Near-infrared fluorescent amphiphilic Aza-BODIPY dye: Synthesis, solvatochromic properties, and selective detection of Cu2+

Article abstract of DOI:10.1016/j.dyepig.2020.108714

A new amphiphilic near-infrared aza-BODIPY dye bearing both hydrophobic alkyl and hydrophilic oligo-ethylene glycol chains was synthesized by facile click reaction. By introducing the electron-donating N,N-disubstituted amino groups into the molecular str

Full text of DOI:10.1016/j.dyepig.2020.108714

Journal Pre-proof
Near-Infrared Fluorescent Amphiphilic Aza-BODIPY Dye: Synthesis, Solvatochromic
2+
Properties, and Selective Detection of Cu
Jiahe Zuo, Hongfei Pan, Yongjie Zhang, Yuanfang Chen, Houchen Wang, Xiang-Kui
Ren, Zhijian Chen
PII:
S0143-7208(20)31411-X
DOI:
https://doi.org/10.1016/j.dyepig.2020.108714
DYPI 108714
Reference:
To appear in: Dyes and Pigments
Received Date: 5 May 2020
Revised Date: 11 July 2020
Accepted Date: 13 July 2020
Please cite this article as: Zuo J, Pan H, Zhang Y, Chen Y, Wang H, Ren X-K, Chen Z, Near-Infrared
Fluorescent Amphiphilic Aza-BODIPY Dye: Synthesis, Solvatochromic Properties, and Selective
2+
Detection of Cu , Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108714.
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Graphical Abstract
2
+
+
Ni
K
1.2
1.0
0.8
0.6
0.4
0.2
0.0
+
Na
2+
Mg
Pb
Mn
2
+
2
+
2
+
Hg
Co
2
+
2
+
Ba
Ca
Cd
Cr
2
+
ε
ε
2+
2
+
2
+
Cu
2+
Cu
2
+
Zn
Pure 1
500
600
700
800
900
λ
/ nm
A new NIR fluorescent amphiphilic aza-BODIPY dye was designed and synthesized, which
exhibited solvatochromic properties as well as potential application in selective detection of
copper(II) ions

Near-Infrared Fluorescent Amphiphilic Aza-BODIPY Dye: Synthesis,
2
+
Solvatochromic Properties, and Selective Detection of Cu
Jiahe Zuo, Hongfei Pan, Yongjie Zhang, Yuanfang Chen, Houchen Wang, Xiang-Kui Ren*,
Zhijian Chen*
School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemistry
Science and Engineering, Tianjin University, Tianjin 300072, China
Email: zjchen@tju.edu.cn (Z. Chen), renxiangkui@tju.edu.cn (X.-K. Ren)
1

Abstract
A new amphiphilic near-infrared aza-BODIPY dye bearing both hydrophobic alkyl and hydrophilic
oligo-ethylene glycol chains was synthesized by facile click reaction. By introducing the
electron-donating N,N-disubstituted amino groups into the molecular structure, the new dye show
outstanding absorption and fluorescence characteristics, specifically, absorption wavelength up to
850 nm and emission up to 950 nm, and fluorescence quantum yield of 0.29. The solvatochromic
properties of the new dye were investigated by UV/Vis absorption and fluorescence spectroscopy.
Furthermore, the electrochemical properties of the dye were studied by electrochemical cyclic
voltammetry. The application of this dye for the detection of metal ions was further studied. The
2
+
results indicated selective binding ability of the dye for Cu ions and revealed its promising
application potential as a near-infrared fluorescence probe.
Keywords：aza-BODIPY; NIR dye; solvatochromism; fluorescence; ion detection
2

