Atom transfer radical polymerization preparation and photophysical properties of polypyridylruthenium derivatized polystyrenes

Article abstract of DOI:10.1021/ic400520m

A ruthenium containing polymer featuring a short carbonyl-amino-methylene linker has been prepared by atom transfer radical polymerization (ATRP). The polymer was derived from ATRP of the N-hydroxysuccinimide (NHS) derivative of p-vinylbenzoic acid, followed by an amide coupling reaction of the NHS-polystyrene with Ru(II) complexes derivatized with aminomethyl groups (i.e., [Ru(bpy)₂(CH₃-bpy-CH₂NH₂)] ²⁺ where bpy is 2,2′-bipyridine, and CH₃-bpy-CH ₂NH₂ is 4-methyl-4′-aminomethyl-2,2′- bipyridine). The Ru-functionalized polymer structure was confirmed by using nuclear magnetic resonance and infrared spectroscopy, and the results suggest that a high loading ratio of polypyridylruthenium chromophores on the polystyrene backbone was achieved. The photophysical properties of the polymer were characterized in solution and in rigid ethylene glycol glasses. In solution, emission quantum yield and lifetime studies reveal that the polymer's metal-to-ligand charge transfer (MLCT) excited states are quenched relative to a model Ru complex chromophore. In rigid media, the MLCT-ground state band gap and lifetime are both increased relative to solution with time-resolved emission measurements revealing fast energy transfer hopping within the polymer. Molecular dynamics studies of the polymer synthesized here as well as similar model systems with various spatial arrangements of the pendant Ru complex chromophores suggest that the carbonyl-amino-methylene linker probed in our target polymer provides shorter Ru-Ru nearest-neighbor distances leading to an increased Ru*-Ru energy hopping rate, compared to those with longer linkers in counterpart polymers.

Full text of DOI:10.1021/ic400520m

Article
pubs.acs.org/IC
Atom Transfer Radical Polymerization Preparation and
Photophysical Properties of Polypyridylruthenium Derivatized
Polystyrenes
†
†
†
†
†
†
†
Zhen Fang, Akitaka Ito, Shahar Keinan, Zuofeng Chen, Zoe Watson, Jason Rochette, Yosuke Kanai,
‡
§
,†
Darlene Taylor, Kirk S. Schanze, and Thomas J. Meyer*
†
Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599, United States
Department of Chemistry, North Carolina Central University, Durham, North Carolina 27707, United States
Department of Chemistry, University of Florida, Gainesville, Florida 32611, United States
‡
§
*
S Supporting Information
ABSTRACT: A ruthenium containing polymer featuring a short carbonyl-amino-methylene linker has been prepared by atom
transfer radical polymerization (ATRP). The polymer was derived from ATRP of the N-hydroxysuccinimide (NHS) derivative of
p-vinylbenzoic acid, followed by an amide coupling reaction of the NHS-polystyrene with Ru(II) complexes derivatized with
2
+
aminomethyl groups (i.e., [Ru(bpy) (CH -bpy-CH NH )] where bpy is 2,2′-bipyridine, and CH -bpy-CH NH is 4-methyl-4′-
2
3
2
2
3
2
2
aminomethyl-2,2′-bipyridine). The Ru-functionalized polymer structure was confirmed by using nuclear magnetic resonance and
infrared spectroscopy, and the results suggest that a high loading ratio of polypyridylruthenium chromophores on the polystyrene
backbone was achieved. The photophysical properties of the polymer were characterized in solution and in rigid ethylene glycol
glasses. In solution, emission quantum yield and lifetime studies reveal that the polymer’s metal-to-ligand charge transfer
(
MLCT) excited states are quenched relative to a model Ru complex chromophore. In rigid media, the MLCT-ground state band
gap and lifetime are both increased relative to solution with time-resolved emission measurements revealing fast energy transfer
hopping within the polymer. Molecular dynamics studies of the polymer synthesized here as well as similar model systems with
various spatial arrangements of the pendant Ru complex chromophores suggest that the carbonyl-amino-methylene linker probed
in our target polymer provides shorter Ru−Ru nearest-neighbor distances leading to an increased Ru*-Ru energy hopping rate,
compared to those with longer linkers in counterpart polymers.
INTRODUCTION
within polymer assemblies featuring long-lived triplet metal-to-
ligand charge transfer (MLCT) excited states.
■
There is an ongoing interest in developing light harvesting
materials that exhibit long-distance exciton and charge-
transport mechanisms operating with efficiencies comparable
to those found in natural photosynthetic light harvesting
systems. Polymers containing transition metal chromophores
are ideal targets for this objective, as they facilitate the study of
In one study, with alkyl-amino-carbonyl linkages of pendant
Ru(II) polypyridyl complexes to polystyrene, the average
nearest neighbor distance between the periphery of Ru
complexes changed by 3 Å depending on whether alternating
polymer structure (P1, 7 Å) or homopolymer structure (P2, 4
4
Å) was used. In the latter case, a dimethylene spacer between
1
−3
photoinduced electron and energy transfer.
References
polystyrene and the amide linker enabled full loading of Ru(II)
to the polymer chains, and fast energy migration on a time scale
therein reported on model systems involving polystyrene
backbones tethered to transition metal complexes via ether or
amide linkages. These studies facilitated an understanding of
the mechanism and dynamics of exciton and charge transport
Received: February 27, 2013
Published: July 16, 2013
©
2013 American Chemical Society
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Figure 1. Structures of previously reported polypyridylruthenium derivatized polystyrenes with alkyl-amino-carbonyl linkages (P1 and P2)
compared with the carbonyl-amino-methylene linkages of P3, P4, and P5.
3
5
of nanoseconds was observed. These studies and others
EXPERIMENTAL SECTION
■
(
suggest that the relative spatial arrangement between the Ru(II)
units is a key parameter in tuning the photophysics of these
materials.
