Photophysical and electrochemical properties of platinum(ii) complexes bearing a chromophore-acceptor dyad and their photocatalytic hydrogen evolution

Article abstract of DOI:10.1039/c2dt30415e

A series of platinum(ii) complexes bearing a chromophore-acceptor dyad obtained by reacting 4-(p-bromomethylphenyl)-6-phenyl-2,2′-bipyridine or 4′-(p-bromomethylphenyl)-2,2′:6′,2′′-terpyridine with pyridine, 4-phenylpyridine, 4,4′-bipyridine, 1-methyl-4-(

Full text of DOI:10.1039/c2dt30415e

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PAPER
Photophysical and electrochemical properties of platinum(II) complexes
bearing a chromophore–acceptor dyad and their photocatalytic hydrogen
evolution†
Gui-Ju Zhang,a Xin Gan,b Quan-Qing Xu,b Yong Chen,a Xi-Juan Zhao,a Biao Qin,a Xiao-Jun Lv,a
Siu-Wai Lai,c Wen-Fu Fu*a,b and Chi-Ming Che*c
Received 21st February 2012, Accepted 30th April 2012
DOI: 10.1039/c2dt30415e
A series of platinum(II) complexes bearing a chromophore–acceptor dyad obtained by reacting
4-(p-bromomethylphenyl)-6-phenyl-2,2′-bipyridine or 4′-(p-bromomethylphenyl)-2,2′:6′,2′′-terpyridine
with pyridine, 4-phenylpyridine, 4,4′-bipyridine, 1-methyl-4-(pyridin-4′-yl)pyridinium hexafluorophosphate
respectively, were synthesized. Their photophysical properties, emission quenching studies by Pt
nanoparticles and methyl viologen, electrochemical properties and photoinduced electron-transfer reactions in
a photocatalytic hydrogen-generating system containing triethanolamine and colloidal Pt without an extra
electron relay, were investigated. A comparison of the rates of hydrogen production for the two photocatalytic
systems, one containing a metal–organic dyad and the other comprising a 1 : 1 mixture of the parental
platinum(II) complexes and the corresponding electron relay, showed that intramolecular electron transfer
improves the photocatalytic efficiency. Compared with cyclometalated platinum(II) complexes, the related
platinum(II) terpyridyl complexes exhibited poor performance for photocatalytic hydrogen evolution. An
investigation into the amount of hydrogen generated by three platinum(II) complexes containing
cyclometalated ligands with methyl groups located on different phenyl rings revealed that the efficiency of
hydrogen evolution was affected by a subtle change of functional group on ligand, and the hydrogen-
generating efficiency in the presence or absence of methyl viologen is comparable, indicating electron
transfer from the excited [Pt(C^N^N)] chromophore to colloidal Pt. 1H NMR spectroscopy of the metal–
organic dyads in an aqueous solution in the presence of excess triethanolamine revealed that the dyad with a
viologen unit was unstable, and a chemical reaction in the compound occurred prior to irradiation by visible
light under basic conditions.
Intensive efforts have been directed towards a solar-driven water
1. Introduction
splitting reaction using semiconductor-based photocatalysts,1 or
Photocatalytic hydrogen evolution from water splitting under
visible light irradiation has been receiving a surge of interest.
metal complexes as a chromophore/catalyst for photochemical
hydrogen and/or oxygen production.2 A method for hydrogen
evolution comprises a multicomponent photochemical system
containing a metal-based photosensitizer such as ruthenium(II)3
and platinum(II)4 complexes or metalloporphyrins,5 an electron
mediator and a proton reduction catalyst in the presence of a
sacrificial electron donor. However, these systems still need to be
improved because many interfacial interactions could affect the
photoinduced electron transfer reactions. We conceive to
combine two of these components into an integrated system to
achieve more efficient hydrogen generation.6,7 A two-electron
mixed-valence dirhodium compound used as an efficient photo-
catalyst of hydrogen production was reported by Nocera and
Heyduk.8 Sakai and co-workers studied a dinuclear metal cata-
lyst composed of a [Ru(bpy)3]2+ derivative and a [(bpy)PtCl2]
moiety for photoinduced hydrogen evolution.9 Studies by these
workers demonstrated that some mono- and dinuclear platinum(II)
complexes can serve as hydrogen-generating molecular catalysts
in the absence of light.10 Rau et al. described a heterodinuclear
aKey Laboratory of Photochemical Conversion and Optoelectronic
Materials, CAS-HKU Joint Laboratory on New Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences,
Peking 100190, P.R. China. E-mail: fuwf@mail.ipc.ac.cn;
Fax: +86 10 62554670; Tel: +86 10 82543519
bCollege of Chemistry and Chemical Engineering, Yunnan Normal
University, Kunming 650092, P.R. China
cDepartment of Chemistry, Institute of Molecular Functional Materials
and CAS-HKU Joint Laboratory on New Materials, The University of
Hong Kong, Pokfulam Road, Hong Kong SAR, P.R. China.
E-mail: cmche@hku.hk; Fax: +85 2 28571586
†Electronic supplementary information (ESI) available: transient absorp-
tion spectra of complexes 2–4 in CH3CN (λexc = 355 nm) at room temp-
erature; emission spectral changes of 1 and 9 as a function of the
colloidal Pt concentration; time-resolved emission spectra of complex 1;
changes in the absorption spectra of 5 upon addition of various concen-
trations of TEOA; 1H NMR spectra of complexes 2–5 in DMSO-d6,
DMSO-d6–D2O (v/v, 2 : 1) with excessive amounts of TEOA;
photocatalytic hydrogen evolution of complexes 6–8. See DOI:
10.1039/c2dt30415e
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8421–8429 | 8421

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Ru–Pd molecular photocatalytic system for hydrogen production
in the presence of triethylamine which acts as both an electron
and proton source, and turnover numbers of more than 50 were
achieved in acetonitrile solution under illumination with 470 nm
light.11 Recently, a new molecular-based system containing a
Ru–DMB–Pd supramolecular complex was used to carry out
photoinduced hydrogen evolution in acetonitrile and triethyl-
amine solutions upon visible-light irradiation at λ ≥ 475 nm.
