Synthesis of azobenzene substituted tripod-shaped bi(p-phenylene)s. Adsorption on gold and CdS quantum-dots surfaces

Article abstract of DOI:10.1016/j.tet.2013.02.054

We report here the synthesis of several tripod-shaped oligo(p-phenylene)s with legs composed of two phenylene units. Each leg is end-capped with a thioacetate group for adhesion to metallic surfaces. An azobenzene chromophore group is present on the functional arm of the tripod. The key step in the synthesis is the Pd-catalyzed Suzuki cross-coupling reaction of the silicon derivative core molecule with substituted phenyl moieties and azobenzene derivatives. Gold surfaces prepared by thermal evaporation and CdS quantum-dots surfaces were covered by the tripod-shaped molecules. Modified surfaces were characterized by atomic force microscopy (AFM), fluorescence, and Kelvin Probe analyses.

Full text of DOI:10.1016/j.tet.2013.02.054

Tetrahedron 69 (2013) 3465e3474
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Tetrahedron
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Synthesis of azobenzene substituted tripod-shaped bi(p-phenylene)s.
Adsorption on gold and CdS quantum-dots surfaces
a
a
a
a
ꢀ
Jesus Hierrezuelo , Rodrigo Rico , María Valpuesta , Amelia Díaz ,
a,
b
c
c
*
ꢀ
J. Manuel LopezeRomero , Martins Rutkis , Jana Kreigberga , Valdis Kampars ,
Manuel Algarra d
a
ꢀ
ꢀ
ꢀ
Dep. de Química Organica, Facultad de Ciencias, Universidad de Malaga, 29071 Malaga, Spain
b Institute of Solid State Physics, University of Latvia, 8 Kengaraga St., Riga LV-1063, Latvia
c Faculty of Material Science and Applied Chemistry, Riga Technical University, 14-24 Azenes St., Riga LV-1048, Latvia
ˇ
ˇ
d
ꢀ
Centro de Geologia, Dep. de Geociencias, Ambiente e Ordenamento do Territorio do Porto, Faculdade de Ciencias, Universidade do Porto, Porto,
Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here the synthesis of several tripod-shaped oligo(p-phenylene)s with legs composed of two
phenylene units. Each leg is end-capped with a thioacetate group for adhesion to metallic surfaces. An
azobenzene chromophore group is present on the functional arm of the tripod. The key step in the
synthesis is the Pd-catalyzed Suzuki cross-coupling reaction of the silicon derivative core molecule with
substituted phenyl moieties and azobenzene derivatives. Gold surfaces prepared by thermal evaporation
and CdS quantum-dots surfaces were covered by the tripod-shaped molecules. Modified surfaces were
characterized by atomic force microscopy (AFM), fluorescence, and Kelvin Probe analyses.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 30 November 2012
Received in revised form 11 February 2013
Accepted 18 February 2013
Available online 26 February 2013
Dedicated to the memory of Professor Inta
Muzikante
Keywords:
Oligo(p-phenylene)s
Chromophores
SAMs
Surfaces
Nanostructures
Kelvin-Probe
Sensors
1. Introduction
by reversible cis4trans photoisomerization of the azo bond, and
the geometric changes that result can be incorporated into poly-
Azobenzenes are versatile molecules that have received great
attention in fundamental and applied research. The strong elec-
tronic absorption band that present these chromophores can be
tuned by ring substitution from ultraviolet to red and visible region,
allowing chemical fine-tuning of color, and because of this they
have been used as dyes and colorants. With appropriate electron-
mers and other materials.7 Molecules containing the azobenzene
functionality are good candidates for converting light into me-
chanical work through cis4trans isomerization, that is, controlled
by ultraviolet (UV) and visible light.8,9
The ability of azo-containing molecules to self-assemble into
monomolecular layers (self-assembled monolayers, SAMs) provides
an additional nanometer-scale mechanical system, combining the
advantages of single-molecule properties with the coherence and
template capabilities of macroscopic structures. These films enable
such applications as sensors, and molecular-level mechanical ma-
nipulators.8 However, due to the packing into a self-assembled film,
steric interactions constraints on the cis4trans switching.10,11 This
fact does not exist in the isolated molecule or bulk disordered films,
thus some slight disorder in the film can be an enabling condition for
the isomerization.12 Moreover, molecular packing also governs the
donor/acceptor ring substitution, the
p-electron delocalization of
the aromatic structure can yield high optical nonlinearity, and azo
chromophores have been extensively studied for nonlinear optical
applications as well.1,2 Azobenzene photoswitches are also valuable
tools to influence protein activity.3,4 For this purpose, they have
been attached to the protein by covalent bonds5 or by affinity in-
teractions.6 The photomechanic properties can be readily induced
ꢀ
* Corresponding author. E-mail address: jmromero@uma.es (J.M. LopezeRomero).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.02.054

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J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
excitonic coupling between chromophores, which can strongly in-
fluence the conversion efficiency.13
because the p-phenylene legs are sufficiently rigid to maintain the
perpendicular orientation of the functional arm at the focal point of
the tripod with respect to the surface.25,26
The most common method to generate a disorder in the func-
tional group distribution on monolayer surfaces prepared by self-
assembly and also to decrease their average density of mono-
dentate adsorbates is by co-deposition of the active molecule with
an inert analog adsorbate. However, it is not possible to avoid non-
randomized mixing on the nanoscale that prevents controlling the
spacing between the functional surface groups.14,15
To generate ordered SAMs with a high control on the orientation
of the functional moieties, several shape-persistent and self-
standing molecules have been developed, including: i) ‘molecular
caltrops’ with four phenylacetylene legs extending from either
a tetrahedral silicon core,16,17 or from an adamantane core,18e20 ii)
conically shaped dendron adsorbates with a functional group at the
core,21,22 and iii) tripod-shaped oligo(p-phenylene)s joined to-
gether by a single silicon atom,23,24 to be used for the functionali-
zation of different surfaces.
In this paper we report, the synthesis of tripod-shaped anchors
functionalized by azobenzene chromophores. Adsorption of these
molecules on gold surfaces was also carried out. Due to the thio-
acetate functional group on the tripod legs, fluorescent nano-
particles, such as CdS quantum-dots (CdS QDs) were capped by the
synthesized tripods, obtaining also a suitable platform of solubili-
zation of this dendron system in aqueous media. Characterization
of the generated surfaces was carried out by AFM, fluorescence, and
Kelvin Probe (KP) measurements. We propose these adsorbates to
control the orientation and spacing of the azo moieties in the or-
ganic thin films in order to improve the optical properties of the
azobenzene derivatives.
