This study aims to synthesize the Cu+/bipyridyl-based complex using Sonogashira cross coupling. The structure of the ligand and the complex were investigated by nuclear magnetic resonance spectroscopy; by single crystal X-ray diffraction and by UV-VIS absorption spectroscopy. The obtained experimental results were in good agreement with the theoretical calculations. The complex is a potential candidate to be the photo sensitizer in dye-sensitized solar cells.

Ha, Nguyen Ngoc

Son, Vuong

Thinh, Tran Quang

Thuy, Luong Thi Thu

Tuan, Mai Anh

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 26, No. 2 (2016), pp. 165-171
DOI:10.15625/0868-3166/26/2/8363
SYNTHESIS OF Cu+/BIPYRIDYL BASED COMPLEX TOWARDS DYE
SENSITIZED SOLAR CELL APPLICATIONS
VUONG SON1, NGUYEN NGOC HA2, LUONG THI THU THUY2, TRAN QUANG THINH1
AND MAI ANH TUAN1,†
1International Training Institute for Materials Science (ITIMS),
Hanoi University of Science and Technology (HUST)
2Faculty of Chemistry, Hanoi National University of Education (HNUE)
†E-mail: tuan.maianh@hust.edu.vn
Received 30 May 2016
Accepted for publication 25 July 2016
Abstract. This study aims to synthesize the Cu+/bipyridyl-based complex using Sonogashira
cross coupling. The structure of the ligand and the complex were investigated by nuclear mag-
netic resonance spectroscopy; by single crystal X-ray diffraction and by UV-VIS absorption spec-
troscopy. The obtained experimental results were in good agreement with the theoretical calcula-
tions. The complex is a potential candidate to be the photo sensitizer in dye-sensitized solar cells.
Keywords: dye sensitized solar cell, Cu+/bipyridine complex.
Classification numbers: 88.40.jr.
I. INTRODUCTION
Solar power is probably one of the most important renewable energy sources today. Solar
cell operates according to the photovoltaics effect [1] in which the cell converts directly the solar
radiation into a direct current. Three generations of solar cells have been introduced so far in
which Si-wafer based cell had a share of around 90% of the total production in 2013 [2]. The first
generation, which is Si-wafer based, and the second one, which is amorphous silicon based and
copper indium gallium selenide (CIGS) based, are technologically matured and it is hard to reduce
the price in fabrication processes. These technologies are also subjected to possible difficulties and
environmental impacts after the solar cell life cycle.
The liquid-based dye sensitized solar cell that belongs to the third generation was first
reported in Ref. [3, 4], and now becomes alternatives solution to the silicon based technology
and its today efficiency is claimed higher than 13% in laboratory and around 10% in module
c©2016 Vietnam Academy of Science and Technology
166 SYNTHESIS OF Cu+/BIPYRIDYL BASED COMPLEX TOWARDS DYE SENSITIZED SOLAR CELL ...
Fig. 1. Structure of a typical DSSC.
scale [5–7]. Numbers of photo-dyes are commercially available and most of which are composed
of Ruthenium as for example N3, N719 and Z907 [8]. The Ruthenium based complexes are now
dominant in dye-sensitized solar cells (DSSC) research and development (R&D). However, using
such expensive and rare earth metal could retain DSSC only at the R&D stage and limit the large-
scale production. The current research trends to develop ideal dyes in order to (i) replace expensive
and toxic Ruthenium based complexes by low cost, transitional metals, environment friendly or
even metalless [9, 10]; (ii) replace the ligand to enhance the radiation adsorption capacity, in
particular, in the visible and infrared regions [11, 12]. Replacing Ru2+ by alternative and low cost
metal ions, such as Mn+, is one of the solutions that brought the DSSC to mainstream use. In this
work, we tried to replace the rare-earth Ru2+ in N3 by Cu+ using polypyridyl based ligand. The
reasons for Cu+ can be in place of Ru+2 are:
(i) Cu is a multivalence transitional metal. The redox process (Cu+ - e ⇔ Cu+2) is
reversible. In addition, its atom radius is close to that of Ruthenium.
(ii) It is capable of forming stable complex with various ligands.
