New self-assemblies with zinc meso -tetra[3-(1H-imidazol-1-yl)phenyl] porphyrin for use in supramolecular solar cells

Article abstract of DOI:10.1080/00958972.2013.867033

In this work, a new zinc meso-tetra[3-(1H-imidazol-1-yl)phenyl]porphyrin (ZnP) was synthesized. Further, the porphyrin ZnP was immobilized by metal-ligand axial coordination (ZnP-A) and a metal-ligand edged binding approach (ZnP-Zn-A) on the nanostructured TiO2 electrode surface modified with coordinating ligand functionality, isonicotinic acid (A). The performances of the assemblies-sensitized solar cells were performed under irradiance of 100 mW cm^-2 AM 1.5G sunlight. Photo-electrochemical studies reveal significantly improved performance of the assembly ZnP-A. These assemblies can afford a fertile base for further design and fabrication of new supramolecular solar cells in future. 2013

Full text of DOI:10.1080/00958972.2013.867033

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New self-assemblies with zinc meso-
tetra[3-(1H-imidazol-1-yl)phenyl]
porphyrin for use in supramolecular
solar cells
Jing Caoa, Dong-Cheng Hua, Jia-Cheng Liua, Ren-Zhi Lib & Neng-Zhi
Jinc
a Key Laboratory of Eco-Environment-Related Polymer Materials
of Ministry of Education, Key Laboratory of Polymer Materials
of Gansu Province, Key Laboratory of Bioelectrochemistry &
Environmental Analysis of Gansu Province, College of Chemistry
and Chemical Engineering, Northwest Normal University, Lanzhou,
PR China
b State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun, PR China
c Gansu Computing Center, Lanzhou, PR China
Published online: 03 Dec 2013.
To cite this article: Jing Cao, Dong-Cheng Hu, Jia-Cheng Liu, Ren-Zhi Li & Neng-Zhi Jin
(2013) New self-assemblies with zinc meso-tetra[3-(1H-imidazol-1-yl)phenyl] porphyrin for
use in supramolecular solar cells, Journal of Coordination Chemistry, 66:23, 4211-4219, DOI:
10.1080/00958972.2013.867033
To link to this article: http://dx.doi.org/10.1080/00958972.2013.867033
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 23, 4211–4219, http://dx.doi.org/10.1080/00958972.2013.867033
New self-assemblies with zinc meso-tetra[3-(1H-imidazol-1-yl)
phenyl] porphyrin for use in supramolecular solar cells
JING CAO†, DONG-CHENG HU*†, JIA-CHENG LIU*†, REN-ZHI LI‡ and
NENG-ZHI JIN§
†Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education, Key
Laboratory of Polymer Materials of Gansu Province, Key Laboratory of Bioelectrochemistry &
Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou, PR China
‡State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun, PR China
§Gansu Computing Center, Lanzhou, PR China
(Received 17 September 2013; accepted 14 November 2013)
In this work, a new zinc meso-tetra[3-(1H-imidazol-1-yl)phenyl]porphyrin (ZnP) was synthesized.
Further, the porphyrin ZnP was immobilized by metal-ligand axial coordination (ZnP-A) and a
metal-ligand edged binding approach (ZnP-Zn-A) on the nanostructured TiO2 electrode surface
modified with coordinating ligand functionality, isonicotinic acid (A). The performances of the
assemblies-sensitized solar cells were performed under irradiance of 100 mW cm−2 AM 1.5G
sunlight. Photo-electrochemical studies reveal significantly improved performance of the assembly
ZnP-A. These assemblies can afford a fertile base for further design and fabrication of new
supramolecular solar cells in future.
Keywords: Self-assembly; Zinc porphyrin; Solar cell
1. Introduction
Dye-sensitized solar cells (DSSCs), as a cost-efficient, easy to fabrication and environmen-
tally friendly product, have attracted considerable attention since Grätzel and co-workers
cost and environmental concern,s reported their groundbreaking work on DSSCs with a por-
ous and nano-crystalline TiO2 electrode modified with an Ru-bipyridyl complex in 1991
[1]. Much effort has been devoted to modify the Ru(II) complexes to improve their conver-
sion efficiencies [2–4]. However, on the basis of cost and environmental concerns, ruthe-
nium dye is not suitable for the development of cost-efficient and environmentally friendly
photovoltaic devices. In nature, photosynthetic antenna reaction centers, such as the porphy-
rin macrocycle as a basic chromophore framework, can use a series of intermolecular
forces, such as coordination bonding, electrostatic hydrogen bonding, and weak van der
*Corresponding authors. Email: hudch@163.com (D-C Hu); jcliu8@nwnu.edu.cn (J-C Liu)
© 2013 Taylor & Francis

