Covalent bonding photosensitizer–catalyst dyads of ruthenium-based complexes designed for enhanced visible-light-driven water oxidation performance

Article abstract of DOI:10.1007/s11243-018-00301-3

Abstract: We have successfully prepared two ruthenium-based covalent bonding photosensitizer–catalyst dyads through a simple procedure. ¹H NMR spectra of both dyads show that only a single stereoisomer was formed for each dyad. The spectroscopic and electrochemical properties and photocatalytic water oxidation activities of both dyads were investigated in detail. The results indicate that there is negligible electron communication between the photosensitizer and catalyst centers, and each component maintains the desired photophysical and electrochemical properties, which would diminish excited-state electron recombination by facilitating the intramolecular electron transfer. In the presence of excess sacrificial electron acceptor, the dyad with iodide ligand shows a 5.5-fold increase in catalytic performance as compared to its chloro analogue, indicating that the iodide ligand plays an important role during the catalytic cycle. Moreover, compared with the multi-component system, the dyad with iodide ligand exhibits a fourfold increase in catalytic turnover number. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11243-018-00301-3

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-00301-3
Covalent bonding photosensitizer–catalyst dyads of ruthenium‑based
complexes designed for enhanced visible‑light‑driven water oxidation
performance
Qihang Chen1 · Qianqian Zhou1 · Ting‑Ting Li1 · Runze Liu1 · Hongwei Li1 · Fenya Guo1 · Yue‑Qing Zheng1
Received: 20 November 2018 / Accepted: 18 December 2018
© Springer Nature Switzerland AG 2019
Abstract
We have successfully prepared two ruthenium-based covalent bonding photosensitizer–catalyst dyads through a simple pro-
cedure. 1H NMR spectra of both dyads show that only a single stereoisomer was formed for each dyad. The spectroscopic
and electrochemical properties and photocatalytic water oxidation activities of both dyads were investigated in detail. The
results indicate that there is negligible electron communication between the photosensitizer and catalyst centers, and each
component maintains the desired photophysical and electrochemical properties, which would diminish excited-state electron
recombination by facilitating the intramolecular electron transfer. In the presence of excess sacrifcial electron acceptor, the
dyad with iodide ligand shows a 5.5-fold increase in catalytic performance as compared to its chloro analogue, indicating
that the iodide ligand plays an important role during the catalytic cycle. Moreover, compared with the multi-component
system, the dyad with iodide ligand exhibits a fourfold increase in catalytic turnover number.
Introduction
four-electron transfer process [3–5]. Hence, the design of
efcient and durable catalysts for catalytic oxygen evolu-
tion reaction (OER) is highly demanded. In this context,
considerable eforts have been made to the fabrication of
three-component systems for photocatalytic water oxidation
[6–8]. Molecular water oxidation catalysts (WOCs) based on
transition-metal complexes have shown great potential for
catalyzing water oxidation due to their facile ligand modif-
cation to tune the electronic structures and their amenability
to mechanistic and kinetic studies [9–11]. However, these
multi-component systems still sufer from difusion-limited
intermolecular electron transfer, which would lead to the
excited-state electron–hole recombination, and thus signif-
cantly lower the catalytic efciency.
A combination of growing energy requirements and increas-
ingly grave environmental pollution has stimulated a con-
siderable interest in the development of new energy sources
[1]. Hydrogen energy is an ideal energy alternative because
it possesses a high calorifc value and the formation of water
as its sole combustion product. Photoinduced water splitting
to produce hydrogen and oxygen provides a most straight-
forward and environmentally benign approach in the con-
version of solar energy into chemical fuels [2]. However, as
one of the two half reactions, water oxidation is deemed as
the main obstacle for water splitting because of its sluggish
One promising approach is the development of supramo-
lecular photosensitizer–catalyst dyads by covalently link-
ing a photosensitizer and a catalyst, such that each compo-
nent retains their original function. Various dimetallic or
trimetallic supramolecular complexes have been reported
by the groups of Sun [12, 13], Meyer [14, 15] and Thum-
mel [16–18], and these complexes showed superior photo-
catalytic water oxidation performance compared with the
non-covalently linked references. In 2012, Thummel and
coworkers reported that the complex [RuII(tpy)(pic)2I]I
(tpy = 2,2′;6,2″-terpyridine, pic = 4-picoline) functions as
an active catalyst for photoinduced water oxidation when
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-00301-3) contains
supplementary material, which is available to authorized users.
