Control of translational and rotational movement at nanoscale
Stacko, Peter
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Chapter II:
Molecular dragsters: Towards controlled
translational motion on surfaces
Molecular dragsters bearing two appending molecular motor units as the propelling wheels have been synthesized. The two dragsters feature different sizes of the rotor units. The motors preserve their rotary function in solution and preliminary experiments regarding movement on a surface under ambient conditions have been performed.
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Introduction
Directional motion of molecules offers prospects for development of unique and unprecedented nanotechnology devices rivaling machinery developed by Nature1,2 such as motor proteins moving along microtubules3, the ATPase rotary
motion4,5 or myosin V walking on actin filaments6,7. Examples of directional
motion in the literature include DNA walkers8,9, photochemically10,11 and
redox-driven12 motors and catenane, and rotaxane translational systems, although the
systems are predominantly studied in solution.13–17
Regarding surface confined motors, directional movement can be divided to rotary or translational motion. Achieving rotary motion is the more straightforward option as the two major conditions to be fulfilled are anchoring the molecules on the surface18–22 and the surface should not interfere with
process of controlling the motion; such as quenching of the excited state for photochemically driven molecules, or redox reaction between the molecules and surface for electrochemically driven processes.
On the other hand, there are many obstacles associated with autonomous directional translational movement on surfaces. For example, the attractive forces between the molecules and the surface have to be strong enough to facilitate physisorption of molecules on the surface, while at the same time, the forces have to be weak enough to allow for propulsion along the surface upon application of stimuli. Perhaps due to the delicate balance necessary to achieve this goal, autonomous directional movement of molecules on surface constitutes a major challenge in contemporary chemistry.
Though, several examples of directional translational movement of molecules; such as fullerenes23, porphyrins24, cyclodextrins25 or molecular landers26, on
surfaces can be found in the literature, however, the majority of them is induced by “pushing” or “pulling” of the molecules with an STM tip (e.g. non-autonomous).27,28 In addition, most of the observed motion is attributed to
sliding or slipping motion, rather than rolling motion and the exceptions to these observations will be discussed here.
In a simple example, Grill et. al. have demonstrated that a molecule consisting of two triptycene units connected with an axle (Figure 1a) can exhibit translational movement when adsorbed on a Cu (110) surface at 25 K.28 Placing the STM tip
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on top of the molecule and moving it perpendicular to the axle of the molecules, 4 nm translational movement was observed (Figure 1b-d).
Figure 1. (a) Two triptycene units connected with an axle. (b) Scheme of manipulation
with the STM tip. (c) STM image of the molecule on Cu (110) prior to manipulation. d) STM image of the molecule after the manipulation. (reproduced from ref. 24)
The group of Tour was able to demonstrate the wheel-assisted rolling motion on gold surface at single molecular level using STM.29–31 For this purpose, the
nanocar 2.1 with four wheels as well as the three-wheel analogue 2.1 has been synthesized (Figure 2). The comparison of movement for the two molecules provided the evidence for wheel-assisted rolling motion. Upon heating the Au surface to 200 °C only the four-wheel nanocar 2.1 showed translational motion perpendicular to the axes, whereas the analogue 2.1 displayed pivoting motion (Figure 3). This behavior demonstrates the expected fullerene-facilitated rolling which is contrast to most examples in the literature where simple pushing is almost universally accepted as the mechanism for large molecules.32–34
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Figure 2. The structure of nanocar 2.1 (left) and the three-wheel analogue 2.2 (right);
(Figure reproduced from ref. 25) In addition, both pushing and pulling movement induced by the STM tip was examined on these molecules. Slightly curved, however preferentially linear movement was clearly observed (Figure 3). This movement could not be reproduced when the tip was positioned on the side of the molecule instead of behind the molecules.
Figure 3. (left) Nanocar 2.1 rolling on Au surface. (a-e). A small section of a sequence of
images taken during annealing at ~200 °C (Vb = -0.95 V, It = 200 pA; image size is 51 × 23
nm2). The orientation of 2.1 is determined easily by the fullerene wheel separation, with
motion occurring perpendicular to the axles. The acquisition time for each image is
| 45
approximately 1 min, with a-e selected from a series spanning 10 min, which shows 80° pivot (a) followed by translation interrupted by small-angle pivot perturbations (b-e). (right) Structure and pivot motion of the trimers. (a) Structure of trimers. A sequence of STM images (b-e) acquired approximately 1 min apart during annealing at ~225 °C show the pivoting motion of 2.2 (both circled molecules) and lack of translation in b-e of any molecules. (Vb = -0.7 V, It ) 200 pA; image size is 34 × 27 nm2). Monatomic step edges in
these images are lined with clustered molecules. (f) A summary of the two methods of motion for the different structures showing that nanocar 2.1 consecutively pivots and then translates perpendicular to its axles, whereas trimer 2.2 pivots but does not translate on the surface. For clarity, both structures are drawn devoid of the alkoxy units (reproduced from ref. 27).
Another attempt from group of Tour and co-workers to construct a “motorized nanocar”, rather than a car being pushed or pulled by STM tip contained
p-carborane as a wheel and a light-driven molecular motor as the engine of the
nanocar 2.3.35,36 The molecule was thought to be propelled by the molecular
motor along the substrate surface as it can perform repetitive unidirectional rotary motion induced by light (Figure 4). The molecule has only been studied by 1H NMR and UV-vis spectroscopy in the solution and unfortunately, no
reports of movement on surface studied by STM have been made. The function of the molecular motor was preserved in the presence of p-carborane units unlike in the case of fullerene-C60 wheels.29
Figure 4. (left) The structure of the nanocar 2.3 with p-carborane units as wheels and
molecular motor in the middle. (right) Irradiation (a) of the motor unit leads to rotation of the motor (b) inducing the forward translational motion (c-d). (Reproduced from ref. 25)
Perhaps the most successful demonstration of unidirectional movement on a surface has been reported by the group of Feringa.37 The molecular nanocar 2.4
features a chassis connected to four molecular motors designated to propel the molecule along the surface when performing the rotational movement (Figure
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5). Upon electronic excitation using an STM tip double bond isomerization occurs followed by a helix inversion resulting in rotary motion pushing the molecule forward. A correct configuration of the stereogenic centers was required as only in the meso isomer both of the motor units rotate in the forward direction from the external point of view. Ultimately, the molecule was shown to undergo a preferentially linear movement showing that the intrinsic motor function is capable of converting external energy input into mechanical action (Figure 6).
