Self-assembled bilayers as an anchoring strategy: Catalysts, chromophores, and chromophore-catalyst assemblies

Article abstract of DOI:10.1021/jacs.9b01044

Anchoring strategies for immobilization of molecular catalysts, chromophores, and chromophorecatalyst assemblies on electrode surfaces play an important role in solar energy conversion devices such as dyesensitized solar cells and dye-sensitized photoelectrosynthesis cells. They are also important in interfacial studies with surface-bound molecules including electron-transfer dynamics and mechanistic studies related to small molecule activation catalysis. Significant progress has been made in this area, but many challenges remain in terms of stability, synthetic complexity, and versatility. We report here a new anchoring strategy based on selfassembled bilayers. This strategy takes advantage of noncovalent interactions between long alkyl chains chemically bound to a metal-oxide electrode surface and long alkyl chains on the molecule being anchored. The new methodology is applicable to the heterogenization of both catalysts and chromophores as well as to the in situ "synthesis" of chromophore-catalyst assemblies on the electrode surface.

Full text of DOI:10.1021/jacs.9b01044

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Communication
Self-assembled bilayers as an anchoring strategy: catalysts,
chromophores and chromophore-catalyst assemblies
Lei Wang, Dmitry E. Polyansky, and Javier J. Concepcion
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 May 2019
Downloaded from http://pubs.acs.org on May 7, 2019
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Self-assembled bilayers as an anchoring strategy: catalysts,
chromophores and chromophore-catalyst assemblies
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Lei Wang, Dmitry E. Polyansky, Javier J. Concepcion*
Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973, United States.
Supporting Information Placeholder
ABSTRACT: Anchoring strategies for immobilization of
molecular catalysts, chromophores and chromophore-catalyst
assemblies on electrode surfaces play an important role in solar
energy conversion devices such as dye-sensitized solar cells and
dye-sensitized photoelectrosynthesis cells. They are also important
in interfacial studies with surface-bound molecules including
electron-transfer dynamics and mechanistic studies related to small
molecule activation catalysis. Significant progress has been made
in this area, but many challenges remain in terms of stability,
synthetic complexity and versatility. We report here a new
anchoring strategy based on self-assembled bilayers. This strategy
takes advantage of non-covalent interactions between long alkyl
chains chemically bound to a metal-oxide electrode surface and
long alkyl chains on the molecule being anchored. The new
methodology is applicable to the heterogenization of both catalysts
and chromophores as well as to the in-situ “synthesis” of
chromophore-catalyst assemblies on the electrode surface.
introduced. These issues become particularly burdensome for
chromophore-catalyst assemblies where the synthesis involves
multiple steps even without introducing anchoring groups. Many of
these anchoring strategies were recently summarized by Brudvig
14
and coworkers. More recent strategies combine chemically bound
17-19
species and non-covalent interactions such as π-π stacking
and
2
0
electrostatic interactions.
We present here a new anchoring strategy using self-assembled
bilayers (SABs) stabilized via non-covalent interactions of long
alkyl chains. This strategy is very versatile, significantly simplifies
synthesis, enhances stability and it is applicable to catalysts,
chromophores and chromophore-catalyst assemblies. Our strategy
builds on the extensively studied self-assembled monolayers
(
SAMs), typically with long thiol-terminated alkyl chains on gold
21-29
electrodes.
Related SAMs were prepared on metal oxide
) using long alkyl chains (R) terminated on
anchoring groups (R−COOH, R−PO(OH) , R−Si(OH) ), Figure 1
left). The resulting SAM@MO was then immersed in a solution
electrodes (MO
x
2
3
(
x
containing a molecular catalyst where at least one of the ligands is
functionalized with a long alkyl chain as well (Cat−R′). The long
alkyl chains of the catalyst self-assembled with those of the
Catalysis plays a fundamental role in many naturally-occurring and
artificial chemical transformations. It has been classified into two
SAM@MO
alkyl chains to give the final self-assembled bilayer
Cat−SAB@MO , Figure 1 (right).
x
stabilized by non-covalent interactions between the
1
major
categories:
heterogeneous
and
homogeneous.
