These lecture notes provide a comprehensive exploration of photochemistry, serving as an essential resource for undergraduate and postgraduate students, and educators. Organized into eight lectures, the content progresses logically from foundational principles to advanced topics, ensuring a thorough understanding of the subject. Beginning with the interaction of radiation with matter, the notes delve into key concepts such as the photon, photochemical laws, the Jablonski diagram, radiative and non-radiative processes, quantum yield, photosensitized reactions, and energy transfer mechanisms like Förster Resonance Energy Transfer (FRET). Each lecture is designed to balance theoretical depth with practical relevance, incorporating historical context, experimental verification, and real-world applications.
Visual aids, such as the Jablonski diagram, are explained in detail, offering clarity on energy states, transitions, and processes in the excited state. Definitions of essential terms, distinctions between related phenomena like fluorescence and phosphorescence, and discussions of factors affecting photochemical reactions further enrich the learning experience. Practical examples, including photosynthesis, solar cells, and ozone layer formation, illustrate the applications of photochemistry in nature and technology.
These notes cater to undergraduate and postgraduate students, educators seeking structured teaching materials, and researchers requiring a concise yet comprehensive overview of photochemical principles. Whether used for sequential learning, quick reference, or exam preparation, these notes aim to foster a deep understanding and appreciation of photochemistry and its critical role in science and technology.
Index
Lecture 1: Introduction to Photochemistry
Interaction of Radiation with Matter
Light as Quanta: The Photon Concept
Characteristics of Photons in Radiation-Matter Interaction
Implications
Historical Significance
Definition and scope of photochemistry
Definition of Photochemistry
Scope of Photochemistry
Difference between thermal and photochemical processes
Importance of photochemistry in nature and technology
Importance in Nature
Importance in Technology
Lecture 2: Laws of Photochemistry 10
Laws of Photochemistry: Overview
1. Grothus-Draper Law
2. Stark-Einstein Law (Law of Photochemical Equivalence)
Combined Importance
Laws of Photochemistry: Experimental Verification and Examples
1. Grotthuss-Draper Law
Experimental Verification
Examples in Real Life
2. Stark-Einstein Law
Experimental Verification
Examples in Real Life
Applications and Broader Examples of Photochemical Laws
1. Photosynthesis
2. Photopolymerization
3. Photovoltaics (Solar Cells)
4. Ozone Formation in the Stratosphere
5. Fluorescence and Phosphorescence
Lecture 3: Introduction to Jablonski Diagram 15
Energy States and Transitions
Electronic Energy States
Transitions
Construction and Interpretation of the Jablonski Diagram
Construction:
Interpretation:
Processes Occurring in the Excited State
Absorption:
Spontaneous Emission:
Stimulated Emission:
Non-Radiative Processes:
Significance of the Jablonski Diagram
Lecture 4: Radiative Processes
Qualitative Description of Fluorescence
Qualitative Description of Phosphorescence
Difference between Fluorescence and Phosphorescence
Lecture 5: Non-Radiative Processes
Internal Conversion: Mechanism and Factors
Intersystem Crossing: Mechanism and Significance
Role of Non-Radiative Processes in Energy Dissipation
Lecture 6: Quantum Yield
Definition and Significance of Quantum Yield
Factors Affecting Quantum Yield
Examples of High and Low Quantum Yield Reactions
Lecture 7: Photosensitized Reactions
Concept of Photosensitization
Examples of Photosensitized Reactions
Energy Transfer Mechanisms in Photosensitized Reactions
Lecture 8: Energy Transfer Processes
Mechanisms of Energy Transfer: Photosensitization and Quenching
Förster Resonance Energy Transfer (FRET)
Dexter Energy Transfer
Applications of Energy Transfer in Photochemical Reactions
Case Studies and Examples
Keywords for the Series:
References:
Dear Learners,
These lecture notes provide a comprehensive exploration of photochemistry, serving as an essential resource for undergraduate and postgraduate students, and educators. Organized into eight lectures, the content progresses logically from foundational principles to advanced topics, ensuring a thorough understanding of the subject. Beginning with the interaction of radiation with matter, the notes delve into key concepts such as the photon, photochemical laws, the Jablonski diagram, radiative and non-radiative processes, quantum yield, photosensitized reactions, and energy transfer mechanisms like Förster Resonance Energy Transfer (FRET). Each lecture is designed to balance theoretical depth with practical relevance, incorporating historical context, experimental verification, and real-world applications.
Visual aids, such as the Jablonski diagram, are explained in detail, offering clarity on energy states, transitions, and processes in the excited state. Definitions of essential terms, distinctions between related phenomena like fluorescence and phosphorescence, and discussions of factors affecting photochemical reactions further enrich the learning experience. Practical examples, including photosynthesis, solar cells, and ozone layer formation, illustrate the applications of photochemistry in nature and technology.
These notes cater to undergraduate and postgraduate students, educators seeking structured teaching materials, and researchers requiring a concise yet comprehensive overview of photochemical principles. Whether used for sequential learning, quick reference, or exam preparation, these notes aim to foster a deep understanding and appreciation of photochemistry and its critical role in science and technology.
Dr. Akhilesh Shende
Lecture 1: Introduction to Photochemistry
● Learning Outcomes:
○ Understand the fundamental principles of photochemistry.
○ Differentiate between thermal and photochemical processes.
Interaction of Radiation with Matter
The interaction of radiation with matter is a cornerstone of modern physics, providing profound insights into the dual nature of light. A pivotal discovery in this realm was the photoelectric effect, which revealed the particle-like behavior of light. This phenomenon demonstrated that light interacts with matter as discrete packets of energy called quanta, each having energy E = hν, where h is Planck's constant and ν is the frequency of light.
