Tripodal pyrene chromophores for semiconductor sensitization: new footprint design

Article abstract of DOI:10.1016/j.tet.2007.05.055

Large footprint tripodal linkers for metal oxide nanoparticle sensitization, substituted with pyrene as the dye, and three COOR binding groups in para or meta position, were prepared to study the effect of the anchoring group position and of the footprint size on the sensitization processes. Two tripods based on tetraphenylsilane, (1-pyrenyl-4-ethynyl-phenyl)-tris(4-carbomethoxyphenyl-4-ethynyl-phenyl)silane, and (1-pyrenyl-4-ethynyl-phenyl)-tris(4-(4-methoxybenzyloxycarbonyl)phenyl-4-ethynyl-phenyl)silane, decomposed during hydrolysis, while the tetraphenyladamantane derivative, 1-(1-pyrenyl-4-ethynyl-phenyl)-3,5,7-(3-carbomethoxyphenyl-4-ethynyl-phenyl)adamantane, was chemically stable and was readily converted into the corresponding acid and bound to TiO₂ films. The FT-IR-ATR spectrum of 1-(1-pyrenyl-p-ethynyl-phenyl)-3,5,7-(3-carboxyphenyl-4-ethynyl-phenyl)adamantane bound to TiO₂ nanoparticles showed bands characteristic of carboxylate bidentate bonds. The UV-vis absorption and fluorescence emission in THF solution at room temperature were typical of pyrenes substituted with oligophenyleneethynylene linkers.

Full text of DOI:10.1016/j.tet.2007.05.055

Tetrahedron 63 (2007) 7550–7559
Tripodal pyrene chromophores for semiconductor sensitization:
new footprint design
Sujatha Thyagarajan, Aiping Liu, Olumide A. Famoyin, Massimiliano Lambertoy
*
and Elena Galoppini
Department of Chemistry, Rutgers University, 73 Warren Street, Newark, NJ 07102, USA
Received 26 March 2007; revised 11 May 2007; accepted 14 May 2007
Available online 18 May 2007
Abstract—Large footprint tripodal linkers for metal oxide nanoparticle sensitization, substituted with pyrene as the dye, and three COOR
binding groups in para or meta position, were prepared to study the effect of the anchoring group position and of the footprint size on the
sensitization processes. Two tripods based on tetraphenylsilane, (1-pyrenyl-4-ethynyl-phenyl)-tris(4-carbomethoxyphenyl-4-ethynyl-
phenyl)silane, and (1-pyrenyl-4-ethynyl-phenyl)-tris(4-(4-methoxybenzyloxycarbonyl)phenyl-4-ethynyl-phenyl)silane, decomposed during
hydrolysis, while the tetraphenyladamantane derivative, 1-(1-pyrenyl-4-ethynyl-phenyl)-3,5,7-(3-carbomethoxyphenyl-4-ethynyl-phenyl)-
adamantane, was chemically stable and was readily converted into the corresponding acid and bound to TiO2 films. The FT-IR-ATR spectrum
of 1-(1-pyrenyl-p-ethynyl-phenyl)-3,5,7-(3-carboxyphenyl-4-ethynyl-phenyl)adamantane bound to TiO2 nanoparticles showed bands charac-
teristic of carboxylate bidentate bonds. The UV–vis absorption and fluorescence emission in THF solution at room temperature were typical of
pyrenes substituted with oligophenyleneethynylene linkers.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
used by us4 and by others5 to bind Ru–bpy complexes and
other groups to the surface of semiconductor nanoparticles.
In this paper we describe a methodology that allows varying
the type and position of the anchoring groups on the tripodal
linker as well as the footprint area (Fig. 1).
TiO2 nanoparticle thin films with anchored chromophores or
redox active group are used in dye-sensitized solar cells and
other devices.1 Linkers that connect a chromophore to the
semiconductor are useful to control the distance and orienta-
tion with respect to TiO2 nanoparticle surfaces for interfacial
electron transfer studies.2,3 In particular, tripodal linkers
with three anchoring groups (COOH, PO3H) have been
In the previously reported procedure, the starting material,
tetrakis-1,3,5,7-(p-iodophenyl)adamantane (1), was con-
verted into linker 2, with fixed footprint size and having
chromophore
HOOC
HOOC
HOOC
HOOC
HO
P
HO
HO
HO
A
A
P
O
O
TiO2
A
Figure 1. Schematic representation of the strategy: footprint size and possible anchoring group modifications.
Keywords: Tripods; Pyrene; Semiconductor nanoparticles; Sensitization; Titanium dioxide.
* Corresponding author. Tel.: +1 973 353 5317; fax: +1 973 353 1264; e-mail: galoppin@rutgers.edu
Present address: Department of Chemistry, Medical Technology and Physics, Monmouth University, West Long Branch, NJ 07764, USA.
y
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.05.055

S. Thyagarajan et al. / Tetrahedron 63 (2007) 7550–7559
7551
R
I
I
I
HOOC
COOH
I
2
1
HOOC
R =chromophore
Scheme 1. Multistep synthesis of tripodal sensitizers.6,7
the COOH groups on the phenyl rings in para position
(Scheme 1).6,7
COOH anchoring groups in meta position (rather than in
para, as in 2, Scheme 1) may enhance the binding of the mol-
ecule to TiO2 anatase (001) by all three legs of the tripods,
because of the more favorable angle for surface anchoring.8
The synthesis of varying tripodal anchoring units will com-
plement ongoing computational studies aiming at matching
anchoring groups with various semiconductor surfaces.8 A
comparison between footprint sizes for various tripodal
linkers is shown in Figure 2.
The possibility to modify the anchoring unit, however, is im-
portant to achieve strong binding from all three anchoring
groups on a variety of surfaces. The binding process influ-
ences most applications, including interfacial charge trans-
fer and excimer effect studies. Also, the use of different
semiconductors (TiO2, ZnO), crystal structures (anatase,
wurtzite), and morphologies (nanoparticles, nanowires) re-
quires a more versatile approach to vary the type and posi-
tion of the anchoring groups.1a For instance, preliminary
theoretical calculations indicate that, in a tripodal linker,
Secondly, tripods with larger footprints can find use to study
dye–dye interactions on the surface of semiconductors, a
phenomenon that influences numerous interfacial processes.