Tour and Cai have described the use of a silicon atom as the core
for constructing tripod-shaped structures.16,23 We already used
ethoxy triphenylsilanes for this purpose, however, we found that
the ethoxy moiety easily decomposed in presence of water.24 Due
to this fact, we chose as the silicon core the more stable tetraphenyl
derivatives 3a and 3b. These molecules are unsymmetrically
substituted with three iodine atoms for the elongation of the tripod
legs, and the less reactive chlorine (3a) or bromine (3b) atoms that
allow the introduction of the active chromophore at the end of the
synthesis.
The preparation of 3a was carried out in two steps. First, the
generation of p-chlorophenylmagnesium bromide at ꢀ78 ꢁC, fol-
lowed by treatment with tetrachlorosilane to produce a tri-
chlorosilane derivative, and then substitution of the chlorine atoms
by etoxy groups to give compound 2a in 35% yield (Scheme 1). We
observed that this compound is more stable that the corresponding
chlorosilane derivative. Second, the treatment of 2a with the
monolithium salt of p-diiodobenzene afforded 3a in 71% yield. This
synthetic pathway provides 3a with a 25% overall yield (Scheme 1).
Compound 3b was prepared from p-dibromobenzene by following
the same reaction sequence (Scheme 1). The smaller overall yield
found for 3b (6%) in comparison to that of 3a, can be attributed to
a higher reactivity of the bromine atom towards the exchange of
lithium under the reaction conditions.
Three ends of the supporting legs in 1a and 1b are derived from
a thioacetate benzyl alcohol group. These groups were introduced
in the last part of the synthesis. Thioacetate moiety as the caltrops
feet has received recently much attemption since it increases the
stability of the thiol group preventing degradation. Moreover, the in
situ deprotection of the acetyl protected thiol under alkaline con-
ditions allows the preparation of high quality SAMs, by controlling
the used base.16,27
2. Results and discussion
2.1. Synthesis
To achieve the thioacetate group derivatization, compound 4
was used for the elongation of the tripod legs. This compound has
a hydroxyl group that will allow its exchange by a sulfur atom.
Compound 4 was prepared by protection of the alcohol group in 3-
(hydroxymethyl) phenylboronic acid pinacol ester by treatment
with tert-butyldimethylsilyl chloride (TBDMSCl) in presence of 4-
dimethylaminopyridine (DMAP) (Scheme 2).
We carried out the synthesis of the tripods 1a and 1b (Fig. 1).
Each tripod leg is composed of two phenylene units and it is end-
capped with a thioacetate group. Their preparation was accom-
plished by coupling azobenzene and phenyl derivatives to a silicon
core, being in both cases the key step of the synthesis the Pd-
catalyzed Suzuki cross-coupling reaction (Schemes 1e5). We
chose tripodal oligo(p-phenylene)s as anisotropic adsorbates
Fig. 1. Oligo-(p-phenylene) tripods substituted by azoderivative chromophores.

J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
3467
Scheme 1. Preparation of 3a and 3b.
the presence of K2CO3, S-Phos (2-dicyclohexylphosphino-20,60-
dimethoxybiphenyl) as ligand and Pd(OAc)2 as catalyst in MeOH, to
provide 9a in 34% yield (Scheme 4).
Removal of the tert-butyldimethylsilyl protecting group was
carried out by treatment of compound 9a with tetra-n-butyl am-
monium fluoride (TBAF) in the presence of AcOH as catalyst and
THF as solvent (Scheme 4). The reaction afforded compound 10a in
67% yield. Finally, the treatment of 10a with PPh3 and diisopropyl
azodicarboxylate (DIAD) followed by thioacetic acid in THF fol-
lowing the Mitsunobo method29 gave the trithioacetate tripod 1a in
46% yield (Scheme 4).
To accomplish the synthesis of tripod 1b we prepared the para-
nitro azoderivative 8. Following the procedure developed by
Miyaura,30 the bromine atom in 7 was replaced with a pinacolbor-
onate group by reaction with 1.1 equiv of bis(pinacolato)diboron, to
provide 8 in 56% yield (Scheme 5).
Scheme 2. Protection of hydroxyl group of 3-(hydroxymethyl) phenylboronic acid
pinacol ester to give 4.
The coupling of 3a or 3b with 4 under standard Suzuki condi-
tions using Pd(PPh3)4 as a catalyst in a mixture of water and tolu-
ene, produced tripods 5a and 5b in yields of 80% and 55%,
respectively (Scheme 3).
The coupling of compound 8 with 5b was carried out in presence
of Pd(PPh3)4 as catalyst and Na2CO3 as the base. The coupling
Scheme 3. Preparation of tripods 5a and 5b.
The incorporation of the chromophore in tripod 1a was made at
this point by using the azoderivative 6 (Scheme 4). The azo-
compound 6 was prepared by following the procedure reported by
Trauner.28 The condensation of commercially available 4-
aminophenylboronic acid pinacol ester with nitrosobenzene, fol-
lowed by hydrolysis in presence of KHF2 of the azobenzene pinacol
ester intermediate, afforded the azobenzene trifluoroborate potas-
sium salt 6. Thus, the triiodide 5a was coupled with compound 6 in
product 9b was obtained in 55% yield (Scheme 6). Compound 9b
vas converted into 1b by following the same reaction sequence as
for 9a. Deprotection of the tert-butyldimethylsilyl protecting group
was carried out by treatment of 9b with TBAF in the presence of
AcOH and THF as solvent. The reaction afforded compound 10b in
89% yield. Finally, the treatment of 10b with PPh3 and DIAD fol-
lowed by thioacetic acid in THF gave the trithioacetate 1b in 17%
yield (Scheme 6).

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J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
Scheme 4. Synthesis of protected tripod 9a, followed by deprotection and thioacetate introduction, to give 1a.
Gold surfaces were prepared on glass slides by thermal evapo-
ration, to give a gold layer of w40 nm thickness (see Experimental).
In order to have a reference substrate of the gold modified glass
slides only half of the slide was immersed in compound 1a or 1b
THF/Et3N solution. Therefore we expected to have half of slide
covered by SAM and half left bare gold layer for further reference.
AFM and KPFM images of the prepared gold surface and the
tripod 1a modified gold surface are shown in Fig. 2.
Fig. 2A shows the AFM topography image of the clean evapo-
rated Au surface (10ꢂ10
seen, while Fig. 2B and C show the work function map and surface
m).
mm), where gold islands can be clearly
potential map of the gold surface covered by 1a (1.0ꢂ1.0
m
The azobenzene derivative films of 1b located on the gold layer,
which was grown on the freshly cleaved glass substrate, were also
investigated by AFM and KPFM (Fig. 3). In Fig. 3A and B are shown
the topography and phase AFM images, respectively, while Fig. 3C
shows the surface potential map.
Scheme 5. Preparation of p-nitro substituted azobenzene boronic acid pinacol ester 8.
The measured work function of azobenzene derivative 1b films
(Fig. 3D), deposited on the gold film, are in the range of 4.7e4.9 eV.