Heterogeneous catalysis takes place at or near the interface of two
different phases (solid-liquid, solid-gas) while homogenous
catalysis takes place in a single phase, usually the liquid phase.
Arguments can be made in favor or against each type.
Homogeneous catalysts are highly selective and highly tunable but
oftentimes lack long-term stability and product separation is
difficult since all the components coexist in the same phase.
Heterogeneous catalysts are usually more robust and allow easy
separation of reactants and products from the catalyst. On the other
hand, tunability and mechanistic understanding for heterogeneous
catalysts are more difficult compared to homogeneous catalysts due
to their structural complexity. In recent years, heterogeneous
catalysts are being “downsized” to the point that they are becoming
“molecular” in order to achieve higher activity and/or better atom
x
Cat
Cat
Cat
Cat-SAB@MO
x
O
O
N
N
Ru
O
N
SAM@MO
x
O
N
HO
HO
P
O
Figure 1. Left: Self-assembled monolayer on a metal oxide
2
-5
economy
heterogenized” on surfaces. For the latter, anchoring strategies are
necessary, including electropolymerization and anchoring groups
while homogeneous catalysts are being
(
(
SAM@MO
Cat−SAB@MO
x
). Right: Self-assembled bilayer on a metal oxide
; Cat = catalyst).
“
x
6
-16
such as carboxylates and phosphonates. Synthetic procedures to
introduce and to de-protect these groups are often challenging and
are required to be tolerated by the molecules where they are being
We have applied the strategy depicted in Figure 1 to water
oxidation catalysts of the type [Ru(tpy)(4,4'-R -bpy)(OH )] ,
2
+
2
2
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2
+
[
Ru(R
2
-bimpy)(4,4'-R
2
-bpy)(OH
2
)] , and [Ru(bda)(L)
2
] (tpy is
chromophore-SAM on the surface of the electrode (SAM-
C@MO ). The resulting SAM-C@MO is then immersed in a
1
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3
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9
1
1
1
1
1
1
1
1
1
1
2
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2
2
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2,2':6',2''-terpyridine;
R
2
-bimpy is 2,6-bis(1-alkyl-1H-
x
x
benzo[d]imidazol-2-yl)pyridine; bpy is 2,2'-bipyridine; bda is 2,2'-
bipyridine-6,6'-dicarboxylate; L is a monodentate ligand such as 4-
alkylpyridine (4-R-py) and 6-alkylisoquinoline (6-R-isq)). In the
solution containing a molecular catalyst where one of the ligands is
functionalized with a long alkyl chain. The long alkyl chains of the
catalyst self-assemble with the long alkyl chains of the
2
+
case of [Ru(R
were easily introduced either in the tridentate (R
bidentate (4,4'-R -bpy) ligand (see Supporting Information). For
catalysts of the type [Ru(bda)(L) ], the long alkyl chains were
2
-bimpy)(4,4'-R
2
-bpy)(OH
2
)] , the long alkyl chains
chromophore in the SAM-C@MO
assembly self-assembled bilayer Cat−SAB-C@MO .
x
to give a chromophore-catalyst
This
2
-bimpy) or the
x
2
methodology allows the easy integration of catalysts and
chromophores where the distance between them is controlled by
the length of the alkyl chains.
2
introduced on the axial ligands using various synthetic procedures.
SAM@FTO (FTO is fluoride-doped tin oxide) were prepared by
immersing FTO slides in a hexanes solution of octylphosphonic
acid (OPA@FTO) or octyltrimethoxysilane/HOAc (OTS@FTO).
The obtained SAMs were then immersed in a solution of
We have synthesized the phosphonated chromophore [Ru(4,4'-
2+
(
2 2 2 9 19 2
PO(OH) ) -bpy) (4,4'-(C H ) -bpy)] and used it to prepare a
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
SAM–C@FTO. Figure 4 shows a CV and a square wave
voltammogram of the SAM–C@FTO in 0.1 M HClO
4
.
