Covalent functionalization of silicon surfaces with a cavitand-modified salen

Article abstract of DOI:10.1002/ejic.201001239

Si(100) and porous silicon substrates have been engineered with cavitand modified salen molecules. A salen derivative, specifically designed for covalently anchoring on silicon, was grafted onto Si(100) surfaces by photochemical hydrosilylation, and onto porous silicon by thermal hydrosilylation. These hybrid systems have been studied by different analytic techniques. Monochromatized X-ray photoelectron spectroscopy (XPS) was used to characterize both flat and porous samples. Atomic force microscopy (AFM) and, in particular, atomic force lithography (AFL) were used to evaluate the monolayer structure and morphology of the flat silicon surface, while Fourier transform infrared spectroscopy (FTIR) was adopted to probe the hydrosilylation process involving porous samples. The uranyl complex of the salen derivative was directly synthesized on the silicon surface by the reaction between Si-anchored salen and uranyl acetate. This surface-based synthesis suggests that the salen molecules are intact and keep their specific properties after silicon anchoring.

Full text of DOI:10.1002/ejic.201001239

FULL PAPER
DOI: 10.1002/ejic.201001239
Covalent Functionalization of Silicon Surfaces with a Cavitand-Modified Salen
Cristina Tudisco,[a] Giuseppe Trusso Sfrazzetto,[a] Andrea Pappalardo,[a]
Alessandro Motta,[a] Gaetano A. Tomaselli,[a] Ignazio L. Fragalà,[a]
Francesco P. Ballistreri,[a] and Guglielmo G. Condorelli*[a]
Keywords: Silicon / Surface chemistry / Monolayers / Metalation / Cavitands
Si(100) and porous silicon substrates have been engineered
with cavitand modified salen molecules. A salen derivative,
specifically designed for covalently anchoring on silicon, was
grafted onto Si(100) surfaces by photochemical hydro-
silylation, and onto porous silicon by thermal hydrosilylation.
These hybrid systems have been studied by different analytic
techniques. Monochromatized X-ray photoelectron spec-
troscopy (XPS) was used to characterize both flat and porous
samples. Atomic force microscopy (AFM) and, in particular,
atomic force lithography (AFL) were used to evaluate the
monolayer structure and morphology of the flat silicon sur-
face, while Fourier transform infrared spectroscopy (FTIR)
was adopted to probe the hydrosilylation process involving
porous samples. The uranyl complex of the salen derivative
was directly synthesized on the silicon surface by the reac-
tion between Si-anchored salen and uranyl acetate. This sur-
face-based synthesis suggests that the salen molecules are
intact and keep their specific properties after silicon anchor-
ing.
the ubiquitous use of silicon in microelectronic devices, the
covalent anchoring of synthetic receptors on Si represents
a key aim in the development of Si-integrated sensing mate-
rials, and some examples of supramolecular receptors
grafted on monocrystalline Si(100) have been recently re-
ported.[9–11]
This paper reports on the synthesis and the covalent an-
choring onto silicon of a quinoxalinic cavitand modified
with N,NЈ-bis(salicylaldehyde) ethylenediimine (salen), re-
ceptor 1, which has been functionalized with an undecyl-
enic group for Si anchoring (Scheme 1). The processes of
covalent anchoring of this cavitand onto both monocrystal-
line Si(100) and porous silicon (PSi) have been investigated
since both of these substrates are of technological interest.
Si(100) is, in fact, the surface of choice for the development
of commercial microelectronic devices, whilst porous silicon
represents an interesting candidate for novel devices due to
its specific properties such as high surface area and lumines-
cence.[12]
1-functionalized monocrystalline silicon (Si-1) and po-
rous silicon (PSi-1) were characterized by XPS, which is an
ideal tool for obtaining molecular information relating to
nanometric layers. AFM and, in particular, AFL were used
to evaluate the structure and morphology of the function-
alized Si(100) surface, while FTIR was adopted to probe
the grafting of 1 onto PSi.
