Terpyridine-containing porphyrin and coordination assembly with fullerene-based pyridine for enhanced electrocatalytic oxygen evolution and photocurrent response

Article abstract of DOI:10.1016/j.poly.2020.114818

A novel terpyridine-modified porphyrin, ZnP-Pr-tpy, has been synthesized and characterized successfully using elemental analysis, mass spectrometry (MS), nuclear magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR) and ultraviolet/visible spe

Full text of DOI:10.1016/j.poly.2020.114818

Journal Pre-proofs
Terpyridine-containing porphyrin and coordination assembly with fullerene-
based pyridine for enhanced electrocatalytic oxygen evolution and photocur‐
rent response
Zhen-Yi Wu, Li-Jing Huang, Rui Zhong
PII:
S0277-5387(20)30475-7
DOI:
Reference:
https://doi.org/10.1016/j.poly.2020.114818
POLY 114818
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
23 July 2020
9 September 2020
22 September 2020
Please cite this article as: Z-Y. Wu, L-J. Huang, R. Zhong, Terpyridine-containing porphyrin and coordination
assembly with fullerene-based pyridine for enhanced electrocatalytic oxygen evolution and photocurrent
response, Polyhedron (2020), doi: https://doi.org/10.1016/j.poly.2020.114818
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Terpyridine-containing porphyrin and coordination assembly
with fullerene-based pyridine for enhanced electrocatalytic
oxygen evolution and photocurrent response
Zhen-Yi Wu，Li-Jing Huang, Rui Zhong,
1Department of Chemistry， College of Chemistry and Chemical Engineering, Xiamen
University, Xiamen, 361005, P.R. China
Abstract
A novel terpyridine-modified porphyrin, ZnP-Pr-tpy, has been synthesized and
characterized successfully using elemental analysis, mass spectrometry (MS), nuclear
magnetic resonance spectroscopy (NMR), infrared spectroscopy (IR) and
ultraviolet/visible spectroscopy (UV-Vis). Studies by steady-state fluorescence and
UV-visible spectroscopies show that a supramolecular dyad is formed by
fullerene-based pyridine (C60-py) and the zinc porphyrin via a coordination bond,
with a binding constant K of 1.18×105 M-1. The photoexcitation of ZnP-Pr-tpy
causes the ground state electrons of the tpy moiety to shift to the ZnP unit via an
electron-exchange process following the Dexter energy transfer mechanism. Thus, a
donor-acceptor composite film, ZnP-Pr-tpy/C60-py, was fabricated by combining the
photosensitizer ZnP-Pr-tpy with C60-py. A comparison of the over-potential at 10
mA/cm2, the Tafel slopes and the photocurrent response under visible light irradiation
showed the electrocatalytic oxygen evolution reaction (OER) and the photocurrent
response to be clearly improved by the covalent modification of the porphyrin with
terpyridine, and further improved by the axial coordination of C60-py to ZnP-Pr-tpy.
These improvements are attributed to increased intra- and intermolecular electron
transfer, the decrease in the electrochemical impedance resulting from the axial
coordination of C60-py to ZnP-Pr-tpy and the electron-exchange in ZnP-Pr-tpy
through the Dexter energy transfer mechanism under light excitation.
Introduction
The energy crisis and environmental pollution are hot issues in modern society
because traditional petrochemical energy is becoming increasingly exhausted and the
*Corresponding author: Tel: +86-13625010329; fax: +86-592-8818087
E-mail address: zywu@xmu.edu.cn
1 / 25

