PHOTOCHEMISTRY By Dr. BHARAT DIXIT Chemistry Department V. P. & R. P. T. P. Science College

1. PHOTOCHEMISTRY By Dr. BHARAT DIXIT Chemistry Department V. P. & R. P. T. P. Science College Vallabh Vidyanagar-388120

2. Photochemistry : It is the study of reactions that are brought about by the action of visible or ultraviolet light. e.g. photosynthesis of glucose in the plants by means of sunlight.

3. Chemical reactions accompanied with light are : By the action of light A → B (i) A new molecule is generated by chemical changes (light induced reactions) (ii) In some reaction, during chemical reaction light is emitted (chemiluminescence)

4. Luminescence • - Chemiluminescence: • (green) • White phosphorus glows ignites in air to form phosphorus pentoxide. • Bioluminescence: - mushrooms • (emission of light • by a living organism) - insects • - fishes

5. Light: Electromagnetic field vibration spreading in quanta(photons). Photon:The smallest amount of light carrying energy. Energy of photons (A. Einstein) SO one particle of a chemical substance can absorb only one photon from a light beam: DE = hn

6. 604 kJ/mol-1 302 151 ULTRAVIOLET VISIBLE INFRARED 200 nm 400 nm 800 nm Chemical bond energies:from 100 – 1000 kJ/mol Light energies: So UV – and VIS region is expected to induce chemical reactions.

7. Laws of Photochemistry • (I) Only light that is absorbed can producephotochemical change (Grotthus, Draper) • (ii) A molecule absorbs a single quantum of lightis becoming excited (Stark, Einstein)

8. Basic Principles of Photochemistry : Photochemistry is based on following fundamentals. 1. Photochemical Energy. 2. Electronic Excitation. 3. Excited State, Modes of Dissipation of Energy. 4. Energy Transfer. 5. Quantum Efficiency.

9. 1. Photochemical energy: For any chemical transformation an activation energy, must be supplied to molecules. Such energy can be provided by different way : • Some molecules undergo spontaneous transformations. (ii) In some cases, energy is supplied by increasing the temperature (thermal condition) as a result molecules present in the system have same amount of energy throughout the chemical transformation.

10. (iii) In another method, molecules present in the system involve the absorption of electromagnetic radiation in the visible or ultraviolet region (photochemical condition). • Such absorption of light excites an individual molecule from ground state to an excited electronic state without effecting the surrounding molecules. • - selective excitation • chemistry of excited molecules differs from the chemistry of those in the ground state • One can change the course of a reaction by activating the reactants by light rather than by heat.

11. Infraredregion is producing Vibrationally or rotationally excited molecules, While light in the visible and ultraviolet region has sufficient energy to cover the range of chemical bond energies and is able to induce chemical changes by exciting molecules to higher electronic states.

12. 2. Electronic excitation: Promotion of an electron from the bonding orbital to the corresponding anti-bonding orbital take place .  - * ;  - * and n- *.

13. Molecular Orbital Diagram CH2 = CH – CH = CH2 In butadiene  - * transition occurs, from Ψ2 to Ψ3

14. 3. Excited States, Modes of Dissipation of Energy : In an organic molecule, even number of electrons are paired in the ground state. Now when absorption of light of the correct energy occurs, than one of the electron excited from ground state () to excited state(*) by retaining the spin, so the electron spins remain paired in the excited state. This state is called excited singlet state (S1). In some cases, spin inversion take places thus giving rise to a new excited state with two unpaired electrons. This state is called an excited triplet state (T1).

15. A triplet state is more stable than the singlet state. Because in triplet state, electrons are unpaired - lesser inter electronic repulsion take place, while in singlet state electrons are paired, -causing inter electronic repulsion and is unstable.

16. Jablonski Diagram: ISC = Intersystem crossing. IC = Internal conversion. F = Fluorescence. P = Phosphorescence. RD = Radiationless decay.

