Preparation, reactivity and controlled release of SAMs of calix[4,6]arenes and calix[6]arene-based rotaxanes and pseudorotaxanes formed on polycrystalline Cu

Article abstract of DOI:10.1039/c0cp01921f

Specific and reversible binding of guest molecules from a solution to a surface pre-treated with host molecules is a recent and active field of research. Self-assembled monolayers may result from supramolecular interactions, adding distinct functionalitie

Full text of DOI:10.1039/c0cp01921f

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PAPER
Preparation, reactivity and controlled release of SAMs
of calix[4,6]arenes and calix[6]arene-based rotaxanes and
pseudorotaxanes formed on polycrystalline Cuw
Alice Boccia,a Valeria Lanzilotto,a Valeria Di Castro,a Robertino Zanoni,*a
Luca Pescatori,b Arturo Arduinib and Andrea Secchi*b
Received 23rd September 2010, Accepted 14th December 2010
DOI: 10.1039/c0cp01921f
Specific and reversible binding of guest molecules from a solution to a surface pre-treated with host
molecules is a recent and active field of research. Self-assembled monolayers may result from
supramolecular interactions, adding distinct functionalities to the surface. In this frame, the first
compared study is given here of the anchoring on the technologically relevant Cu surface of
calix[4]arene receptors and calix[6]arene-based rotaxanes and pseudorotaxanes. These molecules,
which belong to the most representative classes of compounds in supramolecular chemistry, have
been chosen for their synthetic accessibility and versatility, which make them useful building blocks
for the synthesis of new advanced supramolecular structures. Covalent functionalisation of
calix[4,6]arenes on Cu was reached via a dip-coating procedure, optimizing the various synthetic
aspects in order to obtain good coverages and copper passivation. Molecular adhesion has been
demonstrated by the presence and relative quantitation of XPS signals from specific elements in the
molecules. We have successfully tested the combination of different functionalities by producing a
mixed film, prepared by ligand exchange of calix[4]arene with undecanethiol. The availability of the
calix[4]arene cavity to reversibly host further species after anchoring on Cu has been demonstrated by
a sequence of uptake and release cycles with pyridinium salts. Rotaxane and pseudorotaxane species,
composed of a calix[6]arene wheel functionalized with N-phenylurea groups on the upper rim, and a
viologen-containing axle, have been anchored on Cu via the SH-termination of the axle. XPS
demonstrated the successful self-assembly of fully threaded rotaxanes and pseudorotaxanes from their
solutions and the controlled release upon biasing of full rotaxanes and of the pseudorotaxane wheel.
nanoparticles with an organic stabilizing shell, very interesting
Introduction
examples of hybrid materials are obtained.1 2D self-assembled
monolayers (2D-SAMs) or monolayer-protected clusters
(MPCs or 3D-SAMs) can be obtained, respectively. The
enticing aspect of these systems is that the nature of both
the metal and the organic layer can influence the properties of
the resulting hybrid material. During the last decade, it
became progressively clear that supramolecular chemistry,
through the application of a ‘‘bottom-up approach’’, opens
virtually unlimited possibilities to the design of nanomaterials
and nanoscale objects.2 The controlled production of organic
SAMs on metal surfaces started in 1983, and is now a mature
field. Early studies reported the behaviour of alkanethiols
and dialkyl sulfides or disulfides on Au.3 Various examples
of functional species have thereafter been proposed, such as
redox species,4–6 biomolecules or selective receptors.7,8 The use
of model surfaces of a high technological impact, such as Cu,
to investigate molecular recognition reactions can provide
great insights into different classes of phenomena. In molecular
electronics, a critical issue is the establishment of a robust and
In the frame of supramolecular studies, the combination of
suitable molecules and surfaces of inorganic solids which have
been pre-organized to produce a host–guest interaction is
relatively new and may lead to materials with new and tuneable
properties for perspective applications. These hybrid materials
generally own more attractive properties, compared with their
separate counterparts. By covering metal surfaces and metal
a
`
Dipartimento di Chimica, Universita degli Studi di Roma
‘La Sapienza’, p.le Aldo Moro 5, I-00185 Roma, Italy.
E-mail: robertino.zanoni@uniroma1.it; Fax: +39 06 06490324;
Tel: +39 06 49913328
b
`
Dipartimento di Chimica Organica e Industriale and Unita INSTM
Sez. 4-UdR Parma, Universita` di Parma, Parco Area delle Scienze
17/a, I-43124 Parma, Italy. E-mail: andrea.secchi@unipr.it;
Fax: +39 0521 905472; Tel: +39 0521 905409
w Electronic supplementary information (ESI) available: NMR spectra
of new compounds (4, 5, 13ꢀ2TsO, 15ꢀ2TsO and pseudorotaxanes 6*13ꢀ
2TsO and 6*15ꢀ2TsO); XPS spectra of Cu/6*13ꢀ2TsO and Cu/6*15ꢀ
2TsO before and after 1 and 3 min biasing. See DOI: 10.1039/c0cp01921f
c
4452 Phys. Chem. Chem. Phys., 2011, 13, 4452–4462
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reliable metal contact without damage of the organic layer and
contamination of the interface. Cu is a metal of choice for vias.
