1
Background and History

1.1 Introduction

Photochemistry is defined as "a branch of chemistry that deals with the effect of radiant energy in producing chemical changes." [1] In a more formal sense, photochemistry involves any overall process:

where R represents a molecule that absorbs the energy of a photon (h?) to yield an electronically excited molecule (R *), which in turn reacts to yield one or more products (P) [2]. As we will see in Chapter 2 and other parts of this book, there are many other possible fates for electronically excited intermediates, R *. In some cases, the excess energy within R * is dissipated by physical processes that may return the reactant molecule to its ground state. These may be viewed as examples of photophysical processes: [2]

Photochemical reactors include natural and engineered systems in which one or more photochemical processes take place to bring about changes to system composition. Common changes within these systems may involve chemical or microbial components of the system, but in principle any component or phase of the system may be susceptible to photochemical change. In most applications, we are concerned with changes to fluid (i.e. liquid or gas) composition; however, photochemical and photophysical processes may also be induced within solids or on solid surfaces.

As compared with other fields of science, our understanding of the fundamental principles of photochemistry and photochemical reactor theory is relatively young. Much of the foundational material that informs our current understanding of photochemistry and photochemical reactors has been formulated over the last century. As a point of reference, calculus was developed independently by Isaac Newton and Gottfried Leibniz in the mid-17th century. Newton also played critical roles in establishment of many of the basic principles of modern physics and mechanics; he defined the "laws of motion" in 1687 in the Principia Mathematica Philosophiae Naturalis (Latin for "Mathematical Principles of Natural Philosophy").

This first chapter is intended to provide brief introductions to photochemistry and photochemical reactors, as well as the people and personalities involved in the development of the fundamental principles that inform our understanding of these processes. In addition to providing historical context, we will see that the people who developed these principles represented a remarkable cross section of humanity. However, among many (not all) of these individuals, several admirable personal attributes were evident. In general, the people who informed our current understanding of photochemical processes had remarkably clear, profound perceptions of the world that surrounds us and were able to distill their understanding of nature and natural phenomena into clear, concise statements. Some were able to describe these phenomena in mathematical terms, and many had a passion to communicate their understanding to others. These pioneers of photochemistry and photochemical processes were often progressive, courageous individuals who were willing to challenge scientific dogma, sometimes at the cost of significant professional risk.

1.2 Early Applications, Discoveries

In the 5th century BC, Hippocrates prescribed heliotherapy (i.e. sunbathing) for medical and psychological purposes [3]. This practice has continued until today, although a formal understanding of the specific medical conditions that were improved by this practice did not emerge until the 1700s. It was not until the 1800s that adverse human health effects were linked to exposure to sunlight.

Isaac Newton's contributions to science were not limited to the discovery of calculus or the formulation of the laws of motion. In 1666, he showed that white sunlight could be divided into colors; he coined the term "spectrum" to describe this range of colors and developed an instrument that involved optical elements, including a prism, which may have been the first spectrometer [4].

Ancient civilizations understood that sunlight provides visibility, warmth, health, and vitality; however, their understanding of these phenomena was almost entirely empirical. Physical explanations evolved through mythology. The first report of an organic photochemical reaction was attributed to Trommsdorff (1834), who reported that crystals of a-Santonin turned yellow and "burst" when exposed to sunlight [5]. Between the 17th century and early 20th century, scientific principles related to electromagnetic radiation and photochemistry evolved considerably [3, 6].

In 1614, Sala noted that sunlight turned silver nitrate (AgNO3) crystals black. More than a century and a half later (1777), Scheele observed that paper soaked in silver chloride (AgCl) darkened when exposed to sunlight. Though it was not evident at the time, this discovery was related to chemical (film) photography that would emerge in the late 19th century. Subsequent experiments by Ritter (1801) indicated that radiation just beyond the violet portion of the electromagnetic spectrum was more effective than other colors (wavelengths) at promoting this reaction. Radiation in this region of the spectrum was referred to as "deoxidizing" rays; today, we identify this as ultraviolet (UV) radiation.

A seminal contribution was provided by Kirchhoff and Bunsen in 1859 in the form of the spectroscope, an early form of a spectrometer (see Figure 1.1) [7]. This device was used to demonstrate that atoms absorb and emit electromagnetic radiation at characteristic wavelengths. Based on observations made with this device, they speculated that gaps in the spectrum of solar radiation were attributable to selective absorption by constituents of earth's atmosphere. This device and the observations made by Kirchhoff and Bunsen provided the first clear links between chemical composition and spectroscopic behavior. As such, this device represents the foundation for contemporary analytical spectroscopy.

Figure 1.1 Schematic illustration of spectroscope developed by Kirchhoff and Bunsen [7]. (a) was an internally blackened, trapezoidal box; two small telescopes (b and c) were directed at a prism (f). The eyepiece of (b) was removed and replaced by a plate in which a slit formed by two brass edges was adjusted at the focus of the objective lens. Light was provided by a device later identified as a Bunsen burner (d?). A sample of the material of interest was held in the flame by a platinum wire mounted on a stand (e). The prism rested on a brass plate; the prism and a mirror (g) were rotated using handle (h).

Source: Kirchhoff and Bunsen [7], figure 2 (p.91)/with permission of Taylor & Francis.

In 1865, Maxwell proposed that light and sound both belong to a larger energy spectrum that has wave-like properties. The term "electromagnetic" wave was assigned because Maxwell believed that both were generated by the interactions of electric and magnetic fields. Maxwell's theory was later supported by Hertz who demonstrated the existence of microwaves, which fell beyond (i.e. at wavelengths longer than) the UV, visible, and infrared (IR) portions of the spectrum. These findings established the basis for the wave theory of light, which still informs some aspects of our current understanding of the behavior of electromagnetic radiation.

1.3 Development of Modern Principles of Photochemistry

The observations and discoveries leading up to the work of Maxwell and Hertz formed the foundation of modern photochemistry. However, formalization of many of the fundamental principles of photochemistry required a number of other profound insights and discoveries. In many cases, the people who made these discoveries emerged as icons of the scientific world and are recognized among the giants of scientific history.

Max Planck was a German physicist who studied under Kirchhoff and Helmholz [8]. He succeeded Kirchhoff as Professor at the University of Berlin. Like many of the other scientists who participated in the development of principles that define photochemistry, he made numerous, wide-ranging scientific contributions, largely in the areas of thermodynamics and the behavior of electromagnetic radiation. Perhaps his greatest contribution came from his examination of radiation emitted from heated bodies, in which he pointed out that the principles that had been used to describe the behavior of radiation to that point were incapable of describing the emission spectrum from a "black body." This led to the development of quantum theory, and ultimately to Planck being awarded the Nobel Prize in Physics in 1919.

As part of his "miracle year" (1905), Albert Einstein published a paper that addressed the photoelectric effect; when subjected to intense irradiation, electrons are ejected from metal atoms [9]. This observation was inconsistent with the wave theory of light and supported the quantum theory that Planck had promoted. Ultimately, this work earned Einstein the Nobel Prize in Physics in 1922. Interestingly, the Nobel Committee was reluctant to recognize the importance of this work, the only work of Einstein that he himself would describe as "revolutionary." The Nobel Committee never...