Varying the electronic structure of surface-bound ruthenium(II) polypyridyl complexes

Article abstract of DOI:10.1021/ic501682k

In the design of light-harvesting chromophores for use in dye-sensitized photoelectrosynthesis cells (DSPECs), surface binding to metal oxides in aqueous solutions is often inhibited by synthetic difficulties. We report here a systematic synthesis approach for preparing a family of Ru(II) polypyridyl complexes of the type [Ru(4,4′-R₂-bpy)₂(4,4′-(PO₃H₂)₂-bpy)]²⁺ (4,4′(PO₃H₂)₂-bpy = [2,2′-bipyridine]-4,4′-diylbis(phosphonic acid); 4,4′-R₂-bpy = 4,4′-R₂-2,2′-bipyridine; and R = OCH₃, CH₃, H, or Br). In this series, the nature of the 4,4′-R₂-bpy ligand is modified through the incorporation of electron-donating (R = OCH₃ or CH₃) or electron-withdrawing (R = Br) functionalities to tune redox potentials and excited-state energies. Electrochemical measurements show that the ground-state potentials, E^′(Ru^3+/2+), vary from 1.08 to 1.45 V (vs NHE) when the complexes are immobilized on TiO₂ electrodes in aqueous HClO₄ (0.1 M) as a result of increased Ru dπ-π back-bonding caused by the lowering of the π orbitals on the 4,4′-R₂-bpy ligand. The same ligand variations cause a negligible shift in the metal-to-ligand charge-transfer absorption energies. Emission energies decrease from _max = 644 to 708 nm across the series. Excited-state redox potentials are derived from single-mode Franck-Condon analyses of room-temperature emission spectra and are discussed in the context of DSPEC applications.

Full text of DOI:10.1021/ic501682k

Article
pubs.acs.org/IC
Varying the Electronic Structure of Surface-Bound Ruthenium(II)
Polypyridyl Complexes
Dennis L. Ashford, M. Kyle Brennaman, Robert J. Brown, Shahar Keinan, Javier J. Concepcion,
John M. Papanikolas, Joseph L. Templeton, and Thomas J. Meyer*
Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599, United States
*
S Supporting Information
ABSTRACT: In the design of light-harvesting chromophores
for use in dye-sensitized photoelectrosynthesis cells
(
DSPECs), surface binding to metal oxides in aqueous
solutions is often inhibited by synthetic difficulties. We report
here a systematic synthesis approach for preparing a family of
Ru(II) polypyridyl complexes of the type [Ru(4,4′-R -
2
2
+
bpy) (4,4′-(PO H ) -bpy)] (4,4′(PO H ) -bpy = [2,2′-bi-
2
3
2 2
3
2 2
pyridine]-4,4′-diylbis(phosphonic acid); 4,4′-R -bpy = 4,4′-R -
2
2
2
,2′-bipyridine; and R = OCH , CH , H, or Br). In this series,
3
3
the nature of the 4,4′-R -bpy ligand is modified through the
2
incorporation of electron-donating (R = OCH or CH ) or
3
3
electron-withdrawing (R = Br) functionalities to tune redox
potentials and excited-state energies. Electrochemical measurements show that the ground-state potentials, E ′(Ru3+/2+), vary
o
from 1.08 to 1.45 V (vs NHE) when the complexes are immobilized on TiO electrodes in aqueous HClO (0.1 M) as a result of
2
4
increased Ru dπ−π* back-bonding caused by the lowering of the π* orbitals on the 4,4′-R -bpy ligand. The same ligand
2
variations cause a negligible shift in the metal-to-ligand charge-transfer absorption energies. Emission energies decrease from λ
max
=
644 to 708 nm across the series. Excited-state redox potentials are derived from single-mode Franck−Condon analyses of
room-temperature emission spectra and are discussed in the context of DSPEC applications.
INTRODUCTION
dearth of these complexes has inhibited studies exploring the
role of synthetic changes on the key parameters for photoanode
■
Light absorption throughout the visible and near-IR spectra is
required for efficient dye-sensitized solar cells (DSSCs) and
dye-sensitized photoelectrosynthesis cells (DSPECs).
mophores suitable to drive water-splitting reactions in DSPEC
photoanodes must satisfy four design criteria: (1) surface-
binding groups (typically carboxylates or phosphonates), (2)
high molar absorptivity throughout the visible and near-IR
spectra, (3) an excited-state redox potential that is sufficient to
undergo rapid and efficient electron injection into the
conduction band of a metal oxide semiconductor (typically
applications, namely, excited- and ground-state redox poten-
1,10,23−26
1
−7
tials.
We report herein a systematic study of the
Chro-
synthesis of phosphonate-derivatized Ru(II) polypyridyls of the
2
+
general form [Ru(4,4′-R -bpy) (4,4′-(PO H ) -bpy)]
2
2
3
2 2
(
4,4′(PO H ) -bpy = [2,2′-bipyridine]-4,4′-diylbis(phosphonic
3
2 2
acid), 4,4′-R -bpy = 4,4′-R -2,2′-bipyridine, and R = OCH ,
2
2
3
CH , H, or Br; Figure 1) and their electrochemical,
3
spectroscopic, and excited-state properties.
EXPERIMENTAL SECTION
■
anatase TiO ), and (4) the resulting oxidized chromophore
2
Materials. Tetraethyl-[2,2′-bipyridine]-4,4′-diylbis(phospho-
must have the thermodynamic potential sufficient to oxidize an
2
7
28
nate), poly-Ru(1,4-cyclooctadiene)Cl , and [Ru(2,2-bipyri-
2
adjacent water-oxidation catalyst to its most active form by
27
8
−11
dine) ([2,2-bipyridine]-4,4-diyldiphosphonic acid)]Cl (RuP) were
2
2
electron transfer.
Ruthenium polypyridyl complexes have been extensively
synthesized as previously reported. Distilled water was further purified
using a Milli-Q Ultrapure water purification system. All other reagents
were ACS grade and used without further purification. Fluoride-doped
tin oxide (FTO)-coated glass (Hartford Glass; sheet resistance = 15
11−15
studied for use as chromophores in DSSCs and DSPECs.
