Photochemistry is the study of chemical reactions caused by the absorption of visible or ultraviolet light. There are two main types of reactions: primary processes where light is directly absorbed, and secondary processes involving excited states produced in primary processes. Factors like fluorescence, quantum yield, and chain reactions can affect the efficiency of photochemical reactions. Photochemistry plays roles in important natural and industrial processes.
Quantum yield, experimental arrangement, reasons for high and low Quantum yield, problems, photochemical reactions, kinetics of photochemical decomposition of HI, photosensitized reaction, mechanism of photosensitization,
Pericyclic reactions involve the formation and breaking of bonds in a concerted cyclic transition state. They can be classified as cycloadditions, electrocyclic reactions, sigmatropic rearrangements, cheletropic reactions, or group transfers. Examples of important pericyclic reactions discussed include the Diels-Alder reaction, 1,3-dipolar cycloadditions, Claisen rearrangement, and electrocyclic ring openings and closings. These reactions are useful in synthesis and occur in biological systems.
This document provides an overview of the topic of photochemistry. It discusses key concepts like quantum yield, which is a measure of reaction efficiency, and how it is experimentally determined. Diagrams like the Jablonski diagram are also mentioned. Specific photochemical processes covered include fluorescence, phosphorescence, chemiluminescence, and bioluminescence. Finally, some applications of photochemistry are listed, along with a bibliography for further reading.
The document discusses the Von Richter rearrangement and Smiles rearrangement.
The Von Richter rearrangement involves the displacement of a nitro group by cyanide ion on an aromatic compound, with the carboxyl group entering ortho to the displaced nitro group. Evidence supports a mechanism where one oxygen of the carboxyl group comes from the nitro group and one from solvent.
The Smiles rearrangement involves an intramolecular nucleophilic substitution where a leaving group is displaced by a nucleophile activated by an ortho nitro group. Examples are given of substrates that undergo Smiles rearrangement where the linking chain can be aromatic or aliphatic. Electron withdrawing groups para to the nucleophile retard the
This document discusses photochemistry, which is the study of chemical reactions caused by the absorption of light. It covers the history and definition of photochemistry, laws of photochemistry such as the Grotthuss-Draper law and Stark-Einstein law, the photochemical process involving primary and secondary reactions, types of photochemical reactions including rearrangement and substitution reactions, and applications like photosynthesis, vitamin D formation, and bioluminescence. It also discusses how photochemistry is applied to atmospheric chemistry and the formation of photochemical smog, which can have adverse health and environmental effects.
Thermal reactions involve absorption or evolution of heat, while photochemical reactions require light to occur. Thermochemical reactions can take place in dark conditions, while photochemical reactions only occur in the presence of light. Temperature significantly affects thermochemical reaction rates, while light intensity mainly influences photochemical reaction rates. The free energy change of a thermochemical reaction is always negative, but a photochemical reaction's free energy change may not be negative. Photochemical reactions involve electronic excitation from light absorption. Excited states can undergo chemical reactions or transfer energy through intersystem crossing to more stable triplet states. Photochemical reactions include photoreduction, photoaddition, and photo-rearrangement reactions of carbonyl compounds and alkenes.
This document contains lecture notes for a Physical Chemistry VIII course taught by Dr. Fateh Eltaboni at the University of Benghazi. The notes cover topics in photochemistry including characteristics of light, absorption of light, the Beer-Lambert law, photochemical reactions versus thermochemical reactions, quantum yield, and calculations related to photochemistry. Dr. Eltaboni provides examples and illustrations to explain key concepts in photochemistry.
Quantum yield, experimental arrangement, reasons for high and low Quantum yield, problems, photochemical reactions, kinetics of photochemical decomposition of HI, photosensitized reaction, mechanism of photosensitization,
Pericyclic reactions involve the formation and breaking of bonds in a concerted cyclic transition state. They can be classified as cycloadditions, electrocyclic reactions, sigmatropic rearrangements, cheletropic reactions, or group transfers. Examples of important pericyclic reactions discussed include the Diels-Alder reaction, 1,3-dipolar cycloadditions, Claisen rearrangement, and electrocyclic ring openings and closings. These reactions are useful in synthesis and occur in biological systems.
This document provides an overview of the topic of photochemistry. It discusses key concepts like quantum yield, which is a measure of reaction efficiency, and how it is experimentally determined. Diagrams like the Jablonski diagram are also mentioned. Specific photochemical processes covered include fluorescence, phosphorescence, chemiluminescence, and bioluminescence. Finally, some applications of photochemistry are listed, along with a bibliography for further reading.
The document discusses the Von Richter rearrangement and Smiles rearrangement.
The Von Richter rearrangement involves the displacement of a nitro group by cyanide ion on an aromatic compound, with the carboxyl group entering ortho to the displaced nitro group. Evidence supports a mechanism where one oxygen of the carboxyl group comes from the nitro group and one from solvent.
The Smiles rearrangement involves an intramolecular nucleophilic substitution where a leaving group is displaced by a nucleophile activated by an ortho nitro group. Examples are given of substrates that undergo Smiles rearrangement where the linking chain can be aromatic or aliphatic. Electron withdrawing groups para to the nucleophile retard the
This document discusses photochemistry, which is the study of chemical reactions caused by the absorption of light. It covers the history and definition of photochemistry, laws of photochemistry such as the Grotthuss-Draper law and Stark-Einstein law, the photochemical process involving primary and secondary reactions, types of photochemical reactions including rearrangement and substitution reactions, and applications like photosynthesis, vitamin D formation, and bioluminescence. It also discusses how photochemistry is applied to atmospheric chemistry and the formation of photochemical smog, which can have adverse health and environmental effects.
Thermal reactions involve absorption or evolution of heat, while photochemical reactions require light to occur. Thermochemical reactions can take place in dark conditions, while photochemical reactions only occur in the presence of light. Temperature significantly affects thermochemical reaction rates, while light intensity mainly influences photochemical reaction rates. The free energy change of a thermochemical reaction is always negative, but a photochemical reaction's free energy change may not be negative. Photochemical reactions involve electronic excitation from light absorption. Excited states can undergo chemical reactions or transfer energy through intersystem crossing to more stable triplet states. Photochemical reactions include photoreduction, photoaddition, and photo-rearrangement reactions of carbonyl compounds and alkenes.
This document contains lecture notes for a Physical Chemistry VIII course taught by Dr. Fateh Eltaboni at the University of Benghazi. The notes cover topics in photochemistry including characteristics of light, absorption of light, the Beer-Lambert law, photochemical reactions versus thermochemical reactions, quantum yield, and calculations related to photochemistry. Dr. Eltaboni provides examples and illustrations to explain key concepts in photochemistry.
Organometallic compounds contain direct bonds between carbon atoms from organic groups and transition metals. The document discusses several key aspects of organometallic chemistry including:
1) Important early organometallic compounds like Grignard reagents and definitions of terms like hapticity and denticity used to describe ligand bonding.
2) The 18-electron rule which states that transition metal complexes are most stable when the metal has 18 electrons, and examples of applying this rule to determine stability.
3) Characteristics of important classes of organometallics like metal carbonyls and ferrocene, including their structures, bonding models and methods of synthesis.
Photochemistry is the study of chemical reactions caused by light. Key points include:
- Photochemistry involves light interacting with matter, causing physical or chemical changes.
- Photolysis is the process of carrying out a photochemical reaction using light, usually infrared, visible, or ultraviolet light.
- Important natural photochemical reactions include photosynthesis, photography, ozone formation, and solar energy conversion.
- The photochemical process involves light absorption promoting an electron to a higher energy state, followed by primary processes like isomerization, dissociation, or secondary processes like chain reactions.
1) Pericyclic reactions proceed in a concerted, one-step process via a cyclic transition state with high stereo selectivity. They include cycloadditions, electrocyclic reactions, and sigmatropic rearrangements.
2) Cycloadditions are classified as (2+2) or (4+2) depending on the number of pi electrons involved. Diels-Alder reactions are a common example of a (4+2) cycloaddition.