17. When a molecule absorbs a photon of energy, electronic transition occurs (Su or S2 or S1). • A molecule in S1 state undergo one of the following four energy • degrading (decay) processes to the ground state. • The molecule can undergo chemical reaction or return to the • ground state by emission of light by a process called fluorescence • and generally occurs with in 10-9 to 10-6sec. • (ii) It may return to ground state (S0) by non radiative process in • which excess energy of the excited state is shuffled into vibrational • modes. • (iii) S1 may undergo chemical reactions. • (iv) The molecule may undergo spin inversion to the triplet state • (spin unpaired) by a process as ”intersystem crossing”, • Which is a radiation less process.

18. Fluorescence :Emission of a photon from a singlet excited state to a singlet ground state, or between any two energy levels with the same spin, is called fluorescence. Fluorescence, decays rapidly after the excitation source is removed. Lifetime of the electron is only 10–5 to 10–8 s Phosphorescence : Emission between a triplet excited state and a singlet ground state is called phosphorescence. phosphorescence may continue for some time after removing the excitation source. Lifetime for phosphorescence ranges from 10–4 to 104 s.

19. 4. Energy Transfer and Photosensitization: It is a one step radiationless transfer of excitation energy from an electronically excited molecule (donor) to the ground state of another molecule (acceptor).” As a result, the donor molecule returns to the ground state and the acceptor molecule gets excited. For energy transfer the donor molecule should have at least 5 kcal/mole more energy than the acceptor molecule. A typical mechanism for triplet energy (T1) transfer is described below :

21. Energy transfer between butadiene as an acceptor and benzophenone as donor. Butadiene (I) (acceptor) upon direct irradiation leads to produce product (II) and (III) via ring closure. When butadiene is mixed with benzophenone (donor) and is irradiated at 366 nm, than butadiene undergoes photochemical change to yield dimmers (IV) (V) and (VI).

22. In this process light is absorbed by benzophenone (donor) but the reaction is taking place with butadiene (acceptor) . Here benzophenone (donor) is functioning as a photosensitizer, while butadiene is good acceptor as it has 9 kcal/mole less triplet energy than benzophenone.

23. Benzophenone (69 Kcal/mole)(donor) triplet energy transfer to 1,3 - Butadiene (60 Kcal/mole) (acceptor) (Intermolecular energy transfer ).

24. Intramolecular energy transfer in benzophenone (donor) and naphthalene. Irradiation of these compounds with light of about 366 nm wavelength absorb by benzophenone showed that there is an efficient transfer of triplet excitation from the benzophenone moiety to the naphthalene moiety.

25. energy transfer in Cis – Trans Isomers of 4-hexen-2-one. Cis - trans isomerization of 4-hexen-2-one shows intramolecular energy transfer when light is absorbed by the carbonyl group. During this photoreaction energy transfer from the carbonyl group to the carbon-carbon double bond takes place.

26. 5. Quantum Efficiency. It is the number of moles of a reactant disappearing, or the number of moles of a product produced, per einstein of monochromatic light absorbed ɸ = NUMBER OF MOLECULES UNDERGOING THE REACTION / NUMBER OF PHOTONES ABSORBED BY THE PHOTOREACTIVE SUBSTANCE

27. Molecular orbital construction of n - * transition The remaining two electrons of oxygen atom present in p-orbital are known as lone pair or n electrons. In n - * transition, an electron of the lone pair of “O” is prompted to a vacant * orbital as shown below : n - * excitation of the carbonyl group.

28. In terms of V. B. T., the excitation may be represented as, n - * transition is always less intense than  - * transition even though it occurs at longer wavelength.

29. Photoreduction: When a solution of benzophenone in isopropyl alcohol is irradiated with a light of 345 nm it produce a benzpinacol. During this photochemical reaction isopropyl alcohol does not absorb light but benzophenone undergoes an n - * transition.

