Self-assemblies formed by isonicotinic acid analogues axially coordinating with zinc porphyrin via pyridyl unit: Synthesis and application in dye sensitized solar cells

Article abstract of DOI:10.1016/j.tetlet.2016.03.054

A novel zinc porphyrin ZnP bearing a 1,3,5-triazine-2,4-diamine unit has been designed and synthesized in good yields to construct axial-coordinated assemblies (denoted as ZnP-Ai, i = 1, 2, 3 and 4) with different isonicotinic acid analogues (assigned as

Full text of DOI:10.1016/j.tetlet.2016.03.054

Accepted Manuscript
Self-assemblies formed by isonicotinic acid analogues axially coordinating with
zinc porphyrin via pyridyl unit: Synthesis and application in dye sensitized solar
cells
Jun-Xiang Zhang, Fa-Ming Han, Jia-Cheng Liu, Ren-Zhi Li, Neng-Zhi Jin
PII:
S0040-4039(16)30276-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.03.054
TETL 47442
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
12 January 2016
14 March 2016
17 March 2016
Please cite this article as: Zhang, J-X., Han, F-M., Liu, J-C., Li, R-Z., Jin, N-Z., Self-assemblies formed by
isonicotinic acid analogues axially coordinating with zinc porphyrin via pyridyl unit: Synthesis and application in
dye sensitized solar cells, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.03.054
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Self-assemblies formed by isonicotinic acid analogues axially
coordinating with zinc porphyrin via pyridyl unit: synthesis
and application in dye sensitized solar cells
Jun-Xiang Zhang a, Fa-Ming Hana, Jia-Cheng Liu* a, b, Ren-Zhi Li c, Neng-Zhi Jin d
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 730070,
People’s Republic of China.
b
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s Republic
of China
c
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun 130022, People’s Republic of China.
d Gansu Computing Center, Lanzhou 730030, People’s Republic of China.
*Corresponding authors.
Tel: +869317975636, fax: +869317971989.
E-mail addresses: jcliu8@nwnu.edu.cn (J. C. Liu).
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Abstract
A novel zinc porphyrin ZnP bearing a 1,3,5-triazine-2,4-diamine unit has been
designed and synthesized in good yield to construct axial-coordinated assemblies
(denoted as ZnP-Ai, i = 1, 2, 3 and 4) with different isonicotinic acid analogues
(assigned as A1, A2, A3 and A4) through zinc-to-ligand axial coordination approach.
Then these assemblies were grafted onto TiO2 electrode surface to build dye
sensitized solar cells. The performance of these cells also was examined.
Photoelectrochemical measurements illustrate that all the assemblies based cells have
certain capabilities to export photocurrent. Especially, the ZnP-A2 assembly with a
cyanoacrylic acid group based solar cell has the highest conversion efficiency of 0.81%
with a JSC of 3.0 mA/cm2, a VOC of 0.38 V, and a FF of 0.71 among these assemblies
based solar cells. Additionally, the UV–Vis absorption, fluorescence spectra, density
functional theory (DFT) calculations, transmission electron microscopy (TEM) and
electrochemical impedance spectra (EIS) were performed to determine the
photoelectric behavior as well as morphological structure on the TiO2 surface
modified by these assemblies.
Keywords: anchoring molecule; zinc porphyrin; axial coordination; solar cell.
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1. Introduction
Solar energy is highly expected to solve the environmental and climate problems
caused by consumption of fossil fuel, as a kind of clean and renewable energy with its’
abundant natural resource on earth. Among the various ways utilizing solar energy,
dye sensitized solar cells (DSSCs) firstly reported by Grätzel and co-workers in 1991
[1] are promising photovoltaic devices converting solar energy to electricity and
attracted much more attentions. In this type of solar cells, dyes play a dominated role
in photocurrent generating [2-5]. For the past decades, variety of dyes including
ruthenium complexes [6-9], mental-free organic dyes [10,11] and porphyrin based
dyes [2,3,12，13] have been developed and applied in DSSCs and show excellent
conversion efficiency.
