B.Sc. 2nd Year Chemistry – Detailed and Exam-Oriented Notes
Photochemistry is the branch of chemistry that deals with chemical reactions and physical changes produced by the absorption of light.
When a molecule absorbs suitable electromagnetic radiation, it may acquire sufficient energy to move from its ground electronic state to an excited state. The excited molecule may then undergo chemical reactions that are not possible, or are much slower, in the ground state.
Photochemical processes are important in atmospheric chemistry, biological systems, photography, medicine, environmental chemistry and industrial processes.
Light is electromagnetic radiation and possesses both wave-like and particle-like characteristics.
The energy of electromagnetic radiation is related to its frequency and wavelength.
Where:
The energy of one photon is given by:
Thus, shorter-wavelength radiation has higher energy than longer-wavelength radiation.
According to the quantum theory of radiation, electromagnetic energy is absorbed or emitted in discrete packets called photons.
The energy of a photon is:
A molecule can absorb a photon only when the energy of the photon is appropriate for a permitted transition between molecular energy levels.
For one mole of photons:
where NA is Avogadro's constant.
A molecule does not absorb all wavelengths of light. Absorption occurs when the energy of incident radiation corresponds to the energy difference between two allowed molecular states.
The absorption of radiation may produce electronic, vibrational or rotational excitation depending on the energy involved.
| Transition | Relative energy | Typical region |
|---|---|---|
| Rotational | Lowest | Microwave region |
| Vibrational | Intermediate | Infrared region |
| Electronic | Higher | UV-visible region |
When a molecule absorbs a photon of suitable energy, one of its electrons may be promoted to a higher-energy orbital.
The normal, lowest-energy state of a molecule is called its ground state.
If an electron is promoted without changing its spin orientation, the excited state is generally called a singlet excited state.
If the excited electron changes its spin so that the two relevant electron spins become parallel, the state is called a triplet state.
Triplet states are generally longer-lived than corresponding singlet excited states.
A Jablonski diagram represents electronic states and transitions between them.
Important photophysical processes include:
Absorption promotes a molecule from a lower electronic state to a higher electronic state.
An excited molecule may lose excess vibrational energy as heat and move to a lower vibrational level within the same electronic state.
Internal conversion is a non-radiative transition between electronic states of the same spin multiplicity.
Intersystem crossing is a non-radiative transition between states of different spin multiplicity, such as from a singlet state to a triplet state.
Fluorescence is the emission of radiation that occurs when an excited molecule returns from an excited singlet state to a lower singlet state, usually the ground state.
Fluorescence is generally rapid and normally stops almost immediately after the exciting radiation is removed.
Phosphorescence is the emission of radiation from an excited triplet state to a lower-energy singlet state, commonly the ground state.
Because the transition involves a change in spin multiplicity, phosphorescence is generally slower than fluorescence.
| Fluorescence | Phosphorescence |
|---|---|
| Usually singlet → singlet | Usually triplet → singlet |
| Generally rapid | Generally slower |
| Usually stops quickly after excitation | May continue after excitation is removed |
The Grotthuss-Draper law states that only light that is absorbed by a substance can bring about a photochemical change.
The Stark-Einstein law, also called the law of photochemical equivalence, states that each molecule involved in the primary photochemical process absorbs one photon of radiation.
Thus, the primary photochemical event is associated with the absorption of a single photon by a molecule.
The quantum yield of a photochemical reaction is the ratio of the number of molecules undergoing the specified photochemical event to the number of photons absorbed.
Quantum yield is an important parameter for evaluating the efficiency of a photochemical process.
A quantum yield greater than unity can occur in chain reactions because one absorbed photon may initiate a chain that causes many molecules to react.
Quantum yield may be less than unity when competing processes such as fluorescence, phosphorescence, internal conversion or other deactivation pathways reduce the fraction of absorbed photons leading to chemical reaction.
A photochemical reaction begins with absorption of light by a reactant or by another species that transfers energy or electrons to the reactant.
A simplified sequence is:
The excited species may undergo bond breaking, bond formation, electron transfer, rearrangement or reaction with another molecule.
Photodissociation is the breaking of a chemical bond due to absorption of suitable radiation.
In many cases, photodissociation produces highly reactive free radicals.
Photodissociation reactions are important in atmospheric chemistry and photochemical chain reactions.
A photochemical chain reaction is a reaction in which the initial absorption of light generates reactive intermediates that participate in repeated reaction steps.
Chain reactions can produce quantum yields greater than one because a single absorbed photon can initiate several reaction cycles.
The reaction between hydrogen and chlorine is a classic example of a photochemical chain reaction.
The chlorine radical is regenerated and can continue the chain.
Hydrogen peroxide can undergo decomposition when exposed to suitable radiation.