3) Electrocyclic reactions involve the formation or breaking of a ring with the generation or loss of a pi bond. They can be analyzed using frontier molecular orbital theory and orbital symmetry correlation diagrams.
Acid Base Hydrolysis in Octahedral ComplexesSPCGC AJMER
This document discusses acid and base hydrolysis in octahedral complexes. It covers factors that affect the rate of acid hydrolysis, including the charge on the complex, steric hindrance effects, and the strength of the leaving group. A higher positive charge, more steric hindrance, or stronger metal-leaving group bond each decrease the rate of acid hydrolysis according to first-order kinetics through a dissociative SN1 mechanism. Base hydrolysis of octahedral complexes can proceed by either associative SN2 or dissociative SN1 pathways depending on conditions.
The document discusses various photooxidation and photoreduction reactions in organic synthesis. It begins by introducing photochemistry and defining related terms. It then provides examples of photoreduction of ketones and aromatic hydrocarbons. Examples of photooxidation reactions include the conversion of trans-stilbene to phenanthrene and the synthesis of benzoic acids via aerobic photooxidation. The document also describes the mechanism and advantages of using a CdIn2S4 photocatalyst for selective photosynthesis of organic aromatic compounds under visible light.
Sir Cyril Hinshelwood and Nikolaevich received the 1956 Nobel Prize in Chemistry for their research on chemical reaction mechanisms. Hinshelwood modified Lindemann's explanation for unimolecular reactions by proposing that energized molecules (A*) may store energy in various molecular bonds and vibrational degrees of freedom, rather than immediately reacting. This statistical distribution of energy among s degrees of freedom leads to a modified rate constant expression containing an additional term of 1/(s-1) that can account for much higher observed reaction rates. However, Hinshelwood's theory does not fully explain some experimental observations such as the temperature dependence of rate constants and nonlinear plots of 1/k1 versus concentration.
This document provides an overview of photochemistry presented by Dr. Satish Kola. It begins with definitions of photochemistry as chemical reactions caused by light absorption. It then discusses characteristics of light as waves and particles. Key aspects of photochemistry covered include absorbance and color of materials, photochemical reaction mechanisms, quantum yield calculations, and photosensitized reactions where a sensitizer transfers energy to start a reaction. Photochemistry examples discussed are atmospheric reactions, photosynthesis, and determination of absorbed light intensity using spectrophotometry.
This document discusses pericyclic reactions, which are concerted reactions where the transition state involves electrons moving in a cyclic pattern. It describes the key properties of pericyclic reactions, including that they are stereospecific and often light- or heat-activated. It then covers various classes of pericyclic reactions like electrocyclic reactions, cycloadditions like the Diels-Alder reaction, sigmatropic rearrangements including the Cope rearrangement, and group transfer reactions. Examples are provided to illustrate each reaction type.
The document summarizes the Franck-Condon principle, which states that during an electronic transition between two states of a molecule, the transition occurs so rapidly that the positions of the nuclei remain almost unchanged. It describes the different types of molecular energy levels and vibrational transitions. It also provides three cases that illustrate how the Franck-Condon principle determines the relative intensities of vibrational transitions between electronic states based on differences in the equilibrium internuclear distances of the states.
The document discusses the Linear Free Energy Relationship known as the Hammett Equation. It describes how the Hammett Equation can be used to investigate organic reaction mechanisms by studying the effects of substituents on reaction rates. The key aspects are:
1) The Hammett Equation relates the logarithm of reaction rates or equilibrium constants to substituent constants (σ) using the reaction constant (ρ).
2) σ values describe electronic properties of substituents, with electron-withdrawing groups having positive σ and electron-donating groups having negative σ.
3) ρ indicates how sensitive a reaction is to substituents, relating the electronic demand of the reaction transition state. Its sign and magnitude provide insight into
Photochemical reactions are chemical reactions initiated by the absorption of light energy. These reactions involve an organic molecule absorbing a photon which causes electronic excitation from a lower to a higher orbital. The excited molecule may then undergo various chemical reactions, including photoaddition, photocycloaddition, and photo-oxidation reactions. Photochemical reactions differ from thermochemical reactions in their requirement of light to initiate the reaction.
This document discusses organic reactions and mechanisms. It defines key terms like substrate, reagent, products, and mechanism. It describes how factors like inductive and mesomeric effects can influence reactions by altering electron density. It also discusses different types of reaction intermediates that can form, such as carbonium ions, carbanions, free radicals, and carbenes. The document classifies reagents as electrophiles or nucleophiles and describes their behaviors. It explains concepts like activation energy and the transition state that systems must go through for a reaction to occur.
This document discusses applications of frontier molecular orbital theory, including cycloaddition reactions, sigma-tropic reactions, and electrocyclic reactions. Cycloaddition reactions like the Diels-Alder reaction can be predicted by Woodward-Hoffmann rules and frontier molecular orbital theory. Sigma-tropic reactions involve breaking a sigma bond and forming a new sigma bond with pi electron rearrangement. Electrocyclic reactions involve opening or closing a ring through conversion of sigma to pi bonds or vice versa. Frontier molecular orbital theory can be used to understand the orbital interactions in these pericyclic reactions. However, the theory has limitations and may not accurately predict reactivity in all cases.
Photochemistry CECH-509 discusses photochemical reactions and the laws that govern them. Photochemical reactions are chemical reactions initiated by the absorption of light. Key points:
- Photochemical reactions increase free energy while thermal reactions decrease it. The rate of a photochemical reaction depends on the intensity of absorbed light.
- Stark-Einstein law states that each reacting molecule absorbs a single photon, gaining energy to activate and enter the reaction.
- Quantum yield refers to the number of molecules reacted or formed per absorbed photon, indicating the reaction's efficiency. Values below 1 indicate a low yield while above 1 is high. Secondary reactions can result in yields above 1.
This document summarizes Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which describes the rates of unimolecular reactions. RRKM theory originated from Lindemann-Christiansen theory and considers how energy is distributed among vibrational modes. During the 1950s-1952, R.A. Marcus merged transition state theory with RRK theory. RRKM theory accounts for individual vibrational frequencies and classifies energy as either active or inactive. It can explain high pre-exponential factors and has been used to study reactions like isomerization, though limitations include uncertainty in activated complex vibrational frequencies.
The document discusses arenium ions, which are reactive intermediates that form during electrophilic aromatic substitution reactions. Arenium ions, also known as Wheland intermediates, form when an electrophile attacks an aromatic ring, disrupting its aromaticity. This rate-determining step leads to the formation of a carbocation that is stabilized by resonance over three carbon atoms. In the second step, the leaving group departs, allowing the aromatic system to reform and regain stability. The orientation and reactivity of substitution depends on any existing substituents on the aromatic ring.
Photochemistry
ELECTROMAGNETIC SPECTRUM
LAW GOVERNING ABSORPTION OF LIGHT
LAW OF PHOTOCHEMISTRY
Grotthurs-Drapper law.
Einstein Stark law of photochemical equivalence
ELECTRONIC TRANSITIONS
Jablonski Diagram
QUANTUM YIELD
Use Of Photochemistry
Chemistry of vision
Photosynthesis in plant
Formation of Vitamin D
Fluorescent dyes in traffic
Photodynamic therapy
Photochemistry is the study of chemical reactions caused by light. Key points include:
- Photochemistry involves light interacting with matter, causing physical or chemical changes.
- Photolysis is the process of carrying out a photochemical reaction using light, usually infrared, visible, or ultraviolet light.
- Natural photochemical reactions include photosynthesis, photography, ozone formation, and solar energy conversion.
- The photochemical process involves light absorption promoting an electron to a higher energy state, followed by primary processes like isomerization, dissociation, or secondary processes like chain reactions.
Organometallic compounds contain direct bonds between carbon atoms from organic groups and transition metals. The document discusses several key aspects of organometallic chemistry including:
1) Important early organometallic compounds like Grignard reagents and definitions of terms like hapticity and denticity used to describe ligand bonding.
2) The 18-electron rule which states that transition metal complexes are most stable when the metal has 18 electrons, and examples of applying this rule to determine stability.
