Fluorescent ligand design for mononuclear copper(I) complex fluorescence in aqueous solution

Article abstract of DOI:10.1016/j.ica.2019.119368

The fluorescent ligands 3-(2-methylquinolyl)diethylenetriamine (QDETA) and bis(2-pyridylmethyl)(2-quinolylmethyl)amine (BPQA), and these Cu(II) complexes were synthesized. The fluorescence of [Cu(I/II)(qdeta)]^+/2+ and [Cu(I/II)(bpqa)]^+/2+ in aqueous solution was characterized; [Cu(I)(qdeta)]⁺ and [Cu(I)(bpqa)]⁺ were found to fluoresce in aqueous solution, whereas the fluorescence of [Cu(II)(qdeta)]²⁺ and [Cu(II)(bpqa)]²⁺ was almost completely quenched. The chelation enhanced fluorescence effect was observed for both [Cu(I)(qdeta)]⁺ and [Cu(I)(bpqa)]⁺. The pentacoordinated Cu(II) complex, [Cu(II)(qdeta)(H₂O)]NDSA?2H₂O (NDSA = 1,5-Naphthalenedisulfonate anion) was characterized by single crystal X-ray crystallography. The geometry around the copper center is a distorted square pyramid with four nitrogen atoms from the QDETA and an oxygen atom from the aqua.

Full text of DOI:10.1016/j.ica.2019.119368

Inorganica Chimica Acta 502 (2020) 119368
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Fluorescent ligand design for mononuclear copper(I) complex fluorescence
in aqueous solution
a,⁎
b
c
d
Makoto Saga , Genta Sakane , Shigeo Yamazaki , Keiitsu Saito
a
Department of Architecture and Civil Engineering, Faculty of Technology, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi 441–8580,
Japan
b
c
d
Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Kita, Okayama 700–0005, Japan
Department of Applied Science, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Kita, Okayama 700–0005, Japan
Department of Human Environmental Science, Graduate School of Human Development and Environment, Kobe University, 3-11 Tsurukabuto, Nada, Kobe 657–8501,
Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
The fluorescent ligands 3-(2-methylquinolyl)diethylenetriamine (QDETA) and bis(2-pyridylmethyl)(2-quino-
+
/
Fluorescent ligand
Copper complex
lylmethyl)amine (BPQA), and these Cu(II) complexes were synthesized. The fluorescence of [Cu(I/II)(qdeta)]
2+ +/2+ + +
and [Cu(I/II)(bpqa)]
in aqueous solution was characterized; [Cu(I)(qdeta)] and [Cu(I)(bpqa)] were
2+ 2+
Coordinate structure
Fluorescence in aqueous solution
Synthesis
found to fluoresce in aqueous solution, whereas the fluorescence of [Cu(II)(qdeta)] and [Cu(II)(bpqa)] was
+
almost completely quenched. The chelation enhanced fluorescence effect was observed for both [Cu(I)(qdeta)]
+
and [Cu(I)(bpqa)] . The pentacoordinated Cu(II) complex, [Cu(II)(qdeta)(H
2
O)]NDSA⋅2H O (NDSA = 1,5-
2
Naphthalenedisulfonate anion) was characterized by single crystal X-ray crystallography. The geometry around
the copper center is a distorted square pyramid with four nitrogen atoms from the QDETA and an oxygen atom
from the aqua.
1
. Introduction
[11–13]. The flattened MLCT state either returns to the Cu(I) ground
state via radiative decay pathway or forms a pentacoordinate complex
Quenching of fluorescent ligands coordinated to transition metal
with Lewis basic solvents, resulting in the “exciplex” quenching in this
ligated MLCT state [14]. Although many fluorescent Cu(I) complexes
have been reported [4–6], almost all of the reported fluorescent Cu(I)
complexes have been in the solid state or in organic solvents. To our
knowledge, there have been only a few reports for fluorescence of Cu(I)
complexes in aqueous media [15]. In addition, although fluorescence
ions has been often reported [1–3]. One of the reasons for the
quenching is magnetism caused by the spin of unpaired electrons in
transitional metal ions. A Cu(II) ion has an unpaired electron, whereas a
Cu(I) ion does not have an unpaired electron. Therefore, the fluores-
cence of the ligands is not necessarily quenched in Cu(I) complexes. In
fact, many fluorescent Cu(I) complexes have been reported [4–6]. As is
well known, the structure of copper ion complexes depends on the
1
0
enhancement of d complexes containing fluorescent ligands such as
Zn(II) and Cd(II) complexes has been reported [16–19], quenching of a
fluorescent Cu(I) complex in aqueous media has been reported [20].