* Ting-Ting Li
litingting@nbu.edu.cn
* Yue-Qing Zheng
zhengyueqing@nbu.edu.cn
1
Research Center of Applied Solid State Chemistry,
Chemistry Institute for Synthesis and Green Application,
Ningbo University, 818 Fenghua Road, Ningbo 315211,
Zhejiang Province, People’s Republic of China
Vol.:(0123456789)
1 3

Transition Metal Chemistry
irradiated by a blue LED light source. They then incorpo-
rated this complex with the photosensitizer into one mol-
ecule by using 2,6-di-(1′,8′-naphthyrid-2′-yl) pyrazine bridg-
ing ligand [16], but the synthetic route to the preparation of
bridging ligand is complicated, and the resulting dyad has
two stereoisomers, which are extremely difcult to sepa-
rate. Recently, we reported the synthesis and photocatalytic
sulfde oxidation properties of a photosensitizer–catalyst
dyad, and the compound tpy–bpy (bpy = 2,2′-bipyridine)
was used as the bridging ligand [19]. We have come up with
the idea that the tpy–bpy bridging ligand would react with
one equivalent of [Ru(bpy)2Cl2] to give a [Ru(bpy)3–tpy]2+
moiety, and the remaining tpy with meridional tridentate
(η3) chelation provides an open coordination site to aford
bimetallic molecular architecture.
chemical shifts (δ, ppm) were calibrated relative to tetra-
methylsilane. Matrix-assisted laser desorption/ionization
time-of-light (MALDI-TOF) mass spectra were obtained
on a Bruker BIFLEX III mass spectrometer. Elemental
analyses were obtained on a Vario EL III instrument. Elec-
tronic absorption spectra were obtained on a Shimadzu
UV-2501PC spectrophotometer. The photoluminescence
properties were measured at room temperature by using
a Hitachi F-4600 Molecular fluorescence spectrometer.
Cyclic voltammetry measurements were performed on a CHI
660E electrochemical potentiostat equipped with a glassy
carbon (GC) working electrode, a saturated calomel refer-
ence electrode (SCE), and a platinum slice as the counter
electrode. All potentials recorded were converted to normal
hydrogen electrode (NHE) scale according to the equation:
ENHE =ESCE +0.245 V. Prior to each experiment, the glassy
carbon (GC) electrode was successively polished with 3.0
and 1.0 μm aluminum oxide powder and then sonicated in
ion-free water, and then the solution was degassed by bub-
bling with high-purity nitrogen for 30 min.
Herein, we designed and prepared a Ru-based photosen-
sitizer–catalyst dyad and its corresponding iodo analogue.
[Ru(bpy)3]2+ unit is used as the photosensitizer (denoted as
Ruphot), [Ru(tpy)(pic)2Cl]+ (denoted as Rucat–Cl) or [Ru(tpy)
(pic)2I]+ (Rucat–I) unit acts as the catalyst, photosensitizer
and catalyst unit are covalently linked by a single bond to
form Ruphot–Rucat–Cl and Ruphot–Rucat–I, respectively. In
the presence of excess Na2S2O8 as the sacrifcial oxidant,
both dyads were investigated as the catalysts for photocata-
lytic water oxidation, Ruphot–Rucat–Cl requires an induction
period prior to the commencement of oxygen evolution and
Ruphot–Rucat–I achieves a higher turnover number (TON)
than the non-covalently linked reference under the same
conditions.
