Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO2 by Re(I)-Complex-Anchored TiO2 Hybrid Catalyst

Article abstract of DOI:10.1021/acs.inorgchem.7b01963

A series of cationic Ir(III) complexes ([Ir(btp)₂(bpy-X₂)]⁺ (Ir-X⁺: btp = (2-pyridyl)benzo[b]thiophen-3-yl; bpy-X₂ = 4,4′-X₂-2,2′-bipyridine (X = OMe, ^tBu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO₂ reduction to CO using a hybrid catalyst (TiO₂/ReP) prepared by anchoring of Re(4,4′-Y₂-bpy)(CO)₃Cl (ReP; Y = CH₂PO(OH)₂) on TiO₂ particles. Irradiation of a solution containing Ir-X⁺, TiO₂/ReP particles, and an electron donor (1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction of CO₂ to CO with efficiencies in the order X = OMe > ^tBu ≈ Me > H; Ir-CN⁺ has no photosensitization effect. A notable observation is that Ir-^tBu⁺ and Ir-Me⁺ are less efficient than Ir-OMe⁺ at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the Ir-X⁺ sensitizers are commonly superior compared to Ru(bpy)₃²⁺, a widely used transition-metal photosensitizer, and (2) the system comprising Ir-OMe⁺ and TiO₂/ReP is much more efficient than a homogeneous-solution system using Ir-OMe⁺ and Re(4,4′-Y′₂-bpy)(CO)₃Cl (Y′ = CH₂PO(OEt)₂). Implications of the present observations involving reaction mechanisms associated with the different behavior of the photosensitizers are discussed in detail.

Full text of DOI:10.1021/acs.inorgchem.7b01963

Article
pubs.acs.org/IC
Photosensitization Behavior of Ir(III) Complexes in Selective
Reduction of CO by Re(I)-Complex-Anchored TiO Hybrid Catalyst
2
2
Ha-Yeon Cheong, So-Yoen Kim, Yang-Jin Cho, Dae Won Cho, Chul Hoon Kim, Ho-Jin Son,*
Chyongjin Pac,* and Sang Ook Kang*
Department of Advanced Materials Chemistry, Korea University, Sejong 30019, Korea
*
S Supporting Information
+
+
ABSTRACT: A series of cationic Ir(III) complexes ([Ir(btp) (bpy-X )] (Ir-X : btp = (2-pyridyl)benzo[b]thiophen-3-yl;
2
2
t
bpy-X = 4,4′-X -2,2′-bipyridine (X = OMe, Bu, Me, H, and CN)) were applied as visible-light photosensitizer to the
2
2
CO reduction to CO using a hybrid catalyst (TiO /ReP) prepared by anchoring of Re(4,4′-Y -bpy)(CO) Cl (ReP;
2
2
2
3
+
Y = CH PO(OH) ) on TiO particles. Irradiation of a solution containing Ir-X , TiO /ReP particles, and an electron donor
2
2
2
2
(
1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction
t
+
of CO to CO with efficiencies in the order X = OMe > Bu ≈ Me > H; Ir-CN has no photosensitization effect. A notable
observation is that Ir- Bu and Ir-Me are less efficient than Ir-OMe at an early stage of the reaction but reveal persistent
2
t
+
+
+
photosensitization behavior for a much longer period of time unlike the latter. Comparable experiments showed that (1) the
+
2+
Ir-X sensitizers are commonly superior compared to Ru(bpy) , a widely used transition-metal photosensitizer, and (2) the
3
+
+
system comprising Ir-OMe and TiO /ReP is much more efficient than a homogeneous-solution system using Ir-OMe and
2
Re(4,4′-Y′ -bpy)(CO) Cl (Y′ = CH PO(OEt) ). Implications of the present observations involving reaction mechanisms
2
3
2
2
associated with the different behavior of the photosensitizers are discussed in detail.
6
,10−14
and heterogeneous dye sensitization15−22
INTRODUCTION
photosensitization
■
of the CO reduction under visible-light irradiation.
Chart 1 illustrates typical reaction systems related to the pre-
sent investigation on the application of ReC to the visible-light-
2
Development of efficient catalytic systems for visible-light-
induced CO reductions has attracted great attention associated
2
with the greenhouse effect due to the rapid increase of CO con-
centration in the atmosphere by combustion of fossil fuels.
Building up such photocatalytic systems for CO reductions
requires optimum combination of a visible-light-absorbing antenna
2
1
−4
induced reduction of CO to CO; (a) homogeneous-solution
2
1
0,13,14
photosensitization
that is typically initiated by electron
2
transfer from an electron donor (ED) to a photosensitizer (PS)
in the excited state followed by electron donation from the
one-electron reduced species of PS (PS ) to reduction catalyst
ReC), (b) a dye sensitization system
anchoring both PS (Dye) and ReC on semiconductor particles
typically TiO ), in which electrons are injected into the semi-
conductor from excited-state PS and then transported to ReC,
and (c) a mixed system comprising free PS and ReC-anchored
semiconductor particles, which is relevant to the main subject of
and a CO -reduction catalyst that can achieve an efficient visible-
2
−•
light-driven flow of electrons from an electron donor to the catalyst
followed by multielectron reductions of CO . Among various
catalysts used in visible-light reductions of CO , transition-metal
complexes have been receiving much attention associated with
15,16,20−23
(
constructed by
2
2
(
2
their potentialities in high catalytic efficiencies and high chem-
5
−7
ical selectivity.
In particular, Re(I) complexes with a general
n+
formula of Re(L)(CO)3Y (abbreviated as ReC; L = 2,2′-bipy-
ridine or a related ligand, Y = auxiliary ligand, and n = 0 or 1) are
attractive because of the remarkable capability in performing
24
the present investigation.
8
,9
the efficient and selective reduction of CO to CO, being
Received: August 14, 2017
2
applied to versatile systems involving homogeneous-solution
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01963
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Inorganic Chemistry
Article
Chart 1. Schematic Representation of Three Reaction Systems Related to the Present Investigation on the Photocatalytic
a
Reduction of CO to CO
2
a
ReC = Re(I) complex catalyst, PS = photosensitizer, and ED = electron donor; (a) homogeneous system, (b) ternary system, and (c) binary
system.
2
0−23
Recently, we reported that a hybrid system
constructed
some possible candidates for PS, we found that cationic
+
by covalent anchoring of both a visible-light absorbing bithio-
phene (BT)-based dye (Dye) and ReC on TiO particles (i.e.,
ternary system in Chart 1b, PS = Dye) reveals highly improved
durability in the visible-light CO reduction to CO; this system
iridium(III) complexes [Ir(btp) (bpy-X )] (btp = 2-(2-pyridyl)-
2
2
t
benzo[b]thiophene; X = OMe, Bu, Me, H), which have been
2
37
partially reported by Murata et al., effectively work as PS for the
CO reduction using the TiO /ReC binary hybrid catalyst. The
2
2
2
III
is abbreviated as Dye/TiO /ReC (Dye = Ar N-BT-CH
Ir complexes have the absorption maximum at ∼430 nm with
2
2
C(CN)COOH and ReC = ReP = Re(4,4′-Y -bpy)(CO) Cl
their long lifetime (3−4 μs). In this report, we discuss details
2
3
of the photocatalytic CO -reduction behavior of the system
(
Y= CH PO(OH) )). The photocatalytic CO reduction by Dye/
2
2
2
2
III
comprising the Ir -complex-based antenna and the TiO /ReC
binary catalyst, and the photosensitization capabilities of the Ir
complexes are compared with that of Ru(bpy)3 , a widely used
photosensitizer, as well as with the photocatalytic behavior of the
TiO /ReC proceeds by (1) fast electron injection from excited-
singlet Dye into TiO , (2) transport of the injected electrons to
the ReC site, and (3) the selective two-electron reduction of CO2
to CO on ReC. An important aspect of this system is that TiO2
can work as both an electron reservoir and an electron trans-
2
2
III
2
2+
Dye/TiO /ReC ternary system.