From the perspectives described above, it is inferred that quenching
of Cu(I) complexes with fluorescent ligands in aqueous media is due to
the loss of excitation energy caused by the conformational change from
the MLCT excited state to the Cu(I) ground state. Based on this in-
ference, we have explored the fluorescent Cu(I) complexes which are
less flattening distortion in the MLCT excited state through the design
1
0
configuration of d electrons. Cu(I) has a d electronic configuration
9
with tetrahedral geometry, whereas Cu(II) has a d electronic config-
uration with square planar geometry [7–9]. Cu(I) complexes with 1,10-
phenanthroline derivatives are known as typical metal complexes
which undergo a large photoinduced structural change [10]. A d elec-
tron of the central copper ion is transferred to the ligands with metal-to-
ligand charge transfer (MLCT) excitation and the central copper ion is
formally oxidized from Cu(I) to Cu(II) which is subject to a Jahn-Teller
distortion. Fluorescent properties of Cu(I) complexes with 1,10-phe-
nanthroline derivatives have been reported to depend on difference in
energy between the MLCT excited state and the Cu(I) ground state
and synthesis of fluorescent ligands. Recently, we have reported that
+
[Cu(I)(bqdmen)]
(bqdmen
=
N,N’-bis(2-methylquinolyl)dimethy-
+
lethylenediamine) and [Cu(I)(bqdmpn)] (bqdmpn = N,N’-bis(2-me-
thylquinolyl)dimethyl-1,3-propanediamine) are fluorescent in aqueous
⁎
Corresponding author.
E-mail address: ms033@edu.tut.ac.jp (M. Saga).
https://doi.org/10.1016/j.ica.2019.119368
Received 11 September 2019; Received in revised form 10 December 2019; Accepted 13 December 2019
Available online 14 December 2019
0020-1693/ © 2019 Elsevier B.V. All rights reserved.

M. Saga, et al.
Inorganica Chimica Acta 502 (2020) 119368
reduced pressure. QDETA was obtained as yellowish oil.
2
+
solution, whereas the fluorescence of [Cu(II)(bqdmen)]
and [Cu(II)
2
+
(
bqdmpn)]
is completely quenched [21]. We also reported the co-
A solution of copper(II) nitrate trihydrate (2.42 g; 10 mmol) in
50 mL of ethanol was added to a solution of QDETA (2.44 g; 10 mmol)
in 200 mL of 50% ethanol. The pH of the mixture was adjusted to 7–8
with a sodium hydroxide solution. The solution was constantly stirred
at 50 °C for 60 min. After cooling to room temperature, a green algae-
like substance appeared in the reaction solution. The green algae-like
substance was filtered off. The dark blue filtrate was diluted with 1 L of
water, and then passed through a column (3 cm i.d. × 20 cm length)
packed with SP-Sephadex C-25 (GE Healthcare Bio-Sciences AB,
Uppsala, Sweden), followed by washing with 1 L of water. The blue
band was retained on the resin. A 0.2 M sodium nitrate solution was
passed through the column. The dark blue effluent from the column was
collected and concentrated with an evaporator under highly reduced
pressure at room temperature. Blue needle crystals appeared in the
solution after being allowed to stand overnight at room temperature.
The confirmation of molecule structure was conducted by the use of X-
ray structure analysis (Supplementary information 1 and 2).
ordinate structure of [Cu(II)(bqdmpn)](NO
3
)
2
which formed a square
+
planar [22]. Furthermore, we found that [Cu(I)(qdeta)] (qdeta = 3-
+
(
(
2-methylquinolyl)diethylenetriamine)
and
[Cu(I)(bpqa)]
bpqa = bis(2-pyridylmethyl)(2-quinolylmethyl)amine) are fluorescent
2
+
in aqueous solution, whereas the fluorescence of [Cu(II)(qdeta)] and
2
+
[
Cu(II)(bpqa)]
is almost completely quenched.