Figure 5. (a) The structure of nanocar 2.4 with embedded four molecular motors as
wheels. (b) Schematic representation of the concept (Figure reproduced from ref. 33)
Figure 6. (a) STM image (imaging parameters: area 10.2 nm × 39.3 nm, current I = 74 pA,
U = 47 mV) of the initial position. The black area was scanned only after the molecule moved into it. (b) Trajectory depicting the individual steps taken. (c) Final position after ten consecutive voltage pulses. (d) The action spectrum for movement shows a voltage threshold at 500 mV. Each data point represents 8 to 40 manipulations performed on various molecules (I = 30–50 pA). Error bars represent the standard deviation from the probability for successful events. (e) STM frames corresponding to individual steps of the trajectory in (b) excluding starting and final position (reproduced from ref. 33).
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Design of the dragsters
Some of the initial difficulties in the molecular nanocar project included poor volatility of the molecule for surface deposition as well as rather high voltage required for movement of the molecule. Motivated by the success of the electrically-driven molecular nanocar, we sought to address these issues by simplifying the design along with enabling modification of the molecule for future applications such as cargo transport. One could imagine a cargo unit being attached to the motorized part and once at the desired destination, the cargo could be released. For this purpose, we envisioned that molecular dragsters bearing only two rotating motor units (instead of four) could be sufficient (Figure 7). To first prove that the unidirectionality of the molecular dragsters on surface is preserved, only a dummy cargo unit, sharing the same carbazole building block as the other half of the molecule, was attached at this point. In order to assess the influence of wheel “size” on the unidirectional movement, we decided to construct two dragsters with different wheel sizes – one based on simple fluorene units and the second one, larger, based on phenyl substituted fluorene units.
Figure 7. The design of the molecular dragsters consisting of a motorized part with two
48 |
Synthesis of the dragsters and intermediates
Due to similarities in the design, the synthesis of the molecular dragsters is based on that of the molecular nanocar 2.4, although with several crucial modifications in both synthetic and purification procedures.37
The synthesis started with the preparation of 2,7-dibromo-9H-carbazole as it is a common building block for both parts of the molecule (2.11 and 2.18a-b, vide
infra). For this purpose, 4,4’-dibromobiphenyl was nitrated using fuming nitric
acid in glacial acetic acid (Scheme 1).38,39 The resulting mono-nitrated product
2.7 was then subjected to reductive cyclization using PPh3 in o-DCB at high
temperature to facilitate the formation of 2.8 in a good overall yield.40
Scheme 1. Synthesis of 2.8 as the common intermediate for further synthesis.
From this intermediate, both parts of the dragster have been synthesized. The carbazole 2.8 was reacted with a large excess (4 eq.) of 1,4-diiodobenzene in Ullmann coupling catalyzed by CuI and 1,10-phenanthroline as a ligand to provide the N-substituted carbazole 2.9 (Scheme 2).41 The excess of
1,4-iodobenzene prevented the formation of the di-substituted product as well as Ullmann coupling between two molecules of the starting material.
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Scheme 2. Synthesis of the unpropelled part 2.11 of the dragsters from the carbazole 2.8.
Following a procedure for Sonogashira coupling with TMSA using Pd(PPh3)Cl2
and CuI as the catalysts, a TMS-protected acetylene was installed onto the molecule to give compound 2.10. The acetylene would then, upon deprotection, serve as a connection point with the other half of the molecule. It should be stressed that excess of TMSA was avoided as it led to coupling at the bromo-substituted positions even at room temperature. Besides that, a complete selectivity for the iodo-substituted reaction site was observed. Metal-halogen exchange at -78 °C using n-BuLi, followed by a substitution of the di-lithiated carbazole with TMSCl and deprotection of the acetylene in situ with KOH afforded the final building block 2.11 in 84 % yield over the three steps (Scheme 3).
In the next stage, the motorized part 2.18a-b of the dragster was to be synthesized. Unlike in the case of the nanocar 2.4 where the solubilizing hexyl chains were introduced via Sonogashira coupling followed by hydrogenation37,
in this synthesis the alkyl moieties were installed in one step using Pd(dppf)Cl2
50 |
Scheme 3. Synthesis of the diketone 2.14 for double Barton-Kellogg coupling.
Employing the same strategy as for the nanocar, the diketone 2.13 was formed from the carbazole 2.12 by a Friedel-Crafts-Nazarov cyclization sequence with methacrylic acid in PPA (Scheme 4).37 Despite the solubilizing chains, the
compound 2.13 is poorly soluble in most organic solvents. Due to this, the original eluent for flash column chromatography was substituted for dichloromethane, improving the yield to 39% across multiple steps. Compound 2.13 was isolated as a mixture of the two possible diastereomers (1:1, based on
1H NMR spectroscopy) and used as such throughout the rest of the synthetic
sequence. Both the Kumada coupling and the condensation could be conveniently carried out on a multigram scale. Reproduction of the procedure reported for the nanocar 2.4 involving a protection of the carbazole 2.13 with a t-Boc group followed by a thionation using Lawesson’s reagent was unfortunately not met with a success due to constant deprotection of the t-Boc group under various conditions and formation of complex mixtures, presumably due to free NH present in the molecule. It was therefore decided to circumvent the use of a protecting group and shorten the synthetic sequence by performing an Ullmann coupling on the carbazole 2.13 prior to double Barton-Kellogg coupling.
Initially, the Ullmann coupling was planned to be carried out with 1-bromo-4-iodobenzene to avoid di-substitution on the benzene ring. Various conditions and catalysts such as Cu43 or CuI44–46 with L-proline47–49, 1,10-phenanthroline50 as
ligands in both toluene and DMF were screened, however, none of the described methods displayed appreciable selectivity towards the iodine substituent and either no conversion or a mixture of products (1:2 to 1:4, bromo:iodo-substituted
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product) was always observed. We have therefore opted for the same approach as in the case of carbazole 2.8 and used a large excess (6 equivalents) of 1,4-idodobenzene (Scheme X). Conveniently, the unreacted 1,4-iodobenzene can be recycled after the reaction by flash column chromatography due to its low polarity. Finally, this procedure gave the N-substituted carbazole 2.14 in an excellent yield of 91%.