Through coordinated efforts within the Solar Fuels Energy
Frontier Research Center, we are investigating polystyrene
Materials. 4,4′-Dimethyl-2,2′-bipyridine, methyl α-bromoisobuty-
rate, 4-vinyl benzoic acid and 1,1,4,7,7-pentamethyldiethylenetriamine
PMDETA) were purchased from Sigma-Aldrich and used without
purification. Selenium dioxide, 1,4-dioxane, hydroxylamine hydro-
chloride, zinc, ammonium acetate, ammonia, dimethylformamide
DMF), triethylamine, and ammonium hexafluorophosphate were
6
(
based systems with very close Ru−Ru distances. To this end,
the current study explores polystyrenes partly (P3) or fully
used as received from Fisher. Copper(I) bromide was freshly treated
with acetic acid, ethanol, and acetone before use. 4′-Methyl-[2, 2′-
bipyridine]-4-carbaldehyde (1), N-hydroxysuccinimide 4-vinyl ben-
zoate (5) and N-methyl-4,4′-bipyridyl hexafluorophosphate were
(
P4) derivatized by the addition of polypyridyl Ru(II)
complexes based on a new carbonyl-amino-methylene linkage
strategy. We report here the synthesis of these polymers
prepared by atom transfer radical polymerization (ATRP). This
polymerization procedure not only ensures small polydispersity
indexes (PDI) and controlled chain lengths, but it also allows
for incorporating halogen functional groups, notably, −Br, at
the end of the polymer chain for postpolymerization
modification. Application of ATRP and use of N-hydroxysucci-
nimide 4-vinylbenzoate provided polymers containing about 50
repeat units. Postpolymerization deprotection enabled amida-
7,8
synthesized according to previous reports.
The solutions for photophysical measurements were prepared in
HPLC grade acetonitrile and degassed with argon for 20 min. Standard
tris(2,2′-bipyridine)ruthenium(II) hexafluorophosphate [Ru(bpy) ]-
3
(
PF ) ] (Figure 2) was synthesized via a counterion exchange from
6 2
II
2+
tion and attachment of [Ru (bpy) (CH -bpy-CH NH )]
2
3
2
2
units to each polystyrene repeat unit (bpy = 2,2′-bipyridine;
CH -bpy-CH NH = 4-methyl-4′-aminomethyl-2,2′-bipyri-
3
2
2
dine). As an additional structural feature of interest, the
electron transfer N-methyl-4,4′-bipyridyl unit was introduced at
the end of the polymer chain (structure P5 in Figure 1) to
explore its possible role in electron and energy transfer.
In these derivatized polymers an important issue is the role
of local structure on intrachain dynamics. The role of the
relatively short carbonyl-amino-methylene link between the
polystyrene backbone and the Ru-polypyridyl units has been
explored by molecular dynamic simulations on both P3 and P4.
Comparisons with structurally related, derivatized polymers
with linkers of different lengths have provided important
insights into the role of the chromophore link on
interchromophore dynamics.
Figure 2. Structures of PEG-DMA550 and Ru(bpy) (PF ) .
3
6 2
Ru(bpy) Cl . Poly(ethylene glycol)dimethacrylate with 9 ethylene
3
2
glycol units (PEG-DMA550, Mn = 550) was purchased from Sigma-
Aldrich. Samples in PEG-DMA550 rigid film were prepared as
9
described in a previous report.
General Methods and Instrumentation. NMR spectra were
obtained on a Bruker instrument operating at 400 MHz utilizing
deuterated chloroform and/or acetonitrile as solvents. Gel permeation
chromatography (GPC) analysis was conducted on a Waters Alliance
system composed of a 2695 Separations Module and Waters 2414
Refractive Index Detector (Waters Associates Inc., Milford, MA) using
DMF as the eluent. The molecular weight was calibrated by using
8
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polystyrene standards. IR spectra were collected on a Bruker ALPHA
FT-IR spectrometer equipped with an attenuated total reflection
Table 1. Polymers Studied Using UFF MD
(
ATR) sampling accessory. UV−vis absorption spectra were recorded
# of
# of
average nearest-
neighbor Ru−Ru
distance/Å
repeat # of PF6−
solvent
on a Hewlett-Packard 8453 spectrometer. Steady-state emission
spectra were recorded on an Edinburgh Instruments FLS920 emission
spectrometer, equipped with a Xenon light source. Excitation was at
name loading
units
molecules molecules
P3
90%
20
20
20
20
36
40
36
40
1206
1184
1203
1181
10.31 ± 1.30
11.11 ± 1.20
11.68 ± 1.09
11.06 ± 1.09
4
60 nm, with inclusion of a 495-nm long-pass optical filter before the
P4
100%
90%
detector. Emission intensities at each wavelength were corrected for
system spectral response. The emission quantum yields (Φ) of the
complexes in CH CN were determined relative to [Ru(bpy) ](PF )
6 2
P3′
P4′
100%
3
3
10
(
Φ = 0.095 in CH CN). The photon counts of the emission spectra
3
poured into cold water (300 mL) and filtered. Recrystallization from
were corrected in wavenumber scale by using an equation I(v)
̃
= I(λ)
1
methanol yielded 2 as a white solid (2.41 g, 90%). The H NMR is
2
11
×
λ , and the Φ values were calculated by using eq 1,
16
identical to the literature.
4
-Methyl-4′-aminomethyl-2,2′-bipyridine (3). A mixture con-
2
Ssample/Asample ⎛n
⎞
⎟
⎠
sample
taining 2 (2.13 g, 10 mmol), ammonium acetate (1.93 g, 25 mmol),
ammonia (30 mL, 50 mmol), ethanol (20 mL), and water (20 mL)
was heated to reflux. Zinc powder (2.8 g, 50 mmol) was added in
portions over 30 min. After the reaction mixture was heated at reflux
for 3 h, it was cooled and filtered to remove the zinc residue. The
filtrate was concentrated to remove ethanol. NaOH (7 g) was added to
form a white precipitate that changed to a slightly turbid solution. The
mixture was extracted with methylene chloride (3 × 100 mL). After
Φsample = Φstd
⎜
S /A
⎝ n
std
std
std
(1)
In eq 1, the subscripts “sample” and “std” represent the sample and
Ru(bpy) ](PF ) , respectively, and, S, A, and n are the integrated area
of the emission band, the absorbance at an excitation wavelength, and
the refractive index of the solvent used. Time-correlated single photon
counting data were obtained by the same instrument equipped with a
pulsed, 445 nm laser source (Edinburgh Instruments EPL-445, fwhm
1.5 ns, repetition rate = 50,000 Hz). Emission from Ru(II) complex
was observed at 610 or 650 nm. Decay traces were fitted by using the
Edinburgh F900 or Origin 8.1 software package.
