Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/ lecture 2 "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.

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

Stableproducts Scheme of photochemical reaction - analysis of stable products - identification of short-lived reactive intermediates - identification of the excited states responsible for photochemical reactions AA* I B + C hIntermediates

Norrish type II Photoreaction

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. Analysis of stable products Note: Separated products can be used as reference samples in the quantitative analysis AA* I B + C h

Analysis of stable products  example h Norrish type II photoreaction of valerophenone (0.1 mol/dm3) in methanol irr > 300 nm irr > 300 nm C 6 H 5 COCH 2 CH 2 CH 2 CH 3 C 6 H 5 COCH 3 + CH 2 =CHCH 3 + cyclobutanol derivative

Internal standard Retention time [min] Detector's signal

Norrish type II Photoreaction

Photochemistry of Valerophenone in methanol  GC MS results acetophenone C 6 H 5 C(O)CH 3 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): 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

1- excitation source, 2- diaphragm, 3- thermal filter (cell with H 2 O), 4- lens, 5- light filter, 6- merry-goround system

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 AA* I B + C h

ns laser flash photolysis

Benzophenone  (Phenylthio)acetic Tetrabutylammonium Salt Sovent: CH 3 CN

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

Reaction scheme

System studied Sensitizers Benzophenone (BP) 4-Carboxybenzophenone (CB)

Sulfur-Containing Organic Compounds (Quenchers):

methionine-containing di-, tripeptides and polypeptides Met-Gly, Gly-Met Met-Met, Met-Met-Met Met-Met-AlaMet-Gly-MetMet-Enkephalin e.g. methionine derivatives

Motivations Oxidative stress Oxidative stress – Alzheimer’s disease – Biological aging Basic issues Basic issues – Neighboring-group effects – Details of oxidative scheme

Our Traditional Scheme

 (M  1 cm  1 ) Reference Spectra of CB

Intermediates

Fig. Transient absorption spectra following laser flash photolysis recorded at four different delay times. Benzophenone ([CB = 2 mM) and (phenylthio)acetic acid ([C 6 H 5 -S-CH 2 -COOH] = 20 mM) in Ar-saturated aqueous solutions pH = 7.5. Inset: kinetic trace at = 660 nm CB + C 6 H 5 -S-CH 2 -COOH in aqueous solution

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 AA* I B + C h

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: O 2, alkenes, RNO, I 2 Identification of short-lived reactive intermediates 2. Chemical methods - chemical trapping AA* R  R Z h+Z stable product

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  ): 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-2- methylpropane (2 mM), and benzophenone (5 mM), hyperfine splitting constants: a N = mT, a H = mT a N = mT, a H = mT and g = B [mT]  

RZ product analysis: GCMS and NMR, IR 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: 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. Direct capture of free radicals. Repair of damage caused by radicals. Repair of damage caused by radicals. This second mechanism is important for the repair of damage by free radicals in biological systems. This second mechanism is important for the repair of damage by free radicals in biological systems.

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 AA* I B + C h

3. Kinetic methods Example (N.J. Turro, Modern Molecular Photochemistry, p. 261, „Involvement of T 1 (n,  *) of benzophenone as the chemically reactive agent in the photoreduction of benzophenone by benzydrol” „Involvement of T 1 (n,  *) of benzophenone as the chemically reactive agent in the photoreduction of benzophenone by benzydrol” B  B*  I a B*  Bk d [B*] B* + BH 2  BHk r [B*][BH 2 ] 2BH  BH-BHk p [BH] 2 B* + Q  B + Q*k q [B*][Q]

Conclusion: T 1 (n,  *) of benzophenone is the reactive state. 3. Kinetic methods Example (N.J. Turro, Modern Molecular Photochemistry, p. 261, „Involvement of T 1 (n,  *) of benzophenone as the chemically reactive agent in the photoreduction of benzophenone by benzydrol” „Involvement of T 1 (n,  *) of benzophenone as the chemically reactive agent in the photoreduction of benzophenone by benzydrol” Experiments: k d / k r = 0.05 M k q / k r = 500 Taking k q = 1x10 9 M -1 s -1 k d  10 5 s -1   10  s

