Molecular recognition on a cavitand-functionalized silicon surface

Article abstract of DOI:10.1021/ja901678b

A Si(100) surface featuring molecular recognition properties was obtained by covalent functionalization with a tetraphosphonate cavitand (Tiiii), able to complex positively charged species. Tiiii cavitand was grafted onto the Si by photochemical hydrosily

Full text of DOI:10.1021/ja901678b

Published on Web 05/11/2009
Molecular Recognition on a Cavitand-Functionalized Silicon
Surface
†
‡
‡
‡
Elisa Biavardi, Maria Favazza, Alessandro Motta, Ignazio L. Fragalà,
§
⊥
⊥
†
Chiara Massera, Luca Prodi, Marco Montalti, Monica Melegari,
,
‡
,†
Guglielmo G. Condorelli,* and Enrico Dalcanale*
Dipartimento di Chimica Organica e Industriale, UniVersity of Parma and INSTM UdR Parma,
V.le G. P. Usberti 17/A, 43100 Parma, Italy, Dipartimento di Scienze Chimiche, UniVersity of
Catania and INSTM UdR Catania, V.le A. Doria 6, 95100 Catania, Italy, Dipartimento di
Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, UniVersity of Parma,
V.le G. P. Usberti 17/A, 43100 Parma, Italy, and Dipartimento di Chimica “G. Ciamician”,
Latemar Unit UniVersity of Bologna, Via Selmi 2, 40126 Bologna, Italy
Received March 4, 2009; E-mail: enrico.dalcanale@unipr.it; guidocon@unict.it
Abstract: A Si(100) surface featuring molecular recognition properties was obtained by covalent function-
alization with a tetraphosphonate cavitand (Tiiii), able to complex positively charged species. Tiiii cavitand
was grafted onto the Si by photochemical hydrosilylation together with 1-octene as a spatial spectator.
The recognition properties of the Si-Tiiii surface were demonstrated through two independent analytical
techniques, namely XPS and fluorescence spectroscopy, during the course of reversible complexation-
guest exchange-decomplexation cycles with specifically designed ammonium and pyridinium salts. Control
experiments employing a Si(100) surface functionalized with a structurally similar, but complexation inactive,
tetrathiophosphonate cavitand (TSiiii) demonstrated no recognition events. This provides evidence for the
complexation properties of the Si-Tiiii surface, ruling out the possibility of nonspecific interactions between
-
the substrate and the guests. The residual Si-O terminations on the surface replace the guests’ original
counterions, thus stabilizing the complex ion pairs. These results represent a further step toward the control
of self-assembly of complex supramolecular architectures on surfaces.
Introduction
molecular recognition is particularly noteworthy for its profound
impact on biology and materials science. In living organisms,
The introduction of specific functions onto surfaces represents
one of the major themes in contemporary chemistry. Notable
examples of functional surfaces include: self-cleaning surfaces
mimicking the lotus leaf, gecko-foot mimetic adhesive surfaces
featuring carbon nanotubes-decorated silicon wafers, mono-
layers with control of surface wettability via electrically triggered
conformational transitions, high density molecular electronic
memory using bistable rotaxanes as storage elements. Among
the several surface functions identified to be worth pursuing,
multiple recognition events trigger the immune system response
through antibodies, and they promote the adhesion of viruses
5
1
on cell surfaces. In nanotechnology, the controlled positioning
2
of molecules and assemblies on surfaces can be driven by
6
multiple binding events. Another significant field impacted by
3
molecular recognition on surfaces is chemical sensing, in which
7
4
the recognition process is translated into an analytical signal.
8
Organic monolayers hosted on inorganic surfaces represent
the best approach for harnessing the full potential of molecular
†
Dipartimento di Chimica Organica e Industriale, University of Parma
and INSTM UdR Parma.
‡
Dipartimento di Scienze Chimiche, University of Catania and INSTM
(5) Mammen, M.; Choi, S.-K.; Whitesides, G. M. Angew. Chem., Int. Ed.
1998, 37, 2754–2794.
UdR Catania.
§
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica,
(6) (a) Ludden, M. J. W.; Reinhoudt, D. N.; Huskens, J. Chem. Soc. ReV.
2006, 35, 1122–1134. (b) Ludden, M. J. W.; Mulder, A.; Tamp e` , R.;
Reinhoudt, D. N.; Huskens, J. Angew. Chem., Int. Ed. 2007, 46, 4104–
4107.
Chimica Fisica, University of Parma.
⊥
Dipartimento di Chimica “G. Ciamician”, Latemar Unit University of
Bologna.
(
1) (a) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003,
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10.1021/ja901678b CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 7447–7455 9 7447

A R T I C L E S
Biavardi et al.
Chart 1. Structure of the Anchored Receptors and the Tested Guests
9
16
recognition on surfaces. Compared to both thin films and bulk
as well as neutral molecules such as alcohols. This diverse
complexation ability is the result of three interaction modes,
which can be activated either individually or in combination
by the host according to the guest requirements. These include
(i) multiple ion-dipole interactions between the inward facing
materials containing molecular receptors, such hybrid
organic-inorganic materials have the advantage of reducing or
even eliminating nonspecific interactions which often mask the
recognition events. Silicon is a particularly attractive inorganic
platform, as it offers the possibility to make robust and durable
devices by forming stable Si-C covalent bonds. Moreover, the
grafting of molecular receptors on silicon wafers represents an
important step toward the generation of silicon-integrated
devices.
