Introduction

A photochemical reaction is a chemical reaction, which is induced via light. Photochemical reactions are useful in organic and inorganic chemistry since they proceed another way from thermal reactions. Photochemical reactions engage electronic reorganization initiated via electromagnetic radiation. The reactions are numerous orders of magnitude faster than thermal reactions; reactions as fast as 10^-9 seconds and connected procedures as fast as 10^-15 seconds are frequently examined. The light needed for a photochemical reaction may come from many sources. Giacomo Ciamician, regarded as the "father of organic photochemistry", utilized sunlight for much of his research at the University of Bologna in the early 1900's. Depending on the compounds being studied and the information being sought, bright incandescent lamps (chiefly infrared and visible light), low, medium and elevated pressure mercury lamps (185-255 nm, 255-1000 nm & 220-1400 nm correspondingly), high intensity flash sources and lasers have all been utilized. 

The procedure via that a photochemical reaction is carried out is said photolysis. Photolysis is generally initiated via infrared, visible, or ultraviolet light. A primary photochemical reaction is the instant consequence of the combination of light. Subsequent chemical transforms are said secondary procedures.

Photochemical reactions are utilized in synthetic chemistry to generate various organic molecules. In addition, many common procedures are photochemical in nature and contain significant applications. For instance: photosynthesis involves the absorption of light through the chlorophyll in plants to create carbohydrates from carbon dioxide and water. Photography uses the action of light on grains of silver chloride or silver bromide to produce an image. Ozone formation in the upper atmosphere results from action of light on oxygen molecules. Solar cells, which are utilized to power satellites and space vehicles, convert light energy from the sun to chemical energy and then release that energy in the form of electrical energy.

In this chapter we shall focus mainly on the nature and behaviour of the electronic excited states shaped when a photon is absorbed by a chromophoric functional group. As a rule, these excitation consequences in a transform in molecular orbital occupancy enhance in energy and changes in local bonding and charge distribution. 

Features of Photochemical Reactions

1. Photochemical reactions don't happen in dark but occur in the presence of light via absorbing it.

2. Because dissimilar coloured radiations in the range of visible light have dissimilar frequencies and therefore different energies, consequently all radiations might not be proficient to initiate a particular reaction. For instance, a photon of violet light has highest frequency and therefore the highest energy. Thus a reaction that is initiated via violet light might not be initiated via other colored radiations of visible light. On the other hand, a photon of red light has lowest frequency and energy. Thus a reaction that can be initiated via red light can be initiated via all other radiations also.

3. Temperature has extremely little consequence on the rate of a photochemical reaction. Instead, the intensity of light has marked effect on the rate of photochemical reaction.

4. The free energy transform of a photochemical reaction may not be negative. 

5. There are many substances that don't react directly when exposed to light. Though, if another material is added, the photochemical reaction begins

Photochemical reactions interesting 

1. The excited states are rich in energy. Hence reactions might take place that are extremely endothermic in the ground state. Using the equation E = hν we can correlate light of a wavelength of 350 nm with an energy of 343 kJ/mol.

2. In the excited state anti-bonding orbitals are occupied. This might permit reactions that aren't probable for electronic causes in the ground state. 

3. Photochemical reaction can involve singlet and triplet states; thermal reactions generally only illustrate singlet states. In photochemical reaction intermediates may be formed which aren't accessible at thermal 

Essential criteria for all photochemical reactions: 

• Molecule must absorb light
• Radiation energy must match energy difference of ground and excited state

Characteristic absorption range of several significant classes of organic compounds: 