Scheme 4. Double Barton-Kellogg coupling used to install the two molecular motor units.
At this point, the two motor units needed to be introduced in the molecule via double Barton-Kellogg coupling (Scheme 5). The sequence started with a thionation of the ketone 2.14 using Lawesson’s reagent in toluene at 95 °C. The temperature had to be closely monitored as increase above 100 °C led to a quick decomposition of the formed thioketone 2.15 presumably due to enolization, followed by dimerization, trimerization and oligomerization of the thioenol.51,52
This side process could be distinguished by a formation of a yellow spot situated at the start of the TLC plate. Isolation of the dithioketone by a short column, followed by a reaction with 9-diazofluorenone 2.16a or 2.16b at 80 °C in toluene and subsequent desulphurization of the resulting episulfides 2.17a-b with HMPT afforded the building block 2.18a-b in 49% and 44% yield, respectively, over three steps.
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Scheme 5. Connection of the two halves 2.11 and 2.18a-b via Sonogashira coupling
Finally, the unpropelled part 2.11 and the motorized part 2.18a-b were connected in a classical Sonogashira coupling, using a slight excess of the alkyne. The final compounds 2.5a-b were isolated as a mixture of two diastereomers since they were found to be inseparable by flash column chromatography in all steps of the synthesis. It should be realized that only the meso diastereomer can be effective for directional movement on a surface. In case of the (R,R)- and (S,S)-isomers, the motor units rotate in a disrotatory fashion with respect to each other, thus preventing the molecule from moving in a linear manner. For this purpose, the diastereomers can be separated using preparative HPLC or supercritical fluid chromatography (SFC) when required.
Photochemical and thermal isomerization studies in a solution
The photochemical and thermal isomerization behavior of 2.5a-b was examined in solution using both low temperature UV-vis absorption and 1H NMRspectroscopy to demonstrate that the rotary function of the two appending motor units remained uncompromised.
The UV-vis absorption spectra of the dragsters 2.5a and 2.5b in heptane at 293 K show absorption bands centered at 392 and 434 nm, respectively (Figure 8, black solid).
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Figure 8. UV-vis absorption spectra (heptane, 293 K) of stable 2.5a (a) and stable 2.5b (b)
(black solid); irradiation to PSS (365 nm) at 273 K (blue solid); and after heating the samples to 333 K (dashed red).
Irradiation of the sample with UV light (365 nm) at 273 K led to a red-shift of the absorption bands to 401 and 436 nm, respectively, consistent with a formation of a higher energy isomer, and hence species containing a more strained central double bond.53 Throughout the irradiation, a single isosbestic point was
observed in each case, indicating that only a single motor unit undergoes photochemical EZ isomerization at a time and that the irradiation does not produce a double unstable form of 2.5a or 2.5b (based on 1H NMR spectroscopy
and presence of a single isosbestic point in UV-vis experiments). Samples were irradiated until no further changes were observed and the photostationary state (PSS) was reached (Figure 8, blue solid).
After heating the samples to 333 K, a complete recovery of the original UV-vis absorption spectra were observed, consistent with the unstable form of 2.5a-b undergoing thermal helix inversion back to the stable isomers (Figure 8, red dashed).
The rate constants for the thermal helix inversion were measured at five different temperatures (heptane) by following the UV-vis absorption spectra over time. Multivariate analysis was performed on the array of the spectra, providing the rate constant for each temperature. The Eyring plot was then constructed and the activation parameters for the thermal process were derived from the linear fit (Figure 9 and 10).
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∆‡G°THI 88.4 ± 0.0 kJ.mol-1 ∆‡H°THI 65.4 ± 0.2 kJ.mol-1 ∆‡S°THI -79.5 ± 0.5 J.K-1.mol-1 k (293 K) (1.10 ± 0.05) × 10-3 s-1 t1/2 (293 K) 10.7 ± 0.0 min
Figure 9. (left) Thermodynamic data for the thermal helix inversion of the unstable 2.5a
to the stable 2.5a (heptane). (right) Eyring plot for the thermal process.
The Gibbs free energy of activation ∆‡G°THI for isomerization of the unstable 2.5a
was determined to be 88.4±0.0 kJ.mol-1 (∆‡H°THI 65.4±0.2 kJ.mol−1, ∆‡S°THI
-79.5±0.5 J.K−1.mol−1), corresponding to a half-life of 10.7±0.0 min at room
temperature (Figure 9, left). These values compare well to those reported for the nanocar 2.4 which were found to be 88.0 kJ.mol-1 and a half-life of 9.1 min.37
∆‡G°THI 92.6 ± 0.1 kJ.mol-1 ∆‡H°THI 67.2 ± 2.3 kJ.mol-1 ∆‡S°THI -86.4 ± 7.5 J.K-1.mol-1 k (293 K) (1.95 ± 0.08) × 10-3 s-1 t1/2 (293 K) 59.2 ± 2.5 min
Figure 10. (left) Thermodynamic data for the thermal helix inversion of the unstable 2.5b
to the stable 2.5b (heptane). (right) Eyring plot for the thermal process.
Using the same procedure, the activation Gibbs energy ∆‡G°THI for the thermal
isomerization of the unstable 2.5b to the stable 2.5b was found to be 92.6±0.1 kJ.mol-1 (∆‡H°THI 67.2±2.3 kJ.mol−1, ∆‡S°THI -86.4±7.5 J.K−1.mol−1), corresponding
to a half-life 59.2±2.5 min at room temperature (Figure 10a). The increase of ∆‡G°THI is in accordance with introduction of phenyl substituents in the
2,7-position of the fluorenyl rotors, resulting in increased steric hindrance in the fjord region during the thermal helix inversion. One should realize that these ∆‡G°THI values are only relevant for measurements on surfaces at ambient
conditions, as in other instances, such as ultra-high vacuum STM at 7 K, the barriers are thermally insurmountable and electron tunneling excitation by STM tip must be used for induction of the thermal step regardless of the activation barrier.
