Fused dual boron core based BODIPY dyes: synthesis and optical character

Article abstract of DOI:10.1016/j.tetlet.2016.02.109

Here we report that two boron dipyrromethene derivatives (BNB-1 and BNB-2), bearing double boron atomic centers, have been synthesized and characterized. The additional boron atom was introduced into the skeleton of typical BODIPY dyes through the reactio

Full text of DOI:10.1016/j.tetlet.2016.02.109

Tetrahedron Letters 57 (2016) 1605–1609
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Fused dual boron core based BODIPY dyes: synthesis and optical
character
b,
Linlin Wang a, Yi Qu a, Jingli Xu a, Jian Cao a, , Chunchang Zhao
⇑
⇑
a College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, PR China
b Shanghai Key Laboratory of Functional Materials Chemistry, Key Laboratory for Advanced Materials, and Institute of Fine Chemicals, East China University of Science &
Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Here we report that two boron dipyrromethene derivatives (BNB-1 and BNB-2), bearing double boron
atomic centers, have been synthesized and characterized. The additional boron atom was introduced into
the skeleton of typical BODIPY dyes through the reaction of BF3ꢀEt2O and the two ligands, phenol and
Schiff’s base. The maximum emission wavelength of both dyes moved to 584–603 nm with the larger
Received 18 December 2015
Revised 23 February 2016
Accepted 29 February 2016
Available online 3 March 2016
Stokes’ shift (
D
k ꢁ 40 nm). Moreover, less solvent polarity dependence of the Stokes’ shift (40 6 nm)
was found in different organic solvents. Furthermore, fluorescent lifetime and CV methods were also
performed to explore the photophysical and electrochemical behaviors of both dyes.
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
BODIPY dyes
Dual boron core
Red-shift emission
Stokes’ shift
Schiff’s base
Introduction
of compounds with different photophysical properties. Until now,
the aryl-, alkyl-, alkynyl-, and alkoxy-substituent of BODIPY has
It has been a hot investigation in recent years that the design
and synthesis of small-molecular fluorescent dyes are applied for
chemosensors, fluorescent label reagents, and nonlinear optical
materials. Boron dipyrromethene (BODIPY) and its derivatives are
classical organic fluorophores with high luminescent efficiency,
relatively sharp fluorescence peaks, and advantageous photostabil-
ity.1–5 These BODIPY dyes have worked as energy/electron donors
or acceptors in different donor–acceptor sensors. With the
demands of the probes or sensitizers in chemosensor and
corresponding devices, several modifying strategies about BODIPY
dyes were developed: (1) electron-deficient and electron-rich sub-
stitutions on the meso site of BODIPYs provided the photo-induced
electron transfer (PeT) process for designing fluorescent sensors
been reported in different fields;20–22 (5) some researches intro-
duced the N atom into the meso position of the BODIPY to give
the far red/near infrared emission derivatives that could be used
as the contrast agents and photodynamic therapy.23–34
Because the indole group has larger conjugated system than
pyrrole, a series of indole-based BODIPY derivatives have been
designed in our previous work.35 These derivatives with red to
near-infrared emission can work as fluorescent probes for different
species. However, further exploration in expanding structures and
optimizing photophysical properties of these derivatives have not
been put forward. Hence, we described a new type of dual boron
core BODIPY (BNB-1 and BNB-2) on the basis of indole-based BOD-
IPY, which integrated both typical BODIPYs and mini BODIPY ana-
logs. As shown in Scheme 1, the second boron atom was induced in
both dyes to construct the mini BODIPY moiety (blue square),
which was fused on the typical BODIPY ring (yellow circle). The
newly boron core fixated the Schiff’s base and phenol of BODIPY-
OH, which could keep rigidity and enhance the planarity of the
structure. Furthermore, we introduced the methoxy moiety as an
electron donor into the para position of Schiff’s base of BNB-1.
Then, we employed the 1H NMR spectra to study the charge distri-
bution on both dyes. UV–vis and emission spectra were employed
to explore the behavior of the additional boron unit fused on the
BODIPY dyes. The fluorescent quantum yield and lifetime were
determined to confirm the role of an electron-rich group (–OCH3)
and switches;6–11 (2) the
p-extended BODIPY dyes with longer
emission wavelength were obtained by the Knoevenagel reaction
or Heck coupling reaction. The formed fluorophores showed deep
puncturing depth that is needed for optical bioimaging
in vivo;12–16 (3) Först resonance energy transfer was another pow-
erful tool to enhance Stokes’ shift and construct dual channel
detection device. BODIPY can couple with other fluorophores and
sensors in order to construct new optical cassettes;17–19 (4) nucle-
ophilic substitution on the boron core of BODIPY provided plenty
⇑
Corresponding authors.
http://dx.doi.org/10.1016/j.tetlet.2016.02.109
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

