Synthesis of rigid-rod linkers to anchor chromophores to semiconductor nanoparticles

Article abstract of DOI:10.1016/S0040-4020(02)00586-0

Four rigid-rod sensitizers, made of a phenylethynyl spacer substituted with a chromophore and two COOR binding groups, were prepared to study dynamics of electron injection at the interface of metal oxide semiconductor nanoparticles. Dimethyl Ru(bpy)

Full text of DOI:10.1016/S0040-4020(02)00586-0

TETRAHEDRON
Pergamon
Tetrahedron 58 /2002) 6027±6032
Synthesis of rigid-rod linkers to anchor chromophores
to semiconductor nanoparticles
Dong Wang, James M. Schlegel and Elena Galoppinip
Department of Chemistry, Rutgers University, 73 Warren Street, Newark, NJ 07102, USA
Received 16 April 2002; revised 3June 2002; accepted 6 June 2002
AbstractÐFour rigid-rod sensitizers, made of a phenylethynyl spacer substituted with a chromophore and two COOR binding groups, were
prepared to study dynamics of electron injection at the interface of metal oxide semiconductor nanoparticles. Dimethyl Ru/bpy)2/5-/5-1,10-
phenanthrolinyl)ethynyl)isophthalate)21 /4a), dimethyl Ru/bpy)2/5-/4-/2,20-bipyridinyl)ethynyl)isophthalate)21 /4b), dimethyl 5-/1-pyrenyl-
ethynyl)isophthalate /4c), and dimethyl 5-/9-anthracenylethynyl)isophthalate /4d), were synthesized and characterized. Their absorption
spectra, emission spectra, and electrochemical properties have been studied in acetonitrile and hexane solutions at room temperature. q 2002
Elsevier Science Ltd. All rights reserved.
1. Introduction
These are useful to study remote electron transfer processes
and tune molecular±semiconductor interactions.6,7 Analo-
gous donor±spacer±acceptor systems have been success-
fully employed for decades to study electron transfer in
¯uid solutions8 or at the surface of electrodes,9 Scheme
1/b) and /c), respectively.
The functionalization of semiconductor and metal nano-
particles with chromophores and redox-active molecules
is an important step toward the development of devices
operating on the molecular level,1,2 including solar cells,
light-harvesting systems and chemical sensors.1 Typically,
their function is based on interfacial electron transfer
processes. An important example is the light-to-energy
conversion in photoelectrochemical solar cells by sensiti-
zation of nanocrystalline TiO2 to visible light using
RuII-polypyridyl complexes.3 The RuII sensitizers, typically
Ru/dcb)/bpy)221,4 are covalently attached to the TiO2 nano-
particles through the carboxylic acid groups on the dcb
ligand.3,5 A recent area of interest in this ®eld is the develop-
ment of linkers that are rigid and that can distance the
sensitizer from the nanoparticle surface, Scheme 1/a).6,7
Recently, we have prepared tripod-shaped10 linkers for
metal oxide nanoparticles, such as I in Fig. 1.11 These are
1,3,5,7-tetraphenyladamantane and tetraphenylmethane
derivatives having three arms ending with COOR binding
groups and the fourth arm carrying a phen or bpy-based
RuII-polypyridyl complex.11 The three-point attachment
and the structural rigidity of the molecule provide a high
degree of control over the distance of the sensitizer from
10,12
Ê
the metal oxide nanoparticles surface /,15 to 18 A).
The study of the tripodal sensitizers prompted us to explore
other types of linkers. Speci®cally, we were interested in
`rigid-rods',9b,13 such as II in Fig. 1.
In the tripodal linkers, the saturated tetrahedral core
/Tdˆsp3C or adamantane) interrupts the conjugation
between the sensitizer and the semiconductor surface,
while the rigid rods are fully conjugated.13 Second, rigid-
rods have been successfully employed in SAM on gold.9
When bound to nanocrystalline TiO2 by the two COOR
groups, linkers like II may enhance charge injection yields
as the wavefunction is delocalized through the linker and
possibly also enhance charge recombination rates to the
chromophoric center.9,13b Furthermore, well-known cross-
coupling methodologies14 could allow to vary the number
of phenyl and/or ethyne units between the tripod and
the sensitizer and thereby the distance from the surface.
In this paper we describe the synthesis, characteri-
zation and selected properties of the ®rst examples of
Scheme 1.
Keywords: rigid-rod linkers; sensitization; semiconductor; nanoparticle.
Corresponding author. Tel.: 11-973-353-5317; fax: 11-973-353-1264;
e-mail: galoppin@andromeda.rutgers.edu
p
0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020/02)00586-0

6028
D. Wang et al. / Tetrahedron 58 *2002) 6027±6032
Figure 1. Representation of the two types of rigid sensitizers /tripods and rigid rods) and their components.
rigid-rod linkers II substituted with organic and inorganic
sensitizers.
considerably easier to synthesize and to purify than the
tripodal sensitizers and are consistently obtained in good
yields.