environment is seriously damaged. It is therefore crucial for research to develop
energy storage and conversion that can provide clean and renewable energy. Water
electrolysis1-3, carbon dioxide electroreduction4-6 and metal-air batteries7-9 are
important means to achieve energy storage and conversion. The electrocatalytic
oxygen evolution reaction (OER)9,10,11 plays a vital role in such energy conversion
technology with noble metal-based electrocatalysts (e.g. Pt, Ru and Ir) currently
considered as the best electrocatalysts. However, their rarity and high cost remain
great obstacles for their wide application. So far, molecules comprising transition
metal oxides, hydroxides, phosphides and sulfides have been developed as low-cost
alternatives to noble metal catalysts for the OER. However, these compounds have
poor electrical conductivity, which hinders their charge transport, and results in slow
kinetics and a high overpotential required to drive the OER. Accordingly, the design
and preparation of efficient and cheap OER electrocatalysts has become an important
research topic. Additionally, converting abundant solar energy into different forms of
storable energy (such as electric and hydrogen energy12-14) is another important way
to utilize clean energy. The design and development of efficient solar energy
conversion materials is therefore also of great significance for research. Organic
materials have unique advantages: their energy band edges are tunable through
molecular structure design and their redox ability can be adjusted to further improve
photo-charge-carrier separation and the spectral response15,16. Organic materials are
also widely available and safe, and have thus attracted increasing research interest. It
is essential for photoelectric materials to have excellent light absorptivity, with highly
efficient charge separation and electronic transmission. Porphyrin compounds are
non-metal semiconductors (with a similar structure to chlorophyll) whose bandgap
structure is suited to visible light absorption and water splitting - they are widely used
as light-harvesting antenna17,18 and electronic donors in photocatalysis and solar
cells19–21. Their photoelectric properties can be optimized by modulating their
molecular structure and charge distribution via the attachment of functional groups to
the porphyrin ring22. These attachments are also an effective strategy to improve the
charge separation of photovoltaic materials23. For instance, terpyridine (tpy), with
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special σ-electron-donating and π-electron-acceptor abilities, can form a
porphyrin-tpy derivative with a large π conjugated structure, which easily coordinates
with transition metals and has an excellent π stacking ability. Thus the
porphyrin-terpyridine derivative is prone to undergoing intramolecular energy
transfer24 and electron transfer25,26 following the Dexter energy transfer mechanism,
and exhibits unique photophysical24,27 and electrochemical properties28, which gives it
a wide range of applications in photoelectricity27, non-linear optics, catalysis and
optical/chemical sensors29-32. However, the pristine porphyrin-terpyridine derivative
has poor intrinsic charge transfer ability and slow OER kinetics, which greatly limits
its photocatalytic oxygen evolution activity. To meet this challenge, a heterostructure
containing a porphyrin-terpyridine derivative and an OER promoter would improve
the interface charge-carrier separation and accelerate water OER kinetics, greatly
enhancing the photocatalytic oxygen evolution activity. Fullerenes, with a strong
electron-withdrawing ability, can form donor-acceptor (D-A) compounds with
porphyrins by covalent bonding or by supramolecular assembly that are susceptible to
undergoing photo-induced electron transfer reactions under light irradiation33.
Accordingly, when the ligand consisting of a fullerene-based pyridine derivative is
further attached to the central metal of a porphyrin-terpyridine derivative via axial
coordination34, photoexcitation of the porphyrin unit will result in the reductive
electron transfer from the terpyridine to the porphyrin and cause oxidation of the tpy
moiety through Dexter's energy transfer mechanism26,24, catalysing water oxidation.
The generated electrons can be quickly transferred to the C60 unit.
Results and discussions
The synthesis route to ZnP-Pr-tpy is illustrated in Fig. 1. Firstly, H2POH was
synthesized and purified by refluxing a mixture of equal equivalents of
p-hydroxybenzaldehyde and pyrrole in propionic acid, according to a modified Adler
method. Next, bromopropyl ether groups were covalently bonded to the para position
of the mesophenyl group of H2POH using 1,3-dibromopropane as the raw material,
based on Williamson's ether synthesis. To avoid polymerization and crosslinking
between the porphyrins, an excess of 1,3-propane was added (molar ratio of
3 / 25

ZnP-Pr-tpy:1,3-dibromopropane = 1:16), followed by reaction with zinc acetate to
produce ZnP-PrBr. Finally, ZnP-Pr-tpy was synthesized using the Williamson ether
synthesis. C60-py (Fig. 8) was synthesized according to the literature35.
All the synthesized compounds were characterized by various standard analytical
techniques, including mass spectrometry and infrared, ultraviolet/visible (UV-Vis)
and nuclear magnetic resonance spectroscopies. The FTIR spectra of H2POH,
H2P-PrBr, ZnP-PrBr and ZnP-Pr-tpy are presented in Fig. S1. A peak at 3417 cm-1
for the free-base porphyrin H2POH is attributed to the O-H and N-H stretching
vibrations, and the peak at 3430 cm-1 for H2P-PrBr is assigned to the N-H stretching
vibration. The IR spectrum of ZnP-Pr-tpy shows the characteristic bands of the
porphyrin and terpyridine moieties, e.g. bands at 2918 and 1514 cm-1 are respectively
assigned to characteristic C–H and C=C stretching vibrations of the porphyrin ring36.
A band at 1583 cm-1 is assigned to the C=N stretching vibrations of the tpy moiety37.
A band at 1466 cm-1 is ascribed to the C-N stretching vibration of the pyridine group.
Bands at 1664, 1603 and 1514 cm-1 are due to C=C stretching vibrations and bands at
1386, 1243, 790, 733 and 659 cm-1 etc. are attributed to C-H stretching vibrations of
the methylene group.
The UV-Vis spectra of tpy, H2POH, H2P-PrBr, ZnP-PrBr and ZnP-Pr-tpy
measured in dichloromethane are shown in Fig. S2. H2POH exhibits an intense Soret
band at 422.2 nm and four weaker Q-bands of the porphyrin moiety at 519.3, 558.1
596.6 and 653.0 nm. A significant blue shift occurs for the five bands after covalent
attachment of bromopropane. The Soret band is blue-shifted from 422.2 nm for
H2POH to 421.0 nm for H2P-PrBr and the Q bands are blue-shifted from 519.3,
558.1, 596.6 and 653.0 nm for H2POH to 518.5, 554.7, 594.0 and 650.7 nm for
H2P-PrBr. After coordinating with zinc ions to form ZnP-PrBr, the Soret band is red
shifted to 422.2 nm, and the number of Q bands is reduced from 4 for the free base
H2P-PrBr to 2 for ZnP-PrBr, which can be ascribed to the increase in degeneracy as
a result of the enhancement in the symmetry of the porphyrin unit after the
coordination of the zinc ions. At the same time, the two Q bands exhibit a blue-shift.
For ZnP-Pr-tpy, a new intense band observed at 282.1 nm was attributed to the π-π *
transition in the terpyridine moiety, and it also exhibited a Soret band at 422.2 nm and
two weak Q bands at 548.2 and 588.5 nm. Fig. S3 shows the spectral changes upon
adding various concentrations of C60-py. With increasing additions of C60-py to a
solution of pristine ZnP-Pr-tpy, both the Soret band and the two Q bands show a
red-shift and an increase in intensity, which may ascribed to molecular orbital energy
4 / 25

