Synthesis of novel multi-chromophoric soluble perylene derivatives and their photosensitizing properties with wide spectral response for SnO2 nanoporous electrode

Article abstract of DOI:10.1039/b004063k

Two series of new multi-chromophoric perylene-3,4:9,10-tetracarboxylic dianhydride dyes for solar cell sensitizers have been synthesized. One series consists of oxadiazole or naphthaldicarboximide that are linked with perylene units as N,N'-substituted chromophores. The other series has four substituents in the bay-region of the perylene core in addition to N,N'-substituents. These novel routes to highly efficient sensitizers for Gratzel-type solar cells avoid the complication of doping. A wide spectral photoresponse (from 310 to 700 nm) was observed with maximum IPCE (incident photon-to-current efficiency) of 24%. In addition, the sensitization is dependent on the energy match of the energy bands of the semiconductor and the redox potential of the segments in such assembled dyes.

Full text of DOI:10.1039/b004063k

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J O U R N A L O F
Synthesis of novel multi-chromophoric soluble perylene derivatives
and their photosensitizing properties with wide spectral response for
SnO2 nanoporous electrode
He Tian,*a Pei-Hua Liu,a Weihong Zhu,a Enqin Gao,b Da-Jun Wua and Sengmin Caib
C H E M I S T R Y
aInstitute of Fine Chemicals, East China University of Science & Technology, Shanghai
200237, P. R. China. Fax: z86-21-64248311; E-mail: hetian@ecust.edu.cn
bCollege of Chemistry, Peking University, Beijing 100871, P. R. China
Received 22nd May 2000, Accepted 4th September 2000
First published as an Advance Article on the web 26th October 2000
Two series of new multi-chromophoric perylene-3,4:9,10-tetracarboxylic dianhydride dyes for solar cell
sensitizers have been synthesized. One series consists of oxadiazole or naphthaldicarboximide that are linked
with perylene units as N,N'-substituted chromophores. The other series has four substituents in the bay-region
of the perylene core in addition to N,N'-substituents. These novel routes to highly ef®cient sensitizers for
GraÈtzel-type solar cells avoid the complication of doping. A wide spectral photoresponse (from 310 to 700 nm)
was observed with maximum IPCE (incident photon-to-current ef®ciency) of 24%. In addition, the sensitization
is dependent on the energy match of the energy bands of the semiconductor and the redox potential of the
segments in such assembled dyes.
appeared to lower the decomposition temperature. This is due
to the initial decomposition of the alkyl substituent.9b
Introduction
Ruthenium(II) complexes [Ru(L)2(NCS)2]-sensitized nanocrys-
talline TiO2 solar cells (GraÈtzel-type) with light-to-electrical
power conversion ef®ciency as high as 10% at AM 1.5 (arti®cial
sun light spectrum) have been reported,1±3 in which the
photoanode was prepared by sintering nanocrystalline TiO2 on
a conducting glass support. However, compared to other
chromophores, the ruthenium(II) complexes have quite poor
photophysics, relatively low extinction coef®cients for visible
light (y14000 M21 cm21), and luminescence very weakly from
a triplet state with a quantum yield of y0.40% (125 K).2
Furthermore, the stability of complexes such as
[Ru(L)2(NCS)2] over long-term operation in a liquid cell is
unknown, which slows down the full realization of the
applications of solar cells. In addition to TiO2 used in
GraÈtzel-type solar cells, several similar transition metal
oxides (e.g. SnO2)4 with wide band gaps have also been well
studied. The investigation and development of other dye-
semiconductor systems are essential to furthering fundamental
understanding of the electron injection processes, and explora-
tion of specialized and long-term applications of dye-sensitized
solar cell. Dyes based on perylene-3,4:9,10-tetracarboxydii-
mides (PTCAs) have received much attention4 as sensitizers
with their outstanding chemical, thermal and photochemical
stability. Perylene derivatives are highly absorbing in the visible
The bare TiO2 or SnO2 ®lms are transparent and colorless,
displaying the fundamental absorption in the UV region, so a
solar cell with high ef®ciency needs ef®cient sensitization by a
dye. The nanoporous surface of the transparent electrode is
increased so that a large number of dye molecules can be
adsorbed directly on the electrode surface and simultaneously
be in direct contact with the redox electrolyte, which results in
an ef®cient separation and transport of the photogenerated
charge carriers. If the roughness factor of an electrode is
suf®ciently large, even a monolayer of dye could absorb most
of the incident photons.1 Consequently the study of a
microporous electrode with a very high speci®c surface area
and high effective sensitizers is of great interest. However, in
contrast with the massive work focused on the properties of the
ruthenium(II) complexes, less effort has been made so far to
explore new types of organic dye sensitizers. In a new
sensitizing dye±semiconductor system comprised of perylene
derivatives on SnO2, perylenes containing carboxylic acid
groups adsorbed to and ef®ciently photosensitized colloidal
SnO2 ®lms.4 It is crucially important for solar cell sensitizers to
control the valence band since most inorganic semiconductors
such as ZnO, TiO2 and SnO2 exhibit n-type character.2,4 The
excited states of sensitizers must have a suf®ciently long lifetime
in order to undergo the photochemical reaction. The
Yokoyama group found a drastic increase in the photocurrent
density and the power conversion ef®ciency by doping H2 due
to a shift of the Fermi level.10 A simple approach is to utilize
two or more different dyes by a doping method, which shows
strong absorption at a different wavelength and compensatory
absorbency.11,12 However, multilayer adsorption does not
often help, as the inner layer tends to act as an insulator
with respect to the outer ones.