In addition to the RuII-polypyridyl complexes 4a and b, that
will be compared to the tripodal linkers, we prepared 4c and
d, substituted with pyrene and anthracene. It has been
demonstrated that organic chromophores are frequently
excellent test systems for elucidating fundamental photo-
physics of charge injection and recombination processes at
molecule±nanoparticle interfaces.17 In particular, anthra-
cene directly bound to TiO2 surfaces by a COOH group
has been studied by others.17b,c Aromatic chromophores,
with their well-de®ned spin states, vibronic structures and
transition dipole orientations, can provide unique insights
that are dif®cult to obtain from the more robust and widely
used inorganic complex sensitizers. Although PL lifetimes
of RuII±polypyridyl complexes are much longer /t,1 to
3 ms), the ultrafast electron injection rates observed in
sensitizer±TiO2 systems,3 including those prepared from
the tripodal sensitizers,11a,b are an assurance that the charge
injection will be complete even with organic chromophores
such as anthracene and pyrene. We expect that, even at the
2. Results and discussion
Four rigid-rod sensitizers were prepared: 4a and b, substi-
tuted with RuII-polypyridyl complexes, and 4c and d, substi-
tuted with aromatic chromophores. Their synthesis is shown
in Scheme 2.
Sonogashira cross-coupling15 of dimethyl-5-bromo-
isophthalate 1 with trimethylsilylacetylene, followed by
deprotection with TBAF, produced alkyne 2. Suzuki-type
coupling16 of
2
with 5-bromo-9,10-phenanthroline,
4-bromo-2,20-bipyridine, 1-bromopyrene and 9-bromo-
anthracene, yielded 3a, b, 4c and d, respectively. A Suzuki
cross-coupling was employed in this step to avoid the
dimerization of 2, which is observed in Sonogashira
coupling conditions. The compounds substituted with the
phen and bpy ligands, 3a and b, were easily converted
into the corresponding RuII complexes 4a and b by reaction
with Ru/bpy)2Cl2´2H2O. We found that the rigid-rods are
Scheme 2. Reagents and yields: /a) Me3SiCuCH, Cl2Pd/PPh3)2, CuBr, /i-Pr)2NH, re¯ux, 85%. /b) TBAF, THF, rt, 86%. /c) 1. /Me3Si)2NLi, 2788C;
2. 9-BBN, 2788C; 3. ArBr, Pd/PPh3)4, THF, re¯ux, 62±70%. /d) 1. Ru/bpy)2Cl2´2H2O; 2. NH4PF6, 70%.

D. Wang et al. / Tetrahedron 58 *2002) 6027±6032
6029
Table 1. Electrochemical and photophysical properties of rigid-rod sensitizers and reference sensitizers in solution
b
Compound
E1/2 ox /V)a
l
abs /nm) /1, M21 cm21
)
l
P /nm)c
t /ms)
4a
1.22d
451 /1.5£104)
450
610
620
640
626
428
392
475
421
1.79
1.20
3.20
0.80
0.060
0.428
0.012
0.015
1.20d,f
21e
Ru$phen)$bpy)2
4b
Ru$bpy)3
1.23d
462 /1.6£104)
1.26,d,g 1.32h
1.20
1.19
452
21e
4ci
383 /1.0£105)
334 /7.5£104)
£104)
Pyrenei
4di
1.13420 /1.0
1.19, 1.30h
Anthracenei
375 /0.8£105)
a
Half-wave potentials were measured at a Pt working electrode in 0.1 M TBAPF6/CH3CN degassed solution using Ag/AgCl as reference. Data are reported
vs. SCE.
b
c
d
e
f
Measurements were made at room temperature, absorption maximum ^2 nm.
Photoluminescence maximum, ^4 nm. All data were obtained under argon atmosphere.
RuIII/II
.
In CH3CN.
Ref. 19.
Ref. 20.
Ref. 21.
In hexane.
g
h
i
increased distance, the charge injection into TiO2 will be
complete within the S1 state lifetime of 4c and d /60 and
12 ns, respectively, Table 1).
Finally, we are interested in increasing the number of
phenylethynyl spacers, as shown in Fig. 2, and thereby
control the distance between the semiconductor surface
and the sensitizer. To test this possibility, we prepared linker
7 /I with nˆ1) through the route shown in Scheme 3. As in
the case of shorter rods, linker 7 was a very soluble material.
To avoid the dimerization of 6 in step /c), we employed
coupling conditions different than those used in step /b).