[
Ru(bda)(4-C10
overnight. Figure 2 (left) shows a cyclic voltammogram (CV) of
the resulting Cat–OPA@FTO in 0.1 M HClO . Waves for the
redox processes are clearly observed at 0.67 and
.94 V, respectively (all redox potentials reported here are vs NHE)
H
21-py)
2
]
in
1:1
dichoromethane:hexanes
4
III/II
IV/III
Ru
0
and Ru
and the electrochemical behavior observed closely resembles that
of a phosphonated version of the catalyst chemically bound to
11
III/II
metal oxide electrodes. For the Ru
wave the peak to peak
separation of 27 mV is also similar to analogous catalysts directly
bound to the electrode, indicative of a relatively fast heterogenous
1
1
electron transfer rate for the catalyst in the SAB. Similar behavior
can be observed for the silane-based SAB (Cat–OTS@FTO) shown
in Figure 2 (right).
Figure 3. Self-assembled bilayer of a chromophore-catalyst
assembly on a metal oxide (Cat-SAB-C@MO ; C = chromophore).
x
10.0
.0
.0
5.0
10.0
20.0
5.0
0.0
.0
0.0
5.0
The typical reversible RuIII/II surface redox wave at 1.33 V indicates
the presence of the chromophore anchored to the FTO slide. Plots
of log(i ) vs log(ν) for various SAM–C@FTO with different
p
lengths of the chromophore’s long chains display slopes close to 1
as expected for a surface wave, Figure S3.
1
5
1
5
0
-
-
-10.0
-
-15.0
3
0.0
0.0
0.0
0.0
10.0
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
0.0
-20.0
-40.0
Voltage (V vs NHE)
Voltage (V vs NHE)
2
1
-60.0
Figure 2. CV of [Ru(bda)(4-C10
2
H21-py) ] in a Cat–OPA@FTO
-80.0
SAB (left) and in a Cat–OTS@FTO SAB (right). Conditions: 0.1
-
-100.0
-120.0
-1
4
M HClO , 100 mVs .
-20.0
-140.0
1
1, 30
-30.0
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
Voltage (V vs NHE)
Voltage (V vs NHE)
As mentioned above this anchoring approach is very versatile and
applicable to a variety of catalysts. Figure S1 (top) shows a CV for
Figure 4. CV (left) and square wave voltammogram (right) of
2
+
[
Ru(4'-Ph-tpy)(4,4'-(C
9
H
19
)
2
-bpy)(OH
2
)] in a Cat–OPA@FTO
2+
[Ru(4,4'-(PO(OH)
C@FTO. Conditions: 0.1 M HClO
2
)
2
-bpy)
2
(4,4'-(C
9
H
19
)
2
-bpy)]
in
SAM–
SAB in 0.1 M HClO
4
. After 40 consecutive scans (Figure S1,
-1
4
, 100 mVs .
bottom left) the peak current appears to be decreasing but a second
run of 40 consecutive scans (Figure S1, bottom right) shows the
same initial peak current. This apparent decrease in current seems
to be associated with structural reorganization of the SAB or with
counterion diffusion in and out of the bilayer and it is reversible at
pH 1. A related “reorganization” takes place at pH 6.8 after 40
consecutive scans (Figure S2, top right) but in this case the process
is not reversible with the final CV (Figure S2, bottom left) closely
resembling the one for the complex in solution (Figure S2, bottom
right) a clear indication of the integrity of the complex being
retained. These experiments also show the high stability of the
SABs even at pH ~ 7, a significant improvement over directly
anchored catalysts with carboxylate or phosphonic acid anchoring
groups.