level changes aroused by the axial coordination of C60-py to ZnP-Pr-tpy.
The electrospray ionization-mass spectrometry ESI-MS spectrum of ZnP-Pr-tpy
shows a major peak at m/z 2197.0, corresponding to the molecular ion, and maximum
relative intensity peaks at m/z 1225.2 for ZnP-PrBr, 1162.2 for H2P-PrBr, 679.3 for
H2POH and 324.1 for tpy were detected as the molecular ion peaks in the respective
ESI-MS spectra (Fig. S4).
The structures of the synthesized compounds can be confirmed by determination
1
of their nuclear magnetic resonance spectra. The H NMR spectrum of tpy shows
aromatic proton resonance signals at δ 7.79 (d, 2H, J = 8.5 Hz) and 6.97 ppm (d, 2H, J =
8.6 Hz), and a multiplet signal at δ 8.65-8.77 ppm is due to the pyridine ring proton.
For H2P-PrBr, signals at δ 8.88 (s, 4H), 8.14(d, J = 7.8 Hz, 4H) and 7.25 ppm (s, 8H)
are attributed to protons in the meso-phenyl ring. Signals at δ 8.52 (d, J = 7.5 Hz, 2H),
8.46 (s, 2H), 7.52 (d, J = 7.7 Hz, 2H) and 7.25 ppm (d, J = 7.9 Hz, 2H) are ascribed to
protons of the pyrrole ring. Signals at δ 4.50 (t, J = 1.6 Hz, 8H), 3.82 (t, J = 3.2 Hz,
8H) and 2.55 ppm (s, 4H) are attributed to alkane protons and a singlet at δ -2.75 ppm
is attributed to the two porphyrin amine protons. These assignments were obtained by
2D 'H NMR COSY, HMBC and HSQC experiments (Fig. S6). Compared with
H2P-PrBr, the most obvious difference in the 1H NMR spectrum of ZnP-PrBr is the
disappearance of the peak at δ -2.75 ppm for the two porphyrin amine protons. The 1H
NMR spectrum of ZnP-Pr-tpy shows signals of protons in the pyridine, alkyl and
aromatic phenyl groups, e.g. a signal at δ 8.89 ppm is attributed to protons in the
peripheral pyridine ring. Signals at δ 4.44 (t, J = 1.6 Hz, 8H), 3.81 (t, J = 6.4 Hz, 8H)
and 2.45 ppm (s, 4H) are due to the alkyl protons and signals at δ 7.28-7.40 ppm are
due to the phenyl protons (Fig. S5).
The steady-state fluorescence spectrum of ZnP-PrBr (~1×10-6 M in toluene)
excited at 425 nm shows an emission peak at 600.0 nm with an intensity of 883.12
and a fluorescence lifetime of 3.44 ns. After the porphyrin ring was covalently
modified with terpyridyl groups, the position of the emission peak of ZnP-Pr-tpy
(with a fluorescence lifetime of 4.63 ns) remained substantially unchanged at the
same excitation wavelength, but its intensity was quenched by about 43.8% to 496.20
a.u. [Figs. 2(a), (e) and (f)]. Studies have shown that electron transfer to terpyridine is
not energetically feasible38, so the quenching of the fluorescence is likely because of
reduced electrons transfer from the terpyridine moiety to the porphyrin unit via the
Dexter energy transfer mechanism24,39. The fluorescence lifetime of ZnP-PrBr with
radiation-decay is shorter than that of ZnP-Pr-tpy with non-radiation-decay for the
5 / 25

intra-molecular energy transfer. Zinc porphyrins can form supramolecular dyads by
axial coordination with pyridine-based ligands40. Accordingly, the fluorescence
behavior of ZnP-Pr-tpy in the presence of C60-py was subsequently investigated. As
shown in Fig. 3, increasing the content of C60-py significantly quenched the intensity
of the porphyrin fluorescence emission peak at 600 nm. As the ratio of
ZnP-Pr-tpy:C60-py was increased to 1:1, the fluorescence quenching rate of the
porphyrin moiety at 600 nm rose to 38.7% and the fluorescence lifetime slightly
decreased to 4.01 ns, which proves the existence of a supramolecular interaction
between ZnP-Pr-tpy and C60-Py. The calculation, based on the construction of a
Benesi–Hildebrand plot, indicates a 1:1 stoichiometry between ZnP-Pr-tpy and
C60-py and the Zn(II)···py binding constant K was evaluated to be 1.18×105 M-1. To
investigate whether an interaction between ZnP-Pr-tpy and C60-py occurred, the
fluorescence spectra of the tpy solution and the tpy/C60-py mixed solution were
measured at an excitation wavelength of 280 nm, which produced identical emission
spectra for the two solutions [Fig. 2(b)]. Excited at the maximum absorption
wavelength of the porphyrin (425 nm), the fluorescence emission spectra were also
identical for the ZnP-PrBr and the mixed ZnP-PrBr/C60-py solutions [Fig. 2(d)].
However, the emission spectrum of the ZnP-PrBr solution was significantly
quenched after C60-py was added [Fig. 2(c)]. These experiments show that a
supramolecular dyad was formed through coordination of C60-py to the zinc
porphyrin, rather than through π-π stacking between the tpy moiety and C60-py. Thus,
excitement of the porphyrin unit in the supramolecular dyad of ZnP-Pr-tpy/C60-py
results in electron transfer from the terpyridine moiety to the porphyrin unit through
the Dexter mechanism, and further transfer to C60 through the coordinating bond. The
photoexcitation of the porphyrin unit clearly leads to oxidation of the tpy moiety. This
indicates that a photosynthesis center that mimics the natural chlorophyll molecule
P680 could be constructed by linking the tpy moiety to the zinc porphyrin unit and to
further coordinate the tpy-modified zinc porphyrin axially with C60-py. In such
complexes, the tpy moieties, as excellent ligands, can form complexes with metal ions
(such as Ru2+) and act as a photocatalytic oxygen evolution center, with the porphyrin
unit acting as a photosensitizer.
To analyze the electrochemistry, cyclic voltammetry was performed in a mixed
solution of toluene:acetonitrile = 3:1, containing 0.1 M tetrabutylammonium
hexafluorophosphate. To determine the relevant energy levels of the synthesized
compounds, the cyclic voltammetry of ferrocene was measured under identical
6 / 25