3) Characteristics of important classes of organometallics like metal carbonyls and ferrocene, including their structures, bonding models and methods of synthesis.
Photochemistry is the study of chemical reactions caused by light. Key points include:
- Photochemistry involves light interacting with matter, causing physical or chemical changes.
- Photolysis is the process of carrying out a photochemical reaction using light, usually infrared, visible, or ultraviolet light.
- Important natural photochemical reactions include photosynthesis, photography, ozone formation, and solar energy conversion.
- The photochemical process involves light absorption promoting an electron to a higher energy state, followed by primary processes like isomerization, dissociation, or secondary processes like chain reactions.
1) Pericyclic reactions proceed in a concerted, one-step process via a cyclic transition state with high stereo selectivity. They include cycloadditions, electrocyclic reactions, and sigmatropic rearrangements.
2) Cycloadditions are classified as (2+2) or (4+2) depending on the number of pi electrons involved. Diels-Alder reactions are a common example of a (4+2) cycloaddition.
3) Electrocyclic reactions involve the formation or breaking of a ring with the generation or loss of a pi bond. They can be analyzed using frontier molecular orbital theory and orbital symmetry correlation diagrams.
Acid Base Hydrolysis in Octahedral ComplexesSPCGC AJMER
This document discusses acid and base hydrolysis in octahedral complexes. It covers factors that affect the rate of acid hydrolysis, including the charge on the complex, steric hindrance effects, and the strength of the leaving group. A higher positive charge, more steric hindrance, or stronger metal-leaving group bond each decrease the rate of acid hydrolysis according to first-order kinetics through a dissociative SN1 mechanism. Base hydrolysis of octahedral complexes can proceed by either associative SN2 or dissociative SN1 pathways depending on conditions.
The document discusses various photooxidation and photoreduction reactions in organic synthesis. It begins by introducing photochemistry and defining related terms. It then provides examples of photoreduction of ketones and aromatic hydrocarbons. Examples of photooxidation reactions include the conversion of trans-stilbene to phenanthrene and the synthesis of benzoic acids via aerobic photooxidation. The document also describes the mechanism and advantages of using a CdIn2S4 photocatalyst for selective photosynthesis of organic aromatic compounds under visible light.
Sir Cyril Hinshelwood and Nikolaevich received the 1956 Nobel Prize in Chemistry for their research on chemical reaction mechanisms. Hinshelwood modified Lindemann's explanation for unimolecular reactions by proposing that energized molecules (A*) may store energy in various molecular bonds and vibrational degrees of freedom, rather than immediately reacting. This statistical distribution of energy among s degrees of freedom leads to a modified rate constant expression containing an additional term of 1/(s-1) that can account for much higher observed reaction rates. However, Hinshelwood's theory does not fully explain some experimental observations such as the temperature dependence of rate constants and nonlinear plots of 1/k1 versus concentration.
This document provides an overview of photochemistry presented by Dr. Satish Kola. It begins with definitions of photochemistry as chemical reactions caused by light absorption. It then discusses characteristics of light as waves and particles. Key aspects of photochemistry covered include absorbance and color of materials, photochemical reaction mechanisms, quantum yield calculations, and photosensitized reactions where a sensitizer transfers energy to start a reaction. Photochemistry examples discussed are atmospheric reactions, photosynthesis, and determination of absorbed light intensity using spectrophotometry.
This document discusses pericyclic reactions, which are concerted reactions where the transition state involves electrons moving in a cyclic pattern. It describes the key properties of pericyclic reactions, including that they are stereospecific and often light- or heat-activated. It then covers various classes of pericyclic reactions like electrocyclic reactions, cycloadditions like the Diels-Alder reaction, sigmatropic rearrangements including the Cope rearrangement, and group transfer reactions. Examples are provided to illustrate each reaction type.
The document summarizes the Franck-Condon principle, which states that during an electronic transition between two states of a molecule, the transition occurs so rapidly that the positions of the nuclei remain almost unchanged. It describes the different types of molecular energy levels and vibrational transitions. It also provides three cases that illustrate how the Franck-Condon principle determines the relative intensities of vibrational transitions between electronic states based on differences in the equilibrium internuclear distances of the states.
The document discusses the Linear Free Energy Relationship known as the Hammett Equation. It describes how the Hammett Equation can be used to investigate organic reaction mechanisms by studying the effects of substituents on reaction rates. The key aspects are:
1) The Hammett Equation relates the logarithm of reaction rates or equilibrium constants to substituent constants (σ) using the reaction constant (ρ).
2) σ values describe electronic properties of substituents, with electron-withdrawing groups having positive σ and electron-donating groups having negative σ.
3) ρ indicates how sensitive a reaction is to substituents, relating the electronic demand of the reaction transition state. Its sign and magnitude provide insight into
Photochemical reactions are chemical reactions initiated by the absorption of light energy. These reactions involve an organic molecule absorbing a photon which causes electronic excitation from a lower to a higher orbital. The excited molecule may then undergo various chemical reactions, including photoaddition, photocycloaddition, and photo-oxidation reactions. Photochemical reactions differ from thermochemical reactions in their requirement of light to initiate the reaction.
This document discusses organic reactions and mechanisms. It defines key terms like substrate, reagent, products, and mechanism. It describes how factors like inductive and mesomeric effects can influence reactions by altering electron density. It also discusses different types of reaction intermediates that can form, such as carbonium ions, carbanions, free radicals, and carbenes. The document classifies reagents as electrophiles or nucleophiles and describes their behaviors. It explains concepts like activation energy and the transition state that systems must go through for a reaction to occur.
This document discusses applications of frontier molecular orbital theory, including cycloaddition reactions, sigma-tropic reactions, and electrocyclic reactions. Cycloaddition reactions like the Diels-Alder reaction can be predicted by Woodward-Hoffmann rules and frontier molecular orbital theory. Sigma-tropic reactions involve breaking a sigma bond and forming a new sigma bond with pi electron rearrangement. Electrocyclic reactions involve opening or closing a ring through conversion of sigma to pi bonds or vice versa. Frontier molecular orbital theory can be used to understand the orbital interactions in these pericyclic reactions. However, the theory has limitations and may not accurately predict reactivity in all cases.
Photochemistry CECH-509 discusses photochemical reactions and the laws that govern them. Photochemical reactions are chemical reactions initiated by the absorption of light. Key points:
- Photochemical reactions increase free energy while thermal reactions decrease it. The rate of a photochemical reaction depends on the intensity of absorbed light.
- Stark-Einstein law states that each reacting molecule absorbs a single photon, gaining energy to activate and enter the reaction.
- Quantum yield refers to the number of molecules reacted or formed per absorbed photon, indicating the reaction's efficiency. Values below 1 indicate a low yield while above 1 is high. Secondary reactions can result in yields above 1.
This document summarizes Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which describes the rates of unimolecular reactions. RRKM theory originated from Lindemann-Christiansen theory and considers how energy is distributed among vibrational modes. During the 1950s-1952, R.A. Marcus merged transition state theory with RRK theory. RRKM theory accounts for individual vibrational frequencies and classifies energy as either active or inactive. It can explain high pre-exponential factors and has been used to study reactions like isomerization, though limitations include uncertainty in activated complex vibrational frequencies.
The document discusses arenium ions, which are reactive intermediates that form during electrophilic aromatic substitution reactions. Arenium ions, also known as Wheland intermediates, form when an electrophile attacks an aromatic ring, disrupting its aromaticity. This rate-determining step leads to the formation of a carbocation that is stabilized by resonance over three carbon atoms. In the second step, the leaving group departs, allowing the aromatic system to reform and regain stability. The orientation and reactivity of substitution depends on any existing substituents on the aromatic ring.
Photochemistry
ELECTROMAGNETIC SPECTRUM
LAW GOVERNING ABSORPTION OF LIGHT
LAW OF PHOTOCHEMISTRY
Grotthurs-Drapper law.