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J. Cao et al.
Waals interaction, to precisely tune the electronic coupling of electron donor and acceptor
units in biomimetic systems [5, 6]. Inspired by this fascinating feature, porphyrin-based
dyes have been used to mimic such features to construct artificial light-harvesting devices
in solar cells, and the porphyrin chromophore has assured an excellent electron donor group
in natural photosynthetic processes [7–12]. It is worth noting that the new PCE record of
the porphyrin-based solar cell has reached 12.3% under simulated air mass 1.5 global sun-
light [8]. Simultaneously, porphyrin-based self-assemblies have also attracted more attention
in attempting to build artificial light-harvesting devices, with an ultimate goal of generating
efficient charge separation and carrying of the separated charges to their respective elec-
trodes in photosynthesis devices [13–16].
In our previous work, we have reported a series of new self-assemblies based on zinc
porphyrin derivatives appended organic acid via a metal-ligand axial coordination and
metal-ligand edged binding approach to functionalize the nanostructured TiO2 electrode sur-
faces, and their respective photoelectrochemical performances were also studied [17–19]. In
this work, we prepare an extension of our investigations into two new assemblies, organized
by zinc meso-tetra[3-(1H-imidazol-1-yl)phenyl] porphyrin (ZnP) appended isonicotinic acid
(A), by a metal-ligand edged binding approach (ZnP-Zn-A, Zn2+ as the metal-mediating
entity) and metal-ligand axial coordination (ZnP-A). The detailed assembled modes are
shown in scheme 1.
2. Experimental
2.1. Materials and physical measurements
All solvents and reagents were used directly without further purification as commercially
analytical grade. Electronic absorption spectra were measured on a UV-2550 spectrometer.
The luminescence spectrum was measured by a LS-55 (PE USA Inc.) fluorescence
1
spectrophotometer at room temperature. H NMR (400 MHz) was measured on a Varian
(a)
(b)
Scheme 1. The assembled modes of assemblies on TiO2 electrode surfaces. (a) Metal-ligand edged binding
approach; (b) metal-ligand axial coordination.

Supramolecular solar cells
4213
N
Br
Br
N
H
N
N
N
Br
Br
NH
N
N
Zn(OAc)2.2H2O
Br
N
N
N
N
N
N
N
N
Z n
Z n
HN
N
Cu,K2CO3/DMF
N
Br
N
Br
Br
N
N
ZnP
Scheme 2.
Mercury Plus-400 spectrometer. The electrospray ionization (ESI) mass spectrum was
measured on a ESI-TOF mass spectrometer. The detail synthesis processes are shown in
scheme 2.
2.2. Synthesis
5,10,15,20-tetra(m-bromophenyl)porphyrin was prepared according to the corresponding lit-
erature method [20]. A mixture of 5,10,15,20-tetra(m-bromophenyl) porphyrin (0.1 mM)
and Zn(OAc)2·2H2O (12 mM) were dissolved in 10 mL chloroform and 2 mL methanol.
The mixture was refluxed for 8 h. Then the solution was evaporated in vacuo. The resulting
precipitate was washed by methanol solution several times. The product was collected and
dried under vacuum to yield 5,10,15,20-tetra(m-bromophenyl)porphyrin zinc. Yield, 91%.
At the next stage, a mixture of zinc 5,10,15,20-tetra(m-bromophenyl)porphyrin
(0.1 mM), imidazole (0.8 mM), K2CO3 (0.6 mM), and copper powder (0.5 mM) was added
to an anhydrous DMF (10 mL), then the suspended solution was refluxed for 24 h under N2
atmosphere (the reaction was monitored by thin-layer chromatography). After the removal
of DMF, the solid was extracted with chloroform. The organic layer was dried over anhy-
drous sodium sulfate. The product was further purified on a silica gel column with CHCl3/
1
C2H5OH (1/3, v/v) as eluent to afford ZnP. Yield, 6.2%. H NMR (400 MHz, CDCl3): δ
8.89–8.91 (m, 8H, β-pyrrole), 8.74 (s, 4H, 2-imidazoyl), 8.22–8.36 (m, 8H, imidazoyl),
8.03–8.12 (m, 8H, ph), 7.72–7.78 (d, 4H, ph), 7.34–7.53 (d, 4H, ph). ESI-MS: m/z 940 [M
+ H]+.
2.3. Device Fabrication
A screen-printed double layer film of interconnected TiO2 particles was used as the meso-
porous negative electrode. A 7 μm-thick transparent layer of 20 nm-sized titania particles
were first printed on the fluorine-doped SnO2 (FTO) conducting glass electrode and further
coated by a 5 μm-thick scattering layer of 400 nm-sized titania particles. The film thickness
was measured by a benchtop Ambios XP-1 stylus profilometer. The detailed preparation
procedures of TiO2 nanocrystals, pastes for screen-printing, and nanostructured TiO2 film
have been reported by Wang [21]. For the metal-ligand edged binding approach, a cycloidal
TiO2 electrode (~0.28 cm2) was stained by immersing it into a dye solution containing i-
sonicotinic acid (2 mM) as the anchoring molecule in methanol solution overnight after
removal of the unbound molecules (through three ethanol washings), then immersing it into