Electrochemistry was probed by using cyclic voltammetry on a
computer-controlled CHI660A electrochemical workstation, where a
glassy carbon electrode served as the working electrode, a platinum
electrode as the counter electrode, and an AgNO /Ag electrode as the
reference. A solution of tetrabutylammonium hexafluorophosphate
[
3
6 2
drying over MgSO
to yield a white solid (1.50 g, 75%). H NMR (400 MHz, CDCl ) δ
3
8.60 (d, 1H, J = 5.2 Hz), 8.53 (d, 1H, J = 5.2 Hz), 8.34 (s, 1H), 8.24
, the solvent was removed under reduced pressure
4
1
∼
(s, 1H), 7.30 (d, 1H, J = 4.8 Hz), 7.14 (d, 1H, J = 4.4 Hz), 3.99 (s,
2H), 2.44 (s, 3H); 13C NMR (100 MHz, CDCl
) δ 156.31, 155.79,
3
153.01, 149.20, 148.85, 148.09, 124.67, 121.94, 121.89, 119.27, 45.54,
21.10. High resolution Mass, Calc. 199.1109, found, 199.1002.
[Ru(bpy)
Ru(bpy) Cl (0.484 g, 1.0 mmol), 3 (0.20 g, 1.0 mmol), ethanol (5
mL), and H O (5 mL) was refluxed overnight in argon. Ethanol was
2
distilled out under reduced pressure. The residue was filtered and
washed with water (20 mL). To the aqueous solution, NH PF (0.5 g,
3 mmol) was added to yield an orange solid (0.51 g, 55%). H NMR
(400 MHz, CD
= 7.2 Hz), 7.72 (br, 4H), 7.60 (t, 1H, J = 5.8 Hz), 7.52 (d, 1H, J = 5.6
Hz), 7.36−7.40 (m, 5H), 7.22 (d, 1H, J = 6.0 Hz), 4.02 (s, 2H), 2.53
(s, 3H). High resolution Mass, Calc. 321.5999, found, 321.6005
(CH -bpy-CH NH )](PF ) (4). A mixture containing
2 3 2 2 6 2
3
2
2
(
0.1 M) in degassed dry acetonitrile was used as the supporting
electrolyte, and the scan rate was 100 mV·s .
Computational Details. All structures for calculation were
constructed using the Materials Studio suite (Accelrys Software Inc.,
San Diego, 2011). Geometries of the monomer were optimized using
−
1
4
6
1
CN) δ 8.47−8.50 (m, 5H), 8.38 (s, 1H), 8.04 (t, 4H, J
3
12
13
the B3LYP DFT functional, and the Lanl2DZ basis sets, as
1
4
implemented in Gaussian09 version 09a02. The optimization was
done with “Grid = UltraFine” and “Tight” convergence criteria. The
repeat units were constructed by the “Build Polymer” module in
Materials Studio. Gas phase geometries of the polymers were
optimized, and then annealed, using the Universal Force Field
2+
(Ru(bpy) (CH -bpy-CH NH ) ).
2 3 2 2
Precursor Polymer 6. A mixture containing 5 (0.368 g, 1.5
mmol), methyl α-bromoisobutyrate (4 μL), PMDETA (42 μL), and
dimethylsulfoxide (DMSO, 0.5 mL) was degassed via three freeze−
pump−thaw cycles. CuBr (4 mg, 0.028 mmol) was added to the
mixture under argon flow. The reaction mixture was warmed to 80 °C
and stirred overnight under argon. The green mixture was poured into
methanol (20 mL) to yield a solid. The solid was filtered and dissolved
in DMF. The DMF solution was passed through a short neutral
alumina column to remove the copper residue. The solution was then
concentrated and precipitated from methanol to yield a white solid (82
1
5
(
UFF) as implemented in the Forcite module in Materials Studio,
with atomic charges (Mulliken charges) obtained using the QM
calculations. The annealing step included 10 cycles starting at an initial
temperature of 300 K and progressing to a final temperature of 500 K
in a time step of 0.2 fs with 5 heating ramps per cycle and 100
molecular dynamic steps per ramp. The simulation cell was built by
the “Amorphous Cell” module in Materials Studio, and it included the
−
polymer, molecules of PF6 (counterions), and molecules of
4
−1
acetonitrile solvent. The number of acetonitrile molecules was
determined by preparing a simulation cell that was 20 Å larger than
the polymer in each direction with the cell density of acetonitrile
mg, 22%). M : 1.3 × 10 , PDI: 1.2; IR (cm ): 2936 (C−H, phenyl),
n
1773 (carbonyl, imide), 1732 (carbonyl, C(ON)O), 1204 (C−N,
1
imide); H NMR (DMSO-d6, 400 MHz) δ 7.83, 6.82, 4.40, 2.85, 1.52.
(
0.783 g/mL). The simulation cell was annealed using the same
P3. To a solution of 6 (20 mg, 0.081 mmol) in anhydrous DMF (3
mL), triethylamine (0.1 mL, 0.72 mmol) and 4 (80 mg, 0.089 mmol)
were added. After stirring at 40 °C for 2 days, the solution was
conditions described above for the polymer annealing. The annealed
cell then went through a molecular dynamics run of 2 ns, with the
canonical ensemble (NVT, constant number of atoms, constant
volume, and constant temperature of 298K enabled by the Nose
themostat), using a time step of 1 fs. Snapshots of the resulting
structures were collected every 0.5 ps during the second ns of each
molecular dynamic run, resulting in a trajectory of 2000 snapshots for
analysis. Table 1 specifies all the polymers that were studied in this
report. Structures of P3′ and P4′ are shown in the Supporting
Information, both of which have replaced the carbonyl-amino-
methylene linkers with carbonyl-amino-ethylene linkers.
concentrated and poured into a methanol/H O mixture (50:10 mL).
2
The mixture was centrifuged and washed with methanol (20 mL × 5)
until a colorless filtrate was obtained. The resulting orange solid was
−1
isolated as product (28 mg, 75%). IR (cm ): 3422 (O−H, carboxylic
acid), 3083 (N−H, amide); 2924 (C−H, phenyl), 1708, 1659
1
(carbonyl, amide); H NMR (DMSO-d6, 400 MHz) δ 8.83, 8.15,
7.71, 7.52, 7.34, 6.57, 4.66, 1.55.
