Excited state electron transfer from Ra(II) polypyridyl complexes anchored to nanocrystalline TiO2 through rigid-rod linkers

Article abstract of DOI:10.1021/jp047454t

Rigid-rod linkers varying in length were used to bind Ru(II) polypyridyl complexes to the surface of TiO₂ (anatase) and of ZrO₂ nanoparticle thin films. The linkers were made of p-phenyleneethynylene (Ph-E)_n bridges carrying two COOR anchoring groups at the end and were capped with Ru(II) polypyridyl complexes as the sensitizing chromophores. Two series of rigid-rod sensitizers were prepared: Ru complexes having bpy or 4,4a?2-(Cl)₂-bpy as the ancillary ligands. In the first series, the excited state was localized on the rigid-rod linker, in the second series, the excited states were localized on the 4,4a?2-(Cl)₂-bpy ligands. The rigid-rod sensitizers with Ru(bpy)₂ complexes did bind strongly (K_ad ~ 10⁵ M^-1) with high surface coverages (~10^-8 mol/cm²) on the nanostructured metal oxide films, ZrO₂ and anatase TiO₂. The length of the folly conjugated rigid-rod linker influences the photophysical properties of the sensitizer, and nanosecond transient absorption measurements indicated long-lived metal-to-ligand charge-transfer (MLCT) excited states (a??2 ??s) with evidence for delocalization onto the rigid-rod linker. The interfacial electron transfer behavior on TiO₂ was found to be dependent on the Br??nsted acidity or basicity of the surface. On base pretreated TiO ₂, the excited state electron injection yields were low and could be increased by addition of LiClO₄ to an external CH₃CN solution. Under these conditions, a fraction of the injection process could be time resolved on a 10 ns time scale. On acidic TiO₂, ultrafast excited state electron injection was observed for both series. Recombination was found to be second order with average rate constants independent of which rigid-rod sensitizer was excited. For rigid-rod sensitizers with Ru(4,4a?2-(Cl)₂-bpy)₂, there was evidence for a direct interaction between the 4,4a?2-(Cl)₂-bpy ligands and the TiO₂ surface. Fhotophysical and interfacial electron transfer properties of these Cl-substituted complexes were nearly independent of the rigid-rod length.

Full text of DOI:10.1021/jp047454t

16642
J. Phys. Chem. B 2004, 108, 16642-16653
Excited State Electron Transfer from Ru(II) Polypyridyl Complexes Anchored to
Nanocrystalline TiO2 through Rigid-Rod Linkers
Dong Wang, Richard Mendelsohn, and Elena Galoppini*
Chemistry Department, Rutgers UniVersity, 73 Warren Street, Newark, New Jersey 07102
Paul G. Hoertz, Rachael A. Carlisle, and Gerald J. Meyer*
Department of Chemistry and Department of Materials Science and Engineering, Johns Hopkins UniVersity,
3400 North Charles Street, Baltimore, Maryland 21218
ReceiVed: June 12, 2004; In Final Form: August 10, 2004
Rigid-rod linkers varying in length were used to bind Ru(II) polypyridyl complexes to the surface of TiO2
(anatase) and of ZrO2 nanoparticle thin films. The linkers were made of p-phenyleneethynylene (Ph-E)n bridges
carrying two COOR anchoring groups at the end and were capped with Ru(II) polypyridyl complexes as the
sensitizing chromophores. Two series of rigid-rod sensitizers were prepared: Ru complexes having bpy or
4,4′-(Cl)2-bpy as the ancillary ligands. In the first series, the excited state was localized on the rigid-rod
linker, in the second series, the excited states were localized on the 4,4′-(Cl)2-bpy ligands. The rigid-rod
sensitizers with Ru(bpy)2 complexes did bind strongly (Kad ∼ 105 M-1) with high surface coverages (∼10-8
mol/cm2) on the nanostructured metal oxide films, ZrO2 and anatase TiO2. The length of the fully conjugated
rigid-rod linker influences the photophysical properties of the sensitizer, and nanosecond transient absorption
measurements indicated long-lived metal-to-ligand charge-transfer (MLCT) excited states (∼2 µs) with evidence
for delocalization onto the rigid-rod linker. The interfacial electron transfer behavior on TiO2 was found to
be dependent on the Brønsted acidity or basicity of the surface. On base pretreated TiO2, the excited state
electron injection yields were low and could be increased by addition of LiClO4 to an external CH3CN solution.
Under these conditions, a fraction of the injection process could be time resolved on a 10 ns time scale. On
acidic TiO2, ultrafast excited state electron injection was observed for both series. Recombination was found
to be second order with average rate constants independent of which rigid-rod sensitizer was excited. For
rigid-rod sensitizers with Ru(4,4′-(Cl)2-bpy)2, there was evidence for a direct interaction between the 4,4′-
(Cl)2-bpy ligands and the TiO2 surface. Photophysical and interfacial electron transfer properties of these
Cl-substituted complexes were nearly independent of the rigid-rod length.
Introduction
Polypyridine complexes of Ru(II) are classical photosensi-
tizing dyes or “sensitizers” for nanocrystalline TiO2 (Gra¨tzel)
solar cells.1,2 The sensitizers are bound to anatase TiO2
nanoparticles through anchoring functional groups, such as
carboxylic acid or phosphonate groups, on the diimine ligand,1a
and the development of rigid molecular linkers to the surface
is attracting increasing attention.3 Rigid linkers varying in length
and structure that have the shape of tripods were developed in
our laboratories to study interfacial charge injection processes,
Figure 1.4 Tripodal sensitizers, because of the three-point
Figure 1. Schematic illustration of rigid-rod and tripodal linkers bound
to metal oxide (MO) nanoparticles’ surface consisting of a bridge (b)
attachment, stand perpendicularly to the surface and were useful
of variable length, anchoring groups (A), and a sensitizing dye (S).
in studies that required a well-defined binding geometry and
distance of the sensitizer (S) from the semiconductor.5,6 There
conjugated p-phenyleneethynylene, (Ph-E)n, bridge of the rigid-
was evidence that the conjugated bridge (b) played a role in
the injection process,6 but the tetrahedral core (carbon or
adamantane) was likely to act as an insulating unit.
rods considerably enhanced the pyrene extinction coefficient
and red shifted the absorption spectrum. Thus, the photon-to-
electron conversion efficiency at single wavelengths of light
was a function of the bridge length and could be controlled at
the molecular level for optoelectronic applications. However,
for regenerative solar cell applications, Ru(II) polypyridine
complexes are generally preferred to organic chromophores
because of their visible light harvesting efficiency, tunability,
and stability.1a,2
An alternative to tripodal linkers was to use rigid-rod linkers
that provided a fully conjugated bridge to the surface.7 We have
recently communicated studies of rigid-rods substituted with
pyrene sensitizers and two COOH anchoring groups.8 Light
absorption by pyrene resulted in quantitative, sub-nanosecond,
electron injection into the TiO2 nanoparticles. Interestingly, the
10.1021/jp047454t CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/01/2004

Excited State Electron Transfer from Ru(II)
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16643
Figure 2. Structure of rigid-rod complexes and two reference complexes. The rigid-rod linkers are made of (Ph-E)n units and are terminated with
two methyl esters (COOMe) as the anchoring groups (A). The number after ROD in the abbreviated name refers to the number of Ph-E units in
-
the linker. The counterion was PF6 in all cases.
In this paper, we describe the synthesis and studies of two
series of rigid-rod linkers varying in length and substituted with
Ru complexes, Figure 2. In the first series (1a-c and 3), the
ancillary ligands are bpy for the Ru complexes; in the second
series (2a-b) the ancillary ligands are 4,4′-(Cl)2-bpy, Figure
2. The studies were performed on the compounds in solution
as well as anchored to nanocrystalline TiO2 or ZrO2 thin films.
A prime motivation for these studies was to ascertain whether
tuning the sensitizer-nanoparticle electronic interactions by
varying the distance could be used to control the rate constants
for charge injection and/or recombination.9 More specifically,
we wish to find conditions where the injection quantum yield,
φinj, was still unity while the charge recombination rate was
further inhibited. Such conditions would not be expected to
significantly improve efficiencies for optimized sensitizers such
as N3, cis-Ru(dcbpy)2(NCS)2. However, these conditions would
be useful for “black” sensitizers that have more negative Ru-
(III/II) reduction potentials such that the rate constants for charge
recombination and iodide oxidation are competitive leading to
less efficient collection of the injected electrons in the external
circuit.10 To test whether the fully conjugated rigid-rod linker
made of (Ph-E)n units functions as a conduit for electron
transport, we specifically designed compounds where the MLCT
excited state was localized on ancillary 4,4′-(Cl)2-bpy ligands,
rather than on the rigid-rod. This design did indeed provide us
with the opportunity to study “remote” interfacial charge-transfer
processes (vide infra).