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The switching behavior was further probed using 1H NMR spectroscopy. A
sample of 2.5a-b (2 mg) in chloroform-d3 was prepared. 1H NMR spectra were
recorded at 243 K before and after irradiation (at 233 K) with a hand-held UV-lamp or LED using optical fiber inside the NMR spectrometer (both 365 nm) (3-16 h). Unfortunately, in neither case a formation of a new set of NMR absorptions was observed as a consequence of the irradiation, unlike in the case of nanocar 2.4.37 Lowering the temperature further (223 K) did not improve the
situation. It remains unclear why no change was observed using 1H NMR
spectroscopy. Possible a much higher concentration than in the UV-vis spectroscopy experiments, low quantum yield or a combination of thereof is responsible for this.
Examining the photochemical and thermal behavior of 2.5a-b in solution using UV-vis absorption, it can be concluded that the motor function of the overcrowded alkenes is preserved in both molecules. The dragsters described herein are therefore suitable candidate for investigation of a directional movement on surfaces.
Single molecule STM experiments at ambient conditions
With the knowledge that the molecules 2.5a-b retained their molecular motor function, we opted to perform preliminary scanning tunneling miscroscopy (STM) experiments and study their behavior on surfaces under ambient conditions. For this purpose, HOPG and gold were chosen as the surfaces for modification with n-pentacontane. N-pentacontane is known to self-assemble on surfaces and serve as a “ruler” and may exhibit attractive interactions with the hexyl chains of the dragsters 2.5a-b, facilitating their absorption on the surface. Upon preparation of the n-pentacontane modified HOPG, the sample was examined by STM at the liquid-solid interface to ensure that the n-pentacontane domains are large enough. Subsequently, a drop of the ~7.9× 10-5 M solution of
the dragster 2.5a in n-tetradecane was added. The pentacontane adlayer could still be resolved after the addition of the dragster, however no dragsters were observed in these experiments (Figure 13). Measurements on bare HOPG did not lead to adsorption of the dragsters either.
For the experiments were performed on gold on mica substrates, a 400 nm-thick layer of gold was deposited (in a home-built thermal deposition system) on a freshly cleaved mica substrate at a temperature of 375°C and a pressure of 1.8 ×
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10-6M. After flame annealing the substrate, a ~9.6 × 10-4M solution of
n-pentacontane in n-tetradecane was drop-casted on the gold/mica substrate. One hour later, a ~7.9× 10-5M solution of the dragster 2.5a in n-tetradecane was
drop-casted on top of the pentacontane solution. In this system, the STM measurements did not reveal adsorped dragster molecules, neither the pentacontane adlayer. However, it is necessary to carry out more experiments in order to conclude whether the dragsters are suitable for use on surfaces under ambient conditions.
Figure 13. STM images of the n-pentacontane adlayer after deposition of the
dragster 2.5a.
Conclusions
Two molecular dragsters with different wheel size have been synthesized. As anticipated, the motors units in the dragsters have been proven to undergo photochemically driven rotation in a solution using UV-vis spectroscopy. Kinetic experiments were performed and thermodynamic data for the thermal helix inversion step of the rotary cycle have been determined by Eyring analysis. The barriers of THI were found to be 88.4±0.0 kJ.mol-1 and 92.6±0.1 kJ.mol-1 for
2.5a and 2.5b respectively, which corresponds to a half-life of 10.7±0.1 and 59.2±2.5 min, respectively, at room temperature.
No movement of the molecules on the pentacontane modified surface has been observed under UV irradiation (365 nm) in the preliminary STM measurements
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so far, presumably due to strong attractive forces between the dragsters and the surface.
Acknowledgment
The single molecule STM experiments on the surfaces described herein have been performed by G. H. Heideman, who is gratefully acknowledged for this contribution.
Experimental section
General remarks
Reagents were purchased from Aldrich, Merck or Fluka and were used as provided unless otherwise stated. The solvents were distilled and dried, if necessary, by standard methods. Column chromatography was performed on silica gel (Merck type 9385 230-400 mesh) unless stated otherwise using positive pressure, TLC: silica gel 60, Merck, 0.25 mm. High Resolution Mass spectra (HRMS) were recorded on an LTQ Orbitrap XL. NMR spectra were obtained using a a Varian Mercury Plus (1H: 400 MHz, 13C: 100 MHz), a Varian Unity Plus
(1H: 500 MHz, 13C: 125 MHz) or a Varian Innova (1H: 600 MHz,) in CDCl3, d8
-toluene or CD2Cl2. Chemical shifts are reported in δ units (ppm) relative to the
residual deuterated solvent signal of CDCl3 (1H NMR, δ 7.26 ppm; 13C NMR, δ
77.23 ppm), d8-toluene (1H NMR, δ 2.09 ppm), or CD2Cl2 (1H NMR, δ 5.32 ppm; 13C NMR, δ 54.0 ppm). The splitting patterns are designated as follows: s
(singlet), d (doublet), t (triplet), q (quartet), dd (doublet of doublets), td (triplet of doublets), qt (quartet of triplets), m (multiplet) and br (broad). SFC was performed on a Thar SFC system consisting of a fluid delivery module (FDM10-1), an autosampler (a modified Alias 840), a semi-prep column oven, PDA detector, a back-pressure regulator (ABPR20), heat-exchanger, and a fraction collector (modified Thar SFC-FC). UV-vis absorption spectra were measured on a Jasco V-630 or a Hewlett-Packard 8453 spectrometer. CD spectra were measured on a Jasco J-815 CD spectrometer. Solvents used for spectroscopic studies was of spectroscopic grade (UVASOL Merck). Irradiations were performed using a spectroline ENB-280C/FE lamp (λmax = 365 nm), or an LED
(5 W, 365 nm, 10 nm width at half-height), mounted in a modified Nalorac Z-Spec probe in the Varian Innova-600 NMR. Samples irradiated for 1H NMR
58 |
temperatures were performed in standard EtOH/N2 bath. Photostationary states
were determined by monitoring changes in UV-vis spectra or 1H NMR spectra
until no further changes were observed. Kinetic analysis of the thermal isomerization steps was performed by UV-vis spectroscopy. Changes in UV-vis absorptions were monitored at different temperatures. The array of the UV-vis spectra was processed using multivariate analysis (from 200 to 800 nm) to obtain the corresponding rate constants from which an Eyring plot was constructed. ∆‡G°, ∆‡H°, ∆‡S° and t1/2 (20 °C) were extracted from this plot.
