Facile One-Pot Synthesis of Ruthenium(II) Quaterpyridine-Based Photosensitizers for Photocatalyzed Hydrogen Production

Article abstract of DOI:10.1021/acs.inorgchem.7b00771

We present here the efficient microwave-assisted synthesis and photophysical study of a family of ruthenium(II) complexes of the general formula [Ru(bpy)_x(qpy)_3-x]²⁺ (where bpy = 2,2′-bipyridine, qpy = 4,4′:2′,2″:4″,4-quaterpyridine, and x = 0, 1, 2 giving compounds 1 = [Ru(bpy)₂(qpy)₁]²⁺, 2 = [Ru(bpy)₁(qpy)₂]²⁺, and 3 = [Ru(qpy)₃]²⁺). Compared to the standard reference, [Ru(bpy)₃]²⁺ (τ = 870 ns, φ = 9.5%), the complexes display longer-lived excited state lifetimes at room temperature (τ: 1 = 1440 ns, 2 = 1640 ns, 3 = 1780 ns) and improved quantum yields (φ: 1 = 14%, 2 = 19%, 3 = 23%). Theoretical calculations were performed to support the interpretation of these photophysical properties. These complexes are excellent photosensitizers as they absorb light throughout the visible spectrum, have excellent excited state lifetimes at room temperature, and have high quantum yields. In combination with a cobalt dimethylglyoxime catalyst, they exhibit remarkable hydrogen evolution with blue light, and they are far more efficient than the reference in the field, [Ru(bpy)₃]²⁺.

Full text of DOI:10.1021/acs.inorgchem.7b00771

Article
pubs.acs.org/IC
Facile One-Pot Synthesis of Ruthenium(II) Quaterpyridine-Based
Photosensitizers for Photocatalyzed Hydrogen Production
Elodie Rousset,†,‡ Ilaria Ciofini,§ Valerie Marvaud,*,‡ and Garry S. Hanan*,†
́
†Dep
́
artement de Chimie, Universite
́
de Montrea
́
l, 2900 Edouard-Montpetit, Montrea
́
l, Queb
́
ec H3T-1J4, Canada
‡IPCM-CNRS-8232, UPMC-Univ Paris 6, cc 42, 4 place Jussieu, 75252 Paris Cedex 05, France
§Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris, F-75005 Paris, France
S
* Supporting Information
ABSTRACT: We present here the efficient microwave-
assisted synthesis and photophysical study of a family of
ruthenium(II) complexes of the general formula [Ru-
(bpy)x(qpy)3‑x]2+ (where bpy = 2,2′-bipyridine, qpy =
4,4′:2′,2″:4″,4‴-quaterpyridine, and x = 0, 1, 2 giving
compounds 1 = [Ru(bpy)2(qpy)1]2+, 2 = [Ru(bpy)1(qpy)2]2+,
and 3 = [Ru(qpy)3]2+). Compared to the standard reference,
[Ru(bpy)3]2+ (τ = 870 ns, Φ = 9.5%), the complexes display
longer-lived excited state lifetimes at room temperature (τ: 1 =
1440 ns, 2 = 1640 ns, 3 = 1780 ns) and improved quantum
yields (Φ: 1 = 14%, 2 = 19%, 3 = 23%). Theoretical
calculations were performed to support the interpretation of
these photophysical properties. These complexes are excellent
photosensitizers as they absorb light throughout the visible spectrum, have excellent excited state lifetimes at room temperature,
and have high quantum yields. In combination with a cobalt dimethylglyoxime catalyst, they exhibit remarkable hydrogen
evolution with blue light, and they are far more efficient than the reference in the field, [Ru(bpy)3]2+.
many metallic complexes have been studied as photosensitizers
and catalysts, from heterogeneous systems, grafting active
entities on a solid surfaces,8 to homogeneous systems using
molecular compounds in solution.9
In systems containing metallic ions, the formation of
colloidal metal might be induced under irradiation10 and
known as an active catalyst for hydrogen evolution with, for
instance, platinum.11 However, as homogeneous catalysis is
closest to nature’s photosynthetic processes using metal-
loenzymes, this molecular approach is developed in the present
paper.
INTRODUCTION
■
The rise in atmospheric CO2 concentrations has generated a
surge in the search for non-carbon based energy sources.1 In
particular, research focusing on the production, storage, and
efficient use of energy has attracted much interest. The
scientific community has also considered many green
approaches, which use abundant resources such as wind,
water, and sunlight as a source of energy.2 One approach to the
energy problem that is currently in use is nuclear power;
however, this energy source is nonrenewable and nuclear waste
poses an environmental risk.2
One design of hydrogen evolution reaction (HER) catalysts
harvests sunlight by coupling subunits absorbing at different
wavelengths of light, which are then capable of vectorial energy
transfer to the lowest energy excited state.12 The best way to
couple the chromophores is to build supramolecular
architectures of diverse complexity13 from linear arrays14 to
radial metallo dendrimers,15 using so-called “complex-as-metal”
and “complex-as-ligand” strategies. This antenna unit redirects
the energy to a main chromophore, also called a primary
photosensitizer (PS), which acts as a bridge with the catalytic
unit of the system.
