Coupling of a new porphyrin photosensitizer and cobaloxime cocatalyst for highly efficient photocatalytic H2evolution

Article abstract of DOI:10.1039/d1ta04517b

Photocatalytic hydrogen evolution (PHE) is a promising strategy to produce environmentally friendly clean energy with the use of solar power and water. For this, developing an efficient and noble metal free photocatalytic system (PS) comprising high light

Full text of DOI:10.1039/d1ta04517b

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Coupling of a new porphyrin photosensitizer and
cobaloxime cocatalyst for highly efficient
photocatalytic H2 evolution†
Cite this: DOI: 10.1039/d1ta04517b
abc
bc
a
*
*
Govardhana Babu Bodedla,
Wai-Yeung Wong
and Xunjin Zhu
Photocatalytic hydrogen evolution (PHE) is a promising strategy to produce environmentally friendly clean
energy with the use of solar power and water. For this, developing an efficient and noble metal free
photocatalytic system (PS) comprising high light-harvesting and photostable photosensitizers is an
important and challenging task. Herein, a new porphyrin photosensitizer ZnDC(p-NI)PP, containing two
naphthalimide (NI) donor chromophores and 4-carboxyphenyl substituents is developed for PHE. Using
chloropyridinecobaloxime (CoPyCl) as a cocatalyst in phosphate buffer/THF solution at pH 7.4, the
homogeneous PS of ZnDC(p-NI)PP exhibits a very high hydrogen evolution rate (hH ) of 35.70 mmol gꢀ1
2
hꢀ1
, turnover number (TON) of 5958 and apparent quantum efficiency (AQE) of 10.01%. This
performance recorded under the same conditions is significantly higher than that of the PS of ZnDCPP
which lacks the NI moieties (hH of 4.64 mmol gꢀ1
ZnTCPP with four 4-carboxyphenyl substituent groups (hH of 2.43 mmol gꢀ1
h
ꢀ1, TON of 1397 and AQE of 1.30%), the typical PS of
2
h
ꢀ1, TON of 562 and AQE
2
of 1.00%), and previously reported ZnD(p-NI)PP containing only two NI chromophores (hH of 3.8 mmol
2
gꢀ1
h
ꢀ1, TON of 590 and AQE of 1.5%). The noticeable performance of ZnDC(p-NI)PP in PHE is
attributed to the intramolecular energy transfer from the NI donor chromophore to the porphyrin
acceptor that would promote the long-lived photoexcitation states. At the same time, the anionic form
(–COOꢀ) of 4-carboxyphenyl substituents at pH 7.4 enables a faster electron transfer from the porphyrin
group to the cationic Co(III) due to electrostatic force. To the best of our knowledge, the PHE results of
ZnDC(p-NI)PP represent the best one for porphyrin photosensitizers and cobaloxime PSs reported so far.
This work paves a new direction for developing new porphyrin-based materials for efficient PHE through
facile molecular structure engineering.
Received 28th May 2021
Accepted 9th August 2021
DOI: 10.1039/d1ta04517b
rsc.li/materials-a
multicomponent PS design, Lehn and co-workers reported for
the rst time decent PHE results using Ru(bpy)3 as a photo-
Introduction
2+
sensitizer, triethanolamine (TEOA) as a sacricial donor and
colloidal Pt as a cocatalyst.5 Aer this a huge number of multi-
component homogeneous/heterogeneous PSs have been devel-
oped by employing different combinations of photosensitizers
such as Ru-, Ir- and Re-based complexes (noble metals),6–8 inor-
ganic composites,7,9 porous materials,10,11 organic small mole-
cules and polymers,7,12–14 graphitic carbon nitride (g-C3N4)
polymeric materials,15,16 porphyrin derivatives17–20 and their
hybrids with g-C3N4/graphene oxide21 and cocatalysts such as
colloidal Pt,7,22 Ni-, Fe- and Co-based complexes23–27 with the use
of sacricial donors such as triethylamine (TEA), ascorbic acid
(AA) and TEOA. Although the PSs comprising either noble metal
based photosensitizers or cocatalysts (e.g. Pt) produced highly
efficient PHE, they cannot be commercialized in the near future
due to the high cost of noble metals. To tackle this problem,
researchers mainly used noble metal free photosensitizers and
cocatalysts for the preparation of PSs. Among the noble metal
free photosensitizers, recently, porphyrin derived materials have
attracted enormous interest in the PHE owing to their strong
Solar light-driven splitting of water into hydrogen as carbon free
fuel and environmentally friendly clean energy is a promising
strategy for reducing the consumption of natural fossil fuel
resources and greenhouse effect.1–4 To produce photocatalytic
hydrogen evolution (PHE), the photocatalytic systems (PSs)
should mainly contain three components namely a photosensi-
tizer, sacricial electron donor and cocatalyst. Based on this
aDepartment of Chemistry and Institute of Advanced Materials, Hong Kong Baptist
University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China. E-mail: xjzhu@
hkbu.edu.hk
bDepartment of Applied Biology & Chemical Technology, Research Institute for Smart
Energy, The Hong Kong Polytechnic University, Hong Kong, P. R. China. E-mail:
wai-yeung.wong@polyu.edu.hk
