Basic photochemistry

B.Tech / UG (Chemistry) / PG Chemistry students and Faculty for academic purpose

1. Basic Principles of Photochemistry Prof. Harish Chopra, Department of Chemistry, SLIET, Longowal (Pb) INDIA

2. Introduction The photochemistry is the interaction of light with matter . IUPAC has defined it as, “The branch of chemistry concerned with the chemical effects of light (far UV to IR)” . ‹#› The simplest photochemical process is seen with the absorption and subsequent emission of a photon by a gas phase atom such as sodium . When the sodium atom absorbs a photon it is said to be excited. After a short period of time, the excited state sodium atom emits a photon of 589 nm light and falls back to the ground state

3. Importance The photochemistry is very important as life itself depends on photochemical processes like photosynthesis . Photochemistry also determines the composition of Earth’s atmosphere supports life and shields us from damaging UV radiation. Further, photochemistry is a central branch from which many other processes find applications. ‹#›

4. Terminology CHARGE-TRANSFER (CT) TRANSITION: An electronic transition in which a large fraction of an electron charge is transferred from one region of a molecular entity, called the electron donor, to another , called the electron acceptor (intramolecular CT) or from one molecular entity to another (intermolecular CT) . MULTIPLICITY (Spin Multiplicity): The number of possible orientations, calculated as 2 S + 1 , of the spin angular momentum corresponding to a given total spin quantum number ( S ) , for the same spatial electronic wave-function. A state of singlet multiplicity has S = 0 and 2 S + 1 = 1. A doublet state has S = ½ and 2 S + 1 = 2. ELECTRONIC ENERGY MIGRATION (or Hopping): The movement of electronic excitation energy from one molecular entity to another of the same species, or from one part of a molecular entity to another part of the same entity. The migration can happen via radiative or radiationless processes ‹#›

5. Terminology HUND’s RULES Of the different multiplets resulting from different configurations of electrons in degenerate orbitals of an atom those with greatest multiplicity have the lowest energy (multiplicity rule) . Among multiplets having the same multiplicity, the lowest-energy one is that with the largest total orbital angular momentum (angular momentum rule) (valid if the total orbital angular momentum is a constant of motion) . In configurations containing shells less than half full of electrons , the term having the lowest total angular momentum J lies lowest in energy, whereas in those with shells more than half filled , the term having the largest value of J lies lowest (fine structure rule) . ‹#›

6. Terminology INTERNAL CONVERSION: A photo-physical process. Iso-energetic radiation-less transition between two electronic states of the same multiplicity . When the transition results in a vibrationally excited molecular entity in the lower electronic state, this usually undergoes deactivation to its lowest vibrational level, provided the final state is not unstable to dissociation QUENCHING: The deactivation of an excited molecular entity intermolecularly by an external environmental influence (such as a quencher) or intramolecularly by a substituent through a non-radiative process . SELECTION RULE: A selection rule states whether a given transition is allowed or forbidden , on the basis of the symmetry or spin of the wave-functions of the initial and final states. ‹#›

7. Laws of Photochemistry Grothus-Draper Law (I st Law of Photochemistry) “When light is incident on a cell containing a reaction mixture, some portion of the light is absorbed while the other remaining part is transmitted. The photochemical reaction is produced by the absorbed light and transmitted light is not effective in any way for the photochemical transformation . Stark-Einstein law (2 nd Law of Photochemistry) It states that each molecule involved in a photochemical reaction absorbs only one quantum of the radiation that causes the reaction ‹#›

8. Quantum Yield (ჶ) As per IUPAC, quantum yield is the number of defined events which occur per photon absorbed by the system . The integral quantum yield is: (ჶ) = (number of events)/ (number of photons absorbed) For a photochemical reaction, quantum yield can be defined as the number of molecules of the reactant consumed or number of molecules of the product formed per quantum of light absorbed. It is denoted by ჶ and is also known as quantum efficiency . The value of quantum efficiency of a reaction may vary from about zero to 10 6 depending upon the reaction. ‹#›

9. Jablonski Diagram ‹#› Jablonski diagrams are frequently used and are actually state diagrams in which molecular electronic states, represented by horizontal lines displaced vertically to indicate relative energies, are grouped according to multiplicity into horizontally displaced columns. Excitation and relaxation processes that interconvert states are indicated in the diagram by arrows. Radiative transitions are generally indicated with straight arrows, while radiationless transitions are generally indicated with wavy arrows.