Uncovered gold film values are in the range of 4.9e5.0 eV, which is
in close agreement with other experimentally determined gold
work function values.31
Tripod-shaped molecules 1a and 1b were used for the modifi-
cation of surfaces of gold substrates and cadmium sulfide quantum-
dots.
The Kelvin Probe method was applied to obtain the values of the
surface potential of the organic films on gold electrodes.31 The
surface potential measurements were performed in the presence of
air (Figs. 4 and 5).
2.2. Modification of gold surfaces with tripods 1a and 1b
Self-assembled films of tripods 1a and 1b on gold surfaces were
prepared by following the procedure described by Valkenier et al.27
The procedure includes the presence of a base (Et3N) in order to
remove in situ the thioacetate protecting group. In case of molecule
1a, without the presence of the base SAMs could not be prepared.
However, in case of molecule 1b, films formation was observed
even in absence of the base, meaning without protecting group
removal.
As can be seen in Fig. 4, tripod 1a covered surface (right side of
chart) shows a surface potential of wþ180 mV. However, bare gold
surface (left side of chart) presents surface potential of wþ90 mV.
Ontheotherhand, tripod1bcoveredsurface(rightsideofchart,in
Fig. 5)showsasurfacenegativepotentialofwꢀ150mV.Thebaregold
surface (left side of chart) presents a surface potential of wþ80 mV,
similar to that found for the unmodified gold surface in 1a.
It is worth to note, that the effect of the tripod adsorption on the
gold surface for 1a is the opposite of that for 1b. For compound 1a
the SAM surface potential (work function) of gold surface is just
SAMs of both compounds were characterized by AFM and KPFM
(Kelvin Probe Force Microscopy) (Figs. 2 and 3). Surface potentials
were measured by using a Kelvin Probe apparatus.

J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
3469
Scheme 6. Synthesis of protected tripod 9b, followed by deprotection and thioacetate introduction, to give 1b.
Fig. 2. AFM and KPFM images of gold surface, and tripod 1a modified gold surfaces: (A) AFM-topography (10ꢂ10
potential map (1.0ꢂ1.0 m).
m
m), (B) work function map (1.0ꢂ1.0
m
m), and (C) KPFM-surface
m

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J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
Fig. 3. AFM and KPFM images (10ꢂ10
m
m) of tripod 1b modified gold surfaces. (A) Topography, (B) phase, (C) surface potential map, and (D) work function map.
slightly affected, showing an increase of wþ90 mV. The situation
changes to the opposite for tripod 1b. In this case, we found a large
opposite sign (wꢀ150 mV) of the surface potential of the covered
surface, showing a net change of wꢀ230 mV in comparison to the
bare gold surface. The negative value found could be attributed to
a large dipole moment of molecule 1b, which is substituted by an
electron withdrawing group. When the SAM of 1b is formed by
anchoring all three sulfur atoms on the gold surface the dipole
moment component perpendicular to plane defined by these atoms
should be responsible for surface charge buildup. According to our
semi-empirical AM1 modeling data for tripod 1a, the dipole mo-
ment component perpendicular to plane defined by three sulfur
atoms was ꢀ1.44 D. At the same time, this dipole moment com-
ponent for tripod 1b was ꢀ8.32 D.32 This is in agreement with
a perpendicular position of tripods 1a and 1b on the gold surface,
with the three sulfur atoms connected to the surface. The differ-
ences in the net surface potentials between both tripods can be
attributed to the azo dye dipole moment differences.
Fig. 4. Kelvin Probe measurements for tripod 1a.
2.3. Quantum-dots capped by the tripod 1b
Well dispersed colloidal cadmium sulfide quantum-dots (CdS
QDs) were successfully synthesized and surface modified by the
tripod 1b, according to the protocol described in Section 4.5. The
presence of protected thiol terminal functional groups makes
them suitable to be assembled to the surface of selected nano-
particles, such as QDs. Fig. 6 shows the fluorescence spectra of CdS
QDs coated by azobenzene substituted tripod-shaped bi(p-phe-
nylene) 1b.
From the analysis of its fluorescence spectra, it is observed that
the maximum is located at 467 nm, with an emission full width of
half maximum (FWHM) of 102 nm. This relatively high FWHM
suggests that the coated CdS QDs shows a high degree of size
Fig. 5. Kelvin Probe measurements for tripod 1b.

J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
3471
100
80
60
40
20
0
tripod 1b onto the surface of CdS QDs makes the system suitable as
sensor in aqueous media, since CdS QDs act as efficient carriers in
this solvent. This is an important point since one of the limitations
for the use of this kind of molecules in biological media is the low
water solubility.
Currently, we are studying the cis4trans isomerization of the
azobenzene chromophore in compounds 1a and 1b once they are
adsorbed on the surfaces.
4. Experimental
4.1. General
Cadmium chloride (CdCl2, 99.9%, #202908), mercaptosuccinic
acid (MSA, 99%, #88460) sodium sulfide (#407410), p-di-
chlorobenzene (#D56829), p-dibromobenzene (#D39029), p-diio-
dobenzene (#193526), 4-bromothiophenol (#B81956), 3-
(hydroxymethyl) phenylboronic acid pinacol ester (#703001),
nitrosobenzene (#N24609), 4-aminophenylboronic acid pinacol
ester (#518571) were purchased from Aldrich and used without
purification. Deionized water was employed in all of the experi-
ments. Melting points were determined with a Gallenkamp in-
strument. UV spectra were recorded with a HewlettePackard
8452A spectrophotometer, and IR spectra were recorded with
Beckman Aculab IV and PerkineElmer 883 spectrophotometers.
Mass spectrometry was done with a Thermo Finnigan instrument
by using the direct injection and electron ionization (EI) modes.
HRMS were recorded with a Micromass (Autospec-Q) spectrome-
ter. 1H and 13C NMR spectra were recorded with a 400 MHz ARX
400 Bruker spectrometer by using the residual solvent peak in
350
400
450
500
550
600
Wavelength / nm
Fig. 6. Fluorescence spectra of CdS QDs capped by the azobenzene derivative 1b.
heterogeneity; compared with the raw CdS QDs, which show
FWHM <30 nm and emission band at 520 nm when un-coated.33
This is probably due to the fact that they are bounded to different
chemical environments, but also to emission surface defects.34 This
maximum emission wavelength fall into the range observed for CdS
QDs literature values for the maximum emission wavelength with
other thiol capping molecules in the surfaces of CdS nanoparticles,
CDCl3
(
(
dH 7.24 ppm for 1H and dC¼77.0 ppm for 13C) or CD3SOCD3
dH¼2.50 ppm for 1H and dC¼39.5 ppm for 13C). TLC analyses were
such as
and thiol-
L
-cysteine-capped-CdS (495 nm),35 mercaptoacetic acid,36
-cyclodextrin (425 nm).37
performed on Merck silica gel 60F 254 plates, and column chro-
matography was performed on silica gel 60 (0.040e0.063 mm).
