A New Water-Soluble Phosphorus-Dipyrromethene and Phosphorus-Azadipyrromethene Dye: PODIPY/aza-PODIPY

Article abstract of DOI:10.1002/chem.201406535

PODIPY and aza-PODIPY have been successfully prepared by the treatment of dipyrromethene and azadipyrromethene with POCl₃ in the presence of Et₃N. The new PODIPY and aza-PODIPY dyes are found to have photophysical properties. PODIPY

Full text of DOI:10.1002/chem.201406535

DOI: 10.1002/chem.201406535
Communication
&
Fluorescent Probes
A New Water-Soluble Phosphorus-Dipyrromethene and
Phosphorus-Azadipyrromethene Dye: PODIPY/aza-PODIPY
Xin-Dong Jiang,*[a] Jiuli Zhao,[a] Dongmei Xi,[a] Haifeng Yu,[a] Jian Guan,[a] Shuang Li,[a] Chang-
Liang Sun,[b] and Lin-Jiu Xiao[a]
central elements aluminum,[4] gallium,[5] and indium[6] have
Abstract: PODIPY and aza-PODIPY have been successfully
prepared by the treatment of dipyrromethene and azadi-
been successfully introduced into this class of compounds.
Moreover, dipyrromethene compounds with group 14 ele-
pyrromethene with POCl3 in the presence of Et3N. The
ments (germanium,[7] silicon[8] and tin[9]) have been successfully
new PODIPY and aza-PODIPY dyes are found to have pho-
synthesized. Although many kinds of dipyrromethene or azadi-
tophysical properties. PODIPY and aza-PODIPY are water-
pyrromethene complexes have been synthesized, dipyrrome-
soluble, and aza-PODIPY is suited for labeling living Hep-2
thene or azadipyrromethene phosphorus complexes with
cells for imaging assays in the near-infrared region. Molec-
group 15 elements have not been reported to date.[10] We
ular orbital calculations show that the increase in the
have explored the compounds of the group 15 elements.[11]
HOMO–LUMO band gap for the lowest energy absorption
Our recent research interest lies in novel BODIPY/aza-BODIPY
bands is observed in the new phosphorus-containing aza-
family of fluorescent dyes.[12] We herein report the novel phos-
PODIPY, and the HOMO and LUMO energies of aza-
phorus-dipyrromethene (PODIPY) and phosphorus-azadipyrro-
PODIPY are found to be higher than those of aza-BODIPY.
methene (aza-PODIPY), and reversible control of their proper-
ties on the phosphorus atom (Figure 1b).
Dipyrromethenes and azadipyrromethenes, have been attract-
ing increasing interest in recent years as very important biden-
tate ligands because of their complexation with BF2 groups to
produce highly fluorescent boron-dipyrromethene and boron-
azadipyrromethene dyes which have a wide variety of applica-
tions ranging from materials science to medicine (Figure 1a).[1]
Dipyrromethenes or azadipyrromethenes have also been ex-
plored as ligands to complex with different metals to form
metallodipyrrin or metalloazadipyrrin complexes, which exhibit
a variety of coordination geometries and architectures. Since
the discovery of the excellent photochemical properties of
boron-containing dipyrromethenes or azadipyrromethenes,
such derivatives have been widely explored. These so-called
BODIPYs or aza-BODIPYs have found diverse applications, such
as protein markers, selective ion sensors, and solar cell sensitiz-
ers, and so forth.[2] In addition to boron as the most frequently
explored central element of dipyrromethene chelates, there
have been a considerable number of compounds reported in
the literature which involve transition metals (e.g., Co, Ni, Cu,
Fe, and Zn) coordinated by this bidentate ligand system (Fig-
ure 1a).[3] In terms of group 13 elements beyond boron the
Figure 1. The design strategies for phosphorus dipyrromethene and phos-
phorus azadipyrromethene dyes.
The major problem with neutral organic dyes are their low
solubility in polar solvents and their tendency to decrease sol-
vent contact by forming nonemissive aggregates.[13] Further-
more, most of these dyes display fluorescence features sensi-
tive to environmental factors such as solvent polarity or associ-
ation with hydrophobic biomolecules or with organized matter
such as micelles, vesicles and thin films.[14] Both good water
solubility and resistance to the formation of non-fluorescent
higher aggregates are essential to most biological applications
of dyes. So, the introduction of desirable water solubilizing
groups is required. Generally, solubility has been imported by
either introducing ionizable hydrophilic groups or by grafting
the hydrophilic group. We have found that PODIPY/aza-
[a] Dr. X.-D. Jiang, J. Zhao, D. Xi, H. Yu, J. Guan, S. Li, Prof. Dr. L.-J. Xiao
College of Applied Chemistry
Shenyang University of chemical Technology
Shenyang, 110142 (P.R. China)
E-mail: xdjiang@syuct.edu.cn
[b] Dr. C.-L. Sun
Center of Physical and Chemistry Test
Shenyang University of Chemical Technology
Shenyang 110142 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406535.
