the
extension of fused-ring conjugate structure simultaneously. To illustrate
its value, we designed and conveniently synthesized a series of novel
deep red bioimaging dyes (CR1E to CR3E)
by esterification of CR1 to CR3, which could
selectively stain mitochondria. They were superior to the MitoTrackers
for mitochondrial staining in terms of large Stokes shift, excellent
contrast for imaging, high photostability, and low cytotoxicity. Furthermore,
the fluorescence of the coumarin moiety and rhodamine-like fluorophore
could be switched like classical rhodamine. Thus, they could be used
as an effective platform in constructing fluorescence sensors. Based
on this fact, we constructed a novel ratiometric sensor (CR1S) for Hg2+ with good selectivity that could be successfully
applied to the imaging of Hg2+ in living A549 cells. This
design strategy is straightforward and adaptable to various deep red
dyes and sensing platforms by simply introducing different fluorophores.