"Molecular Photochemistry - how to study mechanisms of photochemical reactions?" Bronislaw Marciniak Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2014/2015 - lecture 4 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 Z Laser K M P start C
0.04 1 s 0.04 Absorbance 0.02 Absorbance 0.02 0.00 0.0 2.0x10-7 4.0x10-7 6.0x10-7 time [s] 12 s 45 s 110 s 0.00 150 s 400 600 800 wavelength [nm] 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. Absorbance 0.06 0.04 0.02 Absorbance Absorbance 0.04 0.02 0.00 0.04 0.02 0.00 710 nm 520 nm 0 200 400 600 800 Time [ns] 710 nm 0 50 100 150 Time [ s] after 1 s after 150 s 0.00 400 500 600 700 800 Wavelength [nm] Fig. Transient absorption spectra following triplet quenching of BP (2 mm) by C 6 H 5 -S-CH 2 -COO - N + 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
N N O O O O C CH C 2 CH S 2 CH 2 C O S C O S N C O CO 2 BP PTAAS C OH N (Hofmann elimination) C O + H + N C OH HS + HG
HS + HG Nanosecond flash photolysis Spectra Physics INDI, 266, 355, 532 nm, 10 Hz, 6-8 ns, 450 mj @ 1064 nm Si photodiode, 2 ns rise-time flow 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) Acton Spectra Pro SP-2155 monochromator with dual grating turret Hamamatsu 955 PMT + SS PS-310 power supply LeCroy W 6100A DSO PC (GPIB, NI-DAQ, LabView) opto-mechanics Standa Instrumentation HS + HG OD I log I ref signal ( t) ( t)
Femtosecond transient absorption spectrometer Pump-Probe Femtosecond Laser at Notre Dame University NDL femto lab
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 AMU Physics Department Picosecond Transient Absorption Interfejs IBM PC Interfejs Światło "białe" Opóźnienie zmienne 1.06 m Wiązka analizująca Fotopowielacz Monochromator D 2 0 G enerator harmon icz nych Pikosekundowy Laser YAG: Nd ze wzmacniaczem Liniowy mikropozycjoner 532 nm lub 355 nm lub 266 nm Opóźnienie stałe Wiązka wzbudzająca
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 Picosecond emission spectrometer (TCSPC): excitation: tunable Ti:Sapphire laser (720-1000 nm) pumped by Argon-Ion laser detection: PMT (IF 200 ps) or MCP (IF 25 ps) operated in TCSPC mode SHG (360-500 nm) THG (240-330 nm) FWHM 1.5 ps 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 eference Data 15, 1-150 (1986) http://www.rcdc.nd.edu/compilations/tta/tta.pdf
Methods of Determining Triplet Absorption Coefficients Energy Transfer Method Singlet Depletion Method Total Depletion Method elative Actinometry Energy Transfer (General) Two compounds placed in a cell. Compound 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.
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 T * = * ( OD T / OD ) Note it doesn t matter whether T or is the triplet energy donor. 3 * + 1 T 1 + 3 T* [3 *] = [ 3 *] 0 exp( k obs t) [ 3 T*] = [ 3 T*] {1 exp( k obs t)} k obs = k et [ 1 T] 0 [ 3 T*] = [ 3 *] 0 1.0 Initial Conditions 0.8 0.6 [c] ( M) 0.4 0.2 [ 3 *] [ 3 T*] [ 3 *] 0 = 1 M [ 1 T] 0 = 1 mm k et = 1 10 9 M -1 s -1 0.0-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Time ( s)
Kinetic Corrections (1) Need to account for unimolecular decay of the triplet donor: 3 D* 1 D k D 3 D* + 1 A 1 D + 3 A* k et The probability of transfer (P tr ) is no longer one, but P tr = k et [ 1 A] / (k et [ 1 A] + k D ) A * = D * ( OD A / OD D ) / P tr 3 D* + 1 A 1 D + 3 A* [ 3 D*] = [ 3 D*] 0 exp( k obs t) [ 3 A*] = [ 3 A*] {1 exp( k obs t)} k obs = k D + k et [ 1 A] 0 [ 3 A*] = [ 3 *] 0 P tr 1.0 0.8 Unimolecular 3 D* decay k D = 0.5 10 6 s -1 0.6 [c] ( M) 0.4 0.2 0.0-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Time ( s) [ 3 *] [ 3 T*] [ 3 D*] [ 3 A*] Otherwise same initial conditions as before k et = 1 10 9 M -1 s -1 [ 1 A] 0 = 1 mm
Kinetic Corrections (2) May need to account for the unimolecular decay 3 A* 1 A k 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 1.0 0.8 0.6 [c] ( M) 0.4 [ 3 A*] infinite A [ 3 D*] [ 3 A*] Unimolecular 3 D* decay 3 D* 1 D k D = 0.5 10 6 s -1 Unimolecular 3 A* decay 3 A* 1 A k A = 0.5 10 6 s -1 0.2 0.0-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Time ( s) Energy Transfer 3 D* + 1 A 1 D + 3 A* k et = 1 10 9 M -1 s -1 [ 1 A] 0 = 1 mm
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 disadvantage is the uncertainty in the probability of transfer.