Electronically coupled porphyrin-arene dyads for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2011.04.010

An acetylene-linked porphyrin-perylene anhydride and an acetylene-linked porphyrin-naphthalic anhydride have been synthesized; the highly conjugated acetylenic bridge in these porpyrins efficiently mediates electronic interaction between the porphyrin and

Full text of DOI:10.1016/j.dyepig.2011.04.010

Dyes and Pigments 91 (2011) 317e323
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Electronically coupled porphyrin-arene dyads for dye-sensitized solar cells
,
Cheng-Wei Lee a b, Hsueh-Pei Lu b, Nagannagari Masi Reddya, Hsuan-Wei Lee a,
,
a,
*
Eric Wei-Guang Diau b , Chen-Yu Yeh
*
a Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan, ROC
b Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 300, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
An acetylene-linked porphyrin-perylene anhydride and an acetylene-linked porphyrin-naphthalic
Received 15 February 2011
Received in revised form
7 April 2011
Accepted 7 April 2011
Available online 21 April 2011
anhydride have been synthesized; the highly conjugated acetylenic bridge in these porpyrins efficiently
mediates electronic interaction between the porphyrin and perylene units to extend the p-conjugation of
the porphyrin dye and to cause both broadening and red shifts of both the Soret and Q absorption bands.
This condition is a useful feature for efficient dye-sensitized solar cell applications. The optical, elec-
trochemical and photovoltaic properties of the new linked anhydrides show that the HOMOeLUMO gap
decreased upon extension of
porphyrin and the perylene or naphthalene unit.
p-conjugation, indicating a strong electronic coupling between the
Keywords:
p-Conjugation
Ó 2011 Elsevier Ltd. All rights reserved.
Dye-sensitized solar cells
Electronic coupling
Light-harvesting
Perylenes
Porphyrins
1. Introduction
energy and electrons have illuminated the development of the
conversion of solar energy.
To maintain global economic growth and to diminish global
warming and environmental pollution, the exploration of renewable
energy resources is of great significance. Sunlight is the most abundant
resource that can deliver clean and efficient energy to meet the
increasing demand worldwide. Nature has chosen porphyrin-related
pigments in the light-harvesting antennae of photosynthetic organ-
isms that power biological systems [1]. The chromophores in the
photosynthetic reaction center capture sunlight efficiently and
convert solar energy into usable chemical energy. The synthesis of
porphyrins and related macrocycles has attracted considerable
attention because they are ubiquitous in natural systems and have
prospective applications in mimicking enzymes [2], catalytic reactions
[3], photodynamic therapy [4], molecular electronic devices and
conversion of solar energy [5e15]. In particular, numerous porphyrin-
based artificial light-harvesting antennae, and donoreacceptor dyads
and triads have been prepared and tested to improve our under-
standing of the photochemical aspect of natural photosynthesis
[5e15]. Extensive investigation on these multicomponent systems has
disclosed the reaction parameters and the mechanism of transfer of
The development of solar cells of new types is escalating,
prompted by increasing energy demand. Dye-sensitized solar cells
(DSSC) appear to be a promising approach for the conversion of
sunlight to electricity on a large scale. The most efficient DSSC are
based on ruthenium polypyridine complexes and have attained
efficiencies w11% of power conversion [16,17]. However, the
application of ruthenium complex devices is significantly limited
by the rareness of ruthenium and also by potential environmental
pollution issues. Diverse organic dyes including porphyrin-related
compounds have consequently been synthesized for use in DSSC
[5e15]. Officer et al. have synthesized
porphyrin monomers and multiporphyrin arrays, and investigated
their efficiencies [18]; -substituted monoporphyrin carboxylic-
b-carboxyl-substituted
b
acid derivatives with a conjugated bridge are effective candidates
for DSSC. The best porphyrin dye, which has a butadiene bridge
between a carboxyl and the porphyrin ring, attains an efficiency up
to 7.1% in its power conversion [10]. The conjugated bridge signif-
icantly broadens the absorption bands to increase the light-
harvesting ability. To increase the absorption width of
a porphyrin, Imahori et al. designed and synthesized a naphthyl-
fused porphyrin with elongated
mance of the naphthyl-fused porphyrin is improved by about 50%
relative to the non-fused porphyrin derivative [19].
p-conjugation. The cell perfor-
* Corresponding authors. Fax: þ886 4 22862547.