2
porter so that ReC fixed on TiO is susceptible of on-demand
2
EXPERIMENTAL SECTION
two-electron supply from TiO followed by complex reaction
■
2
General Procedure. All the synthetic procedures were performed
in a dry dinitrogen atmosphere. All the solvents used were distilled
over sodium−benzophenone under nitrogen prior to use. Iridium(III)
chloride hydrate and benzo[b]thien-2-ylboronic acid were purchased
from Sigma-Aldrich and used without further purification. Glassware,
syringes, magnetic stirring bars, and needles were dried in a convection
oven for over 4 h. Reactions were monitored by thin-layer
chromatography (TLC; Merck Co.). The spots developed onto TLC
were identified under UV light at 254 or 365 nm. Column chroma-
tography was performed on silica gel 60 G (particle size 5−40 μm;
processes to complete the selective reduction of CO to CO on
2
20,25
the catalyst center.
This is a major advantage of the hybrid
system in achieving long-term catalytic cycles.
A further activity enhancement can be achieved with the intro-
duction of merits of homogeneous system, which are defined as
(
1) an efficient antenna function without a loss of light harvesting
by scattering effect and (2) fast diffusion-mediated electron
transfer dynamics between components, resulting in a significant
10,14
reduction of induction period in photolysis.
We call this
1
Merck Co.). The synthesized compounds were characterized by H
system a binary hybrid system to discriminate it from the Dye/
13
NMR, C NMR, and high-resolution mass spectrometry (HR-MS).
TiO /ReC “ternary” hybrid catalyst. In the binary hybrid system,
1
13
2
The H and C NMR spectra were recorded on a Varian Mercury
00 spectrometer (operating at 300.1 and 75.4 MHz) and Bruker
excited-state PS should mediate electron transfer from an
3
electron donor to the catalytic TiO /ReC particles in solution
Ascend 400 spectrometer in KBSI Ochang Center (operating at 400.1
and 100.6 MHz), respectively. HR-MS analysis was performed on an
LC/MS/MSn (n = 10) spectrometer (Thermo Fisher Scientific, LCQ
Fleet Hyperbolic Ion Trap MS/MSn Spectrometer). The absorption
and emission spectra at room temperature were recorded on a UV−vis
spectrophotometer (Agilent, Cary-5000) and on a fluorimeter (Varian,
Cary Eclipse), respectively.
2
(Chart 1c). However, the organic dye (Dye) used in the ternary
hybrid catalyst has a serious drawback in using it as PS in
homogeneous solution, because the excited-state lifetime is too
short (<1 ns) to undergo effective collisional electron transfer
2
6
with either TiO or ED. As an alternative PS, the cationic
2
III
Ir -complex is selected due to its merits such as the efficient
+
t
+
+
Synthesis. The Ir(III) complexes (Ir-OMe , Ir- Bu , Ir-Me , and
+
3
7
visible-light absorption properties from single-triplet transition, a
longer lifetime of excited state, and high photochemical sta-
Ir-H ), Re(4,4′-Y -2,2′-bipyridine)(CO) Cl (ReC (Y = CH PO H ),
2
3
2
3
2
2
1
RePE (Y = CH PO(OC H ) )), organic sensitizer (dye = (E)-2-cyano-
2
2
5 2
2
7
III
bility. Some heteroleptic Ir complexes have been successfully
3-(5′-(5″-(p-(diphenylamino)phenyl)thiophen-2″-yl)thiophen-2′-yl)-
38
acrylic acid), and 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole
applied as a visible light harvester in water-splitting H generation
2
3
9
2
7−32
(BIH) were prepared by literature methods.
system.
However, the utilization of Ir complex as sensitizer
+
+
40
[
Ir(btp) (bpy-CN) ] PF (Ir-CN ). A mixture of Ir-dimer complex
2 2 6
41
has been still less explored in photocatalytic CO reduction
2
3
3−35
III
(0.088 g, 0.068 mmol) and 4,4′-dicyano-2,2′-bipyridine (bpy-CN:
process.
Very recently, Ishitani et al. reported that Ir
0
.029 g, 0.14 mmol) in ethylene glycol (5.9 mL) was heated at 150 °C
complex dyad model linked to the tricarbonyl rhenium complex
for 45 h under nitrogen. The reaction mixture was poured into water
(40 mL) and washed with diethyl ether (40 mL × 2). To the aqueous
layer was added ammonium hexafluorophosphate (0.610 g, 3.74 mmol).
has the high, durable photocatalytic CO to CO conversion effi-
2
36
ciency with its enhanced CO selectivity. After having investigated
B
DOI: 10.1021/acs.inorgchem.7b01963
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Inorganic Chemistry
Article
Chart 2. Photosensitization System in the Present Investigation on the CO Reduction to CO Using the Binary Hybrid Catalyst
2
+
2+
(
TiO /ReC), Ir-X or Ru(bpy) as PS, and BIH as ED in DMF
2 3
After treatment of the solution with dichloromethane (40 mL × 2), the
solvent was removed by rotary evaporation under vacuum. The solids
were collected by filtration, washed with water, and vacuum-dried. The
obtained crude product was purified by column chromatography on
silica gel (solvent: methanol/dichloromethane, 1:6 v/v), followed
by recrystallization from dichloromethane through n-hexane vapor
were taken by using a spectroelectrochemical cell consisting of a Pt mesh
(8 mm × 5 mm) working electrode and a Pt wire counter electrode.
+
An Ar-purged CH CN solution containing Ir-X (1 mM) and TBAP
3
(0.1 M) was electrolyzed at −1.6 V with a BAS 100B electrochemical
analyzer using an Ag/AgNO reference electrode.
