Faculty of Science - Papers (Archive)

Signs of life: engraved stone artefacts from neolithic South India

Adam R. Brumm — Tue, 19 Mar 2013 21:46:23 PDT

While exceedingly rare on any given archaeological site, engraved stone artefacts have nonetheless been reported from sites covering a range of periods mid regions across the world. Attempts to interpret such engravings have often focused on potential representational or communicative functions, including their role in notational systems, symbolic depiction, and the development of early forms of writing. Contextual and microscopic investigation Of a number of engraved artefacts discovered in a large assemblage of dolerite artefacts excavated from a Neolithic hilltop habitation and stone-tool production site in south India suggests, however, that an alternative interpretation of engraved stone artefacts is possible. Drawing oil ethnographic evidence concerning the perception of stone, and particularly natural markings on stone, this article argues that the stone pieces on which the marks were engraved were more than just passive surfaces for the creation of unrelated stalls. Instead, engravings appear to draw on natural features within and upon the surface of the dolerite, and to suggest an appreciation for the patterns of nature, as well as a lack of distinction between anthropogenic and natural markings. It is argued that the engravings may have been a response to a perceived 'life-force' within the dolerite. The fact that they were produced and then broken apart by knapping suggests that they may have been made to accentuate or attenuate a power that was perceived as either somehow beneficial or in need of careful control.

Stone Axe Technology in Neolithic South India: New Evidence from the Sanganakallu-Kupgal Region, Mideastern Karnataka

Adam R. Brumm — Tue, 19 Mar 2013 16:19:39 PDT

The transition to agriculture—and to settled village life—occurred at different times in various parts of the world. Even within the Indian subcontinent, the Neolithic transition did not occur simultaneously across the entire region; rather, Neolithic ‘‘pockets’’ developed at di¤erent moments in certain key areas within the subcontinent. One such area is the South Deccan Plateau in South India, where the third millennium b.c. saw the development of a novel Neolithic way of life that di¤ered in crucial ways from Neolithic lifeways in other parts of the subcontinent (Allchin 1963). This tradition was marked by a particular focus on cattle and by the appearance of specific, perhaps ritual practices that featured the burning of large quantities of cow dung and the resultant creation of ashmounds in the landscape (Allchin 1963; Boivin 2004). This unique Neolithic tradition, while still relatively poorly understood compared to Neolithic cultures in Europe and the Near East, has much to o¤er prehistorians attempting to understand the changes that led to and accompanied domestication and sedentarization. It also has much to o¤er South Asian scholars who wish to gain a better appreciation of the changes that led to complexity, political economy, and state-level societies in South India (Boivin et al. 2005; Fuller et al. forthcoming). One key requirement for such studies is a better understanding of the material culture changes that attended the Neolithic transition, as well as the subsequent transition from the Neolithic to the Megalithic or Iron Age (see Table 1 for period designations and chronology). Such understanding is currently poor, and this essay o¤ers an attempt to address this lacuna with respect to one particular form of material culture: stone artifacts.

Injection limitations in a series of porphyrin dye-sensitized solar cells

Tracy Dos Santos — Tue, 05 Mar 2013 18:21:19 PST

We report electron injection dynamics for a series of porphyrin sensitized nanocrystalline TiO2 films, comparing zinc and free base porphyrins with a conjugated or nonconjugated linker group to the carboxylate binding group. Injection dynamics are measured used time correlated single photon counting, using dye sensitized ZrO2 control films. These injection dynamics are correlated with molecular orbital calculations, electrochemical data and device photocurrent efficiencies. The injection dynamics, and overall injection efficiency is found to be increased by linker conjugation and by the use of a zinc central metal. The faster injection dynamics for the Zinc porphyrins is shown to be quantitative agreement with the higher singlet excited state energy of these dyes compared to free base porphyrins. For the most efficient dye studied, addition of a typical redox electrolyte to the dye sensitized film is observed to retard the injection dynamics. Moreover studies of sensitized ZrO2 control films indicated that the redox electrolyte resulted in a reduction of excited state lifetime, indicative of an additional decay pathway competing with electron injection. Overall, a close correlation is found between electron injection dynamics and photocurrent efficiency for this series of porphyrin sensitized solar cells, indicating that for such sensitizer dyes, electron injection is a key factor limiting device performance.