The functionality of the hybrid Si-1 surface was tested
by reacting Si-1 with uranyl acetate solution. In this way a
salen uranyl complex was obtained directly on the Si-1 sur-
face via a 2D surface synthetic route.[2a] It is well known
that chiral uranyl-salen complexes act as receptors for enan-
Introduction
The use of organic molecules to impart specific surface
properties has been a subject of intense research not only
from a fundamental research perspective, but also because
of the many technological applications these systems are
suitable for.[1] In this context, the functionalization of inor-
ganic surfaces with organic monolayers[2] is an attractive
approach to develop hybrid materials, with tuned func-
tional properties, that are suitable for a variety of applica-
tions.[3] Several workers have explored the use of molecular
monolayers as active layers in thin-film interfaces,[4] molec-
ular devices,[5] molecular magnets[6] and various sensors.[7]
In this last field, considerable contributions that have as-
sisted in the development of new receptors have been given
by synthetic organic chemistry, and many systems have been
synthesized with molecular recognition properties based on
specific interactions and shape complementarity.[8] Never-
theless, any synthetic effort remains useless for most practi-
cal applications unless the organic receptors are assembled
in solid devices. For such applications, monolayer-based
sensors have several advantages compared to both thin
films and bulk materials for which the weak signals due to
the interactions between surface layers and analytes are
often covered by the signals due to the bulk layers. Given
[a] Dipartimento di Scienze Chimiche, University of Catania and
INSTM Udr of Catania,
V.le A. Doria 6, 95125 Catania, Italy
Fax: +39-095-580-138
E-mail: guido.condorelli@unict.it
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001239.
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Covalent Functionalization of Silicon Surfaces
Scheme 1. Procedure for the synthesis of the salen derivative 1.
tioselective molecular recognition, due to the fact that they from solution experiments (see Supporting Information).
are able to recognize anions by exploiting the Lewis acidity The second site is a salen-derived moiety based on a bis(sal-
of the uranyl centre that equatorially coordinates with icylidene)-1,2-diphenylethylenediimine group that is active
anions.[13d–13e] The uranyl-salen complex derived from re- for the complexation of metallic cations such as MnIII and
VI [13]
ceptor 1 behaves as a heteroditopic receptor in which the UO2
metal centre is able to coordinate the anion, whereas the π-
.
XPS characterization of Si-1 and PSi-1: Cavitand-salen 1
rich quinoxalinic cavity binds tetraalkylammonium cations was grafted onto H-terminated Si(100) surfaces via photo-
by stabilizing them with CH···π and cation···π interactions. chemical hydrosilylation of the double bonds, while ther-
The synthesis of the uranyl-salen complex on Si-1 was per- mal-driven hydrosilylation was adopted for grafting of 1
formed with a double purpose: (i) to evaluate possible sur- onto PSi.
face routes towards metal-salen complexes, and (ii) to prove
The 1-functionalised monocrystalline and porous silicon-
that after silicon anchoring the salen molecules are intact based materials (Si-1 and PSi-1) were characterized by
and keep their specific molecular properties. To validate the XPS. Table 1 compares the elemental compositions of two
complexation results, control experiments on native silicon reference samples, namely HF-etched silicon (a) and HF-
oxide and alkyl-functionalized surfaces were also per- etched PSi (b), with freshly prepared Si-1 (c), aged Si-1 (d)
formed.
and freshly prepared PSi-1 (e).
Table 1. XPS determined atomic concentrations for (a) HF-etched
Si; (b) HF-etched PSi; (c) fresh Si-1 (d) aged Si-1 and (e) PSi-1.
Results and Discussion
Atomic concentration
Cavitand-salen 1. Synthesis and complexation properties:
A cavitand-modified salen suited for silicon anchoring (1)
was synthesized (according to Scheme 1) from the monofor-
myl cavitand 2.[13,14] The reaction of 3-hydroxysalicylalde-
hyde with 11-bromo-1-undecene leads to compound 3.
Si 2p
O 1s
C 1s
N 1s
–
(a) HF-etched Si
77.0
7.8
7.6
15.2
15.7
23.0
22.0
47.7
(b) HF-etched PSi[a] 76.7
–
(c) Fresh Si-1
(d) Aged Si-1
58.5
47.6
28.4
17.8
29.7
22.0
0.7
0.7
1.8
Condensation of 3 with (1R,2R)-(+)-diphenylethylenedi- (e) PSi-1[a]
amine chloride[13a] affords the monoimine ammonium chlo-
ride derivative 4. Monoformyl cavitand 2 was finally con-
verted into the enantiopure receptor 1 by treatment with
compound 4 in the presence of triethylamine. Compounds
[a] A small amount of residual Ag (Ag Ͻ 0.2%) was found in the
PSi substrates.