to NIR (ey105 M21 cm21
) and emit ¯uorescence with
quantum yields near unity.5 On the other hand, perylene
derivatives can also be used as near-infrared (NIR) absorbers
or luminescent materials, and nowadays the highly photo-
luminescent and also environmentally stable NIR materials are
strongly required. Beside their conventional uses, perylenes are
key chromophores for high-tech applications, from electronic
to biological ®elds, such as optical switches,6 lasers7 and DNA/
RNA probes8 etc. Many types of PTCAs have been
synthesized, but the expected low solubility of these molecules
is a problem for synthesis and puri®cation. Therefore work has
shifted towards more soluble perylenes.5 The solubility of the
perylene dyes may increase with attached bulky aliphatic
groups like tert-butylaryl or long-chain secondary alkyl
groups9 (swallow-tail substituents). However, the longer
alkyl and branched alkyl chain in the N-alkyl compounds
Another possible approach is to incorporate known
chromophoric units into a molecule. Such multi-chromophoric
dyes should have a wide absorptive region, whose photocurrent
could cover the whole visible region. The alternatives leave
much scope for a chemist. In this way, it might be possible to
avoid the complication of doping and to ®nd a novel route to
design appropriate solar cell sensitizers. Scandola et al.13 used
an appropriate antenna-sensitizer assembly to overcome
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This journal is # The Royal Society of Chemistry 2000
DOI: 10.1039/b004063k

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Fig. 1 The molecular structures of multi-chromophoric perylenedicarboximide derivatives.
problems of light harvesting ef®ciency in the spectral
sensitization of wide-band gap semiconductors. As electrical
and optical function rely on extended conjugated systems, two
ways to enlarge the p system of perylene dyes can be
envisaged:5b an arylimidazole introduction and substituents
in the bay-region of the perylene core. The ®rst route generally
yields dyes with fairly poor solubility, and unfortunately the
UV/Vis spectrum of these dyes depends weakly on the
substituents.5c Accordingly we designed and synthesized
several novel soluble multi-chromophoric dyes (perylenetetra-
carboxydiimide derivatives) as shown in Fig. 1. One series
consists of oxadiazole or naphthaldicarboximide units that are
linked with perylene units as N,N'-substituted chromophores.
The other series has four substituents in the bay-region of the
perylene core besides N,N'-substituents. They have long-
wavelength absorption, surprisingly good solubility in organic
solvents and high thermal stability. These novel routes to
design highly ef®cient sensitizers for GraÈtzel-type solar cell can
avoid the complication of doping. Due to the high photo-
stability, broad visible-light absorption and high extinction
coef®ciency (ey105 M21 cm21, 450y600 nm in solution) of
perylenedicarboximide derivatives, they are potentially inter-
esting compounds for solar cells. The improved solubility of
perylenedicarboximide in common organic solvents provided
possible adsorption onto colloidal SnO2 ®lms. Naphthal-1,8-
dicarboximide can absorb blue, green and yellow colors by
adjusting the substituted group at its 4-position. The LUMO
levels14 of the oxadiazole unit and perylene unit are higher than
that of the conduction bond (CB) of nanoporous TiO2 or SnO2
Fig. 2 Schematic representation of the band levels of SnO2 and
sensitizer OXZ-PER.
J. Mater. Chem., 2000, 10, 2708±2715
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reaction was determined by thin layer chromatography (TLC).
The reaction mixture was evaporated and poured into 300 ml
of water. The resulting precipitate was ®ltered and crystallized
from ethanol to give 2.2 g (II) in 67% yield, mp: 169±172 ³C. MS
(EI~70 eV) m/z(%): 445(12.8) [Mz], 156(23.29). C25H23N3O5:
found C 67.40, H 5.14, N 9.40; calcd C 67.42, H 5.17, N 9.44%.
Synthesis of N-[6-(2-
aminophenyl)carbonylaminohexyl]naphthal-1,8-dicarboximide
(III)
A mixture of II (1.6 g, 3.6 mmol), reduction iron powder (5.6 g,
100 mmol), absolute ethanol (150 ml) and water (60 ml) was
agitated and heated to 75 ³C,, then 4 ml hydrochloric acid (conc.
HCl±H2O~1 : 3 (v/v)) was added dropwise for 2 h. TLC
determined the end of the reaction . The resulting mixture was
adjusted to pH~7 by anhydrous sodium carbonate and sodium
sul®de and heat ®ltered. The hot ®ltrate was cooled, ®ltered and
dried under vacuum to obtain 1.2 g (III) in 80% yield, mp 167±
169 ³C. 1H-NMR (in d6-DMSO): d 1.15±2.00(8H, m), 3.19(2H,
t), 4.05(2H, t), 6.49(1H), 6.67(1H), 7.14(1H), 7.44(1H), 7.83(2H),
8.48(4H). MS (EI~70 eV) m/z(%): 416(32.01) [Mz], 121(100).
Synthesis of N,N'-bis[2-[[6-(N''-naphthal-1,8-
dicarboximidyl)hexyl]carbonylamino]phenyl]perylene-3,4:9,10-
tetracarboxydiimide (PCTA2)
A
mixture of N-[6-(2-aminophenyl)carbonylaminohexyl]-
naphthal-1,8-dicarboximide (1.2 g, 2.9 mmol), perylene-
3,4:9,10-tetracarboxylic dianhydride (0.3 g, 0.76 mmol), zinc
acetate (0.5 g) and redistilled quinoline (25 ml) was stirred and
kept at 200±215 ³C, for 7 h under the protection of argon gas.
Absolute ethanol (30 ml) was poured into the resulting mixture
when cooled at 70 ³C, and stirring was continued for 0.5 h, and
then ®ltered to obtain a red solid. The solid was puri®ed by
re¯uxing with dimethylformamide and absolute ethanol, to
give 0.5 g (PCTA2) in 55% yield, mpw300 ³C. IR (KBr):
y~3370, 3060, 2920, 2870, 1700, 1660, 1600, 1590, 1580, 1360,
Scheme 1 Synthetic scheme of PCTA colorants.