Einstein Stark law of photochemical equivalence
ELECTRONIC TRANSITIONS
Jablonski Diagram
QUANTUM YIELD
Use Of Photochemistry
Chemistry of vision
Photosynthesis in plant
Formation of Vitamin D
Fluorescent dyes in traffic
Photodynamic therapy
Photochemistry is the study of chemical reactions caused by light. Key points include:
- Photochemistry involves light interacting with matter, causing physical or chemical changes.
- Photolysis is the process of carrying out a photochemical reaction using light, usually infrared, visible, or ultraviolet light.
- Natural photochemical reactions include photosynthesis, photography, ozone formation, and solar energy conversion.
- The photochemical process involves light absorption promoting an electron to a higher energy state, followed by primary processes like isomerization, dissociation, or secondary processes like chain reactions.
Photochemistry is the study of chemical reactions initiated by light. Light provides the energy needed for photochemical reactions. There are several types of photochemical reactions including photo-oxidation, photo-addition, and photo-fragmentation. Photochemical reactions have specific characteristics - each molecule absorbs only one photon, the rate depends on light intensity, and the change in free energy may be positive or negative. Photochemistry is important for processes like vision, vitamin D formation, photosynthesis, and polymerization.
This document summarizes a seminar on photochemistry presented by Mr. Dinkar B. Kamkhede. The seminar covered topics including the definition of photochemistry, laws of photochemistry, mechanisms of light absorption, electronic transitions, photosensitization, and the Jablonski diagram. It discussed how photochemical reactions are initiated by the absorption of light energy and explained concepts such as quantum yield. The seminar provided an overview of the key concepts and processes in photochemistry.
PHOTOCHEMISTRY BASIC PRINCIPLE AND JABLONSKI DIAGRAMsuriyachem27
Photochemistry is the branch of chemistry in which study of chemical reactions take place by
the absorption of electromagnetic radiation or by molecules absorb light radiation
(electromagnetic radiation) particularly the visible (wavelength from (400-750) and ultra violet region (wavelength from 100-400nm ), the molecules generally get activated due to
electronic excitation. When electromagnetic radiation is absorb in the ultraviolet/visible
region the molecules get excited to higher electronic state. This involves the promotion of an
electron from bonding molecular orbital to antibonding molecular orbital. According to the quantum theory, both matter and light are quantised, and only certain
specific energies of light are absorbed by specific organic molecule for its excitation. The
absorption or emission of light occurs by the transfer of energy as photons. All photochemical and photo physical processes are initiated by the absorption of a photon of visible or ultraviolet radiation leading to the formation of an electronically - excited state. The molar absorptivity (formerly called the extinction coefficient) of a compound constant
that is characteristic of the compound at a particular wavelength. The Jablonski diagram is a pictorial illustrated of different energy states which are absorbed by molecules. This
partial energy diagram represents the energy of a photo luminescent molecule in its different energy states.The life time of singlet excited state S1 is long hence in this state has done many physical and chemical processes. Molecules returns to its ground state, S0 from excited singlet S1,/ S2 state by release energy as heat, but this is generally quite slow because the amount of energy is large between S0 and S1. This process is called internal conversion. When molecules return to its ground state S0 from excited state S1,/ S2 by giving off
energy in the light form within 10-9 seconds. This process is known as Fluorescence.
This pathway is not very common because it is relatively slow. For smaller, diatomic
and rigid molecules (mainly aromatic compounds) show fluorescence. This is because
emitted fluorescent light is of lower energy than absorbance light. Most molecules in the S1 state may drop to triplet state (T1) (S1→T1). This is
energetically slow process. However, if the singlet state S1 is long lived, the S1 →
T1 conversion occurs by a process called intersystem crossing. It is important
phenomenon in photochemistry. For every excited singlet state there exist
corresponding triplet states. Since transition from ground state singlet (S0) to triplet
state (T1) is forbidden, intersystem crossing is the main source of excited triplet
state. This is one way of populating the triplet state. The efficiency in intersystem
crossing depends on the S1 → T1 energy gap.
Photochemical reactions are chemical reactions initiated by the absorption of light energy. When molecules absorb light, they enter transient excited states with different chemical and physical properties. Chain reactions involve reactive intermediates that propagate the reaction by inducing additional reactions. In 1913, Max Bodenstein proposed the idea of chemical chain reactions, and in 1918 Walther Nernst suggested that the reaction of hydrogen and chlorine occurs by a chain mechanism involving reactive radical intermediates.
Phenomena such as fluorescence, phosphorescence must use special teqniques to detect it
Detector must be sensitive
The spectroscopic nature of luminescent radiation using mono chromatic together with a photomultiplier to detection
1. Photochemistry is the study of chemical reactions caused by the absorption of light. It involves photochemical reactions, which require light for initiation, as well as photophysical processes during the de-excitation of excited molecules.
2. Key concepts in photochemistry include Grotthuss-Draper law, Lambert's law, Beer's law, and Stark-Einstein law of photochemical equivalence. Quantum yield determines the efficiency of photochemical reactions.
3. Photochemistry examines differences between photochemical and thermal reactions. It also explores photochemical processes like fluorescence, phosphorescence, internal conversion, and intersystem crossing depicted in Jablonski diagrams.
Photochemistry is the branch of chemistry concerned with chemical reactions caused by light. It involves the absorption, excitation, and emission of photons by molecules. Important examples include photosynthesis, vitamin D formation, and bioluminescence. Photochemical reactions require light absorption by a reactant and proceed through an excited state intermediate before products form.
This document discusses the differences between photochemical and thermal reactions. Photochemical reactions are initiated by light, not heat, and involve excitation of molecules to electronically excited states. Thermal reactions are accelerated by increased temperature and introduce energy randomly. Both methods provide ways to overcome activation energy barriers, but photochemistry allows access to electronically excited states not achievable through thermal means alone.
Photochemical Reactions M Pharm Chemistry.pptxDiwakar Mishra
Photochemical reaction is included in the syllabus Advance Organic Chrmitry M Pharm (Pharmaceutical Chemistry) which discribes thode chemical reactions which are takes place by the help of light
This document discusses chemiluminescence reactions that emit light. It defines chemiluminescence as a light-emitting chemical reaction and distinguishes it from other types of luminescence. Examples of gas-phase and liquid-phase chemiluminescent reactions are provided, such as the reaction of nitric oxide with ozone that produces excited nitrogen dioxide and emits light. The document also explores the chemiluminescent reaction involved in glowsticks and the firefly luciferase reaction.
This document discusses photochemistry, which involves chemical reactions that are accompanied by light. It covers the electromagnetic spectrum and defines a photon as a particle of light. Examples of photochemical reactions discussed include chemiluminescence, bioluminescence, and the formation of ozone in the atmosphere through absorption of ultraviolet light. The document also addresses the mechanisms of light absorption by molecules, photodissociation reactions, photosensitized reactions like photosynthesis, and the process of photography.
Photochemical reactions are chemical reactions initiated by light absorption. The document discusses the basics of photochemistry including its laws and characteristics of photochemical reactions. It describes several types of photochemical reactions including photo-oxidation, where oxygen is incorporated into a molecule upon light absorption. Photo-addition involves the addition of part of one molecule to a double or triple bond of another. Photo-fragmentation causes a molecule to break into smaller fragments upon light absorption. Examples and mechanisms of these different photochemical reaction types are provided.
The document provides an overview of organic reaction mechanisms. It discusses the main types of organic reactions - substitution, addition, elimination, and rearrangement - and describes common reaction intermediates and steps. Reaction mechanisms are explained including radical, polar, nucleophilic, and electrophilic reactions. Activating and deactivating groups are described along with their inductive and mesomeric effects.
Thermal reactions involve absorption or evolution of heat, while photochemical reactions require light to occur. Thermochemical reactions can take place in darkness, while photochemical reactions only occur in the presence of light. Temperature affects thermochemical reaction rates, while light intensity affects photochemical reaction rates.