10. Jablonski Diagram ‹#› Relaxation Mechanism for Excited State Molecules Once a molecule has absorbed energy in the form of electromagnetic radiation it goes to higher energy level (excited state) from ground state as arrow upward pointing S S 1 . There are a number of routes by which it can return to ground state. If the photon emission ( S 1 S ) occurs between states of the same spin state this is termed as fluorescence . If the spin state of the initial and final energy levels is different (T 1 S ), the emission (loss of energy) is called phosphorescence .

11. Franck -Condon Principle ‹#› T he nuclei are enormously heavy as compared to the electrons so, during light absorption (which occurs in femtoseconds) electrons can move, not the nuclei. The much heavier atomic nuclei have no time to readjust themselves during the absorption act, but have to do it after it is over, and this readjustment brings them into vibrations. Hence, Franck-Condon principle states that, “As electrons move much more rapidly as compared to the nuclei, so it is approximated that in an electronic transition, the nuclei do not change their position” .

12. Franck-Condon Principle ‹#› Fluorescence, originates from near the bottom of the upper potential curve , until it strikes the lower potential curve . Again, it does not hit it in its deepest point , so that some excitation energy becomes converted into vibrational energy. The absorption-emission cycle therefore contains two periods of energy dissipation. Because of this, the fluorescence arrow (F) is always shorter (that is, the fluorescence frequency is lower) than that of absorption (A) . In other words, the wavelengths of the fluorescence band are longer than of the absorption band . This displacement of fluorescence bands towards the longer wavelengths compared to the absorption bands is called as Stokes' shift . The Stokes shift is thus displacement of fluorescence band compared to the absorption band of a molecule.

13. Photochemical Energy ‹#› The energy of p hotons is dependent upon the wavelength of the light . Longer wavelength light has low energy and shorter wavelength light has high energy . Photochemistry involves radiation between 2000 nm (near infrared) and <100 nm (soft x-ray). The most important regions for photochemistry are: 700-400 nm (visible), 400-200 nm (Ultraviolet) 200-100 nm (Ultraviolet- visible) The energy range for photochemical dissociation (~ 170-1200 kJ mol -1 )

14. Electromagnetic Spectrum ‹#›

15. Chemistry of Photochemical Excitation ‹#› In photochemistry, the light is absorbed in the wavelength range of 200-800 nm of the spectrum. Such a spectrum measures amount of incident light by the molecule as a function of wavelength. Lambert’s law states that when a monochromatic beam of light is allowed to pass through a transparent medium, the rate of decrease in intensity with the thickness of the medium is directly proportional to the intensity of the incident light. Beer’s Law states that when a monochromatic light is passed through a solution the rate of decrease in intensity with the thickness of the solution is directly proportional to the intensity of the incident light as well as the concentration of the solution.

16. Chemistry of Photochemical Excitation ‹#› The expression in the following equation is termed as mathematical statement for Beer-Lambert’s law , The quantity log (I /I t ) is called as absorbance (A) . The following is the relation between absorbance (A), transmittance (T) and molar absorption coefficient (e) : .

17. Electronic Transitions ‹#› The absorption of UV or visible radiation corresponds to the excitation of outer electrons. There are 3 types of electronic transition which can be considered; (i) Transitions involving 𝝅, 𝛔, and n electrons (ii) Transitions involving charge-transfer electrons (iii) Transitions involving d and f electrons When an atom or molecule absorbs energy, electrons are promoted from their ground state to an excited state . In a molecule, the atoms can rotate and vibrate with respect to each other. These vibrations and rotations also have discrete energy levels, which can be considered as being packed on top of each electronic level .

18. Electronic Transitions ‹#› The Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups ( chromophores ) that contain valence electrons of low excitation energy. The electronic transition is involved in promotion of an electron from one of three ground states ( 𝝅, 𝛔 or n) to one of the antibonding ( 𝝅*, 𝛔* ) molecular orbital. Organic molecules show four important types of transitions:

19. Electronic Transitions ‹#› Most absorption spectroscopy of organic compounds is based on transitions of n or 𝝅 electrons to the 𝝅 * excited state. This is because the absorption peaks for these transitions fall in an experimentally convenient region of the spectrum (200 - 700 nm). These transitions need an unsaturated group in the molecule to provide the 𝝅 electrons .

20. Photochemical Reaction Processes ‹#› A photo-excited species, [X-Y]* , can relax (react) via a variety of pathways as given below .

