The interaction between two TEMPO spin centers connected to a photoswitchable overcrowded alkene ... more The interaction between two TEMPO spin centers connected to a photoswitchable overcrowded alkene changes from noncoupled (three-line EPR spectrum) in the trans state, where the two spin centers are separated by ∼22 Å, to strongly coupled (five-line EPR spectrum) in the cis state, where the separation is ∼7 Å, upon photoswitching. Importantly, the performance of the alkene switching unit is essentially unaffected by the spin centers.
Photochromic sexithiophenes were prepared by oxidative electrochemical coupling of terthiophenes.... more Photochromic sexithiophenes were prepared by oxidative electrochemical coupling of terthiophenes. The redox properties in the open state are typical of sexithiophenes. Ring closure of both photochromic units leads to a decrease in the energy of the LUMO orbitals with little affect on the energy of the HOMO orbitals. The photochemical tuning of the conjugation of a molecular wire is achieved by combining dithienylethene units with a sexithiophene.
The amplification of molecular chirality by liquid crystalline systems is widely applied in inves... more The amplification of molecular chirality by liquid crystalline systems is widely applied in investigations towards enantioselective solvent-solute interactions, chiral supramolecular assemblies, smart materials, and the development of liquid crystal displays. Here we present an overview of recent achievements in the development of new chiral dopant systems for the generation of cholesteric liquid crystalline phases. Based on a distinction between shape-persistent and bistable dopants, several dopant classes will be discussed.
In this Account, recent advances in catalytic asymmetric conjugate addition of Grignard reagents ... more In this Account, recent advances in catalytic asymmetric conjugate addition of Grignard reagents are discussed. Synthetic methodology to perform highly enantioselective Cu-catalyzed conjugate addition of Grignard reagents to cyclic enones with ee's up to 96% was reported in 2004 from our laboratories. Excellent levels of stereocontrol were achieved with Cu(I) halides, alkylmagnesium bromides, and commercially available chiral ferrocenyl diphosphines. Studies carried out during the last 2 years demonstrated that these Cu-catalysts are very effective for the enantioselective conjugate addition of Grignard reagents to acyclic enones, alpha,beta-unsaturated esters, and thioesters. On the basis of this methodology, a diastereo- and enantioselective iterative route to deoxypropionate units was developed and applied to the synthesis of natural products. Finally, we summarize our recently conducted mechanistic investigations and the application of this catalytic system to the enantioselective SN2' substitution reactions of allylic bromides with Grignard reagents.
ABSTRACT The investigation of the electronic conduction through monosulfurdiarylethene (1S-DE) im... more ABSTRACT The investigation of the electronic conduction through monosulfurdiarylethene (1S-DE) immobilized in an insulating dodecanethiol matrix on a gold surface was addressed. Scanning tunneling spectroscopy allows to probe spatially the frontier molecular orbitals of 1S-DE at 77 K. We locally extracted the electronic highest occupied molecular orbital-lowest unoccupied molecular orbital gap of 1S-DE with the value of 1.56 eV. This attempt reveals the importance of scanning tunneling spectroscopy as a tool to measure the charge transport properties of diarylethene towards the realization of a switching-based molecular device.
In this contribution, we show how zinc-5,10,15,20-meso-tetradodecylporphyrins (Zn-TDPs) self-asse... more In this contribution, we show how zinc-5,10,15,20-meso-tetradodecylporphyrins (Zn-TDPs) self-assemble into stable organized arrays on the surface of graphite, thus positioning their metal center at regular distances from each other, creating a molecular pattern, while retaining the possibility to coordinate additional ligands. We also demonstrate that Zn-TDPs coordinated to 3-nitropyridine display a higher tendency to be adsorbed at the surface of highly oriented pyrolytic graphite (HOPG) than noncoordinated ones. In order to investigate the two-dimensional (2D) self-assembly of coordinated Zn-TDPs, solutions with different relative concentrations of 3-nitropyridine and Zn-TDP were prepared and deposited on the surface of HOPG. STM measurements at the liquid-solid interface reveal that the ratio of coordinated Zn-TDPs over noncoordinated Zn-TDPs is higher at the n-tetradecane/HOPG interface than in n-tetradecane solution. This enhanced binding of the axial ligand at the liquid/solid interface is likely related to the fact that physisorbed Zn-TDPs are better binding sites for nitropyridines.
