Hierarchical self-assembly on silicon

Article abstract of DOI:10.1021/ja9099938

A set of modular components was designed, synthesized, and combined to yield an innovative, robust, and reliable methodology for the self-assembly of large supramolecular structures on silicon wafers. Specific host-guest and H-bonding motifs were embedded in a single molecule by exploiting the remarkable complexing properties of tetraphosphonate cavitands toward methylammonium and methylpyridinium salts and the outstanding homo- and hetero-dimerization capability of the ureidopyrimidone moiety. An assembly/disassembly sequence in solution was devised to assess the orthogonality and reversibility of H-bonding and host-guest interactions. The entire process was fully tested and characterized in solution and then successfully transferred to the solid state. The selected binding motifs resulted to be fully compatible in the assembly mode and individually addressable in the disassembly mode. The complete orthogonality of the two interactions allows the molecular level control of each step of the solid-state assembly and the predictable response to precise external stimuli. Complementary surface analysis techniques, such as atomic force microscopy (AFM), ellipsometry, and fluorescence, provided the univocal characterization of the realized structures in the solid state.

Full text of DOI:10.1021/ja9099938

Published on Web 03/10/2010
Hierarchical Self-Assembly on Silicon
Francesca Tancini,† Damiano Genovese,‡ Marco Montalti,‡ Luigi Cristofolini,&
Lucia Nasi,§ Luca Prodi,‡ and Enrico Dalcanale*,†
Dipartimento di Chimica Organica e Industriale, UniVersita` di Parma, and INSTM UdR Parma,
43124 Parma, Italy, Dipartimento “G. Ciamician”, UniVersita` di Bologna,
40126 Bologna, Italy, Dipartimento di Fisica, UniVersita` di Parma, 43124 Parma, Italy, and
Istituto CNR-IMEM, 43124 Parma, Italy
Received November 25, 2009; E-mail: enrico.dalcanale@unipr.it
Abstract: A set of modular components was designed, synthesized, and combined to yield an innovative,
robust, and reliable methodology for the self-assembly of large supramolecular structures on silicon wafers.
Specific host-guest and H-bonding motifs were embedded in a single molecule by exploiting the remarkable
complexing properties of tetraphosphonate cavitands toward methylammonium and methylpyridinium salts
and the outstanding homo- and hetero-dimerization capability of the ureidopyrimidone moiety. An assembly/
disassembly sequence in solution was devised to assess the orthogonality and reversibility of H-bonding
and host-guest interactions. The entire process was fully tested and characterized in solution and then
successfully transferred to the solid state. The selected binding motifs resulted to be fully compatible in the
assembly mode and individually addressable in the disassembly mode. The complete orthogonality of the
two interactions allows the molecular level control of each step of the solid-state assembly and the predictable
response to precise external stimuli. Complementary surface analysis techniques, such as atomic force
microscopy (AFM), ellipsometry, and fluorescence, provided the univocal characterization of the realized
structures in the solid state.
Introduction
because of their tunable strength, selectivity, and directionality.
For H-bonding, we exploited the self-assembly of ureidopy-
The development of self-assembly protocols generating
functional surfaces with well-defined structures and tunable
properties is one of the main goals of modern materials
chemistry.1 The perspective of hybrid materials, held together
by different kinds of noncovalent interactions, presenting distinct
and unrelated association dynamics, is particularly intriguing
because they lead to adaptive materials,2 characterized by
switchable functions. The resulting complexity of these hybrid
materials requires implementing combinations of two or more
different interaction modes, among which hydrogen bonding,
host-guest complexation, and metal-ligand coordination are
pivotal. They have in common a high level of structural
definition and tunable strength, which allow the design of
functional materials at the molecular level. Although these weak
interactions were employed individually to build supramolecular
architectures on surfaces,3-6 few efforts have been made on
the route of their concurrent employment for the generation of
hybrid materials and stimuli-responsive surfaces.7-9
rimidone (UPY) derivatives and 2,7-diamino-1,6-naphthyridine
diamides (NAPY) to generate robust H-bonded heterodimers.10
(3) (a) H-bonding: De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV.
2003, 32, 139–150. (b) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.;
Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029–1031. (c)
Ruben, M.; Payer, D.; Landa, A.; Comisso, A.; Gattinoni, C.; Lin,
N.; Collin, J. P.; Sauvage, J.-P.; De Vita, A.; Kern, K. J. Am. Chem.
Soc. 2006, 128, 15644–15651. (d) Llanes-Pallas, A.; Matena, M.; Jung,
T.; Prato, M.; Sto¨hr, M.; Bonifazi, D. Angew. Chem., Int. Ed. 2008,
47, 7726–7730. (e) Madueno, R.; Ra¨isa¨nen, M. T.; Silien, C.; Buck,
M. Nature 2008, 454, 618–621.
(4) (a) Host-guest: Ludden, M. J. W.; Reinhoudt, D. N.; Huskens, J.
Chem. Soc. ReV. 2006, 35, 1122–1134. (b) Ludden, M. J. W.; Mulder,
A.; Tampe`, R.; Reinhoudt, D. N.; Huskens, J. Angew. Chem., Int. Ed.