cThe Hong Kong Polytechnic University, Shenzhen Research Institute, Shenzhen
518057, P. R. China
† Electronic supplementary information (ESI) available: Materials and methods,
synthesis, photocatalytic systems under different conditions, cyclic
voltammograms, 1H and 13C NMR spectra, and MALDI-TOF spectra. See DOI:
10.1039/d1ta04517b
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solar light absorbing nature in the maximum UV-vis region, long- ZnDC(p-NI)PP bearing two NI donor chromophores and two
lived photoexcited states and possession of suitable HOMO and –COOH groups (Fig. 1) and tested its PHE properties using the
LUMO energy levels for efficient photoinduced charge separation chloropyridinecobaloxime (CoPyCl) cocatalyst under homoge-
and their transfer to the cocatalyst.17,18,28
neous phosphate buffer/THF conditions. We presume that
On the other hand, cobalt complexes, especially, cobaloximes upon light irradiation ZnDC(p-NI)PP porphyrin produces long-
as cocatalysts have received tremendous interest due to their easy lived photoexcited states due to occurrence of energy transfer
synthesis, reasonable photostability and high PHE efficiencies between the NI donor chromophore and porphyrin acceptor
due to low reduction potential.25,29 Importantly, understanding of resulting from the overlap between the emission prole of NI
electron transfer between the components of PSs is the main and the absorption of the porphyrin moiety. As a result, the
criterion to improve the PHE. For this, preparation of PSs under ZnDC(p-NI)PP efficiently accepts electrons from the sacricial
fully homogeneous conditions is required. Thus development of donor and quickly transfers to CoPyCl where proton reduction
new PSs comprising porphyrin photosensitizers and the coba- takes place. Thus, the PS of ZnDC(p-NI)PP porphyrin exhibited
loxime cocatalyst under homogeneous conditions for high a hydrogen evolution rate (hH2) of 35.70 mmol gꢀ1 hꢀ1, which is
performance PHE is not only a prerequisite but also a necessary 8-fold higher than that of the control porphyrin ZnDCPP which
condition to pave this research eld towards our modern society. lacks the NI groups (4.64 mmol gꢀ1 hꢀ1) and even 15-fold higher
However, though a very few homogeneous PSs featuring than that of the typical porphyrin, ZnTCPP (2.43 mmol gꢀ1 hꢀ1).
porphyrin photosensitizers and the cobaloxime cocatalyst for
PHE have been reported so far, they are not much efficient and
stable due to weak light absorbing nature of porphyrins and their
photo instability.30,31 Thus, we aimed to develop highly efficient
Results and discussion
Experimental section
PSs for PHE by enhancing the light absorbing properties and
photostability of porphyrin photosensitizers.
We have recently found that the linear substitution of di-
naphthalimide (NI) moieties at the meso-position of the
porphyrin core greatly improved the light absorption, stability
of photoexcited states, photoinduced charge separation and
photostability of porphyrin photosensitizers and thus PHE.32
The synthetic protocol used for the preparation of ZnDC(p-NI)
PP is shown in Scheme 1. The di-bromozinc derivative,
DiBrZnD(p-NI)PP was synthesized by bromination of D(p-NI)
PPH2 with the NBS reagent followed by reaction with
Zn(OAC)2$2H2O. Subsequently, reaction of DiBrZnD(p-NI)PP
with 4-carboxyphenyl boronic acid by the Suzuki–Miyaura
coupling reaction yielded the target ZnDC(p-NI)PP porphyrin.
The control porphyrin ZnDCPP was synthesized in two steps
This could be attributed to the efficient intramolecular energy
transfer from the NI energy donor chromophore to the
(Scheme S1†). First, the acid catalyzed condensation of phenyl
porphyrin energy acceptor. In this study, we used heteroge-
dipyrrolomethane with 4-formylmethylbenzoate produced the
neous conditions to evaluate the PHE of di-NI conjugated
DMMPPH2 porphyrin and zinc metalation of this gave
porphyrin, ZnD(p-NI)PP with the use of a Pt cocatalyst. Though
ZnDMCPP. In the second step, demethylation of ZnDMCPP
under basic conditions resulted in the ZnDCPP porphyrin.
Zinc(II)-tetracarboxyphenylporphyrin (ZnTCPP) was synthesized
for comparison (Scheme S2†). Both porphyrins were thoroughly
the PS of ZnD(p-NI)PP delivered efficient PHE results, it is not
a cost-effective approach and the intrinsic photocatalytic cyclic
mechanism was also not much fully addressed due to the usage
of Pt and heterogeneous photocatalytic conditions, respectively.
characterized by NMR and MALDI-TOF techniques.
In order to prepare cost-effective, efficient and homogeneous
PSs, herein we have developed a new porphyrin photosensitizer,
Optoelectronic properties
The absorption and emission spectra of the porphyrins are
shown in Fig. 2 and the corresponding data are shown in Table
Scheme 1 Synthetic route for the preparation of ZnDC(p-NI))PP
Fig. 1 Structures of the porphyrins used in this study.
porphyrin.