In the literature, a few different methods are described to this
aim, as producing Cu deposits on self-assembled monolayers
(SAMs) covalently attached to a semiconductor as Si,9 or
depositing Cu nanoparticles on semiconductor surfaces.10 We
are interested in a selective functionalisation procedure of a
substrate as a key-step towards controlled deposition of
suitable metals (also as nanoparticles) on semiconductors. In
the case of Cu and Si, distinct and selective anchoring groups
towards the two moieties are needed in case a bridging bond
should be established between, e.g., Cu nanoparticles and a
pre-patterned Si surface. Moreover, investigations on the
reactivity of polycrystalline Cu surfaces can be considered
as preliminary to Cu single-crystal studies.11 We have been
investigating for several years the reactivity of organics and
organometallics both on Si oriented surfaces, also exploring
molecular recognition reactions, and on single crystals
and polycrystalline copper.12,13 We made use primarily of
photoemission spectroscopies, coupled with EXAFS and
other synchrotron-radiation based techniques, because of their
effectiveness in evidencing formation and stimulated evolution
of chemical bonds. Among the several macrocyclic synthetic
hosts that have been used as receptors in supramolecular
chemistry, calixarenes have found wide application as molecular
platforms, on which a large variety of binding sites can be
inserted and oriented in space, or as tridimensional receptors
able to include into their aromatic cavities either neutral or
charged species.14 Calix[6]arene derivatives functionalized
with N-phenylureido groups have been recently employed
as active components for the preparation of prototypes
of molecular machines.15 Owing to their particular shape
and size, these macrocycles can act as asymmetric wheels for
N,N0-dialkyl-4,40-dipyridinium salts (viologens), to give rise
to self-assembled species belonging to the class of rotaxanes
and pseudorotaxanes.14b–f These latter are supramolecular
complexes characterized by high thermodynamic stability,
which can be reversibly disassembled in solution by external
electrochemical stimuli.2
N-hydroxymethyl-N-phenylurea,17 1,6-bis(toluene-4-sulfonyloxy)-
hexane,18 pentyl tosylate 8,19 and undec-10-enyl tosylate20 were
synthesized according to reported procedures. All other reagents
were of reagent grade quality as obtained from commercial
suppliers and were used without further purification.
Calix[4]arene (4). A solution of calix[4]arene 3 (1.2 g,
1.4 mmol) and N-hydroxymethyl-N-phenylurea (0.23 g,
1.4 mmol) in dry CH2Cl2 (100 mL) was cooled at 0 1C with
an external ice bath. After stirring for 15 min, AlCl3 (0.4 g,
3 mmol) was added and the resulting mixture was stirred
for further 30 min. The external ice bath was removed and
the mixture was left to react at room temperature under
stirring for 2.5 h. After this period, the reaction was quenched
by adding 100 mL of distilled water. The resulting organic
phase was separated, washed with water up to neutrality,
dried over Na2SO4 and evaporated to dryness under reduced
pressure. The residue was purified by column chromatography
(eluent: hexane/ethyl acetate = 7/3) to afford 0.6 g of 4 (40%)
as a white solid (found: C, 72.10; H, 7.59; N, 2.69; S, 6.06.
C62H80N2O7S2 requires: C, 72.37; H, 7.78; N, 2.72;
S, 6.22%); mp 45.5 1C–46.5 1C; dH (300 MHz, CDCl3) 8.33
(s, 1H), 8.27 (s, 1H), 7.3–6.6 (m, 16H), 6.3 (bs, 1H), 4.9
(bt, 1H), 4.4–4.2 (m, 6H), 3.99 (t, 4H, J = 6.5 Hz), 3.37 and
3.35 (2d, 4H, J = 13 Hz), 2.84 (t, 4H, J = 7.2 Hz), 2.30
(s, 6H), 2.2–2.0 (m, 4H), 1.8–1.2 (m, 32H); dC (75 MHz,
CDCl3) 196.5, 156.1, 153.3, 152.6, 151.9, 138.9, 133.4, 133.3,
129.1, 128.9, 128.7, 128.4, 128.1, 128.0, 125.2, 122.7, 119.7,
119.0, 76.7, 43.9, 31.4, 31.3, 30.4, 30.0, 29.6, 29.5
(2 resonances), 29.2, 28.8, 25.9; m/z (ESI) 1051 (M + Na+,
100), 1052 (M + Na+, 70).
Calix[4]arene (5). CH3ONa (0.2 g, 4 mmol) was added to a
solution of calix[4]arene 4 (0.4 g, 0.4 mmol) in a 1 : 1 mixture
of CH3OH : THF (50 mL) kept under argon atmosphere.
After stirring for 15 min, 1,4-dithioerythritol (0.36 g, 2.3 mmol)
was added. After stirring at room temperature for 3 h, 0.5 g of
activated DOWEX-H+ was added to the reaction mixture.
The solution was filtered, evaporated to dryness under
reduced pressure. The solid residue was purified by column
chromatography (eluent: CH2Cl2/ethyl acetate = 95/5) to yield
0.26 g of 5 (70%) as a white solid (found: C, 73.53; H, 8.30; N,
2.86; S, 6.71. C58H76N2O5S2 requires: C, 73.72; H, 8.05; N,
2.97; S, 6.78%); mp 67.5–68.5 1C; dH (300 MHz, CDCl3) 8.32
(s, 1H), 8.25 (s, 1H), 7.3–6.6 (m, 16H), 6.2 (bs, 1H), 4.9 (bt,
1H), 4.4–4.2 (m, 6H), 3.99 (t, 4H, J = 6.5 Hz), 3.37 and 3.35
(2d, 4H, J = 13 Hz), 2.51 (m, 4H), 2.2–2.0 (m, 4H), 1.8–1.2
(m, 16H); dC (75 MHz, CDCl3) 155.6, 153.3, 152.7, 151.9,
138.4, 133.4, 133.2, 129.0, 128.9, 128.7, 128.5, 128.4, 128.1,
128.0, 125.1, 123.5, 120.6, 119.0, 76.7, 44.1, 34.0, 31.4, 31.3,
29.9, 29.6 (2 resonances), 29.5, 29.4, 29.1, 28.3, 25.9, 24.6; m/z
(ESI) 967 (M + Na+, 100), 968 (M + Na+, 65).
We report here the first surface study on the application of
supramolecular systems on Cu. The molecular species here
selected are the very versatile calix[4,6]arenes, which have been
studied together with rotaxanes and pseudorotaxanes as
important applications. The characterization of the bonding
interactions, of the reactivity of the anchored systems towards
host–guest interactions, and of the effectiveness of controlled
release upon biasing have been all conducted by the use
of XPS.
Experimental section
Synthesis
All reactions were carried out under nitrogen; all solvents were
freshly distilled under nitrogen and stored over molecular
6-(3,5-Di-tert-butylphenoxy)hexyl tosylate (7). A solution
of 1,6-bis(toluene-4-sulfonyloxy)hexane (10 g, 0.02 mol), 3,5-
di-tert-butylphenol (4.4 g, 0.02 mol) and K2CO3 (3.2 g,
0.02 mol) in CH3CN (200 mL) was refluxed and stirred for 48 h.
