Reversible photoinduced twisting of molecular crystal microribbons

9-Anthracenecarboxylic acid, a molecule that undergoes a reversible [4 + 4] photodimerization, is prepared in the form of oriented crystalline microribbons. When exposed to spatially uniform light irradiation, these photoreactive ribbons rapidly twist. After the light is turned off, they relax back to their original shape over the course of minutes. This photoinduced motion can be repeated for multiple cycles. The final twist period and cross-sectional dimensions of individual microribbons are measured using a combination of atomic force and optical microscopies. Analysis of this data suggests that the reversible twisting involves the generation of interfacial strain within the ribbons between unreacted monomer and photoreacted dimer regions, with an interaction energy on the order of 3.4 kJ/mol. The demonstration of reversible twisting without the need for specialized irradiation conditions represents a new type of photoinduced motion in molecular crystals and may provide new modes of operation for photomechanical actuators.     

钨离子团簇与N2分子的反应性

利用激光蒸发团簇源产生Wn团簇束,团簇束通过一个充有N2气体分 子的低压反应池,利用飞行时间质谱探测反应产物,在类单次碰撞条件下研究了W+10 -W+50和N2分子的反应性,在室温条件下测量了N2分子与W+n团簇反应的 反应几率。团簇尺寸在10～26原子的团簇与N2分子的反应几率与团簇尺寸有很强的相关性 ,对n=16,22,23的团簇具有比较高的反应性。W+n与N2分子的反应性与Wn与N 2分子的反应性显示出相似的规律性。     

Template assisted synthesis of silica-coated molecular crystal nanorods: From hydrophobic to hydrophilic nanorods

We report a method for the preparation of silica-coated molecular crystal nanorods. A sol-gel method was used to make silica nanotubes inside anodized alumina templates. The nanotubes were then loaded with 9-anthracene carboxylic acid (9-AC) and solvent annealed to produce silica-coated organic nanorods. The core-shell structure was confirmed using electron microscopy, and the highly crystalline organic core was characterized using powder X-ray diffraction and transmission electron microscopy. The silica-coated 9-AC rods had much improved dispersal properties in aqueous solution, and were also able to undergo reversible bending under UV illumination, as observed previously for uncoated 9-AC rods. This work demonstrates that it is possible to make surface-coated molecular crystal nanorods that retain their useful functionalities.     

A weight-function approach for determining watershed initial conditions for combustion waves

In many generic combustion models, one finds that a combustionwave will develop with a specific wave speed. However, thereare possible initial temperature profiles which do not evolveinto such waves, but rather die out to the ambient temperature.There can exist, in some models, a clear distinction betweenthose initial conditions that do evolve into combustion wavesand those that do not; this is sometimes referred to as thewatershed initial condition. When fuel consumption is consideredto be negligible, analytical methods can be used to obtain theexact watershed. In this paper, we consider the problem of determiningpseudo-watersheds and ascertaining the relationship betweenthese pseudo-watersheds and the exact watersheds. In the processa novel weight-function approach for infinite spatial domainsis developed.     

Bridging photochemistry and photomechanics with NMR crystallography: the molecular basis for the macroscopic expansion of an anthracene ester nanorod

Crystals composed of photoreactive molecules represent a new class of photomechanical materials with the potential to generate large forces on fast timescales. An example is the photodimerization of 9-tert-butyl-anthracene ester (9TBAE) in molecular crystal nanorods that leads to an average elongation of 8%. Previous work showed that this expansion results from the formation of a metastable crystalline product. In this article, it is shown how a novel combination of ensemble oriented-crystal solid-state NMR, X-ray diffraction, and first principles computational modeling can be used to establish the absolute unit cell orientations relative to the shape change, revealing the atomic-resolution mechanism for the photomechanical response and enabling the construction of a model that predicts an elongation of 7.4%, in good agreement with the experimental value. According to this model, the nanorod expansion does not result from an overall change in the volume of the unit cell, but rather from an anisotropic rearrangement of the molecular contents. The ability to understand quantitatively how molecular-level photochemistry generates mechanical displacements allows us to predict that the expansion could be tuned from +9% to −9.5% by controlling the initial orientation of the unit cell with respect to the nanorod axis. This application of NMR-assisted crystallography provides a new tool capable of tying the atomic-level structural rearrangement of the reacting molecular species to the mechanical response of a nanostructured sample.