1. Introduction
Over the past decades, organic dyes with absorption and emission in the near-infrared (NIR)
region have attracted considerable interests owing to their potential applications in molecular probes,
biosensors, phototherapy, and others [1,2]. Up to date, a large number of NIR dyes have been
developed, e.g. cyanines [3-5], porphyrins [6,7], phthalocyanines [8-11], squaraines [12], rylene dyes
[
13,14], and others. Among the NIR dyes, BF -chelated azadipyrromethenes (aza-BODIPYs) exhibit
2
highly attractive advantages, such as facile synthesis and chemical modification, outstanding optical
and electrochemical properties, as well as excellent photostability [15-24]. However, many of the
known Aza-BODIPY dyes do not possess absorption and emission above 850 nm [25]. Moreover,
this class of dyes are often poorly soluble in polar solvents [26]. These unfavourable features lead to
the limitation of application of aza-BODIPYs in various environment. To optimize the optical
properties of aza-BODIPY dyes, much efforts have been devoted to shifting the absorption and
emission to longer wavelength by structural modifications [21], including attaching strong
electron-donating groups [27], extending the conjugation length of the π-systems [3], rigidifying the
planarity of dyes [28]. On the other hand, flexible hydrophilic and hydrophobic peripheral chains can
be introduced into the molecular structure of the aza-BODIPY dyes to improve their solubility in a
wide range of solvents [29-32].
In this paper, we report the design and synthesis of an amphiphilic NIR aza-BODIPY dye with
emission wavelength up to 950 nm by introducing N,N-dipropargyl groups at the 3,5-phenyl of the
aza-BODIPY core and subsequently appending multiple oligo-ethylene glycol (OEG) hydrophilic
chains by Cu-catalysed click reaction [33-35]. Noteworthy that such strategy has been successfully in
the synthesis of BODIPY-based fluorescent probes by Ng et al. [33] and Wu et al. [36]. In
comparison with our previously reported aza-BODIPY dyes [30,37], the absorption and emission
bands for the new dye in current work can be largely shifted to longer wavelength by replacement of
oxygen atoms at para-positions of 3,5-phenyls with nitrogen. Furthermore, the optical and
electrochemical properties of the new NIR dye were investigated and the potential application for
metal ion detection was explored.
2. Experimental section
2.1 Materials and Methods
All the reagents and solvents were obtained from commercial suppliers and purified and dried
according to standard procedures. Anhydrous dichloromethane (DCM) was distilled over CaH and
2
3

THF was distilled over sodium and stored under Argon. Chromatographic purification was
performed on silica gel (100 - 200 mesh) columns with the eluents.
1
13
H NMR and C NMR spectra were recorded on Bruker 400MHz NMR spectrometer using
chloroform-d as solvent and TMS as an internal standard for the calibration of chemical shifts.
High-resolution mass spectrum (HRMS) measurements were performed on a miorOTOF-QII mass
spectrometer. UV/Vis absorption spectra were recorded on an Agilent Technologies Cary 300 UV/Vis
spectrometer. The spectra were recorded by using quartz glass cuvettes and the absorption coefficient
(εdye) was calculated according to Lambert-Beer’s law. The electrochemical
The steady state fluorescence spectra and fluorescence quantum yield were measured on an
Edinburgh FLS980 spectrofluorometer equipped with an integrating sphere. All the fluorescence
spectra were corrected. The time-resolved fluorescence spectroscopic measurement for the dye
solution was performed on an Edinburgh FLS980 spectrofluorometer. The instrument response
function was collected by scattering the exciting light with the aqueous suspension of Silica
(
LUDOX). The least square regression analysis of the fluorescence decay curve was performed by
2
the software supplied with the instrument. The quality of the fit was evaluated by analysis of χ (0.9
–
2
1.1), as well as by inspection of residuals and autocorrelation function.
.2 Studies on detection of metal ions
2
+
2+
2+
The stock solutions of metal ions of Cu (CuCl ), Co (Co(NO ) ⋅6H O), Mg (Mg(ClO ) ),
2
3 2
2
4 2
+
2+
2+
2+
+
2+
Na (C H ONa), Ba (Ba(ClO ) ), Cd (Cd(NO ) ), Cr (CrCl ⋅6H O), K (KCl), Mn (MnCl ),
2
5
4 2
3 2
3
2
2
2
+
2+
2+
2+
2+
Hg (HgCl ), Pb (Pb(NO ) ), Zn (ZnCl ), Ca (CaCl ⋅2H O), and Ni (Ni(NO ) ) as well as the
2
3 2
2
2
2
3 2
stock solution of dye 1 in CH CN/H O (1:1 v/v) were prepared and used in the experiments.
3
2
2
+
2+
For the determination of binding constant (K) between Cu and the dye 1, solutions of Cu (1.0
-4
mL) of various concentrations (up to 1.2×10 M) in CH CN/H O (1:1 v/v) were prepared and added
3
2
to the solutions of dye 1 (1.0 mL) in CH CN/H O (1:1 v/v) and spectral changes were monitored by
3
2
absorption and fluorescence spectroscopy. The values of K were determined by linear least-squares
analysis of the absorption spectroscopic data by using equation (1) for 1:2 host–guest complexes
[
38],
ꢃ
[
ꢀ] [ꢂ]
ꢅ
[ꢂ] ([ꢂ] ꢉꢊ[ꢀ] )
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
=
+
(1)
∆
ꢄ
∆ꢆ ꢈ
ꢇ
∆ꢆꢇ
where [H] and [G] are the initial concentrations of the host and guest, respectively, and ΔA and Δε
a
0
0
denote changes in the absorbance and molar absorption coefficient, respectively, of the host upon
complexation with the guest. The binding constant (K) was determined by the linear fitting of
2
[
H] [G] /ΔA against [G] ([G] +4[H] ).
0
0
0
0
0
4

