Photochemistry in ordered physisorbed monolayers of dinitrogen tetroxide

The effects of physisorption and two-dimensional ordering on the photochemistry of N₂O₄ were investigated. Ordered monolayers were prepared by adsorption of NO₂ at 100 K on a water-ice surface. Irradiation with a continuous light source in the wavelength region 300–400 nm or with pulsed laser radiation at 355 nm resulted in exclusive desorption of NO₂. This desorption was induced by electronic absorption directly in the adsorbate via a transition corresponding to the ( )¹B_2u←( )¹A_g transition in N₂O₄, as in the gas phase. However, the subsequent dynamics in the excited state were markedly different from the gas-phase counterpart. Time-of-flight mass spectrometry of NO₂ photodesorbed at 355 nm revealed a most probable fragment translational energy of ca. 17 meV; and the angular distribution of the nascent NO₂ was peaked sharply in a direction around 10° from the normal. It is apparent that, despite the weak interaction with the substrate, significant energy transfer occurs in the ordered physisorbed monolayer to yield nascent NO₂ with very low translational energy and a constrained angle of escape which is consistent with a high degree of adsorbate order and alignment.     

Specific and nonspecific collapse in protein folding funnels

Experiments with fast folding proteins are beginning to address the relationship between collapse and folding. We investigate how different scenarios for folding can arise depending on whether the folding and collapse transitions are concurrent or whether a nonspecific collapse precedes folding. Many earlier studies have focused on the limit in which collapse is fast compared to the folding time; in this work we focus on the opposite limit where, at the folding temperature, collapse and folding occur simultaneously. Real proteins exist in both of these limits. The folding mechanism varies substantially in these two regimes. In the regime of concurrent folding and collapse, nonspecific collapse now occurs at a temperature below the folding temperature (but slightly above the glass transition temperature).     

Singlet and Triplet State Properties and Singlet Oxygen Yield of an Amino-Acyl-Quinoxalinone Yellow Dye

Amino-acyl-quinoxalinone yellow dyes are cyclised analogues of the yellow azomethine dyes developed for, and still used in, silver halide colour photography. Unlike image azomethine dyes, which are rapidly deactivated in their excited states by torsion about the azomethine bond, amino-acyl-quinoxalinone dyes have an interesting photophysics because torsion is not possible due to their cyclised structure. We report results from studies on singlet and triplet state properties, and singlet oxygen yields, of the yellow dye, 7-diethylamino-3-(2,2-dimethyl-propionyl)-5-methyl-1-phenyl-1H-quinoxalin-2-one, in polar and nonpolar solvents. The dye photophysics is characterised by a weak fluorescence, with a solvent dependent emission yield (Φ_F?≈?0.002–0.004), and short singlet state lifetime (τ_expt?≈?20–50 ps), both increasing by a factor of ≈2 in going from polar acetonitrile to non-polar dioxane as solvent. DFT ZINDO calculations show a transition involving significant electron transfer from the diethyl-amino group into the carbonyl region of the molecule. In solution, in the presence of oxygen, the triplet state decays almost exclusively by oxygen quenching, and singlet oxygen is produced in high yield (Φ_??≈?0.5–0.55). The triplet state absorbs across the 450–750 nm region with maxima around 480 and 650 nm, and moderate molar absorption coefficients (ca. 6000–8000 M^?1 cm^?1). In a glass at 77 K, triplet decay gives a red phosphorescence, with λ_max?≈?640–650 nm, and a ?≈?0.25 s lifetime. If singlet oxygen yields are a good indication of triplet yields, then internal conversion and intersystem crossing occur with roughly equal efficiency.

     

Pioneering O.R.

This paper describes the critical start-up period of a new Operational Research unit in industry. Four principles for success evolved and are discussed against a background of practical work.     

Highly Selective Optical Detection of Fe3+ Ions in Aqueous Solution Using Label‐Free Silicon Nanocrystals

High brightness amine‐terminated silicon nanocrystals (Si NCs) have been utilized in a simple and rapid assay for the highly selective and sensitive detection of Fe³⁺ via quenching of their strong blue luminescence, without the need for analyte‐specific labeling groups. Sensitive detection of Fe³⁺ is successfully demonstrated, with a linear relationship observed between luminescence quenching and Fe³⁺ concentration from 5 × 10^?6 to 900 × 10^?6m and a limit of detection of 1.3 × 10^?6m . The Si NCs show excellent selectivity toward Fe³⁺ ions, with no quenching of the luminescence signal induced by the presence of Fe²⁺ ions, allowing for solution phase discrimination between the ionic species in different charge states.     

Greedy sums of distinct squares

When a positive integer is expressed as a sum of squares, with each successive summand as large as possible, the summands decrease rapidly in size until the very end, where one may find two 's, or several 's. We find that the set of integers for which the summands are distinct does not have a natural density but that the counting function oscillates in a predictable way.

     

Graded left modular lattices are supersolvable

We provide a direct proof that a finite graded lattice with a maximal chain of left modular elements is supersolvable. This result was first established via a detour through EL-labellings in [MT] by combining results of McNamara [Mc] and Liu [Li]. As part of our proof, we show that the maximum graded quotient of the free product of a chain and a single-element lattice is finite and distributive.Received May 24, 2004; accepted in final form October 12, 2004.     

Photodynamics of a PV Trimer in High‐Viscosity Solvents and in PMMA Films: A New Insight into Energy Transfer versus Conformational Relaxation in Conjugated Polymers

Non‐Stokes–Einstein relaxation : The rate constant of conformational relaxation of a phenylenevinylene trimer (see picture) in different solvents is proportional to η^?α, with α values decreasing from close to unity (low viscosity) to zero at sufficiently high solvent viscosity. This behaviour is attributed to the flexible methylbutyl side chains of the trimer, which partially screen the solvent friction.

     

Characterization of an immobilized antibody-enzyme complex

Antiserum specific for propanediol dehydrogenase, an enzyme found inNeisseria gonorrhoeae cells, has been immobilized to glass. When mixed withN gonorrhoeae cell lysates, the immobilized antibody (IMA) binds the enzyme. Over 70% of the calculated adsorbed activity can be recovered from the immobilized antibody-enzyme (IMA-E) complex. When mixed with bacterial lysates prepared from different organisms having propanediol dehydrogenase-like activity, the IMA specifically adsorbed the enzyme from theN gonorrhoeae lysate. IMA-E complexes have been prepared and their kinetic, temperature and chemical stability, and antigenic properties investigated. These studies demonstrated the feasibility of using an immobilized antibody in the detection of the propanediol dehydrogenase enzyme.