Photovoltaic properties of new cyanine-naphthalimide dyads synthesized by 'Click' chemistry

Article abstract of DOI:10.1016/j.tetlet.2007.02.034

Two novel cyanine dyads, in which a naphthalimide unit is attached to benzoindole ring of unsymmetric trimethine cyanine, have been synthesized via 'Click' reaction and characterized by ¹H NMR, ¹³C NMR, and MS-ESI. Under the illumina

Full text of DOI:10.1016/j.tetlet.2007.02.034

Tetrahedron Letters 48 (2007) 2461–2465
Photovoltaic properties of new cyanine–naphthalimide
dyads synthesized by ‘Click’ chemistry
Wen-hai Zhan, Wen-jun Wu, Jian-li Hua, Yin-hua Jing, Fan-shun Meng and He Tian*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China
University of Science and Technology, Shanghai 200 237, China
Received 16 January 2007; revised 7 February 2007; accepted 9 February 2007
Available online 13 February 2007
Abstract—Two novel cyanine dyads, in which a naphthalimide unit is attached to benzoindole ring of unsymmetric trimethine
cyanine, have been synthesized via ‘Click’ reaction and characterized by 1H NMR, 13C NMR, and MS-ESI. Under the illumination
of AM 1.5 (75 mW cmꢀ2), the power conversion efficiency of cyanine I reached 4.8% (Jsc = 14.5 mA cmꢀ2, Voc = 0.50, FF = 0.49).
The results show that the two cyanine dyes are promising sensitizers for nanocrystalline dye-sensitized solar cell.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Dye-sensitized solar cells (DSSC) have been actively
investigated since the report of highly efficient ruthe-
nium complex-sensitized TiO2 solar cells by Gra¨tzel
and co-workers. Up to now, the Ru(II) polypyridyl
complexes have reached highly efficient photoelectric
conversion efficiencies of up to 11%.1 However, pure
organic dyes exhibit not only higher extinction coeffi-
cient, but simple preparation and purification procedure
with a low cost. Recently, great progress has been made
in this field and the highest overall photoelectric conver-
sion efficiency of solar cells sensitized by organic dyes
has reached 9%, indicating the promising perspective
of metal-free organic dyes.2–5
unsymmetrical counterparts occurs towards the indole
ring which contains the carboxylic acid group used for
linking the molecule to the semiconductor surface.6 An
unidirectional flow of electron distribution can result
in favorable charge separation and electron injection
in DSSCs. According to the advantages outlined above,
the structure of unsymmetric cyanine dyes was chosen in
this Letter.
Recently, we have succeeded in the synthesis of a series
of hemicyanine derivatives, and we found that through
reasonable design, hemicyanine dyes could perform
excellent spectral sensitization.2c,d In order to enrich
the research field of cyanine dyes for dye-sensitized
and gain more information on the structure–property
relationships, we synthesized the two new cyanine dyes
containing naphthalimide via Cu(I) catalyzed 1,3-dipo-
lar cycloaddition (I–II, Scheme 1). The ‘Click’ reaction
Cyanine dyes have very high absorption extinction coef-
ficients (ꢁ105 Mꢀ1 cmꢀ1). Apparently, the sharp absorp-
tion peaks and shoulder absorption peaks in solution
disable their utility as sensitizers for solar cell, but
cyanine dyes adsorbed on the nanocrystalline TiO2 film
can form J- or H-aggregates, which contributed to hypo-
chromism and bathochromism, respectively. Relative
high incident photo-to-electron conversion efficiency
(IPCE) values are expected over a wide region of
wavelength, which will in turn enhance the conversion
efficiency of light to electricity. Among a variety of
cyanine dyes, structural unsymmetric ones exhibit more
outstanding performance. Compared with symmetric
cyanine dyes, the shift in electron density of their
N
N
O
N
O
N
N
COOH
N
N
I
I
N
N
NH
O
N
O
N
N
N
N
COOH
N
N
I
Keywords: Cyanine dyes; Dye sensitized solar cell; Naphthalimide.
