Journal of the American Chemical Society
Article
by 300 W xenon lamp (LX300F, Japan). Electron spin resonance
spectra (ESR) were collected on a JES-FA200 electron paramagnetic
resonance spectrometer under visible-light irradiation.
reflects that electron transfer, where oxygen adsorption is
crucial as indicated above, is a favorable process in DhaTph-Ni
(Figure 5b). Excitons in DhaTph-Ni are easily dissociated into
Synthesis of DhaTph-M (M = 2H, Ni). The COF sythesis was
performed following previous reports with some minor modifica-
tions.37 Typically, TPh-M (M = 2H or Ni, 0.04 mmol) and Dha (0.08
mmol) in an mixed solvent of o-dichlorobenzene/n-butanol/6 M
HAC (1:1:0.2 mL) were degassed in a glass tube by liquid nitrogen.
The tube was sealed off and heated at 120 °C for 3 or more days. The
solid was isolated by centrifugation and washed with THF and
acetone until the supernatant is colorless. Then the samples were
further dried overnight at 393 K under vacuum for further use.
Synthesis of DhaTph-Zn. The synthetic procedure of DhaTph-
Zn was similar to that of DhaTPh-M (M = 2H, Ni) except for o-
dichlorobenzene/n-butanol/6 M HAC (1:1:0.25 mL) and the amount
of Tph-Zn (0.025 mmol) and Dha (0.05 mmol).
Photoelectrochemical Measurements. Photocurrent measure-
ments were conducted with a CHI 760E electrochemical workstation
(Chenhua Instrument, Shanghai, China) in a standard three-electrode
system with the photocatalyst-coated FTO as the working electrode,
Pt plate as the counter electrode, and an Ag/AgCl as a reference
electrode. A 300 W xenon lamp with a UV cutoff filter (>380 nm) was
used as the light source. EIS was performed with a Zahner Zennium
electrochemical workstation in a conventional three electrode cell,
using a Pt plate as the counter electrode and Ag/AgCl electrode as the
reference electrode. The electrolyte was a 0.1 M Na2SO4 solution.
The 2 mg of catalyst was added into 1 mL of CH3OH and 10 μL of
Nafion mixed solution. Then a 100 μL suspension was dropped on
the surface of a FTO glass and dried at room temperature for
photocurrent measurements, and the signals were recorded under
chopped light at 0.5 V. EIS was performed with a 30 μL suspension
on the working electrode in a frequency range from 10−1 to 105 Hz
with a bias potential of −1.5 V.
TMB Oxidation Measurements. Typically, 5 mg of TMB was
dissolved with 1 mL of H2O and 2 mL of HAc/NaAc buffer solution
(0.2:0.2 M). A total of 200 μL of COF aqueous solution (5 mg/mL)
was then added into the mixture solution with O2 bubbling under
visible light irradiation. The samples were taken at different time
intervals for UV−vis measurements. In order to verify the specific
ROS, various scavengers were added into the TMB solution before
the light irradiation: carotene (10 mg), mannite (2 mg), catalase (2
mg), and superoxide dismutase (SOD, 200 μL), respectively.
NBT Reduction Measurements. Typically, 3.5 mg of nitro blue
tetrazolium (NBT) was ultrasonically dispersed in 1 mL of phosphate
buffer (pH 7). Then 3 mg of Dha-Tph-M (Zn, Ni) was ultrasonically
dispersed in another 1 mL of phosphate buffer (pH 7). The above
two solutions were mixed with O2 bubbling under visible light
irradiation. At different time intervals the samples were taken for
further UV−vis measurements.
charge carriers around the Ni sites, where O2 molecules are
•−
prone to be adsorbed, giving rise to the generation of O2
.
Although O2 molecules are difficult to be adsorbed onto
DhaTph-Zn, the significantly increased concentration of triplet
excitons enables energy transfer, which does not require direct
contact, thereby producing singlet oxygen.