Selected properties of 4a±d in ¯uid solutions and of refer-
ence chromophores /Ru/phen)/bpy)221, Ru/bpy)321, pyrene,
and anthracene) are listed in Table 1. The oxidation waves
for 4a±d were similar to those of the references. Rigid-rod
sensitizers 4a and b displayed reversible RuIII/II waves at
,1.22 V vs SCE and 4c and d displayed oxidation waves
at 1.20 and 1.13V vs SCE. The visible absorption spectra of
sensitizers 4a and b in acetonitrile displayed broad bands
typical of MLCT excited states. The phen-based 4a and
3. Conclusions
21
Four sensitizers /4a±d) containing conjugated rigid-rod
ligands were synthesized and characterized. Rigid spacers
for sensitizers are useful models for photophysical and
electron transfer studies at nanoparticle interfaces. The
synthesis of linker 7, containing two phenyethynyl units,
demonstrates that the length of the spacer can be increased.
The study of the binding properties to TiO2 and other metal
oxide nanoparticle surfaces, and the excited state and redox
properties of surface-bound 4a±d and sensitizers derived
from 7 is in progress.
Ru/phen)/bpy)2 displayed the MLCT band centered at
,451 nm while the bpy-based 4b displayed the MLCT
/lmaxˆ462 nm) red-shifted with respect to Ru/bpy)321, an
observation that is consistent with data obtained for the
tripodal sensitizers.11a The visible absorption spectra of 4c
and d displayed broad bands 49 and 45 nm red-shifted with
respect to pyrene and anthracene,18 respectively. All sensi-
tizers displayed room-temperature photoluminescence
/PL) and the emission maximum followed the same trends
as the absorption. As expected, the PL decays of sensitizers
4a±d in acetonitrile or hexane solutions followed single
exponential kinetics.
Scheme 3. Reagents and yields: /a) Me3SiCuCH, Cl2Pd/PPh3)2, CuBr,
/i-Pr)2NH, re¯ux. 83%. /b) MeLi LiBr, THF, 08C. /c) Pd/dba)2, PPh3,
CuI, THF and Et3N, re¯ux. 60%.
Figure 2. Calculated C to C distances of rigid-rod linkers of various length
/MM2, Spartan, Wavefunction, Inc.).

6030
D. Wang et al. / Tetrahedron 58 *2002) 6027±6032
4. Experimental
puri®ed through silica gel column chromatography /hexane/
ethyl acetate, 90:10) to give 125 mg of dimethyl 5-/tri-
methylsilylethynyl)isophthalate as a white solid /yield:
4.1. Synthetic procedures
1
85%). Mp: 101±1028C. H NMR d: 8.61 /1H, s), 8.30
/2H, s), 3.95 /6H, s), 0.27 /9H, s). 13C NMR d: 165.58,
136.87, 130.84, 130.29, 124.24, 102.72, 96.74, 52.52,
20.20. GC/MS m/z: 290 /M1, 15), 275 /M215, 100), 259
/M231, 10), 201 /M289, 8). Tetrabutylammonium¯uoride
trihydrate /160 mg, 0.52 mmol) was added to a THF solu-
tion of the TMS-protected alkyne /100 mg, 0.35 mmol).
After stirring at room temperature for 2 h, was added
water. After standard workup with chloroform, the crude
product was puri®ed through column chromatography
/hexane/AcOEt, 90:10) to give 65 mg /yield: 86%) of 2 as
General. NMR spectra were obtained on a Varian INOVA
500 spectrometer operating at 499.90 MHz for 1H and
124.98 MHz for 13C and collected in CDCl3, unless other-
1
wise speci®ed. The H spectra were referenced to tetra-
methylsilane and the 13C spectra to the central line of the
solvent. Coupling constants /J) are reported in Hz with a
precision of ^0.1 Hz. High- or low-resolution mass spectra
/HRMS or LRMS) and elemental microanalytical data
were obtained at commercial facilities /Michigan State
University Mass Spectrometry Facility and Robertson
Microlit Laboratories, Madison, NJ, respectively). GC/MS
data were obtained on a HP 6890 gas chromatograph with a
HP 5973MS detector and a capillary column /HP 19091s-
433: 30 m, phenylmethyl siloxane). Major ions are recorded
to unit mass, intensity is parenthetically indicated as a
percentage of the strongest peak. Melting points were
measured with a Fisher melting point apparatus. UV±Vis
absorption spectra were collected on a VARIAN Cary-500,
and ¯uorescence emission spectra on a VARIAN Cary-
Eclipse. IR measurements were made on a Mattson RS1
FT-IR spectrometer using polyethylene cards.