STM visualization of 2.5a on HOPG
Prior to imaging, the n-pentacontane (TCI Europe) molecules were dissolved in n-tetradecane (Sigma-Aldrich) by heating (40°C) and sonication for at least 2 hours. The non-volatile solvent n-tetradecane allows us to perform STM measurements at the solid-liquid interface. A ~9.6 × 10-4 M solution of n-pentacontane in n-tetradecane was drop-casted on a freshly cleaved highly oriented pyrolytic graphite (HOPG) crystal (SPI supplies, SPI-3 grade). The STM tips were prepared by mechanical cutting from Pt/Ir wire (90:10, diameter 0.25 mm, Goodfellow). All experiments were performed at room temperature using a PicoSPM instrument (Molecular Imaging, Scientec) and PicoScan imaging software.
Synthesis of the dragsters and intermediates
4,4'-Dibromo-2-nitro-1,1'-biphenyl (2.7).
Fuming nitric acid (92.5 %, 120 mL, 2.5 mol) was added into a solution of 4,4'-dibromo-1,1'-biphenyl (29.0 g, 93 mmol) in acetic acid (300 mL) over 10 min. The resulting suspension
was heated to 100 °C for 30 min. The solution was cooled down to 0 °C, and the pale yellow solid was filtered through a glass filter. The solid was dried on air and then triturated with ethanol (350 mL) to give the pure product 2.7. Yield: 27.53 g (83%). Pale yellow solid. Mp 124.3127.5 °C. 1H NMR (400 MHz, CDCl3):
δ (ppm) 8.03 (dd, 1H, J1 = 1.7 Hz, J2 = 1.7 Hz), 7.76 (ddd, 1H, J1 = 8.2 Hz, J2 = 1.8
Hz, J3 = 1.8 Hz), 7.57 (dd, 2H, J1 = 8.5 Hz, J2 = 1.6 Hz), 7.29 (dd, 1H, J1 = 8.2 Hz, J2
= 1.5 Hz), 7.16 (dd, 2H, J1 = 8.5 Hz, J2 = 1.6 Hz). 13C NMR (100 MHz, CDCl3): δ
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2,7-Dibromo-9H-carbazole (2.8).
A mixture of the biphenyl 2.7 (28.5 g, 80 mmol) and triphenylphosphine (52.3 g, 200 mmol) in 1,2-dichlorobenzene (160 mL) was heated at 190 °C for 6 h. The solvent was then
distilled out at reduced pressure and the residue was purified by column chromatography on silica gel (pentane : dichloromethane – 5 : 1 to 3 : 1) give the pure carbazole 2.8. Yield: 20.3 g (78%). Tan solid. Mp 230.1231.8 °C. 1H NMR
(400 MHz, CDCl3): δ (ppm) 8.07 (s, 1H), 7.88 (d, 2H, J = 8.5 Hz), 7.58 (d, 2H, J =
1.6 Hz), 7.36 (dd, 2H, J1 = 8.5 Hz, J2 = 1.6 Hz). 13C NMR (100 MHz, CDCl3): δ
(ppm) 138.5, 129.4, 124.2, 123.3, 112.7, 112.2. 2,7-Dibromo-9-(4-iodophenyl)-9H-carbazole (2.9).
A mixture of the carbazole 2.8 (1.95 g, 6.0 mmol), 1,4-diiodobenzene (7.92 g, 24.0 mmol), 1,10-phenanthroline (216 mg, 1.2 mmol), Cs2CO3 (3.91 g, 12.0 mmol) and CuI
(229 mg, 1.2 mmol) in toluene (30 mL) was heated at 110 °C. When the starting material was completely consumed
(67 h, TLC), the solvents were evaporated at reduced pressure and the residue was purified by column chromatography on silica gel (pentane to pentane : dichloromethane – 5 : 1) to give the pure product 2.9. Yield: 2.43 g (77%). White solid. Mp 188.1189.3 °C. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.97 (d, 2H, J = 8.1
Hz), 7.93 (d, 2H, J = 8.3 Hz), 7.47 (s, 2H), 7.41 (d, 2H, J = 8.3 Hz), 7.25 (d, 2H, J = 8.1 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) 141.6, 139.7, 136.3, 129.0, 124.1,
121.9, 121.7, 120.3, 113.0, 93.5.
2,7-Dibromo-9-(4-iodophenyl)-9H-carbazole (2.10).
A mixture of the carbazole 2.9 (2.30 g, 4.36 mmol), ethynyltrimethylsilane (620 l, 4.36 mmol), Pd(PPh3)2Cl2
(153 mg, 0.22 mmol) and CuI (42 mg, 0.22 mmol) in dry THF (50 mL) and Et3N (25 mL) was degassed by four
freeze-pump-thaw cycles. The resulting solution was stirred at
room temperature under N2 atmosphere overnight. The volatiles were
evaporated at reduced pressure and the residue was purified by column chromatography on silica gel (pentane to pentane : dichloromethane – 5 : 1) to give the pure product 2.10. Yield: 1.78 g (82%). White solid. Mp 239.1240.3 °C.
1H NMR (400 MHz, CDCl3): δ (ppm) 7.93 (d, 2H, J = 8.3 Hz), 7.74 (d, 2H, J = 8.5
60 |
1.5 Hz), 0.32 (s, 9H). 13C NMR (100 MHz, CDCl3): δ (ppm) 141.7, 136.4, 134.0,
126.9, 124.0, 123.4, 122.0, 121.7, 120.2, 113.2, 103.9, 96.3, 0.1. 9-(4-Ethynylphenyl)-2,7-bis(trimethylsilyl)-9H-carbazole (2.11). The solution of carbazole 2.10 (710 mg, 1.4 mmol) in dry THF (30 mL) was cooled down to -78 °C under nitrogen atmosphere. Solution of n-BuLi (3.1 mL, 5.0 mmol) was added dropwise over 10 minutes. After stirring for 30 min, TMSCl (730 l, 5.7 mmol) was added at once and the
reaction was left to warm to room temperature. Aq. KOH (3 ml, 2 M) was added and the mixture was stirred for 6 h. The reaction mixture was partitioned between water (60 mL) and ethyl acetate (50 mL). The water layer was washed with ethyl acetate (2 × 50 mL) and the combined organic extracts were dried with MgSO4. The volatiles were evaporated at reduced pressure and the residue
was purified by column chromatography on silica gel (pentane : ethyl acetate – 50 : 1) to give the pure product 2.11. Yield: 505 mg (86%). White solid. Mp 221.6-222.3 °C. 1H NMR (400 MHz, CDCl3): δ (ppm) 8.17 (d, 2H, J = 7.7 Hz), 7.81 (d,
2H, J = 8.3 Hz), 7.617.63 (m, 4H), 7.49 (d, 2H, J = 7.7 Hz), 3.24 (s, 1H), 0.36 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) 140.4, 138.7, 138.4, 133.9, 127.1, 125.1,
124.2, 121.1, 120.0, 114.4, 83.2, 78.3, -0.5. HRMS (ESI+): calcd for C26H30NSi2+ (M +
H+) 412.1911 found 412.1909.