Photochemical reactions include photoreduction of carbonyl compounds, where carbonyl compounds are converted to alcohols or diols in the presence of hydrogen donors and light. They also include photoaddition reactions, where an excited state of one molecule adds to the ground state of another like in the Paterno-Buchi reaction of carbonyl compounds and alkenes forming oxetane rings. Photorearrangement reactions can
Photochemistry is concerned with reactions initiated by electronically excited molecules produced by absorption of visible or near-ultraviolet radiation. Photochemistry played a key role in the origin of life and the synthesis of complex organic molecules from simple gases like methane and ammonia. Photochemistry has various applications including synthesis of chemicals, polymers, fluorescent lights, and laser technology. The two main photochemical processes are photophysical, where light absorption does not cause chemical reactions, and photochemical, where light absorption results in chemical changes. Lambert's law and Beer's law govern the decrease in light intensity through an absorbing medium and solutions, respectively. Only the light absorbed by a system can bring about a chemical change, according to Grotthus-Draper
This document discusses structural theory in organic chemistry. It covers topics like bond fission and its types (homolysis and heterolysis), organic reagents (electrophiles, nucleophiles, free radicals), types of organic reactions (substitution, elimination, addition, rearrangement), inductive effect and its applications, and mesomeric effect and its applications. Examples are provided to illustrate key concepts. Essay and short answer questions are also included at the end to test understanding of topics covered.
This document discusses organic reactions and mechanisms. It defines key terms like substrate, reagent, products and defines the steps in a reaction mechanism. It describes factors that influence reactions like inductive and resonance effects. It also discusses types of reaction intermediates like carbonium ions, carbanions and free radicals. The document outlines different types of organic reactions including substitution, addition, elimination and rearrangement. It provides details on the mechanism of substitution reactions including free radical, electrophilic and nucleophilic substitution reactions.
Colour centres are point defects or defect clusters in crystal lattices that cause the material to change color. They occur when electrons or holes become trapped at defect sites. Common examples are the F-centre in alkali halides, which forms when an electron is trapped at a halide ion vacancy, and the H-centre and V-centre in alkali halides, which involve trapped holes. Defect clusters can also form through the interaction of simple point defects like vacancies and interstitials. Colour centres play an important role in determining the optical and electronic properties of solids like alkali halide crystals and wustite.
This document discusses the Cotton effect, Faraday effect, and Kerr effect.
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2. Content
▪ Dark and Photochemical reaction
▪ Laws of photochemistry, Quantum efficiency
▪ Primary and secondary photochemical processes
▪ Consequences of light absorption
▪ Chemiluminescence, Fluorescence, Phosphorescence
▪ Photosensitization, Photochemical equilibrium
▪ Photochemical and Photophysical processes
▪ Mechanism and kinetics of photochemical reactions
▪ Photochemical chain and non-chain reactions
▪ Photolysis of acetaldehyde
▪ Lasers and its applications in chemistry
3. Photochemistry
• Photochemistry is that branch of science
which deals with the chemical processes that
occur when a material is illuminated by radiation
from an external source
• Photochemistry is the branch of chemistry
concerned with the chemical effects of light. It is
the study of the interaction of electromagnetic
radiation with matter resulting into a physical
change or into a chemical change.
• So, photochemistry is the science of the
chemical effects of radiation whose wavelength
lie in the visible and ultraviolet region, i.e., in the
wavelength range from 200 nm to 800 nm.
4. Types of Chemical Reactions
A chemical reaction is one in which, the identity of
molecules is changed due to the rupture and
formation of chemical bonds. Chemical reactions
are two types, these are:
1) Dark or thermal reactions
2) Photochemical reactions
• 1) Dark or thermal reactions:
• Ordinary reactions occur by absorption of heat
energy from outside.
• The reacting molecules are energised and
molecular collisions become effective. These
bring about the reaction.
5. Dark or Thermal Reactions
• The reactions which are caused by heat and in
absence of light are called dark or thermal
reactions.
• These are the ordinary chemical reactions which
are influenced or induced by temperature,
concentration of reactants, presence of catalyst,
etc. except light radiations. For examples:
N2
+ 3H2
= 2NH3
H2
+ I2
= 2HI
PCl5
= PCl3
+ Cl2
6. 2) Photochemical reactions:
• A photochemical reaction may be defined as any
reaction which is induced or influenced by the action
of light on the system.
• The reactant molecules absorbs photons of light and
get excited. These excited molecules then produce
the reactions.
• All spontaneous reactions are accompanied by a
decrease of free energy, but photochemical
reactions takes place with increase of free energy,
as a result of free energy supplied by light.
H2
+ Cl2
→ 2HCl
6CO2
+ 6H2
O → C6
H12
O6
+ 6O2
Types of Chemical Reactions
7. Photochemical reactions
• A mixture of hydrogen and chlorine remains
unchanged with lapse of time. But when
exposed to light, the reaction occurs with a loud
explosion.
H2
+ Cl2
→ 2HCl
6CO2
+ 6H2
O → C6
H12
O6
+ 6O2
h
ν
h
ν
8. Difference between photochemical and
thermal reactions
Photochemical Reaction Thermal Reaction
These reactions involve
absorption of light radiations.
These reactions involve
absorption or evolution of heat.
Presence of light is the
primary requirement.
These reactions can take place
in light as well as in dark.
Rate of reaction is
independent on temperature.
Rate of reaction depends on
temperature.
ΔG may be +ve or –ve. ΔG is always –ve.
Photochemical activation is
highly selective process.
Thermal activation is not
selective in nature
Energy varies from 23 to 230
kcal/mol.
Energy needed in the range of
100 to 1000 kcal/mol.
9. Laws of Photochemistry
1) Grotthuss-Draper law
This law states that
“ Only those radiation which are absorbed by a
reacting substance or system are responsible for
producing chemical change.”
According to this law, all light radiations are not
bringing the chemical reaction. Some are
increase the kinetic energy of molecule while
some are re-emitted (i.e. fluorescence).
10. 2) Eienstien law of photochemical equivalence
The law of photochemical equivalence states that
“When an atom or molecule absorbs light of a given
frequency, it absorbs one quantum only.”
This law also states that (Stark and Einstein)
“Each molecule which takes part in a chemical reaction
absorb one quantum of light which induces the
reaction.”
Explanation
A molecule acquire energy by absorbing photon as,
A + hν = A*
The energy of one photon is equal to hν, where ν is the
frequency of absorbing photon and h is the plank’s
constant.
11. For activation of one mole ,
E = Nhν
where N is the Avogadro’s number and is equal to one
mole.
This quantity of energy E absorbed per mole of the
substance is called an Einstein. But N = 6.023x1023
molecules, h = 6.624x10-27
erg sec. Therefore,
E = 6.023x1023
x 6.624x10-27
xν ergs
= 6.023x1023
x 6.624x10-27
xν/4.186x107
calories
= 9.53x10-11
xν calories
= 9.53x10-11
x3x1010
/λ calories
where ν = c/λ = 3x1010
/λ
Eienstien law of photochemical equivalence
12. Primary and Secondary Processes
• Photochemical processes are complete with the
formation of photochemical products via deactivating
the excited state.
• Bodenstein pointed out that the photochemical
reaction involve in two distinct processes, namely
primary and secondary processes.
• Primary photochemical processes
Process in which light radiation is absorbed by an atom
or molecule giving rise to the formation of an excited
atom or an excited molecule.
A + hν A*
13. • Secondary photochemical processes
Processes which involves the excited atoms,
molecules or free radicals produced in the
primary process.
• Consequence of light absorption can be
discussed under the following two headings:
• a) Light absorptin by atoms
• b) Light absorption by moloecules
Primary and Secondary Processes
14. Consequences of light absorption
• The consequence of molecules absorbing light is
the creation of transient excited states whose
chemical and physical properties differ greatly from
the original molecules.
• These new chemical species can fall apart, change
to new structures, combine with each other or other
molecules, or transfer electrons, or their electronic
excitation energy to other molecules.
15. • When ultraviolet or visible light is absorbed by an
atom or molecule, an electron is excited.
• In other words, it is raised to an orbital of higher
energy than the one it usually occupies.
• There are several options that could happen
next after absorption of photon, either the
electron returns to the ground state emitting the
photon of light or the energy is retained by
the matter and the light is absorbed.