1
a white solid. Mp: 124±1258C. H NMR d: 8.65 /1H, s),
8.33 /2H, s), 3.96 /1H, s), 3.18 /1H, s). 13C NMR d: 165.47,
137.07, 131.00, 130.69, 123.21, 81.56, 79.16, 52.58. GC/
MS m/z: 218 /M1, 60), 187 /M231, 100), 159 /M259, 35),
144 /M274, 35), 100 /M2118, 15), 74 /M2144, 15). IR
/cm21): 3250, 1728, 1594, 1270. Anal. Calcd for C12H10O4:
C, 66.05; H, 4.62; O, 29.33. Found: C, 65.85; H, 4.57.
4.2. Suzuki-type coupling of 2 with ArBr
In a typical procedure, to a solution of 2 /100 mg, 0.459
mmol) in THF /10 ml) at 2788C was added lithium bis/tri-
methylsilyl)amide /0.63mmol, 0.63ml of 1 M hexane
solution). After 30 min, was added 9-methoxy-9-BBN
/0.63mmol, 0.63ml of 1 M hexane solution). After stirring
for 2 h at 2788C, the solution was transferred via cannula to
a second ¯ask containing Pd/PPh3)4 /31 mg, 0.028 mmol)
and the aryl bromide /ArBrˆ0.386 mmol) in dry THF
/15 ml). The reaction mixture was heated to re¯ux overnight
and cooled to room temperature. After standard workup
with chloroform the crude product was puri®ed by silica
gel column chromatography using the eluent indicated in
each case.
Materials. THF was purchased anhydrous /Aldrich) and
then distilled ®rst from sodium/benzophenone and then
from LiAlH4 immediately prior to use. Benzene was
purchased anhydrous /Aldrich) or dried by removal of the
water azeotrope followed by distillation from sodium. Et3N
and i-Pr2NH were distilled from KOH under nitrogen prior
to use. Spectroscopic grade CH3CN for spectroscopic and
electrochemical measurements was used without further
puri®cation. Organolithium reagents /MeLi´LiBr and
/Me3Si)2NLi) and B-methoxy-9-borobicyclo[3,3,3]nonane
/B-Methoxy-9-BBN) were purchased from Akros or from
Aldrich. Column chromatography was performed using
silica gel /Selecto Silica Gel, 230±600 mesh). TLC was
performed on Whatman silica gel plates, using UV light
as the developing agent. Dimethyl-5-bromoisophthalate is
commercially available /Fluka) or can be easily prepared by
treating the corresponding carboxylic acid with a solution
of diazomethane in ether. Bis-1,4-/trimethylsilylethynyl)-
benzene /5), which is commercially available, was prepared
following the same procedure used to prepare 2.
4.2.1. 3a $ArBrˆ5-bromo-9,10-phenanthroline). Eluent:
1
1% of MeOH in CHCl3. Yield: 68%. Mp: 160±1618C. H
NMR d: 9.26 /1H, dd, J1ˆ4.0 Hz, J2ˆ2.0 Hz), 9.22 /1H, dd,
J1ˆ4.0 Hz, J2ˆ2.0 Hz), 8.83/1H, dd, J1ˆ8.0 Hz, J2ˆ
1.5 Hz), 8.71 /1H, t, Jˆ1.5 Hz), 8.51 /2H, d, Jˆ1.5 Hz),
8.26 /1H, dd, J1ˆ8.0 Hz, J2ˆ1.5 Hz), 8.15 /1H, s), 7.79
/1H, dd, J1ˆ8.5 Hz, J2ˆ4.0 Hz), 7.68 /1H, dd, J1ˆ8.5 Hz,
J2ˆ4.0 Hz), 4.01 /6H, s). 13C NMR d: 165.49, 151.23,
150.85, 146.34, 145.96, 136.60, 135.92, 134.59, 131.38,
131.22, 130.62, 128.08, 127.92, 123.72, 123.56, 123.53,
119.19, 93.09, 87.65, 52.66. MS m/z: 396 /M1, 100), 365
/M231, 10), 322 /M274, 11), 277 /M2119, 10), 262
/M2134, 17), 183 /M2213, 15). HRMS /EI) calcd for
C24H16N2O4: 396.1110, found 396.1114. IR /cm21): 1728,
1598, 1243.
Methods. All reactions, except step /b) in Scheme 2, were
performed under nitrogen atmosphere in glassware that
had been oven-dried and ¯amed under vacuum and using
anhydrous solvents. `Standard workup' refers to extractions
with the indicated organic solvent, followed by washing of
the combined extracts with water or brine, drying over
Na2SO4 and removal of solvents in vacuo on a rotary
evaporator.
