A class of wavelength-tunable near-infrared aza-BODIPY dyes and their application for sensing mercury ion

Article abstract of DOI:10.1016/j.dyepig.2013.12.004

We present the design, synthesis, photophysical properties and application for sensing mercury ion, of a series of excellent near infrared fluorescent aza-boron-dipyrromethene (aza-BODIPY) dyes. The introduction of different aromatic substituents to the a

Full text of DOI:10.1016/j.dyepig.2013.12.004

Accepted Manuscript
A class of wavelength-tunable near-infrared aza-BODIPY dyes and their application
for sensing mercury ion
Shujuan Liu, Zhengjian Shi, Wenjuan Xu, Huiran Yang, Na Xi, Xiangmei Liu, Qiang
Zhao, Wei Huang
PII:
S0143-7208(13)00475-0
10.1016/j.dyepig.2013.12.004
DYPI 4201
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 20 July 2013
Revised Date: 3 December 2013
Accepted Date: 6 December 2013
Please cite this article as: Liu S, Shi Z, Xu W, Yang H, Xi N, Liu X, Zhao Q, Huang W, A class of
wavelength-tunable near-infrared aza-BODIPY dyes and their application for sensing mercury ion, Dyes
and Pigments (2014), doi: 10.1016/j.dyepig.2013.12.004.
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Table of contents:
A new class of NIR aza-BODIPY fluorescent dyes with tuneable emission
wavelength has been used as excellent “turn-off” type fluorescent probes for Hg2+.

ACCEPTED MANUSCRIPT
A class of wavelength-tunable near-infrared aza-BODIPY
dyes and their application for sensing mercury ion
Shujuan Liu,a Zhengjian Shi,a Wenjuan Xu,a Huiran Yang,a Na Xi,a Xiangmei Liu,a
Qiang Zhao,*a and Wei Huang*ab
a Key Laboratory for Organic Electronics & Information Displays (KLOEID) and
Institute of Advanced Materials (IAM), Nanjing University of Posts &
Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210023, China. Fax: +86 25
8586 6396; Tel: +86 25 8586 6396; E-mail: iamwhuang@njupt.edu.cn,
iamqzhao@njupt.edu.cn
b Jiangsu-Singapore Joint Research Center for Organic/Bio- Electronics &
Information Displays and Institute of Advanced Materials, Nanjing University of
Technology, Nanjing 211816, China
Abstract: We present the design, synthesis, photophysical properties and application
for sensing mercury ion, of a series of excellent near infrared fluorescent
aza-boron-dipyrromethene (aza-BODIPY) dyes. The introduction of different
aromatic substituents to the aza-BODIPY core induced red-shifted absorption and
emission wavelengths due to the extension of the π-system. In addition, “turn-off”
fluorescence responses and solution color changes selectively toward mercury ion
have also been realised. The Hg2+-induced fluorescence quenching may be assigned to
the energy or electron transfer from emissive aza-BODIPY core to Hg2+.
Keywords: aza-boron-dipyrromethene, carbazoles, fluorenes, mercury ion, near
infrared, sensing

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1. Introduction
The use of fluorescent probes for the detection of heavy-metal ions plays an
increasingly important role in biology, medicine and chemistry because of their
promising characteristics of operational simplicity, cost-effectiveness, short response
time, easy visualization and high sensitivity [1]. In the past few years, a large number
of fluorescent probes have been reported [2]. Most of the reported fluorescent probes,
however, usually exhibit absorption and emission in the ultraviolet and visible
regions, which can be highly absorbed by biological substances and easily cause
damage to biological substances [3]. Thus, their practical utilization in biological
systems is limited.
It is well-known that biomolecules exhibit low absorption, light scattering and
auto-fluorescence background in the near infrared (NIR) spectral region (650-900 nm)
[4]. Moreover, the NIR fluorescence can penetrate deep into biological tissue.
Therefore, probes with either absorption or emission bands in this region are very
favorable for cell recognition and in vivo imaging. Hence, the exploitation of
excellent NIR fluorescent probes has become a very active research field. In recent
years, several classes of fluorescent probes emitting in the NIR region have been
reported based on different fluorophores, such as boron-dipyrromethene (BODIPY)
[5], cyanines [6], squaraines [7] and rhodamine [8], etc.
BODIPY dyes, as an important class of fluorescent dyes, have been widely used in
various fields that include chemosensors [9], live-cell imaging [10], photodynamic
therapy [11], supramolecular fluorescent gels [12], solar cell [13] and
telecommunication transmission [14]. Furthermore, this class of dye exhibits
fascinating characteristics, such as highly rigid molecular structure, increased
stability, relatively sharp fluorescence emission, high luminescent quantum yields and
large extinction coefficients. Besides, the absorption and emission bands of BODIPY
dyes can be tuned easily because the chemical structures of these dyes are easy to
modify [15]. In recent years, aza-boron-dipyrromethene (aza-BODIPY) dyes, in
which a C atom of the BODIPY core is replaced by a N atom as shown in Scheme 1,
are emerging as a promising class of NIR emitting dyes [16]. By introducing various