Hammarström and co-workers found that the photoinduced
release of Pd from Ru–DMB–Pd plays a crucial part in the
observed hydrogen-generating reaction.12 Eisenberg et al. have
revealed that some platinum(II) complexes for light-driven hydro-
gen evolution can function as photochemical precursors to form
Pt(0) loaded on TiO2 surface as co-catalyst.13 In contrast to
supramolecular photocatalysts, development of a multicompo-
nent system for photocatalytic hydrogen production focused on
the design of “chromophore-quencher” assemblies that comprise
a metal complex chromophore covalently linked to an organic
electron donor or acceptor. In the past few decades, studies invol-
ving both dyad14 and triad15 systems containing a [Ru(bpy)3]2+
Scheme 1 Chemical structures of complexes 1–10.
3
component with well-investigated MLCT excited state have led
400 MHz AVANCE II spectrometer at 298 K with chemical
shifts (δ, ppm) relative to tetramethylsilane (TMS). ESI-MS
spectra were performed by an APEX II Model FT-ICR mass
spectrometer. Elemental analyses were obtained with an Elemen-
tar Vario EL (Germany) instrument. Electronic absorption
spectra were recorded using a HITACHI U-3010 spectro-
photometer. Fluorescence spectral measurements were carried
out with a HITACHI F-4500 fluorescence spectrophotometer
adapted to a right-angle configuration at room temperature.
Time-resolved emission spectra and emission lifetimes of sol-
ution samples were performed on a Flash Photolysis Spectro-
photometer (LP920, Edinburgh) or using single photon counting
on a FL900 Spectrometer (Edinburgh). In the transient absorption
spectroscopy experiments, all samples were purged with nitrogen
for 30 min. The back electron-transfer rate constants were calcu-
lated by the obtained time-resolved spectra. Excitation was pro-
vided by using an Nd:YAG laser (third harmonic, 10 ns) at
355 nm. The detector was a xenon lamp on the Edinburgh
LP920 apparatus from Analytical Instruments. Cyclic voltam-
metric experiments were conducted on a computer-controlled
CHI660C electrochemical workstation with a conventional three-
electrode system comprised of platinum as counter electrode,
mercury-coated glassy carbon working electrode and saturated
calomel electrode (SCE) as the reference. All experiments were
carried out under a nitrogen atmosphere in acetonitrile solution
containing Pt(II) complex (2.0 × 10−4 mol L−1) and n-Bu4NPF6
(0.1 mol L−1).
to many important observations concerning efficient charge sep-
aration and energy transfer processes.16 Eisenberg and co-
workers reported for the first time that platinum(II) terpyridyl
complexes could be used as a chromophore for light-driven
hydrogen generation in a three-component system,17 and also
they reported the structures and photophysical properties of both
triad and dyads (chromophore-acceptor and donor-chromophore)
containing a [Pt(terpy)]2+ unit.18 We have also reported that the
cyclometalated platinum(II) complex, [Pt(C^N^NPhMe)Cl]
(HC^N^NPhMe = 4-(p-tolyl)-6-phenyl-2,2′-bipyridine) is an
efficient photosensitizer for photocatalytic hydrogen evolution in
solution containing methyl viologen, triethanolamine and col-
loidal Pt.19
On the basis of previous reports, the synthesis of a stable
chromophore that contains a photosensitizer covalently linked to
a mediator should promote intramolecular electron transfer with
high hydrogen-generating efficiency. Our efforts are focused
towards using platinum(II) complexes bearing 6-phenyl-2,2′-
bipyridyl (HC^N^N) and terpyridyl (N^N^N) ligands and corre-
sponding chromophore–acceptor dyads as photosensitizers for
hydrogen evolution. In this paper we report the synthesis of a
series of polypyridyl platinum(II) complexes with chromophore–
acceptor dyad ligands as well as their parent complexes
(Scheme 1). These compounds were employed as photocatalysts
in a photocatalytic hydrogen production system containing
triethanolamine (TEOA) and colloidal Pt without an extra elec-
tron relay.
2.2. Syntheses and characterizations
2. Experimental section
4-[(Pyridinium-1-yl)methylphenyl]-6-phenyl-2,2′-bipyridine
hexafluorophosphate. This ligand was synthesized according to
the literature method using 4-(p-bromomethylphenyl)-6-phenyl-
2,2′-bipyridine (0.25 g, 0.60 mmol) instead of 4′-(p-bromo-
2.1. General procedures
All starting materials were obtained commercially and used as
received unless stated otherwise. All solvents were purified
according to standard methods.20 4-(p-Bromomethylphenyl)-6-
phenyl-2,2′-bipyridine, 4′-(p-bromomethylphenyl)-2,2′:6′,2′′-ter-
pyridine and complex 1, 5, 6, 8 and 10 were prepared as pre-
viously described.21 1H NMR spectra were recorded on a Bruker
1
methylphenyl)-2,2′:6′,2′′-terpyridine.18 Yield: 0.18 g (68%). H
NMR (400 MHz, CD3CN, TMS): δ = 5.82 (s, 2H, CH2), 7.59
(m, 6H), 8.01 (d, J = 8.1 Hz, 2H), 8.07 (t, J = 7.1 Hz, 2H), 8.12
(t, J = 7.8 Hz, 1H), 8.20 (s, 1H), 8.30 (d, J = 7.4 Hz, 2H), 8.55
8422 | Dalton Trans., 2012, 41, 8421–8429
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(t, J = 7.8 Hz, 1H), 8.62 (s, 1H), 8.70 (d, J = 8.0 Hz, 1H), 8.74
(d, J = 4.4 Hz, 1H), 8.82 (d, J = 5.5 Hz, 2H); ESI-MS: m/z
400.3 [M − PF6]+. Anal. Calcd for C28H22F6N3P (545.46): C,
61.65; H, 4.07; N, 7.70. Found: C, 61.54; H, 4.09; N, 7.73.
C34H28F12N4P2 (782.54): C, 52.18; H, 3.61; N, 7.16. Found: C,
52.33; H, 3.64; N, 7.15.
