Water-Assisted Highly Efficient Photocatalytic Reduction of CO2to CO with Noble Metal-Free Bis(terpyridine)iron(II) Complexes and an Organic Photosensitizer

Article abstract of DOI:10.1021/acs.inorgchem.0c03503

Photocatalytic CO2 reduction reaction is believed to be a promising approach for CO2 utilization. In this work, a noble metal-free photocatalytic system, composed of bis(terpyridine)iron(II) complexes and an organic thermally activated delayed fluorescence compound, has been developed for selective reduction of CO2 to CO with a maximum turnover number up to 6320, 99.4% selectivity, and turnover frequency of 127 min-1 under visible-light irradiation in dimethylformamide/H2O solution. More than 0.3 mmol CO was generated using 0.05 μmol catalyst after 2 h of light irradiation. The apparent quantum yield was found to be 9.5% at 440 nm (180 mW cm-2). Control experiments and UV-vis-NIR spectroscopy studies further demonstrated that water strongly promoted the photocatalytic cycle and terpyridine ligands rather than Fe(II) were initially reduced during the photocatalytic process.

Full text of DOI:10.1021/acs.inorgchem.0c03503

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Article
Water-Assisted Highly Efficient Photocatalytic Reduction of CO2 to
CO with Noble Metal-Free Bis(terpyridine)iron(II) Complexes and an
Organic Photosensitizer
Yanan Wang, Ting Liu, Longxin Chen, and Duobin Chao*
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ABSTRACT: Photocatalytic CO2 reduction reaction is believed to be a
promising approach for CO2 utilization. In this work, a noble metal-free
photocatalytic system, composed of bis(terpyridine)iron(II) complexes and
an organic thermally activated delayed fluorescence compound, has been
developed for selective reduction of CO2 to CO with a maximum turnover
number up to 6320, 99.4% selectivity, and turnover frequency of 127 min−1
under visible−light irradiation in dimethylformamide/H2O solution. More
than 0.3 mmol CO was generated using 0.05 μmol catalyst after 2 h of light
irradiation. The apparent quantum yield was found to be 9.5% at 440 nm
(180 mW cm−2). Control experiments and UV−vis−NIR spectroscopy
studies further demonstrated that water strongly promoted the photocatalytic
cycle and terpyridine ligands rather than Fe(II) were initially reduced during the photocatalytic process.
complexes have been developed for CO2RR. Initially, they
INTRODUCTION
■
were used for electrocatalytic CO2RR. For example, Meyer and
coworkers have utilized novel terpyridine−Ru(II) complexes
typically as [RuII(tpy)(bpy)(S)]2+ for electrocatalytic reduc-
tion of CO2 since the 1980s.53,54 They found that ligands such
as tpy and bpy instead of RuII were reduced during
electrocatalytic processes. Abruna et al. developed a series of
terpyridine complexes based on first-row transition metals
including Fe(II), Co(II), and Ni(II) in the late 1980s and early
1990s.55−57 Electropolymerized films of metal terpyridine
complexes exhibited moderate catalytic efficiencies toward
the reduction of CO2. Recently, Fontecave and coworkers
investigated the application of [M(tpy)2]2+ complexes (M =
Fe, Co, Ni, Cu, Zn, and Mn) for homogeneously electro-
catalytic CO2RR in 2014.58 They found that [Co(tpy)2]2+ and
[Ni(tpy)2]2+ showed efficient catalytic activity with relatively
low Faradic efficiencies, while [Fe(tpy)2]2+ almost exhibited no
catalytic efficiency. However, Ortiz−Frade et al. found that Fe-
based tpy complex [Fe(tpy)2]2+ was electrochemically active
under a CO2 atmosphere even though it had no open
coordination site in 2017.59 The different results obtained by
them are probably due to the different electrochemical
potential window they used. Lately, solar-driven CO2
Solar-driven reduction of CO2 to value-added chemical fuels
has attracted extensive attention in the past decade.1−9 Among
various catalysts for CO2 reduction reaction (CO2RR),
molecular metal complexes have shown promising catalytic
efficiencies not only under electrochemical conditions but also
upon visible-light irradiation due to their diverse chemical
structures via tuning ligands and metals.10−18 From the point
of view of sustainable development, molecular photocatalysts
based on earth-abundant metals are more appealing than noble
metal complexes.19−26 Recently, non-precious metal complexes
such as Fe,11,27−29 Co,30−33 Ni,34−36 Cu,37 and Mn38−40 have
been widely studied for photocatalytic CO2RR. However,
those non-precious molecular catalysts are usually used
together with precious photosensitizers such as [Ru(bpy)3]2+.
