DOI: 10.1002/chem.201502921
Communication
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Fluorescence |Hot Paper|
Near-Infrared Phosphorus-Substituted Rhodamine with Emission
Wavelength above 700 nm for Bioimaging
Xiaoyun Chai,[a] Xiaoyan Cui,[b] Baogang Wang,[a] Fan Yang,[a, c] Yi Cai,[a, c] Qiuye Wu,[a] and
Ting Wang*[a]
the parent fluorophore. One efficient way is to perturb the
Abstract: Phosphorus has been successfully fused into
skeleton of the xanthene core with other atoms, such as C,[4]
a classic rhodamine framework, in which it replaces the
Si,[5] Ge,[6] Se,[7] and Te.[8] In particular, replacement of the bridg-
bridging oxygen atom to give a series of phosphorus-sub-
ing oxygen atom with a silicon atom elicits a 90 nm batho-
stituted rhodamines (PRs). Because of the electron-accept-
chromic shift compared with the parent rhodamine, yielding
ing properties of the phosphorus moiety, which is due to
near-infrared (NIR) Si-rhodamine with emission maximum at
around 650 nm.[5] Nonetheless, this is still not favorable for
effective s*–p* interactions and strengthened by the in-
ductivity of phosphine oxide, PR exhibits extraordinary
practical bioimaging, because two longer wavelength windows
long-wavelength fluorescence emission, elongating to the
at 700 nm and 800 nm with deeper tissue penetration and
region above 700 nm, with bathochromic shifts of 140
lower background fluorescence are often used.[9] More recently,
and 40 nm relative to rhodamine and silicon-substituted
Nagano and co-workers applied a ring-expansion strategy to
rhodamine, respectively. Other advantageous properties of
the Si-rhodamine scaffold and obtained two NIR fluorescent
the rhodamine family, including high molar extinction co-
dyes with the emission wavelength elongated to 700 nm.[10]
efficient, considerable quantum efficiency, high water sol-
Klµn and co-workers synthesized 9-phenylethynyl Si-pyronine
ubility, pH-independent emission, great tolerance to pho-
with an emission-wavelength maximum at 730 nm.[11] However,
tobleaching, and low cytotoxicity, stay intact in PR. Given
the proposed strategies stated above still have some disadvan-
these excellent properties, PR is desirable for NIR-fluores-
tages, such as restricted structural flexibility or diminished fluo-
cence imaging in vivo.
rescence quantum yield.
The fusion of main-group elements into rhodamine frame-
work to replace the bridging oxygen atom has become an at-
Advances in chemistry, environmental science, biology, and
medicine are often dependent on small-molecule fluorophores
with tailored chemical and photophysical properties to be
used as environmental indicators, biomolecular tags, or cellular
stains.[1] Among the various fluorophores, the well-known xan-
thene dye rhodamine has found extensive use due to favor-
able characteristics, including high absorptivity, excellent fluo-
rescence properties, great photostability, and tunability for
probe design.[2] Although rhodamine is widely used in bio-
imaging, the applications are facing a daunting challenge: the
absorption and emission wavelengths are typically below
600 nm, which are sometimes not suitable in vivo.[3] Many ef-
forts have been made to obtain new rhodamine analogues
with longer emission wavelengths, but retained advantages of
tractive research area, because the replacing atoms can effec-
tively tune the chemical and photophysical properties of the
fluorescent dye at the electronic level, resulting in a revolution
of the traditional rhodamine.[5,12] Surprisingly, the replacing
atoms have so far been mainly limited to groups 16 and 14 el-
ements, whereas atoms in other groups have received little at-
tention. We anticipate that fusing new groups of atom into
the rhodamine framework might boost the development of
rhodamine-inspired fluorophores with intriguing chemical and
photophysical properties. Furthermore, it will provide more in-
sights into the effects exerted by the replacing atoms.
Organophosphorus species have attracted great interest due
to the unique optical and electronic features.[13] Phosphorus
offers a peculiar geometry, which gives rise to s*–p* orbital
coupling when incorporated within conjugated ring systems,
altering the energy level of the LUMO. The electron-accepting
properties of the phosphorus moiety can be further increased
by phosphine oxides (P-oxides), which tune the chemical and
photophysical properties. Because of these attractive features
of the phosphorus moiety, phosphacyclic p-conjugated materi-
als have been widely applied in organic light-emitting
diodes,[14] organic photovoltaic cells,[15] and fluorescent
probes.[16]
[a] Dr. X. Chai, B. Wang, F. Yang, Y. Cai, Prof. Q. Wu, Dr. T. Wang
Department of Organic Chemistry, College of Pharmacy
Second Military Medical University
Shanghai 200433 (P.R. China)
[b] X. Cui
Department of Chemistry, New York University
New York, New York 10003 (USA)
[c] F. Yang, Y. Cai
College of Pharmacy, Yantai University
Yantai, Shandong 264005 (P.R. China)
Inspired by the unique geometrical and electronic character-
istics of phosphorus, we successfully fused a phosphorus atom
into the rhodamine framework to replace the bridging oxygen
Supporting information for this article is available on the WWW under
Chem. Eur. J. 2015, 21, 16754 – 16758
16754
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