II
*
Corresponding author. Tel.: +86 21 64252756; fax: +86 21
64252288; e-mail: tianhe@ecust.edu.cn
Scheme 1. Structures of the cyanine dyes (I and II).
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.02.034

2462
W. Zhan et al. / Tetrahedron Letters 48 (2007) 2461–2465
of azides and acetylenes was widely used in organic syn-
thesis because of its mild reaction conditions, high yield
and simple purification steps.7 To our knowledge, no
organic solar cell sensitizer was reported to be synthe-
sized by ‘Click’ reaction. The introduction of naphthal-
imide moiety whose feature absorption peak is around
410 nm aimed to broaden the absorption field of cyanine
dyes. Triphenylamine moiety on II was expected to
greatly locate the cationic charge from the TiO2 surface
and efficiently restrict recombination between conduc-
tion band electron and oxidized sensitizer.8
1.0
0.8
0.6
0.4
0.2
0.0
I
II
The brief synthetic route to these cyanine dyes (I and II)
is depicted in Scheme 2. Two butyl chains on indole
nucleus can improve the solubility and forꢀm a tightly
packed insulating monolayer blocking the I3 or cations
approaching the TiO2. Varied reaction systems (CuI,
di-i-propylethylamine (DIPEA), CHCl3; CuI, DIPEA,
THF; CuI, DIPEA, DMSO; CuI, DIPEA, DMF; CuBr,
DIPEA, DMF; Cu(PPh3)Br, DIPEA, DMF) have been
explored to pursue the suitable ‘Click’ approach for cya-
nine dyes. Then a standard ‘Click’ protocol has been
established using a catalystic system including CuI as
the copper source, DIPEA as the ligand, including
DMF as solvent. Under the similar conditions, naph-
thalimide moiety was introduced through 1,2,3-triazole
ring via ‘Click’ reaction between alkyl-end aldehyde
and azido derivatives.9 Then the target products were
prepared via Knoevenagel condensation. All the inter-
mediates and cyanine dyes (I and II) were characterized
by standard spectroscopic methods (see Supplementary
data for details).
300
400
500
600
700
Wavelength/nm
Figure 1. Absorption spectra of I and II in CH2Cl2/C2H5OH = 1/1
(10ꢀ5 M).
to the feature absorption band of I at 420 nm; the exis-
tence of triphenylamine–naphthalimide dyad resulted in
the wide absorption band at 350 nm. The extinction co-
efficient of I and II were 0.75 and 1.17 · 105 Mꢀ1 cmꢀ1
,
respectively, which is very high in comparison to
0.139 · 105 Mꢀ1 cmꢀ1 at 541 nm for cis-di(thio-
cyanato)bis(2,20-bipyridyl)-4,40-dicarboxylate)
ruthe-
nium(II) (N3).3b Figure 2 gives the absorption spectra of
2.0
I
Different from the present synthetic route, the original
steps was to finish the synthesis of cyanine dyes firstly,
then clicked with naphthalimide moiety. But the separa-
tion of final dyads from pristine cyanine dyes was unex-
pectedly frustrating due to their similar polarity and
strong adsorption on gel column. The updated plan
avoided this difficulty, spared the commonly used costly
purification by Prep. HPLC and proclaimed the bright
commercial future of these two products.
II
1.5
1.0
0.5
0.0
The absorption spectra of I and II were recorded in a
dilute solution of dichloromethane/ethanol = 1/1 as
shown in Figure 1. Cyanine dyes I and II showed maxi-
mum absorption at 588 and 589 nm with vibrational
shoulder peaks at 555 and 562 nm because of their simi-
lar conjugation systems. Naphthalimide unit attributed
400
500
600
Wavelength/nm
700
Figure 2. Absorption spectra of I and II on TiO2 electrode after
adsorption for 12 h.
N
N
N
N
RN
R N
6
8
R-N3
O
COOH
Ac2O
CuI, DIPEA,
DMF
N
N
N
N
I
16, 17
I, II
13, 15
N
NH
O
N
O
O
N
O
N
13, 16, II R=
15, 17, I R=
N
Scheme 2. Synthetic route of cyanine dyes I and II.