4214
J. Cao et al.
a 2 mM ZnCl2 methanol solution for 1 h, followed by washing with methanol, immersing
into a dye solution containing ZnP (0.2 mM) in DMF for 2 h, and after this, washing by
DMF solution three times. For the metal-ligand axial coordination mode, the first procedure
was the same as that of the metal-ligand edged binding approach followed by immersion
into a dye solution containing ZnP (0.2 mM) in DMF for 2 h and washing by DMF solu-
tion three times. After being washed with ethanol and dried by air flow, the sensitized tita-
nia electrode was assembled with a thermally platinized FTO electrode. The electrodes
were separated by a 35 μm-thick Bynel (DuPont) hot-melt gasket and sealed up by heating.
The internal space was filled with a liquid electrolyte using a vacuum backfilling system.
The electrolyte-injecting hole on the counter electrode glass substrate, made with a sand-
blasting drill, was sealed with a Bynel sheet and a thin glass cover by heating. The electro-
lyte used contained 50 mM LiI and 30 mM I2 in acetonitrile solvent. After all these proce-
dures, the cells were located in the oven for heating post-treatment at 100 °C for 30 min
and cooled to room temperature before photoelectrochemical measurements.
2.4. Photovoltaic Characterization
A LS1000 solar simula-tor (Solar Light Com. Inc., USA) was used to give an irradiance of
100 mW cm−2 (the equivalent of one sun at AM 1.5G) at the surface of a testing cell. The
current–voltage characteristics were obtained by applying external potential bias to the cell
and measuring the dark current and photocurrent with a Keithley model 2602 digital source
meter. This process was fully automated using Labview 8.0. A similar data acquisition sys-
tem was used to control the incident photon-to-collected electron conversion efficiency
(IPCE) measurement. Under full computer control, light from a 1000 W xenon lamp was
focused through a monochromator onto the photovoltaic cell under test. A computer-con-
trolled monochromator (Omni λ300) was incremented through the spectral range
(300–900 nm) to generate a photocurrent action spectra with a sampling interval of 10 nm
and a current sampling time of 2 s. IPCE is defined by IPCE(λ) = hcJsc/eΦλ, where h is
Planck’s constant, c is the speed of light in a vacuum, e is the electronic charge, λ is the
wavelength (m), Jsc is the short-circuit photocurrent density (A m−2), and Φ is the incident
radiative flux (W m−2). Photovoltaic performance was measured by using a metal mask with
an aperture area of 0.158 cm2. A homemade heating-cooling system was used for tempera-
ture-dependent J–V measurements.
3. Results and discussion
3.1. UV–visible and fluorescence spectra
The detailed data of UV–vis absorption and fluorescence spectra of ZnP dissolved in DMF
solution (~10−6 M) and ZnP-Zn-A/ZnP-A on TiO2 thin films are shown in table S1 (see
online supplemental material at http://dx.doi.org/10.1080/00958972.2013.867033) and
figure 1. The results reveal the absorption maxima in the range of 400–480 nm for strong
soret (B) bands and 550–630 nm for Q bands, respectively, ascribed to π–π* transitions
[22]. It can be seen that the B and Q bands of ZnP-Zn-A/ZnP-A are more red-shifted than
those of ZnP, ascribed to the formation of coordination-bond of Zn-to-ligand to extend the
π-conjugated system in ZnP-Zn-A/ZnP-A. Besides, the broadened B bands of ZnP-Zn-A/
ZnP-A are also found, which can be ascribed to the presence of J-type porphyrin-aggrega-
tion on the TiO2 surfaces [23, 24]. The considerably broadened B bands of assemblies

