Near-UV Application

Anthraquinones are nearly perfect sensitizers for the one-electron oxidation of DNA. They absorb light in the near-UV spectral region (350 nm) where DNA is essentially transparent. This permits excitation of the quinone without the simultaneous absorption of light by DNA, which would confuse chemical and mechanistic analyses. Absorption of a photon by an anthraquinone molecule initially generates a singlet excited state however, intersystem crossing is rapid and a triplet state of the anthraquinone is normally formed within a few picoseconds of excitation, see Fig. 1 [11]. Application of the Weller equation indicates that both the singlet and the triplet excited states of anthraquinones are capable of the exothermic one-electron oxidation of any of the four DNA bases to form the anthraquinone radical anion (AQ ) and a base radical cation (B+ ). 

Determination of protein secondary structure has long been a major application of optical spectroscopic studies of biopolymers (Fasman, 1996 Havel, 1996 Mantsch and Chapman, 1996). These efforts have primarily sought to determine the average fractional amount of overall secondary structure, typically represented as helix and sheet contributions, which comprise the extended, coherent structural elements in well-structured proteins. In some cases further interpretations in terms of turns and specific helix and sheet segment types have developed. Only more limited applications of optical spectra to determination of tertiary structure have appeared, and these normally have used fluorescence or near-UV electronic circular dichroism (ECD) of aromatic residues to sense a change in the fold (Haas, 1995 Woody and Dunker, 1996). 

Instrnmentation for UV-vis process analysis falls into fonr categories scanning instruments, diode-array instrnments, photometers, and fiber-optic spectrophotometers with photodiode-array (PDA) and charge-conpled device (CCD) detectors. The former two are more typically enconntered in at-line or near-line applications, whereas the latter two are better snited to actnal on-line analyses. 

A mixture of three isomeric cresols is used in a commercially available cresol-formaldehyde Novolak resin. This mixed Novolak resin, Varcum resin (12), provides adequate properties as a host resin for near-UV- and mid-UV-photoresist applications. Gipstein and his co-workers prepared pure cresol-formaldehyde Novolak resin from each isomeric cresol and compared their spectroscopic and resist characteristics (13). Their data on the UV-absorption spectra of each cresol-formaldehyde Novolak resin together with the commercially available Varcum resin are as follows the absorbances of 0.2 jim thick Novolak films at 250 nm are 0.165(Varcum), 0.096(o-cresol), 0.092(m-cresol), and 0.055(p-cresol). The so-called "window" in the UV absorption at around 250 nm is a maximum with the p-cresol-formaldehyde Novolak resin, while the other isomeric cresol and formaldehyde Novolak resins yielded similar UV absorptions at this wavelength. The smallest UV absorption at 254 nm is an advantage for the p-cresol-formaldehyde Novolak when the resin is used for a deep UV photoresist with a suitable photoactive compound (14). 

Electromagnetic Radiation. Functional dyes interact with electromagnetic radiation in the near-UV (300-400 nm), visible (400-700 nm), and near-IR (700-1500 nm) to produce a variety of effects required for hi-tech applications. The more important effects are described briefly, including their main applications. 

Instead, thanks to its high photoactivity, the anatase type is used in a wide range of applications as photocatalytic coatings on various substrates such as glass and ceramic tiles that can photodegrade various noxious or malodorous chemicals, smoke and cooking oil residues under low-intensity near-UV light. 

As indicated in the introduction to the section on Applications, this last category covers those molecules that are chiral but have no chromophore and are therefore CD inactive. The chromophore would be introduced through some color derivatization reaction. Absorption in the visible would be the desirable end result, but in some instances absorption in the near UV may be the very best that can be accomplished. The category also includes those molecules that are already CD active but only at very short wavelengths where experimental data are hard to measure. 

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