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R1 and R2 substituents into the aza-BODIPY core, their chemical characteristics
together with the absorption and emission bands can be modulated. Up to now,
however, the reports concerning spectral characteristics modulation and applications
of NIR aza-BODIPY dyes are few.
Herein, we designed and synthesized a new class of NIR aza-BODIPY fluorescent
dyes (2a and 2b, as shown in Scheme 2) with a tuneable emission wavelength, which
can be further used as a “turn-off” type fluorescent probe for Hg2+. The introduction
of aromatic substituents (R2) to the aza-BODIPY core by Suzuki coupling reaction
induced a significant change of photophysical properties due to the extension of the
π-system, and consequently the red-shifted absorption and emission wavelengths were
realized successfully. In addition, excellent sensing performances for Hg2+ have also
been realized owing to a well defined binding pocket for mercury ion created by
attaching two thienyl groups at R1 positions (see Scheme 2).
2. Experimental
2.1. Materials and measurements
All starting reagents and materials, unless otherwise specified, were purchased
1
from commercial sources and used without further purification. H (400 MHz) and
13C NMR (100 MHz) spectra were recorded in CDCl3 at room temperature using a
Bruker Ultra Shield Plus 400 MHz instrument with reference tetramethylsilane (TMS)
as internal reference. Melting points (m.p.) were taken on an X-4 microscope
electrothermal apparatus (Taike China). The elemental analyses were performed on a
Vario EL III O-Element Analyzer system. FT-IR spectra were recorded using a
Shimadzu IR prestige-21 spectrometer using a KBr pellet. Mass spectra were
obtained using ion trap (ESI) instruments operating in positive mode. The UV-visible
absorption spectra were recorded on a Shimadzu UV-3600 UV-VIS-NIR
spectrophotometer. Photoluminescent spectra were measured using a RF-5301PC
spectrofluorimeter. Column chromatography was performed on silica gel (200-300
mesh). The fluorescent quantum yields was determined through an absolute method
by employing an integrating sphere.
2.2 DFT calculations

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The structure optimization of model compounds of 2a and 2b (2a' and 2b') before
and after binding with Hg2+ were performed with the Gaussian 03 package using
B3LYP density functional theory (DFT), respectively [17]. The LANL2DZ basis set
was used to treat the mecury atom, and the 6-31G(d) basis set was used to treat all
other atoms. The alkyl-chains were substituted by methyl to reduce computational
cost. The contours of the HOMO and LUMO orbitals of 2a' and 2b' were plotted
using GaussView. On the basis of ground-state optimization, the time-dependent
density functional theory approach was applied to predict their absorption. The
solvent effect (CH2Cl2) was simulated using the polarizable continuum model (PCM)
in which the solvent cavity is regarded as a union of interlocking atomic spheres.
2.3 Synthesis:
Synthesis
of M1:
1-(4-Bromophenyl)ethanone
(9.9
g,
50
mmol),
thiophene-2-carbaldehyde (5.6 g, 50 mmol), and potassium hydroxide (8.4 mg, 1.5
mmol), were dissolved in ethanol / H2O (85:15 v/v, 100 mL) and stirred at room
temperature for 24 h. During the course of the reaction, the product was precipitated
from the reaction mixture. The reaction mixture was filtered and the resulting solid
was washed with cold ethanol as a yellow solid (13.5 g, 92.5%). 1H NMR (400 MHz,
CDCl3) δ (ppm): 7.94 (d, J = 15.3 Hz, 1H), 7.86 (d, J = 8.4 Hz, 2H), 7.63 (d, J = 8.4
Hz, 2H), 7.43 (d, J = 5.0 Hz, 1H), 7.37 (d, J = 3.5 Hz, 1H), 7.27 (d, J = 15.6, 1H),
7.12-7.06 (m, 1H). 13C NMR (100 MHz, CDCl3) δ (ppm): 188.7, 140.2, 137.7, 136.9,
132.4, 131.9, 129.9, 129.1, 128.5, 127.9, 120.1. GC-MS (EI) calcd. for C13H9BrOS:
291.96, found 292.00.
Synthesis of M2: Compound M1 (4.90 g, 16.8 mmol), nitromethane (5.13 g, 84
mmol) and diethylamine (6.15 g, 84 mmol) were dissolved in dry methanol (100 mL)
and heated under reflux for 24 h. The solution was cooled, acidified with 1M HCl and
then extracted with CH2Cl2/H2O. The combined organic layers were dried over
anhydrous Na2SO4. The solvent was evaporated under reduced pressure. The obtained
crude product was recrystallized from dichloromethane and ether. The M2 was
1
obtained as white solid (0.92 g, 64.7%). H NMR (400 MHz, CDCl3) δ (ppm): 7.79
(d, J = 8.2 Hz, 2H), 7.59 (d, J = 8.2 Hz, 2H), 7.22–7.17 (m, 1H), 6.97–6.91 (m, 2H),