2007, 46, 4104–4107.
(5) (a) Metal-ligand coordination: Weissbuch, I.; Baxter, P. N. W.; Cohen,
S.; Cohen, H.; Kjaer, K.; Howes, P. B.; Als-Nielsen, J.; Hanan, G. S.;
Schubert, U. S.; Lehn, J.-M.; Leiserowitz, L.; Lahav, M. J. Am. Chem.
Soc. 1998, 120, 4850–4860. (b) Hatzor, A.; Moav, T.; Cohen, H.;
Matlis, S.; Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am.
Chem. Soc. 1998, 120, 13469–13477. (c) Levi, S.; Guatteri, P.; van
Veggel, F. C. J. M.; Vancso, G. J.; Dalcanale, E.; Reinhoudt, D. N.
Angew. Chem., Int. Ed. 2001, 40, 1892–1896. (d) Busi, M.; Laurenti,
M.; Condorelli, G. G.; Motta, A.; Favazza, M.; Fragala`, I. L.; Montalti,
M.; Prodi, L.; Dalcanale, E. Chem.sEur. J. 2007, 13, 6891–6898. (e)
Li, S.-S.; Northrop, B. H.; Yuan, Q.-H.; Wan, L.-J.; Stang, P. J. Acc.
Chem. Res. 2009, 42, 249–259.
Starting from this premise, we designed a set of molecules
featuring one or two binding motifs, to use as “switching
modules” to control the self-assembly process in the multistep
growth of supramolecular structures on silicon. As binding
motifs, we chose hydrogen bonding and host-guest interactions,
(6) π-Stacking: (a) Bhosale, R.; Perez-Velasco, A.; Ravikumar, V.;
Kishore, R. S. K.; Kel, O.; Gomez-Casado, A.; Jonkheijm, P.; Huskens,
J.; Maroni, P.; Borkovec, M.; Sawada, T.; Vauthey, E.; Sakai, N.;
Matile, S. Angew. Chem., Int. Ed. 2009, 48, 6461–6464. (b) Kishore,
R. S. K.; Kel, O.; Banerji, N.; Emery, D.; Bollot, G.; Mareda, J.;
Gomez-Casado, A.; Jonkheijm, P.; Huskens, J.; Maroni, P.; Borkovec,
M.; Vauthey, E.; Sakai, N.; Matile, S. J. Am. Chem. Soc. 2009, 131,
11106–11116.
† Dipartimento di Chimica Organica e Industriale, Universita` di Parma.
‡ Dipartimento “G. Ciamician”, Universita` di Bologna.
& Dipartimento di Fisica, Universita` di Parma.
§ Istituto CNR-IMEM.
(1) Descalzo, A. B.; Mart`ınez-Ma`o`ez, R.; Sanceno`n, F.; Hoffmann, K.;
Rurack, K. Angew. Chem., Int. Ed. 2006, 45, 5924–5948.
(2) Lehn, J. M. Chem.sEur. J. 1999, 5, 2455–2463.
9
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J. AM. CHEM. SOC. 2010, 132, 4781–4789 4781

A R T I C L E S
Tancini et al.
Chart 1
For the host-guest binding mode, our long-standing interest in
phosphonate cavitand chemistry11 led us to employ these
molecules as efficient hosts for N-methylpyridinium and N-
methylammonium salts.12 The two have in common the fol-
lowing features: (i) remarkable stability of their respective
complexes with Kass above 107 M-1 in chlorinated solvents; (ii)
high fidelity in recognition. The first characteristic is crucial
for generating robust self-assembly protocols, while the second
is essential for the orthogonality of the two interaction modes.
An additional feature pertaining to the host-guest motif is the
presence of specific decomplexation modes, operating either
electrochemically13 or via protonation/deprotonation.12a Silicon
was chosen as inorganic surface, since it is a technologically
relevant platform and it allows the formation of very stable and
durable grafting via Si-C bonds.
In this paper we report a precise and reversible hierarchical
assembly on silicon using a sequence of host-guest and
H-bonding interactions. The entire process has been fully tested
and characterized in solution, and then successfully transferred
to the solid state. The complete orthogonality of the two
interactions allows the molecular level control of each step of
the solid-state assembly and the predictable response to precise
external stimuli.
