Polydopamine nanofilms as visible light-harvesting interfaces for palladium nanocrystal catalyzed coupling reactions

Article abstract of DOI:10.1039/c5cy01330e

Light-harvesting material is of great importance for the efficient use of solar energy to drive catalytic reactions in chemistry. We herein report mussel-inspired polydopamine nanofilms as light-harvesting interfaces for heterogeneous palladium catalyzed coupling reactions under irradiation from visible light. Our strategy includes the in situ growth of palladium nanocrystals on a polydopamine nanofilm to prepare photocatalysts and photocatalytic Suzuki coupling reactions involving a broad range of aryl bromides/iodides and arylboronic acid substrates. The polydopamine based photocatalysts could be supported on various carriers regardless of size or morphology, thus they are easily recycled and reused. A plausible photocatalytic mechanism has been proposed including light-harvesting, photoelectron-hole separation and transfer processes. This strategy also has potential for other noble metals such as platinum, indium and gold photocatalytic organic reactions such as couplings, reductions and oxidations.

Full text of DOI:10.1039/c5cy01330e

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Polydopamine nanofilms as visible light-harvesting
interfaces for palladium nanocrystal catalyzed
coupling reactions†
Cite this: DOI: 10.1039/c5cy01330e
ab
c
*
*
Aming Xie,
Kun Zhang, Fan Wu, b Nana Wang,b Yuan Wangb
ab
*
and Mingyang Wang
Light-harvesting material is of great importance for the efficient use of solar energy to drive catalytic reac-
tions in chemistry. We herein report mussel-inspired polydopamine nanofilms as light-harvesting interfaces
for heterogeneous palladium catalyzed coupling reactions under irradiation from visible light. Our strategy
includes the in situ growth of palladium nanocrystals on a polydopamine nanofilm to prepare photo-
catalysts and photocatalytic Suzuki coupling reactions involving a broad range of aryl bromides/iodides and
arylboronic acid substrates. The polydopamine based photocatalysts could be supported on various carriers
regardless of size or morphology, thus they are easily recycled and reused. A plausible photocatalytic
mechanism has been proposed including light-harvesting, photoelectron-hole separation and transfer pro-
cesses. This strategy also has potential for other noble metals such as platinum, indium and gold photo-
catalytic organic reactions such as couplings, reductions and oxidations.
Received 14th August 2015,
Accepted 12th October 2015
DOI: 10.1039/c5cy01330e
www.rsc.org/catalysis
Polydopamine (PDA) (Fig. 1), a black biopolymer mimic
inspired by mussels, shellfish and geckos, has displayed a
Introduction
Efficient harvesting of solar energy, a clean, abundant, and
sustainable energy source, is of great importance in the future
exploitation of new energies.1 Up to now, there have been
mainly three technologies used for the capture and conver-
sion of solar energy: photovoltaics, solar heating, and solar
thermal electricity.2–4 Apart from these technologies, the use
of sunlight to drive chemical reactions, often called photo-
catalysis, is another important means of harvesting solar
energy and shows great potential in various chemical pro-
cesses, such as the photocatalytic degradation of pollutants
where semiconductors are typically utilized to generate
electrons/holes under light irradiation,5,6 organic reactions
photocatalyzed by Mott–Schottky heterojunctions,7 Au–Pd
alloys,3,8,9 and Pd nanostructures.10,11 Normally, photo-
catalysis processes prefer efficient photogeneration and sepa-
ration of electron–hole pairs which are the key factors for
photosynthesis as well as light-driven chemical reactions.12–14
very broad range of applications in the fields of energy, the
environment, and biomedicine, since its first use as a coating
material in 2007.15–20 As a synthetic eumelanin polymer, PDA
possesses a capability for light absorption under a broadband
spectrum from the UV to visible region.21–24 Its photoconduc-
tivity can be strongly enhanced under the radiation of visible
light, implying an increase of photogenerated free electrons
and electron holes.25 As a catechol derivative, PDA can effec-
tively transfer photoinduced electrons and protons which is
crucial for artificial photocatalysis systems, thus significantly
improving the photocatalytic activity for systems such as
a School of Mechanical Engineering, Nanjing University of Science & Technology,
Nanjing 210094, China. E-mail: aminghugang@126.com, wmyrf@163.com
b State Key Laboratory for Disaster Prevention & Mitigation of Explosion &
Impact, PLA University of Science and Technology, Nanjing 210007, China.