The XPS data show that C 1s signals are observed for
1–4 were fully characterized by NMR and EI-MS tech- both reference samples due to the ubiquitous and adven-
niques. Salen 1 is a receptor that possesses two different titious presence of contaminant species.[16] However, both
binding sites. The first site is a quinoxalinic cavity that is the Si-1 and PSi-1 surfaces show a significant enhancement
active for tetraalkylammonium ions binding,[15] and the of C 1s related signals compared to both the HF-etched
complexation properties of which have been determined Si and PSi samples. This enhancement, and especially the
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presence of N 1s signals due to the quinoxaline and salen bond to oxygen and nitrogen atoms.[9] The broad band de-
moieties, is an indication of the grafting of 1 onto the sub- tectable in the spectrum of PSi-1 at approximately 291.5 eV
strate surfaces. From XPS data (Table 1), the surface den- can be assigned to the π-π* shake-up of aromatic rings.[20]
sity of salen 1 on monocrystalline Si(100) has been esti- The same band in the Si-1 spectrum cannot be discerned
mated[17] to be ca. 6ϫ1012 molecules/cm2, this leads to a from the background due to the much lower absolute
molecular footprint of 16 nm2 for each molecular unit. This amount of 1 on the flat surface of Si compared to number
footprint size is much higher than the density functional of molecules of 1 on the porous surface of PSi. No relevant
theory (DFT) estimated cross-sectional area of 1 (ca. change in the C 1s band shape occurs after aging of the
4 nm2), and therefore this experimentally determined size samples.
indicates a low surface coverage.
Useful information on the nature of the grafted layers
was obtained from high resolution spectra of the relevant
photoemission bands.
The Si 2p spectra of the freshly etched Si and PSi sur-
faces show bands due to a doublet (at 99.2 and 99.8 eV,
respectively) associated with elemental silicon (Si0). After
grafting 1 onto the surfaces, the Si 2p spectra of both Si-1
and PSi-1 show, besides the Si0 2p3/2-1/2 doublet, broad
bands at 103.0 eV[17] that are consistent with the presence
of oxidized silicon in the samples (Figure 1). The intensity
of this band in the spectrum of thermal grafted PSi is
Figure 2. High resolution C 1s XPS spectra of (a) fresh Si-1 and
(b) fresh PSi-1. C 1s spectra of Si-1 and PSi-1 do not change after
sample aging.
slightly higher than the intensity of the same band in the
spectrum of UV grafted Si-1, but after aging (1 week) the
oxidation levels of the two samples became comparable.
High resolution data for the N1s region of the XPS spec-
tra of fresh and aged Si-1 samples and of PSi-1 samples are
shown in Figure 3. The signals in the N 1s spectral regions
give a clear indication of the surface anchoring of 1 in these
samples.
Figure 1. High resolution Si 2p XPS spectra of (a) fresh Si-1 and
1 week aged Si-1, and (b) fresh PSi-1. The Si 2p spectrum of PSi
does not change after aging.
The presence of surface oxidation indicates that the
monolayers are not densely packed, which is in accordance
with the low surface coverage estimated from the XPS data
in Table 1. This result is not unexpected since the larger size
of the cavitand moiety compared to the alkyl foot precludes
a close packing arrangement. The high resolution C 1s
spectra (Figure 2) of Si-1 and PSi-1 are similar, and consist
of three main peaks: (i) a component at 285.0 eV (C0) due
to the aliphatic and aromatic hydrocarbon backbone of 1
Figure 3. (a) High resolution N 1s XPS spectra of aged Si-1 (up-
per), fresh Si-1 (middle) and blank-Si (bottom); (b) High resolution
N 1s XPS spectra of fresh PSi-1 (upper) and blank-PSi (bottom).
The N 1s spectrum of PSi-1 does not change after sample aging.