They typically absorb light in the visible spectrum, have
sufficient excited-state potentials to inject electrons into the
2
Ω/□) was cut into 10 × 40 mm strips and used as the substrate for
ZrO and TiO nanoparticle films. Microwave reactions were carried
conduction band of TiO , and are capable of driving water
2
2
2
o
out using a CEM MARS microwave reactor. A CEM HP-500 Plus
oxidation with E ≥ 1.23 V (vs NHE) in appropriately designed
5
,16−18
complexes.
We have previously demonstrated stable surface linkages
Received: August 26, 2014
Revised: December 2, 2014
Accepted: December 3, 2014
with phosphonate derivatives for a variety of polypyridyl Ru(II)
12,19−22
complexes and assemblies.
The extent of this chemistry
has been limited by difficulties associated with synthesis. The
©
XXXX American Chemical Society
A
DOI: 10.1021/ic501682k
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Article
complexes dissolved in 80:20 CH CN/H O ([complex] = 1 mM )
3
2
deaerated with argon for 5 min with a glassy-carbon working electrode,
a Pt-wire counter, and a Ag/AgNO3 reference. Surface electro-
chemistry was conducted by immersing derivatized TiO working
2
1
9,21,27
electrodes in aqueous HClO (0.1 M).
4
Computational Methods. All molecular geometries were
3
3,34
calculated by density functional theory (DFT) with the B3LYP
functional and the LanL2DZ
3
5,36
basis set. Solvent-environment effects
Figure 1. Structures of RuPOMe, RuPMe, RuP, and RuPBr.
Teflon-coated microwave vessel (100 mL) was used at a power setting
of 400 W. The vessel was rotated and stirred throughout the
microwave procedure. The pressure of the reaction vessel was
monitored throughout the reaction and never exceeded 300 psi.
29
30
Metal-Oxide Films. Nano-TiO2 and nano-ZrO2 films, typically
2
5
−7 μm thick with a coating area of roughly 10 × 15 mm , were
prepared according to the literature. Dye-absorption isotherms on
TiO (Figure S2) were obtained by soaking the films in methanol
2
solutions of [Ru(4,4′-dimethoxy-2,2′-bipyridine) ([2,2′-bipyridine]-
2
4
,4′-diyldiphosphonic acid)]Cl (RuPOMe), [Ru(4,4-dimethyl-2,2-
2
bipyridine) ([2,2-bipyridine]-4,4-diyldiphosphonic acid)]Cl
2
2
(
4
5
RuPMe), and [Ru(4,4′-dibromo-2,2′-bipyridine) ([2,2′-bipyridine]-
2
SYNTHESIS OF LIGANDS AND COMPLEXES
,4′-diyldiphosphonic acid)]Cl (RuPBr) at concentrations of 10, 20, ■4
2
0, 100, 150, and 200 μM. The slides were then removed, rinsed with
,4′-Dibromo-2,2′-bipyridine. 4,4′-Dimethoxy-2,2′-bipyr-
methanol, and dried over a stream of nitrogen.
idine (2.7 g, 12.5 mmol) was dissolved in PBr (20 mL, 212
3
UV−Visible Absorption Spectra. UV−visible absorption spectra
were obtained by placing the dry derivatized films perpendicular to the
detection beam path of a UV−vis-NIR absorption dual-beam
spectrophotometer (HP 8453A). The expression Γ = A(λ)/(ε(λ) ×
mmol) under an atmosphere of argon. The reaction mixture
was heated to 180 °C with vigorous stirring. The reaction was
completed in 3 h and followed by TLC. After cooling the
reaction to room temperature, crushed ice was carefully added,
followed by the addition of concentrated aqueous ammonia
alternated with the addition of ice. (Caution! Addition of ice and
ammonia causes the mixture to heat quickly; take great care when
1
000) was used to calculate surface coverage (Γ) on metal-oxide
electrodes where A is the absorption and ε(λ) is the molar absorptivity
31
at wavelength λ. Maximum surface coverage (Γmax) and surface-
binding constants (K ) on TiO for RuPOMe, RuPMe, and RuPBr
were obtained by evaluation of the Langmuir isotherm (eq 1) with [X]
the concentration of the complex in the loading solutions (Figure
S2). All subsequent measurements were carried out on films loaded
from solutions of a ruthenium complex in methanol (100 μM), which
ad
2
adding them alternately to the PBr solution.) Enough ammonia
3
was added to reach a pH of ∼10, at which point a significant
amount of precipitate formed. The solution was then
transferred to a separatory funnel and extracted with ether (4
32
−8
−2
yielded surface coverages of ∼7 × 10 mol cm .
×
70 mL). The organic layers were combined, dried over
Γ
K [X]
max ad
MgSO , filtered, and the solvent was removed by rotary
Γ = 1
+ K [X]
4
(1)
evaporation. A white solid (1.81 g, 47%) was isolated. The solid
ad
1
appeared clean via H NMR but contained a small phosphorus
Steady-State and Time-Resolved Emission. Measurements
impurity. The impurity was removed by running the sample
through a plug of silica with dichloromethane as the eluent. The
were carried out by inserting derivatized thin films of ZrO at a 45°
angle into a standard 1 cm path length cuvette containing aqueous
HClO (0.1 M). Emission spectra were collected at room temperature
2
4
3
1
characterization matches that previously reported. H NMR
4
using an Edinburgh FLS920 spectrometer with luminescence first
passing through a 495 nm long-pass color filter, then a single-grating
(400 MHz, CDCl ): δ (ppm) 8.59 (d, 2H), 8.465 (d, 2H), 7.49
(dd, 2H).
3
(
1800 L/mm, 500 nm blaze) Czerny−Turner monochromator (5 nm
[
2,2′-Bipyridine]-4,4′-diyldiphosphonic Acid. Tetraeth-
bandwidth), and finally detected by a Peltier-cooled Hamamatsu
R2658P photomultiplier tube. For steady-state experiments, samples
were excited using light output from a combination of a housed 450 W
Xe lamp and a single-grating (1800 L/mm, 250 nm blaze) Czerny−
Turner monochromator with 5 nm bandwidth. The dynamics of
emission decay were monitored using the FLS920 time-correlated
single-photon-counting capability (1024 channels; 1 ns per channel)
with each data set collecting >5000 counts in the maximum channel.