E-mail: wufanjlg@163.com
c School of Chemical Engineering, Nanjing University of Science & Technology,
Nanjing 210094, China
† Electronic supplementary information (ESI) available: Preparation process of
carbonized loofah (CL), catalysts recovery experiments, XRD results, NMR data
and NMR spectra. See DOI: 10.1039/c5cy01330e
Fig. 1 The structure of mussel-inspired PDA.
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photochemical oxidation of water, and the photodegradation
of organic dyes, though independent photoactive reagents
were required.26,27 The photoexcited electron in PDA can be
injected into wide band gap semiconductors such as TiO2
nanoparticles, resulting in enhanced photovoltaic conversion
under solar light.28 All the results above sufficiently demon-
strate that PDA can provide efficient photogeneration, separa-
tion as well as transfer of electron–hole pairs under irradia-
tion of visible light, implying a possibility of electron transfer
to the conduction band of noble metals such as Pd, Pt, Rh,
and Ir because of the zero energy gap. The light energy trans-
formation of PDA may significantly improve the catalytic
activity of noble metals and make PDA a potential light
harvesting material for photocatalysis. However, to date, the
use of PDA to effectively harvest and transform light energy
for photocatalyzed reactions has been largely overlooked.
Recently, by virtue of their facile recyclability and high
efficiency, heterogeneous Pd nanocrystal (PN) catalysts have
been extensively developed for coupling reactions, such as
the loading of PNs on carbon materials including graphene,29
polymers including polyIJN-isopropylacrylamide),30 and metal
oxides including alumina oxides.31 However, elevated temper-
atures and prolonged reaction times are required for these
catalytic systems because the rate of the rate-limiting steps is
restricted by the high activation barrier of the substrates,
thus leading to undesired side products and the instability of
the catalysts. An ideal strategy to circumvent this insuffi-
ciency may be by using the energy of visible light to activate
the reactants or the catalysts at lower operating tempera-
tures.32,33 Herein, we describe a strategy to execute heteroge-
neous photocatalytic coupling reactions using PDA nanofilm
as a light harvesting and transformation interface and PNs as
a noble metal catalyst. In our strategy, photogenerated
electron–hole pairs in the PDA under irradiation of visible
light separate along the PDA networks and then the electron
transfers to the conduction band of the PNs resulting in
improved catalytic activity. To execute this strategy, we first
decorated the PDA nanofilm on various carriers including
carbonaceous materials, inorganic oxides or organic poly-
mers, then supported PNs on the surface of the PDA nano-
film by an in situ growth pathway, and finally used Suzuki
coupling reactions as an example to demonstrate the effi-
ciency and mechanism of this photocatalysis system.
catalyst. Using silica gel (SG) and polyurethane sponge (PS),
respectively, Pd@PDA-SG and Pd@PDA-PS were prepared
according to a similar process. The N and OH groups in PDA
can offer chelating binding sites for noble metals, resulting
in enhanced affinity toward the metal surface. Hence, the
loading amounts of the Pd NPs increased noticeably, indicat-
ing an elevated utilization ratio of the Pd source under the
same conditions (see Table S1 in ESI†). The Fourier trans-
form infrared (FT-IR) spectra showed the appearance of O–H
or N–H (around 3300 cm−1) and N–H (1270 cm−1) groups on
the PDA skeleton and the apparent disappearance of peak
(1410 cm−1) belonging to the C–H bond of CL, implying the
successful modification of the PDA thin film on CL (Fig. 2a). In
Fig. 2b, the band around 3300 cm−1 was designated to the O–H
or N–H group and was significantly strengthened for the PDA-
SG, as well as the appearance of the peak at 1512 cm−1 which
was attributed to the indole ring vibration of the PDA skele-
ton, indicating the existence of PDA on the SG.37 Fig. 2c and d
show the SEM images of Pd@PDA-CL and Pd@PDA-SG,
respectively. It was found that Pd particles distributed on the
surfaces of CL and SG with sizes smaller than 100 nm.
