SalenZn-bridged D-π-A dyes for dye-sensitized solar cells

Article abstract of DOI:10.1002/cjoc.201300924

A series of SalenZn based dyes with triphenylamine derivatives as the donor, benzoic acid as the acceptor, and coplanar Salen complexes as the spacer have been designed and synthesized for dye-sensitized solar cells. The absorption, electrochemical, and photovoltaic properties for all sensitizers have been systematically investigated. When the tail length of the alkyl substituents is increased from C-0 to C-8 on the donor part, the efficiency of its DSSC augments evidently. It is found that the incorporation of bis-carboxyl groups instead of the single carboxyl group as anchoring groups induces a remarkable enhancement of the electron injection efficiency from the excited dyes to the TiO₂ semiconductor and generates higher electron density and voltage. Copyright

Full text of DOI:10.1002/cjoc.201300924

FULL PAPER
DOI: 10.1002/cjoc.201300924
SalenZn-bridged D-π-A Dyes For Dye-Sensitized Solar Cells
Yongjian Jia,a Faliang Gou,a Ran Fang,a Huanwang Jing,*,a,b and Zhenping Zhub
a State Key Laboratory of Applied Organic Chemistry and College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, Gansu 730000, China
b State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,
Taiyuan, Shanxi 030001, China
A series of SalenZn based dyes with triphenylamine derivatives as the donor, benzoic acid as the acceptor, and
coplanar Salen complexes as the spacer have been designed and synthesized for dye-sensitized solar cells. The ab-
sorption, electrochemical, and photovoltaic properties for all sensitizers have been systematically investigated.
When the tail length of the alkyl substituents is increased from C-0 to C-8 on the donor part, the efficiency of its
DSSC augments evidently. It is found that the incorporation of bis-carboxyl groups instead of the single carboxyl
group as anchoring groups induces a remarkable enhancement of the electron injection efficiency from the excited
dyes to the TiO2 semiconductor and generates higher electron density and voltage.
Keywords dye-sensitized solar cells, SalenZn complexes, salicylic acid, anchoring group
Introduction
gained considerable light-to-electron conversion effi-
ciency in spite of their less stabilities.[7] So far, one of
zinc porphyrin dyes (YD-o-C-8) based DSSCs in con-
junction with an organic dye as co-sensitizer and a Co
(II/III)-based redox has gained superior photovoltaic
performance (12.3%)[5a] under full sun illumination,
which might overcome the limit resources of ruthenium
on the Earth.
Generally, the designs of metal-based dyes for high
performance DSSCs obey donor-(π-spacer)-acceptor
(D-π-A) structure principle springing from the push-pull
concept of the photo-induced intramolecular charge
transfer, which adjusts the molecular absorption to
match solar radiation for competent light harvesting.[8]
Salen metal complexes as mimic metalloporphyrins,
in addition to broadly employing in the field of homo-
geneous catalysis,[9] have been paid much less attention
to the sensitizers.[9c] Typical and promising salen metal
complexes-based materials have recently emerged in-
cluding molecular sensors,[10] and useful supramolecular
structures.[11] Along with the syntheses of metallosallens
are widely accessible, relatively and increasingly mature,
they become good alternatives of metalloporphyrins due
to their planar structures.
Along with the increasing energy demand in the
world, sustainable energy, especially the solar energy,
has received widespread attention in various research
fileds. Dye-sensitized solar cells (DSSCs) improved by
Grätzel since 1991,[1] are regarded as prospective alter-
native for renewable clean energy sources in virtue of
their versatility, short energy payback time and low cost
of manufacture compared with conventional silicon-
based solar cells and thin-film photovoltaic technolo-
gies.[2] A characteristic solar cell makes up of three
components: (i) a sensitized photoanode, which is typi-
cally a monolayer of molecular dye sensitized nano-
crystalline TiO2 film with anatase phases on a transpar-
ent fluorine doped tin oxide (FTO) conducting glass; (ii)
an electrolyte solution embracing iodide anion/triiodide
anion or Co (II/III) tris(bipyridyl)-based as a redox cou-
ple; and (iii) a cathode platinized on a FTO conducting
glass or a carbon film as a flexible counter electrode.[3]
Although many factors affect the photovoltaic per-
formance of the DSSCs, the role of dye is obviously a
crucial one. To approach high photovoltaic performance,
numerous focuses are centered on designing and syn-
thesizing new photosensitizers with a panchromatic
light response. DSSC devices with Ru complexes (N719)
as photosensitizers keep the record of validated effi-
ciency of over 11%.[4] Recently, more cheaper dyes of
zinc porphyrin chromophores are also hopeful candi-
dates.[5] The metal-free organic sensitizers[6] have also
Herein, we report a new series of dyes with D
(triphenylamine)-π (SalenZn complexes)-A (benzoic
acids) structure (Figure 1). The detailed investigations
of the performances of their DSSC devices were
achieved in comparison with those of DSSCs of N719.
