Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland

1. "Molecular Photochemistry - how to study mechanisms of photochemical reactions ?" Bronislaw Marciniak Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 2

2. Contents • Introduction and basic principles (physical and chemical properties of molecules in the excited states, Jablonski diagram, time scale of physical and chemical events, definition of terms used in photochemistry). • Qualitative investigation of photoreaction mechanisms - steady-state and time resolved methods(analysis of stable products and short-lived reactive intermediates, identification of the excited states responsible for photochemical reactions). • Quantitative methods(quantum yields, rate constants, lifetimes, kinetic of quenching, experimental problems, e.g. inner filter effects).

3. Contents cont. • 4.   Laser flash photolysis in the study of photochemical reaction mechanisms (10–3 – 10–12s). • 5.   Examples illustrating the investigation of photoreaction mechanisms: • -   sensitized photooxidation of sulfur (II)-containing organic compounds, • -   photoinduced electron transfer and energy transfer processes, • -   sensitized photoreduction of 1,3-diketonates of Cu(II), • -   photochemistry of 1,3,5,-trithianes in solution.

4. 2. Qualitative investigation of photoreaction mechanisms - steady-state and time resolved methods - analysis of stable products - identification of short-lived reactive intermediates -identification of the excited states responsible for photochemical reactions

5. Jablonski diagram

6. h A A* I B + C Scheme of photochemical reaction Stable products Intermediates - analysis of stable products - identification of short-lived reactive intermediates -identification of the excited states responsible for photochemical reactions

7. Norrish type II Photoreaction

8. Analysis of stable products h A A* I B + C • 1. Preparative irradiations • 2. Product analysis: GC, HPLC, TLC, GCMS, LCMS, spectroscopic methods etc. • 3. Separation of products from the reaction mixture: • - prepartative GC, HPLC, TLC, • - column chromatography • - other methods • 4. Identification of separated products:spectroscopic methods: IR, NMR, UV-Vis, Fl, MS, elemental analysis etc. Note: Separated products can be used as reference samples in the quantitative analysis

9. Norrish type II photoreaction of valerophenone (0.1 mol/dm3) in methanol irr > 300 nm C6H5COCH2CH2CH2CH3 C6H5COCH3 + CH2=CHCH3 + cyclobutanol derivative Analysis of stable products - example h

10. Internal standard Detector's signal Retention time [min]

11. Norrish type II Photoreaction

12. Photochemistry of Valerophenone in methanol - GCMS results acetophenoneC6H5C(O)CH3 m/e (relative intensity): 121(3,4), 120(M+,41), 106(8), 105(100), 78(9), 77(83), 51(30), 50(11) 1-phenyl-2-methylcyclobutanol (izomer trans) m/e (relative intensity): 162(M+,3), 135(9), 134(33), 133(25), 120(100), 105(76), 91(15), 78(43), 77,(42), 51(18) 1-phenyl-2-methylcyclobutanol (izomer cis) m/e (relative intensity): 135(8), 134(7), 133(12), 120(100), 105(56), 91(10), 78(40), 77(36), 51(12).

13. Steady-state irradiation systems

15. 1- excitation source, 2- diaphragm, 3- thermal filter (cell with H2O), 4- lens, 5- light filter, 6- merry-goround system

17. h A A* I B + C Identification of short-lived reactive intermediates 1. Spectroscopic methods - flash photolysis - UV-Vis absorption and emission - IR - NMR (CIDNP) - EPR 2. Chemical methods 3. Kinetic methods

18. ns laser flash photolysis

19. Benzophenone-(Phenylthio)acetic Tetrabutylammonium Salt Sovent: CH3CN

20. Fig. Transient absorption spectra of intermediates following thequenching of benzophenone triplet by Ph-S-CH2-COO-N+(C4H9)4 (0.01M). Inset: kinetic trace at 710 nm.

21. Fig. Transient absorption spectra following triplet quenching of BP (2 mM) by C6H5-S-CH2-COO-N+R4(10 mM) after 1 s and 150 s delays after the flash in MeCN solution. Insets: kinetic traces on the nanosecond and microsecond time scales

22. Reaction scheme

23. System studied Sensitizers 4-Carboxybenzophenone (CB) Benzophenone (BP)

24. Sulfur-Containing Organic Compounds (Quenchers):

25. Sulfur-Containing Organic Compounds (Quenchers): methionine derivatives methionine-containing di-, tripeptides and polypeptides e.g. Met-Gly, Gly-MetMet-Met, Met-Met-Met Met-Met-Ala Met-Gly-Met Met-Enkephalin

26. Motivations • Oxidative stress • Alzheimer’s disease • Biological aging • Basic issues • Neighboring-group effects • Details of oxidative scheme

27. Our Traditional Scheme

28. Reference Spectra of CB  (M-1cm-1)

29. Intermediates

30. CB + C6H5-S-CH2-COOH in aqueous solution Fig. Transient absorption spectra followinglaser flash photolysis recorded at four different delay times. Benzophenone ([CB=2mM) and (phenylthio)acetic acid([C6H5-S-CH2-COOH]=20mM) in Ar-saturated aqueous solutions pH=7.5. Inset: kinetic trace at  = 660 nm

31. h A A* I B + C Identification of short-lived reactive intermediates 1. Spectroscopic methods - flash photolysis - UV-Vis absorption and emission - IR - NMR (CIDNP) - EPR 2. Chemical methods 3. Kinetic methods

32. +Z h A A* R RZ stable product Identification of short-lived reactive intermediates 2. Chemical methods - chemical trapping Scavenger (Z) of free radicals: - does not absorb excitation light - selectively react with R with a large rate - does not react with A, A* and RZ - does not affect the mechanism of RZ formation - form RZ easy to detect. Typical scavengers: O2, alkenes, RNO, I2