To further explore the morphologies, sizes and crystals of
the Pd nanoparticles (NPs) supported on the carriers, high-
resolution transmission electron microscopy (HRTEM) imag-
ing was used and the images are shown in Fig. 3. The snail-
shaped PNs distributed evenly on the PDA-modified surface
of the lumpy CL with sizes ranging from 10 to 30 nm
(Fig. 3a and c), while on the SG the PNs were spheres with an
average size of 20 nm (Fig. 3b and d). It can be inferred from
the crystal lattice pattern that the PNs might be composed of
randomly oriented small nanocrystals instead of a single crys-
tal when CL was employed as the carrier (Fig. 3e). Contrast-
ingly, the SG benefited from the formation of regular PNs
(Fig. 3b and d). These results can be attributed to the differ-
ent surface topographies which influenced the growth pro-
cesses of the PNs. The selected area electron diffraction
(SAED) patterns also indicated that the PNs were polycrystals
rather than single crystals, regardless of the carrier
Results and discussion
Through the formation of a thin nanofilm, PDA was a versa-
tile agent for the surface functionalization of various mate-
rials including metals, carbonaceous materials, oxides, and
polymers, regardless of their size or morphology.34,35 As a
result, noble metal PNs could easily nucleate in situ and grow
on the PDA-modified surfaces.36 To simplify the recycling of
the catalysts, we first decorated PDA nanofilm onto the sur-
face of CL, then supported PNs on the resulting PDA surface
through an in situ nucleation and growth process which was
similar to the previous work,3 and finally prepared a Pd@PDA-CL
Fig. 2 (a) FT-IR spectra of CL, PDA-CL and Pd@PDA-CL; (b) FT-IR
spectra of SG, PDA-SG and Pd@PDA-SG; (c) SEM image of Pd@PDA-
CL; (d) SEM image of Pd@PDA-SG.
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light was enhanced enormously for Pd@PDA-SG (Fig. 4b),
indicating an excellent light-harvesting ability for the PDA
nanofilm. The electron paramagnetic resonance (EPR)
showed an increased intensity of Pd@PDA-CL under visible
light irradiation compared to Pd@CL (Fig. S2 in ESI†), indi-
cating a longer lived photogenerated electron–hole pair
inside Pd@PDA-CL than in Pd@CL.
The coupling of iodobenzene with phenylboronic acid was
chosen as a model reaction to explore the role of PDA and
the photocatalytic activity of the Pd@PDA catalyst. The cou-
pling reaction proceeded well under white LED light irradia-
tion whereas only trace product was obtained in the dark
(Table 1, entries 1 and 2). To avoid the photoheating effect, a
control experiment under water bath was employed, but no
apparent difference was observed (Table 1, entry 3). No reac-
tions took place when the PNs were removed (Table 1, entry
4). These results demonstrate a positive photocatalytic effect
emerged during the coupling reaction. Nonplasmonic Pd NPs
can exhibit apparent absorption of visible irradiation through
the bound electrons and resulting excited conduction
electrons, thus as a result they display significant photocata-
lytic activity.11 Nevertheless, supporting PNs on a carbon
material CL to form Pd@CL catalyst severely destroyed the
photocatalytic activity without loss of the thermocatalytic
activity which was similar to the commercial Pd@C catalyst
(Table 1, entries 5–8), whereas Pd NPs supported on ZrO2
powder did not show decreased photocatalytic activity.11 This
phenomenon may have originated from the strong absorp-
tion of light by the black CL which prevents the light being
harvested by the PNs and thus negligible light absorption at
Fig. 3 (a, c, e) HRTEM images, (g) SAED pattern of Pd@PDA-CL; (b, d,
f) HRTEM images, (h) SAED pattern of Pd@PDA-SG.
Table 1 Photocatalytic Suzuki coupling reactions using Pd@PDA
catalysta
(Fig. 3g and h). The X-ray scattering pattern (XRD) showed
that the supported PNs possessed consistent crystal faces
(111), (200), and (220) with single-crystalline Pd nanoparticles
(Fig. S1 in ESI†).38
Entry
X
Catalyst
Light
Yieldb (%)
Next we used UV/vis diffuse reflection (UV/vis DR) spectra
to study the visible light absorption of the Pd@PDA photo-
catalysts. Normally, nonplasmonic Pd NPs show a mild
absorption toward visible light.11 After supporting on PDA,
the absorption intensity was increased slightly for Pd@PDA-
CL (Fig. 4a), because the CL material was black there was
excellent light absorption ability. To avoid the interference of
the carrier, white particle SG, with a negligible absorption in
the visible region, was used and the absorption toward visible
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
Acetyl
Acetyl
Acetyl
Pd@PDA-CL
Pd@PDA-CL
Pd@PDA-CL
PDA-CL
Pd@CL
Pd@C
−
+
+
+
+
+
−
−
+
+
−
+
+
+
+
Tracec
96
94d
0
Trace
17e
92f
90f
86g
92
Pd@PDA-CL
Pd@CL
9
Pd@PDA-CL
Pd@PDA-SG
Pd@PDA-SG
Pd@PDA-PS
Pd@PDA-CL
Pd@PDA-CL
Pd@PDA-CL
10
11
12
13
14
15
58
90
97
68h
0i
a
Reaction conditions: iodobenzene (1 mmol), phenylboronic acid
(1.1 mmol), K2CO3 (1.5 mmol), catalyst (contained 0.6 mg of Pd),
DMF/H2O (3 mL 3 mL−1), white LED lamp (1.2 W cm−2), room
b
c
d
temperature, 2 h. Isolated yields. In the dark, 6 h. Water bath.