Light can promote the formation of reactive intermediates, which participate in the decomposition process.
This is one reason hydrogen peroxide should be stored under suitable conditions and protected from unnecessary light exposure.
Halogen molecules can absorb suitable radiation and form reactive radicals. These radicals can participate in substitution and addition reactions.
Such radical formation is important in the photochemical halogenation of organic compounds.
Photosensitization is a process in which one substance absorbs radiation and transfers the excitation energy to another substance that undergoes the photochemical reaction.
The sensitizer therefore helps the reactant reach an excited state without necessarily being consumed in the overall reaction.
Quenching is the reduction of the lifetime or intensity of an excited state through interaction with another species or through another deactivation process.
A quencher may deactivate the excited molecule before it can undergo the desired photochemical reaction.
Quenching is important in fluorescence studies, photochemical reaction mechanisms and biological systems.
In some photochemical systems, a condition can be established in which the rates of formation and consumption of certain photochemical intermediates become approximately equal.
Such a condition is referred to as a photostationary state.
Photostationary behavior is particularly important in systems where molecules undergo reversible photochemical transformations.
The wavelength determines the photon energy and therefore influences which molecular transitions can occur.
Light intensity affects the rate of photon absorption when other factors remain constant, although the exact effect on reaction rate depends on the reaction mechanism.
Concentration of reactants can affect the probability of collisions and therefore influence the rate of subsequent chemical steps.
Although the initial absorption of a photon is not determined by temperature in the same way as an ordinary thermal activation process, temperature can strongly influence subsequent thermal steps.
Sensitizers can enable molecules to undergo reactions through energy transfer pathways.
Quenchers can reduce the concentration or lifetime of excited species and thereby affect the photochemical reaction.
| Photochemical reaction | Thermal reaction |
|---|---|
| Initiated by absorption of light. | Driven primarily by thermal energy. |
| Involves electronically excited species in many cases. | Usually involves ground-state species. |
| Reaction pathway may differ from thermal pathway. | Reaction pathway is governed by thermal activation and mechanism. |
| Can occur at relatively low bulk temperatures. | Often requires sufficient thermal energy. |
Solar radiation drives many important atmospheric photochemical reactions.
Molecular oxygen absorbs high-energy ultraviolet radiation and can undergo photodissociation:
The oxygen atoms can then react with molecular oxygen:
Ozone also absorbs ultraviolet radiation and participates in further photochemical reactions, forming part of the dynamic ozone layer.
Photosynthesis is one of the most important natural photochemical processes.
Green plants and other photosynthetic organisms absorb light energy using pigments such as chlorophyll and convert it into chemical energy.
A simplified overall representation is:
The actual process involves a complex sequence of light-dependent and biochemical reactions rather than a single elementary reaction.
Traditional photographic processes depend on light-induced chemical changes in photosensitive materials.
Silver halides such as silver bromide have historically been important photosensitive substances.
The formation of metallic silver is associated with image formation in conventional photographic systems.
Photochemical principles are used in phototherapy and in certain light-activated medical treatments.
Photochemical reactions contribute to atmospheric processes, degradation of pollutants and formation of photochemical smog.
Light can initiate reactions such as photochemical additions, substitutions, cyclizations and rearrangements.
Photochemical initiation is used in some polymerization and photo-curing processes.
Photochemical processes are used in coatings, printing, imaging and selected manufacturing processes.
| Term | Meaning |
|---|---|
| Photochemistry | Study of chemical reactions produced by absorption of light. |
| Photon | Discrete packet of electromagnetic radiation energy. |
| Excited state | Higher-energy state produced after absorption of energy. |
| Fluorescence | Rapid radiative emission usually from an excited singlet state. |
| Phosphorescence | Slower radiative emission generally involving a triplet state. |
| Quantum yield | Ratio of specified photochemical events to photons absorbed. |
| Photosensitization | Energy or electron transfer from an excited sensitizer to another molecule. |
| Quenching | Deactivation of an excited state by interaction with another species or process. |
| Photodissociation | Breaking of a chemical bond following absorption of radiation. |
Photochemistry is the branch of chemistry dealing with chemical changes produced by the absorption of light.
Absorption of a photon promotes a molecule to an excited state. Excited molecules can undergo radiative and non-radiative processes such as fluorescence, phosphorescence, internal conversion and intersystem crossing.
The important laws of photochemistry are the Grotthuss-Draper law and the Stark-Einstein law. Quantum yield is used to describe the efficiency of a photochemical process.
Photochemical reactions may involve photodissociation, radical formation, chain reactions, energy transfer and electron transfer. Photosensitization and quenching are important processes that influence excited-state behavior.
Photochemistry has major importance in atmospheric chemistry, photosynthesis, photography, medicine, environmental chemistry, organic synthesis and industrial applications.