Long-range electron transfer across molecule-nanocrystalline semiconductor interfaces using tripodal sensitizers

Article abstract of DOI:10.1021/ja025840n

Four tripodal sensitizers, Ru(bpy)₂(Ad-tripod-phen)²⁺ (1), Ru(bpy)₂(Ad-tripod-bpy)²⁺ (2), Ru(bpy)₂-(C-tripod-phen)²⁺ (3), and Ru(bpy)₂(C-tripod-bpy)²⁺ (4) (where bpy i

Full text of DOI:10.1021/ja025840n

Published on Web 06/06/2002
Long-Range Electron Transfer across
Molecule-Nanocrystalline Semiconductor Interfaces Using
Tripodal Sensitizers
Elena Galoppini,*,† Wenzhuo Guo,† Wei Zhang,† Paul G. Hoertz,‡ Ping Qu,‡ and
Gerald J. Meyer*,‡
Contribution from the Chemistry Department, Rutgers UniVersity, 73 Warren Street,
Newark, New Jersey 07102, and Department of Chemistry, Johns Hopkins UniVersity,
3400 North Charles Street, Baltimore, Maryland 21218
Received February 6, 2002
Abstract: Four tripodal sensitizers, Ru(bpy)2(Ad-tripod-phen)2+ (1), Ru(bpy)2(Ad-tripod-bpy)2+ (2), Ru(bpy)2-
(C-tripod-phen)2+ (3), and Ru(bpy)2(C-tripod-bpy)2+ (4) (where bpy is 2,2′-bipyridine, phen is 1,10-
phenanthroline, and Ad-tripod-bpy (phen) and C-tripod-bpy (phen) are tripod-shaped bpy (phen) ligands
based on 1,3,5,7-tetraphenyladamantane and tetraphenylmethane, respectively), have been synthesized
and characterized. The tripodal sensitizers consist of a rigid-rod arm linked to a RuII-polypyridine complex
at one end and three COOR groups on the other end that bind to metal oxide nanoparticle surfaces. The
excited-state and redox properties of solvated and surface-bound 1-4 have been studied at room
temperature. The absorption spectra, emission spectra, and electrochemical properties of 1-4 in acetonitrile
solution are preserved when 1-4 are bound to nanocrystalline (anatase) TiO2 or colloidal ZrO2 mesoporous
films. This behavior is indicative of weak electronic coupling between TiO2 and the sensitizer. The kinetics
for excited-state decay are exponential for 1-4 in solution and are nonexponential when 1-4 are bound
to ZrO2 or TiO2. Efficient and rapid (kcs > 108 s-1) excited-state electron injection is observed for 1-4/TiO2.
The recombination of the injected electron with the oxidized RuIII center is well described by a second-
order kinetic model with rate constants that are independent of the sensitizer. The sensitizers bound to
TiO2 were reversibly oxidized electrochemically with an apparent diffusion coefficient ∼1 × 10-11 cm2 s-1
.
Introduction
Interest in nanometer-sized semiconductor surfaces has risen
as the tendency toward miniaturization in the electronic industry
continues. The covalent attachment of redox-active and photo-
active molecules to semiconductor surfaces is an important step
toward the development of molecular devices, such as solar cells,
light-emitting diodes, and chemical sensors.1 Thus, fundamental
studies of electronic interactions across molecule-nanoparticle
interfaces are increasingly relevant in several emerging fields
of science.2
The sensitization of nanocrystalline titanium dioxide to visible
light with dye molecules is a field in which tuning molecular-
semiconductor interactions could lead to improvements and
further insights.3 For example, Figure 1a shows some key elec-
tronic transitions that promote and inhibit light energy conver-
Figure 1. (a) Main steps in the sensitization of TiO2 by a surface-bound
RuII-polypyridyl complex: (1) MLCT excitation; (2) charge separation; and
(3) charge recombination. (b) Schematic representation of a surface-bound
molecular tripodal sensitizer, where d is the distance from the Ru center to
the footprint, i.e., the plane defined by the three surface-bound oxygen atoms
and shown as a dotted line.
* To whom corresponcence should be addressed. E-mail: galoppin@
andromeda.rutgers.edu.
sion at a nanocrystalline (anatase) TiO2 interface with Ru(dcb)-
(bpy)22+, where dcb is 4,4′-(COOH)2-2,2-bipyridine. Photoex-
citation of the RuII complex results in the formation of metal-
to-ligand charge-transfer (MLCT) excited states that can inject
electrons into TiO2 to form an interfacial charge-separated state
consisting of an electron in TiO2 and an oxidized dye
(RuIII|TiO2(e-)). It has been shown that under a wide variety
of experimental conditions the injection yield is near unity and
the injection can occur on an ultrafast femtosecond time scale.3
† Rutgers University.
‡ Johns Hopkins University.
(1) (a) Molecular Electronics; Jortner, J., Ratner, M., Eds.; Blackwell: London,
1997. (b) Meyer, G. J. Molecular LeVel Artificial Photosynthetic Ma-
terials; Progress in Inorganic Chemistry; John Wiley & Sons: New York,
1997.
(2) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226.
(3) (a) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49. (b) Kamat, P. V.
Chem. ReV. 1993, 93, 267. (c) Qu, P.; Meyer, G. J. In Electron Transfer in
Chemistry; Balzani, V., Ed.; Wiley: New York, 2001; Vol. IV, Part 2,
Chapter 2, p 355.
9
10.1021/ja025840n CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 7801-7811
7801

A R T I C L E S
Galoppini et al.
Figure 2. Structure of tripodal sensitizers 1-4, reference complexes 5 and 6, and the ligands employed in this study. The distances (d) to the Ru center from
the tripod footprint for 1-4 are d1 ) 17.6 Å, d2 ) 17.3 Å, d3 ) 15.7 Å, and d4 ) 15.4 Å, respectively.
Charge recombination to the RuIII center, which is several orders
of magnitude slower, regenerates the ground state and occurs
with second-order kinetics.4
and sensitizers with flexible alkyl chains bridging the binding
groups and the bpy ligand.8 The former approach is not readily
amenable to systematic studies, and in the latter approach
semiconductor-sensitizer distances cannot be fixed. Clearly,
there exists a need for rigid linkers that can be modified to
systematically regulate the sensitizer-nanoparticle electronic
interaction. Organic linkers that have such properties were
developed recently in our laboratories and are schematically
shown in Figure 1b.9 These are rigid “tripods”,10 having a
tetrahedral core made of tetraphenylmethane or 1,3,5,7-tetraphe-
nyladamantane, three COOR surface binding groups, and a rigid-
rod arm carrying the sensitizer. This design provides a stable,
three-point attachment to the surface of metal oxide nanopar-
ticles and a well-defined position of photoactive and/or redox-
active groups on nanoparticle surfaces. The kinetic rate constants
for remote interfacial electron-transfer processes can be quanti-
fied spectroscopically after selective light excitation of the
sensitizer.
In the state-of-the-art solar cells, the MLCT excited state is
typically localized on a dcb or a terpyridine ligand substituted
with carboxylic acid groups.5 Depending on the surface binding
conditions, the carboxylic acid substituents can react with
surface hydroxyl groups to form ester linkages.6 Such bonds
are thought to provide strong electronic coupling between the
dye and the semiconductor and underlie the ultrafast electron
injection rate constants that have been measured.3 In principle,
however, such injection rates are not needed for efficient solar
energy conversion, and interfacial charge separation yields of
unity are expected when the rate constants for injection are 3-4
orders of magnitude slower.3 In fact, an optimal electronic
interaction may exist wherein the quantum yield for charge
injection is still unity, while charge recombination is further
inhibited. This scenario would be expected to increase the power
output of the regenerative solar cell.3
We recently reported the study of the first tripodal sensitizer,
Ru(bpy)2(Ad-tripod-phen)2+ (1 in Figure 2), in solution and
Recently, several studies have demonstrated that excited states
remote from the semiconductor surface can efficiently inject
electrons into TiO2.7,8 To study these weakly coupled systems,
researchers have prepared bimetallic coordination compounds7
bound to TiO2 thin films.11 We observed rapid (kcs > 108 s-1
)
interfacial electron transfer in Ru(bpy)2(Ad-tripod-phen)2+/TiO2
and efficient conversion of light into electricity when this
material was utilized as photoanode in regenerative solar cells.
In this article, we report additional results with 1 and the
synthesis and study of three new tripodal sensitizers, Ru(bpy)2-
(Ad-tripod-bpy)2+ (2), Ru(bpy)2(C-tripod-phen)2+ (3), and Ru-
(bpy)2(C-tripod-bpy)2+ (4), shown in Figure 2. All four have a
phenyethynyl unit as the rigid spacer and bpy as the auxiliary
ligands, but they differ in the ligand (bpy or phen) and
tetrahedral core (adamantane or an sp3-hybridized carbon). Two
(4) Kelly, C. A.; Thompson, D. W.; Farzad, F.; Stipkala, J. M.; Meyer, G. J.
Langmuir 1999, 15, 7047.
(5) (a) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller,
E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115,
6382. (b) Nazeeruddin, M. K.; Pe´chy, P.; Renouard, T.; Zakeeruddin, S.
M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.;
Shklover, V.; Spiccia, L.; Deacon, G. B.; Kay, A.; Bignozzi, C. A.; Gra¨tzel,
M. J. Am. Chem. Soc. 2001, 123, 1613.
(6) (a) Umpathy, S.; Cartner, A. M.; Parker, A. W.; Hester, R. E. J. Phys.