Photovoltaics have risen to prominence in order to face the
increasing demand for electricity while trying to abandon
nuclear power and fossil fuel sources. The most extensively
studied generation of devices include organic photovoltaic cells
(OPV),3 using organic semiconductors for conversion, and dye-
sensitized solar cells (DSSC),4 which use metal complexes with
a large absorption in the visible and near-infrared (NIR)
spectral regions. The most well-known inorganic dye remains
the black dye,5 a [Ru(4,4′,4″-(COOH)3-tpy)(SCN)3] complex
(tpy = 2,2′:6′,2″-terpyridine) and the global standard that
inspires the field of artificial photosynthesis.
Following this idea, sunlight and water can also be combined
into solar-driven water splitting devices6 to produce O2 and H2,
the latter considered as a possible new fuel.7 These systems
open the field of artificial photosynthesis as they mimic nature’s
capacity to store energy in chemical bonds. For that purpose,
The PSs (usually Ru,16 Ir,17 Pt,18 or Re19) are studied in the
literature as the center part of a triad, coupled with an electron
Received: March 28, 2017
© XXXX American Chemical Society
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donating subunit (usually a water oxidation catalyst as Mn,20
Ru,21 or Ir22) and an electron accepting subunit (usually a water
reduction catalyst as Pd23 or Co24). The PS should present a
large extinction coefficient over a broad spectral range to absorb
available sunlight, as well as stable reduced and oxidized states
and photostability coupled with a high quantum yield and a
sufficiently long excited state lifetime for photoinduced electron
transfer to occur.25 These points maximize the possibility of
multielectron transfers between the subunits, generating H2 and
O2 from water reduction and oxidation respectively, under
sunlight irradiation. A powerful complete system is not
reported yet as both redox processes are not equally efficient,
pushing researchers to focus on the “half” reactions, in which
the production of H2 or O2 are studied separately with a
sacrificial donor or acceptor, for instance, triethanolamine or
sodium persulfate, respectively. The efficiency of the whole
system depends on the activity of the different photosensitizers
and catalysts and precisely in the particular design of those
subunits. Therefore, the conception of a strategy allowing
researchers to rapidly synthesize a variety of PS seems to be of
prime interest. Such a technique would tend to avoid multistep
syntheses and ease the study of a large number of different
complexes.
[Ru(bpy)2(qpy)](PF6)2. [Ru(bpy)2Cl2] (30 mg, 0.06 mmol) and
4,4′:2′,2″:4″,4‴-quaterpyridine (19.0 mg, 0.06 mmol) were dissolved
in ethylene glycol (7 mL). The resulting suspension was heated at 240
°C for 10 min using microwave irradiation (Pmax = 400 W). The
mixture was cooled to room temperature and water was added
followed by a saturated aqueous solution of potassium hexafluor-
ophosphate. The resulting red precipitate was filtered on Celite and
washed with water (2 × 10 mL) and diethyl ether (2 × 10 mL). The
residue was dissolved in acetonitrile and concentrated under a vacuum.
Further purification was achieved by chromatography on silica gel
using MeCN/H2O/KNO3(sat.) (7:2:1, v/v) as the eluent. The fractions
of the second band containing the desired product were combined.
After evaporation of the acetonitrile, water was added followed by a
saturated aqueous solution of potassium hexafluorophosphate,
resulting in a red precipitate, which was filtered over Celite and
washed with water (2 × 10 mL) and diethyl ether (2 × 10 mL). The
resulting product was dissolved in acetonitrile, concentrated under a
vacuum, and precipitated with diethyl ether to afford a red powder (45
mg, 75%). Characterizations were found as previously described.30 1H
NMR (400 MHz, CD3CN) δ = 8.96 (d, J = 2 Hz, 2H), 8.81 (d, J = 6
Hz, 4H), 8.53 (d, J = 8 Hz, 4H), 8.08 (tdd, J = 8, 4, 1.5 Hz, 4H), 7.84
(m, 8H), 7.76 (d, J = 6 Hz, 2H), 7.72 (dd, J = 6, 2 Hz, 2H), 7.43 (m,
4H). IR (KBr pellets, cm−1): 1598, 1533, 1466, 1446, 1404, 1384, 839,
558.
