Surface immobilized copper(i) diimine photosensitizers as molecular probes for elucidating the effects of confinement at interfaces for solar energy conversion

Article abstract of DOI:10.1039/d0cc05972b

Heteroleptic copper(i) bis(phenanthroline) complexes with surface anchoring carboxylate groups have been synthesized and immobilized on nanoporous metal oxide substrates. The species investigated are responsive to the external environment and this work provides a new strategy to control charge transfer processes for efficient solar energy conversion.

Full text of DOI:10.1039/d0cc05972b

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Surface Immobilized Copper(I)diimine Photosensitizers as
Molecular Probes for Elucidating the Effects of Confinement at
Interfaces for Solar Energy Conversion
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
Michael S. Eberhart, Brian T. Phelan, Jens Niklas, Emily A. Sprague-Klein, David M. Kaphan,
b
a,c
a
a
a
DOI: 10.1039/x0xx00000x
David J. Gosztola, Lin X. Chen, David M. Tiede, Oleg G. Poluektov, and Karen L. Mulfort*
Heteroleptic copper(I) bis(phenanthroline) complexes with surface Cu—N bond length, and restricting flattening in the Cu(II)
anchoring carboxylate groups have been synthesized and oxidation state. The light-induced structural dynamics of these
immobilized on nanoporous metal oxide substrates. The species two complexes are well understood from investigations using
investigated are responsive to the external environment and multiple time-resolved optical and X-ray spectroscopy
1
6, 17
3
provide a new strategy to control charge transfer processes for methods.
efficient solar energy conversion.
In the non-coordinating solvent CH
decay for Cu(I)(dmp) is ~90 ns, but increases to 1.9
Cu(I)(dtbp) due to substituents preventing deactivation
2
Cl
2
, the MLCT
2
µ
s for
2
Copper(I) coordination complexes have attracted interest
for integration in solar energy conversion schemes because of
their broad absorption in the visible region and the relative
earth abundance of copper compared to the ruthenium centre
pathways associated with structural distortion in the Cu(II)
MLCT state and minimizing Cu(II)—solvent interaction. These
general design principles are well-established for homoleptic
Cu(I)bis(phen) complexes, and the same trends apply to
2+
of the prototype molecular photosensitizer [Ru(bpy)
,2’-bipyridine).1-6 Recent work focused on ligand designs of
Cu(I) photosensitizers has demonstrated the potential for these
3
]
(bpy =
A
B
18, 19
heteroleptic Cu(I)(phen )(phen ) complexes.
2
Short excited state lifetimes are problematic for diffusional
reactions, but electrode surface immobilization provides a
direct, well-defined pathway for interfacial electron transfer.
Our group has shown that Cu(I)diimine complexes anchored to
earth-abundant complexes to initiate light-driven H
2
catalysis in
multimolecular systems.7 Herein, we describe a fundamentally
new strategy for controlling and directing photochemical
processes through dimensional control of a metal oxide
environment.
-9
TiO
2
substrates inject photogenerated electrons from the
1
MLCT state on the sub-picosecond time scale while charge
recombination occurs on the microsecond timescale, indicating
For Cu(I)bis(phen) (phen = 1,10-phenanthroline) complexes,
substitution of the phen ligands and accompanied distortions to
5, 20
a long lived charge separated state. Indeed, these favourable
charge separation kinetics have led to an interest in deploying
the ideal tetrahedral symmetry profoundly influence their
Cu(I)diimine complexes as the photosensitizer in dye-sensitized
ground and excited state properties.1
0-13
By comparing
1, 21
solar cells.
In the pursuit of long-lived charge separated
2 2
Cu(I)(dmp) and Cu(I)(dtbp) (where dmp = 2,9-dimethyl-1,10-
states from Cu(I) complexes, the immobilization approach
provides more synthetic versatility than imposing the strict
functionalization requirements for stable homoleptic
complexes, and can also exploit what we have learned about
the directional electron transfer in heteroleptic Cu(I)diimine
1
4
phen, dtbp = 2,9-di-tert-butyl-1,10-phen), we know that
increasing the steric bulk at the 2,9-phen position from methyl
to tert-butyl results in a 29 nm blue shift of the absorbance
feature assigned to MLCT from λmax = 454 nm to λmax = 425 nm
15
and a 0.20 V increase in the Cu(II/I) oxidation potential. Both
observations are explained by the bulkier substituents yielding
a more idealized D2d symmetry in the Cu(I) state, increasing the
6, 22
complexes.
