Transcript Document
"Molecular Photochemistry - how to study mechanisms of photochemical reactions ?" Bronislaw Marciniak Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 2 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 – 10–12s). 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. 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 Jablonski diagram S2 IC S1 T2 IC ISC +Q T1 R A S0 F IC +Q Ph ISC R Scheme of photochemical reaction A h A* I B+C Intermediates Stable products - analysis of stable products - identification of short-lived reactive intermediates - identification of the excited states responsible for photochemical reactions Norrish type II Photoreaction Ph Ph O H Ph O* H h O* H ISC R R' R R' R R' (K) (1K) (3K) (1) Ph CH2=CRR' + PhCOCH3 Ph OH (2) OH R' R R' (D) R (3) PhCOCH2CH2CHRR' Analysis of stable products A h 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 Analysis of stable products - example Norrish type II photoreaction of valerophenone (0.1 mol/dm3) in methanol irr > 300 nm C6H5COCH2CH2CH2CH3 h C6H5COCH3 + CH2=CHCH3 + cyclobutanol derivative 0 5 10 15 czas retencji Retention time[min] [min] (1:20) (1:20) CH2 =CHCH 3 sygnał detektora signal Detector's (1:200) PhCOCH3 (1:200) Internal standard standard wewnętrzny CH 3 OH (1:2000) PhCOCH 2 CH 2 CH2 CH3 Ph Ph OH OH CH 3 CH 3 // 20 Norrish type II Photoreaction Ph Ph O H Ph O* H h O* H ISC R R' R R' R R' (K) (1K) (3K) (1) Ph CH2=CRR' + PhCOCH3 Ph OH (2) OH R' R R' (D) R (3) PhCOCH2CH2CHRR' Photochemistry of Valerophenone in methanol GC MS results acetophenone C6H5C(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). Steady-state irradiation systems 6 7 1 2 3 45 1- excitation source, 2- diaphragm, 3- thermal filter (cell with H2O), 4- lens, 5- light filter, 6- merry-goround system A h 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 ns laser flash photolysis Z K Laser M P st art R C Benzophenone- (Phenylthio)acetic Tetrabutylammonium Salt O BP + S CH2 C O Sovent: CH3CN N 1 s Absorbance 0.04 Absorbance 0.04 0.02 0.00 0.0 -7 2.0x10 -7 4.0x10 -7 6.0x10 12 s time [s] 0.02 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-CH2-COO-N+(C4H9)4 (0.01M). Inset: kinetic trace at 710 nm. Absorbance 0.06 0.04 710 nm 0.02 520 nm 0.00 0.02 after 1 s 200 400 600 800 Time [ns] 0.04 0.04 Absorbance Absorbance 0 0.02 710 nm 0.00 0 50 100 Time [s] 150 after 150 s 0.00 400 500 600 700 800 Wavelength [nm] 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 Reaction scheme R R R R N N R R O C O C R R O O CH2 C O CH2 R CH2 C S S O S R C N R O R BP PTAAS (Hofmann elimination) R C R C OH N R R O + H+ R C OH R N R R CO2 System studied Sensitizers 4-Carboxybenzophenone (CB) Benzophenone (BP) O O COO Sulfur-Containing Organic Compounds (Quenchers): carboxylic acids thioethers R S R alcohols R S (CH2)n OH trithianes H R S S R H S R + amino acids H R S (CH2)n COO- NH3 R S (CH2)n CH COONH2 R S (CH2)n CH COO- Sulfur-Containing Organic Compounds (Quenchers): methionine derivatives methionine-containing di-, tripeptides and polypeptides NH R1 CH3 S (CH2)2 CH C R2 O e.g. Met-Gly, Gly-Met Met-Met, Met-Met-Met Met-Met-Ala Met-Gly-Met Met-Enkephalin Motivations • Oxidative stress – Alzheimer’s disease – Biological aging • Basic issues – Neighboring-group effects – Details of oxidative scheme Our Traditional Scheme 3 CB* + [ CB- >S >S ] kesc kbt CB + kCH CB- + >S CBH + CH2-S-CH2- or CH3-S-C H- >S Reference Spectra of CB 4-Carboxybenzophenone Transients 25000 Triplet Radical anion 20000 Ketyl radical 15000 10000 5000 0 400 500 600 Wavelength (nm) 700 Intermediates 3(CB)* (540 nm) CB (660 nm) CBH (570 nm) S+ S+ S S S O + (480 nm) S S + N S + (380 nm) (410 nm) S + CB + C6H5-S-CH2-COOH in aqueous solution 0.16 = 660 nm 0.06 A 125 ns 1.25 s 12.5 s 50 s 0.03 0.00 0.12 50 100 150 A 0 0.08 0.04 0.00 400 500 600 700 800 Wavelength (nm) Fig. Transient absorption spectra following laser flash photolysis recorded at four different delay times. Benzophenone ([CB = 2 mM) and (phenylthio)acetic acid ([C6H5-S-CH2-COOH] = 20 mM) in Ar-saturated aqueous solutions pH = 7.5. Inset: kinetic trace at = 660 nm A h 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 Identification of short-lived reactive intermediates 2. Chemical methods - chemical trapping A h A* R +Z RZ stable product 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 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 ): (CH3)3C-NO + CH(COCH3)2 (CH3)3C-N-CH(COCH3)2 O 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-2methylpropane (2 mM), and benzophenone (5 mM), hyperfine splitting constants: aN = 1.363 mT, aH = 0.315 mT and g = 2.0062. B [mT] 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: C C + CH(COCH3)2 addition products RZ product analysis: GCMS and NMR, IR Conclusion: acac was proved to be the reactive intermediate in the sensitized photoreduction of Cu(acac)2. 