Simple alkene         190 - 200 nm 

Acylic diene            220 - 250 nm 

Cyclic diene            250 - 270 nm 

Styrene                    270 - 300 nm 

Saturated ketones    270 - 280 nm 

α,β-Unsaturated ketones   310 - 330 nm 

Aromatic ketones/aldehydes   280 - 300 nm 

Aromatic compounds    250 - 280 nm 

Factors Determining Outcome of a Photochemical Reaction 

The broad variety of molecular mechanisms of photochemical reactions creates a common discussion of these factors extremely hard. The chemical nature of the reactant(s) is certainly among the most significant factors determining chemical reactivity initiated via light. Nevertheless, a better understanding of this aspect might be gained from a closer examination of the individual groups of chemical compounds. The nature of excited states included in a photoreaction is straight related to the electronic structure of the reactant(s). Ecological variables, for instance, parameters that aren't directly related to the chemical nature of the reacting systems, might as well strongly influence photochemical reactivity. It is helpful to differentiate between variables that are common for thermal and photochemical reactions, and those that are precise for the reactions of excited species. The 1^st group contains reaction medium, reaction mixture composition, temperature, isotope consequences to name the most significant. The distinctive characteristic of photochemical reactions is that such parameters almost always operate under situations when one or more photophysical procedures compete by a photoreaction. The effect of a photo-induced transformation can only be implicit as the interplay of numerous processes analogous to passages on and between at least 2 potential energy surfaces. Reaction medium might straight modify the potential energy surfaces of the ground and excited states and therefore influence the photo-reactivity. The outcome of several reactions changes dramatically when solvent polarity and hydrogen bonding capacity are changed. The protolytic photo-dissociation of 1-naphthol is entirely suppressed in aprotic solvents since of unfavourable solvation energies both for the anion and proton. Under these situations, proton transfer reaction can't compete through the deactivation. Solvent viscosity will strongly influence photoreactions where the encounter of 2 reactants or a substantial structural change is needed. In extremely viscous or solid solutions the loss of excitation by light emission or unimolecular non-radiative deactivation is more probable than a chemical modification of the excited species. On the other hand, slow diffusion in viscous solutions might stop self-deactivation of the triplet state via a bimolecular procedure said triplet-triplet annihilation and increase the competence of a photoreaction from this state. Triplet-triplet annihilation belongs to electronic-energy transfer procedures, which might be classified as quenching of excited states. Quenching rate is an extremely significant factor in discussing results of 

The Photochemical Process

To initiate a photochemical procedure, an atom or molecule must absorb a quantum of light energy from a photon; when this happens, the energy of the atom or molecule enhances above its normal level. The atom or molecule is now in an excited (or activated) state. If a quantum of visible or ultraviolet light is absorbed, then an electron in a comparatively low energy state of the atom or molecule is excited into a higher energy state. If infrared radiation is absorbed via a molecule, then the excitation energy influences the motions of the nuclei in the molecule.

After the initial absorption of a quantum of energy, the excited molecule can undergo a number of primary photochemical procedures. A secondary procedure might take place after the primary step. The absorption step can be symbolized via where the molecule M absorbs a quantum of light of appropriate energy to yield the excited M* molecule.

Primary Photochemical Processes

The figure below indicates the diverse primary processes that the excited M* molecule can undergo: The highly energized - or excited - molecule may return to its initial state according to any of three physical processes: 

1) It can liberate its excitation energy via emitting luminescent radiation through fluorescence or phosphorescence. 

2) It might transfer its energy to several other molecule, C, through that it collides, with no emitting light. The latter energy transfer procedure consequences in a normal molecule, M, and an excited molecule, C*. 

3) As an effect of the initial light absorption step, an electron (e^-) in the atom or molecule might absorb so much energy that it might escape from the atom or molecule, leaving behind the positive M⁺ ion. This process is said photoionization. If the excited M* molecule (or atom) does react, then it might undergo any of the subsequent chemical processes: photodissociation, intramolecular (or internal) rearrangement, and reaction through an additional molecule C. Photodissociation may affect when the excited molecule smashes apart into atomic and/or molecular fragments A and B. A rearrangement (or photoisomerization) reaction involves the conversion of molecule M into its isomer N - a molecule through the similar numbers and kinds of atoms but via a different structural arrangement of the atoms. The conversion of trans-1,2-dichloroethylene into the cis isomer is an instance of intramolecular rearrangement. The reaction is revealed below:

In the trans isomer the chlorine atoms lie on opposite sides of the double bond, whereas in the cis isomer they are on the similar side of the double bond.

Secondary Photochemical Processes

Secondary processes might take place upon completion of the primary step. Numerous instances of these processes are explained below.

Formation of Ozone

Ozone (O₃) is shaped in the upper environment from ordinary oxygen (O₂) gas molecules according to the reaction: After a quantum of ultraviolet light is absorbed (step 1), the excited oxygen molecule separates into 2 oxygen atoms (step 2). An oxygen atom then reacts through O₂ to form ozone (step 3).

Destruction of Ozone in the Upper Stratosphere

Certain chlorofluoromethanes, these as CCl₃F and CCl₂F₂, are utilized as refrigerants and-in several countries-as propellants in aerosol cans. These compounds ultimately diffuse into the stratosphere, where the molecules undergo photodissociation to generate chlorine (Cl) atoms, which then react through ozone molecules according to the formula Cl + O₃ → ClO + O₂. This reduce in the ozone content of the upper atmosphere permits more ultraviolet radiation to reach the surface of the earth.