experimental conditions and then cyclic voltammetry values of the synthesized
compounds were converted to values against E1/2 of the ferrocenium/ferrocene redox
couple. Fig. 4 shows the voltammograms recorded at a scan rate of 200 mV s-1 for
ZnP-Pr-tpy, along with the reference compounds ZnP-PrBr and tpy.
In the potential range -2.67 to 1.33 V (vs Fc/Fc+), ZnP-Pr-tpy shows three
irreversible oxidation waves at Eox = 0.0780 (Eoxpa1), 0.490 (Eoxpa2) and 0.799 V
pa
(Eoxpa3) in the oxidation region. For the reference samples, ZnP-PrBr shows two
reversible oxidation waves at Eox1/2 = 0.0828 (Eox`pa1 = 0.194 V) and 0.497 V (Eox`
=
pa2
··
0.604 V) and tpy shows an irreversible oxidation peak at Eox
= 0.463 V, under
pa
the same experimental conditions. Obviously, the two waves of at Eox and Eox
pa1
pa3
of ZnP-Pr-tpy are ascribed to porphyrin ring oxidation, which is consistent with the
previously reported results41 that there are two ring oxidation waves for porphyrins in
the oxidation region, and the peak of Eoxpa2 is ascribed to oxidation of tpy.
The above experimental data show that after the terpyridine moiety covalently
bonds to the porphyrin unit, the first oxidation peak of the porphyrin is shifted
negatively by 0.116 V from 0.194 V (Eox`pa1) for ZnP-PrBr to 0.078 V (Eoxpa1) for
ZnP-Prtpy, and the oxidation peak of terpyridine is shifted positively by 0.027 V
from 0.463 V (Eox ··) for tpy to 0.490 V (Eoxpa2) for ZnP-Prtpy. The positivity of
pa
the zinc ion in the metal porphyrin unit leads to the electron-withdrawing effect of the
porphyrin unit and affects the π-π interaction in the terpyridine.
In the reduction region, a reversible reduction peak at Ered1/2 = -1.472 V and three
irreversible reduction waves at Eredpc = -2.144, Eredpa = -2.1861 and Eredpa = -1.256 V
were observed for ZnP-Pr-tpy. Two reversible reduction waves at Ered = -1.517
1/2
(Eredpa = -1.468 V) and -2.311 V (Eredpa = -2.218 V) for the ZnP-PrBr reference and
an irreversible reduction peak at Eredpc = -1.445 V for the tpy reference were observed.
Visibly, the two reduction waves at Ered = -1.472 and Ered = -2.186 V for
1/2
pa
ZnP-Pr-tpy resulted from the reduction of the porphyrin ring. The two reduction
peak positions of ZnP-Pr-tpy are similar to those of ZnP-PrBr. According to the
formula HOMO = -[4.75+ Eoxpa], the HOMO of the ZnP unit in ZnP-Pr-tpy is
calculated to be -4.828 eV, and that of the tpy moiety is -5.240 eV. Therefore, from
the fluorescence quenching and electrochemical data, the photoexcitation of
ZnP-Pr-tpy is determined to result in electron exchange between the excited singlet
porphyrin unit and the ground state tpy moiety through the Dexter energy transfer
mechanism, Specifically, the excited state electrons transfer from the HOMO of the
porphyrin unit to the HOMO of the terpyridine moiety and the ground state electron
7 / 25