Emission spectra excitation (295.3e671.7 nm) and emission
(249.7e719.7 nm) spectra were obtained with a Spex 3D lumi-
nescence spectrophotometer equipped with a Xenon pulse dis-
charge lamp (75 W) and a CCD detector and 0.25 mm slits and 1 s
integration time were used in scan mode at room temperature. A
quartz cuvette was used.
b
The system includes the mercaptosuccinic acid (MSA). The
chemical role of MSA is, first, complexation of Cd(II) and, second,
act as stabilizing agent of the CdS QDs fluorescent nanoparticles
when they begin to form in water. Without the presence of MSA, no
fluorescence effect was observed. It is worth to note that, even with
a poor water solubility of this organic molecule, anchorage of 1b
onto CdS QDs can be easily achieved in water, acting these nano-
particles as carriers. This fact opens the possibility of using this
system as sensor in biological media.
2D Potential maps were recorded in a Kelvin Probe apparatus
(KP Technology Ltd., model SKP5050).
3. Conclusions
4.2. Synthesis of tripod 1a
We have carried out the synthesis of two tripod-shaped mole-
cules holding diazobenzene chromophores at the functional arm
(1a and 1b) by following a convergent route. The key step in the
synthesis is the Pd-catalyzed Suzuki cross-coupling reaction of the
unsymmetrical triiodohalotetraphenylsilane as the core of the tri-
pod. The presence of a halogen atom in the apical position of the
tripod allows their functionalization with different active mole-
cules. Due to the presence of the thioacetate group, the three tripod
legs can be used for surface immobilization. Although the caltrop
feet used here are based upon protected thiols, the methodology
would permit the incorporation of numerous surface-binding
moieties.
Gold surfaces have been modified with films of 1a or 1b and
characterized by AFM/KPFM analysis. Based on the surface poten-
tial differences between the covered surfaces and the bare gold
surface, we can conclude that the azobenzene chromophore is
vertically oriented on the surface.
4.2.1. (4-Chlorophenyl)triethoxysilane (2a). Under an argon atmo-
sphere, a solution of distilled SiCl4 (10 mL, 87.11 mmol) in dry THF
(10 mL) was added at ꢀ78 ꢁC over a suspension of p-di-
chlorobenzene (6.40 g, 43.56 mmol) and magnesium (1.38 g,
56.62 mmol) in dry THF (34 mL). The reaction mixture was stirred
at ꢀ78 ꢁC for 1 h. After this period, the mixture was concentrated to
dryness under vacuum. The crude residue was dissolved in dry
cyclohexane (200 mL) and absolute ethanol (12.70 mL,
217.80 mmol) and dry pyridine (11 mL, 135.91 mmol) was slowly
added. The reaction mixture was stirred at 20 ꢁC for 12 h. After this
period, the reaction was filtered and the solution was washed with
1 M HCl (50 mL), saturated NaHCO3 (50 mL) and water (2ꢂ50 mL).
The organic phase was dried over anhydrous MgSO4 and the sol-
vent removed in vacuum. The residue was purified by distillation
under reduced pressure and the silane 2a was obtained as a white
solid (8.35 g, yield: 35%). 1H NMR (400 MHz, CDCl3):
d 1.17 (t, 9H,
J¼7.0 Hz, 3ꢂ CH3), 3.79 (t, 6H, J¼7.0 Hz, 3ꢂ CH2), 7.28 (d, 2H,
Moreover, tripod 1b has been deposited on CdS QDs surfaces,
and characterized by fluorescence spectroscopy. The anchorage of
J¼8.5 Hz, ArH), 7.54 (d, 2H, J¼8.5 Hz, ArH) ppm; 13C NMR (100 MHz,
CDCl3):
d
18.1 (3ꢂ CH3), 58.8 (3ꢂ CH2),128.1 (2ꢂ CH),129.5 (2ꢂ CH),

3472
J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
136.1 (CeCl), 136.7 (CeSi) ppm; IR (KBr):
y
2975, 2888, 1583, 1557,
332 (72), 166 (33); HRMS m/z Calcd for
1053.4692, found: 1053.4680.
C63H79ClO3Si4Na:
1485, 1443, 1389, 1296, 1261 cmꢀ1
.
4.2.2. Tri(4-iodophenyl) (4-chlorophenyl)silane (3a). Under an ar-
gon atmosphere, n-BuLi (1.6 M, 20.8 mL, 33.30 mmol) was added
over a solution of p-diiodobenzene (11.00 g, 33.30 mmol) in dry
4.2.5. p-(4-Azobenzene)phenyl tri(30-tert-butyldimethylsilyloxymethyl
[4,10]biphenyl)silane (9a). Under an argon atmosphere, an oven-
dried round bottom flask was charged with potassium boronate 7
(221 mg, 0.77 mmol), Pd(OAc)2 (1.60 mg, 0.007 mmol), S-Phos
(5.74 mg, 0.014 mmol), and K2CO3 (290 mg, 2.10 mmol). The flask
was evacuated and re-filled with argon three times. Then a solution
of 5a (720 mg, 0.70 mmol) in methanol (10 mL) was added. The
reaction mixture was refluxed under stirring for 12 h. Once the
starting material disappeared (TLC, 12 h) the mixture was left to
reach 20 ꢁC, diluted with EtOAc (20 mL) and filtered through Celite.
The organic solution was concentrated to dryness. The crude re-
action was column chromatographed (cyclohexane/EtOAc, 7:3/3:7)
to obtain 9a as an orange syrup (272 mg, yield: 34%). 1H NMR
ethyl ether (100 mL) at ꢀ63 ꢁC. The mixture was stirred at ꢀ63 ꢁ
C
for 1 h, and after this period, a solution of 2a (1.4 mL,10.09 mmol) in
dry ethyl ether (50 mL) was added via cannula at ꢀ63 ꢁC. The
mixture of reaction was stirred for 30 min at this temperature, and
then for 12 h at 20 ꢁC. After this period, HCl 1 M (30 mL) was added.
