Molecular Turnstile Research Articles

Molecular Turnstiles Featuring Bicyclic Rotators: Solution and Solid‐State Investigation of Steric and Electronic Concerns

AbstractA molecular rotor is created when a 2,1,3‐benzothiadiazole rotator is incorporated into a rigid arylene ethynylene framework supported by pyridine coordination to a metal (Ag+ or PdCl2) guest. Comparisons to a similarly sized naphthyl rotator via 1H NMR spectroscopy provide insights into the movement of these bicyclic rotators relative to the rigid stator framework. Chemical shift increases of 0.3 ppm, or more, upon metal complexation are consistent with through‐space interaction of the central arene with a bound PdCl2 guest. Further study via X‐ray crystallography illustrates that rotation of the 2,1,3‐benzothiadiazole unit in the solid state is likely hampered by relatively strong chalcogen bonding (N⋅⋅⋅S distance of 2.93 Å), forming 2S‐2N squares between benzothiadiazoles of neighboring complexes. Strong π–π interactions (3.29–3.36 Å) between neighboring complexes likewise restrict solid‐state rotation of the potential benzothiadiazole rotator. Modest changes to UV–vis spectra upon metal coordination suggest that electronic properties are mostly independent of stator configuration.

A pyridyl-benzimidazole based molecular luminescent turnstile

A molecular turnstile based on a luminescent pyridyl-benzimidazole stator and a rotor containing a pyridyl coordinating site may be reversibily switched between its open and closed states upon binding/unbinding of silver cations.

Controlled Photoerasable Fluorescent Behaviors with Dithienylethene-Based Molecular Turnstile.

The precise control of molecular interaction and motion is a powerful strategy in the creation and development of intelligent materials. We demonstrate here a simple concept and approach to integrate intramolecular photochromic property with intermolecular aggregation-induced emission behaviors, with the aim to construct a new type of photoswitchable luminescent materials. This strategy is realized by the dithienylethene-bridged bispyridinium salt as photochromic molecular turnstile, and their subsequent fabrication into optically functional materials is reported. By restricting the rotation of central chemical bonds, the obtained molecular turnstile not only exhibits photocontrolled fluorescence emission through solvent exchange but is also capable of transforming into photowritable and photoerasable films in polymeric matrix with good recyclability. This functional molecular turnstile provides convenient routes to construct photochromic nanomaterials with controlled photophysical behaviors.

A Molecular Turnstile as an E‐Field‐Triggered Single‐Molecule Switch: Concept and Synthesis

A new single‐molecule switching concept relying on the E‐field‐dependent orientation of a push‐pull system (via its dipole moment) is presented. As first steps towards realizing the E‐field‐triggered single‐molecule motion, the synthesis of turnstile 1, which is designed to bridge a gap within a carbon nanotube junction, is reported. Turnstile 1 consists of a hexakis(m‐phenyleneethynylene) macrocycle decorated with phenanthrene‐functionalized oligo(phenyleneethynylene) (OPE) wires at opposite ends, as well as a para‐connected push‐pull rod as rotator unit. The similarity of the rotor dimension with the surrounding macrocycle guarantees efficient π‐stacking between both subunits when an E‐field is applied. To introduce a dipole moment into the rotator, the structure is terminally functionalized with a nitrile group on one side and with a dimethylamino group on the opposite end. Synthetic protocols based on Sonogashira–Hagihara couplings were developed to build up the macrocycle scaffold. The highly functionalized open precursor A is the key building block of the sequence as it allows a twofold intramolecular palladium‐catalyzed cyclization reaction to be performed to obtain the target turnstile in a very good yield of 68 %. The target structure was fully characterized by NMR spectroscopy and mass spectrometry. Furthermore, 1H‐1H NOESY NMR experiments pointed to a pseudo‐rotation of the push‐pull rod within the turnstile structure on the time scale of the NMR experiment.

Symmetrical or non-symmetrical luminescent turnstiles based on hydroquinone stators and rotors bearing pyridyl or p-dimethylaminopyridyl coordinating units.