4′-[(4,4′′-Bipyridinium-1′′-yl)methylphenyl]-2,2′:6′,2′′-ter-
pyridine bromate. 4′-(p-Bromomethyl phenyl)-2,2′:6′,2′′-terpyr-
idine (0.4 g, 1.0 mmol) and 4,4′-bipyridine (0.16 g, 1 mmol) in
CH3CN (150 mL) were heated to 60 °C for 60 h under a nitrogen
atmosphere. The pale-yellow precipitate was isolated by fil-
tration, and washed with diethyl ether. The solid was purified
twice by recrystallization with CH3CN and diethyl ether. Yield:
4-[(4-Phenylpyridinium-1-yl)methylphenyl]-6-phenyl-2,2′-
bipyridine hexafluorophosphate. 4-(p-Bromomethylphenyl)-
6-phenyl-2,2′-bipyridine (0.30 g, 0.75 mmol) and 4-phenyl-
pyridine (0.13 g, 0.85 mmol) in CH3CN (50 mL) were heated to
60 °C for 72 h under a nitrogen atmosphere. The pale-yellow
precipitate was isolated by filtration, and washed with diethyl
ether. The solid was redissolved in 10 mL of distilled water. A
saturated aqueous solution of NH4PF6 was added, and the result-
ing solution was stirred at room temperature for 3 h. The white
solid was filtrated and washed with distilled water and diethyl
ether. Yield: 0.30 g (72%). 1H NMR (400 MHz, DMSO-d6,
TMS): δ = 5.95 (s, 2H, CH2), 7.51 (m, 2H), 7.57 (t, J = 7.6 Hz,
2H), 7.66 (broad s, 3H), 7.81 (d, J = 8.2 Hz, 2H), 8.03 (t, J =
6.7 Hz, 1H), 8.09 (m, 4H), 8.31 (s, 1H), 8.36 (d, J = 7.4 Hz,
2H), 8.58 (d, J = 6.0 Hz, 2H), 8.62 (s, 1H), 8.64 (s, 1H), 8.75
(d, J = 4.2 Hz, 1H), 9.30 (d, J = 6.9 Hz, 2H); ESI-MS: m/z
476.4 [M − PF6]+. Anal. Calcd for C34H26F6N3P (621.55): C,
65.70; H, 4.22; N, 6.76. Found: C, 65.47; H, 4.25; N, 6.73.
1
0.32 g (58%); H NMR (400 MHz, DMSO-d6, TMS): δ = 6.00
(s, 2H, CH2), 7.54 (m, 2H), 7.81 (d, J = 8.3 Hz, 2H), 8.05 (m,
6H), 8.67 (d, J = 2.6 Hz, 2H), 8.69 (s, 2H), 8.72 (s, 2H), 8.76
(d, J = 4.8 Hz, 2H), 8.87 (d, J = 4.5 Hz 2H), 9.43 (d, J = 7.0 Hz,
2H). Anal. Calcd for C32H24BrN5 (558.47): C, 68.82; H, 4.33;
N, 12.54. Found: C, 69.05; H, 4.34; N, 12.49.
4-(p-Methylphenyl)-6-(p-methylphenyl)-2,2′-bipyridine. This
ligand was prepared according to the literature method22 employ-
1
ing p-methyl acetophenone instead of acetophenone. H NMR
(400 MHz, CDCl3, TMS): δ = 2.44 (s, 3H, CH3), 2.45 (s, 3H,
CH3), 7.33 (m, 5H), 7.74 (d, J = 8.1 Hz, 2H), 7.86 (t, J = 6.8
Hz, 1H), 7.96 (s, 1H), 8.11 (d, J = 8.2 Hz, 2H), 8.62 (s, 1H),
8.72 (m, 2H). Anal. Calcd for C24H20N2 (336.43): C, 85.68; H,
5.99; N, 8.33. Found: C, 85.60; H, 5.98; N, 8.34.
4-[(4,4′-Bipyridinium-1-yl)methylphenyl]-6-phenyl-2,2′-bi-
pyridine bromate. 4-(p-Bromomethylphenyl)-6-phenyl-2,2′-
bipyridine (1.08 g, 2.70 mmol) and 4,4′-bipyridine (0.47 g,
3.00 mmol) in CH3CN (150 mL) were heated to 60 °C for 72 h
under nitrogen atmosphere. The pale-yellow precipitate was iso-
lated by filtration, and washed with diethyl ether. The solid was
purified twice by recrystallization with CH3CN and diethyl ether.
Yield: 0.97 g (64%); 1H NMR (400 MHz, DMSO-d6, TMS): δ =
6.03 (s, 2H, CH2), 7.53 (m, 4H), 7.84 (d, J = 8.1 Hz, 2H), 8.03
(m, 3H), 8.11 (d, J = 8.3 Hz, 2H), 8.31 (s, 1H), 8.35 (d, J = 7.1
Hz, 2H), 8.62 (m, 2H), 8.69 (d, J = 6.8 Hz, 2H), 8.74 (d, J = 4.7
Hz, 1H), 8.87 (d, J = 6.1 Hz, 2H), 9.48 (d, J = 6.0 Hz, 2H);
ESI-MS: m/z 477.4 (M − Br)+. Anal. Calcd for C33H25BrN4
(557.48): C, 71.10; H, 4.52; N, 10.05. Found: C, 71.51; H, 4.56;
N, 10.07.
Complexes 2–5 and 7–9. These complexes were prepared
according to a modification of the literature method.23
Characterization data for 2. Yield (72%); 1H NMR
(400 MHz, DMSO-d6, TMS): δ = 5.95 (s, 2H), 7.05 (m, 2H),
7.41 (d, J = 7.1 Hz, 1H), 7.71 (d, J = 7.4 Hz, 1H), 7.76 (d, J =
7.7 Hz, 2H), 7.85 (m, 1H), 8.16 (s, 3H), 8.24 (t, J = 6.8 Hz, 2H),
8.31 (m, 1H), 8.44 (s, 1H), 8.65 (m, 2H), 8.80 (s, 1H), 9.29 (d, J
= 4.1 Hz, 2H). ESI-MS: m/z 630.2 [M − PF6]+. Anal. Calcd for
C28H21ClF6N3PPt (774.97): C, 43.39; H, 2.73; N, 5.42. Found:
C, 43.19; H, 2.74; N, 5.56.
