Chemical Science Blog

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.