Applied Photochemistry

This chapter gives an introduction to the key ideas which underpin photochemistry: the nature of electromagnetic radiation, the nature of matter, and the way the two interact. After a discussion of ultraviolet and visible electromagnetic radiation and its interaction with the optical properties of materials, an account is given of the fundamental properties of the four components involved in photochemistry, the protons, neutrons and electrons which make up atoms, and the photon. The ideas of wave mechanics and its application to atomic structure are introduced in a non-mathematical way, with atomic orbitals described in terms of quantum numbers, energies, degeneracies, shapes and symmetries. The role of electron spin in governing orbital occupancy is discussed, along with the structure of many-electron atoms and the use of term symbols to identify the various spin, orbital, and total angular momenta of atomic states. The use of atomic orbitals as constructs for molecular orbitals and molecular bonding is described. Term symbols for small molecules are illustrated briefly using O₂, which is particularly important in photochemistry, as an example. The concepts of a Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) are introduced, and the importance of these orbitals in photochemistry is explained. Bonding in conjugated systems, metals and semiconductors is described. The link between energy levels and electrochemical redox potentials is made. The various energy states in atoms molecules and solids, and the way energy is distributed within these energy levels according to the Boltzmann equation, are described. Timescales for various physical and photochemical processes are given. The interaction of electronic energy states with ultraviolet and visible light is discussed in terms of absorption, emission and stimulated emission, using the Einstein A and B coefficients, transition probabilities, and absorption coefficients. The absorption process and the various selection rules which control the efficiency of absorption, and emission, are described, as are the common types of electronic transitions. Absorption in gas, solution, and solid phases, and the effect of aggregation on absorption in solution, are discussed. Unimolecular radiative and non-radiative excited state deactivation processes are discussed in terms of the Jablonski diagram, and the ideas of, competition between decay routes, and quantum yield, are introduced. Bimolecular interactions, quenching and energy transfer are described, with Förster Resonance Energy Transfer (FRET) and Dexter energy transfer discussed in some detail, and the analysis of bimolecular quenching kinetics using the Stern–Volmer equation is given. The chapter finishes with brief discussions of excimers, exciplexes, delayed fluorescence and proton transfer.

Photochemical reactions are generally easily carried out, at least in laboratory scale, and require no expensive apparatus. Some general reactions, e.g. the cycloaddition of enones to alkenes and various oxygenations have been extensively investigated and represent an excellent choice for preparative applications. Many other possibilities are known—and a few are presented below. This suggests that photochemical steps should be considered more often in synthetic planning.

The fascinating field of inorganic photochemistry is extremely diverse. This chapter discusses some general principles governing light-induced properties of metal-containing molecular compounds. The great variety of excited states of different nature—far greater than those available in organic compounds—accessible in metal-containing species is discussed, and linked to various photochemical transformations. The emphasis is placed on the diversity and open-end possibilities to use wavelength-dependent reactivity of such species for creating a variety of products. Modern methods for the interrogation of excited states on ever faster time scales are briefly outlined. Recent technological advances open up exciting prospects of modulating the outcome of photochemical reactions by altering the earliest photo-events. It is clear that inorganic photochemistry will continue to play a central role in light-driven applications.

In this chapter we discuss some of the typical materials used in photochemistry. We describe, in general terms, how their suitability for application as absorber, emitter, sensitiser, energy acceptor or quencher, depends on the energy states within the material and the routes of interconversion between these states, and also how suitability as a redox or chemical sensitiser/acceptor/trap is determined by specific chemical reactivities. We describe the application of photochemical principles to the design of light sources and displays, and describe the photochemical principles and applications of photochromics and molecular switches. A table giving the structures, characteristics, and uses, of a number of compounds widely used in photochemistry is provided at the end of the chapter.

The main photochemical processes occurring in the Earth’s atmosphere and their effects on its chemistry and structure are described. The solar flux and its interaction with the components of air are discussed in Sect. 5.1. Of these, O₂ is the most photochemically active, UV absorption causing photodecomposition into ground state and excited state O atoms (Sect. 5.2). This causes differential heating of the atmosphere as the solar flux passes through. Without this, the air in thermal equilibrium cools with increasing altitude due to the effect of gravity. Combining the two effects creates the distinct layers (Sect. 5.3) known as the tropo-, strato-, meso- and thermo-spheres. Other designations e.g. the high altitude (D–F) regions containing high charge density are due to photo-ionisation. The detailed photochemistry of each region is discussed in Sects. 5.4–5.7, dominated at high altitude by oxygen atom and ozone reactions, which culminates in the stratospheric ozone layer. Ozone depletion due to the photochemistry involving chlorofluorocarbons is discussed. Comparatively little UV penetrates through to the troposphere, except enough to induce the formation of OH. It is the secondary reactions of OH which set off the oxidative chain reactions which dominate low altitude chemistry and initiates ground level ozone production. The combination of strong sunlight and automobile emissions causes photochemical smog. Aerosols play a vital role in dissolving the soluble reactants and oxidised hydrocarbons formed, thus removing them from the atmosphere by deposition as rain, and completing the cycle of pollutant emission and removal from the atmosphere.

A brief overview on the main photoprocesses applied to the treatment of water and wastewater is presented. The photodegradation methods that have been applied to the oxidation of organic pollutants are described. A review on advanced oxidation processes (AOP’s) and photooxidation mechanisms in homogeneous and heterogeneous solution is presented and some practical applications discussed. Combinations of biological and chemical treatments are considered to be a good approach to improve the removal efficiencies and reduce costs.