Results and Discussion
(7) Langner, A.; Tait, S. L.; Lin, N.; Chandrasekar, R.; Ruben, M.; Kern,
K. Angew. Chem., Int. Ed. 2008, 47, 8835–8838.
Synthesis of the Molecular Components. The compounds used
in the present work are shown in Chart 1, and their preparation
is reported in the Supporting Information (SI). The target
molecule 1 was synthesized in five steps starting from the known
monohydroxy-footed silylcavitand I (Scheme 1).12b The hy-
droxyl group at the lower rim was initially protected by reaction
with chloromethyl methyl ether. The subsequent treatment of
the resulting product II with an aqueous 36% HF solution caused
the selective removal of the dimethylsilyl bridges, affording the
free resorcinarene III, ready for the functionalization with
dichlorophenylphosphine. This latter reaction gave rise to a
tetraphosphonite intermediate,14 which was in situ oxidized by
addition of H2O2,12b to give tetraphosphonate cavitand IV. Only
the isomer featuring all the PdO groups pointing toward the
cavity was formed, due to the stereospecificity of this bridging
reaction. Then, the methylmethoxy protection was removed by
an HCl-catalyzed hydrolysis to give V. Addition of 2(6-
(8) For alternative combinations of orthogonal interaction modes on
surfaces, see: (a) Corbellino, F.; Mulder, A.; Sartori, A.; Ludden,
M. J. W.; Casnati, A.; Ungaro, R.; Huskens, J.; Crego-Calamata, M.;
Reinhoudt, D. N. J. Am. Chem. Soc. 2004, 126, 17050–17058. (b)
Ludden, M. J. W.; Huskens, J.; Reinhoudt, D. N. Small 2006, 1192–
1202.
(9) For the combination of weak interactions in supramolecular polymers
see: Hofmeier, H.; Schubert, U. S. Chem. Commun. 2005, 2423–2432.
(10) (a) Beijer, F. H.; Sijbesma, R. P.; Kooijman, H.; Spek, A. L.; Meijer,
E. W. J. Am. Chem. Soc. 1998, 120, 6761–6769. (b) Corbin, P S.;
Zimmermann, S. C. J. Am. Chem. Soc. 1998, 120, 9710–9711. (c)
Wang, X.-Z.; Li, X.-Q.; Shao, X.-B.; Zhao, X.; Deng, P.; Jiang, X.-
K.; Li, Z.-T.; Chen, Y.-Q. Chem.sEur. J. 2003, 9, 2904–2913. (d)
Park, T.; Todd, E. M.; Nakashima, S.; Zimmermann, S. C. J. Am.
Chem. Soc. 2005, 127, 18133–18142. (e) de Greef, T. F. A.; Ligthart,
G. B. W. L.; Lutz, M.; Spek, A. L.; Meijer, E. W.; Sijbesma, R P.
J. Am. Chem. Soc. 2008, 130, 5479–5486.
(11) (a) Pinalli, R.; Suman, M.; Dalcanale, E. Eur. J. Org. Chem. 2004,
451–462. (b) Pirondini, L.; Dalcanale, E. Chem. Soc. ReV. 2007, 36,
695–706.
(12) (a) Biavardi, E.; Battistini, G.; Montalti, M.; Yebeutchou, R. M.; Prodi,
L.; Dalcanale, E. Chem. Commun. 2008, 1638–1640. (b) Yebeutchou,
R. M.; Tancini, F.; Demitri, N.; Geremia, S.; Mendichi, R.; Dalcanale,
E. Angew. Chem., Int. Ed. 2008, 47, 4504–4507. (c) Biavardi, E.;
Favazza, M.; Motta, A.; Fragala, I. L.; Massera, C.; Prodi, L.; Montalti,
M.; Melegari, M.; Condorelli, G.; Dalcanale, E. J. Am. Chem. Soc.
2009, 131, 7447–7455.
(13) Gadenne, B.; Semeraro, M.; Yebeutchou, R. M.; Tancini, F.; Pirondini,
L.; Dalcanale, E.; Credi, A. Chem.sEur. J. 2008, 14, 8964–8971.
(14) Xu, W.; Rourke, J. P.; Puddephatt, R. J. R. Chem. Commun. 1993,
145–147.
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A R T I C L E S
Scheme 1a
a (i) DIPA, CHCl3/DMF, 40 °C, 48 h, 75%; (ii) HF, DMF, 45 °C, overnight, qt. yield; (iii) PhPCl2, pyridine, 70 °C, 3 h; H2O2/CHCl3, rt, 30 min, qt. yield;
(iv) HCl, CHCl3/MeOH, 60 °C, overnight, 93%; 2(6-isocyanatohexylaminocarbonylamino)-6-methyl-4[1]pyrimidinone, DABCO, CHCl3, reflux, 48 h, 49%.
isocyanatohexylaminocarbonylamino)-6-methyl-4[1]pyrimidi-
none15 at the free hydroxyl function, in the presence of 1,4-
diazobicyclo[2.2.2]octane (DABCO) as catalyst, afforded cavitand
1 in 34% overall yield.
