Neutral Cyclometalated Ir(III) Complexes with Pyridylpyrrole Ligand for Photocatalytic Hydrogen Generation from Water

Article abstract of DOI:10.1021/acs.inorgchem.0c03812

To explore structure-activity relationships with respect to light-harvesting behavior, a family of neutral iridium complexes [Ir(ppy)2(LR)] 1-4 (where ppy = 2-phenylpyridine, and NN = 2-(1H-pyrrol-2-yl)pyridine and its functionalized derivatives) were designed and synthesized. The structural modifications in metal complexes are accomplished through the attributions of electron-donating CH3 in 2, OCH3 in 3, and electron-withdrawing CF3 in 4. The structural analysis displays that the pyridylpyrrole acts as one-negative charged bidentated ligand to chelate the iridium center. The electrochemical and photophysical properties of these complexes were systematically studied. The neutral 1-4 as well as the ionic structurally analogous [Ir(ppy)2(bpy)](PF6) (5) were utilized as PSs in photocatalytic hydrogen generation from water with [Co(bpy)3](PF6)2 as catalyst and triethanolamine (TEOA) as electron sacrificial agent in the presence of salt LiCl. Complex 1 maintains activity for more than 144 h under irradiation, and the total turnover number is up to 1768. The electrochemical properties and the quenching reaction indicate the H2 generation by neutral complexes 1-4 is involved exclusively in the oxidative quenching process.

Full text of DOI:10.1021/acs.inorgchem.0c03812

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Article
Neutral Cyclometalated Ir(III) Complexes with Pyridylpyrrole Ligand
for Photocatalytic Hydrogen Generation from Water
Yi Zhou, Piao He,* Xiu-Fang Mo, Chao Liu, Zhi-Liang Gan, Hai-Xia Tong, and Xiao-Yi Yi*
Cite This: Inorg. Chem. 2021, 60, 6266−6275
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sı Supporting Information
ABSTRACT: To explore structure−activity relationships with respect to light-
harvesting behavior, a family of neutral iridium complexes [Ir(ppy) (L )] 1−4 (where
2
R
̂
ppy = 2-phenylpyridine, and NN = 2-(1H-pyrrol-2-yl)pyridine and its functionalized
derivatives) were designed and synthesized. The structural modifications in metal
complexes are accomplished through the attributions of electron-donating CH in 2,
3
OCH in 3, and electron-withdrawing CF in 4. The structural analysis displays that the
3
3
pyridylpyrrole acts as one-negative charged bidentated ligand to chelate the iridium
center. The electrochemical and photophysical properties of these complexes were
systematically studied. The neutral 1−4 as well as the ionic structurally analogous
[
Ir(ppy) (bpy)](PF ) (5) were utilized as PSs in photocatalytic hydrogen generation
2 6
from water with [Co(bpy) ](PF ) as catalyst and triethanolamine (TEOA) as electron
3
6 2
sacrificial agent in the presence of salt LiCl. Complex 1 maintains activity for more than
44 h under irradiation, and the total turnover number is up to 1768. The
1
electrochemical properties and the quenching reaction indicate the H generation by neutral complexes 1−4 is involved exclusively
in the oxidative quenching process.
2
2
3
24
INTRODUCTION
The cyclometalated Ir(III) complexes [Ir(ppy) (NN)]
where ppy is 4-(2-pyridyl)benzene, NN is bidentated nitrogen
2,2′-bibenzimidazole, and boron dipyrromethene dye
■
(
n+
ligands were used to replace the bpy ligand to form a neutral
Ir(III) complex, of which photocatalytic reactivity was
maintained for 500 h (TON = 2178). However, there are
̂
2
̂
22
donor, n = 0 or 1) as photosensitizers (PSs) have long held the
interest of chemists primarily due to their features of high light
harvest, long excited-state lifetimes, and tunable emission
̂
still few studies on neutral cyclometalated [Ir(ppy) (NN)] for
2
photocatalytic hydrogen generation from water. In this respect,
1
,2
̂
it is worth pursuing to explore the neutral [Ir(ppy) (NN)] PSs
2
wavelengths. Since the pioneering works by Bernhard and
̂
+
with a new anionic NN structural modification aimed at
improving the efficiency and stability of the system for catalytic
H generation from water.
We have long stood on the study of metal complexes based
on a pyridylpyrrole ligand.
anionic pyridylpyrrole LR ligand is a bidentated nitrogen
donor, which is regarded as an analogue of the neutral bpy
ligand. Except for the similar structure and coordination
co-workers based on [Ir(ppy) (bpy)] (bpy = 2,2′-dipyridyl)
2
2
+
as PS, TEOA as electronic sacrificial reagent, and [Co(bpy) ]
3
as catalyst in the photocatalytic H generation from water,
tremendous efforts were devoted to develop a highly efficient
2
2
25−31
As shown in Chart 1, the
three-component catalyst system for H generation by altering
2
−
the catalyst and modifying ppy− and/or NN ligands with
̂
various substituents to tune their structural and electro-
3−13
chemical properties.
A total turnover number (TON, 0.5
H per PS) of more than 17 000 has been reached by using
2
[
Ir(ppy) (bpy-TPS)](PF ) (bpy-TPS = 4,4′-triphenylsilyl-
Chart 1
2
6
2
,2′-bipyridine) as PS in conjunction with K [PtCl ] (catalyst)
2
4
14
and triethylamine (TEA) (electron donor). However, to
improve the photostability over long-term and photoefficiency
1
5−19
of PSs is still an important issue in practical application.
n+
̂
For cationic [Ir(ppy) (NN)] PS, its excited PS (PS*) is
2
usually reductively quenched in the catalytic process, leading to
unstable radical anions that undergo decomposition. The
Received: December 29, 2020
Published: April 19, 2021
̂
̂
neutral [Ir(ppy) (NN)] complexes with an anionic NN ligand
2
in place of a neutral bpy ligand via an oxidative quenching
process is recently identified to be an effective way of
20
preventing PS degradation. For example, the anionic imine
2
1
22
N,N′-diisopropyl-benzamidinate, 2-phenylbenzothiazole,
©
2021 American Chemical Society
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Article
properties, the anionic LR− ligand has flexible regulation of the
electron structure through the π-donor/π*-acceptor capability
of the pyrrole/pyridine nitrogen atom, resulting in stabilizing
product was purified by chromatography on silica with elution of ethyl
acetate/hexane (v/v = 1/8) to give the corresponding pyridylpyrrole
ligand as a white solid.