Synthesis of Ruphot–Rucat–Cl
A mixture of Ruphot–tpy (600 mg, 0.70 mmol) and
RuCl3.3H2O (200 mg, 0.70 mmol) in 20 mL ethanol was
heated at refux for 5 h, and after cooling to room tempera-
ture, the solvent was evaporated by rotary evaporation. The
residue was then dissolved in 5 mL 4-picoline, followed by
addition of 0.2 mL triethylamine, and the resulting solu-
tion was further refuxed overnight. After the reaction was
cooled to room temperature, excess NH4PF6 was added and
brown solid was precipitated; the crude product was fltered
of and washed with ethanol and diethyl ether, respectively.
The product was purified by chromatography on silica
gel, eluting with CH2Cl2/CH3OH (v/v, 50:1), giving a red
solid with a yield of 56%. High-resolution electrospray
ionization–mass spectrometry (HR-ESI–MS) (m/z): calcd
1428.1332 [M–PF6]+, found 1428.1260; calcd 641.5795
[M–2PF6]2+, found 641.5702. 1H NMR (400 MHz, CD3CN):
δ (ppm) 9.24 (d, 2H, J=4.8 Hz), 9.00 (d, 1H, J=1.6 Hz),
8.74 (s, 2H), 8.71 (s, 1H), 8.54 (t, 4H, J=8.2 Hz), 8.52 (d,
2H, J=4.4 Hz), 8.07 (m, 6H), 7.95 (dd, 1H, J=6.0, 1.9 Hz),
7.90 (d, 1H, J=6.0 Hz), 7.83 (d, 1H, J=5.6 Hz), 7.81 (td,
2H, J=6.9, 1.0 Hz), 7.77 (d, 2H, J=5.4 Hz), 7.74 (d, 5H,
J = 6.6 Hz), 7.59 (d, 1H, J = 5.8 Hz), 7.42 (m, 4H), 7.31
(d, 1H, J=5.0 Hz), 6.79 (d, 4H, J=6.2 Hz), 2.59 (s, 3H),
2.12 (s, 6H). 13C NMR (101 MHz, CD3CN): δ (ppm) 158.9,
158.8, 158.5, 158.4, 158.3, 157.0, 156.3, 156.0, 152.5,
152.3, 151.9, 151.8, 151.7, 151.6, 151.0, 150.8, 145.3,
139.8, 138.0, 137.2, 137.0, 136.1, 128.9, 127.1, 126.2,
125.7, 124.9, 124.0, 123.8, 123.7, 123.4, 121.8, 120.2, 24.0,
20.5. Anal. Calcd for C58H49ClF18N11P3Ru2.H2O: C, 43.80;
H, 3.23; N, 9.69. Found: C, 43.65; H, 3.18; N, 9.55.
Experimental section
Materials
4,4′-Dimethyl-2,2′-bipyridine, SeO2, RuCl3.3H2O, 4-pico-
line, KI, 2-acetylpyridine, NH4PF6, and other chemicals
were purchased from commercial sources, unless otherwise
noted. 4-Methyl-2,2′-bipyridine-4′-carbaldehyde, ligands
tpy–bpy, Ruphot–tpy, and Ru(bpy)2Cl2 were prepared accord-
ing to published procedures [19–22], and the purity of each
1
compound was ensured by H NMR spectra based on the
reported data. All solvents used in synthetic procedures and
measurements were analytical grade and used without fur-
ther purifcation. Standard phosphate bufer (0.1 M, pH 7.0)
was prepared from mixing NaH2PO4 and Na2HPO4 aqueous
solutions, and the fnal pH value was confrmed by a pH
meter (Mettler Co. FE20 K).
Characterizations and measurements
1H NMR spectra were recorded on a Bruker Avance II
400 spectrometer operating at 400 MHz, and the proton
1 3

Transition Metal Chemistry
1
NMR and H–1H COSY NMR spectra show that the pro-
Synthesis of Ruphot–Rucat–I
ton peaks are clearly resolved and only a single stereoi-
somer was formed for Ruphot–Rucat–Cl (Fig. 1, Fig. S3).