The N1s band in the spectrum of freshly prepared Si-1
and “adventitious” carbon; (ii) a component at 286.3 eV consists of two components: (i) a main component at
(C+1) due to oxidized carbon bonded to one oxygen atom 399.8 eV (Nqx) that is assigned to the quinoxaline nitrogen
(or double bonded to nitrogen) in the cavitand phenyl atoms,[9,18] and (ii) a less intense component at 398.0 eV
rings,[9] salen moiety,[18] as well as in the Si–O–C frame- (Nsal) that is assigned to the iminic nitrogen atoms[21] of the
works of “adventitious” contaminants,[6,9,19] and (iii) a third salen. The observed intensity ratio Nqx/Nsal is about 3,
component at 287.7 eV due to the quinoxalinic carbons that which is as expected from the molecular structure. After
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Covalent Functionalization of Silicon Surfaces
one week of sample aging, the N 1s band shape changes thermal anchoring of 1 onto the PSi surface. These results
(Figure 3) even though the total amount of nitrogen re- are supported by the evolution of Si–H features in the spec-
mains constant (Table 1). The N 1s spectrum of the aged tra as the grafting reaction progresses. The spectrum of HF-
Si-1 samples consists of the same main components; the etched PSi (Figure 4, b) shows intense bands at 2087 cm–1,
N
qx band is at 399.8 eV whilst the Nsal component (denoted 2106 cm–1 and 2142 cm–1 due to SiH, SiH2 and SiH3
as NsalH) is shifted to a higher binding energy (NsalH at stretches of H-terminated PSi, respectively.[24] After graft-
401.7 eV) than the signal in the spectrum of the freshly pre- ing 1 onto the PSi surface, the intensities of the SiHx bands
pared sample. This energy shift can be explained either by clearly decrease (Figure 4, b, PSi-1) due to the hydro-
the formation of hydrogen bonds or by the protonation[22] silylation reaction between Si–H terminations and the
of the salen nitrogen atoms due to the presence of acidic double bonds of 1. Note that a new broad feature appears
OxSi–OH groups on the oxidized surface (pKa ≈ 4.5).[23] at 2260 cm–1 due to oxygen back-bonded SiH groups
Moreover, the Nqx/NsalH intensity ratio is about 3, which is (mainly O3SiH),[24] which are formed because of silicon oxi-
consistent with the atomic ratio between the quinoxalinic dation. On the other hand, the blank-Si spectrum shows
and salen nitrogen atoms.
an intense band due to SiHx stretches (2150–2080 cm–1),
The N 1s band in the high resolution XPS spectrum of indicating that no hydrosilylation reaction occurred with
PSi-1 is similar to that observed in the spectrum of aged this sample. In addition, two new features are observed
Si-1. It consists of a Nqx component at 399.8 eV and a com- (Figure 4, b, blank-Si) at 2260 cm–1 and 2200 cm–1, which
ponent at 401.8 eV that is due to protonated or H-bonded can be assigned to Si–H stretches of the O3SiH and O2SiH
salen nitrogen atoms (NsalH). The intensity ratio Nqx/NsalH species of the oxidized surface, respectively.[24,25]
is equal to 3, which is equivalent to the ratio observed in
the spectrum of aged Si-1. Note that the surface oxidation
(Figure 1), and hence the availability of acidic OxSi–OH
groups on PSi-1, is comparable to the aged Si-1 surface;
this explains why there are similar interactions (H-bonding
or protonation) involving the salen nitrogen atoms in PSi-
1 as in aged Si-1 samples.
To rule out any possible physisorption and to demon-
strate the covalent anchoring of 1 on both flat and porous
silicon, blank-Si and blank-PSi samples have been obtained
by processing the Si and PSi surfaces with the same treat-
ment adopted for Si-1 and PSi-1, but in the absence of UV
irradiation and heating, respectively. The N 1s regions in
the XPS spectra of these samples are reported in Figure 3.
The spectra do not show any significant signal in the N 1s
region, which indicates the absence of physisorbed 1 in
these samples. This result provides proof that the grafting
of 1 onto the Si and PSi surfaces occurs only under either
UV irradiation or heating (200 °C), processes which pro-
mote the hydrosilylation of the olefin groups and the forma-
tion of Si–C bonds.