Since we initiated these studies on the tripodal4-6 and rigid-
rod7,8 sensitizers, Kilsa˚ et al.11 have reported TiO2 sensitization
studies of rigid-rods of various lengths carrying Ru(II) poly-
pyridyl sensitizers and one COOH anchoring group. The lack
of clear distance dependencies suggested that the rods were
bound to TiO2 with multiple orientations and distances with
respect to the surface.11 Although the rigid-rods reported herein
are very similar to those previously reported,11 they differ in
several important respects. First, the linkers described here have
two anchoring groups instead of one, which may influence the
sensitizer-surface binding mode and orientation. Second, the
bridge is made of p-phenyleneethynylene spacers, (Ph-E)n,
instead of p-phenylene units, (Ph)n. The (Ph-E)n units, as
opposed to (Ph)n, are able to assume a flat, fully conjugated
conformation. The degree to which the excited electron is
dispersed into the bridge is therefore different, and this
could alter both the excited state and the interfacial electron
transfer properties. In fact, we found that the photophysical
properties of the Ru(II) rigid-rod sensitizers described herein

16644 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Wang et al.
SCHEME 1. Conditions and Reagentsa
a Step a: 1. (Me3Si)2NLi, MeO-9-BBN, THF; 2. Pd(PPh3)4. Step b: TBAF, r.t. THF. Step c: Pd(PPh3)4, CuI, piperidine, r.t. Step d:
(bpy)2RuCl2‚2H2O, EtOH, reflux.
shared more commonalties with the oligomeric 4,4′-p-phenyl-
eneethynylenes Ru(II) polypyridyl derivatives studied in con-
siderable detail by Schanze12 for applications in organic light
emitting diodes and sensors.
2.1. Synthesis of 7. TBAF (360 mg, 1.14 mmol) was added
to a solution of 67 (400 mg, 1.03 mmol) in THF. After stirring
at room temperature for 2 h, water (30 mL) was added, and
after standard workup with CHCl3, the crude was purified
through column chromatography (hexanes/AcOEt; 9/1, Rf ) 0.3)
to give 240 mg of 7 as a yellow powder (yield: 74%). mp:
160-163 °C. IR (cm-1): 3256 (V tC-H), 2920 (VC-
H(aliph.)), 2216 (VCtC), 1728 (VCdO), 1594 (VC-C(Ar)),
1444, 1252. 1H NMR δH (CDCl3): 8.60 (1H, t, J ) 1.5,
PhCOOMe), 8.33 (2H, d, J ) 1.5, PhCOOMe), 7.47 (4H, s),
3.93 (6H, s, COOMe) 3.18 (1H, s, CtCH). 13C NMR δC
(CDCl3): 165.51 (COOMe), 136.47, 132.12, 131.58, 130.99,
130.22, 124.02, 122.87, 122.51, 90.59, 89.18, 83.07, 79.22,
52.52 (COOMe). GC/MS m/z: 318 (M+, 100), 287 (M-31, 45),
259 (M-59, 15), 244 (M-74, 25), 200 (M-118, 15), 187 (M-
131, 15). HRMS Calcd for C20H14O4: 318.0892. Found:
318.0891.
Experimental Section
1. Metal Oxide Thin Films. Transparent and mesoporous
thin films of TiO2 or ZrO2 on glass slides were prepared in a
manner very similar to that previously described.13 ZrO2, an
insulator (band gap ≈ 5 eV),1a was used to study the excited
state of bound molecules. The films were pretreated with
aqueous acid or with aqueous base prior to attachment of the
sensitizers.14 Acid pretreated films were prepared by immersing
the films in pH ) 1 H2SO4 (aq), while basic films were
pretreated using pH ) 11 NaOH (aq). After this treatment, the
films were rinsed with CH3CN prior to immersion in an ∼0.1-1
mM CH3CN solution of the sensitizer for 24 h. The sensitized
films were thoroughly rinsed and then immersed in pure solvent
for several hours until desorption of weakly bound sensitizer
molecules was no longer detected (UV-vis of the solution).
2. Synthesis. The reagents, experimental methods, and
instrumentation used are described in the General Section in
the Supporting Information. The syntheses of 1a and 6 have
been published.7 The synthetic strategy for 1b, 3, and 1c is
outlined in Scheme 1. The complexes with Cl-substituted bpy
ligands, 2a and 2b, were prepared from the same ligands used
to synthesize 1a and 1b but using (4,4′-(Cl)2-bpy)2RuCl2‚
2H2O.15
2.2. General Procedure for the Synthesis of Rigid-Rod
Ligands 8, 10, and 11. Lithium bis(trimethylsilyl)amide (0.74
mmol, 0.74 mL of 1.0 M hexane solution) was added to a
solution of 7 (201 mg, 0.632 mmol) in THF (15 mL) at -78
°C. After 30 min, 9-MeO-9-BBN (0.74 mmol, 0.74 mL of 1.0
M hexane solution) was added. The mixture was stirred at -78
°C for 2 h, then transferred via cannula to a second flask
containing Pd(PPh3)4 (36 mg, 0.032 mmol) and 1-bromo-4-
trimethylethynylbenzene (196 mg, 0.773 mmol) for 8, 4-bromo-
2,2′-bipyridine (182 mg, 0.773 mmol)16 for 10, or 5-bromo-
1,10-phenanthroline17(200 mg, 0.773 mmol) for 11 in THF (15
mL). The reaction mixture was refluxed overnight, then allowed

Excited State Electron Transfer from Ru(II)
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16645
to cool to room temperature and water (20 mL) was added. After
standard workup with chloroform, the crude was purified by
silica gel column chromatography (hexanes/AcOEt, 9/1, Rf )
0.2 for 8; AcOEt/CHCl3, 15/85, Rf ) 0.2 for 10; AcOEt/CHCl3,
1/4, Rf ) 0.3 for 11) to give the corresponding product. 8:
Yield: 60% (186 mg, yellow powder). IR (cm-1): 3084 (VC-
H(Ar)), 2924 (VC-H(aliph.)), 2218 (VCtC), 1730 (VCdO),
) 2.0), 7.41 (1H, m), 3.98 (6H, s, COOMe).13C NMR δC
(CDCl3): 165.50 (COOMe), 156.17, 155.36, 149.18, 149.13,
137.00, 136.44, 131.85, 131.68, 131.63, 130.97, 130.17, 128.51,
128.42, 125.15, 124.06, 124.03, 123.69, 123.30, 123.14, 122.53,
122.22, 121.16, 93.44, 91.25, 91.05, 90.82, 89.28, 88.90, 52.51
(COOMe). Anal. Calcd for C38H24N2O4: C, 79.71; H, 4.22; N,
4.89; Found: C, 77.90; H, 4.52; N, 4.26.
1
1601 (VC-C(Ar)), 1440, 1249. H NMR δH (CDCl3): 8.64
2.5. General Procedure for the Synthesis of Complexes 1b,
1c, and 3. A solution of 10, 11, or 12 (0.329 mmol) in THF (5
mL) was added to ethanol (30 mL). To the solution, deaerated
by bubbling nitrogen for 10 min, was added Ru(bpy)2Cl2‚2H2O
(188 mg, 0.362 mmol), and the mixture was refluxed overnight
under nitrogen, then cooled to room temperature and filtered.
Addition of aqueous NaPF6 (7.70 g, 45.8 mmol) to the filtrate
formed precipitate, which was collected and washed with water
to afford the complexes as orange powders. 1b: Yield: 74%.