HOOC
COOH
74 Å2
HOOC
HOOC
197 Å2
COOH
COOH
Si
HOOC
COOH
HOOC
217 Å2
Figure 2. Comparison between the footprint size of various tripodal linkers.

7552
S. Thyagarajan et al. / Tetrahedron 63 (2007) 7550–7559
employed to prevent it include the use of lipophilic coadsor-
bates,19,21 substitution with bulky group,12a,12d,22 and encap-
sulation of the dye in a guest.23–25 There are examples,
however, in which aggregation results in good efficiencies.26
For instance, our studies of closely stacked pyrene-
substituted rigid rods suggest that pyrene excimer could be
a very effective sensitizer.27 The spacing of dyes on the
surface of semiconductors by using large tripods could be
useful to study excimer and monomer effects as shown in
Chart 1.
A
A
A
A A
A
A
A
A
A
TiO2
TiO2
Chart 1.
Finally, the footprint of the adamantane tripod 2 isw0.7 nm2.
Extension of each ‘leg’ by a phenylethynyl unit, for instance,
results in a fourfold increase of the footprint area (Fig. 2).
Model dyes having larger footprint area but with similar dis-
tance of the Ru-chromophore from the semiconductor will
allow to study whether the binding area influences the rates
of injection or recombination.
The study of aggregation effects is important, because of the
increasing interest in organic dyes that possess useful prop-
erties, such as porphyrins,9 coumarins,10,11 perylenes,12 and
other organic chromophores.13–16 Through structural modi-
fications it is possible to tune the position of the excited state
energy level relative to the conduction band edge of the
semiconductor, towiden the range of spectral absorption, and
to increase the extinction coefficient. Also, organic dyes are
excellent model systems for fundamental studies of charge
injection and recombination processes at the molecule–
nanoparticle interface.4b,17
2. Results and discussion
2.1. Synthesis
Organic dyes containing p-conjugated systems tend to stack
through p–p interactions on the surface of the semiconduc-
tor,18 resulting in shifts in the absorption spectra and forma-
tion of excimers. Aggregation on TiO2 influences most
applications, and usually decreases the performance of solar
cells through a variety of mechanisms.19,20 Strategies
In this paper we describe the synthesis of silicon- and ada-
mantane-centered tripods with larger footprints and variable
position of the anchoring groups (Fig. 2). Pyrene was
selected as the chromophore to make comparisons with the
reported pyrene-substituted rigid rods.27 Silicon-centered
tripods 7a and 7b, in Scheme 2, were prepared by modifying
SiMe3
SiMe3
Me3Si
Li
H
COOR
4a,b
(EtO)4Si
i, ii
iii
Si
Si
I
I
ROOC
3
I
COOR
5a,b
iv
6a,b
I
H
ROOC
8
v
vi
R = COOH
Si
a: R =
b: R =
CH3
CH2
ROOC
OMe (PMB)
COOR
7a,b
ROOC
Scheme 2. Reagents and conditions: (i) 1,4-diiodobenzene, n-BuLi, Et2O, ꢀ78 ꢁC, 44%; (ii) Et2O, ꢀ78 ꢁC, 33%; (iii) Pd(dba)2, PPh3, CuI, (i-Pr)2NEt, THF,
rt, 5a 69%, 5b 75%; (iv) TBAF, AcOH, THF, rt, 6a 90%, 6b 52%; (v) Sonogashira: Pd(dba)2, PPh3, CuI, (i-Pr)2NEt, THF, rt, 7a 66%, 7b 76%. Suzuki: (a)
(Me3Si)2NLi, THF, ꢀ78 ꢁC, (b) B-MeO-9-BBN, ꢀ78 ꢁC, (c) 1-iodopyrene, Pd(PPh3)4, THF, rt, 7a 41%; (vi) see text for details.

S. Thyagarajan et al. / Tetrahedron 63 (2007) 7550–7559
7553
the procedure developed by Tour and Yao for the synthesis of
‘Si-caltrops’.28 This route permits to extend the footprint of
‘Si-caltrop’ 328 by adding the anchoring unit (in blue) in
a Pd-catalyzed29 cross-coupling step, in this case a methyl-
or 4-methoxy-benzyl-4-ethynyl-benzoate (4a and 4b). The
PMB ester 4b was prepared by esterification of 4-trimethyl-
silylethynyl-benzoic acid30 with 4-methoxybenzyl alcohol
followed by deprotection with TBAF. The chromophore
was introduced via a Sonogashira31 coupling reaction of
terminal alkyne 6a with 1-iodopyrene at room temperature
to form 7a in 66% yields. A Suzuki-type32 cross-coupling
between iodopyrene and ethynylboronate from 6a gave
lower yields. The last step involved the hydrolysis of the
methyl ester 7a, since carboxylic acids are needed to form
strong bonds to the metal oxide semiconductor surface.33
Bases such as NaOH could not be used, since the silicon
center is readily attacked by nucleophiles. The use of weak
bases,34 hard acid-soft nucleophile systems,35 or mild condi-
tions involving Me3SiCl36 resulted in partial fragmentation
of 7a. We then prepared ester 7b, since the COOPMB group
is readily hydrolyzed by trifluoroacetic acid, and the end of
the hydrolysis is conveniently indicated by a color change.37
Decomposition of 7b, however, occurred within minutes,
most likely by the same mechanism described for the de-
composition of tetraphenylsilanes by acids.38 In conclusion,
esters 7a and 7b had limited chemical stability, could not be
converted into carboxylic acid 8 in satisfactory amounts for
the binding study, and attempts to directly bind the esters to
metal oxide surfaces did not succeed.
Although there are numerous protecting groups and method-
ologies for ester hydrolysis, the instability of Si-tripods on
the surface of the semiconductor to acids, bases, or electro-
lytes would be a major limitation to the photoelectrochemi-
cal studies of sensitized films alone or in working solar cells.
This convinced us to modify the adamantane tripods, be-
cause of their proven stability. The first synthesis of such
compounds, 15, is shown in Scheme 3.
COOMe
I
MeOOC
H
COOMe
9
I
1
i
11
I
MeOOC
COOMe
12
COOMe
COOMe
COOMe
MeOOC
10
COOMe
H
13
COOR
10
ii
ROOC
14: R = Me
15: R = H
iii
COOR
Scheme 3. Reagents and conditions: (i) Pd(PPh3)2Cl2, CuI, (i-Pr)2NEt, THF, rt, 10 22%, 11 28%, 12 7%; (ii) Pd2(dba)3, (o-tolyl)3P, Et3N, THF, 60 ꢁC, 38%;
(iii) NaOH aq, THF, rt, 83%.