We recently reported that the photoisomerization of molecular motors used as chiral dopants in a ... more We recently reported that the photoisomerization of molecular motors used as chiral dopants in a cholesteric liquid crystal film induces a rotational reorganization which can be observed by optical microscopy and produces the motion of microscopic objects placed on top of the film (Feringa, B. L.; et al. ). The mechanism underlying the mesoscopic manifestation of the molecular process was not fully understood, and here we present a joint theoretical and experimental investigation, which provides a detailed insight into the mechanism of texture rotation. This description allows us to identify the interplay between the chemical structure of the chiral dopant and the material properties of the liquid crystal host, and to quantify their role in the observed dynamic phenomenon. We have found that a crucial role is played by the hybrid anchoring of the liquid crystal, with the director parallel to the substrate and perpendicular to the interface with air; in this configuration an almost unperturbed cholesteric helix, with its axis normal to the substrate, is present in most of the film, with strong deformations only close to the free interface. The texture rotation observed in the experiment reflects the rotation of the director during the unwinding of the cholesteric helix, produced by the change in shape of the chiral dopant under photoisomerization. The rotational reorganization is controlled by the photochemical process, via the coupling between the chirality of the dopant and the elastic properties of the liquid crystal host.
The structure of molecular monolayers formed at the interface between atomically flat surfaces an... more The structure of molecular monolayers formed at the interface between atomically flat surfaces and a solution of free-base meso-tetradodecylporphyrins (H2Ps) was examined by scanning tunneling microscopy (STM) at the liquid/solid interface. On the surface of graphite (HOPG), H2Ps form a well-ordered monolayer characterized by an oblique unit cell. On Au , H2Ps form a self-organized monolayer comprised of two distinct domain types. In both types of domains, the density of the porphyrin cores is increased in comparison to the arrangement observed on HOPG. Also, high-resolution STM images reveal that, in contrast to what is observed on HOPG, physisorption on Au(111) induces a distortion of the porphyrin macrocycle out of planarity. By using X-ray photoelectron spectroscopy, we demonstrate that this is likely to be due to the coordination of the lone pairs of the iminic (-CdN-) nitrogen atoms of the porphyrin macrocycle to Au(111).
Molecular memory devices based on dithienylethene switch modified ITO electrodes undergo reversib... more Molecular memory devices based on dithienylethene switch modified ITO electrodes undergo reversible ring opening/closing both photo- and electro-chemically with non-destructive electrochemical readout.
Understanding the structure of chiral self-assemblies at fluid/solid interfaces is not only relev... more Understanding the structure of chiral self-assemblies at fluid/solid interfaces is not only relevant from a fundamental point of view but also crucial for the development of new artificial molecular materials and systems operating at the interface between a conductive surface and a fluid. Moreover, elucidating how the balance between intermolecular and interfacial interactions influences expression of chirality in 2D crystals
Recent advances in synthetic methods and analysis techniques provide a basis for the construction... more Recent advances in synthetic methods and analysis techniques provide a basis for the construction and characterization of organized arrays of molecular switches and motors on surfaces. Among them, molecular systems that can be controlled by light are particularly promising because of their ease of addressability, fast response times and the compatibility of light with a wide range of condensed phases.
A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl grou... more A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl groups have been reported previously to achieve both high turnover numbers and selectivity in the oxidation of alkenes and alcohols, using H 2 O 2 as terminal oxidant. Here we report our recent finding that these ligands decompose in situ to pyridine-2-carboxylic acid and its derivatives, in the presence of a manganese source, H 2 O 2 and a base. Importantly, the decomposition occurs prior to the onset of catalysed oxidation of organic substrates. It is found that the pyridine-2-carboxylic acid formed, together with a manganese source, provides for the observed catalytic activity. The degradation of this series of pyridyl ligands to pyridine-2-carboxylic acid under reaction conditions is demonstrated by 1 H NMR spectroscopy. In all cases the activity and selectivity of the manganese/pyridyl containing ligand systems are identical to that observed with the corresponding number of equivalents of pyridine-2-carboxylic acid; except that, when pyridine-2-carboxylic acid is used directly, a lag phase is not observed and the efficiency in terms of the number of equivalents of H 2 O 2 required decreases from 6-8 equiv. with the pyridin-2-yl based ligands to 1-1.5 equiv. with pyridine-2-carboxylic acid.
Several novel tri-and tetradentate amino alcohol ligands, all derived from (+)-camphor, have been... more Several novel tri-and tetradentate amino alcohol ligands, all derived from (+)-camphor, have been synthesized by using specific N-alkylation procedures. The amino alcohols were employed as chiral ligands in the nickel catalyzed conjugate additions of diethylzinc to chalcone and cyclohexenone as model substrates. For the acyclic enone enantioselectivities up to 83% were achieved. (~) 1997
The interaction between two TEMPO spin centers connected to a photoswitchable overcrowded alkene ... more The interaction between two TEMPO spin centers connected to a photoswitchable overcrowded alkene changes from noncoupled (three-line EPR spectrum) in the trans state, where the two spin centers are separated by ∼22 Å, to strongly coupled (five-line EPR spectrum) in the cis state, where the separation is ∼7 Å, upon photoswitching. Importantly, the performance of the alkene switching unit is essentially unaffected by the spin centers.