The organic phase was washed with water and brine (2ꢂ20 mL),
dried over anhydrous MgSO4 and concentrated to dryness. Com-
pound 3a was isolated by column chromatography eluting with
cyclohexane. White needles (2.83 g, yield: 71%). 1H NMR (400 MHz,
CDCl3):
d
7.17 (d, 6H, J¼8.0 Hz, ArH), 7.35 (d, 2H, J¼8.0 Hz, ArH), 7.39
(d, 2H, J¼8.0 Hz, ArH), 7.72 (d, 6H, J¼8.0 Hz, ArH) ppm; 13C NMR
(400 MHz, CDCl3):
d
0.06 (s, 18H, 3ꢂ Si(CH3)2), 0.90 (s, 27H, 3ꢂ
(100 MHz, CDCl3):
d
97.8 (3ꢂ CeI), 128.5 (2ꢂ CH), 130.7 (2ꢂ CH),
C(CH3)3), 4.72 (s, 6H, 3ꢂ CH2O), 7.26e7.31 (m, 4H, ArH), 7.39 (s, 1H,
ArH), 7.36e7.42 (m, 6H, ArH), 7.47e7.49 (m, 4H, ArH), 7.50e7.60 (m,
6H, ArH), 7.62e7.71 (m, 16H, ArH) ppm; 13C NMR (100 MHz, CDCl3):
132.0 (6ꢂ CH), 136.8 (CeSi), 137.3 (3ꢂ CeSi), 137.4 (CeCl), 137.6 (6ꢂ
CH) ppm; MS m/z (%): 750 (25, Mþþ2), 748 (75, Mþ), 623 (30), 621
(96), 547 (30), 545 (100), 420 (12), 418 (27); IR (KBr):
y
2927, 1565,
d
ꢀ2.3 (3ꢂ Si(CH3)2), 16.1 (3ꢂ C(CH3)3), 30.5 (3ꢂ C(CH3)3), 65.4 (3ꢂ
1540, 1033 cmꢀ1; HRMS m/z Calcd for C24H16ClI3SiNa: 770.7742,
CH2O), 124.9 (2ꢂ CH), 125.9 (3ꢂ CH), 126.2 (2ꢂ CH), 126.6 (3ꢂ CH),
126.8 (8ꢂ CH),128.3 (3ꢂ CH),128.8 (2ꢂ CH),129.2 (4ꢂ C),132.3 (CH),
132.6 (2ꢂ CH), 136.9 (8ꢂ CH), 137.8 (4ꢂ C), 141.2 (3ꢂ C), 141.5 (3ꢂ
CH), 142.4 (3ꢂ C), 142.7 (C), 152.9 (2ꢂ CeN]N) ppm; HRMS m/z
Calcd for C75H88N2O3Si4Na: 1199.5770, found: 1199.5780.
found: 770.7728.
4.2.3. 3-(tert-Butyldimethylsilyloxymethyl)phenylboronic acid pina-
col ester (4). Under an argon atmosphere, DMAP (772 mg,
6.32 mmol) was slowly added over a solution of 3-(hydroxymethyl)
phenylboronic acid pinacol ester (1.0 g, 4.27 mmol) and TBDMSCl
(900 mg, 5.98 mmol) in dry THF (20 mL) cooled at 0 ꢁC. The mixture
was stirred for 24 h at 20 ꢁC. The white solid was filtered off,
washed with cold THF and the organic filtrates were concentrated
to dryness under vacuum. The residue was column chromato-
graphed eluting with CH2Cl2, to obtain compound 4 as a white solid
4.2.6. p-(4-Azobenzene)phenyl tri(30-hydroxymethyl[4,10]biphenyl)
silane (10a). Over a solution of 9a (234 mg, 0.19 mmol) in anhy-
drous THF (30 mL), AcOH (44 mL, 0.78 mmol), and TBAF (0.78 mL,
0.78 mmol) were added. The reaction mixture was stirred at 20 ꢁC
for 28 h. After this period, the mixture was poured into water
(10 mL) and extracted with EtOAc (3ꢂ5 mL). The organic phase was
washed with water (10 mL), brine (10 mL), dried over anhydrous
MgSO4, filtered, and concentrated to dryness under vacuum. The
residue was separated by column chromatography (EtOAc) to give
10a as an orange syrup (109 mg, yield: 67%). 1H NMR (400 MHz,
(1.06 g, yield: 71%). 1H NMR (400 MHz, CDCl3):
d 0.07 (s, 6H,
Si(CH3)2), 0.92 (s, 9H, C(CH3)3), 1.32 (s, 12H, 2ꢂ OC(CH3)2), 4.73 (s,
2H, CH2O), 7.04e7.35 (m, 2H, ArH), 7.51e7.54 (m, 1H, ArH),
7.70e7.73 (m, 1H, ArH) ppm; 13C NMR (100 MHz, CDCl3):
d
ꢀ5.2
(Si(CH3)2), 18.5 (C(CH3)3), 24.9 (4ꢂ CH3), 26.0 (C(CH3)3), 64.9
(CH2O), 83.6 (2ꢂ CeO), 127.6, 129.0, 132.2, 133.1, 140.4 ppm; MS m/z
(%): 348 (1, Mþ), 291 (26), 191 (100); HRMS m/z Calcd for
C19H33BO3SiNa: 371.2190, found: 371.2191.
CDCl3):
d
4.70 (s, 6H, 3ꢂ CH2O), 7.26e7.30 (m, 4H, ArH), 7.37 (s, 1H,
ArH), 7.36e7.40 (m, 6H, ArH), 7.47e7.50 (m, 4H, ArH), 7.50e7.57 (m,
6H, ArH), 7.59e7.70 (m, 16H, ArH) ppm; 13C NMR (100 MHz, CDCl3):
d
65.8 (3ꢂ CH2O), 125.7 (3ꢂ CH), 126.1 (2ꢂ CH), 126.3 (4ꢂ C), 126.6
(C), 126.7 (8ꢂ CH), 128.3 (3ꢂ CH), 129.0 (4ꢂ C), 132.4 (2ꢂ CH), 132.6
(3ꢂ C), 136.2 (3ꢂ C), 136.7 (8ꢂ CH), 136.8 (CH), 137.7 (3ꢂ CH), 141.0
(2ꢂ CH), 141.5 (3ꢂ CH), 142.3 (2ꢂ CH), 152.9 (2ꢂ CeN]N) ppm.
4.2.4. p-Chlorophenyl tri(30-tert-butyldimethylsilyloxymethyl[4,10]bi-
phenyl)silane (5a). An oven-dried round-bottomed flask was fitted
with a condenser, placed under an argon atmosphere and charged
with the pinacol ester 3a (713 mg, 2.05 mmol), the silane 4 (495 mg,
0.66 mmol), Pd(PPh3)4 (38 mg, 0.03 mmol), n-Bu4NBr (53 mg,
0.17 mmol), and KOH (554 mg, 9.90 mmol). The flask was evacuated
and refilled with argon three times and then toluene/methanol
(15:3 mL) was added. The reaction mixture was heated at 85 ꢁC for
12 h. When the reaction was complete (TLC, 12 h), the inorganic
solids were removed by filtration through Celite, which was
washed with several portions of dichloromethane; then the solvent
was evaporated. The residue was absorbed onto silica and subjected
to column chromatography (cyclohexane/EtOAc, 9.8/0.2) to give 5a
as a white solid (538 mg, yield: 80%), mp: 190e195 ꢁC. 1H NMR
4.2.7. p-(4-Azobenzene)phenyl
tri(30-thioacetyloxymethyl[4,10]bi-
phenyl)silane (1a). Over a solution of PPh3 (629 mg, 2.40 mmol) in
THF (5 mL), DIAD (0.48 mL, 2.40 mmol) was added at 0 ꢁC. The
mixturewas stirred at 0 ꢁC for 30 min. Then a solutionof 10a (100 mg,
0.12 mmol), and thioacetic acid (0.17 mL, 2.40 mmol) in THF (2 mL)
was slowly added. The reaction mixture was left to reach 20 ꢁC and
stirred for 4.5 h at this temperature. After this period, the mixture
was poured into water (20 mL) and extracted with EtOAc (2ꢂ10 mL).