In the present paper, we report the fluorescence behavior of [Cu(I/
+
/2+
+/2+
II)(qdeta)]
and [Cu(I/II)(bpqa)]
in aqueous solution.
Additionally, all the Cu(I) complexes were obtained by reacting the
reductant with the Cu (II) complexes in aqueous solution, since we have
not been able to isolate the Cu(I) complexes. We considered that the
reaction of these Cu(II) complexes with reductant are completely
achieve based on these constants in the fluorescence intensity and on
these disappearances in the d-d transition spectra of these complexes
over time. Furthermore, we report the coordination geometry of [Cu(II)
(
qdeta)(H
2
O)]NDSA (NDSA = 1,5-Naphthalenedisulfonate anion) with
X-ray structural analysis.
2
. Experimental
2.2.2. Synthesis of [Cu(II)(qdeta)(H
Cu(II)(NO )(qdeta)]NO (4.32 g; 10 mmol) was dissolved in
disodium 1,5-
2
O)]NDSA⋅2H
2
O
[
3
3
2.1. Materials, chemicals and apparatus
100 mL of water. Then, 10 mL of 0.2 M
Naphthalenedisulfonate solution was added dropwise. Blue precipitate
appeared in the solution after being allowed to stand overnight at room
temperature. After filtration, the blue precipitates were recrystallized
from water. Blue prismatic crystals were obtained. Yield: 1.29 g (20%).
Diethylenetriamine, bis(2-pyridylmethyl)amine, and 2-chlor-
ometylquinoline hydrochloride were obtained from Tokyo Chemical
Industry (Tokyo, Japan). Copper(II) nitrate trihydrate was purchased
from Sigma-Aldrich Japan (Tokyo, Japan). Other chemicals used were
purchased from Wako Pure Chemical Industries (Osaka, Japan).
The absorption spectra were recorded on a Shimadzu UV-1600
spectrophotometer (Kyoto, Japan). The fluorescence and excitation
spectra were recorded on a Shimadzu RF-5300 spectrofluorometer
Found: C, 44.2; H, 5.1; N, 8.7; S, 9.9%. Calcd for C24
H
4
32CuN O
9
2
S : C,
44.5; H, 5.0; N, 8.6; S, 9.9%.
2.2.3. Synthesis of BPQA and [Cu(II)(NO
3
)(bpqa)]NO
3
⋅2·.5H O
2
(
Kyoto, Japan) with a quartz cell (1 cm × 1 cm cross-section) equipped
Although the synthesis of BPQA has already been reported [24], in
the present study, BPQA was prepared using a different method. Bis(2-
pyridylmethyl)amine (2.02 g; 10 mmol) and 2-(chloromethyl)quinoline
with a xenon lamp and dual monochromators. For all experiments, the
bandwidths for both excitation and emission were set at 5 nm.
(
2.14 g; 10 mmol) were dissolved in 200 mL of ethanol. To this solution
2.2. Synthesis
was added anhydrous potassium carbonate (6.91 g; 50 mmol), and the
suspension was heated under reflux with vigorous stirring for 5 d
(Scheme 2). After cooling to room temperature, potassium carbonate
was filtered off. A yellow oily residue remained after evaporating
ethanol under reduced pressure. The residue was dissolved in di-
chloromethane. The organic solution was washed several times with
water and dried over anhydrous sodium sulfate. After sodium sulfate
was filtered off, the filtrate was evaporated under reduced pressure. The
yellow oily residue was dissolved in 200 mL of warmed hexane. A pale
yellow powder appeared in the solution after being allowed to stand
overnight at room temperature.
2.2.1. Synthesis of QDETA and [Cu(II)(NO
3
)(qdeta)]NO
3
1
,5-Diphthalimido-3-aza-pentane was synthesized from diethylene-
triamine and phthalic anhydride according to a previous paper [23]
[
Scheme 1 (1)]. 1,5-Diphthalimido-3-aza-pentane (18.17 g; 50 mmol)
and 2-(chloromethyl)quinoline hydrochloride (10.70 g; 50 mmol) were
dissolved in 150 mL of acetonitrile. To this solution was added anhy-
drous potassium carbonate (20.73 g; 150 mmol), and the suspension
was heated under reflux with vigorous stirring for 3.5 d [Scheme 1 (2)].
