Synthesis and properties of NIR aza-BODIPYs with aryl and alkynyl substituents on the boron center

Article abstract of DOI:10.1016/j.tetlet.2012.08.056

NIR diaryl aza-borondipyrromethene dyes (C-aza-BODIPY) and dialkynyl aza-borondipyrromethene dyes (E-aza-BODIPY) were synthesized from difluoro aza-borondipyrromethene dyes (F-aza-BODIPY) in 45-83% yields. By X-ray analysis the N-B distances (1.603 and 1.606 ?) in C-aza-BODIPY were found to be longer than the corresponding bond of C-BODIPY by 0.02-0.04 ?. C-aza-BODIPYs display remarkable blue shifts and low fluorescence quantum yields, and E-aza-BODIPYs have moderate fluorescence quantum yields (0.16-0.29). C- and E-aza-BODIPYs display good stability in weak acidic and basic conditions. E-aza-BODIPY was dialytic and suitable to staining of living cells.

Full text of DOI:10.1016/j.tetlet.2012.08.056

Tetrahedron Letters 53 (2012) 5703–5706
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Synthesis and properties of NIR aza-BODIPYs with aryl and alkynyl
substituents on the boron center
Xin-Dong Jiang a, , Yantao Fu a, Tao Zhang c, Weili Zhao a,b,
⇑
⇑
a Key Laboratory for Special Functional Materials of the Ministry of Education, Henan University, Henan 475004, China
b School of Pharmacy, Fudan University, Shanghai 201203, China
c Affiliated Huaihe Hospital, Henan University, Henan 475004, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 March 2012
Revised 24 July 2012
Accepted 14 August 2012
Available online 19 August 2012
NIR diaryl aza-borondipyrromethene dyes (C-aza-BODIPY) and dialkynyl aza-borondipyrromethene dyes
(E-aza-BODIPY) were synthesized from difluoro aza-borondipyrromethene dyes (F-aza-BODIPY) in
45–83% yields. By X-ray analysis the N–B distances (1.603 and 1.606 Å) in C-aza-BODIPY were found
to be longer than the corresponding bond of C-BODIPY by 0.02–0.04 Å. C-aza-BODIPYs display remark-
able blue shifts and low fluorescence quantum yields, and E-aza-BODIPYs have moderate fluorescence
quantum yields (0.16–0.29). C- and E-aza-BODIPYs display good stability in weak acidic and basic
conditions. E-aza-BODIPY was dialytic and suitable to staining of living cells.
Keywords:
NIR fluorescent
Aza-BODIPY
Dye
Ó 2012 Elsevier Ltd. All rights reserved.
X-ray
Hypsochromic shift
pH
4,40-Difluoro-4-bora-3a,4a-diaza-s-indacene (abbreviated as
BODIPY) dyes are a family of very widely used fluorescent dyes
with intense absorption, high quantum yields, and tunable emis-
sion wavelength.1 Therefore, they acted as good candidates for bio-
logical labeling applications.2 By modifying the BODIPY core,
conformationally restricted BODIPYs,3 heteroaryl-fused BODIPYs,4
and aza-BODIPYs,5 have been developed to extend the emission
into red/near infrared (NIR) region for improved sensitivity and
deeper penetration into tissues. Due to the hydrophobic nature
of BODIPY dyes, aggregation in aqueous environment often leads
to quenching of fluorescence.2 Very recently, the replacements of
fluoride atom(s) in traditional difluoro-boradiazaindacenes
(F-BODIPY) with alkyl or aryl substituent(s) (formed C-BODIPY),6
and alkynyl derivatives (leading to E-BODIPY) to modulate stabil-
ity, solubility, Stokes shift, and electronic properties of the fluores-
cent dyes became important progresses in BODIPY studies.7 The
replacements of fluoride of F-BODIPY with alcohol or phenol
affording O-BODIPY were also reported to be a useful strategy for
modulating physical properties, as well as fluorescence behavior.8
Very recently, we documented the replacement of F-BODIPY with
acetic acid which led to improved solubility.9 While F-BODIPY
and O-BODIPY are susceptible to acidic and basic conditions,
C-BODIPY and E-BODIPY display good stability.8,10 The reported
studies on boron replacement are almost exclusively on BODIPYs.
Aza-BODIPYs possess a longer emission wavelength than the par-
ent BODIPY, which has great advantages for the detection of bio-
logical substances because of reduced background absorption,
auto-fluorescence, light scattering, and improved sensitivity for
the detection.5 Moreover, NIR fluorescent dyes have deep penetra-
tion to achieve the distinct image, and the realization of live testing
becomes possible.11 Apart from a few studies on O-aza-BODIPYs,8
to the best of our knowledge, no C-(or E)-aza-BODIPY has been
documented. Our recent research interest lies in the novel BODIPY
family of fluorescent dyes.9,12 We are interested in the influence of
aryl and alkynyl groups on the properties of aza-BODIPY dyes.
Herein we report the synthesis and properties of aza-BODIPYs with
aryl and alkynyl groups attached to the boron atom.
F-aza-BODIPY 8 as one of the most studied aza-BODIPY dye was
selected as our target family.5a,12c The modifications of 8 on boron
atom were very straight forward. C- and E-aza-BODIPYs can
be made through the nucleophilic attack of Grignard reagent
(Scheme 1). Compounds 1–7 with various aryl and alkynyl substit-
uents were successfully obtained in 45–83% yields.
The structures of C- and E-aza-BODIPYs 1 and 3 are confirmed
by X-ray analysis. The crystal structure of C-aza-BODIPY 1 in
comparison with C-BODIPY 913 and F-aza-BODIPY 105a is shown
in Figure 1. The N–B distances (1.603 and 1.606 Å) were found to
⇑
Corresponding authors. Tel./fax: +86 0378 3881358 (X.-D.J.); tel./fax: +86 021
51980111 (W.Z.).
E-mail addresses: xdjiang@henu.edu.cn (X.-D. Jiang), zhaow@henu.edu.cn
(W. Zhao).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.056