Excitation was provided by an Edinburgh EPL-445 ps pulsed-diode
laser (444.2 nm, 80 ps fwhm) operated at 200 kHz. Kinetics were
evaluated using either Edinburgh or Origin software.
yl [2,2′-bipyridine]-4,4′-diylbis(phosphonate) (1.0 g, 2.33
mmol) was dissolved in anhydrous CH Cl (∼50 mL) under
2
2
an atmosphere of argon. To the solution was added
bromotrimethylsilane (2.15 mL, 12.1 mmol), and the reaction
was stirred at room temperature under an atmosphere of argon
for 3 days. The solvent was removed under vacuum, and
anhydrous methanol (∼30 mL) was added. The solution was
stirred for 30 min at room temperature, the methanol was
removed under vacuum, and ether (∼60 mL) was added to the
white solid. The suspension was stirred for 2 h, and the white
solid was collected by suction filtration. This compound was
Electrochemical Measurements. Measurements were carried
out with a CH Instruments 660D potentiostat with a Pt-wire counter
electrode and either a Ag/AgNO (0.01 M AgNO /0.1 M tetra-n-
1
used without further purification (0.74 g, 87%). H NMR (400
3
3
MHz, d -DMSO): δ (ppm) 8.85 (t, 2 H), 8.66 (d, 2 H), 7.75
butylammonium hexafluorophosphate (TBAPF ) CH CN; −0.09 V vs
6
6
3
0
/+
(dd, 2H).
Fc ) or a Ag/AgCl (3 M NaCl; 0.198 V vs NHE) reference
o
electrode. E ′ values were obtained from the peak currents in square-
General Procedure for Ru(4,4′-R
-bpy) Cl . In a typical
2 2 2
wave voltammograms. Reductive electrochemistry was conducted with
procedure, poly-Ru(1,4-cyclooctadiene) Cl (0.30 g, 0.97
2
2
B
DOI: 10.1021/ic501682k
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Inorganic Chemistry
Article
mmol) and 4,4′-R -bipyridine (where R = OCH , CH , or Br)
Scheme 1. Synthesis of 4,4′-Br-2,2′-Bipyridine
2
3
3
(
0.97 mmol) were dissolved in 1,2-dichlorobenzene (∼35 mL).
The solution was thoroughly degassed with argon, and the
mixture was heated to 180 °C under an atmosphere of argon
for 2 h. The solution was cooled, ether (∼100 mL) was added,
and the precipitate was isolated by suction filtration, washed
with excess ether, and collected. These complexes were used
without further purification. Yields ranged from 87 to 92%.
silica plug, giving a 47% yield (Experimental Section). The 4,4′-
PO H ) -bpy ligand was synthesized by a simple bromo-
(
3
2 2
General Procedure for [Ru(4,4′-R -bpy) (PO H -bpy)]-
2
2
3
2
trimethylsilane hydrolysis of the esterified ligand (4,4′-
(
Cl) . In a typical procedure, Ru(4,4′-R -bpy) Cl (0.12 mmol)
2
2
2
2
(PO Et ) -bpy), which has been previously reported (Exper-
3
2 2
and [2,2′-bipyridine]-4,4′-diyldiphosphonic acid (0.04 g, 0.12
27,45
imental Section).
mmol) were dissolved in 1:1 EtOH/H O (∼35 mL). The
2
All of the complexes reported herein have the same general
solution was then heated to 160 °C for 20 min in a microwave
oven. The solution was cooled, filtered, and dried by a rotary
evaporator. The crude product was purified by size exclusion
2+
structure: [Ru(4,4′-R -bpy) (4,4′(PO H ) -bpy)] , where R =
2
2
3
2 2
OCH , CH , H, or Br. The complexes were synthesized in good
3
3
yields (87−92%) by a systematic procedure to vary the
chromatography (Sephadex LH-20) with 1:1 H O/MeOH as
2
bidentate ligand, 4,4′-R -bpy. For the precursors, Ru(4,4′-R -
2
2
the eluent. Similar fractions (based on UV−vis absorption
spectra) were combined, and the solvent was removed by rotary
evaporation. The dark-red solids were triturated with ether and
collected.
bpy) Cl , 2 equiv of the 4,4′-R -bpy ligand was reacted with
2
2
2
28
poly-Ru(1,4-cyclooctadiene)Cl2 in o-dichlorobenzene at 180
C for 2 h under an argon atmosphere (Experimental
Section). Poly-Ru(1,4-cyclooctadiene)Cl was used as the
°
27
2
[
Ru(4,4′-Dimethoxy-2,2′-bipyridine) ([2,2′-bipyridine]-4,4′-
2
precursor in the study because it inhibits the formation of
diyldiphosphonic acid)]Cl (RuPOMe). Isolated as a red
powder (0.104 g, 90%). H NMR (600 MHz, D O): δ
2+
27
2
[Ru(4,4′-R -bpy) ] salts in the nonpolar o-dichlorobenzene.
2
3
1
2
Upon the addition of ether, the products, cis-Ru(4,4′-R -
2
(
2
3
ppm) 8.67 (d, 2H), 8.11 (dd, 4H), 7.73 (m, 2H), 7.52 (m,
H), 7.47 (d, 2H), 7.37 (d, 2H), 6.94 (dd, 2H), 6.89 (dd, 2H),
bpy) Cl , precipitate from the solution and were used without
2
2
further purification (Scheme 2). Limited solubility makes the
characterization of cis-Ru(4,4′-R -bpy) Cl complexes difficult.