1606
L. Wang et al. / Tetrahedron Letters 57 (2016) 1605–1609
Scheme 1. Structures of the BNB-1, BNB-2, and control.
O
O
O
O
N
BF3·C4H10
O
NH2
EtOH,rt, 2hr
N
B
N
HO
F
N
triethylamine,stir 0.5hr,0°C
N
F
B
N
HO
O
F
F
N
F
B
F
F
B
F
N
BODIPY-OH
BNB-1
O
BF3·C4H10
O
N
N
B
NH2
EtOH,rt, 2hr
N
F
HO
triethylamine,stir 0.5hr,0°C
N
N
N
F
B
F
HO
O
F
N
F
B
F
F
B
N
F
BODIPY-OH
BNB-2
Scheme 2. Synthesis of BNB-1 and BNB-2.
produced the corresponding Schiff’s bases of BNB-1 and BNB-2,
respectively. Then, both kinds of the Schiff’s bases reacted with
BF3ꢀOEt2 under ice-bath for half an hour to gain the targets
(BNB-1 and BNB-2). The structures of both targets were character-
ized by 1H NMR, 13C NMR, and HRMS spectra in Supporting
information.
Figure 1 described the difference between the 1H NMR spectra
of BNB-1 and BNB-2. BNB-1 gave the double peak at high field that
assigned to the protons on the phenyl moiety (Hb; 6.95 and
6.97 ppm). Without methoxy group, the corresponding signals of
0
BNB-2 were found at 7.556 ppm (Hb ). Moreover, BNB-2 had one
more proton on the phenyl moiety than BNB-1, which could be
found at 7.42–7.45 ppm (blue color in Fig. 1b). The HRMS spectrum
showed a peak at m/z 584.2303 for BNB-1 and 554.2175 for BNB-2
(Figs. S6 and S9), which were consistent with the calculated values
(584.2304 for [MꢂH]ꢂ of BNB-1 and 554.2198 for [MꢂH]ꢂ of BNB-
2).
Compared with the proton Ha of BNB-2 (Fig. 1), the methoxy
group of BNB-1 showed electronic donor property, it caused the
difference between BNB-1 and BNB-2. For the investigation of
the photophysical properties of BNB-1 and BNB-2 in different sol-
vents, UV–vis and fluorescence spectra were performed in Figure 2.
As Figure 2a shown, the absorption peak around 350 nm was
assigned to the Schiff’s base ligand of BNB-1. The absorption peak
of BNB-1 was stronger than the one of BODIPY-OH that indicated
the planarity of BNB-1 was better than BODIPY-OH. The similar
phenomenon could also be found in BNB-2 (Fig. S10).
Figure 1. Low-field 1H NMR of BNB-1 and BNB-2.
in these BODIPY dyes. Finally, the CV test was also performed to
discuss the difference of the electronic properties between BNB-1
and BNB-2.
Results and discussion
In Figure 2b, the maximum absorption bands of BODIPY-OH
shifted from 527 nm to 549 nm with different polarity of solvents.
The similar variation was found in BNB-1 and BNB-2 but in a
longer wavelength region. As shown in Figures 2a and S10e, the
maximum absorption bands of BNB-1 and BNB-2 ranged from
As shown in Scheme 2, BNB-1 and BNB-2 were prepared by a
simple method. The condensation of the BODIPY-OH with the p-
anisidine and aniline in ethanol under room temperature for 2 h

L. Wang et al. / Tetrahedron Letters 57 (2016) 1605–1609
1607
Figure 2. In various solvents (a) absorption spectra of BNB-1; (b) absorption spectra of BODIPY-OH; (c) fluorescence spectra of BNB-1; and (d) fluorescence spectra of
BODIPY-OH.
Figure 3. Solvachromic effects of BNB-1 (a), BNB-2 (b), and BODIPY-OH (c).
540 nm to 568 nm and 538 nm to 565 nm, respectively. These evi-
dences implied that the second boron could enlarge the -exten-
sion of BODIPY dyes. But the methoxy group showed little effect
Table 1
p
Lifetime and rate constants for radiative and non-radiative deactivation of BNB-1 and
BNB-2 in CH2Cl2
s
(ns)
Kr (109 sꢂ1
)
Knr (109 sꢂ1
)
in absorption spectra.
Emission spectra were powerful tools for evaluating the new
fluorescent dyes. The maximum excitation wavelength is at
530 nm. We detected the emission spectra of BNB-1, BNB-2, and
BODIPY-OH in different solvents in Figures 2c, d and S10. BNB-1
BNB-1
BNB-2
2.74
3.55
0.26
0.023
0.11
0.26