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4.82 (dd, J = 12.6 Hz, 6.4 Hz, 1H), 4.70 (dd, J = 12.6 Hz, 7.5 Hz, 1H), 4.59–4.47 (m,
1H), 3.57–3.38 (m, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm):= 195.5, 141.6, 135.0,
132.1, 129.6, 129.0, 127.2, 125.7, 124.8, 79.8, 42.2, 34.7.
Synthesis of M3: Compound M2 (1.24 g, 3.01 mmol) and ammonium acetate (7.70 g,
0.11 mol) in ethanol (30 mL) were heated under reflux for 48 h. The solution was
cooled and the precipitate was filtered. The solid was washed with cold ethanol and
isolated as a black solid. The solid was used directly in the next step.
Synthesis of 1a: A mixture of compound M3 (185 mg, 0.3 mmol), fluorene single
borate (327 mg, 0.69 mmol) and [Pd(PPh3)4] (15.6 mg, 0.014 mmol) as main catalyst
was first added to a round bottomed flask, and then a degassed mixture of toluene (2
mL), ethanol (1 mL) and aqueous 2 M potassium carbonate (1 mL) was injected under
a nitrogen atmosphere. The mixture was stirred at 85 ºC for 24 h. After cooling to
room temperature, the mixture was washed with water and extracted with CH2Cl2.
The combined organic phase was concentrated and then purified by column
chromatography on silica by using ether / dichloromethane (6:1) as eluent to yield a
green solid (152 mg, 41%). m.p. 125 °C. IR (KBr): 3439, 2954, 2925, 2853, 1601,
1509, 1470, 1452, 1387, 1264, 1236, 1116, 1038, 1009, 908, 851, 803, 745, 687 cm-1.
1H NMR (400 MHz, CDCl3) δ (ppm): 8.04 (d, J = 8.4 Hz, 4H), 7.92-7.88 (m, 2H),
7.86 (d, J = 8.4 Hz, 4H), 7.82-7.64(m, 8H), 7.44 (m, 2H), 7.40-7.31 (m, 6H),
7.20-7.17 (m, 2H), 7.17 (s, 2H), 2.10-1.96 (m, 8H), 1.33-1.00 (m, 40H), 0.79 (t, J =
7.0 Hz, 12H) 0.75-0.59 (m, 8H). 13C NMR (100 MHz, CDCl3) δ (ppm):= 154.9,
151.6, 151.1, 149.5, 143.3, 141.2, 140.6, 138.9, 137.0, 136.1, 130.7, 128.0, 127.7,
127.3, 127.2, 127.0, 126.9, 126.0, 125.5, 123.0, 121.2, 120.1, 119.9, 113.4, 55.3, 40.4,
31.8, 30.3, 30.0, 29.2, 23.8, 22.6, 14.1. MALDI-TOF calcd. for C86H100N3S2: 1238.74,
found 1238.80. Elemental analysis calcd. (%) for C86H100N3S2: C, 83.31; H, 8.13; N,
3.39, found C, 83.40; H, 8.27; N, 3.21.
Synthesis of 1b: A mixture of compound M3 (370 mg, 0.3 mmol), carbazole single
borate (250.6 mg, 0.69 mmol) and [Pd(PPh3)4] (15.6 mg, 0.014 mol) as main catalyst
was first added to a round bottomed flask, and then a degassed mixture of toluene (2
mL), ethanol (1 mL) and aqueous 2 M potassium carbonate (1 mL) were injected

