Transcript Document

"Molecular Photochemistry - how to
study mechanisms of photochemical
reactions ?"
Bronislaw Marciniak
Faculty of Chemistry, Adam Mickiewicz University,
Poznan, Poland
2012/2013 - 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 – 10–12s).
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.
3. Laser flash photolysis in the study of photochemical
reaction mechanisms (10–3 – 10–12s).
ns laser flash photolysis
Z
K
Laser
M
P
st art
R
C
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
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
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 R955 PMT + SRS PS-310
power supply
• LeCroy WR 6100A DSO
• PC (GPIB, NI-DAQ, LabView)
• opto-mechanics Standa
Instrumentation
HS + HG
OD  log
I ref (t )
I signal (t )
Femtosecond transient absorption spectrometer
Pump-Probe Femtosecond
Laser at Notre Dame
University
NDRL 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
Inter fejs
Opóźnienie
zmienne
Wiązka analizująca
1.06 m
Światło "białe"
Fotopowielacz
M onochr oma tor
D2 0
L iniowy
mikropozyc joner
532 nm
lub 355 nm
lub 266 nm
Opóźnienie
stałe
G ener ator
har mon icz nych
Pikose kundowy
Lase r YAG: Nd
ze wzm ac niaczem
Wią zka wzbudzają ca
Rys. 6 Schemat pikosekundowego układu do badania absorbcji przejściowej
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 (IRF 200 ps) or MCP
(IRF 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 Reference 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
• Relative Actinometry
Energy Transfer (General)
• Two compounds placed in a cell.
• Compound R 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
eT* = eR* ( ODT / ODR )
Note it doesn’t matter whether T or R is the triplet energy donor.
3R*
[3R*]
=
+ 1T  1R + 3T*
[3R*]0
exp(-kobs t)
[3T*] = [3T*] {1 - exp(-kobs t)}
kobs = ket [1T]0
[3T*] = [3R*]0
Initial Conditions
1.0
0.8
3
[ R*]
3
[ T*]
0.6
[c] (M)
0.4
[3R*]0 = 1 M
[1T]0 = 1 mM
ket = 1 × 109 M-1 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)
Kinetic Corrections
(1) Need to account for unimolecular decay of the triplet donor:
3D*
3D*
 1D
+ 1A  1D + 3A*
kD
ket
The probability of transfer (Ptr) is no longer one, but
Ptr = ket[1A] / (ket[1A] + kD)
eA* = eD* ( ODA / ODD ) / Ptr
3D*
+ 1A  1D + 3A*
[3D*] = [3D*]0 exp(-kobs t)
[3A*] = [3A*] {1 - exp(-kobs t)}
kobs = kD + ket [1A]0
[3A*] = [3R*]0 Ptr
Unimolecular 3D* decay
1.0
kD = 0.5 × 106 s-1
0.8
0.6
[c] (M)
3
[ R*]
3
[ T*]
3
[ D*]
3
[ A*]
0.4
0.2
0.0
Otherwise same initial
conditions as before
ket = 1 × 109 M-1 s-1
[1A]0 = 1 mM
-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
(2) May need to account for the unimolecular decay
3A*
 1A
kA
if the rise time of 3A* is masked by its decay. Then
the growth-and decay scheme can be solved as
[3A*] =W {exp(-kAt) - exp(-ket[1A]t-kDt)}
W =[3D*]0 ket[1A] / (kD + ket[1A] - kA)
the maximum of this concentration profile is at tmax
tmax = ln{kA/(ket[1A] + kD)} / (kA - ket[1A] - kD )
ODA = ODA(tmax) exp(kAtmax)
Kinetics involving decay of both triplets
Unimolecular 3D* decay
3D*
1.0
3
[ A*] infinite A
 1D
kD = 0.5 × 106 s-1
3
0.8
[ D*]
3
[ A*]
Unimolecular 3A* decay
3A*
0.6
[c] (M)
 1A
kA = 0.5 × 106 s-1
0.4
Energy Transfer
0.2
3D*
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)
+ 1A  1D + 3A*
ket = 1 × 109 M-1 s-1
[1A]0 = 1 mM
Uncertainty in Probability of Transfer
If there is a dark reaction for bimolecular deactivation of
3D*
+ 1A  1D + 1A,
kDA
then the true probability of transfer is
Ptr = ket[1A] / (kDA[1A] + ket[1A] + kD)
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.