Photochemistry PDF

# PHOTOCHEMISTRY ## Introduction Ordinary reactions occur by absorption of heat energy from outside. The reacting molecules are energized and molecular collisions become effective. These are called thermal or dark reactions. However, some reactions occur by absorption of light radiations. These b...

# PHOTOCHEMISTRY ## Introduction Ordinary reactions occur by absorption of heat energy from outside. The reacting molecules are energized and molecular collisions become effective. These are called thermal or dark reactions. However, some reactions occur by absorption of light radiations. These belong to the visible and ultraviolet regions of the electromagnetic spectrum (2000 - 8000 Å). The reactant molecules absorb photons of light and get excited. These excited molecules then produce the reactions. A photochemical reaction is called a reaction which takes place by the absorption of ultraviolet radiations. Photochemistry studies photochemical reactions. ## Difference between thermal and photochemical processes | Process | Description | |---------------------------------|------------------------------------------------------------------------------------| | Photochemical Reaction | These involve absorption of light radiations. The presence of light is the primary requirement for reactions to take place. Temperature has a minimal effect on the rate of photochemical reactions. | | Thermal reaction | These reactions involve absorption or evolution of heat. These reactions can take place in the dark as well as in light. Temperature has a significant effect on the rate of a thermochemical reaction. | ## Laws of photochemistry Photochemistry is governed by several fundamental principles and laws that describe the behavior of molecules and the interactions between light and matter. Some of the key laws of photochemistry include: 1. **Grothus-Draper Law:** When light falls on a cell containing a reaction mixture, some light is absorbed and the rest is transmitted. Only the absorbed light is effective in producing a chemical reaction. 2. **Stark-Einstein Law:** Each molecule taking part in the reaction absorbs only a single quantum or photon of light. The molecule that gains photon- equivalent energy is activated and enters into reaction. ## Quantum Yield The number of molecules reacted or formed per photon of light absorbed is called the quantum yield. φ = **Number of molecules that react** / **Number of quanta of radiation absorbed** For a reaction that obeys the Einstein law, one molecule decomposes per photon. The quantum yield φ = 1. If the number of molecules decomposed per photon is less than one, the reaction has a low quantum yield. **Energy of photons:** The energy of a photon is given by the equation: ε = **hv** = **hc/λ** Where: * h = Planck’s constant (6.624 × 10^-27 erg-sec) * v = frequency of radiation * λ = wavelength of radiation * c = velocity of light (3 × 10^10 cm sec^-1) ## Examples of high quantum yield 1. **Decomposition of HI**: The decomposition of hydrogen iodide is brought about by the absorption of light of less than 4000 Å. In the primary reaction, a molecule of hydrogen iodide absorbs a photon and dissociates to produce H and I. 2. **Hydrogen-Chlorine reaction**: This is a well-known example of a photochemical chain reaction. A mixture of hydrogen and chlorine is exposed to light of wavelength less than 4000 Å. The hydrogen and chlorine react rapidly to form hydrogen chloride. In the primary step, a molecule of chlorine absorbs a photon and dissociates into two Cl atoms. ## Reasons of low quantum yield 1. **Deactivation of reacting molecules:** The excited molecules in the primary process may be deactivated before they get an opportunity to react. 2. **Occurrence of reverse of primary reaction:** The product then undergoes a thermal reaction giving back the reactant molecules. 3. **Recombination of dissociated fragments:** The reactant molecules may dissociate to give smaller fragments. These fragments can recombine to give back the reactant. ## Factors affecting on Quantum yield 1. **Wavelength of light:** The quantum yield is often higher for photons with shorter wavelengths because these photons have more energy and are more likely to cause a chemical reaction. 2. **Temperature:** The quantum yield is often higher at lower temperatures because non-radiative processes are slowed down at lower temperatures. 3. **Solvent:** The solvent can affect the quantum yield by interacting with the excited state of the molecule and promoting non-radiative decay. 4. **Phase:** The quantum yield is often higher in the solid phase than in the liquid phase because the solid phase provides a more rigid environment that is less conducive to non-radiative decay. 5. **Presence of Impurities:** Impurities can often quench the quantum yield by absorbing photons or interacting with the excited state of the molecule and promoting non-radiative decay. ## Experimental method for the determination of quantum yield φ = **Number of moles that react** / **Number of Einstein of radiation absorbed** ## Types of photochemical reactions 1. **Photosynthesis:** Photosynthesis is a photochemical process by which green plants, seaweeds, algae, and certain bacteria absorb solar energy and utilize it to convert the atmospheric carbon dioxide to carbohydrates in the presence of water. 6CO2 + 6H2O → C6H12O6 + 6O2 2. **Photolysis:** Also known as photodissociation, is a chemical reaction in which a compound is broken down into smaller molecules or atoms due to the absorption of light energy. This process occurs when molecules absorb photons of sufficient energy to overcome the chemical bonds holding them together. 3. **Photo-catalysis:** This phenomenon relies on the absorption of photons by a photocatalyst material, which then leads to the creation of electron-hole pairs. These separated charges can initiate various chemical reactions that wouldn't occur under normal thermal conditions. 4. **Photosensitization:** In many photochemical reactions, the reactant molecule does not absorb the radiation required for the reaction. However, the reaction may still occur if a foreign species such as mercury vapor is present. The mercury atom absorbs the incident radiation and subsequently transfers its energy to the reactant molecule, which is activated. ## Jablonski Diagram The activated molecules return to the ground state by following processes: 1. **Nonradiative transition:** This transition involves the return of an activated molecule from a higher excited state to a lower state; it does not involve emission of a photon. - **Internal conversion:** The molecular spin state for internal conversion remains the same, whereas it changes for intersystem crossing. - **Intersystem crossing:** When a singlet state nonradiatively passes to a triplet state, or conversely a triplet transitions to a singlet. 2. **Radiative Transition:** This transition involves the return of an activated molecule to the ground state by the emission of photon. - **Fluorescence:** Transition from S₁ to S₀ by emission of a photon is an allowed process and occurs in 10–11 sec. It is an energy-releasing phenomenon. - **Phosphorescence:** Transition from T₁ to S₀ by emission of a photon is an allowed process and occurs in 10–³ sec. It is a time-dependent energy-releasing process. ## Chemiluminescence Some chemical reactions are accompanied by the emission of visible light at ordinary temperature. The reaction is referred to as a chemiluminescent reaction. Such a reaction is the reverse of a photochemical reaction which proceeds by absorption of light. The light emitted in a chemiluminescent reaction is called "cold light" because it is produced at ordinary temperature.