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under a nitrogen atmosphere. The mixture was stirred at 85 ºC for 24 h. After cooling
to room temperature, the mixture was washed with water and extracted with CH2Cl2.
The combined organic phase was concentrated and then purified by column
chromatography on silica by using petroleum ether / dichloromethane (4:1) as eluent
to yield a green solid (122 mg, 40.0%). m.p. 85 °C. IR (KBr): 3439, 2954, 2925, 2853,
1596, 1509, 1471, 1442, 1384, 1264, 1235, 1144, 1110, 1077, 1053, 1004, 956, 803,
1
750, 726, 702 cm-1. H NMR (400 MHz, CDCl3 ) δ(ppm): 8.40 (s, 2H), 8.16 (d, J =
7.6 Hz, 2H), 8.03–7.96 (m, 4H), 7.92–7.84 (m, 6H), 7.78 (d, J = 8.0 Hz, 2H),
7.52–7.39 (m, 8H), 7.25–7.20 (m, 2H) 7.18–7.13 (m, 2H), 7.10 (s, 2H), 4.29 (t, J =
7.0 Hz, 4H), 1.94–1.82 (m, 4H), 1.40–1.18 (m, 20H), 0.87 (t, J = 6.8 Hz, 6H). 13C
NMR (100 MHz, CDCl3) δ (ppm): 158.7, 151.5, 151.1, 145.2, 144.0, 141.2, 140.6,
138.9, 138.0, 135.8, 134.8, 130.2, 130.1, 129.7, 128.3, 127.3, 126.8, 126.0, 125.5,
122.9, 121.6, 120.1, 119.9, 116.8, 55.2, 40.4, 31.8, 30.3, 30.0, 29.2, 23.8, 22.6, 14.1.
MALDI-TOF calcd. for C68H66N5S2: 1016.48, found 1017.03. Elemental analysis
calcd. (%) for C68H66N5S2: C, 80.27; H, 6.54; N, 6.88, found C, 80.32; H, 6.77; N,
6.79.
Synthesis of 2a: Compound 1a (123.8 mg, 0.1 mmol) was dissolved in distilled
dichloromethane (20 mL) and distilled diisopropylethylamine (0.17 mL, 1 mmol) was
added dropwise. The solution was stirred at room temperature for 15 minutes and then
BF3ꢀEt2O (0.19 mL, 1.5 mmol) was added. The solution was stirred at room
temperature for 48 h under a nitrogen atmosphere and the solution was washed with a
saturated brine and water. The organic layer was dried over sodium sulfate and the
solvent was evaporated. The crude product was purified by chromatography on silica
gel (1:6, dichloromethane : petroleum ether) to afford a dark solid (55 mg, 42.8%).
m.p. 136 °C. IR (KBr): 2923, 2851, 1601, 1509, 1476, 1437, 1380, 1298, 1235, 1100,
1053, 1004, 952, 827, 812, 740, 702 cm-1. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.20
(d, J = 8.2 Hz, 4H), 7.97 (d, J = 3.2 Hz, 2H), 7.85–7.71 (m, 8H), 7.69–7.62 (m, 4H),
7.59 (d, J = 4.8 Hz, 2H), 7.40–7.30 (m, 6H), 7.25–7.19 (m, 2H), 7.03 (s, 2H),
2.12–1.93 (m, 8H), 1.33–0.99 (m, 40H), 0.81 (t, J = 6.9 Hz, 12H) 0.74–0.60 (m, 8H).
13C NMR (100 MHz, CDCl3) δ (ppm): 154.7, 149.4, 143.6, 140.9, 140.3, 136.6,

