A Bisamide Ruthenium Polypyridyl Complex as a Robust and Efficient Photosensitizer for Hydrogen Production

Article abstract of DOI:10.1002/cssc.201701543

A photosensitizer based on a ruthenium complex of a bisamide-polypyridyl ligand gives rise to a large improvement in photocatalytic stability, rate of activity, and efficiency in photocatalytic H₂ production compared to [Ru(bpy)₃]²⁺ (bpy=2,2′-bpyridine). The bisamide ruthenium polypyridyl complex combined with a cobaltoxime-based photocatalyst was found to be highly efficient under blue-light (turnover number (TON)=7800) and green-light irradiation (TON=7200) whereas [Ru(bpy)₃]²⁺ was significantly less effective with a TON of 2600 and 1100, respectively. The greatest improvement was under red-light-emitting diodes, with bisamide ruthenium polypyridyl complex and cobaltoxime exhibiting a TON of 4200 compared to [Ru(bpy)₃]²⁺ and cobaltoxime at a TON of only 71.

Full text of DOI:10.1002/cssc.201701543

Accepted Article
Title: A bis-amide ruthenium polypyridyl complex as a robust and
efficient photosensitizer for H2 production
Authors: Garry Shawn Hanan, Amlan K. Pal, Olivier Schott, and Daniel
Chartrand
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ChemSusChem
10.1002/cssc.201701543
COMMUNICATION
A bis-amide ruthenium polypyridyl complex as a robust and efficient photosensitizer for H
2
production
a
a,b
c
a ,
Olivier Schott, Amlan K. Pal, Daniel Chartrand, Garry S. Hanan *
Dedication ((optional))
Abstract:
reveals a large improvement in photocatalytic stability, rate of activity
and efficiency in photocatalytic H production when compared to
] . The photocatalytic system of 1 combined with a
A
bis-amide polypyridyl ruthenium photosensitizer
1
system, the PS and the catalyst must be in close proximity in order
to favour electron transfer, and many assemblies exist that are
2
covalently linked or connected by pendant pyridine on the PS
2
+
[4b, 4c, 4l, 8a, 14]
[
Ru(bpy)
3
which coordinates the catalysts.
Indeed, the better the
cobaltoxime was found to be highly efficient under blue-light (Turnover
number (TON) = 7800) and green-light irradiation (TON = 7200) while
electron transfer, the lesser is the probability of photosensitizer
decomposition induced by ligand dissociation,[15] e.g., multi-
2+
[
Ru(bpy)
3
]
was significantly less effective with a TON of 2600, and
6
metallic supramolecular assembly Ru-Co proves to have better
2
+
+
[8a,
1100, respectively. The greatest improvement was under red LED
efficiency than dissociated [Ru(bpy)
3
] -[Co(dmgH)
2
] system.
1
4d]
irradiation, with 1 and cobaltoxime exhibiting a TON of 4200
Charge-separated states and electronic transfer rates have
2+
compared to [Ru(bpy)
3
]
and cobaltoxime at only 71.
also been investigated through hydrogen bonding or hydrophobic
interactions. [16] In this work, complex 1 was investigated as a PS
2
in combination with [Co(dmgH) (DMAP)Cl], 3, (where dmgH =
dimethylglyoxime and DMAP = 4-dimethylaminopyridine) as the
cobaltoxime to give the pre-HER photocatalyst 4 (Chart 1). The
The future well-being of our society relies on innovative
research to produce clean sources of energy. With global
warming on the rise, new alternatives to fossil fuels are needed,
and of the several options available, the inexhaustible power of
the sun is one of the most convenient clean energy sources.[
Molecular artificial photosynthesis presents a promising avenue
for the conversion and storage of solar energy in chemical bonds
as found in hydrogen gas.[2],[3] Molecular photocatalytic systems
for the hydrogen evolution reaction (HER), half of the water
splitting process, are mainly composed of a photosensitizer (PS),
a catalyst, and a sacrificial electron donor. In the last decade of
photocatalytic system research, Ir(III),[4] Re(I)[5] and Pt(II)[6] have
been tested to improve overall performance of the HER. In the
photocatalytic H
archetypical [Ru(bpy) ] , 2, as the PS and complex 3 as the
3
2
production results were compared with that of
2+
1]
cobaloxime. After the initial Co(III) to Co(II) reduction, the
liberated chloride ion is available to bind in the pocket of PS 1,
thus enhancing it properties as described by Beer (vide infra).