7554
S. Thyagarajan et al. / Tetrahedron 63 (2007) 7550–7559
The starting material, tetrakis-1,3,5,7-(p-iodophenyl)ada-
mantane 1, was prepared in w50% yields and multigram
amounts from 1-bromoadamantane, following a two-step
procedure published by Mathias and Reichert.7 Sonogashira
cross-coupling of 1 with m-ethynyl-benzoate 9,39 produced
a mixture of the product, 10, together with the di-ester
11 (28%), tetra-ester 12 (7%), and a small amount of un-
reacted 1. Compound 10 was isolated in 22% yield after re-
peated silica gel chromatography columns with hexanes/
ethylacetate. Cross-coupling of 10 with ethynylpyrene 13
yielded methyl ester 14 in 38% yield. The overall yield of
14 from 1 was w8%. This methyl ester was readily hydro-
lyzed with base to the corresponding acid 15 as a pale yellow
solid.
compared to pyrene (3372 w200 Mꢀ1 cmꢀ1).27a The bands
at 325 nm were assigned to the p,p* transitions of the
p-phenyleneethynylene units. The UV–vis absorption spec-
trum of 15 bound to TiO2 is shown in Figure 3b. Only the
lowest energy vibronic band is observable for 15/TiO2 as
bands below 380 nm are covered by the absorption of the
semiconductor.
The fluorescence emission spectra of 7a and 14 in THF so-
lutions are shown in Figure 4a. In both cases the emission
lmax are red-shifted compared with those of pyrene.41 Fur-
thermore, 7a and 14 exhibited almost quantitative fluores-
cence quantum yields compared to pyrene (FFl w0.9).
Similar effects, shorter excited state lifetimes (t w2 ns)
and quantum yields close to unity, have been observed in
1-phenylethynyl-pyrenes27a,40 and were attributed to a
switching in the order of the two lowest singlet-lying states
of pyrene upon substitution with phenyleneethynylene
groups.27a The quenching of fluorescence of 15 on anatase
TiO2 films (Fig. 4b) indicates efficient electron injection
into the semiconductor. In conclusion, the solution photo-
physical properties of 14 are similar to those observed for
2.2. Solution properties and binding
The UV–vis absorption spectra of 7a and 14 in THF solu-
tions, Figure 3a, displayed a pattern similar to that observed
in other phenylethynylpyrenes,40 with absorption spectra
red-shifted with respect to the spectrum of pyrene.41
The extended p-conjugation in 7a and 14 resulted in a 20-
nm red shift of the longer wavelength band and higher
extinction coefficients (3393¼36,567 Mꢀ1 cmꢀ1
) when
(a)
500
Si
400
MeOOC
(a)
700
1.4x105
COOMe
COOMe
600
500
400
300
200
100
0
MeOOC
MeOOC
1.2x105
1.0x105
8.0x104
6.0x104
4.0x104
300
COOMe
325 350 375 400 425 450
Wavelength, nm
200
COOMe
Si
MeOOC
MeOOC
MeOOC
COOMe
100
0
COOMe
2.0x104
300
350
400
450
500
550
600
650
0.0
250
Wavelength, nm
300
350
400
450
500
Wavelength, nm
(b)
500
400
300
200
100
0
(b)
2.5
2.0
1.5
1.0
0.5
0.0
15/ TiO2
COOH
HOOC
COOH
TiO2
TiO2
300
350
400
450
500
550
600
400
450
500
550
600
Wavelength, nm
Wavelength (nm)
Figure 3. (a) Absorption spectra of 7a (solid line) and 14 (dotted line) in
THF. Inset: absorption spectrum of pyrene in THF. (b) Absorption spectrum
of 15 bound to TiO2/glass (dotted line) and the reference TiO2/glass (solid
line).
Figure 4. (a) Normalized fluorescence emission spectra of 7a (solid line), 14
(dotted line), and pyrene (dashed line) in solution. In all the cases, the
solvent was THF. lex¼360 nm (for 7a and 14) and 335 nm (for pyrene).
(b) Fluorescence quenching of 15 bound to TiO2. lex¼380 nm.

S. Thyagarajan et al. / Tetrahedron 63 (2007) 7550–7559
7555
the rigid rod pyrene derivatives27a that aggregate on the sur-
3. Conclusions
face of nanoparticles. Comparisons of aggregation effects
and solar cell efficiency between such rigid rod compounds
and 15 bound to metal oxide surfaces are in progress.
Silicon- and adamantane-centered tripodal pyrene sensi-
tizers (7a, 7b, and 14) were synthesized and characterized.
By adding the anchoring unit in a cross-coupling step we
were able to vary the position of the functional groups, as
well as increase the footprint size. The tripods based on tet-
raphenylsilane, 7a, and 7b, decomposed during hydrolysis,
while the tetraphenyladamantane derivative was chemically
stable and was readily converted into the corresponding acid,
15, and bound to TiO2 films. The compounds exhibited spec-
tral properties typical of phenyleneethynylene-substituted
pyrenes. The quenching of fluorescence of 15 on anatase
TiO2 films indicates efficient electron injection into the
semiconductor. Pyrene tripods, with their characteristic
excimer and exciplex emission spectra are good models for
photophysical and electron transfer studies at nanoparticle
interfaces. Photophysical studies of excimer and exciplex
effects, solar cells studies, and a comparison with previously
reported pyrene rigid rods27 are in progress.
The binding was done by immersing the films in 0.5 mM so-
lutions of 15 in THF. Base (pH 11), acid (pH 1) pretreated
TiO2 films (see Section 4)were employed, as well as non-pre-
treated films, as we observed that pretreatment of the film
does influence the coverage.4,27 In this case, the highest cov-
erage was obtained when the films were pretreated with base
and the data of 15/TiO2 reported here were obtained on such
films. The fluorescence emission of 15/TiO2 was fully
quenched, indicating that binding results in electron injection
in the semiconductor. The binding mode of 15 to TiO2
through the COOH groups was characterized by FT-IR-
ATR. Figure 5 shows the FT-IR-ATR spectra of 15 neat (as
powder) and bound to TiO2. The free carboxylic acid 15
shows an intense band at 1697 cmꢀ1 characteristic of the car-
bonyl asymmetric n(C]O) stretch in carboxylic acids, and the
n(C–OH) stretch at 1257 cmꢀ1. Upon binding to TiO2, the
n(C]O) and n(C–OH) stretching modes disappear with the ap-
pearance of broad bands atw1555 cmꢀ1, in the carboxylate
(COꢀ2 ) region. The observed spectral changes are consistent
with carboxylate derivatives bound to TiO2 surfaces through
bidentate binding modes as in Figure 5.27a,42,43 The disap-
pearance of the C]O band suggests that 15 binds to TiO2
through carboxylate bonds, rather than ester-type bonds
and that no detectable free acid is present.