*
E-mail: hwjing@lzu.edu.cn; Tel.: 0086-0931-8912585; Fax: 0086-0931-8912582
Received December 31, 2013; accepted March 20, 2014; published online April 22, 2014.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300924 or from the author.
Chin. J. Chem. 2014, 32, 513—520
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Jia et al.
FULL PAPER
13C NMR (CDCl3, 75 MHz) δ: 21.3, 55.4, 114.5, 121.7,
123.7, 127.1, 138.5, 141.2, 148.3, 155.8. ESI-HRMS
m/z: [M＋H]＋ calcd for C23H25NO: 332.2009, found
332.2011.
R1
R2
Acceptor
anchoring Group
1
N
O
N
O
3c Colorless solid (49%); H NMR (CDCl3, 300
Ar
N
Ar
Ar
N
Ar
Zn
MHz) δ: (s, 18H), 3.78 (s, 3H), 6.82 (d, J＝8.7 Hz, 2H),
6.95 (d, J＝8.4 Hz, 4H), 7.07 (d, J＝8.7 Hz, 2H), 7.21
(d, J＝9 Hz, 4H); 13C NMR (CDCl3, 75 MHz) δ: 31.4,
34.1, 55.4, 114.6, 122.3, 125.8, 126.9, 141.1, 144.3,
145.6, 155.7.
Donor
Coplanar bridge
SLa: Ar = Ph
R1 = H; R2 = COOH
R1 = H; R2 = COOH
1
3d Colorless oil (55%); H NMR (CDCl3, 300
SLb: Ar = m-xylene
MHz) δ: 0.94－0.98 (m, 6H), 1.37－1.41 (m, 8H),
1.63－1.68 (m, 4H), 2.56 (t, J＝8.1Hz, 4H), 3.83 (s,
3H), 6.84－6.86 (m, 2H), 7.00 (d, J＝8.1 Hz, 4H),
7.09－7.11 (m, 6H); 13C NMR (CDCl3, 75 MHz) δ:
14.1, 22.6, 31.3, 31.6, 35.2, 55.4, 114.5, 122.8, 126.6,
128.9, 136.3, 141.2, 146.0, 155.6. ESI-HRMS m/z: [M
＋H]＋ calcd for C29H37NO: 415.29, found 416.2947.
SLc: Ar = 4-tert-butylbenzene R1 = H; R2 = COOH
SLd: Ar = 4-pentylbenzene
SLe: Ar = 4-octylbenze
SLf: Ar = Ph
R1 = H; R2 = COOH
R1 = H; R2 = COOH
R1= R2 = COOH
Figure 1 Structure of the SalenZn-bridged D-π-A dyes.
1
Experimental
3e Colorless oil (50%); H NMR (CDCl3, 300
MHz) δ: 0.88 (t, J＝6.9 Hz, 6H), 1.29－1.39 (m, 20H),
1.56－1.63 (m, 4H), 2.52 (t, J＝7.8 Hz, 4H), 3.78 (s,
3H), 6.81 (d, J＝2.1 Hz, 2H), 6.84 (d, J＝2.4 Hz, 4H),
6.97－7.08 (m, 6H); 13C NMR (CDCl3, 75 MHz) δ:
14.1, 22.7, 29.5, 29.7, 30.3, 31.6, 31.9, 35.2, 55.4, 113.6,
122.2, 126.6, 128.9, 136.3, 141.3, 146.0, 155.6; ESI-MS
[M＋H]＋ calcd for C35H50NO: 499, found 500.6.
Materials
All reagents and solvents were purchased from
commercial resources and used without further purifica-
tion unless otherwise noted. CH2Cl2 was dried over
CaH2 and freshly distilled prior to use. THF, ether,
toluene and triethylamine were dried over sodium/ben-
zophenone and freshly distilled before use. 1,2-Dimeth-
yl-3-propylimidazolium iodide (DMPII, Aldrich), iodine
(I2, Aldrich), 4-tert-butylpyridine (TBP, Aldrich) were
used as received. The N719 dye (cis-diisothiocyanato-
bis(2,2-bipyridyl-4,4'-dicarboxylato) ruthenium(II) bis-
(tetrabutylammonium)) was purchased from Solaronix.