21. Photochemical Reaction Processes ‹#› Dissociation: The excited state species may fragment to a pair of radicals (halogens) or, in the case of nitrogen dioxide to nitric oxide and oxene. 2- Pentanone Propanone

22. Photochemical Reaction Processes ‹#› Direct Reaction: The photo-excited state may undergo reactions unavailable to the ground state species. E.g., a photo-excited ketone can undergo a [2+2] cycloaddition with an alkene to give an oxetane, a Paterno-Büchi reactio n .

23. Photochemical Reaction Processes ‹#› Isomerization: Photo-excited species may undergo isomerization. E.g. trans-stilbene can be photo-excited to a state that allows free rotation around the alkene double bond. This photo-excited species is able to relax back to the ground state to give cis-stilbene .

24. Photochemical Reaction Processes ‹#› Energy transfer: An excited state species can transfer energy to another ground state species. This process is used to produce singlet oxygen and energy transfer in such cases is intermolecular . A dye, usually rose bengal, is photo-excited with UV light to transfer energy to triplet oxygen, which is converted to singlet oxygen . Intramolecular Energy Transfer Intermolecular Energy Transfer

25. Photochemical Reaction Processes ‹#› Quenching: In the liquid state, the excited state species may be quenched in which the energy of the excited state species is converted into vibrational energy (heat) . The quenching is generally affected with solvents . Photoionization: These processes are of great importance high in the atmosphere where pressures are low and short wavelength UV radiation from the sun has a high flux. E.g., photo-ionization of nitric oxide by photon having wavelength 134.3 nm .

26. Selection Rules for Electronic Transitions ‹#› Spin Based Selection Rules Symmetry Based Selection Rules Transitions between electronic states of same spin multiplicity are ALLOWED . Singlet Singlet Triplet Triplet Transitions between electronic states of different spin multiplicity are FORBIDDEN . Singlet Triplet Triplet Singlet Transitions between orbitals of same symmetry are ALLOWED . = ALLOWED transition Transitions between orbitals of different symmetry are FORBIDDEN . = FORBIDDEN transition

27. Primary Photochemical Reactions ‹#› Reactions initiated by 𝛑 𝛑 * state [S 1 state] Reactions involving Carbocations Reactions involving Carbanions Concerted Pericyclic reactions Electron Transfer reactions Cis-trans Isomerization reactions Homolytic Fragmentation Hydrogen atom Abstraction Addition to Unsaturated Bonds Rearrangement of stable carbon centred radicals Reactions initiated by n 𝛑 * state [T 1 state]

28. Photosensitized Reactions ‹#› Reactants in some chemical reactions do not absorb light and no product is formed on exposure of such reactants to radiations. However, if with the addition of another substance, which can absorb radiations, these reactants are converted into products. In actual the substance added absorbs light and becomes excited and then passes this energy to one of the reactants, which gets activated to react with the other reactant(s) to give products. The substance which when added to a reaction mixture to help in initiating a photochemical reaction without undergoing any chemical change is called a photo-sensitizer and these types of reactions are called photosensitization reactions . The process is called as photosensitization. The most commonly used photosensitizers include mercury , cadmium, benzophenone and sulphur dioxide .

29. Mechanism of Photosensitization ‹#› A general donor -acceptor system in which donor (sensitizer) absorbs the incident light and becomes excited. The triple state of the donor is higher than the triple state of the acceptor that is reactant . On absorption of photon donor changes to singlet excited state and then it changes to triplet excited state by intersystem crossing (ISC). This triplet state then collides with acceptor producing triplet excited state of acceptor (T 1 ) and ground state of donor, when triplet state of acceptor gives the desired product then the mechanism is called photosensitization . The triplet excited state of sensitizer must be higher in energy than triplet excited state of the reactant so the energy available is sufficient to raise the reactant molecule to its triplet excited state . .

30. Applications of Photosensitization ‹#› Isomerization of but-2-ene: SO 2 acts as photosensitizer in this isomerization reaction. The cis-but-2-ene and SO 2 vapour are irradiated with light of 𝞴 = 254 nm leading to the formation of trans-but-2-ene Dimerization of Cyclohexane: Hg is used as photosensitizer in t his reactio n. The mixture of cyclohexane and mercury vapou r are irradiated with light of 𝞴 =254 nm to give dimerized products

31. ‹#› References Chemistry For Engineers, Harish Kumar and Anupama Parmar, ISBN No. 978-81-8487-545-4 (2016), Published by Narosa Publishing House Pvt. Ltd., New Delhi. . NPTEL Lectures & Videos Internet sources

32. Thank You ! ‹#›