One-Pot Synthesis of the Complete Series [Ru-
(bpy)3‑x(qpy)x](PF6)2 - General Procedure. [Ru(DMSO)4Cl2],
2,2′-bipyridine (bpy) and 4,4′:2′,2″:4″,4‴-quaterpyridine (qpy) were
dissolved in ethylene glycol (C[Ru] ≈ 6 mmol L−1). The resulting
suspension was heated at 240 °C for 1 h using microwave irradiation
(Pmax = 400 W). The mixture was cooled to room temperature and
water was added followed by a saturated aqueous solution of
potassium hexafluorophosphate. The resulting red precipitate was
filtered on Celite and washed with water (2 × 10 mL) and diethyl
ether (2 × 10 mL). The residue was dissolved in acetonitrile and
concentrated under a vacuum. Separation of the four species was
achieved by chromatography on silica gel using MeCN/H2O/KNO3
(sat.) (7:2:1, v/v) as the eluent. The complexes were eluted in the
following order: [Ru(bpy)3]2+, [Ru(bpy)2(qpy)]2+, [Ru(bpy)-
(qpy)2]2+, and finally [Ru(qpy)3]2+. Each band was treated as follows:
after evaporation of the acetonitrile, water was added followed by a
saturated aqueous solution of potassium hexafluorophosphate yielding
a precipitate, which was isolated by filtration on Celite and washed
with water (2 × 10 mL) and diethyl ether (2 × 10 mL). The resulting
product was dissolved in acetonitrile, concentrated under a vacuum,
and precipitated with diethyl ether to afford a red powder (orange for
[Ru(bpy)3]2+).31
Therefore, we report herein a series of homogeneous Ru and
Co containing systems using ruthenium photosensitizers based
on the 4,4′:2′,2″:4″,4‴-quaterpyridine (qpy) ligand (Figure 1),
[Ru(bpy)2(qpy)](PF6)2. Yield (bpy/qpy ratio): 35% (2:1), 36%
(1.5:1.5), 23% (1:2), characterizations were found as previously
described.30
Figure 1. Generic representation of the targeted complexes 1−3.
[Ru(bpy)(qpy)2](PF6)2. Yield (bpy/qpy ratio): 25% (2:1), 32%
(1.5:1.5), 38% (1:2). 1H NMR (400 MHz, CD3CN) δ = 8.98 (s, 4H),
8.56 (d, J = 8 Hz, 2H), 8.11 (td, J = 8, 1.5 Hz, 2H), 8.03−7.92 (m,
8H), 7.91−7.82 (m, 12H), 7.75 (td, J = 6, 2 Hz, 4H), 7.47 (t, J = 6 Hz,
2H), 7.18−7.14 (m, 2H). IR (KBr pellets, cm−1):1595, 1533, 1471,
1404, 831, 555. HRMS (ESI-TOF) m/z: [M − 2PF6]2+ Calcd for
C50H36N10Ru 439.1085; found 439.1092.
compared to the reference in the field bearing 2,2′-bipyridine
(bpy) ligands, [Ru(bpy)3]2+. To allow the fast screening of
different photosensitizers, a new methodology toward the
efficient synthesis of many chromophores using microwave
irradiation will be presented. The design of the qpy ligand
follows the perspective of grafting other chromophoric or
catalytic units at the periphery since the complexes might be
used as the dendritic core in supramolecular assemblies.
[Ru(qpy)3](PF6)2. Yield (bpy/qpy ratio): 7% (2:1), 17% (1.5:1.5),
32% (1:2), Characterizations were found as previously described.26
DISCUSSION
■
EXPERIMENTAL SECTION
■
Synthesis and NMR Characterization. The efficient
preparation of heteroleptic ruthenium complexes of bpy
remains difficult and the classic way to obtain such ruthenium-
(II) complexes is via the stepwise synthesis of a [Ru(L1)2Cl2]
complex followed by the addition of the ligand L2.32
Furthermore, through the use of precursor ruthenium carbonyl
complexes, [Ru(L1)(L2)(L3)]Cl2 complexes with three different
ligands can also be obtained.33
Instrumentation details are available in Supporting Information
(Section S.I.-1).
Synthesis. 4,4′:2′,2″:4″,4‴-Quaterpyridine (qpy),26 [Ru-
(DMSO)4Cl2],27 cis-[Ru(bpy)2Cl2],28 and [Ru(qpy)3](PF6)2 were
26
synthesized as previously reported. All other chemicals were purchased
from commercial suppliers and used without further purification.