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PODIPY are water soluble and herein also communicate these
results.
a methane in BODIPY/PODIPY systems to narrow the HOMO–
LUMO band gap.[1,2] The properties of 2 were evaluated in
The complexing power between P and N atoms is found to
be weaker than that between B and N atoms in the dipyrrome-
thene system,[15] and the stability of phosphorus-containing di-
pyrromethene dye could be enhanced by the inductive effect
if having a withdrawing group in the meso position. Delightful-
ly, based on the classical dipyrromethene bearing a À(p-NO2)-
C6H4 group in the meso position, in the air system the phos-
phorus-dipyrromethene (PODIPY) 1 can be steadily synthe-
sized, under Et3N, followed by complexation with POCl3 and
then hydrolysis with H2O (Scheme 1a). PODIPY 1 showed the
single phosphorus signal in the 31P NMR spectrum (d=
À50.011 ppm in CDCl3), which are consistent with the reported
literature,[15] indicating the existence of the center atom of
phosphorus at 4-position in dipyrromethene (see the Support-
ing Information). Furthermore, the method was also suitable to
diazapyrromethene, and the corresponding phosphorus-azadi-
pyrromethene dye (aza-PODIPY) 2 was successfully obtained,
with the 31P NMR spectrum (d=À40.346 ppm in CDCl3) in the
relatively low-field region due to the shield from the full effect
of the applied field by its surrounding electrons (Scheme 1b).
To the best of our knowledge, this is a first synthetic example
of phosphorus-containing dipyrromethene and azadipyrrome-
thene dye with group 15 elements.
comparison with tetraphenyl azadipyrromethene 3 (labs
=
640 nm; lem =661 nm; e=7.9ꢁ104 mÀ1 cmÀ1
;
Ff =0.36 in
CH2Cl2; Figure 4).[16] Aza-PODIPY 2 (lem =661 nm) possesses
almost identical emission to aza-BODIPY 3, although the re-
placement of BF2 with PO2 resulted in a hypsochromic shift
(15 nm). Clearly, aza-PODIPY 2 possesses a larger Stokes shift
(871 cmÀ1) than that of aza-BODIPY 3 (496 cmÀ1). Although the
fluorescence quantum yield (Ff) of aza-PODIPY 2 was mea-
sured to be 0.08, dye 2 was bright enough for staining experi-
ments. In addition, the picture of 1 and 2 in CH2Cl2 under
normal conditions shows a vivid color difference (Figure 2).
Figure 2. a) Normalized absorption, and b) fluorescence spectra of 1 and 2
in CH2Cl2 at 298 K.
Scheme 1. Synthesis of PODIPY 1 and aza-PODIPY 2.
To gain insight into the photophysical property of PODIPY/
aza-PODIPY, the absorption and emission spectra for 1 and 2
were measured and are outlined in Figure 2 and Table 1.
PODIPY 1 exhibits the absorption and emission maxima at 510
and 524 nm, respectively, in the visible region, with an extinc-
tion coefficient of 3.49ꢁ104 mÀ1 cmÀ1 and fluorescence quan-
tum yield of 0.17. In contrast, aza-PODIPY 2 absorbs at 625 nm
and emits at 661 nm in the near-infrared (NIR) region. The opti-
cal shift results are in good agreement between BODIPY and
aza-BODIPY in which the electronegative nitrogen atom is in-
troduced at the meso position, due to an imine instead of
Table 1. Photophysical properties of PODIPY 1 and aza-PODIPY 2 in
CH2Cl2 at 298 K.
labs/lem
[nm]
e
Stokes shift
[cmÀ1
]
Fwhm
[nm]
Ff
[mÀ1 cmÀ1
]
1
2
510/524
625/661
34900
33000
523
871
106
72
0.17[a]
0.08[a]
[a] Quantum yields determined with references rhodamine 6G in metha-
nol as standard (Ff =0.78, in air equilibrated water and deaerated solu-
tions)[17] for 1, and Nile blue as standard (Ff =0.27, 0.5% (v/v) 0.1m HCl
in ethanol)[18] for 2.
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PODIPY 1 and aza-PODIPY 2 are water soluble. PODIPY 1 ex-
hibits 13 mgLÀ1 solubility in water (see the Supporting Infor-
mation). In water PODIPY 1 absorbs at 505 nm and emits at
520 nm (Figures S1 and S2 in the Supporting Information), and
its single phosphorus signal in the 31P NMR spectrum shows
d=À50.1 ppm at RT and 608C (Figure S3 in the Supporting In-
formation). Based on the 31P NMR spectrum and the absorption
spectrum in water, PODIPY 1 was stable and can stand for two
weeks in water. The dyes 1 and 2 are more polar than the
BODIPY/aza-BODIPY, thus, have improved water solubility.