Chem. 1990, 94, 1357. (b) Meyer, T. J.; Meyer, G. J.; Pfenning, B.;
Schoonover, J. R.; Timpson, C.; Wall, J. F.; Kobusch, C.; Chen, X.; Peek,
B. M.; Wall, C. G.; Ou, W.; Erickson, B. W.; Bignozzi, C. A. Inorg. Chem.
1994, 33, 3952. (c) Argazzi, R.; Bignozzi, C. A.; Heimer, T. A.; Castellano,
F. N.; Meyer, G. J. Inorg. Chem. 1994, 33, 5741. (d) Finnie, K. S.; Bartlett,
J. R.; Woolfrey, J. L. Langmuir 1998, 14, 2744. (e) Qu, P.; Meyer, G. J.
Langmuir 2001, 17, 6720.
(7) (a) Argazzi, R.; Bignozzi, C.; Heimer, T. A.; Castellano, F. N.; Meyer, G.
J. Inorg. Chem. 1994, 33, 5741. (b) Kleverlaan, C. J.; Indelli, M. T.;
Bignozzi, C. A.; Pavanin, L.; Scandola, F.; Hasselmann, G. M.; Meyer, G.
J. J. Am. Chem. Soc. 2000, 122, 2840. (c) Kleverlaan, C. J.; Alebbi, M.;
Argazzi, R.; Bignozzi, C. A.; Hasselmann, G. M.; Meyer, G. J. Inorg. Chem.
2000, 39, 1342.
(9) Guo, W.; Galoppini, E.; Rydja G. I.; Pardi, G. Tetrahedron Lett. 2000, 41,
7419.
(10) Tripod-shaped linkers of various structure (also termed caltrops, ref 10c)
have been previously reported in studies of redox-active molecules bound
on gold electrodes: (a) Whitesell, J. K.; Chang, H. K. Science 1993, 261,
73. (b) Fox, M. A.; Whitesell, J. K.; McKerrow, A. J. Langmuir 1998, 14,
816. (c) Yao, Y.; Tour, J. M. J. Org. Chem. 1999, 64, 1968. (d) Hu, J.;
Mattern, L. J. Org. Chem. 2000, 65, 2277. (e) Hirayama, D.; Takimiya,
K.; Aso, Y.; Otsubo, T.; Hasobe, T.; Yamada, H.; Imahori, H.; Fukuzumi,
S.; Sakata, Y. J. Am. Chem. Soc. 2002, 124, 532.
(8) (a) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer,
G. J. Inorg. Chem. 1996, 35, 5319. (b) Asbury, J. B.; Hao, E.; Wang, Y.;
Lian, T. J. Phys. Chem. B 2000, 104, 11957.
(11) Galoppini, E.; Guo, W.; Qu, P.; Meyer, G. J. J. Am. Chem. Soc. 2001,
123, 4342.
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7802 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Long-Range Electron Transfer Using Tripodal Sensitizers
A R T I C L E S
model RuII complexes, 5 and 6, with phenylethynyl arms on
the phen and bpy ligands, were prepared: Ru(Ph-E-phen)3
132.27, 129.76, 128.51, 124.64 (2C), 120.16, 83.38 (CtCH), 60.91,
46.62 (2C), 39.58, 39.25, 14.34 (CtCH overlaps with the solvent).
Ad-tripod-phen (12a). The characterization data for this ligand have
been previously reported.9
2+
(5) served as a model for the phen-based tripods 1 and 3, and
2+
Ru(Ph-E-bpy)3 (6) for the bpy-based tripods 2 and 4. Ref-
Ad-tripod-bpy (12b): 1H NMR δ 8.70 (d, 1H, J ) 4.0 Hz), 8.65
(d, 1H, J ) 5.0 Hz), 8.54 (bs, 1H), 8.41 (d, 1H, J ) 8.0 Hz), 8.04 (d,
6H, J ) 8.5 Hz), 7.83 (m, 1H), 7.55 (m, 8H), 7.50 (d, 2H, J ) 8.0
Hz), 7.38 (d, 1H, J ) 5.0 Hz), 7.34 (m, 1H), 3.92 (s, 9H, OCH3), 2.21
(s, 12H, adamantane); 13C NMR δ 166.86, 156.12, 155.48, 153.72,
149.75, 149.15, 136.99, 132.40, 132.08, 129.80, 128.26, 125.19, 125.16,
125.05, 123.99, 123.14, 121.11, 120.30, 93.73, 87.05, 52.07, 46.56,
39.54, 39.32; HRMS (FAB) calcd for C52H45N2O6 (MH+) 793.9392,
found 793.9371.
2+
erence complexes Ru(phen)32+, Ru(bpy)2(phen)2+, Ru(bpy)3
and Ru(bpy)2(deeb)2+ were also prepared.
,
Experimental Section
Characterization Data. For detailed synthetic procedures and
methods, see Supporting Information.
1-(Trimethylsilylethynylphenyl)-3,5,7-tris(4-iodophenyl)adaman-
tane (8, Td ) Ad, X ) I). Characterization data for 8 have been
previously reported.9
1
C-Tripod-phen (12c): mp 148-150 °C; H NMR δ 9.24 (dd, 1H,
J ) 4.5 Hz, J ) 1.5 Hz), 9.20 (dd, 1H, J ) 4.5 Hz, J ) 1.5 Hz), 8.80
(dd, 1H, J ) 8.5 Hz, J ) 2.0 Hz), 8.24 (dd, 1H, J ) 8.5 Hz, J ) 2.0
Hz), 8.10 (s, 1H), 7.97 (d, 6H, J ) 8.5 Hz), 7.73 (q, 1H, J ) 8.5 Hz,
J ) 4.0 Hz), 7.65 (dd, 1H, J ) 8.0 Hz, J ) 4.0 Hz), 7.59 (d, 2H, J )
9.0 Hz), 7.33 (d, 6H, J ) 8.5 Hz), 7.27 (d, 2H, J ) 9.0 Hz), 3.91 (s,
9H, COOCH3); 13C NMR δ 166.55 (COOCH3), 150.91, 150.62, 150.12,
146.07, 145.90, 145.86, 135.79, 134.62, 131.43, 130.85, 130.70, 129.28,
128.47, 128.19, 127.96, 123.46, 123.34, 120.99, 119.71, 94.70, 86.40,
65.41, 52.15 (COOCH3) (one carbon overlaps in the aromatic region);
LRMS (FAB) m/z (relative intensity) 698 (52, M+ + 1), 697 (100,
M+). Anal. Calcd for C45H32N2O6: C, 77.57; H, 4.63; N, 4.02. Found:
C, 74.87; H, 4.62; N, 3.53.
C-Tripod-bpy (12d): mp 98-100 °C; 1H NMR δ 8.69 (d, 1H, J )
4.0 Hz), 8.66 (d, 1H, J ) 5.0 Hz), 8.52 (s, 1H), 8.41 (d, 1H, J ) 7.5
Hz), 7.96 (d, 6H, J ) 7.5 Hz), 7.83 (m, 1H), 7.49 (d, 2H, J ) 7.5 Hz),
7.38 (d, 1H, J ) 5.0 Hz), 7.31 (m, 7H), 7.23 (d, 2H, J ) 7.5 Hz), 3.91
(s, 9H); 13C NMR δ 166.52, 156.14, 155.40, 150.08, 149.14, 149.12,
146.03, 136.96, 132.13, 131.55, 130.76, 130.67, 129.24, 128.41, 125.17,
123.96, 123.06, 121.09, 120.63, 93.16, 87.58, 65.36, 52.12; LRMS
(FAB) m/z (relative intensity) 674 (51, M+ + 1), 673 (100, M+).
Ru(bpy)2(Ad-tripod-phen)(PF6)2 (1, Td ) adamantane, L ) phen,
R ) Et). The characterization data for 1 have been reported.11 IR: 2206
cm-1 (CtC), 1708 cm-1 (CdO).
Ru(bpy)2(Ad-Tripod-bpy)(PF6)2 (2, Td ) adamantane, L ) bpy,
R ) Me): 1H NMR (acetone-d6) δ 8.93 (m, 2H), 8.83 (d, 4H, J ) 8.5
Hz), 8.18-8.26 (m, 6H), 8.08 (m, 5H), 8.03 (d, 6H, J ) 8.5 Hz), 7.80
(m, 8H), ∼7.59-7.65 (m, 8H), 3.88 (s, 9H, OCH3), 2.32 (s, 12H,
adamantane); 13C NMR (acetone-d6) δ 167.15, 158.49, 158.07, 157.73,
155.62, 152.93, 152.65, 139.07, 133.49, 133.00, 130.30, 129.65, 129.06,
128.83, 126.94, 126.47, 125.44, 119.78, 98.88, 86.37, 52.25, 46.97,
40.61; HRMS (FAB) calcd for C72H6F6O6N6PRu (M - PF6) 1351.3392,
found 1351.3367; IR 2206 cm-1 (CtC), 1715 cm-1 (CdO); Raman
shift 2207 cm-1 (CtC).
4-Trimethylsilylethynylphenyl-tris(4-bromophenyl)methane (8,
Td ) C, X ) Br): mp 225 °C (DSC); 1H NMR δ 7.37 (m, 8H), 7.07
(d, 2H, J ) 8.0 Hz), 7.00 (d, 6H, J ) 8.0 Hz), 0.235 (s, 9H, Si(CH3)3);
13C NMR δ 145.66, 144.49, 132.39, 131.50, 131.01, 130.49, 121.40,
120.72, 104.39, 94.99, 63.89, -0.08 (Si(CH3)3). Anal. Calcd for C30H25-
Br3Si: C, 55.15; H, 3.86; Br, 36.69; Si, 4.30. Found: C, 54.90; H,
3.72. The disubstituted product, bis(4-trimethylsilylethynylphenyl)-bis-
(4-bromophenyl)methane, was also isolated as a white solid (540 mg,
26%).16
4-Trimethylsilylethynylphenyl-tris(4-carbomethoxyphenyl)-
methane (10, Td ) C, R ) Me): mp 78-80 °C; 1H NMR δ 7.93 (d,
6H, J ) 8.5 Hz), 7.38 (d, 2H, J ) 8.5 Hz), 7.28 (d, 6H, J ) 8.5 Hz),
7.13 (d, 2H, J ) 8.5 Hz), 3.90 (s, 9H, COOCH3), 0.235 (s, 9H, Si-
(CH3)3); 13C NMR δ 166.63 (COOCH3), 150.22, 145.31, 131.63,
130.74, 130.56, 129.22, 128.42, 121.52, 104.32 (CtCSi), 95.10
(CtCSi), 65.34, 52.18 (COOCH3), -0.14 (Si(CH3)3). Anal. Calcd for
C36H34O6Si: C, 73.19; H, 5.80. Found: C, 73.30; H, 5.75.