27
2-Methyl-6-(1H-pyrrol-2-yl)pyridine (HL 3). Yield of 139.0 mg
lower and higher oxidation state metal complexes. Predict-
ably, it is helpful to improve the stability of its metal complexes
as PSs. Furthermore, the π-donor LR− ligand is quite unique
and usually exhibits attractive features for the design of ligand-
to-metal charge transfer (LMCT), ligand-to-ligand charge
transfer (LLCT), or ligand-centered excitation (LC) PS, which
completely invert the character of the bpy ligand architecture,
leading to the metal-to-ligand charge transfer (MLCT) excited
CH
1
(
88%). H NMR (δ, 400 MHz, CDCl ): 9.97 (s, 1H), 7.52 (t, J = 8.0
3
Hz, 1H), 7.38 (d, J = 8.0 Hz, 1H), 6.89 (dd, J = 13.6, 4.4 Hz, 2H),
6
.72 (s, 1H), 6.31 (dd, J = 5.8, 2.6 Hz, 1H), 2.54 (s, 3H). ESI-MS (m/
+
−1
z): 159.0866, calcd. 159.0917 for [M + H] . IR (KBr, cm ): 3403
s), 3166 (w), 2981 (w), 2857 (w), 1593 (s), 1460 (s), 1164 (m),
103 (m), 1025 (m), 790 (s), 737 (s).
(
1
2
-Methoxy-6-(1H-pyrrol-2-yl)pyridine (HLOCH3). Yield of 134.0 mg
1
+
/0
32−36
(77%). H NMR (δ, 400 MHz, CDCl
): 9.45 (s, 1H), 7.53 (t, J = 7.8
3
̂
state in [Ir(ppy) (NN)]
complexes.
Although the
2
Hz, 1H), 7.14 (d, J = 7.2 Hz, 1H), 6.90 (d, J = 1.2 Hz, 1H), 6.69 (s,
zirconium(IV) and hafnium(IV) complexes with 2,2′-pyridyl-
1
H), 6.51 (d, J = 8.0 Hz, 1H), 6.30 (dd, J = 6.0, 2.8 Hz, 1H), 3.99 (s,
pyrrolide or bispyrrolylpyridine ligand as PS for visible light
+
3H). ESI-MS (m/z): 175.0732, calcd. 175.0866 for [M + H] . IR
32−35
photoredox catalysis
and [Ir(dfppy) (fpy)] (dfppy = 2-
−1
2
(KBr, cm ): 3402 (s), 2950 (w), 2850 (w), 1589 (s), 1572 (s), 1468
(s), 1406 (w), 1328 (m), 1256 (m), 1103 (m), 1027 (m), 794 (s),
733 (s). Evaporation of HLOCH3 solution in ethyl acetate and hexane
(
2,4-difluorophenyl)pyridine; fpy = 2-(2,4-ditrifluoromethyl-
37
pyrrolyl)pyridine) as OLED material were reported, this new
structure family of [Ir(ppy) (NN)] with a pyridylpyrrole
̂
2
mixed solvent afforded needle crystals which were suitable for X-ray
diffraction analysis.
ligand are not as well-studied. Therefore, herein we will
describe the syntheses, characterization, and spectroscopic and
electrochemical properties of a class of neutral Ir(III)
2-Trifluoromethyl-6-(1H-pyrrol-2-yl)pyridine (HLCF3). Yield of
1
1
27.2 mg (60%). H NMR (δ, 400 MHz, CDCl ): 9.64 (s, 1H),
3
complexes [Ir(ppy) (L )] (R = −H (1); −CH (2);
2
R
3
7.77 (t, J = 7.8 Hz, 1H), 7.68 (d, J = 8.0 Hz, 1H), 7.39 (d, J = 7.6 Hz,
1H), 6.96 (d, J = 0.8 Hz, 1H), 6.78 (s, 1H), 6.32 (dd, J = 5.8, 2.6 Hz,
−
OCH (3); −CF (4)) with a pyridylpyrrole ligand and
3
3
−
present the improved performance of these complexes as PSs
1H). ESI-MS (m/z): 211.0482, calcd. 211.0489 for [M − H] . IR
−
1
in the three-component catalyst system for H generation from
(KBr, cm ): 3401 (s), 2950 (w), 1590 (s), 1572 (s), 1467 (s), 1259
2
(
m), 1149 (m), 1104 (m), 1027 (m), 795 (s), 734 (s).
General Procedure for Synthesis of [Ir(ppy) (L )]. The
water. To better understand their photocatalytic activity, the
effect of the ligand in [Ir(ppy) (L )] is studied by the
2
R
2
R
mixture of [Ir(ppy) Cl] (214.0 mg, 0.2 mmol) and HL (R = H,
2
2
R
theoretical calculation.
CH , OCH , or CF ) (0.4 mmol) in the presence of base Na CO
3
3
3
2
3
(
42.4 mg, 0.4 mmol) in degassed 2-methoxyethanol (8 mL) was
EXPERIMENTAL SECTION
refluxed overnight under a nitrogen atmosphere in the dark. The color
of mixture changed from yellow to dark brown. After removing off the
solvent by vacuo, the oily residual was redissolved in CH Cl (10 mL
× 2) and filtered. The filtrate was collected and concentrated to about
2
■
General Considerations. All syntheses were carried under a
nitrogen atmosphere using standard Schlenk technology unless
otherwise stated. Solvents were freshly dried before use. The H
2
2
1
mL. The crude product was purified by chromatography on silica
NMR spectra were recorded on a Bruker AVANCE(III) 400 M
spectrometer. The infrared spectra (in KBr) were recorded on a
Nicolet 6700 spectrometer FT-IR spectrophotometer. The UV−vis
spectra were recorded on an Agilent Technologies Cary 8454 UV−vis
spectrometer at ambient temperature with a 1 cm quartz cell. ESI-MS
was performed in a Bruker Daltonik GmbH, Bremen mass
spectrometer equipped with an electrospray ionization (ESI) source.
Emission spectra were recorded on an F97Pro Fluorescence
spectrometer. The excited-state lifetimes (τ) were performed on an
Edinburgh FLS-920 spectrofluorometer at room temperature. The
hydrogen evolution experiments are performed by the PerfectLight
PCX50C photochemistry system. Cyclic voltammetry was performed
on a CHI Instruments CHI610A electrochemical analyzer. Electro-
chemical measurements were performed in acetonitrile solution
containing 0.1 M Bu NPF electrolyte in a one compartment cell,
with elution of ethyl acetate/hexane (v/v = 1/6) avoiding light to give
complexes 1−4 as yellow solids.