(shown in Fig. 2). We expect that these novel multi-chromo-
phoric sensitizers can improve the electrochemical and
photostability of the GraÈtzel solar cells under high photo-
current density, which was the key problem limiting the
practical application.
1350, 810, 760, 750 cm21 1H-NMR (in CF3CO2D): d 1.1±
.
2.3(16H, m), 3.67(4H, t), 4.16(4H, t), 7.7±8.3(12H), 8.3±
9.7(16H, m).
Other PCTA compounds were synthesized by a similar
procedure. PCTA1: IR(KBr): y~ 3410, 3390, 3060, 2950,
Experimental
1H-NMR spectroscopy: Bruker 500 MHz (relative to TMS).
Mass spectra were obtained with HP5989A, Mariner API time-
of ¯ight (TOF, TIS ion source, PE Corp.) and API2000 (TIS,
PE Corp.) spectrometers. Infrared spectra were measured on a
Shimidzu IR-408. UV-vis were recorded on a Shimidzu UV-
260. Fluorescence spectroscopy were recorded on a HITACHI-
850. Quinoline and 1-methyl-2-pyrrolidone (NMP) were
1700, 1660, 1590, 1550, 1530, 1350, 810, 780, 750 cm21 1H-
.
NMR (in CF3CO2D):
d
3.9(4H, t), 4.63(4H, t),
7.51y9.15(28H, m). PCTA3: 3390, 3070, 2950, 1700, 1660,
1
1590, 1570, 1550, 1350, 810, 780, 760, 750 cm21. H-NMR (in
CF3CO2D): d 3.9(12H, s), 4.08(4H, t), 4.78(4H, t), 7.71(2H, d),
7.91(2H, d), 8.20(4H, m), 8.32(2H, d), 8.43(2H, m),
8.85y9.08(14H, m). OXZ-PER: IR(KBr): y~ 3380, 3040,
2920, 1690, 1660, 1610, 1590, 1500, 1340, 805, 780, 750 cm21
.
Ê
distilled over calcium hydride and molecular sieves (4 A)
respectively, under reduced pressure. Methylene chloride was
distilled over P2O5, and diphenylmethanine was used as an
indicator. See Scheme 1 for representative syntheses.
The synthesis of the T-PTCA series starts with tetrachloro-
perylene tetracarboxylic dianhydride T-PTCA1. Compounds
T-PTCA2 and T-PTCA4 were easily obtained according to the
literature procedure.15 Condensation of T-PTCA2 with the
dyes T-PTCA7, T-PTCA8 occurs cleanly in hot quinoline with
zinc acetate as a catalyst, and both the C-shaped isomers and S-
shaped isomers were obtained as deep red compounds. The
clefts were already highly preorganized, not only by restricted
rotation about the C(aryl)±N(imide) bond but also by ®xing of
the C(acid) or O(-OH)±C(aryl) bond by an internal hydrogen
bond, and they could be identi®ed by the higher polarity of the
C-isomers as evidenced by a lower Rf on silica gel.16 We have
synthesized the compound R1~CH2CH2OH, but its solubility
was not satisfactory, and when R1~H, the solubility was even
worse. Demethylation of T-PTCA5 was achieved in dry
methylene chloride in the presence of BBr3. The best reaction
occurred at room temperature for 12 hours and then re¯ux for a
further 2 h. 1,6,7,12-Tetrakis(4-methylphenoxy)perylene-3,4:9,
Synthesis of N-(6-aminohexyl)naphthal-1,8-dicarboximide (I)
A mixture of naphthalic 1,8-anhydride (6.0 g, 30 mmol),
hexane-1,6-diamine (69.8 g, 600 mmol) and absolute ethanol
(180 ml) was stirred and heated at 80 ³C, for 1.5 h, then ®ltered.
The ®ltered solution was evaporated and poured into 2000 ml
of water. The precipitate was ®ltered and dried in a vacuum to
obtain 3.3 g (I) in 36.8% yield, mp: 78±80 ³C. C18H20N2O2:
found C 72.93, H 6.73, N 9.43; calcd C 72.97, H 6.76, N 9.46%.
Synthesis of N-[6-(2-nitrophenyl)carbonylaminohexyl]naphthal-
1,8-dicarboximide (II)
A mixture of I (3.0 g, 10 mmol), 2-nitrobenzoyl chloride (2.3 g,
12 mmol), absolute THF (90 mml) and re-distilled pyridine
(7 ml) was stirred at room temperature for 20 h. The end of the
1
10-tetracarboxylic dianhydride (T-PTCA2): H-NMR (CDCl3):
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d 8.12(s, 4H, per), 7.11(d, 8H, J~8.23 Hz, Ph), 6.85(d, 8H,
J~8.39 Hz, Ph), 2.35(s, 12H, -CH3). N,N'-Dipropyl-1,6,7,12-
tetrakis(4-methylphenoxy)perylene-3,4:9, 10-tetracarboxydii-
mide (T-PTCA4): 1H-NMR(CDCl3): d 8.13(s, 4H, per), 7.08(d,
8H, J~8.19 Hz, Ph), 6.84(d, 8H, J~8.43 Hz, Ph), 4.07(t, 4H, ±
CH2Nv), 2.33(s, 12H, ±CH3), 1.75(m, 4H, ±CH2±), 0.98(t, 6H,
±CH3).
OH), 2.31(s, 12H, ±CH3); C64H42N2O10 (998.6): found C 76.28,
H 4.11, N 2.60; calcd C 76.97, H 4.21, N 2.81%; Absorption
peaks(in acetone) lAbmax/nm(log e): 568.6(4.61), 529.6(4.40),
436.6(4.15); Fluorescence peaks lmaxem(in acetone, excited at
568.6 nm)~601.4 nm.
