Intramolecular C-H/C-D exchange in cofacially stacked polyfluorenes via electron-induced bond activation

EPR studies in hexamethylphosphoramide, along with DFT studies, reveal that the one-electron reduction of pi-stacked polyfluorenes containing two, three, and four fluorene units leads to the corresponding anion radicals where the odd electron is located exclusively on an outside fluorene moiety. These anion radicals disproportionate to form small concentrations of diamagnetic dianions, wherein there is a nonclassical pz overlap that crosses the C2 axis. The presence of the two extra electrons activates the C-H bonds in the ortho positions, and concomitant overlap of the p-orbitals involving adjacent fluorene moieties results in intramolecular hydrogen exchange at temperatures as low as 90 K.     

A neutron scattering study of the structure and water partitioning of selectively deuterated copolymer micelles

We present a scattering study of a selectively deuterated micelle-forming diblock copolymer. The copolymer comprises a partially deuterated polystyrene (d,h-PS) block and an imidazolium-functionalized PS (IL) block. In toluene solutions, the copolymers assemble into elongated micelles where the IL block forms the micelle core. Through dynamic light scattering (DLS) measurements, we obtain the overall size of the micelles. In our small-angle neutron scattering (SANS) studies, we use contrast matching to characterize the IL core and the PS shell of the micelles independently. The PS block forming the micelle shell exhibits either a starlike or brushlike conformation depending upon the size of the core to which it is tethered. We find the IL block to be in an extended conformation, driving the formation of slightly elongated and relatively stiff micelle cores. The elongated micelle core cross-sectional radius and length depend linearly on the length of the IL block. We find that the micelles can sequester a few water molecules for each IL repeat unit; the addition of water slightly increases the cross section of the elongated micelles.     

Single-electron entrapment of [8]annulyne, biannulenylenes, and an annulenoannulene

The low-temperature (-100 degrees C) dehydrohalogenation of bromocyclooctatetraene followed by immediate electron-transfer yields a stable solution of the [8]annulyne anion radical. If the unstable [8]annulyne is reacted with itself, cyclobutadiene, or benzyne, the respective bi-[8]annulenylene, [6]annuleno[8]annulene, or [6]-[8]annulenylene can be trapped as their anion radicals via one-electron transfer. These condensation products were all obtained from simple [2 + 2] cycloaddition reactions. B3LYP/6-31G geometry optimizations were carried out, and the calculated spin densities were compared to the EPR spectral results obtained for the anion radicals of [6]annuleno[8]annulene, [8]annulyne, bi[8]annulenylene, and [6]-[8]annulenylene, and excellent agreement has been realized. This simple "one-pot" approach should be applicable to a wide range of such systems.     

Demonstrating the feasibility of monitoring the molecular-level structures of moving polymer/silane interfaces during silane diffusion using SFG

In this paper, the feasibility of monitoring molecular structures at a moving polymer/liquid interface by sum frequency generation (SFG) vibrational spectroscopy has been demonstrated. N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (AATM, NH2(CH2)2NH(CH2)3Si(OCH3)3) has been brought into contact with a deuterated poly(methyl methacrylate) (d-PMMA) film, and the interfacial silane structure has been monitored using SFG. Upon initial contact, the SFG spectra can be detected, but as time progresses, the spectral intensity changes and finally disappears. Additional experiments indicate that these silane molecules can diffuse into the polymer film and the detected SFG signals are actually from the moving polymer/silane interface. Our results show that the molecular order of the polymer/silane interface exists during the entire diffusion process and is lost when the silane molecules traverse through the thickness of the d-PMMA film. The loss of the SFG signal is due to the formation of a new disordered substrate/silane interface, which contributes no detectable SFG signal. The kinetics of the diffusion of the silane into the polymer have been deduced from the time-dependent SFG signals detected from the AATM molecules as they diffuse through polymer films of different thickness.     

12]Annulynes

Only one isomer of o-benzyne ([6]annulyne or 1,2-didehydrobenzene) exists, but the dehydro analogue of the "ring-opened double benzene", [12]annulyne, was generated in several isomeric forms. 1,5-Hexadiyne undergoes self-condensation in the presence of potassium tert-butoxide to yield two isomers of [12]annulyne (3,11-di-trans-[12]annulyne and 5,9-di-trans-[12]annulyne), both of which exhibit a weak paratropic ring current in their 1H NMR spectra and are oxygen sensitive. They can be reduced to their respective dianions, which are diatropic. A third isomer (3,9-di-trans-[12]annulyne) was generated via the complete dehydrohalogenation of hexabromocyclododecene and found to be much less stable but can be tamed via one- or two-electron reduction. A tight association of the cation (K+) with the p(y)-orbitals within the alkyne moiety results in an unusually low-field resonance for an adjacent external proton.     

Classification of line-transitive point-imprimitive linear spaces with line size at most 12

In this paper we complete a classification of finite linear spaces with line size at most 12 admitting a line-transitive point-imprimitive subgroup of automorphisms. The examples are the Desarguesian projective planes of orders 4, 7, 9 and 11, two designs on 91 points with line size 6, and 467 designs on 729 points with line size 8.      

SERRS and visible extinction spectroscopy of copper chlorophyllin on silver colloids as a function of pH

A study of sodium copper chlorophyllin adsorbed on silver colloids (CuChl/Ag) is conducted using surface-enhanced resonance Raman scattering (SERRS) and visible extinction spectroscopy to examine how the system changes as a function of pH. Initially at basic pH, SERRS signal is not detected even though CuChl is adsorbed onto the silver surface and deprotonated. Upon decreasing the solution pH slightly, colloidal aggregation is induced, evidenced by the broadening of the visible extinction spectra. The larger aggregates possess a surface plasmon that is in resonance with the laser excitation frequency (633-nm) and SERRS signal is detected. As the acid protonates CuChl, the overall negatively-charged surface approaches neutrality which induces more aggregation. Complete protonation of CuChl by pH 4.6 results in colloidal precipitation. However, when aggregation is halted about pH 5, adding NaOH(aq) to the system maintains the extent of aggregation and an intense SERRS signal is detected at basic pH.     

Permutation groups,simple groups,and sieve methods

We show that the number of integersn≤x which occur as indices of subgroups of nonabelian finite simple groups, excluding that ofA _n−1 inA _n , is ∼hx/logx, for some given constanth. This might be regarded as a noncommutative analogue of the Prime Number Theorem (which counts indicesn≤x of subgroups of abelian simple groups). We conclude that for most positive integersn, the only quasiprimitive permutation groups of degreen areS _n andA _n in their natural action. This extends a similar result for primitive permutation groups obtained by Cameron, Neumann and Teague in 1982. Our proof combines group-theoretic and number-theoretic methods. In particular, we use the classification of finite simple groups, and we also apply sieve methods to estimate the size of some interesting sets of primes. Research partially supported by the Australian Research Council for C.E.P. and by the Bi-National Science Foundation United States-Israel Grant 2000-053 for A.S.     

Practical synthesis of (Z)-polyaromatic and heteroaromatic vinylacetylenes

[reaction: seet text] Two synthetic routes to several (Z)-polyaromatic and heteroaromatic substituted vinylacetylenes are described. The nature of aryl- or heteroaryl-substituted carboxaldehyde used as starting material dictated the choice of Wittig salt employed. A very attractive way to construct polyaromatic and pyridine-containing enynes is the reaction of polyaromatic and pyridine-containing aldehydes with bromomethyltriphenylphosphonium bromide in the presence of potassium tert-butoxide followed by a Sonogashira desilylation procedure (method B).