E-mail addresses: diau@mail.nctu.edu.tw (E.W.-G. Diau), cyyeh@dragon.nchu.
edu.tw (C.-Y. Yeh).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.04.010

318
C.-W. Lee et al. / Dyes and Pigments 91 (2011) 317e323
Similar to porphyrin derivatives, perylenes are attractive molec-
spin-coating drops of H2PtCl6 solution onto ITO glass and heating at
380 ꢂC for 15 min. To prevent a short circuit, the two electrodes
were assembled into a cell of sandwich type and sealed with a hot-
ular components for application in molecular electronic devices
because of their great photostability and unique structures, and their
electrochemical and photophysical properties [20e23]. Several
perylene dyes for DSSC have been rationally designed and synthe-
sized with introduction of electron-donating groups and bulky
substituents to decrease aggregation. The most effective perylene
dyes show a power conversion efficiency of up to 6.8% [24,25].
melt film (SX1170, Solaronix, thickness 25
mm). The electrolyte
solution containing LiI (0.1 M), I2 (0.05 M), PMII (0.6 M), 4-tert-
butylpyridine (0.5 M) in a mixture of acetonitrile and valeronitrile
(volume ratio 1:1) was introduced into the space between the two
electrodes, so completing the fabrication of these DSSC devices.
On the basis of previous work, extension of p-conjugation of the
porphyrin dye appears to cause broadening of the absorption
bands, which is an essential requirement for an efficient sensitizer
[5e15,18,19]. We thus aim to construct electronically coupled
porphyrin-arene dyads as sensitizers for use in DSSC. Here, we
report the synthesis of porphyrin-perylene anhydride dyad 4 and
porphyrin- naphthalene anhydride dyad 5, and their optical, elec-
trochemical and photovoltaic properties. A perylene sensitizer (P1)
was also synthesized for comparison.
2.3. Photovoltaic characterization
The current-voltage characteristics of the devices were
measured with a solar simulator (AM 1.5, SAN-EI, XES-502S, type
class A) calibrated with a Si-based reference cell (VLSI standards,
Oriel PN 91150V). When the device is irradiated with the solar
simulator, the source meter (Keithley 2400, computer-controlled)
sends a voltage (V) to the device, and the photocurrent (I) is read
at each step controlled by a computer via a GPIB interface. The
2. Experimental
efficiency (
this relation,
h) of conversion of light to electricity is obtained with
h
¼ Jsc Voc FF/Pin, in which Jsc (mA cmꢁ2) is the current
All reagents and solvents were obtained from commercial
sources and used without further purification, unless otherwise
noted. CH2Cl2 was dried over CaH2 and freshly distilled before use.
THF was dried over sodium/benzophenone and freshly distilled
before use. Tetrabutylammonium hexafluorophosphate (TBAPF6)
was recrystallized twice from absolute ethanol and further dried for
two days under vacuum. Column chromatography was performed
on silica gel (Merck, 70e230 Mesh ASTM).
density measured at short circuit, and Voc (V) is the voltage
measured at open circuit. Pin is the input radiation power (for one-
sun illumination Pin ¼ 100 mW cmꢁ2) and FF is the filling factor. The
incident monochromatic efficiency for conversion from photons to
current (IPCE) spectra of the corresponding devices was measured
with a system comprising a Xe lamp (PTi A-1010, 150 W), mono-
chromator (Dongwoo DM150i, 1200 g/mm blazed at 500 nm), and
source meter (Keithley 2400, computer-controlled). A standard Si
photodiode (ThorLabs FDS1010) served as a reference to calibrate
the power density of the light source at each wavelength. Photo-
current densities of both the target device and the reference Si cell
were measured under the same experimental conditions (excita-
tion beam size w0.08 cm2) so to obtain the IPCE value of the device
from comparison of the current ratio and the value of the reference
cell at each wavelength.