Trimethylsilylethynylphenyl-3,5,7-tris(4-carbomethoxylphenyl)-
1
adamantane (10, Td ) Ad, R ) Me): H NMR δ 8.60 (d, 2H, J )
5.0 Hz), 8.04 (d, 6H, J ) 8.0 Hz), 7.56 (two doublets overlap, 8H),
7.50 (d, 2H, J ) 8.0 Hz), 7.38 (d, 2H, J ) 5.0 Hz), 3.92 (s, 9H), 2.21
and 2.20 (two overlapping s, 12H); 13C NMR δ 166.83, 153.70, 149.85,
149.71, 132.02, 131.40, 129.80, 128.30, 125.48, 125.20, 125.04, 120.13,
93.75, 86.64, 52.07, 46.58, 39.55, 39.34. The characterization data for
10 (Td ) Ad, R ) Et) have been reported.9
4-Ethynylphenyl-tris(4-carbomethoxyphenyl)methane (11, Td )
C, R ) Me): mp 158-160 °C; 1H NMR δ 7.94 (d, 6H, J ) 8.5 Hz),
7.41 (d, 2H, J ) 8.5 Hz), 7.29 (d, 6H, J ) 8.5 Hz), 7.16 (d, 2H, J )
8.5 Hz), 3.90 (s, 9H, COOCH3), 3.08 (s, 1H, CtCH); 13C NMR δ
166.56 (COOCH3), 150.15, 145.66, 131.78, 130.70, 130.61, 129.23,
128.41, 120.51, 82.96 (CtCH), 77.83 (CtCH), 65.32, 52.14 (COOCH3).
Anal. Calcd for C33H26O6: C, 76.43; H, 5.05. Found: C, 74.85; H,
5.43.
Ru(bpy)2(C-tripod-phen)(PF6)2 (3, Td ) C, L ) phen, R )
Me): decomposes without melting above 250 °C (DSC); H NMR δ
1-(Ethynylphenyl)-3,5,7-tris(4-carbomethoxylphenyl)ada-
mantane (11, Td ) Ad, R ) Me): H NMR δ 8.03 (d, 6H, J ) 8.5
1
1
8.89 (d, 1H, J ) 8.0 Hz), 8.45 (m, 5H), 8.33 (s, 1H), 8.13 (d, 1H, J )
5.0 Hz), 8.08 (d, 1H, J ) 5.0 Hz), 7.93 (m, 11H), 7.80 (m, 3H), 7.58
(d, 2H, J ) 8.0 Hz), 7.54 (d, 1H, J ) 5.0 Hz), 7.51 (d, 1H, J ) 5.0
Hz), 7.45 (m, 2H), 7.30 (m, 10H), 3.88 (s, 9H, COOCH3); 13C NMR
δ 166.56 (COOCH3), 156.63, 152.86, 151.70, 151.39, 150.03, 147.21,
146.76, 138.15, 136.35, 135.44, 131.74, 131.37, 131.97, 130.68, 130.20,
129.33, 128.49, 128.19, 127.99, 127.21, 127.01, 124.20, 122.25, 120.00,
97.81, 84.38, 65.46, 52.18 (COOCH3) (one carbon overlaps in the
aromatic region). Anal. Calcd for the neutral complex: C, 55.76; H,
3.46; N, 6.00. Found: C, 56.03; H, 3.46; N, 5.91. 3, Td ) C, L )
phen, R ) Et: 1H NMR δ 8.89 (d, 1H, J ) 8.0 Hz), 8.41 (m, 5H),
8.32 (s, 1H), 8.15 (d, 1H, J ) 4.5 Hz), 8.09 (d, 1H, J ) 4.5 Hz),
7.93 (m, 11H), 7.83 (m, 3H), 7.55 (m, 4H), 7.47 (m, 2H), 7.31 (m,
10H), 4.35 (q, 6H, J ) 7.0 Hz), 1.36 (t, 9H, J ) 7.0 Hz); 13C NMR
δ 166.08, 156.59, 152.98, 151.84, 151.53, 149.95, 147.20, 146.75,
138.12, 137.96, 136.28, 135.45, 131.70, 131.29, 131.00, 130.65, 130.18,
129.30, 128.86, 128.27, 128.03, 127.26, 127.10, 124.09, 122.33, 119.92,
Hz), 7.54 (d, 6H, J ) 8.5 Hz), 7.49 (d, 2H, J ) 8.0 Hz), 7.43 (d, 2H,
J ) 8.0 Hz), 3.91 (s, 9H, OCH3), 3.06 (s, 1H, CtCH), 2.19 and 2.18
(2s, 12H, adamantane); 13C NMR δ 166.83, 153.74, 149.33, 132.23,
129.78, 128.25, 125.03, 120.13, 83.37, 52.07, 46.57, 39.53, 39.21 (one
C of the ethyne overlaps with the solvent). 11, Td ) Ad, R ) Et: 1H
NMR δ 8.04 (d, 6H, J ) 8.5 Hz), 7.54 (d, 6H, J ) 8.5 Hz), 7.49 (d,
2H, J ) 8.0 Hz), 7.43 (d, 2H, J ) 8.0 Hz), 4.37 (q, 6H, J ) 7.0 Hz,
OCH2CH3), 3.07 (s, 1H, CtCH), 2.21 and 2.19 (2s, 12H, adamantane),
1.39 (t, 9H, J ) 7.0 Hz, OCH2CH3); 13C NMR δ 166.39, 153.65, 149.39,
(12) Wilson, L. M.; Griffin A. C. J. Mater. Chem. 1993, 3, 991.
(13) Eichert, V. R.; Mathias, L. J. Macromolecules 1994, 27, 7015.
(14) Mizuno, T.; Masayuki, M.; Hamachi, I.; Nakashima, K.; Shinkai, S. J.
Chem. Soc., Perkin Trans. 2 1998, 2281.
(15) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa, D.;
Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447.
(16) The di- and trisubstituted byproducts obtained in this step were isolated
and characterized. Their syntheses and the study of di- and trichromophoric
compounds prepared from them will be published elsewhere.
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A R T I C L E S
Galoppini et al.
97.93, 84.29, 65.47, 61.07, 14.31; IR 2208 cm-1 (CtC), 1716 cm-1
(CdO); Raman shift 2204 cm-1 (CtC). Anal. Calcd for the neutral
complex: C, 56.63; H, 3.77; N, 5.83. Found: C, 56.46; H, 3.71; N,
5.76.
22 spectrometer using a Pike Miracle ATR accessory with 2 cm-1
resolution and 64 or 256 scans. IR measurements of the tripods on
TiO2 were made in transmission mode with unsensitized, pH 1
pretreated TiO2/sapphire as the reference. Raman spectra of solid
samples of 1-4 were collected on a Thermo-Nicolet Nexus 670 FT-
Raman Module.
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 cuvette,
immersed in acetonitrile, and argon purged 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. Photolumines-
cence quantum yield measurements were performed using the optically
dilute technique19 with Ru(bpy)3Cl2 in deionized H2O as the actinometer,
and calculated using eq 1,
Ru(bpy)2(C-tripod-bpy)(PF6)2 (4, Td ) C, L) bpy, R ) Me):
mp 224 °C (DSC); 1H NMR (acetone-d6) δ 8.91 (m, 2H), 8.83 (d, 4H,
J ) 8.0 Hz), 8.23 (m, 5H), 8.18 (d, 1H, J ) 5.5 Hz), 8.08 (d, 2H, J )
6.0 Hz), 8.05 (d, 3H, J ) 5.5 Hz) 7.97 (d, 6H, J ) 8.5 Hz), 7.60 (m,
8H), 7.45 (d, 6H, J ) 8.5 Hz), 7.41 (d, 2H, J ) 8.5 Hz), 3.88 (s, 9H);
13C NMR (acetone-d6) δ 166.77, 158.47, 157.99, 157.97, 157.95,
157.90, 157.59, 152.76, 152.67, 152.53, 151.12, 148.33, 139.00, 133.06,
132.64, 131.96, 131.63, 129.98, 129.65, 129.35, 128.99, 128.75, 126.87,
126.69, 125.53, 125.39, 125.29, 125.19, 120.37, 97.92, 86.87, 66.38,
52.39; IR 2208 cm-1 (CtC), 1716 cm-1 (CdO); Raman shift 2204
cm-1 (CtC). Anal. Calcd for the neutral complex: C, 54.99; H, 3.52;
N, 6.11. Found: C, 54.10; H, 3.75; N, 6.01.
1
Ph-E-phen (13):17 mp 149-150 °C; H NMR δ 9.24 (d, 1H, J )
4.5 Hz), 9.20 (d, 1H, J ) 4.5 Hz), 8.85 (d, 1H, J ) 8.5 Hz), 8.24 (d,
1H, J ) 8.5 Hz), 8.11 (s, 1H), 7.75 (q, 1H, J ) 4.5 Hz), 7.68 (m, 3H),
7.44 (m, 3H); 13C NMR δ 150.83, 150.59, 146.05, 145.88, 135.72,
134.67, 131.71, 130.55, 128.92, 128.52, 128.22, 127.98, 123.40, 123.31,
122.52, 119.90, 95.33, 85.76.