4.2.2. 3b $ArBrˆ4-bromo-2,20-bipyridine). Eluent: hex-
1
ane/AcOEt, 80:20. Yield: 62%. Mp: 183±1848C. H NMR
4.1.1. Dimethyl 5-ethynylisophthalate $2). To a solution
of dimethyl-5-bromoisophthalate /140 mg, 0.51 mmol) in
diisopropylamine /15 ml), were added Cl2Pd/PPh3)2
/32 mg, 0.05 mmol), CuBr /110 mg, 0.53 mmol) and
trimethylsilylacetylene /2 ml, or 1.4 g, 14 mmol). The
reaction mixture was re¯uxed overnight, ®ltered and the
solution was concentrated in vacuo. The solid residue was
d: 8.71 /2H, m), 8.69 /1H, m), 8.58 /1H, s), 8.43/1H, m),
8.41 /2H, d, Jˆ1.5 Hz), 7.85 /1H, ddd, J1ˆJ2ˆ7.5 Hz,
J3ˆ2.0 Hz), 7.42 /1H, dd, J1ˆ5.0 Hz, J2ˆ1.5 Hz), 7.35
/1H, ddd, J1ˆ7.5 Hz, J2ˆ4.5 Hz, J3ˆ1.0 Hz), 3.99 /6H, s).
13C NMR d: 165.43, 156.31, 155.29, 149.27, 137.11,
136.78, 131.67, 131.16, 130.90, 125.24, 124.15, 123.33,
121.21, 91.757, 88.67, 52.65. GC/MS m/z: 372 /M1, 100),

D. Wang et al. / Tetrahedron 58 *2002) 6027±6032
6031
341 /M231, 42), 314 /M258, 22), 298 /M274, 11), 253
/M2119, 11), 170 /M2202, 16). Anal. Calcd for
C22H16N2O4: C, 70.96; H, 4.33; N, 7.52; O, 17.19. Found:
C, 70.65; H, 4.23; N, 7.20. IR /cm21): 1733, 1584, 1249.
4.2.6. 1-Ethynyl-4-$trimethylsilylethynyl)benzene $6). To
a solution of 5 /800 mg, 2.96 mmol) in THF /30 ml) cooled
with an water/ice bath, MeLi´LiBr /2.97 mmol, 1.35 ml of
2.2 M solution in diethyl ether) was added dropwise
/,20 min). The solution was stirred for 10 min and then
poured into ice-cold, diluted HCl aq. After standard workup
with ether were obtained 540 mg of a ,1:1 mixture of 5 and
6 /GC/MS). This mixture was dif®cult to separate and it was
used in the next step after puri®cation through a short pad of
silica gel. A small amount of 6 was isolated by silica gel
column chromatography /pentane) for characterization
4.2.3. 4c $ArBrˆ1-bromopyrene). Eluent: CHCl3. Yield:
70%. Mp: 194±1958C. 1H NMR d: 8.67 /1H, t, Jˆ1.5 Hz),
8.65 /1H, s), 8.53/2H, d, Jˆ1.5 Hz), 8.24 /4H, m), 8.14
/2H, m), 8.06 /2H, m), 4.01 /6H, s, COOCH3). 13C NMR d:
165.69 /COOCH3), 136.50, 132.10, 131.69, 131.22, 131.07,
131.03, 130.07, 129.78, 128.68, 128.51, 127.21, 126.34,
125.86, 125.79, 125.34, 124.65, 124.55, 124.45, 124.26,
116.80, 92.95 /Pyr±CuC±), 90.60 /Pyr±CuC±), 52.61
/COOCH3). HRMS /EI) calcd for C28H18O4: 418.1205.
Found: 418.1191. Anal. Calcd for C28H18O4: C, 80.37; H,
4.34; O, 15.29. Found: C, 79.71; H, 4.30. IR /cm21): 2206,
1730, 1592, 1240.
1
purposes: H NMR d: 7.39 /4H, s), 3.14 /1H, s, CuCH),
0.23/9H, s, Si/C H3)3). 13C NMR d: 131.90, 131.81, 123.57,
122.08, 104.33 /CuCSi), 96.46 /CuCSi), 83.19 /CuCH),
78.92 /CuCH), 20.12 /Si/CH3)3). GC/MS m/z: 198 /M1,
30), 183 /M215, 100), 28 /M2170, 30).
4.2.7. Synthesis of 7. To a solution of a mixture of 5 and 6
/amount of 6 estimated by GC/MS is 396 mg, 2.00 mmol),
dimethyl 5-bromoisophthalate /273mg, 1.00 mmol), bis/di-
benzylideneacetone)palladium /30 mg, 0.05 mmol), PPh3
/52 mg, 0.20 mmol), CuI /19 mg, 0.10 mmol) were added
dry THF /20 ml) and Et3N /147 mg, 1.50 mmol). The reac-
tion mixture was re¯uxed for 24 h. The solvent was
removed in vacuo and the crude product was puri®ed by
silica gel column chromatography /hexane/AcOEt, 95:5)
to give 233 mg of 7 as a pale yellow powder /yield: 60%
4.2.4. 4d $ArBrˆ9-bromoanthracene). Eluent: CHCl3.