To construct fully noble metal-free photocatalytic systems for
CO2RR, a few luminescent Cu(I) complexes, Zn(II)
complexes, and organic dyes have been developed as efficient
photosensitizers for photocatalytic CO2RR.27,41−47 Besides
molecular photosensitizers, quantum dots such as CuInS2 and
CdS are also employed as light-absorbing components for
photocatalytic CO2RR by combining molecular metal com-
plexes.34,48 However, most noble metal-free photocatalytic
systems display a moderate turnover number (TON) less than
3000 under optimal conditions even after longtime irradiation.
Therefore, it is still necessary to develop more efficient fully
noble metal-free photocatalytic systems.
Received: November 30, 2020
Published: February 21, 2021
Tridentate ligands based on 2,2′:6′,2″−terpyridine (tpy)
and their metal complexes have been widely studied in a large
range of research fields.49−52 Some terpyridine-based metal
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reduction systems with tpy-based metal complexes have been
developed by the group of Reisner, where they combined
Ni(II) and Co(II) complexes with CdS QDs and Si|TiO2 in
water.34,60 The two light-driven systems displayed TONs less
than 400. We recently found a terpyridine−Fe(III) complex
that showed impressive photocatalytic efficiency with a TON
of 2250 when an organic thermally activated delayed
fluorescence (TADF) compound 4CzIPN was employed as
the photosensitizer.61 However, the apparent quantum yield
(AQY) is relatively low (2.04%). More importantly, further
studies including roles of solvent water and ligands in the
photocatalytic cycle are much in demand for designing more
efficient photocatalysts.
Herein, we report a noble metal-free photocatalytic system
for CO2 reduction using bis(terpyridine)iron(II) complexes as
catalysts and an organic TADF dye 4CzIPN as a photo-
sensitizer in dimethylformamide (DMF)/H2O (v/v = 3:2)
solution (Figure 1). It is found that the Fe(II) complex
Figure 2. (a) UV−vis−NIR absorption spectra of bis(terpyridine)-
iron(II) complexes Fe(Ntpy)2, Fe(Cltpy)2, and Fe(Htpy)2 in DMF
solution at room temperature. CV results of bis(terpyridine)iron(II)
complexes (b) Fe(Ntpy)2, (c) Fe(Htpy)2, and (d) Fe(Cltpy)2 under
N2 (black line) and CO2 (red line) atmosphere at 100 mV s−1 scan
rate, respectively.
transitions of Fe(Ntpy)2, Fe(Cltpy)2, and Fe(Htpy)2,
respectively. The MLCT transitions around 580 nm are similar
to other reported bis(terpyridine)iron(II) complexes.55,56,62−65
Cyclic voltammetry (CV) experiments were further carried
out to examine the catalytic activity of three bis(terpyridine)-
iron(II) complexes toward the reduction of CO2. The reducing
potentials of bis(terpyridine)iron(II) complexes and corre-
sponding terpyridine ligands are collected in Table 1. As
Figure 1. Chemical structures of photosensitizer 4CzIPN and
bis(terpyridine)iron(II) complexes abbreviated as Fe(Ntpy)2, Fe-
(Htpy)2, and Fe(Cltpy)2, respectively.