30. Mechanism In benzophenone, an initial light absorption is followed by rapid intersystem crossing from S1 to T1. n - * triplet state of carbonyl group of benzophenone is capable of abstracting the α - hydrogen of isopropyl alcohol, thus giving rise to produce two radicals A & B.

31. combination of two units of A leads to form benzpinacol. Since the quantum yield () for the formation of benzpinacol is 1, the second unit of ‘A’ does not form as a result of absorption of a fresh quantum of light. However, it is generated by the reaction between benzophenone and unit B. i.e.

32. Similarly benzpinacol is formed during the reaction between benzophenone and benzhydrol in benzene

33. Mechanism:

34. Michler’s ketone does not undergo photoreduction in isopropyl alcohol In michler’s ketones, the lone pair of electrons situated on nitrogen atom is in conjugation with the aromatic rings, which lowers the energy required for the  - * transition in the molecule. Thus  -* transition becomes the lowest triplet of the system and occur more rapidly than the n - * transition. The relative energies of n - * and  - * transition are shown below :

35. Photoreduction of α – ketoester α – ketoesters undergo photoreduction to yield pinacols.

36. lactone from ethyl aceto acetate Ethyl acectoester yields solvent adduct Instead of pinacol. Adduct is than converted into a Lactone. (β – Ketoester)

37. Norrish Type I reaction Photochemical excitation of ketones results into the hemolytic fission of the α - carbon - carbon bond. This process is known as α - cleavage or norrish type I reaction. n - * excited state of acetone undergoes a carbon-carbon cleavage yielding a methyl radical and an acetyl radical. At room temperature two acetyl radical combines to form biacetyl.

38. At temperature above 1000C; acetyl radicals decarbonylate to produce ethane and carbon monoxide. In case of unsymmetrical ketone, splitting takes place in a way to generate the more stable of the two possible radicals.

39. Norrish Type II reaction Ketones having a γ - hydrogen atom undergo photochemical reaction, which results in the formation of an olefin and the enol of a smaller ketone. This process is known as Norrish type II reaction.

40. Norrish type I and II reaction for 2-pentanone.

41. Photochemical reaction of cyclic ketones. The photochemical reaction of saturated cyclic ketones begin with the n - * excitation of the carbonyl group followed by α – fission. The biradical may then undergo hydrogen transfer from the α–carbon atom to the carbonyl group yielding a ketene or it may give rise to an unsaturated aldehyde by hydrogen transfer to the carbonyl group. 2,2| -dimethyl cyclohexanone converted to 1,1| -dimethyl cyclopentane .

42. cyclic ketones may give rise to an unsaturated aldehyde by hydrogen transfer to the carbonyl group.

43. Paterno-Buchi Reaction Photocycloaddition of carbonyl compounds with olefins upon irradiation yields oxetanes, is known as paterno-buchi reaction. Photocycloaddition is performed by irradiation with the light of wave length absorbed only by the carbonyl group, it involves the triplet excited state of the carbonyl comp rather than that of olefin.

44. Mechanism of pateron-buchi reaction Unsymmetrical Olefins: Since the olefin is unsymmetrical, two products will be obtained. The product in which the most stable biradical intermediate forms, will be the major and other will be minor.

45. Mechanism of pateron-buchi reaction

46. (ii) Symmetrical Olefins: Irradiation of benzophenone with either cis or trans-2-butene yields the same mixture of both isomeric oxetanes.

47. Limitation of paterno-buchi reaction Paterno-buchi reaction fails when the triplet excitation energy of ketoneexceeds that of olefins. Under such condition oxetane formation does not take place but energy transfer take place from triplet excitation state of ketone (T1) to the olefins (S0). There by olefin is excited to triplet state and is dimerize. e.g. Irradiation of acetone with Norborene, yields dimmers of norbornene rather than Oxetanes.

49. When a ketone (Benzophenone) having less triplet energy is used will leads to paterno – buchi reaction to form Oxctanes of a nonrborene.

50. Paterno – buchi reaction also fails to occur with conjugated dienes.