The primary goal of this work is to understand whether the
structural factors that dominate the solution phase
photophysical properties of Cu(I)diimine complexes also apply
in heterogeneous environments. To this end, we have
a.Division of Chemical Sciences and Engineering, Argonne National Laboratory,
Lemont, IL 60439
synthesized
three
new
heteroleptic
Cu(I)bis(phen)
b.
(CuHETPHEN) complexes functionalized with carboxylic acid
groups which are capable of binding to metal oxide surfaces
Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL 60439
Department of Chemistry, Northwestern University, Evanston, IL 60208
c.
Electronic Supplementary Information (ESI) available: Materials and methods for
molecular synthesis and immobilization, description of physical characterization
(Chart 1). These complexes were designed to evaluate two
methods, additional spectra, and details of kinetic analysis. See different ligand factors that we anticipate will impact the
DOI: 10.1039/x0xx00000x
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DOI: 10.1039/D0CC05972B
Figure 1. A) UV-Vis absorbance of
3
in CH
2
Cl
2
(red, right axis) and immobilized in
Chart 1. CuHETPHEN complexes used in this study for surface immobilization. Blocking
AAO pores of various pore size (blue traces, left axis), constant path length.
ligand indicated in black, surface anchoring ligand indicated in blue.
2 2
blue shift 10-30 nm as compared to each complex in CH Cl
photophysical properties, particularly when immobilized or solution (Figures S3, S4), which is consistent with increased
4,
15
confined. Complex
connected to the phen ligand. In contrast, the imidazole- observe only a minimal shift in the MLCT band for
containing anchoring ligand of complexes and is the immobilized complex and in solution (Figure 1), which
approximately 6.5 Å longer than that of and should allow suggests that immobilization has less of an impact on the
1
has a short anchor which is directly restriction to geometric changes in the complex.1
We
between
3
2
3
1
studies to investigate the effect of distance and packing spectral response than the increased sterics of the phen ligand
efficiency on the ground and excited state properties. environment. Additionally, we were able to correlate the
Complexes
2 and 3 differ only in the 2,9-phen substitution on density of immobilized 1-3 on the surface of the AAO wafers
the anchor ligand and provide a direct comparison for how the from the UV-Vis absorbance spectra and the AAO pore sizes and
known solution trends apply to immobilized CuHETPHEN densities (Tables S3-S4). We determined that 1-3 were
complexes.
adsorbed on the surfaces at a density of approximately 1-2
Complexes
1
,
2
, and
3
were synthesized by the two-step complexes per square nanometre which correlates to roughly
one-pot HETPHEN approach that yields kinetically stable monolayer coverage based on related immobilized
2
3-26
31
complexes with no formation of the homoleptic analogues.
complexes.
However, we had to refine the standard synthesis methods
To further probe the impact of pore immobilization on
because of the poor solubility of the phenanthroline ligands molecular and electronic structure, we utilized EPR
functionalized with carboxylic acids. We dissolved spectroscopy due to its high sensitivity for paramagnetic
Cu(CH
target complex, then added a stoichiometric amount of 2,9- information of the copper centre in the immobilized complexes.
dimesityl-1,10-phenanthroline ( ) followed by one equivalent of Since Cu(I) is diamagnetic and therefore EPR silent, we
carboxylate-functionalized ligands L1 L2, or L3 to form immobilized
on 10, 20, and 40 nm pore size AAO, oxidized the
complexes , and . The synthesis of was accomplished in surface bound Cu(I) to the corresponding Cu(II) species using a
neat CH Cl but the somewhat more Lewis basic solvent THF solution of AgNO in CH CN, and recorded X-band EPR spectra
was required for synthesis of
To systematically investigate the ground and excited state spectra characteristic of Cu(II) 3d (S = ½),
3 4 6 2 2
CN) PF in de-aerated CH Cl or THF depending on the centres and ability to elucidate structural and electronic
L
,
3
1
,
2
3
1
2
2
3
3
2
and
3
.