[Ir(ppy)
CDCl
1H), 6.95−6.84 (m, 5H), 6.80 (t, J = 7.0 Hz, 2H), 6.56 (t, J = 6.2 Hz,
1H), 6.47 (d, J = 7.2 Hz, 1H), 6.39 (d, J = 7.2 Hz, 1H), 6.30 (s, 1H),
1
(L
)] (1). Yield of 213.8 mg (83%). H NMR (δ, 400 MHz,
2
H
): 7.86−7.77 (m, 3H), 7.63−7.52 (m, 7H), 7.45−7.40 (m,
3
+
6.21 (s, 1H). ESI-MS (m/z): 644.0721, calcd. 644.1548 for [M] . IR
−
1
(KBr, cm ): 3055 (w), 1603 (s), 1519 (s), 1474 (s), 1441 (m), 1383
(m), 1306 (m), 1159 (w), 1040 (m), 756 (s). Elemental analysis for 1
for calcd: C, 57.84; H, 3.60; N, 8.70; Found: C, 57.48; H, 3.68; N,
8.80. Evaporation of complex 1 solution in dichloromethane and
hexane mixed solvent afforded bar crystals which were suitable for X-
ray diffraction analysis.
1
4
6
[Ir(ppy) (L )] (2). Yield of 218.4 mg (83%). H NMR (δ, 400
2
CH3
where a glassy carbon electrode was used as working electrode, Pt
wire as counter electrode, and Ag/AgCl electrode in saturated KCl as
reference electrode. The yield of product (H ) was determined by a
SHIMADZU GC-2014C with a thermal TCD detector, 13X column,
and argon as carrier gas.
MHz, CDCl ): 8.07 (d, J = 5.6 Hz, 1H), 7.81 (dd, J = 12.0, 8.4 Hz,
3
2
1
7
H), 7.65−7.50 (m, 5H), 7.40 (t, J = 7.6 Hz, 1H), 7.21 (d, J = 5.2 Hz,
2
H), 6.91−6.82 (m, 4H), 6.75 (dd, J = 11.2, 6.8 Hz, 2H), 6.55 (d, J =
.2 Hz, 1H), 6.39 (d, J = 7.2 Hz, 1H), 6.12 (d, J = 7.2 Hz, 1H), 1.89
+
3
8
(s, 3H). ESI-MS (m/z): 659.1253, calcd. 659.1783 for [M + H] . IR
(KBr, cm ): 3041 (w), 1604 (s), 1581 (s), 1518 (s), 1474 (m),1450
The ligand 2-(1H-pyrrol-2-yl)pyridine (HL ) and iridium
−1
H
39
precursor [Ir(ppy) Cl]
were prepared according to literature
2
2
(w), 1388 (m), 1306 (m), 1266 (m), 1168 (w), 1043 (m), 753 (s).
methods. All other chemicals were obtained from J&K Scientific
Ltd. and used without further purification.
General Procedure for Synthesis of NN Ligands. The mixture
of substituted bromopyridine (1.0 mmol), N-boc-2-pyrroleboronic
acid (316.5 mg, 1.5 mmol), and catalyst Pd(PPh ) (115.6 mg, 0.1
Elemental analysis for 2·0.5H O for calcd: C, 57.64; H, 3.93; N, 8.40;
Found: C, 57.27; H, 4.02; N, 8.27.
2
̂
1
[
Ir(ppy) (L )] (3). Yield of 167.2 mg (62%). H NMR (δ, 400
2
OCH
3
MHz, CDCl ): 8.10 (d, J = 5.6 Hz, 1H), 7.77 (t, J = 8.0 Hz, 2H), 7.55
3
3
4
mmol) in the presence of base K PO ·3H O (532.5 mg, 2.0 mmol) in
(dd, J = 15.8, 7.4 Hz, 4H), 7.45 (t, J = 7.8 Hz, 1H), 7.22 (t, J = 9.0
Hz, 2H), 6.92−6.81 (m, 3H), 6.81−6.67 (m, 4H), 6.45−6.37 (m,
1H), 6.22 (d, J = 7.6 Hz, 1H), 6.17 (s, 2H), 5.99 (d, J = 8.0 Hz, 1H),
3
4
2
degassed n-BuOH (8 mL) was refluxed overnight under a nitrogen
atmosphere. The color of the mixture changed from yellowish green
to reddish brown. After removing the solvent by vacuo, the oily
residual was redissolved in CH Cl (10 mL × 2) and filtered. The
+
3.07 (s, 3H). ESI-MS (m/z): 674.1420, calcd. 674.1654 for [M] . IR
−
1
(KBr, cm ): 3030 (w), 1602 (s), 1560 (s), 1511 (s), 1473 (s), 1421
(m), 1386 (m), 1306 (m), 1285 (w), 1152 (w), 1048 (m), 955 (w),
2
2
filtrate was collected and concentrated to about 2 mL. The crude
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Article
Scheme 1. Synthesis of the HL Ligands and Complexes 1−4
R
7
3
58 (m), 735 (s). Elemental analysis for 3 for calcd: C, 57.04; H,
nm). The evolving gases in a given time were detected by GC with a
thermal conductivity detector. Each result was reproduced for three
times. There were no significant differences in the hydrogen evolution
yields upon repeated experiments. Control experiments were also
conducted in the absence of LiCl, iridium PS, catalyst, TEOA, or the
irradiation source under conditions mentioned above (Table S6).
.74; N, 8.32; Found: C, 56.50; H, 3.86; N, 8.51.
1
[
Ir(ppy) (L )] (4). Yield of 165.2 mg (58%). H NMR (δ, 400
2
CF
3
MHz, CDCl ): 8.16 (d, J = 6.0 Hz, 1H), 7.81 (dd, J = 16.8, 7.6 Hz,
3
3
1
2
H), 7.62−7.55 (m, J = 14.8, 4H), 7.50 (d, J = 7.6 Hz, 1H), 7.24 (s,
H), 7.15 (d, J = 7.2 Hz, 1H), 6.93−6.79 (m, 5H), 6.77−6.67 (m,
H), 6.32 (d, J = 7.6 Hz, 1H), 6.23−6.16 (m, 1H), 6.08 (s, 1H), 5.99
(d, J = 7.6 Hz, 1H). ESI-MS (m/z): 712.0537, calcd. 712.1422 for
RESULTS AND DISCUSSION
+
−1
■
[M] . IR (KBr, cm ): 3044 (w), 1605 (s), 1522 (s), 1476 (s), 1441
Synthesis and Characterization. As shown in Scheme 1,
(m), 1387 (m), 1298 (m), 1178 (s), 1160 (s), 1108 (w), 1048 (m),
7
54 (s). Elemental analysis for 4·0.5H O for calcd: C, 53.32; H, 3.22;
the pyridylpyrrole ligands HLCH3, HLOCH3, and HLCF3 were
2
N, 7.77; Found: C, 53.28; H, 3.27; N, 7.69.