of the terpyridine moiety transfers to the porphyrin unit (Fig. 5).
Modification of the electrode surface with the catalyst leads to a decrease in the
activation overpotential and to a reduction in the power consumption and waste. Since
the mid- 1970s, considerable research has concentrated on how to attach molecular
species (non-electronic conductors) to electronically conductive electrodes
(sometimes referred to as chemically modified electrodes)42. To study the oxygen
evolution reaction (OER) performance for ZnP-Pr-tpy, ZnP-PrBr and
ZnP-Pr-tpy/C60-py composite films, samples were drop-casted onto a fluorine-doped
tin oxide (FTO) transparent conducting glass to fabricated ZnP-Pr-tpy/FTO,
ZnP-PrBr/FTO and ZnP-Pr-tpy/C60-py/FTO photoelectrodes, respectively. The
morphologies of the different modified FTOs were examined by SEM. As shown in
Fig. 6, ZnP-Pr-tpy, ZnP-PrBr and ZnP-Pr-tpy/C60-py all exhibit a uniform and
dense structure on FTO, however, the ZnP-Pr-tpy and ZnP-Pr-tpy/C60-py films
show an undulating wave-like structure, which greatly increases the contact area
between the film and the solution. The ZnP-PrBr film does not show this structure,
but some of its crystallites are irregularly dispersed and appear aggregated.
The OER activities were determined from the overpotential at 10 mA/cm2 and
the Tafel plots in the overpotential region, as shown in Figs. 7(a) and (b). When the
current density is 10mA/cm2, the overpotential of ZnP-Pr-tpy is 1.860 V, with an
onset potential of 1.665 V, which is lower than that of ZnP-PrBr (1.920 V, with an
onset potential of 1.706 V). The electrocatalytic OER activity of ZnP-Pr-tpy is
clearly enhanced after modification of the tpy moiety because of the improvement in
the intra- and intermolecular electron transfer. The ZnP-Pr-tpy/C60-py composite
film’s OER activity is further enhanced, as evidenced by the lower potential (1.837 V)
at 10 mA/cm2 and onset potential (1.640 V), which can be ascribed to a wider light
absorption range for ZnP-Pr-tpy as a result of the covalent attachment of the tpy
moiety. The OER activities were also evaluated from the Tafel plots in the
overpotential region. The Tafel slopes of ZnP-Pr-tpy/C60-py, ZnP-Pr-tpy and
ZnP-PrBr are 193, 246 and 309 mV per decade, which indicates that
ZnP-Pr-tpy/C60-py and ZnP-Pr-tpy have a higher OER performance.
The photoelectrochemical (PEC) properties of ZnP-Pr-tpy/C60-py, ZnP-Pr-tpy
and ZnP-PrBr were determined in a three-electrode cell in a 0.5 M sodium sulfate
electrolyte with a ZnP-Pr-tpy/C60-py/FTO (or ZnP-Pr-tpy/FTO, ZnP-PrBr/FTO)
composite photoelectrode (1 cm2) as the working electrode, a platinum wire as the
auxiliary electrode and a saturated Ag/AgCl electrode as the reference electrode. The
8 / 25

j-t curves of the photocurrent responses for these photoelectrodes under chopped light
at a bias voltage of 0.275 V are shown in Fig. 7(c). The ZnP-Pr-tpy/FTO electrode
exhibited a photocurrent response of 1.77×10-6 A cm-2 under visible light irradiation,
which is much larger than that of ZnP-PrBr/FTO (6.19×10-7 A cm-2), after adding
C60-py. The photocurrent response of ZnP-Pr-tpy/C60-py/FTO was further increased
to 3.45×10-6 A cm-2. For the ZnP-Pr-tpy/FTO photoelectrode, the photoexcitation of
the porphyrin unit leads to the ground state electrons of the tpy moiety transferring to
the porphyrin unit by an electron-exchange process following the Dexter energy
transfer mechanism, showing a better photocurrent response than that of the
ZnP-PrBr/FTO photoelectrode. For the ZnP-Pr-tpy/C60-py/FTO electrode, with its
donor-acceptor system where the C60-py ligand axial coordinates to the zinc ion of the
porphyrin unit, photoexcitation of the composite film of ZnP-Pr-tpy/C60-py leads to
electrons of the tpy moiety transferring to the porphyrin unit, and the electrons of the
porphyrin unit further transferring to the C60 moiety through the coordination bond
formed by the zinc ion of the porphyrin unit and C60-py, thus demonstrating a greater
photocurrent response than ZnP-Pr-tpy/FTO.
The electrochemical impedance spectra (EIS) of ZnP-Pr-tpy/C60-py/FTO (or
ZnP-Pr-tpy/FTO, ZnP-PrBr/ITO) were further characterized for exploring the
enhanced PEC performance (in the illuminated state). Fig. 7(d) shows the Nyquist
plots of the EIS of ZnP/FTO, ZnP-PrBr/FTO, ZnP-Pr-tpy/FTO and ZnP-Pr-tpy/
C60-py/FTO. The EIS spectra are comprised of two parts, with a semi-circle at high
frequency and a straight slopping line in the low frequency region. The equivalent
circuit was determined using ZView software to fit the spectra (as shown in the inset).
Rs represents the overall ohmic resistance of the cell, Rct and CPE are respectively the
charge-transfer resistance and its constant phase element related double-layer
capacitance between the electrolyte and electrode, and W is the Warburg impedance
related to the diffusion of ions in the electrode.. The EIS results show that the Rct of
ZnP-Pr-tpy/FTO (18149 Ω) is smaller than that of ZnP-PrBr/FTO (24680 Ω) under
light illumination. The addition of C60-py further decreases the Rct value (to 10577 Ω)
for ZnP-Pr-tpy/C60-py/FTO, which leads to a further increase of the photocurrent
response of ZnP-Pr-tpy/C60-py/FTO.
9 / 25