Porphyrins are high-potential candidates of the dyes applied in DSSCs due to
their strong light-harvesting ability in the visible region, high photochemical and
electrochemical stabilities as well as tunable molecular structures [14-18]. In
particular, such porphyrins commonly feature a donor-π bridge-accepter (D-π-A)
structure and are attached onto TiO2 surface through covalently linking via anchoring
groups containing a carboxylic or cyanoacrylic group [19]. However, the complicated
and high-cost syntheses of these asymmetrical porphyrins limit their extensive
application.
Utilization of axial metal-ligand coordination appears to be an effective and
alternative approach to facilely construct porphyrin based donor-acceptor
supramolecular assemblies with remarkable photoelectric properties [20-24]. In this
type of assemblies, ligand coordinated with a mental ion in the porphyrin center at the
axial place as electron acceptor. Sufficient researches indicate that electron transfer
between the porphyrin and axial ligand is capable and feasible [21-25]. Axial-bonding
approach can afford a conveniently method for designing and synthesizing of more
elaborate porphyrin assemblies having diverse structures and functions. Therefore,
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such axial-coordinated assemblies are widely studied and applied in photocurrent
generation and photocatalysis due to their excellent advantages [25-28].
Herein, we have designed and synthesized a novel zinc porphyrin ZnP bearing a
1,3,5-triazine-2,4-diamine unit in good yield to form such axial-coordinated
assemblies (noted as ZnP-Ai, i = 1, 2, 3 and 4) with different ligands or anchoring
groups (signed as A1, A2, A3 and A4). Then these assemblies were grafted on to TiO2
electrode surface and the performance of their corresponding DSSCs are tested. The
molecule structures and coordination mode are shown in Scheme 1. The triazine unit
was employed to the porphyrin due to its’ excellent photoelectric and electrochemical
properties [29-31] in photoelectric devices and the long alkyl chain was introduced to
the meso position of the porphyrin for enhancing solubility and reducing dye
aggregation.
Scheme 1. Structures of axial coordination-bond-assisted self-assemblies on TiO2 electrode
surfaces
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2. Experimental Section
Supplementary Materials completely described the specific synthetic procedures
of ZnP and A2-A4 as well as device fabrication methods. The sensitized titania
electrode was assembled as the following procedures: A circular TiO2 electrode
(∼0.28 cm2) was primarily stained by immersing it into a solution containing
anchoring molecule (~2 mM) in DMF overnight, the unbounded molecules were
washed out by ethanol, then the electrode was immersed into a dye solution
containing ZnP (~0.2 mM) in CHCl3/CH3OH for 2 h and washed by acetonitrile
solution three times. After dried by air flow, the sensitized titania electrode was
fabricated to cells according to the methods specified in Supplementary Materials.
UV-vis spectra were obtained on a UV-2550 spectrometer. Fluorescence spectra
were measured by LS-55 (PE USA Inc) fluorescence spectrophotometer at room
temperature. Transmission electron microscopy (TEM) (Hitachi Model H-900) was
carried out to characterize the morphology and particle feature. The current
density−voltage (J−V) and incident photon-to-collected electron conversion efficiency
(IPCE) were measured under an irradiance of 100 mW∙cm-2 (the equivalence of one
sun at AM 1.5G) by using a metal mask with an aperture area of 0.158 cm2. The
details of photoelectrochemical measurements were illustrated in our previous article
[32]. The electrochemical impedance spectrum (EIS) experiments were performed by
applying a bias voltage at -0.7 V under the dark using a CHI660 electrochemical
workstation, with a frequency range of 0.1Hz −100 kHz and ac amplitude of 10 mV.
The obtained spectra were fitted with ZSimpWin software in terms of an appropriate
equivalent circuit [33].
