Polysilanes ultraviolet

Several classes of polycyclic polysilanes, such as the octasilacubanes and the hex-asilaprismanes, have been synthesized100, but little is known about the photochemical behavior of the silacubanes which appear to be stable toward ultraviolet radiation. The silaprismanes, such as 195, on the other hand, were observed to rearrange to the Dewar benzene isomer 196, using 340-380 nm radiation101. The Dewar benzene reverted to the... 

X(3) values for a number of amorphous polysilanes as measured by the Maker fringe technique are reported in Table I. These values are quite large for a sigma-bonded system as predicted by the highly polarizable nature of the polymer backbone and the intense oa electronic transition in the ultraviolet. In some cases, the numbers are as large as those reported for a number of pi conjugated polymers... 

Yet we know now that polysilane polymers are stable to heat up to almost 300 °C, are inert to oxygen at ordinary temperatures, and are only mildly susceptible to hydrolysis. The principal weakness of polysilanes, as materials, is not any of these properties it is that they become degraded when exposed to ultraviolet light. 

When polysilanes absorb energy in the ultraviolet region, electrons are promoted from the a valence band to the a conduction band.50 Absorption maxima for polysilanes fall between about 290 and 410 nm typical ultraviolet spectra for some polysilanes in solution are shown in Figure 5.5. Because this o-o transition is permitted, the electronic absorptions are intense, with extinction coefficients e between 5,000 and 10,000 per Si-Si bond. Consistent with the splitting portrayed in Figure 5.4, as the number of silicon atoms increases, the energy of the electronic transition decreases. The absorption wavelength therefore increases and eventually reaches a limit when... 

When irradiated with ultraviolet light, most polysilanes undergo chain scission into smaller fragments. When polysilanes (RR Si), are photolyzed with UV light X = 254 nm)... 

The polysilanes can be used as photoresists because they undergo scission when exposed to ultraviolet light, as explained in Section 5.10. Both the extinction coefficient and the wavelength of maximum absorption decrease with increasing exposure, so that polysilanes can be bleached photochemically, as illustrated for ( -HexSiMe) in Figure 5.27. Dialkylsilane polymers are generally better as positive photoresists than... 

The photoconductivity of polysilanes was described in Section 5.8, and their electroluminescence is covered in Section 5.9.2. These properties make polysilanes possible components of polymer light-emitting diodes, either as charge transport layers or as the actual emissive materials.146 A drawback of the polysilanes is their photodegradation under ultraviolet irradiation, a problem which must be overcome if polysilanes are to become commercially useful. 

The organopolysilanes are those compounds containing at least one silicon-silicon bond and one silicon-carbon linkage. This review is mainly concerned with the chemistry of aliphatic derivatives of polysilanes. Consideration of aromatic organopolysilanes is excluded from this review except as far as they are used as intermediates for synthesis and their properties correlate with the aliphatic silicon-silicon compounds, because the aromatic organopolysilanes have recently been well reviewed elsewhere (31,51, 73, 76a, 212). Physical properties of the polysilanes also are excluded from consideration except for spectral properties of ultraviolet absorption and nuclear magnetic resonance, since they are well summarized in earlier excellent reviews and texts (8, 34, 35, 51,131,132). 

Ultraviolet Spectral Properties of Polysilanes Containing jt-Electron Systems... 

Ultraviolet Spectral Properties of Aliphatic Polysilanes Containing Negative Substituents... 

Photochemical behavior of monosilanes has been investigated by mercury-sensitized photolysis (15-19), flash photolysis (20, 21), vacuum ultraviolet photolysis (22-27), and matrix photolysis (28-30). The first examples of the photolysis of permethylated polysilanes were published in 1970 (14). All of the cyclic and acyclic permethylpolysilanes with the exception of hexamethyldisilane readily undergo photolysis on irradiation with ultraviolet light to give shorter chain compounds with the concurrent generation of the divalent silicon intermediate, dimethylsilylene (8). 

Free radical induced cationic polymerization may also be performed by irradiating monomer solutions containing polysilane and pyridinium ions [59]. Polysilanes have strong ultraviolet absorption in the 300-350 nm region and rapid photodegradation occurs upon irradiation at this band [60] ... 

FIGURE 9. Wavelength (Amax) of ultraviolet absorption band of polysilanes as a function of chain length. Me(SiMe2) , (rt-Cj2H25SiMe) , . Reproduced by permission of John Wiley Sons, Inc. 

As explained in Section III.D, polysilanes photolyze in ultraviolet light to produce silyl radicals as well as silylenes. The silyl radicals can add to carbon-carbon double bonds and initiate the formation of polymer chains, so polysilanes can be used as radical photoinitiators112,128. The process is a fairly general one various different polysilanes can be used, as initiators, for those vinyl compounds susceptible to radical polymerization. 

The uses of polysilanes as photoresist materials , and as photoinitiators both depend on the photoscission of the polymers by ultraviolet light. Initial steps in this process are chain cleavage to radicals and elimination of silylene, RR Si ... 

Present studies on the ultraviolet properties of polysilanes have led to the following observations (118, 119, 119a) ... 

---