Nile Red is a benzo[a]phenoxazone dye containing
a diethylamino substituent at the 9-position. In recent years, it
has become a popular histological stain for cellular membranes and
lipid droplets due to its unrivaled fluorescent properties in lipophilic
environments. This makes it an attractive lead for chemical decoration
to tweak its attributes and optimize it for more specialized microscopy
techniques, e.g., fluorescence lifetime imaging or two-photon excited
fluorescence microscopy, to which Nile Red has never been optimized.
Herein, we present synthesis approaches to a series of monosubstituted
Nile Red derivatives (9-diethylbenzo[a]phenoxazin-5-ones)
starting from 1-naphthols or 1,3-naphthalenediols. The solvatochromic
responsiveness of these fluorophores is reported with focus on how
the substituents affect the absorption and emission spectra, luminosity,
fluorescence lifetimes, and two-photon absorptivity. Several of the
analogues emerge as strong candidates for reporting the polarity of
their local environment. Specifically, the one- and two-photon excited
fluorescence of Nile Red turns out to be very responsive to substitution,
and the spectroscopic features can be finely tuned by judiciously
introducing substituents of distinct electronic character at specific
positions. This new toolkit of 9-diethylbenzo[a]phenoxazine-5-ones
constitutes a step toward the next generation of optical molecular
probes for advancing the understanding of lipid structures and cellular
processes.