2.1. Spectral and electrochemical measurements
1H NMR spectra (Varian spectrometer, 400 MHz), UVevisible
spectra (Varian Cary 50), UVevisibleeNIR spectra (Shimadzu UV-
3600), emission spectra (JASCO FP-6000 spectrofluorimeter), High
resolution mass spectra (LTQ Orbitrap XL, Thermo Fisher Scientific)
and FAB mass spectra (JMS-SX/SX102A Tandem Mass spectrometer)
were recorded on the indicated instruments. Electrochemical tests
were performed with a three-electrode potentiostat (CH Instru-
ments, Model 750A) in THF deoxygenated on purging with pre-
purified dinitrogen gas. Cyclic voltammetry was conducted with
a three-electrode cell equipped with a BAS glassy carbon disk
(0.07 cm2) as the working electrode, a platinum wire as auxiliary
electrode, and an Ag/AgCl (saturated) reference electrode; the
reference electrode is separated from the bulk solution with
a double junction filled with electrolyte solution. The working
2.4. Synthesis of dyes 4, 5, and P1
2.4.1. Zinc(II) 5,15-Bis(3,5-di-tert-butylphenyl)-10-(bis(4-
octylphenyl)amino)-20-(3- (perylenedicarboxylic-9,10-anhydride)
ethynyl)porphyrin (4)
To a solution of porphyrin 2 [5] (26.4 mg, 0.02 mmol) in dry THF
(5 mL) was added tetrabutylammonium fluoride (TBAF) 1 M in THF
(0.08 mL, 0.08 mmol). The solution was stirred at 25 ꢂC for 30 min
under dinitrogen. The mixture was quenched with H2O and then
extracted with CH2Cl2. The organic layer was dried over anhydrous
Na2SO4 and the solvent was removed undervacuum. The residue and
9-bromo-perylene-3,4-dicarboxylic anhydride (16.0 mg, 0.04 mmol)
were dissolved in dry THF (5 mL) and NEt3 (1 mL) and degassed with
electrode was polished with aluminium (0.03
mm) on felt pads
(Buehler) and treated ultrasonically for 1 min before each experi-
ment. The reproducibility of individual potential values was within
ꢀ5 mV.
dinitrogen for 10 min; then Pd2(dba)3 (2.2 mg, 2.5 mmol) and AsPh3
2.2. Device fabrication
(6 mg, 0.02 mmol) were added to the mixture. The solution was
heated under reflux for 3 h under dinitrogen. The solvent was
removed under vacuum. The residue was purified on a column
chromatograph (silica gel) using CH2Cl2/hexane ¼ 4/6 as eluent.
Recrystallization from CH2Cl2/CH3OH gave 4 (13.4 mg, 45%). 1H NMR
The porphyrins were sensitized onto TiO2 nanoparticulate films
to serve as working electrodes in DSSC devices. A paste composed
of TiO2 particles (w20 nm) for the transparent active layer was
coated on a TiCl4-treated FTO glass substrate (FTO, 8
repetitive screen printing to obtain the required film thickness
(w10 m). The TiO2 film was annealed according to a programmed
U
/cmꢁ2) with
(CDCl3, 400 MHz)
d
9.49 (d, J ¼ 4.4 Hz, 2H), 9.22 (d, J ¼ 4.8 Hz, 2H),