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. A h 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 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” Ph2CO + Ph2CHOH h OH OH Ph-C C-Ph Ph Ph B BH2 B B* B* B B* + BH2 BH 2BH BH-BH B* + Q B + Q* BH-BH aIa kd[B*] kr[B*][BH2] kp[BH]2 kq[B*][Q] 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” 1 1 kd 1 B a ak r [ BH 2 ] k q [Q] 1 1 kd B a ak r [ BH 2 ] ak r [ BH 2 ] Experiments: kd / kr = 0.05 M kq / kr = 500 Taking kq = 1x109 M-1s -1 kd 105 s -1 10 s Conclusion: T1 (n,*) of benzophenone is the reactive state. A h 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. A h 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) If the photoreaction is wavelenght- independent, the involvement of upper excited states can be neglected. Question: S1 or/and T1 triplet sensitization S1 ISC PS h S0 T1 PT triplet quenching Direct PS PT S1 T1 QS Photosensitized QT S0 Experiment (result) Reactive state (conclusion) 1. Only S1 quenched, reaction inhibited None 2. Only T1 quenched, reaction inhibited T1 3. Only T1 quenched, reaction uninhibited S1 4. Only T1 sensitized, reaction does not occur S1 5. Only T1 sensitized, reaction occurs T1 or S1 + T1 Experimental Methods for Detection of Intermediates and Excited States [Turro] Reactive intermediate Direct methods Indirect 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 Scheme of photochemical reaction A h A* I B+C Intermediates Stable products - analysis of stable products - identification of short-lived reactive intermediates - identification of the excited states responsible for photochemical reactions A h 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. Example: Photochemistry of Phenyl Alkyl Ketones in the Presence of PPh3 PhCOR + PPh3 + CH3OH h > 330 nm Ph3PO + PhCHR OCH3 where: R = CH3, (CH2)2CH3, (CH2)3CH3, (CH2)2CH(CH3)2, (or Ph) Norrish type II photoreaction Ph Ph O H Ph O* H h O* H ISC R R' R R' R R' (K) (1K) (3K) (1) Ph CH2=CRR' + PhCOCH3 Ph OH (2) OH R' PPh3 R R' (D) PPh3 R (3) PhCOCH2CH2CHRR' Kinetic scheme K 3K h ISC = 1.0 1K k2 K + + H -abstraction 3K CH3OH k3 3B 3K 4 + PPh3 k k 5 [K-PPh3] CH3OH k6 [K-PPh3] k7 [K-PPh3] PhCH(OCH3)R + Ph3PO K + PPh3 3B 3B 8 AP + olefina 3B 9 3B 3K K k k CB k 10 + PPh3 K + PPh3 0 1 (k4 T k10 B )[ PPh3 ] k4 T k10 B [ PPh3 ]2 0 1 (k4 T k10 B )[ PPh3 ] 1 1 1 e k4 T [ PPh3 ] 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) T = 1 / (k2 +k3) B = 1 / (k7 +k8 +k9) = k5 / (k5 +k6) = emax 0 5 10 15 czas retencji Retention time[min] [min] (1:20) (1:20) CH2 =CHCH 3 sygnał detektora signal Detector's (1:200) PhCOCH3 (1:200) Internal standard standard wewnętrzny CH 3 OH (1:2000) PhCOCH 2 CH 2 CH2 CH3 Ph Ph OH OH CH 3 CH 3 // 20 Stern-Volmer plot for the valerophenone photolysis in the presence of PPh3 2.6 2.4 valerophenone (0.1 M) 2.2 2.0 1.8 0 / 1.6 1.4 1.2 1.0 0.8 0.00 slope = k4T + k10 B = (34+-3) M 0.01 0.02 [PPh3], M 0.03 -1 0.04 Reciprocal of e vs reciprocal of [PPh3] 50 valerophenone (0.1 M) 45 1/e 40 35 30 -1 intercept/slope = k4T = (30+-5) M 25 20 30 40 50 60 70 80 -1 1/[PPh3], M 90 100 110 Table Summary of kinetic data Keton k4T + k10B k4T T B k410-9 k1010-8 [M-1] [M-1] [ns] [ns] PhCOCH2CH2CH3 8712 9930 70 110 1.4 <2 PhCOCH2CH2CH2CH3 343 305 16 102 1.9 <1 9.41.6 103 97 2.1 < 0.3 914 250 - 1.0 - PhCOCH2CH2CH2(CH3)2 PhCOCH3 4.7 200 [M-1s-1] [M-1s-1] Conclusions • Application of Stern-Volmer relation for AP, CB, and K leads to the same values of rate constants • k4 = 2 109 M-1s -1 (for all aromatic ketones used) • k10 << k4 (reaction of biradical with PPh3 can be neglected) • emax = = k5 k5 +k6 = 0.08 8 % - chemical quenching (reaction) 92 % - physical quenching Norrish type II photoreaction Ph Ph O H Ph O* H h O* H ISC R R' R R' R R' (K) (1K) (3K) (1) Ph CH2=CRR' + PhCOCH3 Ph OH (2) OH R' PPh3 R R' (D) PPh3 R (3) PhCOCH2CH2CHRR'