3. Results and discussion
3.1. UV-visible absorption and fluorescence spectra.
The UV-visible absorption spectra and fluorescence spectra of ZnP in CHCl3
solution and the assemblies ZnP-Ai (i = 1~4) adsorbed onto TiO2 films are shown in
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Fig.1a and b separately, detailed absorption and emission maximum wavelength
numbers are summarized in Table S1. As shown in Fig.1a, these assemblies on TiO2
thin-films (2.5μm) exhibit typical porphyrin spectra [34]: strong Soret bands (B-bands)
appear in the region of 400-500 nm while Q bands locate in the long wavelength
region of 550-650 nm, attributed to π-π* transitions. The absorption bands of these
assemblies on TiO2 surface are slightly shifted to the lower wavelength along with
broadening Soret and enhanced Q bands compared with that of their respective
porphyrin ZnP.
Fig. 1. Normalized UV−vis absorption spectra (a) and emission spectra (b) of ZnP in CHCl3
solution and ZnP-Ai adsorbed onto TiO2 films. The emission spectra were measured by exciting
at 420 nm
When jointing ZnP onto TiO2 surface via anchoring molecules, the fluorescence
spectra of the assemblies are weakened and blue shifted. These shifts should be
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assigned to the side-to-side J-aggregation of porphyrins [35,36]. The weakened
fluorescence intensities indicated the existence of electronic coupling behavior
between ZnP and anchoring molecules through zinc-to-ligand axial coordination.
3.2. Density functional theory (DFT) calculations.
In order to further understand the electron density distribution of ZnP-Ai
assemblies within the frontier and other neighboring orbitals, DFT calculations were
performed using the Gaussian 09 program [37] at the B3LYP/LanL2DZ level of
theory for the ground state geometry optimization. The butoxyl groups in ZnP was
simplified to a methoxy group for a less uncomplicated calculation process. As shown
in Fig. 2, the molecule orbital (MO) patterns of the assemblies are in accordance with
the Gouterman's four-orbital model with a slight deviation [34], suggesting that, from
another respect, the electronic adsorption should attribute to π-π* transitions.
Delocalization of the MO patterns at the HOMO and LUMO orbitals can be assigned
to the lowered symmetry and the extended π-conjugation of the assemblies.
Importantly, the electrons of HOMO and LUMO orbitals concentrated at the top
porphyrin cores and bottom anchoring molecules respectively, which are similar to the
result of the reported [38] based on experiment observations and calculations of a
series of zinc porphyrins. Such an electron-separated state may translate to a
push–pull tendency of the assemblies upon photoexcitation, a favorable merit for
electron transferring. It is worthy noticed that in the LUMO+1 level of ZnP-A2
assembly, electron densities extremely diverged to the carboxylic group in A2
molecule, indicating that electron injection from the excited A2 based assembly to
TiO2 conduction band (CB) can be more effective and guaranteeing a better cell
performance when applied in solar cells [39,40]. This inference can be further
demonstrated by the calculated energy level depicted in Fig. 3.
Fig. 3 shows the calculated energy level diagram of the assemblies, all the
LUMO levels of these assemblies lie above the CB of TiO2, all the HOMO levels are
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lower than the redox energy of I-/I3-. As suggested in the figure, all assemblies should
be efficient to inject electrons to the CB of TiO2 upon excitati on and the
corresponding oxidized molecule can be efficiently reduced by the electrolyte.
Fig. 2. Frontier molecular orbital distributions of ZnP-Ai assemblies under B3LYP/LanL2DZ
level.
Fig. 3. Frontier orbital energy distribution of the assemblies.