IR (cm-1): 2909 (VC-H(aliph.)), 2206 (VCtC), 1733 (VCd
O), 1249, 1004, 841. 1H NMR δH (acetone-d6): 8.97 (2H, m),
8.84 (4H, m), 8.58 (1H, t, J ) 2.0), 8.35 (2H, d, J ) 2.0), 8.23
(6H, m), 8.12 (5H m), 7.70 (4H, m), 7.62 (6H, m), 3.98 (6H, s,
COOMe). 13C NMR δC (acetone-d6): 165.71 (COOMe), 158.60,
158.07, 158.05, 157.98, 157.68, 152.87, 152.82, 152.80, 152.74,
152.64, 139.12, 139.09, 139.01, 136.82, 133.09, 133.07, 132.89,
132.38, 130.77, 129.70, 129.22, 128.84, 126.94, 125.68, 125.37,
124.90, 124.70, 122.58, 97.67, 91.15, 90.56, 88.40, 53.04
(COOMe). HRMS Calcd for C50H36F12N6O4P2Ru: 1031.1480.
Found: 1031.1469. Anal. Calcd for C50H36F12N6O4P2Ru: C,
51.07; H, 3.09; N, 7.15; Found: C, 51.32; H, 3.13; N, 6.93. 3:
Yield: 71%. IR (cm-1): 2210 (VCtC), 1730 (VCdO), 1250,
(1H, t, J ) 1.5, PhCOOMe), 8.37 (2H, d, J ) 1.5, PhCOOMe),
7.53 (4H, m), 7.46 (4H, m), 3.98 (6H, s, PhCOOMe), 0.27 (9H,
s, Si(CH3)3). 13C NMR δC (CDCl3): 165.54 (COOMe), 136.47,
131.91, 131.67, 131.59, 131.40, 130.96, 130.19, 124.08, 123.41,
123.17, 122.91, 122.41, 104.51, 96.48, 91.18, 90.85, 90.79,
89.21, 52.55 (COOMe), -0.13 (Si(CH3)3). Anal. Calcd for
C31H26O4Si: C, 75.89; H, 5.34; Found: C, 75.96; H, 5.51. 10:
Yield: 70% (209 mg as white powder). mp: 156-158 °C. IR
(cm-1): 3027 (VC-H(Ar)), 2211 (VCtC), 1729 (VCdO), 1601
1
(VC-C(Ar)), 1440, 1245. H NMR δH (CDCl3): 8.67 (1H, d,
J ) 4.0), 8.63 (1H, d, J ) 5.0), 8.59 (1H, t, J ) 2.0), 8.51 (1H,
s), 8.38 (1H, d, J ) 8.0), 8.32 (2H, d, J ) 2.0), 7.80 (1H, td,
J1 ) 8.0, J2 ) 2.0), 7.51 (4H, s), 7.35 (1H, dd, J1 ) 5.0, J2 )
1.5), 7.30 (1H, m), 3.93 (6H, s, COOMe). 13C NMR δC
(CDCl3): 165.45 (COOMe), 156.13, 155.31, 149.15, 149.11,
137.00, 136.44, 132.07, 131.88, 131.72, 130.95, 130.22, 125.15,
124.02, 123.95, 123.16, 123.12, 122.55, 121.14, 93.29, 90.64,
89.53, 89.00, 52.49 (COOMe). HRMS calcd for C30H20O4N2:
472.1423. Found: 472.1424. 11: Yield: 63% (197 mg, white
powder). mp: 242-244 °C. IR (cm-1): 3010 (VC-H(Ar)), 2215
(VCtC), 1728 (VCdO), 1246, 1002, 827. 1H NMR δH
(CDCl3): 9.25 (1H, dd, J1 ) 4.5, J2 ) 1.5), 9.21 (1H, dd, J1 )
4.5, J2 ) 2.0), 8.81 (1H, dd, J1 ) 8.5, J2 ) 2.0), 8.61 (1H, t, J
) 1.5), 8.36 (2H, d, J ) 1.5), 8.24 (1H, dd, J1 ) 8.5, J2 ) 1.5),
8.09 (1H, s), 7.76 (1H, m), 7.66 (1H, m), 7.61 (4H, m), 3.95
(6H, s, COOMe). 13C NMR δC (CDCl3): 165.50 (COOMe),
150.50, 150.35, 131.91, 131.88, 131.84, 131.80, 131.76, 131.03,
130.79, 130.30, 128.39, 128.23, 136.84, 136.50, 123.94, 123.88,
123.80, 123.25, 122.65, 120.04, 95.52, 90.60, 89.70, 87.42,
52.60 (COOMe). HRMS Calcd for C32H20O4N2: 496.1423.
Found: 496.1414.
1
1016, 842. H NMR δH (acetone-d6): 9.19 (1H, dd, J1 ) 8.4,
J2 ) 1.2), 8.85 (5H, m), 8.72 (1H, s), 8.56 (1H, t, J ) 1.6),
8.53 (1H, dd, J1 ) 5.5, J2 ) 1.2), 8.49 (1H, dd, J1 ) 5.5, J2 )
1.2), 8.36 (2H, d, J ) 1.6), 8.27 (2H, m), 8.18 (4H, m), 8.05
(1H, m), 7.97 (3H, m), 7.84 (4H, m), 7.65 (2H, m), 7.42 (2H,
m), 3.97 (6H, s, COOMe). 13C NMR δC (acetone-d6): 165.71
(COOMe), 158.38, 158.11, 154.20, 153.08, 153.06, 153.03,
148.73, 148.49, 139.07, 138.93, 137.71, 136.80, 136.34, 133.11,
133.03, 132.88, 132.36, 131.56, 131.52, 130.70, 128.77, 128.64,
127.85, 127.80, 125.34, 125.26, 124.77, 124.49, 123.19, 122.25,
97.73, 91.31, 90.36, 87.00, 53.04 (COOMe). HRMS Calcd for
C52H36F12N6O4P2Ru: 1055.1480. Found: 1055.1494. Anal.
Calcd for C52H36F12N6O4P2Ru: C, 52.05; H, 3.02; N, 7.00;
Found: C, 51.98; H, 3.12; N, 6.81. 1c: Yield: 65%. IR (cm-1):
3088 (VC-H(Ar)), 2957 (VC-H(aliph.)), 2208 (VCtC), 1720
(VCdO), 1605 (VC-C(Ar)), 1446, 1271. 1H NMR δH (acetone-
d6): 8.93 (2H, m), 8.83 (4H, m), 8.56 (1H, t, J ) 1.0), 8.34
(2H, d, J ) 1.0), 8.21 (6H, m), 8.09 (5H, m), 7.66 (14H, m),
3.97 (6H, s, COOMe).13C NMR δC (acetone-d6): 165.83
(COOMe), 158.67, 158.16, 158.13, 158.07, 157.77, 152.93,
152.86, 152.79, 152.70, 139.20, 139.17, 139.09, 136.84, 133.18,
133.05, 132.97, 132.92, 132.78, 132.44, 130.70, 129.77, 129.18,
128.94, 128.92, 127.01, 125.77, 125.48, 125.46, 124.97, 124.18,
123.76, 122.35, 97.90, 92.47, 91.57, 91.53, 90.04, 88.42, 53.08
(COOMe). Anal. Calcd for C58H40F12N6O4P2Ru: C, 54.60; H,
3.16; N, 6.59; Found: C, 54.83; H, 3.14; N, 6.16.
2.3. Synthesis of 9. Deprotection of 8 by TBAF was performed
using same procedure described to prepare 7. Alkyne 9 was
obtained as a yellow powder (yield 90%). IR (cm-1): 3286 (V
tC-H), 3073 (VC-H(Ar)), 2952 (VC-H(aliph.)), 2211 (VCt
1
C), 1728 (VCdO), 1599 (VC-C(Ar)), 1439, 1246. H NMR
δH (CDCl3): 8.54 (1H, t, J ) 1.5, PhCOOMe), 8.27 (2H, d, J
) 1.5, PhCOOMe), 7.44 (4H, m), 7.39 (4H, m), 3.88 (6H, s,
COOMe), 3.11 (1H, s, CtCH). 13C NMR δC (CDCl3): 164.51
(COOMe), 135.45, 131.08, 130.78, 130.68, 130.62, 130.48,
129.98, 129.18, 123.07, 122.37, 121.49, 121.16, 89.98, 89.88,
89.84, 88.26, 82.18, 78.12, 51.53 (COOMe).