at 20 K (Figure 2). For all of the AAO pore sizes, we observe
9
32, 33
but with
properties of immobilized CuHETPHEN complexes, we selected noticeable differences in the linewidth of the parallel, low field
nanoporous anodic aluminium oxide (AAO) wafers as the metal components as a function of AAO pore size. (We assign the
oxide platform. AAO wafers are commercially available in a sharp signal at 338 mT to an organic radical/defect, g
≈ 2.005,
27
variety of pore sizes ranging from 10 nm to 200 nm, and the that we also observed in the bare AAO lacking any immobilized
controlled pore sizes offer a uniform chemical environment copper species, see Figure S3.) The largest pore size of AAO
compared to the larger distribution of pore sizes that are investigated (40 nm) has the smallest linewidth (~290 MHz),
present in more conventional metal oxide surfaces such as TiO
nanoparticle films. Complexes 1, 2, and 3
2
which increases as the pore size decreases from 20 nm (~300
were immobilized on MHz) to 10 nm (~550 MHz). Since there is no evidence of a
AAO wafers by submerging the wafers in a CH Cl or CH OH
chemical change to in the different pore sizes, we interpret
2
8
2
2
3
3
solution of each complex overnight. The AAO wafers adopted the broadening as a response to shorter neighbour-neighbour
the characteristic red-orange colour of the CuHETPHEN distances between Cu(II) ions in the smaller pores, which leads
complexes and upon removal from solution the wafers were
rinsed, soaked in pure solvent, and rinsed again to remove any assumption that there is no substantial contribution to line
residual weakly adsorbed complex.
width from spin-spin interactions in glassy frozen dilute
to stronger magnetic dipole-dipole interaction. Using the
UV-Visible absorption spectra can be used to gather solutions of Cu(II) complexes (see Figure S7 and associated
information on the coordination environment of Cu(I)diimine discussion), we estimate the distance between the immobilized
complexes.1
3, 29, 30
The AAO-immobilized MLCT band of 1 and 2
Cu(II) neighbours to be ~12 Å for the 40 and 20 nm pores, and
2
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Figure 4. Ultrafast transient absorption spectra of
3 on 10 nm AAO in deaerated
Figure 2. Continuous wave X-band EPR spectra of
3
in the Cu(II) oxidation state
CH Cl . Excitation at 500 nm, time delay noted in legend.
2
2
immobilized on 10, 20, and 40 nm AAO recorded at T = 20K.
with two local λ-maxima at 530 and 570 nm within 30 ps (see
for example immobilized on 10 nm AAO in Figure 4 and Figures
S8-S39). As expected from literature precedent, methyl
substitution in results in a significant increase in excited state
3
3
lifetimes compared to
summarized in Table S7.
1
and
2
, and the solution data are
We immobilized complexes
the wafers in CH Cl
and pore size on the excited state kinetics. Both
1
and 3 on AAO and immersed
2
2
to investigate the impact of immobilization
and showed
1
3
significantly longer excited state lifetimes when immobilized on
AAO than in solution, suggesting that immobilization restricts
the ability of the complexes to undergo a flattening distortion
Figure 3. Cyclic voltammograms of
in 0.1 M nBu PF in CH CN.
1, 2, and 3 immobilized on nanoITO substrates
4
6
3
and reduces exposure to solvent following photoexcitation. For
3
complex
1
, the MLCT lifetime is 1.0 ns in solution and increases
3
~
8.5 Å for 10 nm AAO pores.34 This difference in spin-spin to 1.7 ns when immobilized on 20 nm AAO. The average MLCT
interaction likely arises from decreasing distance between lifetime for
neighbouring chromophore tethering sites with changes in pore (51 ns), but the average MLCT lifetime increases to 69 ns for
size and may also be related in part to increased curvature with both 20 nm and 10 nm pores. This trend is similar to that
2 2
3 in CH Cl is 49 ns, similar to that in 40 nm pores
3
decreasing pores size.
previously observed for Cu(I)(dmp) dispersed into a polymer
2
3
Cyclic voltammetry of
1
,
2
, and
3
was performed in solution matrix where the MLCT lifetime increased from 90 ns in CH Cl
2
2
3
5
as well as immobilized on nanoITO which functions as both a to 210 ns with increasing polymer viscosity. We interpret our
substrate and working electrode since AAO is an insulator results in a similar fashion: as
is immobilized in increasingly
Figure 3, S8). The dominant factor in determining the Cu(II/I) smaller pores and in closer proximity to other immobilized Cu
oxidation potential is the substitution at the 2,9-phen position centres (as measured by EPR), the typical flattening distortion is
3
(
18
of the anchor ligand, consistent with previous observations.
As expected, the Cu(II/I) potential for is substantially more
positive than for and
Figure 3, S8, and Table S6). Importantly, the potentials of
restricted which increases the lifetime of the excited state.