The symmetric 2,7-acetamido-1,8-naphthyridine 2 was pre-
pared following a known literature protocol,16 while the
asymmetric 2(1-pyrenebutyricamido)-7-acetamido-1,8-naphthy-
ridine 3 was synthesized by the Pd-catalyzed amidation of
2-chloro-7-acetamido-1,8-naphthyridine,17 according to a pro-
cedure reported by Meijer and co-workers.18
and donor, respectively). The association constant between a
tetraphosphonate cavitand and a methylpyridinium guest was
determined by ITC and fluorescence to be in the 107 M-1 regime
(CH2Cl2, 298 K),12a while complexation of methylalkylammo-
nium salts is much stronger, occurring with constants not directly
measurable via ITC.12b As far as the UPY unit is conceYrned,
Meijer and co-workers showed that DDAA-AADD sequences
have a dimerization constant of 6 × 107 M-1 (CHCl3, 298 K).20
The asymmetric 2,7-diamino-1,6-naphthyridine diamide
(NAPY) 3 was equipped with a pyrenic unit to allow fluores-
cence monitoring of its complexation/decomplexation both in
solution and on surfaces. Moreover, in addition to its ho-
modimerization capability, which is quite weak (Kass) 101-102
M-1, CHCl3, 298 K), 3 is complementary to an ADDA
H-bonding array. This is the typical array shown by UPY
systems, once the self-complementary 4[1H]-pyrimidinone
conformer isomerizes to the noncomplementary 4[3H]-pyrimi-
dinone conformer (Figure 1). For this reason NAPY motifs like
that of molecule 3 are used to dissociate stable UPY-UPY
homodimers and to turn them in robust UPY-NAPY het-
erodimers,21 introducing switchability in H-bonded networks.
Preparation of monotopic guests 4, 5, and 7 was previously
reported,12a as well as that of cavitand 8.19
Properties of the Modular Components 1 and 3. Cavitand 1
is functionalized at the upper rim with four phosphonate bridges
in their all-inward configuration and at the lower rim with a
single ureidopyrimidone (UPY) unit. While the PdO groups
impart molecular recognition properties toward methylpyri-
dinium guests and methylammonium salts, the UPY motif
enables the molecule to dimerize according to a DDAA-AADD
quadruple H-bonding network (A, D: hydrogen bond acceptor
(15) Keizer, H. M.; Van Kessel, R.; Sijbesma, R. P.; Meijer, E. W. Polymer
2003, 44, 5505–5511.
Molecules 4 and 6 are efficiently complexed by tetraphos-
phonate cavitands, thanks to a synergistic combination of
multiple cation-dipole and CH3-π interactions.12c While guest
(16) Alvarez-Rua, C.; Garcia-Granda, S.; Goswami, S.; Mukherjee, R.; Dey,
S.; Claramunt, R. M.; Maria, M. D. S.; Rozas, I.; Jagerivic, N.; Alkorta,
I.; Elguero, J. New J. Chem. 2004, 28, 700–707.
(17) Corbin, P. S.; Zimmerman, S. C.; Thiessen, P. A.; Hawriluk, N. A.;
Murray, T. J. J. Am. Chem. Soc. 2001, 123, 10475–10488.
(18) Lightart, G. B. W. L.; Ohkawa, H.; Sijbesma, R. P.; Meijer, E. W. J.
Org. Chem. 2006, 71, 375–378.
(20) Sontjens, S. H. M.; Sijbesma, R. P.; Van Genderen, M. H. P.; Meijer,
E. W. J. Am. Chem. Soc. 2000, 122, 7487–7493.
(21) (a) Zhao, X.; Wang, X.-Z.; Jiang, X.-K.; Chen, Y.-Q.; Li, Z.-T.; Chen,
G.-J. J. Am. Chem. Soc. 2003, 125, 15128–15139. (b) Li, X.-Q.; Feng,
D.-J.; Jiang, X.-K.; Li, Z.-T. Tetrahedron 2004, 60, 8275–8284.
(19) Bibal, B.; Tinant, B.; Declercq, J.-P.; Dutasta, J.-P. Supramol. Chem.
2003, 15, 25–32.
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Figure 1. Pyrimidinone isomerization between 4[1H] and 4[3H] forms.
Scheme 2. Assembly/Disassembly Sequence in Solution
Figure 2. Section of 1H NMR spectra (10 mM, CDCl3) monitoring of
UPY-NAPY 1·3 formation: diagnostic NH signals of (a) 1·1 (red triangles),
(b) 3·3, (green triangles), (c) 1·1 + 1·3 (1:0.5), and (d) 1·3 (red + green
dots).
4 presents just one binding site, leading to the formation of 1:1
complexes, methylviologen 6 has a ditopic character, allowing
formation of 2:1 complexes.22 For this reason monotopic guest
4 was chosen for the complexation studies in solution, while
ditopic guest 6 was employed to build supramolecular structures
on the silicon surface. Finally, tetraphosphonate cavitands bind
very efficiently methylalkylammonium salts such as N-methyl-
butylammonium iodide 5. The additional, synergistic H-bonding
interactions between the adjacent inward-facing P(O) groups
and the two NH of the guest enhance the complexation. This
results in a higher association constant for this guest family,
which justifies the use of methylalkylammonium salts as
effective competitive guests to replace methylpyridinium species.
Complexation Studies in Solution. In order to test the
orthogonality and reversibility of H-bonding and host-guest
interactions, we performed complexation studies in solution
Figure 3. Section of 1H NMR spectra (10 mM, CDCl3) monitoring
host-guest complexation: signals relative to N-Me pyridinium aromatic
protons of (a) 4 (pink triangles), (b) 1·3 (1:1), and (c) 1·3·4 (1:1:1, pink
dots).