Synthesis of N,N'-dipropyl-1,6,7,12-tetrakis-
(4-methyloxyphenoxy)perylene-3,4:9,10-tetracarboxydiimide
(T-PTCA5)
Synthesis of N,N'-bis(2-carboxyphenyl)-1,6,7,12-tetrakis-
(4-methylphenoxy)perylene-3,4:9,10-tetracarboxydiimide
(T-PTCA7)
Diimide T-PTCA3 (0.5 g, 0.82 mmol), p-hydroxyanisole
(1.01 g, 8.2 mmol) and K2CO3 (1.13 g, 8.2 mmol) were added
to 12 ml NMP. The mixture was stirred under argon at 130 ³C,
for 8 h. After cooling to room temperature, the reaction
mixture was poured under stirring into 100 mL of a 10% (vol)
HCl solution in water. A precipitate appeared, which was
washed with water and dried under vacuum (P2O5). Puri®ca-
tion by chromatography (SiO2, methylene chloride : ace-
tone~30 : 1) and recrystallization from CHCl3±MeOH
afforded 0.56 g of the title compound as a purple red powder
with a yield of 71%. mpw300 ³C; IR(KBr): y~3435, 2960,
1696, 1657, 1588, 1502, 1439, 1353, 1290, 1252, 1204,
T-PTCA2 (0.3 g, 0.37 mmol), o-aminobenzoic acid (0.5 g,
3.7 mmol) and zinc acetate hydrate (0.02 g, 0.2 mmol) were
added to 10 ml of dry quinoline. The reaction mixture was
stirred at 190 ³C, over 8 h in an argon atmosphere. After
cooling to room temperature, the reaction mixture was poured
in 80 mL of a 10% HCl solution in water. A precipitate
appeared, which was washed with water and dried under
vacuum (P2O5). Column chromatography (silica gel, ®rst
methylene chloride : acetone~3 : 1, then methylene: etha-
nol~6 : 1) yielded two isomers, S (higher Rf, 117 mg, 30%)
and C (lower Rf, 113 mg, 29%), as deep red solids. C-isomer:
mpw300 ³C; IR(KBr): y~3447, 2921, 1706, 1671, 1590, 1502,
1
1036 cm21; H-NMR(CDCl3): d 8.10(s, 4H, per), 6.92(d, 8H,
J~9.04 Hz, Ph), 6.83(d, 8H, J~9.06 Hz, Ph), 4.07(t, 4H,
±CH2Nv), 1.69(m, 4H, ±CH2±), 0.95(t, 6H, ±CH3);
C58H46N2O12 (962.2): found C 71.08, H 4.87, N 2.88; calcd
C 72.36, H 4.78, N 2.91%; Absorption peaks(in acetone)
lAbmax/nm(log e): 575.6(4.40), 535.6(4.17), 447.4 (3.90); Fluor-
escence peaks lmaxem(in acetone, excited: 575.6 nm)~
607.4 nm.
1412, 1342, 1320, 1285, 1201 cm21 1H-NMR (500 MHz,
;
acetone-d6): d 8.08(dd, 2H, J~8.03, 1.47 Hz, Ph), 7.88 (s,
4H, per), 7.74(m, 2H, Ph), 7.59(m, 2H, Ph), 7.45(d, 2H,
J~7.86 Hz, Ph), 7.20 (d, 8H, J~8.05 Hz, Ph), 6.96(d, 8H,
J~8.14 Hz, Ph), 2.09(s, 12H, ±CH3); C66H42N2O12 (1054.7):
found C 74.99, H 3.64, N 3.21; calcd C 75.16, H 3.98, N 2.66%;
Absorption peaks (in acetone) lAbmax/nm(log e): 569.8(4.61),
531.6(4.40), 438.0(4.13); Fluorescence peaks lmaxem(in acetone,
excited at 569.8 nm) ~603.4 nm; S-isomer: mpw300 ³C,;
IR(KBr): y~3431, 2920, 1705, 1671, 1591, 1500, 1412, 1342,
Synthesis of N,N'-dipropyl-1,6,7,12-tetrakis-
(4-hydroxyphenoxy)perylene-3,4:9,10-tetracarboxydiimide
(T-PTCA6)
1320, 1285, 1201 cm21 1H-NMR(500 MHz, acetone-d6): d
;
T-PTCA5 (0.2 g, 0.21 mmol) was dissolved in 15 ml dry
methylene chloride and cooled in an acetone±dry ice bath at
280 ³C. Boron tribromide (0.08 mL) was added carefully to the
stirred solution. When the addition was complete, a calcium
chloride tube was ®tted to the top of the air condenser. The
reaction mixture was allowed to attain room temperature
overnight with stirring and then re¯uxed for a further 2 h. After
cooling to room temperature, the reaction mixture was
hydrolyzed by careful shaking with 10 ml of water, and then
extracted with ether. The organic phase was dried over
anhydrous magnesium sulfate. Puri®cation by chromatogra-
phy (SiO2, methylene chloride : acetone~2.5 : 1) afforded
56.2 mg of the title compound as a purple red powder with a
yield of 62%. Mpw300 ³C; IR(KBr): y~3380, 2922, 1691,
1653, 1585, 1502, 1438, 1365, 1298, 1263, 1200, 1095,
8.08(dd, 2H, J~7.91, 1.35 Hz, Ph), 7.85(s, 4H, per), 7.74(m,
2H, Ph), 7.59(m, 2H, Ph), 7.44(d, 2H, J~7.83 Hz, Ph), 7.20(d,
8H, J~8.46 Hz, Ph), 6.96(d, 8H, J~8.39 Hz, Ph), 2.09(s, 12H,
±CH3); C66H42N2O12 (1054.7): found C 75.10, H 3.63, N 3.19;
calcd C 75.16, H 3.98, N 2.66%; Absorption peaks (in acetone)
lAbmax/nm(log e): 569.8(4.61), 531.6(4.40), 438.0(4.13); Fluor-
escence peaks lmaxem(in acetone, excited at 569.8 nm)~
603.4 nm.
