A Correlation between Global Conformation of Polysilane and UV Absorption Characteristics

Abstract

When a mobile backbone conformation of π-and σ-conjugating polymers is strongly coupled with an electronic structure, changes in UV-vis absorption spectra and/or chiroptical properties are observed phenomenologically as thermochromism, solvatochromism, piezochromism, electrochromism, and/or mutarotation. 1,2 Although polysilanes, being σ-type chromophores, exhibit these phenomena to a remarkable degree, their structural origins are controversial because of the limited information available on the relationship among the conformational properties of the Si backbone, excited electronic state, and actual optical characteristics. 2 We found empirically for various polysilanes in THF at 30°C that the lowest excitonic backbone peak intensity per silicon repeating unit, (Si unit/dm 3 ) -1 ‚cm -1 , increases exponentially as the viscosity index, R, increases. The R value relates primarily to the conformational properties of the chain 3 and therefore in polysilanes to the mean free path of a photoexcited electron-hole pair. This correlation would be helpful for discussing the conformational change of polysilane in any condition. It would also be useful for predicting the lower and upper limits of the peak intensity for the respective globular and perfectly extended rod polysilanes.Both optically active and inactive polysilanes, poly(R 1 R 2 Si), have excellent optical characteristics for studying such conformational structures in solution, because it is known that the characteristics of the lowest excitonic Si(σ)-Si(σ*) absorption at 3-4 eV varies sensitively with the helical torsion angle, segmentation, and regularity of the helix. 4-6 On the basis of this understanding, we recently obtained various optically active and inactive polysilanes bearing chiral and achiral alkyl and/or phenyl pendants by Wurtz condensation of the corresponding substituted dichlorosilanes. The main features of these polysilanes in THF at 30°C are that the value is broadly distributed from 5500 to 57 500 and the peak energy ranges from 3.5 to 4.2 eV (λ max ) 290-352 nm), while the R value ranges widely from 0.51 to 1.35.Twenty-one polysilane samples were examined in this study: seven optically active poly(dialkylsilane)s with four different types of chiral -, γ-, or δ-branched alkyl substituents; eleven optically inactive poly(dialkylsilane)s with six different types of achiral -, γ-, or δ-branched substituents; an optically inactive poly(dialkylsilane) copolymer prepared from a racemic chiral monomer; and two optically inactive poly(alkylphenylsilane)s. These polysilanes were classified as having shrink coil, flexible coil, stiff, and rigid rodlike conformations from their R value 3 and 29 Si-NMR line width. 6d A shrink form with restricted segmental motion, which was suggested from the low R value of 0.5-0.6 6b and broad 29 Si-NMR line width of about 50 Hz, 6d is forced by a pair of methyl and -branched alkyl pendants.Although the viscosity measurement needs a wide range of narrowly dispersed molecular weight polymer samples, today's advanced ...

Optically Active Polysilanes. Ten Years of Progress and New Polymer Twist for Nanoscience and Nanotechnology

“…These results led to a semi-empirical relationship between the main chain absorption characteristics and the global conformation of various polysilanes in solution. Figure 2 shows a correlation between the values of ε, fwhm, and α of various poly(dialkylsilane)s and poly(alkylarylsilane)s in THF at 30 • C. 46 The polymers include seven optically active poly(dialkylsilane)s with four different types of chiral β-, γ-, or δ-branched alkyl substituents, twelve optically inactive poly(dialkylsilane)s with six different types of linear, β-, γ-, or δ-branched substituents, and two optically inactive poly(alkylphenylsilane)s. For the polysilanes exhibiting λ max ranging from 290 to 352 nm, it is evident that with increasing α, ε increases exponentially, but fwhm decreases exponentially. From this semiempirical ε-α-fwhm relationship, either value of ε or fwhm renders information on the degree of chain coiling in solution at a given condition.…”

“…In 1996, it was reported that in various polysilanes in THF at 30 • C, the main chain peak intensity per silicon repeat unit, ε (Si repeat unit) −1 dm 3 cm −1 , is closely connected to the viscosity index, α. 46,47 Figure 1 shows the UV absorption spectra of four optically active poly(dialkylsilane)s featuring different chiral side groups in THF at 30 • C. 30k,35a It is evident that as the value of α increases from 0.59 to 1.25 in THF at 30 • C, the UV absorption intensity increases, whereas the full width at half maximum (fwhm) decreases. Thus, the degree of σ-conjugation, global conformation, and UV absorption characteristics in polysilanes are controllable by the appropriate choice of side groups.…”

“…Moreover, even as little as 0.6 mol% of chiral side groups induces a marked preferential screw sense helical conformation in the copolymers at temperatures below 0 • C. Presumably, the reason for no marked "sergeants and soldiers" effects in 8 and 9 is due to the greater flexibility of the di-n-pentylsilane moieties, since the α value of poly(di-n-pentylsilane) (13) may be similar to that of random coil poly(di-n-butylsilane) (14, α ∼0.71) and poly(di-n-hexylsilane) (15, α ∼0.74). 46 Structural similarity between chiral and branched achiral side groups in copolymer systems seems to be vital in the promotion of effective "sergeants and soldiers" cooperativity.…”

Nonclassical forces: Seemingly insignificant but a powerful tool to control macromolecular structures

“…In the last two decades, we have designed and synthesized a wide range of polysilanes comprising chiral and/or achiral side groups. [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] MF realized that polysilanes were gifted with unique features of exhibiting intense and sharp UV absorption, circular dichroism (CD), and photoluminescence (PL) spectra around 300-400 nm, characteristic of the r-conjugation in the helical silicon-catenated main chain. Individual rod-like helical polysilanes in solution afford a highly intense, narrow exciton absorption band along with a sharp mirror-imaged emission band.…”

Nano-Segregation of Squalane between Smectic Layers of Rigid-Rod Polysilane

“…[12] The molecular weights of PDMS were selected so that the smectic layer reflection for the SR-SAXS and smectic banded pattern for AFM can be easily observed. Squalane was purchased from Tokyo Kasei (TCI, Tokyo, Japan) and used without further purifications as its purity has been found sufficiently high.…”