9.00 (d, J ¼ 8.0 Hz,1H), 8.94 (d, J ¼ 4.4 Hz, 2H), 8.77 (d, J ¼ 4.8 Hz, 2H),
8.42e8.36 (m, 2H), 8.18 (d, J ¼ 8.0 Hz, 2H), 8.12 (s, 4H), 7.99
(t, J ¼ 8.0 Hz,1H), 7.83 (s, 2H), 7.71 (br, 2H), 7.52 (br,1H), 7.45 (br,1H),
7.24 (d, J ¼ 8.4 Hz, 4H), 6.95 (d, J ¼ 8.4 Hz, 4H), 2.47 (t, J ¼ 7.6 Hz, 4H),
1.55 (s, 36H), 1.25 (m, 22H), 0.84 (m, 8H); 13C NMR (CDCl3, 100 MHz)
m
procedure: (1) heating at 80 ꢂC for 15 min; (2) heating at 135 ꢂC for
10 min; (3) heating at 325 ꢂC for 30 min; (4) heating at 375 ꢂC
for 5 min; (5) heating at 450 ꢂC for 15 min; (6) heating at 500 ꢂC for
15 min. For each dye P1, 4 and 5, the electrode was immersed in the
dry DMF solution (0.2 mM, 25 ꢂC) containing tetrabutylammonium
hydroxide (TBA, 0.2 mM) for dye loading onto the TiO2 film at
25 ꢂC for 4 h and 8 h. The Pt counter electrodes were prepared on
d
164.9, 152.5, 152.3, 150.6, 150.3, 150.1, 150.0, 148.6, 143.8, 141.4,
137.3, 137.0, 134.8, 134.5, 133.4, 133.3, 133.1, 131.9, 131.1, 130.8, 130.3,
130.1, 129.6, 129.0, 128.8, 128.7, 128.1, 127.8, 126.8, 126.1, 125.4, 124.5,
124.3,123.6,123.5,122.0,121.4,121.3,120.9,120.6,120.5,108.7,102.4,

C.-W. Lee et al. / Dyes and Pigments 91 (2011) 317e323
319
Scheme 1. Synthetic routes for (a) porphyrins 4 and 5, i) Bis(4-octylphenyl)amine, NaH, Pd(OAc)2, DPEphos, THF. ii) TBAF, THF, RT; then 9-Bromo-3,4-perylenedicarboxylic
anhydride for 4 or 4-bromo-1,8-naphthalic anhydride for 5, [Pd2(dba)3], AsPh3, THF, NEt3, reflux; (b) P1, i) [Pd2(dba)3], Di(4-tert-butylphenyl)amine, (t-Bu)3P, t-BuONa, toluene,
reflux. ii) KOH, IPA, reflux.
98.8, 94.4, 35.2, 35.0, 31.8, 31.7, 31.5, 31.2, 29.4, 29.3, 29.2, 22.6, 14.1
35.2, 35.1, 31.7, 31.8, 31.5, 29.7, 29.5, 29.4, 29.2, 22.6, 14.1; UVeVis
UVeVis (CH2Cl2): lmax/nm (
3
/103 Mꢁ1 cmꢁ1) ¼ 436(86), 493(50),
(CH2Cl2): lmax/nm (
3
/103 Mꢁ1 cmꢁ1) ¼ 486(114), 679(38); IR
548(36), 685(43); IR (KBr, cmꢁ1): v ¼ 2957, 2924, 2854, 2173, 1707,
1596, 1568, 1506, 1355, 1293, 1244, 800, 715; HRMS: m/z calcd. for
C100H100N5O3Zn: 1482.7112, found: 1482.6808 ([M ꢁ H]ꢁ).
(KBr, cmꢁ1): v ¼ 2957, 2923, 2854, 2174,1768,1719,1587,1506,1451,
1340, 1250, 1018, 800, 711; HRMS: m/z calcd. for C90H98N5O3Zn:
1360.6956, found: 1360.6982 ([M þ H]þ).
2.4.2. Zinc(II) 5,15-Bis(3,5-di-tert-butylphenyl)-10-(bis(4-
octylphenyl)amino)-20-(4- (naphthalic-1,8-anhydride)ethynyl)
porphyrin (5)
Porphyrin 5 was prepared with a procedure similar to that for 4
except 4-bromo-1,8-naphthalic anhydride instead of 9-bromoper-
ylene-3,4-dicarboxylic anhydride. Recrystallization of the product
from CH2Cl2/CH3OH gave 5 in a yield 55%. 1H NMR (CDCl3,
2.4.3. N-(2,6-diisopropylphenyl)-9-(bis(4-tert-butylphenyl)amino)
perylene-3,4- dicarboximide (3)
A mixture of N-(2,6-diisopropylphenyl)-9-bromoperylene-3,4-