NH signals due to the UPY NH protons. The first set, relative
to the 1·1 homodimer, vanished when the 1:1 ratio was reached,
meanwhile the second set increased in magnitude to become
the only one present (red and green dots in Figures 2c and 2d).
Moreover, a shift for the NAPY NH peaks was observed,
consistent with a slow equilibration of dimers on the NMR
experiment time scale.
(ii) Host-Guest Complexation. Addition of 4 gave rise to
the hybrid H-bonded/host-guest system 1·3·4. Formation of
the host-guest complex was proved by the upfield shift of
HORTHO and HMETA pyridine protons, due to the shielding effect
of the receptor cavity (spectra a and c in Figure 3). On the other
hand, the unchanged position of H-bonded NH protons dem-
onstrated that host-guest complex formation did not interfere
with the previously assembled UPY-NAPY dimer (spectra b
and c in Figure 3).
1
monitored by H NMR spectroscopy.
The following sequential set of experiments was devised: (i)
formation of 1·3 UPY·NAPY heterodimer; (ii) complexation
of the monotopic methylpyridinium guest 4 by 1·3 to give the
ternary complex 1·3·4; (iii) selective disassembly/reassembly
of the H-bond motif in 1·3·4; (iv) guest exchange in 1·3·4 to
give 1·3·5 using methylbutylammonium iodide 5. The entire
sequence is sketched in Scheme 2.
(i) UPY ·NAPY Heterodimer Formation. In chloroform solu-
tion, monomers 1 and 3 exist exclusively as 1·1 and 3·3
1
homodimers, as confirmed by the presence, in the H NMR
spectrum, of signals typical for the hydrogen-bonded NH protons
(red triangles in Figure 2a and green triangles in Figure 2b).
Addition of increasing amounts of 3·3 to a 1·1 solution resulted
in the complete dissociation of 3·3 and in the concurrent
exclusive formation of 1·3 heterodimer. This process was
proved by the disappearance of the NH signals related to the
3·3 H-bonded complex and by the appearance of two sets of
(iii) H-Bonding Motif Disassembly/Reassembly. Addition of
MeOD to the 1·3·4 solution (CDCl3/MeOD, 8:2) produced the
selective disassembly of the H-bonded heterodimer, while the
host-guest system persisted (see SI, spectrum b in Figure S1).
When the added MeOD was removed by vacuum evaporation
(22) Dutasta, J.-P.; et al. Phosphorus, Sulfur Silicon 2002, 177, 1485–1488.
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A R T I C L E S
Figure 4. Absorption spectra of a 2 × 10-6 M solution of 2 and 7 and upon addition of an increasing amount of 1 (0-1 equiv).
Figure 5. Emission spectra (λexc ) 325 nm) of a 2 × 10-6 M solution of 2 and 7 and upon addition of an increasing amount of 1 (0-1 equiv) (a) and upon
subsequent addition of 5 (0-1 equiv) (b).
and the sample was redissolved in CDCl3, the initial 1·3·4
ternary complex was fully restored, demonstrating the solvent-
dependent reversibility of the H-bond motif (see SI, spectra a
and c in Figure S1).
(iv) Guest Exchange. Addition of 1 equiv of 5 to the 1·3·4
complex led to the formation of the new 1·3·5 ternary complex.
The position of the aromatic pyridine protons and the upfield
shifts undergone by the NH and N-CH3 of the ammonium salt
proved that the competitive guest replaced completely the
methylpyridinium moiety inside the cavity (see SI, spectrum d
in Figure S2). This process again did not interfere with the 1·3
heterodimer, as demonstrated by the unchanged position of the
H-bonded NH signals.
connected to a pyrene unit. This latter system was used in this
case since the formation of its complex with tetraphosphonate
cavitands can be monitored through the increase of the
luminescence intensity of the pyrene, due to the decrease of
exoergonicity of the photoinduced electron transfer process from
the pyrene to the methylpyridiniun induced by the formation
of the host-guest complex, as previously reported.12a As a
consequence, to avoid the presence of overlapping signals
coming from chromophores of the same nature, we used 2
instead of 3.
The absorption spectrum of this solution (Figure 4) was the
sum of the spectra of the two components, proving that no
interaction occurred at this stage among NAPY and 7. In
particular in the 270-300 nm range, the spectrum is dominated
by a band typical of the pyrene unit (not shown for clarity),
while in the region between 300 and 400 nm both NAPY
(prevalently in the 300-345 nm region) and pyrene (prevalently
at lower energies) absorb with similar efficiency (violet line in
Fluorescence Complexation Studies in Solution. The above
findings were also supported by photophysical investigations.
In this case, the three-component assembly was proved by
starting from an equimolar solution (2 × 10-6 M) of 2 and 7,
which presents a methylpyridinium unit as recognition moiety
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Tancini et al.
Figure 6. Self-assembly cycle on the Si-surface. The lumps on the Si-wafer indicate SiO2 growth on the surface after cavitand grafting.