Conclusions
A novel tpy-modified porphyrin, ZnP-Pr-tpy, was synthesized and
characterized by MS, NMR, IR and UV-Vis. The synthesized porphyrin has better
electrocatalytic OER activity than ZnP-PrBr and the OER activity is further
enhanced in the composite film formed with C60-py. The results of PEC tests show
that ZnP-Pr-tpy has a better photocurrent performance than ZnP-PrBr and its
photocurrent response is further enhanced by the formation of a ZnP-Pr-tpy/C60-py
composite film.
Experimental methods and materials
In this paper, a new zinc porphyrin modified with terpyridine at the para position
of the meso-position phenyl ring was synthesized. The OER electrocatalytic
performance and photoelectric response properties of the porphyrin and the D-A
composite film formed by the porphyrin with fullerene-based pyridine (C60-py, Fig.8)
were studied. All the reagents were of analytical grade and purchased from
Sinopharm Chemical Reagent Co., Ltd. Neutral alumina (for column chromatography,
200-300 mesh), activated at 180 °C for 2 h before used. Fullerene[60] (C60) was
purchased from Wuda Three-dimensional Carbon Cluster Materials Co., Ltd. in
Wuhan City, and used directly without further processing. The samples were
characterized by various standard analytical techniques, including mass spectrometry
(MS), nuclear magnetic resonance spectroscopy (NMR), ultraviolet/visible
spectroscopy (UV-Vis) and infrared spectroscopy (IR). The MS spectra were recorded
using a Daltonics mass spectrometer of Bruker (Bremen, Germany); 1H NMR spectra
were recorded on a Varian Unity 400 spectrometer (400 MHz); The UV-vis
absorption spectra were measured on a Shimadzu UV-2501 spectrophotometer. The
IR spectra were recorded on a Nicolet Avatar 360 Fourier transform infrared
spectrometer using KBr pellets. The steady-state fluorescence emission was
monitored by a HITACHI F-4500 spectrometer. Elemental analysis was performed on
a Vario EL III Elemental Analyzer (Elementar Germany). Cyclic voltammetry
measurements were performed using a CH Instruments Model CHI650E
Electrochemical Analyzer in a deaerated toluene/acetonitrile (4:1, v/v) solution
10 / 25

containing 0.10 M tetrabutylammonium hexafluorophosphate (n-Bu4NBF4) as the
supporting electrolyte at room temperature. A standard three-electrode cell was
employed, using a 0.07 cm2 glassy carbon (GC) electrode as the working electrode, a
platinum wire as the auxiliary electrode and an Ag/AgCl (KCl saturated) electrode as
the reference electrode. The surface morphology of the modified FTO electrodes was
tested using scanning electron microscopy (SEM) (ZEISS Sigma, backscattered
electron imaging, accelerating voltage of 20 keV). All measurements of the OER
activities and the photoelectrochemical (PEC) properties were performed on the CH
Instruments in 0.5M Na2SO4 solution. A standard three-electrode cell was employed,
with a working electrode, a saturated Ag/AgCl electrode as the reference electrode
and a platinum wire as the counter electrode. The working electrodes were prepared
by deposition of ZnP-Pr-tpy, ZnP-PrBr and ZnP-Pr-tpy/C60-py on FTO glass
slides. Prior to the deposition, the FTO glass slides were cleaned by sonication in
acetone, isopropanol and deionized water for 10 minutes separately and dried in air.
The working electrodes were prepared as follows: Firstly, solutions of 5.5 mg
ZnP-Pr-tpy in 11 mL O-dichlorobenzene (DCB) (A), 4.0 mg ZnP-PrBr in 8 mL
DCB (B) and 2.5mg C60-py in 5 mL DCB (C) were prepared. The solution of
ZnP-Pr-tpy/C60-py （D）was prepared by mixing A and C in the proportions 1:1.3
(v:v) and stirred overnight at room temperature. For fabricating the working electrode,
the mixture of 50 μL of the as-prepared solutions and 10μL of 5 wt% Nafion solution
was carefully drop cast onto the FTO glass electrode and it was allowed to dry in air
at 80 °C. Linear sweep voltammograms (LSV) polarization curves were obtained by
sweeping the potential from 0.5 to 2.0 Vꢀat a sweep rate of 5 mVꢀS-1` in 0.5 M Na2SO4
solution. The impedance spectroscopy (EIS) experiment was performed in the
illuminated state for a test frequency range from 100ꢀKHz to 0.01ꢀHz. The amplitude
of the imposed sinusoidal voltage was 5 mV. The impedance was recorded at a
potential of 0.1 V (vs. Ag/AgCl). The area of the FTO glass substrate was 1 cm2. The
light ON-OFF switches were set as 60s when measuring the I-t curves of the absolute
values under visible light. The bias for the measurement was set as 0.275 V. A 300-W
xenon lamp was used as the light source for the photocurrent measurements. The
11 / 25