3
As a first step toward investigating the effect of nanoporous
1
2
, both in solution and when immobilized confinement on interfacial charge injection, we measured the
and electron transfer kinetics of
immobilized on mesoporous n-
experience a more significant shift on immobilization than for type semiconducting TiO films immersed in CH Cl (Figures
(
2
3
1
3
2
2
2
(230 mV for
1 and 2 vs. 50 mV for 3) which suggests 3 is less S40-S41). Upon photoexcitation at 500 nm, we observe an
influenced by the external environment, consistent with the average lifetime of 75 μs for the recovery of the ground state
observations based on UV-vis spectroscopy.
bleach, which demonstrates that does inject electrons into
The excited state dynamics of the AAO-immobilized TiO . The recombination lifetime is comparable to analogous
3
2
5, 20
complexes were measured by ultrafast and nanosecond cases of other Cu(I) species bound to TiO2.
transient optical spectroscopy. Because of the complicated
Immobilization of Cu(I)diimine complexes in nanoporous
spectral changes and multiple photophysical processes of metal oxide materials presents an interesting opportunity to
Cu(I)diimine complexes,1
3, 17, 29
we used global analysis to fit the tune the photophysical properties of these complexes by
transient absorption data over a broad range of wavelengths external environmental factors. Here we have shown that
with multi-exponential functions (complete fitting details are decreasing the pore size of the AAO framework from 40 to 10
found in the Supporting Information). Similar to the well- nm results in enhanced spin-spin interactions between Cu(II)
3
documented photophysical behaviour for related complexes in centres, as well as a measurable increase in the MLCT lifetime,
3
solution, the transient spectra for the immobilized complexes over twice that observed in solution. The increase in MLCT
are characterized by an initial broad featureless excited state lifetime is consistent with inhibited structural flattening as pore
absorbance centred around 540 nm which evolves to a feature size decreases and ongoing work is focused on restricting pore
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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diameter even further to investigate the impact of extreme 15.
confinement on Cu(I)diimine excited state dynamics. These
O. Green, B. A. Gandhi and J. N. Burstyn, Inorg. Chem.,
View Article Online
2
009, 48, 5704-5714.
DOI: 10.1039/D0CC05972B
16.
17.
18.
M. Iwamura, S. Takeuchi and T. Tahara, Phys. Chem.
Chem. Phys . 2014, 16, 4143-4154.
M. Iwamura, S. Takeuchi and T. Tahara, Acc. Chem. Res.,
results present a fundamentally new strategy for controlling
and directing photochemical processes.
This work was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, Division of
Chemical Sciences, Geosciences, and Biosciences, through
Argonne National Laboratory under Contract No. DE-AC02-
2
015, 48, 782-791.
L. Kohler, D. Hayes, J. Hong, T. J. Carter, M. L. Shelby, K. A.
Fransted, L. X. Chen and K. L. Mulfort, Dalton Trans., 2016,
4
5, 9871-9883.
0
6CH11357. Synthesis and electrochemistry were supported by
1
9.
L. Kohler, R. G. Hadt, D. Hayes, L. X. Chen and K. L.
Mulfort, Dalton Trans., 2017, 46, 13088-13100.
J. Huang, O. Buyukcakir, M. W. Mara, A. Coskun, N. M.
Dimitrijevic, G. Barin, O. Kokhan, A. B. Stickrath, R.
Ruppert, D. M. Tiede, J. F. Stoddart, J.-P. Sauvage and L. X.
Chen, Angew. Chem., Int. Ed., 2012, 51, 12711-12715.
M. Sandroni, M. Kayanuma, A. Planchat, N. Szuwarski, E.
Blart, Y. Pellegrin, C. Daniel, M. Boujtita and F. Odobel,
Dalton Trans., 2013, 42, 10818-10827.
the Early Career Research Program, EPR and TA facilities were
supported by the Solar Photochemistry program. Transient 20.
optical spectroscopy was performed in part at the Center for
Nanoscale Materials. The CNM is an Office of Science user
facility supported by the U.S. Department of Energy, Office of
2
1.
Science, Office of Basic Energy Sciences, under Contract No. DE-
AC02-06CH11357.
2
2
2
2
2.
3.
4.
5.
D. Hayes, L. Kohler, L. X. Chen and K. L. Mulfort, J. Phys.
Chem. Lett., 2018, 9, 2070-2076.
M. Schmittel and A. Ganz, Chem. Commun., 1997, DOI:
10.1039/A701509G, 999-1000.
Conflicts of interest
There are no conflicts to declare.
S. De, K. Mahata and M. Schmittel, Chem. Soc. Rev., 2010,
3
9, 1555-1575.
Y. Pellegrin, M. Sandroni, E. Blart, A. Planchat, M. Evain,
N. C. Bera, M. Kayanuma, M. Sliwa, M. Rebarz, O. Poizat,
C. Daniel and F. Odobel, Inorg. Chem., 2011, 50, 11309-
Notes and references
1
2
3
4
.
