Photoresponsive Self-Assembled Systems Cheng, Jinling
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Towards a Photoswitchable Mesogen
Liquid crystals are promising materials for optical switching and image storage because of their high resolution and sensitivity. In this section, second-generation molecular motors (trans/cis M3) containing an intrinsic 4,4'-disubstituted biphenyl unit, have been synthesized and fully characterized. The correlation between the molecular structure, electronic factors and photoresponsive properties is discussed. Although trans/cis M3 does not show liquid crystal properties (which possibly can be attributed to the steric hindrance), the isomers can be applied as chiral dopants in nematic liquid crystal materials.
7.1 Introduction
The concept of reversibly addressing different states of a molecular or supramolecular system by light has been widely applied in the study of functional materials to access distinct properties.1–3 Select examples include high-density data storage systems, sensors, photonic switches and molecular logic gates.3–6 Among them, systems that combine photochromic and mesogenic units offer bifunctional properties that are useful for many practical applications, such as data storage and display devices. These systems have received a lot of attention in recent years.7–9 Up to now, most of the reported photochromic liquid crystal (LC) materials are polymers embedded with photochromic units, or alternatively photochromic compounds are used as dopants in the LC materials.10–16 However, the synthesis of a small molecule that combines both photochromic and liquid crystalline properties is challenging as the combination usually results in the loss of one of these properties.
Scheme 7.1. The principles of constructing low-molar-mass photochromic liquid crystals (a)
and (b-d) some photochromic liquid crystals systems. (Cr–crystal phase; N–nematic phase; SC–smectic C phase; Ch–cholesteric phase; QLC-quasi-liquid crystal phase; I–isotropic phase).
As the design principle, the photoresponsive mesogen contains three modules (Scheme 7.1a): 1) volume-excluding cores: such as benzene, cyclohexane, bicyclooctene, cubane, and adamantane, to prevent crystallization in the cooling process; 2) photochromic cores, which exert the photoswitchable functions; 3) spacers connecting volume-excluding units and photochromic cores, which are usually alkyl chains. Selected low-molar-mass photochromic liquid crystalline systems that include azobenzenes, spiropyrans, diarylethenes and stilbenes are shown in Scheme 7.1b-d.17–21
Among the families of photochromic compounds, molecular motors based on overcrowded alkenes (which were developed by our research group) have been shown capable of performing unidirectional 360° rotation upon continuous UV irradiation. As shown in Scheme 7.2, when the P-stable-M2 is irradiated with UV light (365 nm), the molecular motor undergoes an E-Z isomerization at central double bond, which gives rise to the formation of the M-unstable M2. After halting the UV irradiation, an irreversible thermal helix inversion step takes place, resulting in the reformation of P-stable-M2. We envisioned that a combination of light-driven molecular motor M2 and 4,4'-disubstituted biphenyls could result in a mesogenic system (as indicated in Scheme 7.3) that exhibit both photochromism and liquid crystallinity properties.
7.2 Results and Discussion
7.2.1 Molecular Design
The LC mesogen in the present study is based on M2 with the lower half modulated with substituents to achieve an intrinsic 4,4'-disubstituted biphenyl unit. Our reasons for selecting 4,4'-disubstituted biphenyls that contain cyano and n-alkoxy-groups (8OCB) are as follows:
1) The biphenyl derivatives have a strong tendency to form LC phase as seen with 5CB and E7.
2) The presence of a cyano-group strongly promotes nematic properties.
3) By implement of n-Alkoxy-groups a highly anisotropic geometry could be generated that could be considered as a rod-shape mesogon.
4) The structural similarity between lower half of M2 and the 4,4'-disubstituted biphenyl favors the synthetic feasibility.