Phosphorus-bridged cavitands are a promising class of
synthetic molecular receptors. The introduction of four P(V)
stereogenic centers as bridging units creates a family of six
diastereomeric cavitands, each with a different orientation of
the P)O groups, i.e. inward (i) or outward (o) with respect to
10
1
4
P)O groups and the positively charged guests, (ii) single or
1
4,17
multiple H-bonding involving the P)O groups,
and (iii)
CH -π interactions between an acidic methyl group present
3
1
6
on the guest and the π-basic cavity of the host. Depending
on the type and number of interactions activated, the measured
Kass in nonpolar solvents can vary between 10 M for short-
chain alcohols to 10 -10 M for N,N-methylalkylammonium
1
1
2
-1
9
10
-1
1
5b,18
salts.
We recently published a protocol for the covalently assembly
of cavitands on silicon surface, along with various aspects of
the coordination chemistry of the resulting cavitand-decorated
assemblies. In the current study we report a comprehensive
19
12
the cavity. Among these molecules, tetraphosphonate cavitands
with an all-inward configuration (Chart 1) are particularly
versatile in their ability to complex positively charged species,
1
3
20
investigation of the molecular recognition properties of a silicon
surface decorated with phosphonate cavitands toward meth-
ylpyridinium and methylammonium guests. The entire process,
consisting of methylpyridinium complexation, guest exchange
with methylammonium salts, base-driven decomplexation of the
latter accompanied by restoration of the initial complex, is fully
reversible. Each step of the process was monitored using two
independent techniques, namely X-ray photoelectron spectros-
copy (XPS) and fluorescence spectroscopy. Control experiments
with a silicon surface covered with a structurally similar, yet
1
4
15
such as inorganic cations, ammonium and pyridinium salts,
(
9) (a) Descalzo, A. B.; Mart `ı nez-M a` o` ez, R.; Sancen o` n, F.; Hoffmann,
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Chem. Commun. 2007, 1050–1052.
(
(
10) For a clearly defined example of the influence of non specific
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Tonezzer, M.; Melegari, M.; Maggioni, G.; Milan, R.; DellaMea, G.;
Dalcanale, E. Chem. Mater. 2008, 20, 6535–6542.
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J.-P.; Dalcanale, E. Chem.-Eur. J. 2008, 14, 5772–5779.
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2007, 3865–3867.
8
4, 1505–1514.
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(
3
L.; Mann, G.; Heyer, U.; Spera, S. J. Org. Chem. 1995, 60, 235–242.
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P. Eur. J. Org. Chem. 2001, 3695–3704.
(
(
(18) Yebeutchou, R. M.; Tancini, F.; Demitri, N.; Geremia, S.; Mendichi,
R.; Dalcanale, E. Angew. Chem., Int. Ed. 2008, 47, 4504–4508.
(19) Condorelli, G. G.; Motta, A.; Favazza, M.; Fragal a` , I. L.; Busi, M.;
Menozzi, E.; Dalcanale, E. Langmuir 2006, 22, 11126–11133.
(20) Busi, M.; Laurenti, M.; Condorelli, G. G.; Motta, A.; Favazza, M.;
Fragal a` , I. L.; Montalti, M.; Prodi, L.; Dalcanale, E. Chem.-Eur. J.
2007, 13, 6891–6898.
(
15) (a) De Zorzi, R.; Dubessy, B.; Mulatier, J.-C.; Geremia, S.; Randaccio,
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7
448 J. AM. CHEM. SOC. 9 VOL. 131, NO. 21, 2009

Molecular Recognition on Silicon
A R T I C L E S
Table 1. XPS Atomic Composition Analysis of (a) HF Freshly
Etched Surface, (b) Pure Si-Tiiii Monolayers, (c) Mixed (ꢀ ) 0.2)
Si-Tiiii/Oct Monolayers (d) Pure Si-TSiiii Monolayers, (e) Mixed
(
ꢀ ) 0.2) Si-TSiiii/Oct Monolayers. Data Are Obtained at a 10°
Electron Takeoff Angle with Respect to the Sample Surface
Si 2p
O 1s
C 1s
P 2p
S 2s
HF etched
pure Si-Tiiii
Si-Tiiii/Oct ꢀ ) 0.2
pure Si-TSiiii
77.0
15.0
15.9
15.0
12.5
7.8
17.3
16.0
14.2
9.4
15.2
65.0
61.2
66.0
74.3
-
2.7
2.0
2.7
2.1
2.1
1.6
Si-TSiiii/Oct ꢀ ) 0.2
18
complexation inactive, thiophosphonate cavitand further vali-
dated the entire complexation cycle.
The active monolayer is constituted by a tetraphosphonate
2
1
cavitand Tiiii[C10
H19, H, Ph] with decyl feet (thereafter Tiiii,
Chart 1) anchored on H-terminated Si(100) surfaces via
22
photochemical hydrosilylation of the double bonds. This same
procedure was likewise used to graft the control monolayer
containing tetrathiophosphonate cavitand TSiiii[C10
H19, H, Ph]
(
thereafter TSiiii Chart 1). Trifluoro-, trichloro-, and bromo-
marked guests 1, 2, 3, 5 were specifically designed and prepared
for XPS detection, while the methylpyridinium guest 4 contains
pyrene as fluorescent probe.