[Ru(bpy)2(qpy)](PF6)2 was prepared in a modified literature
procedure.29
B
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Scheme 1. Synthetic Pathway Towards the Ruthenium(II) Complexes Reported Herein
Figure 2. Zone of interest of the 1H NMR spectrum (CD3CN, 400 MHz) of the [Ru(bpy)x(qpy)y]2+ mixture: [Ru(bpy)3]2+ (0) in gray,
[Ru(bpy)2(qpy)]2+ (1) in green, [Ru(bpy)(qpy)2]2+ (2) in blue [Ru(qpy)3]2+ (3) in red.
We observed from a previous study on the homoleptic
[Ru(qpy)3]2+ complex26 that the yield of the reaction, which
afforded the discrete species, is decreased by the formation of
oligomers through the peripheral pyridine nitrogen atoms. In
fact, the formation of those oligomers can be observed
following purification of the reaction mixture by size exclusion
chromatography. In order to exclusively obtain the discrete
species, microwave irradiation was used to allow an increased
temperature and decreased reaction time compared to regular
thermal methods. This process favors the formation of the
bidentate bonds by a basic chelate effect versus the single bond
through the peripheral pyridine nitrogen atoms. Hence, for the
homoleptic complex, [Ru(qpy)3]2+, the synthesis time was
reduced from hours to 15 min using microwave irradiation at
240 °C.
We can follow the first mentioned procedure to rationally
obtain [Ru(bpy)2(qpy)]2+ from cis-[Ru(bpy)2Cl2] upon the
addition of the qpy ligand as reported in the literature.29 In the
present paper, the procedure used for the synthesis of the
homoleptic complex has been applied to the heteroleptic
[Ru(bpy)2(qpy)]2+ complex, affording the compound after 10
min of microwave irradiation at 240 °C.
In order to simplify the entire process, a one-pot synthesis of
the whole family of complexes, [Ru(bpy)3]2+ (0), [Ru-
(bpy)2(qpy)]2+ (1), [Ru(bpy)(qpy)2]2+ (2), and [Ru(qpy)3]2+
(3), as their racemic mixtures, has been tested under microwave
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irradiation at 240 °C (Scheme 1). The classic ruthenium
synthesis usually does not use this approach due to the
difficulty in purification of the generated species. However, in
the case of the qpy ruthenium(II) complexes, the peripheral
pyridine nitrogen atoms are available to interact with a silica gel
chromatographic support, giving a differential retention for
each complex under chromatographic separation.
By varying the ratio of qpy to bpy, one can modulate the
proportion of each ruthenium(II) complex that is formed in the
reaction. One equivalent of bpy and two of qpy favor the qpy-
containing species [Ru(bpy)(qpy)2]2+ (38% yield) and [Ru-
(qpy)3]2+ (32% yield) in a statistical way (see Table S.I.-1 for
details). On the contrary, two equivalents of bpy and one of
qpy favor the bpy-containing species [Ru(bpy)2(qpy)]2+ (35%
yield) and [Ru(bpy)3]2+ (32% yield).
A
1H NMR study of the crude mixture was used to
determine the proportion of each ruthenium(II) species by
monitoring the integration of the signals at 9.01 ppm
(attributed to six protons of [Ru(qpy)3]2+), 8.98 ppm (four
protons of [Ru(bpy)(qpy)2]2+), 8.95 ppm (two protons of
[Ru(bpy)(qpy)2]2+), and the multiplet between 8.57 and 8.48
ppm presenting three doublets corresponding to two protons
of [Ru(bpy)(qpy)2]2+, four of [Ru(bpy)2(qpy)]2+, and six of
[Ru(bpy)3]2+ (Figure 2). As the peaks in the last multiplet are
overlapped, the total integration was considered equal to 12
protons. The information about [Ru(bpy)3]2+ can be extracted
after the subtraction of the value of the two other complexes
given at 8.98 and 8.95 ppm. In a 1:1:1:1 ratio, the theoretical
integration of the peaks should be 6:4:2:12. Transposing that
ratio to the experimental values gives access to the experimental
ratio of the reaction. The overall yield in terms of ruthenium-
(II) is in all cases quantitative. The pendant pyridine nitrogen
atoms, once a problem for the synthesis of qpy complexes of
ruthenium(II), are now the cornerstone for the rapid synthesis
of families of complexes containing a bidentate ligand and qpy.
X-ray Structure. The X-ray crystal structure of [Ru(bpy)-
(qpy)2]2+ is reported herein, while the rest of the series have
been reported previously by us and others.26,29,34 For
[Ru(bpy)(qpy)2]2+ (2), suitable crystals for X-ray diffraction
were grown by slow diffusion of diethyl ether into a saturated
acetonitrile solution of the compound. The crystal structure is
presented in Figure 3. This particular compound crystallizes in
a centrosymmetric P1 triclinic space group where the
̅
asymmetric unit contains one enantiomer of the complex,
two hexafluorophosphate molecules, acting as counteranions, as
well as five acetonitrile solvent molecules. The other
enantiomer is generated by an inversion center in order to
complete the racemic mixture at the unit cell level.