The progress of reaction was monitored by thin-layer chromatography
(
development solvent: hexane/ethyl acetate = 1/1). After cooling to
A solution of copper(II) nitrate trihydrate (2.42 g; 10 mmol) in
20 mL of ethanol was added to a solution of BPQA (3.40 g; 10 mmol) in
100 mL of ethanol. The solution was constantly stirred for 45 min at
50 °C. After cooling to room temperature, the reaction mixture was
allowed to stand for a while. A precipitate appeared in the solution was
filtered off. A light blue powder was obtained by drying. The light blue
powder was dissolved in 1 L of water, and then passed through a
column (3 cm i.d. × 20 cm length) packed with SP-Sephadex C-25,
followed by washing with 1 L of water. The green band was retained on
the resin. A 0.2 M sodium nitrate solution was passed through the
column. The blue effluent from the column was collected and con-
centrated with an evaporator under highly reduced pressure at room
temperature. Blue plate crystals appeared in the solution after being
allowed to stand overnight at room temperature. Yield: 1.54 g (27%).
room temperature, potassium carbonate was filtered off. After evapor-
ating acetonitrile under reduced pressure, a yellow oily residue was
dissolved in dichloromethane. The organic solution was washed three
times with water and dried over anhydrous sodium sulfate. After so-
dium sulfate was filtered off, the filtrate was evaporated under reduced
pressure. The yellow oily residue was dissolved in warmed methanol. A
white powder of 3-(2-methylquinolyl)-1,5-diphthalimido-3-aza-pen-
tane appeared in the solution after cooling to room temperature. 3-(2-
Methylquinolyl)-1,5-diphthalimido-3-aza-pentane (10.08 g; 20 mmol)
was dissolved in 300 mL of ethanol and 10-fold excess of hydrazine
hydrate (10.01 g, 200 mmol) was added. The mixture was heated under
reflux with stirring for 3 h [Scheme 1 (3)]. The progress of reaction was
monitored by thin-layer chromatography (development solvent:
hexane/ethyl acetate = 1/1). After cooling to room temperature, white
solids were filtered off and washed with cold ethanol. Then, the wash
liquid was combined with the filtrate. The ethanol solution was con-
centrated and cooled until by-products separated out. After the by-
products were filtered off, the filtrate was evaporated under highly
Found: C, 45.9; H, 4.1; N, 14.7%. Calcd for C22
H
25CuN O8.5: C, 46.1; H,
6
4.4; N, 14.7%. The confirmation of coordination structure was con-
ducted by the use of X-ray structure analysis (Supplementary in-
formation 1 and 3).
2

M. Saga, et al.
Inorganica Chimica Acta 502 (2020) 119368
Scheme 1. Synthesis of QDETA.
3. Result and discussion
2.3. X-ray crystallography
Crystals of [Cu(II)(qdeta)(H
2
O)]NDSA⋅2H
2
O suitable for X-ray
Under the conception that quenching of Cu(I) complexes with
fluorescent ligands in aqueous media is due to the loss of excitation
energy caused by the conformational change from the MLCT excited
state to the Cu(I) ground state, we have explored the fluorescent Cu(I)
complexes which are less flattening distortion in the MLCT excited state
through the design and synthesis of fluorescent ligands. The QDETA
and BPQA were designed and synthesized as the fluorescent ligands
which are hard to form square planar geometry in Cu(II) complexes
analysis were obtained by slow recrystallization from water. The X-ray
diffraction data for [Cu(II)(qdeta)(H
3 K on a Rigaku VariMax Saturn724 diffractometer (Tokyo, Japan)
using multi-layer mirror monochromated Mo-K with a radiation wa-
velength of 0.71075 Å. Data were processed using CrystalClear
2
O)]NDSA⋅2H
2
O was collected at
9
α
(
Rigaku) [25]. The structure was solved by direct methods and ex-
panded using Fourier techniques. The non-hydrogen atoms were re-
fined anisotropically. Hydrogen atoms were refined using the riding
model. The entire refinement procedure was based on 6133 observed
reflections and 361 variable parameters, which converged with un-
9
having d electronic configuration. The spectral characteristics of the
Cu(I/II) complexes with these ligands were investigated in aqueous
media. In addition, the coordination geometries of [Cu(II)(qdeta)
(H O)]NDSA·2H O was confirmed with X-ray crystallography.
weighted and weighted agreement factors of R [I > 2σ(I)] = 0.0281
1
2
2
and wR (all data) = 0.0804, respectively.