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Fig. 2 (a) Absorption and (b) emission spectra of the dyes recorded in phosphate buffer/THF (9 : 1 v/v, 10 mM, pH ¼ 7.4)) solution. All the
porphyrins were excited at 420 nm and additionally, ZnDC(p-NI)PP was also excited at 330 nm to understand the intramolecular energy transfer
between the NI donor and porphyrin acceptor.
1. All the porphyrins mainly show two types of peaks. The broad acceptor due to the overlapped emission spectrum of NI and the
and intense peaks located at ca. 420–440 nm correspond to the absorption spectrum of the porphyrin ring.32–34 Accordingly,
Soret band absorption and the less intense peaks positioned at ZnDC(p-NI)PP porphyrin bearing the NI would show FRET
ca. 500–650 nm are attributed to the Q-band absorption. For between NI moieties and the porphyrin ring. In general, the NI
ZnDC(p-NI)PP porphyrin, the additional peaks in the higher chromophore shows a very strong emission peak at ca. 450 nm
energy region (ca. 350 nm) could be ascertained to the NI under 330 nm excitation which corresponds to NI absorption.
absorption peaks. Notably, ZnDC(p-NI)PP porphyrin shows Impressively, the absence of the NI emission peak in the
apparently higher molar extinction coefficient (3) values for ZnDC(p-NI)PP emission spectrum under 330 nm excitation
Soret- and Q-bands than the ZnDCPP which lacks the NI groups clearly indicates efficient FRET between the NI donor and the
and ZnTCPP featuring four carboxylic groups. This could be porphyrin acceptor. The calculated FRET efficiency (FET) of
attributed to the conjugation of NI donor chromophores to the ZnDC(p-NI)PP is 99%. Since the uorescence lifetime (tF) of the
meso-positions of the porphyrin acceptor which might affect the porphyrin is directly related to the stability of porphyrin excited
electronic transition probability between the 1S0–1Eu and states, calculation of tF could give more insight into the
1S0–2Eu transitions of the porphyrin macrocycle.33 These results porphyrin photoexcited states (Fig. 3(a)). The order of calculated
clearly indicate that conjugation of NI donor chromophores to tF of the porphyrins is as follows: ZnDC(p-NI)PP (3.8 ns) >
the meso-position of the porphyrin moiety could enhance the ZnDCPP (2.4 ns) > ZnTCPP (1.8 ns). Among the three porphy-
light-harvesting ability in the UV-visible region and conse- rins, the ZnDC(p-NI)PP bearing the NI moieties showed the
quently could lead to good PHE results. It is noted that ZnTCPP highest tF. This obviously indicates that the ZnDC(p-NI)PP
shows a lower 3 value for the Soret band than ZnDC(p-NI)PP and possesses highly stabilized photoexcited states. The higher tF of
ZnDCPP.
ZnDC(p-NI)PP than the ZnDCPP and ZnTCPP could be ascribed
All the porphyrins show two emission peaks in the region of to the efficient FRET between NI and porphyrin moieties.
600–700 nm under 420 nm excitation which belongs to the Soret Moreover, the control porphyrin, ZnDCPP also shows higher tF
band of the porphyrin moiety. The intense emission peaks at ca. than the commonly used ZnTCPP. This indicates that the
610 nm and weak emission peaks at ca. 660 nm correspond to porphyrin with two carboxylic groups possesses long lived
the Q(0,0) and Q(0,1), respectively of the porphyrin macrocycle. photoexcited states than the congener porphyrin with four
In our previous reports, we have already demonstrated the COOH groups. The calculated photoluminescence quantum
¨
occurrence of intramolecular Forster resonance energy transfer yield (FF) of the porphyrins is in the order of ZnDC(p-NI)PP
(FRET) between NI donor chromophores and the porphyrin
Table 1 Optical properties of the dyes recorded in buffer/THF (9 : 1 v/v) solution at pH 7.4
labs (3 ꢁ 104, Mꢀ1 cmꢀ1) (nm)
lema (nm) FF
tFa (ns) EOx (V)
ERed (V)
E(P /P*) (V) E(P*/P ) (V) E0–0
a
a
b
c
d
e
ꢀ
f
+
Porphyrin
ZnDC(p-NI)PP 350 (3.18), 426 (35.88), 556 (1.82), 597
611, 653
0.18 3.8
1.23, 1.53 ꢀ1.13
1.22, 1.54 ꢀ1.19
ꢀ0.94
ꢀ0.97
1.04
2.17
(0.78)
ZnDCPP
ZnTCPP
424 (19.66), 556 (0.77), 599 (0.26)
429 (7.15), 560 (0.40), 599 (0.27)
609, 655
610, 660
0.13 2.4
0.10 1.8
1.00
1.32
2.19
2.15
1.20, 1.48 ꢀ0.83, ꢀ1.17 ꢀ0.95
a
Phosphate buffer/THF (9 : 1 v/v) solution. b Eox (vs. NHE) ¼ 0.77 + Eox (vs. ferrocene). c Ered (vs. NHE) ¼ 0.77 ꢀ Ered (vs. ferrocene). d E(P /P*) (vs. NHE)
+
¼ EOx ꢀ E0–0; here “P” refers to the porphyrin photosensitizer. e E(P*/P ) (vs. NHE) ¼ ERed + E0–0
.