After this period, the solvent was completely evaporated under
reduced pressure and the solid residue was taken up with
1
sieves for at least 3 h prior to use. H and 13C NMR spectra
were recorded on instruments operating at 300 and 75 MHz,
respectively. Mass spectra were recorded in the ESI mode.
Melting points are uncorrected. Calix[4]arene 3,16 calix[6]arene 6,15f
c
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CH2Cl2 and H2O. The separated organic layer was washed
with H2O, dried over Na2SO4, and evaporated to dryness.
Purification of the solid residue by column chromatography
(silica gel, hexane 70%–ethyl acetate 30%) afforded 7.8 g of
tosylate 7 (85%) as a white solid (found: C, 70.77, H, 8.42, S,
7.22. C27H40O4S requires: C, 70.39, H, 8.75, S, 6.96%); mp
79–80 1C; dH (CDCl3, 300 MHz) 7.79 (d, 2H, J = 8.3 Hz), 7.33
(d, 2H, J = 8.3 Hz), 7.01 (t, 1H, J = 1.5 Hz), 6.73 (d, 2H, J =
1.5 Hz), 4.04 (t, 2H, J = 6.4 Hz), 3.92 (t, 2H, J = 6.4 Hz),
1.8–1.6 (m, 4H), 1.5–1.3 (m, 4H), 1.30 (s, 18H); dC (CDCl3,
75 MHz) 152.1, 129.7, 127.8, 114.8, 108.7, 70.5, 67.3, 34.9,
31.4, 28.7, 29.2, 25.5, 25.1; m/z (ESI) 483 (M + Na+, 100).
30.5, 29.7 (2 resonances), 29.5 (2 resonances), 29.3 (2 resonances),
28.8 (2 resonances), 25.8, 21.3; m/z (ESI) 432 (M + Na+, 100).
General procedure for the synthesis of viologens 12ꢀ2TsO and
14ꢀ2TsO. A solution of the monotosylate salt (9ꢀTsO or
10ꢀTsO, 3.3 mmol) and S-11-(tosyloxy)undecyl ethanethioate
11 (1.4 g, 3.3 mmol) in CH3CN (70 mL) was refluxed for 72 h
at 95 1C in a sealed glass autoclave. After cooling to room
temperature, the reaction mixture was evaporated to dryness
under reduced pressure. Recrystallization of the solid residue
from CH3CN affords the desired viologen salt as a white solid.
12ꢀ2TsO: 1.7 g (50%) (found: C, 67.55, H, 8.53, N, 2.99,
S, 9.10. C57H80N2O8S3 requires: C, 67.23, H, 8.65, N, 2.75,
S, 9.45%); mp 160–162 1C; dH (CD3OD, 300 MHz) 9.27
(bt, 4H), 8.66 (d, 4H, J = 6.7 Hz), 7.70 (d, 4H, J = 8 Hz),
7.23 (d, 4H, J = 8 Hz), 7.03 (t, 1H, J = 1.5 Hz), 6.70 (d, 2H,
J = 1.5 Hz), 4.8–4.7 (m, 4H), 3.99 (t, 2H, J = 6 Hz), 2.86
(t, 2H, J = 7 Hz), 2.38 (s, 6H), 2.31 (s, 3H), 2.2–2.0 (m, 4H),
1.9–1.7 (m, 2H), 1.7–1.3 (m, 38H); dC (CDCl3, 75 MHz) 158.5,
152.0, 148.7, 146.1, 143.3, 139.6, 128.8, 127.3, 125.7, 114.8,
108.6, 67.2, 61.9, 34.9, 31.4, 30.6, 29.4, 29.3, 29.2, 29.0, 28.9,
28.7, 26.0, 25.9, 25.5, 21.2; m/z (ESI) 337 (M ꢁ 2TsO, 100).
14ꢀ2TsO: 1.7 g (65%) (found: C, 63.07, H, 7.12, N, 3.44,
S, 11.87. C42H58N2O7S3 requires: C, 63.13, H, 7.32, N, 3.51,
S, 12.04%); mp 145.0–146.0 1C; dH (DMSO-d6, 300 MHz) 9.38
(d, 4H, J = 5.7 Hz), 8.77 (d, 4H, J = 6.7 Hz), 7.46 (d, 4H, J =
8 Hz), 7.10 (d, 4H, J = 8 Hz), 4.68 (t, 4H, J = 7 Hz), 2.80
(t, 2H, J = 7.2 Hz), 2.31 (s, 3H), 2.28 (s, 6H), 2.1–1.9 (m, 4H),
1.6–1.2 (m, 20H), 0.88 (t, 3H, J = 6.7 Hz); dC (DMSO-d6,
75 MHz) 195.1, 148.3, 145.6, 145.5, 137.5, 127.9, 126.4, 125.3,
60.7, 33.2, 30.7, 30.4, 30.3, 29.0, 28.7, 28.6, 28.4, 28.3, 28.2,
28.0, 27.6, 27.4, 25.3, 23.6, 21.4, 20.6, 13.6; m/z (ESI) 228
(M ꢁ 2TsO, 100).
General procedure for the synthesis of the monotosylate salts
9ꢀTsO and 10ꢀTsO. A solution of the proper tosylate (7 or
8, 0.01 mol) and 4,40-bipyridyl (1.6 g, 0.01 mol) in CH3CN
(300 mL) was refluxed with stirring for 48 h. After cooling to
room temperature, the solvent was completely evaporated
under reduced pressure and the sticky residue was taken up
with hot ethyl acetate. After cooling to room temperature, the
desired pure monotosylate salt was recovered as a white solid
by suction filtration.
9ꢀTsO: 4 g (65%) (found: C, 71.84, H, 7.84, N, 4.54, S, 5.20.