Consequences of light absorption
16. 1) Primary effects of photon absorption by
atoms are:
(a) Electronic excitation: One or more electrons
go into higher energy levels.
(b) Ejection of electrons: If excitaion is high,
one or more electrons may given out by an
atom; causes ionization. Average lifetime of
electronically excited atom is 10-7
to 10-8
s.
Secondary effects follow the primary effects.
Consequences of light absorption by atom
17. 2) Secondary effects of photon absorption by
atoms are:
(a) Fluorescence: Excited electron returns to
ground state or lower enegy levels
instantaneously, emitting a part or whole of
excitation energy.
(b) Resonance fluorescence: Excited electron
return to ground state emitting radiation of
exactly same frequency of incident radiation.
Consequences of light absorption by atom
18. (c) Phosphorescence: Fluorescence stops
immediatly after the incident light is cut off.
In some cases the emission of radiation
persist for some time after the incident light
is cut off, this phenomenon is known as
phosphorescence.
(d) Photochemical reaction: Excited atoms
may take part in chemical reactions with
non-excited atoms or molecules.
(e) Photosensitization: An excited atoms may
collide with a molecule and by energy
exchanges may cause the molecules to
dissociate.
Consequences of light absorption by atom
19. • The above cases of energy transfer can be summarised
as follows:
(a) Primary excitation by photon absorption,
A + hν A* (Excited atom)
(b) The excited atom may activate another atom with
which it collides,
A* + B A + B* (Excited atom)
(c) The excited atom may collide with a molecule and
activate it,
A* + B2
A + B2
*
(d) The excited atom may react with the colliding molecule
A* + B2
AB + B (Reaction)
(e) The excited atom may collide with a molecule which in
turn dissociate
A* + B2
A + 2B (Photosensitization)
Consequences of light absorption by atom
20. Consequences of Light Absorption by Molecules
• A majority of systems consists of molecules than
atoms.
1) Primary effects: The absorption of light may
produce the following primary effects:
(a) Excitation of molecules: includes electronic,
vibrational and rotational excitation.
(b) Isomeric change to form new molecules: direct
photochemical effects.
(c) Dissociation of molecules: direct photochemical
change.
(d) Ionisation of molecules through electron ejection:
known as photoionisation.
21. 2) Secondary effects: If the dissociation or
ionisation of absorbing molecules does not
occur by direct absorption of radiation, then
the energy of excited molecules can give
rise to the following secondary effects:
(a) Photosensitization reactions
(b) Delayed photochemical changes
(c) Rise in temperature
(d) Fluorescence
(e) Phosphorescence
Consequences of Light Absorption by Molecules
22. Photochemical reaction requires absorption of
energy as the first step,
the result is that the reacting molecule is raised
to a higher energy level.
For photochemical reaction an electronic
transition occurs in the visible or ultraviolet
region.
The nature of the resulting primary process the
depends on the relationship between the upper
and lower electronic states of the molecule.
Primary photochemical process and
molecular spectrum
23. Four types of behavior are of interest in
connection with the primary photochemical
process which are depicted in figure.
Type I:
In type I, the nature of the potential energy
curves is such that in the transition indicated the
vibrational energy of the molecule in the upper
electronic state exceeds the maximum value.
After absorption of energy the molecule will
dissociate in its 1st
oscillation.
Transition from lower vibrational levels in the
lower electronic state will not be accompanied by
dissociation.
Primary photochemical process and
molecular spectrum
24. Primary photochemical process and
molecular spectrum
▪The absorption of light in the
continuous region of the
spectrum results in
dissociation of the molecule
is the primary photochemical
process.
▪ In the region of discrete or
banded structure,
dissociation will not occur.
▪ The molecule will be in an
electronically excited state.
25. • Type II:
• In type II, no transition from lower to upper state
can result in dissociation of the molecule.
• The electronic spectrum consists of a series of
bands with no continuous region.
• Absorption of energy can result only in the
formation of an excited molecule.
• The primary photochemical process is then
excitation of the absorbing molecule, just as in
the banded region in case I.
Primary photochemical process and
molecular spectrum
27. • Type III:
• Type III is uncommon, the upper electronic
state is completely unstable and has no
vibrational levels.
• The molecular spectrum then has no
bands, but is continuous throughout.
• In these circumstances the electronic
transition is always accompanied by
dissociation of the molecule.
Primary photochemical process and
molecular spectrum
29. • Type IV:
• Type IV is a combination of type II and III.
• There are two upper electronic levels close together;
one is like that in case II (stable) and other like case
III (unstable)
• Transition from the lower state occurs to the stable
upper state.
• During vibration the excited molecule is changed
into the unstable state, at the point where the two
curves cross one another, the molecule then
dissociates.
• The behavior of this type is called predissociation.
• The electronic spectrum has a banded structure, but
in the region of predissociation the rotational line are
absent, the vibrational bands having a diffuse
appearance.
Primary photochemical process and
molecular spectrum
30. • The reason for this is that
dissociation occurs during
a shorter period than is
required for the molecule
to rotate.
• If absorption occurs in the
diffuse region of the
spectrum, the primary
photochemical process is
dissociation, in the banded
region it is electronic
excitation, as in cases I
and II.
Primary photochemical process and
molecular spectrum
31. Quantum Efficiency or Quantum Yield
Efficiency of a photochemical process is expressed in terms of
quantum efficiency or quantum yield (ϕ). It states that
“the number of molecules reacting per quantum of light
absorbed.”
Energy of one photon, E = hν, and 1 mole = N molecules
Since each molecule absorbs one photon.
32. • Energy absorbed by N molecules = Energy of N photons
E = Nhν
• The energy (E) which activates one mole of reactant, i.e.,
energy corresponding to N photons is called ‘one
einstein’. So,
• ‘Einsteins’ is the unit of photochemical intensity. The
value of φ may vary from 1 to 1010; in some cases it is
less than 1.
Quantum Efficiency or Quantum Yield
33. • For a reaction that obeys strictly the Einstein
law, one molecule decomposes per photon, the
quantum yield, φ = 1.
• When two or more molecules are decomposed
per photon, φ > 1 and the reaction has a high
quantum yield.
• High quantum yield reactions
• When φ > 1
CO + Cl2
→ COCl2
φ = 103
H2
+ Cl2
→ 2HCl φ = 104
to 106
2H2
O2
→ 2H2
O + O2
φ = 7
Quantum Efficiency or Quantum Yield
34. If the number of molecules decomposed is less
than one per photon, the reaction has a low
quantum yield (φ < 1).
Low quantum yield reactions
When φ < 1
H2
+ Br2
→ 2HBr φ = 0.01
2NH3
→ N2
+ 3H2
φ = 0.2
CH3
COCH3
→ CO + C2
H6
φ = 0.1
Quantum Efficiency or Quantum Yield
35. Reasons for Low and High Quantum Yield
Reasons for low quantum yield
• Excited molecules may get deactivated before they
form a product so the value is less than 1.
• Collision of excited molecule with non-excited
molecule.
• Molecules may not receive adequate energy to
enable them to react.
• The photochemical reaction may be reversed.
36. • When one photon decomposes or forms more than one
molecule, the quantum yield φ > 1 and is said to be high.
The main reasons for high quantum yield are :
• One photon absorbed in a primary reaction dissociates
one molecule of the reactant. But the excited atoms that
result may start a subsequent secondary reaction in
which a further molecule is decomposed
AB + hv → A + B Primary
AB + A → A2
+ B Secondary
• Obviously, one photon of radiation has decomposed two
molecules, one in the primary reaction and one in the
secondary reaction. Hence the quantum yield of the
overall reaction is 2.
Reasons for High Quantum Yield
37. • When there are two or more reactants, a molecule of
one of them absorbs a photon and dissociates
(primary reaction). The excited atom that is
produced starts a secondary reaction chain.
A2
+ hv → 2A ...(1) Primary
A + B2 → AB + B ..(2) Secondary
B + A2
→ AB + A ...(3) Reaction chain
• It is noteworthy that A consumed in (2) is
regenerated in (3). This reaction chain continues to
form two molecules each time. Thus the number of
AB molecules formed in the overall reaction per
photon is very large. Or that the quantum yield is
extremely high.