To further get deep insight into the above process, the
calculated charge density difference for DhaTph-Ni (0.19 e)
and DhaTph-Zn (0.15 e) in the presence and absence of O2
suggests that more charges are transferred to O2 over DhaTph-
Ni than DhaTph-Zn (Figure 5c and Table S3).63 Especially,
the depletion of 0.07 e in the Ni center in reference to 0 e in
the Zn center clearly demonstrates that the Ni2+ in the
porphyrin center is prone to promote electron transfer to O2
for the production of O2•−, in good agreement with the above
results.
CONCLUSIONS
■
In summary, the porphyrinic COFs involving diverse metal
ions, DhaTph-M, have been synthesized and exhibit distinctly
different excitonic effects, the regulation of which on COF
photocatalysis has been systematically investigated for the first
time. While the introduction of Zn2+ in the COF gives rise to
the increase of triplet excitons, evidenced by sound
phosphorescence intensity, the Ni2+ promotes exciton
dissociation to charge carriers, as jointly demonstrated by
increased DOS of VBM and photoelectrochemical results. In
line with the above results, the oxidation of α-terpinene in O2
over DhaTph-M affords distinctly different products as
different ROS are evolved with these COFs. The results have
been multiply confirmed by TMB oxidation, NBT reduction
1
and EPR spectra, unambiguously demonstrating that the O2
•−
production by DhaTph-Zn while the generation of O2 by
DhaTph-Ni are preferred via the oxygen activation under light
irradiation. With this knowledge on selective oxygen activation
in mind, the particularly O2 and 1O2-involved catalytic
•−
reactions have been rationally promoted with the help of
DhaTph-Ni and DhaTph-Zn, respectively. This work not only
sheds new light on molecular oxygen activation by porphyrinic
COFs but also provides deep insights into the regulation of
excitonic effects toward improved photocatalysis.
ESR Detection. The ROS generated by COFs have been detected
by ESR in the presence of DMPO and TEMP, respectively. Typically,
30 μL DMPO or TEMP in 1 mL MeOH was mixed with 0.5 mL of
COF/MeOH suspension (2 mg/2 mL). The formed mixture (300−
400 μL) was added into the ESR tube. ESR measurements were
carried out during the light irradiation with a 500 W xenon lamp (λ >
380 nm) under the air conditions.
General Procedure for Photocatalytic Oxidation of α-
Terpinene. Typically, 10 mg of COFs was dispersed in 3 mL of
CH3CN, followed by the addition of 0.1 mmol of α-terpinene in a
two-neck flask (10 mL). The reaction was carried out at ambient
temperature with O2 bubbling under illumination with a 300 W Xe
lamp (λ > 420 nm) and magnetic stirring. Upon reaction completion,
the reaction solution was centrifuged. Then, the conversion and
selectivity were quantified by gas chromatography and the products
were confirmed by GC-MS. The mass of porphyrin linkers was
calculated based on the number of equivalents of Tph units in
DhaTph-M.
MATERIALS AND METHODS
■
Materials and Equipment. All chemicals were purchased from
commercial suppliers. XRD were collected on a Japan Rigaku Miniflex
600 rotation anode X-ray diffractometer equipped with graphite
monochromatized Cu Kα radiation (λ = 1.54 Å). The UV−vis
absorption spectra were recorded on a Shimadzu UV-3600 Plus in the
wavelength range of 200−3600 nm. Nitrogen sorption isotherms were
measured using a Micromeritics ASAP 2020 system at 77 K. Prior to
N2 sorption measurement, the samples were preactivated with
supercritical carbon dioxide. The ICP-AES results were taken on
Thermo scientific iCAP 7400 to confirm the metal contents. The XPS
measurements were performed by a Kratos Axis Supra+ high-
performance electron spectrometer using monochromatized Al Kα
(hν = 1486.6 eV) as the excitation source. The catalytic reaction
products were identified and analyzed by using a Shimadzu gas
chromatograph (GC-2010 Plus with a 0.25 mm × 30 m Rtx-5
capillary column). The products were determined by using the Agilent
technologies 7890A GC and a mass spectrometer Agilent
technologies 5975C as detector. The light irradiation was obtained
General Procedure for Photocatalytic Hydroxylation of
Boronic Acids and Selective Oxidation of Thioanisole.
Typically, 10 mg of COFs was dispersed in 3 mL of CH3CN,
F
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Qian, Yunyang