The design and synthesis of a novel family of molecular turnstiles T1-T5 were achieved. All five turnstiles are based on a stator and a rotor covalently interconnected. While turnstiles T1-T2 are based on a symmetric stator equipped with two coordinating pyridyl units and a rotor bearing either a pyridyl or p-dimethylaminopyridyl coordinating moiety, the two non-symmetric turnstiles T4 and T5 are based on a stator bearing only a single pyridyl unit and the same rotor as T1 and T2 mentioned above. The switching between the open (T1-T5) and the closed (T-M) states of the turnstiles by metal cations (M = Ag+ or Pd2+) was investigated in solution by using 1D and 2D NMR techniques. The locking of the rotational movement leading to the closed state of the turnstile was achieved upon addition of the Ag+ cation through its simultaneous binding by both pyridyl moieties of the stator and the rotor. The unlocking process leading back to the open state was achieved by the addition of Et4NBr. For the symmetric turnstiles T1 and T2, bearing two pyridyl units on the stator, the binding of the Ag+ cation leads to an oscillating phenomenon between the two energetically equivalent closed states. However, in the case of turnstile T1, the oscillating process could be prevented by blocking the rotational movement using PdCl2 as the locking agent. Owing to the emissive nature of the stator, the open and closed states of the turnstiles were investigated by steady state and time-resolved photophysical methods. The photo-excitation of the turnstiles in their open state leads to an intense near-UV to deep-blue emission with short-lived excited states and a singlet intra-ligand charge transfer (1ILCT) character. Upon binding of the Ag+ cation, sizeable bathochromic shifts and a substantial decrease of PLQY were observed. Finally, the coordination of PdCl2, which possesses lower-lying excited states with metal-centered (MC) and ligand-to-metal charge transfer (LMCT) character, completely quenches the photoluminescence.

Flexible, Linear Chains Act as Baffles To Inhibit the Intramolecular Rotation of Molecular Turnstiles.

In artificial molecular devices, flexible, linear chains typically exhibit very weak capability in inhibiting molecular motion. Herein, we describe the dynamic properties of a series of molecular turnstiles consisting of a rigid frame and a phenyl rotator flanked with linear alkoxymethyl substituents. The long, flexible substituents act as elastic baffles to inhibit the rotations of the rotator at medium to fast speeds on the NMR time scale. When the rotator moves slowly, the substituents become more relaxed, thus obtaining an opportunity to completely thread through the cavity of the turnstiles. These findings reveal a basic but missing correlation between steric hindrance and speed of motion for flexible, linear chains in dynamic molecular devices, thus opening up a new direction toward molecular machines with more elaborate dynamic functions.

Phosphorus(V) Porphyrin-Based Molecular Turnstiles.

A new cationic molecular turnstile based on a P(V) porphyrin backbone bearing two pyridyl interaction sites, one at the meso position of the porphyrin and the other on the handle connected to the porphyrin through P-O bonds, was designed and synthesized. The dynamic behavior of the turnstile 2, investigated by 1D and 2D 1H NMR techniques, showed that in the absence of an effector, the turnstile is in its open state and undergoes a free rotation of the rotor (the handle) around the stator (the porphyrin backbone). In the presence of an external effector such as Ag+ cation or H+, the turnstile is switched to its closed states 2-Ag+ and 2-H+, respectively. The locking/unlocking process is reversible and may be achieved by precipitation of AgBr upon addition of Et4NBr in the case of the silver-locked turnstile or by addition of Et3N in the case of the proton-locked turnstile.

Molecular Turnstiles Regulated by Metal Ions.

A family of novel molecular turnstiles 1-3 composed of two stators with pyridyl binding sites and a different-sized triptycene rotor was synthesized. The molecular turnstiles behave in an open state at room temperature in the absence of metal ions but display significantly different closed states in the presence of Ag(+) and Pd(2+). The Ag(+)-mediated turnstiles 1-3Ag exhibited closed states but unreadable bistability at ambient temperature because the Ag(+)-mediated macrocyclic framework is not able to restrict the rotations of the rotors; while temperature was decreased, the macrocyclic frameworks became stable enough to halt the rotations of the rotors, eventually leading to the readable closed states for 1-3Ag. In contrast, Pd(2+)-mediated macrocyclic frameworks are stable, giving rise to a detectable closed state of turnstiles 1-3Pd in a wide range of temperatures. These findings have also been supported by DFT calculations.

