Photodegradation of Polysilanes

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) 

When aryl substituent groups are present, more complex photolytic reactions occur, and some cross-linking takes place as well as chain scission. Groups containing C=C double bonds are even more effective at bringing about cross-linking. This leads directly to the topic of cross-linking in polysilanes. 

For some uses it is important to form bonds that link different polysilane chains, to transform soluble, meltable polysilanes into insoluble resins. This process is vital if the polysilanes are to be used as precursors to silicon carbide ceramics, since, if cross-linking is not carried out, most of the polymer is volatilized before thermolysis to silicon carbide can take place. Several methods have therefore been developed to bring about cross-linking of polysilanes.109 110 

As mentioned in the preceding section, when vinyl or other alkenyl groups are present as substituents, photolysis leads to cross-linking, which dominates over chain scission. Probably the cross-linking results when silyl radicals formed in the initial photolysis add to vinyl carbon atoms on neighboring chains, but other mechanisms are also possible. 

2 Free-Radical Cross-Linking with Polyunsaturated Additives 

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

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). 

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

The photodegradation of polysilanes occurs by the random chain scission of the Si—Si bond with formation of very reactive silyl (4.94) radicals [1050] ... 

Photodegradation of polysilanes has been used for the production of photoresists [172, 2335]. 

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

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. 

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.). 

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. 

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). 

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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