Radical Ions of Polysilynes - American Chemical Society

Chapter 33

Radical Ions of Polysilynes 1

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Akira Watanabe , Minoru Matsuda , Yoichi Yoshida , and Seiichi Tagawa Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 29, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch033

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Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980, Japan Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan

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The radical ions of polysilynes which have silicon network structure were studied by the pulse radiolysis. The polysilynes showed broad absorption and emission spectra due to a quasi-two dimensional Si skeleton. The Si skeleton of polysilynes was investigated by the far-IR spectra. The far-IR spectra were compared with the calculated vibrational bands of cyclic silicon model compounds. As the Si skeleton of the polysilynes, the localized silicon rings connected by linear silicon chains were presumed. The transient absorption spectra of radical ions of poly(η-propylsilyne) and poly (η-hexylsilyne) showed two characteristic absorption bands in the visible and the IR region. A time-dependent spectral change of the radical ions was explained by the formation of the charge resonance (CR) state where the radical ion site is stabilized after the geometric change of the σ-conjugated polysilane chain.

Polysilane is an organometallic polymer that has a Si-Si main chain and organic side chains. The discoveries of soluble polysilanes with high molecular weight have caused considerable attention to the properties of the σ-conjugated polymers (1-3). It is a new type of conjugated system for π-conjugation. For π-conjugated polymers, characteristic absorption bands related to the conduction state were observed. For example, a soliton model, a polaron model, and a bipolaron model were developed for polyacetylene, polyaniline, polypyrrole,polythiophene, and soon (4-10). Figure 1 shows energy-band diagrams of polaron and bipolaron. When conjugated polymer is doped with an electron acceptor, new energy levels are formed between the valence band and the conduction band. Doping with an electron donor also forms new energy levels. In a chemical sense, these states can be interpreted as radical cation and radical anion, respectively (7-10). The bipolaron can be interpreted as dication. The formation of the new energy levels causes low energy transition as shown in Figure 1. These low energy transitions which appear in the IR region are closely related to the conduction state of the conjugated polymer. With an increase in the conductivity, the transition energy of the absorption band in the IR region decreases and the spectral shape changes from sharp one to broad one (10). This type of relationship between the IR band and the conductivity is shown in many π-conjugated polymers. 0097-6156/94/0579-0408$08.00/0 © 1994 American Chemical Society

Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 29, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch033

33. WATANABE ET AL.

Radical Ions of Polysilynes

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No characteristic absorption bands related to the conduction state have been observed for doped polysilane. One of the reasons is the instability of the Si-Si σ-bond of polysilane to doping with a cationic or anionic dopant. In such a case, transient techniques are advantageous to observe unstable species. Pulse radiolysis provides the possibility to observe unstable ionic states of polysilanes (11-16). In the electron beam-induced reaction in solutions, most of the electron beams are absorbed by the solvent and the ionic species of the solvent are produced. In the next step, the electron transfer between the ionic solvent molecules and the solute takes place (17). When the solvent is tetrahyroraran (THF), solvated electrons are produced and some of them are transferred to polysilane, and the radical anion of polysilane is produced. On the other hand, cationic species are produced by the electron beam irradiation to methylene chloride, and electron abstraction from pol ysilane by the cationic species of the solvent forms a radical cation of polysilane. Tagawa et al. have reported on the ionic state of polysilanes which have a linear Si-Si chain (12,13). The radical ions of polysilanes show two characteristic absorption bands in the UV region and the IR region. There are some problems œncerning the polysilane radical ion. The first problem is the radical ion site where charge is mainly located. Is it the side chain type or the main chain type? Tlie second problem is the assignment of the IR band. The third problem is the relaxation phenomena of the IR band. In this paper we report on the radical ion state of polysilynes. Polysilynes have been considered to be silicon network polymers which have a quasi-two-dimensional Si skeleton (18-22). The increase in σ-conjugation along the Si skeleton with an increase in Si-dimensionality is expected, and the absorption band ofthe radical ion must be influenced by the change ofthe σ-conjugation state. In the following, we call polysilanes which have a linear Si-Si chain rx>lysilylene and call polysilanes which have a silicon network structure polysilyne on the basis of the repeating unit. Experimental Polysilynes were synthesized by the Kipping reaction of the trichloro-organosilanes using Na in toluene at 110°Cunder N atmosphere. The molecular weight of polysilynes R 2

_ l ^

18-Crown-6

CI—si—CI ^

_ ^

(—Si—)