Intermolecular peripheral pyridines interact with each other
through π-stacking (Section S.I.-2).
Figure 3. ORTEP representation of the [Ru(bpy)2(qpy)]2+ (1),34
[Ru(bpy)(qpy)2]2+ (2) (this work), and [Ru(qpy)3]2+ (3) com-
plexes.26 Thermal ellipsoids are drawn at a 50% probability level.
Hydrogen atoms, counterions, and cocrystallized solvent molecules
have been omitted for clarity. Ru in red, carbon in black, and nitrogen
in blue.
The crystallographic structure of [Ru(bpy)(qpy)2]2+ can be
compared to the optimized geometry at the DFT level in
solution, and characteristic distances and angles across the
series are compared in Table S.I.-6.
Both crystallographic structures (Figure 3) and DFT
optimized geometries of the complexes show distorted
octahedral coordination polyhedra with angles formed by
ligands and metallic centers at ∼79° instead of 90° in a perfect
Oh geometry. The interpyridyl distances conserve the
conjugation over all the bpy and qpy ligands. The 2,2′-
bipyridine sites present a certain planarity (dihedral angle of
2°), while the 4-pyridyl substituent show twist angles of about
30° as classically found in bpy-based ruthenium complexes.
Consequently, the replacement of the para hydrogen of the
2,2′-bipyridine ligands by a 4-pyridyl substituent does not affect
the global geometry of the molecule when compared to
[Ru(bpy)3]2+. The intrinsic properties of the complexes
presented here are thus attributed to the extended π-system
and not from a change of geometry.
Electrochemical Analysis. The electrochemical properties
of the complexes were investigated in acetonitrile, using
tetrabutylammonium hexafluorophosphate TBAPF6 as electro-
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a
Table 1. Electrochemical Data of Compounds 0−3
Ox
Red1
Red2
Red3
E1/2
E1/2
E1/2
E1/2
band gap (V)
b
b
[Ru(bpy)3]2+ (0)
1.29
−1.33
2.62
2.53
2.49
2.47
[Ru(bpy)2(qpy)]2+ (1)
[Ru(bpy)(qpy)2]2+ (2)
[Ru(qpy)3]2+ (3)
1.33 (80)
1.42 (57)
1.48 (77)
−1.20 (59)
−1.07 (58)
−0.99 (63)
−1.52 (57)
−1.25 (66)
−1.14 (66)
−1.74 (56)
−1.58 (56)
−1.34 (74)
c
c
c
c
c
a
Potentials are given in volts (V) vs SCE. Cyclic voltammetry was performed in a degassed MeCN/TBAPF6 (0.1 M) solution at room temperature
b
c
with a sweep rate of 100 mV·s−1. Reference 32. Reference 26.
lyte. The results will be discussed in terms of increasing the
HOMO and LUMO orbitals computed for the whole series of
compounds and illustrated Figure 5 shows that all HOMOs are
mainly centered on the metal site, while LUMOs are localized
on the ligands. When two different ligands are present, the
lower energy orbitals are found on the qpy ligand, whereas the
higher energy one(s) are localized on the bpy. These
calculations confirm the assignment of the reduction waves
observed in the electrochemical measurements.
Comparison of the simulated and experimental IR spectra
validated the model used, and as seen in Figure S.I.-5, an
acceptable difference is found after applying the literature
scaling factor of 0.94036 to the calculated frequencies. Peaks at
830 and 555 cm−1, attributed to the PF6 counteranion, are
absent from the calculated IR spectra.
2+
proportion of qpy ligand on going from [Ru(bpy)3] (0) to
[Ru(qpy)3]2+ (3) and are presented in Table 1.
Each cyclovoltammogram of complexes 0 to 3 presents a
reversible oxidation process at 1.29, 1.33, 1.42, and 1.48 V,
respectively, corresponding to each RuII/RuIII oxidation. On the
reduction side, the first reduction processes for all compounds,
observed at −1.33, −1.20, −1.07, or −0.99 V, respectively,
correspond to a reduction of a qpy ligand, except for 0 in which
a bpy is reduced. The oxidation potential, attributed to the
oxidation of ruthenium(II) to ruthenium(III), increases in the
series with the increasing number of qpy units, following the
same trend as the reduction potentials. [Ru(qpy)3]2+ is thus the
easiest to reduce and the hardest to oxidize, which is in
accordance with an effect of the substitution of a hydrogen
atom by a more electron withdrawing pyridyl group with a
Hammett parameter of 0.44.35 The band gap, defined as the
difference between the energy levels of the HOMO and LUMO
orbitals, decreased with an increased number of pyridyl
substituents. This is in agreement with the qpy ligand exhibiting
better electron delocalization than the bpy ligand.