4. Experimental
4.1. Metal oxide thin film preparation
Colloidal anatase TiO2 films were prepared by a previously
described sol–gel technique44 that produces mesoporous
films of approximately 10 mm thickness, and that consist
of nanoparticles with an average diameter of w20 nm.
Briefly, the films were prepared by casting the colloidal so-
lutions by the doctor blade technique onto cover glass slides
(VWR Scientific) over an area of 1ꢂ2 cm2, followed by sin-
tering atw450 ꢁC for 30 min. Binding of 15. Basic or acidic
pretreatment of the TiO2 films was tested, as well as non-
pretreated films, but higher coverages were obtained when
the films were pretreated by immersion in a pH 11 aqueous
NaOH solution for 5 min, heated at 150 ꢁC for 30 min, and
cooled to 80 ꢁC prior to immersion in the dye solution.
The sensitization of the films was accomplished by over-
night (w10–12 h) immersion in 0.5 mM THF solutions of
15 at room temperature. The sensitized films were thor-
oughly rinsed and immersed into pure THF until desorption
of weakly bound sensitizer molecules was no longer de-
tected by UV–vis.
(a)
0.015
C
C
15 / TiO2
O
Ti
O
Ti
O
O
Ti
0.010
0.005
0.000
0.02
(b)
4.2. Spectroscopy
C
Fourier-transform infrared attenuated total reflectance (FT-
IR-ATR) measurements were collected on a Thermo Elec-
tron Corporation’s Nicolet 6700 FT-IR spectrometer using
ZnSe crystal. Solution UV–visible absorption measurements
were acquired on a VARIAN Cary-500 spectrophotometer,
in THF (Acros, spectroscopic grade). 15/TiO2 films were
placed diagonally in a 1 cm2 cuvette in air while recording
the spectra. Fluorescence emission spectra were obtained
with a VARIAN Cary-Eclipse fluorescence spectrophoto-
meter using THF as solvent for 7a, and 14 in a four-way trans-
parent 1 cm2 cuvette (lex¼360 nm). Fluorescence quantum
yields for the samples (FFl) were performed in THF solu-
tions deaerated by freeze-pump-thaw, using the optically
dilute technique with pyrene as the reference (FPy¼0.72
in acetonitrile)45, lex¼360 nm, using Eq. 1.
OH
O
COOH
HOOC
15
COOH
0.01
0.00
1500
2000
2500
3000
3500
Wavenumber (cm-1)
Figure 5. FT-IR-ATR spectra of (a) 15 bound to TiO2 (pH 11 pretreated) and
(b) 15 in the solid state. The broad band observed at w3000 cmꢀ1 for the
TiO2 bound 15 is assigned to absorbed moisture on the film.

7556
S. Thyagarajan et al. / Tetrahedron 63 (2007) 7550–7559
À
ÁÀ
ÁÀ
Á
2
(2H, d, J¼8.5 Hz), 7.54 (2H, d, J¼8.5 Hz), 7.39 (2H, d,
FFl ¼ APy=As Is=IPy hs=hPy FPy
ð1Þ
J¼8.5 Hz), 6.93 (2H, d, J¼9.0 Hz), 5.31 (2H, s), 3.83 (3H,
s), 3.23 (1H, s). 13C NMR (CDCl3) d 165.8, 159.7, 132.0,
130.3, 130.2, 129.5, 127.9, 126.7, 114.0, 82.8, 80.0, 66.8,
55.3. IR (cmꢀ1): 3271, 2959, 2157, 1695, 1455, 1314.
LRMS (FAB) calcd for C17H14O3 (M+): 266.09, found
266.20.
where ‘s’ refers to the samples, ‘Py’ refers to reference pyr-
ene, A is the absorbance at the excitation wavelength, I is the
integrated emission area, and h is the solvent refraction
index.
4.3. Synthesis
4.3.5. (4-Trimethylsilylethynyl-phenyl)-tris(4-carbo-
methoxyphenyl-4-ethynyl-phenyl)silane 5a. A flask was
charged with 3 (0.39 g, 0.48 mmol), 4a (0.48 g, 3 mmol),
Pd(dba)2 (0.040 g, 0.07 mmol), PPh3 (0.074 g, 0.28 mmol),
CuI (0.027 g, 0.14 mmol), (i-Pr)2NEt (3 mL), and THF
(7 mL). The reaction mixture was stirred for 3 days at
room temperature under nitrogen. Then the reaction mixture
was poured into distilled water and filtered to remove the
solid. After standard workup with ether (3ꢂ15 mL) the
crude product was purified by silica gel column chromato-
graphy with hexanes/ethyl acetate (88/12) to afford 0.3 g
of 5a as a white solid (69%). Rf5a¼0.68. Mp 176–178 ꢁC.
1H NMR (CDCl3) d 8.01 (6H, d, J¼8.5 Hz), 7.58 (6H, d,
J¼8.5 Hz), 7.55 (6H, d, J¼8.5 Hz), 7.51 (6H, d, J¼
7.5 Hz), 7.27 (4H, m), 3.91 (9H, s), 0.24 (9H, s). 13C
NMR (CDCl3) d 166.5, 136.2, 136.0, 134.0, 133.4, 131.6,
131.4, 131.1, 129.6, 129.5, 127.7, 124.9, 124.4, 104.6,
96.0, 92.0, 90.0, 52.3, ꢀ0.1. IR (cmꢀ1): 2957, 2157, 1721,
1435, 1144. HRMS (FAB) calcd for C59H46O6Si2 (MH+):
907.2912, found 907.2916.
4.3.1. General. NMR spectra were obtained on a Varian
INOVA 500 spectrometer operating at 499.896 MHz for
1H, 125.711 MHz for 13C and collected at ambient probe
temperature using CDCl3 as solvent unless otherwise men-
tioned. The chemical shifts d are reported in parts per
million relative to the residual CHCl3 peak at 7.27 ppm.