Characterization data for 3. Yield (67%); 1H NMR
(400 MHz, DMSO-d6, TMS): δ = 5.94 (s, 2H), 7.13 (m, 2H),
7.48 (m, 1H), 7.66 (m, 3H), 7.78 (d, J = 7.4 Hz, 1H), 7.83 (d, J
= 8.1 Hz, 2H), 7.92 (m, 1H), 8.09 (m, 2H), 8.22 (m, 3H), 8.37
(m, 1H), 8.52 (s, 1H), 8.59 (d, J = 6.7 Hz, 2H), 8.72 (d, J = 8.0
Hz, 1H), 8.88 (s, 1H), 9.32 (d, J = 5.5 Hz, 2H); ESI-MS: m/z
706.4 [M − PF6]+. Anal. Calcd for C34H25ClF6N3PPt (851.10):
C, 47.98; H, 2.96; N, 4.94. Found: C, 48.17; H, 2.99; N, 5.16.
4-[p-(1-Methyl-4,4′-bipyridinium-1′-yl)methylphenyl]-6-phenyl-2,2′-
bipyridine hexafluorophosphate. 4-(p-Bromomethylphenyl)-
6-phenyl-2,2′-bipyridine (1.08 g, 2.70 mmol) and 1-methyl-4,4′-
bipyridine hexafluorophosphate (0.94 g, 2.97 mmol) were
refluxed in CH3CN for 48 h under nitrogen atmosphere. The
yellow precipitate was isolated by filtration, and washed with
diethyl ether. The solid was then dissolved in deionized water,
and the resulting solution was filtered. Addition of excess
NH4PF6 to the stirred filtrate produced pale-yellow precipitates,
which were collected by filtration, and washed with deionized
Characterization data for 4. Yield (70%); 1H NMR
(400 MHz, DMSO-d6, TMS): δ = 5.99 (s, 2H), 7.06 (m, 1H),
7.43 (d, J = 7.2 Hz, 1H), 7.54 (m, 1H), 7.72 (d, J = 7.3 Hz, 1H),
7.85 (m, 3H), 8.03 (d, J = 5.6 Hz, 1H), 8.18 (t, J = 7.2 Hz, 3H),
8.32 (t, J = 7.6 Hz, 2H), 8.45 (s, 1H), 8.68 (m, 3H), 8.81 (d, J =
4.8 Hz, 1H), 8.87 (s, 1H), 9.24 (m, 1H), 9.48 (m, 2H). ESI-MS:
m/z 707.3 [M − PF6]+. Anal. Calcd for C33H24ClF6N4PPt
(852.07): C, 46.52; H, 2.84; N, 6.58. Found: C, 46.41; H, 2.87;
N, 6.61.
1
water, cold ethanol and diethyl ether. Yield: 0.88 g (42%); H
NMR (400 MHz, CDCl3, TMS): δ = 4.40 (s, 3H, CH3), 5.93 (s,
2H, CH2), 7.45 (t, J = 6.0 Hz, 1H), 7.52 (t, J = 7.0 Hz, 1H),
7.58 (t, J = 7.0 Hz, 2H), 7.70 (d, J = 8.0 Hz, 2H), 7.97 (t, J =
7.7 Hz, 1H), 8.05 (d, J = 8.0 Hz, 2H), 8.18 (s, 1H), 8.30 (d, J =
7.6 Hz, 2H), 8.37 (d, J = 5.9 Hz, 2H), 8.42 (d, J = 6.2 Hz, 2H),
8.67 (s, 1H), 8.68 (d, J = 6.0 Hz, 1H), 8.72 (d, J = 4.1 Hz, 1H),
8.85 (d, J = 6.2 Hz, 2H), 9.04 (d, J = 6.4 Hz, 2H); ESI-MS: m/z
491.9 [M − 2PF6]+, 246.3 [(M − 2PF6)/2]+. Anal. Calcd for
Characterization data for 5. Yield (75%); 1H NMR
(400 MHz, DMSO-d6, TMS): δ = 4.44 (s, 3H), 6.06 (s, 2H),
7.27 (m, 3H), 7.90 (d, J = 8.3 Hz, 2H), 8.04 (m, 2H), 8.29 (d, J
= 8.2 Hz, 2H), 8.48 (t, J = 6.9 Hz, 1H), 8.53 (s, 1H), 8.72 (m,
3H), 8.83 (m, 3H), 9.28 (d, J = 6.7 Hz, 2H), 9.47 (d, J = 5.3 Hz,
This journal is © The Royal Society of Chemistry 2012
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Methyl-4,4′-bipyridinium hexafluorophosphate (A3). 4,4′-
Bipyridine was stirred with an equivalent amount of iodo-
methane in acetonitrile at room temperature for 12 h. The
formed yellow precipitate was filtered and washed with chloro-
form followed by diethyl ether. The yellow iodide salt was dis-
solved in water, and then added into an aqueous solution
containing excess ammonium hexafluorophosphate to form a
pale yellow precipitate. Recrystallization with acetonitrile and
1H), 9.57 (d,
C34H27ClF12N4P2Pt (1012.09): C, 40.35; H, 2.69; N, 5.54.
Found: C, 40.14; H, 2.73; N, 5.72.
J
=
6.8 Hz, 2H). Anal. Calcd for
Characterization data for 7. Yield (68%); 1H NMR
(400 MHz, DMSO-d6, TMS): δ = 6.01 (s, 2H, CH2), 7.91 (d, J
= 8.4 Hz, 2H), 8.00 (m, 2H), 8.04 (d, J = 6.2 Hz, 2H), 8.30 (d, J
= 8.4 Hz, 2H), 8.58 (t, J = 7.9 Hz, 2H), 8.70 (d, J = 6.9 Hz, 2H),
8.85 (d, J = 7.6 Hz, 2H), 8.89 (d, J = 6.1 Hz, 2H), 8.99 (d, J =
5.8 Hz, 2H), 9.03 (s, 2H), 9.43 (d, J = 6.9 Hz, 2H). Anal. Calcd
for C32H24ClF12N5P2Pt (999.03): C, 38.47; H, 2.42; N, 7.01.