FTIR characterization of PSi-1: FTIR measurements
provided useful information for monitoring the grafting re-
action of 1 onto PSi substrates by taking advantage of the
high surface area of the porous samples. FTIR spectra of
HF-etched PSi, blank-PSi and PSi-1 samples are compared
in Figure 4. In particular, three spectral regions are re-
ported: (a) the C–H stretching region between 3200 and
2600 cm–1, (b) the Si–H stretching region between 2300 and
1900 cm–1 and (c) the region between 1300 and 600 cm–1 in
Figure 4. FTIR spectral regions between (a) 3200–2600 cm–1 (C–H
which SiOx stretching vibrations and some bending modes
stretching region), (b) 2300–1900 cm–1 (Si–H stretching region) and
are present. The presence in the PSi-1 spectrum (Figure 4,
(c) 1300–600 cm–1 (SiOx stetching and bending modes) of HF-
a) of intense bands due to the CH2 symmetric [νs(CH2)]
etched PSi, blank-PSi and PSi-1. The transmission spectrum of
and antisymmetric [νa(CH2)] stretches at 2858 cm–1 and
Si(100) has been subtracted from these spectra as the background.
2926 cm–1, respectively, and of a shoulder at 2956 cm–1 due
to the CH3 antisymmetric stretch [νa(CH3)], combined with
The 1300–600 cm–1 region of the FTIR spectrum of HF-
the absence of the homologous stretches in the spectrum of etched PSi (Figure 4, c) shows two intense bands at
HF-etched PSi and with the negligible intensity of these 660 cm–1 and 625 cm–1 that are due to the wagging vi-
signals in the blank-PSi spectrum, is consistent with the bration modes of SiH2 and SiH, respectively, and the spec-
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Figure 5. (a) AFM image of Si-1 after AFL, (b) average cross-section profile of Si-1 surface scratch and (c) DFT estimated height of 1.
The large feature on the scratch edge is due to the material removed from the scratch.
trum also shows the SiH2 bending vibrations (δSiH2) at that salen maintains its molecular functionalities after
910 cm–1.[24c] In the PSi-1 spectrum, the intensity of these grafting onto the Si surface. Reactivity towards uranyl ions
features strongly decrease, analogously to the Si–H stretch- has been evaluated for different functionalized surfaces, the
ing signals, due to the hydrosilylation reaction. The 1300– active Si-1 and, as control experiments, native silicon oxide
600 region of the PSi-1 FTIR spectrum is dominated by SiOx/Si and decyl-functionalized Si (Si-decyl), by treating
a broad band centred at 1040 cm–1 that is assigned to the samples with ethanol solutions of uranyl acetate. Figure 6
vibrational modes of SiO2.[26] In the spectrum of the blank- compares high resolution U 4f regions of the XPS spectra
PSi sample, Si–H wagging modes are intense even though of Si-1 and the reference samples (SiOx/Si and Si-decyl)
silicon oxide vibrations centred at 1110 cm–1 are also pres- after uranyl treatment. Peaks associated with U 4f7/2 and
ent.[25]
U 4f5/2 at 382.1 eV and 392.7 eV, respectively, are evident
AFM lithography on Si-1: AFM studies were carried out in the spectrum of uranyl treated Si-1, which indicates the
on HF-etched Si and flat Si-1 samples. AFM topographic presence of UVI species in this sample.[29,30]
images of Si-1 show a uniform surface with measured statis-
tical parameters (mean height Rmean = 1.1 nm and rough-
ness RMS = 0.21 nm) that are slightly higher than mea-
sured for HF-etched Si (Rmean = 0.3 nm and RMS =
0.11 nm). Since AFM images do not have enough resolu-
tion to discern single molecules, AFL (contact mode)[27] has
been used to evaluate the structure of the functionalized
surface.[28] Thus, grafted molecules of 1 were removed by
rastering different surface areas (0.5ϫ1.5 μm2) with the
AFM tip under a suitable constant force (0.24 μN). The
thus obtained scratches have an average depth of
1.1Ϯ0.2 nm (Figure 5). Such a value is lower than that ex-
pected for 1 with an extended configuration. DFT geometry
optimization was performed on 1 with a linear configura-
tion and gave a molecular length of 2.6 nm (Figure 5, c).
Figure 6. High resolution U 4f XPS spectra of various surfaces
treated with uranyl solutions: Si-1 (upper spectrum), SiOx/Si (mid-
dle spectrum) and Si-decyl (bottom spectrum).
These differences can be explained by either assuming a
folded configuration for the molecules. or by considering a
model, previously proposed, of a low density monolayer
with silicon oxidation of the surface.[28] It has been ob-
served that in these systems, SiOx islands (with thicknesses
of around 1 nm) can intercalate organic molecules.[28] Since
the SiOx islands that grow between the salen molecules can-
not be scratched during ALF due to their greater hardness
compared to organic molecules,[28] the overall scratch depth
results obtained in these measurements are lower than the
estimated length of 1.