7.2.2 Synthesis
The synthesis of M3 begins with the alkylation of dihydroxyfluorenone 1. Mono-substituted fluorenone 2 can be obtained in 26% yield. After protection of the remaining hydroxyl moiety with a triflate group, fluorenone 3 was then treated with hydrazine monohydrate at 60 °C for 4 h to afford hydrazone 4 in 90% yield. Oxidation of hydrazone 4 to diazo compound 5 was performed by mixing MnO2 in THF at 0 °C. The obtained diazo compound 5 was used without further purification. The Barton-Kellogg reaction was applied for the coupling of the diazo compound 5 and the freshly prepared thioketone 6, and the mixture was stirred overnight to provide overcrowded alkene 7. The overcrowded alkene 7 was reacted with zinc cyanide in DMF using XPhos/Pd-G3 catalyst.22 The reaction was completed within 3 h, giving 95% yield of the desired M3. The obtained M3 was subsequently recrystallized by slow diffusion of pentane into ethyl acetate, yielding pure trans and cis isomers. The structure of trans and cis M3 was determined by 1H and 13C-NMR and composition by HRMS.
7.2.3 Photochemical and Thermal Isomerization Studies
The photochemical and thermal isomerization behavior of both the trans and cis isomers of
M3 were examined by UV-vis, CD spectroscopy and 1H-NMR. The results confirmed that M3
undergoes a full unidirectional rotary cycle triggered by light (photoisomerization) and heat (thermal isomerization, Scheme 7.2).
UV/vis spectroscopy was used to follow the photochemical isomerization of both isomers of
M3 (Figure 7.1). The push-pull substituents in M3 result in red-shifting of the absorption
spectra for both trans/cis M3 compared to the parent motor M2, as the maximum absorption band is shifted from 375 nm to 400 nm.23 In addition, trans/cis M3 exhibit similar absorption spectra, as an overlay spectrum (Figure 7.1a) clearly shows. Irradiation of
trans-M3 in CH2Cl2 at 20 °C gives rise to a bathochromic shift in the UV/vis spectrum with clear isosbestic point at 421 nm, which is indicative of the formation of the unstable form. This process can be triggered by different wavelengths (365 nm, 385 nm and 420 nm). The photostationary state (PSS) ratio at λmax = 420 nm based on the spectral changes in the UV/vis absorption is quite low (69:31, unstable: stable), since there is a strong absorption for the unstable form in this region.
Figure 7.1 (a) An overlay of UV-vis spectra of cis-M3 and trans-M3 and (b) The spectral
changes of cis-M3 upon the irradiation at 365 nm, 385 nm and 420 nm. (1 × 10-5 M in CH2Cl2). The photochemical isomerizations of trans/cis M3 by irradiation at 365 and 385 nm were also followed by 1H NMR spectroscopy. A solution of M3 in CD2Cl2 was cooled to −40 °C and
irradiated at 365 nm (Figure 7.2). The characteristic shift (Ha: 1.42 ppm →1.78 ppm) of the methyl group at the stereogenic indicates that the methyl moiety adopts a pseudoequatorial position in the unstable isomer. Other notable shifts include the shift of the proton b and b’ (Hb: 2.81 ppm →3.22 ppm) and (Hb′: 3.55 ppm →3.63 ppm) and proton c (Hc: 4.25 ppm →4.01 ppm), due to the formation of the unstable isomer. PSS ratios of 90:10 (unstable state: stable state) and 86:16 are found upon 385 nm and 365 nm irradiation, respectively. The higher PSS ratio is highly desired for the visualization of changes in the bulk LC phase.
Figure 7.2. 1H-NMR spectra of cis isomer M3 in CD2Cl2 at -40 °C upon irradiation of 365 nm. Initial state (bottom spectrum), PSS state (top spectrum).
From the results obtained by UV/vis and 1H-NMR spectroscopies, it can be concluded that
M3 is able to undergo photochemical isomerization. In addition to this, the subsequent
thermal isomerization was studied, the kinetics of the process was followed at five temperatures (0, 5, 10, 15 and 20 °C) in CH2Cl2. The Gibbs free energy (Δ⧧G°) of activation of the thermal helix inversion (THI) was determined by the Eyring equation and was calculated to be 90 ± 1 kJ/ mol for both trans and cis M3 (Figure 7.3), with half-lives 409 s and 398 s at room temperature. Comparison of Δ⧧G° of the THI for M3 with parent motor M2 (Δ⧧G° = 86 ± 1 kJ/mol, and half-live 250 s), reveals that the Δ⧧G° for THI of M3 is larger due to the structural modification. The slightly increasing in the thermal stability might also help the visualization of changed properties of the bulk LC phase.