dicarboximide (112 mg, 0.2 mmol), di(4-tert-butylphenyl)amine
(112 mg, 0.4 mmol), [Pd2(dba)3] (10 mg, 0.01 mmol), tri-tert-
butylphosphine (10 mg, 0.05 mmol), sodium tert-butyl alcohol
(27 mg, 0.27 mmol) and toluene (50 mL) was heated under reflux
under dinitrogen overnight. The solvent was removed under
vacuum, and the residue was purified on a column chromatograph
(silica gel) using CH2Cl2/hexane ¼ 4/6 as eluent. Recrystallization
from CH2Cl2/CH3OH gave 3 (126.2 mg, 83%). 1H NMR (CDCl3,
400 MHz)
d
9.77 (d, J ¼ 4.4 Hz, 2H), 9.35 (d, J ¼ 8.4 Hz, 1H), 9.19 (d,
J ¼ 4.4 Hz, 2H), 8.97 (d, J ¼ 4.4 Hz, 2H), 8.73 (d, J ¼ 8.4 Hz, 2H), 8.58
(br, 2H), 8.34 (d, J ¼ 5.6 Hz, 1H), 8.00 (s, 4H), 7.96 (t, J ¼ 8.0 Hz, 1H),
7.78 (s, 2H), 7.17 (d, J ¼ 8.4 Hz, 4H), 6.93 (d, J ¼ 8.4 Hz, 4H), 2.47
(t, J ¼ 8.0 Hz, 4H),1.53 (s, 36H),1.24 (m, 22H), 0.84 (m, 8H); 13C NMR
400 MHz)
d 8.67e8.63 (m, 2H), 8.47e8.42 (m, 3H), 8.37
(CDCl3, 100 MHz)
d
158.9, 153.0, 152.1, 150.6, 150.4, 150.2149.0,
(d, J ¼ 8.4 Hz, 1H), 8.08 (d, J ¼ 8.4 Hz, 1H), 7.52e7.46 (m, 2H), 7.40 (d,
J ¼ 8.4 Hz, 1H), 7.34 (d, J ¼ 8.0 Hz, 2H), 7.25 (d, J ¼ 8.4 Hz, 2H), 7.01
(d, J ¼ 8.4 Hz, 2H), 2.78 (m, 2H),1.31 (s,18H),1.18 (d, J ¼ 7.2 Hz,12H);
141.0, 135.1, 133.7, 132.8, 132.5, 131.4, 130.5, 129.6, 128.9, 128.6,
126.5, 125.8, 124.0, 122.5, 121.2, 117.6, 115.4, 108.7, 105.4, 97.7, 93.1,
Fig. 1. Molecular structures of dyes P1, 4 and 5.

320
C.-W. Lee et al. / Dyes and Pigments 91 (2011) 317e323
13C NMR (CDCl3, 100 MHz)
d 164.3, 164.2, 147.8, 146.1, 146.0, 145.8,
Table 1
Spectral and electrochemical data for P1, 2, 4, and 5 dyes.a
138.0, 137.8, 132.4, 132.3, 131.7, 131.4, 130.9, 130.0, 129.9, 129.6,
128.2,127.4,127.2,127.0,126.4,125.1,124.4,124.2,122.6,121.1,120.5,
120.3, 119.9, 34.5, 31.7, 29.4, 24.3; FAB-MS: m/z calcd. for
C54H52N2O2: 760; found: 761 ([M þ H]þ).
Dyea Absorption lmax/nm Emission
/103 Mꢁ1 cmꢁ1
lmax/nm (
Oxidation
E1/2/V
Reduction E1/2/V
b
(
3
)
f)
2
428(233), 574(12), 677 (0.0548)
634(27)
þ0.92, þ1.32c ꢁ1.11
P1
4
486(9), 593(20)
436(86), 493(50),
548(36), 685(43)
440(114), 679(38)
604, 743 (0.0404) þ1.23
ꢁ0.71, ꢁ1.29
2.4.4. 9-(bis(4-tert-butylphenyl)amino)perylene-3,4-dicarboxylic
anhydride (P1)
787 (0.0006)
þ0.92, þ1.29c ꢁ0.72, ꢁ0.93,
ꢁ1.47
þ0.94, þ1.32c ꢁ0.81. ꢁ1.00
A mixture of 3 (164 mg, 0.2 mmol), potassium hydroxide
(500 mg, 9 mmol) and isopropanol (50 mL) was heated under reflux
with stirring overnight. The reaction mixture was cooled to 23 ꢂC
and then poured into acetic acid; the solution was stirred at 23 ꢂC
for 6 h. Water was poured into solution and the mixture was
filtered. The crude product was washed with water. Column chro-
matography (silica gel) using CH2Cl2/hexane ¼ 8/2 as eluent.