2.3 Synthesis of aza-BODIPY 1
The synthesis and characterization of all the intermediate compounds 2-7 were summarized in
Supporting Information. Compound 7 (104.8 mg, 0.10 mmol) and CuI (11.4 mg, 0.06 mmol) was
dissolved in N, N-Diisopropylethylamine (DIEA, 20ml). triethylene glycolhydrophilic chains
(151.2mg, 0.80 mmol) was subsequently added to the reaction system. The reaction mixture was
o
heated to 80 C with stirring and refluxed for 24h. The reaction solvent was then removed by vacuum
rotary evaporation. The crude product was purified by silica gel column chromatography
1
(
MeOH/CH Cl = 1/30) to give a fuchsia solid (140.1 mg, 77.6%). H NMR (400 MHz, CDCl ): δ =
2
2
3
8.06 (d, J = 27.5 Hz, 8H, 8ArH), 7.67 (s, 4H,4-CH-), 6.96 (d, J = 7.5 Hz, 10H, 8-ArH, 2-CH-), 4.80
(
s, 8H, 4-CH -), 4.49 (s, 8H, 4-CH -), 4.03 (s, 4H, 2-CH -), 3.82 (s, 8H, 4-CH -), 3.51 (s, 32H,
2 2 2 2
1
0
1
6
2
1
1
6-CH -), 3.29 (s, 12H, 4-CH ), 1.83 (s, 4H, -CH -), 1.49 (s, 4H, -CH -), 1.28 (s, 32H, 16-CH -),
2 3 2 2 2
13
.88 (s, 6H, 2-CH ). C NMR (101MHz, CDCl ): δ = 159.97, 155.93, 149.60, 144.83, 144.37,
3
3
41.06, 131.53, 130.47, 125.46, 123.31, 120.35, 116.44, 114.52, 112.70, 71.79, 70.49, 70.41, 70.39,
9.37, 68.12, 58.89, 50.23, 46.48, 31.90, 29.66, 29.63, 29.61, 29.59, 29.43, 29.34, 29.28, 26.07,
2.67, 14.12. HRMS (ESI): m/z calculated for C H BF N O Na ([M+Na]), 1828.0758; found
96
140
2
17 14
+
828.0669 [M+Na] . Elemental analysis: calculated for C H BF N O : C 63.88%, H 7.82%, N
9
6
140
2
17 14
3.19%; found: C 63.49%, H 7.65%, N 12.96%. UV/Vis (CH Cl ): λ (ε ) = 781 (127000), 540
2
2
max dye
-1
-1
(
55600 ), 424 (11830 ), 350 (34110 ) M cm ；Fluorescence (CH Cl ): λ = 836 nm.
2 2 max
3. Results and discussion
3.1 Synthesis
As shown by the synthetic route of aza-BODIPY 1 in Scheme 1, the intermediate chalcone 4 was
synthesized through a typical aldol condensation reaction of corresponding aldehyde and ketone. To
improve the synthesise and promote reaction yield of 4, an optimized reaction condition with long
reaction time over 48 h was used. The dye precursor 5 was obtained through a Michael addition
reaction of chalcone 4 with nitromethane under basic condition. When potassium t-butoxide was
used as the base, a good yield of 70% for 5 was obtained. In contrast, only yield lower than 10% was
observed for the base diethyl amine, which has been frequently used in other addition reaction with
nitromethane [39]. The compound 5 was then heated with ammonium acetate under the solventless
condition [40] and the crude condensation product was subsequently treated with BF ⋅OEt and
3
2
N,N-diisopropylethylamine (DIEA) in anhydrous CH Cl at room temperature for 24 h to give the
2
2
BF -chelated compound 7. In the last step, the copper(I) catalysed CuAAC "click" reaction between
2
the four terminal alkyne units of 7 and azide-substituted OEG 8 [41] was performed to generate the
5