Table 1. Partial Reducing Potentials of
Fe(Ntpy)2 bearing electron-donating groups displayed the
highest TON up to 6320 with 99.4% selectivity and a TOF of
127 min−1 in the first 20 min. The AQY was estimated to be
9.5%. More importantly, we found that the photocatalytic
process was strongly promoted by water and initiated through
the reduction of terpyridine ligands. To the best of our
knowledge, such a highly efficient non-precious visible-light-
driven system for CO2RR using molecular photocatalysts has
rarely been reported.
Bis(terpyridine)iron(II) Complexes, Ligands, and 4CzIPN
a
in DMF Using SCE as a Reference
b
compounds
E (V) [ΔEp (mV)]
E
red(CO2) (V)
Fe(Ntpy)2
Fe(Cltpy)2
Fe(Htpy)2
Ntpy
−1.34 [60], −1.43 [60], −2.15 [irr.]
−1.22 [60], −1.32 [60], −2.08 [irr.]
−1.24 [60], −1.35 [60], −2.01 [110]
−2.13 [90]
−1.82
−1.75
−1.78
Cltpy
Htpy
−1.97 [irr.], −2.06 [66]
−2.05 [95]
RESULTS AND DISCUSSION
■
4CzIPN
−1.31 [77]
The three bis(terpyridine)iron(II) complexes including Fe-
(Ntpy)2, Fe(Htpy)2, and Fe(Cltpy)2 were readily available
a
Standard conditions: 1 mM compounds, 0.1 M Bu4NPF6, GC
working electrode, Ag/AgCl reference electrode, and platinum plate
·
through the reaction between FeCl2 4H2O and corresponding
counter electrode at 100 mV s−1 scan rate at room temperature in N2-
terpyridine ligands such as Ntpy, Htpy, and Cltpy. They were
designed to be similar in chemical structure but mainly
different in the group at the 4′ position of the terpyridine
skeleton. This is to explore the substituent effect on the
catalytic efficiency of bis(terpyridine)iron(II) complexes. It is
worth noting that bis(terpyridine)iron(II) complexes have no
open coordination sites, which is different from the
terpyridine−Fe(III) complex.61 We studied electronic tran-
sitions of bis(terpyridine)iron(II) complexes at a steady state
by UV−vis−NIR spectroscopy. As shown in Figure 2a, these
Fe(II) complexes exhibit a ligand-based absorption band
around 325 nm and another shoulder band around 370 nm.
Fe(Ntpy)2 has a distinctive absorption peak at 406 nm due to
intraligand charge transfer (ILCT) transition caused by the
N,N-dimethyl group in the ligand Ntpy. The relatively strong
bands at 587, 574, and 572 nm are assigned as metal-to-ligand
charge transfer (MLCT) (FeII to terpyridine ligands)
b
saturated DMF solution. Experiments were carried out in CO2-
saturated DMF solution.
