Transcript Molecular Photochemistry - how to study mechanisms of
"Molecular Photochemistry - how to study mechanisms of photochemical reactions ?"
Bronislaw Marciniak
Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland 2012/2013 - lecture 3
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.
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
2. Quantitative methods
- quantum yields , - rate constants, - lifetimes, - kinetic of quenching, - experimental problems, e.g. inner filter effects
Definition of terms used in photochemistry
Quantum yields
For a photochemical reaction A h B differential quantum yield: x d [ x ] dt I a A dt I a B dt I a
Kinetic scheme
rate A(S 0 ) A(S 1 ) A(S 1 ) A(S 1 ) A(S A(S A(S 1 0 0 ) ) + h f ) + heat A(T 1 ) A(S 1 ) B + C A(S 1 ) + Q A(T 1 ) quenching A(S 0 ) + h p A(T 1 ) A(T 1 ) A(S 0 ) + heat B' + C' A(T 1 ) + Q quenching I a (einstein dm -3 s -1) k f [A(S 1 )] k IC [A(S 1 )] k ISC [A(S 1 )] k r [A(S 1 )] k q [A(S 1 )] [Q] k p [A(T 1 )] k' ISC [A(T 1 )] k' r [A(T 1 )] k' q [A(T 1 )] [Q]
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 f = k f [ A(S 1 )] / I a = k f S IC = k IC S ISC = k ISC S For photochemical reaction from S 1 : R A = k r = B [ A(S 1 )] / I a = 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
Quantum yield measurement
Chemical actinometry:
Uranyl Oxalate Actinometry H 2 C 2 O 4 UO 2 +2 R = 0.602 (for 254 nm) R = 0.561 (for 313 nm) H 2 O + CO 2 + CO 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) acetone = 0.22 (for 313 nm)
Typical dependence of quantum yield
vs
I a t A B ) Ia t b a
Quantum yield of intermediates
Laser flash photolysis: I
=
st
A
p st
/
A
st p 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) 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 ) A(S 1 ) A(S 1 ) A(S A(S A(S 1 0 0 ) ) + h f ) + heat A(T 1 ) A(S 1 ) B + C A(S 1 ) + Q A(T 1 ) quenching A(S 0 ) + h p A(T 1 ) A(T 1 ) A(S 0 ) + heat B' + C' A(T 1 ) + Q quenching rate I a (einstein dm -3 s -1) k f [A(S 1 )] k IC [A(S 1 )] k ISC [A(S 1 )] k r [A(S 1 )] k q [A(S 1 )] [Q] k p [A(T 1 )] k' ISC [A(T 1 )] k' r [A(T 1 )] k' q [A(T 1 )] [Q]
Stern-Volmer equation
0
f
f
1
k q
0 S [Q] 0
R
R
1
k q
0 S [Q] 0 S S 1
k q
0 S [Q] for S 1 1 S 1 S 0
k q
[Q]
k obs
k
0 +
k q
[Q] 0 S
k f
+
k ISC
1 +
k IC
+
k r
S
k f
1 +
k ISC
+
k IC
+
k r
+
k q
[Q]
Stern-Volmer equation
0
p
p
1
k ' q
0 T [Q] ' 0
R
'
R
1
k q '
0 T [Q] 0 T T 1
k ' q
0 T [Q] for T 1 1 T 1 T 0
k q '
[Q] k obs k 0 + k ' q [Q] 0 T 1
k p
+
k ' ISC
+
k r '
T 1
k p
+
' k ISC
+
k r '
+
k q '
[Q]
Quenching of 3 CB* by Met-Gly in aqueous solutions at pH = 6.8
k obs
1 T 0
k q '
[Q] 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 Thiaproline Methionine Alanine
S
-(Carboxymethyl)cysteine Met-Gly L-Met-L-Met Gly-Gly-Met Met-Enkephalin 2.1
2.5
0.0005
0.81
2.1
2.9
1.8
1.9
Rate constants of the order of 10 9 M 1 s 1 indicative of electron transfer 2.6
2.3
0.18
0.75
2.3
1.8
1.9
1.8
Methionine
+ H 3 N C COO C 2 ) 2 S CH 3 pK a = 9.06
H 2 N C COO ( C H 2 ) 2 S CH 3
Traditional Scheme
3 CB* + >S CB + >S [ CB - >S ]
k
esc
k
bt
k
CH CB + >S CBH + or CH 2 S CH 2 CH 3 S C H -
Definition of terms used in photochemistry 2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry 79 , 293–465
Inner-filter effects
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
).
I a [einstein dm 3 s 1 ] I A A a I 0 1 10 ε A c A l I A + Q A(Q) a ε ε A c A A c A ε Q c Q I A(Q) a I Q(A) a ε A c A ε Q c Q I 0 1 10 ( ε A c A ε Q c Q ) l I A(Q) a I A a ε A c ε A A c A ε Q c Q I 0 ( 1 10 ( ε A c A ε Q c Q ) l ) I 0 ( 1 10 ε A c A l )
Corrections for inner filter effect (1) (for the absoprtion of incident light by Q) I A, a corr I A(Q), a obs ε A c A ε ε A c A Q c Q 1 10 ε 1 10 ( ε A c A A c A l ε Q c Q ) l I f corr I f obs ε A c A ε ε A c A Q c Q 1 1 10 ε 10 ( ε A c A A c A l ε Q c Q ) l Corrections for inner filter effect (2) (for reabsorption of fluorescence of A by Q) I f corr I f obs T Q 10 I f obs ε Q [Q] l'
Changes of fluorescence spectra of benzene with various Cu(acac) 2 concentrations
Changes of fluorescence spectra of benzene with various Cu(acac) 2 concentrations
without correction with correction Stern-Volmer plot for the quenching of benzene fluorescence by Cu(acac) 2
Experimental setups for measuring fluorescence spectra
3 2 1
slope = k q
S = (1060 +- 20) M
-
1 taking
S = 29.5 ns k q = 3.6 x 10 10 M
-
1 s
-
1
0 0.0
0.5
1.0
1.5
[Cu(acac) 2 ], mM
2.0
2.5
Stern Volmer plot for quenching of benzene fluorescence by Cu(acac) 2 - front-face technique ( ex =250 nm, f =278 nm)