For the heteroleptic complexes, the pattern observed in the
square-wave measurements (Figure 4) allows us to localize the
first reductions systematically on the qpy ligands: [Ru(bpy)-
(qpy)2]2+ exhibits two close peaks attributed to the qpy and a
second one related to the bpy. Similarly, [Ru(bpy)2(qpy)]2+
presents one peak attributed to the qpy in addition to two
peaks side-by-side attributed to the bpy. These assignments
were corroborated by DFT calculations. Inspection of the
Photophysical Measurements. The absorption spectra of
the Ru complexes are presented in Figure 6.
The different bands in the absorption spectra of the qpy-
containing complexes can be attributed by analogy to the
[Ru(bpy)3]2+ spectrum. Classically for bpy-based ruthenium-
(II) complexes, the low energy bands (>350 nm) are attributed
to metal-to-ligand charge transfer (MLCT) transitions, whereas
the higher energy bands (<350 nm) are assigned to processes
involving the ligands.32
The presence of the qpy ligand is responsible for the two
new bands at ∼350 nm, which are attributed to a MLCT
transition involving the qpy ligand, and 307 nm, corresponding
to the LC transitions centered on the qpy ligand. Moreover, the
latter presents a slight bathochromic shift of about 20 nm
compared to the bpy centered transitions, possibly due to an
increased conjugation in the qpy ligands. This effect is also
observed for the low energy MLCT band (∼475 nm).
Time-dependent density functional theory (TD-DFT) was
used to validate the attribution of these bands. Superimposition
of experimental absorption spectra and calculated spectra can
be found in Figure 7, the theoretical contribution of each being
found in sections S.I.-7, S.I.-8, and S.I.-9 for 0, 1, and 2,
respectively. We previously reported the corresponding data for
complex 3 elsewhere.26 Apart from a consistent energetic shift
of all spectra, the calculations qualitatively fit the experimental
data quite well and confirm the original assignment.
Concerning the two homoleptic complexes, the bands at 287
and 307 nm, for 0 and 3 respectively, were attributed to LC
transitions. These bands are split in two for the heteroleptic
complexes, due to the before mentioned differential LC
contribution of the bpy and qpy ligands. In the case of
complex 2, the TD-DFT calculation identified the band at 292
nm as the superimposition of transitions between orbitals
HOMO-8 and HOMO-9, qpy-centered, to the LUMO and
LUMO+1, also qpy-centered (Table S.I.-12). This band was
therefore attributed as a pure LCqpy. The band at 308 nm
reveals a 75% contribution of a transition between the HOMO-
7 and LUMO+2, both centered on the bpy ligand. The rest of
the contribution includes the HOMO-2 orbital, partially
Figure 4. Square-wave measurements of [Ru(bpy)2(qpy)]2+ (1) (in
green), [Ru(bpy)(qpy)2]2+ (2) (in blue), and [Ru(qpy)3]2+ (3) (in
red), performed in a degassed MeCN/TBAPF6 (0.1 M) solution at
room temperature.
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Figure 5. Representation of the molecular orbitals of interest (isocontour value 0.02 a.u., HOMO, LUMO, LUMO+1, LUMO+2, from bottom to
2+
top) of the [Ru(bpy)x(qpy)3‑x
]
(x decreasing from left to right).
lifetimes follow the same pattern, lengthening from 870 ns for
[Ru(bpy)3]2+ (0) to 1.44 μs, 1.64 μs, and finally 1.78 μs for
[Ru(bpy)2(qpy)]2+ (1), [Ru(bpy)(qpy)2]2+ (2), and [Ru-
(qpy)3]2+ (3), respectively. The efficiency of the intersystem
crossing (ISC) between the first excited state singlet and the
emissive triplet 3MLCT being considered close to unity in these
complexes, we observe a parallel increase of the quantum yield
(Φ) and the excited state lifetimes (τ).
