Toward Exceeding the Shockley−Queisser Limit
A R T I C L E S
dry acetone, the solution was filtered, and the purple solution was placed
in an ether chamber for slow recrystallization (0.46 g, 21%). 1H NMR
δ (CD3CN): 9.38 (2H, s), 9.22 (2H, s), 8.74 (2H, d), 8.51 (2H, d),
7.85 (2H, d), 7.75 (2H, t), 7.60 (4H, m), 7.45 (4H, m), 7.30 (2H, d),
7.15 (4H, m), 7.00 (2H, m), 4.70 (4 H, q, CH2), 4.60 (4 H, q, CH2),
1.60 (6H, t, CH3), 1.50 (6H, t, CH3). Elem. Anal. Calcd: C, 51.68; H,
3.59; N, 6.23. Found: C, 50.47; H, 3.66; N, 6.07.
taking 128-256 scans at 4 cm-1 resolution. For TiO2 samples, the
background was taken using either a blank TiO2/glass or TiO2/FTO
slide. Elemental analysis was performed by Atlantic Microlabs, Inc.
X-ray crystallography was performed by Arnold Rheingold and co-
workers at the University of Delaware. Matrix Assisted Laser Desorp-
tion Ionization Mass Spectrometry (MALDI-MS) measurements were
performed on a Kratos MALDI-TOF model SEQ mass spectrometer
using R-cyano-4-hydroxycinnamic acid as the matrix. Fast Atom
Bombardment Mass Spectrometry (FAB-MS) measurements were
obtained on a VG70S mass spectrometer; samples were suspended in
a p-nitrobenzyl alcohol matrix.
MO2 Preparations. Transparent films of colloidal TiO2 or ZrO2
nanoparticles were prepared by a previously described sol-gel
procedure.27,28 Sensitizer binding was carried out by placing a MO2
film in an approximately millimolar sensitizer acetonitrile solution for
∼24 h.
[Ru(bpy)(deebq)(NCS)2]. (a) [Ru(bpy)(deebq)2](PF6)2 (0.17 g, 0.13
mmol) was dissolved in 80 mL of acetonitrile and irradiated (1000 W
Xe lamp, λ > 570 nm) in a 100 mL borosilicate round-bottomed flask
under argon for 4 days.26 Complete conversion to [Ru(bpy)(deebq)-
(CH3CN)2](PF6)2 was apparent from UV-vis spectra. The acetonitrile
was removed, and the resultant red solid was dissolved in 15 mL of
dry acetone that was treated with 15 mL of H2O. The deebq ligand
was removed by filtration. The filtrate was then treated with 2 M NH4-
PF6 (aq, 5 mL), giving a red precipitate, which was filtered and washed
with H2O followed by ether (0.11 g, 80%). 1H NMR δ (CD3CN): 9.75
(1H, dd), 9.05 (3H, m), 8.80 (1H, s), 8.70 (1H, d), 8.20 (2H, m), 8.00
(4H, m), 7.85 (1H, m), 7.77 (1H, d), 7.70 (1H, m), 7.50 (1H, m), 7.25
(1H, m), 7.00 (1H, d), 4.75 (4H, q, CH2), 4.55 (4H, q, CH2), 2.60 (3H,
s, CH3CN-Ru), 2.40 (3H, s, CH3CN-Ru), 1.60 (6H, t, CH3), 1.50
(6H, t, CH3). UV-vis (CH3CN), nm: 490 (sh), 525. (b) [Ru(bpy)-
(deebq)(CH3CN)2](PF6)2 (40 mg) was dissolved in argon purged EtOH
(5 mL) containing tetrabutylammonium thiocyanate (25 mg) and
refluxed for 4 h, during which time, a blue-green precipitate formed.
The solid was isolated by filtration and rinsed with EtOH and ether
(13 mg, 43%). 1H NMR δ (CD3CN): 9.49 (1H, d), 9.33 (1H, d), 8.98
(1H, dd), 8.89 (1H, s), 8.74 (1H, d), 8.72 (1H, s), 8.14 (1H, d), 8.00
(3H, m), 7.90 (1H, m), 7.75 (1H, m), 7.66 (1H, td), 7.55 (1H, m), 7.36
(1H, d), 7.25 (2H, m), 7.00 (1H, m), 4.66 (2H, q, CH2), 4.54 (2H, q,
CH2), 1.59 (3H, t, CH3), 1.50 (3H, t, CH3). ATR-IR: 2095 cm-1 and
2071 cm-1 (NdCdS).
Photoluminescence. Corrected photoluminescence (PL) spectra were
obtained with a Spex Fluorolog that had been calibrated with a standard
tungsten-halogen lamp using procedures provided by the manufacturer.
Sensitized films were placed diagonally in a 1 cm square quartz cuvette,
immersed in acetonitrile, and purged with nitrogen for at least 15 min.
The excitation beam was directed 45° to the film surface, and the
emitted light was monitored from the front face of the surface-bound
sample and from the right angle in the case of fluid solutions.