Synthesis of N,N'-bis(2-hydroxyphenyl)-1,6,7,12-tetrakis(4-
methylphenoxy)perylene-3,4:9,10-tetracarboxydiimide (T-
PTCA8)
Reaction conditions and workup were similar to those for T-
PTCA7, but o-aminophenol was used instead of o-aminoben-
zoic acid, and the reaction temperature was lowered to
180 ³C. Puri®cation was carried out by chromatography
(silica gel, ®rst methylene chloride : acetone~25 : 1, then
methylene chloride : acetone ~15 : 1). Two isomers were
yielded, S (higher Rf, 103 mg, 28%) and C (lower Rf, 92 mg,
25%), as deep red solids. C-isomer: mpw300 ³C; IR(KBr):
y~3380, 2920, 1705, 1668, 1585, 1460, 1410, 1342, 1286,
1
1038 cm21; H-NMR(acetone-d6): d 8.00(s, 4H, per), 6.95 (d,
8H, J~8.62 Hz, Ph), 6.86 (d, 8H, J~8.63 Hz, Ph), 5.64(s, 4H,
±OH), 4.0(t, 4H, ±CH2Nv), 1.66(m, 4H, ±CH2±), 0.92 (t, 6H,
±CH3); C54H38N2O12(906.5): found C 70.93, H 4.01, N 5.43;
calcd C 71.54, H 4.19, N 5.60%; Absorption peaks (in acetone)
lAbmax/nm(log e): 579.6(4.55), 537.6(4.48), 448.8(4.32); Fluor-
escence peaks lmaxem(in acetone, excited: 579.6 nm)~
613.9 nm. Compounds R-PTCA17 and PPDCA4 shown in
Fig. 1 were used as comparisons in this study.
1200 cm21 1H-NMR(500 MHz, acetone-d6): d 8.05(s, 4H,
;
per), 7.36(s, 2H, Ph), 7.27(m, 2H, Ph), 7.19(d, 8H, J~7.81 Hz,
Ph), 7.02(d, 2H, J~9.83 Hz, Ph), 6.97(d, 8H, J~8.33 Hz, Ph),
6.91(m, 2H, Ph), 5.64(s, 2H, ±OH), 2.31(s, 12H, ±CH3);
C64H42N2O10 (998.6): found C 76.19, H 4.03, N 2.60; calcd C
Photoelectrochemical experiments
As shown in Fig. 3, the liquid junction solar cell for measuring
the photocurrent is composed of the sensitized nanoporous
SnO2 electrode and a counter electrode with a electrolyte
solution containing 0.3 mol l21 LiI and 0.03 mol l21 I2
dissolved in 1,2-propylene carbonate solution. The counter
electrode is saturated calomel electrode (SCE). All the
potentials measured in this article are relative to SCE unless
otherwise stated. SnO2 colloids were prepared according to the
method described previously.18 Colloidal SnO2 solution (ca.
76.97, H 4.21, N 2.81%; Absorption peaks (in acetone) lAb
/
max
nm(log e): 568.6(4.61), 529.6(4.40), 436.60(4.15); Fluorescence
peaks lmaxem(in acetone, excited at 568.6 nm) ~601.4 nm; S-
isomer: mpw300 ³C; IR(KBr): y~3378, 2920, 1705, 1668,
1585, 1460, 1410, 1342, 1286, 1200 cm21; 1H-NMR (500 MHz,
acetone-d6): d 8.04(s, 4H, per), 7.36(s, 2H, Ph), 7.27(m, 2H,
Ph), 7.17(d, 8H, J~7.81 Hz, Ph), 7.02(d, 2H, J~9.83 Hz, Ph),
6.96(d, 8H, J~8.33 Hz, Ph), 6.91(m, 2H, Ph), 5.64(s, 2H, ±
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Fig. 3 Scheme of liquid junction cell based on the SnO2 electrode sensitized by OXZ-PER.
III A) image indicated the morphology of the SnO2 ®lm
deposited on a conducting glass support. It can be estimated
that the SnO2 particles in the ®lm are spherical with the
diameter of about 5 nm, so the characteristics of the SnO2 ®lm
are nanocrystalline, and is composed of interconnected
particles and pores.
Results and discussion
Absorption and ¯uorescence spectra
The absorption spectra of these multi-chromophoric com-
pounds (PCTA series and OXZ-PER) in DMF are shown in
Fig. 4. Comparison with the absorption of 1,3,4-oxadiazole
and perylenetetracarboxydiimide, the high absorption coef®-
cient at the wavelength of ultraviolet region (340 nm)
corresponds to the absorption of the oxadiazole unit, and
the peaks of 439, 529 nm correspond to the absorption of the
perylene unit. That is to say the absorbency of multi-
chromophoric dye OXZ-PER is the exact sum of the
constituent chromophores. This means that there is little or
no interaction between the chromophores connected by
saturated covalent bonds in their ground state, so that their
individual absorption characteristics should be maintained for
OXZ-PER. For the PCTA series, the characteristic absorption
of the perylene unit and naphthal-1,8-dicarboximide units were
also observed. Naphthal-1,8-dicarboximide can absorb blue,
green and yellow colors by adjusting the substituted group at
its 4-position.19,20 When its 4-position substituent was a
dimethylamino group, i.e. PCTA3, the maximum absorption
of the naphthal-1,8-dicarboximide was at around 430 nm,
which overlaps with some of the absorption of perylene. From
this standpoint, it can be shown that multi-chromophoric dyes,
in which the chromophoric units are connected by saturated
bonds, provide an appropriate route to effective sensitizers for
solar cells with a wide spectrum range of absorption.