1
Yield: 66%. Mp: 231±2328C. H NMR d: 8.68 /1H, t,
Jˆ1.5 Hz), 8.64 /2H, d, Jˆ8.5 Hz), 8.57 /2H, d, Jˆ
1.5 Hz), 8.48 /1H, s), 8.04 /2H, d, Jˆ8 Hz), 7.65 /2H,
ddd, J1ˆ8.5 Hz, J2ˆ6.5 Hz, J3ˆ1.0 Hz), 7.54 /2H, dd,
J1ˆ8 Hz, J2ˆ6.5 Hz), 4.02 /6H, s, COOCH3). 13C NMR
d: 165.70 /COOMe), 137.03, 136.42, 132.78, 131.10,
130.11, 128.77, 128.50, 126.99, 126.54, 125.80, 124.70,
116.20, 98.49 /An±CuC), 88.25 /An±CuC), 52.63
/COOCH3). HRMS /EI) calcd for C26H18O4: 394.1205.
Found: 394.1200. Anal. Calcd for C26H18O4: C, 79.17; H,
4.60; O, 16.23. Found: C, 79.39; H, 4.54. IR /cm21): 2196,
1728, 1595, 1246.
1
calculated on 1). Mp: 161±1628C. H NMR d: 8.63/1H, t,
Jˆ1.5 Hz), 8.35 /2H, d, Jˆ1.5 Hz), 7.47 /4H, m), 3.97 /6H,
s, COOCH3), 0.26 /9H, s, Si/CH3)3). 13C NMR d: 165.56
/COOMe), 136.48, 131.97, 131.52, 131.00, 130.20, 124.13,
123.57, 122.45, 104.42, 96.71, 90.84, 89.13, 52.55
/COOCH3), 20.11 /Si/CH3)3). GC/MS m/z: 390 /M1, 70),
375 /M215, 100), 172 /M2218, 15). HRMS /EI) calcd for
C23H22O4Si: 390.1287, found 390.1289. IR /cm21): 2157,
1730, 1597, 1244.
4.2.5. Formation of RuII±polypyridyl complexes. In a
typical procedure, a solution of the ligand 3a or b
/0.311 mmol) in THF /3 ml) was added to a 1:1 mixture
of ethanol/water /15 ml). To the solution, purged with
nitrogen /15 min), was added Ru/bpy)2Cl2´2H2O /177 mg,
0.341 mmol) and the mixture was re¯uxed for 6 h, cooled to
room temperature and ®ltered. Addition of aqueous NH4PF6
/3.0 g, 18.4 mmol) to the ®ltrate formed precipitate, which
was collected and washed with water to afford the complex
4.3. Spectroscopic measurements
UV±Vis absorbance. UV±Vis absorbance measurements
were made on a VARIAN Cary-500 spectrophotometer,
using hexane as solvent for pyrene, anthracene, 4c and
4d and acetonitrile for 4a and 4b. The measurements were
performed in 1 cm2 cuvettes with following set-up para-
meters: 0.5 nm of data interval, 0.6 s of average time and
1.0 nm of SBW.
1
as an orange powder. 4a: yield 70%. H NMR d /acetone-
d6): 9.26 /1H, d, Jˆ3.0 Hz), 8.84 /7H, m), 8.63 /1H, t,
Jˆ2.0 Hz), 8.52 /3H, m), 8.27 /2H, m), 8.18 /4H, m),
8.05 /1H, m), 7.97 /3H, m), 7.65 /2H, m), 7.40 /2H, m),
3.99 /6H, s). 13C NMR d /acetone-d6): 165.67, 158.40,
158.13, 154.37, 154.24, 153.08, 153.03, 148.72, 148.64,
139.09, 138.96, 137.77, 137.23, 136.46, 133.63, 132.56,
131.51, 131.48, 131.45, 128.77, 128.64, 127.89, 127.85,
125.36, 125.27, 124.01, 121.91, 95.80, 86.73, 53.10. Anal.
Calcd for C44H32F12N6O4P2Ru: C, 48.05; H, 2.93; N, 7.64.
Found: C, 46.70; H, 2.77; N, 7.26. IR /cm21): 1725, 1603,
Photoluminescence. Photoluminescence /PL) spectra were
obtained with a VARIAN Cary-Eclipse Fluorescence Spectro-
photometer using hexane as solvent for pyrene, anthracene,
4c and d and acetonitrile for 4a and b in 4-way transparent
1 cm2 cuvettes. The excitation slit was 5 nm and emission
slit was 1.5 nm. The excitation wavelengths for pyrene,
anthracene, 4a±d are 305, 323, 450, 465, 360 and 395 nm,
respectively. The PMT detector voltage was 800 V.