The distinct doublet at 9.24 ppm for Ruphot–Rucat–Cl is
assigned to the tpy protons closest to chloro ligand [19,
22]. In the case of Ruphot–Rucat–I, the doublet is shifted to
downfeld region, appearing at 9.76 ppm, this diference
refects the fact that as the halide ligand becomes larger,
de-shielding efect gets stronger. The mass spectrum of
Ruphot–Rucat–I recorded under a positive ion mode shows
one single-charge peak at m/z = 1483.9432, ascribed to
[Ruphot–Rucat–I−I]+, another two fragments at m/z 678.5180
and 410.0430 corresponding to [Ruphot–Rucat–I–2I]2+ and
[Ruphot–Rucat–I–3I]3+ ions, respectively (Figs. S5–S7).
As for Ruphot–Rucat–Cl, the HR-ESI–MS signals recorded
at m/z = 1428.1260 ([Ruphot–Rucat–Cl–PF6]+, z =1) and
641.5702 ([Ruphot–Rucat–Cl–2PF6]2+, z=2) suggest that the
complex exists as a dimetallic form in homogeneous solu-
tion (Fig. S8, S9).
A mixture of 200 mg Ruphot–Rucat–Cl and 50 mg KI in
EtOH/CH2Cl2 (20 mL, 1:1, v/v) was refuxed overnight, and
after cooling to room temperature, 20 mL hexane was added
to the solution, and the precipitate was collected and washed
with hexane. The crude product was purifed with column
chromatography on silica gel, a mixture of CH2Cl2/CH3OH
(v/v, 10:1) was used as the eluent and aforded Ruphot–Rucat–I
as deep red solid with a yield of 48%. HR-ESI–MS (m/z):
calcd 1483.9393 [M–I]+, found 1483.9432; calcd 678.5174
[M–2I]2+, found 678.5180; calcd 410.0434 [M – 3I]3+, found
410.0430. 1H NMR (400 MHz, CD3CN): δ (ppm) 9.76 (d,
2H, J=4.8 Hz); 9.43 (d, 1H, J=1.8 Hz); 9.26 (s, 1H); 8.97
(s, 2H); 8.88 (d, 2H, J= 8.0 Hz); 8.58 (t, 4H, J= 9.2 Hz);
8.12 (td, 2H, J = 7.9, 1.4 Hz); 8.08 (m, 5H); 7.93 (t, 2H,
J=6.1 Hz); 7.81 (m, 8H); 7.75 (d, 1H, J=5.0 Hz); 7.59 (d,
1H, J=5.8 Hz); 6.78 (d, 4H, J=6.2 Hz); 2.64 (s, 3H); 2.11
(s, 6H). Anal. Calcd for C58H49I4N11Ru2: C, 43.27; H, 3.07;
N, 9.57. Found: C, 43.10; H, 2.99; N, 9.43.
The electronic absorption and the redox properties of
both dyads were investigated, and the related data are sum-
marized in Table 1. Firstly, the UV–Vis absorption spec-
tra were recorded in CH3CN; Ruphot–Rucat–Cl exhibits
strong absorptions in the ultraviolet and visible regions; the
absorption bands in the ultraviolet range centered at 360 and
380 nm are attributed to ligand-dominated π–π* transitions,
and the absorption bands centered at 453 and 535 nm are
originated from the metal-to-ligand charge transfer (MLCT)
absorptions of Ruphot and Rucat–Cl moieties, respectively
[23]. The large diference between the MLCT absorptions
in Ruphot–Rucat–Cl indicates that the photosensitizer and
catalytic metal sites are largely electronically uncoupled.
Compared to Ruphot–Rucat–Cl, the MLCT absorptions of
Ruphot–Rucat–I are blue-shifted to higher energy (λmax at 447
and 520 nm), due to an increase in dπ–π* energy gap caused
by Cl− to I− ligand exchange [24] (Fig. 2). In addition, both
dyads are essentially non-emissive at room temperature,
while the mononuclear complex Ruphot–tpy is a strong emit-
ter with a λmax at 631 nm (Fig. S10), the quenching indicates
the presence of either energy or electron transfer between the
Ruphot and Rucat–Cl portions.