synthesized by the Suzuki coupling reaction of bromopyridine
and N-boc-2-pyrroleboronic acid in the presence of catalyst
Pd(PPh3)4 and base K PO ·3H O. Their corresponding
X-ray Diffraction Studies. Diffraction data were record on a
Bruker CCD diffractometer with monochromatized Mo−Kα radiation
3
4
2
(λ = 0.71073 Å). The collected frames were processed using the
iridium complexes were facilely synthesized by a conventional
reaction between [Ir(ppy) Cl] and HL in the presence of
software SAINT. The absorption correction was processed with
SADABS. The structure was solved by a direct method and refined
using a full matrix least-squares method on F with the SHELXTL
2
2
R
2
base Na CO under refluxing 2-methoxyethanol conditions in
2
3
software package. Atomic positions of non-hydrogen atoms were
refined with anisotropic parameters. All hydrogen atoms were
introduced at their geometrical positions and refined as riding atoms.
Theoretical Calculations. DFT calculations were performed by
using the Gaussian 09 package (see Supporting Information for the
reference). Geometry optimizations were performed on the ground
state structures with the Becke’s three-parameter B3LYP exchange-
58−83% yields. The metal complexes as yellow solids are
soluble in polar solvents such as dichloromethane, acetone,
dmso, and acetonitrile and are very stable in the solid state.
The ligands and complexes were fully characterized. The
ESI-MS spectra of ligands HLCH3 and HLOCH3 display m/z at
+
1
59.0866 and 175.0732 assigned to the [M + H] , respectively.
40,41
1
correlation functional,
together with the 6-31G(d) basis set for H,
Their H NMR spectra show a broad single peak at about δ 9.6
ppm for the imino hydrogen and six groups of peaks in the
range of δ 6.3 pm to δ 7.8 ppm attributed to the hydrogen of
aromatic pyridine and pyrrole rings. The resonance peak of
hydrogen on the 5-position of pyridine significantly is
downfield shifted from δ 6.5 ppm (HLOCH3) to δ 7.4 ppm
42,43
44
C, N, O
and the LANL2DZ basis set for the Ir. Vibrational
frequencies were calculated based on the optimized structures to
confirm the absence of imaginary frequencies. MOs of complexes
were calculated and visualized as well. The excited states calculations
were carried out on the basis of the optimized ground state (S )
0
45
structures via time-dependent DFT (TD-DFT) at the same level.
And the solvent affect was also taken into account using the self-
consistent reaction field (SCRF) and polarizable continuum model
(HLCF3) with an increase of electron-withdrawing of
substituents. For HLOCH3 and HLCH3, the sharp single peaks
at respective δ 2.54 ppm and δ 3.99 ppm is due to the methyl
group.
46
(
PCM) with the acetonitrile solvent. The geometries of the triplet
states were calculated at the spin-unrestricted UB3LYP level with a
spin multiplicity of 3. The rest of the general information is given in
the Supporting Information.
General Procedures of Photocatalytic Hydrogen Produc-
tion. Unless otherwise specified, studies were conducted at room
temperature. The mixture of Ir(III) PS (0.2 μmol, 10 μM), catalyst
1
The H NMR of 1−4 is obviously complicated due to the
−
overlap of resonance peaks of ppy and pyridylpyrrole groups.
The disappearance of the imino hydrogen resonance indicates
the pyridylpyrrole ligand is coordinated to Ir(III) center, which
is consistent with what is observed in their IR spectra where
the N-H stretching band is absent. In complexes 2 and 3, the
resonance of the methyl group is upfield shifted to δ 1.89 ppm
and δ 3.06 ppm. The ESI-MS spectra of 1−4 show parent ionic
peaks at m/z 644.0721, 659.1253, 674.1420, and 712.0537,
[
Co(bpy) ](PF ) (16.3 mg, 1.00 mM), electron sacrificial agent
3 6 2
TEOA (1134.4 mg, 0.38 M), and salt LiCl (228.9 mg, 0.27 M) was
dissolved in mixed dimethyl sulfoxide (dmso)/water (1:1 = v/v)
solvent (20 mL). The pH value was adjusted to 10 by concentrated
hydrochloric acid. After degassed with Ar for 15 min, the solution was
irradiated by a 10 W LED lamp with a given wavelength (400−750
6
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respectively, which are assigned to the formula of the complex
as [Ir(ppy) (L )].
Structure Description. Ligand HLOCH3 and complex 1
have been characterized by X-ray diffraction analysis. As shown
in Figure 1, HLOCH3 is planar with an inter-planar angle of
The structure of complex 1 is very similar to that of
+
[Ir(ppy) (bpy)] . The Ir atom is surrounded by two
2
R
2
−
cyclometalated ligands (ppy) and one ancillary L
The coordinated N atoms of ppy are trans to each other, and
the coordinated N of L ligand is trans to the C atoms of
ppy ligand. The Ir−N
range from 2.036(6) to 2.089(5) Å are slightly shorter than
ligand.
H
−
−
R
−
−
and Ir−N bond distances in the
LH
ppy
+
that of Ir−N
in [Ir(ppy) (bpy)] (2.13 Å) due to the
bpy
2
−
stronger electrostatic interaction between Ir(III) and L in
H
4
8
1
(
.
Presumably, complex 1 is more stable than [Ir-
ppy) (bpy)] in the solid state or in solution. The
+
2
intermolecular π−π stacking interaction (4.450 Å) between
pyridyl groups of neighboring ligands is also observed.
Photophysical Properties. As shown in the Figure 2, the
absorption spectra of 1−4 are very similar to that of related
Figure 2. Absorption spectra of complexes 1−4 in CH CN. The
3
−
5
concentration of complexes is 1.0 × 10 M.
̂
neutral Ir(ppy) (NN) complexes. For complexes 1−3, the
2
strong absorption bands below 300 nm are assigned to the
π−π* ligand-centered (LC) transitions of the aromatic groups
−
of the ppy and pyridylpyrrole ligands. The low-energy band at
Figure 1. Structure of HLOCH3 (A) and complex 1 (B) with thermal
ellipsoids shown at the 40% probability. Selected bond lengths (Å) of
HLOCH3: N1−C5 1.3537(16), N1−C9 1.3197(15), N2−C1
about 370 nm is likely associated with a spin-allowed ligand-to-
ligand charge-transfer ( LLCT) transition mixed with some
metal-to-ligand charge-transfer ( MLCT) transition character
due to the strong donating ability of L . The weak and
1
1
1
1
.3552(16), N2−C4 1.3618(15), C4−C5 1.4565(17), C9−O1
.3573(15), C10−O1 1.4236(15). Selected bond angles (deg) of
−
49
R
3
broad tail bands at ∼450 nm can be assigned to the MLCT
HLOCH3: C1−N2−C4 110.22(10), N2−C4−C5 121.45(11), C4−
50,51
−
transitions.