1
1252, 841. 4b: Yield 72%. H NMR d /acetone-d6): 9.07
/1H, s), 8.95 /1H, d, Jˆ3.0 Hz), 8.84 /4H, d, Jˆ8.5 Hz),
8.63/1H, t, Jˆ1.5 Hz), 8.38 /1H, d, Jˆ1.5 Hz), 8.22 /8H,
m), 8.09 /4H, m), 7.72 /1H, dd, J1ˆ6 Hz, J2ˆ2 Hz), 7.61
/5H, m), 3.98 /6H, s). 13C NMR d /acetone-d6): 165.53,
158.69, 158.10, 158.06, 157.99, 157.69, 152.95, 152.88,
152.81, 152.75, 152.64, 139.13, 139.05, 137.16, 132.63,
132.49, 131.79, 129.80, 129.14, 128.85, 127.19, 125.69,
129.39, 123.38, 95.68, 88.04, 53.15. Anal. Calcd for
C42H32F12N6O4P2Ru: C, 46.89; H, 3.00; N, 7.81. Found: C,
46.16; H, 2.97; N, 7.44. IR /cm21): 1728, 1605, 1260, 840.
Time-resolved photoluminescence. Time-resolved photo-
luminescence decays were acquired on a nanosecond
Nd:YAG /Continuum NY-61) laser. Measurements were
acquired using 355 nm laser pulse for solutions of pyrene,
anthracene, 4c and d in hexane and 532 nm laser pulse for
solutions of 4a and b in acetonitrile, ca. 8 ns, 30±50 mJ.
Samples were degassed by three freeze±pump±thaw cycles.

6032
D. Wang et al. / Tetrahedron 58 *2002) 6027±6032
9. /a) Bakkers, E. P. A. M.; Roest, A. L.; Marsman, A. W.;
Jenneskens, L. W.; de Jong-van Steensel, L. I.; Kelly, J. J.;
Vanmaekelbergh, D. J. Phys. Chem. B 2000, 104, 7266 and
references therein. /b) Yang, G. H.; Qian, Y. L.; Engtrakul, C.;
Sita, L. R.; Liu, G. Y. J. Phys. Chem. B 2000, 104, 9059.
/c) Roncali, J. Acc. Chem. Res. 2000, 33, 147. /d) Vondrak,
T.; Wang, H.; Winget, P.; Cramer, C. J.; Zhu, X. Y. J. Am.
Chem. Soc. 2000, 122, 4700.
The samples concentrations were such that A,0.3at the
corresponding excitation wavelength. Each kinetic trace
was acquired averaging 16 laser shots.
Electrochemistry. Cyclic voltammetry was performed in
0.1 M tetrabutylammonium hexa¯uorophosphate aceto-
nitrile /TBAPF6/CH3CN) degassed solutions with scan
rateˆ100 mV/s. The solutions were ,0.1 mM in the sensi-
tizers. A BAS model CV-50 W potentiostat was used in a
standard three-electrode arrangement consisting of a Pt
working electrode, a Pt wire auxiliary counter electrode,
and Ag/AgCl as the reference electrode. The measurements
were carried out under nitrogen atmosphere and at room
temperature and were recorded in the absence and in
the presence of ferrocene. All half-wave potentials are refer-
enced to the ferrocene/ferrocenium couple and are reported
vs SCE.
10. Tripod-shaped linkers of various structure /also named
caltrops, see Ref. 9c) have been previously reported in studies
of redox active molecules bound on gold electrodes:
/a) 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. /b) Yao, Y.; Tour, J. M. J. Org.
Chem. 1999, 64, 1968. /c) Fox, M. A.; Whitesell, J. K.;
McKerrow, A. J. Langmuir 1998, 14, 816. /d) Hu, J.; Mattern,
L. J. Org. Chem. 2000, 65, 2277. For tetrahedral molecules
with extended arms see: /e) Aujard, I.; Baltaze, J.-P.; Baudin,
Â
Â
J.-B.; Cogne, E.; Ferrage, F.; Jullien, L.; Perez, E.; Prevost, V.;
Qian, L. M.; Ruel, O. J. Am. Chem. Soc. 2001, 123, 8177.
11. /a) Galoppini, E.; Guo, W.; Zhang, W.; Hoertz, P.; Qu, P.;
Meyer, G. J. J. Am. Chem. Soc. 2002 in press. /b) Galoppini,
E.; Guo, W.; Qu, P.; Meyer, G. J. J. Am. Chem. Soc. 2001, 123,
4342. /c) Guo, W.; Galoppini, E.; Rydja, G. I.; Pardi, G.
Tetrahedron Lett. 2000, 41, 7419.