2,7-Dihexyl-9H-carbazole (2.12).
A solution of hexylmagnesium bromide (38.5 mL, 2.0 M in Et2O) was added slowly to a degassed solution of 2.8 (5.0 g,
15.4 mmol) and Pd(dppf)Cl2 (450 mg, 0.62 mmol) in dry
THF (100 mL) at room temperature under N2 atmosphere. The resulting mixture
was then heated at reflux for 4 h. After no more starting material could be observed by TLC, the reaction mixture was cooled down and the reaction was quenched by slow addition of methanol (5 mL). The solvents were evaporated at reduced pressure and the residue purified by column chromatography on silica gel (pentane : dichloromethane – 5 : 1) to give the pure product 2.12. Yield: 4.21 g (81%). White flaky solid. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.93 (d, 2H, J =
7.9 Hz), 7.76 (brs, 1H), 7.17 (s, 2H), 7.07 (dd, 2H, J1 = 8.0 Hz, J2 = 1.2 Hz), 2.78 (t,
4H, J = 7.7 Hz), 1.72 (m, 4H), 1.331.39 (m, 12H), 0.92 (t, 6H, J = 7.0 Hz). 13C NMR
(100 MHz, CDCl3): δ (ppm) 140.8, 140.1, 121.5, 120.4, 119.8, 110.2, 36.7, 32.1, 32.0,
| 61
4,8-Dihexyl-2,10-dimethyl-10,11-dihydro-1H-dicyclopenta[c,g]carbazole-3,9(2H,6H)-dione (2.13).
Methacrylic acid (10.4 mL, 122 mmol) was added to a mechanically stirred mixture of the carbazole 2.12 (4.10 g, 12.2 mmoL) and in PPA (115 %, 100 mL) and heated at 100 °C overnight. The reaction was quenched by addition of ice (300 mL) and the resulting mixture was extracted
with dichloromethane (3 x 100 mL). The combined organic extracts were washed with sat. aq. K2CO3 (100 mL) and dried with MgSO4. The solvents were
evaporated at reduced pressure and the residue purified by column chromatography on silica gel (dichloromethane) to give the pure product 2.13. Yield: 1.94 g (34%). Pale yellow solid. Mp 197.5198.3 °C. 1H NMR (400 MHz,
CDCl3): δ (ppm) 8.91 (brs, 1H), 7.22 (s, 2H), 4.06 (dd, 2H, J1 = 17.0 Hz, J2 = 7.9 Hz), 3.143.36 (m, 6H), 2.802.91 (m, 2H), 1.641.72 (m, 4H), 1.401.46 (m, 10H), 1.281.38 (m, 8H) 0.88 (t, 6H, J = 7.0 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) 209.0, 208.9, 150.07, 150.04, 143.8, 143.0, 127.58, 127.55, 118.65, 118.63, 111.5, 42.3, 42.2, 36.9, 36.8, 32.5, 32.0, 31.2, 29.6, 22.9, 17.2, 17.0, 14.3. 4,8-Dihexyl-6-(4-iodophenyl)-2,10-dimethyl-10,11-dihydro-1H-dicyclopenta[c,g]carbazole-3,9(2H,6H)-dione (2.14).
A mixture of the carbazole 2.13 (250 mg, 0.53 mmol), 1,4-diiodobenzene (1.05 g, 3.2 mmol), 1,10-phenanthroline (38.2 mg, 0.21 mmol), Cs2CO3 (345 mg, 1.1 mmol) and CuI
(40.4 mg, 0.21 mmol) in toluene (2 mL) was heated to 110 °C. When the starting material was completely consumed (67 h, TLC), the solvents were evaporated at
reduced pressure and the residue purified by column chromatography on silica gel (dichloromethane to dichloromethane : ethyl acetate – 40 : 1) to give the pure product 2.13. Yield: 321 mg (90 %). Pale yellow solid. Mp 79.781.6 °C. 1H NMR
(400 MHz, CDCl3): δ (ppm) 8.02 (d, 2H, J = 8.5 Hz), 7.23 (d, 2H, J = 8.4 Hz), 6.96
(s, 2H), 4.09 (dd, 2H, J1 = 16.8 Hz, J2 = 7.9 Hz), 3.303.37 (m, 2H), 3.043.23 (m,
4H), 2.792.89 (m, 2H), 1.58 (m, 4H), 1.261.44 (m, 18H), 0.87 (t, 6H, J = 7.0 Hz).
13C NMR (100 MHz, CDCl3): δ (ppm) 208.7, 208.6, 149.89, 149.85, 145.2, 143.3,
139.8, 136.3, 130.0, 128.1, 128.0, 118.69, 118.67, 110.6, 94.6, 42.33, 42.30, 37.1, 37.0, 32.7, 31.9, 31.5, 29.6, 22.9, 17.2, 17.0, 14.3. HRMS (ESI+): calcd for C38H45INO2+ (M
62 |
General procedure for Barton-Kellogg coupling (2.18a-b). A mixture of the diketone 2.14 (200 mg, 0.30 mmol) and Lawesson’s reagent (480 mg, 1.2 mmol) in dry toluene (20 mL) was heated at 90 °C for 3 h. The mixture was left to cool down to room temperature and passed through a short column of silica gel. The dithioketone was eluted (pentane : ethyl acetate – 10 : 1) as the first red band. The solvents were evaporated at reduced pressure and the residue was
redissolved in dry toluene (20 mL). The solution was heated to 80 °C and diazofluorenone54 or 2,7-biphenyldiazofluorenone (1.05 mmol) was added at
once. The resulting solution was heated at 80°C overnight. HMPT (160 l, 0.89 mmol) was added and the mixture was heated for additional 3 h. The solvents were evaporated at reduced pressure and the residue was purified by column chromatography.
