In his introduction to the series in 1983, Hoffman defined inorganic photochemistry as relating to "the interaction of photons with substances regarded as 'inorganic'". He then proceeded to provide a narrower description that would fit the focus of the symposium: " the interaction of visible and ultraviolet light with metal complexes and organometallic compounds, largely but not exclusively in solution and largely but not exclusively at room temperature". Since the 1983 symposium, research in inorganic photochemistry has continued at a brisk pace and new and exciting areas have evolved. From the understanding of the photophysics and photochemistry of monometallic transition metal complex chromophores sprang the evolution of what are known as supramolecular photochemical systems, discrete molecular ensembles of two or more chromophores designed to perform a function unique to the system (2). In addition, the gap between solid-state photophysics and molecularphotochemistry narrowed as hybrid inorganic systems were developed. One example is the class of nanocrystalline semiconductors, specifically prepared clusters having on the order of a few hundred atoms that exhibit photophysical behavior between that of bulk semiconductors and discrete molecular species. Hybrid materials have also been prepared by covalently linking transition metal complexes to make clusters that exhibit behavior reflecting extended intermetallic interactions. Traditional transition metal complexes having well characterized photophysical and photochemical behavior began to be used in a variety of applications such as luminescence sensors, photoreactive polymers, and semiconductor sensitizers. These changes reflect the fact that the study of photochemical and photophysical properties of inorganic substances has matured to the point that fundamental principles can be exploited in the development of more complex chemical systems and in very particular applications.

Because of these changes we were motivated to conduct a second symposium, which brought together individuals from a broad range of interdisciplinary areas that make use of inorganic photochemistry. The articles collected herein stem from a symposium sponsored jointly by the Division of Chemical Education and the Division of Inorganic Chemistry at the American Chemical Society meeting held in Orlando, Florida, in August 1996. The two-day symposium focused on applications of inorganic photochemistry to three general areas: biological chemistry, luminescence sensors, and advanced materials. In the first two areas, classical coordination complexes have been utilized creatively in a variety of applications ranging from understanding electron transfer reactions in biological macromolecules to evaluating aerodynamic flow over aircraft wings. Inorganic chemists with a variety of specializations, including organometallics, solid-state chemistry, and classical transition metal chemistry have contributed to the development of new high technology materials.

Electron transfer reactions in proteins (Durham et al. p636).

Only one of the authors contributing to this collection wrote an article for the 1983 "State of the Art" issue. However, most of the authors of the articles presented here were either graduate students or postdoctoral associates of one of the earlier contributors. This collection begins with a discussion of particular applications to biological sciences. The article by Bill Durham and co-workers focuses on the use of Ru(II) diimine complexes as sensitizers for triggering electron transfer reactions in proteins. This is followed by an introduction to the use of bimetallic Pt(II) complexes in the cleavage and footprinting of DNA, as presented by Holden Thorp. An area of intense current interest is photoinduced electron transfer reactions of chromophores that intercalate into DNA. Thomas Netzel's article outlines the general arguments and presents a summary of some current results in this area.

Luminescence sensors (Sullivan el al. p 685).

Inorganic photochemistry and nucleic acids (Thorp et al. p 641).

Applications of inorganic photochemistry to chemical sensing (Ellis p. 680).

The focus then switches to applications related to the development of new materials. In his article on light-to-electrical-energy conversion, Gerald Meyer illustrates how various transition metal complex chromophores can be used as sensitizers for charge injection into nanocrystalline semiconductors in photoelectrochemical cells. This is followed by a discussion by Joseph Hupp of fundamental aspects of the charge injection process into nanocrystalline semiconductors. Next, Andrew Bocarsly discusses inorganic approaches to the development of new materials for photolithography and pattern generation at interfaces using complexes of Pt(II) and Fe(II). The focus switches to applications of organometallic complexes and is described here by David Tyler; he illustrates novel approaches to generating polymers that photodegrade to yield environmentally benign products. The final article of this section, by Raymond Ziessel, illustrates fundamental work directed toward the use of supramolecular transition metal complexes in the development of still-unrealized nanoscale electronic devices.

The final section of this discussion of developments in inorganic photochemistry involves applications of luminescent inorganic materials and complexes to chemical sensing. The use of luminescent semiconductors in the detection of various organic vapors is discussed by Arthur Ellis, whose group is largely responsible for defining this chemistry. The final three articles discuss applications of luminescent transition metal complexes of Ru(II) and Re(I) in chemical sensing. In his article, Patrick Sullivan illustrates how ion intercalation into crown ethers covalently linked to luminescent Re(I) complexes can provide a basis for analyzing levels of specific metal ions in solution. James Demas then demonstrates applications of particular Ru(II) diimine complexes in pH and pO₂ measurement. Demas also includes an experiment that illustrates the utility of luminescence quenching by molecular oxygen in sensor development. Finally, Martin Gouterman shows how the oxygen-sensing ability of luminescent Ru(II) complexes is exploited in aircraft design!

Luminescence of pressure sensitive paint for wind tunnel research (Gouterman p. 697).

Oxygen sensing by luminescence quenching (Demas et al. p. 696).

Photo-induced energy or electron transfer in supramolecular systems (Ziessel p. 673).

Overall, the collected works presented here are meant to provide readers with a general picture of the types of applications that have evolved from the fundamental knowledge base of inorganic photochemistry, and no effort has been made to comprehensively cover this research area. Since applications are generally interdisciplinary extensions of the basic science, the range of existing and potential applications of inorganic photochemistry extends far beyond the domains selected as focus points in this collection. We hope that this series of articles will serve to illustrate that as basic science in a particular area matures, it gives rise to applications that could not have been envisioned before the research area was developed.

Generating photodegradable polymers (Tyler p. 668).

Charge injection process (Hupp el al. p. 657).

Photolithography and pattern generation.

Photolithography and pattern generation (Bocarsly et al. p. 663).

Efficient light-to-electrical energy conversion: nanocrystalline TiO₂ films modified with inorganic sensitizers (Meyer p. 652).

Acknowledgment

Finally, we would like to acknowledge those who have supported this endeavor. First are the authors. The persons contributing to this collection took the time, made the effort, and, for the most part, bore the expense of participating in the symposium and preparing thoughtful, well organized manuscripts. We would also like to thank Morton Hoffman, who provided us with guidance in the initial stages of planning. In addition, we were fortunate to receive financial support for the symposium from the Petroleum Research Fund of the American Chemical Society, the Division of Chemical Education, and UVTech Associates. Finally, we would like to express our special thanks to Glenn Crosby of Washington State University for providing an insightful review of the entire symposium in print.

Literature Cited

1.Inorganic Photochemistry: State of the Art; Hoffman, M. Z., Ed. J. Chem. Educ. 1983 , 60, 784-887. Also available as a reprint book from Journal of Chemical Education Subscription and Book Order Department, P. O. Box 606, Vineland, NJ 08360; Order No. IN1.

2.Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood: New York, 1991.