shown in Figure 2 and Table 1, all bis(terpyridine)iron(II)
complexes have three similar reducing processes. The
potentials of Fe(Ntpy)2 are more negative than those of
Fe(Htpy)2 and Fe(Cltpy)2 which have the most positive
potentials. This is probably due to the electron-donating N,N-
dimethyl groups in Fe(Ntpy)2 and electron-withdrawing Cl
atom in Fe(Cltpy)2. For Fe(Ntpy)2, the potentials at −1.34
and −1.43 V are reversible with ΔEp = 60 mV, indicating two
one-electron electrochemical processes. The two waves are not
easy to be recognized through CV, but square wave
voltammetry (SWV) experiment clearly confirms the two
waves (Figure S1). They are attributed to the reduction of
ligand Ntpy in comparison with other metal terpyridine
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Figure 3. Conditions: all reaction solutions were irradiated with visible light (3 W white LEDs, λ = 420−650 nm) irradiation for 120 min under a
CO2 atmosphere at room temperature. (a) TONs of products CO and H2 using Fe(Ntpy)2 (10 μM), 4CzIPN (50 μM), and TEA (0.28 M) in 5
mL of DMF/H2O solution with different fractions of water (0−70%). (b) TONs of products CO and H2 using Fe(Ntpy)2 (10 μM), 4CzIPN, and
TEA (0.28 M) in 5 mL of DMF/H2O (v/v = 3:2) solution with different concentrations of 4CzIPN (10−150 μM). (c) TONs of products CO and
H2 using Fe(Ntpy)2, 4CzIPN (100 μM), and TEA (0.28 M) in 5 mL of DMF/H2O (v/v = 3:2) solution with different concentrations of
Fe(Ntpy)2 (0−40 μM). (d) TONs of products CO and H2 with Fe(Ntpy)2 (10 μM), Fe(Cltpy)2 (10 μM), and Fe(Htpy)2 (10 μM) in the
presence of 4CzIPN (100 μM), TEA (0.28 M) in 5 mL of DMF/H2O (v/v = 3:2) solution. (e) Time course of TON (CO and H2) using
Fe(Ntpy)2 (10 μM), TEA (0.28 M), and 4CzIPN (100 μM, blue) or [Ru(bpy)3]2+ (100 μM, black) in 5 mL of DMF/H2O (v/v = 3:2) solution.
The blue line was carried out with Fe(Ntpy)2 (10 μM), TEA (0.28 M), and 4CzIPN (100 μM) in 5 mL of CH3CN/H2O (v/v = 3:2) solution. (f)
TONCO of irradiated reaction solution containing Fe(Ntpy)2 (10 μM), TEA (0.28 M), and 4CzIPN (100 μM) after adding again various
substances including 4CzIPN (100 μM, black bar), Fe(Ntpy)2 (10 μM, red bar), ligand Ntpy (20 μM, green bar), and 4CzIPN (100 μM, blue bar)
plus ligand Ntpy (20 μM, blue bar) for another 120 min of light irradiation under a CO2 atmosphere at room temperature.
complexes.54−56,66 They are not likely to be assigned as Fe(II)-
centered potentials. Additionally, we found the linear relation-
ship between currents and square root of scan rate for the two
reducing processes from 100 to 800 mV s−1 (Figure S2). The
third irreversible reduction signal at −2.15 V is similar to that
of the free ligand Ntpy. Under a CO2 atmosphere, the current
obviously increased at −1.82 V for Fe(Ntpy)2, suggesting its
catalytic activity toward the reduction of CO2. The potentials
of CO2 reduction for Fe(Cltpy)2 (−1.75 V) and Fe(Htpy)2
(−1.78 V) are more positive than for Fe(Ntpy)2, in line with
their chemical structures. Therefore, all three Fe(II) complexes
could be probably used as efficient catalysts for photocatalytic
CO2RR when cooperating with a suitable photosensitizer.
Nevertheless, the direct reduction of these bis(terpyridine)-
iron(II) complexes around −1.82 V for photocatalytic CO2
reduction might be difficult because such strong reducing
photosensitizer is really rare under light-driven conditions.
More importantly, the ligand could be reduced prior to −1.82
V. Hence, a ligand-based reduction process might play an
important role when bis(terpyridine)iron(II) ligands are
utilized for photocatalytic CO2RR.