Reasons for High Quantum Yield
38. Reasons for high quantum yield
• The product of primary process may collide with
2nd
molecule and transfer energy. 2nd
to 3rd
and
so on. Thus the chain reaction starts and
number of reacting molecule will be high. The
activated molecule reacts with another molecule.
• The radiation absorption includes the production
of the atoms with initiate the chain reaction.
• Due to the formation of intermediate product
which acts as a catalyst.
• After the absorption of radiation, the activated
molecule which is formed may collide with
another active molecule.
39. Luminescence
• When the emission of visible radiation occurs
due to some cause other than temperature, the
phenomenon is known as luminescence.
Luminescence is of the following types:
1) Photoluminescence
2) Chemiluminescence
3) Bioluminescence
4) Cathodoluminescence
5) Electroluminescence
40. Photoluminescence:
• Luminescence caused by light is called
photoluminescence.
• Photoluminescence that ceases immediately
after the cause of excitation is cut off is called
fluorescence.
• Photoluminescence that persists for an
appreciable time after the stimulating process
is cut off is called phosphorescence.
•
Luminescence
41. • Chemiluminescence:
Luminescence resulting from chemical reactions
is called chemiluminescence.
• Bioluminescence:
Observed on living organism
• Cathodoluminescence:
Luminescence caused by bombardment of
electrons is called cathodoluminescence.
• Electroluminescence:
Luminescence resulting from the application of
an electric field to matter is called
electroluminescence.
Luminescence
42. Luminescence
Chemiluminescence is the emission of light, as
the result of a chemical reaction. It is the
generation of electromagnetic radiation as light
by the release of energy from a chemical
reaction. While the light is emitted in the
ultraviolet, or visible region.
Examples:
• The greenish-white glow of yellow
phosphorous to P2
O5
due to oxidation in air at
low temperature
• When alkali metal vapours react with halogen
or organic halides at low temperature
43. Absorption and Emission
• Absorption: When atoms absorb energy, through
heating, from electricity, or by absorbing
electromagnetic radiation, the electrons of ground level is
pushed up to higher levels
• Emission: Emission occurs when the excited electron
returns to a lower electron orbital. The emitted radiation is
termed luminescence. Luminescence is observed at
energies that are equal to or less than the energy
corresponding to the absorbed radiation.
Absorption Emission
44. Electronic transitions
• Following types of electronic transition
takes place
б б* Transitions
n б* Transitions
n π* transitions
π π* transitions
47. • б → б* Transitions Transitions in which as
bonding electron is excited to an antibonding б*
orbital are called б → б* transitions. These
transitions are shown by only saturated
hydrocarbons.
• For example, methane (which has only C-H
bonds, and can only undergo б → б* transitions)
shows an absorbance maximum at 125 nm.
Absorption maxima due to б → б* transitions
• These wavelengths are lesser than 200 nm and
fall in the vacuum UV region.
Electronic transitions
48. • n → б* Transitions: Saturated compounds
containing atoms with lone pairs (non-bonding
electrons) are capable of n → б* transitions.
• These transitions usually need less energy than
б → б* transitions.
• They can be initiated by light whose wavelength
is in the range 150 - 250 nm.
• The number of organic functional groups with n
→ б* peaks in the UV region is small.
• e.g., CH3
Cl.
Electronic transitions
49. • n → π* transitions: These are the transitions in
which an electron in a non – bonding atomic
orbital is promoted to an antibonding π* orbital.
• Compounds having double bonds between
heteroatoms, e.g., C=O, C=S, and N=O.
• For the >C=O group of saturated aldehydes or
ketones exhibit an absorption of low intensity at
about 285 nm.
• These transitions require only small amounts of
energy and takes place with in the range of
ordinary UV spectrophotometer.
• These are generally forbidden transitions.
Electronic transitions
50. • Forbidden transitions
• For n → π* transition of a saturated aldehydes
or ketones exhibit a weak absorption of low
intensity near about 285 nm and having εmax
less than 100 is a forbidden transition.
• The term molar extinction coefficient (εmax
) is a
measure of how strongly a chemical species or
substance absorbs light at a particular
wavelength.
Electronic transitions
51. • π → π* transitions: These are the transitions
in which an electron in a p electron is promoted
to an antibonding π* orbital.
• These transitions require relatively higher
amount of energy than n → π* transitions.
• For the >C=O group of saturated aldehydes or
ketones exhibit an absorption of high intensity at
about 180 nm.
Electronic transitions
52. Effect of excited molecules
The excited molecules can lose their energy in two ways:
1. By nonradiative transition
a) IC, internal conversion
b) ISC, intersystem conversion
2. By radiative transition
a) Fluorescence
b) Phosphorescence
54. Photophysical processes
• If the absorbed radiation is not used to
cause a chemical change, it is re-emitted
as light of longer wavelength. The there
several photophysical processes occur.
For example:
• (a) Fluorescence (b) Phosphorescence (c)
Chemiluminescence
55. Fluorescence
Fluorescence is the emission of light by a substance that
has absorbed electromagnetic radiation.
According to Stoke’s law, during fluorescence light is
absorbed at a certain wavelength and should be emitted
at a greater wavelength.
It is a form of luminescence.
The substance that exhibits fluorescence is called
florescent substance.
In most cases, the emitted light has a longer wavelength,
and lower energy, than the absorbed radiation.
When a beam of light is incident on certain substances,
they emit visible light or radiations and they stop emitting
light or radiation as soon as the incident light is cut off.
This phenomenon is known as fluorescence.
56. Substances which emit radiations during the
action of stimulating light are called fluorescent
substances.
Examples of fluorescent substance
❖ Chlorophyll present in green leaves show the
phenomenon of fluorescence.
❖ Petroleum, vapors of iodine, acetone and
hydrocarbons (paraffin and olefins) have been
found to fluoresce in ultraviolet regions.
Fluorescence
57. Examples of fluorescent substance
• Fluorescence is generally observed in those
organic molecules which have rigid framework
and not many loosely coupled substituents
through which vibronic energy can flow out.
• A functional group which exhibits absorption of
radiations in the visible or ultraviolet region is
called a chromophore.
• In analogy with chromophores, following
structures are termed as fluorophores.
-C=C-, N=O, -N=N, -C=O, -C=N, -C=S
58. Examples of fluorescent substance
• A large number of substances enhance
fluorescence, these are known as fluorochromes
in the same analogy as auxochromes.
• Generally electron donors group (-OH, -NH2
,
-CH3
etc.) act as auxochromes.
• Electron withdrawing group (–COOH, -NO2
) tend
to diminish or inhibit fluorescence completely.
• For example benzoic acid is non-fluorescent
whereas aniline, azophenanthrene are fluorescent
59. Fluorescence occurs when an excited molecule,
atom, or nanostructure, relaxes to a lower
energy state (possibly the ground state) through
emission of a photon.
It may have been directly excited from the
ground state S0
to a singlet state, S1
, from the
ground state by absorption of a photon of energy
(hvex
) and subsequently emits a photon of a
lower energy hvem
as it relaxes to state S0
:
Excitation: So
+ hνex
→ Sn
Mechanism of Fluorescence
60. Fluorescence: S1
→ So
+ hνem
The singlet state, S1
, lose its remaining energy
through further fluorescent emission
and/or non-radiative relaxation in which the
energy is dissipated as heat.
When the excited state is a metastable state,
then that fluorescent transition is rather termed
phosphorescence.
Relaxation from an excited state can also occur
through transferring some or all of its energy to a
second molecule through an interaction known
as fluorescence quenching.
Mechanism of Fluorescence
61. • This phenomenon is instantaneous and starts
immediately after the absorption of light and stops
as soon as the incident light is cut off.
•
• Fluorescence is stimulated by light of the visible or
ultraviolet regions of the spectrum.
• Substances exhibiting fluorescence generally
re-emit excess radiation within 10-6
to 10-4
seconds of absorption.