C37H48N2O4S requires: C, 72.04, H, 8.11, N, 4.24, S, 4.89%);
mp 59–60 1C; dH (CDCl3, 300 MHz) 9.21 (d, 2H, J = 6.5 Hz),
8.73 (d, 2H, J = 5.4 Hz), 8.18 (d, 2H, J = 6.5 Hz), 7.73 (d, 2H,
J = 8 Hz), 7.55 (d, 2H, J = 8 Hz), 7.08 (d, 2H, J = 8 Hz), 6.99
(t, 1H, J = 1.5 Hz), 6.70 (d, 2H, J = 1.5 Hz), 4.75 (t, 2H, J = 6 Hz),
3.87 (t, 2H, J = 6 Hz), 2.25 (s, 3H), 2.0–1.9 (m, 2H), 1.7–1.6
(m, 2H), 1.5–1.4 (m, 2H), 1.28 (s, 20H); dC (CDCl3,
75 MHz) 158.4, 153.1, 152.1, 151.2, 150.6, 145.7, 143.5, 140.9,
139.4, 128.7, 125.8, 125.6, 121.4, 114.8, 108.6, 67.0, 61.5, 34.9,
31.4, 31.1, 29.5, 25.6, 25.4, 21.2; m/z (ESI) 445 (M ꢁ TsO, 100).
10ꢀTsO: 3 g (75%) (found: C, 66.62, H, 7.68, N, 6.91,
S, 8.14. C22H26N2O3S requires: C, 66.30, H, 7.58, N, 7.03,
S, 8.05%); mp 155.5–156.5 1C; dH (CD3OD, 300 MHz) 9.08
(d, 2H, J = 6.8 Hz), 8.79 (d, 2H, J = 5.9 Hz), 8.46 (d, 2H, J =
6.7 Hz), 7.96 (d, 2H, J = 6.2 Hz), 7.66 (d, 2H, J = 8 Hz), 7.19
(d, 2H, J = 8 Hz), 4.65 (t, 2H, J = 7.6 Hz), 2.33 (s, 3H),
2.2–2.0 (m, 2H), 1.5–1.3 (m, 4H), 0.94 (t, 3H, J = 6.6 Hz); dC
(CD3ODCl3, 75 MHz) 155.1, 152.1, 146.8, 144.0, 143.9, 142.0,
130.2, 128.5, 127.4, 127.2, 123.8, 63.0, 32.5, 32.4, 29.6, 23.5,
21.6, 14.5; m/z (ESI) 227 (M ꢁ TsO, 100).
General procedure for the synthesis of the thiolate viologen
axles 13ꢀ2TsO and 15ꢀ2TsO. To a solution of the viologen
salt (12ꢀ2TsO or 14ꢀ2TsO, 2 mmol) in EtOH (150 mL),
toluene-4-sulfonic acid (0.5 g, 2.5 mmol) was added. The
resulting mixture was refluxed under stirring for 48 h. After
this period, the solvent was evaporated under reduced pres-
sure. The solid residue was triturated with hot ethyl acetate.
After cooling to room temperature, the desired thiolate viologen
axle was recovered as a pure white solid by suction filtration.
13ꢀ2TsO: 1.35 g (70%) (found: C, 67.91, H, 7.85, N, 3.18,
S, 10.13. C55H78N2O7S3 requires: C, 67.72, H, 8.06, N, 2.90,
S, 9.86%); mp 189–191 1C; dH (CDCl3, 300 MHz) 9.21
(bt, 4H), 8.79(d, 4H, J = 6.6 Hz), 7.70 (d, 4H, J = 8 Hz),
7.10 (d, 4H, J = 8 Hz), 6.71 (d, 2H, J = 1.5 Hz), 6.99 (t, 1H,
J = 1.5 Hz), 4.7–4.6 (m, 4H), 3.85 (t, 2H, J = 6 Hz), 2.6–2.4
(m, 2H), 2.26 (s, 6H), 1.9–1.8 (m, 4H), 1.7–1.5(m, 4H), 1.5–1.0
(m, 36H); dC NMR (CDCl3, 75 MHz) 158.5, 152.1, 148.7,
145.8, 143.3, 139.6, 128.9, 127.3, 125.7, 114.8, 108.7, 67.2, 61.9,
34.9, 29.6, 29.4, 29.1, 29.0, 28.9, 28.7, 26.0, 25.9, 25.7, 25.5,
21.2; m/z (ESI) 316 (M ꢁ 2TsO, 100).
S-11-(Tosyloxy)undecyl ethanethioate (11). To a solution of
undec-10-enyl 4-methylbenzenesulfonate (5 g, 0.015 mol) and
thioacetic acid (5.9 g, 77 mmol) in dry toluene (200 mL), a tip
of spatula of AIBN was added. After refluxing for 5 h, the
reaction was quenched by addition of water (200 mL). The
separated organic phase was dried over Na2SO4. After the
removal of the solvent under reduced pressure, the solid
residue was purified by column chromatography (eluent:
hexane/ethyl acetate = 9/1) to afford 5.7 g of 11 (95%) as a
yellowish solid (found: C, 59.57; H, 8.15; S, 16.23. C20H32O4S2
requires: C, 59.96; H, 8.05; S, 16.01%); mp 28.0–29.5 1C; dH
(300 MHz, CDCl3) 7.78 (d, 2H, J = 8.3 Hz), 7.34 (d, 2H, J =
8.3 Hz), 4.01 (t, 2H, J = 6.6 Hz), 2.85 (t, 2H, J = 7.2 Hz), 2.44
(s, 3H), 2.32 (s, 3H), 1.8–1.5 (m, 4H), 1.4–1.1 (m, 14H); dC
(75 MHz, CDCl3) 195.0, 144.4, 140.3, 130.5, 128.3, 70.0, 32.5,
15ꢀ2TsO: 1 g (69%) yield (found: C, 63.32, H, 7.51, N, 3.60,
S, 12.46. C40H56N2O6S3 requires: C, 63.46, H, 7.46, N, 3.70,
S, 12.71%); mp 236.0–237.0 1C; dH (CDCl3, 300 MHz) 9.19
(d, 4H, J = 6.3 Hz), 8.85 (d, 4H, J = 6 Hz), 7.74 (d, 4H, J =
8.1 Hz), 7.16 (d, 4H, J = 7.9 Hz), 4.65 (bt, 4H), 2.5–2.4 (m, 2H),
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2.33 (s, 6H), 2.0–1.8 (m, 4H), 1.7–1.5 (m, 2H), 1.5–1.1 (m, 18H),
0.85 (t, 3H, J = 6.7 Hz); dC (CD3OD, 75 MHz) 151.5, 147.3,
144.0, 142.0, 130.2, 128.6, 127.2, 63.5, 40.0, 35.5, 32.9, 32.5, 30.9
(2 resonances), 30.8, 30.6, 30.5, 29.7 (2 resonances), 29.6, 27.5,
25.2, 23.5, 21.6, 14.5; m/z (ESI) 207 (M ꢁ 2TsO, 100).
