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 3

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. 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

5. 2. Quantitative methods - quantum yields, -rate constants, -lifetimes, - kinetic of quenching, - experimental problems, e.g. inner filter effects

6. Definition of terms used in photochemistry Quantum yields  hn For a photochemical reaction AB differential quantum yield:

7. Kinetic scheme rate hn A(S0)A(S1) Ia (einstein dm-3 s-1) A(S1) A(S0) + hnf kf [A(S1)] A(S1) A(S0) + heat kIC [A(S1)] A(S1) A(T1) kISC [A(S1)] A(S1) B + C kr [A(S1)] A(S1) + Qquenching kq [A(S1)] [Q] A(T1)A(S0) + hnp kp [A(T1)] A(T1) A(S0) + heat k'ISC [A(T1)] A(T1) B' + C' k'r [A(T1)] A(T1) + Q quenching k'q [A(T1)] [Q]

8. Steady-state approximation : Ia = (kf + kIC + kISC + kr + kq[Q]) [ A(S1)] = [A(S1)]/S Fluorescence quantum yield: f = kf[ A(S1)] / Ia f = kf SIC = kIC SISC = kISC S For photochemical reaction from S1: R = kr[ A(S1)] / Ia A = B = kr S

9. Phosphorescence quantum yield: p = kp[ A(T1)] / Ia p = ISCkpT For photochemical reaction from T1: 'R = k'r[ A(T1)] / Ia 'A = 'B = ISC k'rT

10. Quantum yield measurement Chemical actinometry: -Uranyl Oxalate Actinometry hv H2C2O4  H2O + CO2 + CO UO2+2 R = 0.602 (for 254 nm) R = 0.561 (for 313 nm) -Benzophenone-Benzhydrol Actinometry (C6H5)2CO + (C6H5)2CHOH  (C6H5)2C(OH) C(OH) (C6H5)2 R = 0.68 (for 0.1M BP and 0.1M benzhydrol in benzene) -2-Hexanone Actinometry (Norrish Type II) acetone= 0.22 (for 313 nm)

11. Typical dependence of quantum yield vs Iat

12. Quantum yield of intermediates Laser flash photolysis: I= st Apst /  Astp Apand  Asttransient absorbances for intermediate and actinometer p and st molar absorption coefficents of intermediate and actinometer st quantum yield of actinometer (using benzophenone equal to ISC= 1) A(ex) for irradiated solution = A(ex) for actinometer

13. Rate constants kr = R/S from S1 k'r = 'R/ (ISC T) from T1 S and T from direct measurement (laser flash photolysis)

14. Kinetic of quenching rate hn A(S0)A(S1) Ia (einstein dm-3 s-1) A(S1) A(S0) + hnf kf [A(S1)] A(S1) A(S0) + heat kIC [A(S1)] A(S1) A(T1) kISC [A(S1)] A(S1) B + C kr [A(S1)] A(S1) + Qquenching kq [A(S1)] [Q] A(T1)A(S0) + hnp kp [A(T1)] A(T1) A(S0) + heat k'ISC [A(T1)] A(T1) B' + C' k'r [A(T1)] A(T1) + Q quenching k'q [A(T1)] [Q]

15. Stern-Volmer equation for S1

16. Stern-Volmer equation for T1

17. Quenching of 3CB* by Met-Gly in aqueous solutions at pH = 6.8 kq = (2.14  0.08)  109 M-1 s -1

18. Triplet Quenchers pH neutral pH basic Triplet Quenchers pH neutral pH basic Quenching Rate Constants (109 M-1 s-1) for quenching of CB triplet state Thiaproline Methionine Alanine S-(Carboxymethyl)cysteine Met-Gly L-Met-L-Met Gly-Gly-Met Met-Enkephalin 2.1 2.5 0.0005 0.81 2.1 2.9 1.8 1.9 2.6 2.3 0.18 0.75 2.3 1.8 1.9 1.8 Rate constants of the order of 109 M-1 s-1 indicative of electron transfer

19. Methionine

20. Traditional Scheme

21. Definition of terms used in photochemistry 2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry 79, 293–465 Inner-filter effects Term used in two different ways: (1) During an irradiation experiment, absorption of incident radiation by a species other than the intended primary absorber is also described as an inner-filter effect.

22. Definition of terms used in photochemistry 2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry 79, 293–465 Inner-filter effects (2) In an emission experiment, it refers to (a) an apparent decrease in emission quantum yield at high concentration of the emitter due to strong absorption of the excitation light (b) an apparent decrease in emission quantum yield and/or distortion of bandshape as a result of reabsorption of emitted radiation (particularly severe for emitters with small Stokes shift).

23. hn A  hn A + Q  Ia[einstein dm-3 s-1]

24. Corrections for inner filter effect (1) (for the absoprtion of incident light by Q) Corrections for inner filter effect (2) (for reabsorption of fluorescence of A by Q)

25. Changes of fluorescence spectra of benzene with various Cu(acac)2 concentrations

26. Changes of fluorescence spectra of benzene with various Cu(acac)2 concentrations

27. without correction with correction Stern-Volmer plot for the quenching of benzene fluorescence by Cu(acac)2

28. Experimental setups for measuring fluorescence spectra

29. Stern Volmer plot for quenching of benzene fluorescence by Cu(acac)2 - front-face technique (ex=250 nm, f=278 nm)