Preface to the Second Edition

In the past 10 years, several textbooks and reference books on photochemistry have been published. However, most of them essentially focus on the photoreactions of organic molecules. In some textbooks, the fundamental bases of excited-state properties are confined in a few pages; in others, theoretical aspects are presented in too much detail, including boring and unnecessary mathematical treatments. Most of the available books ignore, or barely mention, the photochemical and photophysical properties of metal complexes, a class of molecules that is attracting increasing theoretical and applicative interest. No textbook emphasizes the most recent trends in photochemistry and photophysics, such as information processing by reading, writing, and erasing molecules with light signals, the capability of powering and controlling molecular machines by light, the conversion of sunlight into electrical energy by inorganic and organic solar cells, the recent developments in the field of light-emitting diodes, and the first achievements along the road toward artificial photosynthesis.

For all these reasons, we felt there was the need for a book capable of (i) presenting a clear picture of the concepts required to understand the excited-state properties of the most important types of molecules, (ii) showing recent applications concerning photochemistry and photophysics, and (iii) opening the eyes of young researchers toward forefront developments or even futuristic visions of the light-matter interaction. The usefulness of the first edition of this book was testified by the prompt publication of a Chinese edition in 2015. In the following years, the frontiers of photochemistry and photophysics continued to expand with the development of new molecules, new materials, and new processes. There is no doubt that photochemistry and photophysics will play an increasingly important role in the development of science and technology.

Although the organization of this second edition is essentially the same as that of the first edition, several chapters have been revised considerably, others have been almost entirely rewritten, a number of schemes and figures have been added, and the reference list at the end of each chapter has been extended and updated.

We believe that this book, which originates from our long experience in teaching photochemistry and photophysics at the University of Bologna, can be a basic text for graduate and postgraduate courses because of its balanced content. We feel that it can also be useful for scientists who desire to enter photochemistry and photophysics research even if they did not have a chance, during their university training, to get the fundamental bases of this field. Scientist already active in photochemical and photophysical research can find suggestions to undertake novel scientific adventures.

Chapters 1-4 of this book deal with fundamental concepts concerning the nature of light, the principles that govern its interaction with matter, and the formation, electronic structure, properties, chemical reactivity, and radiative and nonradiative decay of excited states. Each concept is illustrated making reference to important classes of molecules. The notion that an excited state is a new chemical species with its own chemical and physical properties compared with the ground state is underlined, leading to the conclusion that photochemistry is a new dimension of chemistry.

Chapter 5 extends the above concepts from molecules to supramolecular (multicomponent) systems where a fundamental role is played by structural organization and component interactions.

Chapter 6 illustrates the fundamental concepts and the theoretical approaches concerning the two most important photochemical and photophysical processes, namely, energy transfer and electron transfer.

Chapters 7 and 8 deal with the photochemical and photophysical properties of organic molecules and metal complexes, respectively. The peculiar light absorption/emission spectra and the photochemical properties of the various families of organic molecules are illustrated by detailed discussions of several examples. For metal complexes, the discussion of the relationship between structure and photochemical and photophysical properties is underlined, with particular emphasis on the nature of the metal(s) involved, the outstanding luminescence properties of some classes of these compounds, and the relationships between luminescence and electrochemical properties.

Chapter 9 offers a detailed presentation of equipment, techniques, procedures, and reference data concerning photochemical and photophysical experiments, including warnings to avoid mistakes and misinterpretations.

Chapter 10 describes the relationships between photochemical, photophysical, and electrochemical properties of molecules that can be exploited for the interconversion between light and chemical energy.

Chapter 11 deals with the mechanisms of homogeneous and heterogeneous photocatalytic processes based on electron and hydrogen transfer reactions, including two-photon-driven photoredox catalysis and applications of photocatalysis for environmental protection.

Chapter 12 concerns the hot topic of light-powered molecular devices and machines. The concepts of exploiting the interaction between molecules and light to read, write, and erase information are illustrated, together with their application in the field of molecular logics. Various molecular devices (e.g., wires, switches, extension cables, pumps, and light-harvesting antennas) based on energy transfer, photoinduced electron transfer, or photoisomerization processes are described and important examples of light-powered molecular machines (e.g., linear and rotary motors) are discussed.

Chapter 13 illustrates in detail the reactions taking place in the natural photosynthetic processes of bacteria and green plants and describes the first achievements along the road toward photochemical water splitting by photocatalytic semiconductor nanoparticles and photoelectrochemical cells.

Chapter 14 illustrates the relationships between light and life, starting from vision and including damages caused by exposure to UV light, benefits deriving from light-based therapeutic processes, fluorescent sensors and their applications, and a brief description of bioluminescence processes.

Chapter 15 deals with applications of photochemistry and photophysics, covering various topics: photochromic compounds, luminescent sensors (including their use in fields as diverse as wind tunnel, thermometers, measuring blood analytes, detecting explosives, and warfare chemical agents), optical brighteners, atmospheric photochemistry, solar cells (PV, OSC, DSSC), electrochemiluminescent materials (LED, OLED, LEC), numerous applications concerning the interaction between polymers and light (e.g., photodegradation, photostabilization, photolitography, and stereolitography), and the photochemical syntheses of industrial products.

After having presented the fundamental concepts of photochemistry and photophysics and described the most important natural and artificial photochemical and photophysical processes, in Chapter 16, we offer the reader the opportunity to make acquaintance with forefront research through the discussion of selected topics taken from recent literature. The choice of the examples has been based not only on their intrinsic interest, but especially on their educational capacity to illustrate connections among fundamental photochemical and photophysical concepts.

In several chapters, additional information on specific topics is presented in boxes interlaced with the text. An important feature of the book is the abundance of illustrations that are essential for an easier understanding of the concepts discussed. References have been updated up to December 2023.

Before closing, we express our feeling concerning science, society, and Earth, the place on which we live. Planet Earth is a very special spaceship that cannot land or dock anywhere for being refueled or repaired. We can only rely on the limited resources available on the spaceship and the energy coming from the Sun. We are concerned about the increasing consumption of natural resources [1], the climate change [2], the energy crisis [3], and the degradation of the environment [4-6], which is accompanied by an increased social disparity. As Pope Francis warns [7-9], we are faced with a complex crisis that is both social and environmental. Strategies for a solution demand an integrated approach to combating poverty and protecting nature.

If we want to continue living on Earth, we must achieve the goals of ecological and social sustainability by implementing three transitions: from fossil fuels to renewable energies, from a linear to a circular economy, and from consumerism to sobriety [10], but we also need to create new resources. In principle, this is possible by exploiting the only abundant, inexhaustible, and well-distributed resource on which we can rely: solar energy. Starting from seawater, the fundamental components of our atmosphere, and mineral resources, by means of sunshine, we need to "fabricate" fuels, electricity, pure water, polymers, food, and other things we need [11].

Until now, humankind has taken from spaceship Earth enormous amounts of resources [12]. Hopefully, future generations will pay back Earth with a capital produced from human intelligence. Photochemistry and photophysics can help. Indeed, science...