Recrystallization from CH2Cl2/CH3OH gave P1 (108.0 mg, 90%). 1H
5
738 (0.0267)
a
Absorption and emission data were measured in CH2Cl2 at 25 ꢂC. Electro-
chemical measurements were performed at 25 ꢂC in THF containing TBAPF6 (0.1 M)
as supporting electrolyte. Potentials measured vs. ferrocene/ferrocenium (Fc/Fcþ
)
couple were converted to normal hydrogen electrode (NHE) by addition of þ0.63 V.
b
The excitation wavelengths were 570, 650 and 620 nm for P1, 4 and 5,
respectively. ZnTPP was used as the reference for the calculation of quantum yields.
c
Irreversible process Epa or Epc.
NMR (CDCl3, 400 MHz)
d
8.37 (d, J ¼ 8.0 Hz, 2H), 8.30 (d, J ¼ 8.0 Hz,
of porphyrin-based dyads involved a Sonogashira coupling reaction
between ethynylporphyrin and the appropriate bromide. As shown
in Scheme 1, deprotection of porphyrin 2 [5] followed by coupling to
perylene bromide using Pd as catalyst gave dyad 4. Porphyrin 5 was
similarly synthesized; these molecular structures are shown in Fig.1.
Reference compound P1 shows a visible spectrum with absorp-
2H), 8.20 (s, J ¼ 8.0 Hz, 1H), 8.15e8.09 (m, 2H), 7.45 (t, J ¼ 8.4 Hz,
1H), 7.35 (d, J ¼ 8.0 Hz, 1H), 7.27 (d, J ¼ 8.4 Hz, 4H), 7.02
(d, J ¼ 8.4 Hz, 4H), 1.31 (s, 18H); 13C NMR (CDCl3, 100 MHz)
d 160.4,
160.3, 148.4, 145.6, 145.5, 138.5, 138.4, 133.1, 132.9, 132.1, 130.7,
129.4, 128.6, 128.5, 126.6, 126.5, 126.1, 125.9, 125.3, 124.7, 124.4,
122.4, 119.6, 119.2, 116.0, 115.2, 34.0, 31.1; UVevis (CH2Cl2): lmax/nm
/103 Mꢁ1 cmꢁ1) ¼ 486(9), 593(20); IR (KBr, cmꢁ1): v ¼ 2958, 2868,
tion maxima at 486
(3
(3
¼
9000 Mꢁ1 cmꢁ1
) and 593 nm
(
3
¼ 20 000 Mꢁ1 cmꢁ1). The absorption spectrum of dyad 4 differs
1699, 1664, 1567, 1499, 1356, 1266, 1100, 1018, 813, 755; HRMS: m/z
calcd. for C42H35NO3: 601.2611, found: 601.2590 ([M]ꢁ).
much from the combined spectrum of the respective porphyrin and
perylene components. As shown in Fig. 2, porphyrin 4 shows broad
absorption bands that cover almost the entire visible region and
extend into the near-IR region; the absorption gap between the Soret
and Q bands is filled through the absorption of the perylene unit. As
expected, an attachment of the perylene to the porphyrin via the
carbonecarbon triple bond significantly perturbs the photophysical
properties of the perylene and the porphyrin. Both the Soret and Q
bands of 4 show a significant red shift, indicating a decreased energy
gap between the HOMO and the LUMO as a consequence of extension
3. Results and discussion
The absorption spectrum of a porphyrin dye generally features an
intense Soret band in the range 400e450 nm and less intense Q
bands in the range 550e650 nm. The gap between 450 and 550 nm is
amenable to structural modification, such that the absorption posi-
tion and width can be tuned in a controllable fashion. To be an effi-
cient dye, an objective would be for the dye to absorb intensely
across the entire visible region, even extending into the near-IR
region. The existence of that absorption gap between the Soret and
Q bands decreases the light-harvesting efficiency of a porphyrin. The
spectral characteristics of perylene dyes that typically have absorp-
tion bands in the range 450e550 nm make them ideal auxiliary
pigments for combination with porphyrins to improve the light-
of p-conjugation [26]. Furthermore, compound 4 exhibits broadening
of the Soret band, indicating a strong electronic coupling between the
porphyrin and perylene units. The band broadening and red shifts for 5
are less than for 4 because the electronic interaction with the
porphyrin is more pronounced for the perylene than for the naph-
thalene. Dyads 4 and 5 notably exhibit significantly intensified Q bands
through the energy splitting between the a1u and a2u orbitals, or the eg
orbitals [26]. Such enhanced absorption would enhance the light-
harvesting efficiency and is thus expected to enable the use of
thinner films to fabricate the devices. This condition might increase
the open circuit voltage and the overall efficiency of power conversion,
and prevent a decreased mechanical strength of the film [27].