Photoluminescence quantum yield measurements were performed using
the optically dilute technique29 with Ru(bpy)3(PF6)2 in acetonitrile as
the actinometer and calculated by eq 1:
φPL ) (Ar/As)(Is/Ir)(ns/nr)2φr
(1)
where Ar and As are the absorbances of the actinometer and sample,
respectively, Ir and Is are the integrated photoluminescence of the
actinometer and sample, respectively, nr and ns are the refractive indexes
of the actinometer and sample solvents, respectively, and φr is the
quantum yield for Ru(bpy)3(PF6)2 in acetonitrile (φr ) 0.062). For
lithium titration experiments, 0.1 M LiClO4 in CH3CN was added via
syringe to the cuvette that was continuously purged with nitrogen.
Time-resolved photoluminescence decays were acquired on a
nitrogen-pumped dye laser (460 nm) apparatus that has been previously
described.30 For solution studies, the samples were optically dilute (A
≈ 0.1 at λmax), and the kinetic traces were fit to a first-order kinetic
model. For TiO2 and ZrO2 studies, the time-resolved photoluminescence
decays were fit to a parallel first- and second-order kinetic model, eq
2,
[Ru(bpy)2(dcbq)]. Ru(bpy)2Cl2‚2H2O (54 mg, 0.10 mmol) and dcbq-
(Na)2 (70 mg, 0.18 mmol) were added to N2-saturated deionized H2O
(5 mL). Refluxing under N2 for 8 h resulted in the formation of a brick-
red precipitate that was filtered and washed with deionized H2O
followed by acetone (69 mg, 88%). 1H NMR δ (d6-DMSO): 8.16 (6H,
m), 8.03 (2H, s), 7.54 (4H, t), 7.32 (4H, t), 6.92 (6H, m), 6.53 (2H,
td), 6.41 (2H, d).
[Os(bpy)2(dcbq)]. Os(bpy)2Cl2‚2H2O (33 mg, 0.052 mmol) and
dcbq(Na)2 (70 mg, 0.20 mmol) were added to N2-saturated deionized
H2O (5 mL). After refluxing under N2 for 8 h, the reaction solution
was filtered (removing a black solid), and acetone (250 mL) was added
to the filtrate. The solution was cooled in a freezer for 4 days, over
which time the desired product precipitates out as a black solid. The
product was isolated by vacuum filtration and washed with deionized
k1 exp(- k1t)
1
PLI(t) ) B
(2)
H2O and acetone (10 mg, 20%). H NMR δ (d6-DMSO): 8.80 (2H,
(
)
k1 + p - p exp(- k1t)
dd), 8.72 (4H, t), 8.59 (2H, s), 7.92 (4H, m), 7.70 (4H, dd), 7.43 (6H,
m), 7.07 (2H, m), 6.75 (2H, d).
where k1 is a first-order rate constant analogous to the solution and B
is a constant.31 The parameter p is the product of the observed second-
order rate constant, k2, and the initial concentration of ruthenium excited
[Ru(bq)2(dcbH2)](PF6)2. [Ru(bq)2(Cl)2] (150 g, 0.21 mmol) was
added to argon-purged DMF (12 mL) and refluxed under argon for 2
h. After cooling to room temperature, the reaction solution was treated
with dcb2- (200 mg, 0.62 mmol) dissolved in 12 mL of argon-purged
H2O, and the solution was refluxed again for 18 h. The solution was
filtered to remove a green precipitate, and the dark red filtrate was
treated with 0.4 M HPF6 (aq, 1 mL) giving a red-brown precipitate
states, [Ru2+*
]t)0. For studies involving the sensitizers on TiO2 or ZrO2,
the excitation beam was directed 45° to the film surface, and the emitted
light was collected at 90°.
Electrochemistry. Cyclic voltammetry was performed in 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6/CH3CN) electrolyte
1
that was filtered and washed with H2O followed by ether. H NMR δ
(CD3CN): 8.98 (4H, q), 8.81 (2H, d), 8.46 (2H, d), 8.25 (2H, s), 8.12
(2H, d), 8.06 (2H, d), 7.81 (4H, m), 7.50 (2H, m), 7.31 (4H, m), 7.04
(4H, m), 6.90 (2H, m).
Characterization. 1H NMR spectra were obtained on a Bruker 300
Hz AMX FT-NMR spectrometer. Attenuated Total Reflectance (ATR)
measurements were obtained using a Golden Gate Single Reflection
Diamond ATR apparatus on a Nexus 670 Thermo-Nicolet FT-IR and
(27) (a) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem. 1990,
94, 8720. (b) Barbe, C. J.; Arendse, F.; Comte, P.; Jirousek, M.; Lenzmann,
F.; Shklover, V.; Gra¨tzel, M. J. Am. Ceram. Soc. 1997, 80, 3157. (c) Heimer,
T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg.
Chem. 1996, 35, 5319.
(28) Qu, P.; Meyer, G. J. Langmuir 2001, 17, 6720 and references therein.
(29) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(30) Castellano, F. N.; Heimer, T. A.; Tandhasetti, T.; Meyer, G. J. Chem. Mater.
1994, 6, 1041.
(31) (a) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Meyer, G. J. Langmuir
1999, 15, 731. (b) Higgens, G. T.; Bergeron, B. V.; Hasselmann, G. M.;
Farzad, F.; Meyer, G. J. J. Phys. Chem. B 2006, 110, 2598.
(26) von Zelewsky, A.; Gremaud, G. HelVetica Chimica Acta 1988, 71, 1108.
9
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