Zinc oxide nanocrystal quenching of emission from electron-rich ruthenium-bipyridine complexes

Article abstract of DOI:10.1039/c4dt03272a

A series of heteroleptic bipyridine ruthenium complexes were prepared using known synthetic methods. Each compound incorporated one electron withdrawing 4,4′-dicarboxylic acid-2,2′-bipyridine and two bipyridines each of which had electron donating dialkylamine substituents in the 4 and 4′ positions. The electronic absorption spectra exhibited absorptions that moved to lower energy as the donor ability of the amine substituent increased. Density functional calculations established that the HOMO was delocalized over the metal and two pyridine groups located trans to the pyridines of the dicarboxylic acid bipyridine. The LUMO was delocalized over the dicarboxylic acid bipyridine. Cyclic voltammetry of the deprotonated compounds exhibit one quasi-reversible oxidation and three reductions. Coupled with the emission data, the excited state reduction potentials were estimated to range from -0.93 to -1.03 V vs. NHE. Monodispersed 3.2 nm diameter nanocrystals (NCs) of zinc oxide were found to quench partially the excited state of the dyes via a static quenching electron transfer process involving the formation of a dyad of the complex and the NC. The magnitude of the partial quenching of complexed dyes was correlated to the distribution of band gaps for the NCs, which is an inverse function of diameter. Dyes attached to the NCs on the small end of the particle size distribution had electron transfer rates that were uncompetitive with radiative and nonradiative decay mechanisms. This journal is

Full text of DOI:10.1039/c4dt03272a

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Zinc oxide nanocrystal quenching of emission
from electron-rich ruthenium-bipyridine
Cite this: DOI: 10.1039/c4dt03272a
complexes†
Ryan J. Hue, Rajan Vatassery, Kent R. Mann and Wayne L. Gladfelter*
A series of heteroleptic bipyridine ruthenium complexes were prepared using known synthetic methods.
Each compound incorporated one electron withdrawing 4,4’-dicarboxylic acid-2,2’-bipyridine and two
bipyridines each of which had electron donating dialkylamine substituents in the 4 and 4’ positions. The
electronic absorption spectra exhibited absorptions that moved to lower energy as the donor ability of the
amine substituent increased. Density functional calculations established that the HOMO was delocalized
over the metal and two pyridine groups located trans to the pyridines of the dicarboxylic acid bipyridine.
The LUMO was delocalized over the dicarboxylic acid bipyridine. Cyclic voltammetry of the deprotonated
compounds exhibit one quasi-reversible oxidation and three reductions. Coupled with the emission data,
the excited state reduction potentials were estimated to range from −0.93 to −1.03 V vs. NHE. Mono-
dispersed 3.2 nm diameter nanocrystals (NCs) of zinc oxide were found to quench partially the excited
state of the dyes via a static quenching electron transfer process involving the formation of a dyad of the
complex and the NC. The magnitude of the partial quenching of complexed dyes was correlated to the
distribution of band gaps for the NCs, which is an inverse function of diameter. Dyes attached to the NCs
on the small end of the particle size distribution had electron transfer rates that were uncompetitive with
radiative and nonradiative decay mechanisms.
Received 23rd October 2014,
Accepted 28th January 2015
DOI: 10.1039/c4dt03272a
www.rsc.org/dalton
In previous work we demonstrated that ZnO NCs quenched
the excited state by electron transfer of two carboxylate-bound
Introduction
Interfacial electron transfer from the photoexcited state of a fluorophores,
sensitizer to the conduction band of the metal oxide semi- porphyrin.
a
terthiophene derivative and
Although capable of binding to the ZnO NCs,
conductor is an important event leading to charge separation in the ruthenium-polypyridyl complex, [Ru(bpy) (dcbpy)](PF )
a
zinc
1
7–19
2
6 2
1
–4
dye-sensitized solar cells. The dynamics of this process have where bpy = 2,2′-bipyridine and dcbpy = 4,4′-dicarboxylic acid-
been examined with a variety of time-resolved and steady-state 2,2′-bipyridine, exhibited fluorescence that was not quenched.
4
–7
spectroscopic methods on metal oxide nanocrystal films.
This was initially unexpected based on the reported fluo-
2
+
The heterogeneous nature of the surface environment, rescence quenching of [Ru(bpy) (dcbpy)] when it was bound
2
2
0
however, complicated electron transfer dynamics leading to to a colloidal ZnO film. Hypothesizing that its excited state
5
,8–14
multiple injection time scales for a given sensitizer.
Sub- was not sufficiently reducing, in this report we describe the
stitution of the nanocrystalline film with monodispersed preparation of a series of ruthenium complexes in which two
metal oxide nanocrystals (NCs) offers an alternative with a amine-substituted bipyridines serve to increase the excited
2
1–24
potentially more uniform environment. This has recently been state reduction potential of the complex.
The third bipyri-
done using TiO dispersions, where injection times were too dine, dcbpy, is also present as a means to attach the dye to the
2
6
,15
fast to be measured accurately.
Zinc oxide (ZnO) offers ZnO NCs. The amines increased the excited state potential by
similar properties to TiO , but has the added advantage that approximately 0.2 V and partial fluorescence quenching by the
2
synthetic routes are available that generate monodispersed NCs was observed. Proton NMR spectroscopy served as an
NCs that are stable in common solvents and are optically independent assessment of binding the dye to the NCs.
1
6
transparent.