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136.2, 131.0, 129.8, 127.8, 127.7, 127.5, 127.0, 126.9, 125.9, 125.0, 123.5, 123.0,
120.5, 119.0, 118.8, 113.1, 109.1, 108.9, 53.4, 31.8, 29.7, 29.4, 29.2, 27.3, 22.6, 14.1.
HRMS-ESI calcd. for C86H98BF2N3S2: 1285.7299, found 1285.7292. Elemental
analysis calcd. (%) for C86H98BF2N3S2: C, 80.28; H, 7.68; N, 3.27, found C, 80.32; H,
7.67; N, 3.39.
Synthesis of 2b: Compound 1b (101.5 mg, 0.1 mmol) was dissolved in distilled
dichloromethane (20 mL) and distilled diisopropylethylamine (0.17 mL, 1 mmol) was
added dropwise. The solution was stirred at room temperature for 15 minutes and then
BF3ꢀEt2O (0.19 mL, 1.5 mmol) was added. The solution was stirred at room
temperature for 48 h under a nitrogen atmosphere and the solution was washed with a
saturated brine and water. The organic layer was dried over sodium sulfate and the
solvent was evaporated. The crude product was purified by chromatography on silica
gel (1:4 dichloromethane: petroleum ether) to afford a dark solid (48 mg, 45.2%). m.p.
97 °C. IR (KBr): 2952, 2928, 2850, 1596, 1538, 1480, 1456, 1384, 1351, 1254, 1153,
1125, 1038, 904, 851, 803, 750, 692 cm-1. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.39
(s, 2H), 8.20 (d, J = 8.5 Hz, 4H), 8.15 (d, J = 7.7 Hz, 2H), 7.96–7.92 (m, 2H), 7.84 (d,
J = 8.5 Hz, 4H), 7.80–7.74 (m, 2H), 7.56 (d, J = 4.8 Hz, 2H), 7.52–7.37 (m, 8H),
7.22–7.18 (m, 2H), 7.01 (s, 2H), 4.29 (t, J = 7.2 Hz, 4H), 1.97–1.75 (m, 4H),
1.51–1.16 (m, 20H), 1.02–0.76 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, CDCl3) δ
(ppm): 158.5, 144.5, 140.9, 140.4, 134.9, 131.0, 130.3, 130.2, 130.0, 129.5, 129.4,
128.2, 128.0, 127.2, 127.1, 125.9, 125.1, 123.4, 123.0, 120.5, 119.1, 119.0, 109.1,
108.9, 53.5, 31.2, 29.7, 29.4, 29.2, 27.3, 22.6, 14.1. HRMS-ESI calcd. for
C68H64BF2N5S2: 1063.4700, found 1063.4798. Elemental analysis calcd. (%) for
C68H64BF2N5S2: C, 76.75; H, 6.06; N, 6.58, found C, 76.82; H, 6.17; N, 6.39.
3. Results and discussion
3.1 Synthesis and characterization
The synthetic route started with an aldol/dehydration reaction of the corresponding
thiophene-2-carbaldehyde and 1-(4-bromophenyl)ethanone, which gave the diaryl
α,β-unsaturated ketone (M1) as shown in Scheme 2. The Michael addition of
nitromethane to M1, with diethylamine (DEA) as base, gave M2 in 64.7% yield,

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which was then converted into M3 by heating under reflux with ammonium acetate in
ethanol for 48 h. The precipitate was filtered from the crude reaction mixture and then
washed with cold ethanol and isolated as a black solid. The intermediate compounds
1a and 1b were subsequently synthesized by Suzuki coupling reaction of M3 with
fluorene and carbazole single boronate, respectively. Then the BF2 moiety was
introduced to the corresponding 1a and 1b by treatment with boron trifluoride etherate
and diisopropylethylamine in dichloromethane at room temperature. The structures of
1
the target complexes were characterized by H NMR spectroscopy, 13C NMR
spectroscopy, FT-IR spectroscopy, elemental analysis, and mass spectroscopy. Details
of the synthesis and characterization data are shown in the Experimental Section.
3.2 Photophysical properties
The photophysical properties of these NIR absorbing dyes in CH2Cl2 solution at
room-temperature were investigated through UV-vis absorption and
photoluminescence (PL) spectra and the corresponding spectra are shown in Fig. 1.
Dye 1a exhibits the absorption maximum at 662 nm with the extinction coefficient (ε)
of 51000 M-1 cm-1. By replacing the fluorene moiety with the carbazole, the resulting
dye 1b shows a red-shifted absorption maximum of 672 nm with the extinction
coefficient of 47000 M-1 cm-1. The red-shift of 10 nm may be attributed to the
stronger electron-donating ability of carbazole than that of fluorene moiety. In
addition, no obvious fluorescent emission of 1a and 1b was observed.
Upon coordination to BF2, significant red-shift of the maximum absorption peak
was observed for both 1a and 1b. The corresponding BF2 chelated derivatives of 2a
and 2b exhibit maximum absorption peaks at 720 and 736 nm, which are 58 and 64
nm longer than those of 1a and 1b, respectively. Obviously, the extension of the
π-conjugation by the coordination to BF2 is responsible for this spectral shift. Similar
to 1a and 1b, the maximum absorption peak of 2b with carbazole substituent is
red-shifted by 16 nm compared with that of 2a containing fluorene substituent.
Moreover, the BF2 chelated derivatives 2a and 2b show the emission maxima of 752
and 848 nm (Fig. 1b), respectively, which are in the NIR region. From this, it can be
seen that 2b based on carbazole substituent exhibits much longer (nearly 100 nm)