A bi-stable Pt(II) based molecular turnstile.

Playing with the competition between H- and coordination-bonds, a bi-stable unsymmetrical organometallic turnstile was designed and its dynamic behaviour was studied in solution using NMR techniques. The turnstile, bearing two differentiated interaction sites, is based on a stator, composed of a square planar Pt(II) centre equipped with one pyridyl group and one phenol moiety, connected to a symmetrical rotor bearing a pyridyl unit as either a H-bond acceptor or as a monodentate coordinating site. Whereas in the absence of a metallic effector, the turnstile is locked by a H-bond formed between the phenol moiety located on the stator and the pyridyl group of the rotor, in the presence of Ag(+) cations, the turnstile switches to another closed state resulting from the simultaneous binding of the cations by pyridyl units belonging to both the stator and the rotor. The switching process was shown to be reversible.

Optical reading of the open and closed states of a molecular turnstile

A molecular turnstile composed of a hydroquinone based rotor and a stator bearing a tridentate coordinating site can be reversibly switched between open and closed states. The locking and unlocking processes may be read optically.

Organometallic turnstiles: acid and base locking and unlocking

A symmetrical organometallic Pt(II) based molecular turnstile 1, composed of a rotor bearing two benzonitrile units as coordinating sites and a stator equipped with a dianionic tridentate coordinating pole, undergoes a reversible switching process between two open and one closed states. In the absence of an effector, the turnstile is in its open state defined by the free rotation of the rotor around the stator. In the presence of Pd(II), the latter is simultaneously complexed by both the rotor and the stator (1-Pd) leading to the first closed state owing to locking of the rotational movement. The turnstile can be unlocked to its second open state 1-Pd-DMAP by addition of para-dimethylaminopyridine (DMAP) behaving as an external competitive ligand replacing the bound benzonitrile in the coordination sphere of the Pd centre. Upon addition of PdCl2(CH3CN)2, a competitive metal complex to remove DMAP, the turnstile is switched back to its closed state. The same process can also be achieved upon addition of MsOH causing the protonation of DMAP into DMAP-H(+) and its decomplexation and replacement by one of the two benzonitrile groups of the rotor. Finally, the deprotonation of DMAP-H(+) by addition of Et3N as a base regenerates the second open state of the turnstile.

A luminescent molecular turnstile.

A molecular turnstile based on a hydroquinone luminescent hinge bearing two divergently oriented pyridyl units behaving as a rotor and equipped with a handle composed of a tridentate coordinating site considered as the stator was synthesized and its structure was studied in the solid state by X-ray diffraction on single crystal. Its dynamic behaviour in solution was investigated by 1- and 2-D NMR experiments which revealed the free rotation of the rotor around the stator. The rotational movement was locked upon addition of Pd(ii) simultaneously complexed by the tridentate moiety of the stator and one of the two monodentate pyridyl sites of the rotor. Interestingly, whereas the open state of the turnstile was luminescent, for its closed state the emission was quenched by the heavy atom effect of Pd(ii).

Geometry and Temperature Dependence of meso-Aryl Rotation in Strained Metalloporphyrins: Adjustable Turnstile Molecules

The rotation of meso-aryl groups in porphyrins depends on the degree of macrocyclic distortion and is also influenced by the surrounding temperature. Dynamic NMR methods and crystal structures of series of nonplanar metalloporphyrins reveal that macrocyclic distortion lowers the rotational barrier by weakening the nonbinding interactions of neighboring groups, while increased temperature allows the rotational barrier to be overcome more readily. Two empirical methods are developed to acquire the rotational barrier. This type of strained molecule can act as an adjustable molecular turnstile through adjusting the degree of macrocyclic distortion and changing the surrounding temperature.

A platinum turnstile with a palladium lock

An organometallic molecular turnstile composed of a stator based on an α,ω-diphosphine bearing, in a symmetric fashion, the 2,6-pyridyl diamide moiety as a central tridentate chelating unit and a rotor composed of Pt(II) equipped with two pyridyl groups in trans configuration was designed. The switching between its open and closed states using Pd(II) was investigated both in solution and in the solid state.