N
O
N
N
H
N
N
N
N
N
N
N
Ru
N
6
(PF )
2
O
O
N
N
N
N
O
O
H
Co
Cl
H
H
N
H
6
case of PS made of d metal complexes, many researchers utilize
O
O
the archetypical [Ru(bpy)
3
]2+ (bpy = 2,2′-bpyridine) complex due
O
N
N
1
3
Co
N
O
to its lower cost and the higher abundance of Ru compared to
other transition metal complexes.[7] To benefit from the availability
of Ru-based PS, however, some of its properties should be
modified, e.g., its absorption maximum at 450 nm should be
shifted to longer wavelength to take advantage of the tailing of the
solar spectrum through the visible and near-IR regions.[7a, 8]
N
O
O
2
+
N
H
N
N
N
N
N
N
N
N
N
H
Ru
N
Cl
O
Ru
N
6
(PF )
2
N
H
N
Among earth-abundant HER catalysts, new advances with
Co catalysts[9] are particularly attractive compared to their Pt,[10]
2
4
Pd,[11] and Rh[12] analogs. Cobaltoxime is one of the most studied
Chart 1. Molecular structures of Ru(II)-based photosensitizers
cobaloxime used in this study;
from to after reduction of the Co(III) complex.
1
, 2 and
3
4 describes the chloride anion transfer
2
molecular catalysts for H production in electrocatalysis and
3
1
photocatalysis.[
4c, 6a, 7d, 9b-d, 13]
For an efficient photocatalytic
In the pioneering work of Beer et al a family of
optoelectronically active bis-amide based polypyridyl Ru(II)
complexes were developed for anion recognition.[17] Acyclic,
macrocyclic and calix[4]arene substituted amide derivatives
were tested to recognize halides.[17-18] In his work, the ligand
[
a]
Olivier Schott, Prof Dr. G.S Hanan
Départment de Chimie,
Université de Montréal
2
900 Edouard-Montpetit, Montréal, Québec H3T-1J4, Canada,
E-mail: garry.hanan@umontreal.ca; Fax: +1 514 343 7586; Tel: +1 514
40 5156.
b] Dr.Amlan K. Pal
N,N′-diphenyl-[2,2′-bipyridine]-4,4′-dicarboxamide,
heteroleptic Ru(II) complex,
bipyridine)( )ruthenium(II) hexafluorophosphate, acts as a
L
, in a
3
[
1
,
bis(2,2′-
Organic Semiconductor Centre,
EaStCHEM School of Chemistry, University of St Andrews,
St Andrews, Fife KY16 9ST, United Kingdom.
L
halide ion receptor and can accommodate a chloride ion by
non-classical hydrogen bonding with a stability constant of
[
c]
Dr. Daniel Chartrand
LAMP – Laboratoire d’Analyse pour les Molécules et Matériaux
Photoactifs – Laboratory for the Analysis of Molecules’ and Materials’
Photoactivity , Université de Montréal,
5
†
-1 [17]
1
4
0 M . Furthermore, molar absorptivity of MLCT (Metal-
to-Ligand Charge Transfer) band and emission intensity of
complex are enhanced as a result of chloride ion
recognition.[17] On this note, we decided to exploit the
155 Chemin de la Rampe, Montréal, Québec H3T 2B1, Canada.
1
Electronic Supplementary Information (ESI) available: Synthetic,
crystallographic, optoelectronic, DFT studies and experimental details of
HER; CCDC 1016847. See DOI: 10.1039/x0xx00000x
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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ChemSusChem
10.1002/cssc.201701543
COMMUNICATION
property of anion recognition[19] of complex
1
and its
The optoelectronic data for
. In electrochemistry (Figure S4 and Table S3 in ESI†) at
positive potential, an anodic shift of 120 mV is observed for
1 and 2 are gathered in Table
subsequent enhancement of photophysical properties for
artificial photosynthesis. Complex was investigated as a
PS in combination with [Co(dmgH) (DMAP)Cl], , (where
dmgH dimethylglyoxime and DMAP 4-
1
1
3
the first oxidation potential (Ru /Ru ) in 1
(E1/2Ox = 1.38 V)
III
II
2
Ox
=
=
compared to that of complex 2 (E1/2 = 1.26 V), in agreement
dimethylaminopyridine) as the cobaloxime photocatalyst.