Results
Ruthenium complexes
Department of Chemistry, University of Minnesota, 207 Pleasant St. SE,
Minneapolis, MN 55455, USA. E-mail: wlg@umn.edu
Fig. 1 shows the structure of the six ruthenium complexes
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt03272a
each of which comprise two different bipyridine ligands.
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Fig. 2 Absorption (black) and emission (blue) of 1 (solid lines) and
−
1-CO
2
(dashed lines) with [Me
4
N][OH] in methanol. When one equivalent
+
of additional H is added, the emission of 1 disappears.
2
+
ent, the corresponding Ru
compound was formed as
expected and subsequently reacted with dcbpy to yield 6. With
the N-methylpiperazino substituent, the initial reaction pro-
ceeded directly to {Ru[(mepipz)
2 3 2
bpy] }Cl , thus precluding its
use for the synthesis of 5. The successful alternative synthesis
6
of 5 involved reaction of {[(η -p-cymene)RuCl]
2
2
(μ-Cl) } with two
equivalents of dcbpy in DMF followed by addition of a slight
Fig. 1 Molecular structure of the substituted tris-bipyridine ruthenium excess of (mepipz) bpy and reflux.
2
complexes (1–6).
The compounds were characterized using mass spectro-
metry and electronic absorption, luminescence, infrared
1
and H NMR spectroscopies, and cyclic voltammetry. Com-
Common to all is the 4,4′-dicarboxylic acid-2,2′-bipyridine pound 1 was also characterized by elemental analysis. Table 1
dcbpy), which binds the complexes to the ZnO NCs through lists the longest wavelength absorption and emission
(
the carboxylate groups. The remaining two bipyridines were maximum for each of the deprotonated dyes. A complete
substituted in the 4 and 4′ positions with strong electron tabular listing of the electronic absorption maxima and extinc-
donating dialkylamino substituents. All syntheses were based tion coefficients for both the protonated and deprotonated
on known methods. Compounds 1–4 were prepared by reac- dyes is available in the ESI Table 1.† Fig. 2 displays the absorp-
−
tion of dcbpy and triethylamine with the paramagnetic bis- tion spectra of 1 and 1-CO
bipyridine) intermediates, [Ru(R bpy) Cl
plexes were prepared using the method of Viala and Coudret,
which was expected to yield the divalent ruthenium com- absorptions from 400–600 nm with molar extinction coeffi-
2
. The complexes have intense
(
2
2
2
]Cl. The latter com- absorptions around 315 nm corresponding to the bipyridine
2
5
π–π* transitions. In the visible region, all complexes have
. Presumably the dialkylamino substitu- cients ranging from 12 000–20 000 M− cm attributed to
1
−1
pounds, Ru(R bpy) Cl
2 2 2
ents help to stabilize the trivalent ruthenium compounds, metal to ligand charge transfer transitions. The lowest energy
which were the observed products. With the methoxy substitu- absorptions in compounds 1–6 exhibit an approximate corre-
Table 1 Spectroscopic and electrochemical properties of deprotonated 1–6
a
a,b
c
III
II
c
2+
3
+
3
c
+
3
0
c
0
/RuL −
d
III
II
Dye anions
λ
abs (nm)
λ
em (nm)
E° (V) Ru /Ru
E° (V) RuL
/RuL
E° (V) RuL
/RuL
3
E° (V) RuL
3
3
E* Ru /Ru
−
1-COO
2-COO
3-COO
4-COO
5-COO
6-COO
515
520
502
499
498
475
748
752
742
723
725
680
0.832
0.779
0.865
0.942
0.940
1.22
−1.40
—
−1.37
−1.33
−1.34
−1.20
−1.71
—
−1.64
−1.55
−1.58
−1.36
−1.92
—
−1.82
−1.76
−1.78
−1.57
−0.99
−1.03
−0.96
−0.94
−0.93
−0.76
−
−
−
−
−
a
b
c
Recorded under argon at 100 mV s−1 using [ Bu
n
Measured in CH
3
OH at 25 °C.
λExcit = λabs
.
4
N][PF
6
] as the supporting electrolyte in
d
dimethylformamide and referenced to a Ag/AgCl electrode. All potentials are listed vs. NHE. Excited state reduction potential.
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Fig. 3 HOMO (left) and LUMO (right) of 1.
lation with the donor characteristics of the bipyridine substitu-
ents. For 1-CO2−, λmax appears at 515 nm, while for 6-CO2− it
shifts to 475 nm.
The push–pull nature of the electronic structures of 1–6 is
evident from density functional calculations. Fig. 3 shows DFT
(B3LYP) results that verify the LUMO resides primarily on the
dcbpy ligand and that the HOMO resides on the ruthenium
metal center and extends onto the amino-substituted bipyri-
dine rings and the amino nitrogen. The ligand-based orbitals
contributing to the HOMO involve one-half of each of the
amino-substituted bipyridine ligands, each of which is
oriented in the xy-plane (defined as the plane of the dcbpy
ligand) for favorable π-overlap with the Ru dxy orbital. The anti-
bonding nature of this interaction emphasizes the strong
π-donor property of the amino-bpy groups, thus raising the
energy of this orbital. The next highest occupied molecular
Fig. 4 Cyclic voltammagrams of 1-CO −
in DMF with 0.1 M [ Bu
n
N]-
2
4
orbitals exhibit a similar interaction between the pyridine [PF
6
] as the supporting electrolyte. The sample was degassed with argon
and referenced to a Ag/AgCl electrode.
groups (perpendicular to the Ru-dcbpy plane) and the Ru dxz
and dyz orbitals. The results for the remaining complexes,
including {Ru[(Et N) bpy] [(Cl) bpy]}(PF ) are similar to the
2
2
2
2
6 2
2
6
results obtained for 1 presented in Fig. 3. As noted by others,
of a zwitterionic structure, in the solid state. Deprotonation of
when push–pull chromophores used in dye-sensitized solar 1 leads to disappearance of the 1700 cm− absorbance, but the
cells are attached to the semiconductor via the functional C–O stretch of the carboxylate is obscured by the intense aro-
group (carboxylates is many cases) on the “pull” end, the matic C–C vibrations.