2
Neutral atom scattering factors were taken from International
Tables for Crystallography (IT), Vol. C, Table 6.1.1.4 [26]. All calcu-
lations were performed using the CrystalStructure (Rigaku) [27] crys-
tallographic software package except for refinement, which was per-
formed using SHELXL Version 2018/3 [28]. Details concerning
collection and analysis are included in Table 1. Full structural data
3.1. Absorption spectra of Cu(I/II) complexes
+/2+
The absorption spectra of [Cu(I/II)(qdeta)]
and [Cu(I/II)
+/2+
(
bpqa)]
at pH 8 (borate buffer) are shown in Figs. 1 and 2, re-
spectively. The Cu(I) complex solutions were prepared by reducing
corresponding Cu(II) complex solutions with excess hydrazinium hy-
drogensulfate in a 100 mM borate buffer solution (pH 8) at 70 °C for
(
atom coordinates, interatomic distances and bond angles) have been
deposited with the Cambridge Crystallographic Data Centre: Deposit
number CCDC-1917067.
3
0 h. After cooling at room temperature, the absorption spectra were
measured.
Scheme 2. Synthesis of BPQA.
3

M. Saga, et al.
Inorganica Chimica Acta 502 (2020) 119368
Table 1
spectra of QDETA were measured by using a ligand substitution reac-
Crystal data and structure refinement for [Cu(II)(qdeta)(H
2
O)]NDSA⋅2H
2
O.
tion. [Cu(II)(NO
)(qdeta)]NO was dissolved in a 100 mM borate buffer
3
3
(
pH 8) to give a 10 μM solution, and then disodium dihydrogen ethy-
Complex
[Cu(II)(qdeta)(H
2
O)]NDSA⋅2H O
2
lenediaminetetraacetate dihydrate was added to the solution to give a
Empirical formula
Formula weight
Color
C
24
H
4 9 2
32CuN O S
1
mM solution. After the mixture was shaken and allowed to stand for 1
648.20
blue
d, the excitation and fluorescence spectra were measured. Fig. 3(a)
shows the excitation and fluorescence spectra of QDETA at pH 8. The
fluorescence spectrum was measured with excitation at 313 nm because
the excitation maximum was observed at 313 nm. The fluorescence
maximum of QDETA was observed at 390 nm.
Temperature (K)
Crystal System
Space Group
Crystal size (mm)
Lattice Type
Lattice Parameters
a (Å)
93
monoclinic
1
P2 /n (#14)
0.300 × 0.200 × 0.150
Primitive
Since BPQA was insoluble in water, a 10 μM BPQA solution was
prepared in ethanol/100 mM borate buffer solution (pH 8) = 1/1 (v/v).
Fig. 3(b) shows the excitation and fluorescence spectra of BPQA at pH
8. The excitation maxima of BPQA were observed at 280 and 340 nm.
The fluorescence spectrum of BPQA was acquired with excitation at
12.2756(19)
14.275(2)
16.108(3)
105.3814(16)
2721.7(7)
4
b (Å)
c (Å)
β (°)
3
V (Å )
2
80 nm. The fluorescence maximum of BPQA was observed at 425 nm.
