ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

Improving the photochemical properties of molecular photoswitches is crucial for the development of light‐responsive systems in materials and life sciences. ortho‐Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ‐electron‐withdrawing F atoms ortho to the N?N unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para‐electron‐withdrawing groups (EWGs) work in concert with ortho‐F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho‐fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.     

Alkylations of tricyclo[5.2.1.0]deca-4,8-dien-3-one by a cuprate reaction

Alkylation at C₆ of tricyclo[5.2.1.0^2,6]deca-4,8-dien-3-one (R=H) was achieved by treatment of 6-bromotricyclo[5.2.1.0^2,6]deca-4,8-dien-3-one with lithium dimethylcuprate and subsequently with an appropriate electrophile. The best results were obtained in THF as the solvent. A wide range of alkyl halides, bromo ketones and esters, and acetyl chloride resulted in C₆-tricyclo[5.2.1.0^2,6]deca-4,8-dien-3-ones in moderate to good yields. This alkylation reaction proceeds via a C₆-carbanionic Cu intermediate, which is likely stabilized by the enone olefinic bond. 6-Bromo-endo-tricyclo[5.2.1.0^2,6]dec-8-en-3-one, which lacks this double bond, behaves differently. Treatment with lithium dimethylcuprate leads to dehydrobromination to give tricyclo[5.2.1.0^2,6]deca-2(6),8-dien-3-one in high yield.     

Alternative reagents for chemical noise reduction in liquid chromatography-mass spectrometry using selective ion-molecule reactions

Reduction of ionic chemical background noise based on selective gas-phase reactions with chosen neutral reagents has been proven to be a very promising approach in liquid chromatography—mass spectrometry (LC-MS). In this study further investigations on alternative reagents including the disulfides (dimethyl disulfide, diethyl disulfide, methyl propyl disulfide), dimethyl trisulfide, ethylene oxide, and butadiene monoxide, for example, have been carried out. Tandem mass spectrometric studies of ion/molecule reactions indicate that—besides dimethyl disulfide—ethylene oxide and butadiene monoxide also exhibit very efficient reactions with background ions. Furthermore, it is confirmed that the reactions are very selective according to the test with some analyte ions. In contrast to its rapid reactions with background ions, ethylene oxide does not react, or reacts much less, with these analytes. Therefore, it can be used as an alternative reagent for noise reduction. Although reactions of the other tested neutral reagents with background ions are evaluated, they are generally not suitable as reagents for this purpose because of lack of reactivity or dramatic ion losses during reactions.     

Electronic coupling and exciton energy transfer in CdTe quantum-dot molecules

Stable dispersions of molecularlike aggregates of CdTe quantum dots are prepared by chemical cross-linking. Cryo-TEM images confirm the presence of cross-linked quantum dots and show that the size of the small aggregates can be controlled by the amount of cross-linker added. Optical measurements reveal two types of interdot interactions within these quantum-dot molecules: exciton energy transfer and electronic coupling. Quantitative information on the energy transfer rates in quantum-dot molecules is obtained by photoluminescence lifetime measurements. The degree of electronic coupling is dependent on the size of the quantum dots, which is supported by quantum mechanical calculations.     

Optimization, standardization, and testing of a new NMR method for the determination of zeolite host-organic guest crystal structures

An optimized and automated protocol for determining the location of guest sorbate molecules in highly siliceous zeolites from (29)Si INADEQUATE and (1)H/(29)Si cross polarization (CP) magic-angle spinning (MAS) NMR experiments is described. With the peaks in the (29)Si MAS NMR spectrum assigned to the unique Si sites in the zeolite framework by a 2D (29)Si INADEQUATE experiment, the location of the sorbate molecule is found by systematically searching for sorbate locations for which the measured rates of (1)H/(29)Si cross polarization of the different Si sites correlate linearly with (1)H/(29)Si second moments calculated from H-Si distances. Due to the (1)H/(29)Si cross polarization being in the "slow CP regime" for many zeolite-sorbate complexes, it is proposed that the CP rate constants are best measured by (1)H/(29)Si cross polarization drain experiments, if possible, to avoid complications that may arise from fast (1)H and (29)Si T(1)rho relaxations. An algorithm for determining the sorbate molecule location is described in detail. A number of ways to effectively summarize and display the large number of solutions which typically result from a prediction of the structure from the CP MAS NMR data are presented, including estimates of the errors involved in the structure determinations. As a working example throughout this paper, the structure of the low loaded p-dichlorobenzene/ZSM-5 complex is determined under different conditions from solid-state (1)H/(29)Si CP MAS NMR data, and the solutions are shown to be in excellent agreement with the known single-crystal X-ray diffraction structure. This structure determination approach is shown to be quite insensitive to the use of relative rate constants rather than absolute values, to the detailed structure of the zeolite framework, and relatively insensitive to temperature and motions.     