All other chemicals were of analytical grade. FTO glass
was purchased from Nippon Sheet Glass Co. Ltd.
General preparation of compound 4
To a solution of compound 3 (7.09 mmol) in dry di-
chloromethane (70 mL) was added boron tribromide
(1.0 mol/L solution in dichloromethane) (10.64 mmol)
at 0 ℃ and stirred for 30 min. The reaction mixture
was stirred at room temperature for 24 h, poured into
water and then extracted with dichloromethane (50 mL
×3). The extract was dried over Na2SO4, and evapo-
rated. The residue was chromatographed on silica gel to
give compound 4.
1
(15/sq). H NMR and 13C NMR spectra were measured
by using Varian 300 MHz spectrometer. The UV-vis
absorption spectra were determined by an UV-3600
spectrophotometer in THF at room temperature.
1
4a Colorless solid (99%); H NMR (CDCl3, 300
MHz) δ: 5.22 (br s, 1H), 6.73 (br s, 2H), 6.90－6.94 (m,
2H), 6.99 (d, J＝7.5 Hz, 6H), 7.20 (d, J＝8.1 Hz, 4H);
13C NMR (CDCl3, 75 MHz) δ: 116.22, 121.82, 122.83,
127.38, 129.04, 140.87, 147.94, 151.57; ESI-HRMS [M
＋H]＋ calcd for C18H15NO: 262.1226, found 262.1219.
4b Colorless solid (99%); MS [M＋H]＋ calcd for
C22H23NO 317, found 317.
General preparation of compound 3
To a stirred solution of p-methoxyaniline (4 mmol),
CuI (0.08 mmol), KOtBu (12 mmol) and 1,10-phe-
nanthroline (0.08 mmol) in toluene was added aryl hal-
ide (12 mmol) under argon. The reaction mixture was
then stirred and refluxed for 14 h. After cooling to room
temperature, the mixture was filtered to remove the base
precipitate. The solution was concentrated to obtain the
crude product, which was chromatographed to get the
pure product.
1
4c Colorless solid (98%); H NMR (CDCl3, 300
MHz) δ: 1.21－1.29 (m 18H), 5.04 (br s, 1H), 6.76 (br s,
1H), 6.94－7.03 (m, 4H), 7.20－7.22 (m, 4H); ESI-
HRMS [M＋H]＋ calcd for C19H17NO: 374.2478, found
374.2470.
1
3a Colorless solid (60%); H NMR (CDCl3, 300
4d Colorless oil (97%); ESI-HRMS [M＋H]＋
calcd for C28H35NO: 401.27, found 402.2786.
MHz) δ: 3.78 (s, 3H), 6.81－6.83 (m, 2H), 6.84－6.95
(m, 2H), 7.04－7.08 (m, 6H), 7.16－7.22 (m, 4H); 13C
NMR (CDCl3, 75 MHz) δ: 55.4, 114.7, 121.8, 122.8,
127.3, 129.0, 140.7, 148.1, 156.1; ESI-HRMS m/z: [M
＋H]＋ calcd for C19H17NO: 276.1383, found 276.1378.
1
4e Colorless oil (99%); H NMR (CDCl3, 300
MHz) δ: 0.858－0.903 (m, 6H), 1.27 (br s, 20H), 1.58
(br s, 4H), 2.50－2.54 (m, 4H), 4.70 (br s, 1H), 6.73 (dd,
J＝6.6, 2.4 Hz, 2H), 7.02 (d, J＝8.4 Hz, 8H), 7.24 (d,
J＝8.1 Hz, 2H); ESI-MS m/z: [M＋H]＋ calcd for
C35H46NO: 486, found 486.6.
1
3b Colorless solid (60%); H NMR (CDCl3, 300
MHz) δ: 2.20 (s, 12H), 3.80 (s, 3H), 6.60 (d, J＝12.3 Hz,
5H), 6.80 (d, J＝8.7 Hz, 2H), 7.01 (d, J＝8.7 Hz, 2H);
514
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 513—520

SalenZn-bridged D-π-A Dyes For Dye-Sensitized Solar Cells
General preparation of compound 5
vent was removed by evaporation and the crude product
was obtained, which was filtrated under vacuum pump
and washed with H2O.
