A BODIPY analogue from the tautomerization of sodium 3-oxide BODIPY

Article abstract of DOI:10.1016/j.cclet.2015.04.018

Abstract The introduction of hydroxy group to the 3- or 5-position of 4,4-difluoro-4-bora-3a,4a-diazaindacene (BODIPY) results in unexpected tautomerization to BDPONa with interesting structural and photophysical properties. The core of BDPONa is an anion

Full text of DOI:10.1016/j.cclet.2015.04.018

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CCLET 3285 1–5
Chinese Chemical Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
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Original article
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A BODIPY analogue from the tautomerization of sodium 3-oxide
BODIPY
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Q1 Peng-Zhong Chen a,b, Hai-Rong Zheng a,b, Li-Ya Niu a, Yu-Zhe Chen a, Li-Zhu Wu a,
Chen-Ho Tung a, Qing-Zheng Yang a,c,
*
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a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
b University of the Chinese Academy of Sciences, Beijing 100049, China
c College of Chemistry, Beijing Normal University, Beijing 100875, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The introduction of hydroxy group to the 3- or 5-position of 4,4-difluoro-4-bora-3a,4a-diaza-indacene
(BODIPY) results in unexpected tautomerization to BDPONa with interesting structural and
photophysical properties. The core of BDPONa is an anion with sodium cation as counter-ion. BDPONa
displays strong fluorescence in organic solvents.
Received 13 March 2015
Received in revised form 1 April 2015
Accepted 2 April 2015
Available online xxx
ß 2015 Qing-Zheng Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute
of Materia Medica, Chinese Academy of Medical Sciences. All rights reserved.
Keywords:
Bodipy
Substitution reaction
Tautomerization
Sodium salt
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1. Introduction
wide applications in fields as fluorescent probes [12–21], bio- 29
imaging [22–29] and photodynamic therapy [30,31].
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The 4,4-difluoro-4-bora-3a,4a-diaza-indacene (BODIPY) dyes
[1–3], firstly discovered as early as 1968 by Treibs and Kreuzer [4],
are valuable class of fluorophore because of their outstanding
attributes, such as good photostability, high molar extinction
coefficients and fluorescence quantum yields, as well as narrow
emission bands. Up to now, thousands of papers were published
that described the multifarious applications of BODIPY dyes. The
diversity of available BODIPYs could be achieved by ingenious
chemical modification and synthetic strategies, such as nucleo-
philic/electrophilic substitutions at the pyrrole rings, replacement
of fluorine atoms with carbon or oxygen centred substituent
Among the BODIPY derivatives reported to date, the construc- 31
tion of 3,5-dichloroBODIPYs, originally introduced by Dehaen and 32
Boens, is an efficient pathway to direct linking of substitution to 33
the BODIPY core. The electron deficiency at the 3,5-postions 34
enables the replacement of chlorine efficiently via nucleophilic 35
aromatic substitution reactions (SNAr) or palladium-catalyzed 36
cross-coupling reactions. Mono- or di-substitution products can be 37
prepared with carbon, nitrogen, oxygen, sulfur, selenium, telluri- 38
um centred nucleophiles by careful tuning the reaction conditions 39
[32,33]. Hence 3,5-dichloroBODIPYs are useful for access to hetero- 40
substituted BODIPYs, which are difficult to obtain by other routes 41
and thus enrich the BODIPY family. On the other hand, introducing 42
new substitution groups at the 3,5-positions by replacement of 43
chlorine has a large effect on the spectroscopic and photophysical 44
properties, shifting both absorption and/or emission spectra, and 45
groups, insertion of donor/accepter groups, extension of
p
conjugated structure and formation of dimers [5–11]. The
versatility of synthetic strategies to the BODIPYs allows the
creation of a perfect fit between the structure of the dyes and their
desired photophysical properties, making BODIPYs attractive for
changing the fluorescence quantum yields [2].
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Efficient preparation and tunable properties make 3,5-dichlor- 47
oBODIPY a good candidate for the design of novel fluorescent 48
sensors. Our group has developed a series of fluorescent sensors for 49
highly sensitive and selective detection of thiols and heavy metal 50
ions based on 3,5-dichloroBODIPY [34–38]. As part of our work on 51
construction of BODIPY based sensors, a hydroxy group was 52
*
Corresponding author at: Key Laboratory of Photochemical Conversion and
Q2
Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing 100190, China.
E-mail address: qzyang@mail.ipc.ac.cn (Q.-Z. Yang).
http://dx.doi.org/10.1016/j.cclet.2015.04.018
1001-8417/ß 2015 Qing-Zheng Yang. Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical
Sciences. All rights reserved.
Please cite this article in press as: P.-Z. Chen, et al., A BODIPY analogue from the tautomerization of sodium 3-oxide BODIPY, Chin. Chem.
Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.04.018