Light as Quanta: The Photon Concept
Albert Einstein extended the quantum theory of light by associating the energy quantum with a particle. He proposed that the quantum, later named the photon, also possesses momentum, defined as p = hν/c , where c is the speed of light. The photon thus has a well-defined energy and momentum, key indicators of its particle-like nature.
Einstein's work on the photoelectric effect, for which he was awarded the Nobel Prize in Physics in 1921, firmly established the photon model. Further experimental confirmation came from A.H. Compton’s scattering experiment 1924, which showed that X-rays, when scattered by electrons, behaved as particles. This phenomenon, known as Compton scattering, validated the particle-like behavior of electromagnetic radiation.
Characteristics of Photons in Radiation-Matter Interaction
The photon picture of electromagnetic radiation is summarized as follows:
1. Particle-like Behavior:
Radiation behaves as if it is made up of particles called photons during interactions with matter.
2. Energy and Momentum:
Each photon has:
○ Energy: E = hν = hc/λ
○ Momentum: p = hν/c = h/λ Photons travel at the speed of light, c.
3. Uniform Energy for a Given Wavelength:
All photons of the same frequency (ν) or wavelength (λ) have identical energy and momentum, regardless of the radiation's intensity. Increasing light intensity increases the number of photons crossing a given area per second but does not alter their energy.
4. Neutrality:
Photons are electrically neutral and are unaffected by electric or magnetic fields.
5. Conservation in Collisions:
In collisions involving photons, such as photon-electron interactions, the total energy and momentum are conserved. However, the number of photons may not remain constant; a photon can be absorbed, or a new photon can be generated.
Implications
These properties highlight the dual nature of light, displaying both wave-like and particle-like characteristics. While wave behavior dominates in phenomena like diffraction and interference, particle nature becomes evident in interactions with matter, such as the photoelectric effect and Compton scattering.
Historical Significance
The discoveries surrounding the photon concept have been transformative. Einstein's Nobel Prize for his explanation of the photoelectric effect and Millikan's Nobel Prize in 1923 for his experimental verification of the photoelectric effect underscore the significance of these findings. Together, these milestones paved the way for quantum mechanics and advanced our understanding of radiation-matter interactions.
Definition and scope of photochemistry
Definition of Photochemistry
Photochemistry is the branch of chemistry that studies the chemical effects of light. It investigates the interaction of electromagnetic radiation with matter, focusing on the processes initiated by light absorption. These processes are different from thermal reactions as they involve electronic excitations, leading to various unique chemical phenomena.
Scope of Photochemistry
Photochemistry has vast applications and significance in both fundamental and applied sciences. Its scope includes:
1. Understanding Natural Processes:
○ Photosynthesis in plants, the basis of life on Earth, is a photochemical reaction.
○ Vision in humans and animals is facilitated by photochemical changes in the retina.
2. Industrial Applications:
○ Synthesis of various chemicals, such as the photochemical production of ozone.
○ Development of light-sensitive materials used in photography and printing.
3. Material Science:
○ Creation of advanced materials like photopolymers and photoresists.
○ Designing photonic devices and solar cells.
4. Environmental Applications:
○ Decomposition of pollutants using photocatalysis.
○ Understanding atmospheric chemistry and ozone layer interactions.
5. Medical and Biological Applications:
○ Photodynamic therapy for cancer treatment.
○ Use of fluorescent markers in biological imaging.
Photochemistry bridges chemistry, physics, and biology making it essential for advancements in energy, health, and environmental sciences
Difference between thermal and photochemical processes
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Importance of photochemistry in nature and technology
Photochemistry plays a crucial role in both natural processes and technological advancements. Its significance lies in its ability to transform radiant energy into chemical energy, enabling different essential and innovative applications.
Importance in Nature
1. Photosynthesis:
○ One of the most critical natural photochemical processes, photosynthesis transform sunlight into chemical energy. Plants, algae, and some bacteria use chlorophyll to absorb light and synthesize glucose, forming the base of the food chain. This process also maintains atmospheric oxygen levels.
2. Vitamin D Synthesis:
○ Photochemical reactions in the skin enable the synthesis of vitamin D when exposed to ultraviolet (UV) light from the sun. This vitamin is vital for calcium metabolism and bone health.
3. Ozone Formation and Protection:
○ In the stratosphere, UV radiation drives the photochemical formation of ozone (O3), which acts as a protective shield by absorbing harmful UV rays, preventing excessive radiation from reaching Earth's surface.
4. Vision:
○ The human eye's ability to detect light relies on photochemical reactions in the retina. Light triggers the isomerization of rhodopsin, initiating a cascade of processes that result in visual perception.
Importance in Technology
1. Photography:
○ Traditional photography relied on photochemical reactions in light-sensitive films. Modern imaging technology continues to draw inspiration from these principles.
2. Solar Energy Conversion:
○ Photochemical principles are harnessed in solar panels and photocatalysts to convert sunlight into usable energy, contributing to renewable energy solutions.
3. Environmental Applications:
○ Photochemical methods are used in water purification and air treatment systems. For example, UV radiation helps disinfect water and degrade pollutants in air and water.
4. Material Science:
○ Photochemical processes are integral to creating advanced materials, such as photopolymers, used in 3D printing and lithography.
5. Medicine and Health:
○ Phototherapy uses light to treat conditions like jaundice in newborns and certain skin disorders. Advances in photodynamic therapy enable targeted treatment of cancers using light-activated drugs.
6. Chemical Synthesis:
○ Photochemistry provides pathways for synthesizing complex molecules that are difficult to achieve using conventional methods, including pharmaceuticals and fine chemicals.