.90 (s, 6H), 3.87 (s, 6H). HR-ESI-MS (MeOH; 20% H O
2
2
2
2
2
+
− 2+
with 1% HCOOH): m/z = 425.0457 = 850.09, [M − 2Cl ]
=
=
The chromophores were isolated as their chloride salts by the
2
+
−
+
2+
850.09, m/z = 849.0903 = 1698.1806, [M − 2Cl − H ]
2
reaction of Ru(4,4′-R -bpy) Cl with 1 equiv of 4,4′-(PO H ) -
2
2
2
3
2 2
1698.16. Anal. found (calcd.) for C H Cl N O P Ru: C
3
5
40
2
6
12
2
bpy in 1:1 EtOH/H O in a microwave oven reactor at 160 °C
2
4
3.53 (43.31), H 4.31 (4.15), N 8.84 (8.66).
for 20 min (Scheme 2). These reactions can be followed via
[
Ru(4,4′-Dimethyl-2,2′-bipyridine) ([2,2′-bipyridine]-4,4′-
2
UV−vis spectroscopy by monitoring the disappearance of
diyldiphosphonic acid)]Cl (RuPMe). Isolated as a red powder
46
2
Ru(4,4′-R -bpy) Cl absorption features and the growth of
2
2
2
1
(0.099 g, 92%). H NMR (600 MHz, D O): δ (ppm) 8.69 (d,
2+
2
[Ru(4,4′-R -bpy) (4,4′-(PO H ) -bpy)] absorption features
2
2
3
2 2
2
7
H), 8.33 (d, 4H), 7.72 (m, 2H), 7.50 (m, 2H), 7.46 (m, 4H),
.17 (m, 4H), 2.44 (s, 6H), 2.43 (s, 6H). HR-ESI-MS (80:20
(
Figure 5 and Table 2). The crude mixtures were each purified
by size exclusion chromatography (Sephadex LH-20) to yield
pure complexes.
2
+
NCMe/H O, 1% HCOOH): m/z = 384.0499 = 768.0996,
2
−
2+
2+
[
M − 2Cl ] = 786.1059, m/z = 785.1042 = 1570.2084, [M
1
The aromatic region of the H NMR spectrum of each
−
+
2+
2
−
2Cl − H ]
= 1570.196. Anal. found (calcd.) for
complex in D O is shown in Figure 2. The complexes have C
2
2
C H Cl N O P Ru: C 47.49 (47.31), H 4.50 (4.31), N 9.58
3
5
38
2
6
7
2
symmetry with a single 2-fold axis bisecting the 4,4′-(PO H ) -
3
2 2
(
9.46).
1
bpy ligand. The C symmetry is apparent in the H NMR
2
[
Ru(2,2′-Bipyridine) ([2,2′-bipyridine]-4,4′-diyldiphospho-
2
spectrum of each complex. There are three distinct resonances
nic acid)]Cl (RuP). Isolated as a red powder (0.086 g, 90%).
2
for the 4,4′-(PO H ) -bpy ligands in each complex, appearing
3
2 2
The characterization matches that of a previously reported
at ∼8.70, 7.75, and 7.50 ppm. The chemical shifts of these
ligands remain relatively unaffected by the variation of the 4,4′-
2
7
sample.
Ru(4,4′-Dibromo-2,2′-bipyridine) ([2,2′-bipyridine]-4,4′-
[
2
R -bpy ligand in the series (Figure 2). As expected, the proton
2
diyldiphosphonic acid)]Cl (RuPBr). Isolated as a red powder
2
resonances of the 4,4′-R -bpy ligands vary significantly through
2
1
(
4
0.120 g, 87%). H NMR (600 MHz, D O): δ (ppm) 8.94 (d,
2
the series, with the more electron-poor 4,4′-(Br) -bpy ligand
2
H), 8.73 (d, 2H), 7.71 (m, 2H), 7.62 (m, 8H), 7.52 (d, 2H).
having resonances shifted downfield relative to those of the
electron-rich 4,4′-(OCH ) -bpy and 4,4′-(CH ) -bpy ligands.
HR-ESI-MS (MeOH; 20% H O with 1% HCOOH): m/z =
2
3
2
3 2
5
(
(
22.83982 = 1045.6796, [M − 2Cl ] = 1045.68. Anal. found
+
− 2+
In addition, as a result of the C symmetry, the 4,4′-R -bpy
2
2
calcd.) for C H Br Cl N O P Ru: C 30.91 (30.79), H 2.52
30
28
4
2
6
9
2
ligands show six unique resonances for the six protons on each
ligand.
2.41), N 7.08 (7.18).
RESULTS AND DISCUSSION
■
Synthesis. 4,4′-Dibromo-bipyridine was synthesized by
modifying a reported procedure starting from commercially
4
4
available 4,4′-dimethoxy-bipyridine. In previous studies,
dimethylformamide (DMF) was used as the solvent for the
reaction between PBr and 4,4′-dimethoxy-bipyridine. Herein,
3
4
,4′-dimethoxy-bipyridine was dissolved directly in PBr , heated
3
to 180 °C, and the reaction was completed after 3 h and
followed by TLC (Scheme 1). Following neutralization and
extraction, purification was completed by passage through a
C
DOI: 10.1021/ic501682k
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Inorganic Chemistry
Article
2
+
Scheme 2. Syntheses of Ru(4,4′-R -bpy) Cl and [Ru(4,4′-R -bpy) (PO H ) -bpy)]
2
2
2
2
2
3
2 2
1
Figure 2. H NMR in D O of RuPBr (blue), RuP (green), RuPMe (orange), and RuPOMe (red).
2
All four complexes exhibit reversible Ru3+/2+ redox couples,
Table 1. Equilibrium Surface-Binding Parameters for
RuPOMe, RuPMe, RuP, and RuPBr
o
E ′ versus NHE, both in solution and on mesoporous TiO (eq
2
+/2+
2
, Table 2). The Ru3
redox potentials, as measured by
square-wave voltammograms (Table 2), are represented as E ′
values measured versus either a Ag/AgCl (0.198 V vs NHE) or
complex
Γmax (mol cm−2)a
Kad (M−1 × 105)
o
8
RuPOMe
RuPMe
6.7 × 10−
6.7 × 10−
8.5 × 10
6.6 × 10−
1.8
8
6.7
a Ag/AgNO (0.40 V vs NHE) reference electrode and are
b
−8
3
RuP
1.3, 0.39
1.5
cited “vs NHE”. They follow the expected trend of increasing
E ′, following the sequence RuPOMe < RuPMe < RuP <
RuPBr, with values ranging from 1.08 to 1.45 V (vs NHE)
8
RuPBr
o
a
Maximum surface coverages are reported on the basis of a per
b
47,48
micrometer thickness for 7 μm films. Previously reported.
when immobilized on TiO (Figure 3). The electronic nature of
2
R in the 4,4′-R -bpy ligand influences the π* acceptor energy
2
levels. In the complexes, the more electron-donating groups (R
1
9
studies. The maximum surface coverages (Γmax) range from
=
OCH or CH ) destabilize the bpy-π* orbitals, decreasing the
.6 × 10−8 (RuPBr) to 8.5 × 10 mol cm (RuP), suggesting
similar packing of the complexes on the TiO network.
