An Overview of Photochemistry
Brad Farris
October 13, 1999
Allen D. Bishop, Jr., Ph.D.
Millsaps College Chemistry Department
ABSTRACT: Photochemistry is the study of the interaction of electromagnetic radiation and matter. In this paper, several processes in photochemistry are discussed such as fluorescence, phosphorescence, and excitation transfer. Reactions which involve photochemistry are also discussed including isomerization, dissociation, and chemiluminescence. High-energy photochemistry, the use of gamma rays, is also discussed.
An Overview of Photochemistry
Introduction
The interaction of light and matter has been a topic of interest to humans for quite some time. Photochemistry is concerned with the study of the interaction between light and matter. As well as being dealing with the chemical changes brought about by the absorption of light, photochemistry also deals with processes which do not involve overall chemical changes. The photochemical processes in which there are no overall chemical changes are fluorescence and phosphorescence. In fluorescence and phosphorescence, light is emitted from chemical species that have absorbed radiation. Chemiluminescence deals with the emission of light as a product of a chemical reaction.
Electromagnetic Radiation
To understand photochemistry, one must first understand electromagnetic radiation. Through the work of Christian Huygens (1629-95), the wave theory of light was developed. The wave theory explained such phenomena as diffraction, polarization, and interference. Sir Isaac Newton (1642-1727) first described the particulate theory of light when he developed the corpuscular theory of light. In the 1860s, James Clerk Maxwell laid the groundwork that would explain how light is propagated. Maxwell came up with the idea of electromagnetic waves in which waves are propagated perpendicularly to planes containing electric and magnetic fields. According to Maxwell, the planes containing the electric and magnetic fields are also perpendicular to each other. Hertz confirmed Maxwell’s theory (ca. 1887-1888) when he showed that the electromagnetic waves explained by Maxwell travel at the same speed as the velocity of light.
Figure 1: The Electromagnetic Spectrum
Max Planck developed the theory which explained that radiation has particulate properties around 1900. He explained that the "particles" of radiation (photons) have a fixed energy (
) given by the equation:
(Equation 1)
where h is Planck’s constant and v is the frequency of the radiation. One can observe that the energy of radiation directly proportional to the frequency of the radiation. Equation 1 can be expressed in terms of wavelength as shown below:
(Equation 2)
where v has been substituted with 
where c is the speed of light and l is the wavelength of the radiation. The wavelength range of 180-1000 nm (10,000 – 50,000 cm-1) is the approximate range which is of interest to photochemistry. This consists of the ultraviolet and visible regions of the electromagnetic spectrum. This range of wavelengths corresponds to an energy range of approximately 150-600 kJ/mole. This wavelength range is similar in energy to the energy of most chemical bonds; therefore, in many cases, bond rupture (leading to chemical change) can be brought about by the photons of these wavelengths.
Particles have discrete states as governed by the Boltzmann distribution law. This law states that the number of particles in two energy levels (n1 and n2) are separated by an energy gap D E as given by the following relationship:
(Equation 3)
Beer’s Law and Absorption Intensity
Molecules may become excited by when they absorb electromagnetic radiation. This is primarily due to an electronic excitation that involves the promotion of electrons to orbitals with higher energy than the ground state. Excitation also may involve the pairing of electrons that are unpaired and in separate orbitals in the ground state. The Grotthus – Draper Law, also called the first law of photochemistry, states that chemical change can only be brought about by light that is absorbed. However, it must be noted that a chemical reaction will not always follow the absorption of light.
Lambert developed a relation to determine the fraction of light transmitted through an absorbing system as shown below:
(Equation 4)
where I0 is the incident light intensity, It is the transmitted light intensity, C is the concentration of the absorbing species, and d is the depth of the absorbing species through which the light has passed, and e a constant of proportionality known as the extinction coefficient. The extinction coefficient (e ) is dependent on the wavelength of radiation and the absorbing substance. Beer developed a law which stated that if C and d are altered but their product was held constant, then the fraction of light transmitted also remained constant. Equation 3 was formerly known as Lambert’s Law, but it is now known as the Beer-Lambert Law (or, more often, just Beer’s Law). It is most commonly expressed in the logarithmic form shown below:
(Equation 5)
where A is the absorbance (optical density) and where T is the transmittance.