Charge transport in dye-sensitized solar cells based on flame-made TiO2 nanoparticles

George Tsekouras — Tue, 05 Mar 2013 18:13:16 PST

The fundamental understanding on charge-transport properties of flame-synthesized TiO2 in dye-sensitized solar cells (DSSCs) is established in this work. By employing a one-step flame spray pyrolysis (FSP), predominantly anatase TiO2 nanoparticles with average nanoparticle sizes between 11 and 36 nm were achieved by varying the rate of combustion enthalpy (through varying liquid precursor feed flow rates) and using either an “openflame” or “enclosed-flame” configuration. Electron diffusion coefficient (D), electron lifetime (τ ), open circuit voltage (Voc ), and capacitance (C) measurements carried out on FSP TiO2 -based DSSCs demonstrated that interband charge trap density decreased with increase in particle size. Compared to earlier studies, interband charge trap density could be controlled more independently of particle size. Under one-sun conditions, relatively high Voc was measured with large particle size due to the lowering of interband charge trap density. This was true despite the associated shorter τ . Comparisons with commercial benchmark Nanoxide-T and Degussa P25 TiO2 were also carried out. The results from the current study have significant implications on the design of TiO2 nanoparticles by flame aerosol techniques, for DSSCs as well as other photoelectrochemical applications. Index Terms—Electron diffusion coefficient

Highly efficient photocathodes for dye-sensitized tandem solar cells

Andrew Nattestad — Tue, 05 Mar 2013 17:11:51 PST

Thin-film dye-sensitized solar cells (DSCs) based on mesoporous semiconductor electrodes are low-cost alternatives to conventional silicon devices1,2. High-efficiency DSCs typically operate as photoanodes (n-DSCs), where photocurrents result from dye-sensitized electron injection into n-type semiconductors. Dye-sensitized photocathodes (p-DSCs) operate in an inverse mode, where dye-excitation is followed by rapid electron transfer from a p-type semiconductor to the dye (dye-sensitized hole injection). Such p-DSCs and n-DSCs can be combined to construct tandem solar cells3 (pn-DSCs) with a theoretical efficiency limitation well beyond that of single-junction DSCs (ref. 4). Nevertheless, the efficiencies of such tandem pn-DSCs have so far been hampered by the poor performance of the available p-DSCs (refs 3, 5–15). Here we show for the first time that p-DSCs can convert absorbed photons to electrons with yields of up to 96%, resulting in a sevenfold increase in energy conversion efficiency compared with previously reported photocathodes7. The donor–acceptor dyes, studied as photocathodic sensitizers, comprise a variable-length oligothiophene bridge, which provides control over the spatial separation of the photogenerated charge carriers. As a result, charge recombination is decelerated by several orders of magnitude and tandem pn-DSCs can be constructed that exceed the efficiency of their individual components.

The effect of molecule size and shape on free charge generation, transport and recombination in all-thiophene dendrimer:fullerene bulk heterojunctions

Attila J. Mozer — Tue, 05 Mar 2013 17:03:59 PST

Charge mobilities of all-thiophene dendrimers were studied using the charge extraction by linearly increasing voltage technique (CELIV). Four dendrimers differing in size, shape and with and without trimethylsilyl (TMS) protecting groups have been compared yielding important structure–property correlations: The hole mobility of the dendrimers with 21 or 42 thiophene units was found to be very similar and around 10^-5 cm² V^-1 s^-1, indicating that shape and size of the studied dendrimers have little effect on their charge transport properties. Hole mobility is decreased by a factor of 3–10 by incorporation of TMS groups, which is attributed to the intrinsically slower electron transfer rates between the TMS-protected dendrimer molecules. Bulk heterojunction solar cells using 1:2 mixtures of the thiophene dendrimers and the electron acceptor [6,6]-phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) were fabricated and their performances were analyzed. The decreased hole mobility in the blend, the unbalanced electron and hole mobility and a rather short charge carrier lifetime measured by charge extraction and transient absorption techniques are thought to be important factors that limit overall power conversion efficiency in theses devices. Improved free charge generation with increasing dendrimer size was observed, which was related to increased separation distance between electron and hole pairs.

