Steady-state absorption and emission and femtosecond time-resolved emission spectroscopy of two benzylidene malononitriles, 2-[4-(dimethylamino)benzylidene])malononitrile (DMN) and julolidinemalononitrile (JDMN), are reported in a variety of room-temperature solvents. Solvatochromic shifts of these molecules are consistent with dielectric continuum descriptions and an S(1)-S(0) dipole moment change of 8.5 D. Time-resolved spectra show modest dynamic Stokes shifts of approximately 1000 cm(-1) occurring independently of fluorescence decay, which takes place in 0.5-5 ps in most room-temperature solvents. Absorption transition moments and fluorescence decay times are used to determine radiative rate constants: k(rad) = 0.32 +/- 0.02 ns(-1) in DMN and 0.28 +/- 0.02 ns(-1) in JDMN, assumed to be independent of solvent. Quantum yield data together with these radiative rates provide the reaction rate constants k(rxn) associated with the internal conversion process of these molecules in 33 representative solvents at 298 K and in several solvents as functions of temperature. Reaction rates of JDMN are systematically lower than those of DMN by a factor of 2.0. Values of k(rxn) in series of homologous solvents or in a single solvent at different temperatures are correlated to solvent viscosity eta and temperature T in the manner k(rxn)/T proportional to eta(-p) with exponents 0.2 < or = p < or = 0.8. Solvent polarity appears to influence these reactions such that for a given viscosity reaction in high polarity solvents is significantly slower than in nonpolar solvents. However, this conclusion is predicated on the assumption that reactive friction is identical in solvents of the same viscosity, which is unlikely to be quantitatively correct. The observed reaction rates and their solvent dependence are discussed in terms of isomerization about the C=C bond occurring on a shelf-like potential.

Phosphorescent transition-metal complexes (PTMCs) have attracted great interest because of their excellent properties which may lead to promising applications in optoelectronics. In recent years, carboranes have been demonstrated to be a novel and effective tool to tune phosphorescence of PTMCs. This Concept article deals with the advances of carborane-functionalized PTMCs for potential optical applications. The discussions are focused on the design strategies and synthetic procedures leading to carborane-functionalized PTMCs, the influence of carboranes on the optical properties of PTMCs, and the promising optical applications of this interesting class of phosphorescent materials.

The results from PFG NMR studies of several tetraalkylammonium−silica mixtures are reported. pH data of these mixtures correlate well with previously reported literature. 1H NMR shows line broadening and shifting of resonances for hydrogen atoms on the periphery of the TAA cations as the silica content is increased. Transverse relaxation measurements show a strong correlation with the pH data in that T 2 decreases strongly until the silica critical aggregation concentration (cac) is reached at which point the T 2 plateaus. These results point to organocation−silica interactions, which were quantified by using pulsed-field gradient NMR. The self-diffusion coefficient of a series of TAA cations in the presence of silica were measured, and a systematic decrease of the diffusion coefficient is observed as the cation/silica ratio decreases. Subsequent analysis gives confidence that the diffusion behavior of the TAA cations can be described as a two-state system, where cations are either free in solution or bound to the silica nanoparticles in the mixture. On the basis of this binding isotherms can be constructed and binding energies can be determined. These values are in the range of −10 to −14 kJ/mol, with tetramethylammonium on the high end and tetrapropylammonium cations on the low end. One conclusion drawn from this work is that the more hydrophilic TMA cations associate more strongly with the silica nanoparticles than TPA. The investigation shows that PFG NMR is a powerful tool for investigating these clear mixtures, and the results are discussed in the context of zeolite nucleation and growth.