.
.
.
C. E. Housecroft and E. C. Constable, Chem. Soc. Rev.,
015, 44, 8386-8398.
M. S. Lazorski and F. N. Castellano, Polyhedron, 2014, 82,
7-70.
M. Sandroni, Y. Pellegrin and F. Odobel, Comptes Rendus
Chimie, 2016, 19, 79-93.
S.-P. Luo, E. Mejía, A. Friedrich, A. Pazidis, H. Junge, A.-E.
Surkus, R. Jackstell, S. Denurra, S. Gladiali, S. Lochbrunner
and M. Beller, Angew. Chem., Int. Ed., 2013, 52, 419-423.
M. W. Mara, D. N. Bowman, O. Buyukcakir, M. L. Shelby,
K. Haldrup, J. Huang, M. R. Harpham, A. B. Stickrath, X.
Zhang, J. F. Stoddart, A. Coskun, E. Jakubikova and L. X.
Chen, J. Am. Chem. Soc., 2015, 137, 9670-9684.
2
1
1322.
2
6.
M. Sandroni, A. Maufroy, M. Rebarz, Y. Pellegrin, E. Blart,
C. Ruckebusch, O. Poizat, M. Sliwa and F. Odobel, J. Phys.
Chem. C, 2014, 118, 28388-28400.
5
2
2
2
7.
8.
9.
InRedox, http://www.inredox.com/product/aao-wafers/,
(
accessed May 14, 2020).
B. O'Regan, J. Moser, M. Anderson and M. Graetzel, J.
Phys. Chem., 1990, 94, 8720-8726.
5
.
G. B. Shaw, C. D. Grant, H. Shirota, E. W. Castner, G. J.
Meyer and L. X. Chen, J. Am. Chem. Soc., 2007, 129, 2147-
2
160.
3
3
0.
1.
M. W. Mara, N. E. Jackson, J. Huang, A. B. Stickrath, X.
Zhang, N. A. Gothard, M. A. Ratner and L. X. Chen, J. Phys.
Chem. B, 2013, 117, 1921-1931.
M. K. Brennaman, A. O. T. Patrocinio, W. Song, J. W. Jurss,
J. J. Concepcion, P. G. Hoertz, M. C. Traub, N. Y. Murakami
Iha and T. J. Meyer, ChemSusChem, 2011, 4, 216-227.
B. J. Hathaway and D. E. Billing, Coord. Chem. Rev., 1970,
6.
7.
8.
D. Hayes, L. Kohler, R. G. Hadt, X. Zhang, C. Liu, K. Mulfort
and L. X. Chen, Chem. Sci., 2018, 9, 860-875.
R. S. Khnayzer, C. E. McCusker, B. S. Olaiya and F. N.
Castellano, J. Am. Chem. Soc., 2013, 135, 14068-14070.
B. J. McCullough, B. J. Neyhouse, B. R. Schrage, D. T. Reed,
A. J. Osinski, C. J. Ziegler and T. A. White, Inorg. Chem.,
3
2.
3.
2
018, 57, 2865-2875.
5
, 143-207.
9
.
S. Saeedi, C. Xue, B. J. McCullough, S. E. Roe, B. J.
3
J. R. Pilbrow, Transition Ion Electron Paramagnetic
Resonance, Oxford University Press, 1991.
R. T. Ross, J. Chem. Phys., 1965, 42, 3919-3922.
A. O. Razgoniaev, C. E. McCusker, F. N. Castellano and A.
D. Ostrowski, ACS Macro Lett., 2017, 6, 920-924.
Neyhouse and T. A. White, ACS Appl. Energy Mater., 2019,
2
, 131-143.
3
3
4.
5.
10.
11.
12.
M. K. Eggleston, D. R. McMillin, K. S. Koenig and A. J.
Pallenberg, Inorg. Chem., 1997, 36, 172-176.
M. T. Miller, P. K. Gantzel and T. B. Karpishin, Inorg.
Chem., 1998, 37, 2285-2290.
J. V. Lockard, S. Kabehie, J. I. Zink, G. Smolentsev, A.
Soldatov and L. X. Chen, J. Phys. Chem. B, 2010, 114,
1
4521-14527.
1
1
3.
4.
M. W. Mara, K. A. Fransted and L. X. Chen, Coord. Chem.
Rev., 2015, 282-283, 2-18.