The initial reduction of the Co(III) species leads to loss of
chloride anion which enhances the photophysical properties
with the lower energy calculated for the highest energy
occupied orbital (HOMO) of complex (EHOMO = -6.24 eV)
compared to that of complex (EHOMO = -6.11 eV) (Figure 2
1
2
of the PS. The photocatalytic H
2
production results were
and Tables S3 and S4). The first reduction potential of
(E1/2Red = -1.06 V) is 270 mV less negative than that of
1
2
2
+
compared with that of archetypical [Ru(bpy)
3
] , 2, as the PS
and complex
3
as the photocatalyst.
(E1/2Red = -1.33 V), which is in line with the fact that the
electron withdrawing amide groups stabilize the lowest
Complex
1
L
(
Chart 1) was prepared from [Ru(bpy)
2
Cl
2
]
unoccupied molecular-orbital (LUMO) of complex
compared to the same of reference complex (calculated
LUMO = -2.92 and -2.54 eV, respectively, for complexes
1
and ligand
as described by Beer et. al.[17] It was
2
1
13
characterized by H and C NMR spectroscopy (Figures
S1a and S2, ESI†), high resolution mass spectrometry and
E
1
and 2).
elemental analysis. The cobaltoxime photocatalyst
3
was
The
synthesized following
a
literature procedure[13d]
.
1
formation of pre-HER catalyst
studies of and
similar to those found by Beer and co-workers.[17] Single
crystals of suitable for X-ray analysis were obtained by slow
vapour diffusion of diethylether into concentrated
acetonitrile solution of complex , prepared directly as its
4
is supported by H NMR
1
3 (see ESI, Figure S1b) with chemical shifts
1
a
1
chloride salt (Figure 1 and Figure S3, ESI† for complete
labelling). Selected crystallographic parameters are
tabulated in Table S1 (ESI†). A comparison of bond angles
and distances obtained crystallographically and by density
functional theory (DFT) calculation of complex
in Table S2 (ESI†). The coordinatively saturated Ru(II)
centre in complex has a distorted octahedral geometry. The
Ru–N distances for the coordinated bpy ligands are
approximately the same for compound (varies from
.063(4) Å to 2.081(3) Å). These values are in line to the
distances observed in Ru-bpy complexes in general (1.96–
.16 Å, average = 2.06(5) Å[20]). The chloride anion was
found to be encapsulated in the amide pocket of the N,N′-
diphenyl-[2,2'-bipyridine]-4,4'-dicarboxamide ligand in
complex by non-classical hydrogen bonding, as shown by
1 is tabulated
1
1
2
2
Figure 2. Calculated frontier MO energies of [1 2
]2+ and [ ]2+, obtained
from TD-DFT (see Figure S5 for details)
1
In UV-vis absorption spectroscopy (Figure 3), several
intense transitions at the high energy region (< 300 nm)
are observed, which can be assigned as the ligand
centered (LC) π-π* transitions, as predicted by the TD-
DFT calculation (Tables S5 and S6). The transitions
between 300-450 nm are mostly constituted by singlet
Beer et al.[17]
1
metal-to-ligand charge transfer ( MLCT) transitions (either
from Ru(dπ) to bpy(π*) or Ru(dπ) to
L(π*)), The
contribution by LC transitions is minor. The lowest energy
1
transition at 476 nm is exclusively contributed by MLCT
transition from Ru(dπ) to
L
(π*) as predicted by TD-DFT.