Fig. 4 Absorption spectra of PCTA1, PCTA2, PCTA3 and OXZ-PER
in DMF (1025 mol l21).
13%) was spread on the OTE (sheet resistancey8 V cm22).
After it was sintered at 250 ³C for 30 minutes and the
temperature dropped to about 100 ³C, it was quickly immersed
into saturated dye solution in DMF, kept for 24 h and then
dried in air. The two-electrode sandwich cells were prepared by
placing the counter electrode, a Pt plate, on top of OTE±SnO2
or OTE±SnO2±Dye with a thin strip of PTFE as spacer and
were used to measure IPCE (Incident photon-to-current
conversion ef®ciency).2 The illuminated area of working
electrode was 0.2 cm2. A high-pass ®lter (w350 nm) was used
to remove ultraviolet radiation. A potentiostat (Model 173
EG&G PARC) served to control potential. The intensities of
monochromatic light were measured with a calibrated radio-
meter/photometer (Model 550-1, EG&G PARC). The photo-
current spectra were normalized after subtracting the
transmittance of the OTE. With our preparation and
measurement system, the IPCE value for cis-(SCN)2 bis(2,2'-
bipyridyl-4,4'-dicarboxylate) ruthenium(II) alone (TiO2 elec-
trode) was 80% in the wavelength range between 510 and
570 nm. The atomic force microscopy (AFM, DI Nanoscope
Absorption spectra of the perylene derivatives of the T-PTCA
series are reported in Table 1. The absorption spectral data of
the two isomers each of T-PTCA7 and of T-PTCA8 are
Table 1 Absorption data for the T-PTCA series in acetone (1025 mol l21) and on solid ®lm
Compounds
Absorption peaks/nm (loge)
Absorption /nm (solid ®lm)
T-PTCA4
T-PTCA3
T-PTCA7
T-PTCA8
569.8 (3.93)
565.2 (4.03)
569.8 (4.61)
568.6 (4.61)
575.6 (4.40)
579.6 (4.55)
530.0 (3.72)
535.4 (3.79)
531.6 (4.40)
529.6 (4.40)
535.6 (4.17)
537.6 (4.48)
441.0 (3.49)
434.6 (3.64)
438.0 (4.13)
436.6 (4.15)
447.4 (3.90)
448.8 (4.32)
485
579.0
580.6
630.4
619.8
580.0
620.4
T-PTCA5
T-PTCA6
R-PTCA9b
PPDCA (NMP)4b
514
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Fig. 6 Absorption spectra of T-PTCA4(1); T-PTCA5(2); T-PTCA7(3);
T-PTCA6(4) on solid ®lm.
Table 3 Fluorescence data for the compounds in solution (1025 M)
Relative
intensity
Fig. 5 Absorption spectra of T-PTCA7(a); T-PTCA4(b); T-PTCA6(c)
in acetone (1025 M).
Compound (Solvent)
T-PTCA4 (CHCl3)
l
ex/nm
l
em/nm
578.6
583.4
569.8
568.6
575.6
579.6
613.1
613.4
603.4
601.4
607.4
613.9
51.71
40.83
52.17
57.80
0.211
0.152
identical, and this is due to their very similar structures. The
expected broadening of their absorption bands compared to T-
PTCA4, which could be measured by the half-height width of the
longest absorption band,4b was not displayed. As seen in Fig. 5,
the shapes of the absorption spectra of the T-PTCA series are
very similar to each other. They all have high molar extinction
coef®ciency. The extension of the p-system in the bay region
induces a dramatic bathochromic shift compared with RPTCA
and PPDCA, non-substituted perylenes in the bay region. In
comparison with T-PTCA5 and T-PTCA6, the maximum
absorption wavelengths of compounds T-PTCA2, T-PTCA4,
T-PTCA7 and T-PTCA8 are similar, a little shorter than the
former. This indicated that the linking of two p-systems through
a single bond did not result in much alteration of the UV/Vis
spectra. Only an appreciable overlap of the chromophore
induced strong absorption at longer wavelengths.17
T-PTCA2 (CHCl3
)
T-PTCA7 (acetone)
T-PTCA8 (acetone)
T-PTCA5 (acetone)
T-PTCA6 (acetone)
absorption of the absorption spectra indicate greater hydrogen
bonding than explained by the small in¯uence of intermolecular
p-conjugation. Because the low concentrations of the solution
and polar solvents both do not favor hydrogen bond
formation,5b,5c no excimers or oligmers were observed in the
UV-Vis absorption. However, the intermolecular interaction
between ±OH and ±H is very strong for compounds T-PTCA7,
T-PTCA8 and T-PTCA6 on solid ®lm, so large bathochromic
shifts occurred (Fig. 6).
Except for T-PTCA5 and T-PTCA6, the other T-PTCAs are
highly ¯uorescent, and high ¯uorescence quantum yields are
obtained for the T-PTCA series. The results of ¯uorescence spectra
of the T-PTCA series (Table 3) exhibit the typical mirror symmetry
between absorption and emission bands. This indicates that little
structural rearrangement occurs between the ground and the
excited state of the compound. The characteristic Q bands of
perylene, i.e. they show a singlet at around 600 and 610 nm together
with a shoulder at around 659 and 660 nm in the Q band region
(500±800 nm), were observed from their emission spectra. For
compounds T-PTCA5 and T-PTCA6, their absorption in the
visible region was strong, but the ¯uorescence emission was very
weak, as seen in Table 3 and Fig. 7.