P5. To a solution of P3 (5 mg) in CH CN (5 mL), N-methyl-4,4′-
3
bipyridyl hexafluorophosphate (10 mg) was added. After refluxing
overnight, the solution was concentrated and poured into methanol/
hexane (20:20 mL) to yield a precipitate. The precipitate was washed
with 1:1 methanol/hexane (20 mL × 3) until no N-methyl-4, 4′-
bipyridyl hexafluorophosphate was visually observed under a 254 nm
UV lamp. The slurry cake was dried in vacuum to yield a yellow solid
Synthesis. 4′-Methyl-[2, 2′-bipyridine]-4-carbaldehyde
Oxime (2). To a solution of 1 (2.5 g, 12.6 mmol) in methanol (30
mL) was added hydroxylamine hydrochloride (3 g, 44 mmol), K CO
2
3
(
8 g, 60 mmol) and water (30 mL). The reaction mixture was stirred
for 1 h at 80 °C. After cooling to room temperature, the mixture was
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Scheme 1
−
1
(
4 mg, 79%). IR (cm ): 3420 (O−H, carboxylic acid), 3081 (N−H,
4-vinylbenzoate in the presence of methyl α-bromoisobutyrate
as the initiator by a typical ATRP procedure. GPC measure-
1
amide); 2922 (C−H, phenyl), 1702, 1654 (carbonyl, amide);
NMR (DMSO-d6, 400 MHz) δ 8.48, 8.35, 8.02, 7.71, 7.53, 7.37, 6.60,
.44.
H
ments on the product show that polymer 6 has a number
1
4
average molecular weight (M ) of 1.3 × 10 g/mol with a PDI
n
of 1.2. The small PDI is a characteristic of ATRP and controlled
radical polymerization. A typical GPC elution curve of 6 is
shown in the Supporting Information, Figure S1. The degree of
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Our group has devel-
oped several polymerization strategies to prepare functionalized
polymerization (DP) is calculated as the ratio of M to the
n
formula weight of monomer 5. The value obtained (DP ∼ 50)
is in good agreement with the loading ratio of monomer to
initiator, consistent with controlled polymerization by ATRP.
polystyrenes, which can be further modified by attaching Ru or
3
,4
Os complexes. A living anionic polymerization of silane-
protected amine derivatized styrene afforded polymers with
narrow PDI and controlled molecular weight. However,
because of the strongly basic conditions used for anionic
polymerization, the reaction is not particularly functional group
tolerant, limiting the scope of the types of functional monomers
that can be used. We also recently developed reversible
addition−fragmentation chain transfer (RAFT) polymerization
combined with the azide-alkyene click chemistry for preparing
1
As shown in the Supporting Information, Figure S2, the H
NMR spectrum of 6 features broad resonances at δ 7.8, 6.8, and
1
.5 ppm, consistent with the polystyrene structure. The protons
for the 2,5-dioxopyrrolidine (NHS-ester) group are located at δ
.85 ppm. A weak resonance at 4.40 ppm is attributed to the
2
terminal chain proton at the carbon next to the bromine atom.
The amidation reaction from 6 to P3 was conducted in DMF
with a catalytic amount of triethylamine to expedite the
exchange. At the end of the reaction, copious amounts of
methanol were added to remove excess free Ru complexes. The
NMR spectrum of P3 is shown in Supporting Information,
Figure S2, with the proton resonances characteristic of the 2,5-
dioxopyrrolidine group absent. New resonances in the aromatic
region δ ∼7.5−9.0 ppm corresponding to the protons of
bipyridine ligands of the Ru complex are present. A loading
level of ∼90% Ru on the polystyrene repeat units is estimated
based on the integration ratio of aromatic to alkyl protons. The
unsubstituted NHS ester groups are hydrolyzed to carboxylic
acids under the amidation reaction conditions and workup, as
confirmed by FTIR as discussed below. The functionalization
of the P3 end group (-Br) with N-methyl-4,4′-bipyridinium
5c
polypyridine Ru(II)-derivatized polystyrenes. Unfortunately,
the −SH end-group arising from the RAFT initiator quenches
the MLCT state by a charge transfer mechanism, leading to
considerable reduction in the lifetime of the MLCT state,
especially in short chain length polymers. In comparison, the
ATRP technique catalyzed with copper(I) affords an effective
method to achieve polymers with narrow PDI and no thiol end-
group functionality. Meanwhile, the bromide functional end
group present in the ATRP polymer makes it possible to create
block copolymers or chain terminated nanoparticles for further
1
7
derivatization.
Scheme 1 shows the synthetic route for the target polymers.
In the first step, 4,4′-dimethyl-2,2′-bipyridine was selectively
oxidized to the monoaldehyde by SeO , followed by the
2
+
−
1
formation of oxime. The oxime is then reduced by zinc in a
weakly basic medium. Using a typical Ru complex preparation
hexafluorophosphate (MV PF ) was confirmed by H NMR.
6
After thoroughly washing the crude product with a 1:1
1
8
+
−
method, Ru monomer 4 was synthesized in moderate yield.
Polymer precusor 6 was prepared from N-hydroxysuccinimide
methanol/hexane mixture until no MV PF6 is observed in
the filtrate, P5 is obtained as a yellow solid. Although NMR
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Inorganic Chemistry
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shows no significant change in the aromatic region, the
resonances at 4.66 ppm attributed to −CHBr in the spectrum
of P3 are absent in that of P5 because of the replacement of
reduction of each of the three bipyridine ligands. In CVs of the
two polymers, anodic−cathodic peak separations, 300 mV at
100 mV/s, were observed likely resulting from slow diffusion
and multiple electron transfer. However, qualitatively the
polymers exhibit similar oxidation and reduction potentials
compared to [Ru(bpy) ](PF ) . These results suggest that
−
Br by N-methyl-4,4′-bipyridinium.
The amidation reaction of 6 to P3 was monitored by FT-IR
as well (Supporting Information, Figure S3). The spectrum of
poly(4-vinyl benzoic acid) (abbreviated as PSCOOH) is shown
for comparison. The phenylene unit of polystyrene shows
3
6 2
there is no significant interaction between the ligands at
adjacent Ru sites, which is in agreement with the observation in
absorption and emission described below. We also note the
absence of a clearly resolved wave because of reduction of the
−1
stretching bands at ∼2930 cm for all polymers. Distinct
carbonyl region bands are present in the spectra of all three
polymers. In particular, an intense band in the spectrum of 6 at
2+
terminal MV unit in P5. The absence of a definable signal is
730 cm− is shifted to 1660 cm in P3. This feature arises
1
−1
1
2+
not surprising, given that the MV end-group is present at a
from the carbonyl stretching mode of the imide in 6 which is
transformed to an amide group in P3. A new band appears at
∼
50-fold lower concentration than the pendant Ru(II) units.