φem ) (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 photoluminescences of the
actinometer and sample, respectively, nr and ns are the refraction indexes
for the solvents used for the actinometer and sample, respectively, and
φr is the quantum yield for Ru(bpy)3Cl2 in deionized H2O (φr ) 0.042).
Time-Resolved Photoluminescence. Time-resolved photolumines-
cence decays were acquired on a nitrogen-pumped dye laser (460 nm)
apparatus that has been previously described.20 For solution studies,
the samples were optically dilute (A ≈ 0.1 at λmax), and the kinetic
traces were fit to a first-order model. Values for kr and knr were
calculated from eqs 2a and 2b
1
Ru(Ph-E-phen)(PF6)2 (5): mp 255 °C (DSC); H NMR (acetoni-
trile-d3) δ 9.02 (d, 1H, J ) 8.0 Hz), 8.58 (d, 1H, J ) 8.0 Hz), 8.50 (s,
1H), 8.08 (m, 2H), 7.77 (m, 2H), 7.72 (m, 1H), 7.64 (m, 1H), 7.52 (m,
3H); 13C NMR (acetonitrile-d3) δ 154.56, 148.96, 148.64, 137.54,
136.57, 136.35, 132.91, 132.51, 132.31, 131.71, 131.50, 130.89, 129.90,
127.28, 122.61, 122.59, 98.40, 84.91. Anal. Calcd for the neutral
complex: C, 58.50; H, 2.95; N, 6.82. Found: C, 57.53; H, 2.80; N,
6.80.
1
Ph-E-bpy (14): mp 90-92 °C; H NMR δ 8.71 (d, 1H, J ) 4.0
Hz), 8.67 (d, 1H, J ) 5.0 Hz), 8.54 (s, 1H), 8.41 (d, 1H, J ) 8.0 Hz),
7.84 (t, d, 1H, J ) 8.0 Hz, J ) 2.0 Hz), 7.57 (m, 2H), 7.39 (m, 4H),
7.34 (m, 1H); 13C NMR δ 156.16, 155.52, 149.19, 149.14, 136.97,
132.42, 131.88, 129.10, 128.45, 125.22, 123.96, 123.15, 122.21, 121.12,
93.89, 87.01.
φem ) kr/(kr + knr)
φem ) krτ
(2a)
(2b)
1
using the measured quantum yields and lifetimes and assuming an
intersystem crossing yield of unity. For studies involving the tripods
on TiO2 or ZrO2, the excitation beam was directed 45° to the film
surface, and the emitted light was collected at 90°.
Ru(Ph-E-bpy)(PF6)2 (6): mp 209 °C (DSC); H NMR δ 8.97 (m,
2H), 8.28 (m, 1H), 8.24 (m, 1H), 8.11 (m, 1H), 7.64 (m, 4H), 7.53 (m,
3H); 13C NMR δ 158.45, 158.36, 157.64, 157.55, 152.86, 152.80,
139.26, 133.59, 132.86, 131.19, 129.85, 129.19, 127.11, 122.03, 98.68,
86.40. Anal. Calcd for the neutral complex: C, 55.92; H, 3.13; N, 7.25.
Found: C, 55.31; H, 3.05; N, 7.20.
MO2 Preparations. Transparent thin films of TiO2 or ZrO2 were
prepared by a modification of published procedures18 that is described
in the Supporting Information.
Electrochemistry. Cyclic voltammetry for solution studies was
performed in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)/
CH3CN electrolyte. The solutions were ∼1 mM in the dyes. A BAS
model CV-50W potentiostat was used in a standard three-electrode
arrangement consisting of a glassy carbon working electrode, a Pt gauze
counter electrode, and Ag/AgCl as the reference electrode. Cyclic
voltammetry of the sensitizers bound to TiO2 was performed in a similar
manner with the sensitizer/TiO2 films deposited on FTO glass as the
working electrodes submerged in 0.1 M tetrabutylammonium perchlo-
rate (TBAClO4) acetonitrile (see Figure S1, Supporting Information).
The CV experiments were carried out under argon atmosphere and at
room temperature.
Spectroelectrochemistry. Solution measurements were carried out
in a 1 mm path-length quartz cuvette consisting of a Pt wire reference
electrode and a Pt gauze working electrode. The counter electrode
consisted of a glass cell containing a Pt wire in TBAClO4/CH3CN
electrolyte separated from the sensitizer solution by a glass frit. A PAR
173 potentiostat was used to control the applied potential, and a Hewlett-
Packard 8453 diode array was used to monitor the absorbance changes
at different applied potentials. The concentration of all solutions was
Spectroscopic Measurements. UV-vis absorbance measurements
were made on a Hewlett-Packard 8453 diode array spectrophotometer.
For TiO2 and ZrO2 studies, transient absorption measurements were
acquired using 532.5 nm laser excitation, ca. 8 ns and 1-20 mJ cm-2
,
from a Nd:YAG (Continuum Surelite II) laser. For transient absorption
experiments in fluid acetonitrile solution, samples were excited with
417 nm (Raman-shifted 355 nm laser light using a D2-filled pressurized
tube). The 5 mm diameter beam was expanded to ensure homogeneous
irradiation of the entire film. The sample was protected from a pulsed
150 W Xe probe beam using a fast shutter and appropriate UV- and
heat-absorbing glass and solution filter combinations. Each kinetic trace
was acquired by averaging 10-160 laser shots (typically 40). Samples
were argon purged and maintained under an acetonitrile premoistened
argon flow.
Infrared and Raman. Attenuated total reflectance (ATR) IR
measurements for solid samples of 1-4 were made on a Bruker Vector
adjusted to obtain less than 0.1 absorbance unit at λmax
Spectroelectrochemistry of derivatized TiO2 (pH 1 pretreated)
.
electrodes was performed in a three-electrode cell compartment using
(17) The synthesis of Ph-E-phen through a different route has been reported:
McGarrah, J. E.; Kim, Y.-I.; Hissler, M.; Eisenberg, R. Inorg. Chem. 2001,
40, 4510.
(19) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(20) Castellano, F. N.; Heimer, T. A.; Tandhasetti, T.; Meyer, G. J. Chem. Mater.
1994, 6, 1041.
(18) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem. 1990,
94, 8720.
9
7804 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Long-Range Electron Transfer Using Tripodal Sensitizers
A R T I C L E S
Scheme 1 a
a Reagents and yields: (a) Me3SiCtH (1.5 equiv), Cl2Pd(PPh3)2, CuBr, (i-Pr)2NH (22-30%); (b) 1. t-BuLi; 2. CO2; 3. H+, H2O (50%); (c) CH2N2 (90%)
(R ) Me) or dicyclohexylcarbodiimide and EtOH (R ) Et); (d) n-Bu4NF (95%); (e) 1. (Me3Si)2NLi; 2. B-methoxy-9-BBN, 4-bromo-2,2′-bipyridine, or
5-bromo-1,10-phenanthroline, Pd(PPh3)4 (36-68%); (f) 1. Ru(bpy)2Cl2‚2H2O; 2. NH4PF6 (55-75%).
Scheme 2 a
a sensitizer/TiO2 film deposited on FTO glass as the working electrode,
Pt gauze as the counter electrode, and Ag/AgCl as the reference
electrode in 0.1 M TBAClO4/CH3CN. Oxidative chronoabsorptometry
measurements were performed on derivatized TiO2 electrodes by
stepping the potential from 1.2 to 1.65 V and taking spectra every 5 s.
Reductive chronoabsorptometry measurements were performed by
stepping the potential from 1.65 to 1.0 V and taking spectra every 5 s.
Results
Synthesis. Tripodal sensitizers 1-4 were prepared as shown
in Scheme 1. Monosubstituted 8 was obtained by Sonogashira
a Reagents and yields: (a) 1. B-methoxy-9-BBN; 2. Pd(PPh3)4 (90%);
(b) 1. RuCl3‚2H2O; 2. NaPF6 (69%).
cross-coupling21 of trimethylsilylacetylene with the tetrabromide
or tetraiodide derivative of tetraphenylmethane12 or 1,3,5,7-
tetraphenyladamantane,13 respectively. Carboxylation of 8 fol-
lowed by esterification of the acid, 9,22 afforded trimethylsi-
lylethyne 10,16 which was deprotected with fluoride to form
alkyne 11. Suzuki-type coupling23 of 11 with 5-bromo-1,10-
phenanthroline or 4-bromo-2,2′-bipyridine produced the series
of four tripodal ligands Ad-tripod-phen (12a), Ad-tripod-bpy
(12b), C-tripod-phen (12c), and C-tripod-bpy (12d), all solid
materials that were soluble in polar organic solvents.
The corresponding RuII-polypyridyl complexes 1-4 were
prepared upon treatment with Ru(bpy)2Cl2‚2H2O and pre-
cipitation with NH4PF6. Similar procedures were used to pre-
pare model complexes 5 and 6 from lithium phenylacetylide
(Scheme 2).
absorbance after soaking the film for 12 h in acetonitrile
solutions with known concentrations of the tripodal sensitizer.
In all cases, the surface coverage saturated at high sensitizer
concentration. The equilibrium binding for 1-4 was well
described by the Langmuir adsorption isotherm model from
which surface adduct formation constants (Kad) were abstracted
using eq 3,24 where [RuII]eq is the equilibrium sensitizer
[RuII]eq
[RuII]eq
1
)
+
(3)
Γ
KadΓ0
Γ0
concentration, Γ0 is the saturation coverage, and Γ is the
equilibrium coverage at a defined molar concentration. The plots
of [RuII]eq/Γ versus [RuII]eq for 1-4 are shown in Figure 3,
Binding Constants. Surface binding was monitored spec-
troscopically by measuring the change in film and solution
insets. The surface adduct formation constants (∼10 × 105 M-1
,
Table 3) are approximately 1 order of magnitude larger than
those of RuII-polypyridyl complexes attached to untreated metal
oxides surfaces via dcb or deeb ligands. The typical equilibrium
(21) Sonogashira, K. In ComprehensiVe Organic Synthesis; Trost, B. M., Ed.;
Pergamon Press: Oxford, 1991; Vol. III, p 551.