Figure 4). Accordingly, also the fluorescence spectrum (black
line in Figure 5a), where the contributions of both NAPY and
7 are present, is the one expected assuming no association and
the lack of any energy transfer process among the two species
in solution.
absorption and emission spectra. In particular, a slight decrease
of the absorbance was observed at 330 and 346 nm, together
with an absorbance increase around 355 nm. These changes are
those expected from the association of UPY and NAPY, since
they were already observed in an analogous experiment
performed in the absence of 7 in solution, and particularly the
band rising at 355 nm is distinctive for the 1·2 complex.
The following step in this experiment was the addition of 1,
which, on the contrary, led to noticeable changes in both the
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A R T I C L E S
The absorption spectroscopy did not provide, however, any
information about the eventual association between the cavitand
unit in 1 and the methylpyridinium in 7, since the absorption
spectrum of this host-guest complex is simply the sum of the
components spectra.
On the contrary, fluorescence spectroscopy revealed that
interesting changes in the luminescence properties of the system
occur during the host-guest complex formation. On one hand,
after a careful correction of the spectra, it was possible to
evidence an increase of the signal in the 380-460 nm region,
where the emission of the pyrene unit of 7 rises upon the tail
of the emission band of NAPY; this emission increase can be
explained by the formation of the host-guest complex between
1 and 7, as previously explained.12a On the other hand, the
fluorescence measurements gave further support to the formation
of the 1·2 complex: Figure 5a displays the characteristic
decrease of emission intensity in the 340-380 nm region that
NAPY features when complexed to an UPY unit; the same
quenching efficiency was observed in the analogous experiment
carried out in the absence of 7. It is noteworthy that in this
case the efficiency of an eventual energy transfer process from
NAPY to the pyrene derivative in 7 in the 1·2·7 ensemble is
negligible, most probably because of the large separation
between these two units; the same energy transfer process is
instead quite efficient in 3, where the same units are connected
by a short spacer.
Figure 7. AFM topography image and surface profile for Si-8 layer (a)
and Si-8·6·1·1 layer (b).
The evidence from both the absorption and the fluorescence
spectra are thus in agreement with the formation of the ternary
1·2·7 adduct. To further support this conclusion, we added an
equivalent of 5 as hexafluorophosphate salt. A decrease of the
fluorescence intensity in the 380-460 nm region was observed
(Figure 5b), as expected, because the more competitive am-
monium salt can expel the methylpyridinium moiety from the
cavity, thus restoring the original, low-fluorescence intensity
of the pyrene group in 7.
It is worth noticing that, as evidenced by NMR spectroscopy
for 1·3·5, the attempt to break the newly formed 1·2·5
host-guest complex by addition of a base able to deprotonate
the ammonium salt, reducing its affinity for the cavitand, failed.
In this case, in fact, DBU interacted before with the H-bonded
network than with the host-guest system, leading to the
dissociation of UPY-NAPY units. Therefore, this approach was
unfit for closing the assembly/disassembly cycle. Moving the
entire protocol on the silicon surface will overcome this problem.
Figure 8. Fluorescent emission spectra of disassembled/reassembled
modular components on the Si-surface.
Compexation Studies on Silicon Surface
geometries, such as through the phosphonate bridges, since the
PdO groups are significantly distant from the surface.12c In this
case, we chose to prepare a pure cavitand monolayer in order
to maximize the amount of receptors on the silicon surface.
Under these conditions, the wafer coverage is not complete,
leading to oxidation of the residual bare Si-surface with
formation of interstitial oxide among cavitands.23
AFM. Atomic force microscopy (AFM) was employed to
interrogate the topology of the formed layers. The AFM topography
image and the surface profile of the starting Si-monolayer are
shown in Figure 7a. As expected, under consideration of the vertical
resolution limit of the AFM technique, a relatively flat surface was
observed, which agrees with the previously proposed scenario.
We then proceeded with a hierarchical construction of a
supramolecular structure, exploiting the self-assembly-driven
Cavitand Grafting and XPS Characterization. To perform
the complexation studies on the surface summarized in Figure
6, the first step was grafting cavitand 8 on a silicon wafer, via
photochemical hydrosilylation on an H-terminated Si(100)
surface. The procedure, described in detail in ref 12c, consists
in first etching with a 2.5% HF solution to remove the native
oxide, followed by surface decoration with cavitand 8 via
photochemical hydrosilylation. The hydrogenated Si-wafer was
soaked in a degassed 50 mM solution of 8 in mesitylene and
irradiated with 254 nm UV radiation, inducing the hydrosily-
lation of the cavitand terminal double bonds. In this way robust
Si-C bonds between the substrate and the organic molecule
formed, affording a cavitand-decorated surface, as proved by
X-ray photoelectron spectroscopy (XPS) measurements (Table
S1, Figure S4, SI), which confirmed the presence of phosphorus
on the wafer. Angular resolved XPS experiments on the O 1s
band (Figure S3, SI) excluded the possibility of different grafting
(23) Condorelli, G. G.; Motta, A.; Favazza, M.; Fragala`, I. L.; Busi, M.;
Menozzi, E.; Dalcanale, E.; Cristofolini, L. Langmuir 2006, 22, 11126–
11133.
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A R T I C L E S
Tancini et al.