Acknowledgements
E. G. and D. W. thank Professor Piotr Piotrowiak for help
with the spectroscopic measurements and for insightful
discussions. The Division of Chemical Sciences, Of®ce of
Basic Energy Sciences, Of®ce of Science, US Department
of Energy and the Petroleum Research Fund /ACS-PRF
35081-G5) are gratefully acknowledged for research support.
12. This is de®ned as the distance from the Ru center to the foot-
print, i.e. the plane de®ned by the three surface-bound oxygen
atoms.
13. For conjugated linkers used to attach redox-active molecules
on gold see for instance /a) Sikes, H. D.; Smalley, J. F.;
Dudek, S. P.; Cook, A. C.; Newton, M. D.; Chidsey, C. E. D.;
Feldberg, S. W. Science 2001, 291, 1519. /b) Davis, W. B.;
Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature 1998,
396, 60.
References
1. /a) Molecular Electronics; Jortner, J., Ratner, M., Eds.; Black-
well: London, 1997. /b) Meyer, G. J. Molecular Level
Arti®cial Photosynthetic Materials; Progress in Inorganic
Chemistry; Wiley: New York, 1997.
14. /a) Coupling Reactions Between sp Carbon Centers. Compre-
hensive Organic Synthesis; Sonogashira, K., Ed.; Pergamon:
Oxford, 1991; Vol. 3, pp 551±561, Chapter 25. /b) Metal-
Catalyzed Cross-Coupling Reactions; Stang, P. J., Diederich,
F., Eds.; Wiley-VCH: Weinheim, 1998. /c) Modern Acetylene
Chemistry; Stang, P. J., Diederich, F., Eds.; Wiley-VCH:
Weinheim, 1995. /d) Bunz, U. H. F. Synlett 1997, 1117.
15. Sonogashira, K. Comprehensive Organic Synthesis; Trost,
B. M., Ed.; Pergamon Press: Oxford, 1991; Vol. 3, p 551.
16. /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. /c) Brown, H. C.; Bhat, N. G.;
Srebnik, M. Tetrahedron Lett. 1988, 29, 2631.
2. Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226.
È
3. /a) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49.
/b) Kamat, P. V. Chem. Rev. 1993, 93, 267. /c) O'Regan,
È
È
B.; Gratzel, M. Nature 1991, 353, 737. /d) Gratzel, M. Hetero-
geneous Photochemical Electron Transfer; CRC: Boca Raton,
Florida, 1989. /e) Gerischer, H. Photochem. Photobiol. 1972,
16, 243.
4. Abbreviations used in this paper: dcbˆ4,40-/COOH)2-2,20-
bipyridine; bpyˆ2,20-bipyridine; phenˆ1,10-phenanthroline.
5. Depending on the surface treatment and the binding con-
ditions, the carboxylic groups bind covalently to the surface
by reacting with surface OH groups and forming ester bonds,
or can form carboxylates that interact with the surface in an
unknown manner. The binding mode is usually determined
by IR /CvO stretching). The ester linkages provide strong
electronic coupling between the complex and the semicon-
ductor and are thought to cause the ultrafast /femtosecond)
electron injection rate constants that have been measured.
6. Meyer, G. J. J. Chem. Ed. 1997, 74, 652.
17. /a) Ramakrishna, S.; Willig, F. J. Phys. Chem. B 2000, 104,
68. /b) Martini, I.; Hodak, J. H.; Hartland, G. V. J. Phys.
Chem. B 1998, 102, 9508. /c) Martini, I.; Hodak, J. H.;
Hartland, G. V.; Kamat, P. V. J. Chem. Phys. 1997, 107,
8064. /d) Hannappel, T.; Burfeindt, B.; Storck, W.; Willig,
F. J. Phys. Chem. B 1997, 101, 6799. /e) Ramakrishna, G.;
Ghosh, H. N. J. Phys. Chem. B 2001, 105, 7000.
18. The data for pyrene and antracene in Table 1 were determined
experimentally and are close to values reported in literature:
Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of
Photochemistry; Marcel Dekker: New York, 1993.
7. /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.
19. Vlcek, A. A.; Dodsworth, E. S.; Pietro, W. J.; Lever, A. B. P.
Inorganic Chemistry 1995, 34, 1906.
8. /a) Piotrowiak, P. Chem. Soc. Rev. 1999, 28, 143and
references therein. /b) Closs, G. L.; Johnson, M. D.; Miller,
J. R.; Piotrowiak, P. J. Am. Chem. Soc. 1989, 111, 3751.
/c) Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Pen®eld,
K. W.; Miller, J. R. J. Phys. Chem. 1986, 90, 3673.
20. Bugnon, P.; Hester, R. E. Chem. Phys. Lett. 1983, 102, 537.
21. Electrochemical Methods; Bard, A. J., Faulkner,
L. R., Eds.; Wiley: New York, 2001; p 811.