The compound 5 was performed referring to a lit-
erature procedure.[13a]
1
1
SLa (60%); H NMR (DMSO-d6, 300 MHz) δ:
5a Yellow solid (90%); H NMR (CDCl3, 300
6.75 (dd, J＝2.7, 6 Hz, 1H), 6.91－6.96 (m, 12H),
7.08－7.19 (m, 3H), 7.21－7.24 (m, 10H), 7.44 (s, 1H),
7.84 (s, 2H), 8.36 (s, 1H), 9.03 (s, 2H); MS
(MALDI-TOF) m/z: [M＋Ag]＋ calcd for C45H32N4O4-
Zn 758.17, found 866.6.
MHz) δ: 6.92－7.05 (m, 8H), 7.22－7.38 (m, 3H), 9.74
(s, 1H), 10.85 (s, 1H); 13C NMR (75 MHz, CDCl3) δ:
118.8, 120.9, 122.6, 123.1, 129.3, 135.1, 140.2, 147.5,
＋
157.8, 176.2; ESI-MS m/z: [M ＋ H] calcd for
C19H15NO2: 289, found 289.
1
1
SLb (63%); H NMR (DMSO-d6, 300 MHz) δ:
5b Yellow solid (90%); H NMR (CDCl3, 300
2.11－2.14 (m, 24H), 6.53－6.55 (m, 12H), 6.73－6.76
(m, 2H), 7.02－7.07 (m, 2H), 7.22 (s, 1H), 7.34 (s, 1H),
7.85 (s, 2H), 8.30 (s, 1H), 8.96 (d, J＝8.4 Hz, 2H); MS
(MALDI-TOF) m/z: [M＋Ag]＋ calcd for C53H48N4O4-
Zn 868.3, found 976.2.
MHz) δ: 2.32 (s, 12H), 6.76 (s, 6H), 7.01 (d, J＝8.7 Hz,
1H), 7.37－7.44 (m, 2H), 9.80 (s, 1H), 10.93 (s, 1H);
13C NMR (75 MHz, CDCl3) δ: 21.3, 118.5, 120.9, 121.2,
124.4, 129.1, 135.0, 140.7, 147.7, 157.5, 196.2.
1
5c Yellow solid (92%); H NMR (CDCl3, 300
1
SLc (64%); H NMR (DMSO-d6, 300 MHz) δ:
MHz) δ: 1.31 (d, J＝1.2 Hz, 18H), 6.89－6.97 (m, 5H),
7.24 (d, J＝7.2 Hz, 4H), 7.31－7.33 (m, 2H), 9.72 (s,
1H), 10.82 (s, 1H); 13C NMR (75 MHz, CDCl3) δ: 29.7,
31.2, 31.4, 34.2, 118.6, 120.9, 122.7, 126.1, 128.8,
134.8, 140.6, 144.9, 145.3, 157.5, 196.3.
1.20 (d, J＝12.9 Hz, 36H), 6.71－6.77 (m, 2H), 6.83 (t,
J＝8.7 Hz, 7H), 6.89 (t, J＝8.7 Hz , 2H), 6.99－7.23 (m,
9H), 7.25 (s, 1H), 7.38 (s, 1H), 7.84 (s, 1H), 8.30 (s, 1H),
9.00 (s, 2H); MS (MALDI-TOF) m/z: [M＋H]＋ calcd
for C61H64N4O4Zn 980.42, found 981.1.
5d Yellow oil (89%); 1H NMR (CDCl3, 300 MHz)
δ: 0.92 (t, J＝6.6 Hz, 6H), 1.32－1.44 (m, 8H), 1.68 (d,
J＝7.2 Hz, 4H), 2.57 (t, J＝8.1 Hz, 4H), 6.92－7.10 (m,
8H), 7.26－7.38 (m, 3H), 9.72 (s, 1H), 10.87 (br s, 1H);
13C NMR (75 MHz, CDCl3) δ: 14.0, 22.5, 32.2, 33.5,
35.2, 118.5, 120.8, 123.2, 128.4, 129.2, 134.3, 137.1,
140.7, 145.3, 157.3, 196.1.