Photochemical water oxidation
Photochemical water oxidation was performed in a single-
necked fask equipped with a Clark oxygen probe (YSI 5331)
under the irradiation of a white LED light source. The Clark
probe was calibrated in oxygen-saturated and oxygen-free
water, respectively. Prior to experiment, 5 mL phosphate
bufer containing 10.0 mM Na2S2O8 was added to the fask
and the Clark probe was immersed in the bufer, and the
bufer was purged with N2 to remove the dissolved oxy-
gen, and 25 μL aqueous solution of the dyad (1.0 mM) was
injected by a syringe through the probe gap. The amount
of the evolved oxygen was recorded as a function of time
and reported as the average value from three independent
measurements.
TON = nO2∕ncat.
Results and discussion
Cyclic voltammetry (CV) of both dyads at glassy car-
bon electrode was recorded in degassed CH3CN (Fig. 3).
The synthetic approach for preparation of the bridging ligand
(tpy–bpy) and two dyads Ruphot–Rucat–Cl and Ruphot–Rucat–I
is shown in Scheme 1. Firstly, the assembly of the bridg-
ing ligand tpy–bpy with Ru(bpy)2Cl2 in 1:1 molar ratio was
used to introduce the photosensitizer moiety. Treatment of
this monomeric complex Ruphot–tpy with one equivalent
RuCl3.3H2O in excess 4-picoline under refuxing condi-
tion, aforded the dyad Ruphot–Rucat–Cl in 56% yield. To
prepare the analogous dyad Ruphot–Rucat–I, the chloro dyad
was treated with excess KI to exchange chloride for iodide.
Ruphot–Rucat–I shows two separated monoelectronic oxidation
ox
waves: the frst reversible oxidation couple at E =0.98 V
1/2
versus NHE corresponding to RuIII/II oxidation of the Rucat–I
site and the second irreversible oxidation wave at Eox =1.23 V
versus NHE is similar to the RuIII/II couple of Ruphot–tpy.
Reductively, Ruphot–Rucat–I possesses a reversible redox cou-
ple at −0.80 V versus NHE, which is assigned to the addition
of an electron to the lowest unoccupied molecular orbital
(LUMO) on the most electronegative polypyridine ligand.
In comparison, the CV plot for Ruphot–Rucat–Cl is similar to
1
Both dyads were characterized on the basis of H
1
NMR spectra, HR-ESI–MS and elemental analysis. H
1 3

Transition Metal Chemistry
Scheme 1 Synthetic procedure for the bridging ligand and two dyads
component maintains the desired photophysical and elec-
trochemical properties, which would diminish excited-state
electron recombination by facilitating the intramolecular
electron transfer. In the light-driven water oxidation systems
catalyzed by molecular photosensitizer–catalyst dyad, the
excited-state reduction potential of the photosensitizer part
must be larger than the ground station oxidation potential
of the dyad [17]. Owing to the luminescent property of the
complex Ruphot–tpy, we can obtain the excited-state redox
potential of the photosensitizer. Similar potential diferences
about +0.24 and +0.18 V are achieved for Ruphot–Rucat–Cl
and Ruphot–Rucat–I, respectively, demonstrating both dyads
are capable to catalyze water oxidation under visible light
irradiation, and they may own similar catalytic performance.
