Photophysical techniques

The U.S. - Australia Symposium on Radiation Effects on Polymeric Materials contained research presentations on fundamental radiation chemistry and physics as well as on technological applications of polymer irradiation. This paper represents a hybrid contribution of these two areas, examining a field of extensive technological importance. Spin casting of radiation sensitive polymer resists for microelectronic fabrication was studied using photophysical techniques that are sensitive to the fundamental radiation response in the ultraviolet range. 

The following experimental setups and instruments were used for the various photophysical techniques. 

The rates of diffusion of solutes and surfactants in and out of micelles have been measured using photophysical techniques. The most commonly used method is to measure the deactivation of excited states of the probe by added quenchers, which are only soluble in the aqueous phase. The measurement of either the decrease in emission intensity or a shortening of the emission lifetime of the probe can be employed to determine exit and entrance rates out of and into micelles 7d). The ability of an added quencher to deactivate an excited state is determined by the relative locations and rates of diffusion of the quenchers and excited states. Incorporation of either the quencher or excited state into a surfactant allows one to determine the rates of diffusion of surfactants. Because of the large dynamic range available with fluorescent and phosphorescent probes (Fig. 3), rates as fast as... 

Biochemists remain the leading exploiters of photophysical techniques and it is necessary to point out that coverage of the biological and medical areas is not comprehensive. 

Various photophysical techniques continue to be used in the study of polymers some particularly interesting work on electrically-conducting pol37mers has been described by Roth and Bleier, inter alia. The photophysics of thin films and colloidal systems, including micelles, continues to be an important and active field of research see e.g. Kalyansundaram Debe. Cyclodextrins have been found to increase the chemiluminescence yields from aqueous peroxyoxalates by up to 300-fold (Woolf and Grayeski). Enzymegenerated excited states of acetone have been found to induce quasi-photochemical behaviour of riboflavin in the dark (Rojas and Silva). From studies of the luminescence of Schmidt and... 

Biochemists continue to be the main exploiters of photophysical techniques,though it should be pointed out that the coverage of these particular aspects is selective rather than comprehensive. Even processes in living cells are now b ing studied (Vigo et al.. inter alia). Some aspects of biological photochemistry are undoubtedly an acquired taste. For example, a three-dimensional presentation of the total fluorescence from urine has enabled Leiner et al. to identify several fluorescent metabolites. 

Proteins provide many problems suitable for study by photophysical techniques. The fluorescence behaviour of sequential... 

The chapters in this book are designed to provide scientists who are engaged in basic and applied polymer research with a clear understanding of the current status of polymer photophysics. The concepts developed in this volume should stimulate others to apply photophysical techniques in their own research. 

One of the basic goals in polymer science is to identify the nature of the local environmental domains (viscosity, hydrophobicity, etc.) domains within a polymer solid or solution. This may be accomplished by using a variety of photophysical techniques all of which involve mixing or covalently attaching a small amount of a luminescent molecule (probe) into a polymer system. The probe molecule is designed such that one or more of its photophysical properties is directly dependent on some aspect of its environment. For example, both the fluorescence lifetime and the relative intensities of the vibrational structure of pyrene are altered when exposed to an aqueous, as opposed to a hydrophobic, medium. Polyelectrolytes such as poly(acrylic acid), under the proper... 

In a recent article Thomas [24] emphasises the prime importance of surfaces in chemical reactions, particularly in the fields of catalysis and corrosion chemistry. Also eluded to in the article is the concept that photochemical and photophysical techniques can yield Information relating to molecules adsorbed on surfaces and the fact that such information "reports back" on the environment of the excited state (i.e. the polarity and nature of surface sites). The technique of laser flash photolysis in diffuse reflectance should be as valuable to the understanding of heterogeneous photoreactions as the transmission mode has been for homogeneous photoreactions. It has already shown great potential in the study of materials adsorbed on surfaces. 

The targets of photooxidative reactions may be proteins, lipids, DNA, RNA, and/or cell membranes (Straight and Spikes, 1985). In vitro tests can be designed to determine the specific sites of damage to the various ocular compartments (i.e., lens and retinal epithelial cells and photoreceptor cells) and the products of those reactions. Table 11.2 presents a summary of additional biochemical and photophysical techniques that can be performed to predict the potential for and extent of in vivo phototoxicity more accurately. 

Therefore, information about the efficiency and excited state intermediates for a phototoxic reaction in the eye obtained by using photophysical techniques (fluorescence, flash photolysis, pulse radiolysis, ESR) can be predictive of phototoxicity in vivo. It has been confirmed that photophysical studies collate well with in vivo data (Roberts et al., 1991a). For instance, tetrasulphonatophenylporphyrin (TPPS), which binds to lens proteins, shows a long-lived triplet in the intact calf and human... 

In Section 3.1.4 we described Kuhn s early studies of energy transfer in monolayers. Their experiments are very well-known. They laid the basis for a rapidly expending area of photophysics concerned with the study of molecular assemblies such as LB-films, various monolayers, and vapor-deposited films. These photophysical techniques will become particularly important in elucidating the molecular organization in self-assembled systems. 

Another photophysical technique used to probe the dynamics of these interactions is steady state luminescence polarization (18, 19). The ruthenium complex bound to the DNA is excited with polarized light. If, on... 

These statements are not meant to be demeaning of the classical tools of supramolecular chemistry, rather to point out that photo-techniques offer researchers in this field another level of information. The previous paragraphs are meant to define the areas where photophysical techniques may be particularly useful, and a major purpose of this article is to alert the supramolecular chemistry coinmunity to the potential of photo-methodology and to provide encouragement for its utilization. 

The first example concerns a molecular dynamic device that undergoes ion triggered conformational changes from a W to a U shape, a mechanical motion that can be followed via photophysical techniques [8]. 

The essentials of a quantum mechanical treatment of the interaction of electromagnetic radiation with molecules is described in the first chapter, and the second one deals with supramolecular photochemistry, with particular emphasis on energy and electron transfer with a description of the Marcus theory. The following chapters are devoted to the different photochemical and photophysical techniques spectrophotometry and spectrofluorimetry, actinometry, absorption and luminescence techniques with polarized light excitation, time-resolved absorption and luminescence spectroscopy, down to femtosecond resolution. Each... 

---