5704
X.-D. Jiang et al. / Tetrahedron Letters 53 (2012) 5703–5706
Scheme 1. Synthesis of C- and E-aza-BODIPYs 1–7.
be slightly longer than that of the corresponding C-BODIPY 9
(1.586 and 1.580 Å) and F-aza-BODIPY 10 (1.562 and 1.561 Å) by
0.02–0.04 Å, respectively. These are attributed to the steric repul-
sion between the phenyl groups at the boron atom and the phenyl
groups in the dipyrromethene. The other bond lengths for 1 were
very similar to those found for 9 and 10. The sp3 hybridized boron
center for 1 appears as a more distorted tetrahedron with angles
N1–B–N2 of 102.6° and C5–B–C6 of 121.1°, which are slightly
different from 9 (104.6 and 115.0°) and 10 (108.1 and 110.1°).
For E-aza-BODIPY 3 in comparison with E-BODIPY 11,7a the sp3
hybridized boron center appears as a slightly distorted tetrahedron
with angles N1–B–N2 of 104.94° and C5–B–C6 of 113.71°, compar-
ing to those (106.3° and 111.5°) of 11 (Fig. 1).
To gain insight into the photophysical property of C- and E-aza-
BODIPYs, the absorption and emission spectra for 1–7 are mea-
sured and outlined in Figure 2 and Table 1. While replacement of
F atoms with two aryl groups in 1 and 2 (both kabs = 618 nm) re-
sulted in a remarkable hypsochromic shift (32 nm) compared to
that of the parent F-aza-BODIPY 8 (kabs = 650 nm), the emission
maximum (662 nm for 1; 666 nm for 2) was slightly blue shifted
compared with 8 (672 nm).5b It is due to the increase in the
HOMO–LUMO band gap for the lowest energy absorption bands
observed for 1 relative to that of 8 by MO calculations (see Supple-
mentary data). Thus the Stokes shift notably increased after aryl
modification on boron (44 nm for 1; 48 nm for 2; 21 nm for 8),
which may favor detection. The unexpected low quantum yields
for 1 (0.018) and 2 (0.027) compared to 8 (0.34) is presumably
attributed to the
p–p interaction between the phenyl groups at
the boron atom and the phenyl groups in the dipyrromethene
(see Fig. S1 and MO calculations in Supplementary data). In
E-aza-BODIPYs 3–7 with two alkynyl groups, the alkyne replace-
ment resulted in only minor changes of the physical properties.
Very minor hypsochromic shifts for absorption and emission were
found for compounds 3–7 compared to 8. Slightly increased Stokes
shifts were observed after modification. E-aza-BODIPYs 3–7 dis-
plays moderate fluorescence quantum yields (0.16–0.29).
Figure 1. ORTEP diagrams of 1 and 3 showing the thermal ellipsoids at the 30%
probability level. All hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and angles (deg) for 1: N1–C1, 1.383(4); N1–C2, 1.369(5); N2–C3, 1.396(4);
N2–C4, 1.366(5); N1–B, 1.603(6); N2–B, 1.606(6); C5–B, 1.618(6); C6–B, 1.629(6);
C1–N1–C2, 106.8(3); C1–N1–B, 124.7(3); C2–N1–B, 127.8(3); C3–N2–C4, 106.0(3);
C3–N2–B, 122.8(3); C4–N2–B, 128.1(3); N1–B–N2, 102.6(3); N1–B–C5, 104.6(3);
N2–B–C6, 114.8(3); C5–B–C6, 121.1(3). Selected bond lengths (Å) and angles (deg)
for 3: N1–C1, 1.392(3); N1–C2, 1.352(3); N2–C3, 1.395(3); N2–C4, 1.363(3); N1–B,
1.561(6); N2–B, 1.591(3); C5–B, 1.577(3); C6–B, 1.597(3); C1–N1–C2, 106.72(18);
C1–N1–B, 123.63(18); C2–N1–B, 129.58(18); C3–N2–C4, 106.99(18); C3–N2–B,
120.95(18); C4–N2–B, 132.06(18); N1–B–N2, 104.94(16); N1–B–C5, 110.71(19);
N2–B–C6, 107.99(18); C5–B–C6, 113.71(18).
The pH dependent stability of the prepared C-aza-BODIPY 1 and
E-aza-BODIPY 3 at room temperature was then followed by