The Law of Photochemical Equivalence and the Quantum Yield
The law of photochemical equivalence, also known as the second law of photochemistry, also deals with the absorption of light. Developed by Einstein, the law of photochemical equivalence (also known as the Stark-Einstein law) states that only one molecule or atom is activated by a quantum of light. The amount of energy absorbed by the atom or molecule is dependent on the Planck equation (Equation 1). Equation 1 can be altered to express the amount of energy absorbed per mole of the absorbing substance to give the following:
(Equation 6)
where NA is Avagadro’s number. This can also be referred to as the excitation energy per mole. The amount of energy in a mole of photons of a certain wavelength is called an einstein. The efficiencies of photochemical processes are expressed in terms of quantum yields. The quantum yield 
of a particular photochemical process is the relationship between the number of molecules entering a reaction and the number of quanta absorbed as shown below:
(Equation 7)
Quantum yields may vary from zero to 106 (for the combination of chlorine and hydrogen described later). According to the Einstein equivalence law, the primary step that initiates a photochemical change is the absorption of one quantum of energy by a molecule of atom. However, the processes that occur after the absorption of this quantum of energy are beyond the concern of the Einstein equivalence law. Since the Einstein equivalence law says that only one molecule can be activated by a quantum of light, a quantum yield greater than one indicates the possibility that a reaction is occurring through one or more secondary reactions (reactions among molecules which were not produced in the primary excitation step). A quantum yield greater than one suggests the occurrence of a chain mechanism. Since there are other fates for molecules that have been excited other than to undergo chemical reactions, quantum yields are often less than one (even for reactions in which there is a chain mechanism). For certain photochemical reactions, the distinction between the primary quantum yield and the overall quantum yield is often made. According to the Einstein equivalence law, the sum of the quantum yields for all primary processes must be one. Measuring quantum yields often requires the measurement of the number of quanta of radiation absorbed, and an absolute measure of a number of photons is often difficult. However, methods do exist to do this. If 
, then it is not likely that a great deal of chemical change is occurring in the absorbing molecule. In this case, the molecule is either emitting radiation (e.g. fluorescence) or intermolecular or intramolecular energy transfer is taking place.
Excited States of Molecules
Absorption of visible or ultraviolet light can result in the excitation of electrons. There are two possible excited states according to the electronic orbital configuration. One state involves the pairing of electron spins (antiparallel spins), and the other state involves electron spins which are unpaired (parallel). In the presence of a magnetic field, paired electrons remain in one state, and this state is called a singlet state (often referred to as S0 for the ground singlet state and S1 for the excited singlet state with the lowest energy). A state with unpaired spins splits into three quantized states in the presence of an electromagnetic field, and this is called a triplet state (often abbreviated as T1). A state energy diagram (also known as a Jablonski diagram) is shown in Figure 2. The diagram shows the energies of the ground state, the excited singlet state, and excited triplet state of a molecule. The photophysical processes (both radiative and radiationless) are also shown in Figure 2.
Figure 2: State Energy Diagram Showing the Energies of the Excited States and the Radiative and Radiationless Photochemical Processes
The Franck-Conden Principle
The Franck-Conden Principle says that electronic transitions occur so much faster than the vibration of molecules that no change in internuclear separation occurs during the electronic transition. As a consequence of the Franck-Conden Principle, the probability that a transition will occur in a small range of internuclear distance will depend on the product of the probability of a molecule in a certain electronic state. The total transitional probability is the probability integrated over all internuclear distances.
The Franck-Conden Principle allows the prediction of the relative intensities of the transitions to vibrational levels. When absorption takes place, transitions are most likely to originate from the point with the highest probability in the lowest state. The intensities of transitions to higher vibrational levels will depend on the probability of encountering an upper state with a certain internuclear separation. The most probable transitions will have the highest intensity.
Figure 3: Illustration of the Franck-Condon Principle
Primary Photochemical Processes
The photochemical processes (which follow electronic excitation by the absorption of light) are shown in Figure 4 (as shown by Wayne) and are discussed in the following text.
Figure 4: Routes to the Loss of Electronic Excitation
Fluorescence
When an atom is excited, it must lose its energy through the emission of radiation or through the deactivation of the energy through collisions. Because chemical degradation of atoms is not possible and an increase in translational energy is not probable, fluorescent emission of radiation is expected from all atoms at low pressures. Fluorescence is represented as path (vi) in Figure 4 under the category of luminescence. Overall, fluorescence involves an absorption of radiation followed by a vibrational relaxation and then a readmission at longer wavelengths than the initial radiation. Fluorescence may also involve the emission of the wavelength of light which was absorbed when an excited electron returns to its initial state. This is referred to as resonance fluorescence. Resonance fluorescence is only observed in gases of atoms or small molecules at low pressures.