No significant signals are detected in the spectra of
uranyl-treated SiOx/Si and decyl-Si, proving that salen is
required for uranyl coordination. The theoretical atomic ra-
tio between U and N in the uranyl complex is 1/8, hence
the complexation yield can be estimated from the measured
U/N atomic ratio (ca. 1/15) as follows:
Surface synthesis of uranyl complexes: Si-1 surface reac-
tions have been performed to test 2D surface routes for the
The thus obtained complexation yield was found to be
preparation of metal-salen complexes and to demonstrate in the 50–60% range.
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Covalent Functionalization of Silicon Surfaces
H, ArH), 7.14 (d, J = 8.0 Hz, 1 H, ArH), 6.94 (t, J = 8.0 Hz, 1 H,
ArH in the para-position relative to OH), 5.81 (m, 1 H, CH=CH2),
4.96 (dd, J = 17.0, 10.0 Hz, 2 H, CH=CH2), 3.39 (t, J = 7.0 Hz, 2
H, O-CH2-), 2.04 (q, J = 7.0 Hz, 2 H, -CH2-CH=CH2), 1.84 (m, 2
H, -CH2-), 1.36–1.43 (m, 4 H, -CH2-), 1.29 (m, 8 H, -CH2-) ppm.
13C NMR (125 MHz, CDCl3, 27 °C): δ = 25.7, 28.8, 29.0, 29.30,
29.37, 30.0, 33.7, 77.4, 114.1, 120.9, 121.7, 124.7, 129.1, 139.0,
148.7, 149.6, 189.7 ppm. EI-MS: m/z = 291 [M + H]+. C18H26O3
(290.00): calcd. C 74.45, H 9.02; found C 74.42, H 9.04.
Conclusions
New hybrid organic-inorganic materials consisting of
quinoxaline cavitand-salen monolayers covalently bonded
to flat and porous silicon have been synthesized. The suc-
cess of the grafting protocol has been demonstrated by
combining different analytic techniques (XPS, FTIR and
AFM) with control experiments performed in the absence
of UV or thermal activation. The results from this work
indicate that molecules of 1 are covalently grafted to the
silicon surface through a hydrosilylation reaction, but after
anchoring further interactions occur between the salen ni-
trogen atoms s and the Si–OH termination groups of the
oxidized surfaces through nitrogen protonation or H-bond-
ing.
In addition, a surface-based synthetic route has been de-
veloped for the preparation of metal-salen complexes di-
rectly on a functionalized surface. This synthetic route is
based on the reaction between the Si-1 surface and uranyl
acetate solution. The results from this experiment proved
that the preparation of a system with different recognition
properties was achieved, and it also showed that the salen
reactivity, and hence its structural integrity, are kept after
it is covalently anchored to the Si surface.
Synthesis of the Mono-imino-amine Derivative 4: (1R,2R)-Diphenyl-
ethylenediamine chloride (0.52 g, 2.1 mmol) was dissolved in a 1:1
mixture of ethanol/methanol (20 mL). To this diamine solution, a
solution of 3 (0.49 g, 2.1 mmol) in a 1:1 mixture of ethanol/meth-
anol (20 mL) was added dropwise. The mixture was stirred for 24 h,
and then the solvent was removed under reduced pressure. The so-
lid residue was washed with water and diethyl ether to give 0.88 g
1
of 4 (90% yield). H NMR (500 MHz, CDCl3, 27 °C): δ = 8.17 (s,
1 H, CHN), 7.17–7.25 (m, 10 H, ArH), 6.69 (m, 2 H, ArH), 5.81
(m, 1 H, CH=CH2), 4.96 (dd, J = 17.0, 10.0 Hz, 2 H, CH=CH2),
4.79 (s, 2 H, CH methine), 3.40 (t, J = 7.0 Hz, 2 H, O-CH2-), 2.04
(q, J = 7.0 Hz, 2 H, -CH2-CH=CH2), 1.86 (m, 2 H,-CH2-), 1.36–
1.44 (m, 4 H, -CH2-), 1.30 (m, 8 H, -CH2-) ppm. 13C NMR
(125 MHz, CDCl3, 27 °C): δ = 166.3, 149.9, 145.1, 139.2, 139.0,
128.6, 127.9, 127.6, 122.5, 118.6, 117.7, 117.5, 114.1, 79.1, 33.9,
33.7, 32.8, 29.3, 29.0, 28.9, 28.7, 28.1 ppm. EI-MS: m/z = 485
[M]+. C32H41ClN2O2 (520.47): calcd. C 73.75, H 7.93, N 5.38;
found C 73.71, H 7.89, N 5.35.
