Chemical Science Blog

In this Chemical Science Edge Article, the Anderson group and colleagues at the Universitiy of Oxford describe ultra-fast light harvesting materials which function in a similar way to various natural light harvesters, like, for example, those found in the chlorophyll assemblies of purple bacteria. These materials represent excellent candidates for use in next generation carbon based solar cells. 

The materials, which may contain up to 24 porphyrin units separated by conjugating butadiyne bridges, can measure up to 10nm in diameter. Recent advances in template directed synthesis mean these molecules have become more accessible.

Barriers to energy delocalisation are overcome due to distortions that occur in the molecular structure. A rigidifying template was used to probe the effect of distortions – without a coordinating constraint present, significantly different behaviour was observed, underlying the importance of flexibility to the behaviour observed.

24 prophyrin containing nanoring, and an example of a 6 unit ring containing a rigidifying template

Physical techniques were used to characterise the complex phenomena being observed, including time resolved photoluminescence spectroscopy, using femtosecond LASERs and steady state fluorescence. Further information about electronic structure was gained by comparing spectra of the ring structures with those of  linear oligomeric analogues. 

The authors describe synthetic materials which show a level of light harvesting and rapid energy delocalising ability, usually only seen in natural systems. The promise of technological applications which exploit these properties will drive the study of the fundamental physics and chemistry of such fascinating systems. 

Read this Chemical Science Edge Article today: 

Ultrafast Delocalisation of excitation in synthetic light-harvesting nanorings
Chaw-Keong Yong, Patrick Parkinson, Dmitry V. Kondratuk, Wei-Hsin Chen, Andrew Stannard, Alex Summerfield, Johannes K. Sprafke, Melanie C. O’Sullivan, Peter H. Beton, Harry L. Anderson and Laura M. Herz.
DOI: 10.1039/C4SC02424A

In this Chemical Science Edge article, Michael Ward, Christopher Hunter and co-workers at the Department of Chemistry, University of Sheffield report tunable host-guest chemistry of an octanuclear cobalt(II) coordination cage with a range of organic guests. The resultant non-covalently bound guests have very different binding constants in different pH environments, making binding tunable and completely reversible.   

The team found that the coordination cage provided a good guest space for a range of organics, including substituted adamantanes. Substitutents which could be charged, such as carboxylic acids and amines, were particularly useful to drive and rationalize the binding observed. NMR also played a powerful role in this work. In this context, the paramagnetic complex host resulted in clear distinguishable signals, differentiating free or bound cage species, and thereby acting as a shift reagent. This effect could also be quantified, allowing the strength and extent of the host-guest binding to be shown. 

Coordination cage and NMR study of effect on chemical shift and binding extent with changing pH

An example with potentially high importance is described using 1-aminoadamantane, a prescription drug used to treat Parkinson’s disease and as an influenza anti-viral. Complete reversible binding was achieved with a binding constant difference of three orders of magnitude between the protonated and neutral forms of the drug. Similar behaviour was observed with other N-basic materials like nicotine and the anaesthetic substituted imidazole drug, demotidine. Mechanistic considerations were further examined with experiments using carboxylic acids, where solvation effects were also shown to play a key role.  

In this article, a practical example of pH controlled, reversible guest binding of functional organic molecules is described. An elegant and practical application of NMR spectroscopy is also shown. Applications for delivery and controlled release of suitable drug, catalyst or other functional organic material cannot be far off.   

Read this Chemical Science Edge article today!   

pH dependent binding of guests in the cavity of a polyhedral coordination cage: reversible uptake and release of drug molecules
William Cullen, Simon Turega, Christopher A. Hunter and Michael D. Ward.,
doi 10.1039/c4sc02090a

Dye sensitized solar cells (DSSCs) are photovoltaic devices which use a sensitizer in combination with a photosensitive material and a wide band gap semiconductor to produce an electric current. Thin-film DSSCs can offer packaging and application opportunites, have potential as functional coatings, and exhibit high power conversion efficiencies (PCE) of up to 12-13%.

Current state-of-the-art dye sensitized solar cells are usually solid phase systems, removing any issues surrounding leakage of components or the corrosive nature of some electrolytes. In such configurations a constituent known as a hole transport material (HTM) is now commonly used instead of a liquid electrolyte.

In this Chemical Science Edge article, authors from the Energy Research Institute, Nanyang, and the Nanyang Technological University, Singapore, give details of a group of triptycene based molecules which, they claim, perform on a comparable level with the current best in class HTM, (spiro-OMeTAD). The materials described were prepared from a starting tri-iodotryptycene material which was derivatised using a variety of transformations including Stille and Suzuki coupling reactions. The materials contain diphenylamine moieties separated from the triptycene core by aromatic spacers, enabling charge transfer.

These materials were fabricated in a complex process into a typical DSSC, containing fluorine doped tin oxide, semiconducting titanium dioxide and lead iodide perovskite dye sensitizer material. Power conversion efficiencies are comparable to devices comprimising the best performing HTMs. For the T102 andT103 derivatives, PCEs were reported as 12.24 and 12.38%, comparable to devices fabricated with spiro-OMeTAD (12.78%). Without a HTM present the number falls to less than 5%.

This article presents details of promising HTM materials which may give rise to significant economic advantages for the production and commercialisation of solid state DSSCs, with current best in class performance levels.