overpotential (η) was calculated according to following formula: η (V)ꢀ=ꢀE-1.23.
Synthesis of 4 `-(4-hydroxyphenyl)-2,2`,6`,2``-terpyridine (tpy-OH)43
A mixture of 4-hydroxybenzaldehyde (7.32 g), 2-acetylpyridine (6.8 ml), potassium
hydroxide (15.6 g) and concentrated ammonia (50 mL) in anhydrous ethanol (100 mL)
was stirred at room temperature under an air atmosphere for 24 hours. Concentrated
ammonia (10 mL 25%) was added and the resulting mixture was stirred at 60 °C for
another 12 hours to give a yellow precipitate. The precipitate was separated by
centrifugation, washed three times with deionized water (5 mL) and anhydrous
ethanol (5 mL), and crystallization from ethanol yielded tpy-OH. Yield: 41%. FTIR
(KBr) ν, cm-1: 3450, 3065, 1584, 1567, 1469, 835. 1H NMR (400 MHz, CDCl3, 20 °C)
δ, ppm: 8.65-8.77 (m, 6H), 8.00-8.04 (m, 2H), 7.79 (d, 2H, J = 8.5 Hz), 7.50-7.54 (m,
2H), 6.97 (d, 2H, J = 8.6 Hz). UV/Vis (CH2Cl2) λmax, nm: 284.8. MS (ESI), m/z calcd
for C21H15ON3, [M]+: 325.3; found: 324.1. Elemental anal. Calcd. (%) for C21H15ON3:
C, 77.47; H, 4.61; N, 12.91; found: C, 77.01; H, 4.41; N, 12.85.
Synthesis of 5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin (H2POH)44
In a 250-ml two necked round-bottom flask, 15 ml of propionic acid, 15 ml of glacial
acetic acid and 15 ml of nitrobenzene were mixed and heated to reflux, to which a
solution of 1.4700 g of 4-hydroxybenzaldehyde in 20 ml of propionic acid was added
gradually, followed by the dropwise addition of 0.832 mL of freshly distilled pyrrole
in 20 ml of nitrobenzene. The reaction mixture was refluxed for 1 hr. The color of the
solution gradually changed from light yellow to dark yellow, to reddish brown and
finally to dark green. Next, 25 ml of methanol was added and the mixture was left to
stand overnight at room temp, then vacuum filtered to give purple crude product. The
crude product was dissolved in 100 mL of 5% sodium hydroxide solution and vacuum
filtered to obtain a purple-green solution; the pH of the solution was adjusted to
neutral with concentrated hydrochloric acid and vacuum filtered to give a blue-purple
sludge-like product. The crude product was subjected to chromatography on neutral
alumina (200–300 mesh) with dichloroform/methanol (19:1 v/v) as the eluent to give
a purple product. Yield: 35%. FTIR (KBr) ν, cm-1: 3417, 1606, 1509, 1468, 1346,
1266, 1169, 967, 801. 1H NMR (400 MHz, CDCl3, 20 °C) δ, ppm: 9.97 (s, 4H), 8.86
(s, 8H), 8.00 (d, 8H, J = 8.8 Hz), 7.21 (d, 8H, J = 8.4Hz), -2.70 (s, 2H, NH). UV/Vis
(CH2Cl2) λmax, nm: 422.2, 519.3, 558.1, 596.6, 653.0. MS (ESI), m/z calcd for
C44H30O4N4, [M+H]+: 678.7; found: 679.3. Elemental anal. Calcd. (%) for
12 / 25

C44H30O4N4: C, 77.80; H, 4.42; N, 8.25; found: C, 77.45; H, 4.19; N, 8.55.
Synthesis of 5,10,15,20-tetra-(4-bromopropoxyphenyl) porphyrin (H2P-PrBr)
To a solution of H2POH (0.1000 g, 0.147 mmol) in anhydrous DMF (2 ml) was
added anhydrous K2CO3 (0.1223 g) under a nitrogen atmosphere. The solution was
dispersed with ultrasonic treatment and then heated to 60 °C in an oil bath for 0.5 h.
To this mixture was added 1,3-dibromopropane (0.95 ml, 9.36 mmol) and the
resulting solution was magnetic stirred for 3 days to yield a dark purple
solution. After distilling most of the DMF under reduced pressure, the crude product
was subjected to chromatography on neutral alumina (200–300 mesh) with
dichloroform as the eluent to give a purple product. Yield: 15%. FTIR (KBr) ν, cm-1:
1
3430, 2918, 2365, 1605, 1505, 1465, 1248, 1169, 510. H NMR (400 MHz, CDCl3,
20 °C) δ, ppm: 8.88 (s, 4H), 8.52(d, J = 7.5 Hz, 2H), 8.46 (s, 2H), 8.14 (d, J = 7.8 Hz,
4H), 7.52 (d, J = 7.7 Hz, 2H), 7.26 (d, J = 7.9 Hz, 2H), 7.25 (s, 8H), 4.50 (t, 8H, J =
1.6 Hz), 3.82 (t, 8H, J = 3.2Hz), 2.55 (s, 4H), -2.75 (s, 2H). UV/Vis (CH2Cl2) λmax
,
nm: 421.0, 518.5, 554.7, 594.0, 650.7. MS (ESI), m/z calcd for C56H50O4N4Br4,
[M+H]+: 1162.6; found: 1163.1. Elemental Anal. Calcd. (%) for C56H50O4N4Br4: C,
57.80; H, 4.30; N, 4.82; found: C, 57.55; H, 4.15; N, 4.98.
Synthesis
of
5,10,15,20-tetrakis(4-bromopropoxyphenyl)porphyrin
zinc(II)
(ZnP-PrBr）
To a solution of H2P-PrBr (0.0400 g) in anhydrous DMF (1.5 ml) was added
ZnAc2·2H2O (0.0057 g) dissolved in anhydrous DMF (1.5 ml) under magnetic stirring
at room temperature. The mixture was stirred for 2 days to yield a blue purple
solution. After evaporating most of the DMF by rotary distillation, the crude product
was subjected to chromatography on neutral alumina (200–300 mesh) with
dichloroform as the eluent to give a purple product. Yield: 19%. FTIR (KBr) ν, cm-1:
1
3440, 2918, 2372, 1650, 1505, 1379, 1227, 1170, 1058, 542. H NMR (400 MHz,
CDCl3, 20 °C) δ, ppm: 8.97 (s, 4H), 8.00 (s, 4H), 7.28-7.31 (m, 16H), 4.43 (t, 8H, J =
1.6 Hz,), 3.81 (t, 8H, J = 6.4 Hz), 2.60 (s, 4H). UV/Vis (CH2Cl2) λmax, nm: 422.2,
550.9, 591.3. MS (ESI), m/z calcd for ZnC56H50O4N4Br4, [M]+: 1228.0; found:
1227.2. Elemental anal. Calcd. (%) for ZnC56H50O4N4Br4: C, 54.72; H, 4.07; N, 4.56;
found: C, (54.17; H, 3.98; N, 4.76.
Synthesis of 5,10,15,20-tetra-(4-(2,2':6',2"-teridopyridyl)propyloxyphenyl)porphyrin
zinc(II) (ZnP-Pr-tpy).
13 / 25