2.4. Synthesis of 12. CuI (10 mg, 0.052 mmol) and Pd(PPh3)4
(25 mg, 0.022 mmol) were added under argon to a deaerated
solution of 4-iodo-2,2′-bipyridine18 (74 mg, 0.26 mmol) and 10
(107 mg, 0.256 mmol) in piperidine (7 mL). The reaction
mixture was stirred overnight at room temperature. The solvent
was removed in vacuo and the solid residue was purified by
silica gel column (CHCl3/MeOH, 97/3, Rf ) 0.2). Compound
12 was obtained as an orange-brown powder (60 mg, yield:
41%). IR (cm-1): 3060 (VC-H(Ar)), 2950 (VC-H(aliph.)),
2205 (VCtC), 1729 (VCdO), 1582 (VC-C(Ar)), 1437, 1245.
1H NMR δH (CDCl3): 8.76 (1H, d, J ) 4.0), 8.72 (1H, d, J )
5.0), 8.64 (1H, t, J ) 2.0), 8.63 (1H, s), 8.53 (1H, d, J ) 8.0),
8.38 (2H, d, J ) 2.0), 7.92 (1H, ddd, J1 ) J2 ) 8.0, J3 ) 2.0),
7.57 (4H, d, J ) 2.0), 7.55 (4H, s), 7.47 (1H, dd, J1 ) 5.0, J2
3. Spectroscopy. 3.1. UV-Vis Absorbance. Ground state
UV-vis absorbance measurements were performed on a
Hewlett-Packard 8453 diode array spectrophotometer. The
sensitized films were placed diagonally in a 1 cm square quartz
cuvette with acetonitrile. An unsensitized film was used as the
reference. Transient absorption measurements were acquired
with an apparatus that has been previously described.13 A Xe
lamp was used to probe, and 417 nm light from a D2-filled
Raman shifter pumped by a Nd:YAG Continuum Surelite II

16646 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Wang et al.
TABLE 1: Surface Adduct Formation Constants and
laser was used as the excitation beam. Samples were nitrogen
purged for at least 15 min or freeze-pump-thawed prior to flash
photolysis studies. Sensitized films were immersed in solutions
of acetonitrile, neat or with LiClO4. Each kinetic trace was
acquired averaging 60-150 laser pulses. The quantum yields
for excited state electron injection into TiO2 were quantified
by comparative actinometry as previously described.19 A Ru-
(bpy)3(PF6)2 doped poly(methyl methacrylate), PMMA, thin
film, whose optical absorption and physical dimensions were
very similar to the sensitized TiO2 films, was used as the
actinometer. The amplitudes of photoinduced absorption changes
were converted to concentration changes through Beer’s law.
A literature value for the difference of (-1.00 ( 0.09) × 104
M-1cm-1 at 450 nm between the extinction coefficients of the
excited state and of the ground state was used.20 Quantum yields
for injection were measured most accurately by probing at
wavelengths that corresponded to isosbestic points between the
ground and excited states. The isosbestic points were determined
on ZrO2 films where only the ground and MLCT excited states
were observed. The extinction coefficients and absorption
spectra of the oxidized rigid-rods were determined by spectro-
electrochemical measurements.
Limiting Surface Coverages for Selected Ru Sensitizers
sensitizer
10-5Kad, M-1
Γo, mol/cm2
2+
(deeb)Ru(bpy)2
2.0 ( 1.0
1.9 ( 1.0
3.7 ( 1.2
5.7 ( 1.3
5 ( 2 × 10-8
ROD2-bpyRu(bpy)22+ (1b)
7.3 ( 0.2 × 10-8
3.9 ( 0.3 × 10-8
4.8 ( 0.4 × 10-8
ROD2-bpyRu(4,4-Cl2bpy) 22+ (2b)
Ru(4,4-(Cl)2-bpy)2(bpy)2+ (5)
respectively, were calculated from eqs 2 and 3 with the measured
quantum yields and lifetimes.
φPL ) kr/(kr + knr)
φPL ) krτ
(2)
(3)
4. Electrochemistry. Cyclic voltammetry was performed in
0.1 M tetrabutylammonium perchlorate (TBAClO4) CH3CN
electrolyte in a standard three-electrode arrangement with a
glassy carbon or sensitized TiO2 working electrode, a Pt gauze
counter electrode, and Ag/AgCl or SCE as the reference
electrode. A BAS model CV-50W potentiostat was used, and
the CV experiments were carried out at room temperature under
argon.
5. Adsorption Isotherms. Surface binding was monitored
spectroscopically by measuring the change in film and solution
absorbance after soaking the film for 12 h in acetonitrile
solutions with known concentrations of the sensitizers. In all
cases, the surface coverage saturated at high sensitizer concen-
tration. The equilibrium binding for all Ru-rod sensitizers were
well described by the Langmuir adsorption isotherm model24
from which surface binding constants (Kad) were abstracted
using eq 4,
3.2. Infrared Spectra. IR spectra of the sensitizers were
obtained by depositing two drops of a 1 mM CH3CN solution
of the sensitizer on a Ge or CaF2 window, allowing the solvent
to evaporate. The IR spectra of the TiO2-bound sensitizers were
obtained in a transmission mode with films cast on Ge or
sapphire windows or by attenuated total reflectance (ATR) with
a Golden Gate Single Reflection Diamond ATR apparatus. In
all cases, an unsensitized film acted as the background. The
spectra were collected with a Nexus 670 Thermo-Nicolet FTIR
or a Mattson Research Series 1 FTIR spectrometer at 4 cm-1
resolution.
3.3. Raman Spectra. Raman spectroscopy was carried out by
using a HoloLab 5000/Raman Rxn1 Confocal Raman Spec-
trometer (Kaiser Optical Systems, Inc.) with an Invictus 785
nm laser diode. A Teflon sample holder was used to measure
the sensitizers (as powders), and the spectra of the TiO2-bound
sensitizers were obtained on films cast on a Sapphire window.
3.4. Photoluminescence. Corrected photoluminescence (PL)
spectra were obtained with a Spex Fluorolog that had been
calibrated with a standard tungsten-halogen lamp using pro-
cedures 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 a right angle in the case of fluid
solutions. Photoluminescence quantum yield measurements, φPL,
were performed using the optically dilute technique with Ru-
(bpy)3(PF6)2 in acetonitrile as the actinometer, eq 1.21
[RuII]eq
[RuII]eq
1
)
+
(4)
Γ
KadΓ0
Γ0
where [RuII]eq is the equilibrium sensitizer concentration, Γ0 is
the saturation surface coverage, and Γ is the equilibrium surface
coverage at a defined molar concentration. Plots of [RuII]eq/Γ
versus [RuII]eq were fitted linearly to obtain the binding constants
Kad and surface coverages Γo.
Results
1. Binding to TiO2 Films. Adsorption isotherms for sensitizer
binding to TiO2 in acetonitrile were measured at room temper-
ature. All equilibrium binding was well described by the
Langmuir adsorption isotherm model from which surface adduct
formation constants and limiting surface coverages were ob-
tained.24 The surface coverages for 1a-c and 2a-b were about
10-8 mol/cm2 and were comparable to those obtained for
tripodal sensitizers5 and for Ru complexes directly bound
through a dcb (4,4′-(CO2H)2-bpy) ligand.1a,c The binding
constants, approximately 2 × 105 M-1 for 1a-c, were consis-
tently smaller than those observed for the tripodal sensitizers
(∼10 × 105).5 The surface binding constants for ROD1-bpyRu-
(bpy)22+ (1b) and ROD1-bpyRu(4,4′-(Cl)2-bpy)22+ (2b) on pH
) 1 pretreated TiO2 films are reported in Table 1. The reference
complex for Cl-substituted rods 2a and 2b, Ru(4,4′-(Cl)2-bpy)2-
(bpy)2+ (5), had no obvious anchoring groups. However, the
equilibrium binding constant for 5 was almost the same as that
for Ru complexes that have COOMe binding groups, such as
φPL ) (Ar/As)(Is/Ir)(ns/nr)2φr
(1)
Ar and As were the absorbances of the actinometer and sample,
respectively, Ir and Is were the integrated photoluminescence
of the actinometer and sample, respectively, nr and ns were the
refraction indexes for the solvents used for the actinometer and
sample, respectively, and φr was the quantum yield for Ru(bpy)3-
(PF6)2 in acetonitrile (φr ) 0.062).22
Time-resolved photoluminescence decays were acquired in
a similar optical arrangement with excitation from a nitrogen-
pumped dye laser that has been previously described.23 For
solution studies, the traces were fit to a first order kinetic model.