Z
3
D
calc. (g/cm )
1.582
Radiation (Å)
0.71075
10.156
μ(Mo K
α
) (cm−1)
3.3. Fluorescence spectra of Cu(I/II) complexes
F (0 0 0)
1348.00
40,622
No. of reflections collected
The fluorescence spectra of [Cu(I)(qdeta)]+ and [Cu(I)(bpqa)]
+
No. of independent reflections
6133
shown in Fig. 4 were considered as fluorescence from Cu(I) complexes
for the following reason. The emission maximal wavelengths of fluor-
escent spectra band from the reaction products of the Cu(II) complexes
with reductive reagent differed from those of the free ligands, ad-
R
int
θ
0.0253
2
max (°)
55.0
2
Goodness-of-fit on F
1.097
R indices [I > 2σ(I)]
R indices (all data)
1
R = 0.0281
R = 0.0295
wR = 0.0804
1,917,067
+
ditionally the fluorescence intensities of [Cu(I)(qdeta)] and [Cu(I)
2
+
CCDC deposition number
(bpqa)] at the maximal emission wavelengths are stronger than those
of QDETA and BPQA, respectively. Furthermore, we also confirmed that
the fluorescence spectrum of a mixed solution containing fluorescent
ligand, reducing agent, and borate buffer after heated for sufficient time
was rarely different from the fluorescence ligand at pH 8. These ex-
perimental facts suggest that the fluorescence of the Cu (II) complex
reduced product in aqueous solution is fluorescence from the Cu (I)
complex.
Fig. 1 shows the absorption spectra of Cu(II) complexes. The ab-
2
+
2+
sorption maxima of [Cu(II)(qdeta)]
and [Cu(II)(bpqa)]
in the
visible region were observed at 650 and 848 nm, respectively. The
2
+
molar absorption coefficients of [Cu(II)(qdeta)]
and [Cu(II)
2
+
−1
−1
(
bpqa)]
were 90 and 180 cm
M
at the maximal absorption
wavelengths, respectively. These weak absorption bands are assignable
to d-d transition of metal center because of the small molar absorption
coefficients. The absorption maximum and molar absorption coefficient
Fig. 4(a) shows the excitation and fluorescence spectra of 10 μM [Cu
+
/2+
+
(I/II)(qdeta)]
solutions at pH 8. The [Cu(I)(qdeta)] solution was
2
+
prepared by reducing [Cu(II)(qdeta)]
with excess hydrazinium hy-
2
+
2+
of [Cu(II)(qdeta)] and [Cu(II)(bpqa)] in the ultraviolet region are
shown in Table 2, respectively.
drogensulfate in a 100 mM borate buffer solution (pH 8) at 70 °C for
27 h. After N bubbling and cooling at room temperature, the excitation
2
Fig. 2 shows the spectra of Cu(I) complexes. These absorbance of
and fluorescence spectra were measured. The excitation maxima of [Cu
+
+
+
[
Cu(I)(qdeta)] and [Cu(I)(bpqa)] increased as the wavelength be-
(I)(qdeta)] were observed at 256, 308, and 340 nm. The fluorescence
+/2+
came shorter. The absorption shoulder band and absorption maxima of
spectra of [Cu(I/II)(qdeta)]
were acquired with excitation at
+
+
+
+
[
Cu(I)(qdeta)] and [Cu(I)(bpqa)] were observed at 248–252 and
58 nm, respectively. The molar absorption coefficients of [Cu(I)
340 nm. The emission maximum of [Cu(I)(qdeta)] was observed at
2
450 nm. The emission maximum of [Cu(I)(qdeta)] was shifted to
+
+
−1
−1
(
qdeta)] and [Cu(I)(bpqa)] were 11,000 and 17000 cm
M
at
longer wavelengths by comparison with that of the free ligand.
Fig. 4(b) shows the excitation and fluorescence spectra of 10 μM [Cu
the shoulder and maximal absorption wavelengths, respectively.
+/2+
+
(
I/II)(bpqa)]
solutions at pH 8. The [Cu(I)(bpqa)] solution was
2
+
3.2. Fluorescence spectra of QDETA and BPQA
prepared by reducing [Cu(II)(bpqa)]
with excess hydrazinium hy-
drogensulfate in a 100 mM borate buffer solution (pH 8) at 70 °C for
30 h. After N bubbling and cooling at room temperature, the excitation
Since QDETA could not be isolated as a solid state, the fluorescence
2
Fig. 1. Absorption spectra of (a) [Cu(II)(qdeta)]2+ and (b) [Cu(II)(bpqa)]2+ at pH 8 (borate buffer solution).
4

M. Saga, et al.
Inorganica Chimica Acta 502 (2020) 119368
Fig. 2. Absorption spectra of (a) [Cu(I)(qdeta)]+ and (b) [Cu(I)(bpqa)]+ at pH 8 (borate buffer solution).