Estimated from the intersection of the normalized
f
ꢀ
absorption and emission spectra.
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Fig. 3 (a) Lifetime decay spectra of the porphyrins recorded in phosphate buffer/THF (9 : 1 v/v, 10 mM at pH 7.4) solution at room temperature
and (b) energy level alignment of the porphyrins, sacrificial donor and water reduction catalyst.
+
(18%) > ZnDCPP (13%) > ZnTCPP (10%) which is also in line photoexcited porphyrins. And the (E(P /P*)) values of porphyrins
with their tF trend.
are more negative than the reduction potential (Eox) of the
The thermodynamic viability of electron transfer during the CoPyCl cocatalyst, suggesting an effective transfer of electrons
PHE cycle depends mainly on the excited state oxidation from photoexcited porphyrin to CoPyCl where proton reduction
+
ꢀ
potentials (E(P /P*)) and excited state reduction potentials (E(P*/ takes place. The relatively high-lying (E(P*/P )) value of ZnDC(p-
ꢀ
35
P )) of the porphyrins. Since such relative energies cannot be NI)PP than the ZnTCPP should be helpful for more efficient
evaluated directly, rst, we calculated the rst oxidation electron transfer from AA to photoexcited porphyrins and thus
potential (EOx) and rst reduction potential (ERed) values which expectably higher PHE performance for ZnDC(p-NI)PP than that
correspond to the HOMO and LUMO of the porphyrins, of ZnTCPP.
respectively, by performing the cyclic voltammetric experiments
in THF solution (Fig. S1†). The calculated EOx values of ZnDC(p-
PHE studies
NI)PP, ZnDCPP and ZnTCPP are 1.23, 1.22, and 1.20 V, respec-
tively. And the values of ꢀ1.13, ꢀ1.19, and ꢀ0.83 V correspond
to the ERed of ZnDC(p-NI)PP, ZnDCPP and ZnTCPP, respectively.
According to the Rehm–Weller equations (see the footnote in
In order to evaluate the PHE properties of porphyrins, a series of
homogeneous PSs were prepared in buffer/THF solution by
employing porphyrins as photosensitizers, AA as the sacricial
electron donor and CoPyCl as the cocatalyst. The optimized
PHE properties of PSs are shown in Fig. 4 and the correspond-
ing data are shown in Table 2. Fig. 4(a) shows the hydrogen
evolution rate (hH2) of the PSs under the illumination of an
OLED light source for 5 h. From Fig. 4(a), it can be noted that
the ZnDC(p-NI)PP bearing the NI donor chromophores and
–COOH groups displayed higher hH2 than the ZnDCPP which
lacks the NI moieties and well known ZnTCPP. The hH2 order of
+
Table 1), the (E(P /P*)) values of the ZnDC(p-NI)PP, ZnDCPP and
ZnTCPP were calculated to be ꢀ0.94, ꢀ0.97, and ꢀ0.95 V,
ꢀ
respectively and (E(P*/P )) values of the porphyrins ZnDC(p-NI)
PP, ZnDCPP and ZnTCPP were calculated to be 1.04, 1.00, and
1.32 V, respectively. As seen in Fig. 3(b), the (E(P*/P )) values of
the porphyrins are more positive than the oxidation potential
(Eox) of the AA sacricial electron donor indicating a favourable
thermodynamic driving force for electron transfer from AA to
ꢀ
Fig. 4 (a) H2 production rate of photocatalytic systems under irradiation for 5 h and (b) H2 production of photocatalytic systems under irradiation
(white LED light (148.5 mW cmꢀ2)) for 50 h: (porphyrin (10 mM) + CoPyCl (2 mM) + AA (0.4 M) + buffer/THF (9 : 1 v/v) at pH 7.4).
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Table 2 Amount of H2 (hH ), turnover number (TON), and apparent
quantum yield (AQE) of the photocatalytic systems
output of all the experiments discloses that the PS conditions in
Fig. 4 are the best to produce efficient PHE.
2
To further understand the molecular design of ZnDC(p-NI)
Porphyrin
hH a (mmol gꢀ1 hꢀ1
)
TONb
AQE%c
2
PP, we evaluated the hH of our recently reported efficient ZnD(p-
2
NI)PP which contains two NI moieties and lacks the COOH
ZnTCPP
ZnDCPP
ZnDC(p-NI)PP
ZnD(p-NI)PP
2.43
4.64
35.70
3.80
562
1397
5958
590
1.00
1.30
10.01
1.50
groups. The PS of ZnD(p-NI)PP exhibited an hH of 3.8 mmol gꢀ1
2
hꢀ1 (Fig. S2,† Table 2), which is 10-fold lower than that of the PS
of ZnDC(p-NI)PP. Similarly, the PHE of methylated ZnDCPP and
ZnTCPP porphyrins such as ZnDMCPP and ZnTMCPP, respec-
a
Calculated under irradiation for 5 h. b Calculated under irradiation for
50 h. c Calculated under irradiation for 1 h.
tively was also tested for better comparison. The hH of
2
ZnDMCPP (1.2 mmol gꢀ1 hꢀ1) and ZnTMCPP (0.9 mmol gꢀ1
hꢀ1) is lower than that of their congener porphyrins, ZnDCPP
and ZnTCPP possessing the COOH groups (Fig. S2†). All these
results clearly suggest that there would be close contact between
COOH of ZnDC(p-NI)PP and CoPyCl in the solution of PS.