Figure 1. High-resolution O 1s spectra from pure Si-Tiiii. a) 45° electron
takeoff angle b) 10° electron takeoff angle and from pure Si-TSiiii. c) 45°
electron takeoff angle d) 10° electron takeoff angle.
intensity ratio of these components (I + /I ) upon the electron
1
0
Results and Discussion
C
C
takeoff angle has been observed in angular resolved XPS
experiments. This is compatible with the position of the oxidized
carbons between the aryl and alkyl moieties (both responsible
Cavitand-Functionalized Si Surface. Cavitand grafting through
photochemical hydrosilylation was optimized according to a
19
previous study. Pure and mixed monolayers of cavitands with
-octene (Oct) as spectator spacer were prepared. The use of
0
of the C signals) on the upper rim and the bottom feet of the
1
cavitands, respectively (Chart 1). Moreover, one should note
that additional contributions to these two components are always
present due to adventitious hydrocarbons and the Si-O-C
the Oct spectator spacer in the mixed monolayers improves the
passivation of the Si surface, thus minimizing substrate oxidation
19
due to aging. Mixed Si-Tiiii/Oct and Si-TSiiii/Oct surfaces were
prepared by Si grafting of 1:4 cavitand/Oct mixtures (cavitand
mole fraction ꢀ ) 0.2).
1
9
framework.
The O1s band (Figure 1) of a pure silicon surface with
covalently assembled Si-Tiiii monolayer consists of two main
components centered at 533.5 eV (Ocav) and at 532.5 eV (OSiOx).
They are due to, respectively, phosphorus-bonded oxygen atoms
of the cavitand and the SiOx formed from partial oxidation of
the substrate. As expected for a carbonaceous overlayer on the
silicon substrate, the OSiOx component decreases at low takeoff
angles, while the Ocav component increases at low takeoff angles.
This behavior is consistent with the expected grafting of the
cavitand feet on the silicon surface, leaving the P)O bridges
at the upper rim significantly distant from the surface oxide.
Similar trends are observed for the mixed Si-Tiiii /Oct mono-
layers and for the Si-TSiiii analogues (Figure 1).
The elemental depth distribution and the bonding states of
the Si-Tiiii/Oct and Si-TSiiii/Oct monolayers were characterized
by XPS analyses. Table 1 compares the elemental compositions
obtained at 10° takeoff angle of (a) a HF etched surface, (b)
pure Si-Tiiii monolayers, (c) mixed (ꢀ ) 0.2) Si-Tiiii/Oct
monolayers (d) pure Si-TSiiii monolayers, and (e) mixed (ꢀ )
.2) Si-TSiiii/Oct monolayers.
These data reveal an enhancement of the C 1s and O 1s
signals as compared to the freshly etched silicon surface. This
enhancement, together with the observation of P 2p signal, and,
for the Si-TSiiii/Oct monolayers, an S 2s signal, indicates the
presence of cavitand molecules on the silicon surface.
0
The high resolution C 1s region (Supporting Information
Figure S1) of Si-Tiiii and Si-TSiiii grafted monolayers (pure
and mixed) always included two main components: (i) a C0
component at 285.0 eV, due to the aliphatic and aromatic
The presence of the P 2p band is indicative of the presence
of Tiiii and TSiiii cavitands on the surface. In this context, a
clear distinction of phosphorus on silicon signals is not trivial,
as the 2p and 2s bands of the phosphorus nearly overlap with
plasmon loss bands of the silicon. To obtain a clear spectrum
and a rigorous quantification of the P concentration, it is
necessary to acquire XPS measurements at low takeoff angles.
2
3,25
1+
hydrocarbon backbone;
and (ii) a C component at 286.4
eV, due to the aromatic carbons bearing the bridging oxygens
of the resorcinarene skeleton. No significant dependence of the
(
21) For the nomenclature adopted for phosphonate cavitands see reference
1b.
1
(23) (a) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L.; Viscuso, O.;
Condorelli, G. G.; Fragal a` , I. L. Mater. Sci. Eng., C 2003, 23, 989–
994. (b) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Mater. Sci.
Eng., C 2003, 23, 253–257. (c) Condorelli, G. G.; Motta, A.; Fragal a` ,
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Chem., Int. Ed. 2004, 43, 4081–4084.
(
22) (a) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey,
C. E. D. Appl. Phys. Lett. 1997, 71, 1056–1058. (b) Sung, M. M.;
Kluth, G. J.; Yauw, O. K.; Maboudian, R. Langmuir 1997, 13, 6164–
6
168. (c) Sun, Q.-Y.; de Smet, L. C. P. M.; van Lagen, B.; Giesbers,
M.; Th u¨ ne, P. C.; van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.;
Sudh o¨ lter, E. J. R. J. Am. Chem. Soc. 2005, 127, 2514–2523. (d)
Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L.; Condorelli, G. G.;
Fragal a` , I. L.; Giorgi, G.; Sgamellotti, A.; Re, N. Appl. Surf. Sci. 2005,
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M. P., Eds.; WILEY-VCH: Weinheim, Germany, 1995; Vol. 1, p 444.
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The Scienta ESCA300 Database; Wiley & Sons: New York, 1992.
2
46, 52–67.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 21, 2009 7449

A R T I C L E S
Biavardi et al.
Figure 2. Typical angular dependence of high-resolution P 2p spectra from
Si-Tiiii monolayer. a) 45° electron takeoff angle b) 10° electron takeoff
angle. The tentative deconvolution reported in a) shows band overlapping.