Figure 9. Average thickness of the different self-assembled layers on Si measured by ellipsometry. The Si-8 layer thickness was set to 0 for clarity.
growth. The cavitand-decorated Si-surface was dipped into a 1
mM solution of methyl viologen 6 in acetonitrile, in order to
obtain the guest complexation. After removal of the physisorbed
guest by extensive rinsing with CH3CN and CHCl3, the Si-wafer
was exposed to a 1 mM solution of 1·1 homodimer in
chloroform. The surface was rinsed again to remove the
physisorbed species; then the AFM image of the grown
Si-8·6·1·1 structure was collected.
As shown in Figure 7b, a well-defined profile was traced,
featuring peaks of different height, in a range between 1.5 and
3 nm. These data demonstrated the growth of nanometric
structures on the surface. In particular, the bigger peaks
correspond to the length of the elongated 1·1 dimer linked at
the surface due to host-guest interactions between the cavity
of 1 and the still free pyridinium moiety of the previously
anchored dimethyl viologen. As far as the smaller peaks are
concerned, they can be attributed either to a looped structure
where the 1·1 dimer is anchored to two adjacent Si-8·6 units
(see Figure S5 in SI) or to a partial bending over the surface of
Si-8·6·1·1 to minimize the surface free energy.24
Fluorescence Spectroscopy. In order to follow precisely the
next steps and the successive assembly disassembly processes,
we turned to fluorescence measurements on the surfaces. The
starting point was the fluorescence-silent structure Si-8·6·1·1
(Figure 6 and Figure 8, trace a, black). Upon exposure to a 1
mM solution of 3·3 homodimer in chloroform, Si-8·6·1·3
structure formed, according to the exchange mechanism previ-
ously reported in solution (Figure 2). As result, a pyrenic unit
was anchored on the surface, affording an emission band typical
for the monomeric form of pyrene in the recorded surface
fluorescence spectrum (Figure 8, trace b, red).
Selective host-guest exchange was obtained by simply
dipping Si-8·6·1·3 into a 1 mM solution of butylmethylam-
monium iodide 5 in chloroform, with the consequent disap-
pearance of the pyrene emission band in the collected spectrum
(Figure 8, trace e, blue).
Immersion of Si-8·5 in a DBU solution restored the initial
Si-8 surface by removing guest 5 (Figure 6, trace f, cyan),
ready to repeat the cycle again. The whole assembly cycle is
highly reproducible, as shown by the fluorescence spectrum of
the Si-8·6·1·3 reassembled system (Figure 8, trace g, ma-
genta).
Ellipsometry. Another independent way to track the self-
assembly process relies on ellipsometry, which provides highly
accurate average thickness measurements.25 The same self-
assembly cycle depicted in Figure 6 and tracked by surface
fluorescence has been monitored via null ellipsometry at the
single wavelength λ ) 633 nm, with an incidence angle of 70°.26
Inversion of the data (ellipsometric angles ∆ and Ψ) to yield
the film thickness was performed in the Drude approximation.27
In this approximation, the angle Ψ is not affected by the film,
and its constant value can be used as a check of the overall
alignment. The variation δ∆ of the phase angle ∆ is linearly
proportional to the film thickness via a coefficient depending,
among other things, on the refractive index of the layer, which
is not known exactly in our case because of its composite nature.
To get the most reliable estimate of film thickness, the
measurements were repeated on a grid of 3 × 3 or 4 × 4 points
on the surface of the Si-wafer. The average value of those
independent measurements and its standard deviations were
taken as the best estimate for ∆ and Ψ. The error bars in Figure
9 arise from the combination of two factors: spread on the
Dipping the Si-8·6·1·3 wafer in a 8:2 CHCl3/MeOH mixture
resulted in the disassembly of unit 3, as proved by the disappearance
of the pyrenic band (Figure 8, trace c, green, and Figure 6). The
subsequent treatment with a 1 mM solution of 3·3 homodimer
restored the destroyed H-bonded network, as confirmed by the
reappearance of the pyrenic peak in the fluorescence spectrum
(Figure 8, trace d, yellow, and Figure 6).
(24) Chen, T.; Ferris, R.; Zhang, J.; Ducker, R.; Zauscher, S. Prog. Polym.
Sci. 2010, 35, 94–112.
(25) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light;
North Holland: Amsterdam. 1977.
(26) Harke, M.; Teppner, R.; Schulz, O.; Motschmann, H. ReV. Sci. Instrum.
1997, 68, 3130–3134.
(27) Tompkins, H. A User’s Guide to Ellipsometry; Academic Press:
Boston, 1993.
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Hierarchical Self-Assembly on Silicon
A R T I C L E S
measured values and uncertainty in the film refractive index,
which is assumed to be in the range n ) 1.45-1.60, typical for
non-mesogenic organic materials. The absorption of the organic
components is negligible at λ ) 633 nm.