3
Time-Resolved Emission-Lifetime Measurement. Phosphor-
+
+
diffusion to yield Ir-CN as dark-green crystals (0.056 g, 0.058 mmol,
escence lifetime measurements of Ir-X in CH CN in the absence of
3
8
5% yield). 1H NMR (300 MHz, deuterated dimethyl sulfoxide
TiO were made with an ICCD (Andor, iStar) attached on a mono-
2
(
(
DMSO-d )) δ 9.54 (s, 2H), 8.13 (d, J = 7.2 Hz, 2H), 8.03−8.07
chromator (DongWoo Optron, Monora500i). The excitation light was
6
m, 4H), 7.95 (m, 4H), 7.77 (d, J = 5.7 Hz, 2H), 7.23 (t, J = 7.5 Hz, 2H),
the 416 nm beam generated through the H Raman shifter seeded with
2
7
.08 (t, J = 6.9 Hz, 2H), 6.88 (t, J = 8.0 Hz, 2H), 5.86 (d, J = 8.1 Hz, 2H).
the third harmonic output (355 nm) of a Q-switched Nd:YAG laser
(Continuum, Surelite II). Comparable measurements of the phosphor-
1
3
C NMR (100.6 MHz, DMSO-d ) δ 163.17, 155.69, 151.87, 150.89,
6
1
1
45.64, 144.61, 142.10, 140.69, 136.47, 132.10, 128.63, 125.84, 124.49,
24.15, 123.57, 122.67, 122.21, 119.99, 115.65. Electrospray ionization
escence decays in the presence and absence of TiO dispersions were
2
undertaken with a picosecond fluorescence lifetime measurement
system using a Hamamatsu streak camera (C11200). An ultrashort laser
pulse was generated with a Ti:sapphire oscillator (Spectra Physics,
MaiTai SP) pumped with a diode solid-state laser, and high power pulses
at 430 nm were generated using a Ti:sapphire regenerative amplifier
(Spectra Physics, Spitfire Ace) and an optical parametric amplifier
(Spectra Physics, TOPAS prime).
mass spectrometry (ESI-MS) calcd. for C H F IrN PS , 819.0977
44
36
6
6
2
+
+
−
−
[
M−PF ] ; found 819 [M−PF ] .
6
6
+
Crystal Structure Determination. Fine crystals of Ir-H and
+
Ir-OMe obtained from a CH Cl /n-hexane solution were sealed in
2
2
glass capillaries under argon and mounted on a diffractometer. The
preliminary examination and data collection were performed using a
Bruker SMART CCD detector system single-crystal X-ray diffrac-
tometer equipped with a sealed-tube X-ray source (50 kV × 30 mA)
using graphite monochromated Mo Kα radiation (λ = 0.710 73 Å). The
preliminary unit-cell constants were determined using a set of 45 narrow
frame (0.3° in ω) scans. The double pass method of scanning was used
to exclude noise. The collected frames were integrated using an orienta-
tion matrix determined from the narrow-frame scans. The SMART
software package was used for data collection, and SAINT was used for
frame integration. The final cell constants were determined through
global refinement of the xyz centroids of the reflections harvested from
the entire data set. Structure solution and refinement were performed
using the SHELXTL-PLUS software package. Crystallographic data
were deposited with the Cambridge Crystallographic Data Centre as
−•
Measurement for Decay of Photogenerated PS . The decay
−•
profiles of PS (Figure 8 and S6) were monitored at 529 ± 15 nm (λmax
+
−•
2+ −•
of [Ir-X ] or [Ru(bpy)
Cary-5000 spectrometer operated at an averaged sampling time of
] ) with a bandwidth of 4 nm on an Agilent
3
−
•
33 ms, after PS had been generated by 2 min of irradiation of an
Ar-purged 3 mL DMF/H O mixture solvent (2.5 vol % H O) con-
) and 2.5 mM BIH in the absence
or a 12 μm thick mesoporous TiO film
2
2
+
2+
taining 25 μM PS (Ir-X or Ru(bpy)
or presence of 5 mg of TiO
3
2
2
42
(7 mm × 20 mm); it was confirmed by preliminary experiments that the
irradiation time of 2−5 min is sufficient for the complete conversion of
−•
PS to PS (see Figures 9 and S7). In the experiment on the enhanced
bleaching of the 529 nm absorption in the presence of 5 mg of TiO2
particles (Figure 8), both irradiation and monitoring were undertaken
without agitation of the sample after complete precipitation of visible
42
+
supplementary publications (CCDC-1481192 (Ir-OMe ) and CCDC-
1
+
510579 (Ir-H )). Additional crystallographic data are available in the
TiO particles dispersed in solution by Ar bubbling.
2
Supporting Information.
Preparation of TiO /ReC Catalyst. Commercially available TiO
2
2
Electrochemical Measurements. Cyclic voltammetry (CV) was
performed on a BAS 100B electrochemical analyzer with a three-
electrode cell system containing a platinum disk working electrode, a
particles (Hombikat UV-100, Huntsman) with specific Brauner−Emmet−
2
Teller (BET) surface areas of greater than 250 m /g were thoroughly
washed with distilled water, ultrasonically treated in water, separated by
centrifugation, and then dried in an oven under N . The TiO particles
platinum wire counter electrode, and a Ag/AgNO (0.1 M) reference.
3
2
2
All data were obtained for an Ar-purged dimethylformamide (DMF) or
(100 mg) were stirred overnight in a 50 mL solution of ReC (fac-
CH CN solution containing 0.1 M tetrabutylammonium perchlorate
[Re(4,4′-bis(dihydroxyphosphorylmethyl)-2,2′-bipyridine)(CO) Cl])
3
3
−1
+ −•
(
TBAP) at a scan rate of 50 mV s . Absorption spectra of [Ir-X ]
(1 μmol) in CH CN/tert-butanol, and then subjected to centrifugation.
3
C
DOI: 10.1021/acs.inorgchem.7b01963
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Inorganic Chemistry
Article
+
+
Figure 1. Structure of (a) Ir-H and (b) Ir-OMe obtained by X-ray crystallographic analysis. Displacement ellipsoids are drawn at the 30% probability
level, and H atoms were omitted for clarity.
Table 1. Photophysical Properties of Ir-X+
λabs (nm) (ε (1 × 10 M cm−1))
a
3
−1
λem (nm)
b
ΦPL
c
τ (μs)
d
e
kr (1 × 104 s−1
)
f
knr (1 × 105 s−1
)
Ir-OMe+
Ir- Bu+
268(38), 328(21), 427(6)
591, 640, 701
589, 638, 702
590, 638, 703
590, 641 712
g
0.107
0.116
0.064
0.009
g
5.64
6.47
4.39
0.255
g
1.90
1.79
1.46
3.53
g
1.58
1.37
2.13
38.9
g
t
272(34), 309(26), 327(19), 430(6)
Ir-Me+
272(31), 308(24), 326(18), 427(6)
Ir-H+
Ir-CN+
Ru(bpy)32+
277(33), 311(25), 335(18), 431(6)
284(39), 310(31), 327(26), 401(8), 429(7), 560(0.3)
h
244(25), 287(84), 451(14)
621
0.095
0.855
10.6
10.2
a
d
h
b
c
Absorption maxima (molar extinction coefficient). Phosphorescence maxima. Phosphorescence quantum yield measured in deaerated acetonitrile.
e
f
g
Phosphorescence lifetime measured in deaerated acetonitrile. Radiative rate constant. Nonradiative rate constant. No luminescence.
Reference10. All experiments are measured at room temperature.
The collected solids were washed with the solvent and then dried in an
X-ray crystallographic analysis, revealing monoclinic P2 /n and
1
oven under N . The successful anchoring of ReC on TiO was confirmed
2
2
P2 /c space group with reliability factors of R = 0.0315 and
1
1
by the IR absorption bands characteristic of the CO ligands at 2025,
0
.0286, respectively. Details of the crystal structures are sum-
−1
1
920, and 1910 cm . The complete loading of ReC was confirmed
marized in Tables S1−S5. As shown in Figure 1 for ORTEP
drawings, the (2-pyridyl)benzo[b]thiophen-3-yl ligands are
commonly bonded to the iridium(III) center with cis-C,C and
by absorbance comparison before and after adsorption step of ReC:
the supernatant, which is separated after centrifugation of the ReC
treated suspensions, showed the negligible absorption intensity of ReC
44
trans-N,N dispositions.