Obviously, the co-ligand L substituted −H,
R
C5−N1 115.95(10), C5−N1−C9 117.01(10), N1−C9−O1
−
CH , and −OCH in 1−3 has a marginal impact on the
absorption. In contrast, the absorption band for 4 has an
obvious blue shift, which is likely a result of the strong
electron-defect −CF unit in the ancillary ligand. This
conclusion is supported by the result of theoretic calculation
(Figure 4). The almost identical HOMO-LUMO energy gaps
(3.32 eV for 1, 3.23 eV for 2, and 3.33 eV for 3) are observed,
and the higher energy gap of HOMO-LUMO for 4 (3.49 eV)
due to the lower HOMO energy causes the main absorption
blue shift.
3
3
1
19.08(10), C9−O1−C10 118.14(9). Selected bond lengths (Å) of
complex 1: Ir1−C11 2.028(6), Ir1−C12 2.044(4), Ir1−N1 2.036(6),
Ir1−N2 2.079(4), Ir1−N3 2.089(5), Ir1−N4 2.085(5). Selected bond
angles (deg) of complex 1: N1−Ir1−N2 87.9(2), N1−Ir1−N3
3
9
5.2(2), N1−Ir1−N4 173.8(2), N3−Ir1−N4 89.8(2), C11−Ir1−N1
0.1(2), C11−Ir1−N2 97.3(2), C11−Ir1−N3 174.3(2), C11−Ir1−
8
N4 95.1(2), C11−Ir1−C12 88.0(2), C12−Ir1−N1 97.1(2), C12−
Ir1−N2 173.34(19), C12−Ir1−N3 95.8(2), and C12−Ir1−N4
7
8.7(2).
6
.62° between pyrrole and pyridine rings. The exterior bond
Upon excitation at λ > 330 nm into the MLCT bands, as
shown in Figure 3 and Table 1, complexes 1−4 display
identical and unstructured emission at ∼510 nm, indicating
this emission is quite insensitive to the nature of the co-ligands.
This luminescence is quenched by oxygen, suggesting a
angles between pyrrole and pyridine rings are 132.21° and
1
1
21.85°, which are consistent with the HLH (131.83° and
21.18°), and larger than what is observed in bipyridine
4
7
(
120.30°). An intramolecular hydrogen bond between the
52
hydrogen atom from imino N-H and the pyridine nitrogen
atom and intermolecular π−π stacking interaction (5.247 Å)
between pyridyl groups of neighboring ligands are observed.
phosphorescent emission property (Figures S3−S6). The
quantum yields (Q ) of 1−4 are rather low, which is
PL
n+
̂
comparable to that in [Ir(ppy) (NN)] complexes and
2
6
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Table 2. Electrochemical and Excited-State Redox
Properties of Complexes 1−4 in Nitrogen-Saturated
a
CH CN at Room Temperature
3
−
+
E(PS*/PS )
E(PS /PS*)
complex Eox (V) Ered (V) E0‑0 (eV)
(V)
(V)
1
2
3
4
5
0.98
0.94
0.91
1.10
1.48
−1.92
−1.97
−2.05
−1.93
−1.12
2.62
2.62
2.61
2.66
2.49
0.70
0.65
0.56
0.73
1.37
−1.64
−1.68
−1.70
−1.56
−1.01
20
a
E0‑0 values were estimated from the intersection of the normalized
absorption and emission spectra in CH CN at room temperature.
E(PS*/PS ) = E + E0‑0; E(PS /PS*) = E − E0‑0
3
−
+
.
red
ox
of the electron-withdrawing effect of the CF group leading to
3
the lower electron densities on the metal center, complex 4 has
a higher Ir(IV)/Ir(III) oxidation potential at 1.10 V. The
irreversible reduction peaks (−1.93 to −2.05 V) of the
Figure 3. Normalized emission spectra of complexes 1−4 in CH CN.
3
The concentration of complexes is 1.0 × 10−5 M.
−
complexes are attributed to the py/py redox, which is also
consistent with the change of the push-pull electron ability of
̂
n+
obviously lower than that in [Ir(ppy) (CC)] complexes and
−
2
the co-ligand L .
R
̂
n+
higher than that in [Ru(bpy) (CN)] complexes. The QPL,
radiative rate constant (k ), and nonradiative rate constant
knr) are affected by the substituted group of the pyridylpyrrole
co-ligand. The QPL value of 3.10% in 2 with the electro-
donating CH group is obviously larger than that of 1, 3, and 4
with QPL of 0.99%, 0.60%, and 0.02%, respectively. Complex 1
2
Theoretical Calculations. In an effort to gain an insight
into the electrochemical and photophysical properties of these
complexes, density functional theory (DFT) calculations were
performed at the B3LYP/(6-31G**/LANL2DZ) level on the
molecular structures and electronic configurations of com-
plexes 1−4. The structure of complex 1 was solved by X-ray
diffraction analysis, and the optimized bond distances and
bond angles by calculation are in good agreement with those
experimentally determined (see Table S1).
The frontier orbitals and energy levels for these complexes
are displayed in Figure 4. In all cases, the HOMO with energy
values ranging between −4.86 and −4.58 eV is composed of a
mixture of Ir(III) dπ orbitals and π orbitals of auxiliary ligands,
especially the pyrrole ring. The higher stabilization of the
HOMO in complex 4 is related to the electron-withdrawing
r
(
3
5
−1
7 −1
has the highest k of 1.16 × 10 s and k of 1.16 × 10 s .
r
nr
3
−1
The lowest k of 1.52 × 10 s is observed in 4, and its k is 3
r
nr
orders of magnitude higher than k , and in the case of 2 with an
r
−
5
−1
LCH3 ligand, its k (1.07 × 10 s ) shows the smallest
difference between rate constants (k is only 31 times higher
than its k ). The k values of complexes 3 and 4 are remarkably
smaller than those of the other complexes. The complex 3 has
the largest lifetime of excited state among these complexes,
indicating of higher percentage of phosphorescent emission
property. The possible intersystem-crossing process leads to
r
nr
r
r
effect of the CF group increasing the energy gap, which in
3
larger knr and smaller k . For complex 4, the electron-
good agreement with the experimental values. Similarly, the
LUMO is located mainly on the pyridine rings of the
cyclometalating ligands with a small contribution of the phenyl
groups of the ligands and the Ir atom. The distribution of the
electron cloud on the HOMO and LUMO orbitals of
complexes 1−4 is opposite to that of the classical cationic
r
withdrawing CF group decreases the HOMO energy level
3
and increases the HOMO-LUMO energy gap, leading to the
lowest fluorescence quantum yield (0.02%) and smaller kr.