1
electron in the excited state is conveniently localized near the
point of attachment. This is expected to increase the efficiency Table 1 for the deprotonated dyes, was used to analyze the
of electron transfer. redox processes in the complexes. Compounds 1–6 show
Cyclic voltammetry, shown in Fig. 4 for 1-CO − and listed in
2
III
II
Deprotonation of 1–6 using [Me N][OH] causes a blue-shift reversible Ru /Ru oxidations between 0.80 and 1.37 V versus
4
in the MLCT bands. The deprotonated form of each com- NHE and irreversible reductions between −0.84 and −1.92 V.
pound exhibits a weak emission with a λmax listed in Table 1. Because it is the deprotonated state of the dyes that bind to
For 1-CO2−, the emission is also plotted in Fig. 2. The very the NCs, we have focused our attention primarily on these.
−
−
weak emission observed from solutions of pure 1 disappears Compounds 1-CO
2
to 6-CO
2
become easier to oxidize as the
III
II
following addition of acid suggesting that it may be due to Ru /Ru couple is shifted to lower potentials (0.78–1.22 V).
residual 1-CO2−
.
27
The infrared spectra in KBr (ESI Fig. 2†) of Also, upon deprotonation, three reversible reductions are now
the amino-substituted compounds 1, 3 and 4 exhibit distinct evident for the three bipyridine ligands of each of the com-
−
1
absorptions at 1700 cm attributable to the carboxylic acid. pounds. The difference between the first and second bpy
For 2 and 5, the lack of a specific absorption at 1700 cm− is reduction potential is slightly larger then that potential differ-
likely due to strong H-bonding, or even the possible existence ence between the second and third reduction. This is consist-
1
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ent with assigning the least negative reduction to the dcbpy
2
8
ligand. Also consistent with this assignment, the second and
third reduction potentials are more sensitive to changing the
dialkylamino substituents. Typically, for heteroleptic Ru(II)
coordination compounds the first ligand reduced electro-
chemically is also the same ligand on which the excited state
2
9
is localized in the thermally equilibrated excited state. Based
on the cyclic voltammetry, the relaxed MLCT excited state is
III
− 2+
2
attributed to [Ru (aminobpy) (dcbpy )] *, where the negative
charge is localized upon the ligand that binds to ZnO.
In the absence of direct measurement of the energy differ-
ence between the singlet and triplet excited states of the com-
plexes, the excited reduction potentials were estimated using
3
0
the method reported by Thomson and coworkers. This
involves adding the energy of the λmax value for room tempera-
3
+
ture emission to the standard reduction potential for the Ru
complex and subtracting from this result a correction constant
of 0.17 V, which was derived from more detailed studies of six
tris-(substituted-bipyridine) ruthenium complexes. In addition
a smaller correction of 0.03 V was added due to the entropy
change at 300 K associated with the change in spin multi-
plicity. Values for E* range from −1.03 to −0.76 V for 2 to 6,
respectively (Table 1). Considering that the size-dependent
conduction band minimum (CBM) of 3.2 nm ZnO NCs is
approximately −0.5 V vs. NHE, the ΔG° for quenching the
excited state will be negative for all of the dyes.
Probing complex binding to ZnO NCs
NMR spectroscopy was used to quantify the extent of dye
attachment to the surface of the NCs. Upon binding, the dye
resonances broaden and disappear into the baseline. This can
be attributed to the slow rotation of the dyad, hindered
31
motion of the molecule on the surface and, possibly, to hetero-
3
2–34
geneity in the binding sites on the surface of the NC.
1
Fig. 5 H NMR spectra of the aromatic region of 1 : ZnO (top) and
1
1
Fig. 5 shows the aromatic region of the H NMR spectra of
pure 1 and 1-CO , as well as in the presence of increasing
2
-Cl : ZnO (bottom) dyads at the given ratios.
−
amounts of ZnO NCs. The concentration of 1 is held constant
as the concentration of ZnO NCs is increased in successive
samples. With increasing NC concentration the resonances of
the protonated form decrease in intensity and shift upfield to
To verify that the broadening and eventual disappearance
−
−
2 2
of the aromatic signals of 1-CO and 6-CO was the result of
attachment to the surface of the NCs, the same set of measure-
ments were made using {Ru[(Et N) bpy] [(Cl) bpy]}(PF )
6 2
−
peaks characteristic of 1-CO
2
. The three resonances furthest
2
2
2
2
where the carboxylic acid groups were replaced with chloro
downfield represent the aromatic hydrogens of the dcbpy
ligand, with the resonance at 9 ppm attributed to the 3 and 3′
hydrogens ortho to the acid group. These hydrogens are
expected to be the most sensitive to the protonation state of
the complex. Resonances associated with the [(Et N) bpy]
−
2
substituents. As with 1-CO , the concentration of the ruthe-
nium complex was held constant while the concentration of
the ZnO was increased with each successive sample. The
spectra in Fig. 5 of 20 : 1 through 2 : 1, exhibited no change in
the intensity or width of the peaks as the NCs were added.