2
+
Table 2
Fig. 4 shows that the fluorescence of [Cu(II)(qdeta)]
and [Cu(II)
Absorption data of [Cu(I/II)(qdeta)]+/2+ and [Cu(I/II)(bpqa)]+/2+.
2+
+
(bpqa)]
was almost completely quenched, whereas [Cu(I)(qdeta)]
+
ε/cm−1 M−1
and [Cu(I)(bpqa)] are fluorescent in aqueous media. Fluorescence
+ +
Complex
λ
abs/nm
Solvent (pH)
Temperature
intensities of [Cu(I)(qdeta)] and [Cu(I)(bpqa)] at the maximal
emission wavelengths are 21 and 14 times stronger than those of
QDETA and BPQA, respectively, suggesting that Cu(I) ions induce
fluorescence enhancement. These fluorescence quantum yields (ϕ) were
calculated using follow equation:
[
Cu(II)(qdeta)]2+
204
52,000
44,000
6000
Water (8.0)
r.t.
2
2
3
3
6
31
73
01
15
50
4900
4300
90
[
Cu(II)(bpqa)]2+
202
59,000
44,000
10,000
5500
Water (8.0)
r.t.
F
F
R
s
R
s
C
R
C
s
s =
R
2
2
2
3
8
34
60
92
17
48
F is area of fluorescence spectrum. ε is molar absorbance coefficient
4800
in excitation wavelength. C is molar concentration. The subscript
R
180
+
+
refers to the reference (quinine sulfate) fluorophore of known fluores-
[
[
Cu(I)(qdeta)]
Cu(I)(bpqa)]
248–252*
258
11,000
17,000
Water (8.0)
Water (8.0)
r.t.
r.t.
cence quantum yield. The subscript
S
refer to the Sample solutions ([Cu
+
+
(
(
I)(qdeta)] or [Cu(I)(bpqa)] ). The emission quantum yields of [Cu(I)
+
+
*
Shoulder.
qdeta)] and [Cu(I)(bpqa)] in aqueous solution at room temperature
were 0.037 and 0.022, respectively (Table 3). Compared with previous
study [21], the fluorescence intensity of Cu(I) complex in aqueous so-
lution were almost same or upper than the copper complex containing
bisquinoline-based ligand. As a result, we succeeded to design and
synthesize the fluorescent ligands which are low the loss of excitation
energy caused by the conformational change from the MLCT excited
state to the Cu(I) ground state than previous ligand.
and fluorescence spectra were measured. The excitation maxima of [Cu
+
(
I)(bpqa)] were observed at 244 and 317 nm. The fluorescence
+
/2+
spectra of [Cu(I/II)(bpqa)]
were acquired with excitation at
+
3
3
17 nm. The emission maximum of [Cu(I)(bpqa)] was observed at
+
97 nm. The emission maximum of [Cu(I)(bpqa)] was shifted to
shorter wavelengths by comparison with that of the free ligand.
Fig. 3. Excitation and fluorescence spectra of fluorescent ligands at pH 8 (borate buffer solution); (a) 10 μM QDETA and (b) 10 μM BPQA.
5

M. Saga, et al.
Inorganica Chimica Acta 502 (2020) 119368
+/2+
and (b) 10 μM [Cu(I/II)(bpqa)]+/
Fig. 4. Excitation and fluorescence spectra of Cu(I/II) complexes at pH 8 (borate buffer solution); (a) 10 μM [Cu(I/II)(qdeta)]
2
+
.
Table 3
Fluorescence behavior data of [Cu(I)(qdeta)]+ and [Cu(I)(bpqa)]+.
ex/nm (ε/cm−1
M
−1
)
λem/nm
ϕ
Solvent (pH)
Temperature
Complex
λ
+
+
[
[
Cu(I)(qdeta)]
Cu(I)(bpqa)]
340* (3500), 308, 256
317* (5100), 244
450
397
0.037
0.022
Water (8.0)
Water (8.0)
r.t.
r.t.
*Used excitation wavelength.
3
.4. Coordination structure of [Cu(II)(qdeta)(H
2
O)]NDSA⋅2H
2
O
Table 4
Selected bond distances (Å) and angles (°) for [Cu(II)(qdeta)(H
2
O)]NDSA·2H O.