1
SLd (61%); H NMR (DMSO-d6, 300 MHz) δ:
0.81－0.88 (m, 12H), 1.24－1.29 (m, 18H), 1.51－1.52
(m, 8H), 2.26－2.53 (m, 8H), 6.71－6.75 (m, 2H), 6.81
(t, J＝7.6 Hz, 7H), 7.01－7.05 (m, 9H), 7.25 (s, 1H),
7.38 (s, 1H), 7.84 (s, 1H), 8.30 (s, 1H), 8.98 (s, 1H); MS
＋
(MALDI-TOF) m/z: [M ＋ Ag]
calcd for
5e Yellow oil (95%); 1H NMR (CDCl3, 300 MHz)
δ: 0.86－0.90 (m, 6H), 1.27－1.30 (m, 20H), 1.60－
1.61 (m, 4H), 2.51 (t, J＝8.1 Hz, 4H), 6.91 (d, J＝2.7
Hz, 4H), 7.03 (d, J＝8.4 Hz, 4H), 7.26 (d, J＝3.9 Hz,
2H), 7.31 (d, J＝2.4 Hz, 1H), 9.73 (s, 1H), 10.80 (s, 1H);
13C NMR (CDCl3, 75 MHz) δ: 14.1, 22.7, 29.27, 29.40,
29.48, 31.54, 31.88, 35.3, 118.5, 120.8, 123.2, 128.5,
129.2, 134.3, 137.3, 140.8, 143.3, 145.3, 157.3, 196.1.
C65H72N4O4Zn 1036.48, found 1144.3.
1
SLe (63%); H NMR (benzene-d6, 300 MHz) δ:
0.59－0.64 (m, 12H), 0.97 (s, 48H), 1.32 (s, 8H), 2.23 (t,
J＝7.5, Hz, 8H), 6.47 (s, 8H), 6.60 (s, 6H), 6.97－6.95
(m, 14H), 7.15 (s, 2H), 7.43 (s, 1H); MS (MALDI-TOF)
m/z: [M]＋ calcd for C77H96N4O4Zn 1207, found 1207.4.
1
13 (62%); H NMR (DMSO-d6, 300 MHz) δ: 3.81
(s, 6H), 6.76 (d, J＝9.3 Hz, 1H), 6.90－6.99 (m, 13H),
7.09－7.11 (m, 1H), 7.22 (t, J＝8.1 Hz, 8H), 7.37 (s,
1H), 8.21 (s, 1H), 9.08 (s, 1H).
Preparation of dimethyl 4,5-diaminophthalate (12)
Compounds 9, 10, 11, and 12 were synthesized fol-
lowing the literature procedures[13b] and characterized
with NMR spectra that were consistent with the reported
data.
A solution of dimethyl-4-amino-5-nitrophthalate (1.0
g) and 10% Pd/C (0.2 g) in methanol (30 mL) was
placed into a hydrogenation apparatus for 1 d. The sol-
vent was removed by evaporation and the product 12
was obtained. 1H NMR (CDCl3, 300 MHz) δ: 3.57 (br s,
4H), 3.77 (s, 6H), 6.96 (s, 2H); ESI-MS m/z: [M＋H]＋
calcd for C19H46NO: 224, found 225.3.
Preparation of the dye SLf
The compound 13 was prepared by using the method
as the same as that of making SLa.
A solution of 13 (1 mmol) and NaOH (4 mmol) in
water (5 mL) and THF (5 mL) was refluxed for 6 h. Af-
ter cooling, the mixture was acidified with 0.012 mol/L
HCl to get the precipitate, which was filtrated, washed
with CHCl3, and dried under vacuum oven.
1
SLf (61%); H NMR (DMSO-d6, 300 MHz) δ:
6.75 (d, J＝2.7 Hz, 1H), 6.91－6.99 (m, 12H), 7.08－
7.09 (m, 3H), 7.22－7.27 (m, 10), 7.30－7.44 (m, 2H),
7.84－7.91 (m, 2H), 8.36 (s, 1H), 9.03 (s, 1H), 9.08 (s,
Preparation of the dyes SLa－SLe
＋
1H); MS (MALDI-TOF) m/z: [M]
C46H32N4O6Zn 800.16, found 800.00.
calcd for
Et3N (2 mL) was added to a solution of salicylalde-
hyde (0.5 mmol), 3,4-diaminobenzoic acid (0.25 mmol)
and Zn(OAc)2•2H2O (0.65 mmol) in ethanol (10 mL).
The reaction mixture was heated to reflux for 5 h under
argon. The mixture was filtrated under vacuum pump
and washed with Et2O to obtain 7.