However, many molecules do not exhibit fluorescence, or they fluoresce only weakly. There are many factors that help decide whether a molecule will fluoresce or not. Absorption must take place at a wavelength long enough to prevent chemical dissociation from occurring. If the molecule is transferred to an unstable state, fluorescence will probably not take place. If the absorption maximum is at a greater energy than the energy required to cleave the least stable bond, fluorescence will not occur. In addition to this requirement, intramolecular energy transfer must be slow as compared to the rate of radiation. As the conjugation in hydrocarbons increases, the first (p ® p *) absorption maximum is shifted towards longer wavelengths, and this tends to favor phosphorescence over decomposition. Likewise, rings with a high density of p bonds have higher levels of fluorescence. For example, the following molecules exhibit fluorescence:
Figure 5: Molecules Possessing Fluorescence
The fluorescence yield 
for fluorene is 0.54, and the 
for biphenyl is 0.23. This is due to the geometrical factors. The phenyl groups of the biphenyl molecule are free to move in and out of the plane of the paper, and this breaks up the conjugation of the two phenyl groups. The conjugation in fluorene is more stable because it is more rigid and planar, making it more fluorescent.
Phosphorescence
Phosphorescence involves an absorption followed by the crossing into another excited state, a vibrational relaxation, and then reemission at longer wavelengths. In phosphorescence, the second state must be a forbidden transition to the ground state, and the lifetime of the excited state must be long (longer than in fluorescence). Phosphorescence is represented in path (vi) of Figure 4 under the general category of luminescence. Phosphorescent species may often emit radiation for relatively long periods of time, often for some time after the excitation source has been extinguished. This is due to the fact that phosphorescent emissions arise from a triplet state. The transition form the triplet state is referred to as a "forbidden" transition, and this transition is normally long-lived. The triplet transition has been confirmed by electron paramagnetic resonance (EPR) and other techniques. The triplet state is of lower energy than the singlet state, and the phosphorescent emission is almost exclusively a longer wavelength than the fluorescent emission. Phosphorescence is often observed in organic molecules trapped in a rigid glassy medium. It has also been observed in solutions of dyes in gelatin. Phosphorescence is now studied in mixtures of various solvents (such as ether, ethanol, and isopentane) frozen at liquid nitrogen temperature (77K). The longest phosphorescence lifetimes are around 30 sec (whereas the longest fluorescence lifetimes are around 10-6 sec).
Excitation Transfer
Excitation transfer occurs after an absorption when the excitation is passed from one excited state to another until the lowest state is reached. In a molecule, this occurs when a molecule has at least two conjugated chromophores that can absorb independently. The chromophore group with the lowest excitation energy will possess the lowest excited state of the molecule.
Quenching
Physical quenching occurs when an excited molecule is relieved of its excess energy by another molecule. Physical quenching cannot be confused with intermolecular energy transfer, the process whereby one excited molecule transfers its energy to another molecule, inducing it to be involved in a variety of chemical processes. In physical quenching, the electronic excitation leads to the vibrational or translational excitation of another molecule. Quenching is represented as path (vii) in Figure 4. In Figure 4, the excited molecule’s excess energy is relieved by another molecule (M). For example, an excited atom can activate a molecule when it collides with it as shown below:
Cd* + H2 ® Cd + H2* (Equation 8)
Photosensitized Reactions
When dissociation or a reaction occurs in a species other than that which originally absorbed the radiation, the reaction is referred to as a photosensitized reaction. In photosensitized reactions, one of the molecules or atoms in the reaction mixture absorbs light and becomes excited, and after this, this molecule passes energy to a reactant, activating it for reaction. The substance which absorbs the light originally is called a photosenstitizer. An example of this type of reaction is the combination of mercury vapor, hydrogen, and carbon monoxide as shown below:
(a) Hg + hv ® Hg*
(b) Hg* + H2 ® 2 H + Hg
(c) H + CO ® HCO
(d) HCO + H2 ® HCHO + H
(e) 2 HCO ® HCHO + CO
(f) 2 HCO ® HCO—CHO (glyoxal) (Equations 9(a)-(f))
The following reactions are examples of photosensitized gas reactions:
(a) 2 O3 ® 3 O2 (sensitized by Cl2, quantum yield of 2-30 at l = 4300 Å)
(b) 2 H2 + O2 ® 2 H2O (sensitized by Cl2, quantum yield of 2 at l = 4300 Å)
(c) 2 CO + O2 ® 2 CO2 (sensitized by Cl2, quantum yield of 1000 at l = 4050-4360 Å)
(Equations 10(a)-(c))
Reaction Examples
There are many examples of processes in which chemical reactions follow the excitation of molecules by radiation. The reaction may be intermolecular or intramolecular. Intramolecular reactions include isomerizations, intramolecular reductions, or additions. Intermolecular reactions are those which occur with reactants which are added to the excited species or with the solvent in which the absorbing substance is in solution. Photochemical gas reactions are important photochemical processes in which additions or decompositions can take place.