1
13
amphiphilic aza-BODIPY dye 1 [42, 43]. The dye 1 were fully characterized by H NMR, C NMR,
as well as high-resolution mass spectroscopy (Supporting Information).
Scheme 1. The synthetic route for aza-BODIPY dye 1. Reagents and conditions: (i) K CO , DMF, rt,
2
3
o
1
8h, 79%; (ii) K CO , DMF, N , 80 C, 60%; (iii) Sodium ethoxide, ethanol, rt, 48 h, 78%; (iv)
2 3 2
o
CH NO , ethanol, Potassium tert-butoxide, 80 C, reflux, 12–24 h, 70%; (v) Ammonium acetate, 120
3
2
o
o
C, 24 h, 21%; (vi) BF ·Et O, DIEA, CH Cl , rt, 12 h, 57%; (vii) CuI, DIEA, CH Cl , CH CN, 50 C,
3
2
2
2
2
2
3
6
h, 77.6%.
3.2 UV/Vis absorption and emission spectroscopic properties
As shown in Figure 1a and Table 1, the UV/Vis absorption spectrum of aza-BODIPY dye 1
5
exhibited an intensive NIR band up to 850 nm with molar absorption coefficient higher than 10
-1
-1
M cm , which could be ascribed to the S –S transition of aza-BODIPY 1. As compared with the
0
1
absorption band (∼ 700 nm) of the aza-BODIPY dyes with alkoxy substituents at the para-positions of
,5-phenyls [30], the absorption band of 1 is largely shifted to the longer wavelength. Obviously, the
3
large bathochromic shift (ca. 80 nm) was caused by the strongly electron-donating amino groups at
the para-position of 3,5-phenyls. Meanwhile, the dye 1 exhibited longer wavelength of absorption
maximum than that for aza-BODIPY 7. In addition, the dye 1 displayed the second and lower
6

absorption band between 500 nm and 600 nm, which could be assigned as the S –S transition of the
0
2
aza-BODIPY dyes [44].
Furthermore, the emission in NIR region was observed for aza-BODIPY dye 1. The fluorescence
spectrum of 1 in dichloromethane have an approximate mirror-image relationship with their S –S1
0
absorption bands (Figure 1a). The emission maximum of dye 1 was observed at the 836 nm.
Meanwhile, a fluorescence quantum yield of 0.29 was determined for the dyes 1. In the
time-resolved fluorescence spectroscopic studies (Figure 1b), the dye 1 showed a bi-exponential
fluorescence decay with an averaged fluorescence lifetime (<τ >) of 2.7 ns (Table 1).
(
a)
(b)
104
1
1
0
0
0
.5
.2
.9
.6
.3
0
1.5
1.2
0.9
0.6
0.3
0
Fluorescence decay of 1
Fitted curve of 1
< τ > = 2.70ns
1
1
1
03
02
01
100
400 500 600 700 800 900 1000
0
20
40
60
80
100
Wavelength / nm
Time /ns
Figure 1. (a) UV/Vis absorption (black line) and fluorescence spectra (red lines) of aza-BODIPY
-6
dyes 1 in CH Cl , λex = 540 nm, [1] = 5.0 × 10 M. (b) Fluorescence decay of aza-BODIPY 1 in
2
2
-6
CH Cl , λex = 540 nm. λem = 838 nm. [1] = 5.0 × 10 M.
2
2
Table 1. The absorption maximum (λA), absorption coefficient (εdye), emission maximum (λF),
fluorescence quantum yields ( ), and fluorescence lifetimes ( ) of the aza-BODIPY dye 1 and 7 in
dichloromethane, [1] = [7] = 5.0×10 M.
Φ
τ
-6
-
1
-1
a
b
c
c
dye λA/nm εdye / M cm
λF / nm
836
Φ
<
τ
> / ns
τ1 /ns
τ2 /ns
5
1
7
781
743
1.3 × 10
1.1 × 10
0.29
0.28
2.7
3.6
1.9 (92.81%) 12.4 (7.19%)
2.7 (87.85%) 10.2 (12.15%)
5
809
a
b
c
Determined by fluorescence spectrometer equipped with integrating sphere.
Averaged fluorescence lifetimes.
Bi-exponential fluorescence lifetimes and pre-exponential factors (in brackets).
7

