Controlling forward and backward rotary molecular motion on demand.

Synthetic molecular machines hold tremendous potential to revolutionize chemical and materials sciences. Their autonomous motion controlled by external stimuli allows to develop smart materials whose properties can be adapted on command. For the realisation of more complex molecular machines, it is crucial to design building blocks whose properties can be controlled by multiple orthogonal stimuli. A major challenge is to reversibly switch from forward to backward and again forward light-driven rotary motion using external stimuli. Here we report a push-pull substituted photo-responsive overcrowded alkene whose function can be toggled between that of a unidirectional 2nd generation rotary motor and a molecular switch depending on its protonation and the polarity of its environment. With its simplicity in design, easy preparation, outstanding stability and orthogonal control of distinct forward and backward motions, we believe that the present concept paves the way for creating more advanced molecular machines.

Experimental verification of the chemical sensitivity of two-site double core-hole states formed by an x-ray free-electron laser.

We have performed x-ray two-photon photoelectron spectroscopy using the Linac Coherent Light Source x-ray free-electron laser in order to study double core-hole (DCH) states of CO2, N2O, and N2. The experiment verifies the theory behind the chemical sensitivity of two-site DCH states by comparing a set of small molecules with respect to the energy shift of the two-site DCH state and by extracting the relevant parameters from this shift.

Rotational diffusion of the 7-diethylamino-4-methylcoumarin C1 dye molecule in polar protic and aprotic solvents.

Fluorescence anisotropy decays of the 7-diethylamino-4-methylcoumarin C1 in various polar solvents of different viscosities and hydrogen bond donor/acceptor character have been recorded by means of the fluorescence upconversion and time-correlated single photon counting techniques. The resulting characteristic times for the rotational diffusion fall into two classes with regards to the viscosity-dependency: n-alcohols and "other" solvents. This deviation from the simple Stokes-Einstein-Debye model may be interpreted in terms of rotation of the coumarin molecule under two different hydrodynamic boundary-conditions ("stick" or "slip") in the two solvent classes. Possible explanations for this behaviour are discussed, and in particular solvent attachment and additional dielectric friction. Both these phenomena may in fact, under certain conditions, explain our findings. Our opinion, however, is that the dielectric friction model offers a more realistic picture of the additional rotational friction experienced by C1 in n-alcohols.

Studies of the fifth-order nonlinear susceptibility of ultraviolet-grade fused silica.

Femtosecond transient absorption pump-probe spectroscopy has been used to study the third- and fifth-order nonlinear susceptibility of UV-grade fused silica. Theoretical simulations support the experimental evidence of interference between the third- and fifth-order contributions to the signal. Approximate values of the second-order nonlinear refractive index n(4) and of the three-photon absorption coefficient gamma are obtained.

Performance of external fixation devices in femoral fractures; the ultimate challenge? A laboratory study with plastic rods.

Whether or not external fixation should be applied to the femur, and on what grounds, will depend upon a variety of factors. It is rarely considered to be the treatment of first choice. External fixation can, however, be indicated in certain circumstances. Looking for optimal rigidity in such cases we tested and compared the stability of 14 different femoral external fixators in an experimental model. It was found that the weight of the different frames varied from 400 to 2000 g. The comparative stability also varied widely. Movements of between 1 mm and 4 cm and rotations varying from almost 0 degrees to 16 degrees were measured at the (experimental) fracture site, based upon the geometry of an adult patient. In view of this finding primary bone-healing would not be expected to occur, since it demands more stability than external femoral fixation can offer. A relatively simple two-dimensional (unilateral) frame can be as rigid as a three-dimensional one, in response to all but transverse loading.

Fast Field Echo imaging: an overview and contrast calculations.

Current fast imaging techniques are based on gradient echo sequences with reduced flip angle excitation pulses and very short repetition times TR. Practical T2 values may be of the order of TR or longer. In this situation, a different image contrast can be obtained, depending on details of the sequence. Four essentially different versions of the basic Fast Field Echo (FFE) sequence can be distinguished and are described systematically in this article. For these sequences, image contrast formulas are presented. Practical imaging should tolerate small field inhomogeneities. This requirement can be satisfied by only three of the four versions. Numerical simulations are used to study the influence of a modified phase alternation scheme on image contrasts of two of the remaining sequences. The results of the calculations are verified by phantom studies on a 1.5-T whole-body imager. Implications for contrast in clinical images are discussed in relation to head images obtained on the same machine.

Very fast MR imaging by field echoes and small angle excitation.

An MR imaging technique has been developed producing head and body images of diagnostic quality in only a few seconds acquisition time. The Fourier type imaging technique uses excitation with relatively small excitation angels, echoes produced by gradient inversion, and extremely fast profile repetition. A typical result at 0.5 T is an artifact-free head image of 128 x 128 resolution, 10 mm slice thickness in an acquisition time of 2 seconds.