Figure 7.4 The CD spectrum of P-trans-M3 in CH2Cl2 upon irradiation at 365 nm.
Photochemical isomerization and the subsequent thermal helix inversion can be monitored by CD spectroscopy (enantiomers were separated by SFC, and the configurations were determined by comparing the CD spectra with DFT calculations). In the CD absorption spectrum, the P helicity of trans-M3 gave rise to a negative Cotton signal in the visible region (Figure 7.4 up, black curve). Upon irradiation, the sign of the CD absorption inverted (bottom, purple curve), indicating that a helical inversion occurred in the photoisomerization process. This transformation was consistent with the formation of the unstable isomer with M helicity. When the sample was kept in the dark, the signal of the stable isomer was observed. To conclude, compound trans/cis M3 showed a high PSS ratio in favor of unstable form for the photochemical isomerization, and a half-life of 400 s at room temperature for the THI step. The good thermal stability and high PSS ratio are essential features for the compound to be used as photoswitchable mesogen.
7.2.4 Liquid Crystalline Properties
The liquid-crystalline properties of compounds trans/cis M3 were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and Wide-angle X-ray scattering (WAXS). The heating and cooling scans at 5 °C/min of M3 preheated to 200 °C with subsequent cooling to -30 °C were performed with DSC and the results are compiled in Figure 7.5. Unfortunately, during the heating and cooling process, neither the trans-isomer nor the cis-isomer exhibits any mesophase properties. Only one phase transition around 50 °C can be observed during the heating process, which might be attributed to the glass transition (Tg), and a very weak endothermic peak correlative to phase transition is obtained during the cooling process. The phase transition process was further investigated by POM. When examined by POM, the cis-isomer sample showed to be slightly birefringent. When the sample was heated above Tg, a slightly higher birefringence was obtained (60 °C). Further heating the sample to 100 °C results in the isotropic point where the birefringence is completely lost. During the cooling process with a speed of 1 °C/min, no significant phase transition can be observed. This observation is in agreement with the DSC result, where a weak endothermic peak was obtained during the annealing process. Next, the trans-isomer
was also monitored by POM. Upon heating to 60 °C, the sample started to lose the birefringence, until an isotropic phase could be observed at 100 °C. The cooling process was also followed by the POM, and no phase transfer could be observed either.
Figure 7.5 Differential scanning calorimetric heating and cooling scans of cis-M3 and trans-M3 at 5 °C /min of samples, and the corresponding POM images. The sample was preheated
above their clearing points and then followed by cooling to -30 °C.
The two isomers exhibit different phase transitions during the heating process. WAXS experiments were carried out on the samples which have been annealed (Figure 7.6). The diffraction patterns for both isomers are quite broad at the low angle. Interestingly, the scattering pattern of trans-isomer only shows a single broad peak at q of 4.7391 nm-1 at 25 °C with d-spacing of 1.326 nm. On the contrary, the WAXS pattern of the cis isomer reveals characteristic Bragg spacing of (1:√3) at q=4.0648 nm-1 and 7.3294 nm-1 with corresponding space of 1.546 nm and 0.857 nm, respectively. Based on DFT calculations, we hypothesized that the cis isomer, which bears the donor–acceptor moieties at the lower half, might have a monolayer packing where a head to tail interaction is present.
Figure 7.6 (a) WAXS patterns pattern of cis-M3 (black curve) and trans-M3 (blue curve), and
(b) Schematic model of the proposed packing model for cis-M3. Note: the DFT calculation shows that the cis isomer is about 1.8 nm in length, and the bulk upper half is 0.8 nm in length.