3,9-Di(9H-fluoren-9-ylidene)-4,8-dihexyl-6-(4-iodophenyl)-2,10-dimethyl-2,3,6,9,10,11-hexahydro-1H-dicyclopenta[c,g]carbazole (2.18a).
Prepared according to the general procedure from the ketone 2.14 (200 mg, 0.30 mmol) and diazafluorenone (228 mg, 1.2 mmol). The product was purified by column chromatography on silica gel (pentane : dichloromethane - 7:1 to 5:1) to give 2.18a as a mixture of diastereomers (~ 1:1). Yield: 132 mg (46 %). Orange solid. Mp >250 °C (decomp.). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.04 (dd, 2H, J1 = 8.3 Hz, J2 = 2.4 Hz), 7.93 (dd, 2H, J1 = 7.2 Hz, J2 = 7.2 Hz), 7.81 (dd, 2H, J1 = 7.6 Hz, J2 = 7.6 Hz), 7.75 (d, 1H, J = 7.6 Hz), 7.71 (d, 1H, J = 7.5 Hz), 7.61 (d, 1H, J = 7.9 Hz), 7.49 (dd, 2H, J1 = 8.2 Hz, J2 = 8.2 Hz), 7.45 (d, 1H, J = 8.3 Hz), 7.307.40 (m, 4H), 7.26 (dd, 2H, J1 = 7.7 Hz, J2 = 7.4 Hz), 7.20 (d, 1H, J = 7.9 Hz), 7.14 (s, 1H), 7.12 (s, 1H), 7.08 (dd, 1H, J1 = 7.5 Hz, J2 = 7.5 Hz), 7.02 (dd, 1H, J1 = 7.6 Hz, J2 = 7.6 Hz), 4.30 (m, 2H), 3.87 (dd, 1H, J1 = 14.9 Hz, J2 = 5.6 Hz), 3.75 (dd, 1H, J1 = 15.2 Hz, J2 = 5.6 Hz), 3.41 (d, 1H, J = 15.0 Hz), 3.20 (d, 1H, J = 15.0 Hz), 3.07 (m, 2H), 2.64 (m, 2H), 1.50 (d, 3H, J = 6.5 Hz), 1.331.43 (m, 7H), 0.881.11 (m, 12H), 0.68 (t, 3H, J = 7.4 Hz), 0.64 (t, 3H, J = 7.4 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) 153.0, 152.9, 143.4, 143.2, 141.5, 141.4, 140.5, 140.2, 139.99, 139.95, 139.8, 139.6, 139.5, 139.15, 139.11, 138.3, 138.1, 137.5, 137.4, 133.6, 133.5, 130.0, 129.7, 128.6, 128.5, 127.05, 127.01, 126.99, 126.95, 126.6, 126.59, 126.54, 123.8, 122.8, 119.82, 119.80, 119.4, 119.33, 119.30, 119.2, 108.9, 108.8, 93.4, 93.1, 44.9, 44.3, 43.3, 43.1, 35.1, 35.0, 32.8, 32.5, 31.8, 31.7, 28.76, 28.75, 22.66, 22.61, 20.0, 19.6, 14.19, 14.15. HRMS (ESI+): calcd for C64H61IN+ (M + H+) 970.3843 found 970.3823.
| 63
3,9-Bis(2,7-diphenyl-9H-fluoren-9-ylidene)-4,8-dihexyl-6-(4-iodophenyl)-2,10-dimethyl-2,3,6,9,10,11-hexahydro-1H-dicyclopenta[c,g]carbazole (2.18b). Prepared according to the general procedure from the ketone 2.14 (200 mg, 0.30 mmol) and 2,7-diphenyldiazafluorenone (360 mg, 1.08 mmol). The product was purified by column chromatography on silica gel (pentane : dichloromethane - 5:1 to 3:1) to give 2.18b a mixture of diastereomers (~1:1). Yield: 195 mg (52 %). Orange solid. Mp >250 °C (decomp.). 1H NMR (400 MHz, CDCl3): δ (ppm) 8.19
(d, 2H, J = 8.4 Hz), 8.10 (d, 2H, J = 8.2 Hz), 7.96 (s, 1H), 7.88 (dd, 2H, J1 = 7.2 Hz, J2 = 7.2 Hz), 7.82 (m, 3H), 7.73 (dd, 4H, J1 = 7.8 Hz, J2 = 7.8 Hz ), 7.607.64 (m, 4 H), 7.287.52 (m, 18H), 7.22 (s, 1H), 7.17 (s, 1H), 4.40 (m, 2H), 3.89 (dd, 1H, J1 = 15.0 Hz, J2 = 5.7 Hz), 3.80 (dd, 1H, J1 = 15.1 Hz, J2 = 5.7 Hz), 3.45 (d, 1H, J = 15.2 Hz), 3.22 (m, 3H), 2.712.76 (m, 2H), ), 1.59 (d, 3H, J = 6.5 Hz), 1.381.45 (m, 7H), 0.891.12 (m, 12H), 0.69 (t, 3H, J = 6.9 Hz), 0.63 (t, 3H, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3): δ (ppm) 153.56, 153.42, 143.66, 143.37, 142.37, 142.23, 141.87, 141.76, 141.74, 141.68, 140.92, 140.87, 140.53, 140.42, 140.11, 140.06, 139.69, 139.62, 139.36, 139.31, 139.24, 139.09, 138.71, 138.00, 137.95, 137.62, 133.65, 133.50, 129.89, 129.48, 129.10, 128.75, 128.71, 128.52, 128.46, 127.42, 127.36, 127.29, 127.27, 127.12, 127.02, 126.97, 126.1, 125.95, 125.92, 122.82, 122.78, 121.50, 121.47, 120.3, 120.2, 119.86, 119.76, 119.52, 119.3, 108.94, 108.89, 93.6, 93.1, 44.7, 44.3, 43.3, 43.2, 35.4, 35.3, 33.0, 32.8, 31.84, 31.77, 28.82, 28.81, 22.67, 22.60, 20.12, 19.7, 14.19, 14.14. HRMS (APCI+): calcd for C88H76IN+ (M+) 1273.5011 found 1273.5017.
64 |
General procedure for Sonogashira coupling of 2.18a-b and 2.11.