e
g
f
Commercial 5 wt% Pd@C catalyst (12 mg). Carried at 100 °C.
Catalyst (contained 0.3 mg of Pd). Equivalent TEMPO was added,
h
Fig. 4 IJa) UV/vis DR spectra of CL, PDA-CL and Pd@PDA-CL; (b) UV/
vis DR spectra of SG, PDA-SG and Pd@PDA-SG.
54% yield of 1-([1,1′-biphenyl]-4-yl)ethanone and 14% yield of 1,1′-
([1,1′-biphenyl]-4,4′-diyl)diethanone. i Equivalent DIPEA was added.
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wavelengths longer than 370 nm. It can also be inferred that
the black CL cannot generate efficient photoexcited electrons
under irradiation of visible light. The catalytic effect
decreased with the reduction of the amount of catalyst
(Table 1, entry 9). We also used another carrier, SG, to sup-
port the PNs, and obviously, the employment of PDA signifi-
cantly enhanced the light absorption and transformation for
the PN catalyzed coupling reaction (Table 1, entries 10 and
11), which indicates enhanced photoexcitation and transfer
of electron–hole pairs on the interface between PDA and the
PNs. Under the same conditions, PS with three dimensional
networks can also be used as the carrier for the PNs, imply-
ing a minor influence due to the carriers (Table 1, entry 12).
In addition, some control experiments were performed,
employing the use of radical and hole scavenger reagents, to
better understand the mechanism of the photocatalytic cou-
pling (Table 1, entries 13–15). To distinguish the homo-
coupled products, 1-(4-iodophenyl)ethanone was taken as the
model substrate instead of iodobenzene. The coupling reac-
tion was largely blocked with the addition of a radical
scavenger 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and a
small amount of homocoupled product 1,1′-([1,1′-biphenyl]-
4,4′-diyl)diethanone appeared, suggesting aggregation of the
1-(4-iodophenyl)ethanone at the interface of the PDA which
can preferentially accept active electrons from the PNs
(Table 1, entry 14). This aggregation effect can be observed
by stirring the reaction mixture for 5–10 min. Via addition of
the hole scavenger reagent diisopropylethylamine (DIPEA),
where the hole was blocked, no target product 1-([1,1′-
biphenyl]-4-yl)ethanone was obtained as well as no homo-
coupled side product (Table 1, entry 15). This phenomenon
may be attributed to the stronger affinities of DIPEA to the
holes which prevents the combination of aryl arylboronic
acid with the hole.
positive charges on the PDA) due to the smaller work func-
tion of PDA than most of the noble metals including Pd.39
Due to the excellent affinity of PDA toward organic and inor-
ganic materials, iodobenzene molecules aggregate near the
PDA interface and can be easily attacked by the PN’s active
center which leads to the formation of an aryl complex with
Pd. Meanwhile, the holes inside the PDA film activate the
C–B bond of the electronically negative BIJOH)3− species,
which is produced from the capture of OH− in the basic reac-
tion medium by phenylboronic acid, and another aryl Pd
complex is formed. Finally, the coupling product is formed
via the reductive elimination and transmetalation processes
which take place in typical Pd-catalyzed Suzuki reactions.3,40
To investigate the reaction scope, a series of aryl halides
bearing a broad range of substituents have been employed to
test the photocatalytic activity of Pd@PDA. As shown in
Table 2, the substrates bearing electron-donating groups such
as –OCH3 and electron-withdrawing groups such as –COCH3
can react with phenylboronic acid smoothly to furnish the
corresponding coupling products in excellent yields (Table 2,
entries 1–3). Prolonged time was required for the substrate
1-(2-iodophenyl)ethanone to obtain a high yield of product
(Table 2, entry 4). This phenomenon can be attributed to the
increased steric effect of the –COCH3 at the ortho-position.
