Photoactive Additives

Photoactive Additives.—Ferric compounds, in particular, the chloride, continue to attract much interest as photosensitizers for thermoplastics. From e.s.r. work the mechanism appears to involve a redox reaction resulting in the formation of active hydroxy-radicals. Photodegradable polyethylene film has been developed by doping it with radiation-modified atactic polypropylene and hydroxyethyl-ferrocene. Several workers have studied the dye-sensitized photo-oxidation of polyisoprene and di-n-butyl sulphide embedded in Augustyniak and 

Wojtczak have found that the photosensitized degradation of polyethylene glycols decreases in the order triethylene glycol polyethylene glycol 400 mol. wt. polyethylene glycol 4000 mol. wt. Sastre and Gonzalez have shown that bromoalkanes are powerful sensitizers for the photo-oxidation of polystyrene, and Rabek and Ranby have found that polynuclear aromatics are photosensitizers for polybutadiene. Aromatic carbonyls have been shown to induce free-radical formation in cellulosic materials. 

---

Meisel etal. [18-20] were the first to investigate how the addition of a polyelectrolyte affects photoinduced ET reactions. They found that charge separation was enhanced as a result of the retardation of the back ET when poly(vinyl sulfate) was added to an aqueous reaction system consisting of tris(2,2 -bipyridine)ruthenium(II) chloride (cationic photoactive chromophore) and neutral electron acceptors [21]. More recently, Sassoon and Rabani [22] observed that the addition of polybrene (a polycation) had a significant effect on separating the photoinduced ET products in an aqueous solution containing cir-dicyano-bis(2,2 -bipyridine)ruthenium(II) (photoactive donor) and potassium hexacyano-ferrate(III) (acceptor). These findings are ascribable to the electrostatic potential of the added polyelectrolytes. 

In addition to cercosorin, other photoactive members of class B include calphostins A-D (4a-d), isolated from Cladosporium cladosporioides [10] phleichrome (5), isolated from Cladosporium phlei [11] and the elsinochromes (6) [12], isolated from several species of the genus Elsinoe. In this chapter, background on the syntheses and biological activity of the calphostins and phleichrome is included, because of their structural similarity to hypocrellin and cercosporin. 

Felekis and Tagmatarchis (2005) used this cycloaddition process to prepare SWNT derivatives possessing photoactive components, such as the addition of ferrocene groups. They used a short PEG-type spacer on the glycine to impart water solubility at the same time. 

The photoactive ligands can be phenylpyridines, quinoline derivatives, or benzimidazoles and the ancillary ligand can be selected from the following structures in addition to the photoactive ligand itself (Scheme 3.78) [291]. 

The workhorse of the VLSI industry today is a composite novolac-diazonaphthoquinone photoresist that evolved from similar materials developed for the manufacture of photoplates used in the printing industry in the early 1900 s (23). The novolac matrix resin is a condensation polymer of a substituted phenol and formaldehyde that is rendered insoluble in aqueous base through addition of 10-20 wt% of a diazonaphthoquinone photoactive dissolution inhibitor (PAC). Upon irradiation, the PAC undergoes a Wolff rearrangement followed by hydrolysis to afford a base-soluble indene carboxylic acid. This reaction renders the exposed regions of the composite films soluble in aqueous base, and allows image formation. A schematic representation of the chemistry of this solution inhibition resist is shown in Figure 6. 

Several different methods of sidewall functionalisation, such as fluorination, radical addition, nucleophilic addition, electrophilic addition and cycloaddition, have been developed (Tasis et al., 2006). The sidewalls of vertically aligned CNTs have been functionalised with DNA using azide units as photoactive components. The azi-dothymidine reacted photochemically with sidewalls of CNTs utilising [2+1] cycloaddition. The oligonucleotides were grown in situ on the sidewalls of CNTs and the DNA-modified CNTs were obtained after the deprotection of the nucleic acid (Moghaddam et al., 2004). 

There are two basic considerations when attempting SDM. One is to determine the amino acids that should be mutated and the other is to decide what to replace them with. The first question is, of course, dependant upon information gathered from previous experimentation in order to target residues that are appropriate. Such information may be derived from biochemical techniques. For instance, in our binding site studies, we have specifically mutated amino acids that had previously shown to be covalently labeled by photoactive ligands. Additionally, we have used comparisons between the sequences of different receptor subunits that correlate with receptor function to identify domains of interest. Chimeragenesis, the technique described in the first half of this chapter, can provide important information in this regard. Obviously, those proteins for which a detailed structural model is available will lend themselves to more rational substitutions. 

Finally, it should be considered that additional non-natural functional groups, which are often incorporated in the lipidated peptides for biological studies such as fluorophores or photoactive groups, typically lead to additional restrictions for the synthesis protocols. 

PE, RIE and IM resistances for an extensive list of commercial photoresists are included as well for comparison with the vinyl systems and amongst themselves. Although the exact com-osition of these systems is not public information, the generic type of base resin or polymer binder is generally known. In addition, the photoactive components are all known to be aromatic azides or azo-compounds. 

In general, the photoresists exhibit greater dry-process resistance than the vinyl polymers of Table II. The greater dry-etch resistances of photoresists is attributed to the aromatic nature of the crosslinking agents, photoactive components, and novolac resins (positive photoresists only). In addition, the... 

Chlorinated Novolak Resins. Mixtures of a cresol formaldehyde Novolak resin and a photoactive compound cross-link at electron doses far smaller than the dose required for the Novolak resin alone (11). The reason for this accelerated cross-linking is the reactions between the ketene (an intermediate formed from the photoactive compound upon irradiation) and the Novolak resin. This reaction may be reduced by using a Novolak resin modified for this purpose, or by using certain additives. The rationale for developing a halogen-substituted Novolak resin is the control of the reaction between the intermediate ketene and the Novolak. 

---