Photophysical Properties in Solution. The absorption
3
080 cm− due to the N−H stretching mode of the secondary
1
and emission spectra of monomer 4, P3, P5, and [Ru(bpy) ]-
3
amide in P3. Compared to the spectrum of PSCOOH, P3
displays features consistent with some residual −COOH units
in the infrared spectrum. For example, a broad band at 3420
(
PF ) in CH CN are shown in Figure 4. All solutions show
6 2 3
MLCT absorption bands at λ ∼ 450 nm, which is typical for
Ru-polypyridine complexes. There is a slight red-shift (∼6 nm,
−1
cm in P3 appears for the O−H group, which features typical
−1
1
61 cm ) in the band maximum for 4, P3, and P5 compared
stretching characteristics of −COOH in PSCOOH. Also, the
with [Ru(bpy) ](PF ) . The red-shifts for 4, P3, and P5 are a
−1
3
6 2
band at 1710 cm in P3 features an intense band from the
CO stretching mode of a −COOH group, as shown in the
spectrum of PSCOOH.
consequence of the substituent effects of the electron donating
19
methyl and aminomethyl groups in the ancillary ligand. The
emission maxima of 4, P3, and P5 exhibit similar red-shifts
Electrochemistry. The electrochemical properties of [Ru-
−1
(
∼11 nm, 226 cm ) compared to [Ru(bpy) ](PF ) . In
3
6 2
(
(
bpy) ](PF ) , P3, and P5 were studied by cyclic voltammetry
3 6 2
comparing the emission quantum yields it can be seen that P3
has a quantum yield (Φ = 0.08) that is comparable to that of
monomer 4 (Φ = 0.09).
CV). As shown in Figure 3, [Ru(bpy) ](PF ) and both
3
6 2
It is well-known that association of polyelectrolyte strands
2
0
occurs and varies with the nature of polymer backbone,
21
22
counterions, solvent environment, and addition of electro-
23
lytes or surfactants. To examine the possible impact of
association on photophysical properties, a concentration
dependent study was undertaken for P3. Within the
concentration range of 5 μM to 50 μM, P3 exhibits identical
absorption and emission spectra when corrected for the
concentration dependence of absorption and intensity (see
Supporting Information, Figure S4). Similarly, the excitation
spectrum of P3 in acetonitrile exhibits a smooth and
symmetrical pattern which mirrors the emission spectrum
(
see Supporting Information, Figure S5). Taken concentration
Figure 3. Cyclic voltammogram of [Ru(bpy) ](PF ) , P3, and P5 in
3
6 2
dependent and excitation spectra together, the fact that the
absorption and emission spectra of 4 and P3 are essentially
identical indicates that the metal complex chromophore units in
P3 are dispersed as single molecules without substantial
aggregation or strong interchromophore interaction. A detailed
discussion about polymer structure is presented in the
modeling calculation section below.
0
s .
.1 M Bu NPF in deoxygenated CH CN with a scan rate of 100 mV
4 6 3
−1
polymers feature a reversible anodic wave at ∼0.92 V vs
AgNO /Ag, attributed to the Ru(III/II) couple. [Ru(bpy) ]-
3
3
(
PF ) exhibits three reversible cathodic waves at −1.64, −1.86,
6
2
and −2.12 V vs AgNO /Ag, from sequential one-electron
3
Figure 4. Absorption (left) and corrected emission spectra (right) of Ru(bpy) PF , monomer 4, polymer P3, and P5 in CH CN (λ = 460 nm;
3
6
3
ex
[
Ru] ∼ 10 μM.). The emission spectra have been normalized based on the absorbance at 460 nm, and the absolute intensities reflect the emission
quantum yields.
8
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Inorganic Chemistry
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2
+
The ability of MV to oxidatively quench the MLCT excited
because of enhanced aggregation (see Supporting Information,
2
+
state of [Ru(bpy) ] , resulting in efficient emission quenching,
Figure S8). A linear fit to the emission quenching data of P3 at
3
MV2+ concentrations <0.9 mM yielded KSV ∼ 2.6 × 10 M ,
2
−1
is well-established. Stern−Volmer emission quenching shows
that the electron transfer quenching process is diffusion-
controlled, and transient absorption spectroscopy confirms that
the quenching produces the reduced acceptor, MV . In the
present work, we examined the effect of MV on the emission
which is 1 order of magnitude smaller than that for
3
−1 16
[Ru(bpy) ](PF ) (K ∼ 2.1 × 10 M ). The reduced
3
6 2
SV
+•
24
quenching efficiency for the polymer relative to the monomeric
2
+
complex is likely due to increased electrostatic repulsion
2
+
2+
of the polymeric RuL3 chromophores in P3 by adding N,N′-
between the polycation and the cationic MV quencher ion.
dimethyl-4,4′-bipyridyl hexafluorophosphorate (MV(PF ) ) to
Time-correlated single photon counting (TCSPC) was
6
2
3
solutions of P3 (Figure 5).
applied to time resolve the decay of the MLCT* states
2
+
following excitation in the absence and presence of MV .
Multiphoton effects are unimportant at the low excitation
5
b
irradiances used in the TCSPC experiments (∼ nJ/pulse).
Thus, herein emission decay studies were able to further probe
the quenching mechanism of MV2+ on the MLCT excited state
decay kinetics for the polymeric chromophores. Thus, emission
2
+
decays of P3 were monitored at 610 nm as a function of MV
2
+
concentration. As shown in Figure 6a, addition of MV slightly
alters the decay kinetics. A biexponential fit to the emission
decay afforded median lifetimes (τ) that were subsequently
2
+
used to construct a lifetime vs concentration of added MV
plot (Figure 6b), where the biexponential fitting results in
2
reasonable reduced R (∼0.997−0.998 for the 5 decays). This
plot is similar to one derived from the quantum yield
2
+
Figure 5. Emission spectra of P3 ([Ru] = 10 μM) in the presence of
MV2 in deaerated acetonitrile. Inset illustrates the Stern−Volmer plot
quenching plot (Φ /Φ vs [MV ]), suggesting the emission
0
+
2+
quenching of P3 by MV is dynamic.