The study and application of photochemistry have significantly influenced our understanding of natural phenomena and driven technological innovation. Its interdisciplinary nature makes it invaluable in addressing modern challenges, including energy sustainability, environmental protection, and health care.
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Lecture 2: Laws of Photochemistry
● Learning Outcomes:
○ Describe the key laws governing photochemistry.
○ Apply the laws to simple photochemical reactions.
Laws of Photochemistry: Overview
Photochemistry involves the interaction between light and matter, leading to chemical reactions initiated by photons. Two fundamental laws govern this domain:
1. Grothus-Draper Law
● Overview: The Grothus-Draper law states that “ only light absorbed by a substance can bring about a photochemical change ”. This means that the light must interact with the molecules of the reacting species to initiate a reaction.
Illustrations are not included in the reading sample
While the Grothus-Draper law does not involve specific equations, it implies the following:
Illustrations are not included in the reading sample
where A is the photochemical change, and Iabs is the intensity of light absorbed.
● Mechanism:
○ A photon is absorbed by a molecule, exciting it from a ground state to an excited state.
○ If the absorbed photon does not excite the molecule, it cannot participate in a photochemical reaction.
● Example:
○ Chlorophyll in plants absorbs light in the red and blue regions of the spectrum, driving photosynthesis. The reflected green light does not contribute to the process because it is not absorbed.
● Significance:
○ It emphasizes the role of selective light absorption in photochemical reactions.
○ This law lays the foundation for understanding why certain wavelengths of light are effective in driving photochemical processes, while others are not.
○ It explains the selective nature of photochemical changes, aiding in the design of processes like photosynthesis and artificial photochemical reactions.
2. Stark-Einstein Law (Law of Photochemical Equivalence)
● Explanation: The Stark-Einstein law states that “ each photon of light absorbed by a molecule activates one molecule in a photochemical reaction. ” This law highlights the quantum nature of light and the molecular interactions it induces. The law is also known as the photochemical equivalence law. This defines a one-to-one correspondence between photons and activated molecules.
● Implications:
○ Establishes a quantitative relationship between the number of photons absorbed and the number of molecules reacting, ensuring energy conservation at the molecular level.
○ Forms the basis of quantum yield calculations in photochemical processes.
○ Provides insights into the efficiency of photochemical reactions, crucial for optimizing industrial processes like polymerization, photography, and dye-sensitized solar cells.
● Quantum Yield (ϕ):
ϕ=Number of molecules reacting/ Number of photons absorbed
For an ideal reaction, ϕ is close to 1, as one molecule reacts per photon absorbed.
● Quantum Nature:
○ Each photon has energy: E = hν = hc/λ where h is Planck's constant, ν is the frequency of light, c is the speed of light, and λ is the wavelength.
● Implications:
○ Only photons of appropriate energy (matching the energy gap between ground and excited states) can cause a reaction.
○ Inefficiencies (e.g., side reactions or relaxation of excited states) can lead to quantum yields less than 1.
● Example:
○ In the photochemical decomposition of hydrogen iodide
2HI → H2 + I2
○ one photon excites an HI molecule, breaking the bond. Quantum yield deviations occur if the recombination of H and I atoms reduces efficiency.
Combined Importance
● Grothus-Draper Law ensures we understand which wavelengths drive the reaction.
● Stark-Einstein Law quantifies the reaction’s efficiency and energy usage.
Laws of Photochemistry: Experimental Verification and Examples
1. Grotthuss-Draper Law
Experimental Verification
● Absorption Spectroscopy: The absorption spectrum of a substance is measured to determine the specific wavelengths of light that it can absorb. Experiments show that chemical reactions occur only at these wavelengths, validating the law.
● Example Experiment: In the photodecomposition of silver halides used in photography, the silver halide absorbs light in specific wavelengths, resulting in the formation of free silver atoms.
○ Procedure: A silver halide crystal (e.g., AgBr) is exposed to white light through a spectrometer to isolate specific wavelengths.
○ Observation: Reaction is observed only at the absorbed wavelengths, such as the visible and UV regions.
Examples in Real Life
● Photography: Silver bromide absorbs light during exposure to form a latent image, which is later developed into a visible photograph.
● Chlorophyll in Photosynthesis: Chlorophyll absorbs light primarily in the blue and red regions of the spectrum to initiate the process of photosynthesis, leading to the synthesis of glucose and oxygen.
2. Stark-Einstein Law
Experimental Verification
● Quantum Yield Measurement: The quantum yield (Φ) is determined by measuring the ratio of molecules reacting to photons absorbed. Ideally, for single-step reactions, Φ = 1, indicating one photon activates one molecule.
● Example Experiment: In the decomposition of hydrogen iodide (HI) under UV light:
○ Procedure: HI gas is irradiated with monochromatic UV light of a specific wavelength.
○ Observation: The number of iodine and hydrogen molecules formed matches the number of photons absorbed.
Examples in Real Life
● Hydrogen-Iodine Reaction:
○ Reaction: HI + hν → H• + I•.
○ Only molecules absorbing UV photons undergo dissociation.
● Vision Process:
○ In the human eye, light absorbed by rhodopsin molecules initiates the biochemical reactions responsible for vision.
Applications and Broader Examples of Photochemical Laws
1. Photosynthesis
● Process: Chlorophyll absorbs light, primarily in the blue and red regions of the spectrum. The absorbed energy drives the conversion of CO2 and H2O into glucose (C6H12O6) and oxygen (O2).
● Verification of Laws:
○ Grotthuss-Draper Law: Only light absorbed by chlorophyll pigments triggers the reaction.