Electrochemistry. The electrochemical properties of each
complex in solution (80:20 CH CN/H O with 0.1 M TBAPF
supporting electrolyte, TBA = tetrabutylammonium) and
immobilized on TiO2 in aqueous HClO4 (0.1 M) were
investigated by cyclic and square-wave voltammetry. The
CH CN/H O mixture (80:20) was used to investigate the
ligand-based reduction potentials (Ru ) under conditions
similar to those achieved using aqueous media without having a
−8
−2
3 3
6
II
extent of dπ−π* back-bonding from Ru to the 4,4′-R -bpy
2
2
6
ligand. Decreased back-bonding destabilizes the dπ core,
resulting in lowered Ru3
+/2+
redox potentials (Table 2). In
3
2
6
contrast, the electron-withdrawing 4,4′-Br-bpy ligand stabilizes
the π*(bpy) orbitals, increasing dπ−π* back-bonding, stabiliz-
6
o
3+/2+ 17,49−51
ing the dπ configuration, and increasing E ′(Ru
).
3
2
III
3+
2
+/+
[Ru (N−N)2(4,4′‐(PO3H2)2‐bpy)]
−
+e
II
2+
⎯
⎯ →⎯ [Ru (N−N) (4,4′‐(PO H ) ‐bpy)]
(2)
significant background H O reduction at the electrode.
2
3
2 2
2
D
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Inorganic Chemistry
Article
Table 2. Summary of Photophysical, Electrochemical, and Surface-Binding Properties for RuPOMe, RuPMe, RuP, and RuPBr
o
o
o
o
o
E ′
E ′
(Ru
absorbance
emission
ΔG
E ′
E ′
E ′
ES
(eV)
a
b
c
3+/2+
d
3+/2+
e
2+/+
e
3+/2+*
g
2+*/+
h
complex
λ (ε)
λmax
(Ru
)
(Ru
)
(Ru
)
(Ru
)
)
RuPOMe
477
708
685
667
644
1.97
1.08
1.19
1.28
1.45
1.05
1.16
1.27
1.40
−1.33
−1.33
−1.29
−1.09
−0.89
−0.82
−0.80
−0.69
0.64
0.68
0.80
1.05
(
11 800)
RuPMe
RuP
461
2.01
2.09
2.14
(12 800)
458
(12 00)
f
RuPBr
465
(13 400)
a
Absorbance λ represented in nanometers. Dominant metal-to-ligand charge transfer (MLCT), which is an absorption feature in H O, represented
2
−1
−1
b
c
in mol L cm . Emission λ represented in nanometers. Sample loaded onto ZrO in argon-deaerated aqueous HClO (0.1 M) at 23 °C. Values
for ΔG are from a Franck−Condon analysis of emission spectra on ZrO in aqueous HClO (0.1 M); see text. Values reported versus NHE from
square-wave voltammograms in aqueous 0.1 M HClO . Measurements were carried out using an FTO|TiO derivatized with a Ru-complex working
electrode, a Pt-wire counter, and a Ag/AgCl reference electrode (0.198 V vs NHE). Values reported versus NHE from square-wave
voltammograms; samples were dissolved in a 80:20 CH CN/H O mixture deaerated with argon. Measurements were carried out using a glassy-
max
2
4
d
ES
2
4
4
2
e
3
2
carbon working electrode, a Pt-wire counter, and a Ag/AgNO reference electrode (0.40 V vs NHE). Irreversible redox couple. E ′(Ru3+/2+*) =
f
g
o
3
o
3+/2+
h
o
2+*/+
o
2+/+
E ′(Ru
) − ΔG . E ′(Ru
) = E ′(Ru ) + ΔGES.
ES
II
2+
[
Ru (4,4′‐Br ‐bpy) (4,4′‐(PO H ) ‐bpy)]
−
2
2
3
2 2
+
e
II
·−
+
⎯
⎯ ⎯→ [Ru (4,4′‐Br ‐bpy )(4,4′‐Br ‐bpy)(4,4′‐PO H ) ‐bpy)]
2
2
3
2 2
(4)
Each complex shows multiple reduction waves within the
potential window of the experiments with scans extended to
−
1.8 V versus NHE. As an example, three ligand-based
reduction waves appear for RuPMe between −0.8 V and −1.8
V (vs NHE), Figure 4, arising from reduction at 4,4′-(PO H ) -
3
2 2
bpy followed by reduction at both of the 4,4′-(CH ) -bpy
3
2
ligands.
Figure 3. Square-wave voltammograms of RuPOMe (blue), RuPMe
(
green), RuP (black), and RuPBr (red), using derivatized FTO|TiO2
as the working electrode, a Pt counter, and a Ag/AgCl reference
electrode (0.198 V vs NHE) in aqueous HClO (0.1 M).
4
The first ligand-based reduction potential (E ′(Ru2+/+)) for
o
each complex in solution (in 80:20 CH CN/H O, 0.1 M
3
2
TBAPF6 supporting electrolyte, Pt-wire counter, and Ag/
AgNO reference electrode) is listed in Table 2. The first
3
reduction of the complexes follows a trend similar to that of the
o
3+/2+
E ′(Ru
) couples with the most electron-withdrawing ligand
Figure 4. Square-wave voltammogram for RuPMe, using 1.0 mM of
(
4,4′-Br -bpy in RuPBr) resulting in the most positive
2
the complex dissolved in an 80:20 mixture of CH CN/H O, a 0.1 M
3
2
reduction potential. The first reduction of RuPBr (−1.09 V
TBAPF supporting electrolyte, a Pt-wire counter, and a Ag/AgNO
6
3
vs NHE) is significantly more positive than those of RuP
reference (0.40 V vs NHE).