Figure 4. Molecular structures of a) Tiiii[C
CH
the H atoms involved in interactions are shown. In the case of complex b
2
H
5
, H, Ph]•1 and b) Tiiii[H,
3
, Ph]•6. H-bonding in structure b is represented by green dots. Only
the solvent molecules and the second orientation of guest 6 are not shown.
-
the lower rim. This position is stabilized by C-H· · ·I interac-
2
tions with the CH groups of the chains, which range in length
-
from 3.077(1) to 3.246(1) Å. The distance between I and the
positive nitrogen of the guest is of 8.743(6) Å. All the above
values are in good agreement with previously reported findings
1
5a,26
in the literature.
This represents the optimal arrangement
for the counterion in order both to stabilize the ion-pair and to
experience aliphatic CH-anion interactions. The localization of
the iodide counterion is important not only for the stabilization
of the complex in solution, but also for the surface complexation
mode (as it will be discussed later).
Figure 3. a) P 2p region and b) S 2s region from Si-TSiiii monolayer at
1
0° electron takeoff angle.
In fact, the substrate plasmon loss contribution decreases at low
3
Crystal Structure of the Tiiii[H, CH , Ph]•6 Complex. A
takeoff electron angles and becomes negligible at 10° (Figure
second crystal structure was pursued to demonstrate the presence
of H-bonding interactions between the tetraphosphonate cav-
itands and the secondary ammonium ions in the solid state.
2
b).
In the XPS spectrum of Si-TSiiii, along with the presence of
15b
17
the P 2p band (Figure 3a), the presence of S 2p and S 2s signals
clearly demonstrate the grafting of TSiiii. Overall, the S 2s XPS
region (Figure 3b) provides clearer information compared to
the S 2p band, which partially overlaps with a plasmon loss
band of the Si substrate.
Solution and gas phase data already indicate that H-bonding
interactions strongly enhance complexation in the case of
protonated ammonium salts. However, structural evidence of
the number and geometry of these H-bonding interactions is
3
currently lacking. The crystal structure of Tiiii[H, CH , Ph]•6
The atomic ratio (P/O/C) of pure Si-Tiiii was inferred from
the intensity of the P 2p signal obtained at a 10° takeoff angles,
along with the Ocav component of the O 1s band and the C 1s
signal (Table 1). The measured 1 /3.1/24.1 ratio is very close
to the theoretical value of 1/3/23 expected from the molecular
structure. The analogous P/O/C ratio observed for mixed Si-
Tiiii is more carbon-rich (1/3.2/30.7), as expected when grafting
from a mixed Tiiii /Oct (ꢀ ) 0.2) solution (solution atomic ratio:
complex reported here fills this gap (Figure 4b). The cavitand
receptor employed here does not bear alkyl chains at the lower
rim. In the complex, two hydrogen bonds are present between
adjacent P)O and the NH
2
protons (dotted green lines in Figure
4b). The NH moiety of the guest is disordered over two
2
positions, each showing an occupancy factor of 0.5. This allows
the formation of two sets of energetically and geometrically
equivalent hydrogen bonds, which increase the entropic stabi-
lization of the complex (see SI for details). Further stabilization
1
/3/31).
Similarly, the P/S/O/C ratios estimated by XPS for pure and
is provided by several C-H· · ·π interactions among the N-CH
3
mixed Si-TSiiii (1/0.8/2.4/24.3 and 1/0.8/2.5/35.5, respectively)
are consistent with the expected values calculated from pure
TSiiii (1/1/2/23) and from the mixed Tiiii/Oct (ꢀ ) 0.2) grafting
solution (1/1/2/31).
hydrogensandthearomaticringsofthecavity(theC-H· · ·centroids
distances span from 2.923(9) to 2.992(9) Å and the angles span
from 136.6(3) to 147.4(4)°). Due to the disorder of the nitrogen,
also the methyl hydrogens adopt two different orientations. The
iodide counterion is located below the cavity at 3.188(3) Å from
Crystal Structure of the Tiiii[C
reported crystal structure reveals in the solid state the interaction
modes between guest 1 and Tiiii[C , H, Ph] (the short feet
2 5
H , H, Ph]•1 Complex. The
the least-squares plane defined by the four CH
of the lower rim, roughly aligned with the cation in the cavity
(in the Tiiii[C , H, Ph]•1 complex this value is 3.449(2) Å).
2
carbon atoms
2
H
5
version of the grafted cavitand, Figure 4a). The inclusion of 1
into the cavity is driven by strong dipolar interactions between
the positively charged nitrogen atom and the P)O groups of
2 5
H
This is the optimal position of the iodide counterion in the solid
state, independent from the presence of alkyl chains at the lower
+
-
Tiiii (the shortest distance for P)O· · ·N is 2.845(5) Å). Further
rim. The distance between I and the disordered nitrogen atom
stabilization is provided by a C-H· · ·π interaction between a
of the guest is of 7.199(6) Å (N1) and of 7.269(9) Å (N1′).
These values indicate that the guest in the Tiiii[H, CH , Ph]•6
3
complex is located deeper within the cavity, with respect to the
methyl hydrogen of the cation and an aromatic ring of the cavity
(
the C-H· · ·centroid distance is of 3.575(2) Å, with an angle
of 158.46(1)°). The iodide counterion is located below the cavity
at 1.904(1) Å from the least-squares plane defined by the four
(
26) Mansikkam a¨ ki, H.; Nissinen, M.; Rissanen, K. Chem Commun. 2002,
CH
2
carbons, roughly in the middle of the four alkyl chains at
1902–1903.