Values for radiative and nonradiative constants, kr and knr
2+
(deeb)Ru(bpy)2 and rigid-rods 1a and 1b (See Supporting
Information, Figure S1). Since control experiments showed that
[Ru(bpy)3]2+ was weakly adsorbed, adsorption in the case of
[Ru(4,4′-(Cl)2-bpy)2(bpy)]2+ must have been assisted by the
presence of the Cl-bpy ligands.

Excited State Electron Transfer from Ru(II)
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16647
Figure 5. Attenuated total reflectance IR spectra for 2b on TiO2 thin
films that were pretreated at pH ) 1 (s) and pH ) 11 (- - -).
bidentate coordination modes as in Figure 4. Interestingly, the
rigid-rod sensitizers bound strongly to TiO2 samples that were
not pretreated with aqueous acid or basic solutions, and yielded
IR spectra with signatures for both the carbonyl and the
carboxylate band indicative of a mixture of binding modes.
The IR spectra of the rigid-rods obtained on TiO2 thin films,
cast on germanium or sapphire windows, were substrate
independent. Germanium was preferred because it combined
resistance to aqueous acidic treatments with transparency in the
IR region of interest. The presence of a single carbonyl band
after binding to an acidic TiO2 surface suggested that both
groups were anchored in a monodentate ester-type bond as in
Figure 4.
Figure 3. (a) Raman spectra of ester 1a (s) as a solid and 1a on TiO2
(- - -). The TiO2 film was cast on sapphire and was pretreated with
acid. (b) IR spectra of the carboxylic acid prepared from ester 1a (s)
on a Ge and 1a on TiO2 (- - -). The TiO2 film was cast on Ge and was
pretreated with acid.
3. Electrochemistry. All rigid-rod sensitizers displayed
quasireversible RuIII/II electrochemistry in CH3CN electrolyte
solution, Table 2. Complexes 1a, 1b, and 3 showed RuIII/II
reduction potentials at ∼1.3 V versus SCE that were, within
experimental error, the same as those reported previously for
tripodal sensitizers with (Ph-E)n bridges.5 These waves were
shifted to slightly more positive potentials compared to
Ru(bpy)32+. A similar effect was observed in 2a-b and in 5,
as the presence of Cl-substituted ligands resulted in a measurable
positive shift in the RuIII/II potential to ∼1.40 V. Ligand-based
reduction potentials for the sensitizers in CH3CN solution are
provided in Table 2.
Figure 4. Three possible binding modes for the attachment of a
carboxylic group to a metal oxide surface.
2. IR and Raman Spectroscopy. The Raman spectra of 1a
before and after binding differed only from contributions by
the characteristic anatase bands, Figure 3a. Both spectra
exhibited bands typical of the aromatic ring and of the CtC
bond, but the carbonyl band was too broad and too weak to
give reliable information about the surface interaction(s). This
was consistent with previous Raman studies of Ru sensitizers
on TiO2.25 IR measurements were performed for the rigid-rods
before and after binding. The pH pretreatment was found to
significantly influence the binding process.14 All spectra of the
nonbound esters displayed intense bands at ∼1720 cm-1
(νCdO) for the carbonyl and at ∼1600 cm-1 (νC-C(Ar)) for
the phenylene groups. The IR and Raman spectra of 1a before
and after binding to acid-pretreated pH ) 1 TiO2, shown in
Figure 3, were representative of the IR and Raman spectra
obtained for the rigid-rods. Upon binding to TiO2 films, the IR
spectra exhibited a small (7-12 cm-1) shift to lower wave-
numbers of the carbonyl stretch, while the 1600 band due to
the aromatic rings remained unchanged, Figure 3b.
The first reduction, E°(Ru2+/+), of 1a-c and 3 occurred at
potentials more positive than the corresponding reference
complexes, that is, [Ru(bpy)3]2+ or [Ru(bpy)2(phen)]2+, respec-
tively. By comparison with the tripods5 and with other literature
reports on Ru complexes substituted with electron-withdrawing
groups,12,26 we assigned this wave to reduction of the bpy
attached to the rigid-rod linker. The presence of 4,4′-(Cl)2-bpy
ligands in 2a-b and in 5 resulted first in reduction potentials
that were ∼50 mV more positive, consistent with the reduction
of the Cl-substituted bpy. The excited state reduction potentials,
E1/2(RuIII/II*), were calculated from the ground state potentials
and from the free energy stored in the thermally equilibrated
MLCT excited state, ∆Ges, using eq 5:
E1/2(RuIII/II*) ) E1/2(RuIII/II) - ∆Ges
(5)
Conversely, the IR spectra of the rods bound to basic (pH )
11 pretreated) TiO2, shown in Figure 5, displayed a broad
carboxylate band at ∼1550-1650 cm-1, most consistent with
∆Ges was estimated by drawing a tangent line to the high-energy
side of the corrected photoluminescence spectra. The excited
state reduction potentials calculated for unsubstituted bpy-based

16648 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Wang et al.
TABLE 2: Electrochemical Data for 1-3 and Reference Compounds in Solutiona
complex
E1/2(RuIII/II), mV
E1/2(Ru2+/+), mV
E1/2(Ru+/0), mV
E1/2(RuIII/II*), mV
ROD1-bpyRu(bpy)22+ (1a)
ROD2-bpyRu(bpy)22+ (1b)
ROD3-bpyRu(bpy)22+ (1c)
ROD1-bpyRu(4,4-Cl2bpy)22+ (2a)
ROD2-bpyRu(4,4-Cl2bpy)22+ (2b)
Ru(4,4-(Cl)2-bpy)32+ (5)
ROD2-phenRu(bpy)22+ (3)
Ru(bpy)32+ b (4)
1300
1300
1287
1400
1390
1430
1300
1260
1230
-1210
-1230
-1223
-1160
-1150
-1080
-1220
-1340
-1370
-1480
-1480
-1429
-1290
-1280
-1190
-1480
-1520
-1530
-820
-830
-850
-750
-760
-730
-930
-860
-910
[Ru(bpy)2(phen)]2+ b
a Data are reported vs SCE. All measurements were performed in 0.1 M TBAClO4/CH3CN. b From Galoppini and co-workers.5
TABLE 3: Photophysical Properties of 1-5 and Other Ru Complexes in CH3CN Solutionsa
b
complex
λ
abs, nm (ꢀ, M-1 cm-1
)
λ
PL, nmc
τ, µsd
∆Ges, eVf
102φPL
10-4kr, s-1
10-5knr, s-1
ROD1-bpyRu(bpy)22+ (1a)
ROD2-bpyRu(bpy)22+ (1b)
ROD3-bpyRu(bpy)22+ (1c)
ROD1-bpyRu(4,4-(Cl)2-bpy)22+ (2a)
ROD2-bpyRu(4,4-(Cl)2-bpy)22+ (2b)
ROD2-phenRu(bpy)22+ (3)
[Ru(bpy)3]2+
462 (1.6 × 104)
465 (2.0 × 104)
465 (2.0 × 104)
465 (1.6 × 104)
466 (1.9 × 104)
450 (1.8 × 104)
450
640
640
639
645
645
606
610
620
690
645
1.9
2.3
2.5
0.69
0.75
1.3
0.80
1.2
0.93
0.70
2.12
2.13
2.14
2.15
2.15
2.23
2.24
2.14
2.01
2.16
11
13
9.5
7.0
6.9
8.5
6.2
5.1
5.4
4.7
10
9.2
6.5
7.8
5.4
4.0
4.3
13
12
7.0
12
Ru(bpy)2(phen)2+
450
[Ru(bpy)2(deeb)]2+ (4)
475 (1.6 × 104)
466
4.4
6.1
4.7
8.7
10
13
[Ru(4,4-Cl2-bpy)3]2+
[Ru(4,4-Cl2-bpy)2(bpy)]2+ (5)
[Ru(bpy)2(bpy-(E-Ph)-AdTripod )]2+g
[Ru(bpy)2(phen-(E-Ph)AdTripod )]2+g
461 (1.3 × 104)
461 (1.9 × 104)
452 (1.6 × 104)
646
624
2.0
1.4
2.14
2.23
10
8.0
5.1
5.7
4.5
6.6
a All measurements were performed at room temperature. b Absorption maxima, (2 nm. c Corrected photoluminescence maxima, (5 nm. d Excited
state lifetime in CH3CN, (5%. The solutions were deaerated by freeze-pump-thaw or by sparging with nitrogen e Half-wave potentials ((20 mV)
were measured at a glassy carbon working electrode in 0.1 M TBAClO4/CH3CN solution using Ag/AgCl as the reference. Data are reported vs
SCE. f Gibbs free energy stored in the thermally equilibrated excited state. g Data from Galoppini et al.5b
rods were nearly identical to those obtained for related bpy-
based tripods.5 Similarly, the potentials for phen-substituted
ligand 3 were nearly identical to those obtained for the phen-
based tripods.5 Rigid-rods containing 4,4′-(Cl)2-bpy were weaker
excited state reductants by ∼70 mV than those with unsubsti-
tuted bpy.