2
The X-ray structural characterization of [Cu(II)(qdeta)(H
2
O)]
O)]
2+
Atom
Atom
Distance(Å)
Atom
Atom
Distance(Å)
NDSA·2H
2
O revealed a discrete complex cation [Cu(II)(qdeta)(H
2
and a 1,5-naphthalenedisulfonate anion. The ORTEP view of [Cu(II)
Cu1
Cu1
Cu1
N1
N2
N3
2.0646(10)
2.0176(12)
2.0047(12)
Cu1
Cu1
N4
O1
2.2134(14)
1.9914(9)
2
+
(
qdeta)(H O)]
2
is shown in Fig. 5(a) [29]. The geometry around the
copper center is a distorted square pyramidal with QDETA and an aqua
ligand. The copper atom is coordinated with four nitrogen atoms and
one oxygen atom (Fig. 5(b) [30]), namely the tertiary amine-nitrogen
N1, primary amine-nitrogen N2 and N3, quinolyl-nitrogen N4 atoms of
QDETA, and the aqua ligand-oxygen atom O1. Relevant interatomic
bond lengths and angles are presented in Table 4. The deviation from
ideal square pyramidal geometry is manifested by the deviation of basal
bond angles from 90°, that is, the bond angles of N1–Cu1–N2,
N1–Cu1–N3, O1–Cu1–N2, and O1–Cu1–N3 are 85.40(4)°, 85.89(4)°,
Atom
Atom
Atom
Angle(°)
Atom
Atom
Atom
Angle(°)
N1
N1
N1
N2
N2
Cu1
Cu1
Cu1
Cu1
Cu1
N2
N3
N4
N3
N4
85.40(4)
85.89(4)
81.31(5)
156.77(5)
99.22(5)
N3
O1
O1
O1
O1
Cu1
Cu1
Cu1
Cu1
Cu1
N4
N1
N2
N3
N4
100.69(5)
169.72(5)
92.21(4)
92.55(4)
108.95(5)
9
2.21(4)°, and 92.55(4)°, respectively. Furthermore, on the basis of the
to be about 0.41 Å, indicating that the Cu1 atom also, is not located in
the plane N2N1N3. The dihedral angle formed between the plane
N1Cu1N3 and the plane O1Cu1N2 is calculated to be about 158.7˚,
indicating that the five atoms (N1, N2, N3, Cu1, O1) are not sub-
stantially coplanar. The bond angles of N4–Cu1–N1, N4–Cu1–N2,
coordinates shown in Table S6 (Supporting Information 4), the shortest
distance from the O1 to a plane N2N1N3 is calculated to be about
0
.44 Å, indicating that the O1 atom is not located in the plane N2N1N3.
The shortest distance from the Cu1 to the plane N2N1N3 is calculated
Fig. 5. (a) ORTEP view of [Cu(II)(qdeta)(H
2
O)]2+. Thermal ellipsoids are shown at the 50% probability level. (b) Coordinate structure of [Cu(II)(qdeta)(H
2
O)]2+
.
6

M. Saga, et al.
Inorganica Chimica Acta 502 (2020) 119368
N4–Cu1–N3, and N4–Cu1–O1 are 81.31(5)°, 99.22(5)°, 100.69(5)°, and
Appendix A. Supplementary data
1
08.95(5)°, respectively, showing that the N4 atom occupies the apical
position of distorted square pyramid. The Cu1–N4 distance (2.2134(14)
Å) is somewhat longer than the Cu1–N1, Cu1–N2, Cu1–N3, and Cu1–O1
distances (1.9914(9)-2.0646(10) Å), showing that the quinolyl-nitrogen
atom is bonded to the copper atom somewhat more weakly.
CCDC 1917067 contain the supplementary crystallographic data for
[Cu(II)(qdeta)(H2O)]NDSA·2H2O. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: de-
posit@ccdc.cam.ac.uk. Supplementary material associated with this
article can be found, in the online version, at https://doi.org/10.1016/
j.ica.2019.119368.
4
. Conclusion
In order to create the Cu(I) complex that fluoresces in aqueous so-
lution, we designed and synthesized the fluorescent ligands (QDETA
and BPQA) which reduce the excited-state distortions in Cu(I) com-
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7