Photochemical conversion of solar photons is one of the most promising and sought after solutions to the current global energy problem. It combines the advantages of an abundant and widespread source of energy, the Sun, and Earth-abundant and environmentally benign materials, to produce other usable forms of energy such as electricity and fuels, without the negative impact of CO₂ or other greenhouse gas release into the atmosphere. Dye-sensitised solar cells (DSSC) and organic bulk heterojunction (BHJ) solar cells are two examples of such systems, allowing the conversion of visible sunlight into electricity by inorganic or organic semiconductor materials, which are inexpensive and easy to process on a large scale. Photocatalytic (PC) and photoelectrochemical (PEC) water splitting systems offer a solution to the problem of diffuse and intermittent sunlight irradiation, by storing the energy of solar photons in the form of clean energy vectors such as H₂. This chapter presents an overview of the technologies based on photochemical solar energy conversion and storage.

This chapter discusses a variety of free radicals and other reactive oxygen species that are biologically and medically relevant. Radiolytic and/or photochemical methods of production for each reactive oxygen species are shown and for each type of reactive oxygen species some antioxidant and/or biomolecule interactions are discussed. Additionally, the techniques of laser flash photolysis and pulse radiolysis are described in detail and a comparison of the two techniques is made.

This chapter discusses the various modalities of photomedicine, an interdisciplinary branch of medicine that involves the study and application of light with respect to health and disease. The following main concepts are covered: Photodynamic Therapy (PDT) for the treatment of cancer, PDT for bacterial infections, vascular PDT, photochemical internalisation, photochemical tissue bonding and the use of lasers in medicine.

Photochemicals play a leading role in the diagnosis of disease, being widely used in assays of whole cells, of protein levels and in genetic analysis. This chapter looks at a number of selected examples of photochemical diagnostics, aiming to give an overview of the principles of their use through case studies of the techniques most commonly employed within the clinical setting. These cover blood based diagnostics, fluorescence assays using immunochemistry and the photochemical analysis of DNA. The chapter concludes with a look to the future and consideration of the role of novel nanocrystal photochemicals in the burgeoning field of Nanomedicine.

Photochemical imaging is dominated by silver halide technology. The early history of photography is described while introducing the essential ingredients of modern photography. The technology is described by reference to modern materials and current understanding of the photochemical processes involved. At the core is the photolysis of silver halide crystals leading to the formation of latent images. Their manufacture and chemical and spectral sensitisation are briefly described. The development of the silver images is explained. Variations on the technology of development lead to the variety of types of familiar and less familiar photographic products. Non-silver photographic systems have also provided significant commercial imaging systems, for example, Blueprints, Diazotypes and dichromated colloid/polymer systems. The last was important in the early development of photography and is still exploited today. The principles of electrophotographic systems are also briefly described.

Optical sensors and probes have emerged as valuable analytical tools for the detection of a variety of biologically and chemically important analytes in the last three decades. Our aim for this chapter is not simply to provide a catalogue of results from the literature, but rather to discuss the fundamental principles behind optical sensing and to provide a suitable entry point for new researchers in the field. We take a bottom-up approach to the design of an optical sensor, starting with the different optical parameters available for use in sensing and the various response mechanisms shown by different classes of optical probes. We then consider the various approaches available for translation of a molecular probe into an optical sensor platform, including the current state-of-the-art and future trends in sensor design.

Photochemistry plays a critical role in modern semiconductor electronics, primarily through the use of photoactive polymers or photoresists in the lithographic processes used to fabricate semiconductor devices. Photoactive polymers have been extensively researched in order to develop resists that are chemically robust and that are able to produce sharp, well defined, high resolution features through photolithography. This chapter introduces photolithography and photoresists, and presents review of the photochemistry of some of the more important commercial photoresists. Miniaturisation of semiconductor devices for consumer electronics and sensors now places increasing demands on lithography processes. This has lead to the development of sub-micrometer and now nanometer scale devices. A review of electron beam lithography and other high-resolution lithography techniques concludes this chapter.

In this chapter we describe the basic photochemical instrumentation, instrument components and consumables, which make up a general photochemical laboratory. We consider factors such as sample preparation, optical properties of the sample, and contributions from background interferences, which can all affect the data obtained. We discuss the different accessories available, to optimise or perform more complex measurements such as fluorescence anisotropy and quantum yields. We do not consider in detail the more expensive systems required for specialised experiments, which are discussed in Chap.​ 15, although we do describe the general principles of these methods. Finally, we describe a Photochemical Library, a reference to useful books, journals, organisations, websites, programs, and conferences for researchers in the field.

The characterisation of the excited state of a molecule implies the determinations of the different quantum yields and lifetimes. Additionally, complex kinetic systems are frequently observed and need to be solved. In this contribution, we give our particular way of studying systems of organic molecules where we describe how a quantum yield of fluorescence (in fluid or rigid solution, or in film), phosphorescence, singlet oxygen and intersystem crossing can be experimentally determined. This includes a brief description of the equipments routinely used for these determinations. The interpretation of bi- and tri-exponential decays (associated with proton transfer, excimer/exciplex formation in the excited state) with the solution of kinetic schemes (with two and three excited species), and consequently the determination of the rate constants is also presented. Particular examples such as the excited state proton transfer in indigo (2-state system), the acid–base and tautomerisation equilibria in 7-hydroxy-4-methylcoumarin (3-state system), together with the classical examples of intramolecular excimer formation in 1,1’-dipyrenyldecane (2-state system) and 1,1’-dipyrenylpropane (3-state system) are given as illustrative examples.