(
Figure S9).
+
Photocatalyzed CO2 Reduction. Suspensions of TiO /ReC
Photophysical and Electrochemical Properties of Ir-X .
2
+
particles (0.1 μmol ReC on 10 mg TiO ) in 3 mL of DMF containing
The UV−visible absorption and emission spectra of Ir-X were
2
+
2+
1
mM PS (Ir-X or Ru(bpy)3 ), 0.1 M BIH, and 2.5 vol % H O were
2
measured in acetonitrile. The absorption maxima and molar
extinction coefficients are listed in Table 1. As shown in Figure 2,
all complexes commonly reveal intense absorptions at ∼250 and
placed in a pyrex cell (∼1 cm pass length; 6.0 mL total volume), bubbled
with CO for 30 min, sealed with a septum, and then irradiated under
2
stirring with visible light at greater than 400 nm emitted from a light-
emitting diode (LED) lamp (60 W, Cree Inc.). Homogeneous-solution
photoreactions were performed in 3 mL of DMF solutions containing
∼
350 nm assignable as the π−π* transitions of the X -bpy and
2
btp ligands, respectively, and a less intense absorption band at
430 nm attributable to a transition with a dominant contribu-
+
∼
Ir-X (0.5 mM), RePE (0.5 mM), and BIH (0.1 M). The amounts of
III
37
+
CO evolved in the overhead space of the cell were determined by gas
chromatography (HP6890A GC equipped with a TCD detector) using a
tion of Ir -to-btp charge transfer. In the case of Ir-CN , an
extra weak band appears around 560 nm, which is attributable to
an insignificant contribution of metal-ligand-to-ligand charge
5
Å molecular sieve column. The liquid phase of the irradiated samples
was subjected to HPLC analysis using a Waters 515 pump, a Waters 486
UV detector operated at 210 nm, a Rspak KC-811 Column (Shodex),
and 0.05 M H PO aqueous solution eluent.
1
transfer ( MLLCT) transition related to the bipyridine ligand
and lowest unoccupied molecular orbital (LUMO) level lowered
by two electron-withdrawing CN substituents as reported for
3
4
+
37
another Ir-X complex (X = CF ).
3
RESULTS AND DISCUSSION
+
■
The emission spectra of Ir-X taken in degassed acetonitrile at
room temperature are shown in Figure 2b, and the data are listed
in Table 1. The phosphorescence diagram and data at 77 K are
shown and summarized in Figure S10 and Table S7, respectively
(see Supporting Information). The Ir(III) complexes reveal
similar phosphorescence spectra with the maxima at ∼590 and+
∼640 nm accompanied by a shoulder at ∼701 nm, while Ir-CN
is virtually nonemissive even at 77 K. As shown in Table 1, the
quantum yields (Φ) and lifetimes (τ) of phosphorescence for the
four emissive complexes uniquely depend on the substituent (X)
of the bpy ligand. From the observed values of Φ and τ, the rate
Synthesis. Chart 2 illustrates a schematic representation of
the present photosensitization system and the structures of the
III
molecules used in this work. The cationic Ir -complexes denoted
+
−
t
as Ir-X PF6 (X = OMe, Bu, Me, H, and CN) were synthesized
from dimeric Ir (btp) Cl and 4,4′-X -2,2′-bipyridines (X -bpy) in
2
4
2
2
2
3
7
2+
3
−
moderate yields. The preparation of Ru(bpy) (PF ) used
6
2
as a comparative photosensitizer was performed according
4
3
+
−
to the published method. The structures of Ir-X PF6 were
confirmed by spectroscopic and elemental analyses. In particu-
+
−
+
−
lar, Ir-H PF6 and Ir-OMe PF gave fine crystals suitable for
6
D
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Inorganic Chemistry
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significantly vary with X, suggesting that the X -bpy ligands
2
significantly contribute to the nonradiative decay process (or
processes) from the emissive state.
A possible assumption is that vibrational modes in the X -bpy
2
ligands would be more or less coupled with the crossing from the
emissive state to other nonemissive state(s), for example, a
3
47
metal-centered triplet state ( MC). Alternatively, the electron-
t
donating OMe, Bu, and Me substituents would more or less
enhance the electron density on the Ir(III) center in the excited
state to result in an increase of the barrier for the crossing to the
+
nonradiative state(s). In the case of Ir-CN , the strong electron-
withdrawing effect of the two CN substituents should cause a
significant reduction of the electron density on the metal center
3
to result in a barrierless crossing to the putative MC state.
+
Alternatively, the lowest-excited singlet state of Ir-CN different
from that of the other complexes would undergo direct inter-
system crossing to a nonemissive triplet state. At any rate, if the
nonemissive state would be coupled with a chemical change of
+
Ir-X , the Ir(III) complexes are not attractive as PS. Fortunately,
it was confirmed that all the complexes are totally stable under
long-term irradiation in DMF (Figure S11), an observation
+
suggesting that Ir-X can work as potential photosensitizer for
the present CO -reduction investigation.
The electrochemical properties of Ir-X were examined by
2
+
cyclic voltammetry (CV), the data of which are summarized
+
in Table 2. Typical CV scans of Ir-X are shown in Figure 3,
Figure 2. Absorption spectra of Ir-X+ in acetonitrile (a). (inset)
Expanded view for the low-energy spectral region. Emission spectra of
+
Ir-X (10 μM) taken by excitation at 300 nm in acetonitrile at room
+
temperature (b). (inset) Phosphorescence decay profiles of Ir-X in
acetonitrile at room temperature.
constants of the radiative (k ) and nonradiative pathways (k )
r
nr
were calculated. While the k values are similar for the emissive
r
complexes with minor differences, k increases in the order
nr
+
t
+
+
+
Ir-OMe ≈ Ir- Bu < Ir-Me ≪ Ir-H , an interesting dependence
on the bulkiness and/or electron-donating character of X. Since
the emissive complexes reveal almost identical phosphorescence
spectra and similar k values independently of the X -bpy ligand,
+
Figure 3. CVs of Ir-X (1 mM) in the oxidation (left) and reduction
(
right) sides in CH CN in the presence of 0.1 M TBAP; scan rate at
3
−1
50 mV s .
r
2
the btp ligand should be dominantly involved in the emissive
excited state accompanied by a small or negligible contribution of
revealing quasi-reversible behavior with distinct anodic and
cathodic waves in both the positive (oxidation) and negative
(reduction) sides, from which the half-wave oxidation and
the X -bpy ligand. On the one hand, the phosphorescence of
2
+
ox
red
ox
Ir-X should commonly occur from a state with a dominant or
reduction potentials, E1/2 and E1/2 , were estimated. The E1/2
3
major contribution of the btp-centered triplet ( LC) mixed with
values are almost identical within 30 mV differences except for
3
+
a metal-to-ligand charge-transfer ( MLCT) character, as
Ir-CN where the potential is more positive by 70−100 mV.