3.3 Solvatochromic Properties
The dye 1 is well-soluble in a number of common organic solvents. To investigate the
solvatochromic properties of dye 1, their absorption and fluorescence spectra were measured in
various solvents. Upon the variation in solvent polarity, obvious shift of the absorption and emission
bands of 1 was observed (Figure 2a, b). On the other hand, aza-BODIPY 1 display different
light-emitting color coordinates in CIE 1931 chromaticity diagram in various polar solvents (Figure
S17), indicating that the emission properties of dyes 1 are sensitive to the solvent polarity. In the
most polar solvent MeOH, the shortest wavelength of the absorption maximum of 773 nm and the
emission maximum of 832 nm were observed (Table 2). With decrease in the solvent polarity, the
absorption and emission maxima of the dye 1 were shift bathochromically to longer wavelengths,
implying that the dipolemoment of ground state is larger than that of excited state. This result is
comparable with the negative solvatochromism reported for BODIPY dyes [45,46]. Based on the
optical spectroscopic data summarized in Table 2, the solvatochromism of dye 1 was further
quantified by linear free energy relationships (LFER) with Lippert-Mataga equation (2) [47,48],
ꢃ
(
ꢋ ꢍꢋ )
ꢌ ꢎ
hc(ν -ν ) = 2∆f
+ C
(2)
A
F
a3
-1
where ν and ν are the wavenumbers (cm ) of the absorption and emission maxima of dye 1,
A
F
respectively, μ and μ are respectively the dipolemoments of the fluorophore in the ground state
G
E
and the excited state, h is Planck's constant, c is the speed of light, a is the radius of the cavity in
which the fluorophore resides, and Δf is the solvent parameter and can be defined as [(ε–1)/(2ε + 1)]–
2
2
[
(n –1)/(2n + 1)]. As shown in Figure 2c, an approximate linear relationship of the Stokes shifts and
the solvent parameter was observed for dye 1. According to the linear fitting, the slope (2/hc)[(µ -
E
2
3
µ ) /a ] was obtained.[48] The cavity radius of the chromophore a was estimated to be 0.58 nm
G
based on molecular modelling. Accordingly, the dipole moments changes between excited and
ground states were obtained as -2.1 D for dyes 1.
-1
-6
Table 2. Stokes’ Shifts (νA
-
νF / cm ) in various polar solvents (aza-BODIPY 1 = 5.0 ×10 M).
8

[
a]
-1
Solvent
Methanol
ε
n
Δf
λA / nm λF / nm νA
-
νF /cm
917
896
873
855
862
803
837
λ - λ /nm
F A
32.66 1.328 0.308
773
776
780
782
785
786
790
832
834
837
838
842
839
846
59
58
57
56
57
53
56
Chloroform
4.89
8.93
6.02
1.446 0.150
1.424 0.218
1.372 0.200
Dichloromethane
Ethyl acetate
Acetonitrile
Toluene
35.94 1.344 0.305
2.38
1.497 0.015
Tetrahydrofuran
7.58
1.407 0.210
a
2
2
Δ
f
= [(
ε–1)/(2
ε
+ 1)]–[(
n
–1)/(2
n
+ 1)]
Moreover, the solvatochromism of dye 1 was also investigated by using empirical solvent
polarity parameters, such as ET(30), π*, and χ (Figure S18). The latter is less common and was first
R
reported by Brooker et al. based on their solvatochromic studies for a polymethine dye [49]. Among
these polarity scales, the χ exhibit good linear free energy relationship for all the 7 solvents
R
(correlation coefficient R = 0.917). In comparison with the fitting obtained from the Lippert equation,
the correlation is largely improved.
(
a)
MeOH CHCl DCM EtOAc CH CN Toluene THF (b)
MeOH EtOAc CHCl DCM Toluene CH CN THF
1
1
1
0
0
0
0
0
.4
.2
.0
3
3
3
3
1
1
1
0
.4
.2
.0
.8
1
.0
1
.0
0.8
0.8
0
0
0
.6
.4
.2
.8 0.6
0.6
0.4
0.2
7
20
760
800
.6
810
825
840
ε
.4
.2
.0
0.0
600
4
00
500
600
700
800
650
700
750
800
850
λ / nm
λ
/ nm
(
c) 9.0
(d) 9.0
MeOH
MeOH
8
8
8
8
8
.8
.6
.4
.2
.0
CHCl3
8.8
8.6
DCM
THF
CH CN
3
CHCl3
CH CN
3
DCM
THF
υ
υ
8.4
8.2
8.0
υ
υ
EtOAc
EtOAc
Toluene
Toluene
0
.0
0.1
0.2
0.3
43
44
45
46
47
48
2
2
-1
∆
f = [(ε-1)/(2ε+1)]-[(n -1)/(2n +1)]
χ / kcal mol
R
Figure 2. UV/Vis absorption (a) and fluorescence spectra (b) of aza-BODIPY 1 in various solvents,
including chloroform (CHCl ), methanol (MeOH), Acetonitrile (CH CN), dichloromethane (DCM),
3
3
9