Inspired by the catalytic activities of bis(terpyridine)iron(II)
complexes toward CO2 reduction, we then employed them as
photocatalysts for photocatalytic CO2RR. The organic TADF
compound 4CzIPN was used here as the photosensitizer (P)
due to its suitable excited-state lifetime and redox poten-
tials.67,68 Triethylamine (TEA) was used as the sacrificial
electron donor, which could reduce the excited state of
4CzIPN (P*) to generate reduced 4CzIPN (P−) under visible-
light irradiation. Light sources are white LEDs (3 W, λ = 420−
650 nm). The potential of reduced 4CzIPN is close to ligand-
based potentials of bis(terpyridine)iron(II) complexes, which
is helpful for electron-transfer process between them. As can be
seen from Figure 3a, a higher TONCO could be obtained in the
mixed DMF/H2O solution than in DMF. Therefore, it can be
concluded that water strongly promoted the catalytic process
and the highest TONCO was reached in the mixed DMF/H2O
(v/v = 3:2) solution. When the fraction of water became
higher, we observed insoluble aggregates of the photosensitizer
4CzIPN and the TONCO decreased sharply. The amounts of
4CzIPN and Fe(Ntpy)2 also had a big effect on the catalytic
efficiency of the photocatalytic process (Figure 3b,c). It is
worth noting that H2 evolution was very low no matter what
the reaction conditions were. The optimal concentration of
4CzIPN was 100 μM when the used Fe(Ntpy)2 was 10 μM.
Under optimal conditions, TONCO was estimated to be 6320
and TON of H2 was 35 upon visible-light irradiation for 120
min. Only a trace amount of formic acid was detected by
HPLC. Selectivity of this photocatalytic system was then
estimated to be 99.4% (CO over H2), which is encouraging for
fully noble metal-free photocatalytic CO2RR in the presence of
water. Control experiments showed that light, Fe(Ntpy)2,
4CzIPN, and TEA are all indispensable for efficient photo-
catalytic reduction of CO2 to CO in this work. Isotope labeling
experiment with 3CO2 confirmed the generation of 13CO
during the photocatalytic process (Figure S3).
Fe(Cltpy)2 and Fe(Htpy)2 also displayed good selectivity
toward CO generation but TONs were found to be 2780 and
4500, respectively (Figure 3d). This seems to contradict the
CV potentials of Fe(Ntpy)2, Fe(Cltpy)2, and Fe(Htpy)2
(Table 1). Since Fe(Cltpy)2 has the most positive reduction
potentials, it should be more feasible to receive electrons from
reduced photosensitizer (P−) than Fe(Ntpy)2, spontaneously
resulting in higher catalytic efficiency. However, we indeed
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Figure 4. Conditions: all solutions were irradiated with white LEDs (3 W, λ = 420−650 nm) at room temperature. (a) UV−vis−NIR absorption
spectra of DMF/H2O (v/v = 3:2) solution containing Fe(Ntpy)2 (10 μM) upon light irradiation for 0−9 min under a N2 atmosphere. (b) UV−
vis−NIR absorption spectra of DMF/H2O (v/v = 3:2) solution containing 4CzIPN (50 μM) and TEA (93 mM) upon light irradiation for 0−120 s
under a N2 atmosphere. (c) UV−vis−NIR absorption spectra of DMF solution containing Fe(Ntpy)2 (10 μM), 4CzIPN (50 μM), and TEA (93
mM) upon light irradiation for 0−300 s under a N2 atmosphere. (d) UV−vis−NIR absorption spectra of DMF/H2O (v/v = 3:2) solution
containing Fe(Ntpy)2 (10 μM), 4CzIPN (50 μM), and TEA (93 mM) upon light irradiation for 0−220 s under a N2 atmosphere. The yellow line
is the spectrum of irradiated solution that was put in the dark for another 480 s (e) UV−vis−NIR absorption spectra of DMF solution containing
Fe(Ntpy)2 (10 μM), 4CzIPN (50 μM), and TEA (93 mM) upon light irradiation for 0−340 s under a CO2 atmosphere. (f) UV−vis−NIR
absorption spectra of DMF/H2O (v/v = 3:2) solution containing Fe(Ntpy)2 (10 μM), 4CzIPN (50 μM), and TEA (93 mM) upon light irradiation
for 0−340 s under a CO2 atmosphere.
found that Fe(Ntpy)2 is much more efficient for CO2RR.