As described above, polymer P5 results from the substitution
of the -Br end-group in P3 with the MV electron acceptor
2+
(
Φ /Φ vs [MV ]), constructed by monitoring the emission intensity
0
at 460 nm excitation.
2+
unit at the terminus. As shown in Figure 4, the absorption and
emission maxima of P5 are essentially indentical to those of P3,
indicating that the presence of the MV2+ end-group does not
strongly influence the properties of the Ru chromophores.
However, interestingly we find that the emission quantum yield
of P5 (Φ = 0.054) is reduced by approximately 30% relative to
that of P3, suggesting that the MV2+ end-group is active as an
intrachain quencher of MLCT excited states. Given that the DP
Figure 5 illustrates the results of a typical quenching study
where P3 ([Ru] = 10 μM) was quenched by the addition of
2+
MV with emission quenching reaching 20% upon the addition
2
+
of about 1.0 mM MV . (Note that this concentration
2+
corresponds to ∼1:100 ratio of Ru chromophores to MV .)
2+
At 1.5 mM MV the emission maximum of P3 red-shifted by 4
−1
nm (81 cm ) and absorption spectra of the solution mixtures
exhibited scattering at longer wavelength (>700 nm, Support-
2+
value of P5 is ∼50, the approximate Ru: MV ratio in the
polymer chains is 50:1. Thus for a solution in which the P5
polymer repeat unit concentration is 10 μM (e.g., [Ru] = 10
μM), the approximate concentration of the MV2+ end-groups is
0.2 μM. In this system, we see 30% emission quenching
corresponding to an effective Stern−Volmer quenching
2
+
ing Information, Figure S6), suggesting that MV induces a
change in polymer conformation, or possibly leads to
aggregation arising from the high ionic strength. These results
are supported by dynamic light scattering experiments that
reveal that the hydrodynamic radius of P3 increased from 100
2
+
6
−1
to 150 nm upon the addition of MV (see Supporting
Information, Figure S7). An emission quenching study by
constant of ∼2.1 × 10 M which is 10,000-fold more efficient
2+
in the P3/MV bimolecular quenching system. The efficient
quenching by the MV2+ end-group in P5 suggests that there is
an active channel by which MLCT excitons can migrate among
adding LiClO into P3 solution shows that increasing the ionic
4
strength impacts both emission maxima and quantum yields
Figure 6. (a) Emission decay of P3 (λ = 445 nm, λem = 610 nm) and (b) Stern−Volmer plot for emission quantum yield quenching (Φ /Φ, black
ex
0
square, λex = 460 nm), and median lifetime decay (τ /τ, red circle, λ = 445 nm, λ = 610 nm) in the presence of MV2 in degassed acetonitrile
+
0
ex
em
solution.
8
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Article
the pendant Ru sites in the polymer. This type of intrachain
exciton migration has been well-documented in previous
studies of polystyrenes that contain pendant Ru-chromophores,
where the mechanism has been attributed to MLCT exciton
dimethylvaleronitrile) (Vazo 52 from DuPont) as the thermal
polymerization initiator. The solid state film described here is
9
different from a neat film formed by casting, spraying, or
dipping because the polymer is diluted in a film free of
interstrand interactions. Both intermolecular and intramolecu-
lar energy transfer typically play role in neat films. In the diluted
PEG-DMA films photophysical properties are dominated by
intrastrand events and the distribution of excited state energies
and dynamics for individual polymer strands.
3
,5
transport by a site-to-site hopping mechanism.
In general, small molecules including Ru(bpy) (PF ) and
3
6 2
monomer 4 show single exponential emission decay kinetics,
while P3 and P5 feature multiple exponential decays as shown
in Figure 7. Individual decay profiles are shown in the
Figure 8 compares the steady-state absorption and emission
spectra of P3 in CH CN and PEG-DMA550 films. The
3
Figure 7. Emission decay profiles of Ru(bpy) (PF ) (black),
3
6 2
monomer 4 (green), P3 (red), and P5 (blue) in CH CN: excitation
3
wavelength = 445 nm and emission wavelength = 610 nm. [Ru] ∼ 50
μM.
Figure 8. Absorption and corrected emission spectra of P3 in CH CN
3
Supporting Information, Figure S9. The emission lifetimes of
(solid curves) and PEG-DMA550 film (broken curves). Excitation
Ru(bpy) (PF ) and 4 were determined to be ∼800−900 ns,
3
6 2
wavelength, 460 nm.
25
typical for Ru-polypyridine complexes (Table 2). On the
emission spectrum in PEG-DMA550 film showed a slight blue
Table 2. Emission Decay of Ru(bpy) (PF ) , 4, P3,, and P5
shift compared to the spectrum measured in CH CN solution
3
6 2
3
in CH CN
while there was no significant difference in the MLCT
absorption band. The blue shift in the emission spectrum
compared to the spectrum in solution arises from the frozen
nature of the medium. In solution, MLCT excitation is followed
by solvent relaxation to a local polarization environment
appropriate to the excited state dipole. In frozen media, the
local polarization is largely frozen resulting in an enhanced
3
⟨
τ⟩/
ns
a
compounds
τ/ns
Ru(bpy) (PF )
6 2
806
945
3
4
P3
P5
τ = 281 (0.33); τ = 808 (0.67)
634
1
2
τ = 82 (0.19); τ = 393 (0.43); τ = 900 (0.38) 527
1
2
3
26
excited-to-ground state energy gap.
a
Note: Number in parenthese is the fraction amplitude of the decay
Figure 9 shows emission decay profiles for Ru(bpy) (PF ) ,
3
6 2
components.
monomer 4 and P3 in PEG-DMA550 films monitored at 650
other hand, P3 and P5 exhibit short-lived decay components in
addition to relatively long-lived components typical of the
isolated ruthenium chromophores. The rapid decay component
is due to energy migration by Ru* to Ru hopping to low energy
sites on the polymer which undergo rapid decay as has
3
observed in previous studies. When the polymer is end-capped
with MV2 in P5, the electron trap site introduces an additional
+
∼80 ns component compared to P3 arising from energy
transfer to the terminus and being oxidatively quenched.