N. A. Gothard, M. W. Mara, J. Huang, J. M. Szarko, B.
Rolczynski, J. V. Lockard and L. X. Chen, The Journal of
Physical Chemistry A, 2012, 116, 1984-1992.
4
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Karen L. Mulfort, Ph.D.
Chemist, Solar Energy Conversion Group
Chemical Sciences and Engineering
Argonne National Laboratory
9
700 South Cass Avenue, Bldg. 241
Lemont, IL 60439
phone: 1-630-252-3545
email: mulfort@anl.gov
September 3, 2020
web: blogs.anl.gov/solar-energy
Dr. Alan Holder
Publishing Editor, Journals (ChemComm)
Royal Society of Chemistry
Thomas Graham House
Science Park, Milton Road
Cambridge, UK
Dear Dr. Holder,
I have re-submitted a revised manuscript in response to the decision of CC-COM-06-2020-004014,
Surface Immobilized Copper(I)diimine Photosensitizers as Molecular Probes for Elucidating the Effects
“
of Confinement at Interfaces for Solar Energy Conversion”. My co-authors and I thank you for handling
this manuscript, as well as the referees for their time evaluating the work and their insightful comments.
We have revised the manuscript to address the referees’ comments and have attached a point-by-point
response to their comments below.
This work has not been submitted to another journal and has not been published elsewhere in any
medium. All authors have approved of the manscript’s contents and conclusions. In the revised
manuscript, we have added David M. Kaphan as an author because of his contribution to the collection
and analysis of the solid state absorbance spectra which were suggested by referee 1.
This work was orginally submitted in response to an invitation for the special issue of Chemical
Communications on (Photo)electrocatalysis for renewable energy. The work presented in this
manuscript describes a potentially seminal change in thinking about how to manipulate and optimize
Cu(I) photophysics and photochemistry so that they may be ultimately deployed in photocatalytic
systems. Given the overall positive reviewer comments on the novelty, broad interest, and scholarly
presentation of the work, and that we have adequately addressed both referees’ concerns with the
original manuscript despite multiple constraints due to the ongoing coronavirus pandemic, we strongly
advocate for publication within this special issue of Chemical Communications.
Sincerely,
Karen Mulfort
A U.S. Department of Energy laboratory managed by UChicago Argonne, LLC

ChemComm
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Response to referee comments: CC-COM-06-2020-004014
Referee: 1
Comments to the Author
The manuscript by Karen L. Mulfort and co-workers deals with an important and interesting topic, i.e. the
immobilization of Cu(I) photosensitizers for later application in photochemical devices. The manuscript
itself is well written, but the publication requires some improvements prior to publication in Chem.
Comm. The following points need attention:
1
.) Unfortunately, the presented complexes 1-3 are quite similar to a publication by Y. Pellegrin and F.
Odobel et al. (c.f. Dalton Trans., 2013, 42, 10818-10827). The authors should comment on the novelty of
their complexes and also compare with the previous one.
We are aware of this important paper which also provides relevant prior work for our immobilized Cu(I)
complexes and we therefore cited this paper both in the original manuscript (ref. 22) and in the revised
version (ref. 21). We acknowledge the reviewer’s point that the complexes described by Sandroni et al.
and those we describe are similar in that both manuscripts use carboxylate groups to immobilize Cu(I)
complexes to metal oxide surfaces. However, this work differs from this previous literature in that we
are specifically investigating heteroleptic Cu(I)bis(phenanthroline) complexes. While the complexes
Pellegrin and Odobel discuss are also heteroleptic Cu(I) complexes, they feature a 2,2′-biquinoline-4,4′-
dicarboxylic acid ligand which serves to anchor the complex to metal oxide surfaces. The pairing of two
different phenanthroline ligands around the Cu(I) center in our work prohibits any rotational flexibility
that might be possible in the bpy-coordinated complexes described by Sandroni et al. Additionally,
pairing two phenanthroline ligands allows for intramolecular pi stacking between the two ligands (this
manuscript), which is absent in Cu(I)(phen)(bpy) complexes (Sandroni et al. Dalton 2013). The
substantial differences in intramolecular ligand interactions in response to bpy v. phen as well as outer
coordination sphere effects have been established by our group (Dalton Trans., 2016, 45, 9871-9883;
Dalton Trans., 2017, 46, 13088-13100) and Gordon (Inorg. Chem., 2013, 52, 2980-2992) and have
significant photophysical and photochemical consequences. Therefore, the complexes we describe here
do have important differences from those in the cited paper, but we also acknowledge the importance
of the previous work.