This indicates that the acid functionality is necessary for
binding to the surface of the NCs.
2
2
ligands are not affected significantly. At ratios of 20 : 1 and
−
1
0 : 1 signals due to free 1-CO
2
are still detectable, however, at
2
: 1 all resonances are indistinguishable from the baseline
indicating all dyes are bound. Based on estimates of the van
der Waal radii of the complexes, we suggest that a maximum
Excited state quenching using ZnO NCs
of six dyes could bind to a 3.2 nm NC. Conducting this set of Steady state Stern-Volmer quenching analysis was performed
experiments using 6 produced similar results, although dis- on the dyads described above using a constant concentration
appearance of the dye resonances occurred at a lower dye to of dye and an increasing amount of ZnO nanocrystals and
NC ratio.
comparing the emission to that of the deprotonated dye. Fig. 6
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Table 2 Stern-Volmer association constants for the corresponding dye
on 3.2 nm ZnO nanocrystals in methanol
Dye
SV (M− × 10−4
1
2
3
4
5
1
K
)
3.1
2.6
2.2
1.7
1.5
Fraction quenched at 2 : 1 dye : NC 0.51 0.50 0.54 0.43 0.39
−0.76 V, and does not show any quenching. Because we know
6
attaches to the surface of the ZnO NCs, the excited state
must not be of sufficient energy for electron transfer to be
competitive with competing radiative and nonradiative decay
mechanisms.
While these Stern-Volmer experiments prove that ZnO is
capable of quenching the emission of the excited dye, they do
not establish the mechanism. We assign the mechanism to
electron transfer based on three factors. First, in previous
reports of dyads comprising ZnO NC with terthiophene or por-
phyrin chromophores bound via carboxylates, ultrafast spectro-
scopic methods provided direct evidence for the formation of
an oxidized dye thus confirming the electron transfer quench-
Fig. 6 Emission from methanol solutions of pure 1-CO −
(solid line),
n
ZnO dispersions with subsequent additions of ZnO NCs at
2
−)
(
1-CO
2
constant dye concentration. As the concentration of ZnO increases the
emission intensity decreases to the final sample of 2 : 1 (λex = 415 nm).
1
7,18
ing mechanism.
Second, given the large energy differences
between the emission wavelengths of the dyes in this study
and the ZnO NC absorption, resonant energy transfer can be
ruled out as a quenching mechanism. Finally, there was no evi-
dence that concentration quenching is significant, as has been
3
5
found in related systems. It is possible that electron injection
could occur into surface trap states that exist on metal oxide
nanoparticles instead of into the empty conduction band.
In studies of excited state charge injection from N3 into ZrO2,
surface trap states were shown to serve as the electron
3
6
acceptor.
Discussion
While we are ultimately interested in measurement of the rates
of electron transfer to ZnO NCs from a series of structurally
similar donors, we discuss here a noteworthy correlation
between the fraction of static quenching and the excited state
energy of the dyes. Proton NMR spectroscopy established that
Fig. 7 Stern-Volmer plot of 1 (diamonds) and 2 (circles).
−
shows the emission of 1-CO
2
at various ZnO ratios from at a 2 : 1 molar dye to NC ratio all dyes present in the mixture
1
20 : 1 to 2 : 1. As the NC concentration increases, the emission were bound to the ZnO NCs. Further, by comparison of spectro-
−
intensity decreases by a factor of 0.51 of the 1-CO
2
alone. scopic changes in 1–6 vs. the lack of any observed changes
From the slope of the Stern-Volmer plot, Fig. 7, the association for {Ru[(Et N) bpy] [(Cl) bpy]}(PF ) the NMR data confirm
2
2
2
2
6 2
4
−1
constant (K ) was calculated to be 2.4 × 10 M . While the that the carboxylates present in 1–6 are integral to dye binding
sv
complexes exhibit different amounts of quenching, the and that the process involves static quenching.
relationship of I
0
/I versus concentration was linear in all cases.
Despite NMR evidence that all dyes were bound to NCs, at
Table 2 lists the Stern-Volmer association constants for the the 2 : 1 dye to NC ratio at most half of the excited states were
amino-substituted bipyridine complexes and Fig. 7 shows the quenched. The fraction of quenching correlated with the
−
−
Stern-Volmer plot for complexes 1-CO
2
and 2-CO
2
.
excited state potential. No quenching was observed for the
As the value of Ksv decreases, the fraction of excited states methoxy-substituted bpy derivative 6, which had the least
that are quenched also decreases in a pattern that correlates negative E* (−0.76 V). For the dialkylamino-substituted com-
roughly with excited state potential. The complexes with pounds, the fraction quenched ranged from 0.39 for 5 (E* =
amine substituents (1–5) have excited state potentials clustered −0.93 V) to 0.54 for 2 (E* = −1.03 V). Two possible explanations
around −1 V. Complex 6 has the lowest excited state value, at could explain the lowered quenching yields of bound dyes.