Photochromic sexithiophenes were prepared by oxidative electrochemical coupling of terthiophenes.... more Photochromic sexithiophenes were prepared by oxidative electrochemical coupling of terthiophenes. The redox properties in the open state are typical of sexithiophenes. Ring closure of both photochromic units leads to a decrease in the energy of the LUMO orbitals with little affect on the energy of the HOMO orbitals. The photochemical tuning of the conjugation of a molecular wire is achieved by combining dithienylethene units with a sexithiophene.
The amplification of molecular chirality by liquid crystalline systems is widely applied in inves... more The amplification of molecular chirality by liquid crystalline systems is widely applied in investigations towards enantioselective solvent-solute interactions, chiral supramolecular assemblies, smart materials, and the development of liquid crystal displays. Here we present an overview of recent achievements in the development of new chiral dopant systems for the generation of cholesteric liquid crystalline phases. Based on a distinction between shape-persistent and bistable dopants, several dopant classes will be discussed.
In this Account, recent advances in catalytic asymmetric conjugate addition of Grignard reagents ... more In this Account, recent advances in catalytic asymmetric conjugate addition of Grignard reagents are discussed. Synthetic methodology to perform highly enantioselective Cu-catalyzed conjugate addition of Grignard reagents to cyclic enones with ee's up to 96% was reported in 2004 from our laboratories. Excellent levels of stereocontrol were achieved with Cu(I) halides, alkylmagnesium bromides, and commercially available chiral ferrocenyl diphosphines. Studies carried out during the last 2 years demonstrated that these Cu-catalysts are very effective for the enantioselective conjugate addition of Grignard reagents to acyclic enones, alpha,beta-unsaturated esters, and thioesters. On the basis of this methodology, a diastereo- and enantioselective iterative route to deoxypropionate units was developed and applied to the synthesis of natural products. Finally, we summarize our recently conducted mechanistic investigations and the application of this catalytic system to the enantioselective SN2' substitution reactions of allylic bromides with Grignard reagents.
ABSTRACT The investigation of the electronic conduction through monosulfurdiarylethene (1S-DE) im... more ABSTRACT The investigation of the electronic conduction through monosulfurdiarylethene (1S-DE) immobilized in an insulating dodecanethiol matrix on a gold surface was addressed. Scanning tunneling spectroscopy allows to probe spatially the frontier molecular orbitals of 1S-DE at 77 K. We locally extracted the electronic highest occupied molecular orbital-lowest unoccupied molecular orbital gap of 1S-DE with the value of 1.56 eV. This attempt reveals the importance of scanning tunneling spectroscopy as a tool to measure the charge transport properties of diarylethene towards the realization of a switching-based molecular device.
In this contribution, we show how zinc-5,10,15,20-meso-tetradodecylporphyrins (Zn-TDPs) self-asse... more In this contribution, we show how zinc-5,10,15,20-meso-tetradodecylporphyrins (Zn-TDPs) self-assemble into stable organized arrays on the surface of graphite, thus positioning their metal center at regular distances from each other, creating a molecular pattern, while retaining the possibility to coordinate additional ligands. We also demonstrate that Zn-TDPs coordinated to 3-nitropyridine display a higher tendency to be adsorbed at the surface of highly oriented pyrolytic graphite (HOPG) than noncoordinated ones. In order to investigate the two-dimensional (2D) self-assembly of coordinated Zn-TDPs, solutions with different relative concentrations of 3-nitropyridine and Zn-TDP were prepared and deposited on the surface of HOPG. STM measurements at the liquid-solid interface reveal that the ratio of coordinated Zn-TDPs over noncoordinated Zn-TDPs is higher at the n-tetradecane/HOPG interface than in n-tetradecane solution. This enhanced binding of the axial ligand at the liquid/solid interface is likely related to the fact that physisorbed Zn-TDPs are better binding sites for nitropyridines.