Found: C, 38.62; H, 2.43; N, 7.18.
1
diethyl ether was carried out to afford a white precipitate. H
NMR (400 MHz, CD3CN, TMS): δ = 4.32 (s, 3H), 7.78 (d, J =
6.0 Hz, 2H), 8.29 (d, J = 6.2 Hz, 2H), 8.70 (d, J = 6.6 Hz, 2H),
8.85 (d, J = 6.0 Hz, 2H); ESI-MS: m/z 170.9 [M − PF6]+. Anal.
Calcd for C11H11F6N2P (316.18): C, 41.79; H, 3.51; N, 8.86.
Found: C, 41.75; H, 3.53; N, 8.85.
Characterization data for 8. Yield (73%); 1H NMR
(400 MHz, DMSO-d6, TMS): δ = 4.44 (s, 3H), 6.06 (s, 2H),
7.27 (m, 3H), 7.90 (d, J = 8.3 Hz, 2H), 8.04 (m, 2H), 8.29 (d, J
= 8.2 Hz, 2H), 8.48 (t, J = 6.9 Hz, 1H), 8.53 (s, 1H), 8.72 (m,
3H), 8.83 (m, 3H), 9.28 (d, J = 6.7 Hz, 2H), 9.47 (d, J = 5.3 Hz,
2.3. Photoinduced hydrogen production experiments
Photoinduced hydrogen generation was carried out in degassed
acetonitrile–water (v/v, 1 : 1) solutions in the presence of TEOA,
chromophore and colloidal Pt. A 400 W metal–halogen lamp
was used as a light source and an optical filter was employed to
cut off light with wavelengths below 390 nm. Each sample was
prepared in a 50 mL quartz cuvette with total volume of 20 mL
solution. Concentrations of TEOA, platinum(II) chromophore
and colloidal Pt are 2.2 × 10−2 M, 4.4 × 10−5 M and 8.7 × 10−5 M,
respectively. The cuvette was sealed with a rubber septum after
degassed by bubbling N2 for 30 min under atmospheric pressure.
All the experiments were conducted at room temperature. The
generated hydrogen from the systems was measured by GC-14C
(Shimadzu) which was equipped with a 5 Å molecular sieve
column (3 m × 2 mm), thermal conductivity detector and
N2 carrier gas. The amounts of hydrogen were quantified by
external standard method, and the turnovers were calculated
versus the amount of platinum(II) chromophore in the systems.
1H), 9.57 (d,
C33H27ClF18N5P3Pt (1159.02): C, 34.20; H, 2.35; N, 6.04.
Found: C, 34.32; H, 2.42; N, 6.16.
J
=
6.8 Hz, 2H). Anal. Calcd for
Characterization data for 9. Yield (76%); 1H NMR
(400 MHz, CDCl3, TMS): δ = 2.12 (s, 3H, CH3), 2.45 (s, 3H,
CH3), 6.78 (d, J = 7.8, 1H), 7.10 (d, J = 7.8 Hz, 1H), 7.27 (s,
1H), 7.34 (m, 4H), 7.53 (d, J = 1.3, 1H), 7.60 (d, J = 8.1, 2H),
7.93 (m, 2H), 8.77 (d, J = 5.1, 1H). Anal. Calcd for
C24H19ClN2Pt (565.95): C, 50.93; H, 3.38; N, 4.95. Found: C,
50.75; H, 3.40; N, 5.13.
Compounds A1–A3. These compounds were prepared accord-
ing to the literature method.16,24
Methyl-pyridinium hexafluorophosphate (A1). Pyridine was
stirred with a slight excess (1.5 equiv.) of iodomethane in aceto-
nitrile at room temperature for 12 h. The formed pale yellow pre-
cipitate was filtered and washed with chloroform followed by
diethyl ether. The pale yellow iodide salt was dissolved in water,
then added into an aqueous solution containing excess
ammonium hexafluorophosphate to form a gray precipitate.
Recrystallization with acetonitrile and diethyl ether was carried
3. Results and discussion
3.1. Spectral characterization
1
1
All complexes were synthesized and characterized by H NMR
out to afford a white precipitate. H NMR (400 MHz, CD3CN,
spectroscopy, mass spectrometry and elemental analyses. The
absorption spectra of 1–10 were measured in acetonitrile–water
(v/v = 1 : 1) solutions (Fig. 1). These complexes display a broad
low-lying dπ(Pt) → π* metal-to-ligand charge transfer transition
(MLCT) from 390 nm to 500 nm with extinction coefficients (ε)
around 103 dm3 mol−1 cm−1, and they have absorption tails
extending to 550 nm. Complex 1 exhibits a λmax emission at
533 nm (τ = 59 ns), while 5 is almost non-emissive.6 Complexes
2–4 show a similar emissive behavior to the parent chromophore
1 but at a significantly reduced intensity (Fig. 2; τ = 18–30 ns,
Table 1). The transient absorption spectra for complexes 2–4 in
CH3CN were measured after nanosecond-pulsed excitation at
355 nm and no obvious charge-separated state signals between
chromophore and acceptor were observed (Fig. S1–S3†).
TMS): δ = 4.29 (s, 3H), 8.00 (d, J = 6.4 Hz, 2H), 8.48 (t, J = 7.9
Hz, 1H), 8.61 (d, J = 6 Hz, 2H); ESI-MS: m/z 94.0 [M − PF6]+.
Anal. Calcd for C6H8F6NP (239.10): C, 30.14; H, 3.37; N, 5.86.
Found: C, 30.09; H, 3.39; N, 5.87.
Methyl-4-phenyl-pyridinium
hexafluorophosphate
(A2).