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3
7
First, based on studies of nanocrystalline ZnO films adsorp- using the values for the change in the band gap as a function
3
9
tion of dye molecules can lead to precipitates that are not elec- of size reported by Sarma and coworkers and adding it to the
tronically coupled to the core of the ZnO NCs, thus reducing bulk ZnO band gap and the work function measured on a
4
0
the electron transfer yield. While the nature of the precipitates nanocrystalline ZnO film. This calculation assigns the entire
has not been defined, as long as they lead to line broadening size-dependent change in the band gap to a change in the
in the NMR spectrum without quenching emission from the CBM; an assumption that over estimates its impact. This
dye, this could contribute to the low quenching yield.
relationship allowed the conversion of the Gaussian NC size
An alternative explanation that is not mutually exclusive distribution to a NC CBM distribution, which is plotted as a
with the possible formation of precipitates is that the trend in dashed line. Integration of the distribution (dotted line) shows
quenching yield correlates to the distribution of diameters the fraction of the NCs on the x-axis that have a conduction
associated with the ZnO NC synthesis. The ZnO NCs used in band minimum above the energy shown on the y-axis. The
this study had a mean diameter of 3.2 nm. Assuming a curve is offset from the experimental data by 0.44 V. Although
Gaussian distribution of NC diameters with a standard devi- the overestimate of the CBM will contribute a small amount to
1
6
ation of 0.32 nm (±10ꢀ), we expect an associated distri- this offset, we suggest that the reorganization energy (λ) associ-
bution of energies for the conduction band minima (CBM). ated with the electron transfer is the major factor. Reorganiz-
This means that a fraction of the dyes will be bound to smaller ation energies for electron transfer to and from
NCs where the corresponding decreased rate of electron trans- nanocrystalline metal oxide films or colloidal dispersions
1
fer becomes uncompetitive with the rates of emission and/or range from 0.3–2 V.
nonradiative decay. Although we use the term conduction
band minimum, we recognize that as the NCs decrease in dia-
meter the density of states decreases and the energy of the
Conclusions
lowest empty state rises. In quantum confined NCs, the lowest
3
8
empty state is the S state.
A series of electron-rich, heteroleptic tris-bipyridine ruthenium
e
Fig. 8 attempts to summarize the factors contributing to complexes were synthesized and characterized. Two of the bpy
this explanation. For each of the dyes the experimental fraction ligands were substituted with electron releasing dialkylamino
of unquenched dye is plotted on the x-axis as a function of E* substituents in the 4 and 4′ positions while the third ligand
on the y-axis. The solid line shows the energy of the CBM as a was dcbpy, which allowed the complex to be anchored to the
function of the NC diameter (top axis). This was calculated surface of ZnO NCs. The dialkylamino groups raised the
excited state reduction potential of the complex by at least
2
+
0
2
.2 V compared to [Ru(bpy) (dcbpy)] . This increase was
sufficient to allow a significant fraction of the excited states to
be quenched by charge injection into the ZnO NCs having a
mean diameter of 3.2 nm. The fraction of quenched dyes was
correlated to the NC size distribution as a result of the impact
of quantum confinement on the energy of the conduction
band minimum. The excited states of dyes bound to NCs on
the smaller end of the distribution continued to decay by fluo-
rescence and nonradiative pathways. This observation may be
important in constructing ZnO-based dye-sensitized solar cells
from NC dispersions. While smaller ZnO particles can have
the beneficial effect of increasing the effective surface area per
mole of ZnO and therefore the dye loading, particles that are
too small would reduce the yield of charge separation and
potentially the overall cell efficiency. Further work is in pro-
gress to measure the effect of ZnO NC diameter on the charge
injection yield and the rate of charge injection using ultrafast
spectroscopic methods.
Fig. 8 The solid black line shows the ZnO NC conduction band
minimum (CBM) energy as a function of NC diameter. The dashed red
line shows the CBM energy distribution for a 10% Gaussian distribution
in NC diameter centered at 3.2 nm. Integration of this distribution
Experimental
Materials
(
dotted line) shows the fraction of particles (top axis) having a CBM
Ruthenium(III) chloride trihydrate (RuCl ·3H O) was purchased
3
2
energy above the value shown on the vertical axis. The + symbols show
the experimental fraction of unquenched dye as a function of E*. The
solid blue line shows the integrated distribution (dotted line) raised in
energy by 0.44 V.
from Pressure Chemical Company and used as received.
Spectroscopic grade methanol, 2,2′-bipyridine, 4,4′-dimethyl-
2,2′-bipyridine, diethylamine, pyrrolidine, piperidine, morpho-
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6
line, N-methylpiperazine, tetramethylammonium hydroxide correct the reported potentials. In 0.1 M [TBA][PF ]/DMF its
4
5
(
NMe OH·5H O), and zinc acetate (Zn-(CH COO) ·2H O), were E° was 0.515 V (literature 0.495 V versus Ag/AgCl).
4 2 3 2 2
purchased from Sigma Aldrich and used as received. All
additional solvents were dried (unless water was used as a co-
Density functional calculations
solvent) and deoxygenated before use, and all procedures were Compounds 1, 3 and 5 were fully optimized at the density
performed using standard Schlenk techniques. 4,4′-Dicarb- functional level of theory using the B3LYP functional and the
oxylic acid-2,2′-bipyridine (dcbpy), 4,4′-dichloro-2,2′-bipyri- 6-31G* basis set for all elements except ruthenium. The
dine-N,N′-dioxide, 4,4′-dichloro-2,2′-bipyridine [(Cl) bpy], 4,4′- 6-31G** basis set was used for 2, 4 and {Ru[(Et N) -
2
2
2
bis(diethylamino)-2,2′-bipyridine [(Et
dino-2,2′-bipyridine [(pyrrol) bpy],
bipyridine [(pip) bpy], 4,4′-dimorpholino-2,2′-bipyridine the Gaussian 09 software suite.