, the lowest
In comparison to the prototype complex,
2
energy transition, which is HOMO→LUMO in nature, is
red-shifted by 26 nm as a result of stabilization of both the
HOMO and LUMO in presence of electron withdrawing
Figure 1. ORTEP view of complex
probability level. Hydrogen atoms and one of the two chloride counter
1. Ellipsoids correspond to a 50%
amide functionality compared to those in
2, thus reducing
anions have been omitted for clarity
.
the overall HOMO-LUMO gap compared to the latter in
2
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ChemSusChem
10.1002/cssc.201701543
COMMUNICATION
(
Figure S5). The emission maximum of complex
57 nm) in degassed MeCN solution at r.t. is red shifted
by 49 nm compared to that of complex em = 608 nm)
Table 1 and Figure S7, ESI†). Triplet spin density
distribution (Figure S8, ESI†) by unrestricted DFT
calculation of complex suggests that the excited state is
predominantly distributed on ligand and the Ru(II) ion as
em
1 (λ =
6
2 (λ
(
1
L
opposed to bpy and thus, the nature of emission can be
3
attributed to MLCT transition from Ru(II) to
The photoluminescence quantum yield (
L.
Φ
PL) and the
excited-state lifetime (τPL) values of complex 1 are
enhanced to 13.3 % and 1200 ns, respectively, in
deaerated acetonitrile compared to those of reference
complex
Considering its enriched optoelectronic proprieties,
appears as promising candidate to act as
2 (Φ PL = 9.5 % and τ PL = 870 ns, Table 1).
1
a
a
Figure 4. H
2
evolution of (
1
-
3
) + 4 equiv. of dmgH
2
(blue) and reference
system ( ) + 4 equiv. of dmgH
2
-
3
2
(red). 1 M TEOA as SED and 0.1 M HBF
4
photosensitizer for the HER.
as proton source were used for both systems in DMF. Solid line: TON,
dashed line: TOF. Irradiation at 452 nm (blue light)
Table 1. Optoelectronic data of 1 and 2 in deaerated MeCN.
[
Co(dmgH)
2
(py)Cl] in N,N′-dimethylformamide (DMF).[13d]
Cmpd
) / V (mV)
(ΔEp) / V (mV)
1
2
However, the DMAP derivative demonstrates no
photocatalytic activity in acidic aqueous media due to the
strong electron-withdrawing character of the protonated
ox
1.38(91) b
c
E
E
∆
λ
λ
(ΔE
p
1.26
-1.33
2.59 V
red
-1.06(61) b
c
[
9d,
25]
dimethylamino moiety.
In our studies, the
E
redox / V
2.44
3
-1
-1
d
photoirradiation conditions used in DMF-TEOA (where
TEOA = triethanolamine) induce a basic medium (pHapparent
abs / nm (ε x10 M cm ) 476 (15.7) 450 (14.6)
d
em / nm
657
13.3
1200
608
9.5
870
.
= 8.9), thus, the DMAP substituent would exhibit a
nucleophilic character. TEOA is used as a sacrificial electron
donor (SED). As we recently reported for a Ru-Co system,
e
Φ
PL / %
d
τ
PL/ nbs
a
6
At r.t. Potentials are in volts (V) vs. SCE, 0.1 M in [n-Bu
recorded at a sweep rate of 100 mV/s. The difference between
cathodic, Epc, and anodic, Epa, peak potentials, , (millivolts) is given
4 6
N]PF ,
photocatalytic activity and decomposition pathways of the
reagents and catalysts depend on the wavelengths of
irradiation.[8a]
∆E
p
c [21] d [22]
.
[23].
e
in parentheses.
.
In our studies, light emitting diodes (LED) with
λem at 452,
Complex
1 was tested vs. complex 2 in parallel
5
23, and 630 nm were used for H production (hydrogen
2
experiments for photocatalytic hydrogen evolution (see ESI†
for experimental details). Recent studies demonstrated the
influence of pyridine on the catalyst
quantum yield calculations are described in Figures S9-S11
of the ESI). An overlap of the emissions of LEDs and the
absorption spectra of PS
Just after turning the blue light on,
catalytic activity of catalyst with a peak turnover frequency
TOF) of 36000 mmolH2.molPS 1.min-1
decreases to exhibit a Turn-over Number (TON) of 7800 after
0 h (Figure 4). The reference system, with , provides a
H2=7.5%) and reaches
a TON of 2600 after 20 h, one third that of with
Under green light irradiation ( em = 523 nm), the overlap
of the emission of LED and the absorption spectra of PSs
and are lower compared to that under blue light irradiation,
although under green light irradiation higher spectral overlap
is expected for PS compared to that of PS . (Figure S10
ESI†) For PS , the photocatalytic reaction rate and stability
1
and
2
were observed in all cases.