(22) While the acids are insoluble materials, the esters are soluble in organic
solvents and can be purified by column chromatography. Ethyl esters were
more soluble than methyl esters, but the latter were obtained in considerably
higher yields.
surface coverage for 1-4 is (3 ( 2) × 10-8 mol cm-2
,
(23) (a) Soderquist, T. A.; Matos, K.; Rane, A.; Ramos, J. Tetrahedron Lett.
1995, 36, 2401. (b) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
(24) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361.
9
J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002 7805

A R T I C L E S
Galoppini et al.
dation wave corresponding to the second ligand reduction
appears as a sharp anodic peak for 1-6, which is characteristic
of anodic desorption of the neutral species from the glassy
carbon surface.25
Cyclic voltammetry performed on derivatized TiO2 films
(Table 3) showed reversible oxidation of the RuII center and
RuIII/II potentials that did not significantly deviate from values
obtained in fluid solution. The electroactive surface coverage,
estimated by integration of the anodic or cathodic waves at scan
rates of 20-200 mV/s, like that shown in Figure S1 (Supporting
Information), was a fraction of that measured spectroscopically,
consistent with previous observations reported by us11 and by
others.27 Complete oxidation of all the surface-bound complexes
was accomplished by stepping the potential positive of E1/2(RuIII/II
)
for about 1 h, Vide infra.
The excited-state reduction potentials were calculated from
the ground-state potentials and the free energy stored in the
thermally equilibrated MLCT excited state, ∆Ges, using eq 4.
∆Ges (in eV) was estimated by drawing a tangent line to the
E1/2(RuIII/II*) ) E1/2(RuIII/II) - ∆Ges
(4)
high-energy side of the corrected emission spectra. Ru(bpy)2-
(Ad-tripod-phen)2+ and Ru(bpy)2(C-tripod-phen)2+ had identical
excited-state reduction potentials (-910 mV), and those of Ru-
(bpy)2(Ad-tripod-bpy)2+ and Ru(bpy)2(C-tripod-bpy)2+ (∼-815
mV) were also the same within experimental error. The excited-
state reduction potentials for phen-based tripodal sensitizers 1
and 3 were the same as that observed for phen-based model
complex 5, Ru(Ph-E-phen)32+, and the excited-state reduction
potentials for bpy-based tripodal sensitizers 2 and 4 were about
the same as that observed for bpy-based model complex 6, Ru-
(Ph-E-bpy)32+. For the model complexes 5 and 6, the potentials
Figure 3. Absorption spectra of 1-4 bound to TiO2 films and immersed
in CH3CN. Insets: Plots of [RuII]eq/Γ vs [RuII]eq with overlaid linear fits.
comparable to that obtained from RuII-polypyridyl complexes
that are directly attached to the surface through a dcb or deeb
ligand ((5 ( 4) × 10-8 mol cm-2).
Solution and Surface-Bound Electrochemistry. The tripodal
sensitizers displayed quasi-reversible25a RuIII/II waves in aceto-
nitrile solution at 1.32 V vs SCE (Table 1). The RuIII/II reduction
potentials for phen- as well as bpy-based tripods were 60-90
mV more positive than those observed for Ru(bpy)2(phen)2+
and Ru(bpy)32+. Similarly, the RuIII/II reduction potentials for
model complexes Ru(Ph-E-phen)32+ and Ru(Ph-E-bpy)32 were
100 and 170 mV more positive than those observed for Ru-
2+
were ∼60 mV more positive than those for Ru(phen)3 and
Ru(bpy)32+, respectively.
Chronoabsorptometry measurements were performed on the
tripods and Ru(bpy)2(deeb)2+ anchored to pH 1 pretreated TiO2
films on FTO electrodes. Apparent diffusion coefficients (Dapp
)
were obtained from linear fits of absorption changes versus t1/2
according to the Cottrell equation, eq 5,
2+
(phen)3 and Ru(bpy)32+, respectively.
∆A ) (2AmaxDapp1/2t1/2)/(dπ1/2)
(5)
Ligand-based reduction potentials for the tripodal sensitizers
in solution are shown in Table 2. The first ligand reductions
for 1-4 occur at potentials that are more positive (∼85-100
mV) than those observed for the other RuII-polypyridyl com-
where ∆A is the change in absorbance at time t, Amax is the
absorbance at which absorbance changes cease, Dapp is the
apparent diffusion coefficient in cm2/s, and d is the thickness
of the film in cm. Plots of ∆A versus t1/2 maintained linearity
for ∼70% of either the oxidative (RuII f RuIII) or reductive
(RuIII f RuII) process. For both the tripods and Ru(bpy)2-
(deeb)2+, values for Dapp for either the oxidative or reductive
process were ∼10-11 cm2/s.
2+
plexes listed. Similarly, in model complexes Ru(Ph-E-phen)3
2+
(5) and Ru(Ph-E-bpy)3 (6), a ligand is first reduced at a
potential that is 230 and 180 mV more positive than those
observed for Ru(phen)32+ and Ru(bpy)32+, respectively. There-
fore, we assigned this wave in 1-6 to the phen or bpy ligand
connected to the phenylethynyl spacer.26 Single cathodic pre-
peaks were observed for 1-6 at ∼ -850 mV on the initial scans
but were absent after multiple scans. Furthermore, the reoxi-
Solution and Surface-Bound Photophysics. The visible
absorption spectra of tripodal sensitizers 1-4 displayed broad
bands typical of MLCT excited states (Figure S2, Supporting
Information). The phen-based tripods, Ru(bpy)2(Ad-tripod-
(25) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; Wiley-Interscience: New York, 1980. (b) Del Guerzo,
A.; Leroy, S.; Fages, F.; Schmehl, R. H. Inorg. Chem. 2002, 41, 359.
(26) For a similar example, where electron-withdrawing substituents in RuII
complexes lead to a more positive shift in a ligand reduction wave, see:
(a) Albano, G.; Belser, P.; De Cola, L.; Gandolfi, M. T. Chem. Commun.
1999, 1171. (b) Kalyanasundarum, K. Coord. Chem. ReV. 1982, 46, 159.
(c) Juris, A.; Balzani, V.; Barigelletti, F.; Belser, P.; Von Zelewsky; A.
Coord. Chem. ReV. 1988, 84, 85.
(27) (a) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer,
G. J. Inorg. Chem. 1996, 35, 5319. (b) Bonhote, P.; Gogniat, E.; Tingry,
S.; Barbe, C.; Vlachopoulos, N.; Lenzmann, F.; Comte, P.; Gra¨tzel, M. J.
Phys. Chem. B 1998, 102, 1498. (c) Farzad, F. Molecular Level Energy
and Electron Transfer Processes at Nanocrystalline Titanium Dioxide
Interfaces. Ph.D. Thesis, Johns Hopkins University, 1999. (d) Trammell,
S. A.; Meyer, T. J. J. Phys. Chem. B 1999, 103, 104.
9
7806 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Long-Range Electron Transfer Using Tripodal Sensitizers
A R T I C L E S
Table 1. Electrochemical and Photophysical Properties of 1-6 and Other RuII-Polypyridyl Sensitizers in Solution
a
b
III/II d
λ
abs, nm
λPL
τc
(µs)
E
1/2(Ru
)
E
1/2(RuIII/II*)
(V)
ΦPL
k
knr
ν(CdO)e
(cm-1
)
r
sensitizer
(ꢀ, M-1 cm-1
)
(nm)
(V)
(×10-2
)
(×104 s-1
)
(×105 s-1
)
∆Ges (eV)
Ru(bpy)2(Ad-tripod-phen)2+ (1)
452
624
1.4
2.0
1.0
2.2
1.32
-0.91
8.0
5.7
5.1
7.9
4.8
6.6
2.23
1709
1715
1716
1716
(1.6 × 104)
461
Ru(bpy)2(Ad-tripod-bpy)2+ (2)
Ru(bpy)2(C-tripod-phen)2+ (3)
Ru(bpy)2(C-tripod-bpy)2+ (4)
646
626
650
1.32
1.32
1.32
-0.82
-0.91
-0.81
10
7.9
10
4.5
9.2
4.1
2.14
2.23
2.13
(1.9 × 104)
450
(1.7 × 104)
461
(2.0 × 104)
Ru(Ph-E-phen)32+ (5)
Ru(Ph-E-bpy)32+ (6)
Ru(bpy)2(phen)2+
449
605
605
620
626
- -
1.1
1.1
1.2
0.80
0.30
0.93
1.37
1.43
1.23
1.26
1.27
1.39
-0.88
-0.82
-0.91
-0.86
-0.92
-0.62
6.4
13
6.1
13
8.9
8.2
2.25
2.25
2.14
2.12
2.19
2.01
468
450
452
447
2+ f
Ru(bpy)3
2+ f,g
Ru(phen)3
Ru(bpy)2(deeb)2+ h
475
690
4.4
4.8
10
1732
(1.6 × 104)
a Measurement were made at 22 ( 2 °C, absorption maximum (2 nm. The molar extinction coefficients, ꢀ, were obtained from CH3CN solutions. A TiO2
film was used as the optical reference. b Photoluminescence maximum, (4 nm. All data were obtained from CH3CN solutions under an argon atmosphere.
c Excited-state lifetime (5%. Data were obtained from CH3CN solutions. d Half-wave potentials ((20 mV) were measured at a glassy carbon working
electrode in 0.1 M TBAPF6/CH3CN solution using Ag/AgCl as reference. Data are reported vs SCE. e IR-ATR was performed on solid samples. f These
complexes have unsubstituted phen and bpy as the ligands and cannot be bound to metal oxide surfaces. g Reference 39. h Reference 4.