Immersion of the SAM–C@FTO in a solution of [Ru(bda)(4-
] in 1:1 dichoromethane:hexanes results in the SAB
10 2
C H21-py)
Cat–SAM–C@FTO due to non-covalent interactions between the
nonyl long chains in the chromophore and the decyl long chain in
the axial ligands of the catalyst. Figure 5 shows a CV and a square
4
wave voltammogram of the Cat–SAM–C@FTO in 0.1 M HClO .
0.0
2
0.0
.0
20.0
40.0
-60.0
-20.0
40.0
-60.0
80.0
100.0
120.0
0
-
-
-
-
-
-
1
.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2
Applications in dye-sensitized photoelectrosynthesis cells
Voltage (V vs NHE)
Voltage (V vs NHE)
(
DSPEC) and other potential applications in photocatalysis require
1
0, 15, 31-40
the combination of chromophores and catalysts.
The new
Figure 5. CV (left) and square wave voltammogram (right) of Cat–
SAM–C@FTO (C is [Ru(4,4'-(PO(OH) -bpy) (4,4'-(C
]). Conditions: 0.1 M
anchoring strategy based on SABs can also be used to prepare
chromophore-catalyst assemblies as depicted in Figure 3. In this
case, a chromophore containing both anchoring groups and long
alkyl chains is loaded onto the metal oxide electrode creating a
2
)
2
2
9 19 2
H ) -
2
+
bpy)] ; Cat is [Ru(bda)(4-C10H21-py)
2
-1
4
HClO , 100 mVs .
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New RuIII/II and RuIV/III surface redox waves for the catalysts are
clearly observed at 0.67 and 1.13 V, respectively, in addition to the
Ru surface wave of the chromophore at 1.35 V. These waves are
Table 2. k for Cat–SAM–C@FTO (C is [Ru(4,4'-
s
2
+
1
2
3
4
5
6
7
8
9
(PO(OH) ) -bpy) (4,4'-(C H ) -bpy)] (1) or [Ru(4,4'-
2
2
2
6
13 2
III/II
2+
(PO(OH)
2
)
2
-bpy)
2
(4,4'-(C
9
H )
19
2
-bpy)] (2); X is H O)
2
indicative of the successful generation of a chromophore-catalyst
III/II
assembly on the FTO slide. Plots of log(i
p
) vs log(ν) for the Ru
-1
β (Å-1)
C
1
2
1
2
1
2
Cat.
Ru(bda)(py-C
21 2
Ru(bda)(py-C H )
s
k (s )
wave of the catalyst for various Cat–SAM–C@FTO display slopes
between 0.5 and 1, with the catalysts with longer chains
approaching 0.5, Figures S4 and S5. This behavior is consistent
with a diffusive component likely associated with in and out
counterions diffusion through the hydrophobic bilayer as the
overall charge of the catalyst changes between redox states.
5
H
11
)
2
1.7
0
0
0
.28
.16
.41
1
0
0.4
Ru((C
6
H
13
)
2
bimpy)(bpy)(X)2+
bimpy)(bpy)(X)2+
2
15.6
8.4
Ru((C10
H
21
)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
We have measured the heterogeneous electron transfer rates, k
s
, for
Ru((CH
3
3
)
)
2
2
bimpy)(bpy(C
6
9
H
13
19
)
)
2
2
)(X)2+
)(X)2+
51.7
18.3
various SAM–C@FTO and Cat–SAM–C@FTO using trumpet
Ru((CH
bimpy)(bpy(C
H
4
1
plots based on Laviron’s equation (Tables 1 and 2). For SAM–
C@FTO using [Ru(4,4'-(PO(OH) -bpy) (4,4'-(C
bpy)] (n = 1, 6, 7, 9), k decreases by an order of magnitude from
2
)
2
2
n (2n+1) 2
H ) -
2+
-1
-
s
β values for Cat–SAM–C@FTO varied between -0.16 and -0.41 Å
-1
3
01 s for n = 1 to 31 s for n = 9, Table 1. This is surprising
1
.
