Chapter 4. Calix[n]arene-based 2D Self-Assembled Monolayers
4.3 Calix[n]arene-based polycrystalline Cu 2D SAMs
4.3.3 Calix[6]arene-based rotaxanes and pseudorotaxanes on Cu surface
Following the strategy adopted in paragraph 4.2.3 for the functionalization of Si(100) surfaces with viologen-calix[6]arene based pseudorotaxanes, we designed and used a monostoppered axle 31 and a not stoppered axle 33 (see figure 4.20) for the formation of pseudorotaxanes by threading reactions with the calix[6]arene “wheel” 25 (see Figure 4.23).
Thanks to the thiol groups present on one ending of the axles, the respective pseudorotaxanes can be covalently anchored on the polycrystalline copper surface.
Figure 4.20 Viologen based axles 31, 33.
Synthesis of the viologen-based axle
The synthesis of the axles 31 and 33 is depicted in scheme 4.5. For the synthesis of axle 31, compound 28 was prepared in 85% yield by refluxing 3,5-di-tert-butylphenol and 1,6-bis(toluene-4-sulfonyloxy)hexane in acetonitrile in the presence of K2CO3 as base.
Successively, 4,4’-bipyridyl was refluxed with compound 28 in acetonitrile to obtain 29 in 65% yield after precipitation of the pure compound from ethyl acetate. After this step, compound 29 was reacted with S-11-(tosyloxy)undecyl ethanethioate 8 (already described in chapter 3) in acetonitrile to afford 30 in 50% yield. Finally the removing of the acetic protection group from the thiolic functionality was accomplished by refluxing 30 in ethanol in the presence of p-toluensulfonic acid. Pure 31, was obtained in 70% yield after precipitation from ethyl acetate.
The 1H NMR in CDCl3 at 300MHz of 31 (see Figure 4.21) shows all the characteristic signals of an asymmetric 4,4’-bipyridinium compound. At δ = 9.19 and 8.79 ppm is possible to see the signals of the bipyridinium unit, in particular the peak at δ = 9.19 is not a doublet but a more complicated signals due to the asymmetry of compound 31. The signals of the stoppers are visible at δ = 6.99 and 6.71 ppm. Diagnostic is also the presence of the multiplet at δ = 2.59 ppm due to the CH2 in α position with respect to the thiol group.
Scheme 4.5 Synthesis of the viologen based axles 31 and 33.
Axle 33 was synthesized with a similar synthetic pathway used to synthesize 31 (see Scheme 4.5). Initially, 4,4’-bipyridyl was refluxed with 1-pentyl tosylate to afford the monoalkylated bipyridine 23 in 75% yield. Successively, 23 was reacted in a sealed tube with 8 using acetonitrile as solvent. After cooling, 32 crystallizes as pure product with 65% yield.
As last step, the deprotection of thiol functionality was accomplished by refluxing 32 in ethanol in presence of p-toluenesulfonic acid. After precipitation from ethyl acetate, 33 was obtain as pure product in 65% yield.
Its 1H NMR in CDCl3 at 300MHz (see Figure 4.22) shows all the characteristic signals of a dialkylated viologen salt. At δ = 9.19 and 8.85 ppm is possible to see the signals of the bipyridinium unit and at δ = 7.74, 7.16 and 2.33 ppm are present the signals of protons attributable to tosilate anions. Diagnostic is the presence of the typical multiplet at δ = 2.52
Figure 4.21 1H-NMR spectra in CDCl3 at 300MHz of compound 31.
Figure 4.22 1H-NMR spectra in CDCl3 at 300MHz of compound 33.
Functionalization and characterization of the Cu surface functionalized with pseudorotaxanes As reported for the functionalization of the Si(100) surfaces, the pseudorotaxanes were preformed in toluene solution and then anchored on the Cu surface through a dip-coating method (see figure 4.23). Using the “monostoppered” axle 31 and the wheel 25 the “oriented”
pseudorotaxane 25⊃31 was obtained thanks to a selective threading process that, in low polar solvents, always occurs from the upper rim of the calix[6]arene with the axle having its stopper oriented above the cavity.11 Using the not stoppered axle 33, a mixture of the two isomers of pseudorotaxane 25⊃33 (up and down) was obtained.
