Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/ lecture 3 "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.

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

2. Quantitative methods - quantum yields, - rate constants, - lifetimes, -kinetic of quenching, - experimental problems, e.g. inner filter effects

differential quantum yield: Definition of terms used in photochemistry Quantum yields  For a photochemical reaction A  B h

A(S 0 )  A(S 1 ) I a (einstein dm -3 s -1) A(S 1 )  A(S 0 ) + h f k f [A(S 1 )] A(S 1 )  A(S 0 ) + heatk IC [A(S 1 )] A(S 1 )  A(T 1 ) k ISC [A(S 1 )] A(S 1 )  B + C k r [A(S 1 )] A(S 1 ) + Q  quenching k q [A(S 1 )] [Q] A(T 1 )  A(S 0 ) + h p k p [A(T 1 )] A(T 1 )  A(S 0 ) + heatk' ISC [A(T 1 )] A(T 1 )  B' + C' k' r [A(T 1 )] A(T 1 ) + Q  quenching k' q [A(T 1 )] [Q] rate h Kinetic scheme

Steady-state approximation : I a = (k f + k IC + k ISC + k r + k q [Q]) [ A(S 1 )] = [A(S 1 )]/  S Fluorescence quantum yield:  f = k f [ A(S 1 )] / I a  f = k f  S  IC = k IC  S  ISC = k ISC  S For photochemical reaction from S 1 :  R = k r [ A(S 1 )] / I a  A =  B = k r  S

Phosphorescence quantum yield:  p = k p [ A(T 1 )] / I a  p =  ISC k p  T For photochemical reaction from T 1 :  ' R = k' r [ A(T 1 )] / I a  ' A =  ' B =  ISC k' r  T

 acetone = 0.22 (for 313 nm) Quantum yield measurement  Uranyl Oxalate Actinometry Chemical actinometry: hv H 2 C 2 O 4  H 2 O + CO 2 + CO UO 2 +2  R = (for 254 nm)  R = (for 313 nm)  Benzophenone-Benzhydrol Actinometry (C 6 H 5 ) 2 CO + (C 6 H 5 ) 2 CHOH  (C 6 H 5 ) 2 C(OH) C(OH) (C 6 H 5 ) 2  R = 0.68 (for 0.1M BP and 0.1M benzhydrol in benzene)  2-Hexanone Actinometry (Norrish Type II)

Typical dependence of quantum yield vs I a t

Quantum yield of intermediates  A p and  A st transient absorbances for intermediate and actinometer  p and  st molar absorption coefficents of intermediate and actinometer  st quantum yield of actinometer (using benzophenone equal to  ISC = 1) Laser flash photolysis:  I =  st  A p  st /  A st  p A( ex ) for irradiated solution = A( ex ) for actinometer

Rate constants k r =  R /  S from S 1 k' r =  ' R / (  ISC  T ) from T 1  S and  T from direct measurement (laser flash photolysis)

Kinetic of quenching A(S 0 )  A(S 1 ) I a (einstein dm -3 s -1) A(S 1 )  A(S 0 ) + h f k f [A(S 1 )] A(S 1 )  A(S 0 ) + heatk IC [A(S 1 )] A(S 1 )  A(T 1 ) k ISC [A(S 1 )] A(S 1 )  B + C k r [A(S 1 )] A(S 1 ) + Q  quenching k q [A(S 1 )] [Q] A(T 1 )  A(S 0 ) + h p k p [A(T 1 )] A(T 1 )  A(S 0 ) + heatk' ISC [A(T 1 )] A(T 1 )  B' + C' k' r [A(T 1 )] A(T 1 ) + Q  quenching k' q [A(T 1 )] [Q] rate h

for S 1 Stern-Volmer equation

for T 1

Quenching of 3 CB* by Met-Gly in aqueous solutions at pH = 6.8 k q = (2.14  0.08)  10 9 M -1 s -1

Quenching Rate Constants (  10 9 M  1 s  1 ) for quenching of CB triplet state Triplet Quenchers pH neutral Thiaproline Methionine Alanine S-(Carboxymethyl)cysteine Met-Gly L-Met-L-Met Gly-Gly-Met Met-Enkephalin pH basic Rate constants of the order of 10 9 M  1 s  1 indicative of electron transfer indicative of electron transfer

Methionine

Traditional Scheme

Term used in two different ways: (1) During an irradiation experiment, absorption of incident radiation by a species other than the intended primary absorber is also described as an inner-filter effect. Definition of terms used in photochemistry 2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry 79, 293–465 Inner-filter effects

(2) In an emission experiment, it refers to (a) an apparent decrease in emission quantum yield at high concentration of the emitter due to strong absorption of the excitation light (b) an apparent decrease in emission quantum yield and/or distortion of bandshape as a result of reabsorption of emitted radiation (particularly severe for emitters with small Stokes shift). Definition of terms used in photochemistry 2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry 79, 293–465 Inner-filter effects

I a [einstein dm  3 s  1 ] A  h A + Q  h

Corrections for inner filter effect (1) (for the absoprtion of incident light by Q) (for the absoprtion of incident light by Q) Corrections for inner filter effect (2) (for reabsorption of fluorescence of A by Q)

Changes of fluorescence spectra of benzene with various Cu(acac) 2 concentrations

Stern-Volmer plot for the quenching of benzene fluorescence by Cu(acac) 2 without correction with correction

Experimental setups for measuring fluorescence spectra

Stern Volmer plot for quenching of benzene fluorescence by Cu(acac) 2 - front-face technique ( ex =250 nm, f =278 nm)