W. Zhan et al. / Tetrahedron Letters 48 (2007) 2461–2465
2463
I and II on TiO2 films after 12 h adsorption. Upon
60
50
40
30
20
10
0
adsorption on TiO2 film the spectra were broadened
compared with those in solution (cf. Fig. 1). It was
found that the spectra were broadened on both sides
(Fig. 2), that is, blue shifted and red shifted due to the
correspondent H- and J-aggregates formed on TiO2 film.
I
II
The amount of I and II adsorbed on a TiO2 electrode
per unit area was tested by spectroscopic measurement
of the concentration change of the dye solution before
and after adsorption. As shown in Table 1, dye I
(2.33 · 10ꢀ7 mol cmꢀ2) has higher value than dye II
(1.76 · 10ꢀ7 mol cmꢀ2). This may be explained by the
size of the molecule. Large molecule occupies more
space on the TiO2 nanoparticle surface and the larger
steric hindrance is unfavorable for the dye molecule
diffusion into the smaller pores of TiO2 film, so the
electrode can adsorb more dyes with smaller size, which
resulted in a wider region and higher intensity of
absorption spectra (Fig. 2).
400
500
600
700
800
Wavelength/nm
Figure 3. Photocurrent action spectra of the TiO2 electrodes sensitized
by I and II.
Two dyes display a strong fluorescence emission at room
temperature. Interestingly, the fluorescence emission
bands of the dyes in the visible region disappear when
the dyes are adsorbed onto the nanocrystalline TiO2
electrodes. The phenomena can be explained by the effi-
cient electron injection from the excited singlet state of
the dyes to the conduction band of TiO2, and suggesting
that products are good sensitizers for nanocrystalline
TiO2 electrodes.
tra of I indicated that I sensitized TiO2 electrode would
generate a higher conversion yield.
Under irradiation of AM 1.5 (75 mW cmꢀ2) or a Xe
lamp (20 mW cmꢀ2), the photo-electrochemical proper-
ties of I and II sensitized TiO2 electrodes are listed in
Table 2, while the corresponding photocurrent–voltage
curves are shown in Figure 4. The dye-coated TiO2 film,
as working electrode, was placed on top of an FTO glass
as a counter electrode, on which Pt was sputtered. The
redox electrolyte was introduced into the inter-electrode
space by capillary force. Because the amount of I
adsorbed on the electrode is much more than that of
II, the short-circuit photocurrent, open-circuit photo
voltage, and power conversion efficiency of I were also
higher than II sensitized solar cell. Unlike the stable
performance of silicon solar cell, DSSC is sensitive to
variable illumination. When the irradiation intensified,
the transport of I3ꢀ=Iꢀ to and from the counter elec-
trode is not fast enough to fully regenerate the oxidized
dye.10 The diffusion kinetics in the electrolyte become
To judge the possibility of electron transfer from the
excited dye to the conduction band of TiO2, the highest
occupied molecular orbital (HOMO) levels of ꢀ5.93 eV
for I, ꢀ5.94 eV for II were measured by cyclic voltamme-
try (CV) in DMF using Ag/AgCl as a reference electrode.
With reference to the absorption maxima, corresponding
to the gap between the HOMO and LUMO levels for I
and II, the energy levels of the LUMO can be estimated
to be ꢀ4.01 and ꢀ4.00 eV, respectively. Obviously, the
excited-state energy levels for the products are higher
than the bottom of the conduction band of TiO2
(ꢀ4.4 eV),2c clarifying that the electron injection should
be possible thermodynamically.
