Synthesis and photovoltaic performances of donor-p-acceptor dyes utilizing 1,3,5-triazine as p spacers

Article abstract of DOI:10.1016/j.tetlet.2011.09.116

A series of new-type of donor-p-acceptor dyes (TCT-1-6) utilizing 1,3,5-triazine as p spacers were synthesized. These dyes were characterized by ¹H NMR, ESI-MS, EA, and X-ray crystallography. Their photovoltaic performances were also investigat

Full text of DOI:10.1016/j.tetlet.2011.09.116

Tetrahedron Letters 52 (2011) 6492–6496
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and photovoltaic performances of donor–
1,3,5-triazine as spacers
p–acceptor dyes utilizing
p
Jian Liu a,b, Kai Wang a,b, Feng Xu a,b, Zekun Tang c, Wei Zheng a,b, Jiyuan Zhang c, Chenghui Li a,b,
,
⇑
a,b,
Tao Yu c, , Xiaozeng You
⇑
⇑
a State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures,
Nanjing University, Nanjing 210093, PR China
b Academician Work Station of Changzhou Trina Solar Energy Co., Ltd, Changzhou 213022, Jiangsu Province, PR China
c Eco-Materials and Renewable Energy Research Center (ERERC), Department of Physics, National Laboratory of Solid State Microstructures,
Nanjing University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new-type of donor–p–acceptor dyes (TCT-1–6) utilizing 1,3,5-triazine as p spacers were syn-
thesized. These dyes were characterized by 1H NMR, ESI-MS, EA, and X-ray crystallography. Their photo-
voltaic performances were also investigated. An overall photon-to-electron conversion efficiency of 1.8%
was achieved with the DSSC based on the dye TCT-1(Jsc = 3.33 mA/cm2, Voc = 757 mV, FF = 71.8%) under
AM 1.5G illumination (100 mW/cm2).
Received 17 August 2011
Revised 19 September 2011
Accepted 23 September 2011
Available online 29 September 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Triazine
Triphenylamine
Dye-sensitized solar cells
Introduction
Triazine has a long history of widespread use as precursors to
construct functional materials such as antibacterial agents, pesti-
Dye-sensitized solar cells (DSSCs) have been intensively inves-
tigated in the past decades as the third generation solar cell due
to their simple fabrication process and low production cost.1
DSSCs with ruthenium complexes such as N3 and N749 as photo-
sensitizers have been shown to exhibit high photoelectric conver-
sion efficiencies.2 However, ruthenium is a rare and expensive
metal, making DSSCs based on ruthenium photosensitizers not
sustainable and cost-effective in large-scale application. It is
therefore very important to investigate metal-free organic dyes
cides, active dyes, encapsulation resins of integrated circuits, and
structural adhesives for advanced carbon–fiber composites in the
aerospace industry.6 Recently, some new
p-conjugated compounds
containing 1,3,5-triazine as the central core have been reported.
Such kind of compounds exhibit good optical and electrical proper-
ties and are useful for liquid crystalline materials, magnetic mate-
rials, and organic light emitting diodes (OLED).7 Previous studies
show that the introduction of 1,3,5-triazine unit into the skeleton
of conjugated compounds can improve their electron-injection
and electron-transportation abilities.8 Based on these results, we
envisage that the triazine ring might be a good electron transporter
for DSSCs in practical applications. Recently, donor–
p–acceptor
(D– –A) dyes have been reported as excellent photosensitizers
p
for DSSCs due to their large molar extinction coefficient, tunable
absorption wavelength, facile synthesis, and lower cost.3
Lipophilic triphenylamine and hydrophilic cyanoacrylic acid or
merocyanine are frequently employed as donor and acceptor,
as
photosensitizers for DSSCs. In this Letter, we designed and synthe-
sized a series of organic sensitizers utilizing 1,3,5-triazine as
spacers (Scheme 1). The photovoltaic performances of DSSCs using
these organic dyes as photosensitizers were investigated.
p spacers in D–p–A systems, which would be interesting in
p
respectively.4 It has been shown that different
p
spacers can
significantly change the photovoltaic performances of D– –A dye-
p
The organic dyes TCT-1 and TCT-2 were prepared by the stepwise
synthetic protocol illustrated in Scheme 2. The protection of the
aldehyde group of 4-bromobenzaldehyde was achieved by reacting
with 2 molecular equivalents of glycol in the presence of 4-methyl-
benzenesulfonic acid (PTSA) as a catalyst, resulting in the formation
of the acetal 1 as a crystalline solid in 95% yield.9 1 was coupled with
cyanuric chloride through Grignard reaction at a moderate temper-
ature to obtain the intermediate 2,8 which subsequently underwent
s.3a,e
p spacers with planar configuration can effectively improve
the electron-transportation from donors to acceptors.5
⇑
Corresponding authors. Tel.: +86 25 83592969; fax: +86 25 83314502 (C.L. and
X.Y.); tel.: +86 25 83686304; fax: +86 25 83686632 (T.Y.).
E-mail addresses: chli@nju.edu.cn (C. Li), yutao@nju.edu.cn (T. Yu), youxz@n-
ju.edu.cn (X. You).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.116