Electrochemical Properties. The redox behaviors of
complexes 1−4 are studied by cyclic voltammetry experiments
5
(
2
Figure S1), and their electrochemical data are listed in Table
. Irreversible peaks at 0.98 V for 1, 0.94 V for 2, and 0.91 V
photosensitizer [Ir(ppy) (bpy)](PF ) 5. We attribute it to the
2
6
2
7
π “lone pair” feature of the pyridylpyrrole ligand, which has a
larger electron cloud density than phenylpyridine and
bipyridine ligands. Therefore, the electron cloud of the
HOMO orbital of the complexes 1−4 tends to distribute on
the pyridine pyrrole ligand rather than the ppy ligand.
for 3 vs NHE are generally assigned to the one-electron
Ir(IV)/Ir(III) oxidation process, which is comparable to that
n+
4
̂
of [Ir(ppy) (NN)] complexes. The little difference of
2
oxidation potential in 1−3 indicates that the substituents
−
(
−H, −CH , −OCH ) on the ancillary L ligand do not
In order to investigate the nature of the excited state, the
low-lying triplet states of these complexes were calculated
3
3
R
significantly alter the ligand field strength of the metal. Because
Table 1. Photophysical Properties of Complexes 1−4 in Nitrogen-Saturated CH CN at Room Temperature
3
kr (s−1)a
knr (s−1
)
a
complex
λexc (nm)
λem (nm)
ΦPL (%)
τ (ns)
1
2
3
4
365
373
373
337
371
506
511
516
505
587
0.99
3.10
0.60
0.02
4.91
85
290
365
132
370
1.16 × 105
1.07 × 105
1.64 × 10
1.52 × 103
1.33 × 105
1.16 × 107
3.34 × 106
2.72 × 10
7.57 × 106
2.57 × 106
4
6
,
b 7,20
5
a
b
Assuming an intersystem crossing efficiency of 1, the k is calculated by k = Φ /τ, and the k is calculated by k = (1 − Φ )/τ. Complex 5 is
r
r
PL
nr
nr
PL
the classical cationic photosensitizer [Ir(ppy) (bpy)](PF ), which is structurally analogous to complex 1.
2
6
6
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Figure 4. HOMO and LUMO distributions (A) and energy diagram for the frontier orbitals (B) of complexes 1−4.
starting from the optimized geometries of the ground state
using the time-dependent TD-DFT approach. The vertical
excitation energies calculated for the first three triplets together
with their molecular orbital description and electronic nature
are shown in Table S2. In most of these complexes, the two
lowest-lying triplet states (T1 and T2) have relatively similar
energy. T1 is mainly described from the HOMO to higher
LUMO excitation, whereas more variability is observed for T2
that has a multiconfigurational character. These triplet states
for T1 are described as MLCT states and LC states involving
Hydrogen Production. Before the H -generation experi-
2
ment, the photostability of 1−4 and analogous 5 in various
solvents (thf, acetonitrile, dmf, dmso) was studied. As shown
in Figures S12−S16, complex 5 is more photostable than 1−4.
Complexes 2−4 are obviously deactivated in coordinating
solvent, while 1 is photostable in in dmso solvent. Poor
photostability of the complexes 2 and 3 is mainly due to the
strong steric effect of the substituting group at the α-position
−
of pyridine on the LR ligand strongly weakening the Ir−
−
Npyridine bond. It leads to that the LR ligand is readily replaced
the L ligands. The ppy ligand involved in the LLCT state in
R
by coordinating solvent (Table S3 and Figure S17). Thus,
the LUMO+3 in 1, 3, and 4 and LUMO+5 for 2 only has a
small contribution. The high participation of the metal in the
HOMO orbital for T2 involved in the case of 1, 2, and 3 may
explain the nonstructured emission band with a higher MLCT
complex 1 has the highest photostability. Furthermore, the
−
HOMO of 1−4 is mainly located on the L ligand based on
R
the calculation result. Upon excitation, the electron density of
−
the LR ligand is decreased. The strength of the covalent bond
character, while the participation of both ppy and L ligands in
−
R
between the LR and iridium center would be weakened; it is
the orbitals of 4 with a large LC and LLCT character is in
accordance with the structured emission band.
Using the spin-unrestricted DFT approach, the lowest triplet
states for 1−4 were further examined by optimizing their
geometries. Figure 5 shows the unpaired-electron spin-density
the possible reason to lead to the less photostability of 1−4
than 5.
For ease of comparison studies, [Co(bpy) ](PF ) as
3
6 2
catalyst, TEOA as electronic sacrificial reagent, and complex
as PS were chosen to optimize the H -generation solvent
1
2
system. As shown in Table S4, the mixed dmso/water solvent
with a ratio of 1:1 caused the highest TON of H generation;
2
both the increase and the decrease of water content in the
system led to lower catalytic activity.
The pH value in the photocatalytic H -generation system
2
also plays a key role in determining the proton concentration
and the state of existence of the electron donor agent which is
usually a base. The pH range of 6−11 was chosen to
investigate the impact of the pH value. As shown in Figure 6,
when the pH value reached 10, the maximum TON of H2
generation based on 1 was achieved. The activity decreased in
an either more acidic or more basic medium, which was in
49
agreement with the literature. Presumably, increasing the pH
I
value can facilitate the formation of the intermediate Co
Figure 5. Unpaired-electron spin-density contours (0.0004 au)
calculated for the fully relaxed T1 state of complex 1.
species, but it also led to a decreased proton concentration,
I
that is not favorable for protonation of the Co species to form
III
the Co hydride, which is believed to be one of the crucial
intermediates for proton reduction in the similar system. On
the other hand, when the pH of the solution was less basic than
distributions of complex 1 calculated for its fully relaxed T1
state (see Figures S9−S11 for complexes 2, 3, and 4). The
unpaired electrons in the T1 or T2 states for these complexes
are mostly concentrated in the metal and ppy ligand
perpendicular to the page as well as partly in other ligands.
1
0, the amount of hydrogen generation also decreased, due to
I
the fact that the rate of Co species formation is apparently
lower, and it is difficult for TEOA to be oxidized at the lower
pH to provide electrons.
For complex 4, the electron-withdrawing CF substituents of
The plots of amount of H
concentration range of 1−50 μM are shown in Figure 7.