The organic phase was washed with water (1ꢂ5 mL), brine (1ꢂ5 mL),
dried over anhydrous MgSO4, filtered, and concentrated to dryness
under vacuum. The residue was separated by column chromatogra-
phy (cyclohexane/EtOAc 9:1) to give 1a as an orange syrup (56 mg,
(400 MHz, CDCl3):
d
0.11 (s, 18H, 3ꢂ Si(CH3)2), 0.94 (s, 27H, 3ꢂ
C(CH3)3), 4.80 (s, 6H, 3ꢂ CH2O), 7.32 (d, 2H, J¼8.0 Hz, ArH), 7.39 (t,
yield: 46%). 1H NMR (400 MHz, CDCl3):
d
2.34 (s, 9H, 3ꢂ CH3), 4.17 (s,
3H, J¼8.0 Hz, ArH), 7.50 (d, 2H, J¼8.0 Hz, ArH), 7.58 (d, 6H, J¼8.4 Hz,
6H, 3ꢂ CH2S), 7.26e7.28 (m, 4H, ArH), 7.35 (s,1H, ArH), 7.37e7.40 (m,
ArH), 7.65 (m, 15H, ArH) ppm; 13C NMR (100 MHz, CDCl3):
d
ꢀ5.2
6H, ArH), 7.47e7.50 (m, 4H, ArH), 7.53e7.57 (m, 6H, ArH), 7.59e7.66
(3ꢂ Si(CH3)2), 18.4 (3ꢂ C(CH3)3), 26.0 (3ꢂ C(CH3)3), 65.0 (3ꢂ CH2O),
125.4 (3ꢂ C), 125.8 (3ꢂ CH),126.7 (6ꢂ CH),128.3 (3ꢂ CH),128.7 (3ꢂ
CH), 132.3 (3ꢂ C), 132.7 (3ꢂ CH), 136.2 (CeCl), 136.8 (6ꢂ CH), 137.8
(m,16H, ArH) ppm; 13C NMR (100 MHz, CDCl3):
d
30.3 (3ꢂ CH3), 33.4
(3ꢂ CH2S), 124.0 (2ꢂ CH), 124.1 (2ꢂ CH), 124.7 (2ꢂ CH), 125.0 (2ꢂ
CH),126.1 (3ꢂ CH),126.7 (8ꢂ CH),127.7 (3ꢂ CH),128.0 (3ꢂ CH),129.1
(3ꢂ CH), 131.0 (CH), 132.9 (3ꢂ C), 136.8 (8ꢂ CH), 137.1 (4ꢂ C), 138.2
(3ꢂ C),141.2 (2ꢂ C),142.0 (3ꢂ C),151.3 (2ꢂ CeN]N),194.9 (3ꢂ C]O)
ppm; UV (CDCl3) lmax: 248, 310, 430 nm; MS m/z (%): 1031 (22,
(2ꢂ CH), 140.6 (4ꢂ C), 142.0 (2ꢂ CH), 142.6 (3ꢂ C) ppm; IR (KBr):
y
3280, 2952, 2927, 2855, 1620, 1580, 1252, 1108, 1079 cmꢀ1; MS m/z
(%): 1030 (1, Mþ), 594 (54), 537 (100), 507 (50), 463 (64), 376 (30),

J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
3473
MþþNa). HRMS m/z Calcd for C63H52N2O3S3SiNa: 1031.2807, found:
a solution of 5b (716 mg, 0.66 mmol), and Pd(PPh3)4 (76 mg,
0.07 mmol) in toluene (20 mL) was added over a solution of 9
(280 mg, 0.79 mmol) and Na2CO3 (140 mg, 1.32 mmol) in ethanol
(20 mL). The reaction mixture was stirred at 100 ꢁC for 12 h. After
this period, the reaction mixture was left to reach 20 ꢁC, and then
was filtered through Celite. The organic filtrates were neutralized
with 1 M HCl and diluted with dichloromethane (3ꢂ10 mL). The
organic extracts were washed aq saturated NaHCO3 (1ꢂ10 mL),
water (1ꢂ10 mL), and brine (1ꢂ10 mL), dried over anhydrous
MgSO4 and concentrated to dryness under vacuum. Compound 9b
was isolated by column chromatography (cyclohexane/dichloro-
methane, 6:4) as an orange syrup (444 mg, yield: 55%).1H NMR
1031.2815.
4.3. Synthesis of tripod 1b
4.3.1. (4-Bromophenyl)triethoxysilane (2b).38 Following the pro-
cedure outlined for 2a, from p-dibromobenzene (14.02 g,
59.39 mmol), SiCl4 (15 mL, 130.67 mmol), magnesium (1.88 g,
77.21 mmol), dry THF (65 mL), cyclohexane (250 mL), pyridine
(14.92 mL), and ethanol (17.31 mL), compound 2b was obtained by
column chromatography (cyclohexane/EtOAc, 9:1) as a colorless
liquid (5.95 g, 31%). 1H NMR (400 MHz, CDCl3):
J¼7.0 Hz, 3ꢂ CH3), 3.85 (q, 6H, J¼7.0 Hz, 3ꢂ CH2), 7.55 (m, 4H, ArH)
ppm; 13C NMR (100 MHz, CDCl3):
d 1.22 (t, 9H,
(400 MHz, CDCl3):
d
0.11 (s, 18H, 3ꢂ Si(CH3)2), 0.94 (s, 27H, 3ꢂ
d
18.1 (3ꢂ CH3), 58.8 (3ꢂ CH2),
C(CH3)3), 4.80 (s, 6H, 3ꢂ CH2O), 7.31 (d, 3H, J¼8.0 Hz, ArH), 7.41 (t,
3H, J¼7.4 Hz, ArH), 7.52 (d, 3H, J¼8.4 Hz, ArH), 7.60e7.84 (m, 21H,
ArH), 8.05 (m, 4H, ArH), 8.38 (d, 2H, J¼9.2 Hz, ArH) ppm; 13C NMR
125.3 (CeBr), 130.3 (2ꢂ CH), 131.0 (2ꢂ CH), 136.3 (CeSi) ppm.