It has been reported that the 8OCB shows good liquid crystalline properties in a wide temperature range (Cr 55°C SA 67°C N 82°C I),24 which is due to dipole-dipole interactions between the mesogenic units. However, when 8OCB is embedded in the lower half of the motor, as in M3, the loss of liquid crystalline properties is observed. This might be due to the steric hindrance that force the upper half of the out of plane, and as a result, the interaction between the molecules is weakened as indicated in Figure 7.6.
7.2.5 Molecular Motor as Chiral Dopants
Although compound M3 does not show a liquid crystalline phase which is attributed to the steric hindrance, the two isomers still can be used as chiral dopants for nematic LC materials. A cholesteric LC (CLC) material is obtained by doping 1.0 wt% of enantiomeric pure compound into E7 mixture. The mixture was heated up to 60 °C and injected into a wedge cell with a wedge angle θ of 0.45° (tan θ = 0.00785) and subsequently the sample was allowed to slowly cool down to 25 °C with a 1 °C/min cooling speed. The Cano’s lines (distance, R) of the induced CLCs in the wedge cell could be clearly observed by POM. The HTP values (β/μM-1) and rotary behaviors (degree) are summarized in Table 7.1.
Table 7.1 Characteristic parameters of chiral dopants within E7 under UV irradiation.
Dopanta P-cis-M3 P-trans-M3
βinitial/μM-1 +94 +95 βPSS /μM-1 365 nm 79 80 385 nm 82 77 420 nm 38 41 Rotation /degree 365 nm 930 1040 385 nm 980 1020 420 nm 540 650
a Measured at room temperature (25 °C), doped in E7 mixture. Note: The rotational motion of the class rod on the LC film is also influenced by the thickness of the LC film.
Although the packing in the bulk phase for trans and cis isomer is quite different, the helical twisting power (HTP) that is induced by the two isomers is almost the same with helical pitches of 128 μm and 120 μm for the cis and trans isomers respectively, indicating a HTP of approximately 95 μM-1. Subsequent irradiation with 365 nm increases the cholesteric pitch to 150 μm and 144 μm, respectively, indicating a HTP of 80 μM-1. The distance between the Grandjean-Cano lines first expanded then disappeared, and upon extended irradiation appeared again. When irradiating the sample with 385 nm light instead of 365 nm, similar HTP can be obtained at the PSS. When using 420 nm irradiation, the helical pitch at the PSS is 300 μm, where the value for HTP is calculated to be 40 μM-1. This might be due to the low PSS ratio at 420 nm (86:14 for 365 nm, 90:10 for 385 nm, 69:31 for 420 nm, unstable: stable). Subsequently, when the irradiation was ceased, the distance between the lines firstly expanded, then disappeared and reappeared. This shows the reversibility of the process.
Figure 7.7. POM images of 1.0 wt% P-cis-M3 in E7 upon UV (385 nm) irradiation and thermal
Figure 7.8. Clockwise rotational motion of a micro glass rod on the surface of the CLC
mixture film (1 wt% of motor P-cis-M3 in E7) upon continuous UV light (385 nm) irradiation for 5 min from (a) the initial state to (i) PSS385 nm. The length of the glass rod is 15 µm and diameter is 5 µm.
When the mixture was placed on top of a glass slide covered with a unidirectionally rubbed polyimide layer, a fingerprint texture was observed, which is typical for alignment of the cholesteric helix axis parallel to the surface. During irradiation of the film at λ = 365 nm, 385 nm and 420 nm under the microscope, the fingerprint textures reorganized in a rotational (clockwise) fashion, which corresponds to the photochemical isomerization of P-cis-M3. The lines of the texture rotated several turns, and eventually faded out. After prolonged irradiation, the rotating lines reappeared and kept rotating in clockwise until the PSS state was reached (Figure 7.8). When the sample is exposed to irradiation at different wavelength (365 nm, 385 nm and 420 nm), the rotational motion still can be observed. After the halting of irradiation, the cholesteric textures started to rotate in the opposite direction (counter-clockwise) immediately. During this process, the lines faded out and reappeared again. In addition, glass rods (15 µm long, 5 µm in diameter) were placed on the doped CLC films. Upon irradiation, the rotation of the glass rods in the same direction as the rotating cholesteric textures is observed. Figure 7.8 shows the typical rotary motion for one of these rods. As indicated in Table 7.1, the rotation of the glass rod induced by 365 nm and 385 nm UV light has a similar effect, while the rotations under 420 nm UV light are much less which is attributed to the low PSS ratio. After the halting of irradiation, the glass rod started to rotate in counter-clockwise fashion immediately, again following the direction of rotation of the cholesteric texture on which they were placed.