(iii) It is able to absorb strongly the solar radiation in wide range of spectrum.
(iv) The highest occupied molecular orbital (HOMO) of the obtained complex is basically
formed from the AOs of Cu+.
II. CALCULATION
Models and computational methods
The theory calculation, including the time-dependent density functional theory (TDDFT),
generalized gradient approximation (GGA), Perdew,Burke,Ernzerhof (PBE) functional theory; the
double zeta polarization (DZP) basis set and the norm-conserving pseudopotentials [13] were used
for all atoms. The plane wave cutoff of 150 Ryd for the grid has been chosen for all calculations.
The ultra violet – visible (UV-VIS) spectra were calculated by using configuration interaction
singles (CIS), in the CIS theory. The absorption energies are the difference between the Hartree–
Fock (HF) ground state and CIS excited state energies.
Mayer bond order calculated as:
BAB = ∑
a∈A
∑
b∈B
[(PS)ab (PS)ba+(P
sS)ab (P
sS)ba]
VUONG SON, NGUYEN NGOC HA, LUONG THI THU THUY, TRAN QUANG THINH AND MAI ANH TUAN 167
where P and Ps are the total density and spin-density matrices, respectively; S is the overlap
integral. By using these tools, the calculation would predict the compatibility of the alternative
materials to replace the Ru+2 in dye complex.
III. EXPERIMENT
Preparation of intermediates
4,4’-dibromo-6,6’-dimethyl-2,2’-bipyridine (d) was prepared via three steps, adapted in
[14], from 6,6’-dimethyl-2,2’-bipyridine (a). The intermediates (d) were obtained in 55% yield as
a white solid.
N N N N
O2N NO2
O O
N N
Br Br
O O
1) H2O2 / HAc
2) H2SO4 / HNO3
3) H2O
CH3COBr
HAc
(a) (b) (c)
N N
Br Br
PBr3
(d)
Synthesis of ligand
The alternative ligand was synthesized from the intermediates (d) via Sonogashira cross-
coupling reaction [15]. 4,4’-Dibromo-6,6’-dimethyl-2,2’-bipyridine (d) (0.25 mmol); Pd(Ph3P)4
tetrakis(triphenylphosphine)palladium (0) (0.025 mmol) catalyst; CuI (0.05 mol); ethynylbenzene
(0.3 mmol) were added to a degassed solution of triethylamine (1 ml) and toluene (4 ml) under an
argon atmosphere.
N N
Br Br
N NPd(Ph3P)4 / CuI
Toluene
Triethylamine(d) Ligand
The mixture was heated at 100˚C for 4 h. The product of Sonogashira reaction was purified
by silica gel column chromatography. The ligand was obtained in 55% yield as a white solid.
Synthesis of copper (I) complex
Copper (I) complex with the obtained ligand was prepared via the ligand exchange reaction
from tetrakis (acetonitrile) copper (I) hexafluorophosphate [Cu(CH3CN)4][PF6] which is a widely
used precursor for the synthesis of other Cu(I) complexes.
[Cu(CH3CN)4][PF6] (18.6 mg, 0.05 mmol) dissolved in CH3CN (2 ml) was added to a
solution of the ligand (0.10 mmol) in CHCl3 (5 ml), and the colorless solution turned red. A red
solid precipitated upon addition of ether to the solution. After that the solid was filtered and rinsed
with water and ether. The structure of the Cu(I) complex was recorded by single crystal X-ray
diffraction.
168 SYNTHESIS OF Cu+/BIPYRIDYL BASED COMPLEX TOWARDS DYE SENSITIZED SOLAR CELL ...
IV. RESULTS AND DISCUSSION
Theoretical calculation
The modification of Cu(I) complex by using new ligands to enhance the photon absorp-
tion capacity requires the support of the computational methods. In this work, the structure of
[Cu(L)CH3CN)] complex was built as Fig. 2 which contains a three-coordinate Cu(I) atom coor-
dinating with a ligand and a molecule of acetonitrile.
(a)
(b)
Fig. 2. The computational UV-VIS spectra of [Cu(L)CH3CN]+.
By employing the models and computational methods above for this structure, the compu-
tational UV-VIS spectra of [Cu(L)CH3CN]+ was also obtained. [Cu(L)CH3CN]+ exhibits a dual
absorption in the region from 200 to 400 nm in which the spectra are dominated by intensity of
oscillator strength in smaller wavelength one.
VUONG SON, NGUYEN NGOC HA, LUONG THI THU THUY, TRAN QUANG THINH AND MAI ANH TUAN 169
The structure of ligand
The structure of ligand is confirmed by using Nuclear Magnetic Resonance (NMR) of which
the obtained data is listed in Table 1.