-6
ethyl acetate (EtOAc), tetrahydrofuran (THF), and toluene. [1] = 5.0 ×10 M, λex = 540 nm. (c)
Lippert plots for aza-BODIPY 1 in various solvents. (d) Plot of (ν -ν ) of aza-BODIPY 1 vs.
A
F
empirical solvent polarity parameter χ of different solvents.
R
For further understanding the exited state properties of these dyes, their redox properties were
studied by cyclic voltammetry (CV). The cyclic voltammogram of dye 1 exhibited one reduction
peaks at -0.88 V and the first and second oxidation peaks at 0.72 V and 0.98 V (Figure 3). According
to the oxidation and reduction potentials obtained from CV measurements, the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the
aza-BODIPYs were calculated (Table 3) based on the energy levels of reference Fc/Fc+ (4.78 eV vs.
the vacuum level). HOMO/LUMO bandgaps of dye 1 was thus obtained to be 1.60 eV which are
generally smaller than the most of previously reported dyes. In comparison with the unsubstituted
OX
red
1
,3,5,7-tetraphenyl aza-BODIPY ( E1/2 = 0.90 V, E1/2 = -0.77 V) [50], the first oxidation potential of
aza-BODIPY 1 was cathodically shifted about 0.35 V and the reduction potential was anodically
shifted about 0.11 V. Accordingly, a smaller HOMO/LUMO bandgap of 1.21 eV was obtained for the
dye 1. The oxidation potential of dye 1 is also lower than that for 1,3,5,7-tetraphenyl aza-BODIPY
bearing methoxygroups [51] or the aza-BODIPYs bearing extended conjugated systems [39,52,53].
Obviously, the electrochemical oxidation of dye 1 was facilitated by the electron-donating amino
substituent at the para-position of the 3,5-phenyls.
The small HOMO/LUMO bandgap of dye 1 implies that intramolecular charge transfer may
easily occur for this dye molecule. On the basis of redox potentials measured by cyclic
voltammetry, the free energy change (∆G) for the photoinduced charge transfer of dye 1 could be
calculated according to the Weller equation (3) [54],
*
ΔG = E (D) - E (A) - E (D )-C
(3)
ox
red
00
where E (D) is the oxidation potential of donor, E (A) is the reduction potential of acceptor,
ox
red
E (D*) (eV) is the 0-0 transition energy of the electron donor, which was determined to be 1.53 eV
0
0
for 1 by [55], and C (0.06 eV or 5.85 KJ/mol) is the stabilization energy of ion solvation. Based on
the data in Table 3, the free energy changes of intramolecular charge transfer of 1 was calculated as
∆G = - 0.30 eV. The negative value of ∆G indicates that charge transfer processes is
thermodynamically feasible upon photoexcitation, which may lead to the longer absorption
wavelength of dye 1 in comparison with that for 1,3,5,7-tetraphenyl aza-BODIPYs with the less
electron-donating alkoxy substituents at 3,5-phenyls.
Table 3. Oxidation potentials (E (D)), reduction potentials (E (A)), free energy change (∆G),
ox
red
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
1
0

energy levels of the dye 1.
OX
a
red
a
b
b
c
E
1/2 (D)
E
1/2 (A)
HOMO
LUMO
ΔE
g
/
V
/V
/eV
/eV
/eV
0
.55, 0.81
-0.66
-5.33
-4.12
1.21
a
b
First reduction/oxidation potential relative to an internal ferrocene reference. Calculated from the
c
oxidation potential and the reduction potentials by using ferrocene as a reference (-4.78 eV). ΔE =
g
LUMO - HOMO.
D - e-
D+
2
5µA
0
.55 V
0
.81 V
-0.66 V
A + e-
A-
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (V) versus Fc/Fc+
-
1
Figure. 3 Cyclic voltammogram of aza-BODIPY 1 in dichloromethane. The scan rate is 100 mV s ,
-3
[
1] = 5.0 ×10 M.
2
+
3
.4 Studies on selective detection of Cu ions
It has been reported in literatures [33] that the bis(1,2,3-triazole) amino moieties in BODIPY
dyes could be used as a ligand for metal ion detection. Accordingly, the dye 1 was investigated as a
new near-infrared fluorescence sensor for the detection of metal ions. As shown in Figure 4a, various
-4
2+
metal ions in CH CN/H O (1:1 v/v) with a concentration of 2.0×10 M, including Cu (CuCl ),
3
2
2
2
+
2+
+
2+
2+
Co (Co(NO ) ⋅6H O), Mg (Mg(ClO ) ), Na (C H ONa), Ba (Ba(ClO ) ), Cd (Cd(NO ) ),
3
2
2
4 2
2
5
4 2
3 2
2
+
+
2+
2+
2+
2+
Cr (CrCl ⋅6H O), K (KCl), Mn (MnCl ), Pb (Pb(NO ) ), Zn (ZnCl ), Hg (HgCl ),
3
2
2
3 2
2
2
2
+
2+
-6
Ca (CaCl ⋅2H O), and Ni (Ni(NO ) ), were mixed with the dye 1 solution (5.0×10 M) in equal
2
2
3 2
2
+
volume and spectral changes were monitored by absorption and fluorescence spectroscopy. For Cu ,
the absorption band was significantly blue shifted by 50-55 nm from 784 nm while for the other ions
1
1