Supramolecular solar cells
4215
ZnP
ZnP-Zn-A
ZnP-A
0.4
0.2
0.0
420
480
540
600
660
Wavelength/nm
Figure 1. UV–vis spectra of ZnP in DMF solution and ZnP-Zn-A/ZnP-A on TiO2 thin films.
indicate that the minimized gaps between the B and Q bands of the assemblies may yield
significantly enhanced photoelectronic responses [25]. Notably, the absorption bands of
ZnP-A are slightly red-shifted compared to those of ZnP-Zn-A, consistent with the varia-
tion of HOMO-LUMO gaps of ZnP-A and ZnP-Zn-A. As illustrated in figure 2, the results
show that the emission spectrum of ZnP in the DMF solution is red-shifted relative to those
of ZnP-Zn-A/ZnP-A. In addition, the emission spectra of ZnP-A show slight blue shifts
compared to those of ZnP-Zn-A. These features may be ascribed to the presence of
J-aggregation [23, 24] of porphyrin molecules on the TiO2 surfaces, especially for the
assembly ZnP-A.
3.2. Molecular orbital patterns and energy levels
To further understand the electron density distribution within the frontier and other neigh-
boring orbitals of ZnP-Zn-A/ZnP-A, we performed calculations with density functional
800
ZnP
ZnP-Zn-A
ZnP-A
600
400
200
0
570
600
630
660
690
Wavelength/nm
Figure 2. Fluorescence spectra of ZnP in DMF solution and ZnP-Zn-A/ZnP-A on TiO2 thin films.

4216
J. Cao et al.
theory (DFT) using the Gaussian 09 program [26] at the B3LYP/LanL2DZ level. As shown
in figure S1, the molecular orbital (MO) patterns of ZnP-Zn-A from HOMO-1 to LUMO+2
are consistent with those of Gouterman’s four-orbital model, wherein the HOMO-1 and
HOMO levels match those of the a1u and a1u (π) orbitals, the LUMO and LUMO+1 are
similar to those of the eg (π*) orbitals. These features show that the electronic adsorption
can be assigned to π–π* transitions. Due to the decreased symmetry leading to minor devia-
tion from Gouterman’s four-orbital model, HOMO and LOMO levels depict partially delo-
calized behavior from the porphyrin center to the adjacent donor units. The electrons of
LUMO+2 are concentrated at the acceptor groups, consistent with the reported results [17,
24]. However, for ZnP-A, the MO patterns of HOMOs are mainly localized at porphyrin
cores along with slight delocalization to the adjacent donor groups. Whereas, the LUMOs
display an electronic distribution on isonicotinic acid groups, indicating occurrence of
excellent electron-separated status in the ZnP-A-sensitized device. Figure 3 shows and
compares the energy-levels diagram of ZnP-Zn-A/ZnP-A, the valence band (VB) of TiO2,
–
and the electrolyte I–/I3 [27]. The results show that the energy gap between the HOMO
and LUMO levels is 2.83 eV for ZnP-Zn-A and 2.58 eV for ZnP-A. The LUMO levels of
ZnP-Zn-A/ZnP-A are above that of the conducting band (CB) of TiO2, while the HOMO
levels are below the oxidation potential of electrolyte I–/I3À, indicating that electrons of
LUMO from the assemblies can efficiently inject the CB of TiO2, and the excited state of
the assemblies can be efficiently reduced by the electrolyte.
3.3. Performance characterization of solar cells
The assemblies of ZnP-Zn-A/ZnP-A were fabricated into DSSC devices and their
respective photovoltaic behavior measured under irradiance of 100 mW cm−2 AM 1.5G
sunlight. The photocurrent action spectra of ZnP-Zn-A/ZnP-A-sensitized solar cells in the
wavelength range 300–800 nm are presented in figure 4. The key responses of IPCE spectra
depict the energy conversion of B and Q bands of porphyrin. The features of photocurrent
responses are roughly similar to their respective absorption spectra and HOMO-LUMO
ZnP-A
Dye
ZnP-Zn-A
LUMO
-3
-4
-5
-6
TiO
2
CB
2.58
2.83
-
-
I /I3
Redox
HOMO
Figure 3. Orbital energy-levels of the assemblies obtained by theoretical calculation, TiO2, and electrolyte I–/I3
.
–