Nevertheless, the Soret band peak of ZnDC(p-NI)PP is gradually
red-shied and splits with increasing the concentration of
CoPyCl by titration, attributed to the electrostatic interactions
between the cationic Co(III) and the negatively charged ZnDC(p-
NI)PP molecules (Fig. S6†).38 Thus under light irradiation, the
higher performance of ZnDC(p-NI)PP in PHE is attributed to the
FRET from the NI donor chromophore to the porphyrin
acceptor that would promote the long-lived photoexcitation
states. At the same time, the anionic form (–COOꢀ) of 4-car-
boxyphenyl substituents at pH 7.4 enables a faster electron
transfer from the porphyrin group to the cationic Co(III) due to
electrostatic force.
In order to get more insight into the effect of CoPyCl and AA
concentrations on hH2, we have prepared a series of PSs con-
taining ZnDC(p-NI)PP and variable concentrations of CoPyCl
and AA. From Fig. S7,† it was observed that the concentration of
both CoPyCl and AA has also played an important role to opti-
mize the PSs. For achieving high hH2, the PSs must be prepared
using 2.0 mM of CoPyCl and 0.4 M of AA. Since PHE of PSs is
strongly dependent on pH, the PHE of the ZnDC(p-NI)PP PS has
been evaluated at different pH values. As seen in Fig. 5, the PS of
ZnDC(p-NI)PP exhibited the highest hH2 of 35.70 mmol gꢀ1 hꢀ1
at pH 7.4, under likely slightly basic conditions. In contrast, the
PS of ZnDC(p-NI)PP showed a gradual decrease in hH2 when pH
the PS of the porphyrins is ZnDC(p-NI)PP (35.70 mmol gꢀ1 hꢀ1
)
> ZnDCPP (4.64 mmol gꢀ1 hꢀ1) > ZnTCPP (2.43 mmol gꢀ1 hꢀ1).
The calculated apparent quantum yield (AQE) values of ZnDC(p-
NI)PP, ZnDCPP and ZnTCPP PSs are 10.01%, 1.30% and 1.00%,
respectively. The AQE values of PSs also match well with their
hH2 order. Furthermore, under the optimized conditions, the
photostability of the three porphyrin PSs was also evaluated by
measuring their PHE up to 50 h of light irradiation. As seen in
Fig. 4(b), the PHE of the three porphyrins gradually increased
with time indicating the good photostability of all components.
The appearance of similar absorption and emission spectra of
the PSs before and aer light irradiation also attests to their
good photostability under light irradiation (Fig. S11 and S12†).
The order of turnover number (TON) of the three PSs is as
follows: ZnDC(p-NI)PP (5958), ZnDCPP (1397) and ZnTCPP (562)
which is also in line with their hH2 and AQE trends. More
importantly, the hH2 and TON of ZnDC(p-NI)PP PS are superior
to those of the PSs comprising porphyrin photosensitizers and
cobaloxime catalysts so far in the literature (Table S1†).30,31,36,37
Particularly, the hH2 of the ZnDC(p-NI)PP PS is 3.3 times higher
than that of the most efficient PS containing the water soluble
porphyrin photosensitizer, PorFN (10.9 mmol gꢀ1 hꢀ1) and Pt
cocatalyst reported recently.38 Moreover, the hH2 of ZnDC(p-NI)
PP is also much higher than that of our recently reported
iridium-conjugated porphyrins39 and far better than that of the
NI-conjugated porphyrins under heterogeneous conditions with
the Pt cocatalyst.32–34 The superior PHE properties of ZnDC(p-NI)
PP possessing the 5,15-di(NI) substituted porphyrin donor
chromophores compared to the ZnDCPP which lacks the NI
could be explained by the following factors: (i) good light-
harvesting properties in the UV-vis region of the solar spec-
trum, (ii) efficient FRET from the NI donor to the porphyrin
acceptor would stabilize the photoexcited states of the
porphyrin ring and thus long-lived photoexcited electron life-
time which further enhances the electron transfer from the
photo-excited porphyrin moiety to the CoPyCl through COOH
groups, and (iii) an efficient acceptance of electrons from AA
and subsequent transfer to CoPyCl (vide infra). Under the opti-
mized PHE conditions of porphyrins, we also tested the hH2 of
the PSs of porphyrins using the Pt cocatalyst (2 wt%) instead of
the CoPyCl cocatalyst (Fig. S3†). The results indicate that the Pt
cocatalyst-based PSs showed lower hH2 than the PSs of the
CoPyCl cocatalyst. Nevertheless, the PSs of porphyrins were
tested by varying the concentration of porphyrins (Fig. S4†) and
the nature of sacricial donors such as TEA (Fig. S5†). The
Fig. 5 H2 production rate of the photocatalytic systems at different pH
under irradiation for 5 h: (ZnDC(p-NI)PP (10 mM) + CoPyCl (2.0 mM) +
AA (0.4 M) + buffer/THF (9 : 1 v/v) at pH 7.4).