NMR crystallography establishes absolute unit-cell orientations relative to the shape change, revealing the atomic-resolution mechanism for the nanorod''s photomechanical response.     

Quantitative analysis of quinidine analogs using ion-pairing HPLC

Quinidine, a useful antiarrhythmic compound, is usually contaminated with dihydroquinidine, a compound that itself shows potent antiarrhythmic activity. Complete hydrogenation of quinidine followed by conversion to dihydroquinidine derivatives was explored as a basis for eliminating the analytical problems inherent in the quality control of quinidine products and for determining their pharmacological potency and pharmacokinetic parameters without interfering impurities. Attempts to resolve the quinidine analogs, dihydrocupreidine, and its benzoyloxy ester failed with normal and reversed-phase chromatography. Ion-pairing chromatography using n-octanesulfonate in methanol:water proved successful. Using 9-hydroxy-4-methoxy acridine as internal standard, separation and quantitation of the dihydroquinidine analogs from spiked plasma samples was achieved with 92 to 95% efficiency.     

General method for the synthesis of crystalline organic nanorods using porous alumina templates

Nanorods composed of a variety of conjugated organic molecules were synthesized using an anodized alumina template and solvent annealing; detailed study of 200 nm thick 2,7-di-t-butylpyrene rods showed they are crystalline, with single domains extending over several microns.     

The photophysical properties of chromophores at high (100 mM and above) concentrations in polymers and as neat solids

The absorption, fluorescence, and photostability of five conjugated chromophores: perylene, 2,5,8,11-tetra-t-butyl perylene (TTBP), perylene orange (PO), perylene red (PR), and a zwitterionic Meisenheimer complex (MHC), are studied as a function of concentration in poly(methyl methacrylate) (PMMA). At 1 mM concentrations, all five molecules exhibit properties consistent with unaggregated chromophores. At higher concentrations, perylene and PO both exhibit excimer formation, while TTBP, PR, and the MHC retain their monomeric fluorescent lineshapes. In these three molecules, however, the fluorescence decay times decrease by 10% (TTBP) to 50% (MHC) at concentrations of 100 mM in PMMA. The fluorescence properties of these highly concentrated samples are sensitive to the sample preparation conditions. In the neat solid where the effective concentration is on the order of 1 M, all three molecules exhibit very fast fluorescence decays, on the order of 150 ps or less, despite the fact that they retain their basic monomeric fluorescence lineshape. In addition to the enhanced nonradiative decay at high concentrations, these three molecules also undergo a concentration-dependent photobleaching. The combined effects of intermolecular nonradiative decay channels and photobleaching appear to be a general obstacle to achieving highly concentrated dye-doped solids.     

Experimental and theoretical study of temperature dependent exciton delocalization and relaxation in anthracene thin films

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton's coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.     

Using light intensity to control reaction kinetics and reversibility in photomechanical crystals

4-Fluoro-9-anthracenecarboxylic acid (4F-9AC) is a thermally reversible (T-type) photomechanical molecular crystal. The photomechanical response is driven by a [4 + 4] photodimerization reaction, while the photodimer dissociation determines the reset time. In this paper, both the chemical kinetics of dimer dissociation (using a microscopic fluorescence-recovery-after-photobleaching experiment) and mechanical reset dynamics (by imaging bending microneedles) for single 4F-9AC crystals are measured. The dissociation kinetics depend strongly on the initial concentration of photodimer, slowing down and becoming nonexponential at high dimer concentrations. This dose-dependent behavior is also observed in the mechanical response of bending microneedles. A new feature in the photomechanical behavior is identified: the ability of a very weak control beam to suppress dimer dissociation after large initial dimer conversions. This phenomenon provides a way to optically control the mechanical response of this photomechanical crystal. To gain physical insight into the origin of the nonexponential recovery curves, the experimental results are analyzed in terms of a standard first-order kinetic model and a nonlinear Finke–Watzky (FW) model. The FW model can qualitatively reproduce the transition from exponential to sigmoidal recovery with larger initial conversions, but neither model can reproduce the suppression of the recovery in the presence of a weak holding beam. These results highlight the need for more sophisticated theories to describe cooperative phenomena in solid-state crystalline reactions, as well as demonstrating how this behavior could lead to new properties and/or improved performance in photomechanical materials.

The chemical and mechanical recovery rates of crystalline 4-fluoro-9-anthracenecarboxylic acid, a thermally reversible photomechanical material, can be controlled by both the intensity of the photodimerization pulse and the use of a weak hold beam.