Molecule molecular rotors

Loutfy and coworkers [29, 30] assumed a different mechanism of interaction between the molecular rotor molecule and the surrounding solvent. The basic assumption was a proportionality of the diffusion constant D of the rotor in a solvent and the rotational reorientation rate kOI. Deviations from the Debye-Stokes-Einstein hydrodynamic model were observed, and Loutfy and Arnold [29] found that the reorientation rate followed a behavior analogous to the Gierer-Wirtz model [31]. The Gierer-Wirtz model considers molecular free volume and leads to a power-law relationship between the reorientation rate and viscosity. The molecular free volume can be envisioned as the void space between the packed solvent molecules, and Doolittle found an empirical relationship between free volume and viscosity [32] (6),... 

In (8), the solvent-independent constants kr, kQnr, and Ax can be combined into a common dye-dependent constant C, which leads directly to (5). The radiative decay rate xr can be determined when rotational reorientation is almost completely inhibited, that is, by embedding the molecular rotor molecules in a glass-like polymer and performing time-resolved spectroscopy measurements at 77 K. In one study [33], the radiative decay rate was found to be kr = 2.78 x 108 s-1, which leads to the natural lifetime t0 = 3.6 ns. Two related studies where similar fluorophores were examined yielded values of t0 = 3.3 ns [25] and t0 = 3.6 ns [29]. It is likely that values between 3 and 4 ns for t0 are typical for molecular rotors. 

Most of the molecules introduced in this chapter are hydrophobic. Even those molecules that have been functionalized to improve water-solubility (for example, CCVJ and CCVJ triethyleneglycol ester 43, Fig. 14) contain large hydrophobic structures. In aqueous solutions that contain proteins or other macromolecules with hydrophobic regions, molecular rotors are attracted to these pockets and bind to the proteins. Noncovalent attraction to hydrophobic pockets is associated with restricted intramolecular rotation and consequently increased quantum yield. In this respect, molecular rotors are superior protein probes, because they do not only indicate the presence of proteins (similar to antibody-conjugated fluorescent markers), but they also report a constricted environment and can therefore be used to probe protein structure and assembly. 

Two of the cytoskeletal components, the actin filaments and the microtubules have been studied with molecular rotors. The main component of the actin filaments is the actin protein, a 44 kD molecule found in two forms within the cell the monomeric globulin form (G-actin) and the filament form (F-actin). Actin binds with ATP to form the microfilaments that are responsible for cell shape and motility. The rate of polymerization from the monomeric form plays a vital role in cell movement and signaling. Actin filaments form the cortical mesh that is the basis of the cytoskeleton. The cytoskeleton has an active relationship with the plasma membrane. Functional proteins found in both structures... 

Norbornadiene. Hamers and co-workers first studied the adsorption of norbornadiene (bicyclo[2.2.l]hepta-2,5-diene) [43] on Si(100) this molecule and a sample STM image are shown in Fig. 6 along with a functionally modified norbornadiene analogue [50] containing a silyl molecular rotor, which we have also investigated. Norbornadiene was in fact the first hydrocarbon containing multiple C=C bonds to be studied in this fashion. Its two double bonds are separated by 2.4 A and are chemically independent moieties, and hence this molecule could react twice with silicon via a double... 

Nonpolar and dipolar altitudinal rotors (compounds 2 and 3 in Fig. 17.3) have been synthesized. 19F NMR spectroscopy showed that the barrier to rotation in 3 was extremely low in solution. Both systems have then been immobilized on Au(l 11) surfaces and studied with a variety of techniques.57 The results obtained indicated that for a fraction of molecules the static electric field from the scanning tunneling microscopy (STM) tip could induce an orientation change in the dipolar rotor but not in the nonpolar analog (for a recent example of an azimuthal molecular rotor controlled by the STM tip, see Reference 58). Compound 3 can exist as three pairs of helical enantiomers because of the propeller-like conformation of the tetra-arylcyclobutadienes. For at least one out of the three diastereomers, an asymmetric potential energy surface can be predicted by molecular dynamics simulations on application of an alternating electric field.55... 

A family of molecular rotors (e.g., compound 4 in Fig. 17.4 a) has been designed to perform rotation under electrochemical stimulation.59 60 The molecules have a piano-stool structure with a stator meant to be grafted on an oxide surface and a rotor bearing redox-active groups, so that addressing the molecule with nanoelectrodes would trigger rotation (Fig. 17.4 b). To avoid intramolecular electron transfer between two electroactive units, which would compete with rotation, insulating... 

The syn and anti conformations of pyrimidine ribonucleosides have an opposite sign for certain transitions (e.g., the B2u transition) (67B843 69JA831 71JA(93)1600 72JCP2736 72MI3). Such relations are empirical correlations, since the molecular structures are too complicated to allow theoretical calculations of the rotational strengths. A combination of DNMR, CD spectroscopy, and molecular mechanics calculations has been applied to derivatives of indole 77 and thiazoline-2-thione 78 substituted by chiral rotors (Scheme 58). In these molecules the rotor adopts one of the bisected... 

Fig. 1. Example of a hexa-ferf-butyldecacyclene (HBDC) molecular rotor, which is supposed to rotate around the X-X axis. Isolated in the gas phase, the quantum state of this molecule can be prepared in a rotational wave packet to start the described rotation before quantum dilution of the wave packet...

Fig. 2. Example of an HBDC molecular rotor chemically bound to a polyene chain holding a metal cluster. As in Fig. 1, X-X is the rotation axis. This molecule must be prepared in a quasi-classical state to ensure lifting of the cluster load using the polymer chain as a string...

Owing to the development of supramolecular chemistry and a series of efficient synthetic methods such as template synthesis and dynamic covalent synthesis, artificial molecular machines have exploded since the late 1980s. In this chapter, we mainly discuss mechanical molecular machines that is, the different components among the molecules or supramolecular aggregates, which are linked by noncova-lent interactions and are usually called mechanical bonds. Mechanical bonds, in which two or more molecular components become mechanically interlocked, one with another," play a dominant role in the development of artificial molecular machines. Threaded structures, such as pseudorotax-anes, rotaxanes, and catenanes, are excellent precursors in the construction of molecular machines, for the mechanical bonds in these supramolecular systems are easy to be altered by external stimuli, causing relative movements among different components. Molecular rotors and propellers are also discussed because of their great importance in the development of motorized machines. 

However, a dominant disadvantage of the above first generation of unidirectional, light-driven molecular rotors is that the upper part of the molecules is too close to the lower part, that is, the gap of the Qord region is too small. Thermal isomerization of the molecule will meet a considerable steric hindrance, making it take several hours to complete the rotation process and limit the speed of the rotation. In the following studies, Feringa et al. proposed two solutions ... 

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