The Chemistry of Small-Molecule Fluorogenic Probes - Progress in Molecular Biology and Translational Science - page 17

caged rhodamines can be accessed directly from fluorescein derivatives
using Pd-catalyzed cross-coupling. 79 Compound 57 has been used for
photoactivatable localization microscopy (PALM) imaging of cellular DNA. 67
Because of the problems encountered with the synthesis of N- acyl caged
rhodamines, other design strategies have been developed for photolabile
rhodamines. As mentioned above, amidation of the ortho -carboxyl group
of rhodamines yields derivatives that adopt a nonfluorescent, UV-absorbing
lactam form under certain conditions. Illumination with short-wavelength
light can cause the molecule to open, transiently generating a fluorescent
species. Photochromic rhodamines such as compound 58 have been used
to perform super-resolution imaging. 92 Another strategy to generate caged
rhodamine dyes is exemplified in compound 59 . Here, the molecule is
locked in a nonfluorescent form through formation of a spiro-carbocycle,
which remains closed regardless of nitrogen substitution. Illumination causes
a photochemical rearrangement of the diazo moiety, ultimately yielding a
fluorescent rhodamine species. 93

7.4. Indicators
Given the tunability of wavelengths, the rhodamines serve as useful
fluorogenic sensors across a large spectral range. Like the fluoresceins,
the addition of an aniline-containing chelation motif can yield fluo-
rogenic sensors that are governed by PeT. An example is the calcium in-
dicator Rhod-2 ( 59 ). One caveat with rhodamine-based sensors is their
lower sensitivity than the fluorescein-based Fluo dyes 9 and 48 . 21 This
difference stems from the efficiency of PeT being based on the l max

and redox potential of the dye. 94 The PeT process is less favorable with
dyes exhibiting high max values. Ion indicators built from newer

rhodamine isologs that exhibit optimal redox potentials show higher sen-
sitivity while retaining relatively long wavelengths. 95 In addition to
PeT-modulated probes, the open-closed equilibrium of rhodamines
can be used to generate sensors. Rhodamine lactams find wide use as ir-
reversible “turn-on” probes. 9,85 For example, compound 60 remains
nonfluorescent until interaction with Cu. 96 Likewise, rhodamine 61
adopts a nonfluorescent form because of the nucleophilicity of the
thiol group. Oxidation of the thiol to a sulfonate by hypochlorous acid
yields a species

l

that exists in the open fluorescent state, allowing

sensing of this ROS. 97