7.3 Conclusion
In summary, the second-generated molecular motors (trans/cis M3), containing an intrinsic 4,4'-disubstituted biphenyl unit have been synthesized and fully characterized. Both isomers exhibit photochromic properties upon irradiation with 365 nm, 385 nm and 420 nm UV light, and the isomers obtained upon irradiation can be reversed to the original form thermally. Although compound M3 does not show a liquid crystalline phase (which is possibly attributed to the steric hindrance in the packing of the mesogen) the two isomers can be used as chiral dopants in nematic LC materials.
7.4 Acknowledgement
Jasper Pol and Wojciech Danowski are gratefully acknowledged for the synthesis of the compounds. Dr. Franco King-Chi Leung and Dr. Giuseppe Portale are acknowledged for performing the WAXS measurements.
7.5 Experimental Section
3.4.1 General Remarks
Materials. Nematic LCs E7 mixture was purchased from Merck Japan Ltd. Its average
molecular weight is 274 g/mol. Polyimide used as alignment layers for LC cells was purchased from JSR MICRO NV. All of the solvent for extraction and the starting materials for synthesis were obtained from commercial suppliers and used as received without purification. All solvents used for the reactions were freshly distilled from drying agents or obtained from a MBraun solvent purification system. All the synthesis was performed under N2, unless stated otherwise. Micro-glass rods were purchased from Nippon Electric Glass with an average length of 25 µm and average diameter of 5 µm (model PF-50s).
Preparation of CLCs. The CLCs were obtained by dissolving the dopant into E7, and the
mixture was heated to 60 °C and kept for 30 min, then slowly cooled to room temperature, and finally capillary-filled into a cell. Two types of cells, namely, the wedge cell of KCRK-03 with a wedge angle of 0.45° (tan θ = 0.0078) and the parallel rubbed cell (with d=10 μm) were used to measure the helical pitches.
A microscope slide was thoroughly cleaned with detergent, and then sonicated in acetone and ethanol. The slide was dried and dipped into piranha solution for two hours. (Caution!
Piranha solution is highly corrosive and reactive toward organics). After rinsing with water
and other organic solvent and dried over with N2 flow, the slide was spin-coated with polyimide and allowed to harden overnight in a vacuum oven (170 °C, 200 mbar). The surface was then rubbed with a polyester fabric to induce parallel alignment for the liquid crystalline samples.
Equipment. CD spectra were recorded at room temperature with a Jasco J-810
spectropolarimeter (Tokyo, Japan). The CD spectra from 600 to 250 nm were recorded at a scan speed of 300 nm/min with a cell path length of 1 cm. Each spectrum represents the average of three measurements. UV−vis absorption spectra were recorded at different temperatures on a HP8454 spectrophotometer with the temperature controller (FP90) was used to control the temperature of the sample with thermal stability of ± 0.1 ◦C.
The WAXS measurements were performed using at the MINA instrument at the University of Groningen, with an X-ray scattering instrument built on a Cu rotating anode source (λ= 1.5413 Å). 2D patterns were collected using a Vantec 500 detector (1024 × 1024 pixel array
with pixel size 136 × 136 μm) located 93 mm away from the sample. Differential scanning calorimetry (DSC) measurements were performed on a TA instruments Q20 DSC, purged with nitrogen. Samples of approx. 2 mg were sealed in aluminum crucibles (Tzero, 40 µL). The scan rate was 5 °C·min-1 for temperatures ranging from -30 °C – 200 °C. Three heating-cooling cycles were run to check the reproducibility.