OMeO
O HN
HN O
O
O NH NH O
OMe OMe HN
HN O
O
O
25
OMeO
O HN
HN O
O
O NH NH O
OMe OMe HN
HN O
O
O
25
Polycrystalline Cu Polycrystalline Cu
N N
HS
++
8 O
N N
HS
++
8 O
Dip-coating 25⊃31 “oriented”
Toluene
2 2
N N
SH +
+ TsO
-TsO -8 O
31
2
N N
SH +
+ TsO
-TsO -8 O
31
2
N N
SH +
+ TsO
-TsO
-8
33
N N
SH +
+ TsO
-TsO
-8
33
OMeO
O HN
HN O
O
O NH NH O
OMe OMe HN
HN O
O
O
25
OMeO
O HN
HN O
O
O NH NH O
OMe OMe HN
HN O
O
O
25
25⊃31 “up” and “down”
25⊃33
OMeO
O HN
HN O
O
O NH NH O
OMe OMe HN
HN O
O
O
25
OMeO
O HN
HN O
O
O NH NH O
OMe OMe HN
HN O
O
O
25
Polycrystalline Cu Polycrystalline Cu
N N
HS
++
8 O
N N
HS
++
8 O
Dip-coating
N N
SH +
+
8
N
N SH
+ +
8
N N
SH +
+
8
N
N SH
+ +
8
25⊃31 “oriented”
2 2
N N
SH +
+ TsO
-TsO -8 O
31
2
N N
SH +
+ TsO
-TsO -8 O
31
2
N N
SH +
+ TsO
-TsO
-8
33
N N
SH +
+ TsO
-TsO
-8
33
OMeO
O HN
HN O
O
O NH NH O
OMe OMe HN
HN O
O
O
25
OMeO
O HN
HN O
O
O NH NH O
OMe OMe HN
HN O
O
O
25
25⊃31 “up” and “down”
25⊃33
+ +
Toluene
+
Dip-coating Dip-coating
The oriented pseudorotaxane 25⊃31 and the mixture of the two isomers 25⊃33 were used for the functionalization of the polycrystalline copper surface using the same dip-coating procedure adopted for the anchoring of thiolated calix[n]arene derivatives (see previous paragraph). After the coating, the two copper surfaces functionalized with pseudorotaxanes 25⊃31 (25⊃31@Cu) and 25⊃33 (25⊃33@Cu) were analyzed through XPS spectroscopy and the experimental results are depicted in figures 4.24 and 4.25 respectively.
In Figure 4.24a it has been depicted the XPS S 2p spectral region of the 25⊃31@Cu surface. Two large S 2p signals are visible. After deconvolution the low BE signal is splitted in two pairs of peaks (blue and red lines in Figure 4.24a). The “blue” components, at lower BE, correspond to sulphur species bonded to copper (S-Cu), while the red components, at higher BE, correspond to sulphur atoms in not anchored SH groups (physisorbed pseudorotaxane). The large signal at high BE is composed by only two components (purple lines) and it was assigned to sulphate species, probably derived from the tosylate anions. The amount of this signal is high probably because an excess of tosylate is physisorbed on the copper surface. The N 1s region of the spectrum (figure 4.24b) shows the presence of two peaks: one at high BE which was assigned to the N+ species present in the axle 31, and one at lower BE attributable to the NH groups of the phenylureas of 25. The BE of N 1s referenced to the signal of C 1s are 400.0 and 402.3 eV, respectively. The elemental ratios reported in table 4.4 evidences that the calculated C/N and NH/N+ ratios are in good agreement with theoretical values.
Table 4.4 Elemental ratios of Cu surfaces functionalized with pseudorotaxanes 25⊃31 and 25⊃33 determined through XPS spectroscopy (error ± 10%).
Entry Designation C/N
(exp)
C/N (theor)
NH/N+
(exp)
NH/N+
(theor)
N/Cu (exp)
1 25⊃31@Cu 13.7 18.1 3.5 3 1.5
2 25⊃33@Cu 11.6 14.5 2.8 3 0.09
The results obtained from the XPS study of the polycrystalline copper surface functionalized with the pseudorotaxane 25⊃33 are depicted in figure 4.24 c and d. Also in this case the deconvoluted of the S 2p signal shows that the amount of anchored pseudorotaxane (figure 4.24c, blue line) is lager than the physisorbed one (figure 4.24c, red line). In the same spectral region is also present a signal attributable to the sulphur specie of the tosylate anions.
The deconvolution of the complex signal of the N 1s spectrum (figure 4.24d) shows the presence of three components: an higher BE signal attributable to the N+ atoms of the axle 33,
a halfway BE signal attributable to the NH groups of the phenylureas of 25 and a lower BE signal whose origin has not been fully disclosed yet. The BE of N 1s referenced to the signal of C 1s are 398.0, 399.6, and 401.8 eV, respectively. The elemental ratios reported in table 4.4 evidences that the calculated C/N and NH/N+ ratios are in good agreement with theoretical values.
Figure 4.24 XPS spectral regions of the polycrystalline copper surface functionalized with the pseudorotaxane 25⊃31: a) S 2p region and b) N 1s region and 25⊃33 : c) S 2p region and d) N 1s region.
The comparison of the N/Cu ratios reported in Table 4.4 can be used as an index of the covering degree of each copper surface. The high value calculated for 25⊃31@Cu, that is 1.5, with respect the very low result obtained with 25⊃33@Cu can be explained with a higher degree of covalent functionalization of the first surface. Works are in progess to understand which are the factors reducing the covalent anchoring of the pseudorotaxane mixtures.