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either increased or decreased with respect to pH ¼ 7.4. The reductive quenching rate order of porphyrins is ZnDC(p-NI)PP >
declined PHE results of the ZnDC(p-NI)PP PS under acidic ZnTCPP > ZnDCPP whereas the order, ZnDC(p-NI)PP > ZnDCPP
conditions could be attributed to the degradation of AA which > ZnTCPP represents the oxidative quenching rates of the
hampers the reducing nature of the photoexcited photosensi- porphyrins. These quenching rate orders reveal that the
tizer, whereas the rate of formation of the active cobalt hydride ZnDC(p-NI)PP porphyrin can efficiently accept electrons from
catalytic intermediate is low under basic conditions and AA, which are further quickly donated to CoPyCl than the
consequently reduces proton reduction.40
control porphyrins ZnDCPP and ZnTCPP. This result is also well
Generally, the electron transfer mechanism involved in consistent with higher PHE results for ZnDC(p-NI)PP PS than
homogeneous PHE systems comprising a photosensitizer, the PSs of ZnDCPP and ZnTCPP porphyrins.
sacricial donor and cocatalyst proceeds through either oxida-
Based on the optoelectronic, PHE and Stern–Volmer
tive or reductive quenching pathways. The dominance of the quenching studies, we have proposed a schematic illustration of
quenching type dictates the multistep electron transfer pathway the electron transfer mechanism involved in the photo-redox
in PSs. The oxidative quenching mechanism involves the cycle for hydrogen production (Fig. 7). Upon light irradiation
transfer of photoexcited electrons from the photosensitizer to on NI containing ZnDC(p-NI)PP porphyrin (denoted as NI–P–
the cocatalyst followed by the reduction of the oxidized photo- COOꢀ), the photoexcited NI moiety transfers energy to the
sensitizer back to the original ground state by taking electrons porphyrin ring through the IET process, followed by the
from the sacricial donor. In the case of the reductive formation of the photoexcited porphyrin complex [NI–P–
quenching mechanism, the sacricial donor reduces the excited COOꢀ]*. This complex then accepts an electron from AA
photosensitizer which further returns to the ground state by resulting in reduced [NI–P–COOꢀ]ꢀ species which further
transferring electrons to the cocatalyst where proton reduction donates two electrons to Co(III)PyCl to give reduced Co(I)PyCl.
takes place. Thus, to understand the type of the electron Finally, the resulting Co(I) species ultimately reduces protons to
transfer mechanism involved in our PSs, we have performed the hydrogen.
quenching studies using CoPyCl and AA as quenchers (Fig. S8–
S10†). For instance, the uorescence emission intensity of
ZnDC(p-NI)PP is gradually decreased with increasing the
concentration of either CoPyCl or AA (Fig. S4†). This indicates
that the photoexcited ZnDC(p-NI)PP could either efficiently
accept electrons from AA or transfer electrons to CoPyCl. The
calculated Stern–Volmer quenching constants (Kq) of ZnDC(p-
NI)PP are 1.9 ꢁ 102 Mꢀ1 sꢀ1 and 4.2 Mꢀ1 sꢀ1 for quenchers
CoPyCl and AA, respectively (Fig. 6). However, given the high
concentration of AA (0.4 M) than the CoPyCl (2.0 mM) used in
the PSs, the calculated oxidative and reductive quenching rates
of ZnDC(p-NI)PP are 38 ꢁ 102 sꢀ1 and 168 ꢁ 102 sꢀ1, respec-
tively. Since the reductive quenching rate is 5 times higher than
the oxidative quenching rate, the reductive quenching mecha-
nism can be assigned to the current PSs. Accordingly, the
calculated oxidative quenching rates of ZnDCPP and ZnTCPP
are 1.6 ꢁ 10ꢀ2 and 4.6 ꢁ 10ꢀ2 sꢀ1, respectively whereas the
Fig. 7 Schematic illustration of the photo-redox cycle mechanism for
values 0.57 ꢁ 10ꢀ4 and 0.15 ꢁ 10ꢀ4 sꢀ1 correspond to the
PHE with ZnDC(p-NI)PP, where NI–P–COOꢀ represents ZnDC(p-NI)
quenching rates of ZnDCPP and ZnTCPP, respectively. The
PP.
Fig. 6 Stern–Volmer plot of porphyrins (10 mM) with (a) CoPyCl and (b) AA as quenchers in the buffer/THF solution at pH 7.4.