Coupling constants (J) are reported in Hertz with a precision
of ꢃ0.1 Hz. The 1H spectra were referenced to tetramethyl-
silane and the 13C spectra are referenced to the central line of
the solvent. Melting points were measured with a Fisher
melting point apparatus. High- or low-resolution mass
spectra (HRMS-FAB or LRMS-FAB) were obtained at
a commercial facility (Michigan State University Mass
Spectrometry Facility). GC/MS data were obtained on
a HP 6890 gas chromatograph with a HP 5973 MS detector.
Major ions are recorded to unit mass with the percentage of
the strongest peak given in parenthesis.
4.3.2. Materials. Tetrahydrofuran (THF) was purchased
anhydrous (Acros) and then distilled under nitrogen from
sodium benzophenone ketyl. Et3N and (i-Pr)2NEt were
distilled from KOH under nitrogen prior to use. Bulk grade
hexanewas distilled prior to use for column chromatography.
Ethyl acetate was HPLC grade and used without further
purification. Flash chromatography was carried out using
230–600 mesh silica gel (Sorbent Technologies). Thin-layer
chromatography was performed using aluminum backed sil-
ica gel TLC plates (Sorbent Technologies) and visualization
with UV light. The following compounds were prepared
accordingtothepublished literature procedures:1,3,5,7-tetrakis
(4-iodophenyl)adamantane 1,7 tris(4-iodophenyl)-(40-(tri-
methylsilylethynyl)phenyl)-silane 3,28 methyl 4-ethynyl-
benzoate 4a,39 1-iodopyrene,46 methyl 3-ethynylbenzoate
9,39 and 1-ethynylpyrene 13.47
4.3.6. (4-Trimethylsilylethynyl-phenyl)-tris(4-(p-meth-
oxybenzyloxycarbonyl)phenyl-4-ethynyl-phenyl)silane
5b. A flask was charged with 3 (0.078 g, 0.096 mmol), 4b
(0.1 g, 0.38 mmol), Pd(dba)2 (0.01 g, 0.017 mmol), PPh3
(0.021 g, 0.081 mmol), CuI (7 mg, 0.036 mmol), (i-Pr)2NEt
(5 mL), and THF (10 mL). The reaction mixture was stirred
for 2 days at room temperature under nitrogen, then poured
into distilled water and filtered. After standard workup of the
filtrate with ether (3ꢂ10 mL) the crude residue was purified
by silica gel column chromatography with hexanes/ethyl
acetate (80/20) to give 0.088 g of 5b as a brown sticky solid
(75%). Rf5b¼0.33. Mp 66–68 ꢁC. 1H NMR (CDCl3) d 8.05
(6H, d, J¼9.0 Hz), 7.60–7.49 (22H, m), 7.41 (6H, d,
J¼9.0 Hz), 6.94 (6H, d, J¼9.0 Hz), 5.32 (6H, s), 3.83 (9H,
s), 0.27 (9H, s). 13C NMR (CDCl3) d 165.9, 159.7, 136.2,
136.0, 134.0, 131.6, 131.4, 131.2, 131.1, 129.8, 129.6,
127.9, 127.8, 127.7, 124.9, 124.4, 114.0, 104.7, 96.0, 92.0,
90.0, 66.8, 55.3, ꢀ0.1. IR (cmꢀ1): 2955, 2156, 1715, 1305,
1144. HRMS (FAB) calcd for C80H64O9Si2 (M+):
1224.4090, found 1224.4096.
4.3.3. Methods. All air and moisture sensitive reactions
were carried out under nitrogen or argon atmosphere, in
glassware that had been oven-dried and flamed under vac-
uum, and using anhydrous solvents. Standard workup proce-
dures involved extractions with the indicated organic
solvent, washing the combined extracts with brine, drying
over Na2SO4, and removal of solvent in vacuo on a rotary
evaporator.
4.3.7. (4-Ethynyl-phenyl)-tris(4-carbomethoxyphenyl-4-
ethynyl-phenyl)silane 6a. A solution of 5a (0.233 g,
0.257 mmol) in THF (10 mL) was stirred at room tempera-
ture. A mixture of TBAF (1.33 mL, 1.33 mmol, 1.0 M in
THF) and acetic acid (0.075 mL, 1.33 mmol, 17.4 mol/L)
was added dropwise to the solution. The solution was stirred
for 45 min after the addition was completed. The reaction
mixture was poured into water and after usual workup
with ether (4ꢂ15 mL), the crude was purified by silica gel
column chromatography with hexanes/ethyl acetate (70/
30) to afford 0.181 g of 6a as a white solid (90%).
Rf6a¼0.59. Mp 140–143 ꢁC. 1H NMR (CDCl3) d 8.01 (6H,
d, J¼8.0 Hz), 7.58 (6H, d, J¼8.5 Hz), 7.55 (6H, d,
4.3.4. p-Methoxybenzyl-4-ethynylbenzoate 4b. To 4-
trimethylsilylethynyl-benzonate-4-methoxy-benzylester30
(0.039 g, 0.12 mmol) in THF (3 mL) was added TBAF
(0.18 mL, 0.18 mmol, 1.0 M in THF). The resulting brown
solution was stirred for 1 h at room temperature and then
poured into water. After standard workup with ether
(3ꢂ5 mL), column chromatography with hexanes/ethyl ace-
tate (90/10) afforded 0.026 g of 4b as a colorless solid
1
(86%). Rf4b¼0.41. Mp 66–69 ꢁC. H NMR (CDCl3) d 8.02

S. Thyagarajan et al. / Tetrahedron 63 (2007) 7550–7559
7557
J¼7.5 Hz), 7.53–7.48 (10H, m), 3.91 (9H, s), 3.14 (1H, s).
13C NMR (CDCl3) d 166.5, 136.2, 136.1, 134.0, 133.9,
131.7, 131.6, 131.2, 129.7, 129.5, 127.7, 124.4, 123.9,
92.0, 90.0, 83.3, 78.6, 52.3. IR (cmꢀ1): 2949, 2144, 1721,
1489, 1191. HRMS (FAB) calcd for C56H38O6Si2 (M+):
834.2438, found 834.2437.
chromatography with hexanes/ethyl acetate (80/20) gave
0.14 g of 7a as a pale yellow solid (41%).
