Photo-active groups

Fig. 44. Common sites for attaching redox- and photo-active groups to nucleosides C5 pyrimidine, C7-deaza-purine, and C8-purine.

Fig 46. On-column attachment of redox- and photo-active groups to DNA oligomers. Reproduced with permission from Ref. (159). Copyright 1999, American Chemical Society. 

One of the most prevalent cell adhesion peptides is the tripeptide arginine-glycine-aspartate (RGD) from fibronectin.f This tripeptide has been used extensively to promote adhesion and spreading of many cell types. For example, polyurethane surface can be activated and coupled to RGD via the carboxy terminus. Surfaces containing photo-activable groups, such as benzophenone or aryl azide, can be modified easily with RGD by UV radiation. This tripeptide and other... 

In the case of resins for photoresists the polymer may also contain photosensitive functionalities, such as sites of unsaturation in the case of negative-working photoresists, or have photo-active groups such as diazonaphthoquinone (DNQ) attached, as in positive-working photoresists. Although some polymers are formulated with such functionalities present, in the most commonly used polymers such groups are absent and the main function of the polymer is as a carrier for the other resist additives (section 2.4.1). 

Though novolacs have been used in formulating both positive and negative-working photoresists, the vast majority are used in the former. A few examples are shown in Table 2.4. In the case of positive-working resists the desired amount of a photo-active group can be added to the novolac resin as a preformed compound (polymers 1 and 2 in Table 2.4) or... 

In order to manufacture environmental friendly qualities of polymers, a photo-active group may be introduced into the polymer. For example, light sensitive polymers are obtained by the co- pol nnerisation of ketonic species with hydrocarbons. The photodegradation of resulting pol nneric material involves. Norrish type II process of the pol mier chain rather than a radical mediated photooxidation. 

Therefore, the graft copolymerization of vinyl monomers onto macromolecules having active an pendant group can be achieved either by redox initiation with a Ce(IV) ion or by photo-induced charge-transfer initiation with BP, depending on the structure of the active groups. 

Organometallic dendrimers have been constructed to act as potential electro-or photo-active materials, the synthesis of which will be discussed in the following section. Apart from the examples discussed above, surface modification of dendrimers with a variety of functional groups has afforded novel redox active materials [110-116]. 

Recently, Kimura et al. have reported the encapsulation of electro chemically and photo chemically active groups such as a porphyrin unit (51) or a ruthe-nium(II) bis(terpyridyl) (52) unit in the interior of a 1,3,5-polyphenylene-based dendritic structure (Scheme 21). The dendritic porphyrins have been synthesiz-... 

Light absorption causes formation of an electron/hole (e h ) pair in the interfacial region of the solid and, in the presence of an electric field (e. g. when the solid is held in an electrolyte), the electrons migrate inwards towards the bulk of the solid and the holes move towards the surface and react with the FeOH groups, i.e. the charges separate. The surface reaction is, Fe-OH + hye Fe(OH)s where s = surface and hvB is a hole. A feature of the iron oxides is electron/hole pair recombination - many electrons recombine with the holes and are neutralized - which decreases the photo-activity of the solid. The extent of recombination depends to some extent on the pH of the solution and its effect on the proportion of FeOH groups at the surface (see Chap. 10 and Zhang et al., 1993). 

The O" ion is much more reactive than either OJ or OJ and the M=0 species reacts as O- when y- or photo-activated (see sections on reactivity in Ref. 7). Hydrogen reacts rapidly and irreversibly with O- even at low temperatures to form OH- groups on the surface and also with Ojc on some systems, but not with 02. Oxygen reacts with O- to form O3 on many different oxides in a reversible reaction (7). This reaction is thought to account for isotopic exchange reactions in which the isotopes can be scrambled among all three nuclei. 

All cationically polymerizable monomers can be potentially used in this process however, the main study has been focused so far on the most reactive oxirane and vinyl ethers [4], Alkoxysilane derivatives - the most common acid-sensitive monomers for the synthesis of siloxane materials through the use of sol-gel methods - were not used extensively. Only a few examples of their application in photo-activated cross-linking can be noted, mainly in co-reaction with oxirane sites [5]. Typically, alkoxysilanes are subjected to an acid- or base-catalyzed process involving hydrolysis of an =SiOR group and then condensation of the formed silanol with another molecule bearing an =SiOH or =SiOR function to give a siloxane linkage [6]. It was of interest to combine the properties of cross-linked silicone materials with the ones provided by sterically overloaded... 

Scheme 1. Formation of Tsi-siloxane network by photo-activated, acid-catalyzed hydrolysis of alkoxysilyl groups.

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