Optically Active Polysilanes

Optically Active Polysilanes with Diastereomeric Helidties Containing Opposite Screw Senses and Different Screw Pitches... 

Helical polysilanes whose optical activity is induced by chiral side chains are particularly suitable chiroptical polymers for elucidating the inherent nature of the polymer helix since they embody a fluorophoric and chromophoric main chain, exhibiting intense UV, CD, and FL bands due to the Sia-Sia ... 

Chart 4.2 Wurtz-type reduction preparation of optically active polysilanes. 

The cut-and-paste technique has two steps (i) M (or P) screw-sense-selective photolysis at the longer kmax region in CC14 to form P (or M) screw sense telomers with Si-Cl termini, and (ii) Na-mediated recondensation of the telomers in hot toluene. Formation of an optically active helical polysilane with an almost single screw sense was evidenced by significant changes in the UV and CD spectra after the cut-and-paste technique, as shown in Figure 4.6b. 

This knowledge and understanding may be helpful to characterize local conformations of other optically active polysilanes in solution. For example, poly(methyl-(-)-(3-pinanylsilane) [(+)-7 Mw = 10,200] prepared by Shinohara and co-workers.281 showed a bisignate CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at 15°C. Since the spectroscopic features are quite similar to those of i,28d-28e it is possible that the main chain in 7 may contain diastereomeric helical motifs with opposite screw senses and different screw pitches. 

Since for the diarylpolysilanes, as for all polysilanes, the main chain is chromophoric due to the electronic transition between the delocalized silicon a and a orbitals, the electronic spectroscopies of CD, UV-Vis, and FL are particularly powerful probes of the structures of these materials. The magnitude of dimensionless quantity, gabs, is perhaps the most useful in comparing the CD spectra of optically active poly(diarylsilane)s. 

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