1
photosensitizes the
3
-
(
(ΦH2=18.4%) which
2
2
3
TOF of 14000 mmolH2.molPS 1.min-1
(Φ
-
1
3.
λ
1
2
1
2
,
1
with green light irradiation are approximatively the same as
for blue-light irradiation. The hydrogen evolution under green
Figure 3. UV-vis absorption spectra of
1
(blue) and
2
(red), in MeCN at
light irradiation of both (
a maximum TOF of 30100 mmolH2.molPS-1.min-1 (
1
-
3
) and (
2
-
3
) systems are different,
H2=45.6 %)
while this value reduces to 3500
H2=9.5 %) for Figure 5). The TON
and a
value of 1100 after 19 h of irradiation for 2. Moreover,
2
98 K. Inset: magnified spectra in the region from 500 to 650 nm
Φ
[
24]
activity of cobaloximes, e.g., the donating effect of DMAP
on cobaloxime induces a 5-fold higher rate for electro-
catalytic activity than pyridine (py) in complex
is reached for
mmolH2.molPS 1.min-1
1
-
(
Φ
2 (
attains a value of 7200 after 20 h of irradiation for
1
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ChemSusChem
10.1002/cssc.201701543
COMMUNICATION
considering the quantity of TEOA used as SED ([TEOA] = 1
M) and the fact that two electrons are given per oxidized
TEOA molecule,[26] the photocatalytic system with [
1
] = 0.1
mM converts, 78% and 72% of TEOA to H
green light irradiation, respectively.
2
under blue and
Figure 6. H
2
evolution of (
1
-
3
2
) + 4 equiv. of dmgH (blue) and reference
system ( ) + 4 equiv. of dmgH
2
-
3
2
(red). 1 M TEOA as SED and 0.1 M HBF
4
as proton source were used for both systems in DMF. Solid line: TON,
dashed line: TOF. Irradiation at 630 nm (red light).
Figure 5.
system (
H
2
evolution of (
1
-
3
) + 4 equiv. of dmgH
2
(blue) and reference
2
-
3
) + 4 equiv. of dmgH
2
(red). 1 M TEOA as SED and 0.1 M HBF
4
In conclusion, we report a photocatalytic system with an amide
as proton source were used for both systems in DMF. Solid line: TON,
dashed line: TOF. Irradiation at 523 nm (green light).
based
Ru(II)-PS, and
1
a
cobaloxime
catalyst,
[
Co(dmgH)
2
(DMAP)Cl], 3 that displays high photocatalytic
stability and rate of activity for HER. The photocatalytic system (1-
3) was found to be highly efficient under both blue and green LED
irradiation conditions, and also found to be operative under red
LED irradiation, albeit with low quantum yields for the latter. The
In the case of PS
irradiation is 5-fold slower than the same conditions under
blue light irradiation with only half the TON.
2, the rate of HER under green light
Under red-light irradiation (λem = 630 nm), PS
1
exhibits
H2=5.3 %) to
reach a TON of 4200, while TON of hydrogen production for
PS attains a maximum value of 71 and TOF of only 18
H2=2.3 %) after 80 h of irradiation (Figure 6). Both systems
are still found to be active under irradiation after 80 hours. To
the best of our knowledge, the TOF and TON of PS are the
best values under red light irradiation condition for molecular
-
1
-1
results were compared with [Ru(bpy)
3
]2+, 2, in combination with
a maximum TOF of 3050 mmolH2.molPS .min (
Φ
catalyst 3. Under each irradiation condition, 1 proves to be a more
efficient PS than 2. Further investigations are underway to
evaluate the impact of stronger H-bonding and tuning of
2
(Φ
2
photocatalyst for efficient H production.
1
[
8a, 14d]
photocatalytic HER.
spectra (Figure 3 inset) of PS
molar absorptivity for both the PS
region (λ> 620 nm). Considering the lower absorptivity in red
energy region and the TOF, conversion of the absorbed Acknowledgements
photons to H is still efficient compared to those in blue and
green irradiation for PS and . Concerning a Ru-Pd dyad
The tailing of the absorption
and exhibits negligibly low
and in the red energy
Experimental Section: Please see supporting information.
