Control of the site and potential of reduction and oxidation processes in pi-expanded quinoxalinoporphyrins

Quinoxalino[2,3-b']porphyrins are pi-expanded porphyrins, having a quinoxaline fused to a beta,beta'-pyrrolic position of the porphyrin. They are used as components in systems proposed as 'molecular wires'. Knowledge of their redox properties is of value in the design of electron- or hole-conduction systems. In particular, the location of the charge density in the radical anions of quinoxalinoporphyrins can be modulated by peripheral functionalization. New theoretical treatments of electrochemical potentials are developed that identify the site of reduction in both the anions and the dianions of 33 quinoxalinoporphyrins. These molecules include free-base and metallated macrocycles substituted on the quinoxaline with electron-withdrawing groups (NO2, Cl, Br) and/or electron-donating groups (NH2, OCH3). Spectroelectrochemistry, density-functional theory calculations, and substituent-parameter models are used to verify the analysis. Five distinct patterns are observed for the locations of the first and second reductions; some of these patterns involve delocalized charges. Nitroquinoxalinoporphyrins with the nitro groups at the 5- and 6-quinoxaline positions are found to have quite different properties owing to distortions caused by peri interactions that force the nitro group of the 5-nitro regioisomer out of conjugation. Charge localization on the nitroquinoxaline fragment is found for some molecules, and this is attributed to ion-pairing with the 0.1 M tetrabutylammonium perchlorate electrolyte used, leading to the verified prediction that electron-paramagnetic resonance spectra of these molecules taken without the electrolyte yield delocalized anions. These properties enable the control of conduction through molecular wires synthesised from quinoxalinoporphyrins.     

Porphyrin-naphthodiimide interactions as a structural motif in foldamers and supramolecular assemblies

pi-pi Stacking interactions between electron deficient naphthalenediimides (NDI) and electron-rich porphyrins (POR) leading to charge transfer are shown to be prevalent in linked NDI-POR and POR-NDI-POR structures. For flexibly-linked systems, intramolecular interactions lead to S-shaped foldamers in solution, whereas intermolecular association is predominant in more rigid systems. The foldamer structures can be interrupted by competing aromatic solvents, by six-coordination of metallated porphyrin derivatives, by protonation of the free base porphyrin in non-metallated structures, and in facially sterically hindered porphyrins.     

Monitoring the thermodynamically-controlled formation of diimide-based resin-attached rotaxanes by gel-phase HR MAS 1H NMR spectroscopy

The thermodynamically controlled self-assembly of rotaxane and pseudorotaxane systems consisting of (i) a naphthodiimide thread unit terminated at one end with a pyridine ligand, and covalently linked at the other to a gel-phase polystyrene resin support, (ii) a dinaphtho-crown ether shuttle unit, and (iii) a ruthenium carbonyl metalloporphyrin stopper unit, is investigated by high resolution magic angle spinning proton (HR MAS 1H) NMR spectroscopy. The effects of variable concentration of the solution-phase components, the temperature, and added Li+ and Na+ ions are described, and the limitations of the technique are addressed. The dynamic behaviour is compared directly to the solution-phase analogues, where a bulky stopper group is substituted for the polystyrene resin bead.     

Portable, low-cost NMR with laser-lathe lithography produced microcoils

Nuclear Magnetic Resonance (NMR) is unsurpassed in its ability to non-destructively probe chemical identity. Portable, low-cost NMR sensors would enable on-site identification of potentially hazardous substances, as well as the study of samples in a variety of industrial applications. Recent developments in RF microcoil construction (i.e. coils much smaller than the standard 5mm NMR RF coils), have dramatically increased NMR sensitivity and decreased the limits-of-detection (LOD). We are using advances in laser pantographic microfabrication techniques, unique to LLNL, to produce RF microcoils for field deployable, high sensitivity NMR-based detectors. This same fabrication technique can be used to produce imaging coils for MRI as well as for standard hardware shimming or "ex-situ" shimming of field inhomogeneities typically associated with inexpensive magnets. This paper describes a portable NMR system based on the use of a 2 kg hand-held permanent magnet, laser-fabricated microcoils, and a compact spectrometer. The main limitations for such a system are the low resolution and sensitivity associated with the low field values and quality of small permanent magnets, as well as the lack of large amounts of sample of interest in most cases. The focus of the paper is on the setting up of this system, initial results, sensitivity measurements, discussion of the limitations and future plans. The results, even though preliminary, are promising and provide the foundation for developing a portable, inexpensive NMR system for chemical analysis. Such a system will be ideal for chemical identification of trace substances on site.     