Chain Reactions

If the primary photochemical procedure includes the dissociation of a molecule into radicals (unstable fragments of molecules), then the secondary procedure might involve a chain reaction. A chain reaction is a cyclic procedure whereby a reactive radical attacks a molecule to create another unstable radical. This new radical can now attack another molecule, thereby reforming the original radical that can now start a new cycle of events.

The hydrogen-chlorine reaction is an example of a chain reaction. The overall reaction between hydrogen and chlorine gases in the presence of violet or ultraviolet light forms hydrogen chloride; it is given by This reaction actually proceeds according to the subsequent series of steps:

According to the above mechanism, an appropriate quantum of light dissociates a chlorine molecule into atoms (step 1). The reactive Cl atom attacks a hydrogen molecule to yield hydrogen chloride and a hydrogen atom (step 2). The reactive hydrogen atom attacks a chlorine molecule that regenerates the Cl atom (step 3). This chlorine atom can then react through another H₂ molecule according to step 2, beginning a new cycle of steps. Steps 2 and 3 will take place many times until either of the 2 reactants, H₂ and Cl₂, is entirely consumed or until the H or Cl radicals attack a new material that has been introduced into the reaction chamber.

Types of Photoreactions

There survives a plethora of photoreactions virtually for each class of chemical compounds. Such reactions might be categorized according to chemical composition and structure. They might as well be classified under dissimilar kinds via using theoretical models for the explanation of the excited state(s) or structure of the potential energy surface. Though, for our introductory discussion it seems to be more appropriate just to consider several illustrations classified via common reaction kinds (Figure). 

Figure: Multiple reaction pathways for electronically excited species. 

Linear addition to an unsaturated molecule

The pyrimidine base, thymne, in DNA can combine with the amino acid residue, cysteine, in proteins. This is a model for the photochemical crosslinking of DNA and proteins by UV radiation

Fig: 5-5-cysteinyl

Cycloaddition of unsaturated molecules

Two thymines can react to shape a ring product, the thymine dimer, an significant class of products formed in DNA via UV radiation. 

Fig: thymine dimer

Photofragmentation

The side chain of riboflavin can divide off to form lumiflavin.

Fig: Photofragmentation

Photooxidation

Singlet oxygen is a simply available reagent. It can be produced from triplet oxygen in many solvents via a broad variety of sensitizers. The reaction of organic compounds through singlet oxygen can lead to reactive molecules, such as hydro peroxides, 1, 2-dioxetanes and endoperoxides. These compounds are helpful for subsequent transformations, for example, the ring formation of cholesterol can add a peroxy group.

Fig: Photooxidation  

Photohydration 

Uracil can add a molecule of water to it 5-6 double bond whenever UV irradiated.A photohydrolysis reaction in aqueous solution (substitution by OH^-) was utilized to give the rapid light-controlled liberate of biologically active molecules, these as aminoacids, nucleotides, and so forth. Biologically inert compounds affording these liberates upon photoirradiation are termed to as 'caged' compounds. Two-photon photochemistry is of huge interest for these studies, because one can use red light or IR radiation that isn't absorbed via biomolecules, and is biologically benign. 

Fig: Photohydration 

Cis-Trans Isomerization

All-trans retinal can be changed to 11-cis retinal.

Fig: Cis-Trans Isomerization

Rearrangements of electronically excited molecules present one of the most exciting chapters in photochemistry in the sense that they go behind reaction pathways that are usually inaccessible for the ground state (activation barriers in the ground state are extremely elevated the cis-trans isomerization of double bonds belongs to these reactions. The azobenzene reaction provides an instructive instance. Scheme 1 illustrates photoinduced rearrangements of stilbene that has been extensively studied. In addition to double bond isomerization, cis-stilbene undergoes as well cyclization through a lower quantum yield to form dihydrophenanthrene. The cis-trans isomerization of stilbene happens through rotation around the double bond. In the ground state this rotation encounters a large barrier, for example, there is a maximum on the ground-state potential energy surface at the geometry analogous to a twist angle of about 90o. In contrast, both the 1^st singlet excited state and triplet state have a minimum approximately at the similar geometry. The close proximity of the minimum and maximum facilitates a jump to the ground state. The cis-trans isomerization of azobenzene may proceeds not only through rotation, but as well through nitrogen inversion, for instance in-plane motion of the phenyl ring. 