The distance between Cano’s lines and its variation process were observed by LV100NPOL polarized microscope, purchased from Nikon equipped with Nikon DS Fi-3 camera and Microscope Hot-stage System HS82. The NIS Elements D software was connected for acquisition of images and videos.
For irradiation, a M365F1, M385F1 and M420P1 LED purchased from THORLABS with 1.0 mW was used. The light source was held at an angle of 60° with respect to the sample plane, to allow irradiation under the microscope. The distance between the light source and the sample was approximately 5 cm.
1H and 13C NMR spectra for products characterization were recorded on a Varian AMX400 (400 and 100 MHz, respectively) with CDCl3, CD2Cl2 or DMSO as solvent. NMR irradiation experiments were recorded on a Varian Unity Plus Varian-500 (500 MHz) with M365F1 and M385F1 LED coupled to a 600 µm optical fiber, which guided the light into the NMR tube inside the spectrometer.25 High Resolution Mass spectra (HRMS) were recorded on an LTQ Orbitrap XL.
Enantiomers were separated by supercritical fluid chromatography (SFC), performed on a Thar Techonologies, Inc. Flash chromatography was performed on silica gel Merck Type 9385 230-400 mesh or on a Reveleris X2 Flash Chromatography system. Enantiomeric excesses were determined by chiral HPLC using Shimadzu LC-10ADVP HPLC equipped with a Shimadzu SPD-M20AVP diode array detector.
Calculations. All the calculations were performed using Gaussian09 package. The stationary
geometries (anti-folded) were optimized by using the density functional theory with the hybrid Becke-3−Lee−Yang−Parr (B3LYP) functional with 6-31G(d, p) basis set. The optimized structures are evaluated by frequency analysis to make sure the conformations have the minima energy.
3.4.2. Synthesis.
2-Hydroxy-7-(octyloxy)-9H-fluoren-9-one, 2
Octyl bromide (2.04 mL, 11.8 mmol) was added to a mixture of 2,7-dihydroxy-9H-fluoren-9-one 1 (2.50 g, 11.8 mmol), K2CO3 (4.88 g, 35.4 mmol), and TBAI (0.435 g, 1.18 mmol) in MeCN (100 mL) at 80 oC over 1 h. After stirring at reflux overnight, the reaction mixture was cooled down to room temperature and then acidified with 200 mL 4M aq. HCl. The mixture was extracted with 200 mL EtOAc. The organic layer was collected and subsequently washed with sat. NaHCO3 (aq.), brine and sat. NH4Cl (aq.) solution and dried over Na2SO4. After evaporation of solvent under reduced pressure, the crude product was purified by column chromatography (SiO2, DCM: MeOH= 10:1) to yield pure product 2 (1.02 g) as a red solid with a yield of 26 %.
1H NMR (400 MHz, DMSO) δ: 7.37-7.49 (m, 2H), 7.0 (m, 2H), 6.83 –6.92 (m, 2H), 3.96 (t, J = 6.3, Hz 2H,), 1.60 – 1.75 (m, 2H), 1.15 – 1.45 (m, 10H), 0.78 – 0.88 (m, 3H).
13C NMR (101 MHz, DMSO) δ: 196.2, 161.9, 161.0, 140.2, 138.4, 138.4, 138.1, 124.5,124.1, 123.8, 114.2, 113.0, 71.1, 34.4, 31.8, 31.8, 31.7, 28.6, 25.2, 17.1.