• Na, toluene, 110 °C

^

n

R= phenyl, π-propyl, A?-hexyl was determined by GPC using a monodispersed polystyrenes as standards. The values of the molecular weight are summarized in Table I. Fractional reprecipitation Table I. The Molecular Weight and the Molecular Weight Distribution of Polysilynes Polymer Poly(phenylsilyne) Poly(rt- propylsilyne) Poly(/i- hexy lsilyne) Poly(/î- hexy lsilyne)

Mw

Mw/Mn

4,000 10,900 12,000 119,000

2.04 4.91 1.23 2.66


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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

from THF solution using methanol as a precipitant was carried out to obtain a high and a low molecular weight poly(n-hexylsilyne). Absorption and emission spectra were measured by a Hitachi U-3500 and a Shimadzu RF-502A, respectively. The IR and far-infrared absorption spectra were measured using a JEOL100 FT-IR spectrophotometer. The Far-IR spectra in the region from 700 to 250 cm" were measured using a KBr disk. The Far-IR spectra in the region from 570 to 50 cm* were measured using a sample prepared by dispersing a polysilyne into a paraffin matrix and pasting it on a polyethylene film (thickness 0.02 mm). The pulse radiolysis system has been described in previous papers (23,24). The electron pulse was with 2 ns durationfroma 35 MV electron linear accelerator. The dose per pulse is evaluated from the absorbance of the hydrated electron at 500 nm in neat deaerated water and determined as 4 - 5 krad per 2 ns pulse. Solvents, THF and CH C1 , were dehydrated and degassed up to 10 Torr, and transferred to a quartz cell ( l x l cm square and 2 cm long) on a vacuum line. The calculations of vibrational bands were done by MOPAC with the MNDO approximation (25). 1


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Results Absorption and Emission Spectra of Polysilynes. Figure 2 shows the absorption and emission spectra of polyphenylsilyne) and poly(n-propylsilyne). Poiy(phenylsilyne) shows an intense absorption below 300 nm, which is attributable to the π-π* absorption of the phenyl group. The absorption above 300 nm can be assigned to the σ-σ* absorption of Si skeleton. The emission of polyphenylsilyne) and poly(rt-propylsilyne) can be also assigned to the emission from the Si skeleton. Polysilynes show broad absorption and emission spectra in a lower energy region compared to poiysilylene. This suggests an increase in σ-conjugation due to the silicon network structure. The effect of the molecular weight on the absorption and emission spectra was investigated for poly(n-hexylsilyne). Figure 3 shows the absorption spectra and emission spectra of the poly(n-hexylsilyne)s with the low molecular weight (Mw = 12,000, Mw/Mn = 1.23) and the high molecular weight (Mw = 119,000, Mw/Mn = 2.66). The absorption shoulder at 300 nm increases slightly with an increase in the molecular weight. In the emission spectra, the maximum wavelength shifted from 450 to 466 nm with an increase in the molecular weight. The emission spectra show a significant effect of the molecular weight compared to the absorption spectra because the emission occurs from the trap site following the energy migration along the Si-Si chain. Si skeleton of Polysilynes Studied by Far-IR Spectra. Figure 4 shows the far-IR spectra of polysilanes by KBr disk method. Poly(methylphenylsilylene) which has a linear Si-Si chain shows two sharp bands assigned to asymmetric (461 cm" ) and symmetric vibrations (382 cm" ) of the silicon-silicon bond. Polysilynes show broad absorption bands compared to poly(methylphenylsilylene). Poly(phenylsilyne) shows a broad absorption band at 495 cm" with a peak at 461 cm" . The 461 cm" absorption can be assigned to the asymmetric vibration of a linear Si-Si chain. Poly(w-propylsilyne) shows a broad absorption band at 453 cm" . Poly(rt-hexylsilyne) also shows a broad band at 393 cm" with shoulders at 496 and 463 cm" . The far-IR spectra of polysilanes in the lower energy regionfrom570 to 50 cm* were observed using a paraffin matrix (Figure 5). The polysilanes showed no absorption band below 200 cm" . The far-IR spectra are quite different from amorphous silicon (26-28). Figure 6 shows the calculated vibrational spectra for silicon model compounds. The structure of the model compounds was optimized by MNDO calculation. The asymmetric (498 cm" ) and symmetric (395 cm" ) vibrations of the silicon-silicon bond were obtained using a linear hexamer model. The calculated spectrum well 1

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WATANABE ET AL.

Radical Ions of Polysilynes

Conduction Band Forbidden Band


%

Band

#

p-dope (with acceptor) (radical cation)

Figure 1.

p-dope

n-dope

(with acceptor)

(with donor) (radical anion)

(dication)

Energy-band diagram of polaron and bipolaron.

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Radical Ions of Polysilynes - American Chemical Society

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