The functional flat and porous materials described in this
work represent a useful addition to the available pool of
hybrid systems that are suited for silicon integration. More-
over, these results show the potential of covalent grafting
of organic molecules for the synthesis of a variety of func-
tional materials based on specifically designed receptors.
Synthesis of 1: In a round-bottomed flask monoformyl cavitand
2[14] (360 mg, 0.293 mmol) and mono-imino-amine chloride 4
(150 mg, 0.293 mmol) were dissolved in absolute ethanol (100 mL).
An excess of triethylamine (150 μL) was added to this mixture and
the reaction was stirred at room temperature overnight. The reac-
tion was monitored by TLC (hexane/EtOAc, 70:30), and the sol-
vent was removed after TLC indicated that all the starting cavitand
had disappeared from the reaction solution. The solid residue was
purified by flash chromatography (hexane/EtOAc, 70:30) to afford
80 mg of receptor 1 (17% yield). 1H NMR (500 MHz, CDCl3,
27 °C): δ = 15.55 (br. s, 1 H, OH), 10.77 (s, 1 H, OH), 10.74 (s, 1
H, OH), 8.43 (s, 1 H, CHN), 8.34 (s, 1 H, CHN), 8.01 (dd, J = 8.0,
1.5 Hz, 1 H, ArH), 7.98 (d, J = 5 Hz, 1 H, ArH), 7.95 (s, 1 H,
ArH), 7.79–7.84 (m, 2 H, ArH), 7.73 (dd, J = 8.0, 1.0 Hz, 1 H,
ArH), 7.71 (s, 1 H, ArH), 7.51–7.64 (m, 11 H, ArH), 7.50 (s, 1 H,
ArH), 7.44 (m, 2 H, ArH), 7.35 (s, 1 H, ArH), 7.23 (s, 1 H, ArH),
7.18 (d, J = 1.5 Hz, 1 H, ArH), 7.10–7.13 (m, 2 H, ArH), 7.06 (d,
J = 8.5 Hz, 1 H, ArH), 7.02–7.04 (m, 1 H, ArH), 6.92 (dd, J = 8.0,
2.5 Hz, 1 H, ArH), 6.79 (dd, J = 8.0, 2.5 Hz, 1 H, ArH), 6.66 (d,
J = 7.0 Hz, 1 H, ArH), 5.69 (t, J = 8.0 Hz, 1 H, CH methine), 5.65
(t, J = 8.0 Hz, 1 H, CH methine), 5.57 (m, 2 H, CH=CH2), 4.97
(dd, J = 17.0, 10.0 Hz, 2 H, CH=CH2), 4.81 (t, J = 8.0 Hz, 1 H,
CH methine), 4.51 (d, J = 7.0 Hz, 1 H, CH methine), 4.55–4.70
(m, 10 H, -CH2-), 3.94 (t, J = 8.0 Hz, 1 H, CH methine), 2.28–2.41
(m, 18 H, -CH2-), 2.20 (m, 2 H, -CH2-), 2.12 (m, 2 H, -CH2-), 1.30–
1.50 (m, 24 H, -CH2-), 0.90 (t, J = 7.5 Hz, 9 H, -CH3), 0.71 (t, J
Experimental Section
General: All reactions were carried out under nitrogen, and dry
ethanol was used. All chemicals were reagent grade and were used
without further purification.
The water used for porous silicon and monolayer preparations was
of Milli-Q grade (18.2 MΩcm) and was filtered through a 0.22 μm
filter.
NMR experiments were carried out at 27 °C on a Varian UNITY
Inova 500 MHz spectrometer (1H NMR at 499.88 MHz, 13C NMR
at 125.7 MHz, samples in CDCl3) equipped with pulse field-gradi-
ent module (z axis) and a tunable 5 mm Varian inverse detection
probe (ID-PFG). Chemical shifts (ppm) were referenced to tet-
ramethylsilane (TMS). EI-MS were obtained on an EI-MS
Thermo-Finnigan LCQ-DECA spectrometer equipped with an ion
trap analyzer.