Read this HOT Chemical Science Edge Article for free* today:

Novel hole transporting materials based on triptycene core for high efficiency mesoscopic perovskite solar cells

Anurag Krishna, Dharani Sabba, Hairong Li, Jun Yin, Pablo P. Boix, Cesare Soci, Subodh G. Mhaisalkar and  Andrew C. Grimsdale
DOI: 10.1039/C4SC00814F

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Carbon nanotube (CNT) fragments are exciting materials for the fields of supramolecular chemistry and nanotechnology. This is due to their tunable optical and physical properties, as well as their potential for host-guest chemical interactions. The authors from Boston and Drexel Universities and co-workers, report here for the first time, the synthesis of the radical cation of [8]-cycloparaphenylene, prepared by reaction of [8]-cycloparaphenylene with the oxidant triethyloxonium hexachloroantimonate (Et3O+SbCl6-
). A vivid colour change, from yellow to orange to deep purple, accompanied the oxidation. The product remained stable when dry for several days, and was readily reduced back to [8]-cycloparaphenylene on reaction with zinc dust.

Surprisingly, electron paramagnetic resonance (EPR) experiments on solutions of the radical cation, did not give detailed information, other than a characteristic signal for one unpaired electron.  The material also proved difficult to crystallise in a pure form. Therefore, the focus shifted to photophysical, electrochemical and theoretical properties. As seen in the figure above, on the right, the radical cation of [8]-cycloparaphenylene has two major absorptions at at 535 and 1115nm, which follow closely the values determined by density functional calculations (DFT), and are characteristically different to the parent neutral material.

Theoretical calculations also suggest a change to a highly delocalised structure in the radical cation and its dimer with the neutral compound, compared to benzene like character in [8]-cycloparaphenylene. This should prove useful for potential applications in electronic and photovoltaic devices. Detailed results from computational studies on the electronic structures of the radical cation of [8]-cycloparaphenylene ([8]-CPP) and its resonance dimer, as well as the 6, 10 and 12 ring-containing ‘hoops’ are given. This article sheds valuable new light on the properties of  intra- and inter-molecularly delocalised systems based on cycloparaphenylenes.

Read this HOT Chemical Science Edge Article today!

Photophysical and theoretical investigations of the [8]cycloparaphenylene radical cation and its charge-resonance dimer
Matthew R. Golder, Bryan M. Wong and Ramesh Jasti
Chem. Sci., 2013, Advance Article
DOI: 10.1039/C3SC51861B

Kevin Murnaghan is a guest web-writer for Chemical Science. He is currently a Research Chemist in the Adhesive Technologies Business Sector of Henkel AG & Co. KGaA, based in Düsseldorf, Germany. His research interests focus primarily on enabling chemistries and technologies for next generation adhesives and surface treatments. Any views expressed here are his personal ones and not those of Henkel AG & Co. KGaA.

In this Chemical Science Edge article, researchers from the Center for Research in Biological Chemistry and Molecular Materials (CIQUS) and Department of Organic Chemistry at the University of Santiago de Compostela, describe a highly elegant method of controlling and changing the helical sense and elongation of poly(phenylacetylene)s.  This is achieved via modifying the polymer backbone with a chiral group which responds to different solvent environments in a reliable way.  The system seems ideal for chemical sensor technology development.

The polymer highlighted in the article is a poly(phenylacetylene) derivatised with a chiral methoxytrifluoro-phenylacetic acid (MTPA) group.  The monomer was prepared from the alkyne 4-ethynylaniline, (R)-α-methoxy-α-(trifluoromethyl)phenylacetic acid and oxalyl chloride in two steps.  Subsequent polymerization was carried out using a rhodium norbornadiene dimer catalyst, under argon, and the polymer was precipitated from a tetrahydrofuran (THF) solution using methanol and hexane.

The key to the effects described in changes to the polymer properties is due to the chemical structure of the pendant group, which contains an amide and a chiral centre, giving rise to four stereoisomers.  The amide group function can result in cis or trans geometry, and the carbonyl and chiral centre yield syn (sp) or anti-periplanar (ap) conformers.

In a range of solvents, the resultant 4 states of this moiety have profound and different effects on the elongation and the sense (or direction), and tightness (compression or elongation), of the helical polymer.  The effect is shown here:

Solvent donor effects which destabilise the amide group to a cis orientation were proven by UV-Vis spectroscopy to elongate and change the sense of the helicate.  A bathochromic shift was observed.  The authors suggest that solvent polarity plays a greater role with the carbonyl-methoxy group in changing the sense (or direction) of the helicate, via circular dichroism measurements. Overall, any destabilisation in the pendant group is accomodated in the polymer backbone, via a change in the orientation and elongation of the helicate, resulting in a new stable state.

Sequential stimulation of the two functional groups was performed, as were experiments using thin films of the polymer. A large amount of analytical data: IR, NMR, UV-Vis, Raman spectroscopy and differential scanning calorimetry (DSC) is presented, as are atomic force microscopy (AFM) measurements and findings from molecular mechanics calculations.

This work should prove of interest to polymer chemists and sensor researchers alike, and to the wider scientific community.

Controlled Modulation of the Helical Sense and Elongation of Poly(phenylacetylene)s by Polar and Donor Effects
Ricardo Riguera, Felix Freire, Seila Leiras, José Manuel Seco and Emilio Quiñoá
Chem. Sci., 2013, Accepted Manuscript
DOI: 10.1039/C3SC50835H

Kevin Murnaghan is a guest web-writer for Chemical Science. He is currently a Research Chemist in the Adhesive Technologies Business Sector of Henkel AG & Co. KGaA, based in Düsseldorf, Germany. His research interests focus primarily on enabling chemistries and technologies for next generation adhesives and surface treatments. Any views expressed here are his personal ones and not those of Henkel AG & Co. KGaA.