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L. Wang et al. / Tetrahedron Letters 57 (2016) 1605–1609
Figure 4. Cyclic voltammogram of BNB-1 (a) and BNB-2 (b) in CH2Cl2 (at 20 mV sꢂ1).
were carried out in the three electrode configuration consisting
Table 2
of a Pt button (working electrode), platinum wire (auxiliary elec-
trode), and saturated calomel (reference electrode) electrodes. All
potentials were calibrated by the addition of ferrocene as an inter-
nal standard, taking E1/2 (Fc/Fc+) = 0.42 V, versus saturated calomel
electrode. The supporting electrolyte was 0.1 M [(n-Bu)4N][PF6]
(TBAPF6) that was purified by recrystallization three times from
hot ethanol before being dried for three days at 100–150 °C under
active vacuum.39 The quasi-reversible oxidation processed with
half-wave potentials at 1.42 V and 1.43 V were attributed to the
oxidation of the BNB-1 and BNB-2, respectively. The irreversible
reduction peaks of the BNB-1 were found at ꢂ0.68/ꢂ0.94 V and
BNB-2 were found at ꢂ0.66 V. Compared to BNB-2, BNB-1 showed
an additional reduction peak at ꢂ0.94 V (star marker in Fig. 4a),
which was considered as another evidence to the individual effect
of the methoxy group.
Electrochemical data for BNB-1 and BNB-2 in CH2Cl2
Compound
Eox,
(V) vs ferrocene
Ered,
(V) vs ferrocene
1/2
1/2
BNB-1
BNB-2
1.42
1.43
ꢂ0.68/ꢂ0.94
ꢂ0.66
exhibited the maximum emission bands, which ranged from
585 nm to 603 nm. BNB-2 also exhibited the similar variation from
584 nm to 603 nm. The emission bands of BODIPY-OH were from
569 nm to 587 nm that showed little hypochromatic shift
(D
k ꢁ 15 nm) than BNB-1 and BNB-2.
Furthermore, fluorescence quantum yield (FQY) of both new
dyes were determined with Rhodamine-6G (U = 0.94 in ethanol).
The FQY value of BNB-1 reached 0.71 in dichloromethane, which
was much brighter than BNB-2 (U = 0.08). Considered the struc-
tures of both dyes, the methoxy group may be responsible for
the enhancement of FQY.
Conclusions
In this work, two new BODIPY derivatives, BNB-1 and BNB-2,
were designed and synthesized. The photophysical and electronic
properties of both compounds were investigated. The solvent
polarity dependence of the fluorophores was explored by UV–vis
and stead-state fluorometric spectra. The fluorescent quantum
yield, time-resolved fluorometric spectra, and cyclic voltammetry
experiments were performed to discuss the role of the electron
donor (–OCH3) in the new dyes. The FQY value and rate constant
for radiative deactivation indicated that the electron donor can
optimize the fluorescent property of BODIPY dyes.
The solvent polarity dependence of the fluorophores is more
important in fluorescent field than others. The energy of emitting
state is more susceptible to the polarity effect that has been known
as the solvent relaxation. In order to apply the fluorescent tools in
different testing conditions, homeostatic control has been an
important parameter for developing the new dyes. Herein, we
tested the relation of Stokes’ shift (in mabs–mem form) and solvent
polarity parameter (normalized ET valve). As shown in Figure 3,
both BNB-1 and BNB-2 showed the smaller variations in Stokes’
shift than the control BODIPY-OH with the same solvent polarity
scales. This consequence illustrated the dual boron core dyes were
more beneficial for fluorescent detection in the extensive scales of
circumstance.
Acknowledgements
This work was partially supported by the NSFC/China
(21404068), the talent programs from Shanghai Education Com-
mission (ZZGCD15029), and SUES (A1-5300-14-020303).
The fluorescence decay curves of BNB-1 and BNB-2 in dichloro-
methane were given in Figure S11. Both dyes showed the monoex-
ponential decay processes. Based on these curves, the fluorescent
lifetime (s) and related rate constants for radiative and non-radia-
Supplementary data
tive deactivation were calculated in Table 1. The fluorescent life-
time of BNB-1 was 2.74 ns, BNB-2 showed a little longer value as
3.55 ns than BNB-1, and the Kr value of BNB-1 was 11 times more
than BNB-2. These results also proved that the FQY value of BNB-1
was higher than BNB-2.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.02.
109.
The cyclic voltammetry test has been performed to explore the
impact of methoxy group on these BODIPY dyes (Fig. 4 and
Table 2).36–38 Electrochemical studies of the solutions were per-
formed from ꢃ0.1 M analyte compound solutions in dry dichloro-
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