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attempted to introduce to the 3- or 5-position of the BODIPY core.
In the previous reports, 8-hydroxy-BODIPY was obtained by Hg2+
promoted hydrolysis of 8-methylthio-BODIPY [39]. Zhao and co-
workers reported 6-hydroxyindole based BODIPY for developing
fluorescent sensors [40–42]. Gabba¨ı and co-workers found boron
hydroxide BODIPY derivatives in which the hydroxyl group could
be easily substituted by fluoride under acidic condition [43,44].
However, as far as we know, no literature has ever reported such a
structure that hydroxy group is directly attached to the 3- or 5-
position of BODIPY core. Herein, we report an interesting BODIPY
analogue which was obtained from the 3-hydroxy-BODIPY under
basic condition.
shorter than that of C–O single bond (ꢀ1.43 A), indicating the C55O
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double bond character. Unlike the common BODIPY derivatives
p system within the pyrrole rings,
the bond lengths involving two pyrrole rings of BDPONa behave
more like an alternation between C55C double bond and C–C single
bond (Fig. 1), confirming the rearrangement of the chromophoric
system of BODIPY core, which is consistent with the result
reflected in the 1H NMR spectroscopy. The pyrrole–pyrrole
dihedral angle is 4.478, similar to that of common BODIPYs
reported in the literature, indicating that the rearrangement of
BODIPY core causes no structural distortion of the planar core
a slightly disordered
tetrahedral geometry, with the two fluorine atoms being arranged
orthogonal to the BODIPY cores. And the B–N bonds, ranging from
1.52 A to 1.53 A, are similar to the common BODIPYs.
Both in acetone/n-pentane and ethyl acetate/n-hexane systems,
BDPONa crystallizes in the triclinic space group P-1. Sodium atoms
were doped in the crystal lattice, adopting six-coordinated with
octahedron geometry. Fluorine and oxygen atoms in each BODIPY
ligand participated in coordination, as well as the carbonyl
oxygen atoms of the solvent molecules (Fig. 2). The bond lengths
for both Na–O and Na–F bonds are ranging from 2.2 A to 2.7 A,
comparable with the sum of ionic radius of the corresponding
elements. Four BDPONa constitute one unit cell, packing in a head-
to-head way. The BODIPY cores are connected via sodium atoms to
form polymer chain along the a-direction for the crystal obtained
from ethyl acetate (Fig. 3). However, the packing structure
performs in a quite different way for the crystal from acetone/n-
pentane. Every four BDPONa are connected via sodium atoms,
forming one group independently and packing along the b-
direction (Fig. S5 in Supporting information). In both crystal
lattices, two adjacent BODIPY cores adopt unparallel packing
behaviour, with dihedral angle 6.718 for crystal from acetone and
which have a fully delocalized
p
structure. The boron centre displays
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2. Experimental
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The reaction precursor 3,5-dichloroBODIPY was synthesized
according to the reported literature methods [45,46]. The hydroxy
substituted BODIPY was expected by the reaction of 3,5-
dichloroBODIPY with an excess of sodium hydroxide in methanol
or dimethylformamide (DMF) (Scheme 1). After reaction overnight,
TLC analysis of crude reaction mixture indicated the formation of
one major product. The crude product was subjected to silica gel
column chromatographic purification by using petroleum ether/
acetone as eluent to afford pure product in ꢀ70% yield. To our
surprise, the product had poor solubility in chloroform, dichlor-
omethane and toluene, but easily dissolved in acetone, ethyl
acetate and tetrahydrofuran, which was significantly different
from the common BODIPY dyes.
For purposes of comparison, 3-chloro-5-methoxy-BODIPY
(BDPOMe) was also prepared (details please see the Supporting
information). BDPOMe was deemed to possess similar skeleton to
the product. But they displayed quite different signals for pyrrole
rings in the 1H NMR spectroscopy. The four pyrrole hydrogen
atoms of BDPOMe exhibited four well-defined signals in aromatic
region in acetone-d6. However, for the product, these signals
shifted to upfield and coalesced into two sets (Fig. S1 in Supporting
information). This observation implied different conjugation
properties involving the pyrrole fragments between the product
and BDPOMe.
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19.848 for crystal from ethyl acetate. The distances between the
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closest overlapping
p
-conjugation systems are 7.566 A and
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3. Results and discussion
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The reaction between 3,5-dichloroBODIPY and sodium hydrox-
ide most probably proceeds by the proposed process as shown in
Scheme 1. A tautomerization reaction of the product was occurred
subsequently to the substitution of chlorine by hydroxy group and
deprotonation of hydroxy group to result in the keto-form product
BDPONa, in which the BODIPY core was negatively charged with
sodium ion as counter-ion. The formation of keto-form product
was further supported by high-resolution mass spectrum (Fig. S4
in Supporting information) and X-ray crystallographic analysis.
Single crystals of BDPONa were successfully grown by slow
diffusion of n-hexane into ethyl acetate and n-pentane into
acetone respectively over a period of one week (CCDC 1003302 and
1003303). Bond lengths involving the two pyrrole rings and the C–
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O bond lengths were examined. The distance of C9–O is 1.259 A,
Fig. 1. ORTEP view of BDPONa showing 50% probability ellipsoids. The benzyl and
pyrrole ring hydrogen atoms and solvent molecules are omitted for clarity. Selected
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bond lengths (A) are C9–O, 1.259(3); C9–C8, 1.460(3); C8–C7, 1.344(3); C7–C6,
1.447(3); C6–C5, 1.365(3); C5–C4, 1.443(3); C4–C3, 1.389(3); C3–C2, 1.406(4); C2–
C1, 1.365(4); C1–N1, 1.368(3); C9–N2, 1.365(3); B–N1, 1.533(3); B–N2, 1.527(3); B–
F1, 1.401(3) and B–F2, 1.415(3).
Scheme 1. The proposed reaction process for the formation of BDPONa.
Please cite this article in press as: P.-Z. Chen, et al., A BODIPY analogue from the tautomerization of sodium 3-oxide BODIPY, Chin. Chem.
Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.04.018