4. Photophysical Studies. Selected photophysical properties
for 1-5 in argon-saturated acetonitrile at room temperature are
listed in Table 3 together with data for the reference complexes
and two tripodal sensitizers.
anchored to TiO2 and ZrO2 were largely preserved, with only
some minor broadening. The UV spectrum of the reference
complex 5b, however did show an additional shoulder upon
binding, possibly due to the Cl-TiO2 interactions (See Sup-
porting Information, Figure S2).
All the rigid-rod sensitizers displayed room-temperature
photoluminescence (PL) in acetonitrile and when anchored to
TiO2 or ZrO2. The coincident PL maxima for 2a and 2b were
centered at 645 nm and were identical to that of the [Ru(4,4′-
(Cl)2-bpy)3]2+*. The PL decays were well described by a first-
order kinetic model in fluid solution, and the excited state
lifetimes (τ), along with the PL quantum yields, are listed in
Table 3. As the number of (Ph-E) increased, for instance from
1a to 1c, the τ increased slightly from 1.9 µs (n ) 1) to 2.3 µs
(n ) 2) and 2.5 µs (n ) 3). The excited state lifetime for the
The UV-visible absorption spectra for 1a-c, and for 2a,
2b, and 5, shown in Figure 6, parts a and b, respectively,
displayed a broad band in the visible region (400-500 nm) that
is typical of metal-to-ligand charge-transfer (MLCT) transitions.
These heteroleptic complexes show broad visible absorption
bands and the individual Ru f bpy, Ru f 4,4′-(Cl)2-bpy, or
Ru f rigid-rod linker charge transfer bands could not be
spectrally resolved. The narrow band at higher energy at ∼290
nm was assigned to the π,π* transition of the ancillary ligands
and the ∼350 nm band was assigned to the π,π* transition of
the rigid-rod (Ph-E)n linker. As the number of (Ph-E) units
increased, the 350 nm band increased in intensity and shifted
to longer wavelengths of light. As expected, this band is absent
in the spectrum of 5. The spectral changes were most significant
when the number n of (Ph-E)n units was increased from 1 to 2.
As expected, the π,π* transition at ∼290 nm due to the ancillary
bpy was insensitive to the number of (Ph-E) units, Figure 6,
parts a and b. Similar spectral features have been observed for
the tripodal complexes4a,5 and oligomeric complexes of Ru.12,26
A comparison between the absorption maxima for 1a-c and
2+
nonsubstituted complex, Ru(bpy)3 is 0.8 µs. Time-resolved
PL decays on TiO2 and on ZrO2 were nonexponential.19c
The transient absorption spectra of the rigid-rods in aceto-
nitrile solutions were assigned to the MLCT excited state. The
kinetics were first order, and the rate constants agreed well with
the time-resolved PL data at all wavelengths monitored. A
comparison of the excited state absorption spectra of 1a-c
uncovered distinct differences due the number of (Ph-E) units;
1b exhibits a stronger, red shifted excited state absorbance
beyond 500 nm compared to that of 1a, Figure 7. The spectra
show a bleach of the MLCT absorption, and for 1b and 1c, a
bleach of the bridge π,π* transition(s).
The excited state absorption spectra for the Cl-substituted 2a
and 2b did not display an intense absorption band in the red
spectral region and agreed well with the reference complex that
lacked the rigid-rod linker, [Ru(4,4′-(Cl)2-bpy)3]2+, Figure 8.
The excited state absorption spectra obtained on ZrO2 thin films
were within experimental error, the same as that in CH3CN.
Transient absorption spectra on rigid-rod sensitized TiO2
2+
the reference Ru(bpy)3 showed an ∼10 nm red shift of the
MLCT band in the presence of the linker, while the absorption
maxima for 2a-b and the reference complex Ru(4,4′-(Cl)2-
bpy)32+ were identical.27 The absorption spectra of 1a-c and 3

Excited State Electron Transfer from Ru(II)
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16649
Figure 6. Ground state absorbance spectra in acetonitrile (a) Spectra
of 1a (s), 1b (‚‚‚), and 1c (- - -). (b) Spectra of 2a (‚‚‚), 2b (- - -), and
[Ru(4,4′-(Cl)2-bpy)2(bpy)]2+ 5 (s).
contained contributions from the MLCT excited state and from
a RuIII/TiO2(e-) charge separated state as described below.
5. Interfacial Electron Transfer. Nanosecond transient
absorption was used to quantify interfacial electron transfer
dynamics and yields. These processes were influenced by the
pH. This is expected, since the conduction band edge shifts 59
mV/pH unit.28,1a On acidic TiO2, injection occurred within our
instrument response (probably on a femto to picosecond time
scale6), and the transient spectra were mainly from the RuIII/
TiO2(e-) charge separated states, while on base pretreated TiO2
the MLCT excited state was predominately observed. To
eliminate possible contributions from the excited state, the
measurements were made at wavelengths corresponding to the
ground-excited state isosbestic point. In this way, the formation
and loss of the interfacial injection and recombination processes
could be cleanly observed. On pH ) 1 pretreated TiO2 samples,
excited state electron injection into TiO2 occurred faster than
could be time resolved with a nanosecond laser for all samples.
On basic TiO2, the injection yield was very low and could be
increased by the addition of LiClO4 to the acetonitrile solution
in which the film was immersed. Figure 9 shows absorption
changes monitored at the ground-excited state isosbestic point
for base pretreated 1a/TiO2. The rise time observed corresponds
Figure 7. Transient absorbance spectra (λex ) 417 nm) in CH3CN of
(a) 1a. The data were recorded at 10 ns (9), 0.5 µs (b), 1.0 µs (2), 2.0
µs (1), and 10 µs ([) delays after the laser pulse. (b) 1b. The data
were recorded at 10 ns (9), 1.0 µs (b), 2.5 µs (2), 5 µs (1), and 20 µs
([) delays after the laser pulse. (c) 1c. The data were recorded at 0.1
µs (9), 0.5 µs (b), 1.0 µs (2), 2.0 µs (1), and 4.0 µs ([) delays after
the laser pulse.
of nanocrystalline TiO2. The injection yield were found to
decrease with irradiance, and extrapolation of the data to zero
irradiance gave the φinj reported in Table 4.19 Injection quantum
yields were measured for 1a-c on pH ) 1 pretreated TiO2 and
on pH ) 11/Li+ pretreated TiO2 with comparable surface
coverages, Γ(RuII). In general, φinj was above 0.60 for Ru-
rigid-rod complexes that were bound to pH ) 1 pretreated films,
while φinj for pH ) 11 pretreated films were lower and decreased
with rigid-rod length, Table 4.
The kinetics for charge recombination between the electron
in TiO2 and the oxidized sensitizer were quantified on pH ) 1
pretreated films at similar surface coverages (∼2.3 × 10-8 mol/
cm2). Transient data were obtained at the ground state-excited
to relatively slow electron injection, kinj ) 1.0 × 107 s-1
.
Typically, 10-30% of the injection process could be observed
over the first 100 ns for 1a-c/TiO2. Injection dynamics were
not observed for the rigid-rods anchored to acidic pretreated
surfaces, for the reference [Ru(bpy)2(deeb)]2+, or for both Cl-
substituted rigid-rod compounds (2a and 2b). The injection is
now being studied by femtosecond laser spectroscopy.
Comparative actinometry was used to measure the quantum
yields (φinj) for excited state electron transfer to the empty states

16650 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Wang et al.
slow component, and ks was the recovery rate constant of the
slow component. The average rate constants for charge recom-
bination were found to be independent of the length of rigid-
rod sensitizer used and were within experimental error the same
as that for the model complex Ru(bpy)2(dcb)2+. The weights
of the two components, ∼70% and ∼30% for the fast and slow
components, respectively, were also independent of the length
of the rigid-rod sensitizer, with kobs ) 2.5 ( 0.3 × 107 s-1
.
