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absorption above 700 nm, and their relatively straightforward synthesis, re-
cent advances in chemistry of squaraine rotaxanes, and recently discovered
chemiluminescence properties of squaraine rotaxanes (see
Section 6.2
), all
together make this class of fluorophores promising candidates for a broad
variety of applications.
4. BORON DIPYRROMETHENE AND RELATED
FLUOROPHORES
4.1. General characterization
Another class of organic fluorophores potentially useful for
in vivo
imaging
are boron dipyrroethenes.
120-122
BDP (4,4-difluoro-4-bora-3
a
4
a
-diaza-
s
-
indacene, BODIPY) are a class of neutral organic fluorophores containing
a conjugated system of two pyrrole rings linked by methine bridge and
complexed by the difluoroboron moiety. Boron dipyromethenes exhibit
strong absorption and emission in the visible and near-IR spectral
window, and their absorption and emission are relatively insensitive to
the solvent polarity and pH. BDPs show also remarkable photo- and
chemostability compared to cyanines. The rigid BDP molecular
framework makes BDP derivatives less prone to nonradiative decay of the
excited state; thus their fluorescence quantum yield is usually high.
Synthesis of BDP typically entails an acid-catalyzed condensation
between pyrrole and aldehyde, oxidation of resulting dipyrromethane,
and finally,
complexation of
the
resulting dipyrrin with boron
trifluoride.
120,122
BDPs with their strong absorption in visible region, high fluorescence
quantum yields, relatively high chemical and photochemical stability, and
the very rich chemistry that allows their versatile structural modification
and enables further fine-tuning of their chemical and optical properties rep-
resent an excellent platform to develop fluorescence probes for
in vitro
and
intracellular imaging (for the most recent review of the application of BDP as
a fluorescent probe, see Ref.
121
). However, there are fewer examples of
in
vivo
applications of BDP derivatives.
123-125
One of the major reasons is that simple BDPs exhibit rather short-
wavelength absorption and emission bands (
500-600 nm), which are
unsuitable for
in vivo
applications. Therefore, several strategies have been
developed to shift the absorption and emission toward longer wavelengths.
Representative examples of red- and near-IR-emitting BDP derivatives
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