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The photoinduced hole–electron pair generation and sepa- Scheme (SRFS2021-5S01), the Hong Kong Polytechnic Univer-
ration of porphyrins also plays a tremendous role in the transfer sity (1-ZE1C), Research Institute for Smart Energy (RISE) and Ms
of electrons from the excited porphyrins to the cocatalyst and Clarea Au for the Endowed Professorship in Energy (847S).
thereby evolves H2. Thus, additionally, we have performed the
photocurrent response studies for the porphyrins. As seen in
Fig. S12,† the photocurrent response of the porphyrins is in the
following order: ZnDC(p-NI)PP > ZnDCPP > ZnTCPP. Among all,
Notes and references
ZnDC(p-NI)PP porphyrin displayed higher photocurrent
response than the congener porphyrins ZnDCPP and ZnTCPP
suggesting the efficient photoinduced hole–electron pair sepa-
ration and migration of electrons from the former porphyrin.
This result is also in line with the higher PHE of ZnDC(p-NI)PP
than the ZnDCPP and ZnTCPP.
1 A. Fujishima and K. Honda, Nature, 1972, 238, 37.
2 Y. Tachibana, L. Vayssieres and J. R. Durrant, Nat. Photonics,
2012, 6, 511.
3 N. Armaroli and V. Balzani, Angew. Chem., Int. Ed., 2007, 46,
52–66.
4 B. Zhang and L. Sun, Chem. Soc. Rev., 2019, 48, 2216–2264.
5 M. Kirch, J.-M. Lehn and J.-P. Sauvage, Helv. Chim. Acta,
1979, 62, 1345–1384.
6 I. N. Mills, J. A. Porras and S. Bernhard, Acc. Chem. Res., 2018,
51, 352–364.
Conclusions
In summary, we designed and synthesized a new porphyrin
ZnDC(p-NI)PP bearing two NI energy donor chromophores and
two 4-carboxyphenyl substituents. Absorption and emission
spectra revealed that the introduction of two NI moieties at the
meso-position of the porphyrin moiety enhanced the light har-
7 X. Zhang, T. Peng and S. Song, J. Mater. Chem. A, 2016, 4,
2365–2402.
8 O. J. Stacey and S. J. A. Pope, RSC Adv., 2013, 3, 25550–25564.
9 D. Li, J. Shi and C. Li, Small, 2018, 14, 1704179.
vesting properties, tF and FF values. Stern–Volmer quenching 10 Y. Shi, A.-F. Yang, C.-S. Cao and B. Zhao, Coord. Chem. Rev.,
studies suggested that the electron transfer rate from the 2019, 390, 50–75.
porphyrin center to CoPyCl was tremendously enhanced for 11 H. Wang, H. Wang, Z. Wang, L. Tang, G. Zeng, P. Xu,
ZnDC(p-NI)PP. This could be ascribed to the stabilized photo-
excited states of ZnDC(p-NI)PP due to efficient energy transfer
M. Chen, T. Xiong, C. Zhou, X. Li, D. Huang, Y. Zhu,
Z. Wang and J. Tang, Chem. Soc. Rev., 2020, 49, 4135–4165.
from NI moieties to the porphyrin ring. In addition, the anionic 12 T.-X. Wang, H.-P. Liang, D. A. Anito, X. Ding and B.-H. Han, J.
form of the 4-carboxyphenyl substituents also facilitated faster
Mater. Chem. A, 2020, 8, 7003–7034.
electron transfer between the porphyrin centre and the cationic 13 J. Jayakumar and H.-H. Chou, ChemCatChem, 2020, 12, 689–
Co(III) due to electrostatic force. Photocurrent response studies 704.
also revealed that the ZnDC(p-NI)PP showed higher photoin- 14 C. Xu, W. Zhang, J. Tang, C. Pan and G. Yu, Front. Chem.,
duced charge carrier generation and separation than the 2018, 6(592).
ZnDCPP. As a consequence, the homogeneous PS of ZnDC(p-NI) 15 J. Wen, J. Xie, X. Chen and X. Li, Appl. Surf. Sci., 2017, 391,
PP exhibited improved PHE properties such as hH2 of 35.6 mmol 72–123.
gꢀ1 hꢀ1, TON of 5958 and AQE of 10.01% than those of ZnDCPP 16 S. W. Cao, J. X. Low, J. G. Yu and M. Jaroniec, Adv. Mater.,
PS (hH2 of 4.64 mmol gꢀ1 hꢀ1 TON of 1397 and AQE of 1.3%) and
2015, 27, 2150–2176.
the typical ZnTCPP porphyrin bearing four COOH (hH2 of 17 S. Fukuzumi, Y.-M. Lee and W. Nam, J. Porphyrins
2.43 mmol gꢀ1 hꢀ1 TON of 562 and AQE of 1.0%). Since ZnDC(p-
Phthalocyanines, 2020, 24, 21–32.
NI)PP porphyrin produced very high PHE results, the studies of 18 K. Ladomenou, M. Natali, E. Iengo, G. Charalampidis,
ZnDC(p-NI)PP and graphitic carbon nitride (g-C3N4) hybrid
materials for H2 evolution and CO2 reduction are under inves-
tigation in our laboratory.
F. Scandola and A. G. Coutsolelos, Coord. Chem. Rev., 2015,
304–305, 38–54.
