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 4

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. 3. Laser flash photolysis in the study of photochemical reaction mechanisms (10–3 – 10–12s).

6. ns laser flash photolysis

7. 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.

8. 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

10. HS + HG

11. Nanosecond flash photolysis HS+HG • Spectra Physics INDI, 266, 355, 532nm, 10Hz, 6-8ns, 450 mJ @ 1064nm • Si photodiode, 2ns rise-time • flow cell + temperature controlled holder • fibre coupled 150W Xe lamp (Applied Photophysics) with pulser, 500ms plateu (or alternatively 175W Xe Cermax CW lamp) • Acton Spectra Pro SP-2155 monochromator with dual grating turret • Hamamatsu R955 PMT + SRS PS-310 power supply • LeCroy WR 6100A DSO • PC (GPIB, NI-DAQ, LabView) • opto-mechanics Standa

12. Instrumentation HS + HG

13. Femtosecond transient absorption spectrometer Pump-Probe Femtosecond Laser at Notre Dame University

14. NDRL femto lab

15. Femtosecond transient absorption spectrometer: • time resolution < 100 fs • sensitivity better than OD=0.005 • excitation: tunable Ti:Sapphire laser (750-840 nm at fundamental) • detection: time-gated CCD camera • SHG (375-420 nm) • THG (250-280 nm) AMU Center for Ultrafast Laser Spectroscopy

16. AMU Physics DepartmentPicosecond Transient Absorption

17. Sub-nanosecond emission spectrometer IBH System 5000 • excitation: nanoLEDs (295, 370, 408, 474 nm) • FWHM 200 ps • detection: PMT operated in TCSPC mode • PC based MCA: 6 ps/channel (50 ns time window / 8196 channels) • emission and fluorescence anisotropy measurements

18. Picosecond emission spectrometer (TCSPC): • excitation: tunable Ti:Sapphire laser (720-1000 nm) pumped by Argon-Ion laser • detection: PMT (IRF 200 ps) or MCP (IRF 25 ps) operated in TCSPC mode • SHG (360-500 nm) • THG (240-330 nm) • FWHM 1.5 ps AMU Center for Ultrafast Laser Spectroscopy

19. Long Lifetime Sample

20. Triplet-Triplet Absorption Spectra of Organic Moleculesin Condensed Phases Ian Carmichael and Gordon L. Hug Journal of Physical and Chemical Reference Data 15, 1-150 (1986) http://www.rcdc.nd.edu/compilations/Tta/tta.pdf

21. Methods of DeterminingTriplet Absorption Coefficients • Energy Transfer Method • Singlet Depletion Method • Total Depletion Method • Relative Actinometry

22. Energy Transfer (General) • Two compounds placed in a cell. • Compound R has a known triplet absorption coefficient. • Compound T has a triplet absorption coefficient to be determined. • Ideally, the triplet with the higher energy can be populated. • Thus triplet energy of one can be transferred to the other.

23. Energy Transfer (General) • If the lifetimes of both triplets are long in the absence of the other molecule, then • One donor triplet should yield one acceptor triplet. • In an ideal experiment eT* = eR* ( DODT / DODR ) Note it doesn’t matter whether T or R is the triplet energy donor.

24. 3R* + 1T  1R + 3T* kobs = ket [1T]0 [3R*] = [3R*]0 exp(kobst) [3T*] = [3R*]0 [3T*] = [3T*] {1  exp(kobst)} Initial Conditions [3R*]0 = 1 mM [1T]0 = 1 mM ket = 1 × 109 M-1 s-1

25. Kinetic Corrections (1) Need to account for unimolecular decay of the triplet donor: 3D* 1D kD 3D* + 1A 1D + 3A* ket The probability of transfer (Ptr) is no longer one, but Ptr = ket[1A] / (ket[1A] + kD) eA* = eD* ( DODA / DODD ) / Ptr

26. 3D* + 1A  1D + 3A* [3D*] = [3D*]0 exp(kobst) kobs = kD + ket [1A]0 [3A*] = [3A*] {1  exp(kobst)} [3A*] = [3R*]0 Ptr Unimolecular 3D* decay kD = 0.5 × 106 s-1 Otherwise same initial conditions as before ket = 1 × 109 M-1 s-1 [1A]0 = 1 mM

27. Kinetic Corrections (2) May need to account for the unimolecular decay 3A* 1A kA if the rise time of 3A* is masked by its decay. Then the growth-and decay scheme can be solved as [3A*] =W {exp(-kAt) - exp(-ket[1A]t-kDt)} W =[3D*]0ket[1A] / (kD + ket[1A] - kA) the maximum of this concentration profile is at tmax tmax = ln{kA/(ket[1A] + kD)} / (kA - ket[1A] - kD ) DODA = DODA(tmax) exp(kAtmax)

28. Kinetics involving decay of both triplets Unimolecular 3D* decay 3D*  1D kD = 0.5 × 106 s-1 Unimolecular 3A* decay 3A*  1A kA = 0.5 × 106 s-1 Energy Transfer 3D* + 1A  1D + 3A* ket = 1 × 109 M-1 s-1 [1A]0 = 1 mM

29. Uncertainty in Probability of Transfer If there is a dark reaction for bimolecular deactivation of 3D* + 1A 1D + 1A, kDA then the true probability of transfer is Ptr = ket[1A] / (kDA[1A] + ket[1A] + kD)

30. Energy TransferAdvantages and Disadvantages • The big advantage is over the next method which depends on whether the triplet-triplet absorption overlaps the ground state absorption. • The big disadvantage is the uncertainty in the probability of transfer.