7
450 J. AM. CHEM. SOC. 9 VOL. 131, NO. 21, 2009

Molecular Recognition on Silicon
A R T I C L E S
Figure 5. a) F 1s XPS region of Si-TSiiii/Oct after complexation with guests
1
(red curve), Si-Tiiii/Oct before complexation (blue curve) and Si-Tiiii/Oct
complexed with guest 1 (black curve); b) Cl 2p XPS regions of Si-TSiiii/
Oct after complexation with guests 2 and 3 (red curve), Si-Tiiii/Oct before
complexation tests (blue curve); Si-Tiiii/Oct complexed with guest 2 (pink
curve) and Si-Tiiii/Oct complexed with guest 3 (black curve).
Figure 6. Proposed ion-pair formation for Si-Tiiii complexes on the silicon
surface.
2 5
Tiiii[C H , H, Ph]•1 case, drawn in by the formation of the
two H-bonds. Therefore, the reported crystal structures validate
the higher affinity of Tiiii cavitands for N,N-methylalkyl
ammonium salts over methylpyridinium salts.
efficiency. This is due to the fact that the presence of chlorine,
unlike fluorine, cannot be confused with contaminants resulting
from the HF etching process.
Complexation on Si-surface: XPS Detection. The affinity of
the Si-Tiiii toward guests 1-3 was evaluated by XPS. The guests
were specifically functionalized with three halogen atoms to
allow for XPS quantification. The Cl 2p and F 1s peaks were
monitored as the main indicators of guest complexation; they
have a significant advantage with respect to the other pyridinium
bands (namely N 1s at 402 eV, and C 1s component at 289.6
eV) in that they are located in a less crowded spectral region
and possess a higher intensity. Complexation was performed
by treating both mixed (ꢀ ) 0.2) Si-Tiiii/Oct (active sensing)
Given the 1:1 stoichiometry of all the complexes reported,
the theoretical atomic ratio between P and Cl for the complexed
species is 3/4. Hence, the efficiency of complexation (i.e., yield
of the complex formed) on the surface can be calculated as
follows:
%
% P 4
Cl 3
Yield of complexation % )
/
× 100
The complexation yield was estimated in the range 60- 70%
for both guests 2 and 3. Similar results were obtained for guest
5 (see next paragraph). No remarkable differences were observed
by changing the solvent. The independence of surface com-
plexation from both the guest and solvent nature, together the
lower efficiency compared to solution data, suggests that the
binding is related to the percentage of surface bonded cavitands
whose cavity is available for complexation.
and mixed (ꢀ ) 0.2) Si-TSiiii/Oct (control experiment) substrates
with guests 1-3 (10- M solution in CH
3
Cl
2
for 1 and in
2
3
CH CN for 2 and 3).
Figure 5 compares the F 1s and the Cl 2p regions of mixed
(
ꢀ ) 0.2) Si-Tiiii/Oct and Si-TSiiii/Oct after the complexation
reaction with either (a) guest 1 or (b) guests 2 and 3.
Precomplexation spectra are also reported for reference. The
presence of halogen bands in the Si-Tiiii/Oct grafted samples
follwing exposure to the guests demonstrates surface complex-
ation. At the same time, the absence of halogen signals for inert
Si-TSiiii/Oct precludes the possibility of physisorption, due to
nonspecific interactions between the charged guest and the
surface.
Complexation on Si-surface: Fluorescence Detection. To
confirm the complexation properties of Si-Tiiii/Oct silicon
surface, we synthesized a pyridinium salt connected to a pyrene
1
5b
probe via a diester tether. Guest 4 has the peculiar property
of activating the fluorescence emission of this pyrene probe upon
complexation. The inclusion of the electron-poor methylpyri-
dinium moiety within the electron-rich cavity of Tiiii decreases
the exoergonicity of the electron-transfer process from pyrene
to the methylpyridinium unit, leading to a thirty-fold increase
An interesting observation regards the role played by the
counterions: in all cases, no significant signals were observed
for the I- and PF
-
counterions. The kinetics of iodide
15b
6
in fluorescence emission. Figure 7 (curve a) shows the typical
disappearance on the surface was monitored via XPS for guest
. Immediately following exposure of the surface to the guest,
structure of the fluorescence band of the pyrene moiety with
λmax ) 384 nm for the Si-Tiiii/Oct complexed with guest 4. No
signal is evident for Si-TSiiii/Oct under the same conditions.
These data validate the XPS results, i.e. pyridinium salts are
captured on the surface only in the presence of the Tiiii cavities.
Moreover, the absence of the excimeric band in spectra (a) and
(c) indicates that the complexes on the surface are isolated and
not clustered.
2
the I 3d5/2 signal was observed. After several minutes, however,
the intensity of the signal began to decrease, disappearing
entirely in about one hour (see Figures S2 and S3, SI).
The counterions, positioned between the alkyl chains as
indicated by the crystal structures, are sufficiently close to the
surface to react with the residual Si-OH surface terminations.
The reaction leads to the formation of HF plus PF
of guest 3, and HI in the case of guests 1 and 2. These molecules
are sufficiently volatile to escape from the surface. The Si-O
terminations on the surface become the new counterions of the
Si-Tiiii complexes, sufficiently close to the guest to stabilize
the ion-pair (Figure 6). This anion replacement by a charged
surface is unexpected and unprecedented.