19 X. Fang, Q. Shang, Y. Wang, L. Jiao, T. Yao, Y. Li, Q. Zhang,
Y. Luo and H.-L. Jiang, Adv. Mater., 2018, 30, 1705112.
20 Y. Qian, D. Li, Y. Han and H.-L. Jiang, J. Am. Chem. Soc., 2020,
142, 20763–20771.
Conflicts of interest
There are no conicts to declare.
21 P. Kumar, R. Boukherroub and K. Shankar, J. Mater. Chem. A,
2018, 6, 12876–12931.
22 J. Yang, H. Yan, X. Zong, F. Wen, M. Liu and C. Li, Philos.
Trans. R. Soc., A, 2013, 371, 20110430.
Acknowledgements
The research was supported by the General Research Fund 23 C. Lentz, O. Schott, T. Auvray, G. S. Hanan and B. Elias,
(HKBU 12304320) from the Hong Kong Research Grants Dalton Trans., 2019, 48, 15567–15576.
Council, and Initiation Grant for Faculty Niche Research Areas 24 F. Gartner, S. Denurra, S. Losse, A. Neubauer, A. Boddien,
¨
(IG-FNRA) (2020/21)-RC-FNRA-IG/20-21/SCI/06 from Research
Committee of Hong Kong Baptist University. W.-Y. W.
acknowledges the nancial support from the Science, Tech-
A. Gopinathan, A. Spannenberg, H. Junge, S. Lochbrunner,
M. Blug, S. Hoch, J. Busse, S. Gladiali and M. Beller,
Chem.–Eur. J., 2012, 18, 3220–3225.
nology and Innovation Committee of Shenzhen Municipality 25 D. Dolui, S. Khandelwal, P. Majumder and A. Dutta, Chem.
(JCYJ20180507183413211), the RGC Senior Research Fellowship
Commun., 2020, 56, 8166–8181.
This journal is © The Royal Society of Chemistry 2021
J. Mater. Chem. A

View Article Online
Journal of Materials Chemistry A
Paper
26 J.-W. Wang, W.-J. Liu, D.-C. Zhong and T.-B. Lu, Coord. 35 G. B. Bodedla, D. N. Tritton, X. Chen, J. Zhao, Z. Guo,
Chem. Rev., 2019, 378, 237–261.
K. C.-F. Leung, W.-Y. Wong and X. Zhu, ACS Appl. Energy
Mater., 2021, 4, 3945–3951.
27 Z. Han, L. Shen, W. W. Brennessel, P. L. Holland and
R. Eisenberg, J. Am. Chem. Soc., 2013, 135, 14659–14669.
28 P. Zhang, J. Hu, B. Liu, J. Yang and H. Hou, Chemosphere,
2019, 219, 617–635.
´
36 N. Queyriaux, E. Giannoudis, C. D. Windle, S. Roy, J. Pecaut,
A. G. Coutsolelos, V. Artero and M. Chavarot-Kerlidou,
Sustainable Energy Fuels, 2018, 2, 553–557.
29 A. Fihri, V. Artero, M. Razavet, C. Baffert, W. Leibl and 37 E. Giannoudis, E. Benazzi, J. Karlsson, G. Copley,
M. Fontecave, Angew. Chem., Int. Ed., 2008, 47, 564–567.
30 P. Zhang, M. Wang, C. Li, X. Li, J. Dong and L. Sun, Chem.
Commun., 2010, 46, 8806–8808.
S. Panagiotakis, G. Landrou, P. Angaridis, V. Nikolaou,
C. Matthaiaki, G. Charalambidis, E. A. Gibson and
A. G. Coutsolelos, Inorg. Chem., 2020, 59, 1611–1621.
31 T. Lazarides, M. Delor, I. V. Sazanovich, T. M. McCormick, 38 X. Yang, Z. Hu, Q. Yin, C. Shu, X.-F. Jiang, J. Zhang, X. Wang,
I. Georgakaki, G. Charalambidis, J. A. Weinstein and
J.-X. Jiang, F. Huang and Y. Cao, Adv. Funct. Mater., 2019, 29,
A. G. Coutsolelos, Chem. Commun., 2014, 50, 521–523.
1808156.
32 G. B. Bodedla, J. Huang, W.-Y. Wong and X. Zhu, ACS Appl. 39 D. N. Tritton, G. B. Bodedla, G. Tang, J. Zhao, C.-S. Kwan,
Nano Mater., 2020, 3, 7040–7046.
K. C.-F. Leung, W.-Y. Wong and X. Zhu, J. Mater. Chem. A,
2020, 8, 3005–3010.
40 B. B. Beyene and C.-H. Hung, Sustainable Energy Fuels, 2018,
2, 2036–2043.
33 G. B. Bodedla, L. Li, Y. Che, Y. Jiang, J. Huang, J. Zhao and
X. Zhu, Chem. Commun., 2018, 54, 11614–11617.
34 G. B. Bodedla, G. Tang, J. Zhao and X. Zhu, Sustainable
Energy Fuels, 2020, 4, 2675–2679.
J. Mater. Chem. A
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