4.3.10. (1-Pyrenyl-4-ethynyl-phenyl)-tris(4-(p-methoxy-
benzyloxycarbonyl)phenyl-4-ethynyl-phenyl)silane 7b.
A flask was charged with 6b (0.030 g, 0.026 mmol), 1-iodo-
pyrene
(0.128 g,
0.039 mmol),
Pd(dba)2
(1 mg,
4.3.8. (4-Ethynyl-phenyl)-tris(4-(p-methoxybenzyloxy-
carbonyl)phenyl-4-ethynyl-phenyl)silane 6b. A mixture
of TBAF (0.36 mL, 0.36 mmol, 1.0 M in THF) and acetic
acid (21 mL, 0.36 mmol) was added to a solution of 5b
(0.088 g, 0.072 mmol) in THF (2 mL) under nitrogen, stirred
for 1 h, and then poured into water. The mixture was washed
with water (2ꢂ5 mL). After usual workup with dichlorome-
thane (3ꢂ5 mL), silica gel column chromatography of the
solid crude with hexanes/ethyl acetate (80/20) afforded
0.0428 g of 6b as a yellow sticky solid (52%). Rf6b¼0.19.
Mp 72–76 ꢁC. 1H NMR (CDCl3) d 8.05 (6H, d, J¼8.0 Hz),
7.60–7.53 (22H, m), 7.41 (6H, d, J¼8.5 Hz), 6.94 (6H, d,
J¼9.0 Hz), 5.32 (6H, s), 3.83 (9H, s), 3.17 (1H, s). 13C
NMR (CDCl3) d 165.9, 159.7, 136.2, 134.8, 133.9, 131.6,
131.5, 131.2, 131.1, 130.2, 129.8, 129.6, 127.9, 127.7,
124.4, 123.9, 114.0, 92.0, 90.1, 83.3, 78.6, 66.8, 55.3. IR
(cmꢀ1): 2925, 2156, 1714, 1305, 1143. HRMS (FAB) calcd
for C77H56O9Si (MH+): 1153.3730, found 1153.3767.
0.002 mmol), PPh3 (2 mg, 0.007 mmol), CuI (0.7 mg,
0.004 mmol), (i-Pr)2NEt (10 mL), and THF (5 mL). The re-
action mixturewas stirred at room temperature for 24 h under
nitrogen, and then pouredintowater. After usual workup with
dichloromethane (2ꢂ5 mL), purification by silica gel col-
umn chromatography of the crude residue with hexanes/ethyl
acetate (70/30) gave 0.028 g of 7b as a brown sticky solid
(76%). Rf 7b¼0.59. Mp 66–69 ꢁC. 1H NMR (CDCl3) d 8.68
(1H, d, J¼8.5 Hz), 8.26–8.20 (4H, m), 8.17–8.12 (2H, m),
8.09–8.05 (9H, m), 7.77 (2H, d, J¼8.0 Hz), 7.64–7.59
(19H, m), 7.41 (6H, d, J¼8.5 Hz), 6.94 (6H, d, J¼8.5 Hz),
5.32 (6H, s), 3.83 (9H, s). 13C NMR (CDCl3) d 166.2,
159.9, 138.0, 136.6, 136.5, 136.2, 134.3, 133.5, 131.8,
131.7, 131.5, 131.4, 131.3, 131.2, 130.4, 130.0, 129.9,
129.2, 128.7, 128.6, 128.2, 128.0, 127.5, 126.8, 126.5,
126.0, 125.7, 125.6, 124.9, 124.8, 124.7, 124.5, 117.7,
95.0, 92.3, 90.4, 90.3, 67.0, 55.5. IR (cmꢀ1): 2955, 2156,
1714, 1514, 1246. HRMS (FAB) calcd for C93H64O9Si
(M+): 1352.4320, found 1352.4328.
4.3.9. (1-Pyrenyl-4-ethynyl-phenyl)-tris(4-carbomethoxy-
phenyl-4-ethynyl-phenyl)silane 7a. Sonogashira: A flask
was charged with 6a (0.031 g, 0.037 mmol), 1-iodopyrene46
(0.023 g, 0.072 mmol), Pd(dba)2 (3 mg, 0.005 mmol), PPh3
(4 mg, 0.015 mmol), CuI (3 mg, 0.018 mmol), (i-Pr)2NEt
(5 mL), and THF (10 mL). The reaction mixture was stirred
at room temperature for 2 days under nitrogen and then
poured into water. After usual workup with ether
(3ꢂ25 mL) the crude product was purified by silica gel col-
umn chromatography with hexanes/ethyl acetate (80/20) to
afford 0.025 g of 7a as a pale yellow solid (66%).
Rf 7a¼0.38. Mp 135–138 ꢁC. 1H NMR (CDCl3) d 8.65 (1H,
d, J¼9.0 Hz), 8.23–8.18 (4H, m), 8.13 (1H, d, J¼8.0 Hz),
8.10 (1H, d, J¼8.5 Hz), 8.05 (2H, d, J¼9.0 Hz), 8.01 (6H,
d, J¼7.5 Hz), 7.75 (2H, d, J¼8.0 Hz), 7.61–7.58 (20H, m),
3.92 (9H, s). 13C NMR (CDCl3) d 166.5, 136.3, 136.2,
134.0, 133.3, 132.0, 131.6, 131.4, 131.3, 131.2, 131.1,
131.0, 129.8, 129.7, 129.5, 128.5, 128.3, 127.7, 127.2,
126.3, 125.8, 125.7, 125.4, 125.3, 124.6, 124.5, 124.4,
124.3, 117.4, 94.8, 92.0, 90.2, 90.0, 52.3. IR (cmꢀ1): 2925,
2255, 1721, 1434, 1190. HRMS (FAB) calcd for
C72H46O6Si (MH+): 1035.3143, found 1035.3139. Suzuki:
A solution of 6a (0.28 g, 0.33 mmol) in THF (15 mL) was
cooled to ꢀ78 ꢁC with stirring. Lithium bis(trimethylsilyl)
amide (0.36 mL, 0.36 mmol, 1.0 M in hexane) was added
dropwise to the solution via a syringe pump (1.96 mL/h).