○ Stark-Einstein Law: One photon excites one electron in the reaction center of the photosystem.
2. Photopolymerization
● Process: Monomers are converted into polymers upon exposure to UV light. This process is widely used in 3D printing, coatings, and adhesives.
● Verification of Laws:
○ The reaction occurs only at UV wavelengths absorbed by the photoinitiator molecules (Grotthuss-Draper).
○ Each photon activates a single photoinitiator molecule, initiating the polymerization process (Stark-Einstein).
3. Photovoltaics (Solar Cells)
● Process: Absorbed sunlight excites electrons in semiconductor materials, generating electric current.
● Verification of Laws:
○ Only absorbed sunlight contributes to electron excitation (Grotthuss-Draper).
○ One photon excites one electron to the conduction band (Stark-Einstein).
4. Ozone Formation in the Stratosphere
● Process: UV light causes the dissociation of oxygen molecules into free oxygen atoms, which combine with O2 to form ozone (O3).
○ Reaction: O₂ + hν → 2O•,
○ followed by O• + O₂ → O₃.
● Verification of Laws:
○ Absorption of UV light by oxygen initiates the reaction (Grotthuss-Draper).
○ Each absorbed photon dissociates one oxygen molecule (Stark-Einstein).
5. Fluorescence and Phosphorescence
● Process: Molecules absorb light and re-emit it as fluorescence (short-lived) or phosphorescence (long-lived).
● Verification of Laws:
○ Emission occurs only after light absorption (Grotthuss-Draper).
○ One photon excites one molecule to an excited state (Stark-Einstein).
The laws of photochemistry form the foundation of our understanding of how light interacts with matter to produce chemical changes. These principles have not only theoretical significance but also have vast practical applications in fields ranging from energy production to medicine. Experimental verification through spectroscopy, quantum yield measurements, and photochemical reactions reinforces the validity of these laws and underscores their universal applicability in both natural and technological processes.
Lecture 3: Introduction to Jablonski Diagram
● Learning Outcomes:
○ Explain the basic processes in excited states.
○ Construct and interpret Jablonski diagrams.
Energy States and Transitions
The Jablonski diagram represents the electronic energy levels and transitions of a molecule, illustrating how it absorbs and dissipates energy. The key elements are:
Electronic Energy States
● Ground State (S0): The lowest energy state where most molecules reside under normal conditions.
● Excited Singlet States (S1, S2, etc.): Higher energy states where the spins of electrons are paired (singlet multiplicity). The molecule can reach these states upon absorbing energy.
● Triplet State (T1): An excited state where the electron spin changes, resulting in unpaired spins (triplet multiplicity). This state typically has lower energy than singlet excited states due to spin pairing energy considerations.
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Figure: Electronic Energy States
Transitions
● Absorption:
○ When a molecule absorbs a photon, it moves from the ground state (S₀) to an excited state (e.g., S₁ or S₂).
○ The energy of the absorbed photon corresponds to the difference between the energy levels, i.e., ΔE = hν, where h is Planck's constant and ν is the frequency of the light.
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Figure: Absorption Process (blue)
● Non-Radiative Transitions:
○ Internal Conversion (IC):
■ A non-radiative process where a molecule transitions between two electronic states of the same spin multiplicity (e.g., S₂ → S₁, T₂ → T₁).
■ Energy is dissipated as heat, usually through molecular vibrations.
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Figure: Internal Conversion (Red)
○ Intersystem Crossing (ISC):
■ A non-radiative transition between states of different spin multiplicities (e.g., S₁ → T₁).
■ This occurs when spin-orbit coupling enables the electron spin to flip, making this transition partially allowed.
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Figure: Inter Syatem Crossing (Pink)
● Radiative Transitions:
○ Fluorescence: A molecule in the excited singlet state (S1) relaxes to the ground state (S0) by emitting a photon. This is a fast process (nanoseconds).
○ Phosphorescence: A molecule in the triplet state (T1) relaxes to the ground state (S0) by emitting a photon. This process is slower (microseconds to seconds) due to the spin-forbidden nature of the transition.
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Figure: Fluorescence (green) and Phosphorescence (blue)
Construction and Interpretation of the Jablonski Diagram
The Jablonski diagram is a graphical representation of molecular energy states and the transitions between them.
Construction:
● Vertical Arrangement: Energy states (S0, S1, S2, T1) are shown on a vertical scale, with higher states representing higher energy.
● Horizontal Lines: Represent vibrational sublevels within each electronic state. Vibrational relaxation (non-radiative energy dissipation) occurs very quickly, typically within picoseconds.
● Arrows:
○ Straight vertical arrows: Represent the absorption of photons, which are almost instantaneous processes.
○ Wavy arrows: Indicate non-radiative transitions such as IC and ISC.
○ Downward straight arrows: Represent radiative processes such as fluorescence or phosphorescence.
Interpretation:
● Absorption and Relaxation Pathways:
○ A molecule absorbs energy and is promoted to a higher electronic state (e.g., S₁ or S₂).
○ From the excited state, it may relax via radiative (fluorescence or phosphorescence) or non-radiative pathways (IC or ISC).
● Timescales:
○ Absorption occurs in femtoseconds (10⁻¹⁵ s).
○ Vibrational relaxation (within an electronic state) takes picoseconds (10⁻¹² s).
○ Internal Conversion (10⁻14 - 10⁻11 s)
○ InterSystem Crossing (10-8 - 10-3 s)
○ Fluorescence occurs in nanoseconds (10⁻⁹ - 10-6 s).
○ Phosphorescence occurs on a much longer timescale, from microseconds (10⁻3 s) to seconds.