4,4′-di(N-methylpiperazino)-2,2′-bipyridine, Synthesis of {Ru[(Et N) bpy]
bpy] 4,4′-dimethoxy-2,2′-bipyridine [(MeO) bpy], {Ru[(Et N) bpy] Cl }Cl (0.30 g, 0.38 mmol, 1 eq.), dcbpy
Syn- (0.27 g, 1.1 mmol, 3 eq.) and triethylamine (0.90 mL,
2
N)
2
2 2 6 2
bpy], 4,4′-dipyrroli- bpy] [(Cl) bpy]}(PF ) . For all compounds the SDD basis set
2
4,4′-dipiperidino-2,2′- was used for ruthenium. All calculations were performed using
46
2
4
7
[
[
(morph)
(mepipz)
2
bpy],
2
2
2 6 2
dcbpy}(PF ) , 1. A mixture of
2
2
1–43
2
2
2
2
4
were prepared according to literature procedures.
1
thetic details and H NMR spectroscopic data for the methoxy- 6.4 mmol, 17 eq.) was added to 25 mL of ethanol–water (4 : 1)
and amino-substituted bipyridines are available in the ESI.† and heated at reflux for 3 h. The solvent was evaporated and
Preparation of the intermediate bis-substituted bipyridine the resulting complex was chromatographed on a Sephadex
ruthenium complexes was based on the report by Viala and LH-20 column using acetonitrile–isopropanol (90 : 10) as
2
5
Coudret,
however, for the amino-substituted bipyridine eluent. The first band to elute yielded a red solid following
ligands the reducing agents (glucose and ascorbic acid) were removal of the solvent. This was dissolved in a minimum of
not strong enough to form the divalent ruthenium dichloro water and the desired complex was precipitated with the
6
complexes. Following chromatographic purification the for- addition of excess NaPF and isolated by filtration to give
3
+
1
mation of the paramagnetic Ru complex ions was confirmed 0.37 g (80ꢀ) of 1. H-NMR (300 MHz; CD CN): δ 9.48 (s br,
3
by mass spectrometry, and the powders were used in sub- 2H), 8.07 (d, J = 5.9 Hz, 2H), 7.82 (dd, J = 5.9, 1.3 Hz, 2H), 7.40
sequent preparation of the target dye molecules. Details are (d, J = 2.7 Hz, 2H), 7.36 (d, J = 2.7 Hz, 2H), 7.19 (d, J = 6.8 Hz,
available in the ESI.†
2H), 6.90 (d, J = 6.8 Hz, 2H), 6.61 (dd, J = 6.8, 2.7 Hz, 2H), 6.40
dd, J = 6.9, 2.7 Hz, 2H), 3.54 (q, J = 7.0 Hz, 8H), 3.47 (q, J =
.1 Hz, 8H), 1.19 (t, J = 7.0 Hz, 12H), 1.13 (t, J = 7.0 Hz, 12H).
(
7
Methods
2
+
HR ESI-MS m/z (obs) 471.1930 (M ), m/z (calcd): 471.1921.
Unless otherwise noted, all NMR spectra of the complexes and
ligands were taken on a Varian Inova 300 MHz spectrometer
and chemical shifts were referenced to the residual solvent
peak. All NMR spectra of dye:ZnO dyads were taken on a
Varian Inova 500 MHz spectrometer and chemical shifts were
referenced to the residual solvent peak. Mass spectral data
were taken on a Bruker BioTOF II in ESI-TOF mode. Electronic
absorption spectra were collected on an Ocean Optics
DH-2000-BAL spectrometer. The steady-state emission spectra
were recorded on a Spex Fluorolog 1680 0.22 m double spectro-
meter equipped with a xenon source. Front face fluorescence
spectra were corrected to compensate for changes in the
instrument’s sensitivity at longer wavelengths. UV-visible and
fluorescence spectroscopic measurements were performed in
quartz cuvettes with a 3 mm path length unless otherwise
noted. The IR spectra of ruthenium complexes (KBr pellet)
were recorded on a Nicolet MAGNA-IR 560 spectrometer.
Elem. Anal. Obs. C, 46.41; H, 4.78; N, 11.25. Calcd C 46.79; H,
4.91; N, 11.37.
Synthesis of {Ru[(pyrrol) bpy] dcbpy}(PF ) , 2. Using the
2
2
6 2
2 2 2
same procedure as above, {Ru[(pyrrol) bpy] Cl }Cl (0.15 g,
0.18 mmol) was reacted with dcbpy (0.14 g, 0.56 mmol, 3 eq.),
and triethylamine (0.50 mL, 3.2 mmol, 17 eq.). Compound 2
was the first band to elute from a Sephadex LH-20 column
using acetonitrile–methanol (90 : 10) as eluent. The isolated
1
yield was 0.17 g (75ꢀ). H-NMR (300 MHz; CD OD): δ 8.95 (s
3
br, 2H), 8.00 (d, J = 5.8 Hz, 2H), 7.73 (dd, J = 5.8, 1.5 Hz, 2H),
7
6
2
2
.55 (d, J = 2.5 Hz, 2H), 7.51 (d, J = 2.5 Hz, 2H), 7.27 (d, J =
.6 Hz, 2H), 7.01 (d, J = 6.6 Hz, 2H), 6.53 (dd, J = 6.7, 2.5 Hz,
H), 6.42 (dd, J = 6.7, 2.5 Hz, 2H), 3.56–3.37 (m, 16H),
2
+
.16–2.00 (m, 16H). HR ESI-MS m/z (obs) 467.1614 (M ), m/z
(
calcd) 467.1608.