Discussion
The photophysical and interfacial electron transfer properties
of Ru(II) rigid-rod sensitizers were quantified at room temper-
ature in acetonitrile solution. The interfacial pH was found to
be important for both sensitizer binding and for photoinduced
electron transfer. Efficient electron injection was observed on
acidic TiO2, while long-lived excited states were observed on
basic TiO2. Back electron transfer rate constants were found to
be independent of the length of the rigid-rod excited, consistent
with a previously proposed mechanism wherein transport of the
injected electron to the oxidized sensitizer was rate limiting.30
These studies provide new details on surface binding, molecular
excited states, and interfacial electron injection. Below we
discuss interpretations and implications of these new findings.
1. Surface Binding. Adsorption isotherms and IR studies of
the rigid-rod sensitizers 1-3 have shown that all bind strongly
to nanocrystalline TiO2 films. In most studies, the methyl esters
of the Ru rigid-rods were reacted with the TiO2 surface in
acetonitrile at room temperature. The observation of strong
binding was interesting, considering that identical rigid-rod
linkers capped with an organic chromophore (pyrene) bound
only when it was first converted into a carboxylate salt or a
carboxylic acid.8 Tripodal sensitizers behaved similarly: tripods
capped with organic chromophores such as pyrene or perylene
did not bind strongly under conditions where Ru tripods did.31,8
The fact that noncharged, aromatic sensitizers required different
binding conditions than the charged Ru complexes may have
been due to a variety of factors, such as differences in the
solubility in organic solvents and/or electrostatic interactions
with the TiO2 surface.
Figure 8. Transient absorbance spectra (λex ) 417 nm) of Cl-
substituted complexes in acetonitrile at room temperature Ru(4,4′-(Cl)2-
bpy)3(PF6)2 (9), 2a (b), and 2b (2). The data were recorded 10 ns
after the laser pulse.
Figure 9. Single wavelength kinetics monitored at the ground state-
excited state isosbestic point for 1a on pH ) 1 pretreated TiO2 (black
line, top) and pH ) 11 pretreated TiO2, 0.1 M LiClO4 (red line, bottom).
Infrared measurements showed that both ester groups reacted
with TiO2 to yield a single binding mode whose chemical nature
was dependent on the Brønsted acidity of the surface. On acidic
TiO2 surfaces, a single asymmetric CO stretch was observed at
higher energy than the methyl ester, which we assigned to ester
linkages. On basic TiO2, a broad band centered at ∼1550-
1575 cm-1 was observed at lower energy and was assigned to
a carboxylate binding mode. The fact that only a single binding
mode was present on acidic and on basic TiO2 indicated that
both anchoring groups were in a similar environment. Interest-
ingly, on surfaces that were not pretreated with acid or base,
spectroscopic signatures of both binding modes were present.
We should emphasize, however, that the IR and surface binding
studies do not demonstrate that the rigid-rods are perpendicular
to the surface or that they assumed a single orientation.
Ruthenium model compounds that contained the 4,4′-(Cl)2-
bpy ligand(s) without obvious anchoring groups were found to
bind surprisingly well to TiO2. The rigid-rods that contain 4,4′-
(Cl)2-bpy ligands, that is, 2a and 2b, may bind with the Ru-
(4,4′-(Cl)2-bpy)2 “head-down” and the anchoring groups of the
linker, all interacting with the TiO2 surface. This geometry
clearly raises issues in the study of remote MLCT states, as
discussed later. In addition, the appearance of this unexpected
kind of Cl-TiO2-surface binding stresses the need to carefully
evaluate the role of “nonbinding” groups introduced on sensitiz-
ers to tune excited state and energetic properties.
TABLE 4: Excited State Electron Yields, Oinj,a for Rigid
Rods Bound to TiO2 Films Pretreated with Acid or Base
sensitizer
pH ) 1b
pH ) 11b
ROD1-bpyRu(bpy)2+ (1a)
ROD2-bpyRu(bpy)22+ (1b)
ROD3-bpyRu(bpy)22+ (1c)
1.0
0.89
0.86
0.77
0.20
<0.05c
a All injection yields were measured spectroscopically at room
temperature, λexc ) 532.5 nm. b The TiO2 thin film pretreatment prior
to binding of the rigid-rod sensitizers. pH ) 1 pretreated samples were
studied in neat acetonitrile; pH ) 11 pretreated films were studied in
0.1 M LiClO4 in acetonitrile. c Li+ addition caused some desorption,
so this value was measured in neat acetonitrile.
state isosbestic point to avoid possible contributions from the
excited states. The transient data was well described by a bi-
second-order kinetic model, eq 6:19c
∆A0 - ∆As
∆As
∆A )
+
(6)
1 + (kf/∆ꢀl) t(∆A0 - ∆As) 1 + (ks/∆ꢀl) t(∆As)
where ∆A was the absorbance change at time t, ∆ꢀ was the
molar extinction coefficient, l was the optical path length, ∆A0
was the initial amplitude (equal to the sum of the contributions
from the fast and slow components), kf was the recovery rate
constant for the fast component, ∆As was the amplitude of the

Excited State Electron Transfer from Ru(II)
J. Phys. Chem. B, Vol. 108, No. 43, 2004 16651
3
2. MLCT Excited State in Solution and Surface Bound.
coordinated to Ru(II) and have also attributed this to delocal-
ization of the excited state.12
The Ru rigid-rods have metal-to-ligand charge-transfer (MLCT)
excited states.27 For the parent compound Ru(bpy)32+, the
preponderance of data supports a “localized” formalism for the
thermally equilibrated excited state in fluid solution, that is,
RuIII(bpy-)(bpy)2
compounds that contain different diimine ligands, the excited
state is expected to localize on the ligand that is most easily
reduced.27 It was therefore of interest to consider which ligand-
(s) the emissive excited state was localized upon. A second and
not completely unrelated issue concerns the extent of charge
transfer into the rigid-rod linker.
3. Excited State Electron Injection into TiO2. The quantum
yield for excited state injection from the Ru rigid-rods to TiO2
(φinj), listed in Table 4, was measured on acid (pH ) 1) and
base (pH ) 11) pretreated TiO2 films on the nanosecond time
scale and therefore may not be the true injection yields as any
sub-nanosecond (geminate) recombination of the injected
electron with the oxidized dye were unaccounted for. Therefore,
the φinj values reported here are best considered as lower limits
of the true injection yields. The φinj on acid (pH ) 1) pretreated
TiO2 were near unity, consistent with negligible geminate
recombination. The φinj values for 1a-c decreased by about
15% as the number of (Ph-E) units in the bridge increased. This
effect may be due to the increasing distance of the complex
from the surface.
not RuIII(bpy-1/3 2+* 2,32
) . Furthermore, for
2+*
The (Ph-E)n bridge in the rigid-rod linker and the 4,4′-(Cl)2-
bpy are known to be electron withdrawing and were therefore
expected to stabilize the MLCT excited states.12,26 This expecta-
tion was realized and it is clear that the π* orbitals of the ligands
decrease in energy: bpy > rigid-rod bpy ∼ rigid-rod phen >
4,4′-(Cl)2-bpy. Therefore, the emissive excited state of Ru rigid-
rod compounds that contain the 4,4′-(Cl)2-bpy ligand was
localized on this ligand, and this explains why their photo-
physical properties were nearly independent of the Ph-E bridge.
For the unsubstituted bpy based Ru rigid-rods, the excited state
was localized on the bpy (or phen) attached to the Ph-E bridge.
The spectroscopic data reveal the same localized excited states
for the rigid-rods in fluid acetonitrile solution and when
anchored to ZrO2 or TiO2 nanoparticles. Therefore, for the Ru-
(bpy)2 rigid-rods, the thermally equilibrated excited state was
localized on rigid-rod linkers bound to the semiconductor
surface.
At present, however, is not possible to determine whether
this is a “distance dependent” effect, since the orientation of
the rods with respect to the surface is unknown. Even if both
COOR anchoring groups bind to the surface, the axis of the
rod may not be perpendicular to the surface and the molecules
may bind at an angle (or angles) with respect to the surface. In
conclusion, the effective injection distance is unknown and it
is probably less than the Ru-center-to-O-bound-to-TiO2 distance
calculated assuming that the molecules are perpendicular (d(1a)
∼ 12 Å, d(1b) ∼ 19 Å, d(1c) ∼ 25 Å).