Processes Occurring in the Excited State
When a molecule absorbs energy, it enters an excited state, where several processes can occur:
Absorption:
● The molecule absorbs a photon, resulting in a transition from the ground state (S₀) to an excited singlet state (S₁ or S0).
● The wavelength of the absorbed light is characteristic of the molecule and depends on the energy gap (ΔE) between the states.
Spontaneous Emission:
● The molecule in the excited state may spontaneously release a photon and transition to a lower energy state:
○ Fluorescence: It occurs when the molecule returns from S1 to S0 and emits light. Due to vibrational relaxation, the emitted photon has a longer wavelength (lower energy) than the absorbed photon.
○ Phosphorescence: It occurs when the molecule transitions from T1 to S0, emitting light. This process is slower due to the spin-forbidden nature of the transition.
Stimulated Emission:
● If a photon of the same energy as the excited state transition interacts with the molecule, it can stimulate the molecule to emit a photon of the same wavelength and phase.
● Stimulated emission forms the basis of laser operation, where light amplification occurs due to the coherent release of photons.
Non-Radiative Processes:
● Vibrational Relaxation: The excited molecule dissipates energy within the same electronic state by transferring it to molecular vibrations.
● Internal Conversion (IC): Energy is dissipated as heat through a transition to a lower electronic state of the same multiplicity.
● Intersystem Crossing (ISC): The molecule transitions to a state with different spin multiplicity, such as from T1 to S0.
Significance of the Jablonski Diagram
The Jablonski diagram is crucial for understanding molecular photophysics and photochemistry. It provides insight into:
1. The mechanisms of light absorption and emission.
2. Pathways for energy dissipation in molecules.
3. Applications such as fluorescence spectroscopy, phosphorescence, and the design of lasers and photosensitizers.
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Lecture 4: Radiative Processes
● Learning Outcomes:
○ Differentiate between fluorescence and phosphorescence.
○ Describe radiative decay mechanisms in photochemistry.
Qualitative Description of Fluorescence
Fluorescence is a physical phenomenon where certain substances absorb light or electromagnetic radiation of a specific wavelength and re-emit it almost instantaneously. This process occurs in two primary stages:
● Excitation: The electrons in the molecule absorb energy from an external light source (e.g., ultraviolet light) and jump to a higher energy level, typically an excited singlet state.
● Emission: Almost immediately, the electrons return to their ground state by releasing the absorbed energy in the form of light, typically at a longer wavelength due to the loss of some energy as heat during the relaxation process.
Key Characteristics of Fluorescence:
● Speed of Emission: The entire process is extremely fast, occurring within nanoseconds (10⁻⁹ seconds) to microseconds (10⁻⁶ seconds).
● Dependence on Excitation Source: Fluorescence ceases as soon as the excitation source is removed, as there is no delay in the relaxation process.
● Wavelength Shift: The emitted light is usually of a longer wavelength than the absorbed light, a phenomenon known as the Stokes shift.
Examples in Practice:
● Fluorescent dyes used in biological imaging.
● Fluorescent minerals such as fluorite.
● Day-glow highlighter inks.
Applications:
Fluorescence has widespread applications in analytical chemistry, medical diagnostics (e.g., fluorescence microscopy), and industrial quality control.
Qualitative Description of Phosphorescence
Phosphorescence is another form of luminescence, but it differs fundamentally from fluorescence in the way the energy transitions occur. When a material absorbs light, its electrons are excited to higher energy levels, similar to fluorescence. However, instead of directly returning to the ground state, the electrons undergo an intersystem crossing to a metastable triplet state. This state has a longer lifetime due to the "forbidden" nature of the transition back to the ground state, as per quantum mechanical selection rules.
Key Characteristics of Phosphorescence:
● Speed of Emission: Phosphorescence is a slow process; the emission of light can last from milliseconds (10⁻³ seconds) to hours or even longer after the excitation source is removed.
● Triplet State: The delay in emission arises because the transition from the triplet state to the ground state is quantum mechanically forbidden, making it a slow process.
● Long-Lasting Glow: Materials exhibiting phosphorescence can "store" light energy and emit it gradually over time, which is why they are often referred to as "glow-in-the-dark" materials.
Examples in Practice:
● Glow-in-the-dark toys and paints.
● Zinc sulfide doped with copper, often used in phosphorescent materials.
Applications:
Phosphorescence is utilized in safety signage, watch dials, emergency lighting, and forensic analysis.
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The primary distinction between fluorescence and phosphorescence lies in the duration of light emission and the quantum mechanical states involved. Fluorescence is a short-lived phenomenon associated with singlet state transitions, while phosphorescence involves longer-lived, "forbidden" transitions from a triplet state. This difference makes fluorescence more suitable for real-time applications and phosphorescence ideal for applications requiring sustained light emission.
Lecture 5: Non-Radiative Processes
● Learning Outcomes:
○ Understand and explain non-radiative decay mechanisms.
○ Analyze their role in photochemical systems.
Internal Conversion: Mechanism and Factors
Mechanism:
Internal conversion (IC) is a non-radiative electronic relaxation process where an excited molecule transitions between two electronic states of the same spin multiplicity (e.g., S2 → S1). This process occurs when the molecule’s energy is redistributed into vibrational modes, allowing it to return to the ground state without emitting photons. The primary mechanism involves vibronic coupling, where the electronic states mix through vibrational overlap, facilitating a rapid transfer of energy. The energy released during IC is dissipated as heat into the surrounding medium.
For instance, in highly conjugated systems, internal conversion acts as a protective mechanism, enabling molecules to handle excess excitation energy without damage.