(
(
−1.29 V vs NHE), RuPMe (−1.33 V vs NHE), and RuPOMe
−1.33 V vs NHE). The positive shift from −1.33 V (RuPOMe
and RuPMe) to −1.09 V (RuPBr) is due to the lowering of the
UV−Vis Absorption Spectra. The absorption spectra of all
of the complexes in aqueous solution feature intense π → π*
energy of the π*-acceptor orbitals of 4,4′-Br -bpy relative to
2
those of 4,4′-(OCH ) -bpy or 4,4′-(CH ) -bpy; this is
3
2
3 2
4
4
−1
absorptions below 350 nm (ε ≈ 4.3 × 10 −5.7 × 10 M
attributable to the electron-withdrawing Br atoms in the
bipyridine framework. RuPOMe, RuPMe, and RuP have
similar first-reduction potentials, which is consistent with a
largely 4,4′-(PO H ) -bpy-based assignment (eq 3). In
−
1
cm ) and lower-energy metal-to-ligand charge-transfer
MLCT) absorptions from 400 to 500 nm (Figure 5 and
(
Table 2, single spectra are available in the Supporting
Information). Although there are slight variations in MLCT
λmax,abs values in the series, there is no obvious correlation
between the electron-donating or -withdrawing nature of the
3
2 2
contrast, the first reduction of RuPBr is significantly more
positive, pointing to a reduction at 4,4′-(Br) -bpy (eq 4).
2
II
2+
[
Ru (4,4′‐R ‐bpy) (4,4′‐(PO H ) ‐bpy)]
4,4′-R -bpy ligands and these values. The lack of correlation
2
2
3
2 2
2
−
+
e
shows that although the dπ orbitals are stabilized by the
II
·− +
⎯
⎯ →⎯ [Ru (4,4′‐R ‐bpy) (4,4′‐(PO H ) ‐bpy )]
(3)
2
2
3
2 2
electron-withdrawing 4,4′-R -bpy ligands resulting in an
2
E
DOI: 10.1021/ic501682k
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Inorganic Chemistry
Article
Figure 5. UV−visible absorption spectra of RuPOMe, RuPMe, RuP, and RuPBr dissolved in unbuffered H O, pH ≈ 6.5, at 23 °C.
2
Table 3. TD-DFT-Calculated MLCT Absorption Energies and Oscillator Strengths in Water
chromophore
excitation (nm)
460
oscillator strength
orbital contribution
RuPOMe
0.18
0.1
Ru dπ → π* (PO H ) -bpy
3
2 2
4
09
455
11
443
11
431
29
Ru dπ → π* OMe-bpy
RuPMe
RuP
0.126
0.155
0.179
0.153
0.18
0.2
Ru dπ → π* (PO H ) -bpy
3
2 2
4
Ru dπ → π* Me-bpy
Ru dπ → π* (PO H ) -bpy
3
2 2
4
Ru dπ → π* bpy
Ru dπ → π* Br-bpy + π* (PO H ) -bpy
Ru dπ → π* (PO H ) -bpy + π* Br-bpy
3 2 2
RuPBr
3
2 2
4
increase in E ′(Ru3+/2+) there is a compensating stabilization in
o
the energies of the π*-acceptor orbitals.
TD DFT calculations were performed to justify and to
quantify the spectral assignments associated with the electronic
transitions (Figure S6). Complex geometries were optimized
using DFT (B3LYP/LanL2DZ functional/basis set), and
optimized geometries were used in the TD DFT (PBE0/
LanL2DZ functional/basis set) calculations with a continuum
model to account for solvation by H O. The computed spectra
2
are blueshifted relative to the experimental spectra, which is
likely caused by the inherent TD DFT overestimation of
MLCT energies in Ru polypyridyl complexes as well as solvent
effects that are not adequately described by the PCM used
Figure 6. UV−visible spectrum of RuPMe at 23 °C in H O (black
2
1
6,52
line) and calculated TD DFT transitions (vertical red bars, the heights
of these illustrate the oscillator strengths red-shifted by 0.15 eV).
here.
Nevertheless, the experimental and computed spectra
correlate well with the strong π → π* absorptions predicted
below 300 nm and MLCT absorptions at longer wavelengths
transition energies are shown as vertical bars whose heights
reflect their relative oscillator strengths. The calculations show
the split in the MLCT manifold between the MLCT transitions
to π*(4,4′-(CH ) -bpy) and to π*(4,4′-(PO H ) -bpy) with
(
Table 3 and Figure S6). The calculations verify the origins of
the visible absorptions as excitations arising from dπ → π*
transitions to either the 4,4′-R -bpy ligand (eq 5) or the 4,4′-
2
3
2
3
2 2
(
PO H ) -bpy ligand (eq 6). Furthermore, TD DFT predicts
3 2 2
the higher-energy MLCT Ru dπ → π* (4,4′-(CH ) -bpy) (eq
3
2
that MLCT transitions to the ancillary 4,4′-R -bpy ligand in
2
5
) and the lower-energy MLCT Ru dπ → π*(4,4′-(PO H ) -
3
2 2
RuP, RuPMe, and RuPOMe are higher in energy than those to
the 4,4′-(PO H ) -bpy ligand (Table 3).
bpy) (eq 6). Figure S7 shows the orbital contribution for both
transitions.
3
2 2
II
2+
Steady-State Emission Spectra. All four complexes
exhibit broad emission spectra at room temperature when
surface-bound to ZrO in deaerated aqueous HClO (0.1 M).
