Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/ lecture 4 "Molecular Photochemistry - how to study mechanisms of photochemical reactions ?" Bronislaw Marciniak Bronislaw Marciniak

Contents 1.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). 2.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). 3.Quantitative methods (quantum yields, rate constants, lifetimes, kinetic of quenching, experimental problems, e.g. inner filter effects).

Contents cont. 4. Laser flash photolysis in the study of photochemical reaction mechanisms (10 –3 – 10 –12 s). 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.

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

ns laser flash photolysis

Fig. Transient absorption spectra of intermediates following the quenching of benzophenone triplet by Ph-S-CH 2 -COO-N + (C 4 H 9 ) 4 (0.01M). Inset: kinetic trace at 710 nm.

Fig. Transient absorption spectra following triplet quenching of BP (2 mM) by C 6 H 5 -S-CH 2 -COO - N + R 4 (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

HS + HG

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

HS + HG Instrumentation

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

NDRL femto lab

time resolution < 100 fstime resolution < 100 fs sensitivity better than OD=0.005sensitivity better than OD=0.005 excitation: tunable Ti:Sapphire laser ( nm at fundamental)excitation: tunable Ti:Sapphire laser ( nm at fundamental) detection: time-gated CCD cameradetection: time-gated CCD camera SHG ( nm)SHG ( nm) THG ( nm)THG ( nm) Femtosecond transient absorption spectrometer: AMU Center for Ultrafast Laser Spectroscopy

AMU Physics Department Picosecond Transient Absorption

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

excitation: tunable Ti:Sapphire laser ( nm) pumped by Argon-Ion laserexcitation: tunable Ti:Sapphire laser ( nm) pumped by Argon-Ion laser detection: PMT (IRF 200 ps) or MCP (IRF 25 ps) operated in TCSPC modedetection: PMT (IRF 200 ps) or MCP (IRF 25 ps) operated in TCSPC mode SHG ( nm) SHG ( nm) THG ( nm)THG ( nm) FWHM 1.5 psFWHM 1.5 ps Picosecond emission spectrometer (TCSPC): AMU Center for Ultrafast Laser Spectroscopy

Long Lifetime Sample

Triplet-Triplet Absorption Spectra of Organic Molecules in Condensed Phases Ian Carmichael and Gordon L. Hug Journal of Physical and Chemical Reference Data 15, (1986)

Methods of Determining Triplet Absorption Coefficients Energy Transfer Method Energy Transfer Method Singlet Depletion Method Singlet Depletion Method Total Depletion Method Total Depletion Method Relative Actinometry Relative Actinometry

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

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

3 R* + 1 T  1 R + 3 T* k et = 1 × 10 9 M -1 s -1 [ 3 R*] 0 = 1 M [ 1 T] 0 = 1 mM k obs = k et [ 1 T] 0 [ 3 R*] = [ 3 R*] 0 exp(k obs t) [ 3 T*] = [ 3 T*]  {1  exp(k obs t)} Initial Conditions [ 3 T*]  = [ 3 R*] 0

Kinetic Corrections (1) Need to account for unimolecular decay of the triplet donor: 3 D*  1 Dk D 3 D* + 1 A  1 D + 3 A*k et P tr = k et [ 1 A] / (k et [ 1 A] + k D ) The probability of transfer (P tr ) is no longer one, but  A * =  D * ( OD A / OD D ) / P tr

3 D* + 1 A  1 D + 3 A* k obs = k D + k et [ 1 A] 0 [ 3 D*] = [ 3 D*] 0 exp(k obs t) [ 3 A*] = [ 3 A*]  {1  exp(k obs t)} [ 3 A*]  = [ 3 R*] 0 P tr k D = 0.5 × 10 6 s -1 k et = 1 × 10 9 M -1 s -1 [ 1 A] 0 = 1 mM Unimolecular 3 D* decay Otherwise same initial conditions as before

Kinetic Corrections (2) May need to account for the unimolecular decay 3 A*  1 Ak A if the rise time of 3 A* is masked by its decay. Then the growth-and decay scheme can be solved as [ 3 A*] =W {exp(-k A t) - exp(-k et [ 1 A]t-k D t)} W =[ 3 D*] 0 k et [ 1 A] / (k D + k et [ 1 A] - k A ) the maximum of this concentration profile is at t max t max = ln{k A /(k et [ 1 A] + k D )} / (k A - k et [ 1 A] - k D ) OD A = OD A (t max ) exp(k A t max )

Kinetics involving decay of both triplets k D = 0.5 × 10 6 s -1 k et = 1 × 10 9 M -1 s -1 [ 1 A] 0 = 1 mM Unimolecular 3 D* decay k A = 0.5 × 10 6 s -1 Unimolecular 3 A* decay 3 D* + 1 A  1 D + 3 A* 3 D*  1 D 3 A*  1 A Energy Transfer

Uncertainty in Probability of Transfer If there is a dark reaction for bimolecular deactivation of 3 D* + 1 A  1 D + 1 A,k DA then the true probability of transfer is P tr = k et [ 1 A] / (k DA [ 1 A] + k et [ 1 A] + k D )

Energy Transfer Advantages 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 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. The big disadvantage is the uncertainty in the probability of transfer.