37,44
45,46
III
reported for similar heteroleptic
and homoleptic
Ir
Since the highest-occupied molecular orbital (HOMO) can be
49
analogues with the btp ligand. On the other hand, the k values
considered to dominantly populate on the Ir(III) center, the
nr
Table 2. Electrochemical Data and Related Energy Levels for PSs
E1/2ox [V]
a
E1/2red [V]
a
E0−0 [eV]
b
HOMO [eV]
c
LUMO [eV]
c
Ered
* [V]
d
E * [V]
e
ox
Ir-OMe+
Ir- Bu+
1.06
1.04
1.05
1.07
1.14
1.25
−1.41
−1.42
−1.40
−1.31
−0.74
−1.34
2.13
2.13
2.13
2.13
2.12
−5.48
−5.46
−5.47
−5.49
−5.56
−5.67
−3.01
−3.00
−3.02
−3.11
−3.68
0.72
0.71
0.73
0.82
1.19
0.80
−1.07
−1.09
−1.08
−1.06
−0.79
−0.89
t
Ir-Me+
Ir-H+
Ir-CN+
Ru(bpy)32+
f
2.14
−3.08
a
b
Epa = anodic peak potential, E = cathodic peak potential, and E1/2 = (Epc + E )/2 versus SCE. E denotes triplet energy estimated from the
phosphorescence data at 77 K. HOMO and LUMO levels were determined using the following equations: E
pc
pa
0−0
c
ox
1/2
(eV) = −e(E
+ 4.42); ELUMO
HOMO
red
/2
d
red
e
(
eV) = −e(E1
+ 4.42). Excited-state reduction potential estimated using E * = E
+ E0−0. Excited-state oxidation potential estimated using
red
1/2
ox
1/2
f
E * = E
− E0−0. Reference 48.
ox
E
DOI: 10.1021/acs.inorgchem.7b01963
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
energy levels would be not significantly changed by the OMe
and alkyl substituents of X -bpy. In the case of X = CN, however,
2
the strong electron-withdrawing effect of CN would cause an
appreciable decrease of electron density on the metal center to
result in a slight positive shift of the HOMO level. On the one
red
hand, with respect to the reduction potential, the E
values in
1
/2
t
the cases of X = OMe, Bu, and Me are again similar within 20 mV
differences but more negative by ∼100 mV than that of Ir-H .
On the other hand, Ir-CN reveals a substantially more positive
shift of E1/2 by 570−680 mV. These results show that the
LUMO level of Ir-X is more sensitive to the electron-donating
+
+
red
+
and electron-withdrawing character of the substituent X on the
bpy ligand than the HOMO level.
Photocatalytic CO Reduction. The hybrid catalyst (TiO /
2
2
ReC) was prepared by anchoring of Re(4,4′-Y -bpy)(CO) Cl
2
3
(
ReC = ReP; Y = CH PO H ) on TiO particles as described
2 3 2 2
20,21
Figure 5. Plots of TN vs irradiation time for the binary systems of
in our previous papers.
in CO -saturated DMF containing PS (Ir-X or Ru(bpy)
1
greater than 400 nm using an LED lamp (60 W, Cree Inc.). With
Ir-X (X = H, Me, Bu, and OMe) as PS, the photoreactions gave
Suspensions of TiO /ReC particles
CO
2
+
2+
+
2+
Ir-OMe −TiO /ReC (black ■) and Ru(bpy) −TiO /ReC (inverted ▼),
,
2
2
2
3
+
the Dye/TiO /ReC ternary system (blue ▲), the Ir-OMe −RePE
2
mM), BIH (0.1 M), and 2.5 vol % H O were irradiated at
2
homogeneous system (red ●), and the TiO /ReC catalyst in the
2
absence of PS (pink ◆).
+
t
CO as the major CO -reduction product accompanied by small
2
Table 3 show CO formation (TNCO and μmol) versus irradiation
time for these systems compared with that for the binary hybrid
amounts of HCOOH and H . In the absence of either PS or BIH,
2
formation of CO was much less efficient or not detected at all
(Table S8). Figure 4 shows plots of TNCO (= molar ratio of CO
Table 3. Comparison of Four Different Reaction Systems in
Visible Light Driven CO Production
amount of CO produced @3 h
PS/
μmol
ReC/
μmol
system
TON
+
Ir-OMe −TiO /ReC
3
0.1
0.1
0.1
1.5
163
54
2
Ru(bpy)32+−TiO /ReC
3
2
Dye/TiO /ReC
1.5
1.5
96
2
+
Ir-OMe −RePE
35
+
+
system using Ir-OMe as PS (Ir-OMe −TiO /ReC). In the case
2
of Ru(bpy)32+−TiO /ReC, leveling-off behavior of TN
2
CO
appears even at an early stage to give a low TN (∼50) after
CO
+
3
h of irradiation, considerably lower than those of Ir-X −TiO /
2
+
ReC, demonstrating that Ir-X works as a much more efficient and
Figure 4. Comparable plots of TNCO versus irradiation time for the four
2+
+
persistent photosensitizer than Ru(bpy)3 . For the Ir-OMe −
RePE homogeneous system, the CO formation is leveled off even
+
Ir-X sensitizers; irradiation at greater than 400 nm for dispersions of
1
0 mg TiO /ReC (0.1 μmol) in CO -saturated DMF involving 1 mM
2
2
+
at 1 h of irradiation to give a very low TN (∼30; Figure S12).
17,20−23,25
Ir-X and 0.1 M BIH and 2.5 vol % H O. The vertical error bar at each
data point is based on experimental data repeated four times.
CO
2
As previously stated,
the hybridization of ReC with
TiO enables the catalyst to persistently work, providing a
2
formed/ReC used) versus irradiation time. After 4 h of irradia-
potential strategy for designing efficient catalytic systems based
on molecular catalysts. It is of interest to note that TNCO of the
+
tion, TNCO reaches ∼180 with Ir-OMe and 120−140 with the
+
+
others except for Ir-CN ; Ir-CN is inactive as PS. On the one
Dye/TiO
the Ir-OMe −TiO /ReC binary system and significantly lower
2
2
/ReC ternary system is ∼1.8-fold lower than that of
+
+
hand, it is of significance to note that Ir-OMe reveals the highest
+
photosensitization efficiency at an initial stage of the reaction but
shows leveling-off behavior after 2 h of irradiation (Figure 4),
than those of the binary system using the other Ir-X sensitizers
t
(X = Bu, Me, and H), perhaps due to light-scattering effects in
+
probably due to net chemical changes of Ir-OMe during the
the ternary system. The apparent quantum yield (AQY) of CO
+
photosensitization. On the other hand, the TN plots for
production at 436 nm for the binary hybrid system (Ir-OMe −
CO
t
+
+
Ir- Bu and Ir-Me are linear up to 10 h to give ∼300 of TNCO,
demonstrating that both Ir- Bu and Ir-Me are less efficient but
TiO /ReC) in the presence of 2.5 vol % water was determined to
2
t
+
+
−2
be (3.7 ± 0.2) × 10 , which is ∼2 times higher than the ternary
system (Table S8). An C isotope tracer experiment was
performed for the binary hybrid system (Ir- Bu −TiO
CO
+
13
much more stable as PS than Ir-OMe .
t
+
For comparison, the other photosensitization systems
Chart 1a,b) were applied to the photocatalytic reduction of CO2
2
/ReC) in
13
(
2
−saturated DMF-d
7
−D
2
O mixed solvent (2.5 vol % D O)
2
1
3
under similar conditions, that is, (1) a binary hybrid system using
containing 0.1 M BIH. A sharp signal at 184.5 ppm and the C
Ru(bpy)32 as PS in place of Ir-X (abbreviated as Ru(bpy)
+
+
2+
−
isotope abundance (>98.7%) of the produced CO gas analyzed
3
+
13
TiO /ReC), (2) a homogeneous system using Ir-OMe as PS
and RePE as CO reduction catalyst (Ir-OMe −RePE), and
3) the Dye/TiO /ReC ternary hybrid system. Figure 5 and
by C NMR spectrum (DMF-d ) and gas chromatography−
2
7
+
mass spectrometry (GC-MS), respectively, indicate that the CO
2
(
originated from injected CO gas (Figures S13 and S14).