A mixture of aryl iodide 2.18a-b (1 eq.), ethyne 2.11 (1.6-1.8 eq), Pd(PPh3)2Cl2 (5
% mmol) and CuI (5 % mmol) in dry THF (5 mL) and Et3N (5 mL) was degassed
by four freeze-pump-thaw cycles. The resulting solution was stirred at room temperature under N2 atmosphere overnight. The volatiles were evaporated at
reduced pressure and the residue was purified by column chromatography.
6-(4-((4-(2,7-Bis(trimethylsilyl)-9H-carbazol-9-yl)phenyl)ethynyl)phenyl)-3,9- di(9H-fluoren-9-ylidene)-4,8-dihexyl-2,10-dimethyl-2,3,6,9,10,11-hexahydro-1H-dicyclopenta[c,g]carbazole (2.5a).
Prepared according to the general procedure from 2.18a (194 mg, 0.20 mmol) and 2.11 (154 mg, 0.37 mmol). The product was purified by column chromatography on silica gel (pentane : dichloromethane - 7:1 to 5:1) to give 2.18a as a mixture of diastereomers (~ 1:1). Yield: 236 mg (94 %). Orange wax. 1H
NMR (400 MHz, CDCl3): δ (ppm) 8.11 (d, 2H , J = 7.6 Hz), 7.687.95 (m, 12H), 7.597.65 (m, 3H), 7.407.51 (m, 3H), 7.167.39 (m,10H), 7.08 (dd, 1H, J1 = 7.6 Hz, J2 = 7.6 Hz), 7.02 (dd, 1H, J1 = 7.7 Hz, J2 = 7.7 Hz), 4.30 (m, 2H), 3.87 (dd, 1H, J1 = 15.3 Hz, J2 = 5.5 Hz), 3.77 (dd, 1H, J1 = 15.1 Hz, J2 = 5.6 Hz), 3.42 (d, 1H, J = 14.9 Hz), 3.20 (d, 1H, J = 15.0 Hz), 3.06 (m, 2H), 2.63 (m, 2H), 1.50 (d, 3H, J = 6.5 Hz), 1.331.43 (m, 8H), 0.881.11 (m, 12H), 0.67 (t, 3H, J = 7.4 Hz), 0.63 (t, 3H, J = 7.4 Hz), 0.30 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) 153.12, 152.97, 143.50, 143.31, 141.52, 141.51, 140.61, 140.50, 140.29, 140.02, 139.98, 139.85, 139.17, 139.12, 138.79, 138.35, 138.29, 138.27, 138.20, 137.80, 137.74, 134.89, 133.73, 133.66, 133.61,
| 65
133.53, 129.29, 128.62, 128.56, 128.15, 127.84, 127.24, 127.06, 127.01, 126.96, 126.68, 126.61, 126.55, 125.19, 124.54, 124.31, 123.90, 123.21, 122.92, 122.87, 121.96, 121.94, 120.51, 120.10, 119.83, 119.56, 119.40, 119.37, 119.33, 114.48, 109.11, 109.06, 90.36, 90.27, 89.76, 89.70, 44.97, 44.37, 43.38, 43.22, 35.13, 35.04, 32.82, 32.56, 31.83, 31.75, 29.92, 28.79, 28.77, 27.14, 22.67, 22.62, 20.08, 19.64, 14.20, 14.15, -0.57. HRMS (ESI+): calcd for C90H88N2Si2+ (M+) 1253.6481 found 1252.6472.6-(4-((4-(2,7-Bis(trimethylsilyl)-9H-carbazol-9-yl)phenyl)ethynyl)phenyl)-3,9- bis(2,7-diphenyl-9H-fluoren-9-ylidene)-4,8-dihexyl-2,10-dimethyl-2,3,6,9,10,11-hexahydro-1H-dicyclopenta[c,g]carbazole (2.5b).
Prepared according to the general procedure from 2.18b (190 mg, 0.15 mmol) and 2.11 (98 mg, 0.24 mmol). The product was purified by column chromatography on silica gel (pentane : dichloromethane - 5:1 to 3:1) to give 2.18b as a mixture of diastereomers (~ 1:1). Yield: 195 mg (84 %). Orange wax.
1H NMR (400 MHz, CDCl3): δ (ppm) 8.19 (d, 2H, J = 9.9 Hz), 8.14 (d, 2H, J = 7.7 Hz), 7.978.01 (m, 3H), 7.777.93 (m, 6 H), 7.217.75 (m, 35H), 4.41 (m, 2H), 3.92 (dd, 1H, J1 = 15.0 Hz, J2 = 5.6 Hz), 3.83 (dd, 1H, J1 = 15.0 Hz, J2 = 5.5 Hz), 3.48 (d, 1H, J = 15.2 Hz), 3.22 (m, 3H), 2.74 (m, 2H), ), 1.59 (d, 3H, J = 6.6 Hz), 1.361.47 (m, 7H), 0.821.12 (m, 12H), 0.69 (t, 3H, J = 6.9 Hz), 0.63 (t, 3H, J = 6.8 Hz), 0.33 (s, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) 153.63, 153.49, 143.73, 143.43, 142.44, 142.39, 142.26, 141.89, 141.77, 141.73, 141.68, 140.94, 140.89, 140.53, 140.50, 140.42, 140.14, 140.08, 139.35, 139.30, 139.27, 139.11, 138.80, 138.72, 138.35, 138.32, 138.01, 137.95, 137.84, 137.80, 133.71, 133.68, 133.53, 129.16, 129.11, 128.95, 128.79, 128.74, 128.49, 128.45, 128.14, 128.03, 127.57, 127.44, 127.39, 127.31, 127.27, 127.16, 127.05, 126.99, 126.04, 125.95, 125.21, 124.32, 123.26, 122.98, 122.84, 122.80, 121.94, 121.52, 120.97, 120.25, 120.19, 120.11, 119.86, 119.76, 119.59, 119.38, 114.49, 109.15, 109.07, 90.54, 90.45, 89.70, 89.67, 44.63, 44.27, 43.37, 43.23, 35.39, 35.28, 33.02, 32.82, 31.87, 31.79, 29.92, 28.85, 27.14, 22.69, 22.61, 20.15, 19.73, 14.15, -0.56. HRMS (ESI+): calcd for C114H105N2Si2+ (M + H+) 1557.7811 found 1557.7751.
66 |
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