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emission wavelength than 2a based on the fluorene substituent. This is consistent with
the stronger electron-donating ability of carbazole than fluorene moiety. The
fluorescent quantum yields of 2a and 2b have been measured to be 14% and 3%,
respectively. In addition, the Stokes shifts of 2a and 2b have been calculated to be 32
and 112 nm, respectively.
In order to further investigate the spectral characteristics, the dependence of the
emission spectra of 2a and 2b on the solvents was carried out. As seen from Fig. 2,
complex 2a exhibit an emission band at 749 nm in toluene, which is slightly blue
shifted compared to that in CH2Cl2 solution. In contrast, the emission wavelength
centers at 768 nm in DMF (dimethylformamide), which is red shifted by nearly 18 nm
compared to 752 nm in CH2Cl2 solution. Compared to 2a, 2b exhibits more evident
dependency on the solvent polarity, which has an emission band at 787, 852 and 881
nm in toluene, CH2Cl2, and DMF solution, respectively. Thus, we deduce that the
intramolecular charge transfer participates in the excited states for 2a and 2b.
3.3 Theoretical calculation
In order to better understand the optical properties of 2a and 2b, theoretical
calculation for model compounds of 2a' and 2b', in which the alkyl-chains were
replaced by methyl to reduce computational time, was carried out using the Gaussian
03 package at the B3LYP level. The electron density distributions of frontier orbitals
for 2a' and 2b' are very similar and the corresponding molecular orbitals are shown in
Fig. 3. The calculated results of the lowest singlet states are summarized in Table 1,
associated with their calculated oscillator strengths, energies, wavelengths and
assignments.
Fitted Gaussian type absorption curves are shown in Fig. 4 to intuitively
understand the absorption properties of these compounds. It can be seen that the
calculated absorption spectra are similar to the experimental results. Moreover, the
calculated absorption peaks of 716 nm and 717 nm for 2a' and 2b', respectively, are
close to the experimental data. The intense absorption band of 2a' at 720 nm mainly
originates from the S1 state, which is derived from HOMO→LUMO (62%) transition.
Similar to 2a', the absorption around 736 nm of 2b' also results from the excitation of

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HOMO→LUMO (62%) with the calculated oscillator strength of 0.96. As seen from
Fig. 3, The LUMO distributions of 2a' and 2b' mainly reside on the central
aza-BODIPY core, thiophene, and phenyl rings. The HOMO distributions are similar
to those of LUMO with additional contributions from fluorene or carbazole moieties.
Thus, the excited states of 2a and 2b can be assigned to π-π* transition mixed with
intramolecular charge transfer process in this system. This is in accordance with the
experimental results of the dependence on the solvent polarity for the
photoluminescence spectra of 2a and 2b.
3.4 Hg2+ sensing performance
As there is a well-defined S^N^S binding pocket for transition metal ions [18], we
then investigated the sensing ability of 2a and 2b for metal cations.
The spectral titrations of 2a and 2b (10 ꢁM) with Hg2+ were conducted through
absorption and photoluminescence spectra in CH2Cl2 solution. As shown in Fig. 5a,
significant decreases of the absorption peaks at 720, 525 and 321 nm are observed
with the gradual addition of Hg2+ to a solution of 2a, which results in an evident
change of solution color from purple to blue. Similarly, the UV-visible absorption
spectrum of 2b is also strongly influenced by Hg2+, in which the intensity of
absorption band at 736, 580, 340 and 290 nm gradually decreased with increasing
Hg2+ concentration, accompanied with a color change from purple to green (Fig. 5b).
The frontier orbitals of 2a and 2b after binding with Hg2+ have been calculated (Fig.
S4 in Supporting Information). There is evident difference of both HOMO and
LUMO for the dyes before and after binding Hg2+, which is responsible for the
significant absorption spectra change.
Furthermore, Job’s Plot (see Fig. S1 in Supporting Information) for the absorbance
was carried out to confirm the stoichiometry of the 2b-Hg2+ complex. 2b was
dissolved in CH2Cl2 and Hg2+ was dissolved in CH3CN at appropriate concentrations,
which were then mixed to control the concentrations. The total concentration of 2b
and mercury ion, namely [Hg]+[M], was maintained to be constant (1×10-5 M) and
the molar fraction of Hg2+ ([Hg]/[Hg]+[M]) was changed from 0.1 to 0.9. After
recording the UV-visible absorption spectrum of 2b in the absence (A) and presence