Table 2. Electrochemical Data for 1-6 and Other RuII-Polypyridyl
on ZrO2 or TiO2 surfaces showed no measurable spectral
changes with respect to the solution spectra. IR measurements
Sensitizers in Solutiona
III/II
III/II
E
1/2(Ru
)
E
1/2(Ru2+/+
)
E
1/2(Ru+/0
)
E
1/2(Ru /)
of 1-4 bound to TiO2 revealed a single CdO stretch at ∼1720
sensitizer
(mV)
(mV)
(mV)
(mV)
cm-1 28
.
Ru(bpy)2(Ad-tripod-phen)2+
Ru(bpy)2(Ad-tripod-bpy)2+
Ru(bpy)2(C-tripod-phen)2+
Ru(bpy)2(C-tripod-bpy)2+
1320
1320
1320
1320
1430
1370
1260b
1270b
1230b
1240b
1230b
1190d
-1306
-1246
-1270
-1237
-1159
-1140
-1477
-1485
-1463
-1497
-1321
-1301
-910
-820
-910
-810
-820
-880
-860b
-920b
-910b
-870b
-870b
The PL decays of the sensitizers in acetonitrile solutions
followed single-exponential kinetics, and the excited-state
lifetimes are listed in Table 1, together with the PL quantum
yields. The PL lifetimes for the phen-based tripodal sensitizers
1 and 3 (1.4 and 1.0 µs) were comparable to the PL lifetime of
Ru(bpy)2(phen)2+ (1.2 µs). The PL lifetimes of bpy-based
tripodal sensitizers 2 and 4 (2.0 and 2.2 µs) however, were
2+
Ru(Ph-E-bpy)3
Ru(Ph-E-phen)3
2+
-1340c -1520c
-1370c -1520c
-1370c -1530c
-1320c
2+
Ru(bpy)3
2+
Ru(phen)3
Ru(bpy)2(phen)2+
Ru(bpy)2(4,7-dpphen)2+
2+
considerably longer than that of Ru(bpy)3 (800 ns). The PL
Ru(bpy)2(4,4′-dpbpy)2+
-1310c
2+
-1270d
lifetimes for the two model complexes, phen-based 5 and bpy-
based 6, were nearly identical (∼1.1 µs), and the latter
compound has a notably high emission quantum yield.
Time-resolved PL decays for the tripods bound to ZrO2 and
TiO2 were nonexponential and were well described by a parallel
first- and-second-order kinetic model, eq 6.29
Ru(4,4′-dpbpy)3
a Data are reported vs SCE. All measurements were performed in 0.1 M
TBAPF6/CH3CN. Data obtained from the literature were performed in 0.1
-
-
M TAAX/CH3CN, where X ) ClO4 or PF6 and TAA ) (N(Et)4)+ or
(N(nBu)4)+. E1/2(Ru2+/+) and E1/2(Ru+/0) are the midpoint potentials for the
first and second ligand reductions, respectively. b Reference 39. c Reference
40. d Reference 41. (V)
Table 3. Electrochemical, Adsorption, and IR Properties of 1-4
Ck1 exp(-k1t)
Bound to TiO2
PLI )
(6)
k1 + k2C - k2C exp(-k1t)
surface
c
binding
constant K
(×105 M-1
coverage
III/II a
b
E
1/2(Ru
)
(×10-8
ν(CdO)d
(cm-1
ad
Here C is the excited-state concentration, k1 is the first-order
rate constant, and k2 is the observed second-order rate constant.
Typical data are shown in Figure 4 for [Ru(bpy)2(C-tripod-
phen)2+] (4) on TiO2 and ZrO2 with surface coverages near
saturation, approximately 2 × 10-8 mol/cm2. The first-order
rate constants on TiO2 and ZrO2 were 1.6 × 106 and 6.9 × 105
s-1, respectively, while the second-order components were 4.3
× 107 and 3.0 × 106 s-1, respectively. Consistent with previous
studies for Ru(deeb)(bpy)22+/TiO2,29 the first-order rate constant
was typically k1 ) (3 ( 2) × 106 s-1, and the second-order
-2
sensitizer
(V)
)
mol cm
)
)
Ru(bpy)2(Ad-Tripod-phen)2+
Ru(bpy)2(Ad-Tripod-bpy)2+
Ru(bpy)2(C-Tripod-phen)2+
Ru(bpy)2(C-Tripod-bpy)2+
1.34
1.35
1.37
1.34
30 ( 20
10 ( 5
10 ( 5
10 ( 5
3.1
3.9
4.2
2.0
1720
1718
1717
1718
a Half-wave potentials ((20 mV) were measured at a sensitizer/TiO2/
FTO working electrode in 0.1 M TBAClO4/CH3CN solution using Ag/
AgCl as reference. Data are reported vs SCE. b Estimated from Langmuir
adsorption isotherm measurements at 22 ( 2 °C. c From Langmuir
adsorption isotherm measurements. d Obtained for samples of sensitizer/
TiO2/sapphire in the transmission mode.
component was k2 ) (9 ( 5) × 107 s-1
.
phen)2+ and Ru(bpy)2(C-tripod-phen)2+, displayed MLCT bands
centered at ∼451 nm, while the bpy-based tripods, Ru(bpy)2-
(Ad-tripod-bpy)2+ and Ru(bpy)2(C-tripod-bpy)2+, were red-
shifted and centered at 461 nm. All complexes displayed room-
temperature photoluminescence (PL) in fluid solution, and the
emission maximum followed the same trends as the absorption
(Table 1). Absorption and emission spectra of 1-4 anchored
Transient Absorption. The spectral features of the transient
absorption spectra of 1-4 in acetonitrile solution and on
(28) (a) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227. (b)
Umpathy, S.; Cartner, A. M.; Parker, A. W.; Hester, R. E. J. Phys. Chem.
1990, 94, 1357. (c) Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Langmuir
1998, 14, 2744.
(29) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Meyer, G. J. Langmuir 1999,
15, 731.
9
J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002 7807

A R T I C L E S
Galoppini et al.
Figure 4. Normalized time-resolved photoluminescence decays for Ru-
(bpy)2(C-tripod-bpy)(PF6)2 adsorbed on either (a) ZrO2 or (b) TiO2 (solid
lines). Overlaid are fits to the parallel first- and second-order models (dashed
lines). Inset: Residuals for fits. The samples were illuminated with pulsed
460 nm light.
Figure 6. Time-resolved absorption difference (∆A) spectra observed after
pulsed 532.5 nm laser light excitation (∼14 mJ cm-2, 8 ns fwhm) of 1-4
bound to nanocrystalline TiO2 films in CH3CN. The data were recorded at
10 ns (9), 100 ns (b), 500 ns (2), and 2 µs (1) delays after the laser pulse.
Overlaid are the absorption difference spectra from spectroelectrochemical
experiments of sensitized TiO2/FTO films (dashed line) obtained by
subtracting the absorption spectrum of RuII from that of RuIII (for details,
see Supporting Information, Figure S3). Insets: Transient absorption signals
of 1/TiO2, 2/TiO2, 3/TiO2, and 4/TiO2 monitored at 503, 509, 510, and 515
nm, respectively, after 532.5 nm laser excitation (∼14 mJ cm-2, 8 ns fwhm)
displayed on a logarithmic time scale from 10-8 to 0.1 s.
difference spectra for [Ru(bpy)2(Ad-tripod-bpy)2+]* and [Ru-
(bpy)2(C-tripod-bpy)2+]* have a bleach centered at ∼340 nm,
absorbance bands centered at ∼310 and ∼380 nm, isosbestic
points at ∼325, 358, and ∼405 nm, and intense absorption bands
beyond 600 nm. The kinetics were first-order in acetonitrile
solution and followed the parallel first- and second-order kinetic
model on ZrO2, with rate constants that agreed well with the
time-resolved PL data.
Time-resolved absorption difference spectra of the four
tripodal sensitizers bound to TiO2 are shown in Figure 6. In all
cases the normalized difference spectra recorded at different
delay times were the same within experimental error. The spectra
are assigned to an interfacial charge-separated state with an
electron in TiO2 and an oxidized RuIII center, RuIII|TiO2(e-).
The absorption difference spectra obtained from spectroelec-
trochemical data, i.e., Abs(RuIII/TiO2) - Abs(RuII/TiO2), agree
well with the difference spectra measured by transient absorp-
tion. At wavelengths greater than 600 nm, the spectroelectro-
chemical data underestimate the measured spectra slightly due
Figure 5. Time-resolved absorption difference (∆A) spectra, obtained after
pulsed 532.5 nm laser light excitation (∼14 mJ cm-2, 8 ns fwhm), of the
tripodal complexes 1-4 bound to nanocrystalline ZrO2 films in argon-purged
CH3CN electrolyte at 25 °C. The data were recorded at 10 ns (9), 100 ns
(b), 500 ns (2), and 5 µs (1) delay after the laser pulse.
insulating ZrO2 immersed in acetonitrile were the same within
experimental error and were assigned to the MLCT excited
state.