A JASCO V-560 UV/Vis spectrophotometer
equipped with a 1 cm path-length cell was used for the UV/Vis
titrations.
Synthesis of Aldehyde 3: To a stirred solution of 3-hydroxysalicylal-
dehyde (300 mg, 2.16 mmol) and K2CO3 (147 mg, 1.07 mmol) in
dry acetonitrile (150 mL) a solution of 11-bromo-1-undecene
(336 mg, 2.16 mmol) was added dropwise over a 4 h period at room
temperature. The mixture was allowed to stir at room temperature
for 2 d and then refluxed for 24 h. Solvent was removed under re-
duced pressure and the solid residue was purified by flash
chromatography (CHCl3/EtOAc, 95:5) to afford 144 mg (23%
yield) of compound 3. 1H NMR (500 MHz, CDCl3, 27 °C): δ =
11.08 (br. s, 1 H, OH), 9.88 (s, 1 H, CHO), 7.18 (d, J = 8.0 Hz, 1
=
7.5 Hz, 3 H, -CH3) ppm. EI-MS: m/z = 1743 [MH +
C2H5OH]+. C109H116N8O10 (1698.16): calcd. C 77.09, H 6.89, N
6.60; found C 77.13, H 6.87, N 6.57.
Porous Silicon Preparation: Porous silicon (PSi) has been obtained
by wet metal-assisted chemical etching according to the procedure
reported by Chartier et al.[31] Briefly, Si(100) slides were dipped for
5 min in an aqueous solution of HF (0.14 m) and AgNO3
(5ϫ10–4 m) to deposit Ag particles on the surface. The slides were
then etched in aqueous solutions containing HF, H2O2 and ultra-
Eur. J. Inorg. Chem. 2011, 2124–2131
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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G. G. Condorelli et al.
FULL PAPER
pure H2O (40% HF, 30% H2O2 and H2O in a 25:10:4 ratio) for
1 min, and after etching the samples were rinsed with ultra-pure
water and dried with prepurified N2.
methylammonium ion. Figure S1: UV/Vis titration between recep-
tor 1 and N,N,N-trimethyl-α-methyl-benzylammonium iodide. Fig-
ure S2: SEM cross-section of PSi before and after grafting of recep-
tor 1 onto the surface.
The thickness and morphology of the etched layer were checked by
SEM cross-section analysis (Figure S2a), and are consistent with
those reported in literature for Ag-assisted chemical etching.
Acknowledgments
Monolayer Preparation: Compound 1 (34 mg) was dissolved in me-
sitylene (4.0 mL, 0.005 m). The solution was placed in a quartz cell
and deoxygenated by stirring in a dry box for at least 1 h. To graft
1 on flat silicon, Si(100) slides were first cleaned in “piranha“ solu-
tion (concd. H2SO4 and 35% H2O2, 70:30 v/v) at room temperature
for 12 min, rinsed in ultra-pure water for 2 min, etched in 1.0%
hydrofluoric acid for 90 s, washed with ultra-pure water for 10 s,
dried with N2, and then immediately placed in the solution of 1.
The cell remained under UV irradiation (254 nm) for 2 h in the dry
box.
This work was finacially supported by Consorzio Interuniversitario
Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM)
through PRISMA “Molecular systems grafted on silicon for inte-
grated sensing devices” project and by Ministero dell’ Istruzione,
dell’ Università e della Ricerca (MIUR) through the FIRB “ITAL-
NANONET” RBPR05JH2P project. We acknowledge the CI-
NECA award N. HP10B0RIE4 for the availability of high perform-
ance computing resource and support. Also we thank University
of Catania for financial support.
To graft 1 onto porous substrates, PSi was etched in 10% HF solu-
tion for 1 h, washed with ultra-pure water for 20 s, dried with N2,
and immediately placed in the solution of 1. The solution was then
refluxed at 200 °C for 5 h, with a slow stream of N2 bubbling
through the solution to prevent bumping.
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Received: November 25, 2010
Published Online: March 15, 2011
Eur. J. Inorg. Chem. 2011, 2124–2131
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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