Julian R. Koe

Chiroptical inversion in helical Si-Si bond polymer aggregates.

To elucidate the factors involved in the chiroptical properties of polymer aggregates composed of helical building blocks, a series of rigid rod helical poly[alkyl-(S)-2-methylbutylsilane]s (achiral alkyl side chains = ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl) have been investigated. It was found that the chiroptical sign in the circular dichroism (CD) spectra of the polysilane aggregates depends on the achiral side chain length and cosolvent fraction. Concerning the achiral side chains, the n-propyl group was of a critical length for solvent-dependent chiroptical inversion on aggregation. This unique side chain length-dependent chiroptical inversion was theoretically predictable by using the novel approach of combining the cholesteric hard-core model and exciton chirality method. The latter was also investigated theoretically by Gaussian 03 (TD-DFT, B3LYP, 6-31G(d) basis set) calculations applied to two spatially arranged helical Si-Si bonded decamer models.

Thermo-driven chiroptical switching polysilane featuring 2-cyclopentylethyl side group

A new rod-like helical polysilane, poly{(S)-3,7-dimethyloctyl-(2-cyclopentylethyl)silane}, was found to undergo a thermo-driven, helix-helix transition at –33 ° C in isooctane associated with the discontinuous changes in the Siσ -Siσ *transition energy and intensity in the transition temperature region. This is the first example of a helix-helix transition polysilane with a cycloalkyl group. A similar rod-like polysilane derivative, poly{(S)-3,7-dimethyloctyl-(1-cyclopentylmethyl)silane}, however, did not undergo any helix-helix transition between –61 and 80 ° C.

Optically Active Silicon-Containing Polymers

Optically active chiral macromolecules have been around since the dawn of time and indeed our whole universe, from atoms upwards, is chiral [1] In biological systems, at least, it is not the presence of optical activity which is remarkable, but rather its absence. DNA is a classic example of a chiral macromolecule, its chirality deriving from two features: (i) the incorporation of chiral sugars (to which are attached chromophoric bases such as adenine, guanine, cytosine and thymine) and (ii) the macromolecular helical conformation arising from base stacking in hydrogen-bonding solvents (a helix is a chiral motif). The task of covalently linking small molecules to form well defined, single screw sense, rigid helical rod polymers with a single molecular weight is a longstanding issue in modern polymer stereochemistry [2]. Such polymers are usually produced only during the course of precisely controlled polymerisation reactions using very specialised monomers and stereospecific catalysts [3]. The synthesis and quantitative conformational analysis by direct spectroscopic characterisation of such ideal polymers, therefore, are very challenging [4]. Synthetic polymers are non-ideal, however, comprising a mixture of molecular weights and stereoisomers and the most prominent properties of the ideal polymer remain a challenge. Synthetic polymers containing enantiopure chiral side groups including polyisocyanides [5], polyisocyanates [6], polyacetylenes [7], polythiophenes [8] poly(p-phenylenevinylene)s [9] and polysilanes, [10] may also adopt preferential screw sense (PSS) helical backbone conformations because of side group interactions. Concerning the analysis of optical active materials, there are several techniques available: optical rotation (rotation of the plane of linearly polarised light on passing through the sample), ellipticity (almost never measured directly), single crystal X-ray crystallography (when crystals can be grown) and circular dichroism (CD; differential absorption of left and right circularly polarised light). For the purposes of structure elucidation, the last two techniques provide the most information, but in the case of most macromolecules, X-ray crystallography is not feasible due to the lack of suitable crystals. Thus, the most appropriate technique for the analysis of optically active polymers is CD spectroscopy, which permits the direct analysis of chiral backbone physical and electronic structures.

Temperature-dependent helix-helix transition of an optically active poly (diarylsilylene)

The poly(diarylsilylene) copolymer mainchain helix in (Ar*2Si)x(Ar2S i)1−x [Ar* = 3-(S)-2-methylbutylphenyl, Ar = 4-butylphenyl, x = 0.2] undergoes a thermally driven inversion of helical screw sense with a transition temperature of −10 °C.