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3.3. Morphological characterization of the TiO2 nanoparticles modified by
assemblies
To gain further insight into the architecture of the assemblies attached onto TiO2,
transmission electron microscopy (TEM) was used to investigate the morphologies of
ZnP-Ai modified TiO2 surfaces. Fig. 4 shows the TEM images of TiO2 nanoparticles
modified with ZnP-Ai, as presented in the figure, the surface of TiO2 nanoparticles
were effectively overlaid by a layer of assemblies, demonstrating the well-organized
joint of porphyrin ZnP onto the TiO2 surface through axially coordination with
anchoring molecules. Simultaneously, the layer thickness was identified to be 1.3±
0.1 nm for ZnP-A1, 1.2±0.1 nm for ZnP-A2, 1.6±0.1 nm for ZnP-A3 and 1.7±0.1
nm for ZnP-A4 modified TiO2 respectively. The layer thicknesses almost match to the
molecule heights obtained by computational simulation displayed in Fig. S1 in
supplementary materials, ulteriorly verifying the assembly patterns.
Fig. 4. TEM images of TiO2 nanoparticles modified with ZnP-A1 (a), ZnP-A2 (b), ZnP-A3 (c)
and ZnP-A4 (d).
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3.4. Photovoltaic Properties.
The photovoltaic performance of devices with ZnP-Ai assemblies was measured
using double-layer TiO2 films as photoanodes, in combination with I-/I3- as electrolyte,
under standard AM 1.5G illumination and under the dark. IPCE spectra of the cells
fabricated with ZnP-Ai in the wavelength range of 350~650 nm are shown in Fig. 5.
The photocurrent responses roughly resemble their corresponding absorption spectra
and computational energy gaps. The highest IPCE value of 70%, 44%, 33%, 30%
were achieved for cells based on assemblies ZnP-A1, ZnP-A2, ZnP-A3 and ZnP-A4,
respectively. Meanwhile, the IPCE values rank in a sequence of ZnP-A2 > ZnP-A1 >
ZnP-A3≈ZnP-A4, suggesting that electrons can feasibly transfer to the anchoring
molecule upon photoexcitation then bound to TiO2 surface, this result consists with
the DFT calculations. Our previous research [41] revealed that the device without
anchoring molecule exhibits obviously lower performance than the assembly-based
device. Moreover, A2 based cell shows the better performance than A1 based cells
under the same testing condition, where A1 is the best anchor we have found before.
Fig. 5. IPCE values of ZnP-Ai based solar cells.
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Fig. 6. Current density–voltage (J-V) characteristics for solar cells based on the assemblies under
irradiance of 100 mW∙cm-2 AM 1.5G sunlight (symbols) and in the dark (dotted lines).
The current density-voltage (J-V) plots of the assemblies based solar cells are
given in Fig. 6. The detailed parameters are collected in Table 1. The solar cell based
on ZnP-A2 exhibits an noticeable conversion efficiency of 0.81% with a JSC of 3.0
mA/cm2, a VOC of 0.38 V, and a FF of 0.71. The explanation for the higher
short-current density of ZnP-A2 based solar cells can be mainly attributed to the
appropriate HOMO-LUMO level and charge separate state as well as more feasible
electron transmission along the porphyrin-anchor-TiO2 pathway compared to other
assemblies. The A3 and A4 with amide groups based devices showed lower
efficiencies due to their faintish photocurrent response. Compared to the present
high-efficiency DSSCs [12-15], the lower overall performances of ZnP-Ai -sensitized
solar cells can be mainly attributed to the small amount of porphyrin loaded on TiO2
surface, reflected by the faint color of modified electrodes and the weak UV–vis
absorbing intensities on TiO2 film.
Table 1. Photovoltaic parameters of ZnP-Ai -sensitized solar cell.
Dye
JSC/mA∙cm-2
VOC/V
0.32
0.38
0.33
0.28
FF
η/%
0.49
0.81
0.40
0.32
2.20
3.00
1.73
1.69
0.69
0.71
0.70
0.67
ZnP-A1
ZnP-A2
ZnP-A3
ZnP-A4
We also investigated the dark current of these cells to inspect the charge
recombination on the TiO2 surface. As presented in Fig. 6, an obviously increased
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dark current for the cells based on ZnP-A1, ZnP-A3 and ZnP-A4 at a given potential
bias was observed, suggesting a facilitated charge recombination of electrons in
titania with I-/I3- electrolyte. As is well known, seriously charge recombination can
result in a lower VOC [42], the higher VOC value of ZnP-A2 was caused by the lower
charge recombination rate at the dye/TiO2/electrolyte interface, which was further
surveyed by EIS measurements.