XPS analyses allowed to estimate the efficiency of host-
guest complexation. In this context, the chlorine-marked guests
5
in the case
Molecular Recognition on Si-surface: Reversible Guest
Exchange. Considering the potential application of tetraphos-
phonate cavitand-decorated silicon surfaces as sensor ele-
ments in integrated devices, a fundamental issue to be
addressed regards the capability of these systems to perform
reversible guest exchange processes. The reversibility of the
association process was tested by performing cyclic guest
exchange between methylpyridinium and N,N-methylalkyl
ammonium salts. The proposed complexation cycle replicates
the one previously studied by us in solution, monitored via
-
2
and 3 are better probes than guest 1, both to monitor the
1
5b
success of the complexation and to evaluate quantitatively its
fluorescence spectroscopy. XPS measurements were used
J. AM. CHEM. SOC. 9 VOL. 131, NO. 21, 2009 7451

A R T I C L E S
Biavardi et al.
recognition steps (complexation, guest exchange, and decom-
plexation) occurring on the surface of a Si-Tiiii wafer.
Conclusion
In this paper, we have described the preparation, charac-
terization, and molecular recognition properties of a func-
tional surface featuring phosphonate cavitands as receptors.
The relevant findings can be summarized as follows: (i) The
photochemical hydrosilylation protocol is compatible with
the presence of P)O/P)S bridging units on the cavitands,
allowing the formation of both pure and mixed monolayers
on the silicon surface. The mixed monolayers improve the
coverage of the surface, minimizing substrate oxidation. (ii)
The noteworthy molecular recognition properties of this class
of synthetic receptors were successfully transferred to a
technologically relevant silicon platform. (iii) The preferential
positioning of the counterions between the alkyl chains at
Figure 7. Fluorescence spectra of: (a) Si-Tiiii/Oct surface after complex-
ation test with guest 4, (b) after addition of the ammonium ion 6, and (c)
after treatment with DBU and subsequent treatment with a solution of 4.
The spectrum resulting from a pristine Si-Tiiii/Oct surface is superimposable
to curve (b).
-
the lower rim allows for their replacement by the Si-O
groups generated on the surface.
A unique feature of this study has been the possibility to
record independent sets of measurements by monitoring the
surface complexation experiments with two different tech-
niques, namely XPS and fluorescence spectroscopy. The
resulting cross validation provides a highly reliable, com-
prehensive picture of the complexation cycle on the Si-Tiiii
surface. Moreover, control experiments with the structurally
similar, but complexation inactive, Si-TSiiii surface excluded
the possibility of nonspecific interactions between the
substrate and the guests. The sequence complexation-guest
exchange-decomplexation demonstrated that all three inter-
action modes of the host available for complexation are
operative on the silicon surface without interference. In
particular, the two additional H-bonds experienced by guests
5 and 6 upon complexation, inferred from the crystal
structure, are sufficient to induce the complete guest exchange
on the Si-Tiiii surface. Removal of these guests by depro-
tonation resets the original surface cleanly and efficiently.
to monitor the steps of this cycle, specifically by tracking
the presence of the chlorine-marked guest 2 and the bromine-
marked guest 5.
The Si-Tiiii/Oct•2 surface (henceforth referred to as Si-Tiiii•2),
was taken as the starting point of the cycle; it shows a prominent
signal in the Cl 2p region (as described above) and, obviously,
no signal in the Br 3d region (Figure 8a). After treatment of
Si-Tiiii•2 with a solution of guest 5 (10- M in CH
3
CN), a
3
negligible signal is present in the Cl 2p region, while the Br 3d
region presents an evident Br 3d signal (Figure 8b). These results
are diagnostic of the exchange of the guest 2 with the guest 5
in the cavity of the tetraphosphonate cavitand, forming the Si-
Tiiii•5 system. On silicon, as in solution, the higher affinity of
1
5b,18
the guest 5 for tetraphosphonate cavitands
drives the
exchange process. The obtained complexation yield expressed
as Yield )(%Br)/(%P)/(1)/(4) × 100 was similar (70%) to that
calculated for guests 2 and 3.
1
6
27
Besides sensing and synthetic applications, the func-
tional surface described in this work represents a useful
addition to the available pool of molecular printboards for
the self-assembly of complex supramolecular architectures
Treatment of the Si-Tiiii•5 surface with a hindered Brønsted
-
3
base (such as 1,8-diazabicyclo[5.4.0]undec-7-ene, DBU; 10
M in CH CN), led to complete removal of guest 5 from the
6
3
on surfaces. Orthogonality of the Si-Tiiii interaction mode
surface, as demonstrated by the absence of both halogen
signatures in Cl 2p and Br 3d regions (Figure 8c).
DBU, by extracting a proton from guest 5, induces the
dissociation of the Si-Tiiii•5 complex, restoring the pristine Si-
Tiiii surface. Exposure of the resulting Si-Tiiii surface to a
solution of guest 2 led to the original Si-Tiiii•2 complex (Figure
with the H-bonding motifs for the layer-by-layer assembly
on silicon has likewise been demonstrated in our lab, and it
will be reported in due course.
Experimental Section
28
29
Materials. Cavitands Tiiii[C10
H
19, H, Ph], Tiiii[C
2
H
5
, H, Ph]
8
d). Figure 9 illustrates the fully reversible cycle consisting of
10
30
15b
3
and Tiiii[H, CH , Ph] and guests 1 and 4
were prepared
(
i) the complexation of the guest 2, (ii) the exchange of 2 with
following published procedures. The syntheses of the phosphonate
the protonated guest 5, and (iii) the regeneration of the original
surface by removal of guest 5 with DBU.