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. AA* I B + C h

Determination of the reactive state in a photoreaction: 1. Direct methods ( A, F, P, EPR) 2. Indirect methods (sensitization and quenching) AA* I B + C h

If the photoreaction is wavelenght- independent, the involvement of upper excited states can be neglected. Question: S 1 or/and T 1

1. Only S 1 quenched, reaction inhibited None 2. Only T 1 quenched, reaction inhibited T 1 3. Only T 1 quenched, reaction uninhibited S 1 4. Only T 1 sensitized, reaction does not occur S 1 5. Only T 1 sensitized, reaction occurs T 1 or S 1 + T 1 Experiment (result)Reactive state (conclusion) (conclusion)

Experimental Methods for Detection of Intermediates and Excited States [Turro] Reactive DirectIndirect methods intermediatemethods S 1 F, ACIDNP, KINETICS, PRODUCTS T 1 P, A, EPRCIDNP, KINETICS, PRODUCTS R 3 C + A, F, PMI, CHEM, PRODUCTS R 3 C  A, F, PMI, CHEM, PRODUCTS R 3 C A, F, EPRMI, CHEM, PRODUCTS BiradicalA, F, P, EPRCIDNP, MI, CHEM, PRODUCTS

Stableproducts Scheme of photochemical reaction - analysis of stable products - identification of short-lived reactive intermediates - identification of the excited states responsible for photochemical reactions AA* I B + C hIntermediates

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. AA* I B + C h

where: R = CH 3, (CH 2 ) 2 CH 3, (CH 2 ) 3 CH 3, (CH 2 ) 2 CH(CH 3 ) 2, (or Ph) Example: Photochemistry of Phenyl Alkyl Ketones in the Presence of PPh 3

Norrish type II photoreaction

K  1 K  3 K 3 K  K +  + H -abstraction 3 K  3 B 3 K + PPh 3  [K-PPh 3 ] [K-PPh 3 ]  PhCH(OCH 3 )R + Ph 3 PO [K-PPh 3 ]  K + PPh 3 3 B  K 3 B  AP + olefina 3 B  CB 3 B + PPh 3  K + PPh 3 h  ISC = 1.0 k 2 CH 3 OH k3k3k3k3 k4k4k4k4 k 5 CH 3 OH k6k6k6k6 k7k7k7k7 k8k8k8k8 k9k9k9k9 k 10 Kinetic scheme

 T = 1 / (k 2 +k 3 )  B = 1 / (k 7 +k 8 +k 9 )  = k 5 / (k 5 +k 6 ) =  e max  0,   quantum yields of acetopnenone (AP) in the absence and presence of PPh 3 (  AP,  CB,  K )  e  quantum yield of ether (PhCH(OCH 3 )R (or Ph 3 PO)

Internal standard Retention time [min] Detector's signal

Stern-Volmer plot for the valerophenone photolysis in the presence of PPh 3

Reciprocal of  e vs reciprocal of [PPh 3 ]

Table Summary of kinetic data k 4  T + k 10  B k 4  T  T  B k 4  10  9 k 10  10  8 [M  1 ] [M  1 ][ns][ns] [M  1 s  1 ][M  1 s  1 ] PhCOCH 2 CH 2 CH 3 87   < 2 PhCOCH 2 CH 2 CH 2 CH 3 34  330  < 1 PhCOCH 2 CH 2 CH 2 (CH 3 )   < 0.3 PhCOCH 3 91   1.0  Keton

Conclusions Application of Stern-Volmer relation for  AP,  CB, and  K leads to the same values of rate constants Application of Stern-Volmer relation for  AP,  CB, and  K leads to the same values of rate constants k 4 = 2  10 9 M  1 s  1 k 4 = 2  10 9 M  1 s  1 (for all aromatic ketones used) k 10 << k 4 (reaction of biradical with PPh 3 can be neglected) k 10 << k 4 (reaction of biradical with PPh 3 can be neglected)  e max =  = = 0.08  e max =  = = %  chemical quenching (reaction) 8 %  chemical quenching (reaction) 92 %  physical quenching k 5 k 5 +k 6

Norrish type II photoreaction