• It is a general phenomenon and is exhibited by
gases, liquids and solids. No fluoresces will be
observed in gases, unless the pressure is low.
Characteristics of Fluorescence
62. • Different substances fluoresce with light of
different wavelengths. Thus fluorspar fluoresces
with blue light, chlorophyll with red light, uranium
glass with green light and so on.
• The fluorescent light from solutions is polarized
and the degree of polarization depends in some
cases upon the concentration of the solution.
Characteristics of Fluorescence
63. • The extent of fluorescence depends upon the
nature of the solvent and the presence of certain
anions in solution. Thus thiocyanate, iodide and
bromide ions show a marked quenching effect.
• The quantum efficiency of fluorescence
increases in proportional to the wavelength of
absorbed radiation. Then after reaching its
maximum value in a certain interval of λmax
, the
efficiency drops rapidly to zero upon a further
increase in wavelength.
Characteristics of Fluorescence
64. • The phenomenon of fluorescence is a well
established analytical too. The tool is known as
fluorimetry. A large number of application are
known. We describe some of them as follows:
• 1) Fluorescence is used to determine uranium in
salts.
• 2) It is used to determine the ruthenium ion in the
presence of other metals.
• 3) It is used to determine aluminium in alloys.
Applications of Fluorescence
65. • 4) It is used to estimate trace of boron in steel.
• 5) It is used to determine vitamin B1
and B2
in
food samples.
• 6) It is used to determine condition of food-stuffs.
• 7) Ringworm can be detected by this tool.
• 8) This method has been used in the quantitative
analysis of drugs and dyes, textile and paper
industry, medicine, fuels and chemicals
Applications of Fluorescence
70. Phosphorescence
When a beam of light is incident on certain substances,
they emit light continuously even after the incident light
is cut off. This type of delayed fluorescence is called
phosphorescence and the substances are called
phosphorescent substances.
Characteristics
The life time of phosphorescence (10-4
to 20 sec) is
much longer than fluorescence (10-6
to 10-4
sec).
Phosphorescence is mainly caused by ultraviolet and
visible light.
It is generally shown by solids.
The magnetic and dielectric properties of
phosphorescent substances are different before and
after illumination.
71. Examples of phosphorescent substances
(a) Sulphates of calcium, barium and strontium
exhibit phosphorescence.
(b) Many dyes which fluoresce in ordinary light in
aqueous solution, it exhibits phosphorescence
when dissolved in fused boric acid.
c) Ruby, emerald
d) Certain fungi shows the phosphorescence
72. Factors Affecting Fluorescence and
Phosphorescence
a) All molecules cannot show the phenomena of
fluorescence and phosphorescence. Only such
molecules show these phenomena that are able to
absorb ultraviolet or visible radiation.
B) Substituents often exhibit a marked effect on the
fluorescence and phosphorescence molecules.
i) Electron donating group (-OH, -NH2
) often enhance
fluorescence.
ii) Electron withdrawing group (–COOH, -NO2
) tend to
diminish or inhibit fluorescence completely.
iii) If a high atomic number atom is introduce into a
–electron system, it enhances phosphorescence and
decrease fluorescence.
c) pH exhibits a marked effect on the fluorescence of
compounds.
73. Applications of Phosphorescence
The phenomenon of phosphorescence is a well
established analytical too. The tool is known as
phosphorimetry. A large number of application are
known. We describe some of them as follows:
• 1) Phosphorescence is used to determine aspirin in
blood serum.
• 2) It is used to determine cocaine and atropine in
urine.
• 3) It is used to determine procaine, cocaine,
phenobarbitol in blood serum.
74. Photochemical Equilibrium
A state of photochemical equilibrium is said to exist in a reaction
when the rates of opposing reactions, of which at least one is
light sensitive, become equal under the influence of light
radiation.
Two types:
First category: only one reaction is light-sensitive
Examples,
❖ Dissociation of Nitrogen Dioxide:
Dimerisation of Anthracene:
75. • Second category: both the reactions are
light-sensitive
• Examples,
– Formation of Sulphur Trioxide:
– Isomerisation of Maleic acid into Fumaric acid:
76. Photosensitization
Certain reaction are known which are not
sensitive to light.
These reactions can be made light sensitive by
adding a small amount of foreign material which
can absorb light and stimulate the reaction
without itself taking part in the reaction.
Such an added material is known as
photosensitizer and the phenomenon as
photosensitization.
Photosensitization is different in nature from
ordinary catalysis.
77. Role played by a photosensitizer
Function of a photosensitizer:
absorb light
become exited
transfer energy to reactants
activate them for reaction without taking part in
reaction.
A photosensitizer acts as a carrier of energy.
Examples are:
Photosensitization
78. Photosensitization
1. Reactions sensitized by Mercury atoms
Dissociation of H2
into atom at 253.7 nm in the presence
of Hg vapour was studied by Carrio and Frank (1922).
Hg(g) + H2
→ 2H +Hg(g) at 253.7 nm
Hg atom absorbs light → Hg* collide with H2
→
dissociate into atoms.
Mechanism:
Hg + hν → Hg*
Hg* + H2
→ H2
* + Hg
H2
* → 2H
79. Photosensitization
2. Chlorine as a photosensitizer
Decomposition of ozone in presence of chlorine in UV.
Rate of reaction independent of concentration but proportional to
intensity of light.
Mechanism:
Excited ClO3
* may absorbed on the wall to form Cl2
O6
, Cl2
and O2
.
ClO3
* may react to form ClO2
and O2
80. 3. Bromine as a photosensitizer
Bromine acts as a photosensitizer in the
conversion of maleic acid into fumaric acid.
4. Cadmium vapour as a photosensitizer
Cadmium vapour acts as a photosensitizer for
the polymerisation of ethylene.
nC2
H4
(-C2
H2
-)n
Photosensitization
h
ν
C
d
81. 5. Uranyl ion as a photosensitizer
Uranyl ion acts as a photosensitizer in the
photolysis of formic acid
UO2
2+
+ hν → [UO2
2+
]*
[UO2
2+
]* + HOOC-COOH → CO2
+ CO + H2
O + UO2
2+
6. Chlorophyll as a photosensitizer
Chlorophyll acts as a photosensitizer in the
photosynthesis of carbohydrate from CO2
and H2
O.
Chlorophyll + hν → [Chlorophyll]*
6CO2
+ H2
O + [Chlorophyll]* → C6
H12
O6
+ O2
+
Chlorophyll
Photosensitization
103. Lasers and its Applications
A laser is a device that emits light (electromagnetic
radiation) through a process of optical amplification
based on the stimulated emission of photons.
The term "laser" originated as an acronym for light
amplification by stimulated emission of radiation.
104. Lasers and its Applications
Types of lasers
1. Gas lasers
2. Chemical lasers
3. Dye lasers
4. Metal vapour lasers
5. Semiconductor lasers
6. Free electron lasers
105. Lasers and its Applications
Gas lasers
Laser Type Applications
Helium-Neon
laser
Spectroscopy, barcode scanning,
alignment, optical demonstrations.
Argon laser Retinal phototherapy, lithography
Nitrogen laser Pumping of dye lasers, measuring air
pollution, scientific research.
CO2
laser Material processing (welding),
photoacoustic spectroscopy
106. Lasers and its Applications
Chemical lasers
Laser Type Applications
Hydrogen
Fluoride laser
Used in research for laser weaponry by
the U.S
Oxygen-iodine
laser
Laser weaponry, scientific and materials
research
All gas-phase
iodine laser
Scientific, weaponry, aerospace.
Dye lasers: Research, laser medicine, spectroscopy,
birth mark removal, isotope separation
107. Lasers and its Applications
Metal-vapour lasers
Laser Type Applications
He-Cd vapour
laser
Printing and type setting applications,
fluorescence excitation examination,
scientific research
Cu vapour laser Dermatological uses, high speed
photography, pump for dye lasers
108. Lasers and its Applications
Free electron lasers: Atmospheric research,
material science, medical application
Semiconductor lasers: It uses for telecommunication,
printing, holography, weapons, machining, welding