Upon increase of concentration, the amount of H generation
2
generation for 1 in a
3
the L ligand cause a slight decrease of the electron density
R
over the iridium atom. The energy difference of T1 and T2 for
complexes 1, 2, and 3 is small (0.07 eV), whereas it is relatively
bigger (0.17 eV) for 4. Thus, two emitting states for these
complexes are totally possible.
2
reaches the maximum of respective 142.6 μmol (TON = 1426)
at a concentration of 1 with 10 μM for 72 h, and then with the
increase of concentration, the H2 generation decreases
6
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Figure 8. Photocatalytic H generation by complexes 1−5. Reaction
2
conditions: total volume of 20 mL containing 10 μM PS, 0.38 M
TEOA, 1 mM [Co(bpy) ](PF ) , and 0.27 M LiCl in 1:1 dmso/H O
3
6
2
2
Figure 6. Effect of the pH value on the photogeneration of hydrogen
in a system of 1-[Co(bpy) ](PF ) -TEOA. Reaction conditions: total
solution at a pH 10 under LED lamp irradiation (λ = 400−750 nm).
3
6 2
volume of 20 mL containing 10 mM 1, 0.38 M TEOA, 1 mM
Co(bpy) ](PF ) , and 0.27 M LiCl in 1:1 dmso/H O solution;
irradiation by LED lamp (λ = 400−750 nm) for 20 h. The pH value
was adjusted by addition of concentrated HCl (36%).
TON of 592 (59.2 μmol) for 2, 722 (72.2 μmol) for 3, and 651
[
3
6
2
2
(
65.1 μmol) for 5, and complex 4 yielded only 12.0 μmol of H2
(TON = 120). The H -generation capability of these neutral
2
complexes followed the order of 1 > 3 > 2 > 4. Comparing
complex 1 and 5, it was found that the initial hydrogen
evolution rate of complex 5 is fast, and the amount of
hydrogen evolution does not increase significantly after 30 h,
whereas the initial hydrogen evolution rate of 1 is slow, but it
can still maintain good hydrogen evolution after 72 h. It
indicates that the photostability of neutral complex 1 as PS is
obviously higher than that of cationic analogous 5 over long-
term. A continuous experiment was performed to show that
the complex 1 is still capable for hydrogen evolution under
irradiation for 144 h; the TON is up to 1768 (Figure S19). The
catalytic activity of 1 is much higher than that of other
complexes. One of the reasons is due to its higher
photostability of 1 as mentioned above. Furthermore, based
on the quenching experiment by catalyst and TEOA (Figures
S20−S26), it was found that complexes 1−3 were only
quenched by catalyst [Co(bpy) ](PF ) (the emission strength
3
6 2
Figure 7. Photocatalytic H generation at various concentrations of
complex 1. Reaction conditions: total volume of 20 mL containing
2
of complex 4 is too weak to detect), whereas complex 5 was
quenched by both catalyst and TOEA. The quenching
0
.38 M TEOA, 1 mM [Co(bpy) ](PF ) , and 0.27 M LiCl in 1:1
3 6 2
11
−1 −1
constant 2.04 × 10
M
s
of 1 is significantly larger than
dmso/H O solution at a pH 10 under LED lamp irradiation (λ =
2
6
−1 −1
10
that of other complexes (5.14 × 10 M
s
to 5.39 × 10
400−750 nm).
−
1
−1
M
s ). It indicates complex 1 has more efficiency in
transferring electrons to the catalyst than other complexes.
gradually. Meanwhile, a yellow precipitate of 1 that was
identified by H NMR (Figure S18) began to appear in the
The mechanism of photocatalytic H generation by iridium
2
1
1,5,20
complexes has been well-documented.
In the reductive
reaction mixture. Interestingly, the amount of produced H2
does not completely increase with the increase of the
concentration of 1, which is different from the previous
reports. The possible reason is that the increase of the
concentration of 1 results in aggregation to form the
precipitation of 1, which not only reduces the effective
concentration of 1 but also reduces the absorption of light,
quenching process, the redox potentials of the PS, catalyst, and
−
+
SR should exhibit the following trends: E(PS*/PS ) > E(SR /
−
−
+
SR) and E(PS/PS ) < E(cat/cat ) < E(H /H ), whereas, in
2
the oxidative quenching process, these relationships are
+
−
followed by the order of E(PS /PS*) < E(cat/cat ) <
+
+
+
E(H /H ) and E(SR /SR) < E(PS /PS). On the basis of the
2
excited-state redox properties of complexes 1−4 (Table 2), it
resulting in the decrease of H -evolution efficiency. Under the
was found that 1−4 have strong and relatively similar reducing
2
+
optimized condition mentioned above, the H -generation
potentials, of which E(PS /PS*) in the range of −1.56 to
2
performance of complexes 1−4 and analogous 5 was studied.
Complex 5 was chosen as a classical cationic photosensitizer to
compare the hydrogen evolution efficiency of a neutral
photosensitizer system.
As shown in Figure 8, upon irradiation for 72 h, the catalytic
activity of 1 is much higher than that of other complexes.
−1.70 V is lower than that of catalyst [Co(bpy) ](PF )
3
6 2
53
(−0.95 V vs NHE). It suggests that the excited state of
these PSs has sufficient driving force to allow for electron
transfer to the Co centers. The E(PS*/PS ) of the excited
state of 1−4 with a range of 0.57−0.74 V is smaller than the
oxidation potential of TEOA (0.84 V vs NHE). The PS*
could not oxidize the SR reagent TEOA. Significantly, the H2
II
−
54
Complexes 2, 3, and 5 had almost a similar yield of H with a
2
6
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Article
generation by iridium complexes is involved exclusively in the
oxidative quenching process. This result was also supported by
the quenching experiment. As shown in Table S5, the excited
states of 1−3 were quenched by catalyst [Co(bpy) ](PF )
character of 1−4 offers the potential to expand the palette of
̂
metals suitable or study exploring different CN ligands for
photosensitizer development.
3
6 2
1
0
11
−1 −1
with a 3.24 × 10 to 2.04 × 10 M
s
quenching constant,
ASSOCIATED CONTENT
■
but not quenched by TEOA. In contrast, the excited state of
−
*
sı Supporting Information
complex 5 has a higher reduction potential [E(PS*/PS ) =
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03812.
1
.37 V vs NHE] and was potentially quenched by TEOA. It
indicated that the cationic complex 5 can also be quenched in a
reductive mechanism which represented an obvious distinction
relative to the neutral complex system studied herein.