2
2
F
DOI: 10.1021/acs.inorgchem.7b01963
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. Schematic representation showing photoexcitation to a singlet excited (S ), intersystem crossing (ISC) to a triplet excited state (T ), long-
1
1
+
+
−•
+ −•
lived OERS of [Ir-X ] ([Ir-X ] ) formed with fast reductive quenching process, and a slow collisional electron injection from [Ir-X ] to TiO /ReC.
2
Note that energy levels in this diagram are not determined based on an experimental evidence and do not reveal the exact values.
5
0
DISCUSSION
hydride transfer suggested by the recent report of Bao and
co-workers. However, we think the reductive quenching in our
hybrid system is preferably taking place following electron
■3
−•
+
PS* + BIH → PS + BIH
+
2+
transfer mechanism rather than hydride transfer, since previous
(
PS = Ir‐X or Ru(bpy)3
)
(1)
51−53
reports by Ishitani and co-workers,
which resemble our PSs
−
•
−
closely, presented the detailed experimental evidence supporting
the reductive quenching of PS* (Ru or Ir ) by electron transfer
PS + TiO /ReC ⇄ PS − − − TiO (e )/ReC
2
2
II
II
−
•
→
PS + TiO /ReC
(2)
mechanism (Figure 6). An alternative initiation process would be
2
3
the direct electron injection from PS* into TiO . However, this
2
−
•
−•
•
−
TiO /ReC (L(CO) ReCl ) → TiO /L(CO) Re + Cl
t
+
2
3
2
3
process is unlikely to occur, because decay profiles of the Ir- Bu
(3)
phosphorescence are unchanged in the presence of dispersed
•
•
TiO particles (Figure S17). In accord with this, the excited-
2
TiO /L(CO) Re + DMF ⇄ TiO /L(CO) Re (DMF)
2
3
2
3
3
state oxidation potential of PS* (E * = −(0.79−1.09 V)) is
ox
(4)
considerably less negative than the flat-band potential (E =
fb
−
•
−1.50 V) of a TiO nanoparticle film in DMF in the presence of
2
TiO (e )/L(CO) Re (DMF) + CO
2
3
2
18
3
% water. For the homogeneous photosensitization, electron
•
3
+
⇄
TiO /L(CO) Re (CO ) + DMF
(5)
(6)
transfer from Ir-X * to RePE would be another possible process
2
3
2
t
+
apart from eq 1. However, quenching of the Ir- Bu phospho-
rescence by RePE was very inefficient; a Stern−Volmer plot gave
−
•
+
TiO (e )/L(CO) Re (CO ) + 2H → →
2
3
2
6
−1 −1
a rate constant of ∼1 × 10 M
s (Figure S18 and Table S10),
→
TiO /ReC + CO + H O
2
2
which is smaller by 2 orders of magnitude than that for quenching
by BIH.
Equations 1−6 show essential reaction processes for dis-
cussing structure−reactivity relationships in the present photo-
sensitized CO reduction based on the binary system. The initia-
tion process for the photosensitized CO reduction in either the
binary hybrid system or the homogeneous system should be
The second important process is collisional electron injection
from PS− into TiO followed by transport of the injected elec-
•
2
2
−
tron (TiO (e )) to ReC (eq 2). On the one hand, the electron
2
2
injection should depend on the differences between the
3
3
+
conduction-band edge of TiO and the reduction potentials
electron transfer from BIH to triplet-state PS ( PS* = Ir-X * or
2
red
3
2+
(
E1/2 ) of PS. On the other hand, the conduction-band edge of
Ru(bpy)3 *) to generate the one-electron reduced species
−•
TiO is known to be close to the experimentally determined flat-
(
OERS) of PS (PS ; eq 1). This is supported by the com-
plete phosphorescence quenching by 0.1 M BIH for each of
2
band potential (E ), which is ca. −1.50 V versus SCE for a TiO
fb
2
20
nanoparticle film in the presence of 3 vol % water in DMF. If the
E value is applicable to the present TiO particles dispersed in
PS. Figure S15 shows a Stern−Volmer plot for quenching of
t
+
the Ir- Bu phosphorescence by BIH as a typical example; the
fb
2
−•
DMF, the electron injection from PS into TiO should be
quenching rate constant was calculated from the slope of the plot
and the phosphorescence lifetime to be 5.9 × 10 M s . This is
in accord with the sufficiently negative free-energy changes of
2
8
−1 −1
endergonic by 0.08−0.19 eV (Figure 7).
Under such conditions, the electron injection might be slow,
probably proceeding in equilibrium with electron reversal from
eq 1 (ΔG = −(0.46−0.94) eV) calculated from the excited-state
1
−
−•
3
reduction potentials of PS* (E * = 0.72−1.19 V vs standard
TiO (e ) to PS. It should be, however, noted that each PS
2
red
+
2+
calomel electrode (SCE) for Ir-X and 0.80 V for Ru(bpy)
)
generated by irradiation of PS in the presence of BIH in DMF can
3
ox
22
+ −•
and the oxidation potential of BIH (E1/2 = 0.25 V). Fur-
thermore, irradiation of a DMF solution containing Ir-X and
survive with half lifetimes (t1/2) of 10−19 min for [Ir-X ] and
+
2+ −•
5.5 min for [Ru(bpy)
]
in the absence of TiO (Figure S6 in
3
2
BIH for 5 min gave a new absorption spectrum with the maxi-
Supporting Information), long-lived enough for effective elec-
tron injection into TiO to occur. Such a long lifetime of OERS is
very unusual, although Fukuzumi and co-workers
previously a photoinduced long-lived charge separation (∼a few
hundred milliseconds to seconds) in a series of dopant−host
dyad systems created from either covalent bond or electrostatic
mum at 510−540 nm, which is essentially identical with the spec-
2
+
−•
54−56
trum of [Ir-X ] taken by spectroelectrochemical measurements
reported
+
of Ir-X (Figures S7 and S16). Note that there might be two
3
different reduction pathways for the PS* by BIH; one is related to
the electron transfer suggested by this work and the other to the
G
DOI: 10.1021/acs.inorgchem.7b01963
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
TiO /ReC particles are fully dispersed in solution by stirring. In
2
t
+ −•
fact, the absorption of [Ir- Bu ] was not observed at all after 1 h
of irradiation of TiO -dispersed solution containing Ir- Bu and
t
+
2
BIH (Figure S21).