The rationale behind using ZrO2 is that it is a metal oxide
substrate that does not participate in interfacial electron-transfer
processes and therefore affords MLCT excited-state character-
ization. Data for 1-4/ZrO2 are shown in Figure 5. The absorb-
ance difference spectra for [Ru(bpy)2(Ad-tripod-phen)2+]*, [Ru-
(bpy)2(C-tripod-phen)2+]*, Ru(Ph-E-phen)32+ *, and Ru(b-
py)32+ * are qualitatively very similar, with absorbance bands
centered at ∼310 and ∼370 nm, an isosbestic point at ∼397
nm, and weak bands beyond 600 nm. In contrast, the absorbance
9
7808 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Long-Range Electron Transfer Using Tripodal Sensitizers
A R T I C L E S
to the weak absorption of the TiO2(e-). Experimental procedures
and data are provided in the Supporting Information and in
Figure S3.
Table 4. Comparison of Photophysical Properties for
Ru(bpy)2(C-Tripod-bpy)2+ (4) and Ru(bpy)2(deeb)2+ in CH3CN
Solution and Bound toTiO2
sensitizer
λabs (nm)
λPL (nm)
E
1/2(RuIII/II) (V)
E
1/2(RuIII/II*) (V)
The recovery of the ground-state absorption spectra, measured
at a ground-excited-state isosbestic point, are shown as insets
in Figure 6. The solid lines superimposed on the kinetic data
represent fits to a sum of two second-order equal concentration
processes (bi-second order), eq 74
4
461
461
475
487
655
660
690
704
1.32
1.34
1.39
1.35
-0.81
-0.78
-0.62
-0.76
4/TiO2
Ru(bpy)2(deeb)2+
Ru(bpy)2(deeb)2+/TiO2
will be very useful to study excited states that are weakly
coupled to the semiconductor surface.
∆A0 - ∆As
∆As
∆A )
+
(7)
1 + (kf/∆ꢀl)t(∆A0 - ∆As) 1 + (ks/∆ꢀl)t(∆As)
Localization of the MLCT Excited State. Time-resolved
resonance Raman experiments have demonstrated that the
excited state of RuII-polypyridine complexes are localized on
one ligand in the metal-to-ligand charge-transfer (MLCT)
excited state on a nanosecond time scale.31 DeArmond correlated
spectroscopic and electrochemical data on heteroleptic RuII
compounds and has convincingly shown that the first ligand
reduced is the “optical” orbital relevant to the photoluminescent
MLCT excited state at room temperature.32 Based on DeAr-
mond’s correlation, the electrochemical data reported here are
consistent with the excited state being localized on the surface-
bound tripodal ligand for all compounds studied. The phenyl-
ethynyl substituent is electron withdrawing and lowers the π*
orbitals and reduction potential relative to those of unsubstituted
bpy or phen ligands.26 Furthermore, for bpy-based tripodal
sensitizers Ru(bpy)2(Ad-tripod-bpy)2+ and Ru(bpy)2(C-tripod-
bpy)2+, the absorbance and PL spectra are significantly red-
shifted relative to that observed for Ru(bpy)32+, consistent with
the bpy-based tripodal ligand being lower in energy. A
comparison of the excited-state absorption difference spectra
of the tripodal sensitizers with other RuII-polypyridine com-
plexes also reveals that the excited state is localized on the
tripodal ligand. An interesting observation is that the bpy-based
tripods give rise to longer excited-state lifetimes than do the
phen-based tripods, despite the fact that the bpy compounds
have a smaller energy gap.
where ∆A is the absorbance change at time t, ∆ꢀ is the molar
extinction coefficient, l is the optical path length, ∆Ao is the
initial amplitude (equal to the sum of the contributions from
the fast and slow components), kf is the recovery rate constant
for the fast component, ∆As is the amplitude of the slow
component, and ks is the recovery rate constant of the slow
component. Typical observed rate constants for charge recom-
bination for the two components (in units of absorbance) are 4
× 108 and 5 × 106 s-1, respectively. The rates were independent
of the tripodal sensitizer studied and were, within experimental
error, the same as those observed for Ru(bpy)2(deeb)2+/TiO2.
The weights of the two components, ca. 70% and 30% for the
fast and slow components, respectively, were also independent
of the sensitizer. Conversion of the rate constants to units of
concentration is difficult due to the heterogeneity of the sample
and the resulting ill-defined nature of the optical path length, l.
Discussion
The synthetic methodologies described have allowed us to
prepare a new class of molecular sensitizers, with novel
semirigid tripodal ligands for binding to metal oxide surfaces.10
These synthetic procedures can be extended to other sensitizers
and can be used to control the coupling between the sensitizer
and the semiconductor. The excited-state properties of tripods
1-4 anchored to colloidal ZrO2 thin films as well as their
electron-transfer dynamics on nanocrystalline (anatase) TiO2 thin
films provide insights into sensitizer-sensitizer and sensitizer-
surface electronic interactions. Below we discuss the implica-
tions of the photophysical and electron-transfer behavior and
compare it with recent literature reports.
1. Photophysical Behavior. The absorption and emission
spectra of the surface-bound tripodal sensitizers 1-4 immersed
in acetonitrile are, within experimental error, the same as those
measured in fluid acetonitrile solution. This behavior is indica-
tive of weak electronic coupling between the sensitizer and the
semiconductor, and it differs considerably from that observed
for inorganic coordination compounds bound to semiconductors
through the dcb or the deeb ligand.3 For instance, in the case
of Ru(deeb)(bpy)22+, the semiconductor surface and acid-base
surface chemistry have a significant effect on the absorption
and emission (Table 4).30,34 Therefore, the tripodal sensitizers
Excited-State Relaxation Kinetics. An important difference
between the excited states of the tripodal sensitizers in fluid
solution relative to those attached to metal oxide surfaces is
that, in the case of 1-4/ZrO2 and 1-4/TiO2, relaxation kinetics
are nonexponential. Tripods 1-4 bound to the ZrO2 and TiO2
surfaces are well described by a parallel first- and second-order
kinetic model. The appearance of a second-order component in
the excited-state relaxation is indicative of excited state-excited
state annihilation processes that result from fast intermolecular
energy transfer (kEnT) across the metal oxide interface (Scheme
3).29 Direct evidence for intermolecular energy transfer has been
previously reported for RuII and OsII sensitizers bound to
nanocrystalline TiO2.33 The observed second-order rate constant
is a function of the excited-state concentration that is unknown
under these experimental conditions. Nevertheless, the appear-
ance of the second-order process reveals significant excited-
state interaction between the surface-bound tripodal compounds.
2. Electron Transfer. Electron-transfer processes can be
quantified in considerable molecular detail in the case of
(30) (a) Giordano, P. J.; Bock, C. R.; Wrighton, M. S.; Interrante, L.; Williams,
R. F. X. J. Am. Chem. Soc. 1977, 99, 3187. (b) Ferguson, J.; Mau, A.
W.-H.; Sasse, W. H. F. Chem. Phys. Lett. 1979, 68, 21. (c) Shimidzu, T.;
Iyoda, T.; Izaki, K. J. Phys. Chem. 1985, 89, 642. (d) Mesmaeker, A. K.-
D.; Jacquet, L.; Nasielski, J. Inorg. Chem. 1988, 27, 4451. (e) Nazeeruddin,
M. K.; Kalyanasundaram, K. Inorg. Chem. 1989, 28, 4251. (f) Nazeeruddin,
M. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska, P.;
Vlachopoulos, N.; Shklover, V.; Fischer, C.-H.; Gra¨tzel, M. Inorg. Chem.
1999, 38, 6298.
(31) Dallinger, R. F.; Woodruff, W. H. J. Am. Chem. Soc. 1979, 101, 4391.
(32) DeArmond, M. K.; Hanck, K. W.; Wertz, D. W. Coord. Chem. ReV. 1985,
64, 65.
(33) Farzad, F.; Thompson, D. W.; Kelly, C. A.; Meyer, G. J. J. Am. Chem.
Soc. 1999, 121, 5577.
9
J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002 7809

A R T I C L E S
Galoppini et al.
Scheme 3
the tripodal sensitizers, which have the Ru centers located ∼17
Å from the surface.
The values of Dapp abstracted from the Cottrell equation with
nearly saturated surface coverages were 10-11 cm2/s for reduc-
tion and oxidation of the tripods. Measurements for Ru(bpy)2-
(dcb)2+/TiO2 also gave Dapp ≈ 10-11 cm2/s, indicating that
intermolecular charge-transfer rates are not significantly influ-
enced by the presence of the tripodal ligands or the increased
distance from the semiconductor surface. The footprint of the
tripodal ligands (∼70 Å2) is comparable in size to those of RuII
tris-chelates, and we anticipate that the packing density, and
hence sensitizer-sensitizer distance, will be similar to those
observed for other RuII sensitizers. Therefore, the values of Dapp
should not be influenced by the sensitizer-sensitizer distance,
and the fact that they are insensitive to the tripodal linkage and
spacer suggests that proximity to the surface and specific ion
adsorption are not significant factors under these experimental
conditions.
mesoporous TiO2 films, which can be characterized both
spectroscopically and electrochemically.3 The tripodal linker
reported here provides a more well-defined semiconductor-
molecular distance and, in some cases, orientation than has been
previously possible. For instance, in the case of cis-Ru(dcb)2-
(NCS)2, at most three of the four carboxylic acid groups can
simultaneously interact with the semiconductor surface, resulting
in a distribution of possible surface orientations.3 The single
asymmetric CO stretch in the IR spectrum of the surface-bound
tripods (Table 3) indicates that all three carboxylic acid groups
interact with the surface in an equivalent manner and are
consistent with the idealized geometry of attachment shown in
Figure 1b.28 It was therefore of interest to quantify molecular
electron-transfer processes with this new class of photosensi-
tizers. Below we discuss our results on three types of electron-
transfer processes and contrast them with previously published
work.
Intermolecular Charge Transfer. We and others have found
that redox-active molecules bound to mesoporous nanocrystal-
line TiO2 films can be electrochemically oxidized and reduced
in a reversible fashion.27 Since the reduction potentials of the
molecules exist near mid-band gap, the process does not in-
volve the valence or conduction bands of the semiconductor.