Experimental and X-ray crystallography details; 1H
NMR, IR, ESI-MS, and computational data (PDF)
Also worth noting is that photocatalysis of H generation in
2
this three-component catalyst system is TEOA concentration
dependent (Figure S29), which compares well with what is
observed in the [Ir(f-mppy) (dtbbpy)](PF ) and [Rh-
Accession Codes
CCDC 2047698 and 2047699 contain 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.
2
6
(
dtbbpy) ](PF ) (f-mppy = 5-methyl-2-(4-fluorophenyl)-
3 6 3
pyridine; dtbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) catalytic
6
reaction. The H generation by 1 increases with the increase
2
of concentration of TEOA and rapidly reaches a maximum
until 0.38 M TEOA, and then decreases slightly with
continuous increase of the concentration of TEOA. This
result indicates that the electron donation from TEOA in the
catalytic system of 1 is the possible rate-determining step at
low concentration of TEOA. The reaction is almost not
affected at relatively high concentration of TEOA. The
dependency on TEOA is also observed in the catalytic system
of 5. The catalytic efficiency by 5 is higher than that of 1 at
lower TEOA concentration, whereas it is lower at higher
TEOA concentration. The possible reason is that complex 5
involved in the main reductive mechanism combined with the
oxidative mechanism leads to higher efficiency; however, when
the TEOA concentration was increased, the unstable radical
AUTHOR INFORMATION
■
Corresponding Authors
Piao He − College of Chemistry and Chemical Engineering,
Hunan Provincial Key Laboratory of Chemical Power
Sources, Hunan Provincial Key Laboratory of Efficient and
Clean Utilization of Manganese Resources, Central South
University, Changsha, Hunan 410083, People’s Republic of
China; orcid.org/0000-0003-2500-7578;
Email: piaohe@csu.edu.cn
Xiao-Yi Yi − College of Chemistry and Chemical Engineering,
Hunan Provincial Key Laboratory of Chemical Power
Sources, Hunan Provincial Key Laboratory of Efficient and
Clean Utilization of Manganese Resources, Central South
University, Changsha, Hunan 410083, People’s Republic of
China; orcid.org/0000-0001-5482-9496; Email: xyyi@
csu.edu.cn
−
anionic intermediate PS from the reaction of PS* and TEOA
−
is generated in large quantity. The quick decomposition of PS
caused a significant decrease of catalytic efficiency.
CONCLUSIONS
■
A new family of neutral Ir(III) [Ir(ppy) (L )] complexes,
2
R
−
where co-ligand L is pyridylpyrrole substituted by electron-
Authors
R
donating −H in 1, −CH in 2, and −OCH in 3 and electron-
Yi Zhou − College of Chemistry and Chemical Engineering,
Hunan Provincial Key Laboratory of Chemical Power
Sources, Hunan Provincial Key Laboratory of Efficient and
Clean Utilization of Manganese Resources, Central South
University, Changsha, Hunan 410083, People’s Republic of
China
Xiu-Fang Mo − College of Chemistry and Chemical
Engineering, Hunan Provincial Key Laboratory of Chemical
Power Sources, Hunan Provincial Key Laboratory of Efficient
and Clean Utilization of Manganese Resources, Central South
University, Changsha, Hunan 410083, People’s Republic of
China
3
3
withdrawing −CF in 4 on the α-position of the pyridyl group,
3
were successfully synthesized and fully characterized. Com-
plexes 1−4 have similar structures as analogous 5, of which
−
coordinated N atoms of ppy are trans to each other and
−
coordinated N of the L ligand is trans to the C atoms of the
R
−
ppy ligand. Complex 1 has better photostability mainly due to
−
the less steric group at the α-position of pyridine on the LR
ligand. On the basis of theoretical calculation, complexes 1−4
as PS* with LLCT and LC character are distinguished with 5
with MLCT excitation, mainly due to the π “lone pair” donor
feature of the pyridylpyrrole ligand. The H -generation
2
capability of these neutral complexes as PSs followed the
order of 1 > 3 > 2 > 4. Complex 1 maintains activity for more
than 144 h under irradiation, and the total turnover number
Chao Liu − College of Chemistry and Chemical Engineering,
Hunan Provincial Key Laboratory of Chemical Power
Sources, Hunan Provincial Key Laboratory of Efficient and
Clean Utilization of Manganese Resources, Central South
University, Changsha, Hunan 410083, People’s Republic of
China; orcid.org/0000-0002-2289-3095
was up to 1768. The H -generation efficiency and PS
2
photostability over the long term are obviously higher than
those of well-known ionic structurally analogous 5 under the
same condition. This advantage of 1 for H generation is
Zhi-Liang Gan − College of Chemistry and Chemical
Engineering, Hunan Provincial Key Laboratory of Chemical
Power Sources, Hunan Provincial Key Laboratory of Efficient
and Clean Utilization of Manganese Resources, Central South
University, Changsha, Hunan 410083, People’s Republic of
China
2
mainly due to the oxidative quenching mechanism, which is
different from the mixture of oxidative and reductive
quenching process in 5. Although the relatively poor quantum
yield of [Ir(ppy) (L )] complexes as PS leads to limited H -
2
R
2
generation catalysis, the alterative LLCT and LC excitation
6
273
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Article
Hai-Xia Tong − School of Chemistry and Food Engineering,
Changsha University of Science and Technology, Changsha,
Hunan 410114, People’s Republic of China
water reduction systems based on cyclometalated iridium(III)
complexes. ChemSusChem 2013, 6 (1), 92−101.
(11) Wang, Y. J.; Chang, G.; Chen, Q.; Yang, G. J.; Fan, S. Q.; Fang,
B. Orchestrated photocatalytic hydrogen generation using surface-
adsorbing iridium photosensitizers. Chem. Commun. 2015, 51 (4),
685−688.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03812
(12) Yuan, Y.-J.; Yu, Z.-T.; Liu, X.-J.; Cai, J.-G.; Guan, Z.-J.; Zou, Z.-
Notes
G. Hydrogen photogeneration promoted by efficient electron transfer
from iridium sensitizers to colloidal MoS catalysts. Sci. Rep. 2015, 4
1), 4045.
13) Yuan, Y. J.; Zhang, J. Y.; Yu, Z. T.; Feng, J. Y.; Luo, W. J.; Ye, J.
The authors declare no competing financial interest.
2
(
(
ACKNOWLEDGMENTS
■
H.; Zou, Z. G. Impact of ligand modification on hydrogen
photogeneration and light-harvesting applications using cyclometa-
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We are thankful for the support of this work by the National
Nature Science Foundation of China (21571190, 22005345)
and the Hunan Provincial Science and Technology Plan
Project, China (No. 2018JJ2492).
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