Under such circumstances where the electron injection pro-
cess (eq 2) is slightly endergonic, it can be presumed that the net
efficiency of electron transport to ReC would be sensitively
red
affected by various factors such as the small differences in E1/2
,
Figure 7. Relative energy levels (V vs SCE) of the flat-band potential
red
steric properties of PS, and distributions of the odd electron in
PS . Provided the photosensitization efficiencies in the early
(
E ) of TiO , the reduction potentials (E
) of PS, and the reduction
fb
2
1/2
red
−•
potential of ReC (represented by that of RePE (E(RePE)1/2 )).
stage of the reaction are related to the amounts of injected
+
interaction. However, it is probable that the OERS of PS can
survive for a long period of time when there are no quenching
pathways available in the presence of strong two-electron reduc-
tant, BIH. This was confirmed by the observation that the decay
electrons, it is of interest to note that Ir-OMe is significantly
t
+
+
more efficient as photosensitizer than Ir- Bu and Ir-Me even
red
though the E1/2 differences are only 10−20 mV. Presumably,
the strong electron-donating effect of the two OMe substituents
would prevent significant population of the odd electron on the
bpy ligand to push the odd electron toward the btp ligand. The
t
+ −•
of [Ir- Bu ] is considerably accelerated in the presence of
TiO , as shown in Figure 8. The similar decay acceleration in the
2
+
−•
particular electronic character of [Ir-OMe ] might be
indicated by the broad spectrum different from the common
+
−•
sharp spectra for other [Ir-X ] (Figures S7d and S16d).
The reduction of CO to CO requires net two-electron
2
−
transfer to ReC from TiO (e ), which should sequentially pro-
2
ceed. The simultaneous transfer of two electrons as an alternative
process is unlikely to occur, because E of TiO is considerably
fb
2
less negative than the two-electron reduction potential of ReC
5
7
(
(
−1.72 V vs SCE). The one-electron reduced species of ReC
−•
−•
ReC = L(CO) ReCl ) generated by the initial one-electron
3
−
transfer from TiO (e ) to ReC gives the 17-electron species
2
•
−
(
(
L(CO) Re ) as a key intermediate after the liberation of Cl
3
5
8−63
eq 3) followed by coordination of a solvent molecule (eq 4).
64,65
This species is known to interact with CO2,
probably by the
coordination of CO to the metal center (eq 5). Although the
2
follow-up processes have not been fully explored, the second
electron transfer should occur with electrons deposited in TiO2
Figure 8. Normalized decay profiles of the absorption at 529 nm (λ of
max
after the coordination of CO to complete the CO reduction
t
+
−•
2
2
[
Ir- Bu ] ) in the dark after 2 min of irradiation of 3 mL of DMF
under participation of protons (eq 6). These chemical processes
t
+
solution of 25 μM Ir- Bu , 2.5 mM BIH, and 2.5 vol % H O in the
2
(
(
eqs 3−6) might be slower than the electron-transfer processes
absence of TiO (red line), with 5 mg of TiO particles placed in the
2
2
17,66
eqs 1 and 2)
to result in a situation with a mismatch between
bottom of the cell (green line), and with a mesoporous TiO film dipped
2
the electron flow and the chemical processes. As the con-
in the solution (blue line).
sequence, PS− might be increasingly accumulated in solution
•
+
presence of TiO particles is also apparent for the other Ir-X
with elapsing of irradiation time after TiO
2
has been filled with
electrons. In cases where PS− would undergo chemical changes
during the CO -reduction catalytic cycles, the efficiency of
2
•
sensitizers (X = OMe, Me, and H; Figures S19 and S20). After
t
+
DMF solutions of Ir- Bu and BIH in the absence and presence
2
of 5 mg of TiO particles placed in the bottom of the cell had
photosensitized CO formation should start to drop after PS has
been significantly consumed. This might lead to the leveling-off
behavior of photosensitized CO formation observed in a later
stage of the reaction. The lower the chemical stability of PS , the
sooner the leveling-off behavior would appear.
2
been irradiated for 5 min, the absorption at 529 nm (λ of
max
t
+ −•
[
Ir- Bu ] ) grown by the irradiation was monitored in the dark.
−•
During the irradiation and monitoring, the solutions were kept
under stationary conditions to minimize dispersing of the TiO2
particles. Otherwise, dispersed TiO particles should extensively
Figure 9 shows changes of absorption spectra following+
2
scatter the incident light to disturb reliable optical measurements.
irradiation time for Ar-purged DMF solution containing Ir-X
t
+ −•
t
Even under such conditions, the decay of the [Ir- Bu ] absorp-
(X = OMe or Bu) and BIH in the absence of TiO
2
. Upon irradi-
+
tion was considerably faster in the presence of TiO than in its
ation of the solution for 5 min, the spectrum of Ir-X was com-
2
absence. Probably, the accelerated bleaching of the 529 nm
absorption should occur as the consequence of electron injection
pletely changed to a new one with the longest absorption maximum
at ∼529 nm, which is the same as that of [Ir-X ] observed by
spectroelectrochemistry. This indicates the complete conversion of
+
−•
t
+ −•
from [Ir- Bu ] into a small amount of colloidal TiO nanopar-
2
+
+ −•
+
ticles (e.g., 4−5 nm size primary particles) floating in solution
Ir-X to [Ir-X ] . In the case of Ir-OMe , further irradiation
resulted in substantial spectral changes accompanied by decreases
of absorbance at 529 nm, a result showing sharp contrast to little
and, perhaps, into the TiO agglomerate particles situated in the
2
bottom of the cell. Another experiment using a TiO mesoporous
2
t
+
t
+
film immersed in solution of [Ir- Bu ] and BIH revealed again
enhanced bleaching of the 529 nm absorption. While the
decay profile in Figure 8 is not relevant to kinetic analysis, the
spectral change in the case of Ir- Bu .
These observations demonstrate that [Ir- Bu ] is stable
under visible-light irradiation, whereas [Ir-OMe ] undergo
photochemical changes. The different photochemical behavior of
t
+ −•
−•
+
t
+ −•
electron injection from [Ir- Bu ] into TiO2 should be
+
−•
complete under the CO -reduction conditions, where 10 mg of
[Ir-X ] should have a possible relationship with net chemical
2
H
DOI: 10.1021/acs.inorgchem.7b01963
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Finally, it is of significance to note that the present photo-
sensitized reactions gave formic acid in small, but not negligible,
amounts in contrast to the lack of HCOOH formation in
the CO reduction by the ternary system (Dye/TiO /ReC).
2
2
Figure S23 shows plots for the formation of CO and HCOOH
t
+
versus irradiation time in the [Ir- Bu] −TiO /ReC case, reveal-
2
ing that the formation of HCOOH is not increased with irradia-
tion time from the initial amount in contrast to the continuous
increase of CO formation. It appears that the major source of
t
+
HCOOH formed is not CO , because irradiation of [Ir- Bu] −
2
TiO /ReC under N in the presence of 2.5% H O for 4 h gave
2
2
2
(
5.4 ± 0.4) μmol of HCOOH, in a significantly greater amount
compared to the photoreaction under CO (∼1.4 μmol of
2
t
+
+
Figure 9. Changes of absorption spectra for Ir- Bu (a) and Ir-OMe
b) following irradiation time for Ar-saturated DMF solution containing
HCOOH; Table S11). On the one hand, the hydrolysis of DMF
appears to be a major consequence of the HCOOH formation
(
+
0
4
.05 mM Ir-X and 0.25 mM BIH; LED irradiation at greater than
t
+
occurring under irradiation in the presence of [Ir- Bu] , because
00 nm.
t
+
HCOOH was not detected after keeping [Ir- Bu] −TiO /ReC
2
in DMF in the presence of 2.5% H O for several hours in the
2
stability of PS during the photosensitized CO reduction.