[
Ru (4,4′‐R ‐bpy) (4,4′‐(PO H ) ‐bpy)]
2
2
3
2 2
hv
→
II
·−
2+*
[Ru (4,4′‐R ‐bpy )(4,4′‐R ‐bpy)(4,4′‐(PO H ) ‐bpy)]
2
2
3
2 2
2
4
(5)
The emission spectrum for each complex is shown in Figure 7,
and the emission energies are listed in Table 2. The emission
II
2+
[
Ru (4,4′‐R ‐bpy) (4,4′‐(PO H ) ‐bpy)]
4
2
2
3
2 2
energies decrease from RuPBr (λ = 644 nm, 1.55 × 10
max
1 4
−
−1
hv
→
II
·− 2+*
cm ) to RuPOMe (λ
= 708 nm, 1.41 × 10 cm ).
max
Emission from these complexes occurs from the lowest-lying
[Ru (4,4′‐R ‐bpy) (4,4′‐(PO H ) ‐bpy )]
(6)
2
2
3
2 2
3
Figure 6 compares the calculated and the experimental
MLCT excited states, following intersystem crossing from the
1
16,17,53,54
electronic absorption spectra for RuPMe in H O; the calculated
initial MLCT excited states that dominate absorption.
2
F
DOI: 10.1021/ic501682k
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
treated classically and included in the bandwidth at half height,
Δv , with Δv defined in eq 7. In eq 7, λ0,L is the sum of the
1
̅
/2
1/2
̅
solvent reorganization energy, λ , and reorganization energy
0
from low-frequency modes, λ ; E is the 0−0 energy gap, the
L
0
energy of the excited state above the ground state with both
states in the v = 0 vibrational levels; k is the Boltzmann
B
constant; and T is the temperature (298 K).
(
Δν1/2)2
6k T ln 2
ΔGES = E + λ = E +
0
0,L
0
1
(7)
B
Results of the spectra-fitting analysis are summarized in
Table 4. The free-energy content of the excited states (ΔGES)
Figure 7. Normalized emission spectra of RuPBr (black), RuP
(
green), RuPMe (red), and RuPOMe (blue); samples were loaded
−8
−2
onto ZrO (Γ ≈ 8 × 10 mol cm ) in argon-deaerated aqueous
HClO (0.1 M) at 23 °C following excitation at 450 nm.
2
Table 4. Emission-Spectra Fitting Parameters Derived from
MLCT Photoluminescence of RuPOMe, RuPMe, RuP, and
RuPBr Loaded onto ZrO in Deaerated Aqueous HClO (0.1
4
2
4
Trends in the emission energies (λmax,em, v ) follow those of
̅
em
o
3+/2+
M) at 25 °C
E ′(Ru
), with more positive values resulting in higher
emission energies. This is illustrated in Figure 8 by the linear
E
Δv
ℏω
ΔG
ES
0
1
̅
/2
)
M
−1
−1
−1
complex
(cm−1)
(cm
(cm
)
SM
(cm )
RuPOMe
RuPMe
RuP
14 300
14 700
15 200
15 700
1920
1850
1930
1870
1350
1350
1350
1350
0.89
0.86
0.79
0.90
15 900
16 200
16 800
17 300
RuPBr
were calculated by using eq 7. As shown in Tables 2 and 4,
trends in ΔG mirror those of the emission energies across the
ES
series. Both the free-energy content of the excited state (ΔG )
ES
o
3+/2+
and the 0−0 energy gap (E ) increase as E ′(Ru
) increases
0
(
Figure 9). This trend is expected because the emission energy
is dependent on the energy of the dπ levels rather than on the
π* levels (see above and Figure 8).
Figure 8. Dependence of emission energy (λmax,em, v m) on
e
̅
o
3+/2+
E ′(Ru
) in aqueous HClO (0.1 M) at 25 °C; samples were
4
o
3+/2+
bound to a metal oxide surface (TiO for E ′(Ru
) and ZrO for
2
2
v ).
em
̅
dependence of the emission energy on E ′(Ru3+/2+). This
suggests that variations in excited-state energies with ligand
changes are mainly a consequence of variations in the energy of
o
1
6,55
the metal-based dπ orbitals.
There is no correlation
o
2+/+
between emission energies and the ligand-based E ′(Ru
)
values (Figure S8).
EMISSION-SPECTRA FITTING: CORRELATION OF
EXCITED-STATE PROPERTIES
■
Figure 9. Dependence of the free-energy content of the excited state
(
ΔG , blue circles) and the 0−0 energy gap (E ) on the ground-state
ES
0
Emission spectra for all complexes bound to ZrO in aqueous
o
3+/2+
2
oxidation potential (E ′(Ru
RuPBr.
)) for RuPOMe, RuPMe, RuP, and
HClO (0.1 M) at 25 °C were analyzed by use of a single-mode
4
1
6,19,56−60
Franck−Condon analysis.
In this analysis, the
contributions from medium-frequency (v) modes (bpy) are
treated as a single averaged mode with low-frequency modes
and the solvent included in the band widths. Emission spectra
were fit to a series of vibronic lines centered on the 0−0
EXCITED-STATE REDOX POTENTIALS
■
A motivation for synthesizing and characterizing this series of
complexes was to explore the role of ancillary ligand variation
on the light-absorption and redox properties of a series of
surface-bound complexes that could be used for possible
photoelectrochemical applications. As noted in the Introduc-
tion, key properties are broad light absorption in the visible
spectrum, excited-state electron injection into the conduction
bands of high-band-gap semiconductors, and sufficient
component at energy E and separated by a vibrational
0
quantum spacing of ℏω . Only the transitions from the v′ =
M
0
level in the excited state to level v in the ground state were
included in the summation.
In the spectral fits, relative intensities of the vibronic lines are
determined by the electron-vibrational coupling constant, SM,
which is related to the equilibrium displacement change, ΔQ ,
eq
1
2
3+
by / (ΔQ ) . As noted above, additional vibrational
contributions from low-frequency modes and the solvent are
potential to drive water-oxidation catalysis as Ru . In the
current series of complexes, the dominating MLCT absorptions
2
eq
G
DOI: 10.1021/ic501682k
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
in the visible spectrum remain relatively constant across the
the series with variations in E ′(Ru3+/2+) that are induced by
o
series (Figure 5), even with significant variations in
varying the 4,4′-R -bpy ligand from 1.08 to 1.45 V (vs NHE).