4.3.2. Tri(4-iodophenyl) (4-bromophenyl)silane (3b).23 Following
the procedure outlined for 3a, from compound 2b (5.95 g,
18.65 mmol), p-diiodobenzene (20.30 g, 61.55 mmol), n-BuLi (1.6 M,
38.47 mL, 61.55 mmol), dry Et2O (350 mL), and column chroma-
tography (cyclohexane), compound 3b was obtained as a white
solid (3.00 g, yield: 20%), mp: dec 280 ꢁC. 1H NMR (400 MHz,
(100 MHz, CDCl3):
d
ꢀ5.2 (3ꢂ Si(CH3)2), 18.4 (3ꢂ C(CH3)3), 26.0 (3ꢂ
C(CH3)3), 65.0 (3ꢂ CH2O), 123.4 (2ꢂ CH), 124.0 (2ꢂ CH), 124.7 (3ꢂ
CH), 124.8 (3ꢂ CH), 125.4 (2ꢂ CH), 125.8 (3ꢂ CH), 126.7 (8ꢂ CH),
128.0 (2ꢂ CH), 128.7 (3ꢂ CH), 132.6 (3ꢂ C), 136.9 (8ꢂ CH), 137.1 (3ꢂ
C), 140.7 (3ꢂ C), 142.0 (2ꢂ C), 142.5 (3ꢂ C),144.9 (C), 148.7 (CeNO2),
151.7 (CeN]N), 155.8 (CeN]N) ppm; HRMS m/z Calcd for
C75H87N3O5Si4Na: 1244.5620, found: 1244.5620.
CDCl3):
d
7.17 (d, 6H, J¼8.0 Hz, ArH), 7.31 (d, 2H, J¼8.0 Hz, ArH), 7.51
(d, 2H, J¼8.0 Hz, ArH), 7.72 (d, 6H, J¼8.0 Hz, ArH) ppm; 13C NMR
(100 MHz, CDCl3):
(2ꢂ CH), 131.9 (6ꢂ CH), 137.4 (4ꢂ CeSi), 137.6 (6ꢂ CH) ppm.
d
97.8 (3ꢂ CeI),125.4 (CeBr),131.3 (2ꢂ CH),131.5
4.3.6. p-(4-(40-Nitroazobenzene))phenyl tri(30-hydroxymethyl[4,10]
biphenyl)silane (10b). Following the procedure outlined for 10a,
from 9b (430 mg, 0.35 mmol), TBAF (1.41 mL, 1.41 mmol), AcOH
4.3.3. p-Bromophenyl tri(30-tert-butyldimethylsilyloxymethyl[4,10]bi-
phenyl)silane (5b). Following the procedure outlined for 5a, com-
pound 3b (1.00 g, 1.26 mmol) was coupled with 4 (1.32 g,
3.78 mmol) in the presence of Pd(PPh3)4 (73 mg, 0.063 mmol), KOH
(1.06 g, 18.90 mmol), n-Bu4NBr (102 mg, 0.32 mmol), toluene
(30 mL), and water (6 mL). Compound 5b was isolated by column
chromatography (cyclohexane) as a yellowish syrup (734 mg, 54%).
(80
by column chromatography (EtOAc/cyclohexane, 6:4) as an orange
syrup (274 mg, yield: 89%). 1H NMR (400 MHz, CDCl3):
1.24 (br s,
3H, 3ꢂ OH), 4.76 (s, 6H, 3ꢂ CH2O), 7.36 (d, 3H, J¼7.6 Hz, ArH), 7.44
(t, 3H, J¼7.8 Hz, ArH), 7.56 (d, 3H, J¼8.0 Hz, ArH), 7.64e7.83 (m, 21H,
ArH), 8.03e8.07 (m, 4H, ArH), 8.38 (d, 2H, J¼8.8 Hz, ArH) ppm.
mL, 1.41 mmol) in dry THF (30 mL), compound 10b was isolated
d
1H NMR (400 MHz, CDCl3):
d
0.11 (s, 18H, 3ꢂ Si(CH3)2), 0.95 (s, 27H,
4.3.7. p-(4-(40-Nitroazobenzene))phenyl tri(30-thioacetyloxymethyl
[4,10]biphenyl)silane (1b). Following the procedure outlined for 1a,
from 10b (270 mg, 0.31 mmol), DIAD (1.23 mL, 6.14 mmol), PPh3
(1.61 g, 6.14 mmol), CH3COSH (0.44 mL, 6.14 mmol) in dry THF
(27 mL) compound 1b was obtained by column chromatography
(dichloromethane/cyclohexane, 8:2) as an orange solid (57 mg,
3ꢂ C(CH3)3), 4.80 (s, 6H, 3ꢂ CH2O), 7.32 (d, 2H, J¼8.0 Hz, ArH), 7.41
(t, 3H, J¼8.0 Hz, ArH), 7.50e7.60 (m,11H, ArH), 7.64 (d, 6H, J¼8.4 Hz,
ArH), 7.68 (d, 6H, J¼8.4 Hz, ArH) ppm; 13C NMR (100 MHz, CDCl3):
d
ꢀ5.2 (3ꢂ Si(CH3)2), 18.4 (3ꢂ C(CH3)3), 26.0 (3ꢂ C(CH3)3), 65.0 (3ꢂ
CH2O), 124.9 (CeBr), 125.4 (3ꢂ C), 125.8 (3ꢂ CH), 126.7 (6ꢂ CH),
128.7 (3ꢂ CH), 131.2 (3ꢂ CH), 132.3 (3ꢂ C), 133.3 (3ꢂ CH), 136.8 (6ꢂ
CH), 138.0 (2ꢂ CH), 140.6 (4ꢂ C), 142.0 (2ꢂ CH), 142.6 (3ꢂ C) ppm;
yield: 17%), mp: 180e181 ꢁC. 1H NMR (400 MHz, CDCl3):
d 2.35 (s,
9H, 3ꢂ CH3), 4.19 (s, 6H, 3ꢂ CH2S), 7.29 (d, 3H, J¼7.6 Hz, ArH), 7.38
(t, 3H, J¼7.8 Hz, ArH), 7.51 (d, 3H, J¼7.6 Hz, ArH), 7.56 (s, 3H, ArH),
7.64 (d, 6H, J¼8.0 Hz, ArH), 7.73 (d, 8H, J¼8.4 Hz, ArH), 7.77e7.84
(m, 4H, ArH), 8.03 (d, 2H, J¼8.8 Hz, ArH), 8.09 (d, 2H, J¼8.8 Hz, ArH),
IR (KBr):
y ;
3300, 2950, 2930, 1630, 1580, 1270, 1150, 1100 cmꢀ1
HRMS m/z Calcd for C63H79BrO3Si4Na: 1097.4187, found: 1097.4180.