The emission spectra are presented normalized and propor-
tional to the quantum yield in Figure 8. The emission bands are
red-shifted for all qpy-containing species as compared to the
classic [Ru(bpy)3]2+. The increase in the excited state lifetimes
can be explained by a lower energy LUMO orbital in qpy-
containing complexes, therefore increasing the difference in
3
energy with the metal centered triplet state MC, known to
deactivate the luminescence of the 3MLCT state of
polypyridine ruthenium complexes.37
Photocatalytic Hydrogen Evolution. As the complexes
absorb in an extended visible light range when compared to
[Ru(bpy)3]2+, with higher quantum yields and longer excited
state lifetimes, the [Ru(bpy)x(qpy)3‑x]2+ family were good
candidates to be tested in a molecular homogeneous photo-
catalytic hydrogen evolution system. Using the ancillary
pyridine groups of the quaterpyridine ligand to support the
photosensitizer on TiO2 surfaces, complexes similar to 1 have
been previously tested in photoelectrochemical cells for water
reduction.38 However, the family of complexes has not been
evaluated in homogeneous photocatalysis yet. In addition, the
homoleptic complex [Ru(qpy)3]2+ was described previously by
our group in this type of photocatalysis.26 In the design of
bigger assemblies for hydrogen production, the system should
be designed with a LUMO orbital centered on the bridging
ligand in order to accelerate the electron transfer between the
metallic centers acting as photosensitizer and the catalyst. As
Figure 6. Absorption spectra of [Ru(bpy)3]2+ (0) (in dash gray),
[Ru(bpy)2(qpy)]2+ (1) (in green), [Ru(bpy)(qpy)2]2+ (2) (in blue),
and [Ru(qpy)3]2+ (3) (in red) in acetonitrile.
centered on the metal, demonstrating the mixed character of
certain transitions. However, as the major contribution only is
taken into consideration, that band was attributed to a LCbpy
.
Following the same reasoning for the heteroleptic complex 1,
the band at 288 nm is attributed to a transition predominantly
centered on the qpy ligand (LCqpy), whereas the one at 307 nm
was assigned to a pure LCbpy (Table S.I.-10).
Emission data for the complexes are summarized in Table 2.
The presence of the qpy ligand enhances the quantum yield in
a regular fashion, from [Ru(bpy)3]2+ (0) at 9.5% to
2+
[Ru(bpy)2(qpy)]2+ (1) at 14%, then [Ru(bpy)(qpy)2] (2)
at 19% and finally [Ru(qpy)3] 2+ (3) at 23%. The excited state
F
DOI: 10.1021/acs.inorgchem.7b00771
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. Superimposition of experimental absorption spectrum (in red) and computed TD-DFT spectrum (black lines) for [Ru(bpy)3]2+ (0),
[Ru(bpy)2(qpy)]2+ (1), [Ru(bpy)(qpy)2]2+ (2), and [Ru(qpy)3]2+ (3).
Table 2. Emission Data of Complexes 1, 2, and 3 in
Degassed Acetonitrile (C = ∼10−5 M) at Room Temperature
λmax em.
60832
Φ
9.532
τ
[Ru(bpy)3]2+ (0)
87032
[Ru(bpy)2(qpy)]2+ (1)
[Ru(bpy)(qpy)2]2+ (2)
[Ru(qpy)3]2+ (3)
643
14
1440
635
62826
19
2326
1640
178026
previously demonstrated, the mixed systems bpy/qpy are
fulfilling this requirement and therefore good candidates for
supramolecular hydrogen production. The primary evaluation
of the building blocks 1 and 2 as photosensitizers for
photocatalytic hydrogen evolution is reported here, under
blue light irradiation (λmax = 460 nm, Δλ = ∼150 nm), against
the reference [Ru(bpy)3]2+ (0).
As many parameters are to be considered, we attempted
conditions as close as possible to the literature. Thus, each
experiment was carried out with the [Ru(bpy)3]2+ in parallel as
a direct reference, using the classic [Co(dmgH)2]2+ complex as
the catalyst, formed in situ as the tetrafluoroborate salt. To
study only the half equation of hydrogen evolution reaction,
triethanolamine (TEOA) was used as a sacrificial electron
donor and aqueous tetrafluoroboric acid was the source of
protons.
All the complexes present a hydrogen evolution profile,
although a difference in efficiency is observed. To make sure
that the observed process is photocatalyzed, the system is first
measured in the dark, defining the background of the gas
Figure 8. Emission spectra of [Ru(bpy)3]2+ (0) (in dash gray),
[Ru(bpy)2(qpy)]2+ (1) (in green), [Ru(bpy)(qpy)2]2+ (2) (in blue),
and [Ru(qpy)3]2+ (3) (in red) in degassed acetonitrile. Intensities are
normalized and proportional to quantum yields.
chromatographic (GC) analysis. The lamp is then turned on,
defining t = 0. No induction period was noticed after turning on
the lamp with maximum TOFs obtained at the start. This
process is followed by a decrease, classically attributed to the
decomposition of the photosensitizer.
G
DOI: 10.1021/acs.inorgchem.7b00771
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Inorganic Chemistry
Article
Under blue-light irradiation (460 nm) (Figure 9), the
When the TOF of the systems reaches zero, an injection of
the cobalt catalyst does not restart the hydrogen evolution.