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(A0) of Hg2+, respectively, the appropriate absorbance values were plotted against the
mole fraction of the Hg2+. It displays that the A-A0 value goes through a maximum at
a molar ratio of about 0.5, confirming the 1:1 stoichiometry between 2b and Hg2+.
The fluorescence titrations of 2a and 2b in the presence of different concentrations
of Hg2+ were then examined (Fig. 6). As expected, upon treatment with Hg2+, a
complete quenching in fluorescence intensity at 752 and 848 nm was observed for 2a
and 2b, respectively, which may be due to the energy or electron transfer from
emissive aza-BODIPY core to Hg2+ ion. The detection limits for Hg2+ of 2a and 2b
were calculated to be 2.5 × 10-6 M and 1.0 × 10-7 M, respectively.
Using the fluorescence titration data, the association constants (K) of 2a and 2b
with Hg2+ in CH2Cl2 solution (see Fig. 7) are calculated to be 1.28 × 103 and 1.36 ×
105 M-1, respectively. Interestingly, 2b gives a much higher association constant than
2a with Hg2+, which may be attributed to the higher electron density of the thienyl
moieties due to the higher electron-donating ability of the carbazole group. Moreover,
Fig. S2 also indicates that 2b possesses the stronger binding ability with Hg2+ than 2a,
because only 1 equiv of Hg2+ is required for 2b to be completely quenched in
fluorescence intensity compared to 2a (about 60 equiv).
To explore the application of 2a and 2b as cation-selective fluorescent probes for
Hg2+, the fluorescence responses of 2a and 2b (10 ꢁM) to other important metal ions
were performed respectively. As displayed in Fig. 8a and 8b, probe 2a and 2b
produced no significant changes in emission intensities with other competing metal
ions except for Cu2+ and Fe3+, which induced a slight decrease of fluorescent
intensity. The fluorescence of 2a and 2b was quenched markedly by the subsequent
addition of Hg2+ to solutions in the presence of other metal ions. These results
indicate the good selectivity of 2a and 2b for Hg2+.
From these results we can find that carbazole-substituted 2b possesses more
favorable fluorescence properties than fluorene-substituted 2a for the detection of
mercury ion in terms of a higher sensitivity and selectivity. Besides, considering the
longer absorbance and emission maxima, 2b is more likely to be an excellent probe
applied in biological systems.

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4. Conclusion
In conclusion, we have successfully designed and synthesized a new series of NIR
fluorescent probes with tuneable absorption and emission bands over a wide range by
introducing different aromatic substituents into an aza-BODIPY unit. The
aza-BODIPY dyes 2a and 2b displayed emission quenching and solution color
changes in the presence of Hg2+. Carbazole-substituted 2b possesses more favorable
fluorescence properties than fluorene-substituted 2a for the detection of mercury ion
in terms of a higher sensitivity and selectivity. It is clear that these dyes, especially
2b, are a class of promising NIR fluorescent dyes and excellent probes for Hg2+.
Future investigations will explore the detection in aqueous solution and the further
application in bioimaging by incorporating water-soluble groups into this fascinating
class of compounds.
Acknowledgment
This work was financially supported by the National Natural Science Foundation
of China (21171098, 21174064 and 61274018), the National Basic Research Program
of China (2012CB933300), Program for New Century Excellent Talents in University
(NCET-12-0740), Natural Science Foundation of Jiangsu Province of China
(BK20130038), the Ministry of Education of China (IRT1148), and A Project Funded
by the Priority Academic Program Development of Jiangsu Higher Education
Institutions.
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Scheme 1. Skeletons of BODIPY and aza-BODIPY.
(a)
Br
Br
S
S
O
KOH
NH4OAC
C2H5OH
N
CH3NO2 EDA
CH3OH
O
+
NH
N
EtOH/H2O
O
S
O
H
NO2
Br
S
S
M2
Br
Br
M1
M3
S
S
S
S
Hg2+
N
S
S
Hg2+
N
N
B
O
CH2Cl2
BF3OEt2
Ar
B
NH
N
N
N
N
N
O
B
F
F
diisopropylethylamine
F
F
P(Ph3)4Pd
Ar
Ar
Ar
Ar
Ar
Ar
2
3
1
Ar
=
Ar
=
(b)
(a)
C8H17
N
C8H17
C8H17
(b)
S
S
S
S
N
B
N
N
N
N
N
B
F
F
F
F
N
N
2a'
2b'
Scheme 2. (a) Synthetic route of dyes 1 and 2. (b) Structures of model compounds 2a'
and 2b'.
Table 1. Calculated energy levels of the lowest singlet states of 2a' and 2b'
Dyes State
E (ev)[a] λ (nm)[b] f[c]
Excitation
HOMO
(62%)
HOMO
2
S
S
1.73
71
0
0
→
→
LUMO
LUMO
6
.95
a'
1
1.73
71
2