4-Phenyl-pyridine was stirred with a slight excess (1.5 equiv.)
of iodomethane in acetonitrile at room temperature for 12 h. The
formed pale yellow precipitate was filtered and washed with
chloroform followed by diethyl ether. The pale yellow iodide salt
was dissolved in water, and then added into an aqueous solution
containing excess ammonium hexafluorophosphate to form a
gray precipitate. Recrystallization with acetonitrile and diethyl
ether was carried out to afford a white precipitate. 1H NMR
(400 MHz, CD3CN, TMS): δ = 4.28 (s, 3H), 7.65 (m, 3H), 7.92
(d, J = 6.4 Hz, 2H), 8.23 (d, J = 6.6 Hz, 2H), 8.60 (d, J = 6.6
Hz, 2H); ESI-MS: m/z 170.0 [M − PF6]+. Anal. Calcd for
C12H12F6NP (315.19): C, 45.73; H, 3.84; N, 4.44. Found: C,
45.65; H, 3.86; N, 4.43.
The cyclometalated complexes 9 and 10 have similar absorp-
tion properties as 1–4. However, compared with those of the ana-
logous complexes, 1, 4 and 5, the MLCT absorption bands of
6–8 with related N^N^N ligands are blue-shifted to 370–450 nm
in acetonitrile–water solutions (Fig. 1).25 Complexes 9 and 10
8424 | Dalton Trans., 2012, 41, 8421–8429
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3.2. Cyclic voltammetry
display a long-lived emission compared with 1 (Table 1). Com-
plexes 6–8 are non-emissive; presumably their low-lying d–d
excited states are close in energy to the MLCT states resulting in
fast internal conversion from MLCT to d–d excited states.26
Cyclic voltammograms of all the complexes as well as of methyl
viologen (MV2+) were recorded in acetonitrile solutions contain-
ing 0.1 M nBu4NPF6. Irreversible oxidation waves at 0.90–1.11 V
were observed and these are attributed to the oxidation of Pt(II)
to Pt(III) (Table 1). Two irreversible reduction waves from
−0.74 V to −0.90 V and −1.26 V to −1.44 V for these com-
plexes were observed and these are due to the reduction of the
−C^N^N and N^N^N ligands, respectively. For the methyl violo-
gen appended to complexes 5 and 8 the reduction waves at
−0.4 V are assigned to the first viologen reduction (MV2+/MV+),
whereas those at −0.71 V and −0.66 V correspond to the MV+/
MV0 of 5 and 8, respectively.6,25 Similarly, the reduction wave at
about −0.6 V for bipyridium appended to complexes 4 and 7 is
due to reduction of MV+.
3.3. Photophysical properties of 1–3, 9 and 10
Previous studies showed that the emission of complex 1 is
quenched by MV2+ in degassed acetonitrile solution following
Stern–Volmer behavior with a kq value of 6.3 × 109 M−1 s−1 6
.
Fig. 1 Absorption spectra of complexes 1–10 (4.4 × 10−5 M) in aceto-
nitrile–water (v/v = 1 : 1).
Present investigations of the luminescence quenching by Pt
nanoparticles in degassed acetonitrile–water solutions for com-
plexes 1, 9 and 10 exhibiting a strong emission reveal that
excited state electron transfer from chromophore to colloidal Pt
occurs in the photocatalytic reaction system and the kq values are
3.51 × 1010, 3.87 × 1010 and 6.58 × 1010 M−1 s−1, respectively
(Fig. 3, S4 and S5†). The time-resolved emission spectra of 1
and 3 in mixed acetonitrile and water solvents in the absence
and presence of Pt nanoparticles were measured upon nanose-
cond pulsed 355 nm excitation (Fig. 4 and S6†). The results
show that the emission of the former followed a single-exponen-
tial decay, whereas that of the latter showed a second-order
dependence of the decay. The back electron-transfer rate con-
stants of the excited complexes 1 and 3 are in the range of
108 s−1 in the presence of the colloidal Pt.6
3.4. Photocatalytic hydrogen evolution
Fig. 2 Emission spectra of complexes 1–4 (4.4 × 10−5 M) in aceto-
nitrile–water (v/v = 1 : 1) with excitation at 390 nm.
Photocatalytic hydrogen evolution in multicomponent system
containing complex 1, various electron-acceptor and Pt
Table 1 Photophysical properties and electrochemical dataa for MV2+ and complexes 1–10
Reduction, E1/2/V
MV2+/+
Oxidation Epa/V
Compounds
λmax (nm)
τ0 (ns)
Pt3+/2+
MV+/0
L0/−, L−/2−
MV2+
1
2
3
4
5
6
7
8
−0.48
−0.85
533
538
539
538
59
30
27
18
0.99
1.09
1.02
0.90
0.98
1.10
1.09
1.11
1.01
1.06
−0.86, −1.35
−0.76, −1.26
−0.74, −1.26
−0.87, −1.36
−0.90, −1.36
−0.82, −1.33
−0.79, −1.44
−0.84, −1.36
−0.83, −1.32
−0.80, −1.33
−0.58
−0.71
−0.40
−0.40
−0.62
−0.66
9
10
563b
567b
145
151
a Potential in V vs. saturated calomel electrode (SEM) in CH3CN/nBu4NPF6 (0.1 M) at room temperature. L = ligand. b Emission in CH3CN solution.
This journal is © The Royal Society of Chemistry 2012
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Fig. 3 Emission spectral changes of 10 (1.6 × 10−5 M) in degassed
CH3CN–H2O (v/v, 1 : 1) at room temperature as a function of the col-
loidal Pt concentration. Inset: Stern–Volmer plot for emission quenching
of 10.
Fig. 5 Hydrogen production from system containing TEOA (2.2 ×
10−2 M), complex 1 (4.4 × 10−5 M), compounds A1–A4 (4.4 × 10−5 M)
and colloidal Pt (8.7 × 10−5 M) in CH3CN–H2O (v/v, 1 : 1) upon
irradiation (λ > 390 nm) at pH 7.0. TONs are calculated based on
complex 1.
Fig. 4 Time-resolved emission spectra of complex 1 (2.5 × 10−5 M)
with excitation at 355 nm in the mixed CH3CN–H2O (v/v, 1 : 1) in the
presence of Pt nanoparticles (4.3 × 10−5 M).
Fig. 6 Hydrogen production from system containing TEOA (2.2 ×
10−2 M), complexes 1–5 (4.4 × 10−5 M) and colloidal Pt (8.7 × 10−5 M)
in CH3CN–H2O (v/v, 1 : 1) upon irradiation (λ > 390 nm) at pH 7.0.