We have recently described efforts directed toward delineating structure-activity relationships for cocaine in an effort to identify a possible antagonist of cocaine's action at the dopamine re-uptake transporter.' During these efforts we discovered that this cocaine-recognition site is rather promiscuous in ita ability to accept groups substantially different from cocaine's C-2 carbomethoxy substituent. The carbethoxyisoxazole 1, for example, was found to be about 2-fold more potent than cocaine in both [3H]mazindol binding and L3H1 dopamine uptake studies. (-)-Cocaine 1This finding and others made us question the idea that specific hydrogen bond donor groups are present within the cocaine recognition site for binding to the carbomethoxy group (Figure 1).2 To explore this point further, we chose to investigate the activity of the four new cocaine analogues, 2a-d, in which the C-2 ester group was replaced by a vinyl group incapable of any strong hydrogen bonding.Chemistry. The vinyl compounds 2a and 2b were synthesized readily from the unstable aldehyde 4, a compound obtained by Swern oxidation of the alcohol 3, which is available in optically pure form from (-)-cocaine as shown in Scheme 1.' After the Wittig reaction of 4, the silyl group is cleaved by the action of fluoride ion, and the free hydroxyl group then is benzoylated.To obtain compounds 2c and 2d, @-chloropheny1)-magnesium bromide was added to anhydroecgonine methyl + Mayo Foundation for Education and Research. t University of Texas. (1) Kozikowski,A. P.;Xiang,L.;Tanaka, J.;Bergmann, J.S.; Johnson, K. M. Use of Nitrile Oxide Cycloaddition (NOC) Chemistry in the Synthesis of Cocaine Analogues; Mazindol Binding and Dopamine Uptake Studies. Med. Chem. Res. 1991,1, 312-321. (2) Carroll, F. I.; Lewin, A. H.; Boja, J. W.; Kuhar, M. J. Cocaine Receptor: Biochemical Characterization and Structure-Activity Relationships of Cocaine Analogues at the Dopamine Transporter. J. Med. Chem. 1992,36,969-981. 0022-2623/92/1835-4764$03.00l0Figure 1. Illustration of possible hydrogen bonding to the C-2 ester group. Scheme I a d c OTBDMS a c (-)-Cocdnm 4 28, X-H 2b. X-CI Reagents and conditione: (a) conc. H2SO4, MeOH, reflux, 18 h; (b) TBDMSC1, imd., rt, 20 h; (c) Dibal-H, CHzClz, -78 OC, 4 h; (d) Swem oxidation; (e) XCH=PPhs; (0 P, (9) PhCOCl, pyr. ester (5) as described by Carrol et al. in their preparation of additional analogues of the "Win" series of compounds?The 2@ubstituted isomer was separated from the 2u-isomer, and the ester converted to aldehyde in two steps by reduction to alcohol followed by Swern oxidation. Lastly, reaction of this aldehyde with methylenetriphenylphosphorane gave the vinyl analogue 20, whereas reaction with (chloromethy1ene)triphenylphoephorane afforded 2d (Scheme II).* Pharmacological Results. The four new analogues were tested for their ability to displace [SHlmazindol bindine from rat striatal membranes as well as to inhibit high-affinity uptake of 13H]dopamine into striatal nerve (3) (a) Carroll, F. I.; Gao, Y.; Ftahman, M. A.; Abraham, P...

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In the past few decades, a variety of rehabilitation robotic exoskeletons have been developed for patients with stroke and traumatic brain injury, which can assist therapists and potentially improve the functional outcomes. Many robotic exoskeleton systems adopted series elastic actuators, which are known to offer a number of advantages over rigid actuators, such as high force/torque fidelity, low output impedance, intrinsic compliance, and tolerance to shocks. While several control schemes have been proposed for robotic exoskeletons driven by series elastic actuators, existing methods are limited at high level, by assuming a perfect inner control loop. Due to the nonlinearity in robot dynamics, changing interaction forces, etc., the stability of the overall system consisting of both the robot dynamics and the actuator dynamics may not be guaranteed. This paper presents a multi-modal control scheme for rehabilitation robotic exoskeletons. Three control modes are smoothly integrated into the controller, where the robot-assisted mode allows the human to exert voluntary efforts within a desired region, the robot-dominant mode is activated to correct the movement of the human when the robot is outside the region, and the safety-stop mode is to stop the robot whenever the interaction force is too large which may result in injuries. By using the regional position and force feedback, the proposed controller achieves the paradigm of “assist-as-needed” and also guarantees the safety of the human. The development of the proposed controller follows the singular perturbation approach, and the stability of the overall system is rigorously proved by using Tikhonov’s theorem. Experimental results in both upper-limb and lower-limb exoskeleton systems are presented to demonstrate the effectiveness of the proposed control method.

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CdTe quantum dots (QDs) were synthesized by a modified hydrothermal method with Na 2 TeO 3 as the Te source. Thioglycolic acid (TGA), thiolactic acid (TLA), 3mercaptopropionic acid (MPA), 3-mercaptobutyric acid (3MBA), 4-mercaptobutyric acid (MBA), and 5-mercaptovaleric acid (MVA) were used. Their effects on the growth and fluorescence of as-synthesized QDs were investigated: Detrimental precursor aggregation when using TGA-like molecules (TGA and L-cysteine) due to their special secondary coordination was observed and further confirmed by comparing with DL-homocysteine and N-acetyl-L-cysteine. The aggregation could be suppressed by using a bulky methyl group as a side chain to confine the carboxyl group, that is, TLA. Moreover, MPA was confirmed as a better stabilizer compared with linear TGA, MBA, and MVA probably because of its suitable balance between hydrophilicity and hydrophobicity. Accordingly, we designed and synthesized 3MBA that has the same main chain length as MPA but with a side methyl group as TLA. 3MBA-modified CdTe QDs exhibited an excellent optical property with a quantum yield of 71%, much higher than that of MPA and TGA. Preliminary results of CdSe QDs confirmed the versatility of 3MBA. Our results uncovered the possible origin of the advantages of mercapto acids with a methyl side chain and a suitable chain length.