Dipolar and Nonpolar Altitudinal Molecular Rotors Mounted on an Au(111) Surface

463

325

Light excitation powers the reversible shuttling movement of the ring component of a rotaxane between two stations located at a 1.3-nm distance on its dumbbell-shaped component. The photoinduced shuttling movement, which occurs in solution, is based on a ''four-stroke'' synchronized sequence of electronic and nuclear processes. At room temperature the deactivation time of the high-energy charge-transfer state obtained by light excitation is Ϸ10 s, and the time period required for the ring-displacement process is on the order of 100 s. The rotaxane behaves as an autonomous linear motor and operates with a quantum efficiency up to Ϸ12%. The investigated system is a unique example of an artificial linear nanomotor because it gathers together the following features: (i) it is powered by visible light (e.g., sunlight); (ii) it exhibits autonomous behavior, like motor proteins; (iii) it does not generate waste products; (iv) its operation can rely only on intramolecular processes, allowing in principle operation at the single-molecule level; (v) it can be driven at a frequency of 1 kHz; (vi) it works in mild environmental conditions (i.e., fluid solution at ambient temperature); and (vii) it is stable for at least 10 3 cycles. molecular machine ͉ nanoscience ͉ photochemistry ͉ rotaxane ͉ supramolecular chemistry T he miniaturization race is encouraging scientists to design and construct motors on the nanometer scale, that is, at the molecular level (1-5). Such a daring goal finds its scientific origin in the existence of natural molecular motors (6-9).Natural molecular motors, however, are extremely complex, and any attempt to construct systems of such a complexity, using the bottom-up molecular approach (10), would be challenging. What can be done, at present, is to construct simple prototypes of artificial molecular motors and machines (1-5, 11-19), consisting of a few components capable of moving in a controllable way, and to investigate the associated problems posed by interfacing them with the macroscopic world (20-25), particularly as far as energy supply is concerned.Natural motors are ''autonomous'': they keep operating, in a constant environment, as long as the energy source is available. By contrast, apart from a few recent examples (26-28), the fuel-powered artificial motors described so far are not autonomous because, after the mechanical movement induced by a chemical input, they need another, opposite chemical input to reset, which also implies generation of waste products. Addition of a fuel, however, is not the only means by which energy can be supplied to a chemical system. In fact, nature shows that, in green plants, the energy needed to sustain the machinery of life is ultimately provided by sunlight. Energy inputs in the form of photons can indeed cause mechanical movements by reversible chemical reactions without formation of waste products (13,14,16,17).In a previous work (29), we reported on the rotaxane 1 6ϩ (Scheme 1) that was carefully designed and synthesized to perform as a linear molecular...

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confidence: 99%

Autonomous artificial nanomotor powered by sunlight

Balzani

Clemente‐León

Credi

et al. 2006

463

325

“…By comparison, the development of artificial molecular machines over the last few decades has evolved from the design of molecules that emulate the structure and motion of macroscopic objects (2,3), to elegant examples of photo-(4) and electroactive rotary molecular motors (5), chemoactive linear motors (6) and actuators (7), among many others (8,9). Although there is much important knowledge to be gathered from the syntheses and dynamics of molecules in solution, lessons from biological systems and potential materials applications have led to subsequent efforts toward the design and characterization of artificial molecular rotors linked to surfaces (10)(11)(12) and in the bulk of crystalline solids (13,14).…”

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confidence: 99%

Symmetry and dynamics of molecular rotors in amphidynamic molecular crystals

Karlen¹,

Reyes²,

Taylor³

et al. 2010

109

123

Rotary biomolecular machines rely on highly symmetric supramolecular structures with rotating units that operate within a densely packed frame of reference, stator, embedded within relatively rigid membranes. The most notable examples are the enzyme FoF1 ATP synthase and the bacterial flagellum, which undergo rotation in steps determined by the symmetries of their rotators and rotating units. Speculating that a precise control of rotational dynamics in rigid environments will be essential for the development of artificial molecular machines, we analyzed the relation between rotational symmetry order and equilibrium rotational dynamics in a set of crystalline molecular gyroscopes with rotators having axial symmetry that ranges from two-to fivefold. The site exchange frequency for these molecules in their closely related crystals at ambient temperature varies by several orders of magnitude, up to ca. 4.46 × 10 8 s −1 .solid-state NMR | line shape analysis | spin-lattice relaxation | molecular design | crystal engineering

“…The VT 1 H NMR study in CDCl 3 showed that 3a, 3b, 4a, and 4b within 1a⅐2a rotate rapidly even at 218 K, whereas guest rotation is almost inhibited or too slow on the NMR time scale for 3h and 3i even at 323 K. In this respect, 4b with large dipole moment is a good candidate for the rotator of 1a⅐2a (3)(4)(5). For 3c-3g, the enthalpic (⌬H ‡ ) and entropic (⌬S ‡ ) contributions to the free energy of activation (⌬G ‡ ) for the guest-rotational steric barriers within 1a⅐2a were obtained, wherein the most sterically hindered 3g showed the largest ⌬H ‡ ϭ 29.0 kcal mol Ϫ1 and exceptionally large positive ⌬S ‡ ϭ 38.9 cal mol Ϫ1 K Ϫ1 .…”