Due to the –NH2 group’s ability to attack the catechol groups
in PDA,35 the substrate 4-iodoaniline was not suitable for our
photocatalytic system (Table 2, entry 5). Similar to DIPEA, –
NIJCH3)2 can also capture the photoexcited holes preferen-
tially, thus preventing the reaction of phenylboronic acid
with 4-iodo-N,N-dimethylaniline (Table 2, entry 6). Apart from
the aryl iodides, various aryl bromides bearing substitutes
regardless of electron-donating groups or electron-
withdrawing groups can be used as the substrates in our
photocatalytic system (Table 2, entries 7–14). –F cannot be
replaced by an aromatic ring under the irradiation of visible
light because only 4-fluoro-1,1′-biphenyl was isolated (Table 2,
entry 7). The existence of naphthalene moieties did not influ-
ence the photocatalytic coupling reaction (Table 2, entries 13
and 14). The substrates bearing N-heterocycle moieties
showed lower reaction activities toward phenylacetylene, par-
tially owing to the self-substitution- reactions between the N
and Br atoms (Table 2, entries 15 and 16).
We proposed a plausible mechanism for the photo-
catalyzed Suzuki coupling reactions based on the observa-
tions from the experiments above and previous research
(Fig. 5). As a semiconductor with a narrow band gap,28 PDA
film first absorbs visible light and generates electron–hole
pairs. The photoexcited electrons in the PDA then diffuse to
the PNs and lead to the separation of the excited charges
(accumulation of negative charges on the Pd metal whereas
In addition, various aryl boronic acids have been used to
explore the scope of our photocatalytic system. As shown in
Table 3, aryl boronic acids bearing electron-rich groups and
electron-deficient groups smoothly react with iodobenzene to
form the corresponding coupling products in excellent yields
(Table 3, entries 1–6). –F and –Cl can be retained under the
reaction condition, indicating that this technique was suit-
able for aryl fluorides/chlorides (Table 3, entries 7–8). Fur-
thermore, 1,4-phenylenediboronic acid was an appropriate
substrate in our photocatalytic system, implying tremendous
potential in noble metal catalyzed polymerization (Table 3,
entry 9). The Pd@PDA-CL catalyzed coupling reaction cannot
be prevented when bromobenzene was employed instead of
iodobenzene (Table 3, entries 1, 3, 5, 7 and 10). For
Fig. 5 Proposed mechanism for the photocatalytic Suzuki coupling at
the interface of the PDA and Pd NPs.
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Table 2 The scope of aryl halides for photocatalytic Suzuki coupling reactionsa
Entry
1
Aryl halide
Product
Yieldb (%)
95
2
3
96
90
4
98c
5
6
7
8
0d
0d
99
92
9
93
95
10
11
99
12
13
14
96
91
99
15
16
30
0
a
Reaction conditions: aryl halide (1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol), Pd@PDA-CL (20 mg, 3 wt% Pd content), DMF/
H2O (3 mL 3 mL−1), white LED lamp (1.2 W cm−2), room temperature, 2 h. Isolated yields. 10 h. No coupling product was monitored by
b
c
d
TLC.
S-heterocycle boronic acids a moderate yield of coupling
product can be isolated (Table 3, entry 10). However, the reac-
N-heterocycle moiety cannot take place because the reduced
electron density prevents the combination of boric acid with
the photogenerated holes (Table 3, entry 11). After carrying
tion of bromobenzene with
a
substrate bearing an
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Table 3 The scope of aryl boronic acids for photocatalytic Suzuki coupling reactionsa
Entry
1
Aryl boronic acid
Product
Yieldb (%)
93c, 90d
2
88c
3
4
99c, 96d
93c
5
6
90c, 94d
98c
7
8
91c, 88d
95c
9
97c
75d
10
11
0d
a
Reaction conditions: aryl halide (1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol), Pd@PDA-CL (20 mg, 3 wt% Pd content), DMF/
H2O (3 mL 3 mL−1), white LED lamp (1.2 W cm−2), room temperature, 2 h. Isolated yields. Iodobenzene was used as the substrate.
b
c
d
Bromobenzene was used as the substrate.
out the photocatalytic reaction, the Pd@PDA-CL catalyst can
be easily recycled by filtration and washing with solvents
without significant loss of Pd (Table S2 in ESI†). The recycled
catalyst can be reused without apparent reduction of photo-
catalytic activity no less than five times (Table S2 in ESI†).