J. Liu et al. / Tetrahedron Letters 52 (2011) 6492–6496
6493
O
OH
OH
O
O
N
S
S
N
N
N
N
N
N
N
N
N
N
N
TCT-1
TCT-2
O
OH
OH
O
O
N
S
S
N
N
N
N
N
N
N
N
N
HO
HO
TCT-3
TCT-4
TCT-6
O
OH
OH
O
O
N
S
S
N
N
N
N
N
N
N
N
N
N
N
TCT-5
Scheme 1. Molecular structure of TCT dyes.
Suzuki coupling with 4-diphenylaminophenylboronic acid to gener-
ate the double triphenylamino compound 3. The treatment of 3 with
4 N HCl aqueous solutions affords aldehyde 4. Finally, the TCT-1 and
TCT-2 were obtained from 4 with cyanoacetic acid (for TCT-1) or
rhodanine-3-acetic acid (for TCT-2) through the Knoevenagel reac-
tion.10 TCT-3, TCT-4, TCT-5, and TCT-6 were prepared in the way
similar to that developed for TCT-1 and TCT-2 (Schemes S1 and
S2). Detailed experimental conditions can be found in the Supple-
mentary data.
With these triazine derivate dyes in hand, we drew our atten-
tion to investigate the geometry structure of their crystals. Unfor-
tunately, only single crystals of TCT-4 and intermediate 9 (Fig. S1)
suitable for X-ray crystallography were obtained by slow diffusion
of Et2O into their DMF solution. Crystals of TCT-4 belong to triclinic
space group P-1 and compound 9 belong to monoclinic space
group C2/c. Their crystal structures are shown in Figures 1 and
S1 respectively, while their crystallographic data are reported in
Table S1. It is noteworthy that the triazine moieties in these two
compounds are almost coplanar to the phenyl rings on both sides,
which is similar to other triazine derivate compounds reported
previously.11
290–308 nm (e
= 28,780–41,250 MÀ1 cmÀ1), respectively. Substitu-
tion of cyanoacetic acid by rhodanine-3-acetic acid leads to in-
creases of absorption intensity in the low energy band but
decreases in the high energy band. On the other hand, substitu-
tions of triphenylamine by hydroxyl or dimethylamine signifi-
cantly reduce the molar absorption coefficient and shift the
absorption energy. The absorption spectra of adsorbed TCT dyes
on TiO2 films are very similar to TCT dyes in DMF while the absorp-
tion peak wavelengths are red-shifted by 40–70 nm, relatively
(Fig. 3). Such a red-shift is attributable to anchoring of the dyes
on TiO2 electrode as reported previously.
The electrochemical properties of TCT dyes were studied by
cyclic voltammetry (CV) in DMF containing 0.1 M tetrabutylammo-
nium tetrafluoroborate as a supporting electrolyte. CV curves
(Fig. S2) of these compounds show two redox waves at similar
potentials irrespective of the sort of the dyes. On the basis of the
spectral analyses and CVs, we estimated the energy gap and HOMO
energy levels of these six dyes, while the LUMO energy level was
obtained by the difference between HOMO energy level and energy
gap.12 The optical properties and energy levels of TCT dyes are
shown in Table 1.
The absorption spectra of TCT dyes in DMF are presented in Fig-
ure 2 and their detailed parameters are collected in Table 1. The
absorption spectra of all these dyes exhibit two peaks with maxi-
Density functional theory (DFT) calculations manifest that the
HOMO orbital of these dyes are located on triphenylamine while
LUMO orbitals are located on cyanoacetic acid or rhodanine-3-ace-
tic acid (Fig. S3).
mum at around 374–403 nm (e
= 23,170–78,940 MÀ1 cmÀ1) and