X.-D. Jiang et al. / Tetrahedron Letters 53 (2012) 5703–5706
5705
Figure 3. Normalized transmission (A/A0) of 1 (d), 3 (j), and 8 (N) in response to
pH range from 1 to 14 in DMSO/H2O (4: 1, v/v) after 8 h at ambient temperature.
Figure 2. Absorption (upper) and emission (bottom) spectra of 1–8 in CHCl3 at
298 K.
Figure 4. Fluorescence microscope imaging of human oral epithelial cells following
incubation with 2 ꢀ 10–5 M in PBS + 0.5% (v/v) DMSO aqueous 3 (red color) for
20 min at 37 °C.
monitoring the changes of absorbance after 8 h (Fig. 3). We found
that 1 and 3 were slightly more stable in acidic condition compared
to F-aza-BODIPY 8, however, much more stable in basic
condition.9,15
corresponding bond of C-BODIPY by 0.02–0.04 Å. While aryl mod-
ification on boron resulted in large blue shifts of the absorption
maxima and low fluorescence quantum yields, alkynyl modifica-
tions resulted in only slight changes of absorption maxima and
high fluorescence quantum yields. C-aza-BODIPY 1 and E-aza-BOD-
IPY 3 were much more stable than F-aza-BODIPY 8 in basic condi-
tion. E-aza-BODIPY 3 was dialytic and suitable to pigmentation for
living cells. The current studies may provide useful hints for the
functionalization of aza-BODIPY dyes and reduction in aggregation
induced quenching of fluorescence for the development of
aza-BODIPY based on NIR probes. Further efforts for the water-sol-
uble version of modifications on boron atom on aza-BODIPY dyes
as potential probes and sensors are ongoing in our lab.
Due to the observed unusual stability of E-aza-BODIPY in basic
environment, with long absorption and high fluorescent quantum
yield, the pigmentation of living cells with 3 for imaging assay
was carried out. By incubation of human oral epithelial cells with
2 ꢀ 10ꢁ5 M in PBS + 0.5% (v/v) DMSO aqueous dye 3, the imaged
cells displayed an uptake of 3 that appeared to be localized to
the cytoplasm ( Fig. 4). NIR E-aza-BODIPY 3 was found to be
dialytic, which may have potential applications in the staining of
living cells.
In conclusion, C and E-aza-BODIPYs 1–7 were successfully syn-
thesized in 45–83% yields. The structures of 1 and 3 were con-
firmed by X-ray analysis. The N–B distances (1.603(6) and
1.606(6) Å) in C-aza-BODIPY 1 were found to be longer than the
Table 1
Spectroscopic data of 1–8 in CHCl3 at 298 K
Entry
Dye
kabs (nm)
kem (nm)
Stokes shift (nm)
Fwhm (nm)
e
(Mꢁ1 cmꢁ1
)
Ufa
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
618
618
645
642
644
643
643
650
662
666
670
670
670
671
671
671
44
48
25
28
26
28
28
21
59
58
49
54
49
54
51
45
50,000
53,000
87,000
62,000
51,000
52,000
69,000
79,000
0.018
0.027
0.29
0.16
0.18
0.27
0.26
0.34
a
Nile blue with Uf = 0.27 in ethanol with 0.5% (v/v) 0.1 M HCl was used as a standard.14

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X.-D. Jiang et al. / Tetrahedron Letters 53 (2012) 5703–5706
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This work was supported by the National Natural Science Foun-
dation of China (20872026), the Hi-Tech Research and Develop-
ment Program of China (863 Plan, 2009AA02Z308), Shanghai
Pujiang Talent Plan Project (09PJD008), and Sino Swiss Science
and Technology Cooperation (SSSTC, EG 30-032010).
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.08.
056. These data include MOL files and InChiKeys of the most
important compounds described in this article.
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