TONs are calculated based on the platinum(II) chromophore.
nanoparticles. The amounts of hydrogen generated during
irradiation of the reaction systems containing the parent chromo-
phore 1, electron-acceptors A1–A4 (A1–A4 denote methyl-
pyridinium hexafluorophosphate, 1-methyl-4-phenyl-pyridinium
hexafluorophosphate,
hexafluorophosphate
1-methyl-4-(pyridin-4′-yl)pyridinium
and 1,1′-dimethyl-4,4′-bipyridinium
electron through different pathways such as from chromophore
to Pt nanoparticles.29
hexafluorophosphate, respectively; 1/A, at a 1 : 1 molar ratio)
and colloidal Pt were examined in degassed acetonitrile–water
solutions using TEOA as sacrificial electron donors (Fig. 5). It
can be seen from Fig. 5 that the hydrogen evolution amount
increased sharply during the initial irradiation and the limiting
values were reached after 5 h with the overall order being 1 <1 +
A1 < 1 + A2 < 1 + A3 < 1 + A4. However, the system containing
A4 exhibited an induction period27 as shown in Fig. 5, indicating
that it is responsible for the number of positive charges in the
acceptor in these systems. With reference to the previous work,
the observed results are related to aggregate formation of radical
cations during the initial stage of light irradiation,28 thereby
retaining the electron transfer from MV˙+ to colloidal Pt. In
addition, it is also possible to directly transfer excited-state
Photocatalytic hydrogen production catalysed by cyclometa-
lated complexes 2–5 with a chromophore-acceptor dyad. Com-
pared with the photocatalytic hydrogen generating capability of
multicomponent photochemical reaction, the studies of photo-
catalytic hydrogen production in the system containing complexes
2–5 with a chromophore-acceptor dyad and Pt nanoparticles
demonstrate that the amount of hydrogen production increased
almost linearly with the irradiation time for 2–5 and followed the
order 2 ≈ 3 > 4 > 5 (Fig. 6). For 1 the rate of hydrogen evolution
was very quick at the onset of irradiation but decreased after 3 h
of irradiation, and became almost zero after 5 h because of
photosensitizer degradation.13 Complexes 2 and 3 performed
almost equally during the first 10 h of irradiation whereas
8426 | Dalton Trans., 2012, 41, 8421–8429
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Fig. 7 The durability of hydrogen production for complexes 2 (a) and
3 (b): After 10 h of irradiation on the first day, the sample solutions were
exposed to air and degassed again by bubbling N2 for 30 min, then fol-
lowed by another 10 h of irradiation on the second day; the procedure
was repeated on the third day.
Fig. 8 1H NMR spectra of complexes 2–5 (a) in DMSO-d6–D2O (v/v,
2 : 1) and (b) in DMSO-d6–D2O (v/v, 2 : 1) in the presence of excessive
amounts of TEOA and after 0.5 h of irradiation (λ > 390 nm, except for
5 being measured before irradiation).
complex 2 (Fig. 7a) exhibited a far better durability than 3
(Fig. 7b) over longer irradiation times, and retained most of its
activity during 20 h of irradiation. In general, intramolecular
electron transfer from the photoexcited platinum(II) chromophore
to the pyridinium moiety produces a charge-separated species,
which subsequently decays to the ground state by oxidative and
reductive processes involving a sacrificial agent and colloidal Pt
leading to hydrogen evolution. The metal–organic dyad is, there-
fore, considered to be a promising chromophore in the present
hydrogen generation system. Nevertheless, in this work, the rate
of hydrogen generation with 5 was the lowest. And the anoma-
lous result shown in Fig. 6 reveals that other complicated factors
can affect the efficiency of hydrogen production.
aromatic rings divided by the methylene protons is almost invari-
able and is consistent with the theoretical value (Table 2). The
results demonstrate that 2, 3 and 4 are stable relative to 5.
Complex 5 achieved effective charge-separation but it is
unstable in basic solution.6 This poor hydrogen-generating
efficiency observed with 5 probably comes from a photocatalytic
inactive species produced by OH− substitutional reaction at
carbon atom for the viologen moiety following the addition of
TEOA.25,30,31
Photocatalytic generation of hydrogen catalysed by 7 and
8. The hydrogen evolution results of complexes 1–4 in two
photocatalytic systems involving chromophore–acceptor dyads
and a bimolecular reaction showed that intramolecular electron
transfer is crucial in affecting the photocatalytic efficiency. We
also used complexes 7 and 8 with the chromophore–acceptor
dyad based on the terpyridyl platinum(II) complex 6 as sensi-
tizers for photocatalytic hydrogen generation (Fig. S10†). The
obtained results showed that the hydrogen generation efficiency
was extremely poor for the terpyridyl platinum(II) chromophores
compared with the cyclometalated platinum(II) chromophores,
which is consistent with our primary research results.19 The total
amount of hydrogen was no more than 2 TONs after 12 h of
irradiation. During the first 6 h to 8 h of irradiation the amount
of hydrogen was too small to be detected by GC analysis. The
poor performance of the terpyridyl derivative platinum(II) chro-
mophores probably originate from a facile non-irradiation decay
through a thermally accessible d–d excited state. Previous reports
clearly showed photocatalytic hydrogen evolution using
platinum(II) terpyridyl complexes containing acetylide ancillary
ligands with an electron-donating ability as photosensitizers.17
This favors an increase in the energy of the highest occupied
molecular orbital (HOMO) of the metal center, which overcomes
the above-mentioned disadvantage and results in a considerably
better hydrogen evolution efficiency. On the other hand, the
stability of the pyridinium appended to complex 8 in aqueous
solution is also a crucial factor in the poor hydrogen generation
performance.25
Photochemical stability of the photosensitizer. Various factors
need to be considered in order to understand the observed order
of photocatalytic hydrogen generation for 2–5. For complex 4,
the free nitrogen atom of 4,4′-bipyridine can be protonated, this
would reduce the efficiency of hydrogen production when com-
pared to that with 3 as a photocatalyst. On the other hand, the
stability of the metal–organic dyad is also vital for hydrogen
evolution since the benzyl linkage of dyads 2–5 is possibly sen-
sitive to basic conditions (Fig. S7†).25,30 Therefore, the stability
of 2–5 in aqueous solutions in the presence of excessive TEOA
was investigated by NMR spectroscopy. The 1H NMR spectra of
2–5 in mixed solvents of DMSO-d6 and D2O (v/v, 2 : 1) are
slightly different from those recorded in pure DMSO-d6 (Fig. 8a
and S8†). However, when an excessive amount of TEOA was
added to the sample solutions, the resonance of the protons on
the benzyl carbon atom disappeared rapidly for 5 (Fig. 8b and
S9†) while the signals for 2–4 were still present even after 0.5 h
of irradiation, revealing that a chemical reaction occurs before
the visible light irradiation of a solution of 5 in DMSO-d6–D2O
containing TEOA,25 thereby leading to the decrease in the
amount of photocatalytic hydrogen evolution.