harvesting efficiency. As mentioned, an extension of p-conjugation
in porphyrin would lead to broadening and red shift of the absorp-
tion bands. We therefore designed a dyad 4 with a perylene anhy-
dride connected to the porphyrin via an acetylenic link; the highly
conjugated acetylenic bridge would efficiently mediate electronic
interaction between the porphyrin and perylene units. The synthesis
The emission data for dyes P1, 4, 5, and the component subunit 2
in CH2Cl2 at 23 ꢂC are listed in Table 1. The emission of porphyrin 2
Fig. 2. Absorption spectra of porphyrins 4 (plain), 5 (bold), 2 (dash), and P1 (dot).
Fig. 3. Energy levels of P1, 4 and 5.

C.-W. Lee et al. / Dyes and Pigments 91 (2011) 317e323
321
Fig. 4. HOMO ꢁ 1, HOMO, LUMO and LUMO þ 1 for dicarboxylic acids of P1, 4 and 5 in the gaseous phase.
is dominated by the Q band at 673 nm. In the case of dyads 4 and 5,
the emission was observed in the range 700e800 nm. The signifi-
calculations on these dyes using density-functional theory (DFT) at
the B3LYP/6-31G(d) level (Spartan 08 package). To simplify the
computations, some alkyl groups on phenyl rings were replaced with
hydrogen atoms or methyl groups. Fig. 4 shows an energy-level
diagram and the corresponding molecular orbitals for these
porphyrin dyes. The electronic distribution of HOMO and HOMO ꢁ 1
for P1 is delocalized over both diarylamine and perylene units
whereas that of LUMO and LUMO þ 1 is localized on the perylene. At
the HOMO level of 4 and 5, the electronic density is distributed mainly
on the diarylamine and porphyrin. The distribution of electronic
density of the LUMO is located primarily at the porphyrin, and the
perylene or naphthalene unit. Comparison of 4 with 5 shows that the
cant red shift is consistent with the elongation of
p-conjugation,
but the emission intensity of 4 and 5 is decreased. This result might
be diagnostic of efficient transfer of energy or electrons from the
excited porphyrin to the acceptor element in the dyads.
Appropriate HOMO and LUMO energy levels of the dye are
ꢁ
required to match the oxidation potential of the Iꢁ/I3 electrolyte
and the conduction-band (CB) edge level of the TiO2 electrode.
Electrochemical tests were performed to determine the HOMO
levels of the dyes, which correspond to the first oxidation potentials
of the sensitizers. The cyclic voltammetry measurements for P1, 4
and 5 in CH2Cl2 were conducted using tetrabutylammonium hex-
afluorophosphate (TBAPF6, 0.1 M) as supporting electrolyte; the
potentials are listed in Table 1. Perylene P1 shows reversible
oxidation at E1/2 ¼ þ1.23. The first oxidations for 4 and 5, which are
reversible, appear at E1/2 ¼ þ0.92 and þ0.94 V, respectively,
whereas the second oxidations are irreversible and occur at
Epa ¼ þ1.32 and þ1.29 V. As shown in Fig. 3, the excited-state
oxidation potentials (E*0-0) of P1, 4 and 5 are ꢁ0.62, ꢁ0.80
and ꢁ0.80 V, respectively, as calculated from the oxidation poten-
tial (Eox) and the optical band gap (E0-0) determined from the
intersection of the corresponding absorption and emission spectra.
The E*0-0 values for these dyes are more negative than the
conduction-band level of TiO2 (wꢁ0.5 V versus NHE) ensuring
a sufficient driving force for electron injection.
HOMOeLUMO gap is decreased upon extension of
p-conjugation
because considerable electronic coupling occurs between the link and
the porphyrin core. This effect is in agreement with the more
pronounced red shift and broadening in the absorption bands for 4
than 5. The electronic distributions of the frontier orbitals indicate that
these dyes are suitable for use in DSSC devices.