the absorption spectra displayed almost no changes in comparison with that for pure dye 1.
Moreover, the fluorescence spectra indicated that significant fluorescence quenching took place upon
2
+
the addition of Cu while the fluorescence intensity was remained for the other ions. These results
2
+
are indicative for the selective sensitivity of dye 1 towards Cu .
2
+
(
a) 1.2
1.1
Ni
K
+
+
2
Na
Mg
Pb
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
+
2
+
2
+
Mn
ε
2
+
Hg
Co
Ba
Ca
Cd
Cr
2
+
1.0
7
80 785 790 795
2
+
λ
/ nm
ε
2+
2+
2
+
Cu2+
2
+
Cu
Zn
2
+
Pure 1
5
00
600
700
/ nm
800
900
λ
(
b)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2+
Cu
K
Zn
+
2
+
2
.6
+
Na
Ca
2
+
2
+
Mg
Cd
Co
2.4
2
+
+
2
2
+
8
30
835
λ
840
Pb
Cr
2
+
/ nm
2+
Mn
Hg
2
+
Cu2+
2
+
Ni
Ba
2
+
Pure 1
7
00
725
750
775
800
/ nm
825
850
875
λ
Figure 4. (a) UV/Vis absorption spectra of 1 in CH CN/H O (1:1 v/v) upon addition of 20
3
2
-6
-4
equivalents of different metal ions. [1] = 5.0 × 10 M, [metal ion] = 1.0 × 10 M. Inset: Local
enlargement of UV/Vis absorption spectra. (b) Fluorescence spectra of 1 in CH CN/H O (1:1 v/v)
3
2
-6
upon addition of 20 equivalents of different metal ions. λex = 540 nm. [1] = 5.0 × 10 M, [metal ion]
-4
=
1.0 × 10 M. Inset: Local enlargement of fluorescence spectra.
1
2

2+
To further quantitative study the binding properties of dye 1 with Cu ions, the dye 1 solution in
2+
CH CN/H O (1:1 v/v) was titrated with Cu of various concentrations (Figure 5a). Upon gradual
3
2
2+
addition of Cu up to 24 equivalents, the absorption band of aza-BODIPY 1 is gradually
blue-shifted from 785 to 733 nm. Note that the absorption wavelength of 733 nm after the binding of
2
+
Cu is comparable to that for the previously reported aza-BODIPYs dyes without amino substituents
[
30], indicating that the electron-donating effect of amino group on the absorption was eliminated by
2
+
the Cu binding. Meanwhile, the fluorescence intensity of 1 decreased gradually upon addition of
2
+
o
o
Cu at both 20 C (Figure 5b) and 40 C (Figure S19). It can be deduced that each of the
2
+
bis(1,2,3-triazole)amino moiety of 1 binds to one Cu [33]. This was further evidenced by a Job’s
2
+
plot in Figure S20, which indicated clearly a binding stoichiometry of 1:2 between dye 1 and Cu .
2
+
By applying the 1:2 binding model in literature [38], the binding constant between Cu and dye 1
9
-2
was calculated to be K= (1.6 ± 0.8)×10 M through linear least-squares fitting of the change in
2
+
absorbance at 781 nm. Moreover, the limit of detection of dye 1 for Cu was determined to be 1.38
μM, as shown in Figure S21.
o
o
The fluorescence quenching data at 20 C and 40 C was further analysed by using the
Stern-Volmer equation (4) [47],
2
+
I
f
0
/I = 1 + k τ [Cu ]
(4)
f
q 0
where I
f
0
and τ are the intensity and lifetime of dye 1, and I is the fluorescence intensity after
0
f
2
+
addition of Cu . As shown in the figure 5b, linear relationship was obtained for the quenching data
for different temperatures. In general, for dynamic collision quenching, the slope of Stern-Volmer
curve increases with increase in temperature. On the other hand, opposite trend has been observed for
static chelation quenching[48] since the binding strength between metal ion and ligand become lower
upon elevation of temperature. In the case of dye 1, the higher slope of the Stern-Volmer curve at 20
o
2+
C confirmed the static fluorescence quenching by chelation interaction between Cu and the
2
+
bis(1,2,3-triazole) amino groups. In addition, the competition experiments between Cu and other
metal ions were monitored by fluorescence spectroscopy upon addition of 10 equiv. of other metal
-
6
2+
-5
ions into a solution containing dye 1 (5.0×10 M) and Cu ions (5.0×10 M , 10 equiv.) in
2
+
CH CN/H O (1:1 v/v). As shown in Figure S22, the Cu could not be replaced by other metal ions
3
2
from the complex. This result confirmed the highly selectivity in the recognition and detection of
2
+
Cu by dye 1.
1
3