Figure 3 shows the action spectra of nanocrystalline
TiO2 solar cells sensitized with these cyanine dyes. The
dye-sensitized solar cells efficiently convert visible light
to photocurrent efficiently in the range from 400 to
680 nm, which corresponds well with the absorption of
the electrode. The IPCE of I reached maximum (60%)
at 540 nm; however, the highest IPCE value of II
(44%) was obtained at 600 nm. The broader action spec-
Table 2. Parameters of dye-sensitized solar cells
Dye (light
intensity/mW cmꢀ2
Jsc (mA cmꢀ2
)
Voc (V) FF
g (%)
)
I (75)
I (20)
II (75)
II (20)
14.5
4.34
9.7
0.50
0.54
0.51
0.56
0.49 4.8
0.49 5.8
0.52 3.5
0.53 5.0
3.33
Table 1. Optical properties and redox potentials of I and II
Dye
k
(nm)
eb (Mꢀ1 cmꢀ1
)
Adsorptionc (mol cmꢀ2
)
HOMOd (eV)
LUMOd (eV)
ab
max
a
I
588
589
0.75 · 105
2.33 · 10ꢀ7
1.76 · 10ꢀ7
ꢀ5.93
ꢀ5.94
ꢀ4.01
ꢀ4.00
II
1.17 · 105
a Absorption maximum in CH2Cl2/C2H5OH = 1/1 (10ꢀ5 M).
b Molar extinction coefficient at k
.
ab
max
c Adsorption amount per unit area of TiO2 film.
d Redox potential. Scan rate: 20 mV sꢀ1, electrolyte: tetrabutyl ammonium perchloride.

2464
W. Zhan et al. / Tetrahedron Letters 48 (2007) 2461–2465
Supplementary data
a 16
I
14
12
10
8
Supplementary data (experimental details, synthetic and
spectroscopic data) associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.02.034.
75 mW cm-2
References and notes
6
1. Gra¨tzel, M. J. Photochem. Photobiol. A 2004, 164,
3–14.
4
20 mW cm-2
2. (a) Yao, Q. H.; Shan, L.; Li, F. Y.; Yin, D. D.; Huang, C.
H. New J. Chem. 2003, 27, 1277–1283; (b) Chen, Y.; Li,
C.; Zeng, Z.; Wang, W.; Wang, X.; Zhang, B. J. Mater.
Chem. 2005, 15, 1654–1661; (c) Yao, Q. H.; Meng, F. S.;
Li, F. L.; Tian, H.; Huang, C. H. J. Mater. Chem. 2003,
13, 1048–1053; (d) Meng, F. S.; Yao, Q. H.; Shen, J. G.;
Li, F. L.; Huang, C. H.; Chen, K. C.; Tian, H. Synthetic
Met. 2003, 137, 1543–1544; (e) Chen, Y.; Zeng, Z.; Li, C.;
Wang, W.; Wang, X.; Zhang, B. New J. Chem. 2005, 29,
773–776.
3. (a) Schmidt-Mende, L.; Bach, U.; Humphry-Baker, R.;
Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Gra¨tzel, M.
Adv. Mater. 2005, 17, 813–815; (b) Horiuchi, T.; Miura,
H.; Uchidab, S. Chem. Commun. 2003, 3036–3037; (c)
Horiuchi, T.; Miura, H.; Uchida, S. J. Photochem.
Photobiol. A 2004, 164, 29–32; (d) Horiuchi, T.; Miura,
H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126,
12218–12219; (e) Ito, S.; Zakeeruddin, S. M.; Humphry-
Baker, R.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin,
2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
V/V
b
10
8
II
75 mWcm-2
6
4
20 mWcm-2
´
M. K.; Pechy, P.; Takata, M.; Miura, H.; Uchida, S.;
2
Gra¨tzel, M. Adv. Mater. 2006, 18, 1202–1205.
4. (a) Hara, K.; Sayama, K.; Ohga, Y.; Shinpo, A.; Suga, S.;
Arakawa, H. Chem. Commun. 2001, 6, 569–570; (b) Hara,
K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.;
Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J.
Phys. Chem. B 2003, 107, 597–606; (c) Hara, K.; Tachi-
bana, Y.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.;
Sugihara, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells
2003, 77, 89–103; (d) Hara, K.; Dan-oh, Y.; Kasada, C.;
Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H.
Langmuir 2004, 20, 4205–4210; (e) Hara, K.; Wang, Z.;
Sato, T.; Furube, A.; Katoh, R.; Sugihara, H.;
Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S. J. Phys.