Fig. 7. Impedance spectra of ZnP-A1 and ZnP-A2 fabricated solar cells, the lines of a and b show
theoretical fits using the equivalency circuits c.
EIS measurement was conducted to deeply understand the VOC variation in solar
cells based on ZnP-A1 and ZnP-A2 at a forward bias of −0.70 V in the dark. The
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Nyquist plots (a), Bode plots (b) and equivalent circuit (c) are shown in Fig. 8. The RS,
RCT, and C represent series resistances, charge transfer resistances at the dye/TiO2/
electrolyte interface and double layer capacitance, respectively. In the Nyquist plots,
the larger semicircle located in the middle-frequency region is refer to the charge
transfer process at the dye/TiO2/electrolyte interface [43]. ZnP-A2 based cells display
higher charge recombination resistance (RCT) value than the ZnP-A1, indicating a
slow rate of charge recombination at the dye/TiO2/electrolyte interface. The larger RCT,
in other words, represented the restraining charge recombination between the injected
electron in the TiO2 and oxidized species in the electrolyte. Usually, a high charge
recombination resistance will lead to a long electron lifetime reflected by the phase
degree peaks in bode plots moved to lower frequency. It can be noticed in Fig. 8b that
ZnP-A2 based solar cells have longer electron lifetime in associated with its’ higher
VOC value compared to ZnP-A1 based cells, in agreement with its’ electron density
distribution, HOMO-LUMO levels and photocurrent responses.
4. Conclusions
we have successfully synthesized
a
novel porphyrin ZnP bearing
1,3,5-triazine-2,4-diamine unit and jointed onto TiO2 electrode surfaces through
axially coordinating with anchoring groups (A1, A2, A3 and A4) bonded on the TiO2
surfaces. The effects of different anchoring molecules in the solar cells fabricated by
these self-assemblies were investigated, and the results suggest that the ZnP-A2
assemblies immobilized onto nanostructured TiO2 surfaces using Zn-to-ligand axial
coordination significantly improved photovoltaic performance. Additionally, the
optical properties, molecular orbital (MO) distributions, HOMO-LUMO energy gaps
of these assemblies are also considered and further demonstrate the experimental
results. The morphological characteristics of the TiO2 nanoparticles modified by these
assemblies were also investigated by transmission electron microscopy (TEM) and
computational simulations. EIS measurements indicate that ZnP-A2 based cells
exhibit higher charge recombination resistance and longer electron lifetime than the
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ZnP-A1 based cells, resulting a higher VOC value. Overall, this work provides a new
type of self-assembly different from other porphyrin-based sensitizers and has
developed a facile method for fabricating solar cells.
Appendix A. Supplementary material
Supplementary material of this article can be found, in the online version, at…
Acknowledgments
The National Natural Science Foundation of China (No. 21461023) has
supported this work. 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 acknowledge the support of Gansu
Computing Center of China.
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Graphical abstract
A novel zinc porphyrin ZnP bearing a 1,3,5-triazine-2,4-diamine unit have been
designed and synthesized to construct axial-coordinated assemblies with different
anchoring groups. Then these assemblies were grafted onto TiO2 electrode surface and
their corresponding solar cells were successfully fabricated. The performance of these
devices were studied and discussed.
17 / 18

Highlights
1. Axial-coordinated self-assemblies based on a novel zinc porphyrin (ZnP).
2. Isonicotinic acid analogues (A1, A2, A3 and A4) act as anchoring molecules.
3. Their performance in dye sensitized solar cells was tested.
4. All the assemblies based cells have certain capabilities to export photocurrent.
5. Improved photocurrent generation of ZnP-A2 with
acrylic acid group.
a
cyano-
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