Therefore, we speculate that electron donating N,N-dimethyl
groups in Fe(Ntpy)2 are probably beneficial to enhance the
nucleophilicity of Fe(II) center, which is more essential for the
whole catalytic cycle.
the irradiated solution, suggesting that Fe(II) still existed even
though the photocatalytic cycle stopped. These assumptions
were further confirmed by adding reagents to the irradiated
reaction solution for another 120 min of irradiation. As shown
in Figure 3f, adding the photosensitizer 4CzIPN into the
irradiated solution resulted in a TONCO of 36 due to the
decomposition of Fe(Ntpy)2. The TONCO was 1021 when
Fe(Ntpy)2 was added, indicating that partial photosensitizer
remained active after 120 min of irradiation. It should be noted
that the addition of ligand Ntpy also rebooted the photo-
catalytic cycle with a TONCO of 493. This result strongly
revealed the existence of Fe(II) after the reaction ceased and
the decomposition of Fe(Ntpy)2 was probably caused by the
side reaction of ligand Ntpy such as a reduction reaction which
made Ntpy lose the ability to coordinate with Fe(II) and
accept electrons from the reduced photosensitizer. As a result,
we witnessed a higher TONCO of 3667 when both 4CzIPN
and ligand Ntpy were added into the irradiated solution
together.
It was further evidenced that both photosensitizer and
solvent were important to achieve efficient photocatalytic
TONs. As shown in Figure 3e, TONCO was only 19 when
[Ru(bpy)3]2+ was used instead of 4CzIPN. This result suggests
that 4CzIPN could be used as a more efficient photosensitizer
than [Ru(bpy)3]2+. It is probable that the reduced photo-
sensitizer (P−) with negative charge may surround the positive
complex Fe(Ntpy)2 through electrostatic interaction, making
electron transfer more feasible. When the solvent DMF was
replaced by CH3CN, TONCO decreased to 992. However,
CH3CN has been used as a solvent for efficient photocatalytic
CO2RR with various iron complexes, suggesting that CH3CN
may hinder a certain crucial step of the photocatalytic cycle
which still requires further investigation in detail.17,69
Although this photocatalytic system displayed relatively high
TON up to 6320, the catalytic reaction mostly ceased after 120
min of irradiation. The TOF was estimated to be 127 min−1 in
the first 20 min under optimal conditions (Figure S4).
Additionally, the AQY was estimated to be 9.5% under blue
light (180 mW cm−2, 440 nm) irradiation (see the Supporting
Information), which is much higher than our previous work.61
To know more about quick shutoff of this catalytic process, we
recorded the UV−vis−NIR spectrum of the irradiated reaction
solution and found that the MLCT absorption band of
Fe(Ntpy)2 and characteristic absorption band of 4CzIPN
decreased significantly (Figure S5). Hence, decomposition of
Fe(Ntpy)2 and 4CzIPN is responsible for the low durability of
this photocatalytic system. However, the MLCT band at 587
nm obviously rose after the ligand Ntpy was added again into
To further elucidate the photocatalytic process, UV−vis−
NIR absorption spectroscopy was employed to monitor the
reaction under a N2 or CO2 atmosphere (Figure 4). We
recorded the absorption spectra of Fe(Ntpy)2 in DMF and
DMF/H2O (v/v = 3:2), respectively. As shown in Figure 4a,
the MLCT absorption band at 587 nm of Fe(Ntpy)2 changed
a little after 9 min of irradiation. A new tiny peak was observed
at 960 nm in the mixed DMF/H2O solution, while no such
peak appeared in the DMF solution (Figure S6). Hence, it can
be assumed that the new peak at 960 nm was originated from
the interaction between H2O and Fe(Ntpy)2. A similar result
was found when TEA was added into DMF/H2O solution of