The absorption spectral data for the T-PTCA series in
different solvents are shown in Table 2. The maximum
absorption wavelengths were longer in chloroform than in
acetone for all the perylene derivatives. This showed that the
less polar solvents are bene®cial for the bathochromic shifts for
the perylenes, i.e. they produce a larger bathochromic shift.
The shapes of the absorption spectra of compound T-PTCA4
and T-PTCA5 in solution and on solid ®lm are very similar.
However, the spectra of T-PTCA2, T-PTCA7, T-PTCA8 and T-
PTCA6 on solid ®lm show only an unstructured broad band. It is
of interest that the differences of the maximum absorption
wavelength of T-PTCA4, T-PTCA2 and T-PTCA5 between
solution and solid ®lm are small, yet remarkably bathochromic
shifts occur for compounds T-PTCA7, T-PTCA8 and T-PTCA6
in acetone and on solid ®lm: 60, 50 and 40 nm, respectively. This
phenomenon could not be explained only by the p interaction or
intermolecular overlap in the solids. Compared with T-PTCA4
and T-PTCA2, the T-PTCA7, T-PTCA8 have bulky groups at
the nitrogen atoms, which could lower the intermolecular
interaction.9b Since±COOH or ±OH groups are present in
compounds T-PTCA7, T-PTCA8 and T-PTCA6, we believe
that, at least in part, changes in the structures and maximum
Photosensitization on SnO2 nanoporous ®lm
The absorption spectra of sensitized and unsensitized SnO2
electrodes were determined (shown in Fig. 8). The spectra show
that the unsensitized electrode (bare SnO2 ®lm) exhibits the
fundamental absorption at 306 nm (band gap energy, 3.8 eV4)
in the ultraviolet region. However, the multi-chromophoric dye
sensitized electrode has a strong absorption in the UV and
visible region. It indicates that the OXZ-PER dye has been
adsorbed onto the nanoporous ITO/SnO2 electrode. The
nature of the interaction leading to adsorption of the dye on
the surface may be the interaction between the carbonyl group
and SnO2 resulting from either bonding electrostatically or
forming an ester linkage by a hydrolysis reaction with a Sn±
OH.4 This is similar with that between a carboxy group and
TiO2. The absorption peaks at 340, 439 and 529 nm indicate
that there is little or no interaction between the oxadiazole and
Table 2 The solvent effect on the absorption spectra of compounds
synthesized
T-
PTCA4 PTCA2 PTCA7 PTCA8 PTCA5 PTCA6
T-
T-
T-
T-
T-
Dyes
CHCl3 578.6
Acetone 569.8
583.4
565.2
586.2
569.8
585.6
568.6
590.2
575.0
595.0
579.6
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energy conversion at the maximum IPCE for the dyed nano-
SnO2 should in fact be higher. After one month storage in dark,
the cell sensitized by OXZ-PER still shows short-circuit
photocurrent of 500 mA cm22
.
The above special sensitization effect of photoelectrical
performance of the cell can be explained in terms of the band
models shown in Fig. 2. The band bending resulting from the
organic±inorganic p±n heterojunction, facilitates electron
injection from the excited state of OXZ-PER into the
conduction band of the SnO2 semiconductor. The band
energies of SnO2,4 oxadiazole unit14,19 and perylene unit5,15
can be taken from the appropriate references. The LUMO level
of the oxadiazole unit and perylene unit in OXZ-PER dye is
higher than the conduction band of SnO2. This means that the
excited level (LUMO) of the oxadiazole unit and perylene unit
matches the conduction band position of nanocrystalline SnO2.
When irradiating under white light, the electrons were excited
from the HOMO orbital (25.9 eV) to the LUMO orbital
(23.9 eV) of the perylene unit and then injected from the
LUMO orbital of the perylene unit into the conduction band of
SnO2 (24.73 eV). For the oxadiazole unit, a similar electron
transfer exists. However, the electrons of the LUMO orbital of
oxadiazole unit have two possible approaches to inject into the
conduction band of SnO2. One is direct electron injection from
the LUMO orbital of the oxadiazole unit (22.5 eV) into the
conduction band of SnO2. Another is indirect electron injection
from the LUMO orbital of the oxadiazole unit via the LUMO
orbital (23.9 eV) of the perylene unit into the conduction band
of SnO2. That is, there exist the energy transfer and/or electron
transfer from the oxadiazole unit to the perylene unit at ®rst.
Then, the electrons inject from the LUMO orbital of the
perylene unit into the conduction band of SnO2. In addition, a
redox electrolyte (I32/I2) is used to mediate charge transfer
between the electrons and to regenerate the sensitizer. As a
consequence, a drastic photoresponse with wide spectral
sensitization (from 310 to 700 nm) is achieved. In fact, the
role of SnO2 in the liquid junction cell based on the OXZ-PER
is merely to conduct the injected electrons.
Fig. 7 Fluorescence emission spectra of compounds T-PTCA4 in
CHCl3(a); T-PTCA5 in acetone(b); T-PTCA7 in acetone(c); T-PTCA8
in acetone(d).
perylene chromophores even in the adsorbed state. Thus the
responding photocurrent of the nanocrystalline SnO2 sensitiza-
tion by the novel multi-chromophoric OXZ-PER dye could
cover a wide range of absorption.