The perylene P1 and porphyrins 4 and 5 were sensitized onto
TiO2 nanoparticulate films to serve as working electrodes of DSSC
devices for photovoltaic characterization. Fig. 5a presents absorp-
tion spectra of the sensitized samples as thin films and Fig. 5b
shows the corresponding IPCE action spectra of the devices. Fig. 5c
shows the current-voltage characteristics of these three dyes on
TiO2 films of thickness 10 mm under the same conditions of device
fabrication; the corresponding photovoltaic parameters are
summarized in Table 2. The Soret bands of 4 and 5 adsorbed on TiO2
films show a saturated feature but the Q bands are significantly
To acquire insight into the electronic distribution at the frontier and
close-lying molecular orbitals, we performed quantum-chemical

322
C.-W. Lee et al. / Dyes and Pigments 91 (2011) 317e323
structure of 5, one should consider a structural modification that
results in the filling of the IPCE spectral gap between Soret and Q
bands and to further reduce dye aggregation so as to reach higher
value of JSC for better cell performance.
4. Conclusion
Two novel electronically coupled porphyrin-arene dyads 4 and 5
have been synthesized and applied in dye-sensitized solar cells. A
perylene sensitizer P1 was synthesized and tested for comparison.
As a consequence of an extended p-conjugation, both Soret and Q
bands of 4 exhibit a significant red shift and a Soret band broader
than that of 5, because the electronic interaction with the porphyrin
is more pronounced for the perylene than for the naphthalene. The
results clearly show that
p-conjugation with a highly conjugated
acetylenic bridge unit can improve the light-harvesting capability
of porphyrin dyes. Of the two porphyrin dyes, 5 exhibits a greater
efficiency of power conversion, 3.9%, which is 7.8 times the value
for P1 under similar conditions. The IPCE spectra and JSC shows
a trend for 5 > 4 > P1. The overall efficiencies of P1, 4 and 5 for
immersion periods 4 and 8 h are 0.5, 1.8 and 3.9%, and 0.3, 1.1 and
3.7% respectively. The poor photovoltaic performance of dye 4 is
due to the formation of aggregates. To improve the performance of
solar cells, further structural modification of the meso-substituted
porphyrins to suppress dye aggregation is under investigation in
our laboratory.
Fig. 5. (a) Absorption spectra of films, (b) IPCE action spectra of devices, and (c)
current-voltage characteristics of devices fabricated with P1, 4 and 5 sensitized on TiO2
films.
Acknowledgement
National Science Council of Taiwan and Ministry of Education of
Taiwan, R.O.C., under the ATU program, provided support for this
project.
blue-shifted with respect to those shown in solution (Fig. 2). This
blue-shifted feature was more pronounced for P1, indicating that
pep stacking of the dye on TiO2 was in an H-type manner. The IPCE
Appendix. Supplementary data
spectra show a systematic trend 5 > 4 > P1, which is consistent
with the variation of JSC showing the same order (5 > 4 > P1). The
overall efficiencies of P1, 4 and 5 are 0.5, 1.8 and 3.9%, respectively,
for an immersion period 4 h. The amounts of dye-loading are
determined to be 184, 119 and 143 nmol cmꢁ2 for P1, 4 and 5,
respectively. When the immersion period was increased to 8 h, the
efficiencies of P1, 4 and 5 decreased to 0.3, 1.1 and 3.7%, respec-
tively. Even though the amounts of dye-loading increased with
increasing immersion period, the corresponding cell performances
decreased. In particular, 4 shows a decrease in cell performance of
ca. 40% when the immersion period was increased from 4 h to 8 h.
Because of the planar feature of the molecule, we expect that dye
aggregation was a major problem for the poor performance of 4. On
the other hand, the extent of dye aggregation for 5 is less signifi-
cant. We have shown that aggregation of porphyrin dyes on TiO2
surface results in rapid intermolecular energy transfer that
substantially reduces the efficiency of electron injection and leads
to poor cell performance [28,29]. Even though less dye molecules
were adsorbed on the TiO2 film, 5 showed much better cell
performance over that of 4. To design a new dye based on the
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.dyepig.2011.04.010.
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