We recently reported that the photoisomerization of molecular motors used as chiral dopants in a ... more We recently reported that the photoisomerization of molecular motors used as chiral dopants in a cholesteric liquid crystal film induces a rotational reorganization which can be observed by optical microscopy and produces the motion of microscopic objects placed on top of the film (Feringa, B. L.; et al. ). The mechanism underlying the mesoscopic manifestation of the molecular process was not fully understood, and here we present a joint theoretical and experimental investigation, which provides a detailed insight into the mechanism of texture rotation. This description allows us to identify the interplay between the chemical structure of the chiral dopant and the material properties of the liquid crystal host, and to quantify their role in the observed dynamic phenomenon. We have found that a crucial role is played by the hybrid anchoring of the liquid crystal, with the director parallel to the substrate and perpendicular to the interface with air; in this configuration an almost unperturbed cholesteric helix, with its axis normal to the substrate, is present in most of the film, with strong deformations only close to the free interface. The texture rotation observed in the experiment reflects the rotation of the director during the unwinding of the cholesteric helix, produced by the change in shape of the chiral dopant under photoisomerization. The rotational reorganization is controlled by the photochemical process, via the coupling between the chirality of the dopant and the elastic properties of the liquid crystal host.
The structure of molecular monolayers formed at the interface between atomically flat surfaces an... more The structure of molecular monolayers formed at the interface between atomically flat surfaces and a solution of free-base meso-tetradodecylporphyrins (H2Ps) was examined by scanning tunneling microscopy (STM) at the liquid/solid interface. On the surface of graphite (HOPG), H2Ps form a well-ordered monolayer characterized by an oblique unit cell. On Au , H2Ps form a self-organized monolayer comprised of two distinct domain types. In both types of domains, the density of the porphyrin cores is increased in comparison to the arrangement observed on HOPG. Also, high-resolution STM images reveal that, in contrast to what is observed on HOPG, physisorption on Au(111) induces a distortion of the porphyrin macrocycle out of planarity. By using X-ray photoelectron spectroscopy, we demonstrate that this is likely to be due to the coordination of the lone pairs of the iminic (-CdN-) nitrogen atoms of the porphyrin macrocycle to Au(111).
Molecular memory devices based on dithienylethene switch modified ITO electrodes undergo reversib... more Molecular memory devices based on dithienylethene switch modified ITO electrodes undergo reversible ring opening/closing both photo- and electro-chemically with non-destructive electrochemical readout.
Understanding the structure of chiral self-assemblies at fluid/solid interfaces is not only relev... more Understanding the structure of chiral self-assemblies at fluid/solid interfaces is not only relevant from a fundamental point of view but also crucial for the development of new artificial molecular materials and systems operating at the interface between a conductive surface and a fluid. Moreover, elucidating how the balance between intermolecular and interfacial interactions influences expression of chirality in 2D crystals
Recent advances in synthetic methods and analysis techniques provide a basis for the construction... more Recent advances in synthetic methods and analysis techniques provide a basis for the construction and characterization of organized arrays of molecular switches and motors on surfaces. Among them, molecular systems that can be controlled by light are particularly promising because of their ease of addressability, fast response times and the compatibility of light with a wide range of condensed phases.
A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl grou... more A number of manganese-based catalysts employing ligands whose structures incorporate pyridyl groups have been reported previously to achieve both high turnover numbers and selectivity in the oxidation of alkenes and alcohols, using H 2 O 2 as terminal oxidant. Here we report our recent finding that these ligands decompose in situ to pyridine-2-carboxylic acid and its derivatives, in the presence of a manganese source, H 2 O 2 and a base. Importantly, the decomposition occurs prior to the onset of catalysed oxidation of organic substrates. It is found that the pyridine-2-carboxylic acid formed, together with a manganese source, provides for the observed catalytic activity. The degradation of this series of pyridyl ligands to pyridine-2-carboxylic acid under reaction conditions is demonstrated by 1 H NMR spectroscopy. In all cases the activity and selectivity of the manganese/pyridyl containing ligand systems are identical to that observed with the corresponding number of equivalents of pyridine-2-carboxylic acid; except that, when pyridine-2-carboxylic acid is used directly, a lag phase is not observed and the efficiency in terms of the number of equivalents of H 2 O 2 required decreases from 6-8 equiv. with the pyridin-2-yl based ligands to 1-1.5 equiv. with pyridine-2-carboxylic acid.
Several novel tri-and tetradentate amino alcohol ligands, all derived from (+)-camphor, have been... more Several novel tri-and tetradentate amino alcohol ligands, all derived from (+)-camphor, have been synthesized by using specific N-alkylation procedures. The amino alcohols were employed as chiral ligands in the nickel catalyzed conjugate additions of diethylzinc to chalcone and cyclohexenone as model substrates. For the acyclic enone enantioselectivities up to 83% were achieved. (~) 1997
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Papers by Ben Feringa