33. 2. Chemical methods - example Y.L. Chow, G. Buono-Core, J. Am. Chem. Soc. 108, 1234, (1986) „Role of the Acetylacetonyl Radical in the Sensitized Photoreduction of Bis( acetylacetonato)copper( II)” Spin trapping of acetylacetonyl radicals (acac ):   EPR spectrum of the benzophenone-sensitized photoreduction of Cu(acac)2, in the presence of 2-nitroso-2-methylpropane measured after two-minute irradiation of a methylene chloride solution of Cu(acac)2 (1mM), 2-nitrozo-2-methylpropane (2 mM), and benzophenone (5 mM), hyperfine splitting constants: aN= 1.363 mT, aH= 0.315 mT and g = 2.0062. B [mT]

34. 2. Chemical methods - example Y.L. Chow, G. Buono-Core, J. Am. Chem. Soc. 108, 1234, (1986) „Role of the Acetylacetonyl Radical in the Sensitized Photoreduction of Bis( acetylacetonato)copper( II)” Trapping of acac with alkenes: RZ product analysis: GCMS and NMR, IR Conclusion:acac was proved to be the reactive intermediate in the sensitized photoreduction of Cu(acac)2.

35. Different Actions of Scavengers • Direct capture of free radicals. • Repair of damage caused by radicals. • This second mechanism is important for the repair of damage by free radicals in biological systems.

36. h A A* I B + C Identification of short-lived reactive intermediates 1. Spectroscopic methods - flash photolysis - UV-Vis absorption and emission - IR - NMR (CIDNP) - EPR 2. Chemical methods 3. Kinetic methods

37. 3. Kinetic methods Example (N.J. Turro, Modern Molecular Photochemistry, p. 261, „Involvement of T1 (n,*) of benzophenone as the chemically reactive agent in the photoreduction of benzophenone by benzydrol” B  B* aIa B*  B kd[B*] B* + BH2 BH kr[B*][BH2] 2BH  BH-BH kp[BH]2 B* + Q  B + Q* kq[B*][Q]

38. 3. Kinetic methods Example (N.J. Turro, Modern Molecular Photochemistry, p. 261, „Involvement of T1 (n,*) of benzophenone as the chemically reactive agent in the photoreduction of benzophenone by benzydrol” Experiments: kd/kr = 0.05 M kq/kr = 500 Taking kq = 1x109 M-1s -1 kd 105s -1   10 s Conclusion: T1 (n,*) of benzophenone is the reactive state.

39. h A A* I B + C Kinetic methods in the study of the mechanism of photochemical reactions Procedure: - assumption of the kinetic scheme - appropriate equations should be derived, e.g. dependence of R vs. [A] or [Q] - experiments, rate constants determnation and the interpretation of the results Kinetic methods are so-called indirect methods and must confirmed by direct methods.

40. h A A* I B + C Determination of the reactive state in a photoreaction: 1. Direct methods ( A, F, P, EPR) 2. Indirect methods (sensitization and quenching)

41. If the photoreaction is wavelenght- independent, the involvement of upper excited states can be neglected. Question: S1 or/and T1

42. Experiment (result) Reactive state (conclusion) 1. Only S1 quenched, reaction inhibited None 2. OnlyT1 quenched, reaction inhibited T1 3. OnlyT1 quenched, reaction uninhibited S1 4. Only T1 sensitized, reaction does not occur S1 5. Only T1 sensitized, reaction occurs T1 or S1 + T1

43. Experimental Methods for Detection of Intermediates and Excited States [Turro] Reactive DirectIndirect methods intermediate methods S1 F, A CIDNP, KINETICS, PRODUCTS T1 P, A, EPR CIDNP, KINETICS, PRODUCTS R3C+ A, F, P MI, CHEM, PRODUCTS R3C- A, F, P MI, CHEM, PRODUCTS R3C• A, F, EPR MI, CHEM, PRODUCTS Biradical A, F, P, EPR CIDNP, MI, CHEM, PRODUCTS

44. h A A* I B + C Scheme of photochemical reaction Stable products Intermediates - analysis of stable products - identification of short-lived reactive intermediates -identification of the excited states responsible for photochemical reactions

45. h A A* I B + C Kinetic methods in the study of the mechanism of photochemical reactions Procedure: - assumption of the kinetic scheme - appropriate equations should be derived, e.g. dependence of R vs. [A] or [Q] - experiments, rate constats determnation and the interpretation of the results Kinetic methods are so-called indirect methods and must confirmed by direct methods.

46. Example: Photochemistry of Phenyl Alkyl Ketones in the Presence of PPh3 where: R = CH3, (CH2)2CH3, (CH2)3CH3, (CH2)2CH(CH3)2, (or Ph)

47. Norrish type II photoreaction

48. Kinetic scheme FISC = 1.0 hn K 1K3K 3K K + + H -abstraction 3K 3B 3K + PPh3  [K-PPh3] [K-PPh3] PhCH(OCH3)R + Ph3PO [K-PPh3] K + PPh3 3B  K 3B  AP + olefina 3B  CB 3B + PPh3 K + PPh3 k2 CH3OH k3 k4 k5 CH3OH k6 k7 k8 k9 k10

49. 0,  - quantum yields of acetopnenone (AP) in the absence and presence of PPh3 (AP , CB, K) e- quantum yield of ether (PhCH(OCH3)R (or Ph3PO) B = 1/(k7 +k8 +k9) T = 1/(k2 +k3)  = k5/(k5 +k6) = emax

50. Internal standard Detector's signal Retention time [min]