Synthesis of {Ru[(pip)
2
bpy]
2
dcbpy}(PF
6 2
) , 3. Using the same
procedure as above, {Ru[(pip)
2
bpy] Cl }Cl (0.13 g, 0.15 mmol)
2
2
was reacted with dcbpy (0.11 g, 0.46 mmol, 3 eq.), and triethyl-
amine (0.37 mL, 2.6 mmol, 17 eq.). Compound 3 was the first
Electrochemistry
Cyclic voltammograms were obtained on a BAS-100B electro- band to elute from a Sephadex LH-20 column using aceto-
4
4
chemical analyzer using methods previously described. All nitrile–isopropanol (90 : 10) as eluent to yield 0.13 g (68ꢀ
1
solutions were prepared in acetonitrile or dimethylformamide yield). H-NMR (300 MHz; CD
with 0.1 M tetrabutylammonium hexafluorophosphate [TBA]- J = 5.9 Hz, 2H), 7.82 (d, J = 5.9 Hz, 2H), 7.67 (d, J = 2.5 Hz, 2H),
PF ] as the supporting electrolyte and degassed with argon. 7.63 (d, J = 2.3 Hz, 2H), 7.21 (d, J = 6.8 Hz, 2H), 6.93 (d, J =
3
CN): δ 9.10 (s br, 2H), 8.14 (d,
[
6
Reported E° potentials were referenced to a Ag/AgCl electrode 6.8 Hz, 2H), 6.76 (dd, J = 6.8, 2.6 Hz, 2H), 6.56 (dd, J = 6.7,
and are reported versus NHE. The E° ferrocenium/ferrocene 2.3 Hz, 2H), 3.69–3.42 (m, 16H), 1.80–1.51 (m, 24H). HR
2
+
couple was measured under identical conditions and used to ESI-MS m/z (obs) 495.1940 (M ), m/z (calcd) 495.1921.
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Synthesis of {Ru[(morph)
2
bpy]
2
dcbpy}(PF
6
)
2
, 4. Using the water. The precipitate was isolated by filtration and was
crude mixture of {Ru[(morph) bpy] Cl }Cl and an excess of washed with water and then acetone to yield 0.35 g (70ꢀ) of 5.
2
2
2
1
dcbpy, the reaction was completed as above and 4 was purified
on a Sephadex LH-20 column using acetonitrile–methanol 2H), 7.68 (d, J = 2.8 Hz, 2H), 7.63 (d, J = 2.7 Hz, 2H), 7.45 (d, J =
90 : 10) as eluent to yield 0.18 g (20ꢀ yield based on the 5.8 Hz, 2H), 7.21 (d, J = 6.6 Hz, 2H), 6.99 (d, J = 6.6 Hz, 2H),
amount of RuCl ·3H used for the preparation {Ru- 6.68 (dd, J = 6.5, 1.1 Hz, 2H), 6.49 (dd, J = 5.3, 0.9 Hz, 2H),
(morph) bpy] Cl }Cl). H-NMR (300 MHz; CD CN): δ 9.64 4.24–3.96 (m, 8H), 3.58–3.37 (m, 8H), 3.38–3.14 (m, 8H),
s br, 2H), 7.99 (d, J = 5.8 Hz, 2H), 7.80 (d, J = 6.0 Hz, 2H), 7.68 3.13–2.93 (m, 8H), 2.80 (s, 6H), 2.78 (s, 6H). HR ESI-MS m/z
2
H-NMR (300 MHz; D O): δ 8.59 (s br, 2H), 7.84 (d, J = 5.5 Hz,
(
3
2
O
1
[
2
2
2
3
(
(
2
2
+
d, J = 2.2 Hz, 2H), 7.65 (d, J = 2.4 Hz, 2H), 7.28 (d, J = 6.7 Hz, (obs) 525.2150 (M ), m/z (calcd) 525.2139.
H), 7.06 (d, J = 6.7 Hz, 2H), 6.75 (dd, J = 6.7, 2.3 Hz, 2H), 6.58
dd, J = 6.7, 2.6 Hz, 2H), 3.79 (t, J = 4.7 Hz, 8H), 3.74 (t, J =
Deprotonation of compounds 1–6
(
4
.7 Hz, 8H), 3.50 (t, J = 4.7 Hz, 8H), 3.43 (t, J = 4.4 Hz, 8H). HR Complexes 1–6 were dissolved in methanol to which a solution
2
+
ESI-MS m/z (obs) 499.1517 (M ), m/z (calcd) 499.1506.
Synthesis of {Ru[(MeO) bpy] dcbpy}(PF ) , 6. Using the was added. The solution was stirred for 30 minutes at room
of aqueous tetramethylammonium hydroxide (10 equivalents)
2
2
6 2
same procedure as above, the crude mixture containing Ru- temperature and concentrated causing the ruthenium complex
(MeO) bpy] Cl (0.47 g, 0.74 mmol) was reacted with dcbpy to precipitate. The complexes were isolated by filtration and
[
2
2
2
(
1
0.55 g, 2.2 mmol, 3 eq.), and triethylamine (1.50 mL, washed with water and dried under vacuum. The deprotonated
1.1 mmol, 15 eq.). Due to solubility, 6 was first converted to dyes will be designated as 1-CO2− to 6-CO2−
.