Instead, the accepted mechanism involves initial oxidation of
compounds bound to the tin oxide substrate followed by
intermolecular charge transfer across the nanoparticle surfaces,
as shown in Figure S4 (Supporting Information). For the entire
film to be oxidized, this mechanism requires electronic com-
munication between all the surface-bound complexes. In fact,
Gra¨tzel and co-workers have quantified the percolation threshold
necessary for complete oxidation of amines bound to related
TiO2 films.27b Chronoamperometry experiments with optical
detection have allowed the apparent diffusion coefficient for
intermolecular hopping with RuIII/II(bpy)2(dcb)3+/2+/TiO2 to be
quantified, Dapp ) 1.4 × 10-9 cm2/s in 0.1 M TBAPF6
acetonitrile.27c
Charge Separation. Interfacial charge separation at dye-
sensitized TiO2 surfaces has been the subject of many
studies.3 For RuII sensitizers, electron transfer generally occurs
from the π* orbitals of a coordinated dcb ligand to the
empty states of the semiconductor, and there is some spec-
tral evidence that the dcb ligands provide strong electronic
coupling to the semiconductor surface.3 Recent ultrafast spec-
troscopic studies have revealed femtosecond electron injection
rates from RuII* excited states under a variety of experimental
conditions.35
In contrast to other inorganic sensitizers, the absorption and
emission properties of 1-4 in fluid solution and 1-4/MO2 are,
within experimental error, the same, consistent with weak
electronic coupling to the surface. We therefore expect charge
separation to occur from the thermally equilibrated excited state
localized on the surface-bound tripodal ligand for 1-4. In all
cases, the rate constant for charge separation was faster than
could be time-resolved with our instrumentation, kcs > 108 s-1
.
Since the radiative and nonradiative rate constants for these
compounds in fluid solution are several orders of magnitude
slower, ∼104 and 105 s-1, respectively, a quantum yield for
electron injection near unity would be expected for an injection
rate of ∼108 s-1. This expectation is consistent with the transient
absorption data that reveal no clear evidence for excited states
for the compounds bound to TiO2 (see Supporting Information).
Electron transfer from excited states that are weakly coupled
to the semiconductor have been previously reported.36 Research-
ers have introduced (CH2)n spacers between the carboxylic acid
groups and a bpy ligand to attenuate the electronic coupling to
the surface.8 Lian and co-workers quantified the decrease in
excited-state electron injection rate constant as the alkyl spacer
increased in two complexes of the type fac-Re(CO)3Cl(L), where
L ) 4,4′-[HO2C-(CH2)n]-2,2′-bpy, with n ) 1 and n ) 3.8b The
injection rate decreased by a factor of 12.6 as n increased from
n ) 1 (5.3 × 1010 s-1) to n ) 3 (4.2 × 109 s-1). If the flexible
In previous work, an unexpected ionic strength dependence
for intermolecular electron hopping was discovered.34 The
presence of small cations, such as H+ or Li+, at the TiO2
interface, results in more rapid and efficient intermolecular
charge transfer. These observations suggested that previously
reported apparent diffusion coefficients might, at least in part,
reflect the rate of counterion movement. Ion motion may be
significantly influenced by specific surface adsorption effects,
and it was therefore of interest to quantify these processes with
(35) (a) Tachibana, Y.; Moser, J. E.; Gra¨tzel, M.; Klug, D. R.; Durrant, J. J.
Phys. Chem. 1996, 100, 20056. (b) Hannappel, T.; Burfeindt, B.; Storck,
W.; Willig, F. J. Phys. Chem. B 1997, 101, 6799. (c) Heimer, T. A.;
Heilweil, E. J. J. Phys. Chem. B 1997, 101, 10990. (d) Ellingson, R. J.;
Asbury, J. B.; Ferrere, S.; Ghosh, H. N.; Sprague, J. R.; Lian, T.; Nozik,
A. J. J. Phys. Chem. B 1997, 101, 6455. (e) Benko, G.; Kallioinen, J. E.;
Korppi-Tommola, J. E. I.; Yartsev, A. P.; Sundstrom, V. J. Am. Chem.
Soc. 2002, 124, 489.
(34) Qu, P.; Meyer, G. J. Langmuir 2001, 17, 6720.
(36) Qu, P.; Thompson, D. W.; Meyer, G. J. Langmuir 2000, 16, 4662.
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7810 J. AM. CHEM. SOC. VOL. 124, NO. 26, 2002

Long-Range Electron Transfer Using Tripodal Sensitizers
A R T I C L E S
spacer was fully extended for the n ) 3 case, the approximate
distance from the pyridyl nitrogen to the oxygen in the
carboxylic acid would be ∼6.5 Å. Extrapolation of these data
to distances of ∼17 Å would suggest that the rates for charge
separation should be time resolved with ∼10 ns time resolution.
However, the energetics for the ReI excited states are different
than those for the heteroleptic RuII compounds. In addition, the
excited states in 1-4 may delocalize over the phenylethynyl
spacer, resulting in a significantly decreased charge separation
distance compared to that for the Re compounds. Studies with
a homologous series of tripodal sensitizers as a function of
distance are under way in our laboratories and will provide
valuable insights into this behavior.
Charge Recombination. Recombination of the injected
electron with the oxidized dye required milliseconds for
completion, a result that is consistent with previous studies of
other RuII sensitizers.3 The recombination process follows
second-order kinetics with rate constants that are independent
of the tripod studied. Since the RuIII/II potentials and the
semiconductor-RuIII distance are very similar for all four tripods,
the fact that the rate constants are the same is not surprising.
However, we had previously noted that the rate of charge
recombination was faster for Ru(bpy)2(Ad-tripod-bpy)3+|TiO2-
(e-) than for Ru(bpy)2(dcb)3+|TiO2(e-) under conditions of
constant irradiance, temperature, and electrolyte.11 In examining
a larger data set and the newly synthesized tripods reported here,
we again find that the time scale, and hence rate, for charge
recombination is consistently slower for Ru(bpy)2(dcb)3+|TiO2(e-).
However, the weighted-average observed rate constants ab-
stracted from the bi-second-order model are, within a factor of
3, the same for all the tripods and for Ru(bpy)2(dcb)3+|TiO2(e-).
Uncertainty in the path length and extinction coefficients
preclude us from quantifying the true second-order rate con-
stants, and a small difference in ∆ꢀ at the isosbestic point for
Ru(bpy)2(dcb)3+|TiO2(e-) could account for the different rate.
We therefore conclude that the charge recombination rate
constants for the tripods and Ru(bpy)2(dcb)3+|TiO2(e-) are,
within experimental error, the same.
force, the sensitizer geometry, the number of carboxylic acid
groups, and the nature of the metal center (Ru, Os, or Re).37
The results suggested that the charge recombination was rate
limited by a process other than interfacial charge recombination.
Nelson and Durrant have provided strong evidence that diffusion
of the injected electron in the TiO2 film is the rate-limiting
process.38 If diffusion is rate-limiting here, then slowing down
charge recombination by further increasing the semiconductor-
RuIII distance should ultimately lead to a change in the
mechanism for charge recombination. Studies of this type are
underway in our laboratories.
Conclusions
Four RuII-polypyridyl compounds (1-4) containing tripodal
ligands were synthesized and characterized for photophysical
and electron-transfer studies at nanoparticle interfaces. The Ru
centers are ∼15-17 Å from the surface and are attached through
a bridge that is not completely conjugated. As a result, the redox
and steady-state optical properties of the compounds are
unchanged upon attachment to the nanoparticles, suggesting
weak sensitizer-surface electronic coupling. This behavior has
not been previously reported for other RuII sensitizers.3 There-
fore, the tripodal compounds are useful to prepare sensitizers
that are not altered by surface chemistry. Whereas the structural
differences among 1-4 are relatively small, this first series
provides a useful data set for tripodal sensitizers that can be
compared to data available for other RuII-polypyridine dyes.3
Systematic variations of the spacer and the sensitizer and
numerous other structural changes will be explored in future
studies.
Acknowledgment. The Division of Chemical Sciences, Office
of Basic Energy Sciences, Office of Science, U.S. Department
of Energy, and the Petroleum Research Fund (ACS-PRF 35081-
G5) are gratefully acknowledged for research support by E.G.
G.J.M. acknowledges the Division of Chemical Sciences, Office
of Basic Energy Sciences, Office of Science, U.S. Department
of Energy.
Supporting Information Available: Detailed synthetic pro-
cedures for the synthesis of 1-6, the method for TiO2
nanoparticles preparation, a description of the spectroelectro-
chemical oxidation of 1-4/TiO2/FTO, cyclic voltammograms
for 1-4, solution absorption spectra for 1-4, and a schematic
for the electron hopping mechanism by which oxidation of
sensitized TiO2 films occurs (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Previous work in our37 and in others’ 38 laboratories has
established that the charge recombination rate constants can be
remarkably insensitive to the apparent thermodynamic driving
(37) Hasslemann, G. M.; Meyer, G. J. J. Phys. Chem. B 1999, 103, 7671.
(38) (a) Nelson, J. Phys. ReV. B 1999, 59, 15374. (b) Nelson, J.; Haque, S. A.;
Klug, D. R.; Durrant, J. R. Phys. ReV. B 2001, 63, 205321.
(39) Vlcek, A. A.; Dodsworth, E. S.; Pietro, W. J.; Lever, A. B. P. Inorg. Chem.
1995, 34, 1906.
(40) Bugnon, P.; Hester, R. E. Chem. Phys. Lett. 1983, 102 (6), 537.
(41) De Cola, L.; Barigeletti, F. HelV. Chim. Acta 1988, 71, 733.
JA025840N
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