However, a new addition of the studied ruthenium complex
revives the catalysis (Figure S.I.-12), confirming that the PSs are
the limiting factors in such systems. These conditions allow the
correct study of the PS itself, not limited by the efficiency of the
catalyst. All the results presented herein demonstrate that all of
the qpy-containing species are more efficient than the reference
[Ru(bpy)3]2+. Concerning the relative efficiency of those PSs,
the superiority of 2 compared to 1 could be attributed to the
highest quantum yield and elongated lifetime of the excited
state. However, further studies will be needed to quantify the
importance of other parameters such as the potential
interaction(s) in solution between the cobalt catalyst with the
peripheral pyridine functions, the kinetic of the electron
transfers or the stability, and the quenching pathway of the
excited PS in such a reductive environment.
complex [Ru(bpy)2(qpy)]2+ (1) reaches a TOF of 2350
CONCLUSION
■
We report here an efficient microwave assisted one-pot
synthesis of a series of ruthenium(II)-based photosensitizers
bearing quaterpyridine and bipyridine ligands. As the
proportion of quaterpyridine increases among the series, the
photophysical properties are linearly enhanced compared to the
parent [Ru(bpy)3]2+, with quantum yields from 9.5% up to 23%
as well as elongated excited state lifetimes from 870 to 1780 ns
for 0 and 3, respectively. The photosensitizers were tested in a
hydrogen evolution reaction and proved to be more efficient
and robust than the archetypal [Ru(bpy)3]2+. The facile
synthesis of this series of complexes could therefore be
effective in the development of larger families of efficient
photosensitizers adaptable in multimetallic supramolecular
arrangements for artificial photosynthesis.
Figure 9. Hydrogen evolution of [Ru(bpy)2(qpy)]2+ (1) (in blue),
[Ru(bpy)3]2+ (0) (in red) in degassed DMF containing 10−4 M of the
photosensitizers, 1.2 × 10−3 M of [Co(dmgH)2]2+, 1 M of (TEOA)
and 0.1 M of HBF4 (48% in water). Turnover frequencies (TOF,
dashed line) and turnover number (TON, plain line) are presented,
measured under the same conditions using blue-light irradiation (460
nm).
mmol·molPS−1·min−1, whereas only 1240 mmol·molPS−1·min−1
is observed for reference 0. After 8 h, complex 1 presents a
higher activity that the reference with TON of 180 versus 70
for 1 and the reference 0, respectively.
Finally, as depicted in Figure 10, complex 2 is revealed to be
the most efficient of the series with a TOF reached at the start
of 2900 mmol·molPS−1·min−1 versus 2040 mmol·molPS−1·min−1
for the reference. After 14 h, when the [Ru(bpy)3]2+ (0)
presents a TON of 75, while the heteroleptic complex 2 reaches
a TON of 320.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00771.
Experimental details, crystallographic details, as well as
the full computational studies (PDF)
Accession Codes
CCDC 1556965 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*(G.S.H.) Fax: +1 514 343 7586. Tel: +1 514 340 5156. E-mail:
garry.hanan@umontreal.ca.
■
*(V.M.) Fax: +33 (0)1 44 27 38 41. Tel: +33 (0)1 44 27 32 77.
E-mail: valerie.marvaud@upmc.fr.
Figure 10. Hydrogen evolution of [Ru(bpy)(qpy)2]2+ (2) (in blue),
[Ru(bpy)3]2+ (0) (in red) in degassed DMF containing 10−4 M of the
photosensitizers, 1.2 × 10−3 M of [Co(dmgH)2]2+, 1 M of (TEOA)
and 0.1 M of HBF4 (48% in water). Turnover frequencies (TOF,
dashed line) and turnover number (TON, plain line) are presented,
measured under the same conditions using blue-light irradiation (460
nm).
ORCID
Ilaria Ciofini: 0000-0002-5391-4522
Garry S. Hanan: 0000-0001-6671-5234
Funding
The research has been supported by the CNRS, UPMC
(University Pierre and Marie Curie), the French Ministry of
H
DOI: 10.1021/acs.inorgchem.7b00771
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Article
(15) (a) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.;
Venturi, M. Designing Dendrimers Based on Transition-Metal
Complexes. Light-Harvesting Properties and Predetermined Redox
Research, ANR (Switch-2010-Blan-712), ANR E-storic (14-
CE05-0002), and the Natural Sciences and Engineering
Research Council (NSERC). E.R. thanks the Chemistry
Department of the University of Montreal, MITACS Globalink
Patterns. Acc. Chem. Res. 1998, 31, 26−34. (b) Plevoets, M.; Vogtle, F.;
̈
De Cola, L.; Balzani, V. Supramolecular dendrimers with a
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1999, 23, 63−69.
Research Award − Sorbonne Universite
financial support.
́
and NSERC for
Notes
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The authors declare no competing financial interest.
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