Fe(Ntpy)2, suggesting no redox reaction between Fe(Ntpy)2
and TEA (Figure S7). We then investigated the absorption
spectra of photosensitizer 4CzIPN in the presence of TEA
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upon visible-light irradiation (Figure 4b). The absorption
bands at 368 and 448 nm diminished quickly in 120 s, which
was resulted from the reduction of excited-state 4CzIPN by
TEA. We next studied UV−vis−NIR spectra of the solution
containing Fe(Ntpy)2, 4CzIPN, and TEA under a N2
atmosphere. In the DMF solution (Figure 4c), the absorption
bands decreased significantly below 500 nm, which were
majorly due to the reduction of 4CzIPN by comparison with
absorption spectra in Figure 4b. The MLCT peak of
Fe(Ntpy)2 at 587 nm disappeared and two new absorption
bands at a longer wavelength (626 and 810 nm) rose,
indicating that the MLCT transition between Fe(II) and
neutral ligand Ntpy diminished. The two new bands at 626
and 810 nm are ascribed to π*−π* transitions of radical anion
ligand (Ntpy•−) which was generated through the reduction of
neutral ligand Ntpy by the photosensitizer P−, consistent with
the reported spectroelectrochemical experiments and theoreti-
cal calculations on [Fe(tpy) (tpy•−)]+,64 [Ni(tpy)(tpy•−)]+,70
anion ligand (Ntpy•−) to CO2, resulting in efficient reduction
of CO2.
On the basis of the above results, it can be proposed that the
photosensitizer 4CzIPN is reduced by electron donor TEA
upon light irradiation. The reduced 4CzIPN is a moderate
reductant which could further reduce terpyridine-based ligands
in bis(terpyridine)iron(II) complexes. For example, the ligand
Ntpy is reduced to generate radical anion Ntpy•−, which is
consistent with UV−vis−NIR absorption spectra in Figure 4c−
e. The electron localized on the reduced ligand may be further
consumed for the reduction of CO2. Such phenomenon that
electrons on the reduced ligands were consumed for the
reduction process has been studied in electrocatalytic CO2
reduction reactions with polypyridyl−Ru(II) complexes by
Meyer and coworkers.53 The significance of water was revealed
by replacing H2O with D2O in the photocatalytic reaction
(Figure S8). We found the TOF in DMF/H2O is a little higher
than that in DMF/D2O (TOFH2O/TOFD2O = 1.38).
However, more experiments are required to elucidate the
reaction mechanism in detail.
71
and [Cr(tpy)(tpy•−)]2+ complexes by Braterman, Klein, and
Wieghardt, respectively. The transformation of neutral ligand
(Ntpy) to radical anion ligand (Ntpy•−) upon light irradiation
also explains why the MLCT peak of Fe(Ntpy)2 at 587 nm
disappeared. More importantly, this further demonstrates that
reduction of Fe(II) to Fe(I) by reduced photosensitizer (P−) is
not available in this photocatalytic system, which is in accord
with the assignments of ligand-based potentials in Figure 2 and
Table 1. In the mixed DMF/H2O (v/v = 3:2) solution (Figure
4d), absorption spectra changed similarly to that in the DMF
solution and the MLCT band gradually decreased along with
the rising bands at 626 and 810 nm, revealing the reduction of
neutral ligand (Ntpy) and simultaneous generation of radical
anion ligand (Ntpy•−). It should be noted that the spectrum
kept a similar shape and MLCT peak could not recover after
the irradiated solution was placed in the dark for another 480 s.
Therefore, the radical anion ligand (Ntpy•−) is relatively stable
and the electron transfer between anion ligand and Fe(II) is
negligible.
Under a CO2 atmosphere, both spectra in DMF and DMF/
H2O changed less than under a N2 atmosphere. However, the
spectra in DMF are similar for both atmospheres (Figure 4c,e).