(
a)
(b)
3
2
1
1
1
5
0
5
0
10
8
2+
o
[Cu ]
2
+
20 C
[
Cu ]
1
0
8
6
4
2
0
2
4 eq
0
6
4
o
40 C
2
0
0
2
4
6
8 10 12 14
-9
0
2
[
4
6
8
[
G] ([G] +4[H] ) (×10 )
0
0
0
2+
-5
24 eq
Cu ] / 10 M
ε
0
7
0
5
50
775
800
λ / nm
825
850
00
600
700
800
λ
/ nm
Figure 5. (a) UV/Vis absorption spectra of aza-BODIPY 1 in CH CN/H O (1:1 v/v) upon the
3
2
2
+
-6
addition of Cu , [1] = 5.0×10 M. Inset: Linear fitting to calculate the binding constant K between
2
+
Cu and 1. (b) Fluorescence spectra of aza-BODIPY 1 in CH CN/H O (1:1 v/v) upon the addition of
3
2
2
+
o
-6
Cu at 20 C, [1] = 5.0 ×10 M, λ = 540 nm. Inset: Stern-Volmer plots of the fluorescence
ex
o
o
quenching at 20 C (black) and 40 C (red).
4. Conclusion
In summary, a new amphiphilic NIR fluorescent aza-BODIPY dye 1 bearing two dodecyloxy
hydrophobic chains and four oligo-ethylene glycol hydrophilic chains was facilely synthesized by
copper-catalysed click reaction and fully characterized. By introducing the electron-donating
N,N-disubstituted amino groups at the para-position of the 3,5-phenyls, the dye 1 exhibited
absorption wavelength up to 850 nm and emission wavelength up to 950 nm. The solvent-dependent
UV/Vis and fluorescence spectroscopic studies indicated the absorption and emission wavelengths of
the dye 1 are sensitive to the environmental polarity and the negative solvatochromism was observed.
Electrochemical studies revealed that intramolecular charge transfer is feasible for dye 1 and may
cause the longer absorption wavelength of this dye than that for the aza-BODIPYs bearing alkoxy
substituents at 3,5-phenyls. Furthermore, the bis(1,2,3-triazole) amino moiety in dye 1 can
2
+
selectively response and binding to Cu among the studied 15 different metal ions. Accordingly, the
2
+
dye 1 exhibited promising application potential as a new near-infrared fluorescence probe for Cu .
Acknowledgements
We acknowledge National Natural Science Foundation of China (No. 21676186 and No. 21875157),
the Tianjin Research Program of Application Foundation and Advanced Technology (No.
1
4

15JCZDJC31400) for financial support.
Supplementary data
Supplementary data to this article can be found online.
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1
9

Highlights
ò
A new amphiphilic NIR aza-BODIPY dye with absorption up to 850 nm and emission
up to 950 nm was synthesized.
ò
ò
The solvatochromic properties of the new dye were investigated.
2+
The dye can be used for the selective detection of Cu ions.

CRediT author statement
Jiahe Zuo: Investigation, Methodology, Writing - original draft. Hongfei Pan:
Data curation, Validation. Yongjie Zhang: Conceptualization. Yuanfang
Chen: Formal analysis. Houchen Wang: Writing - review & editing. Xiang-Kui
Ren: Resources, Writing - review & editing, Funding acquisition. Zhijian
Chen: Conceptualization, Supervision, Resources, Writing - review & editing,
Supervision, Funding acquisition.

Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests: