Applications of Photophysics

Photophysical processes have been adapted to provide systems for the measurement of temperature, critical micellar concentrations, micropolarity and microviscosity of organized media, and sol-gel transition points. 

Applications of fluorescence probes in cellular biology have been reviewed while labelled polypeptides and proteins have been described. Detection of DNA fragments has been considered by several authors, including the quantitative estimation of low concentrations of DNA in drinking water. A phosphorescence-based method has been used to study refolding of disulfide reduced RNase T i. 

Photoswitchable materials could be important components in molecular-scale electronic devices and several new systems have been reported. There have been other investigations of phototropic systems in which light is used to drive a reversible conformational change or, in the case of liquid crystals, a phase transformation. Such photosystems are of interest for the engineering of microscopic photoactive devices but the subject is still in its infancy and practical devices remain elusive. Much more subtle is the use of light to trigger a change in 

Molecular photophysics, especially the use of steady-state and time-resolved luminescence spectroscopy, have many important applications and there has been a progressive emergence of a new field of analytical chemistry based on these principles. It has been known for many decades that the excited state properties of certain molecules are highly sensitive to the local environment but it is only recently that a concerted effort has been made to use this sensitivity in a practical way. The main approaches to employing variations in photophysical properties as an analytical tool can be divided into two areas namely, (i) development of luminescent probes that respond to changes in the environment and (ii) identification of molecular systems for which the emission 

A new solvatochromic probe, based on 3,6-diethyltetrazine, has been proposed that extends the solvent acidity scale to highly acidic organic solvents. A co-polymerizable dansyl monomer has been suggested as an indicator of solvent polarity, since the fluorescent TICT state is very sensitive to this parameter. A method has been developed to prepare fluorescent labelled natural sediment for use as a measure of sediment transport 

---

Figure 15.7 Application of photophysical and photochemical processes in biology, biomedicine, and medicine...

Biological Systems.—The application of photophysical principles and methods to biological systems has been steadily developed and fluorescence spectroscopy is now one of the most important techniques in physical biochemistry. 

It has become apparent over the last few years that photophysical research has been largely concerned with the very detailed analysis of particular systems. The economics of research funding are reflected in the style and extent to which groups in di+ferent parts of the world are able to investigate specific problems. The application of photophysical processes in areas such as molecular electronics is still in an embryonic state and much more work needs to be done before substantial progress towards useful devices will be achieved. 

Applications of photophysics in biology and medicine are very extensive and only a few topics can be mentioned in this review. A survey of the use of lanthanide ions as luminescent probes of biomolecular structure and a general account of long distance electron transfer in proteins and model systems are very helpful. The methods applicable to the synthesis and activation of a number of photoactivable fluoroprobes have been described and photoactivation yields measured . 

There are various potential applications of photophysical phenomena in analytical chemistry. The relatively short lifetimes of most excited states, however, is a serious drawback to the construction of practical devices but studies which focus on finding ways to extend triplet lifetimes have now been described by Harriman et al. Kneas et al. have examined new types of luminescent sensor on polymer supports, and both Neurauter et al. and Marazuela et al. have designed sensors based on the ruthenium(II) polypyridine complex for the detection of carbon dioxide. A system, based on the formation of twisted intramolecular charge transfer states, has been devised for measuring the molecular weight of polymeric matrices (Al-Hassan et a/.), and the chemical reactivity at the interface of self-assembled monolayers has been assessed using fluorescence spectroscopy (Fox et al). 

A basic problem which can be encountered in the application of photophysics to colloidal systems are difficulties involved in the measurement of true luminescence spectra and determination of luminescence quantum yields of molecules in light scattering media. Gade and Kaden have produced a theory for this effect which can be used to take account of readsorption and re-emission effects in suspensions. 

It is known that some insects are able to detect a single molecule of a pheromone. The reported detection of single molecules of a dyestuff by repetitive laser pulsing shows that photochemists are now catching up with insects (Shera et al.). Luminescence-activated barometry in wind tunnels represents an unusual application of photophysics (Kavandi et al.). Wild and Renn have provided a useful review of new ideas for high-density information storage. 

Biologically Related Systems.—The application of photophysical methods to biochemical problems is not new, but increased activity is now apparent. Equipment and techniques are now widely available and it happens that the time scales especially amenable to study by photophysical methods are those in which many of the most important biochemical processes occur. 

Fluorescent probes are widely used to study the microstructure of micelles and microemulsion, providing information on microstructural aspects of the aggregates. Zana has recently reviewed static and dynamic fluorescence techniques and pointed out the advantages and disadvantages of these techniques [24]. For the application of photophysical and photochemical techniques in the study of surfactants, the reader is also referred to the review [25]. 

---