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7
.96
(62%)
b'
1
[a] calculated energy. [b] calculated wavelength. [c] calculated oscillator strength.

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Figure captions
Fig. 1. (a) Absorption spectra of 1a, 1b, 2a and 2b in CH2Cl2 solution (10 ꢁM). (b)
Fluorescence spectra of 10 ꢁM 2a (excitation at 650 nm) and 2b (excitation at 690
nm) in CH2Cl2 solution.
Fig. 2. (a) The dependence of the emission spectra of 2a (excitation at 650 nm) on the
solvents. (b) The dependence of the emission spectra of 2b (excitation at 690 nm) on
the solvents.
Fig. 3. The HOMO and LUMO distributions of 2a' and 2b'.
Fig. 4. Calculated absorption spectra of 2a' and 2b' in CH2Cl2 solution.
Fig. 5. (a) Changes in the UV-vis absorption spectra of 2a (10 ꢁM in CH2Cl2) upon
titration by Hg2+. (b) Changes in the UV-vis absorption spectra of 2b (10 ꢁM in
CH2Cl2) upon titration by Hg2+. Inset: photos of solution color change before and after
adding Hg2+.
Fig. 6. (a) Emission spectra (excitation at 650 nm) of 10 ꢁM 2a in the presence of
various concentrations of Hg2+ ranging from 0 to 650 ꢁM. These spectra were
measured in CH2Cl2 solution. (b) Emission spectra (excitation at 690 nm) of 10 ꢁM
2b in the presence of various concentrations of Hg2+ ranging from 0 to 12 ꢁM. These
spectra were measured in CH2Cl2 solution.
Fig. 7. PL emission titration analysis of 2a (a) and 2b (b) at different Hg2+
concentrations.
Fig. 8. (a) Fluorescence responses of 2a (10 ꢁM) to various metal ions, including Ag+,
Cd2+, Co2+, Cu2+, Fe3+, Ni2+, Pb2+, Zn2+, Cr3+, Na+, Ca2+, Mg2+ (650 ꢁM). Excitation
was at 650 nm and emission was at 752 nm. (b) Fluorescence responses of 2b (10
ꢁM) to various metal ions, including Ag+, Cd2+, Co2+, Cu2+, Fe3+, Ni2+, Pb2+, Zn2+,
Cr3+, Na+, Ca2+, Mg2+ (10 ꢁM). Excitation was at 690 nm and emission was at 848
nm. The blue bars represent the emission intensity of 2a (2b) in the presence of other
cations. The red bars represent the emission intensity that occurs upon the subsequent
addition of Hg2+ to the above solutions.

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Fig. 1.

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Fig. 2.
Fig. 3.

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Fig. 4.
Hg2+
Hg2+
Fig. 5.

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Fig. 6.

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Fig. 7.

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Fig. 8.

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Research highlights
ꢀ A new series of NIR aza-BODIPY derivatives was successfully designed and
synthesized.
ꢀ These aza-BODIPY derivatives exhibited tunable absorption and emission bands
over a wide range by introducing different aromatic substitutes.
ꢀ Excellent NIR probes for Hg2+ based on aza-BODIPY derivatives have been
realised.