Thermochromism of Poly(di-n-hexylsilane) in Solution Revisited

Abstract

Solutions of poly(di-n-hexylsilane) in hexane, after aging for 5 days, exhibit on cooling two long-wavelength absorption bands at 357 and 368 nm, in addition to the normal high-temperature band at 320 nm. These UV absorptions probably represent different conformations of the polymer chain and so provide evidence for the simultaneous presence in solution of three different conformational forms. This is the first time such an observation has been made for a polysilanes and perhaps for any polymer.The UV thermochromism of polysilane polymers in solution, first reported in 1986, 1,2 has been the subject of numerous experimental and theoretical studies. 2-10 For many polysilanes, as the temperature is decreased, the prominent σ-σ* absorption band near 320 nm is replaced by one at longer wavelength. These different bands are believed to represent different polymer chain conformations, which exist in equilibrium in polysilane solutions.This thermochromism was first observed for poly(din-hexylsilylene), PDHS, which has become probably the best studied of all polysilanes. According to previous reports, 1-4 solutions of PDHS at room temperature exhibit a broad absorption near 320 nm (band I). As the temperature is decreased to -50°C, this band decreases in intensity and a new absorption (band II) grows in at ∼357 nm, indicating straightening of the polysilane chain leading to greater σ-conjugation. The thermochromism is reversible upon warming.We now find that under some circumstances PDHS may exhibit an absorption at 368 nm (band III) in addition to those at 320 and 357 nm. The 368 nm absorption band probably represents an even more extended form of the polymer. This is the first time that three different conformations of a polysilane, and perhaps of any polymer, have been observed simultaneously in solution. 11 Experimental SectionPDHS was prepared by the usual sodium condensation of di-n-hexyldichlorosilane 3 and purified by reprecipitation from toluene with 2-propanol. The purified polymer was monomodal with Mw 1 708 000 and polydispersity 2.3 (GPC, polystyrene standards). A solution of the polymer, between 10 -5 and 10 -6 mol (Si unit) L -1 in hexane, was placed in a specially constructed, hermetically sealed, temperature-controlled metal cell with quartz windows and optical path ) 50 mm. The temperature was controlled with a Pt resist (R ) 100 ohm), placed directly into the solution. The temperature was maintained with an accuracy of (1°C in the interval from +20 to -60°C. UV spectra were determined with a computerized Carl Zeiss M40 spectrophotometer. Figure 1 shows the evolution of the electronic spectrum of a freshly prepared solution of PDHS on fast cooling. Between +20 and -40°C, band I shifted gradually to longer wavelength, from 317.5 to 322 nm ( Figure 1a, Table 1). Upon further cooling from -40 to -55°C, band I disappeared and band II grew in ( Figure 1b). The change was rapidly reversible upon warming. This is the classical "type 3" behavior for polysilanes in solution. 3,4 Band II is quite narrow, with ha...

Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain

“…[1][2][3][4] They are sensitive to the backbone conformation and as ar esult lead to phenomena such as thermochromism, [5][6][7][8][9] piezochromism, [10,11] electrochromism, [12] ionochromism, [13,14] and solvatochromism. [15] Also their single-molecule electrical conductivity appearst od epend on conformation.…”

Hydroxy‐Substituted Oligosilane Dendrimers: Controlling the Electronic Properties through Hydrogen Bonding

“…[1] Their intense electronic absorption in the near UV was found to be sensitive to the electronic nature of the substituents and the conformation of the silicon backbone, as reflected in the thermochromism, [2] solvatochromism, [3] and ionochromism [4] of some oligosilanes. Recent studies on linear derivatives with discrete conformations have shown that sconjugation is more effective in an anti-conformation (Si-SiSi-Si dihedral angle w = 1808) than in conformations with small dihedral angles, such as syn, cisoid, or even gauche (w = 0-608).…”

“…MeSi(SiMeCl 2 ) 3 (1) was treated with 3 equivalents of KSi(SiMe 3 ) 2 Me at À78 8C. The reaction proceeds regio-and diastereoselectively yielding the l,l-form of 2 as the main product.…”

Hydroxysubstituierte Oligosilandendrimere – Kontrolle der elektronischen Eigenschaften durch Wasserstoffbrücken

“…MeSi(SiMeCl 2 ) 3 (1) wurde zunächst mit 3 ¾quivalenten KSi(SiMe 3 ) 2 Me bei À78 8C in einer regiound diastereoselektiven Reaktion zu l,l-2 umgesetzt. [7] Die anschließende Hydrolyse in Gegenwart von NH 4 (NH 2 COO) liefert racemische Mischungen der beiden möglichen Diastereomere l,u-3 und l,l-3 (siehe die Hintergrundinformationen).…”

“…[10] Ungewöhnlich ist das unterschiedliche UV-spektroskopische Verhalten beider Diastereomere. Die Absorptionskurven von l,u-3, l,l-3 und dem Strukturanalogon MeSi[SiMe 2 Si-(SiMe 3 ) 2 Me] 3 (4) sind in Abbildung 3 gezeigt. Das Absorptionsmaximum von l,u-3 bei 282 nm (e = 3.7 10 4 ) mit einer Schulter bei ca.…”