Fabrication of the DSSCs
Fabrication of the DSSCs was prepared following a
modified procedure reported in the literature.[14] With a
starting compound of titanium(IV) n-butoxide, TiO2
nanoparticles were prepared via a hydrothermal synthe-
A solution of 7 in THF was dropped with 0.03 mol/L
HCl (2 mL) at 0 ℃ and stirred for 4 h. The most sol-
Chin. J. Chem. 2014, 32, 513—520
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515

Jia et al.
FULL PAPER
Results and Discussion
sis. The practicable TiO2 paste invloving polyethylene
glycol (molecular weight 20000) in a proportion of 50%
weight was coated on a commercial fluorine-tin-oxide
(FTO) using vacuum spincoater method. The FTO glass
supported TiO2 film (thickness 15 μm and active area
0.16 cm2) was then sintered at 500 ℃ for 1 h. The
electrode was immerged immediately into the dye solu-
tion (SL series, 1 mmol/L, V(EtOH)/V(THF)＝1/1, 12 h)
when the oven temperature was cooled to 100 ℃. The
TiO2 electrode was then taken out, flushed with CH3CN,
and dried with nitrogen stream. The solar cell was pre-
pared by agglutinating the platinum counter electrode
and the TiO2 electrode. Electrolyte was injected and
penetrated to the TiO2 film via vacuum perfusion. The
electrolyte solution was constitutive of 0.6 mol/L
DMPII, 0.05 mol/L I2, and 0.5 mol/L TBP in a mixture
solution of acetonitrile and valeronitrile (85∶15, V/V).
The solar cell was finally sealed by hot glue.
Synthesis of the target compounds
The synthetic route of the new dyes named SL is
dipicted in Scheme 1. With a starting compound of
4-methoxyaniline 1 and various aryl iodides 2, C－N
bond formation was achieved via Ullmann reaction
catalyzed by CuI, and gave 4-methoxy-N,N-di-
phenylaniline derivates 3. By dint of BBr3 removing the
methoxy group, hydroxyl was exposed and the series
compounds 4 were formed, which were readily con-
verted into their corresponding salicylaldehydes 5 via
Grignard reagent and paraformaldehyde, respectively.
The addition of an excess of triethylamine protected the
free carboxyl group from facilitating inner salt with
amine group resulting in mono-Schiff base. In the pres-
ence of dilute aqueous solution of HCl, the triethyl-
amine was removed to generate the target molecules
SLa－SLe.
To enhance the efficiency of electron injection and
increase effective contact area between TiO2 film and
dyes, the second carboxyl group was introduced into
ortho-position of carboxyl group and meta-position of
3-amino-group (Scheme 2). Treatment of the intermedi-
ate 8 with acetic anhydride and CH3COOH afforded
amide 9 for protecting the free amine. The phthalic acid
derivative 10 was oxidized with potassium permanga-
nate. Reaction of 10 with CH3OH via a one pot esterifi-
cation and deprotection reaction gave dimethyl-4-
amino-5-nitrophthalate 11, which carried through hy-
drogenation reaction using H2 and Pd/C to give the
molecule 12. The installation of backbone bond on dyes
was disposed by template synthesis of the compound 12
with 5a, and then Schiff-base metal complex 13 was
formed, which underwent hydrolysis reaction with
Photoelectrochemistry and electrochemistry
The working performance of DSSCs was examined
via recording the photocurrent-voltage (J-V) curves with
a Keithley Series 2601A System Source Meter under
irradiating from a solar simulator (San-Ei XES-301S)
giving AM 1.5 G illumination. The light intensity was
100 mW/cm2 calibrated by a standard Si solar cell. The
measurement of the incident photon-to-current conver-
sion efficiency (IPCE) was carried out with a Crown
instrument. Cyclic voltammetry curves were obtained
by a CHI 660D electrochemical workstation with a tra-
ditional three-electrode system. The working electrode,
reference electrode, and auxiliary electrode were a
glassy carbon electrode, Ag/AgCl and Pt-wire electrode,
respectively.
Scheme 1 Synthetic route of the SL dyes
without Et3N
-
COO
R
O
OH
N
OH
N
O
N
R
CHO
CuI, t-BuOK
BBr3, CH2Cl2
0 oC
NH3+
(HCHO)n, Mg, I2, EtBr
Toluene, THF, Reflux
+
R
I
OH
1,10-Phenanthroline
Toluene, Reflux
2
COOH
N
N
NH2
1
R
R
R
R
R
R
3
5
4
+
-
COOH
COO Et3NH
H2N
NH2
Et3N, CH3OH
Zn(OAc)2 2H2O
THF, HCl
.