Section: Discussionmentioning

confidence: 96%

“…Areas for further investigation are (i) the preparation of a self-assembled monolayer of 4b@(1⅐2) with unidirectional orientation on a gold surface using 2 with side chains R ϭ alkylthioether (18) and 1 with R ϭ chiral alkyl group (26) and (ii) the study of its guest rotation behavior in alternating applied electric fields directed toward formation of a supramolecular gyroscope (1,4,5).…”

Section: Discussionmentioning

confidence: 99%

“…One of the important topics is that of molecular gyroscopes (1,3), which consist of a rotator with a spinning axis and a stator framework, wherein a rotator is encased and protected by a stator with a frictionless environment. It is expected that rotators with electric dipole moments can rotate unidirectionally in alternating applied electric fields (1,4,5). On the basis of this concept, molecular gyroscopes have so far been synthesized by using several types of covalent-bonded strategies (3,(6)(7)(8)(9)(10)(11) and by a metal-coordination strategy such as a metal-centered gyroscope (12,13) and a self-assembled solid system (14).…”

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confidence: 99%

See 1 more Smart Citation

Encapsulated-guest rotation in a self-assembled heterocapsule directed toward a supramolecular gyroscope

Kitagawa

Kobori²,

Yamanaka³

et al. 2009

The self-assembled heterocapsule 1⅐2, which is formed by the hydrogen bonds of tetra(4-pyridyl)-cavitand 1 and tetrakis(4-hydroxyphenyl)-cavitand 2, encapsulates 1 molecule of guests such as 1,4-diacetoxybenzene 3a, 1,4-diacetoxy-2,5-dimethylbenzene 3b, 1,4-diacetoxy-2,5-dialkoxybenzenes (3c, OCH 3; 3d, OC2H5; 3e, OC3H7; 3f, OC 4H9; 3g, OC5H11; 3h, OC6H13; 3i, OC8H17), 1,4-diacetoxy-2,5-difluorobenzene 4a, and 1,4-diacetoxy-2,3-difluorobenzene 4b. The X-ray crystallographic analysis of 3c@(1⅐2) showed that the acetoxy groups at the 1,4-positions of 3c are oriented toward the 2 aromatic cavity ends of 1⅐2 and that 3c can rotate along the long axis of 1⅐2. Thus, the 1⅐2 (stator) with the encapsulation guest (rotator) behaves as a supramolecular gyroscope. A variable temperature (VT) 1 H NMR study in CDCl 3 showed that 3a, 3b, 4a, and 4b within 1⅐2 rotate rapidly even at 218 K, whereas guest rotation is almost inhibited for 3h and 3i even at 323 K. In this respect, 4b with a large dipole moment is a good candidate for the rotator of 1⅐2. For 3c-3g, the enthalpic (⌬H ‡ ) and entropic (⌬S ‡ ) contributions to the free energy of activation (⌬G ‡ ) for the guest-rotational steric barriers within 1⅐2 were obtained from Eyring plots based on line-shape analysis of the VT 1 H NMR spectra. The value of ⌬G ‡ increased in the order 3c < 3d < 3e < 3f < 3g. Thus, the elongation of the alkoxy chains at the 2,5-positions of 3 puts the brakes on guest rotation within 1⅐2.encapsulation ͉ self-assembly M olecular motors with a rotor function have attracted considerable attention in the field of mechanical molecular devices (1, 2). One of the important topics is that of molecular gyroscopes (1, 3), which consist of a rotator with a spinning axis and a stator framework, wherein a rotator is encased and protected by a stator with a frictionless environment. It is expected that rotators with electric dipole moments can rotate unidirectionally in alternating applied electric fields (1,4,5). On the basis of this concept, molecular gyroscopes have so far been synthesized by using several types of covalent-bonded strategies (3, 6-11) and by a metal-coordination strategy such as a metal-centered gyroscope (12, 13) and a self-assembled solid system (14). As an alternative strategy, a self-assembled molecular capsule with an encapsulated guest could behave as a supramolecular gyroscope in which an encapsulated guest is a rotator, and a self-assembled capsule is a stator (15). Molecular self-assembly based on noncovalent interactions offers great advantages in minimization of synthetic effort by use of modular subunits and through thermodynamic equilibration (16). Recently, we have reported the self-assembly of tetra(4-pyridyl)-cavitand 1a and tetrakis(4-hydroxyphenyl)-cavitand 2a (R ϭ (CH 2 ) 6 CH 3 ) into a heterocapsule 1a⅐2a in a rim-to-rim fashion via 4 pyridyl⅐⅐⅐phenol hydrogen bonds, wherein 1 molecule of 1,4-diacetoxybenzene 3a (Y ϭ H) is encapsulated to form a ternary complex 3a@(1a⅐2a), and the exchange of 3a in and out of 1a⅐2a in s...