On base (pH ) 11) pretreated TiO2, the injection yields were
very low, φinj < 0.05, Table 4. This is expected, as the
conduction band edge shifts 59 mV/pH unit, resulting in poor
orbital overlap with the excited state sensitizer.28 It is known,
however, that the addition of “potential determining ions”, such
as Li+, can shift the conduction band edge and promote electron
injection under these conditions.33
It was of interest to compare Ru(bpy)2 rigid-rods where the
excited state was localized on a bpy with a variable length
phenyl ethyne bridge, 1a-c. The visible absorption and photo-
luminescence spectra of these rigid-rods were within experi-
mental error the same. Interestingly, the excited state lifetime
increased for the two longer rigid-rods relative to the shortest
one. For a series of closely related Ru compounds, an increase
in lifetime is generally accompanied by a blue shift in the
emission spectra in accordance with the energy gap law.27 The
increased lifetime observed here must have a different origin
and may reflect the phenylethyne substituents whose extended
π conjugation allows for greater electron delocalization. Excited
state delocalization disperses the electron into the (Ph-E) bridge,
thereby decreasing the average bond displacement energy, which
decreases vibrational overlap and hence the nonradiative rate
constant.29
A curious detail of the injection process is that the quantum
yield decreased with increased incident irradiance. This behavior
was first reported by Kay and Gra¨tzel and was attributed to
trap filling that shifts the quasi-Fermi level of TiO2 toward the
vacuum level, making electron injection less favorable.34 If this
explanation were correct, the photoluminescence quantum yield
should increase as φinj decreases. However, here, and in previous
studies, we find that both quantum yields decrease with
irradiance, indicating that some other fast process is lowering
the injection yield.19c
The electron injection rate constants on acid pretreated TiO2
films could not be time resolved with the nanosecond laser used
in this study (kinj . 108 s-1). This result was expected, since
the rates of photoinduced interfacial electron transfer for Ru-
(II) sensitized TiO2 are known to occur on the femto to
picosecond time scale.35 Recent ultrafast measurements of
tripodal sensitizers 15-24 Å long bound to TiO2 revealed that
the injection occurs on a femto to picosecond time scale.6,35
We note that injection rate constants kinj . 108 s-1 with (kr +
knr) ∼ 5 × 105 s-1 for the rods imply that the injection quantum
yields should be near unity, consistent with the actinometry
measurements.
An alternative explanation for the long-lived “red” emissive
excited state is that mixing between MLCT and intraligand
excited state(s) of the rigid-rod imparts more triplet character
and hence, a longer lived excited state. A mechanism where
the MLCT excited state transfers energy to a lower-lying triplet
state of the rigid rod is ruled out by the wavelength independent
lifetimes measured by transient absorption spectroscopy and the
MLCT-like steady state PL spectrum. Nevertheless, excited state
delocalization and/or mixing between the MLCT manifold and
intraligand triplet states likely explain the long-lived excited
state behavior that does not follow the energy gap law.
A fraction of the injection process for the rigid-rods with bpy
as the ancillary ligand could be time resolved on base pretreated
TiO2 films immersed in 0.1 M LiClO4 acetonitrile solution. As
mentioned above, at basic pH the electron injection is less
favorable. Under these conditions, approximately 70-90% of
the signal was instrument response limited, while the remaining
fraction could be quantified (kinj ∼ 1 × 107 s-1). This is in
agreement with the quantum yields measurements and strongly
Indirect evidence for delocalization into the phenylethyne
bridge is seen by the strong excited state absorption in the
>600 nm region. The first extension of the bridge (from 1a
to 1b) resulted in significant changes in the intensity and
spectrum, while further elongation resulted in smaller differ-
ences. Previous researchers have also observed intense near-IR
absorption bands for phenylethyne substituted bpy ligands

16652 J. Phys. Chem. B, Vol. 108, No. 43, 2004
Wang et al.
suggests that the density of acceptor states for basic TiO2 is
significantly lower than for acidic TiO2.
resulting in delocalization of the MLCT state on the linkers, as
indicated by measurable spectral shifts in the absorption, PL,
and transient absorption spectra. When the ancillary ligands
contain Cl groups, there is clear evidence that the MLCT is
localized on the 4,4′-Cl2-bpy. An unexpectedly strong binding
between the 4,4′-Cl2-bpy and the TiO2 was observed, so that
while the rigid-rods bind exclusively through the carboxylic
group, the rigid-rods with Cl-substituted ligands may also bind
with the Ru complex “head down”. Ester-like or carboxylate
binding modes to the TiO2 surface were observed, and condi-
tions to obtain only one type of binding mode were found. The
rigid-rods were expected to exhibit diminished electronic
coupling with the TiO2 acceptor states relative to complexes
that are directly attached. This behavior was realized under
conditions where the density of TiO2 acceptor states was
intentionally decreased by surface pretreatments with basic
aqueous solutions. Future studies will focus on ultrafast
spectroscopic measurements aiming at quantifying the electron
injection process and on the application of these new Ru rigid-
rod sensitizers in regenerative solar cells.
The injection rate could not be time resolved, kinj > 108 s-1
,
for the rigid-rods containing the 4,4′-(Cl)2-bpy ancillary ligand
bound to acidic as well as basic TiO2, despite the fact that they
are weaker excited state photoreductants than rigid-rods 1a-c.
This finding supports the notion that the 4,4′-(Cl)2-bpy ligands
interact directly with the TiO2 surface, and that the molecules
may bind “head down”, thereby providing a new pathway for
electron injection.
In summary, a small fraction of the injection process could
be observed on the nanosecond time scale when the TiO2 was
base pretreated and when the excited state was localized on a
rigid-rod linker. All other experimental conditions led to
instrument response limited electron injection rate constants with
nearly quantitative yields. The faster and more efficient electron
injection into acidic TiO2 was attributed to a higher density of
unfilled states that overlap with the excited state sensitizer
orbitals. Femtosecond laser spectroscopy studies aimed to
quantify the injection rates for 1a-c to determine whether there
are differences due to the linker’s length are in progress.
4. Back Electron Transfer. A specific goal of this study
was to find conditions where the injection yield was unity while
the back electron transfer rate constant decreased with distance.
The recombination between the electrons in TiO2 and the
oxidized dye can lower the efficiencies of solar cells. Hence,
an attractive way to minimize this process would be to increase
the distance between the TiO2 surface and the metal center,
thereby decreasing the rate of recombination. This effect was
not realized. Back electron transfer rate constants were found
to be independent of the length of the rigid-rod excited. We
note that a similar observation was made for the tripodal
sensitizers, where the recombination rates between the electrons
in TiO2 and the oxidized dye were found to be independent of
the linker’s length.5,6 Recombination of the injected electron
with the oxidized dye required milliseconds for completion, and
the kinetics were well described by a sum of two second order
rate constants. Average rate constants for the rigid-rods 1a-c
were within experimental error the same. This is not unexpected
in view of literature reports proposing a mechanism wherein
transport of the injected electron to the oxidized sensitizer is
the rate limiting step.30 The main process responsible for
controlling the recombination rates is usually assumed to be
the electron transport by diffusion between trap sites on a
semiconductor nanoparticle surface (the Random Flight Model30).
It is generally concluded that the recombination reaction is
governed by the energy redistribution of the electrons trapped
in the semiconductor, rather than by spatial diffusion. However,
Clifford et al.36 have recently observed slower recombination
dynamics with single exponentials in dye cations that are
physically separated from the surface, an effect that is being
studied by Tachiya et al.37 The recent interest in the processes
that regulate the recombination rates shows the importance of
having available model linkers such as the ones studied here.
Based on these data,36,37 we anticipate observing a distance
dependence with TiO2 materials that have a lower density of
trap states and at higher excitation irradiances. Studies of this
type will be performed soon.
Acknowledgment. E.G. and G.J.M. are grateful to the
Division of Chemical Sciences, Office of Basic Energy Sciences,
Office of Energy Research, U.S. Department of Energy for grant
support (DE-FG02-01ER15256 and DE-FG02-96ER14662,
respectively).
Supporting Information Available: General Section for the
experimental procedures, surface adduct formation plots for 1b,
2b and 5, absorbance spectra of 1a and 5 in acetonitrile solutions
and bound to TiO2 films. This material is available free of charge
via the Internet at http://pubs.acs.org.
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