Chem. B 2005, 109, 15476–15482; (f) Wang, Z.; Hara, K.;
Dan-oh, Y.; Kasada, C.; Shinpo, A.; Suga, S.; Arakawa,
H.; Sugihara, H. J. Phys. Chem. B 2005, 109, 3907–
3914.
5. (a) Hara, K.; Kurashge, M.; Ito, S.; Shinpo, A.; Suga, S.;
Sayama, K.; Arakawa, H. Chem. Commun. 2003, 252–253;
(b) Kitamura, T.; Ikeda, M.; Shigaki, K.; Inoue, T.;
Anderson, N. A.; Ai, X.; Lian, T.; Yanagida, S. Chem.
Mater. 2004, 16, 1806–1812; (c) Hara, K.; Sato, T.; Katoh,
R.; Furube, A.; Yoshihara, T.; Murai, M.; Kurashige, M.;
Ito, S.; Shinpo, A.; Suga, S.; Arakawa, H. Adv. Funct.
Mater. 2005, 15, 246–252; (d) Hagberg, D. P.; Edvinsson,
T.; Marinado, T.; Boschloo, G.; Hagfeldt, A.; Sun, L.
Chem. Commun. 2006, 2245–2247.
0.0
0.1
0.2
0.3
0.4
0.5
V/V
Figure 4. Photocurrent–voltage curve of I (a) and II (b) sensitized TiO2
electrodes under different light intensities.
the limiting step in the current production, which
has a dramatic influence on the efficiency of I and II,
which decreases from 5.8% to 4.8%, 5.0% to 3.5%,
respectively.
In conclusion, two new cyanine dyes with carboxylic
acid group were synthesized via ‘Click’ reaction and
characterized by 1H NMR, 13C NMR and MS-ESI.
Through investigation of their absorption and photo-
electrochemical properties, I is found to be a better sen-
sitizer, which has a conversion yield of 4.8%, with a
short-circuit photocurrent of 14.5 mA cmꢀ2, an open-
circuit voltage of 0.5 V and a fill factor of 0.49 under
illumination of AM 1.5 (75 mA cmꢀ2). The optimal
measurement of their photovoltaic properties is in
progress.
6. Alex, S.; Santhosh, U.; Das, S. J. Photochem. Photobiol. A
2005, 172, 63–71.
7. (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew.
Chem., Int. Ed. 2001, 40, 2004–2021; (b) Rostovtsev, V. V.;
Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew.
Chem., Int. Ed. 2002, 41, 2596–2599; (c) Tornøe, C. W.;
Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67, 3057–
3064; (d) Kolb, H. C.; Sharpless, K. B. Drug Discovery
Today 2003, 8, 1128–1137; (e) Hang, H. C.; Yu, C.; Kato,
Acknowledgements
This work was supported by NSFC (60508011)/China
and Scientific Committee of Shanghai. The authors
thank Professor Fuyou Li at Fudan University for the
solar cells’ tests.

W. Zhan et al. / Tetrahedron Letters 48 (2007) 2461–2465
2465
D. L. Proc. Natl. Acad. Sci. 2003, 100, 14846–14851; (f)
Helms, B.; Mynar, J. L.; Hawker, C. J.; Frechet, J. M. J. J.
Am. Chem. Soc. 2004, 126, 15020–15021; (g) Englert, C.;
Bakbak, S.; Bunz, U. H. F. Macromolecules 2005, 38,
5868–5877.
Eur. J. 2004, 10, 595–602; (b) Robertson, N. Angew.
Chem., Int. Ed. 2006, 45, 2338–2345.
9. Zhan, W.; Barnhill, H. N.; Sivakumar, K.; Tian, H.;
Wang, Q. Tetrahedron Lett. 2005, 46, 1691–1695.
`
10. Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.;
8. (a) Hirata, N.; Lagref, J.; Palomares, E. J.; Durrant, J. R.;
Nazeeruddin, M. K.; Gra¨tzel, M.; Censo, D. D. Chem.
Lenzmann, F.; Shklover, V.; Gra¨tzel, M. J. Am. Ceram.
Soc. 1997, 80, 3157–3171.