(KEs) from the relation: BE = hn ꢁ KE, after spectrometer
energy calibration with metal standards. XPS atomic ratios for
the bipyridine-functionalized hybrids were estimated from
experimentally determined area ratios of the relevant core lines,
corrected for the corresponding theoretical atomic cross-sections
and for a square-root dependence of the photoelectrons kinetic
energies. All reported spectra were acquired at a photoelectron
take-off angle of 111 measured from the surface normal. Surface
coverage was estimated by making use of eqn (1) and (3) in the
model for the substrate attenuation of core line intensity exerted
by an alkanethiol monolayer on Au.21 The values for S 2p and Cu
2p inelastic mean free path (IMFP) at the photon energy
employed were taken from ref. 20.
General procedure for the synthesis of pseudorotaxanes
6*13ꢀ2TsO and 6*15ꢀ2TsO. To a stirred solution of calix[6]-
arene 6 (0.01 g, 0.007 mmol) in toluene (10 ml), an excess of
desired axle (0.02 mmol) was added. After few minutes, the
colour of the toluene solution turns orange-red. The reaction
mixture was stirred at room temperature for further 30 min
and then filtered off to remove the solid suspension. The
resulting toluene solutions containing the pseudorotaxanes
(6*13ꢀ2TsO or 6*15ꢀ2TsO) were used for the monolayer
preparation without any further purification.
Cyclic voltammetry. Cyclic voltammetry was performed in
0.1 M NBut4ClO4 (tetrabutylammonium perchlorate, TBAP) in
dry CH3CN. The electrolyte solution contained no deliberately
added electroactive species. All electrochemical measurements
were performed with a three-electrode cell using an Autolab
Electrochemical Analyzer (model PGSTAT 12, Eco Chemie BV,
The Netherlands). The counter electrode was a platinum coil
wire, and a silver wire immersed in 0.01 M AgNO3/0.1 M TEAP
in CH3CN, separated from the main solution by a porous fritted
glass + agar plug, served as a reference electrode. All potentials
reported will be henceforth referred to this reference.
Monolayer preparation on copper. The polycrystalline Cu
substrates (99.95%) were cleaned by immersion into the 32.5%
nitric acid solution for 30 s followed by the immersion into 3.7%
hydrochloric acid solution for 7 min. The samples were
eventually rinsed with large amounts of distilled water, and
immediately dipped into the appropriate thiol solution for 4 h.
Toluene and CH2Cl2 were the solvents used for surface adsorp-
tion. We used a 0.2 mM solution for calixarenes, 0.4 mM for
rotaxane and pseudorotaxane, in order to maintain the same
concentration of sulfur. As a reference, a copper surface was
covered with undecanethiol (45 mM solution for 17 h, to form a
self-assembled monolayer). A mixed film was prepared by ligand
exchange: the copper substrate functionalized with calix[4]arene
was immersed in the undecanethiol solution for 4 h. XPS analysis
was accomplished to evaluate the monolayer composition.
The reactivity of the rotaxane and pseudorotaxane was
investigated in toluene and dichloromethane, in order to possibly
evidence the influence of a different polarity of the solvent on the
threading of the adducts and their assembling on the Cu surface.
The kinetics of formation of the adducts is favored in a dichloro-
methane solution,15e but the larger solubility of the charged axle
may favor the anchoring of the bare axle directly to the surface.
The overall behavior of the adducts in the two solvents was
found closely comparable. In toluene we observed a closer match
between theoretical and experimental atomic ratios for both
NH/N+ and C/S, but the surface coverage was much lower.
Results and discussion
Production and characterization of calix[4,6]arenes on Cu
The functionalization of polycrystalline copper surfaces with
calixarene-based receptors requires the introduction on the
calixarene macrocycle of suitable ‘‘anchoring points’’. The
high affinity of sulfur for copper has been largely documented;22
therefore, calix[4]arene 1 and calix[6]arene 2, characterized by
the presence of two and three o-alkylthiolate chains on their
lower rims, respectively, were chosen as active coating agents
of the copper surface.23
X-Ray photoelectron spectroscopy. XPS results were
obtained on an experimental apparatus in UHV consisting of
a modified Omicron NanoTechnology MXPS system, with an
XPS chamber equipped with a dual X-ray anode source
(Omicron DAR 400) and an Omicron EA-127 energy analyzer,
and an attached VT-atomic force and scanning tunneling
microscope. Sample transfer between the various experimental
areas was conducted by means of linear magnetic transfer rods or
manipulators. All measurements were conducted in the least
possible time after sample preparation. Samples were produced
directly on cleaned polycrystalline Cu sample holders. No sizeable
sign of sample degradation under the acquisition times under the
X-rays was observed for the samples. MgKa and AlKa photons
were employed (hn = 1253.6 and 1486.6 eV, respectively),
generated operating the anode at 14–15 kV, 10–20 mA. BE values
were derived from experimentally determined kinetic energies
The chemisorption experiments were systematically conducted
on polycrystalline Cu surfaces, which were preliminarily
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characterized by XPS. The relevant region of adsorbed
calixarenes 1 and 2 is S 2p, which is reported in Fig. 1, together
with the spectrum of undecanethiol (C11SH), which we took as
the reference compound for the present investigation, since it
is known to form a nearly ideal SAM on Cu.24 Two S 2p
spin–orbit split doublets were always evident, which refer to
the formation of thiolate and, at a higher binding energy (BE),
to the presence of residual thiol groups. These last could
belong to calixarenes chemisorbed via a different thiol group
and/or to purely physisorbed molecules. The amount of this
component did not exceed B10% for calixarenes 1 and 2, and
it was much smaller in the case of C11SH (3%).