Factors Influencing Internal Conversion:
1. Energy Gap:
○ The rate of IC increases when the energy difference between the electronic states (S1 and S0) is small. This is explained by the energy gap law, which states that closely spaced vibrational levels enhance non-radiative decay.
2. Molecular Structure:
○ Molecules with larger and more flexible structures tend to exhibit faster IC. For example:
■ Heavy Atoms Effect: The presence of heavy atoms (e.g., iodine, bromine) enhances spin-orbit coupling, indirectly promoting IC.
■ Conjugated Systems: Planar and conjugated molecules, such as aromatic hydrocarbons, often show efficient IC.
3. Solvent Effects:
○ The polarity and viscosity of the solvent can modulate IC. For instance, in highly polar solvents, stabilization of the excited state may reduce IC efficiency.
4. Temperature:
○ Higher temperatures enhance vibrational relaxation, allowing better overlap of vibrational levels between the two states. This facilitates more effective energy transfer during IC.
5. Presence of Functional Groups:
○ Molecules with functional groups such as carbonyls or halogens can exhibit faster IC due to their electronic properties, which promote vibrational coupling.
Intersystem Crossing: Mechanism and Significance
Mechanism:
Intersystem crossing (ISC) is a radiationless transition where a molecule moves from an excited singlet state (e.g., S1) to a triplet state (T1). Unlike IC, ISC involves a change in spin multiplicity, which is quantum mechanically forbidden but becomes possible due to spin-orbit coupling. Spin-orbit coupling allows interaction between electronic and spin wavefunctions, enabling the conversion.
The probability of ISC increases significantly when:
1. Heavy Atoms Are Present: Elements like iodine or bromine increase spin-orbit coupling, facilitating ISC.
2. Vibrational Coupling Occurs: Overlapping vibrational states of singlet and triplet energy levels enhance the rate of ISC.
Once in the triplet state, the molecule has a longer lifetime compared to singlet states, as transitions back to the ground state (phosphorescence) are spin-forbidden, resulting in delayed emission.
Significance:
1. Photochemical Transformations:
○ ISC is vital in photochemical reactions where triplet states serve as reactive intermediates. For example, in photosensitized oxidation, the triplet state of a sensitizer reacts with molecular oxygen to generate singlet oxygen, a highly reactive species.
2. Biological Applications:
○ In photodynamic therapy (PDT), ISC is exploited to generate triplet states in photosensitizers, which produce reactive oxygen species to kill cancer cells.
3. Optoelectronic Applications:
○ In materials science, ISC plays a role in organic light-emitting diodes (OLEDs), where efficient harvesting of triplet states improves device performance.
○ ISC also facilitates upconversion processes, enabling the absorption of low-energy photons and the emission of higher-energy light.
4. Protection Mechanism:
○ ISC protects molecules from excessive energy absorption by rerouting energy into the triplet state, which is less likely to undergo harmful reactions.
Role of Non-Radiative Processes in Energy Dissipation
Non-radiative processes are crucial for dissipating excess energy in molecular systems. These processes include:
● Internal Conversion (IC):
○ Molecules return to the ground state without emitting light by converting electronic energy into vibrational energy.
○ Example: IC protects DNA from UV damage by dissipating absorbed light energy as heat.
● Intersystem Crossing (ISC):
○ ISC transitions molecules to the triplet state, where energy is released over longer timescales, typically through phosphorescence or quenching.
● Vibrational Relaxation:
○ Once in a vibrationally excited state, molecules rapidly lose excess vibrational energy to the surrounding medium through collisions, resulting in thermal dissipation.
Significance of Non-Radiative Processes:
1. Protective Mechanisms:
○ Non-radiative processes prevent photochemical degradation by dissipating energy as heat rather than engaging in bond-breaking reactions.
2. Fluorescence Modulation:
○ Non-radiative pathways compete with fluorescence, affecting the quantum yield. For instance, quenching by non-radiative processes explains why some molecules exhibit weak fluorescence.
3. Thermal Management:
○ Energy dissipation via non-radiative processes is vital for maintaining thermal stability in dyes and pigments exposed to prolonged light.
4. Photosynthesis:
○ Non-radiative energy dissipation mechanisms prevent overexcitation in photosynthetic systems, protecting against photo-oxidative damage under high-light conditions.
5. Energy Transfer and Storage:
○ By dissipating unwanted energy, non-radiative processes enable efficient energy management in biological and artificial systems, ensuring their longevity and functionality.
Lecture 6: Quantum Yield
● Learning Outcomes:
○ Calculate quantum yield for given reactions.
○ Discuss the significance of quantum yield in photochemistry.
Definition and Significance of Quantum Yield
Definition:
Quantum yield (Φ) is the ratio of the number of specific events occurring to the number of photons absorbed by the system. In simpler terms, it measures how effectively absorbed light is utilized in producing a specific result, such as a photochemical reaction or a photophysical process. Mathematically, it is given by:
Φ=Number of molecules of product formed/ Number of quanta of radiation absorbed
This equation highlights that quantum yield represents the efficiency of converting absorbed light into the desired outcome.
Significance: Quantum yield is a crucial parameter in photochemistry and photophysics for several reasons:
● Efficiency Measurement: It helps in determining the efficiency of photochemical and photophysical processes. A high quantum yield indicates that a large fraction of absorbed photons leads to the desired event, making the process efficient.
● Material and Reaction Optimization: By knowing the quantum yield, scientists can optimize materials and reaction conditions to maximize efficiency.
● Applications in Technology: Quantum yield plays a significant role in designing solar cells, LEDs, and other light-sensitive technologies, as high quantum yield materials can improve device performance.