HRMS (ESI+): calcd. for C21H25O3 [M+H]+: 325.1725, found =325.1791. 7-(Octyloxy)-9-oxo-9H-fluoren-2-yl trifluoromethanesulfonate, 3
A solution of Tf2O (0.6 mL) was added to a solution of 2 (0.97 g, 3 mmol) in pyridine (0.96 mL) and DCM (20 mL), dropwise at r.t. After stirring for 1 h, the solution was washed with H2O (1 x 50 mL) and brine (1 x 50 mL). The organic layers were collected and dried over Na2SO4. The crude product was purified by column chromatography (SiO2, pentane: DCM = 1: 1) to yield the pure compound 3 as an orange solid (1.3 g, yield 96%). 1H-NMR (400 MHz, CDCl3) δ: 7.34 - 7.47 (m, 3H). 7.27 – 7.34 (m, 1H), 7.14-7.20 (m, 1H), 6.94 – 7.02 (m, 1H), 3.98 (t, J = 6.5 Hz, 2H), 1.78 (q, 2H), 1.20 – 1.52 (m, 10H), 0.83 – 0.95 (m, 3H). 13C-NMR (101 MHz, CDCl3) δ:162.9, 162.3, 151.2, 150.2, 146.4, 145.6, 144.1, 142.6, 142.3, 140.9, 134.7, 134.4, 133.6, 132.3, 124.7, 124.2, 123.6, 123.6, 122.9, 122.4, 121.2, 119.2, 117.3, 116.4, 115.6, 108.4, 71.3, 71.0, 34.5, 32.0, 31.9, 31.9, 28.7, 28.7, 25.3, 16.8.
HRMS (ESI+): calcd. for C22H24F3O5S [M+H]+:457.1218, found =457.1275 9-Hydrazineylidene-7-(octyloxy)-9H-fluoren-2-yl trifluoromethanesulfonate, 4
Compound 3 (1.45 g, 3.2 mmol) and hydrazine monohydride (5mL) were stirred in a solution of DCM/MeOH (1:1, 100 mL) at 60 °C for 4 h. The reaction mixture was cooled down to r.t. and subsequently concentrated to afford the crude hydrazone. The crude product was purified by column chromatography (SiO2, pentane: DCM = 1: 1) to provide compound 4 (1.35 g, 90% yield) as a yellow solid.
1H-NMR (400 MHz, CDCl3) δ: 9.34 (d, J = 8.4 Hz, 1H), 7.87 (d, J = 8.1 Hz, 1H), 7.76 (d, J = 8.3 Hz, 1H), 7.64 (ddd, J = 8.4, 6.9, 1.4 Hz, 1H), 7.51 (ddd, J = 8.1, 6.8, 1.3 Hz, 1H), 7.33 (d, J = 8.3 Hz, 1H), 5.28 (d, J = 52.3 Hz, 2H), 3.38 – 3.18 (m, 2H), 2.61 -2.31 (m, 12H), 1.26 (d, J = 6.9 Hz, 3H). HRMS (ESI+): calcd. for C22H27N2F3O5S [M+H]+: 471.1559, found 471.1560
9-diazo-7-(octyloxy)-9H-fluoren-2-yl trifluoromethanesulfonate, 5
To a solution of hydrazine 4 (75 mg, 0.16 mmol) in THF (5 mL), MnO2 (200 mg, 23 mmol) was added at 0°C and the reaction mixture was stirred for 15 min. After filtration, the product 5 (71 mg, 0.15 mmol) was obtained in 95% yield as red needles and can be used without further purification.