We evaluated the photocatalytic water oxidation per-
formances of both dyads by monitoring the catalytic TON
as a function of reaction time in deoxygenated phosphate
bufer. As shown in Fig. 4, Ruphot–Rucat–I shows superior
catalytic activity compared with that of Ruphot–Rucat–Cl, a
modest TON of 66 was achieved within a period of 130 s
while Ruphot–Rucat–Cl only gives a low TON of 12. The
control experiments show that no water oxidation activity
1
Fig. 1 Downfeld regions of the H NMR spectra for Ruphot–Rucat–Cl
and Ruphot–Rucat–I in CD3CN
III/II
that of Ruphot–Rucat–I and exhibits a reversible Ru
couple
cat
III/II
at 0.95 V versus NHE and an irreversible Ru
wave at
phot
1.30 V versus NHE [25]. Based on the above results, there
is negligible electron communication between the photosen-
sitizer and catalyst centers, indicating that each individual
1 3

Transition Metal Chemistry
Table 1 Electronic absorption
ox
red
1/2
Compound
λmax/nm (ε/M−1 cm−1 a
)
E
(ΔE)b
Er1e/d2 (ΔE)b
cE*
1/2
and redox properties of Ruphot
Rucat–Cl and Ruphot–Rucat–I and
their analogous mononuclear
complexes in deoxygenated
CH3CN solution
–
Ruphot–tpy
355 (44,000)
380 (46,000)
385 (42,000)
360 (41,000)
380 (42,000)
360 (44,000)
380 (45,000)
465 (17,000)
540 (8900)
525 (8000)
453 (9800)
535 (10,500)
447 (12,000)
520(10,000)
1.26 (60)
0.95 (85)
1.00(80)
0.95 (70)
1.30 (63)
0.98 (78)
1.23 (ir)
−1.14(73)
−1.18 (85)
−1.22 (80)
−0.77 (75)
−1.10 (ir)
−0.80 (60)
−1.15 (ir)
[Ru(tpy)(pic)Cl]+
[Ru(tpy)(pic)I]+
RUphot–RuCat–Cl
1.19
1.16
RUphot–Rucat–I
aMeasured in CH3CN (1.0×10−5 M) at room temperature. bThe redox properties of these compounds
(1.0 mM) in degassed CH3CN containing 0.1 M NBu4PF6 as supporting electrolyte were performed on
a CHI 660E electrochemical potentiostat, E1/2 =(Eox +Ered)/2 in volts, and ΔE=Eox −Ered in millivolts,
ir=irreversible. cE*red =E1re/2d +Eem
1/2
undergo a diferent catalytic step. Thummel’s work has
revealed that the complex [Ru(tpy)(pic)2Cl]+ is the WOC
precursor and a H2O–chloride ligand exchange process is
involved as the initial step. However, [Ru(tpy)(pic)2I]+ is
not requiring an original H2O–halogen exchange process,
the iodide ligand would be retained during the catalytic
cycle [16, 24]. To make a comparison of intra- and inter-
molecular catalytic activities toward water oxidation, the
efciency of a three-component system (5 mL) containing
[Ru(tpy)(pic)2I]+ (0.01 mM), [Ru(bpy)3]2+ (1.0 mM) and
Na2S2O8 (10.0 mM) was investigated. The three-compo-
nent system shows a considerably slower initial oxygen
evolution rate and a lower TON value of 15 compared to
those obtained from the supramolecular system (0.01 mM)
Fig. 2 UV–Vis spectra of Ruphot–Rucat–Cl and Ruphot–Rucat–I in deox-
under identical conditions (with a TON of 60) (Fig. 5).
This result indicates that the assembly of photosensitizer
and catalyst via covalent bonds results in superior water
oxidation activity as it avoids difusion-limited intermo-
lecular electron transfer and thus enables accelerated intra-
molecular electron transfer from the catalyst to photosen-
sitizer moiety [26–28].
ygenated CH3CN solution (1.0×10−5 M) at room temperature
was detected in the absence of the dyad or Na2S2O8. In
addition, as for Ruphot–Rucat–Cl, an induction period of
15 s is required prior to the commencement of oxygen evo-
lution. It seems likely that the chloro and iodo dyads may
Fig. 3 Cyclic voltammograms of Ruphot–Rucat–I with positive scan (a) and negative scan (b) in degassed CH3CN containing 0.1 M NBu4PF6 as
supporting electrolyte. Conditions: the concentration of Ruphot–Rucat–I is 1.0 mM, the scan rate is 100 mV s−1
1 3

Transition Metal Chemistry
Acknowledgements This work was fnancially supported by the
National Natural Science Foundation of China (21603110) and K.
C. Wong Magna Fund in Ningbo University. Y. Z. thanks the support
from K. C. Wong Education Foundation, Hong Kong.
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