1
It is clear that the H NMR signals of 2–5 became broad and
were overlapped in the presence of D2O (Fig. 8a and S8†),
especially upon the addition of TEOA. For complexes 2–4 the
ratio of protons calculated by the total number of protons on the
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Table 2 The ratio of proton numbers on the aromatic rings and methylene
Complexes
Theoretical value
Metrical valuea
Metrical valueb
Metrical valuec
Metrical valued
2
3
4
5
9.5
11.5
11
9.3
11.6
11.7
11.3
9.45
11.6
11.8
11.3
9.6
12.0
12.1
—
9.3
12.0
12.3
—
11
a DMSO-d6. b DMSO-d6–D2O (v/v, 2 : 1). c DMSO-d6–D2O (v/v, 2 : 1), in the presence of excessive amounts of TEOA. d DMSO-d6–D2O (v/v, 2 : 1),
in the presence of excessive amounts of TEOA, after 0.5 h of irradiation (λ > 390 nm).
hydrogen evolution was achieved after 12 h of irradiation 10,
which is comparable to the 146 TONs obtained for the same
system without MV2+. The results also indicate that the presence
of MV2+ did not have a profound effect on the total amount of
hydrogen evolution.
4. Conclusion
A series of complexes bearing various chromophore–acceptor
dyads and their parental complexes were synthesized. The inves-
tigation of hydrogen evolution of the complexes used as sensi-
tizers revealed that the hydrogen-generating efficiency of the
systems containing the metal–organic dyad was much higher
than that of the bimolecular systems with a separate parent
complex and a corresponding electron relay. The new complexes
Fig. 9 (a) Hydrogen production from system containing TEOA
2–4 are very stable dyads upon visible-light irradiation. For 5, a
chemical reaction occurs in basic TEOA solutions, thereby
(2.2 × 10−2 M), complexes 1, 9 and 10 (4.4 × 10−5 M) and colloidal Pt
(8.7 × 10−5 M) in CH3CN–H2O (v/v, 1 : 1) upon irradiation (λ >
showing that it must be kept in the mind for the photosensitizer
390 nm) at pH 7.0. TONs are calculated based on the platinum(II)
being converted into a photocatalytic active or inactive form
chromophore. (b) Hydrogen generation capability of complexes 1, 9 and
during hydrogen photogeneration. The hydrogen-generating
efficiency was found to be extremely poor for the analogous ter-
pyridyl platinum(II) complexes 6–8. An investigation into a
bimolecular system containing 1 and MV2+ showed that negli-
gible hydrogen generation was observed initially for the photoca-
talytic reaction, indicating that excited-state electron transfers
from chromophore to colloidal Pt. This is supported by results of
emission quenching and time-resolved luminescence exper-
iments of the platinum(II) complexes in the presence of Pt nano-
particles. Furthermore, the cyclometalated platinum(II)
complexes 9 and 10 that contain the same [Pt(C^N^N)] moiety
as complex 1 gave a much better performance for photocatalytic
hydrogen evolution than 1 under the same conditions of the pres-
ence of triethanolamine (TEOA) and Pt nanoparticles but
without an extra electron relay. Complex 10 may be a promising
lead structure for future development of robust catalysts with
improved hydrogen evolution efficiencies.
10 under same conditions as (a) except that MV2+ (4.4 × 10−5 M) was
added into the system.
Photocatalytic hydrogen evolution with complexes 1, 9 and 10
in the absence of an electron relay. Emission quenching studies
by Pt nanoparticles and MV2+ revealed that transfer processes of
an excited-state electron to Pt nanoparticles proceed via different
pathways. For comparison, the hydrogen generation capabilities
of 1, 9 and 10 were investigated in the absence or presence of an
electron relay. In the case of the former, these results show that
the efficiencies (10 > 9 > 1) of hydrogen evolution for 1, 9 and
10 exhibited similar changes in that they increased almost line-
arly at the beginning of light irradiation, slowed down after 4 h
and then reached limiting values after 12 h (Fig. 9a). The results
are well in agreement with kq values of their emission quenching.
Complexes 1, 9 and 10 all have the same 6-phenyl-2,2′-bi-
pyridine ligand and with a square planar coordination geometry
around the platinum atom. However, the electronic and steric
effects of substituent groups on the phenyl ring and 4′-position
of pyridine ring play an important role in determining the proper-
ties of the complexes. Complex 10 gave better hydrogen evol-
ution performance than 1 and this complex could be a more
promising candidate for further work in improving hydrogen
evolution efficiency.
Acknowledgements
This work was financially supported by the Ministry of Science
and Technology (2012DFH40090). We thank the Natural
Science Foundation of China (NSFC Grant Nos. 21071123,
21001110 and U1137606) and the Program for Changjiang
Scholars and Innovative Research Team in University
(IRT0979). We acknowledge the support of a grant from the Uni-
versity Grants Committee of the Hong Kong Special Administra-
tive Region, China (Project No. [AoE/P-03/08]).
In the multicomponent systems containing MV2+, the amount
of photocatalytic hydrogen evolution using complexes 1, 9 and
10 was similar to that of the systems without MV2+ and follows
the same order and development trend (Fig. 9b). A TON 166 for
8428 | Dalton Trans., 2012, 41, 8421–8429
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