Functional materials research is one of the high priority strategic areas of development in science and technology in the 21st century. Amongst the variety of functions, the interaction of matter with light to generate light-driven or photoresponsive properties has always been one of the most appealing and attractive areas. Recent advances in the exploitation of transition metal complexes in bringing about photo-induced functions have attracted growing attention, particularly in areas related to materials, energy, and biomedical research. Selected examples include the development of molecular triplet emitters for organic light-emitting devices (OLEDs), optical and photo-switches, photochemical energy storage, dye-sensitized solar cells, photochemical molecular devices (PMD) and machines, optical and luminescence probes and chemosensors, luminescent labels and tags for biomolecules, and luminescence signaling and imaging.
This volume serves to provide the readers with some fundamentals of luminescent transition metal complexes and the recent exciting developments of a selected variety of functions and potential applications that transition metal complexes can offer for the betterment of the society in areas related to materials, energy, and biomedical research.
The first chapter of this volume by Balch discussed the current progress in two-coordinate luminescent gold(I) complexes. This class of complexes is well-known to show weak metal... metal interactions that lead to the isolation of novel architectures and polymorphism from relatively simple building blocks and the appearance of unique electronic absorption and emission spectroscopic features. The effect of the environment, such as solvents and counter ions, on the luminescence behavior of a number of two-coordinate gold(I) complexes was discussed.
The ability to generate machines and devices at the molecular level and setting them into motion via light excitation has always been a fascinating topic of research. The second chapter by Sauvage highlighted the importance of generating long-lived charge-separated states through the harvesting of light to mimic the natural photosynthetic reaction centers, and the design and assembly of multi-component systems using metal complex building blocks for light-driven molecular machines and devices.
Photochromic transition metal complexes derived from photoisomerization represents another important branch of photofunctional materials. Several classes of photochromic complexes with photoisomerizable moieties were presented in the chapter by Nishihara. The effect of the transition metal complex systems on the photochromic moieties and the corresponding changes in physical properties of these materials were discussed. The importance of the introduction of transition metal complexes into organic photochromic units was highlighted.
Apart from using light to drive molecular motions and switching in the desired fashion as well as to induce charge separation as demonstrated in the previous chapters, another important consequence of photo-induced charge separation is to harvest light for conversion into electrical energy. This area of research has attracted fast growing attention as a major global issue that the world is facing today is the upcoming depletion of fossil fuels, the energy crisis and the urgent need for clean and renewable sources of energy. The chapter by Grätzel began with a brief historical background and the working principles of dye-sensitized solar cells followed by the maximization of quantum efficiencies through rational design of transition metal complex sensitizers. The second part of this chapter presented some recent examples of luminescent iridium(III) complexes used in organic light emitting devices. Organic lightemitting devices (OLEDs) have been identified as a promising candidate for the generation of lighting systems that are more energy-efficient so as to reduce the energy and environmental cost. The tuning of emission color and luminescence quantum yields of these highly emissive cyclometalated iridium(III) complexes were emphasized.
Instead of redox and light-driven molecular machines which generate mechanical motions, electron transfer through a "molecular wire" is also of prime interest. A number of wire-type metal diimine complexes that are capable of binding selectively to different enzymes through hydrophobic interactions were described in the chapter by Gray. This strategy not only provides a handle for the study of protein mechanism, photo-generation of reactive enzyme redox states and parameters controlling substrates binding, but also opens up a new avenue for the development of promising luminescent sensors, electrochemical probes, and crystallographic tools for enzyme study.
The last chapter by Lo described the luminescent properties of some recent examples of transition metal complexes that can be employed as luminescent labels and probes for bio-molecules. The structural design of the metal complexes, the labeling and probing strategies, the spectroscopic and luminescent properties of the complexes and their bio-conjugates as well as their biological and analytical applications were presented.
Last but not least, I am indebted to the authors for their immense contributions on these important and exciting topics and I hope that the readers will find this volume useful, stimulating and inspirational to their research. I would also like to acknowledge my coworker, Dr. C. H. Tao, for providing assistance in the preparation of this volume.
Hong Kong, May 2007