General procedure for the growth of PNs on the PDA nanofilm
Taking the CL carrier as an example: CL powder was added
to a solution of dopamine (2 mg mL−1) in 10 mM Tris-HCl
(pH = 8.5) with stirring for 24 hours at room temperature.
After filtration and washing with deionized water and ace-
tone, a PDA nanofilm decorated CL material (PDA-CL)
was obtained. Then, PDA-CL (1 g) was dispersed in water
(100 mL) and mixed with an acetone solution (50 mL) of
PdIJOAc)2 (100 mg). After stirring at 90 °C for 3 h, the
resulting Pd@PDA-CL photocatalyst was obtained by filtra-
tion, washing with hot water and acetone and drying
under vacuum. Other photocatalysts such as Pd@PDA-SG
using silica gel (SG) instead of CL was produced through a
similar process. ICP analysis: 3.2 wt% Pd for Pd@PDA-CL
and 4.7 wt% Pd for Pd@PDA-SG.
Experimental
Chemicals and materials
All reagents and solvents were purchased from GENERAL-
REAGENT, Titan Scientific Co., Ltd., Shanghai, China and
used without purification. Carbonized loofah (CL) was pre-
pared by the process detailed in the ESI.† Distilled water was
obtained from Direct-Q3UV, Millipore. Visible light was
obtained for photocatalysis using two white LED lamps (12 W)
purchased from Philips Co., Ltd.
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General procedure for the photocatalytic Suzuki coupling
reaction by Pd@PDA photocatalysts
Impact (DPMEIKF201310) for financially supporting this
research.
Taking Pd@PDA-CL as an example: a mixture of iodobenzene
(1 mmol), phenylboronic acid (1.1 mmol), K2CO3 (1.5 mmol) Notes and references
and 20 mg Pd@PDA-CL (3.2 wt% Pd content) in 3 mL of
1 N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A.,
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sphere with two white LED lamps (12 W) for a certain period
of time. After the reaction, the catalyst was recycled by filtra-
tion and the organic phase of the filtrate was extracted with
EtOAc, washed three times with water and dried over Na2SO4.
The pure product was then isolated by silica chromatography
using petroleum ether/EtOAc mixtures as the eluent.
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UV-vis diffuse reflectance spectra were recorded at room temper-
ature on a Thermo Fisher Scientific EV220 spectrophotometer.
NMR measurements were recorded on Bruker AVANCE 500 (III)
systems using CDCl3 as the solvent. FT-IR spectra were recorded
on a Nicolet iS10 FTIR instrument (Thermo Fisher Scientific,
USA). The detailed morphologies of the photocatalysts were
observed with a field emission scanning electron microscope
(FE-SEM, S4800, Hitachi) and a field emission high resolution
transmission electron microscope (FE-HRTEM, Tecnai G2 F20,
FEI). Inductively coupled plasma analysis (ICP) was performed
on a spectrometer (Optima 7300 V, PerkinElmer). The crystal
structure data of the as-synthesized samples was obtained from
an X-ray diffractometer (XRD, D8 Advance, Bruker AXS) from
10° to 80°, using Cu Kα (λ = 1.54 Å) radiation. The electron para-
magnetic resonance (EPR) spectra were obtained on a spectro-
meter (A300-10/12, Bruker). Flash column chromatography was
performed by employing 200–300 mesh silica gel. Thin-layer
chromatography (TLC) was performed with silica gel HSGF254.
Conclusions
In summary, mussel-inspired polydopamine nanofilm has been
developed as a light-harvesting interface for heterogeneous pal-
ladium catalyzed coupling reactions under irradiation of visible
light. Photocatalysts have been prepared by an in situ growth of
palladium nanocrystals on polydopamine nanofilms and used
for photocatalytic Suzuki coupling reactions involving a broad
range of aryl bromides/iodides and arylboronic acid substrates.
The polydopamine based photocatalysts could be supported on
various carriers regardless of their size or morphology, and are
easily recycled and reused. A plausible photocatalytic mecha-
nism has been proposed including light-harvesting, photoelec-
tron–hole separation and transfer processes. This strategy also
has potential for other noble metals such as platinum, indium
and gold photocatalytic organic reactions such as couplings,
reductions and oxidations.
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