Photophysics in Rigid Medium. To obtain additional
evidence for intrastrand energy migration, we conducted
photophysical measurements on P3 in a semirigid polymer
film. Since the MLCT excited states in the Ru complexes are
sensitive to the local environment, a distribution of excited state
energies and dynamics exists along an individual polymer
strand. In a homogeneous solution environment solvent
fluctuations lead rapid redistributions of local environments
while local environments are, at least, partly frozen in polymer
films. To explore the role of the film environment on energy
decay dynamics and time-resolved emission, the polymers were
dispersed in rigid PEG-DMA550 films. The samples were
prepared by using a literature procedure; with 2,2′-azobis(2,4-
Figure 9. Emission decay profiles of Ru(bpy) (PF ) (black),
monomer 4 (green), and P3 (red) in PEG-DMA550 film: excitation
wavelength = 445 nm and emission wavelength = 650 nm.
3
6 2
nm. The decay profile for P3 deviates from a single, exponential
decay, and the longer-lived decay component is comparable to
the emission lifetime of Ru(bpy) (PF ) and monomer 4 (∼1.4
3
6 2
μs) in CH CN. Nevertheless, all three samples display longer
3
lifetimes in PEG-DMA550 films than those in CH CN
3
solution. This is a manifestation of the “rigid medium effect”
8
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Figure 10. Corrected time-resolved emission spectra (left), normalized theoretical fits by one-mode Franck−Condon analysis (center) at 0−2 μs
after the excitation (black to green), and time profile of the decrease in E (right) for P3 in PEG-DMA550 film. Excitation at 445 nm.
0
and the influence of the increased excited-to-ground state
(see Supporting Information, Figure S10). These results point
to a more inhomogeneous environment for P3 in PEG-
DMA550 compared to solution with significant variations of
the excited-state energies and the time dependence of the
emission spectrum on intrastrand energy migration to low
energy sites in the frozen medium where nonradiative decay is
enhanced. Based on this interpretation, the average rate
constant for energy migration (⟨kmig⟩) was determined to be
18
energy gap on nonradiative decay. As discussed above, the
short-lived decay component observed for P3 represents a
heterogeneous distribution of excited-state energy sites and
rapid intrastrand energy migration.
We previously noted that the appearance of multi
exponential kinetics in multichromophore polymers is often
accompanied by time-dependent emission spectral shifts to
1
2,27
red.
The time-resolved emission spectrum of P3 in CH CN
6
−1
3
5 × 10 s with an average E decay time of ∼200 ns.
0
solution is shown in Supporting Information, Figure S10. These
data show that the emission maximum gradually shifts from 617
to 623 nm on a 2 μs time scale without significant change in the
emission profile. In comparison, Figure 10 shows the time-
resolved emission spectra of P3 in a PEG-DMA550 film. The
emission spectrum gradually shifts from ∼600 to ∼630 nm, a
Structure Modeling by Molecular Dynamics Simu-
lations. In terms of microscopic insight, energy transfer
dynamics depend on polymer structures, and the rates vary
exponentially with the distance between chromophores, in this
case, distance between Ru pendants. To illustrate this point, the
polymers investigated here were simulated by using 20
monomer repeat units for each of the four polymer structures
larger shift than that observed in CH CN. At or near room
3
temperature emission from the polymers is structureless. In
(
see Table 1). There are two main differences between the
fitting these spectra a one-mode, Franck−Condon analysis was
polymers. First, the number of pendant Ru-complexes per
polymer repeat unit is varied between full loading (P4) vs 90%
loading (P3) (for the latter there is a benzoic acid unit for each
9 Ru pendant units, optimized structure shown in Figure S11 in
the Supporting Information). The second difference is in the
length of the linker between the amino and Ru pendant unit,
with methyl linker for P3 and P4 and ethyl linker for P3′ and
P4′ (the latter polymers were not studied experimentally).
2
8
used as eq 2, with low frequency bpy-ring torsional and
metal−ligand stretching modes treated classically and included
in the bandwidth. In these fits the single mode is the average of
2
9
8
−10 medium frequency υ (bpy) modes. With the
appearance of vibronic structure at 77 K, a two-mode fit was
utilized which included both averaged medium and low-
frequency modes.
4
∞
3
Note that P4′ resembles the previously studied P2, with the
⎛
⎝
⎞
m
E0 − mℏω ⎛ S ⎞
⎟ ⎜
⎟
only difference being the position of the amino group. All four
polymers were generated and simulated according to the
procedure in the Materials section. Typical structures of P3′
and P4′ are shown in the Supporting Information. The average
nearest-neighbor Ru−Ru distances are summarized in Table 1.
As can be seen in Table 1, there is interplay between linker
length and higher loading that determines which specific
polymer gives a shorter average nearest-neighbor Ru−Ru
distance. Figure 11 compares the radial distribution function of
P3 and P4, and the position of the first peak in this figure
represents the closest Ru−Ru distance that each polymer can
reach. While P3 (Ru−Ru nearest-neighbor average distance
I(v)
̃
= ∑ ⎜
E0
⎠
⎝ m ⎠
!
m=0
2
⎡
⎤
⎥
⎛
⎞
⎟
⎠
v − E + mℏω
̃
⎢
0
exp −4 ln 2 ⎜
⎢
v
1/2
⎥
⎦
⎝
̃
⎣
(2)
̃ ̃
In eq 2, I(v) is the emission intensity at the energy v
in
−1
wavenumber (cm ), relative to the intensity of the 0→0
transition. E0 is the energy gap between the zero−zero
vibrational levels of the ground- and excited-states. m, ℏω
and S are vibrational quantum number, the quantum spacing,
and the Huang−Rhys factor reflecting the degree of distortion
in the single, average mode as the difference in equilibrium
1
0.31 Å) has a shorter Ru−Ru nearest-neighbor average
distance than P4 (Ru−Ru average distance 11.11 Å), it is also
evident from the position and size of the first peak in Figure 11
that P3 features these short distances much more frequently
than P4, where P3 has a closest Ru−Ru distance of ∼9 Å. It is
interesting to note that in most cases the nearest-neighbor pairs
of Ru-pendants are not adjacent on the polystyrene backbone.
Rather, in some cases they can be quite far separated along the
chain. Figure S12 in the Supporting Information shows, for
instance, a map of how different Ru-pendants find their nearest
Ru neighbor in P3 by plotting the arrows for the nearest
displacements. v1 is the full width at half-maximum (fwhm)
̃
/2
for individual vibronic lines. The observed time-resolved
emission spectrum at each delay time after excitation was
30
adequately fit by eq 2 as described in our recent related paper.