6494
J. Liu et al. / Tetrahedron Letters 52 (2011) 6492–6496
O
N
O
N
O
N
O
N
O
H
O
O
a
b, c
d
N
Br
N
N
Br
Cl
N
Cl
1
2
3
O
N
H
N
e
f or g
TCT-1, TCT-2
N
N
N
4
Scheme 2. Synthesis of TCT-1 and TCT-2: Reagents and conditions: (a) glycol, PTSA, toluene, 120 °C; (b) Mg, THF, 60 °C; (c) cyanuric chloride, 0 °C–rt; (d) 4-
diphenylaminophenylboronic acid, K2CO3, PddppfCl2, dioxane, H2O, 105 °C; (e) HCl aq, THF, rt; (f) cyanoacetic acid, ammonium acetate, HOAc, 90 °C; (g) rhodanine-3-acetic
acid, ammonium acetate, HOAc, 90 °C.
Figure 1. Crystal structure of TCT-4. Hydrogen atoms are omitted for clarity.
Table 1
Optical properties of TCT 1-6 in DMF and their energy levels of HOMO and LUMO
1.8
a,b
TCT-1
TCT-2
TCT-3
TCT-4
TCT-5
TCT-6
Dye
kmax
/
emax
kedge/Eg
HOMO
(eV)
LUMO
(eV)
(nm/dm3 tmolÀ1 cmÀ1
)
(nm/eV)
1.5
1.2
0.9
0.6
0.3
0.0
TCT-1
TCT-2
TCT-3
TCT-4
TCT-5
TCT-6
396/54410,300/41250
403/78940,292/38080
397/23170,313/41410
396/51690,291/28780
374/27520,308/40660
395/60030,290/32550
501/2.48
518/2.39
509/2.44
522/2.38
468/2.65
498/2.49
À1.49
À1.37
À1.36
À1.38
À1.52
À1.52
0.99
1.02
1.08
1.00
1.13
0.97
a
Versus normal hydrogen electrode (NHE).
Eg was estimated by the absorption threshold for the dyes in DMF.4a
b
300
350
400
450
500
550
600
that these dyes can inject electrons to the conduction band of
TiO2 electrode. Sandwich-type DSSCs consisting of transparent
FTO conducting glass electrode, Pt counter electrode, and liquid
electrolyte were assembled and tested. The liquid electrolytes were
composed of 0.5 M lithium iodide, 0.05 M iodine, and 0.05 M 4-
tert-butylpyridine in acetonitrile and tert-butanol (1:1, v/v). The
Wavelength/nm
Figure 2. Absorption of TCT dyes 2 Â 10À5 M in DMF.
The LUMO energy levels for these dyes are higher than the en-
ergy level of TiO2 conduction band (À0.5 eV vs NHE), indicating

J. Liu et al. / Tetrahedron Letters 52 (2011) 6492–6496
6495
(%)
Table 2
0.4
0.3
0.2
0.1
0.0
Photovoltaic performance of DSSCs based on TCT dyes
Dye
Voc (V)
Jsc (mA cmÀ2
)
FF (%)
g
TCT-1
TCT-2
TCT-3
TCT-4
TCT-5
TCT-6
TCT-1
TCT-2
TCT-3
TCT-4
TCT-5
TCT-6
0.757
0.648
0.550
0.510
0.688
0.543
3.33
1.67
2.81
0.96
1.45
1.37
71.8
75.9
78.5
70.8
77.7
64.1
1.81
0.82
1.21
0.35
0.78
0.48
acceptor moiety exhibits better photovoltaic performance com-
pared to which with rhodanine 3-acetic acid(TCT-2, TCT-4, TCT-
6), probably owing to their good electron coupling with TiO2.3b,5
The onset wavelengths of the IPCE spectra for DSSCs based on
TCT-1 to TCT-6 dyes are around 550 nm. IPCE values higher than
50% were observed in the range of 410–460 nm with a maximum
value of 57% at 438 nm for the DSSC based on TCT-1. The observed
IPCE values are consistent with the absorption spectra of the dyes
anchored on TiO2 films.
350
400
450
500
550
600
650
700
Wavelength(nm)
Figure 3. Absorption of TCT dyes absorbed on TiO2 film.
5
To summarize, a new series of donor–p–acceptor organic sensi-
TCT-1
TCT-2
TCT-3
TCT-4
TCT-5
TCT-6
tizers with high molar extinction coefficient were designed and
synthesized. The photovoltaic performances of dye-sensitized solar
cells based on these dyes were investigated. Due to their narrow
absorption band, the efficiencies of the resulting coplanar dyes
molecule are somewhat lower in comparison to other photosensi-
tizers such as ruthenium complexes and metal-free organic dyes.
However, this work considers a new strategy to construct novel do-
4
3
2
1
0
nor–
p–acceptor organic sensitizers by using triazine as p-spacers.
Especially, the open-circuit voltage of TCT-1 reached 757 mV,
which is interestingly high among D–p–A systems reported previ-
ously.4a We believe that the development of highly efficient organ-
ic sensitizers is possible through molecular engineering based on
the triazine derivate dyes, and work to extend the spectrum
response of these sensitizers is now in progress.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Photovoltage(V)
Figure 4. Photocurrent density–voltage curves of DSSCs based on TCT dyes.
Acknowledgments
60
TCT-1
This work was supported by the National Basic Research
Program of China (2011CB933303, 2007CB925100 and 2011
CB808704), the National Natural Science Foundation of China
(21021062, 91022031), and the funding from Academician Work-
station in Changzhou Trina Solar Energy Co. Ltd, Jiangsu Province.
TCT-2
50
TCT-3
TCT-4
40
30
20
10
0
TCT-5
TCT-6
Supplementary data
Supplementary data (detailed experimental procedures, synthe-
sis and characterisation of all compounds as well as the cyclic vol-
tammetry curves and TD-DFT calculation results of TCT dyes)
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2011.09.116.
300
400
500
600
700
Wavelength/nm
Figure 5. IPCE values for DSSCs based on the TCT dyes.
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