Photocurrent±voltage characteristics under white light
irradiation and action spectra of the photocurrent were
measured with the two-electrode system shown in Fig. 9 to
characterize the photoenergy conversion ef®ciency. Fig. 9
shows the photocurrent action spectra of the sensitized and
unsensitized SnO2 electrode. As shown in Fig. 9, the open
circuit voltage (Voc) and short-circuit photocurrent (Jsc) of the
sensitized electrode by OXZ-PER dye were 0.17 V and 180 mA
(i.e. 900 mA cm22), respectively. The ®ll factor (FF) was 0.34.21
The dye sensitized nano-SnO2 electrode showed relatively large
Jsc although Voc was relatively small. The photoresponse was
drastically broadened to the visible light region, covering a
wide range of spectrum (310 to 700 nm). The photocurrent of
the unsensitized electrode (ITO/SnO2) was very low around the
400 nm region and decreased to almost zero at 500 nm. A little
photocurrent generated by the light with wavelength over
400 nm may be related to the surface states or inner localized
states of bare SnO2 nanostructured ®lm. These surface states or
inner localized states may absorb a photon of energy smaller
than its energy gap and make a contribution to produce very
small photocurrent in this wavelength region. When sensitized
by OXZ-PER, the maximum IPCE of the cell in the UV region
reached 12% (at 370 nm). In the visible region the maximum
IPCE at 540 nm reached 10.5%. The photocurrent was
drastically broadened to a wide visible light region covering
400 to 650 nm. Since IPCE values are not corrected for the loss
owing to incident light scattering by the glass support, the
Because of the improved solubility of the T-PTCA series in
common organic solvents, the adsorption of dyes onto the
SnO2 ®lm should be improved. The absorption spectra of the
T-PTCA7 dye sensitized SnO2 electrode were very similar to
that of dye ®lm. This means that there is little or no interaction
between chromophores connected with saturated covalent
bonds in their ground state even in the adsorbed state, so that
their individual absorption characteristics should be main-
tained for T-PTCA7. On the other hand, because of steric
constraints, the carboxylic acid groups on the phenyl ring are
oriented perpendicular to the perylene ring system. The poor
photosensitization (a relative small photocurrent density of
Fig. 9 Photocurrent action spectra of OXZ-PER dye-sensitized (curve
a) and unsensitized SnO2 porous ®lms (curve b). Insert ®gure:
photovoltage±current characteristics of the cell based on dye-sensitized
SnO2 nanostructured electrode.
Fig. 8 Absorption spectra of SnO2 nanostructured porous ®lm
deposited on conducting glass unsensitized (curve a) and dye-sensitized
by OXZ-PER (curve b).
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Fig. 10 Perylene T-PCTA7 adsorbed on nanoporous SnO2 electrode.
Fig. 11 IPCE spectra of perylene T-PCTA8 adsorbed on nanoporous
TiO2 electrode.
y100 mA cm22) obtained for this kind of dye (T-PTCA series)
may be related to its orientation on the surface of the SnO2
particles. When sensitized by T-PTCA7, the maximum IPCE of
the cell in the visible region reached 24% (at 500 nm). The
photocurrent was drastically broadened to a wide visible light
region covering 400 to 600 nm, as shown in Fig. 10. As the
comparison system, the photocurrent response of the T-
PTCA8 dye sensitized nano-structured TiO2 electrode was
determined, which were shown in Fig. 11. As seen in Fig. 11,
the photon-to-current quantum yields in the visible region for
T-PTCA8 sensitized TiO2 ®lm were very small. This may result
from the poor adsorption of the dye on the surface of the TiO2.
The results indicated that only the carboxylic acid group is
effective for the adsorption onto the surface of SnO2 and TiO2
nanoparticles. The other groups such as hydroxy would
in¯uence the adsorption, so consequently result in a poor
photosensitization. In Fig. 10 and 11, the photoresponses of
the cells shown by the thick lines were obtained after two hours
exposure to white light (200 W Hg lamp). The results show
relative better photostability of the perylenes synthesized in the
study.
In addition, due to the good solubility of the T-PTCA series
compounds in organic solvents, the oxidation potential of these
compounds can be obtained by the measurements of solution
electrochemistry. The oxidation potential of a commercially
available N,N'-bis(2,5-di-tert-butylphenyl)perylene-3,4:9,10-
tetracarboxydiimide is z1.66 V vs. SCE in CH2Cl2. The very
positive oxidation potential is thermodynamically capable of
driving the water oxidation reaction at appropriate pH. The
oxidation potential of T-PTCA series synthesized in this study
were between z1.00 and z1.60 V (SCE), which should be a
substantial over-potential for water oxidation (z0.63 V vs.
SCE at pH 7).4 The perylene dyes presented here may be useful
in water-splitting dye-sensitized solar cells.
Acknowledgements
This project was supported by NSFC/China and partially
sponsored by the Shanghai Scienti®c Committee. Authors are
indebted to Professor Dr K. MuÈllen (MPI for Polymer
Research, Mainz/ Germany) and Professor K. C. Chen
(ECUST, Shanghai/ China) in the synthesis of the compounds.
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21 The ®ll factor FF~Emax/ (Jsc? Voc), where Emax is the maximum
output power of the cell, Jsc is the short circuit current, Voc is the
open-circuit voltage.
In summary, we have found a novel route to design highly
ef®cient sensitizers for GraÈtzel-type solar cells, which can avoid
the complication of doping. Two series of new multi-
chromophoric perylene-3,4:9,10-tetracarboxylic dianhydride
dyes for solar cell sensitizer have been synthesized. One
series consists of oxadiazole or naphthaldicarboximides linked
by perylene units as N,N'-substituted chromophores. The other
series has four substituents in the bay-region of the perylene
core besides N,N'-substituents. The latter series of T-PTCA
compounds have surprisingly good solubility in common
organic solvents, in addition to good photo- and thermal-
stability. In the dye sensitized SnO2 system, a wide spectral
photoresponse (from 310 to 700 nm) was observed for these
novel multi-chromophoric dyes. In addition, the sensitization is
dependent on the energy match of the energy bands of the
semiconductor and the redox potentials of the segments in such
assembled dyes.
J. Mater. Chem., 2000, 10, 2708±2715
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