Measurements of electron-transfer rates of charge-storage molecular monolayers on Si(100). Toward hybrid molecular/semiconductor information storage devices

Redox kinetics were measured for two electroactive molecules attached to Si(100) surfaces, a ferrocene (Fc-BzOH) and a Zn(II) trimesitylporphyrin (Por-BzOH). Each molecule was derivatized with a benzyl alcohol linker for attachment to the Si surface via the formation of a Si-O bond. A complete protocol was developed for the preparation of stable Si(100) surfaces derivatized with the electroactive molecules. The redox-kinetic measurements were performed on the resulting Fc-BzOH and Por-BzOH monolayers to probe (1) the rate of electron transfer (k0) for oxidation in the presence of applied potentials and (2) the rate of charge dissipation after the applied potential is disconnected (in the form of a charge-retention half-life t1/2). The k0 values for the two types of monolayers were found to be similar to one another as were the t1/2 values. Perhaps more importantly, the electron-transfer rates for both the Fc-BzOH and the Por-BzOH monolayers differ from the charge-dissipation rates by approximately 6 orders of magnitude and are strongly dependent on the surface concentration of the electroactive species. For the Por-BzOH monolayers on Si(100), the k0 and t1/2 values and their trends as a function of surface coverage were determined to be similar to those previously measured for the analogous thiol-derivatized molecule assembled on Au(111). In contrast, the Fc-BzOH monolayers on Si(100) were found to exhibit much slower electron-transfer and charge-dissipation rates than those in the corresponding thiol-Au(111) case. Two alternative hypotheses are advanced to explain both the diminution in rates with increased surface coverage and the contrasting behavior with the analogous thiols on Au, one based on space-charge effects at the monolayer-solution interface, and a second relying on changes in distance of the redox centers from the surface as modulated by the orientation of the linking chains. Collectively, the ability to prepare and study stable, electroactive molecular media on Si(100) is likely to be key in the development of hybrid molecular/semiconductor devices.     

Single side strapping: a new approach to fine tuning the anion recognition properties of calix[4]pyrroles

Three calix[4]pyrroles bearing m-orcinol-derived diether straps of different lengths on one side of the tetrapyrrolic core have been synthesized and characterized. Structural information for an analogous diester bridged strapped system reported previously (Yoon, D. W.; Hwang, H.; Lee, C. H. Angew. Chem., Int. Ed. Engl. 2002, 41, 1757-1759) is also provided as are bromide and chloride anion affinities for all four systems determined by Isothermal Titration Calorimetry (ITC) in acetonitrile. Although both sets of the strapped calix[4]pyrroles displayed enhanced affinities for chloride and bromide anion, differences were seen among the various receptors that support the conclusion that the anion binding ability of calixpyrrole-type systems can be effectively tuned by modifying the length and nature of the bridging straps. In the specific case of the diether systems, the largest chloride affinity was seen with the shortest strap, whereas the largest affinity for bromide anion was recorded in the case of the longest strap. On the basis of these findings, as well as supporting (1)H NMR spectroscopic studies, it is postulated that not only cavity size per se, but also the ability of the aryl portion of the strap to serve as a CH hydrogen bond donor site are important in regulating the observed anion affinities.     

Transformations of cyclic nonaketides by Aspergillus terreus mutants blocked for lovastatin biosynthesis at the lovA and lovC genes

Two mutants of Aspergillus terreus with either the lovC or lovA genes disrupted were examined for their ability to transform nonaketides into lovastatin 1, a cholesterol-lowering drug. The lovC disruptant was able to efficiently convert dihydromonacolin L 5 or monacolin J 9 into 1, and could also transform desmethylmonacolin J 15 into compactin 3. In contrast, the lovA mutant has an unexpectedly active beta-oxidation system and gives only small amounts of 1 upon addition of the immediate precursor 9, with most of the added nonaketide being degraded to heptaketide 22. Similarly, the lovA mutant does not accumulate the polyketide synthase product 5 and rapidly degrades any 5 added as a precursor via two cycles of beta-oxidation and hydroxylation at C-6 to give 20. The possible involvement of epoxides 21a and 21b in the biosynthesis of 1 was also examined, but their instability in fermentation media and fungal cells will require purified enzymes to establish their role.     

A new method for the stereoselective synthesis of alpha-substituted serine amino acid analogues

[reaction: see text]. A new method was developed for the stereoselective synthesis of alpha-substituted serine amino acids. The strategy utilizes a common enantiomerically enriched intermediate obtained through an enzymatic desymmetrization. A variety of amino acids were synthesized in good ee's through nucleophilic acetylide addition reactions and palladium-catalyzed Sonogashira couplings.     

Dearomatizing cyclization of arylsulfonylalkoxymethyl lithiums: a route to the podophyllotoxin skeleton

[reaction: see text] The phenylsulfonyl group promotes the dearomatizing cyclization of tethered organolithiums onto aromatic rings. With an ether tether, the cyclizations create a new tetrahydrofuran ring, and both cyclization and subsequent electrophilic quenches proceed with high levels of diastereoselectivity. The sulfonyl group can be removed from the cyclized products oxidatively or reductively. The dearomatizing cyclization of a naphthyl sulfone was used in the synthesis of a close structural analogue of podophyllotoxin.     

Randomised circular means of Fourier transforms of measures

We explore decay estimates for circular means of the Fourier transform of a measure on in terms of its -dimensional energy. We find new upper bounds for the decay exponent. We also prove sharp estimates for a certain family of randomised versions of this problem.

     

Strict convexity of some subsets of Hankel operators

We find some extreme points in the unit ball of the set of Hankel operators and show that the unit ball of the set of compact Hankel operators is strictly convex. We use this result to show that the collection of lower triangular Toeplitz contractions is strictly convex. We also find some extreme points in certain reduced Cowen sets and discuss cases in which they are or are not strictly convex.

     

Analytic models for commuting operator tuples on bounded symmetric domains

For a domain in and a Hilbert space of analytic functions on which satisfies certain conditions, we characterize the commuting -tuples of operators on a separable Hilbert space  such that is unitarily equivalent to the restriction of to an invariant subspace, where is the operator -tuple on the Hilbert space tensor product  . For the unit disc and the Hardy space , this reduces to a well-known theorem of Sz.-Nagy and Foias; for a reproducing kernel Hilbert space on such that the reciprocal of its reproducing kernel is a polynomial in and  , this is a recent result of Ambrozie, Müller and the second author. In this paper, we extend the latter result by treating spaces for which ceases to be a polynomial, or even has a pole: namely, the standard weighted Bergman spaces (or, rather, their analytic continuation) on a Cartan domain corresponding to the parameter in the continuous Wallach set, and reproducing kernel Hilbert spaces for which is a rational function. Further, we treat also the more general problem when the operator is replaced by ,  being a certain generalization of a unitary operator tuple. For the case of the spaces on Cartan domains, our results are based on an analysis of the homogeneous multiplication operators on , which seems to be of an independent interest.

     

Some Generic Results on Nonattaining Functionals

We prove that (a) in a reflexive space, for any linearly bounded but unbounded closed convex subset the nonsupport functionals are a dense G subset of the polar set, and (b) any nonsemicoercive proper convex lsc [weak^*-lsc] function in a [dual] Banach space has a generic [dense G ] set of L -perturbations which do not attain their infimum. We also characterize the proper convex functions that have inf-nonattaining L -perturbations. This results also in a criterion for reflexivity.