Supramolecular solar cells
4217
45
30
15
0
ZnP-Zn-A
ZnP-A
400
500
600
700
/nm
Figure 4. Photocurrent action spectra of ZnP-Zn-A/ZnP-A-sensitized solar cells.
gaps, especially for the broadened B bands. The maximum value of the ZnP-A device
above 40% is significantly larger than that of the ZnP-Zn-A device. As shown in figure 5
and table S2, the short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), fill
factor (FF), and overall conversion efficient (η) at AM 1.5G conditions (100 mW cm−2) are
1.12 mA cm−2, 0.33 V, 0.70, and 0.26% for the ZnP-Zn-A device; and 1.93 mA cm−2,
0.30 mV, 0.67, and 0.40% for the ZnP-A device. The ZnP-A device shows a significantly
enhanced conversion efficient, which could be ascribed to the formation of effective trans-
mission channels for electron and energy transfer in the device. Besides, a larger Voc is
observed in the ZnP-Zn-A device. Notably, compared to our previous reported assemblies
(ZnP1-A) based on zinc meso-tetra[4-(1H-imidazol-1-yl)phenyl]porphyrin (ZnP1) [17], the
ZnP-A device reveals enhanced photovoltaic properties under the same assembled and
2
1
ZnP-Zn-A
ZnP-Zn-A (d)
ZnP-A
ZnP-A(d)
0
0.0
0.1
0.2
0.3
V/mV
Figure 5. J-V characteristics of ZnP-Zn-A/ZnP-A-sensitized SC measured under irradiance of 100 mW cm−2 AM
1.5G sunlight (ZnP-Zn-A/ZnP-A) and in the dark (ZnP-Zn-A/ZnP-A(d)).

4218
J. Cao et al.
measured conditions (figures S2, S3 and table S2), but, it is worse than other acetohydraz-
ide Zn(II) porphyrin devices reported in our previous work [18, 19]. The lower overall per-
formances of ZnPx-A-sensitized cells reported in this article can be mainly assigned to the
small amount of adsorbed porphyrin-sensitizers on TiO2 electrode surfaces to capture the
inefficient light harvesting, reflected by the faint color of sensitized electrodes and the low
UV–vis absorbing properties.
4. Conclusions
In summary, we have successfully synthesized and characterized two new self-assemblies
(ZnP-Zn-A and ZnP-A) based on zinc meso-tetra[3-(1H-imidazol-1-yl)phenyl]porphyrin
(ZnP) appended isonicotinic acid ligands (A) by metal-ligand axial coordination and a
metal-ligand edged binding approach. Photo-electrochemical studies reveal that the assembly
ZnP-A shows a significantly improved photocurrent response, which also outperforms the
performance of our previous reported assembly with zinc meso-tetra[4-(1H-imidazol-1-yl)
phenyl]porphyrin under the same conditions. However, compared with other reported
sensitizers of polymeric metal complexes containing 8-hydroxyquinoline [28], although the
values of their Voc are larger than that of ZnP-A, a significantly enhanced Jsc is observed for
ZnP-A. This work affords a fertile base for further investigation into supramolecular solar
cells.
Acknowledgements
We are very grateful to Prof. Peng Wang (Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences) for supplying device fabrication and measurement of solar cells. We also
thank the support of Gansu Computing Center of China.
Funding
This work was supported by the National Natural Science Foundation of China [grant number
20871099]; Gansu provincial Natural Science Foundation of China [grant number
0710RJZA113].
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