Polysilanes photodegradation

Scheme 1. Possible Reactions tor the Photodegradation of Polysilane Polymers,...

During photolysis, the double bond content of the polysilane(P-l)(15mol% in this experiment) decreased to 10mol%, as measured by 1H-NMR spectroscopy. However, the ratio, quantum yield of scission(Q(S))/quantum yield of crosslinking(Q(X)), was not affected by the reaction of the double bond. West and his coworkers have reported that poly((2-(3-cyclohexenyl)-ethyl)methylsilane-co-methylphenylsilane) crosslinked upon irradiation(55). The difference between our results and West s may lie in the amount of the double bond and inhibitation of the radical closslinking by the phenol moiety. Polysilane with a halogen moiety, P-8, photodecomposed rapidly, compared with P-1 or P-3. The introduction of a chloride moiety was effective for the sensitization of the photodegradation. Similar results has already been reported(55). 

Figure 5. Mechanistic hypothesis for the photodegradation of high molecular weight polysilane derivatives.

Microlithography, Xerography. Because of their photosensitivity, polysilanes are under intense investigation for use as positive photoresist materials (94) (see Lithographic resists). They are particularly attractive because both wet and dry development techniques can be used for imaging (131,132). The use of polysilanes for xeroprinting has been reported (133). Thermal and optical sensors based on the photodegradation of polysilanes have been developed (134). 

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. 

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] ... 

The polysilane core part within the shell cross-linked micelles was photodegraded by UV irradiation (>280nm) and dialysis against water produced nanometer-sized... 

Whereas oxygen can play a role when polysilanes are photodegraded in air, photobleaching with attendant molecular weight reduction occurs even when poly silane derivatives are irradiated in an inert atmosphere or under vacuum. 

Polysilanes are important as photoresists. I.M.T.Davidson et.al. have identified three pathways in the photodegradation of these species. Sekiguchi et al. have described a potentially valuable new photochemical synthesis of tetramethyldisilene (44) based on irradiation of its 1,4-adduct with benzene. An interesting contrast between silicon and carbon chemistry is provided by the photoisomerisation of the trisilacycloheptene (45) to the corresponding trans isomer (46) (Shimizu et al.). 

SCHEME 4. Possible reactions for the photodegradation of polysilane polymers... 

It should be mentioned that these silicon-backbone polymers are highly absorptive, and must therefore be used in very thin films ( 30 nm), which can be prone to defects. Also, the principal electronic transition in these polysilane (with two-dimensional silicon backbone) and polysilane (three-dimensional silicon backbone) polymers when irradiated at 193 nm is most likely a (t-ct transition involving the all-silicon backbone. Due to this excitation, reaction pathways leading to scission of the Si-Si bonds lead to photodegradation of the polymer. 

The PL from substituted silsesquioxanes is efficient, fast and, because it is due to excitation from non-bonding states at the oxygen, does not show photodegradation, unlike that from the majority of polysilanes. This phenomenon makes them interesting for possible applications. 

In addition to their other interesting properties, polysilanes are photoconduc-tive [41] (see Chapter 2) and, therefore, are attractive v th regard to practical applications [42, 43]. However, to the detriment of their technical applicability, polysilanes show a pronounced trend to suffer photodegradation. Light absorption induces main-chain scission and extrusion of silylene, as depicted in Scheme 7.17. 

The photodegradation and photo-oxidative degradation of different poly-silicones include two main groups polysilanes (section 4.15.1) and poly-siloxanes (section 4.15.2). 

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