To a solution of ZnP-PrBr (0.0274 g) in anhydrous DMF (2.0 ml) was added
anhydrous K2CO3 (0.0401 g) and 4-hydroxyphenyl-2,2′:6′,2″-tripyridine (tpy-OH),
dispersed with ultrasonic treatment in anhydrous DMF (8 ml). The mixture was
stirred at 60 °C for 5 days. After evaporating most of the DMF by rotary distillation,
the crude product was subjected to chromatography on alkaline alumina (200–300
mesh) with a mixed solvent of dichloromethane:ethylenediamine = 3:1 as the eluent
to give a purple product. Yield: 5.1%. FTIR (KBr) ν, cm-1: 3450, 2918, 1664, 1603,
1
1583, 1514, 1466, 1386, 1243, 1174, 1060, 991, 790, 733, 659, 618, 502. H NMR
(400 MHz, CDCl3, 20 °C) δ, ppm: 8.89 (s, 4H), 8.65-8.90 (m, 24H), 7.80-7.95 (m,
12H), 7.28-7.40 (m, 24H), 7.17 (d, 8H, J = 8.4 Hz), 4.44 (t, 8H, J =1.6 Hz), 3.81 (t,
8H, J = 6.4 Hz), 2.45 (s, 4H). UV/Vis (CH2Cl2) λmax, nm: 282.1, 422.8, 549.8, 592.0,
637.5. MS (ESI), m/z calcd for ZnC140H104O8N16, [M]+: 2201.7; found: 2200.1.
Elemental anal. Calcd. (%) for ZnC140H104O8N16: C, 76.30; H, 4.72; N, 10.17; found:
C, 75.95; H, 4.38; N, 10.68.
C60-py was synthesized and purified according to the method reported in the
literature35
Acknowledgments
The authors gratefully acknowledge the financial support from the National Natural
Science Foundation of China (No. 21771149) and the Natural Science Foundation of
Fujian Province of China (No. 2018J01012, 2013J01057)
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Fig.1. Outline of the synthetic route to ZnP-Pr-tpy
Fig.2. (a) Steady-state fluorescence spectra of ZnP-PrBr, ZnP-Pr-tpy and a 1:1 mixture of
ZnP-Pr-tpy and C60-py in toluene (excited at 425 nm, c = 1.0 × 10-6 mol L-1).(b) Steady-state
fluorescence spectra of tpy and a 1:1 mixture of tpy and C60-py in toluene (excited at 280 nm, c = 1.0 ×
10-6 mol L-1). (c) Steady-state fluorescence spectra of ZnP-PrBr and a 1:1 mixture of ZnP-PrBr and
C60-py in toluene (excited at 425 nm, c = 1.0 × 10-6 mol L-1).(d) Steady-state fluorescence spectra of
18 / 25
HP-PrBr and a 1:1 mixture of HP-PrBr and C60-py in toluene (e)The fluorescence decay lifetime of
ZnP-PrBr in toluene.(f) The fluorescence decay lifetime of ZnP-Pr-tpy in toluene.

Fig. 3. The fluorescence spectra of ZnP-Pr-tpy (10
μM) upon adding various concentrations of C60-Py in
toluene solution. Spectra were measured after 15 min
reaction. [C60-Py] = 0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0,
10.0 μM, respectively. Inset: Benesi–Hildebrand plot.
Fig.4. Cyclic voltammograms of ZnP-PrBr (a),
ZnP-Pr-tpy (b) and tpy (c), potential vs Fc/Fc+, in a
mixed solution of toluene:acetonitrile = 3:1.
19 / 25

Fig.5.schematic representation for the energy
transfer and electron exchange of
ZnP-Pr-tpy
Fig. 6. SEM images of the electrodes of
ZnP-Pr-tpy/C60-py/FTO (A), ZnP-Pr-tpy/FTO (B) and
ZnP-PrBr/FTO (C and D)
20 / 25

Fig.7. (a) LSV curves of the ZnP-Pr-tpy/C60-py mixture,
ZnP-Pr-tpy and ZnP-PrBr in 0.5M Na2SO4 solution. (b)
Tafel plots of the ZnP-Pr-tpy/C60-py mixture, ZnP-Pr-tpy
and ZnP-PrBr. (c) Photocurrent response of the
ZnP-Pr-tpy/C60-py mixture, ZnP-Pr-tpy and ZnP-PrBr.
(d)
Nyquist
plots
of
the
as-prepared
Fig.8. Structure of C60-py
CRediT author statement
Zhen-Yi Wu: Supervision, Writing- Reviewing and Editing. Investigation
21 / 25

Li-Jing Huang: Investigation,
Rui-Zhong: Original draft preparation
Declaration of interests
The authors declare that they have no known competing
financialinterestsor personal relationships that could have appeared to
influence the work reported in this paper.
22 / 25

23 / 25

24 / 25

A novel terpyridine-modified porphyrin , ZnP-Pr-tpy, had been synthesized and fully
characterized, and the activity of the electrocatalytic oxygen evolution and the
photocurrent response for the composite film composed
fullerene-based pyridine was studied.
by ZnP-Pr-tpy and
Highlights


A novel terpyridine-modified porphyrin, ZnP-Pr-tpy, had been synthesized.
The electrocatalytic oxygen evolution activity for a composite film composed
from ZnP-Pr-tpy and fullerene-based pyridine, ZnP-Pr-tpy/C60-py, was studied.

The photocurrent response of the composite film of ZnP-Pr-tpy/C60-py was
studied.
25 / 25