Nanoelectrodes: energy conversion and storage

Gordon G. Wallace — Tue, 05 Mar 2013 16:52:50 PST

Nanosized materials are known to take on peculiar properties compared to the bulk material. Their electronic and mechanical properties are known to improve e.g. higher electrical conductivity and greater strength. Their electrochemical redox properties can change dramatically, e.g. in the case of Ag°, the E° value for Ag°→Ag⁺ + e can change by up to half a volt as the particle size decreases. Nanodimensional materials also have an extraordinarily high surface area to volume ratio. All of these properties would bring beneficial effects if they could be retained when the material is assembled into a structure capable of being used as an electrode – nanostructured electrodes. Here we consider selected examples illustrating the importance of nanostructured electrodes in energy conversion (organic solar cells and fuel cells) and storage (batteries and capacitors). These examples involve the use of inorganic as well as organic conducting and semiconducting materials.

Zn-Zn porphyrin dimer-sensitized solar cells: toward 3-D light harvesting

Attila J. Mozer — Tue, 05 Mar 2013 16:34:51 PST

The emulation of the extraordinary photosynthetic light harvesting apparatus has inspired researchers to investigate the assembly and chemistry of a wide variety of covalent and noncovalent porphyrin arrays,1 but surprisingly there have been very few studies of their use in solar cells.2 While the enhancement of solar-toelectrical energy conversion efficiency by porphyrin arrays has been theoretically demonstrated,3 there is little experimental data to support this. In this communication, we demonstrate for the first time that each porphyrin in an array can contribute to current generation in a solar cell.

Synthetic and spectroscopic studies on the structures of uniflorines A and B: structural revision to 1,2,6,7-tetrahydroxy-3-hydroxymethylpyrrolizidine alkaloids

Andrew S. Davis — Mon, 04 Mar 2013 15:32:08 PST

The diastereoselective synthesis of the C-2 epimer and the C-1, C-2 di-epimers of the putative structure of the alkaloid uniflorine A has been achieved. The synthesis of the latter di-epimers employed a novel pyrrolo[1,2-c]oxazin-1-one precursor to allow for the reversal of π-facial diastereoselectivity in an osmium(VIII)-catalyzed syn-dihydroxylation (DH) reaction. The NMR spectral data of these epimeric compounds and that of related isomers did not match that of the natural product. From a comparison of the NMR data of uniflorines A and B with that of casuarine and the known synthetic 1,2,6,7-tetrahydroxy-3-hydroxymethylpyrrolizidine isomers we concluded unequivocally that uniflorine B is the known alkaloid casuarine. Although we cannot unequivocally prove the structure of uniflorine A, without access to the original material and data, the published data suggest that the natural product is also a 1,2,6,7-tetrahydroxy-3-hydroxymethylpyrrolizidine with the same relative C-7–C-7a–C-1–C-2–C-3 stereochemistry as casuarine. We thus suggest that uniflorine A is 6-epi-casuarine.

The components and anticancer activity of the volatile oil from Steblus asper

Weerachai Phutdhawong — Sun, 03 Mar 2013 21:34:24 PST

The volatile oil from fresh leaves of Streblus asper Lour. was isolated by hydrodistillation and analysed through a combination of gas chromatography with FID (GC–FID) and gas chromatography–mass spectrometry (GC–MS). The essential oil was obtained in 0.005% yield as a brown liquid. The major constituents of the volatile oil of S. asper were phytol (45.1%), α-farnesene (6.4%), trans-farnesyl acetate (5.8%), caryophyllene (4.9%) and trans-trans-α-farnesene (2.0%). In addition, the volatile oil showed signiﬁcant anticancer activity (ED₅₀ << 30 µg/ml) from cytotoxicity primary screening tests with P388 (mouse lymphocytic leukaemia) cells but no signiﬁcant antioxidant activity (IC₅₀ values of >>100 µg/ml) in a DPPH radical scavenging assay.

Synthesis of (+/-)epipentenomycin I and III

Weerachai Phutdhawong — Thu, 28 Feb 2013 22:15:50 PST

A synthesis of (±) epipentenomycin I and III is reported from a regioselective epoxidation of racemic 3-hydroxy- and 3-acetoxy-2-methylene-4-cyclopentenone, respectively, with dimethyldioxirane followed by hydrolytic ring-opening of the resulting epoxide.

Chemo-enzymatic synthesis of (-)-epipentenomycin I

Tawesin Klomklao — Thu, 28 Feb 2013 21:46:04 PST

A chemo-enzymatic synthesis of (−)-epipentenomycin I is reported using a lipase-catalysed kinetic resolution of the racemic pentacyclic alcohol 8. Flash vacuum pyroloysis of (−)-8 so obtained gave (−)-(4R)-4-hydroxy-5-methylene-2-cyclopentenone. Epoxidation of this compound with dimethyldioxirane followed by hydrolytic ring-opening of the resulting epoxide gave (−)-epipentenomycin I.