These β values are significantly smaller than the values of -0.70
considering that in all cases the distance from the Ru center to the
electrode is the same because the complexes are anchored through
the phosphonic acid groups. We believe that this behavior is also
due to the different rates of diffusion of counterions through the
hydrophobic SAM as a function of the chain length.
-1
to -0.79 Å discussed above (in absolute values). This is not
surprising considering that for the SABs there is no direct chemical
bond between the alkyl chains and the electrode allowing for more
fluxional behavior of the chains. The measured ET rates are likely
the average value for multiple conformations at various distances
potentially allowing ET to occur with a slower falloff in rate. More
detailed experimental studies and Molecular Dynamics simulations
Table 1. k
s
for SAM–C@FTO (C is [Ru(4,4'-(PO(OH) ) -
2
2
2
2
+
bpy) (4,4'-R -bpy)] )
2
44
with Quantum Mechanics/Molecular Mechanics are underway.
-
1
R
Transfer coefficient (ꢀ)
k
s
(s )
The results of this study demonstrate the versatility and the breadth
of potential applications of the new anchoring strategy presented
here. The ability to easily combine chromophores and catalysts on
the surface of metal oxide electrodes with control of the distance
between them have the potential to provide tunability of the rates
of electron transfer between catalysts and chromophores as well as
back electron transfer from the metal oxide to the catalyst. These
are key variables in the performance of DSPEC and extension of
this strategy to high surface area mesoporous electrodes is currently
under investigation.
CH
3
0.42
0.57
0.48
0.51
301
164
115
31
C
6
C
7
C
9
H
13
H
15
H
19
When the electron transfer is dominated by the electronic coupling
matrix element, HDA, the electron transfer is expected to decay
exponentially with the electron transfer distance (r) according to
equation 1. The distance attenuation factor β reflects how efficient
ASSOCIATED CONTENT
Supporting Information
the coupling is per unit of length and k
at r = 0.
0
is the electron transfer rate
The Supporting Information is available free of charge on the ACS
Publications website. Additional methods, including syntheses,
characterization and additional electrochemical data, including
Figures S1−S47.
−ꢄꢅ
ꢁꢂ = ꢁ0ꢃ
eq. 1
For two related systems that only differ by the electron transfer
distance and where the electron transfer mechanism is the same, β
is constant and can be calculated according to equation 2.
1
ꢈꢉꢇ
AUTHOR INFORMATION
ꢆ = (
ln( ) eq. 2
ꢅ2−ꢅꢇ)
ꢈꢉ2
Corresponding Author
Electron transfer (ET) rate measurements as a function of distance
for SAMs on gold (SAM-Au) with a variety of configurations and
electroactive species abound in the literature. Finklea and
Handshew reported ET rates as a function of distance for
jconcepc@bnl.gov.
Notes
2+
42
The authors declare no competing financial interests.
[
3 5
Ru(NH ) (py)] -terminated SAM-Au. Becka and Miller used
SAM-Au of ω-hydroxythiols as tunneling barriers to measure ET
rates of several electroactive species in solution.43 McLendon et al.
ACKNOWLEDGMENT
studied
Langmuir-Blodgett
monolayers
of
16-
This work was carried out at Brookhaven National Laboratory and
it was supported by the U.S. Department of Energy, Office of
Science, Division of Chemical Sciences, Geosciences,
Biosciences, Office of Basic Energy Sciences under contract DE-
SC00112704. We thank Dr. Renato Neiva Sampaio for helpful
discussions.
4
4
ferrocenylhexadecanoic acid on SAM-Au of various distances. In
all cases mentioned above, the heterogeneous electron transfer rates
&
follow equation 1 with β values of -0.96 to -1.08 per –CH
2
spacer
≈ -0.70 to -0.79 Å ). In the case of Cat–SAM–C@FTO, the
general trend is that k deceases as the length of the chains increase,
-1 44
(
s
in agreement with the structure depicted in Figure 3 and with
equation 1 (Table 2). For a given chain length of the catalyst using
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