The correlation coefficients (r) for the one-mode fits are
shown, together with the fitting parameters, in the Supporting
Information, Tables S2 and S3. The theoretical fits and the time
profile of the E value are also shown in Figure 10. From the
0
−
1
fits, the E value shifted from 17,000 to 16,000 cm within 2 μs
after excitation compared to 16200 to 16050 cm in CH CN
0
−1
3
8
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Inorganic Chemistry
Article
polymer, intrachain energy transfer hopping to the terminal
2
+
chromophore and electron transfer quenching by MV occur.
2
+
Intrastrand lifetimes are characteristic of [Ru(bpy)3] (∼1 μs),
but decays are nonexponential with time-dependent emission
maxima. These observations are consistent with intrastrand
energy migration by Ru* to Ru hopping to low energy sites
where nonradiative decay is enhanced.
Time-resolved emission spectra in rigid medium demonstrate
a significant variation in the distribution of excited state
energies with intrastrand migration, with the effect enhanced
compared to those in solution. Following excitation, the energy
gap between ground state and excited state shifts significantly
from 17000 to 16000 cm− on the microsecond time scale
1
−
1
compared to a shift of ∼16200 to 16050 cm in CH CN
3
solution. The amplified effect for the polymer in the semirigid
film is consistent with an enhanced heterogeneous distribution
of excited state sites with rapid intrastrand energy migration
Figure 11. Calculated radial distribution function of the distance
between Ru−Ru centers for P4 (blue line) and P3 (red line). Only
distributions up to 25 Å are shown.
between sites with a rate constant of 5 × 10− s . Enhanced
heterogeneity in the film compared to solution is attributed to
its frozen nature in contrast to solution where rapid fluctuations
lead to local equilibration. Suppression of molecular motion in
the films increses the distribution of excited state sites with the
energy migration among sites to low energy environments
where nonradiative decay is enhanced. Calculations show that
close packing of the chromophore occurs in the polymers with
∼9 Å minimal Ru−Ru distances calculated by MD simulations
with the relatively close contact distances supporting rapid
intrastrand energy migration among the Ru chromophores.
6
−1
connections that exist for more than 50% of the snapshots from
MD simulations.
We find that there is no apparent pattern in how the nearest
Ru-pendant is located for any of the polymers. The average
theoretically determined nearest-neighbor Ru−Ru distance and
radial distribution distances calculated here (for both P3 and
P4) are significantly shorter than that in P2 and Ru−Os
1a,3
random polymers reported previously. This is probably due
to the differences in the computational methods employed,
with previous studies assuming that Ru(bpy) units are hard
3
spheres, while in this work we use a full atomistic model, which
allows the Ru(bpy) units to be closer. At the same time, the
ASSOCIATED CONTENT
3
■
differences in the polymer length and loading level of P2
compared to P3 or P4 could also contribute to such observed
differences. The dense loading of Ru-chromophores in P2
causes the polymer to adopt an extended rod-like structure that
alleviates the steric and Coulombic repulsions between adjacent
chromophores. Meanwhile, the close proximity of the Ru
pendant units in P3 and P4 facilitates faster energy transfer
hopping among the Ru complexes. An experimentally measured
energy transfer rate on the nanosecond time scale was observed
for P3 (see Supporting Informations, Figure S10). An
important aspect of the computational results is the similarity
in average Ru−Ru distance and Ru−Ru radial distribution
between P3 and P4 even though P3 is only 90% loaded and P4
is 100% loaded. Based on these computational results and the
Ru−Ru distances, rapid energy migration can still be achieved
with lower loading of 90% as demonstrated here experimen-
tally.
S
*
Supporting Information
1
GPC elution curve of 6, H NMR spectra of 6, P3, and P5, IR
spectra of 6 and P3, excitation spectrum of P3, absorption
2
+
spectra of P3 upon the addition of MV , dynamic laser
scattering spectra of P3 without and with MV , absorption and
emission spectra of P3 upon the addition of LiClO , emission
decay profiles of monomer and polymers, time-resolved
2
+
4
emission profiles of P3 in CH CN, figures of computational
3
results, tables of spectral fitting parameters of P3 in CH CN
and PEG-DMA550 film. This material is available free of charge
via the Internet at http://pubs.acs.org.
3
AUTHOR INFORMATION
Corresponding Author
E-mail: tjmeyer@email.unc.edu.
■
*
Notes
The authors declare no competing financial interest.
CONCLUSION
■
ACKNOWLEDGMENTS
Polypyridylruthenium derivatized polystyrenes have been
synthesized by ATRP with narrow PDI and moderate
molecular weights, followed by derivatization by transamidation
in high yields. Although a relatively short linker tethers the Ru
polypyridyl units to the polystyrene backbone, a high loading
level of Ru units on the polystyrene backbone was achieved.
The polymers have good solubility in polar solvents, for
■
We acknowledge support from the UNC EFRC: Center for
Solar Fuels, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Award Number DE-SC0001011
supporting Z.F., J.R., Z.C. and Z.W. A.I. acknowledges support
from the U.S. Department of Energy, Office of Science, Office
of Basic Energy Sciences, under Award Number DE-FG02-
06ER15788. S.K. acknowledges support under U.S. DOE
SciDAC-e award DE-FC02-06ER25764. We also acknowledge
support for the purchase of instrumentation from the UNC
EFRC Center for Solar Fuels (award DE-SC0001011) and
from UNC SERC (“Solar Energy Research Center Instrumen-
tation Facility” funded by the U.S. Department of Energy,
example, DMF, THF, and CH CN. The polymeric chrom-
3
phores show similar absorption and emission spectra compared
to Ru(bpy) (PF ) , indicating that there are, at best, only weak
3
6 2
interactions between the polymer-bound chromophores,
despite their close proximity. Polymer P5, with an electron
acceptor methyl viologen (MV ) end-group exhibited a
suppressed emission quantum yield and lifetime. For this
2
+
8
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Inorganic Chemistry
Article
Office of Energy Efficiency & Renewable Energy, under Award
Number DE-EE0003188).
(25) Kim, H.-B.; Kitamura, N.; Tazuke, S. J. Phys. Chem. 1990, 94,
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P.; Brennaman, M. K.; Cardolaccia, T.; Pandya, A.; DeSimone, J. M.;
Meyer, T. J. J. Phys. Chem. B 2011, 115, 64.
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dx.doi.org/10.1021/ic400520m | Inorg. Chem. 2013, 52, 8511−8520