1
2
1
2
2
1
2
The authors thank the Natural Sciences and Engineering
Research Council of Canada (NSERC) and Centre for Self-
Assembled Chemical Structure (CSACS) for financial support.
reported in the literature, wavelength dependence studies for
HER show that catalytic efficiency or the TON is correlated
with the direct population of the excited state upon excitation
on MLCT [
11a]
In our study, excitation in the RGB energy
3
region of PS
1
promotes a MLCT electronic transition from
Keywords : • solar fuel generation• supramolecular photo-
Ru(II) to ligand
density distribution of PS
L
, the fact also corroborated by triplet spin
at its excited state (Figure S8
chemistry • transition metals • photophysics • catalysis
1
,
ESI†). In this case of green light irradiation for PS 1, the
quantum yield is particularly impressive: ΦH2=45.6 %. Upon
[
1]
2]
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free to bind in the amide pocket of in PS . Once the
chloride ion is in the pocket of PS displays enhanced
photophysical properties as described by Beer. The transfer
of chloride ion to PS would contribute to the enhanced
photocatalytic H evolution rate for PS compared to PS
At this point, however, the PS and cobaltoxime
would not be in a supramolecular assembly.
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ChemSusChem
10.1002/cssc.201701543
COMMUNICATION
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10.1002/cssc.201701543
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Fast and stable : A heteroleptic bisamide Ru
Author(s), Corresponding Author(s)*
photo-sensitizer in association with Co(dmgH)
2
2+
Page No. – Page No.
(DMAP)Cl is studied versus Ru(bpy)
3
for
light-driven hydrogen evolution under blue,
green and red LED irradiation sources. Here,
we describe the most productive system to
date under red light irradiation.
Title
Layout 2:
COMMUNICATION
Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
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ChemSusChem
10.1002/cssc.201701543
Chart 1. Molecular structures of Ru(II)-based photosensitizers
transfer from to after reduction of the Co(III) complex.
1,
2
and cobaloxime
3
used in this study;
4
describes the chloride anion
3
1
Figure 1. ORTEP view of complex 1. Ellipsoids correspond to a 50% probability level. Hydrogen atoms and one of the two chloride counter anions
have been omitted for clarity.
Figure 2. Calculated frontier MO energies of [1 2
]2+ and [ ]2+, obtained from TD-DFT (see Figure S5 for details)
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ChemSusChem
10.1002/cssc.201701543
Table 1. Optoelectronic data of 1 and 2 in deaerated MeCN.
Cmpd
) / V (mV)
(ΔEp) / V (mV)
1
2
ox
b
c
E
E

(ΔE
p
1.38(91)
1.26
-1.33
2.59 V
red
-1.06(61) b
2.44
c
E
redox / V
abs / nm ( x10 M cm ) 476 (15.7) 450 (14.6) d
3
-1
-1




em / nm
PL / %
PL/ ns
657
13.3
1200
608 d
9.5
870 d
e
Figure 3. UV-vis absorption spectra of
1 (blue) and 2 (red), in MeCN at 298 K. Inset: magnified spectra in the region from 500 to 650 nm
Figure 4. H
HBF
2 2 2
evolution of (1-3) + 4 equiv. of dmgH (blue) and reference system (2-3) + 4 equiv. of dmgH (red). 1 M TEOA as SED and 0.1 M
4
as proton source were used for both systems in DMF. Solid line: TON, dashed line: TOF. Irradiation at 452 nm (blue light)
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ChemSusChem
10.1002/cssc.201701543
Figure 5.
H
2
evolution of (
1
-
3
) + 4 equiv. of dmgH
2
(blue) and reference system (
2
-
3
) + 4 equiv. of dmgH
2
(red). 1 M TEOA as SED and
0
4
.1 M HBF as proton source were used for both systems in DMF. Solid line: TON, dashed line: TOF. Irradiation at 523 nm (green light).
Figure 6. H
.1 M HBF
2 2 2
evolution of (1-3) + 4 equiv. of dmgH (blue) and reference system (2-3) + 4 equiv. of dmgH (red). 1 M TEOA as SED and
as proton source were used for both systems in DMF. Solid line: TON, dashed line: TOF. Irradiation at 630 nm (red light).
0
4
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