Photochemical Gas Reactions
One of the most classical photochemical gas reactions is the combination of hydrogen and chlorine gas after the absorption of light. This reaction follows the following mechanism:
(a) Cl2 + hv ® 2Cl
(b) Cl + H2 ® HCl + H
(c) H + Cl2 ® HCl + Cl
(d) Cl (at walls) ® ˝ Cl2 (Equations 11(a)-(d))
As many as 106 molecules of HCl are formed per absorbed quantum of light based on experimental observation. This is due to the fact that this reaction follows a photochemical chain reaction. The following rate expression has been determined by Bodenstein and Unger:
(Equation 12)
where Ia is the intensity of the absorbed light and k is a constant.
Isomerization
Photochemical processes can bring about a number of isomerizations and rearrangements. The isomerizations brought about by photochemical processes can include cis-trans isomerizations and other structural isomerizations. The following is an example of a geometrical cis-trans isomerization brought about by a photochemical process is shown below:
Figure 6: Cis-Trans Isomerization of cis-2,2 dichloroethylene to the trans Isomer by a Photochemical Process
Another important category of photochemical reactions includes electrocyclic processes. In these reactions, bond-breaking and bond-making occurs in a single reaction. The commercial manufacture (shown in the figure below) of vitamin D includes a photochemical step in which an electrocyclic ring opening occurs. A similar process is thought to occur in the body.
Figure 7: The Process Used in the Commercial Manufacture of Vitamin D including a Photochemical Electrocyclic Ring Opening Step
2,5-Dimethylhexa-1,3,5-triene can undergo an electrocyclic ring formation (in the formation of 1,4-dimethyl-cyclohexa-1,3-diene) or a cis-trans isomerization depending on the wavelength of incident radiation as shown below:
Figure 8: Photochemical Reactions Occurring to 2,5-Dimethylhexa-1,3,5-triene Depending on the Incident Radiation
Otther isomerizations include hydrogen abstractions and sigmatropic shifts. In sigmatropic shifts, bonding electrons are reorganized in a continuous cyclic array as shown below:
Figure 9: Sigmatropic Shift of a Conjugated Triene Induced by a Photochemical Process
Dissociation
Dissociation of molecules into fragments can be induced by photochemical processes. The primary products of a dissociation brought about in a photochemical process can often be hard to determine, and the dissociative mechanisms can also be hard to elucidate. In organic compounds, the photochemical process is followed by decomposition, rearrangement, or a reaction of the intermediates formed first. For example, nitrogen dioxide dissociates in the following reaction:
NO2 + hv ® NO + O2 (Equation 13)
The photodissociation of nitrogen dioxide has a quantum yield of approximately one at a wavelength less than 370 nm. At a wavelength of 404.7 nm, the quantum yield is only 0.36. The quantum yield for this reaction tends to increase with increasing temperature.
Photodissociation is also called photolysis. Some additional photolysis reactions are shown below:
(a) 2 NH3 + hv ® N2 + 3 H2 (quantum yield of 0.25 at a wavelength of 2100 Å)
(b) 2 H2O2 ® 2 H2O + O2 (quantum yield of 7-80 at a wavelength of 3100 Å in H2O)
(Equations 14(a)-(b))
Chemiluminescence
Chemiluminescence refers to the emission of radiation following excitation by a chemical reaction. A thermal process is not involved in the emission of light. There are many natural examples of chemiluminescence such as fireflies, the glow of rotting fish, many bacteria, and the light of glow-worms. Chemiluminescence involves the general reaction sequence D G ® * ® hv where the chemiexcitation step (D G ® *) is the most important in the process. The * ® hv is the common process in which an excitation is followed by the emission of radiation. Photochemistry is often considered as the reverse of luminescence in which the fundamental process is hv ® * ® D G.
