A Statistical Model for the Cooperative Thermochromic Transition of Polysilanes

Sanji, Takanobu; Sakamoto, Kenkichi; Sakurai, Hideki; Ono, Katsumichi

doi:10.1021/ma981466t

Cited by 31 publications

(28 citation statements)

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“…According to previous reports, [1][2][3][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.…”

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confidence: 91%

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

2001

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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...

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confidence: 91%

“…[2][3][4][5][6][7][8][9][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.…”

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confidence: 99%

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

2001

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“…[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.…”

Section: Introductionmentioning

confidence: 99%

“…The delocalization of s electrons is more complicated and less well understood than the delocalization of p electrons, primarily because more than one orbitalp articipates at most atomic centers.I ts effects dominate the optical properties of sbondeds tructures and extends to many others, such as ionization potential, chargea nd energy transfer,s pin density propagation,a nd chemical reactivity.Astriking example of the effects of s-electron delocalization is provided by the optical properties of peralkylated linear polysilanes, fully saturated chains with an all-silicon backbone. [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.…”

Section: Introductionmentioning

confidence: 99%

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

Jovanović

Antic

Rooklin

et al. 2017

Chemistry An Asian Journal

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Conformational effects on the s-electron delocalization in oligosilanes are addressed by Hartree-Fock and time-dependent density functional theory calculations (B3LYP,6 -311G**) at MP2 optimizedg eometries of permethylated uniformly helical linear oligosilanes (all-w-Si n R 2n + 2 )u p to n = 16 and forb ackboned ihedral angles w = 55-1808.T he extent of s delocalization is judged by the partition ratio of the highest occupiedm olecular orbital and is reflected in the dependence of its shape and energy and of UV absorption spectra on n. The results agree with knowns pectra of all-transoid loose-helix conformers (all-[AE 165]-Si n Me 2n + 2 )a nd revealatransition at w % 908 from the "s-delocalized" limit at w = 1808 toward and closetot he physically non-realizable "s-localized" tight-helix limit w = 0w ith entirely different properties. The distinction is also obtainedi nt he Hückel Ladder Ha nd Cm odelso fs delocalization. An easy intuitive way to understand the origin of the two contrasting limits is to first view the linear chain as two subchains with alternating primary and vicinal interactions (s hyperconjugation), one consisting of the odd and the other of the even s(SiSi) bonds, andt hen allow the two subchainst oi nteract by geminal interactions (s conjugation).

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“…UV absorption, [1][2][3][4][5][6][7][8][9] luminescence, [5][6][7][8]10] nonlinearp olarizability, [11,12] and photoconductivity [13] )a nd reviewed. [14][15][16][17][18][19][20][21] Ac omplete understandingo fs delocalization in simple terms is still far away, due to its strong conformational dependence, as exemplified by the thermochromism [22][23][24][25][26] of oligosilanes. [27] Flexibleu nconstrained permethylated polysilane chains favor three SiSiSiSi dihedral angles, transoid (1658), gauche (558), and ortho (908).…”

Section: Introductionmentioning

confidence: 99%

Electronic Transitions in Conformationally Controlled Peralkylated Hexasilanes

et al. 2016

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The photophysical properties of oligosilanes show unique conformational dependence due to σ-electron delocalization. The excited states of the SAS, AAS, and AEA conformations of peralkylated n-hexasilanes, in which the SiSiSiSi dihedral angles are controlled into a syn (S), anti (A), or eclipsed (E) conformation, were investigated by using UV absorption, magnetic circular dichroism (MCD), and linear dichroism spectroscopy. Simultaneous Gaussian fitting of all three spectra identified a minimal set of transitions and the wavenumbers, oscillator strengths, and MCD B terms in all three compounds. The results compare well with those obtained by using the symmetry-adapted-cluster configuration interaction method and almost as well with those obtained by time-dependent density functional theory with the PBE0 functional. The conformational dependence of the transition energies and other properties of free-chain permethylated n-hexasilane, n-Si Me , was also examined as a function of dihedral angles, and the striking effects found were attributed to avoided crossings between configurations of σσ* and σπ* character.

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A Statistical Model for the Cooperative Thermochromic Transition of Polysilanes

Cited by 31 publications

References 44 publications

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

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

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

Electronic Transitions in Conformationally Controlled Peralkylated Hexasilanes

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