Moreover, the interaction between the p-orbital of the phos-
phorus atom and the lone pair electrons of the nitrogen
atom,[19] and many resonators of the limit structure of charge
separation in dye 1 (Figure 3), may also enhance the stability
in water. In addition, like the BODIPY dyes, solutions of dye
1 are stable toward light and air for one day as well as toward
extended UV irradiation (365 nm) in the presence of air.
The molecular geometries of aza-PODIPY 2 and aza-BODIPY
3 were optimized using density functional theory (DFT) at the
B3LYP/6-31G(d) level (see the Supporting Information). The cal-
culated HOMO and LUMO orbital energy levels are summarized
in Figure 4. Aza-PODIPY 2 (labs =625 nm) resulted in a remark-
able hypsochromic shift compared to that (labs =640 nm) of
the aza-BODIPY 3. This is due to the increase in the HOMO–
LUMO band gap for the lowest energy absorption bands ob-
served for 2 relative to that of 3 as determined by molecular
orbital (MO) calculations (Figure 4). Significantly higher HOMO
and LUMO energies for PODIPY 2 were found compared to
those of aza-BODIPY 3 (Figure 4). In addition, the stabilization
of both HOMO and LUMO energies of PODIPY 1 was observed
and most probably can be attributed to the attachment of the
electron-withdrawing group at the meso position (Figure S4 in
the Supporting Information), which is similar to that of the re-
ported literatures.[1d,20]
Figure 4. Frontier molecular orbitals of aza-PODIPY 2 and aza-BODIPY 3 at
the B3LYP/6-31G(d) level with Gaussian03. HOMO/LUMO [eV]=À5.36/À3.15
for 3; HOMO/LUMO [eV]=À3.99/À1.29 for 2.
Figure 5. Fluorescence microscope imaging of Hep-2 cells following incuba-
tion with 2ꢁ10À5 m of 2 in PBS (red color) for 20 min at 378C; dark area rep-
resents the cell nuclei. Scale bar: 10 mm.
To test the effect of labeling in live-cell imaging assays, we
chose dye 2 as a representative compound, due to its water
solubility and its moderate fluorescence quantum yield in the
NIR region, which would enable facile visualization with fluo-
rescence microscopy. For the cell-staining experiment, 2ꢁ
10À5 m of dye 2 in PBS was incubated with Hep-2 cells for
20 min at 378C and excess dye was removed by washing with
PBS prior to visualization. An ArrayScanꢂ VTI HCS reader invert-
ed fluorescence microscope was employed for the fluores-
cence imaging, with the excitation wavelength of the laser at
590 nm. The obtained images indicated that 2 was efficiently
internalized by living cells. Dye 2 was membrane permeable
and was found to localize exclusively to the cytoplasm (red
color) with nuclei intact (dark area; Figure 5).
In conclusion, PODIPY 1 and aza-PODIPY 2 were successfully
synthesized by the treatment of dipyrromethene and azadipyr-
romethene with POCl3 in the presence of Et3N. The new
PODIPY and aza-PODIPY dyes, like the classical BODIPY and
aza-BODIPY, were found to have photophysical properties, with
large Stokes shifts and moderate fluorescence quantum yields.
The new PODIPY and aza-PODIPY are water soluble, and aza-
PODIPY dye 2 was bright enough to be suitable for labeling
living Hep-2 cells and imaging in the NIR region. By using MO
calculations the increase in the HOMO–LUMO band gap for
the lowest energy absorption bands was observed in the phos-
Figure 3. Resonators of the limit structure of PODIPY 1.
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Acknowledgements
This work was supported by the Public Research Foundation of
Liaoning Province for the Cause of Science (2014003009), the
Scientific Research Foundation for the Returned Overseas Chi-
nese Scholars, State Education Ministry, and the start-up funds
from Shenyang University of Chemical Technology. We also
thank Dr. Guo-Tao Sun (Henan University) for his help with the
cell imaging assay.
Keywords: fluorescent probes · phosphorus · photophysical
properties · PODIPY · water soluble
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Received: December 17, 2014
Revised: February 18, 2015
Published online on && &&, 0000
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COMMUNICATION
Splitting color: PODIPY and aza-PODIPY
(1 and 2) have been successfully pre-
pared by the treatment of dipyrrome-
thene and azadipyrromethene with
POCl3 in the presence of Et3N. The new
PODIPY and aza-PODIPY dyes are found
to have photophysical properties (see
figure). Molecular orbital calculations
show that the HOMO and LUMO ener-
gies of aza-PODIPY are higher than
those of aza-BODIPY.
&
Fluorescent Probes
X.-D. Jiang,* J. Zhao, D. Xi, H. Yu, J. Guan,
S. Li, C.-L. Sun, L.-J. Xiao
&& – &&
A New Water-Soluble Phosphorus-
Dipyrromethene and Phosphorus-
Azadipyrromethene Dye: PODIPY/aza-
PODIPY
Chem. Eur. J. 2015, 21, 1 – 5
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