Asymmetric synthesis of polyfunctionalized pyrrolidines and related alkaloids

Stephen G. Pyne — Thu, 28 Feb 2013 21:33:38 PST

This account describes our recent studies on the development of a general method of preparing polyfunctionalized pyrrolidine, indolizidine, pyrrolizidine and pyrrolo[1,2-a]azepines and their related alkaloids.

Phytochemical and larvicidal studies on Stemona curtisii: Structure of a new pyriodo[1,2-a]azepine Stemona alkaloid

Pitchaya Mungkornasawakul — Thu, 28 Feb 2013 21:00:08 PST

A new pentacyclic Stemona alkaloid, stemocurtisinol (3), with a pyrido[1,2-a]azepine A,B-ring system, and the known pyrrolo[1,2-a]azepine alkaloid oxyprotostemonine (4) have been isolated from a root extract of S. curtisii. The structure and relative stereochemistry of stemocurtisinol was determined by spectral data interpretation and X-ray crystallography. This compound is a diastereoisomer of oxystemokerrin and has the opposite configuration at C-4 and C-19. The individual alkaloid components showed significant larvicidal activity (IC₅₀ 4−39 ppm) on mosquito larvae (Anopheles minimus HO).

Stemocurtisine, the first pyrido[1,2-a]azapine Stemona alkaloid

Pitchaya Mungkornasawakul — Thu, 28 Feb 2013 20:29:33 PST

A new pentacyclic stemona alkaloid, stemocurtisine (2), with a novel pyrido[1,2-a]azapine A,B-ring system, has been isolated from a root extract of Stemona curtisii. The structure and relative stereochemistry was determined by spectral data interpretation and X-ray crystallography.

Synthesis of chiral allylic amines via palladium(0) catalysed allylations of allylic carbonates with chiral sulfinamide anions

Zemin Dong — Thu, 28 Feb 2013 20:11:46 PST

The palladium(0) catalysed allylation reactions of allylic carbonates with chiral sulfinamide anions to give unstable allylic sulfinamide products are described. These products are readily converted to stable, chiral N-benzoyl or N-tosyl allylic amine derivatives with poor to modest enantiomeric purities (ee 23-41%).

Phytochemical studies on Stemona burkillii Prain: Two new dihydrostemofoline alkaloids

Pitchaya Mungkornasawakul — Thu, 28 Feb 2013 19:51:54 PST

Two new dihydrostemofoline alkaloids, 11(S),12(R)-dihydrostemofoline (3) and stemoburkilline (4), along with stemofoline (1) and 2’-hydroxystemofoline (2) have been isolated from a root extract of Stemona burkillii Prain. The structure and relative configuration of (3) have been determined via spectroscopic data and from comparison with synthetic 11(S),12(S)-dihydrostemofoline (5). The configuration of the exo-cyclic alkene group in (4) is tentively assigned as (E) based upon mechanistic considerations.

Protecting group effects on the efficiency of the ruthenium-catalyzed Alder-ene reaction

Joseph Hartley — Thu, 28 Feb 2013 19:20:56 PST

The efficiency of the ruthenium-catalysed Alder-ene reaction of hydroxy alkenes depends heavily on the nature of the O-protecting groups employed as well as the length of the carbon spacer between the hydroxy and alkene group.

Synthesis of (+)-(1R,2S,9S,9aR)-octahydro-1H-pyrrolo-[1,2-a]azepine-1,2,9-triol: a potential glycosidase inhibitor

Karl B. Lindsay — Thu, 28 Feb 2013 18:28:56 PST

The title compound was prepared as a potential glycosidase inhibitor. Key steps in the synthesis are vinyl epoxide aminolysis, ring-closing metathesis, cis-dihydroxylation and then ring closure.

Studies on the synthesis of croomine: synthesis of the tricyclic B,C,D-ring core structure

Karl B. Lindsay — Thu, 28 Feb 2013 18:16:24 PST

A convergent and asymmetric synthesis of the tricyclic B,C,D-ring core structure of croomine has been achieved, using aminolysis reactions of chiral vinyl epoxides and the RCM reaction.