2
.) The basic structural characterization of the complexes 1-3 is not complete in the Supporting
Information. There are no 13C-NMR and no elemental analysis. Hence, it is very difficult to judge on the
purity and quality of this compounds. Also IR or Raman spectra might be helpful to gain information
about the carboxylic acid groups (on page 2 the authors speculate about the nature of the anchor group
and suggest zwitterions, but strong evidences are missing).
1
We demonstrate the purity of 1, 2, and 3 by H NMR and we have additionally definitively established
13
their composition by high resolution mass spectrometry. Furthermore, we have now added C NMR
spectra for the new organic ligands of 2 and 3, as well as for complexes 2 and 3 to the Supporting
Information to further establish the identity and purity of the new complexes (the synthesis of the
1
13
carboxylated ligand of 1 has been previously reported). We chose to use H NMR, C NMR, and HR-MS
rather than elemental analysis to establish purity since the complexes are likely to co-crystallize with
solvent molecules owing to their carboxylate and imidazole functional groups. Therefore, quantification
of results from any CHN analysis would likely require the inclusion of some (arbitrary) quantity of solvent
molecules. Work is in progress to obtain X-ray crystallographic data which would more clearly elucidate
information about solvent species which co-crystallize with these complexes.

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We thank the reviewer for the suggestion about investigating carboxylic acid anchoring groups by IR or
Raman spectroscopies; however, carboxylic acid anchoring groups for metal oxides are fairly well
understood (from Grätzel cells and related work, as well as the Pellegrin and Odobel paper referenced in
point #1 above). More in depth investigation of metal oxide anchoring is certainly important for the field
in general, but it is more suitable subject matter for another manuscript and outside the scope of this
work.
Following the reviewer’s suggestion, we have removed the discussion of zwitterionic ligands from the
main text because of its speculative nature.
3
1
.) A very important point in my eyes is the (missing) comparison of the properties of the pure complexes
-3 in the solid state (i.e. without solvent) with the immobilized samples. This comparison would clarify
the effect of solvent, which can also cause deactivation of the excited states. Therefore, the authors
should measure absorption and emission of their complexes in the solid state (powdered samples). For
instance, diffuse reflectance absorption spectroscopy is a convenient method. This would help to explain,
if the change in the UV-vis absorbance is caused by the solvent or by immobilization.
We thank the reviewer for this suggestion to help delineate the effect of solvent on the complexes from
the effects of immobilization on the heterogeneous metal oxide. For the revised manuscript, we have
measured the absorbance spectra of the solid state powders using diffuse reflectance and added this
data to the Supporting Information as Figures S4 and S5. Additionally, in the revised manuscript we have
presented all of the photophysical data in CH
2
Cl , including for our surface immobilized samples, to
2
make a consistent comparison between the surface and solution data. We note that the immobilized
surfaces also open up a wide variety of solvents for investigation that would otherwise be impossible
due to solubility constraints. Our preliminary results in that regard are that the effects of
coordinating/non-coordinating solvents on the photophysics and photochemistry of immobilized
Cu(I)bis(phenanthroline) complexes parallel what is known from solution phase work, and we are
looking forward to publishing more data on this subject in the future.
4
.) It is confusing that the UV-vis spectra (Fig. 1) and the amount of immobilized complex were presented
for complex 1, but then the authors continue the EPR experiments with complex 3. 1 and 3 differ
significantly from each other (one vs. two carboxyl groups). Therefore, the amount of immobilized 3
should also be determined by UV-vis.
On further reflection, we agree with the referee’s comment regarding Figure 1. Therefore, we have
moved (old) Figure 1 that compares the UV-Vis spectra of 1 in CH
2
Cl with that immobilized on AAO to
2
the Supporting Information as Figure S3. Figure 1 in the revised manuscript compares the absorbance
spectra of complex 3 with AAO-immobilized complex 3, which correlates better with the EPR and TA
results presented later in the manuscript.
5
.) The photophysical and electrochemical properties (i.e. absorption an emission maxima, excited state
lifetimes, redox potentials etc.) should be summarized in a table for all complexes. Some values are
discussed in the text, but without a table it is more difficult to follow and to compare.
We have added Table S5 which summarizes the optical properties and Table S6 which summarizes the
redox potentials to the Supporting Information. The kinetic data are summarized in Tables S7 and S8.
6
.) Where are the cyclic voltammograms of 2 and 3 in solution? Only the CV of 1 in CH3CN is presented
(
Fig. S4). Furthermore, the UV-vis spectrum of 1 is given in solution, but not for 2 and 3. These spectra
should be added (to the SI).