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P.-Z. Chen et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
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Fig. 2. The coordinated modes of sodium with the BODIPY ligands in the crystal structure from acetone/n-pentane (a) and ethyl acetate/n-hexane (b) respectively. Part of
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bonds involving sodium in (b) are artificially omitted for clarity. The selected bond lengths (A) are (a) Na1–F2, 2.253; Na1–F3, 2.313; Na1–F4, 2.717; Na1–O1, 2.384; Na1–O2,
2.317; Na1–O5, 2.309; Na2–F1, 2.431; Na2–F4, 2.414; Na2–O1, 2.367; Na2–O1, 2.409; Na2–O2, 2.451, Na2–O11, 2.303; (b) Na1–F2, 2.375; Na1–O1, 2.452; Na1–O1, 2.350;
Na1–O2, 2.568; Na1–O4, 2.290; Na1–F4, 2.353; Na2–O1, 2.363; Na2–O2, 2.512; Na2–O2, 2.307; Na2–F1, 2.233; Na2–F3, 2.268; Na2–F4, 2.399.
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Fig. 3. Views of the packing structure of crystal obtained from ethyl acetate (top, side view; bottom, view along a-direction). H atoms were omitted for clarity. Colour coding:
nitrogen, blue; oxygen, red; boron, yellow; fluorine, green; chlorine, dark green; sodium, blue-green. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Fig. 4. (a) Normalized emission spectra of BDPONa in different solvents at the concentration of 20
mmol/L at 298 K. (b) Normalized emission spectra of BDPONa (red line) and
BDPOMe (black line) in acetone at room temperature. The inset shows the fluorescence of both compounds in acetone under UV lamp illumination at 365 nm. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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13.650 A respectively, indicating the presence of negligible
stacking interactions in the solid state.
p
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are typical for BF2 compounds. The emission spectra (Fig. 4a) of 146
BDPONadisplaybroadand structurelessbandsbetween475 nmand 147
505 nm, with high fluorescence quantum yields (0.35 in THF). Both 148
absorption and emission spectra ofBDPONahavea remarkableblue- 149
shift property compared to the common BODIPY dyes. With UV 150
lamp illumination at 365 nm, the BDPONa in acetone solution 151
The spectroscopic and photophysical properties of BDPONa were
investigated. In different solvents, BDPONa exhibits an absorbance
(Fig. S6 in Supporting information) in the range from 410 nm to
440 nm, with molar extinction coefficients ꢀ104 MÀ1 cmÀ1, which
Please cite this article in press as: P.-Z. Chen, et al., A BODIPY analogue from the tautomerization of sodium 3-oxide BODIPY, Chin. Chem.
Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.04.018

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exhibits blue-green fluorescence, while green fluorescence of
BDPOMe in acetone is observed (Fig. 4b). The blue shift is more
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depicted in X-ray crystallographic analysis, BDPONa is a cross-
conjugated structure, where the push–pull interaction of the
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In conclusion, we discovered a new blue-emitting BODIPY
analogue with interesting structural property. The introduction of
hydroxy group to the 3- or 5-position of BODIPY resulted in an
unexpected structural rearrangement in the BODIPY core, leading
to the keto-form product BDPONa. The core of BDPONa is an anion
with sodium ion as counter-ion. BDPONa with significant blue-
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Acknowledgments
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This work was financially supported by the 973 program (Nos.
2013CB933800, 2013CB834505), National Natural Science Foun-
dation of China (Nos. 21222210, 21102155, 91027041), and the
Chinese Academy of Sciences (100 Talents Program).
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Appendix A. Supplementary data
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193
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2015.04.018.
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