Table 1. NMR data of the ligand.
NMR δ (chemical shift, ppm)
1H NMR (500 MHz, CDCl3)
δ = 8.35(s, 1H); 7.35 (m; 2H); 7.48 (m, 3H);
7.28 (s, 1H); 2.65 (s, 3H).
13C NMR (125 MHz, CDCl3)
C-aryl: δ = 158.2, 155.6, 132.5, 120.6;
CH-aryl: δ = 120.6, 132.0, 128.6, 125.0, 120.4;
C-ethynyl: δ = 93.3, 87.5; C-methyl: δ = 24.6.
s = singlet peak; d = doublet peak; m = multiplet peak. This data evidences the structure as
predicted by theoretical calculation.
Structure of copper (I) complex
In our synthetic experiment, the ligand based on a derivative of bipyridine was synthe-
sized by Sonogashira cross-coupling reaction which chemical structure was demonstrated by NMR
spectra. The Copper (I) complex with this ligand was also obtained via the ligand exchange reac-
tion from [Cu(CH3CN)4][PF6].
(a)
Cu
N
N C CH3
N
P
F
F
F
F
F
F
Cl
C
Cl
Cl
H
CH3
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
(b)
Fig. 3. a) Single crystal X-ray diffraction of [Cu(L)CH3CN]+and b) the illustrative formula.
Single crystal X-ray Diffraction is a non-destructive analytical technique that provides de-
tailed information about crystal structures and atomic spacing. As the result in Fig. 3, Copper (I)
170 SYNTHESIS OF Cu+/BIPYRIDYL BASED COMPLEX TOWARDS DYE SENSITIZED SOLAR CELL ...
complex with the ligand crystallized in the asymmetric unit which contains a molecule of the lig-
and and a molecule of acetonitrile coordinating the copper (I) atom through their nitrogen atoms
(CuN3). Interestingly, the copper (I) cation in this complex has trigonal planar geometry which
is more uncommon than the tetrahedral geometry of the other copper (I) complexes, for example
[Cu(CH3CN)4][PF6] precursor. However, the electrochemical behavior of the other similar three-
coordinate CuN3 complexes has been confirmed. As the result, the Cu(II)/Cu(I) redox couple
corresponding with the CuN3 complex exhibits a fully reversible with a positive redox potential
which is a necessary requirement for dye-sensitizer [16].
Experimental absorption spectra
The UV-VIS absorption spectra of [Cu(L)CH3CN][PF6] was recorded in acetonitrile so-
lutions at room temperature, as shown in Fig. 4. This synthetic complex exhibits characteristics
that are well matched with those obtained by the theoretical calculation. The spectra compose
dual absorption in the certain range and the smaller intense absorption of the longer wavelength
one. Especially, the experimental absorption spectra of [Cu(L)CH3CN][PF6] is dominated by an
intense absorption band in the visible light range (from 350 nm to 550 nm).
Fig. 4. The UV-VIS absorption spectra of [Cu(L)CH3CN][PF6].
Thus, the current synthetic copper (I) complex shows bathochromic shift in comparison
with the computational UV-VIS spectra. This result can be explained by the presence of the solvent
in the media from which the spectra data was recorded (the theoretical calculation was conducted
without any solvent). Although the acetonitrile solvent (used in UV-VIS spectra examination) shift
the absorption bands but the similar spectra shape was obtained. It is noted that the acetonitrile is
also employed in the triiodide/iodide electrolyte of the DSSC that we are developing.
VUONG SON, NGUYEN NGOC HA, LUONG THI THU THUY, TRAN QUANG THINH AND MAI ANH TUAN 171
V. CONCLUSION
This study developed an alternative dye for the replacement of Ruthenium based complexes
which are dominant as the photosensitizer in dye sensitized solar cell. Cu+was chosen as the
central ion because its transitional multivalence allows the reversible redox reaction as well as it
is possible to form chemically stable complex. The ligand, bipyridine derivative, was prepared by
the Sonogashira cross coupling that was employed for the complex synthesis. The data given by
the theoretical calculation as well as the experimental results revealed that [Cu(L)CH3CN)]+ is
potential to be the photosensitizer in DSSC.
ACKNOWLEDGEMENT
Colleagues and PhD students at ITIMS, HUST and Faculty of Chemistry, HNUE are ac-
knowledged for their contributions.
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