−
the PF
6
6
salt with NaPF and purified on a Sephadex LH-20
1
6
ZnO nanocrystal (NC) synthesis
column using acetonitrile–methanol (90 : 10) as eluent to yield
1
0
1
7
6
3
.62 g (80ꢀ yield). H-NMR (300 MHz; CD CN): δ 9.02 (d, J =
To a 0.1 M solution of Zn(O CCH ) ·2H O (25 mL, 2.51 mmol)
2
3 2
2
.2 Hz, 2H), 8.03–8.00 (m, 6H), 7.80 (dd, J = 5.9, 1.7 Hz, 2H),
.50 (d, J = 6.5 Hz, 2H), 7.36 (d, J = 6.5 Hz, 2H), 7.00 (dd, J =
.5, 2.7 Hz, 2H), 6.89 (dd, J = 6.6, 2.7 Hz, 2H), 4.01 (s, 6H), 3.97
in DMSO, a 0.55 M solution of [NMe
4
3
4
]OH·5H
.25 mmol) in ethanol was added dropwise over the course of
0–180 s and allowed to stir at room temperature (ca. 24 °C)
2
O (7.73 mL,
2
+
(s, 6H). HR ESI-MS m/z (obs) 389.0683 (M ), m/z (calcd)
for up to 30 minutes. The longer reaction times led to larger
particles. The diameters were determined using electronic
3
89.0662.
Synthesis of {Ru[(Et
3
9
2
N)
2
bpy]
2
[(Cl)
2
bpy]}(PF . Using the
6
)
2
absorption spectra. To induce precipitation, an aliquot of the
solution was removed and added to three times the volume of
ethyl acetate. The turbid mixture was centrifuged using a LW
Scientific Ultra-8 V centrifuge. After the liquid was decanted,
the white pellet was dispersed in a minimum of ethanol and
precipitated a second time. The purified ZnO NCs were mixed
with methanol to yield an optically clear dispersion. To obtain
a mass of ZnO, a known volume of the dispersion was evapor-
ated and then heated to 500 °C in air for one hour to remove
organics. Based on the initial mass of zinc acetate used a
typical yield for the ZnO NC preparation was 78–80ꢀ.
same procedure as above, {Ru[(Et
0
3
product was purified on a Sephadex LH-20 column using
acetonitrile–isopropanol (90 : 10) as eluent to yield 0.16 g (52ꢀ
yield). H-NMR (500 MHz; CD
2
N) bpy] Cl }Cl (0.21 g,
2
2
2
.26 mmol) was reacted with (Cl) bpy (0.18 g, 0.78 mmol,
eq.), and triethylamine (0.73 mL, 5.2 mmol, 17 eq.). The
2
1
2
Cl
2
): δ 8.24 (d, J = 2.1 Hz, 2H),
7
.88 (d, J = 6.1 Hz, 2H), 7.41 (dd, J = 6.2, 2.2 Hz, 2H), 7.23–7.19
m, 6H), 6.96 (d, J = 6.8 Hz, 2H), 6.56 (dd, J = 6.8, 2.9 Hz, 2H),
.44 (dd, J = 6.8, 2.8 Hz, 2H), 3.50 (q, J = 7.1 Hz, 8H), 3.45 (q,
J = 7.1 Hz, 8H), 1.25 (t, J = 7.1 Hz, 12H), 1.21 (t, J = 7.1 Hz,
(
6
2
+
1
2H). HR ESI-MS m/z (obs) 461.1641 (M ), m/z (calcd)
4
61.1633.
Sample preparation: steady state absorption, emission and
2 2 6 2
Synthesis of {Ru[(mepipz) bpy] dcbpy}(PF )
, 5. The glucose NMR
reaction was unsuccessful at producing {Ru[(mepipz) bpy] Cl }-
2
2
2
In all samples the amount of dye was held constant through-
out the experiment and the amount of ZnO was varied in sub-
Cl as all ruthenium went directly to the tris-(mepipz)
ruthenium complex when (mepipz)
confirmed through NMR and ESI-MS as only the {Ru-
2
bpy
2
bpy was added. This was
sequent samples. In a typical experiment aliquots of 3.0 ×
−4
10
M ZnO nanocrystals (3.2 nm in diameter) dispersed in
2
+
[
(mepipz)
alternative route started with {[(η -p-cymene)RuCl]
which was prepared according to literature procedures.
2
bpy]
3
}
complex ion was observed. The following
methanol were added to methanol solutions of dye (1.0 ×
6
(μ-Cl)
2
},
A
−3
2
10
M) that had been deprotonated with ten equivalents of
4
8,49
[
Me N][OH]. The samples were then diluted to a total volume
4
6
mixture of {[(η -p-cymene)RuCl]
eq.) and dcbpy (0.19 g, 0.00076 mol, 2 eq.) in 80 mL DMF
was heated to 65 °C for 4 h. Then excess (mepipz) bpy (0.60 g,
2
(μ-Cl)
2
} (0.23 g, 0.00038 mol,
of 1.2 mL with methanol. Samples were allowed to equilibrate
1
for 2 h before any measurements were taken. Methanol-d
used for the NMR experiments.
4
was
2
0
.0017 mol, 4.5 eq.) was added and the reaction was refluxed
for 4 additional hours, cooled to room temperature, and the
solvent evaporated. The crude reaction mixture was dissolved
in water, washed with methylene chloride (3 × 50 mL), and
Acknowledgements
then metathesized to the hexafluorophosphate salt by adding This material is based upon work supported by the U.S.
NaPF (0.015 mol, 20 eq.) dissolved in a minimum volume of Department of Energy, Office of Science, Office of Basic Energy
6
Dalton Trans.
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Sciences under Award Number DE-FG02-07ER15913. This work 21 M. J. Cook, A. P. Lewis, G. S. G. Mcauliffe, V. Skarda,
was carried out in part using hardware and/or software
provided by the University of Minnesota Supercomputing
Institute.
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000, 39, 1621.
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