Figure 10 shows absorption spectra after the photosensitized
2
dark. On the other hand, irradiation of a DMF solution con-
t
+
taining [Ir- Bu] , BIH, and 2.5% H O for 4 h under either CO or
2
2
N (i.e., without TiO /ReC) gave (2.0 ± 0.1) μmol of HCOOH
2
2
(
Table S11). A possible speculation is that DMF would be
photochemically hydrolyzed after electron transfer from BIH to
3
t
+
[
Ir- Bu] *. However, further investigation was not undertaken,
because the HCOOH is not a “product” from CO2.
CONCLUSIONS
■
+
t
The present investigation reveals that Ir-X (X = OMe, Bu, Me,
and H) commonly works as effective visible-light photosensitizer
for the selective reduction of CO to CO by the TiO /ReC
2
2
binary catalyst that proceeds via electron transfer from BIH to
+
+ −•
excited-state Ir-X followed by electron injection from [Ir-X ]
+
to TiO /ReC. The Ir-X −TiO /ReC binary system is more effi-
cient than the Dye/TiO /ReC ternary system. While the elec-
tron injection from [Ir-X ] into TiO is supposed to be slightly
2
2
2
t
+
+
Figure 10. Absorption spectra of Ir- Bu (a) and Ir-OMe (b) after
+ −•
2
stirring for 1 h in the dark (black line) and after photosensitized
endergonic unlike exergonic electron injection from excited-state
CO reduction for 30 min (red line) and for 1 h (blue line); 3 mL of
2
Dye, the higher efficiencies of the binary system is attributed to
CO -saturated DMF/H O mixture solution (2.5 vol % water) of
2
2
+
+
more efficient light harvesting by Ir-X in solution compared
1
BIH.
mM Ir-X , 10 mg of TiO /ReC(0.1 μmol) particles, and 0.1 M
2
with Dye fixed on TiO . The photosensitizers used in the CO
2
2
reduction based on the binary catalyst show different efficiencies
+
t
+
+
+
in the order of Ir-OMe > Ir- Bu ≈ Ir-Me > Ir-H ≫
+
CO reduction by the Ir-X −TiO /ReC binary systems for
2
2
2+
Ru(bpy)3 , whereas the long-term stability in the photosen-
3
0 min and for 1 h compared with those after stirring of the solu-
t
+
+
+
sitization cycle is in the order of Ir- Bu ≈ Ir-Me > Ir-H >
tions in the dark for 1 h; the spectra were taken by 10-fold
dilution of the solutions separated by centrifugation of the
+
2+
3
+
Ir-OMe ≫ Ru(bpy) ; the Ir-X photosensitizers are consid-
2
+
+
erably more efficient and more robust than Ru(bpy) , a com-
3
irradiated samples. In the case of Ir-OMe , significant spectral
monly used photosensitizer. The higher photosensitization effi-
changes occurred during the photosensitized CO reduction,
2
+
t
+
2+
ciency of Ir-OMe appears to be attributed to more efficient
whereas Ir- Bu was relatively stable. In the case of Ru(bpy)
,
3
+ −•
electron injection from [Ir-OMe ] into TiO associated with
2
still faster spectral changes occurred during the photosensi-
the distribution of odd electron toward btp ligand (relatively high
energy level) by the electron-donating OMe substituents. The
tized CO reduction (Figure S22), a result in line with the early
2
appearance of leveling-off behavior of the photosensitized CO
2
t
+
t
+
persistent photosensitization behavior of Ir- Bu should arise
reduction. The long-term photosensitization capability of Ir- Bu
t
+ −•
+
2+
from the photochemical stability of [Ir- Bu ] , while the photo-
compared with Ir-OMe as well as with Ru(bpy) is mainly
3
+
−•
2+ −•
t
+ −•
chemical labile property of [Ir-OMe ] and [Ru(bpy)3
]
attributable to the photochemical stability of [Ir- Bu ] . Prob-
ably, on the one hand, the very bulky and chemically stable Bu
substituents would block [Ir- Bu ] against unwanted chemical
t
leads to the leveling-off behavior in the photosensitized CO
2
+
t
+ −•
reduction. The present investigation proves that Ir-X can work
in solution as effective and robust photosensitizer for the CO
reduction based on the TiO /ReC binary catalyst and implies
changes during the photosensitization cycle. In the case of
2
+
Ir-OMe , on the other hand, the strong electron-donating effect
2
of the OMe substituent would be divergent; the localization of
the odd electron of PS on the btp ligand would help the
that the structure−reactivity relationships explored in this work
might give a strategic basis for molecular design of Ir-complex-
based photosensitizers directing to the construction of efficient
−•
electron injection into TiO but enhance the chemical reactivity
2
−
•
of PS .
and persistent CO reduction systems.
2
I
DOI: 10.1021/acs.inorgchem.7b01963
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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7) Alibabai, L.; Luo, H.; House, R. L.; Hoertz, P. G.; Meyer, T. J.; et al.
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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.
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AUTHOR INFORMATION
Corresponding Authors
E-mail: hjson@korea.ac.kr. (H.-J.S.)
E-mail: jjpac@korea.ac.kr. (C.P.)
E-mail: sangok@korea.ac.kr. (S.-O.K.)
■
*
*
*
Photons, Protons and Earth-Abundant Metal Complexes for Molecular
Catalysis of CO Reduction. ACS Catal. 2017, 7, 70−88.
2
(13) Tamaki, Y.; Ishitani, O. Supramolecular Photocatalysts for the
Reduction of CO . ACS Catal. 2017, 7, 3394−3409.
(14) Elgrishi, N.; Chambers, M. B.; Wang, X.; Fontecave, M. Molecular
polypyridine-based metal complexes as catalysts for the reduction of
CO . Chem. Soc. Rev. 2017, 46, 761−796.
(
Reisner, E. Dye-sensitized semiconductors modified with molecular
catalysts for light-driven H production. Chem. Soc. Rev. 2016, 45, 9−23.
(
2
ORCID
2
Dae Won Cho: 0000-0002-4785-069X
Ho-Jin Son: 0000-0003-2069-1235
Sang Ook Kang: 0000-0002-3911-7818
15) Willkomm, J.; Orchard, K. L.; Reynal, A.; Pastor, E.; Durrant, J. R.;
2
16) Woolerton, T. W.; Sheard, S.; Reisner, E.; Pierce, E.; Ragsdale, S.
Notes
W.; Armstrong, F. A. Efficient and Clean Photoreduction of CO to CO
by Enzyme-Modified TiO Nanoparticles Using Visible Light. J. Am.
2
The authors declare no competing financial interest.
2
Chem. Soc. 2010, 132, 2132−2133.
ACKNOWLEDGMENTS
■
(17) Windle, C. D.; Pastor, E.; Reynal, A.; Whitwood, A. C.; Vaynzof,
Y.; Durrant, J. R.; Perutz, R. N.; Reisner, E. Improving the Photocatalytic
This work was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education (NRF-2014R1A6A1030732),
the Functional Districts of the Science Belt support program,
Ministry of Science, ICT and Future Planning (2015K000287),
and the Basic Science Research Program through the NRF funded
by the Ministry of Education (NRF-2016R1D1A3B03934670 and
NRF-2016R1D1A3B03936414).
Reduction of CO
to CO through Immobilisation of a Molecular Re
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Catalyst on TiO . Chem. - Eur. J. 2015, 21, 3746−3754.
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