N
N
O
N
O
N
R
N
R
R
R
Zn
Zn
R
R
R
N
R
O
N
O
6
N
7
SLa-SLe
R =
C5H11
C8H17
b
a
c
d
e
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Chin. J. Chem. 2014, 32, 513—520

SalenZn-bridged D-π-A Dyes For Dye-Sensitized Solar Cells
Seheme 2 Synthetic route of absorption group & SLf dye
O
O
O2N
NH
O2N
MgSO4
O2N
H2N
Ac2O
OH
OH
KMnO4, H2O
CH3COOH
O
NH
MeOH
H2SO4
O
8
10
9
O
O
H2N
H2N
H2N
O2N
Pd/C, H2
MeOH
O
O
O
OH
O
CHO
11
12
O
O
HOOC
COOH
MeOOC
COOMe
N
NaOH
THF/H2O
5a
N
O
N
O
N
O
N
.
Zn(OAc)2 2H2O
CH3OH Reflux
Zn
Zn
N
N
N
O
N
14
SLf
NaOH to afford the desired dye SLf.
dyes.
The electrochemical properties of the dyes were
measured by cyclic voltammetry and presented in Table
1. The lowest unoccupied molecular orbital (LUMO) of
all dyes lies lower than that of N719 (−3.64 eV) and
Photophysical and electrochemical properties
The UV-vis absorption spectra of SL series dyes are
depicted in Figure 2a & Table 1. In THF solution, all of
dyes reveal two obvious bands between 350－600 nm,
which is fulfilling for solar light harvesting. The peak at
a wavelength region (300－350 nm) and with a high
molar absorption coefficient is ascribed to the triphe-
nylamine (TPA) and 3,5-diaminobenzoic acid (Anchor),
which have wavelength peaks at 297 nm and broad ab-
sorption from 270 to 400 nm respectively (see SI). The
other peak at 400－650 nm is a characteristic absorption
peak of SalenZn complex, which shows a significant
bathochromic shift (ca. 50 nm) due to the elongation of
the π-conjugation compared with SalenZn complex
without diphenylamine as donor (CON). Upon adsorp-
tion onto TiO2 (Figure 2b), most of dyes show different
levels of hypsochromic effect compared with that meas-
ured in solution, and display wavelength peak at ca. 420
nm, which exihibts bathochromic shift compared to the
absorption of nanocrystalline TiO2 (380 nm). SLf based
on TiO2, of which the peak has a little change, gives a
broader and higher absorption in comparison with other
Table 1 Absorption and electrochemical properties of SL series
dyes
λmaxa/nm
[ε/(104 L•mol−1•cm−1)]
b
Dye
Eox/Ered
Eg HOMOd LUMOe
c
SLa 303 (8), 470 (1.3)
SLb 305 (7.7), 473 (1.4)
SLc 303 (12), 470 (1.7)
SLd 303 (14), 452 (1.6)
SLe 292 (7.7), 470 (8.9)
0.67/−0.73 1.40 −4.91 −3.51
0.59/−0.72 1.33 −4.83 −3.50
0.66/−0.75 1.41 −4.90 −3.49
0.93/−0.82 1.75 −5.17 −3.15
0.62/−0.74 1.36 −4.86 −3.50
SLf 303 (11), 470 (1.8)
0.69/−0.83 1.52 −4.93 −3.41
a
b
Absorption spectra were measured in THF solutions. The oxi-
dation and reduction potentials of dyes were measured in THF
with 0.1 mol/L tetrabutylammonium hexafluorophosphate (TBA-
PF6) with a scan rate of 50 mV•s−1. Band gaps derived from the
c
difference between onset potentials of oxidation and reduction.
Fe/Fe＋
d HOMO＝−(Eox＋4.8−Eox
). e LUMO＝HOMO＋Eg.
Figure 2 UV-vis absorption spectra of SL series dyes in THF (a) and on TiO2 film (b).
Chin. J. Chem. 2014, 32, 513—520
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517

Jia et al.
FULL PAPER
above the conduction band minimum (CB) of TiO2
(−4.26 eV). The highest occupied molecular orbital
(HOMO) of the dyes is much higher than the valence
band maximum (VB) of TiO2 (−7.46 eV).
by SLf was higher than that of SLa and lower than that
of SLe in the range of visible light.