We could still see the bands assigned to the radical anion
ligand (Ntpy•−) at 626 and 810 nm under a CO2 atmosphere
in Figure 4e, suggesting that the electrons located in the anion
ligand hardly transferred to reduce CO2 and the photocatalytic
cycle proceeded very slowly in DMF. This was further
confirmed by the subsequent analysis of the headspace gas of
the irradiated solution in Figure 4e through gas chromatog-
raphy (GC) and only trace amount of CO was detected. On
the contrary, no absorption bands of radical anion ligand
(Ntpy•−) were observed in the DMF/H2O solution under a
CO2 atmosphere (Figure 4f), indicating that the electrons
located in the anion ligand (Ntpy•−) transferred to reduce
CO2 and the catalytic cycle proceeded smoothly. In fact, we
found a significant generation of CO after analyzing the
headspace gas in Figure 4f by GC. Although the MLCT band
of Fe(Ntpy)2 at 587 nm gradually decreased in Figure 4f, it still
kept most of the absorbance and did not disappear as shown in
Figure 4c−e. The minor diminishment of the MLCT band at
587 nm could be ascribed to the binding with H2O or CO2 as
well as partial cleavage of the Fe−N bond. The dramatically
different results of the control experiments in Figure 4e,f show
that water is beneficial for the electron transfer from radical
CONCLUSIONS
■
In summary, we have developed a noble metal-free photo-
catalytic system for the selective reduction of CO2 to CO using
bis(terpyridine)iron(II) complexes and an organic TADF
photosensitizer in DMF/H2O solution. It is found that
Fe(Ntpy)2 bearing electron-donating groups is most efficient
(TONCO = 6320, TOFCO = 127 min−1, 99.4% selectivity, and
AQY = 9.5% at 440 nm) under optimal conditions,
demonstrating substituent effect on the catalytic efficiency of
molecular photocatalysts for CO2 reduction. The TADF
compound 4CzIPN was proved to be a promising photo-
sensitizer and showed much better performance than the
popular ruthenium photosensitizer [Ru(bpy)3]2+ in this work.
We believe TADF photosensitizers will contribute more to the
development for photocatalytic CO2 reduction in the future.
Terpyridine-based ligands were found to be reduced during
photocatalytic process. It is also revealed that the catalytic
reaction is strongly promoted by water. These results shed new
light on designing molecular photocatalysts for CO2 reduction.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03503.
■
sı
Experimental details; general synthetic route for bis-
(terpyridine)iron(II) complexes Fe(Ntpy)2, Fe(Htpy)2,
and Fe(Cltpy)2; SWV result and CVs of Fe(Ntpy)2 in
DMF under a N2 atmosphere; isotopic labeling experi-
ment with 13CO2; TOF determination for photocatalytic
CO2RR with Fe(Ntpy)2; UV−Vis−NIR absorption
1
spectra; H NMR spectrum of Fe(Ntpy)2 in DMSO−
d6, Fe(Htpy)2 in DMSO−d6, and Fe(Cltpy)2 in
DMSO−d6; and HR−ESI−MS spectrum of Fe(Ntpy)2,
Fe(Cltpy)2, and Fe(Htpy)2 (PDF).
AUTHOR INFORMATION
Corresponding Author
■
Duobin Chao − School of Materials Science and Chemical
Engineering, Ningbo University, Ningbo, Zhejiang 315211,
China; orcid.org/0000-0001-6424-3683;
Email: chaoduobin@nbu.edu.cn
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Inorganic Chemistry
Authors
Yanan Wang − School of Materials Science and Chemical
Engineering, Ningbo University, Ningbo, Zhejiang 315211,
China
Ting Liu − School of Materials Science and Chemical
Engineering, Ningbo University, Ningbo, Zhejiang 315211,
China
Longxin Chen − School of Materials Science and Chemical
Engineering, Ningbo University, Ningbo, Zhejiang 315211,
China
pubs.acs.org/IC
Article
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03503
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21605013) and Natural Science
Foundation of Ningbo (202003N4077).
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