The resulting pale yellow solution was stirred at ꢀ78 ꢁC
for 30 min, followed by the addition of B-methoxy-9-BBN
(0.36 mL, 0.36 mmol, 1.0 M in hexane), dropwise via a
syringe pump (1.96 mL/h). The solution was stirred for 2 h
after the addition was completed and then transferred via
cannula to a second flask containing a mixture of Pd(PPh3)4
(0.129 g, 0.11 mmol) and 1-iodopyrene (0.14 g, 0.04 mmol)
in THF (15 mL) at room temperature. The reaction mixture
was refluxed for 2 days, then allowed to cool to room
temperature and poured into water. After workup with
ether (3ꢂ25 mL), purification by silica gel column
4.3.11. 1-(4-Iodophenyl)-3,5,7-(3-carbomethoxyphenyl-
4-ethynyl-phenyl)adamantane 10. A flask was charged
with 1 (0.25 g, 0.27 mmol), 9 (0.44 g, 2.73 mmol),
Pd(PPh3)2Cl2 (6 mg, 0.008 mmol), CuI (2.4 mg,
0.013 mmol), (i-Pr)2NEt (10 mL), and THF (5 mL). The re-
action mixture was stirred for 2 days at room temperature
under nitrogen, then poured into distilled water, and filtered.
After standard workup of the filtrate with dichloromethane
(3ꢂ10 mL) the crude residue was purified by silica gel col-
umn chromatography with hexanes/ethyl acetate (80/20) to
give 0.039 g of 10 as a white solid (22%). Rf10¼0.14. Mp
1
88–92 ꢁC. H NMR (CDCl3) d 8.25 (2H, s), 8.03 (2H, d,
J¼6.5 Hz), 7.73–7.68 (5H, m), 7.59–7.57 (7H, m), 7.50–
7.46 (8H, m), 7.29–7.22 (4H, m), 3.97 (9H, s), 2.16 (12H,
two s). 13C NMR (CDCl3) d 166.4, 149.4, 148.6, 137.5,
135.7, 132.7, 131.8, 131.7, 130.4, 129.2, 128.5, 127.2,
125.1, 123.7, 120.8, 90.1, 88.2, 52.3, 46.7, 39.3, 39.1, 29.7.
IR (cmꢀ1): 2923, 2208, 1723, 1509, 1257. HRMS (FAB)
calcd for C64H49IO6 (MH+): 1041.2649, found 1041.2659.
Also isolated were 11 and 12 both as white solids. Compound
11: 0.074 g (28%). Rf11¼0.17. Mp 83–85 ꢁC. 1H NMR
(CDCl3) d 8.23 (2H, s), 8.01 (2H, d, J¼7.5 Hz), 7.72–7.68
(5H, m), 7.55 (4H, d, J¼8.0 Hz), 7.47–7.42 (8H, m), 7.22
(3H, d, J¼8.5 Hz), 3.95 (6H, s), 2.11 (12H, two s). 13C
NMR (CDCl3) d 166.4, 149.2, 148.5, 137.4, 135.6, 132.7,
131.7, 130.4, 129.1, 128.5, 127.1, 125.1, 125.0, 123.7,
120.8, 90.1, 88.2, 52.3, 46.7, 46.6, 39.2, 39.0, 29.6. IR
(cmꢀ1): 2924, 2206, 1720, 1508, 1255. LRMS (FAB) calcd
for C54H42I2O4 (MH+): 1009.1, found 1009.3. Compound
12: 0.021 g (7%). Rf12¼0.11. Mp 97–99 ꢁC. 1H NMR
(CDCl3) d 8.23 (4H, s), 8.00 (8H, d, J¼7.5 Hz), 7.73 (8H,
d, J¼7.5 Hz), 7.57 (4H, d, J¼8.5 Hz), 7.51 (4H, d,
J¼8.0 Hz), 7.44 (4H, t, J¼8.0 Hz), 3.95 (12H, s), 2.19
(12H, s). 13C NMR (CDCl3) d 166.4, 149.4, 135.7, 132.7,
131.8, 130.4, 129.1, 128.5, 125.2, 123.8, 120.8, 90.2, 88.2,
52.3, 46.8, 39.3. IR (cmꢀ1): 2925, 2204, 1720, 1501, 1249.

7558
S. Thyagarajan et al. / Tetrahedron 63 (2007) 7550–7559
LRMS (FAB) calcd for C74H56O8 (M+): 1072.3, found
1072.2.
H. B. J. Phys. Chem. A 2003, 107, 3379; (e) Anderson,
N. A.; Ai, X.; Chen, D.; Mohler, D. L.; Lian, T. J. Phys.
Chem. B 2003, 107, 14231.
4. (a) Galoppini, E.; Guo, W.; Zhang, W.; Hoertz, P. G.; Qu, P.;
Meyer, G. J. J. Am. Chem. Soc. 2002, 124, 7801; (b)
Lamberto, M.; Pagba, C.; Piotrowiak, P.; Galoppini, E.
Tetrahedron Lett. 2005, 46, 4895.
5. (a) Loewe, R. S.; Ambroise, A.; Muthukumaran, K.; Padmaja,
K.; Lysenko, A. B.; Mathur, G.; Li, Q.; Bocian, D. F.; Misra, V.;
Lindsey, J. S. J. Org. Chem. 2004, 69, 1453; (b) Long, B.;
Nikitin, K.; Fitzmaurice, D. J. Am. Chem. Soc. 2003, 125, 5152.
6. (a) Wei, Q.; Galoppini, E. Tetrahedron 2004, 60, 8497; (b) Guo,
W.; Galoppini, E.; Rydja, G. I.; Pardi, G. Tetrahedron Lett.
2000, 41, 7419.
7. See for instance the attempts to prepare meta derivatives by
Mathias and co-workers: Reichert, V. R.; Mathias, L. J.
Macromolecules 1994, 27, 7015.
8. Petter Persson, University of Lund, Chemical Physics
Department, Private communication.
9. (a) Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley,
K. W. Coord. Chem. Rev. 2004, 248, 1363 and references cited
therein; (b) Hasobe, T.; Kamat, P. V.; Troiani, V.; Solladie, N.;
Ahn, T. K.; Kim, S. K.; Kim, D.; Kongkanand, A.; Kuwabata,
S.; Fukuzumi, S. J. Phys. Chem. B 2005, 109, 19; (c) Wang, Q.;
Campbell, W. M.; Bonfantani, E. E.; Jolley, K. W.; Officer,
D. L.; Walsh, P. J.; Gordon, K.; Humphry-Baker, R.;
4.3.12. 1-(1-Pyrenyl-4-ethynyl-phenyl)-3,5,7-(3-carbo-
methoxyphenyl-4-ethynyl-phenyl)adamantane 14.