(E/Z)-9-(2-methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-ylidene)-7-(octyloxy)-9H-fluoren-2-yl trifluoromethanesulfonate, 7
A mixture of diazo 5 (71 mg, 0.15 mmol) and freshly prepared thioketone 6 (100 mg, 0.47 mmol) in DCM (10 mL) was stirred in 10 mL of THF at r.t. overnight. The solvent was removed in vacuo and the crude product was purified by column chromatography (SiO2, pentane: DCM, 5:1) to yield a mixture of motor and episulfide. The mixture was then dissolved in THF (10 ml) to which HMPT (0.5 mL) was added. After stirring at r.t. overnight, the residue was purified by column chromatography (SiO2, pentane: DCM = 1: 1), and the desired motor 7 (49 mg, 0.08 mmol) was obtained as a yellow solid with a yield of 52%. The compound was directly used for the next step without further separating the E/Z isomers.HRMS (ESI+): calcd. for C36H36F3O4S [M+H]+: 621.2281, found 621.2261
(E/Z)-9-(2-methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-ylidene)-7-(octyloxy)-9H-fluorene-2-carbonitrile, M3
Compound 7 (100 mg, 0.16 mmol, 1.0 eq), tBuXPhos Pd G3 (19 mg, 25 μmol, 0.15 eq), tBuXPhos (20 mg, 47 μmol, 0.29 eq), and Zn(CN)2 (12 mg, 106 μmol, 0.66 eq) were stirred in 2 mL of wet DMF (prepared by adding 1 mL of DI H2O to 100 mL of DMF and the mixed solvent was degassed at 60 °C for 2 h under an argon atmosphere). The residue was purified with column chromatography (SiO2, pentane: Et2O = 4: 1) to give the desired
M3 (77 mg, 0.15 mmol) as an orange oil with a yield of 96%. The E-and Z-isomers were
separated by recrystallization from EtOAc by slow diffusion of pentane. cis-M3 1H-NMR (400 MHz, CDCl3) δ: 8.15 (s, 1H), 7.97 (d, J = 8.3 Hz, 2H), 7.74 (m, 2H), 7.63 (m, 3H), 7.48 (t, J = 8.0 Hz, 1H), 7.39 (t, 8.0 Hz, 1H), 6.83 Hz (dd, J = 8.6, 2.0 Hz, 1H), 6.30 (d, J =2.3 Hz, 1H), 4.30 (m, 1H), 3.60 (dd, J = 15.4, 5.9 Hz, 1H), 3.05 (m, 1H), 2.83 (m, 2H), 1.05-1.50 (m, 15H), 0.93 (t, J =7.1 Hz, 3H). 13C-NMR (101 MHz, CDCl3) δ: 162.0, 156.6, 150.9, 146.7, 142.2, 142.0, 138.2, 135.3, 134.4, 133.4, 133.3, 132.3, 131.5, 131.5, 130.2, 129.9, 129.7, 128.1, 126.8, 123.5, 123.1, 121.8, 119.1, 112.9, 110.9, 69.9, 47.9, 44.6, 34.5, 31.9, 31.8, 31.5, 28.39, 25.4, 22.2, 16.9. trans-M3 1H-NMR (400 MHz, CDCl3) δ: 7.97 (d, J = 7.9 Hz, 2H), 7.82 (d, J = 8.4 Hz, 1H), 7.70 (d, J = 8.1 Hz, 1H), 7.61 (d, J = 8.3 Hz, 2H), 7.55 (d, J = 1.9 Hz, 1H), 7.51 (t, J = 7.7 Hz, 1H), 7.44 (dd, J =7.9, 1.2 Hz, 1H), 7.00 (dd, J = 8.7, 1.8 Hz, 1H), 6.86 (s, 1H), 4.33 (m, 1H), 4.11 (t, J = 6.4 Hz, 2H), 3.61 (dd, J = 15.5, 5.2 Hz, 1H), 2.82 (d, J = 15.1 Hz, 1H), 1.87 (t, 7.0 Hz, 2H,), 1.20 - 1.60 (m, 15H), 0.92 (t, J =7.1 Hz, 3H). 13C-NMR (101 MHz, CDCl3) δ: 162.9, 156.9, 150.8, 146.0, 144.8, 139.5, 138.0, 135.5, 134.7, 134.0, 133.1, 132.1, 131.9, 131.9, 131.1, 129.8, 129.3, 128.5, 126.6, 124.2, 122.5, 121.2, 116.6, 113.3, 110.0, 71.2, 48.1, 44.7, 34.5, 32.1, 32.0, 32.0, 28.8, 25.4, 21.8, 16.8.
HRMS (ESI+): calcd. for C36H36NO [M+H]+: 498.2791, found 498.2774. 7.6 References
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