Degenerate molecular shuttles with flexible and rigid spacers

The preparation and dynamic behavior of degenerate rotaxane molecular shuttles are described in which a benzylic amide macrocycle moves back and forth between two naphthalimide-glycine units along a diphenylethyne spacer or an aliphatic spacer consisting of a C(9), C(12), or C(26) alkyl chain. Subtle differences in the (1)H NMR spectra of the rotaxanes can be related to the presence of conformers in which the macrocycle interacts simultaneously with both glycines, especially in the case of the C(9) spacer. The kinetic data of the shuttling behavior in the C(26) rotaxane were obtained from dynamic NMR spectroscopy. The Eyring activation parameters were found to be ΔH(?) = 10 ± 1 kcal mol(-1), ΔS(?) = -6.5 ± 2.0 cal mol(-1) K(-1), ΔG(?)(298) = 11.9 ± 0.2 kcal mol(-1). For the systems with the shorter spacers, the shuttling rates were higher. Also in the diphenylethyne, rotaxane shuttling is rapid on the NMR time scale, indicating that the rigid unit does not impose a large barrier to the translocation of the macrocycle.     

Self-Organized Forest-Fires Near the Critical Time

We consider a forest-fire model which, somewhat informally, is described as follows: Each site (vertex) of the square lattice is either vacant or occupied by a tree. Vacant sites become occupied at rate 1. Further, each site is hit by lightning at rate λ. This lightning instantaneously destroys (makes vacant) the occupied cluster of the site.This model is closely related to the Drossel-Schwabl forest-fire model, which has received much attention in the physics literature. The most interesting behaviour seems to occur when the lightning rate goes to zero. In the physics literature it is believed that then the system has so-called self-organized critical behaviour.We let the system start with all sites vacant and study, for positive but small λ, the behaviour near the ‘critical time’ t _c , defined by the relation 1 − exp(− t _c ) = p _c , the critical probability for site percolation.Intuitively one might expect that if, for fixed t > t _c , we let simultaneously λ tend to 0 and m to ∞, the probability that some tree at distance smaller than m from O is burnt before time t goes to 1. However, we show that under a percolation-like assumption (which we can not prove but believe to be true) this intuition is false. We compare with the case where the square lattice is replaced by the directed binary tree, and pose some natural open problems.Part of vdB’s research is supported by BRICKS project AFM 2.2.     

N2O5 photolysis products investigated by fluorescence and optoacoustic techniques

Pulsed laser photolysis of N₂O₅ near 290 nm coupled with fluorescence detection (calibrated by NO₂ photolysis) showed that the O(³P) quantum yield is ≤0.1. A pulsed laser optoacoustic technique in a flow tube (ca. 6 torr of N₂) was tested by photolysis of NO₂ and then applied to N₂O₅. Nitric oxide was added to react with NO₃ free radical and the resulting increase in the optoacoustic signal confirmed the presence of NO₃ free radicals. Based on the relative optoacoustic signals observed for NO₂ and N₂O₅, the quantum yield for NO₃ production is 0.8 ± 0.2.     

Thermal isomerization of azulene to naphthalene in shock waves

The thermal isomerization of azulene was studied in shock waves over the range 1300–1900 K. Monitoring azulene and naphthalene light absorptions in the UV, a complete conversion azulene → naphthalene was observed. After correction for some falloff effects, a limiting high pressure rate constant k_x = 10^12.93 exp(?263 kJ mol^?1/RT) s^?1 was derived. Based on this k_x, specific rate constants k(E) for photoexcitation experiments were constructed.