Structural induced control of energy transfer within Zn(II)-porphyrin dendrimers

We report on a study of singlet-singlet annihilation kinetics in a series of Zn(II)-porphyrin-appended dendrimers, where the energy transfer efficiency is significantly improved by extending the molecular chain that connects the light-harvesting chromophores to the dendrimeric backbone with one additional carbon. For the largest dendrimer having 64 Zn(II)-porphyrins, only approximately 10% of the excitation intensity is needed in order to observe the same extent of annihilation in the dendrimers with the additional carbon in the connecting chain as compared to those without. Complete annihilation, until only one chromophore remains excited, now occurs within subunits of seven chromophores, when half of the chromophores are excited. The improvement of the annihilation efficiency in the largest dendrimer with 64 porphyrins can be explained by the presence of a the two-step delayed annihilation process, involving energy hopping from excited to nonexcited chromophores prior to annihilation. In the smallest dendrimer with only four chromophores, delayed annihilation is not present, since the direct annihilation process is more efficient than the two-step delayed annihilation process. As the dendrimer size increases and the chances of originally exciting two neighboring chromophores decreases, the delayed annihilation process becomes more visible. The additional carbon, added to the connecting chain, results in more favorable chromophore distances and orientations for energy hopping. Hence, the improved energy transfer properties makes the Zn(II)-porphyrin-appended dendrimers with the additional carbon promising candidates as light-harvesting antennas for artificial photosynthesis.     

Exhaustive glycosylation, PEGylation, and glutathionylation of a [G4]-ene(48) dendrimer via photoinduced thiol-ene coupling

We report in this paper the use of free-radical thiol-ene coupling (TEC) for the introduction of carbohydrate, poly(ethylene glycol), and peptide fragments at the periphery of an alkene functional dendrimer. Four different sugar thiols including glucose, mannose, lactose and sialic acid, two PEGylated thiols and the natural tripeptide glutathione were reacted with a fourth generation alkene functional dendrimer [G4]-ene(48) upon irradiation at λ(max) 365 nm. In all cases, the (1)H NMR spectra of the crude reaction mixture revealed the complete disappearance of alkene proton signals indicating the quantitative conversion of all 48 alkene groups of the dendrimer. With one exception only, all dendrimer conjugates were isolated in high yields (70-94%), validating the high efficiency of multiple TEC reactions on a single substrate. All isolated and purified compounds were analyzed by MALDI-TOF spectrometry and gave spectra consistent with the assigned structure.     

Rapid method for determination of 228Ra in water samples

A new rapid method for the determination of ²²⁸Ra in natural water samples has been developed at the SRNL/EBL (Savannah River National Lab/Environmental Bioassay Laboratory) that can be used for emergency response or routine samples. While gamma spectrometry can be employed with sufficient detection limits to determine ²²⁸Ra in solid samples (via ²²⁸Ac), radiochemical methods that employ gas flow proportional counting techniques typically provide lower minimal detectable activity levels for the determination of ²²⁸Ra in water samples. Most radiochemical methods for ²²⁸Ra collect and purify ²²⁸Ra and allow for ²²⁸Ac daughter ingrowth for ~36 h. In this new SRNL/EBL approach, ²²⁸Ac is collected and purified from the water sample without waiting to eliminate this delay. The sample preparation requires only about 4 h so that ²²⁸Ra assay results on water samples can be achieved in <6 h. The method uses a rapid calcium carbonate precipitation enhanced with a small amount of phosphate added to enhance chemical yields (typically >90 %), followed by rapid cation exchange removal of calcium. Lead, bismuth, uranium, thorium and protactinium isotopes are also removed by the cation exchange separation. ²²⁸Ac is eluted from the cation resin directly onto a DGA Resin cartridge attached to the bottom of the cation column to purify ²²⁸Ac. DGA Resin also removes lead and bismuth isotopes, along with Sr isotopes and ⁹⁰Y. La is used to determine ²²⁸Ac chemical yield via ICP-MS, but ¹³³Ba can also be used instead if ICP-MS assay is not available. Unlike some older methods, no lead or strontium holdback carriers or continual readjustment of sample pH is required.     

Rhodium-catalysed hydroboration employing new Quinazolinap ligands; an investigation into electronic effects

As part of an ongoing effort to improve the efficiency and substrate scope of our Quinazolinap ligand series in the rhodium-catalysed asymmetric hydroboration of vinyl arenes, 2-(p-trifluoromethylphenyl)-Quinazolinap and 2-(p-methoxyphenyl)-Quinazolinap have been synthesised and resolved in good yield. These, along with the previously reported 2-(2-pyridyl)-Quinazolinap and 2-(2-pyrazinyl)-Quinazolinap, form part of an electronic series of Quinazolinap ligands synthesised in order to explore electronic effects in this ligand class. The application of this series of ligands to the rhodium-catalysed asymmetric hydroboration of a range of vinylarenes is described. Good conversions and regioselectivities as well as excellent enantioselectivities up to 97% were obtained. 2-(p-Methoxyphenyl)-Quinazolinap demonstrated consistently high enantioselectivities in the hydroboration of sterically demanding vinylarenes.