● Environmental Impact: Efficient photochemical processes with high quantum yields can lead to more sustainable and environmentally friendly technologies, reducing waste and energy consumption.
Factors Affecting Quantum Yield
Several factors influence the quantum yield of a photochemical reaction, including:
1. Nature of the Reacting Substance:
○ Different substances have varying abilities to absorb light and convert it into chemical energy. The chemical structure, presence of specific functional groups, and electronic properties of the molecules affect the quantum yield.
○ For example, aromatic compounds often have higher quantum yields due to their conjugated systems, which can efficiently absorb and utilize light energy.
2. Wavelength of Incident Light:
○ The energy of absorbed photons depends on their wavelength. Light of different wavelengths can lead to different reaction pathways and efficiencies.
○ For instance, shorter wavelengths (higher energy) can excite electrons to higher energy states, potentially leading to more efficient photochemical reactions.
3. Presence of Quenchers:
○ Quenchers are substances that deactivate the excited state of the reactants without producing the desired product, thus reducing the quantum yield.
○ Common quenchers include oxygen, which can react with excited states to form non-radiative products.
4. Reaction Environment:
○ The solvent, temperature, and pressure of the reaction environment can significantly impact the quantum yield.
○ Solvents can stabilize or destabilize the reactants, intermediates, or products, influencing the efficiency of the photochemical process.
5. Concentration of Reactants:
○ High concentrations of reactants can lead to self-quenching or aggregation, where the excited states interact with each other, reducing the quantum yield.
○ Proper optimization of reactant concentrations is essential to maximize the quantum yield.
Examples of High and Low Quantum Yield Reactions
High Quantum Yield Reactions:
● Fluorescence of Organic Dyes:
○ Fluorescent dyes such as Rhodamine B exhibit high quantum yields, where nearly every absorbed photon results in emitted light. This makes them useful in various applications, including fluorescence microscopy and flow cytometry.
● Photosynthesis:
○ In the process of photosynthesis, plants efficiently convert absorbed light energy into chemical energy, resulting in a high quantum yield. This efficiency is crucial for the survival and growth of plants.
Low Quantum Yield Reactions:
● Phosphorescence in Certain Materials:
○ Phosphorescence involves transitions from triplet excited states to the ground state, which are often forbidden transitions, resulting in low quantum yields. Materials with low phosphorescence quantum yields are used in applications where prolonged emission is not desired.
● Photoreduction of Carbon Dioxide (CO2):
○ The conversion of CO₂ to formic acid (HCOOH) through photoreduction typically exhibits a low quantum yield due to competing side reactions and the involvement of complex multi-electron processes.
The number of quanta absorbed can be calculated using the formula:
Illustrations are not included in the reading sample
Where:
● I is the intensity of light
● tt is the time
● λ is the wavelength of light
● h is Planck's constant
● c is the speed of light
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Lecture 7: Photosensitized Reactions
● Topics:
○ Concept of photosensitization
○ Examples of photosensitized reactions (e.g., photodecomposition, photosynthesis)
○ Energy transfer mechanisms in photosensitized reactions
● Learning Outcomes:
○ Explain the concept and applications of photosensitized reactions.
○ Discuss the role of energy transfer in such reactions.
Concept of Photosensitization
Photosensitization is a process in photochemical reactions where the reactant molecule does not absorb the radiation required for the reaction directly. Instead, a foreign species, known as a photosensitizer, absorbs the radiation and subsequently transfers its energy to the reactant molecule, activating it and allowing the desired chemical reaction to occur. This is particularly useful in cases where the reactant itself is unable to absorb the necessary radiation.
The role of a photosensitizer is to act as an intermediary. Upon absorbing radiation, the photosensitizer becomes excited and then transfers this excitation energy to the reactant molecule, which becomes activated and can then undergo the desired reaction. A common example of a photosensitizer is mercury vapor.
The general process can be summarized by the following reactions:
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Examples of Photosensitized Reactions
Here are some examples of photosensitized reactions to illustrate the concept:
1. Reaction between Hydrogen and Oxygen:
○ This reaction is photosensitized by mercury vapor, leading to the formation of hydrogen peroxide (H₂O₂).
Illustrations are not included in the reading sample
2. Reaction between Hydrogen and Carbon Monoxide:
○ Mercury vapor is used as a photosensitizer, resulting in the formation of formaldehyde (HCHO).
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● Additionally, some glycol (CHO-CHO) is formed by the dimerization of formaldehyde radicals (HCO).
3. Photodecomposition of Diphenylamine (C12H11N):
○ Diphenylamine undergoes decomposition when irradiated in the vapor form in the presence of benzophenone, which acts as a photosensitizer.
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Here, Bz represents benzophenone.
4. Photosynthesis:
○ In plants, chlorophyll acts as a photosensitizer. It absorbs light and transfers the energy to carbon dioxide and water to produce glucose and oxygen.
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Energy Transfer Mechanisms in Photosensitized Reactions
In photosensitized reactions, the energy transfer mechanism involves several steps:
1. Primary Absorption: The photosensitizer absorbs the incident radiation and becomes excited.
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2. Energy Transfer: The excited photosensitizer collides with the reactant molecule and transfers its excitation energy to it.
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3. Reaction: The activated reactant molecule undergoes the desired chemical reaction.
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After transferring the energy, the photosensitizer returns to its original unactivated state, ready to absorb more radiation and facilitate further reactions.