A
flask was charged with 10 (0.068 g, 0.065 mmol), 1-ethynyl-
pyrene 1347 (0.030 g, 0.130 mmol), Pd2(dba)3 (0.030 g,
0.032 mmol), (o-tolyl)3P (0.060 g, 0.196 mmol), Et3N
(10 mL), and THF (20 mL). The reaction mixture was stirred
at 60 ꢁC for 24 h under nitrogen, then cooled, poured into
distilled water, and filtered. After usual workup with di-
chloromethane (3ꢂ10 mL) the residue was purified by silica
gel column chromatography using hexanes/ethyl acetate
(80/20) to give 0.028 g of 14 as a pale yellow solid (38%).
1
Rf14¼0.28. Mp 130–134 ꢁC. H NMR (CDCl3) d 8.69 (1H,
d, J¼9.0 Hz), 8.24–8.00 (14H, m), 7.75–7.72 (5H, m),
7.59–7.51 (9H, m), 7.46–7.43 (8H, m), 3.86 (9H, s), 2.12
(12H, two s). 13C NMR (CDCl3) d 166.4, 149.5, 149.3,
135.7, 132.7, 131.9, 131.8, 131.7, 131.3, 131.2, 131.1,
130.4, 129.6, 129.3, 129.2, 128.5, 128.3, 128.1, 127.3,
126.2, 125.7, 125.6, 125.3, 125.2, 125.1, 124.5, 124.3,
123.8, 121.5, 120.9, 117.9, 95.0, 90.2, 88.6, 88.2, 52.3,
52.1, 46.8, 39.4, 39.3. IR (cmꢀ1): 2923, 2208, 1721, 1510,
1257. HRMS (FAB) calcd for C82H58O6 (MH+):
1139.4309, found 1139.4315.
€
Nazeeruddin, M. K.; Gratzel, M. J. Phys. Chem. B 2005, 109,
15397.
4.3.13. 1-(1-Pyrenyl-4-ethynyl-phenyl)-3,5,7-(3-carboxy-
phenyl-4-ethynyl-phenyl)adamantane 15. To methyl ester
14 (0.02 g, 0.017 mmol) in THF (4 mL) was added 1.0 N
NaOH (1 mL) and the solution was stirred at room temper-
ature overnight. The basic aqueous layer was extracted
with dichloromethane, followed by the dropwise addition
of 1.0 N HCl. Upon acidification, a precipitate was formed.
This was filtered to afford 15 as a pale yellow solid (0.018 g,
10. 7.7% Efficiencies have been obtained with Arakawa’s coumar-
ine dyes. Hara, K.; Kurashige, M.; Dan-oh, Y.; Kasada, C.;
Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. New J.
Chem. 2003, 27, 783.
11. (a) Wang, Z. S.; Hara, K.; Dan-oh, Y.; Kasada, C.; Shinpo, A.;
Suga, S.; Arakawa, H.; Sugihara, H. J. Phys. Chem. B 2005,
109, 3907; (b) Furube, A.; Katoh, R.; Hara, K.; Sato, T.;
Murata, S.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B
2005, 109, 16406; (c) Ramakrishna, G.; Ghosh, H. N.
J. Phys. Chem. A 2002, 106, 2545.
12. (a) Wang, L.; Ernstorfer, R.; Willig, F.; May, V. J. Phys.
Chem. B 2005, 109, 9589; (b) Ferrere, S.; Zaban, A.; Gregg,
B. A. J. Phys. Chem. B 1997, 101, 4490; (c) Nolde, F.;
Pisula, W.; Muller, S.; Kohl, C.; Mullen, K. Chem. Mater.
2006, 18, 3715; (d) Burfeindt, B.; Hannappel, T.; Storck, W.;
Willig, F. J. Phys. Chem. 1996, 100, 16463.
13. (a) Khazraji, A. C.; Hotchandani, S.; Das, S.; Kamat, P. V.
J. Phys. Chem. B 1999, 103, 4693; (b) Sayama, K.;
Tsukagoshi, S.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe, Y.;
Suga, S.; Arakawa, H. J. Phys. Chem. B 2002, 106, 1363.
1
83%). Mp 250–255 ꢁC. H NMR (THF) d 8.69 (1H, d,
J¼9.0 Hz), 8.28–7.98 (14H, m), 7.73–7.66 (5H, m), 7.58–
7.56 (9H, m), 7.48–7.45 (8H, m), 2.27 (12H, two s). 13C
NMR (THF) d 167.3, 150.1, 149.6, 137.2, 135.8, 133.7,
133.2, 132.9, 132.6, 132.4, 131.9, 131.1, 130.4, 129.1,
128.7, 128.5, 128.0, 127.6, 127.2, 126.7, 125.8, 125.6,
125.3, 125.1, 125.0, 124.7, 124.4, 124.0, 122.9, 121.8,
118.6, 94.9, 91.4, 89.0, 88.6, 48.3, 45.4, 39.9, 38.7. IR
(cmꢀ1): 3038, 2925, 2211, 1697, 1510, 1257. HRMS
(FAB) calcd for C79H52O6 (MH+): 1097.3839, found
1097.3848.
Acknowledgements
€
14. (a) He, J.; Benko, G.; Korodi, F.; Polivka, T.; Lomoth, R.;
˚
€
Akermark, B.; Sun, L.; Hagfeldt, A.; Sundstrom, V. J. Am.
Chem. Soc. 2002, 124, 4922; (b) Inabe, T.; Tajima, H. Chem.
Rev. 2004, 104, 5503.
Support of this research by the National Science Foundation,
NIRT-0303829 is gratefully acknowledged.
15. (a) Pitigala, P. K. D. D. P.; Seneviratne, M. K. I.; Perera, V. P. S.;
Tennakone, K. Langmuir 2004, 20, 5100; (b) Ziolkowski, L.;
Vinodgopal, K.; Kamat, P. V. Langmuir 1997, 13, 3124; (c)
Zhang, H.; Zhou, Y.; Zhang, M.; Shen, T.; Li, Y.; Zhu, D.
J. Phys. Chem. B 2002, 106, 9597; (d) Ramakrishna, G.; Das,
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^
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