The bioluminescence of the firefly (Phontius pyralis) is reported to be the most efficient chemiluminescent system. This bioluminescence is brought about through the action of a lumophore and enzymes which regulate the chemiexcitation step. Luciferin is the compound associated with the lumophore, and luciferin (a substrate) is triggered by luciferase (the enzyme). The measured quantum yield 
of this reaction is 1.0.
Figure 10: Bioluminescent Reaction in the Firefly (Phontius pyralis)
Interestingly, there are number of other ways in which electronic excitation may occur to produce the emission of radiation. These include the following: pyroluminescence, the excitation by heat (e.g. in NO2); electroluminescence, the excitation by an electric field (e.g. in solid ZnS); cathodoluminescence, excitation by the impact of electrons on solid phosphors (as in television tubes); triboluminescence, the excitation brought about by the crushing of crystals (e.g. uranyl nitrate); the excitation brought about by the electron impact on gases (e.g. in discharge lamps), and crystalloluminescence, the excitation brought about by rapid crystallization from solution (e.g. strontium bromate). Chemiluminescence may also exist when wood and vegetation decay in places such as marshes.
Kinetics and the Study of Photochemical Reactions
For photochemical processes, the primary processes initiated the absorption of light are often followed by secondary processes (reactions) that must be determined before a rate law can be derived. To determine a rate law, the rate of disappearance of all of the reactants can be analyzed. Also, the effect of various intensities of light can also be useful in determining the rate law. Using these methods, the primary step can often be discovered. Photochemical reactions come in a variety of different forms, and it would be difficult to outline all of the different kinetic expressions for photochemical reactions. Here is an example of a general type of photochemical reaction:
Overall reaction: A2 ® 2A
(a) 
(activation)
(b) 
(dissociation)
(c) 
(deactivation) (Equations 15(a)-(c))
The rate of formation of A is given by the following:
(Equation 15(d))
The rate of formation of the excited A* is proportional to the intensity of absorbed light (Ia). This gives us the following relationship:
(Equation 15(e))
The rate of disappearance of A2 is given by:
(Equation 15(f))
Equation 15(e) and 15(f) can be equated using the steady state approximation to yield:
(Equation 15(g))
(Equation 15(h))
Equation 15(h) can be substituted into equation 15(e) to obtain:
(Equation 15(i))
For every two molecules of A that are formed, one molecule of A2 reacts. Therefore, the photochemical efficiency of the process is given by the following:
(Equation 15(j))
High Energy Photochemistry
High energy photochemistry deals with the chemical activation brought about by a ,b , and g rays, protons, neutrons, deuterons, and high-voltage electric discharges. Primarily, this high-energy radiation produces a great deal of ionization. The ionization is brought about by the direct impact of these particles and by the secondary radiation produced by the initial ionization. Lighter and faster g rays and electrons do not produce ionization as readily as heavier particles such as a particles, neutrons, protons, and deuterons. The ions formed can go on to be involved in reactions, or they may be discharged by electrons. The products formed by the reactions induced in this manner are often of a lower molecular weight than the molecules which were initially bombarded. The bombardment of a monomer mixture can induce polymerization. The properties of polymers which are bombarded by these particles often change, and these polymers often behave like substances with a higher molecular weight than the original substances.
The use of gamma rays in high energy photochemistry is particularly important. Reactions induced by gamma rays (high energy photons) are photonuclear reactions. High energy photons are obtained in three ways: by slowing down electrons accelerated to 10-100 MeV by tungsten or other targets, by radiative capture nuclear reactions, or by targets bombarded with high energy electrons. Photonuclear reactions can lead to an excitation of a nucleus followed by a deexcitation and radiative emission. These reactions can also be followed by the emission of a variety of nucleons. In some cases, photonuclear reactions can lead to the fragmentation of a nucleus into several nuclei of similar masses.
Conclusion
As one can see, photochemistry is an extremely important topic. Photochemical processes are crucial to many natural phenomena, synthetic methods, and everyday applications. The study of photochemistry will continue to be an important research area because the use of light energy is a very efficient way to do useful work through chemical and physical processes.
References