Table 1 XPS peak positions and quantitative ratios (ꢂ10%), and
values of surface coverage (s) for Cu/calixarene, Cu/C11SH and mixed
film
S 2p (eV) SCu/Cu s/nmꢁ2 SH/SCu C/S (theor.)
1
2
162.7
163.9
162.2
163.3
0.45
0.47
0.51
0.49
3.1
3.2
3.8
3.4
0.12
0.10
0.03
0.12
28 (25)
34 (34)
12 (11)
17
C11H23SH 162.2
163.3
2 + C11
a
162.3
163.3
a
Mixture of 2 and C11H23SH.
The surface coverage, s, was estimated by means of a
literature model,21,25 by making use of the SCu(thiolate)/Cu
ratio (columns 2 and 3 in Table 1). The atomic ratios of
calixarenes are, within the experimental error, closely similar
to the case of C11SH. The coverages fall in the same range,
hinting at a close-to-ideal monolayer coverage. These results
are consistent with the passivation of Cu surfaces. In fact, only
Cu(0) and Cu(I) components are present in the Cu LMM
Auger spectra, while a Cu(II) component was never found in
the Cu 2p spectrum (Fig. 2).
A clear indication that molecules adsorb intact on the
surface with no added contamination comes from inspection
of column 5 in Table 1, where a superposition can be seen
between theoretical and experimental C/S ratios. We took this
encouraging result as a reference in our study of a mixed
Fig. 1 S 2p XPS spectra taken at 111 from the surface normal of the
Cu/thiolate ligand. The main spin–orbit split component is due to
thiolate. The cross-hatched areas identify the component due to SH.
(a) 1, (b) 2 and (c) undecanethiol (C11SH).
Fig. 2 Cu 2p and Cu LMM Auger lines of Cu/calixarenes 1 and 2.
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monolayer, which we produced in order to demonstrate the
feasibility of a double functionalization. Such a monolayer
was obtained by ligand exchange, starting from a Cu/calix[4]-
arene
1 surface, which was eventually contacted with
C11SH. The experimental C/S value is 17, right in between
calix[4]arene 1 and C11SH, which hints at a 1 : 1 monolayer
on Cu.
Recognition reactions of N-methylpyridinium ion-pairs towards
anchored heteroditopic calix[4]arene receptors
We tested the reactivity of heteroditopic calixarenes anchored
on Cu with a series of experiments. To this aim, we selected the
heteroditopic calix[4]arene 5 in order to extend to surfaces our
solution studies on its reactivity towards N-methylpyridinium
and N,N0-dimethylviologen salts.26 5 was synthesized in ca.
30% of the overall yield following the pathway described in
Scheme 1.
In previous studies,27 we evidenced how a phenylureido
group in the upper rim of a calixarene macrocycle allows for
a combined effect (a cooperative heteroditopic effect28) on
the recognition of a salt. The combination of a hard H-bond,
which can be established by the urea group with an anion, with
a soft calix-cation p-bond,29 through the methyl group of the
pyridinium, reduces the ionic interaction in the salt, as testified,
e.g., by the fact that N-methyl pyridinium chloride (NMPCl),
when it acts as a guest, is a ligand-separated ion pair, in the solid
state.26b By testing a series of NMP salts with different anions,
such as tosylate, Clꢁ, Iꢁ, trifluoroacetate, we determined the
binding constants at RT for the formation of 1 : 1 adducts in
CHCl3, which evidenced that the phenylurea group increases
the affinity for the NMPX salt by up to two orders of magnitude
with respect to the monotopic receptor.26a
A sequence of N 1s XPS spectra of Cu/calix[4]arene 5 is
reported in Fig. 3. They were obtained after a first cycle of
immersion in a CH2Cl2 solution of N-methyl pyridinium
iodide (NMPI), followed by rinsing in CH3OH acidified with
HCl and by a second immersion in the solution of the
Fig. 3 Sequence of N 1s XPS spectra taken at 111 from the surface
normal of Cu/5, after immersion in a CH2Cl2 solution of NMPI (a),
after subsequent rinsing in a CH3OH solution, acidified with HCl (b),
after a new immersion in the above solution (c) (the gray areas identify
the component due to N+).
pyridinium salt.z Each N 1s spectrum of this sequence is
complex, and three components result from curve fitting.
The main component, also present in the initial sample, before
complexation, is assigned to the calixarene phenylureido
group, on the basis of the literature.30 A second peak
z The N 1s XPS spectra for Cu/calix[4]arene 5 before and after
+
immersion in the pyridine solution are closely similar, as a result of
the presence of adventitious NH4 ions (presumably coming from
DOWEX-H+ resin used in the deprotection step) in the former. In this
sample, the rinsing procedure applied to the sequence of samples in
Fig. 3 was not effective in removing ammonium, because of the poor
solubility of this cation in CH3OH. We recall here that water rinsing
cannot be applied. Notice that the N+ component in the two cases
comes from distinct species, ammonium and pyridinium, respectively.
Scheme 1 Reactions and conditions: (i) AlCl3, CH2Cl2, 15 min, T = 0 1C;
(ii) 3 h, T = 25 1C; (iii) CH3OMe, THF/MeOH, T = 25 1C, 15 min;
(iv) 1,4-dithioerythritol, THF/MeOH, T = 25 1C, 3 h; (v) activated
DOWEX-H+, THF/MeOH.
c
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component at a higher BE, characteristic for N+,31 appears
after each immersion cycle, while it is absent upon rinsing with
acidified CH3OH. Such component necessarily relates to the
coordination of the pyridinium cation by the calix[4]arene
(Fig. 3a and c). The minor component at B398 eV is assigned
to a direct coordination to the Cu surface of neutral pyridine
molecules,32 an impurity which may accompany the synthesis
of methylpyridinium. This assignment of the component is
further supported by its disappearance upon acid rinsing in
CH3OH.
expected because of the presence of the phenylureido group. A
coverage of 0.43 nmꢁ2 can be calculated from the SCu/Cu
ratio.21,25
The reduction in surface coverage with respect to the case of
calix[4]arene 1 could result from a larger surface disorder, due
to the combined effects of polarity and flexibility of the
phenylureido group.