Photo rearrangement 

Two illuminating instances of photoinduced rearrangements of substituted benzaldehydes are presented in Scheme 2. Intramolecular hydrogen transfer in 2-hydroxybenzaldehyde is an extremely fast reaction in the singlet excited state. Though, the procedure is entirely reversed upon a jump to the ground state. Overall, no chemical conversion is observed and excitation energy is either dissipated as heat or released as light, but by a longer wavelength. This performance is trait for aromatic carbonyl compounds via ortho-hydroxy groups, and they originate application as UV protectors, in sunscreens for illustration. Molecules acting as UV protectors absorb light, which is dangerous for biological molecules, and change light into heat or radiation that is biologically benign. In contrast, an intramolecular hydrogen transfer in 2-nitrobenzaldehyde initiates a succession of the ground-state reactions that leads to 2-nitrosobenzoic acid. The latter molecule is a reasonably strong acid, and dissociates in aqueous solutions so that the photochemistry of 2-nitrobenzaldehyde can be utilized to generate a quick pH-jump in solution. Many biological macromolecules, these as proteins and nucleic acids, demonstrate pH-dependent conformational changes. Those transforms can be monitored in real time via using the light-induced pH-jump.

Scheme 2 

Another significant instance is the conversion of 7-dehydrocholesterol to vitamin D3. 

Energy Transfer (Photosensitization)

When a 2^nd molecule is located near an electronically excited molecule, the excitation can be transferred from one to the other through space. If the second molecule is chemically different, there can be a substantial transform in the luminescence. For instance, the chemi-luminescence of a jellyfish is in fact blue, but, since the energy is transferred to GFP, the examined fluorescence is green. Photosensitized molecular oxygen is a powerfully oxidative species that severely hampers the photosynthetic competence of plants and reasons health difficulties these as cataracts in humans. The ground state of molecular oxygen is extremely strange in, which it is a triplet; therefore it can admit electronic energy from more-energetic triplet states of other molecules in a procedure said quenching (as in the case of the space shuttle wing explained above). When this happens, the donor molecule starts in its triplet state and undergoes a change in spin to its singlet ground state. The molecular oxygen starts in its triplet ground state and as well changes spin to a singlet excited state. Since the total spin between the 2 molecules is unchanged, the shift of energy can take place quickly and proficiently. The consequential molecular oxygen singlet state phosphoresces in the far red and the near infrared. Furthermore, it is together a strong oxidant and peroxidant and, if formed, might chemically attack (oxidize) a nearby molecule, frequently the similar molecule that sensitized the molecular oxygen. The oxidation reaction often modifies the molecule to a form without colour. This light-induced bleaching (one kind of photodamage) can be examined in nearly any coloured substance left in sunlight.

In fact, the photosynthetic systems in plants must be continuously dismantled, renovated, and rebuilt since of photodamage (primarily from singlet molecular oxygen). Several organisms utilize photodamage to their benefit. A remarkably effective plant-pathogenic fungus, Cercospora, generates a pigment that proficiently sensitizes singlet molecular oxygen. Peroxidation of the plant cell membrane reasons the cells of the infected plants to burst, giving nutrients to the fungus.

Carbonyl compounds 

The n → π* excited states of carbonyl compounds display a rich chemistry in their own right.

Fig: Carbonyl compounds 

Because the oxygen has an unpaired electron, it performs in much the equivalent way as an alkoxy radical. Hydrogen abstraction and addition to double bonds are characteristic reactions. Cleavage of neighboring carbon-carbon bonds might as well take place, the 2 most general of such being designated Type I and Type II. An important primary photo process of carbonyl compounds is cleavage, as well recognized as a Norrish Type I reaction (Scheme 3). Besides recombination, the acyl and the alkyl radicals shaped in the primary reaction can undergo various secondary reactions that are responsible for the multitude of final products. 

Fig: secondary reactions

Scheme 3 

Intramolecular hydrogen abstraction is a general photoreaction of carbonyl compounds through a hydrogen atom connected to the 4^th carbon atom (Scheme 4). The consequential diradical can form cycloalkanol or undergo C-C bond fission to provide an alkene and enol. The latter is generally thermodynamically unfavorable and transfers to a ketone. Intramolecular abstraction of a -hydrogen is recognized as a Norrish Type II procedure.

Scheme 4 

Fig: Norrish Type II process

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