The same complexation cycle was monitored by fluorescence
spectroscopy using guests 4 and 6, respectively (Figure 10). In
this case, the addition of the guest 6 to the Si-Tiiii•4 system
cavitand Tiiii[C H , H, Ph], thiophosphonate cavitand TSiiii[C H ,
2
5
10 19
H, Ph], and guests 2, 3, 5, 6 are described in detail in the Supporting
Information.
Crystal Structures. The crystal structure of the complexes
+
-
Tiiii[C
2
H
5
, H, Ph] · CF
3
C
5
H
4
NCH
3
I
3
and Tiiii[H, CH , Ph] ·
(
Figure 7, trace a) led to the complete disappearance of the
(
27) Yebeutchou, R. M.; Dalcanale, E. J. Am. Chem. Soc. 2009, 131, 2452–
fluorescence signal on the surface, due to the formation of
fluorescence silent complex Si-Tiiii•6 (Figure 7, trace b).
Addition of DBU, followed by the treatment of the silicon
surface with a solution of 4, restored the initial fluorescent Si-
Tiiii•4 complex (Figure 7, trace c).
2
453.
(
28) Bibal, B.; Tinant, B.; Declercq, J.-P.; Dutasta, J.-P. Supramol. Chem.
2003, 15, 25–32.
(29) Cantadori, B.; Betti, P.; Boccini, F.; Massera, C.; Dalcanale, E.
Supramol. Chem. 2008, 20, 29–34.
(
30) Timperley, C. M.; Bird, M.; Heard, S. C.; Notman, S.; Read, R. W.;
Tattersall, J. E. H.; Turner, S. R. J. Fluorine Chem. 2005, 126, 1160–
1165.
The combination of XPS and fluorescence spectroscopy
provides a consistent and reliable picture of the three consecutive
7
452 J. AM. CHEM. SOC. 9 VOL. 131, NO. 21, 2009

Molecular Recognition on Silicon
A R T I C L E S
Figure 8. Cl 2p and Br 3d XPS region after treatment of the Tiiii-grafted silicon surface with a) a solution of guest 2, b) a solution of guest 5, c) a solution
of DBU, and d) a solution of guest 2, again.
Figure 9. Complexation cycle of Si-Tiiii, as monitored by XPS. The Si-O- counterions are omitted for clarity.
C
4
9
H NH
2
CH
3
+I- were determined by single crystal X-ray dif-
of diffuse solvent which could not be sensibly modeled in terms
fraction methods. Crystallographic and experimental details for
the structures are summarized in Table S1 in the Supporting
Information. Intensity data and cell parameters were recorded
of atomic sites. Their contribution to the diffraction pattern was
2
removed and modified F
o
written to a new HKL file. The number
of electrons thus located, 191 per unit cell, are included in the
formula, formula weight, calculated density, µ and F(000). This
residual electron density was assigned to six molecules of acetone
solvent per unit cell (six molecules of acetone would give 192 e).
All the non-hydrogen atoms were refined with anisotropic
atomic displacements, with the exclusion of one of the fluorine
atoms of the guest 1, of two carbon atoms of the guest 6 and of
+
-
at room temperature for Tiiii[C NCH
2
H
5
, H, Ph] · CF
3
C
5
H
4
3
I
+
-
and at 173 K for Tiiii[H, CH , Ph] · C H NH CH I on a Bruker
3
4 9 2 3
AXS Smart 1000 single-crystal diffractometer, equipped with a
CCD area detector using graphite monochromated Mo KR
radiation. The structures were solved by direct methods using
3
1
2
the SIR97 program and refined on F
o
by full-matrix least-
32
squares procedures, using the SHELXL-97 program. Both
programs were used in the WinGX suite. The data reduction
was performed using the SAINT and SADABS programs.
The PLATON SQUEEZE procedure was used to treat regions
one carbon atom of the phenyl group in the Tiiii[H, CH , Ph]
3
3
3
host. The hydrogen atoms were included in the refinement at
3
4
35
3
6
(
(
33) WinGX: Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.
34) SAINT, Software Users Guide, 6.0; Bruker Analytical X-ray Systems:
Madison, WI, 1999.
(
(
31) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115–119.
32) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refine-
ment; University of G o¨ ttingen: G o¨ ttingen, Germany, 1997.
(35) Sheldrick, G. M. SADABS Area-Detector Absorption Correction, 2.03;
University of G o¨ ttingen: G o¨ ttingen, Germany, 1999.
(36) SQUEEZE: Sluis, P. V. D.; Spek, A. L. Acta Crystallogr., Sect. A
1990, 46, 194–201.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 21, 2009 7453

A R T I C L E S
Biavardi et al.
Figure 10. Complexation cycle of Si-Tiiii, as monitored by fluorescence. The Si-O- counterions are omitted for clarity.
idealized geometries (C-H 0.95 Å) and refined “riding” on the
corresponding parent atoms. In the guest 6, the nitrogen atom
was found disordered over two positions, each showing an
occupancy factor of 0.5. The weighting scheme used in the last
mean diameter of 279.4 mm. The analyses were carried out at
various photoelectron angles (relative to the sample surface) in
the 10°-80° range with an acceptance angle of (7° (the
acceptance angle was fixed high to avoid photoelectron diffrac-
tion effects).