As for the Iꢁ anion, its quantitative ratio to N+ was lower
than that expected for the anchored calix[4]arene 5. The
N+/Iꢁ ratio further diminished after rinsing, and a Clꢁ peak
became visible in the XPS spectrum. We interpret this as a
result of the anion exchange, which takes place during the
rinsing cycle in a CH3OH solution acidified with HCl to free
the aromatic cavity of 5.
We have established the extent of such a recognition reac-
tion by taking the experimental relative atomic ratios between
the two N 1s components, reported in Table 2. Such ratios
differ from the value expected from stoichiometry, indicating
that the filling of the calix sites by the pyridinium amounted to
60–70%. The establishment of the recognition reaction and
the molecular integrity are confirmed by the close match,
within the experimental error, of experimental and theoretical
C/N values (Table 2).
Self-assembly of rotaxane and pseudorotaxane species on Cu
The recognition in a CH2Cl2 solution of N,N0-dialkylviologen-
based ‘‘axles’’ 13ꢀ2TsO and 15ꢀ2TsO, synthesized as depicted
in Scheme 2, with triphenylureidocalix[6]arene-based ‘‘wheel’’
6,15f followed by immersion of Cu in the above solution,
resulted in the first self-assembly example, to our knowledge,
of rotaxane and pseudorotaxane species on Cu. To this aim,
two similar though conceptually different pre-formed systems
were used. As shown in previous studies15b,f and here verified,
the pseudorotaxane 6*13ꢀ2TsO that originates by threading,
in low polar solvent such as toluene, the wheel 6 with the
The S 2p spectrum of Cu/5 is reported in Fig. 4, where the
two spin–orbit split doublets, already assigned above, are
present in a SH/SCu 0.27 ratio. The larger amount of physi-
sorbed molecules, with respect to the simpler calix[4]arene, is
Table 2 XPS peak positions and NH/N+ quantitative ratios (ꢂ10%)
for Cu/5 as prepared and as a function of progressive NMP+ uptake
from a solution and rinsing
S 2p
(eV)
N 1s
(eV)
NH/N+
(theor.)
C/N
(theor.)
Cu/5 + NMPIa (1st
cycle)
161.9
163.2
398.0
3.0 (2)
21.2 (21.3)
399.3
401.8
398.0
399.3
401.8
398.0
After rinsing
161.9
163.2
—
27.4 (29)
Cu/5 + NMPIa (2nd
cycle)
161.9
163.2
2.6 (2)
24.1 (21.3)
399.3
401.8
a
CH2Cl2 solution of NMPI.
Scheme
2 Reactions and conditions: (i) 3,5-di-tert-butylphenol,
Fig. 4 S 2p XPS spectrum taken at 111 from the surface normal of
Cu/5. The main spin–orbit split component is due to thiolate. The
cross-hatched areas identify the component due to SH.
K2CO3, CH3CN, reflux, 48 h; (ii) CH3COSH, AIBN, toluene, reflux,
5 h; (iii) CH3CN, reflux, 48 h; (iv) CH3CN, reflux, 72 h (autoclave);
(v) TsOH, EtOH, reflux, 48 h.
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NH/N+ ratio is in the range of 3, as expected from the
stoichiometric value in the absence of dethreading (Table 3).
We also found no sizeable dethreading of the pseudorotaxanes
from the values of the NH/N+ and C/N ratios, which fall in
the range expected for intact chemisorbed rotaxanes. The S 2p
photoemission region displays the two couples of spin–
orbit components with the same meaning and assignment
reported above, apart from the presence at high BE of the
peak component related to the tosylate counterion. The
relative intensity of this last peak to the rest of the S 2p signal
follows the stoichiometry of the salt.
Controlled release of rotaxane and pseudorotaxane species from
Cu surfaces
We investigated in detail the controlled release of rotaxanes
and pseudorotaxanes by application of a suitable potential.
It is known from solution studies15e that the reduction of
bipyridinium N+ induces the dethreading of the wheel from
the viologen axle in pseudorotaxanes. We first measured the
reduction potentials of pseudorotaxane 6*15ꢀ2TsO chemi-
sorbed on an Au microelectrode via CV. An Au micro-
electrode was preferred to Cu because of the high reduction
current displayed by the latter. We found two well separated
peaks at ꢁ0.7 and ꢁ1.2 V (vs. Ag/Ag+ in acetonitrile), which
we assigned, on the basis of the literature,15e,33 to the two N+
reductions of the viologen-based axle 15ꢀ2TsO. We compared
the behaviour of rotaxane 6*13ꢀ2TsO and pseudorotaxane
6*15ꢀ2TsO on Cu by keeping the two samples biased at
ꢁ1.7 V (vs. Ag/Ag+ in acetonitrile) for 1 min.
XPS results before and after polarization can be compared
in Table 4. The diminishing of the thiolate component of S 2p
in the case of rotaxane 6*13ꢀ2TsO (see ESIw) is a clear
indication of the release of approximately half of the rotaxanes,
induced by the breaking of Cu–S bonds. This is accompanied
by the drastic fall of intensity displayed by the tosylate anion,
likely due to its reduction, in addition to the loss of rotaxane.
The nearly constant ratio between the NH group from the
triphenylurea and the S of the anchored species, NH/(SH +
S–Cu) in Table 4 indicates the residual presence of intact
rotaxanes at the surface. The reduction of the viologen N+
appears in the N 1s spectrum as a new component at B398.9 eV
(see ESIw). Its energy separation with respect to triphenylureido
N is consistent with the electron-withdrawing effect exerted on
N by the CQO group in the latter.