2
o
3+/2+
o
2+/+
3+
E ′(Ru
) and E ′(Ru ) (Figure 1 and Table 2).
The enhanced oxidative-ground-state potential for Ru comes
To quantify the impact of ligand variation on excited-state
at the price of a decrease in the excited-state oxidation
potential, with Eo′(Ru3+/2+*) increasing in the same series from
o
redox potentials, E ′ values for the excited state acting as an
oxidant, Ru2
+*/+
(eq 8), and reductant, Ru
3+/2+*
(eq 9), were
−0.89 to −0.69 V (vs NHE).
As a particular example, E ′(Ru3+/2+) for RuPBr immobilized
o
calculated from the ground-state potentials in Table 1 and the
free energies of the excited state above the ground state, ΔG ,
on TiO in aqueous HClO (0.1 M) is 1.45 V (vs NHE) with
ES
2
4
1
6,18,23,55
determined by emission-spectra fitting in Table 2.
light-absorption properties comparable to those of RuP (Figure
o
5
). An E ′ of this magnitude provides the thermodynamic basis
Eo′(Ru2+*/+) = E ′(Ru2+/+) + ΔG
o
(8)
(9)
ES
for driving water-oxidation catalysis. However, the exchange of
bpy for 4,4′-Br -bpy increases E ′(Ru3+/2+*) from −0.80 to
0.69 V (vs NHE), lowering the thermodynamic driving force
o
Eo′(Ru3+/2+*) = Eo′(Ru3+/2+) − ΔG
2
ES
−
Similar to the correlations previously reported for complexes
for electron injection and likely resulting in slower and less
2
+
II
II
5,24,64
of the type [M(bpy) (L)] (where M = Ru or Os and L is a
efficient electron injection.
Analysis of electron injection
2
bidentate, neutral four-electron-donor ligand), metal-based
efficiencies and kinetics is currently under investigation for this
series of complexes.
3
+/2+
5
6
potentials for both ground-state Ru
(dπ /dπ ) and
excited-state Ru2
+*/+
(dπ π* /dπ π* ) redox couples decrease
5
1
6
1
6
1−63
CONCLUSIONS
linearly with emission energy.
centered ground-state Ru
In contrast, the ligand-
■
2
+/+
6
6 1
(dπ /dπ π ) and excited-state
We describe here the study of a series of polypyridyl complexes
in which ligand variations were used systematically to modify
excited- and ground-state properties of the complexes. A target
was the synthesis of a series of Ru(II) polypyridyl
chromophores for potential applications in DSPEC devices.
The approach taken was to prepare a family of chromophores
with the common 4,4′-(PO H ) -bpy ligand for surface-binding
Ru3
+/2+*
(dπ /dπ π* ) couples decrease by only half the
5
5
1
magnitude of Ru2
+*/+
and Ru3+/2+ potentials (Figure 10). These
observations reinforce the conclusion that variations in Ru-dπ
levels (and not variations in π*(bpy)) are the major factor
influencing excited-state redox potentials.
3
2 2
to oxides, with variations in the remaining ligands being used to
modify the electronic structure and thus alter light-absorption
and excited-state properties, including redox properties.
Variations in the polypyridyl ligand have been shown to result
in a related series of complexes in which the ground-state
o
3+/2+
E ′(Ru
) values vary from 1.08 to 1.45 V (vs NHE) without
significant loss in visible light absorption, as observed by UV−
visible spectroscopy and analyzed by TD DFT calculations. The
insensitivity of light absorption to ligand changes is due to the
changes in ligand π* acceptor levels being compensated for by
changes in dπ levels, resulting in a nearly constant energy gap.
This electronic-compensation effect results in enhanced
ground-state oxidizing strength and a parallel decrease in
excited-state reducing strength, with the latter decreasing the
driving force for electron injection.
Figure 10. Variation of ground- and excited-state redox potentials with
variation of the emission energy for each complex.
ASSOCIATED CONTENT
■
Figure 11 illustrates the variation of E ′(Ru3+/2+*) with
o
*
S
Supporting Information
o
3+/2+
variation of E ′(Ru
) across the series. An important feature
in the data is the increase in oxidizing strength of Ru across
3+
UV−visible spectra, square-wave voltammograms, TD DFT
results, surface-loading isotherms, orbitals contributing to the
MLCT exitations for RuPMe, and plot of variation of
v (λmax,em) for RuP, RuPBr, RuPMe, and RuPOMe. This
material is available free of charge via the Internet at http://
pubs.acs.org.
̅
em
AUTHOR INFORMATION
■
Corresponding Author
*
E-mail: tjmeyer@unc.edu.
Present Address
J.J.C.: Chemistry Department, Building 555, Brookhaven
National Laboratory, P.O. Box 5000, Upton, New York
1
1973-5000, United States.
Figure 11. Variation of the excited-state reduction potential (Ru3+/2+*)
Notes
with variation of the ground-state oxidation potential (Ru3 ).
+/2+
The authors declare no competing financial interest.
H
DOI: 10.1021/ic501682k
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Inorganic Chemistry
Article
(
28) Doi, T.; Nagamiya, H.; Kokubo, M.; Hirabayashi, K.; Takahashi,
ACKNOWLEDGMENTS
■
T. Tetrahedron 2002, 58, 2957.
29) Lee, S.-H. A.; Abrams, N. M.; Hoertz, P. G.; Barber, G. D.;
Halaoui, L. I.; Mallouk, T. E. J. Phys. Chem. B 2008, 112, 14415.
30) Song, W.; Glasson, C. R. K.; Luo, H.; Hanson, K.; Brennaman,
This work was primarily supported by the UNC Energy
Frontier Research Center (EFRC) for Solar Fuels, an EFRC
funded by the U.S. Department of Energy, Office of Science,
and Office of Basic Energy Sciences (DOE BES) under award
DE-SC0001011, supporting M.K.B., R.J.B., S.K., and J.J.C. We
thank the Department of Energy Office of Science Graduate
Fellowship Program (DOE SCGF) for a fellowship for D.L.A.,
made possible by the American Recovery and Reinvestment
Act of 2009, administered by ROISE-ORAU under contract
number DE-AC05-06OR23100.
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