4.3.4. 40-Nitroazobenzene-4-boronic acid pinacol ester (8). Under an
argon atmosphere, a solution of 7 (300 mg, 0.98 mmol), bis(pina-
colato)diboron (299 mg, 1.18 mmol), Pd(dppf)2Cl2 (160 mg,
0.20 mmol), KOAc (288 mg, 2.94 mmol) in dry DME (30 mL) was
stirred at reflux for 12 h. After this period, the mixture of reaction
was cooled at 20 ꢁC, filtered, and diluted with CH2Cl2 (15 mL). The
organic solution was washed with water (2ꢂ10 mL) and brine
(1ꢂ10 mL), dried over anhydrous MgSO4 and concentrated to dry-
ness under vacuum. The residue was separated by column chro-
matography (cyclohexane/EtOAc, 9:1) to give compound 8 as red
crystals (193 mg, 56%), mp: 197e200 ꢁC. 1H NMR (400 MHz, CDCl3):
8.38 (d, 2H, J¼9.2 Hz, ArH) ppm; 13C NMR (100 MHz, CDCl3):
d 30.3
(3ꢂ CH3), 33.4 (3ꢂ CH2S), 123.4 (2ꢂ CH), 124.0 (2ꢂ CH), 124.7 (2ꢂ
CH), 125.0 (2ꢂ CH), 126.1 (3ꢂ CH), 126.7 (8ꢂ CH), 127.7 (3ꢂ CH),
128.0 (3ꢂ CH), 129.1 (3ꢂ CH), 132.8 (3ꢂ C), 136.8 (8ꢂ CH), 137.0 (3ꢂ
C), 138.2 (3ꢂ C), 141.2 (2ꢂ C), 142.0 (3ꢂ C), 144.8 (C), 148.7 (CeNO2),
151.7 (CeN]N), 155.8 (CeN]N), 194.9 (3ꢂ C]O) ppm; UV (CDCl3)
lmax: 248, 320, 450 nm; HRMS m/z Calcd for C63H51N3O5S3SiNa:
1076.2658, found: 1076.2658.
4.4. Preparation of self-assembled monolayers on gold
d
1.36 (s, 12H, 4ꢂ CH3), 7.93 (d, 2H, J¼8.4, Hz, ArH), 7.98 (d, 2H,
J¼8.4, Hz, ArH), 8.03 (d, 2H, J¼8.8 Hz ArH), 8.37 (d, 2H, J¼8.8 Hz,
ArH) ppm; 13C NMR (100 MHz, CDCl3):
24.9 (4ꢂ CH3), 84.2 (2ꢂ C),
122.4 (2ꢂ CH), 123.5 (2ꢂ CH), 124.7 (2ꢂ CH), 135.7 (2ꢂ CH), 148.8
The procedure described by Valkenier et al.27 was followed to
obtain SAMs on gold of molecules 1a and 1b. In a typical experi-
ment, compound 1a (or 1b) was dissolved in THF with 15% (v/v) of
Et3N. Concentrations of 1a (or 1b) solutions were w0.5 mg/mL.
Glass slides with thermally evaporated gold layer (thickness
w40 nm) were used as substrates for SAMs preparation. Half of the
gold substrate was immersed in the solution of the tripods at room
temperature for 48 h. Therefore half of the gold slide was covered
SAMs of the organic molecules, and half was left as a bare gold layer
d
(CeNO2), 154.0 (CeN]N), 155.7 (CeN]N) ppm; IR (KBr):
y 2931,
2160, 1605, 1523, 1356, 1341, 1143, 1087, 853 cmꢀ1; MS m/z (%): 353
(50, Mþ), 203 (100).
4.3.5. p-(4-(40Nitroazobenzene))phenyl tri(30-tert-butyldimethylsily-
loxymethyl[4,10]biphenyl)silane (9b). Under an argon atmosphere,

3474
J. Hierrezuelo et al. / Tetrahedron 69 (2013) 3465e3474
7. Mahimwalla, Z.; Yager, K. G.; Mamiya, J.-i; Shishido, A.; Priimagi, A.; Barrett, C. J.
Polym. Bull. 2012, 69, 967.
8. Tirosh, E.; Benassi, E.; Pipolo, S.; Mayor, M.; Valasek, M.; Frydman, V.; Corni, S.;
for further reference. Afterward, samples were taken out from the
solution and intensively rinsed with chloroform.
ꢀꢁ
The Kelvin Probe (KP) method was then applied to obtain the
values of the surface potential of the organic films on gold elec-
trodes.31 The surface potential measurements were performed
under air conditions, to obtain a 2D surface map of half covered by
SAMs of 1a or 1b slides.39
Cohen, S. R. Beilstein J. Nanotechnol. 2011, 2, 834.
9. Beharry, A. A.; Sadovski, O.; Andrew-Woolley, G. J. Am. Chem. Soc. 2011, 133,
19684.
ꢂ
10. Wang, Z.; Nygard, A.-M.; Cook, M. J.; Russell, D. A. Langmuir 2004, 20, 5850.
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€
M.; Rampi, M. A.; Samori, P. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 9937.
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13. Gahl, C.; Schmidt, R.; Brete, D.; McNellis, E. R.; Freyer, W.; Carley, R.; Reuter, K.;
Weinelt, M. J. Am. Chem. Soc. 2010, 132, 1831.
4.5. Preparation of CdS coated with 1b
21.9 mg (1.19ꢂ10ꢀ1 mmol) of CdCl2 were dissolved in H2O
(170 mL) and gently, 114.6 mg (0.767 mmol) of MSA was added and
left to stabilize during 24 h. Equi-molar sodium sulfide
(1.29ꢂ10ꢀ1 mmol) was added and left for 72 h, to afford a yellowish
transparent solution of CdS QDs nanoparticles. This was dialyzed at
room temperature for 12 h against deionized water, by using
14. Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282.
15. Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev.
2005, 105, 1103.
16. Yao, Y. X.; Tour, J. M. J. Org. Chem. 1999, 64, 1968.
17. Shirai, Y.; Guerrero, J. M.; Sasaki, T.; He, T.; Ding, H.; Vives, G.; Yu, B. C.; Cheng,
L.; Flatt, A. K.; Taylor, P. G.; Gao, Y.; Tour, J. M. J. Org. Chem. 2009, 74, 7885.
18. Thyagarajan, S.; Liu, A.; Famoyin, O. A.; Lamberto, M.; Galoppini, E. Tetrahedron
2007, 63, 7550.
19. Lamberto, M.; Pagba, C.; Piotrowiak, P.; Galoppini, E. Tetrahedron Lett. 2005, 46,
a
cellophane membrane (MW CO 12e14 kDa, Medicell In-
4895.
ternational). After this period, 10 mg of 1b was added to the solu-
tion, in order to be absorbed onto the surface of the CdS
nanoparticles. This suspension was again dialyzed (room temper-
ature) for 12 h against deionized water by using a cellophane
membrane (MW CO 12e14 kDa, Medicell International). The yellow
purified suspension was used for analysis.
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