2
2
2
cycle of refinement was w ) 1/[σ F + (0.0865P) + 0.7176P]
2
2
o
2
2
2
and w ) 1/[σ F
Tiiii[C H, Ph] · CF
Ph] · C
o
+ (0.0927P) ] where P ) (F
o
+ 2F
and Tiiii[H, CH
9 2 3
H NH CH I , respectively. CCDC-731069 and -731070
c
)/3, for
Atomic concentrations have been evaluated from the intensity
of both standard Mg and monochromatic Al spectra after removing
of a Shirley background and correction for the Wagner sensitivity
+
-
2
H
5
,
3
C
5
H
4
NCH
3
I
3
,
+
-
4
3
8
contain the supplementary crystallographic data. These data can
be obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html [or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-
factors. All high-resolution XPS spectra were recorded adopting
only monochromatic Al radiation. Experimental uncertainties in
3
9
binding energies lie within (0.28 eV. Some spectra were
deconvoluted by fitting the spectral profiles with a series of
symmetrical Gaussian envelopes after subtraction of the background.
1
223/336-033]. Geometric calculations were performed with the
37
2
2 1/2
PARST97 program.
The agreement factor, R ) [Σ(F
o
- F
c
) /Σ-(F
) converged to R values e0.04.
XPS B.E. scale was calibrated by centering the C 1s peak of the
o
) ] , after mini-
2
Cavitand Grafting on Si. For grafting monolayers, pure cavitand
ꢀcav ) 1.0) and cavitand/1-octene mixtures (ꢀcav ) 0.2) were
mization of the function Σ(F
o
- F
c
(
4
0,41
dissolved in mesitylene (solution concentration ) 0.05 M). Cavitand
solutions (2.0 mL) were placed in a quartz cell and deoxygenated
by stirring in a drybox for at least 1 h. A Si(100) substrate was
adventitious carbon at 285.0 eV.
Fluorescence Measurements. Fluorescence spectra on Si-Tiiii
and Si-TSiiii surfaces were obtained using a spectrofluorimeter
ISA FLUOROLOG 3 in a front-face geometry, with a He-Cd
continuum laser at 325 nm, 50 mW power, as excitation source.
dipped in H
contaminants, then it was etched in a hydrofluoric acid solution
1% v/v) for 90 s and quickly rinsed with water. The resulting
2 4 2 2
SO /H O (3:1) solution for 12 min to remove organic
(
Acknowledgment. This paper is dedicated to Prof. J. Rebek,
Jr. on the occasion of his 65th birthday. This work has been
supported by the EU through NoE MAGMANet (3-NMP
515767-2), MIUR through FIRB 2003-2004 LATEMAR (http://
www.latemar.polito.it) (B.O.), INSTM through PRISMA “Mo-
hydrogenated silicon substrate was immediately placed in the
mesitylene solution. The cell remained under UV irradiation (254
nm) for two hours. The sample was then removed from the solution
and sonicated in dichloromethane for 10 min to remove residual
physisorbed material.
Cavitand Complexation Tests. To test the complexation
properties of Si-Tiiii and Si-TSiiii, samples were dipped in a solution
-3
(38) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond,
1
CH
0
M of guests 1-6 in CH
3
CN for 30 min and then sonicated in
CN for 10 min. The same treatment was followed for the DBU
R. H.; Gale, L. H. Surf. Interface Anal. 1981, 4, 211–225.
3
(
39) (a) Cerofolini, G. F.; Galati, C.; Renna, L.; Viscuso, O.; Camalleri,
M.; Lorenti, S.; Condorelli, G. G.; Fragalà, I. L. J. Phys. D.: Appl.
Phys. 2002, 35, 1032–1038. (b) Cerofolini, G. F.; Galati, C.; Renna,
L. Surf. Interface Anal. 2002, 34, 583–589.
decomplexation.
XPS Characterizations. The XPS spectra were run with a PHI
ESCA/SAM 5600 Multy technique spectrometer equipped with
a monochromated Al KR X-ray source, a standard dual-anode
Mg/Al source and a spherical capacitor analyzer (SCA) with a
(40) Swift, I. L. Surf. Interface Anal. 1982, 4, 47–51.
(
41) Cerofolini, G. F.; Galati, C.; Lorenti, S.; Renna, L.; Viscuso, O.;
Bongiorno, C.; Raineri, V.; Spinella, C.; Condorelli, G. G.; Fragal a` ,
I. L.; Terrasi, A. Appl. Phys. A: Mater. Sci. Process. 2003, 77, 403–
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37) Nardelli, M. J. Appl. Crystallogr. 1996, 29, 296–300.
454 J. AM. CHEM. SOC. 9 VOL. 131, NO. 21, 2009
7

Molecular Recognition on Silicon
A R T I C L E S
lecular systems grafted on silicon for integrated sensing devices”.
The instrumental facilities at the Centro Interfacolt a` di Misure
G. Casnati of the University of Parma were utilized. We thank
Dr. M. Busi for growing X-ray quality crystals of one phos-
phonate complex.
XPS measurements and kinetics; X-ray crystallographic data
and CIF files; ORTEP drawing of the two crystal structures.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Supporting Information Available: Preparation and charac-
terization of host TSiiii and guests 2, 3, 5, 6; high-resolution
JA901678B
J. AM. CHEM. SOC. 9 VOL. 131, NO. 21, 2009 7455