As a starting point we characterized the initial Si-8 surface
(leftmost blue point in Figure 9), which provides the reference
initial thickness of 5.5 nm. This value accounts for the native
silicon oxide and the cavitand layer. Given the similarity in the
refractive index of silicon oxide (n ) 1.46) and of the organic
layer, it is not possible to discriminate between the two; however
the typical value for equilibrium oxide is 2.5 nm,25 which allows
estimating the organic layer thickness at about 3 nm.28
on the surface resulted from the combined use of orthogonal
host-guest and H-bonding interactions, embedded in selected
modular components. The two interactions are fully compatible
in the assembly mode and individually addressable in the
disassembly mode, enabling the molecular level control of each
step on the surface and their complete reversibility under precise
external stimuli. The entire process has been fully tested and
characterized in solution and then successfully transferred to
the solid state. The transfer of the protocol to the silicon surface
requires (i) the covalent grafting of cavitand 8 on a silicon wafer
as reactive layer to initiate the building process and (ii) the
ditopic methyl viologen 6 for the multistep growth.
Each single self-assembly step was then monitored. The
complexation of methyl viologen led to a small but detectable
increase of layer thickness. A much larger increase was obtained
in the subsequent formation of Si-8·6·1·1, quantified at about
1.5 nm. The measured thickness of the layer fully supports the
AFM image (Figure 7b), indicating the prevalent formation of
1.5 nm high objects. The Si-8·6·1·1 structure is flexible
enough to bend over the surface whenever possible, in order to
minimize the surface free energy.24 The alternative formation
of looped structures (see Figure S5, SI) cannot be excluded a
priori. The Si-8·6·1·1 to Si-8·6·1·3 conversion led only to
a tiny thickness variation, as expected for structures of
comparable dimensions. The truly remarkable changes were
observed in the two orthogonal disassembly modes. Solvent-
driven removal of the fluorescent NAPY probe resulted in a 9
Å decrease of layer thickness, fully recovered upon exposure
of the wafer to a 3·3 solution. Even more compelling is the
guest exchange leading to the Si-8·5 coverage, substantiated
by a 1.4 nm drop in layer thickness. The complexation of 5
does not determine a sizable increase of Si-8·5 dimensions
with respect to the original Si-8 layer, since the small guest is
encased within the cavity. The whole process was repeated to
assess its reproducibility. Treatment with DBU restored the
initial Si-8 surface, which underwent complexation to give
Si-8·6. Exposure of the wafer to a solution of 1·1 brought
the layer thickness back to the Si-8·6·1·1 original value. The
same holds for the final conversion of Si-8 · 6 · 1 · 1 to
Si-8·6·1·3 (rightmost green point in Figure 9).
The design of the building blocks and of the entire protocol
sequence was finalized to allow univocal surface characterization
and to achieve complete reversibility. Ellipsometry and fluo-
rescence measurements provided compelling evidence of each
step of the self-assembly process on the surface respectively
from the topographical and structural point of view. The
Si-8·6·1·1 to Si-8·6·1·3 conversion allowed the introduction
of the pyrene moiety for fluorescence detection, while the
Si-8·6·1·3 to Si-8·5 guest exchange, followed by basic
treatment, reset the starting surface. The problem related to the
only nonorthogonal disassembly process in solution, namely,
the DBU treatment of 1·3·5, has been overcome on silicon,
where a full cycle has been realized (Figure 6).
This work demonstrated that H-bonding and host-guest
interactions can be concurrently employed to realize stimuli-
responsive surfaces and complex hybrid organic-inorganic
materials.
Acknowledgment. This work was supported by the EC through
the ITN Project FINELUMEN (PITN-GA-2008-215399) and the
Project BION (ICT-2007-213219). Financial support from INSTM
to F.T. is gratefully acknowledged. We acknowledge the Centro
Interfacolta` di Misure “G. Casnati” of the University of Parma for
the use of NMR and HR ESI-MS facilities, and Prof. G. G.
Condorelli of the University of Catania for the preparation and XPS
characterizations of the Si-8 surface. M.M. and L.P. acknowledge
the fundamental financial support from Cassa di Risparmio in
Bologna and MIUR (PRIN and FIRB). Finally, we are grateful to
Prof. J. de Mendoza for helpful discussions and to one referee for
suggesting the use of ellipsometry.
A bare silicon wafer underwent the same treatment, to exclude
physisorption phenomena. Ellipsometry did not evidence any
thickness variation at any stage of this control experiment.
Supporting Information Available: Preparation and charac-
terization of compounds II, III, IV, V, 1, and 3, fluorescence
measurements details, XPS atomic composition analysis of HF
Conclusions
In this paper we have introduced a robust and reliable
methodology for the self-assembly of complex architectures on
silicon. The multistep growth of such supramolecular structures
1
freshly etched surface, additional H NMR spectra, and pure
Si-8 monolayers (Table S1), and picture of the possible
arrangements of 1·1 homodimers on the Si-surface (Figure S5).
This material is available free of charge via the Internet at http://
pubs.acs.org.
(28) This value is higher than expected for the cavitand layer (2.0 nm).
The presence of adventitious carbon has been shown to increase the
organic layer thickness in silicon wafers. See: Subramanian, V.;
Bhattacharya, P. K.; Memon, A. A. Int. J. Electron. 1995, 78, 519–
525.
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