1. Equation for the formation and dissociation of ozone in the atmosphere:
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2. Rate of change of ozone concentration:
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3. Photodissociation of Diphenylamine (C12H11N):
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4. Energy Transfer Mechanism:
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5. Quantum Yield of Fluorescence:
Φ=Number of molecules of product formed/ Number of quanta of radiation absorbed
Lecture 8: Energy Transfer Processes
● Topics:
○ Mechanisms of energy transfer
■ Forster Resonance Energy Transfer (FRET)
■ Dexter Energy Transfer
○ Applications of energy transfer in photochemical reactions
○ Case studies and examples
● Learning Outcomes:
○ Explain energy transfer mechanisms with examples.
○ Understand their importance in biological and chemical systems.
Mechanisms of Energy Transfer: Photosensitization and Quenching
Energy transfer in photochemical reactions involves the movement of energy from an excited donor molecule to an acceptor molecule. This transfer can occur through various mechanisms, primarily photosensitization and quenching.
Photosensitization: In photosensitization, a photosensitizer absorbs radiation and then transfers this energy to a reactant molecule, allowing the reactant to undergo the desired chemical reaction. The photosensitizer acts as an intermediary, facilitating reactions that would not occur under normal conditions because the reactant molecules themselves cannot absorb the required radiation. Examples include the reactions between hydrogen and oxygen, and hydrogen and carbon monoxide, both photosensitized by mercury vapor.
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Figure Photosensitization Process
Quenching: Quenching is the process by which another molecule, called a quencher deactivates the excited state of a molecule. This deactivation can occur through various pathways, including collisional quenching, where the quencher physically interacts with the excited molecule, and static quenching, where the quencher forms a non-fluorescent complex with the excited molecule.
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Förster Resonance Energy Transfer (FRET)
FRET is an energy transfer mechanism between two light-sensitive molecules (donor and acceptor). It occurs through dipole-dipole interactions without the emission of a photon. The efficiency of FRET depends on:
● Distance Dependency: FRET efficiency decreases with the inverse sixth power of the distance between the donor and acceptor molecules, making it effective over distances of 1-10 nm.
● Spectral Overlap: There must be a significant overlap between the donor's emission spectrum and the acceptor's absorption spectrum.
● Orientation Factor: The relative orientation of the donor and acceptor dipoles affects energy transfer efficiency.
Abbildung in dieser Leseprobe nicht enthalten
Figure Fluorescence and FRET: (A) Photon absorption excites an electron, emitting a photon of longer wavelength. (B) In FRET, energy from the donor excites a nearby acceptor, emitting a photon. (C) FRET efficiency depends on the distance between fluorophores, with the Förster radius (R0) where 50% transfer occurs. (Hochreiter et al.)
Dexter Energy Transfer
Dexter Energy Transfer involves the exchange of electrons between the donor and acceptor molecules. This mechanism requires direct overlap of molecular orbitals and is effective over very short distances (typically less than 1 nm). Key features include:
● Electron Exchange: Energy transfer occurs through a simultaneous exchange of electrons between donor and acceptor molecules.
● Short Range: Dexter transfer is a short-range process, and it is less dependent on spectral overlap compared to FRET.
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Figure FRET and Dexter Energy Transfer Process (Strieth-Kalthoff)
Applications of Energy Transfer in Photochemical Reactions
Energy transfer mechanisms play a crucial role in various photochemical processes and applications, including:
1. Photosynthesis: In photosynthesis, energy transfer between chlorophyll molecules enables the efficient capture and conversion of light energy into chemical energy. The light-harvesting complexes use FRET to transfer energy from antenna pigments to the reaction center.
2. Fluorescence Quenching: FRET is widely used in fluorescence quenching studies to measure molecular interactions and distances, helping to understand the dynamics and conformational changes of biomolecules.
3. Light-Harvesting Systems: Energy transfer mechanisms are employed in the design of light-harvesting systems, such as organic solar cells, to improve performance by enhancing the absorption and conversion of light into electrical energy.
Case Studies and Examples
1. Photosynthesis: In the light-harvesting complexes of plants, FRET is used to transfer energy from antenna pigments to the reaction center, allowing plants to capture and utilize solar energy efficiently.
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2. Fluorescence Resonance Energy Transfer (FRET) in Biological Imaging: FRET is extensively used in biological imaging to study protein-protein interactions, conformational changes, and the dynamics of cellular processes. It provides a powerful tool for visualizing and measuring molecular events in living cells.
3. Organic Solar Cells: In organic photovoltaics, energy transfer mechanisms are employed to enhance the absorption and conversion of light into electrical energy. Efficient energy transfer between donor and acceptor materials improves overall solar cell efficiency.
Equations
1. Equation for the formation and dissociation of ozone in the atmosphere:
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2. Rate of change of ozone concentration:
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Keywords for the Series:
● Radiation-matter interaction
● Thermal vs. photochemical processes
● Grothus-Draper Law, Stark-Einstein Law
● Jablonski diagram
● Fluorescence, phosphorescence
● Non-radiative processes
● Quantum yield
● Photosensitized reactions
● Energy transfer mechanisms
References:
[Hochreiter et al.] Hochreiter, Bernhard & Garcia, Alan & Schmid, Johannes. (2015). Fluorescent Proteins as Genetically Encoded FRET Biosensors in Life Sciences. Sensors. 15. 26281-26314. 10.3390/s151026281.
[Strieth-Kalthoff] Strieth-Kalthoff, F., James, M. J., Teders, M., Pitzer, L., & Glorius, F. (2018). Energy transfer catalysis mediated by visible light: principles, applications, directions. Chem. Soc. Rev., 47, 7190–7202. doi:10.1039/C8CS00054A
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- Akhilesh Shende (Author), 2025, Photon Diaries: Unlocking the Secrets of Photochemistry, Munich, GRIN Verlag, https://www.grin.com/document/1674568