We have measured the UV-Vis and CV of 2 and 3 in solution and added this to the Supporting
Information. The UV-Vis data for 3 are in the main text as Figure 1, the UV-vis data for 1 and 2 are in

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Supporting Information as Figures S3 and S4. The cyclic voltammograms for 2 and 3 in solution have
been added to Figure S8 in the Supporting Information. The comparison of solution UV-Vis and CV to the
immobilized response supports our conclusions regarding restrictions on geometric distortion imposed
in the nanoscale confined environments, and how those plus immobilization impacts the electronic
properties of the molecular species.
7
.) I am very interested in the stability of the anchoring. Is any leaching of the immobilized Cu(I) complex
from the heterogeneous support observed? The authors should state something about this issue in the
manuscript, because it is essential for the samples immersed in CH2Cl2.
We did not observe desorption of the complexes from the metal oxide substrates during the optical,
EPR, or electrochemical measurements performed in this work. We anticipate that the stability of the
carboxylic acid anchoring groups on metal oxides parallels that of other carboxylic acid functionalized
species such as N3 or other Ru(II) dyes for DSSCs which have been extensively discussed in the literature.
In general, carboxylic acid binding to metal oxides is very stable under organic solvent conditions, less
stable under aqueous conditions, and unstable in the presence of base. However, like the precise nature
of carboxylate binding to metal oxide surfaces mentioned above in point #2, a detailed investigation of
surface anchoring stability is outside the scope of this work and is more appropriate for a different
study.
Referee: 2
Comments to the Author
The manuscript submitted by Mulfort and coworkers requires very little commentary as it is a work of
excellent scholarship and I cannot really find fault with any of it... I really enjoyed reading it and after
multiple times couldn't even find any typos!
The work is extremely thorough and well executed and the SI is clear and should allow for easy
reproducibility. The compounds are characterized well, one minor change I would suggest is that in
complexes 2 and 3, the HRMS data is given after the NMR data whereas in complex 1 it is given prior to
the NMR data. I'd simply recommend consistency in presentation.
We thank the reviewer for these positive comments and have adjusted the SI for consistent
presentation.
My only hesitation with the manuscript is that it seems to follow on closely from earlier work. For
example, as the authors themselves point out, charge injection characteristics of complex 3 are in
agreement with the work of Chen and Stoddart (refs 5 and 21). Also, the lifetime is extended as a result
of restricted flattening inside the AAO pores - similar to effects achieved in solution through the use of
bulky substituents at the 2,9-position of the phen groups. Although significantly the lifetime is doubled in
the pores versus the solution state lifetime which is a nice feature. It would have been nice for the casual
reader to know the significance of these values... the manuscript starts by describing the motivation from
moving away from Ru(bipy)3 - how do the copper complexes stack up in comparison to the more well-
established ruthenium systems?
We believe ligand modification is an important tool for chemists, but it is only one of the many tools
available that we can use to modulate a molecule’s photophysical and photochemical properties. For
example, changes to the greater chemical environment are also very important, particularly for Cu(I)
complexes that are extremely responsive to their chemical environment. In this work, we are
investigating the role of chemical environment beyond the ligand and beyond solvent interactions in the
first coordination sphere to introduce an entirely new dimension to the factors that can be used to
direct charge transfer processes. The observation that immobilizing complex 3 in pores of decreasing

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diameter results in a two-fold increase in the excited state lifetime, without any other structural
modification, is quite a remarkable result! We acknowledge the reviewer’s point that the excited state
2+
lifetimes of the complexes reported here do not compare with those reported for [Ru(bpy) ] and its
3
analogs, but that is not the goal of this work. The Cu(I) complexes reported here behave like “molecular
probes” since the photophysical properties of the Cu(I)bis(phenanthroline) complexes are more
responsive to their environment that the more common Ru(II)poly(pyridyl) chromophores.
Thus, based on the reviewer guidelines, I do not believe the manuscript meets the criteria to warrant
publication in Chem Comm
I would recommend publication in Dalton Trans after the very minor points are addressed.
We respectfully disagree with the reviewer’s assessment of the merit of this work and its meeting the
high standards required to publish in Chemical Communications. As the reviewer stated, our “work is of
excellent scholarship.” The results in this manuscript demonstrate a fundamentally new strategy for
controlling and directing photochemical processes. As such, we are eager to publish these exciting
results in a timely manner and anticipate that these results will lead to a significant shift in thinking
within the chemical community and inspire follow up research that will build on this early publication in
an emerging field. Therefore, we are confident that Chemical Communications is the best place to
publish these results.