The more complex case represented by pseudorotaxane
6*15ꢀ2TsO can be deduced by inspection of Table 4 and
Fig. 7. A clear evidence for dethreading of the wheel as the
largely prevailing effect of biasing comes from the sizeable
lowering of the ratio NH/(SH + S–Cu) in Table 4. Notice the
constancy in the S–Cu/Cu atomic ratio, which excludes loss of
the whole pseudorotaxane. We still notice the drastic loss of
the tosylate anion, with the same meaning as above (see
Fig. 7). This general trend is further confirmed by extending
the biasing to 3 min. Inspection of Fig. 7b and Table 4
evidences that the low BE component in the N 1s spectrum
increases with respect to 1 min biasing. The absence of any
further dethreading of the wheel after 1 and 3 min comes from
the comparable decrease of the NH/(SH + S–Cu) ratio.
Fig. 5 Schematic representation of the threading reaction between
‘‘wheel’’ 6 and axles 13ꢀ2TsO and 15ꢀ2TsO. In toluene, axles 13ꢀ2TsO
and 15ꢀ2TsO give rise to solutions of oppositely oriented pseudo-
rotaxanes 6*13ꢀ2TsO and 6*15ꢀ2TsO, respectively. These solutions
resulted in different coverages of polycrystalline Cu surfaces.
mono-stoppered axle 13ꢀ2TsO is an oriented pseudorotaxane
bearing the phenyl stopper at the upper and the –SH function
at the lower rim (see Fig. 5).
The threading reaction carried out by using the axle
15ꢀ2TsO, that lacks the stopper, yields the pseudorotaxanes
6*15ꢀ2TsO, which have the SH function oriented towards the
upper rim of the macrocycle (see Fig. 5). This means that the
anchoring of these two different systems yields in one case an
oriented polyrotaxane, in which the copper surface acts as a
stopper, and a reversely oriented polypseudorotaxane in the
other (see Fig. 5).
The assembly on Cu of preformed pseudorotaxanes was
preferred to a 2-step reaction (adsorption of axles on Cu
followed by threading of the calix by reacting Cu with its
solution) because of the poor adsorption of N,N0-dialkylviolo-
gen on the surface, likely due to repulsive interactions between
the positive charges of the axle. This same approach was
followed for closely related molecular systems on Si(100).13g
Fig. 6 reports the N 1s and S 2p peaks of the rotaxane
Cu/6*13ꢀ2TsO and pseudorotaxane Cu/6*15ꢀ2TsO adducts,
respectively. NH and N+ peak components are again present,
indicating the anchoring of these species at the Cu surface. The
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Fig. 6 S 2p and N 1s XPS spectra taken at 111 from the surface normal of Cu/6*13ꢀ2TsO (a and c) and Cu/6*15ꢀ2TsO (b and d). For the S 2p
spectra, the main spin–orbit split component is due to thiolate and the cross-hatched areas identify the component due to SH; the doublet at high
BE is assigned to tosylate. For the N 1s spectra, the experimental curve is displayed as dotted line, with the NH and N+ peak components,
resulting from curve-fitting, as solid lines.
Table
3
XPS peak positions and experimental and theoretical
Table 4 XPS quantitative ratios (ꢂ10%) for rotaxane Cu/6*13ꢀ
2TsO and pseudorotaxane Cu/6*15ꢀ2TsO before and after biasing
(reported in brackets) quantitative ratios (ꢂ10%) and surface coverage
for rotaxane Cu/6*13ꢀ2TsO and pseudorotaxane Cu/6*15ꢀ2TsO
OTsꢁ/Cu S–Cu/Cu NH/(SH + SCu) N+/(SH + SCu)
S 2p
(eV)
N 1s NH/N+ C/N
(eV) (theor.) (theor.)
SCu/Cu y/nmꢁ2
Cu/6*13 0.17
Cu/6*13a 0.03
Cu/6*15 1.1
Cu/6*15a 0.10
Cu/6*15 0.96
Cu/6*15b 0.10
0.18
0.11
0.44
0.43
0.40
0.38
3.8
3.0
4.5
1.9
5.3
2.8
0.89
0.74
1.3
0.57
1.5
Cu/6*13 162.5 400.0 2.8(3)
163.8 402.3
167.7
Cu/6*15 161.8 399.8 3.4(3)
163.1 402.0
15.3(18.1) 0.32
1.6
16.4(16.2) 0.44
2.8
0.64
a
b
1 min bias. 3 min bias.
167.7
We interpret this last result as due to the second reduction of
+
combination of different functionalities by producing a mixed
film, prepared by ligand exchange of calix[4]arene with
undecanethiol. The availability of the calix[4]arene cavity to
reversibly host further species after anchoring on Cu was
demonstrated by a sequence of uptake and release cycles with
pyridinium salts.
the 15ꢃ radical cation axle, from which the wheel has
departed after the first reduction.15e
Conclusions
We have reported the first study on supramolecular systems
self-assembled on polycrystalline Cu surfaces. SAMs of calix-
arenes, rotaxanes and pseudorotaxanes were produced and
characterized by means of XPS. Covalent functionalization of
calix[4,6]arenes on Cu was reached using a dip-coating proce-
dure which allowed for good surface coverages and copper
passivation. Molecular adhesion was demonstrated by the
presence and relative quantitation of XPS signals from
specific elements in the molecules. We successfully tested the
Pseudorotaxane species, composed of a calix[6]arene wheel
derivatized with N-phenylureido groups on the upper rim, and
viologen-containing axle, were anchored on Cu via the SH-
termination of the axle. XPS demonstrated the successful self-
assembling of fully threaded rotaxanes and pseudorotaxanes
and the effectiveness of controlled release upon biasing of full
rotaxanes and of the pseudorotaxane wheel. This will allow for
more complex species to be produced on Cu, by the use of
suitably substituted calixarenes.
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Fig. 7 S 2p and N 1s XPS spectra taken at 111 from the surface
normal of Cu/6*15ꢀ2TsO after 3 min biasing. The main spin–orbit
split S 2p component in (a) is due to thiolate and the cross-hatched
areas identify the component due to SH.
Acknowledgements
Universita degli Studi di Roma ‘‘La Sapienza’’, Universita di
Parma and MIUR (Ministero dell’Istruzione, Universita e
Ricerca) PRIN project no. 2008HZJW2L are thanked for
financial support. We thank CIM (Centro Interdipartimentale
di Misure) ‘‘G. Casnati’’ of the University of Parma for NMR
measurements.
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