Zinc– and palladium–porphyrin based turnstiles

The design and synthesis of two new Zn(II) and Pd(II) strapped-porphyrin based molecular turnstiles have been achieved. These molecules are based on a porphyrin backbone as a stator and a handle as a rotor. The junction between the two parts is ensured by covalent bonds using two opposite meso positions. Both compounds have been characterised in solution by NMR spectroscopy and in the solid state by X-ray diffraction on a single crystal. The dynamic of the two systems have been investigated in solution by 1- and 2-D NMR techniques such as ROESY. The switching between the open and closed states is described. Whereas for the Pd(II) complex, the handle freely rotates around the stator, for the Zn(II) analogue owing to the presence of a water molecule bound to the zinc atom and hydrogen bonded to the pyridyl moiety of the rotor, the rotation is blocked. The unlocking of the rotational movement was achieved using an external agent, mainly dimethylaminopyridine as a strong coordinating ligand.

Strapped‐Porphyrin‐Based Molecular Turnstiles

The synthesis of a series of molecular turnstiles that contained both H-bond-donor and -acceptor sites was achieved. Their structures were based on tetra-aryl X(2)Sn(IV) porphyrins (X=Cl or OH) as H-bond-acceptor sites that were equipped with a rotor that contained a pyridyldiamide moiety as a H-bond donor. In the solution phase, 1D and 2D NMR spectroscopic analysis showed that switching between the closed state, which resulted from the formation of intramolecular H-bonds, and the open state of the turnstile was achieved by using external H-bond-acceptor molecules, such as DMSO. The solid-state structure of the closed state of the turnstile was established by single-crystal X-ray diffraction.

Porphyrin‐Based Switchable Molecular Turnstiles

The design, synthesis and structural characterisation, in solution, of two new molecular turnstiles based on Sn-porphyrin derivatives are described. The system is composed of a stator (5-(4-pyridyl)-10,15,20-triphenylporphyrin), a hinge (Sn(IV)) and a rotor (handle equipped with 2,6-pyridinedicarboxamide as a tridentate coordinating site or its Pd(II) complex). The presence of interaction sites, both on the stator and the rotor, offers the possibility of switching between an open state (free rotation of the handle around the porphyrin) and a closed state (blockage of the rotation) by either establishment of hydrogen bonds between the stator and the rotor or by the simultaneous binding of Pd by both coordinating groups.

Open and closed states of a porphyrin based molecular turnstile

The molecular turnstile 1 composed of a stator based on an Sn(IV)-porphyrin bearing two sets of monodentate coordinating sites (pyridyl and benzonitrile) and a handle bearing a 2,6-pyridinediamide moiety as a tridentate unit was synthesised. In the absence of metal behaving as a blocking agent, the handle freely rotates around the stator (open state). In the presence of Pd(II), the closed state of the turnstile 1-Pd resulting from the simultaneous binding of the metal centre by both the dianionic tridentate site and one of the two pyridyl units is generated. The reaction of 1-Pd with the organometallic 2,6-diphenylpyridine Pt(II) complex 11 leads to the heterotrinuclear (Pt, Sn, Pd) species 13, resulting from the binding of the platinum complex by 1-Pd.

An oscillating molecular turnstile

An oscillating molecular turnstile based on a stator composed of a porphyrin core bearing two trans coordinating pyridyl units at the meso positions and a rotor equipped with a pyridyl group was designed. Whereas, in solution, the rotor freely rotates around the stator, in the presence of a silver cation behaving as an effector, the system oscillates between two stations. The oscillatory movement may be frozen upon cooling at -70 °C generating thus the closed state of the turnstile. The switching of the system between the open and closed states may be achieved using an external stimulus such as addition of Et(4)NBr.

Porphyrin-Based Molecular Turnstiles

The controlling of movements in molecular machines has been gaining considerable interest over the past years. Herein, the authors report the synthesis and characterization of a molecular turnstile. The Sn-O bonds form the hinge allowing for free rotation of the handle about it (1), while the binding of palladium(II) impedes this movement resulting in the closed state 2.