Photovoltaic performance of DSSCs
DSSCs with an effective area of 0.159 cm2 were fab-
ricated combined with a redox electrolyte composed of
0.6 mol/L 1-butyl-3-methylimidazolium iodide (BMII),
0.05 mol/L LiI, 0.03 mol/L I2, 0.5 mol/L 4-tert-
butylpyridine, and 0.1 mol/L guanidinium thiocyanate in
a mixture of acetonitrile and valeronitrile (85/15, V/V).
Photovoltaic characteristics of the DSSCs based on the
six dyes as well as the standard N719 dye under stan-
dard global AM1.5G solar irradiation condition (100
mW/cm2) are exhibited in Table 2.
Table 2 Photovoltaic performance of DSSCs based on SL dyes
Figure 4 J-V curves of DSSCs sensitized with SL series dyes.
Sensitizer
SLa
Vocb/V
0.55
0.52
0.58
0.60
0.56
0.62
0.75
Jsc/(mA•cm−2)
FF/% η/%
The short circuit photocurrent density and overall
conversion efficiency of DSSCs sensitized by the five
dyes were in the following order: SLe＞SLd＞SLc＞
SLa＞SLb. These phenomena were attributed to their
abilities of prohibition for the back current from TiO2 to
the HOMO of dye molecule. When increasing the length
of the alkyl substituents attaching on the donor part, the
back current of the DSSC device reduced. Strangely, the
device sensitized by SLf exhibited the highest JSC value
(0.81%) under the lower IPCE in the range of visible
light. This phenomenum was attributed to its good elec-
tron injection from LUMO of exited dye molecules to
the VB of TiO2 via tetradentates anchoring group (Fig-
ure 5) and consistent with our recent results of dia-
zoporphyrinZn dyes.[5c]
0.91
56.5
67.5
61.5
62.4
70.4
70.5
64.1
0.28
0.19
0.39
0.50
0.60
0.81
6.35
SLb
0.55
SLc
1.08
SLd
1.32
SLe
1.52
SLf
1.85
N719
13.2
The incident photon to current conversion efficiency
(IPCE) spectra of the devices based on each dye, which
is directly related to the short-circuit current, reveals one
obvious band in the spectral region ranging from 400 to
800 nm. Figure 3 indicated the IPCE spectra of SLa,
SLe and SLf, among which SLa showed the worst result.
When the substituted branch R group increased to C-8,
the IPCE of cell sensitized by SLe enhanced almost
double in all range of solar spectrum implying the less
back current inhibited by the C-8 branches under the
similar absorption spectra/light harvesting. And the al-
kyl groups regarded as spatial hindrance chains on the
sp3-carbon bridge of the core could mitigate undesirable
dye aggregation. In contrast, the IPCE of cell sensitized
O
O
Zn
N
N
e-
O
e-
O
O
O
Figure 5 The sketch map of binding modes.
DFT calculations and energy levels
Geometries and energies of the stationary points
found herein were fully optimized by hybrid density
functional theory (DFT) utilizing the GAUSSIAN 09
program suite.[15] According to our calculations, the hy-
brid gradient corrected exchange functional of B3LYP
was used.[16] The standardized 6-31G basis set was used
together with the polarization (d) functions basis set[17]
for all atoms except Zn that was depicted by the effec-
tive core potential LANL2DZ basis set.[18] The MO pat-
terns of HOMO and LUMO of zinc Salen dyes were
shown in Figure 6. Our DFT calculations demonstrated
that the electrons staying in the frontier molecular or-
Figure 3 IPCE of DSSCs sensitized with SL series dyes.
518
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Chin. J. Chem. 2014, 32, 513—520

SalenZn-bridged D-π-A Dyes For Dye-Sensitized Solar Cells
bital (HOMO) easily delocalized from the zinc Salen
skeleton to the framework of benzene and the anchoring
group of carboxylic acid (LUMO). Figure 7 displayed
that the energy level of conduction band of TiO2 was
located between that of the LUMO and the HOMO of
SalenZn dyes. These results confirmed that the electron
injection from the LUMO of dyes to the conduction
band of TiO2 was quite available.
Key Laboratory of Coal Conversion (No. J14-15-913-1).
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Conclusions
In summary, we synthesized a class of sensitizers for
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essential π-spacer in the D-π-A structure. Their physical
properties and device performances were tested and
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Acknowledgement
The authors acknowledge the financial support of
this work from the National Natural Science Foundation
of China (No. 21173106) and the Foundation of State
Chin. J. Chem. 2014, 32, 513—520
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