Faculty of Chemistry, Adam Mickiewicz University, Faculty of Chemistry, Adam Mickiewicz University, Poznan, PolandPoznan, Poland

2012/2013 - lecture 42012/2013 - lecture 4

"Molecular Photochemistry - how to "Molecular Photochemistry - how to study mechanisms of photochemical study mechanisms of photochemical

reactionsreactions ? ?""

BronisBronisllaw Marciniakaw Marciniak  

ContentsContents

1.1. Introduction and basic principles Introduction and basic principles (physical and chemical properties of molecules in the excited states, (physical and chemical properties of molecules in the excited states, Jablonski diagram, time scale of physical and chemical events, Jablonski diagram, time scale of physical and chemical events, definition of terms used in photochemistry).definition of terms used in photochemistry).

2.2. Qualitative investigation of photoreaction mechanisms - Qualitative investigation of photoreaction mechanisms - steady-state and time resolved methodssteady-state and time resolved methods(analysis of stable products and short-lived reactive intermediates, (analysis of stable products and short-lived reactive intermediates, identification of the excited states responsible for photochemical identification of the excited states responsible for photochemical reactions).reactions).

3.3. Quantitative methodsQuantitative methods(quantum yields, rate constants, lifetimes, kinetic of quenching, (quantum yields, rate constants, lifetimes, kinetic of quenching, experimental problems, e.g. inner filter effects).experimental problems, e.g. inner filter effects).

Contents cont.Contents cont.

4.   Laser flash photolysis in the study of photochemical 4.   Laser flash photolysis in the study of photochemical reaction mechanisms (10reaction mechanisms (10–3–3 – 10 – 10–12–12s).s).

5.   Examples illustrating the investigation of photoreaction 5.   Examples illustrating the investigation of photoreaction mechanisms:mechanisms:

      sensitized photooxidation of sulfur (II)-containing organic sensitized photooxidation of sulfur (II)-containing organic compounds,compounds,

      photoinduced electron transfer and energy transfer processes, photoinduced electron transfer and energy transfer processes,

      sensitized photoreduction of 1,3-diketonates of Cu(II),sensitized photoreduction of 1,3-diketonates of Cu(II),

      photochemistry of 1,3,5,-trithianes in solution.photochemistry of 1,3,5,-trithianes in solution.

3. 3. Laser flash photolysis in the study of photochemical Laser flash photolysis in the study of photochemical reaction mechanisms (10reaction mechanisms (10–3–3 – 10 – 10–12–12s).s).

start

Z

K

C

R

P

M

Laser

ns laser flash photolysisns laser flash photolysis

FigFig. Transient absorption spectra of intermediates following the. Transient absorption spectra of intermediates following the quenching quenching of benzophenone triplet by Ph-S-CHof benzophenone triplet by Ph-S-CH22-COO-N-COO-N++(C(C44HH99))44 (0.01M). (0.01M).

Inset: kinetic trace at 710 nm.Inset: kinetic trace at 710 nm.

400 600 800

0.00

0.02

0.04

0.0 2.0x10-7

4.0x10-7

6.0x10-7

0.00

0.02

0.04

Abs

orba

nce

time [s]

150 s

110 s

45 s

12 s

1 sA

bsor

banc

e

wavelength [nm]

Fig. Fig. Transient absorption spectra following triplet quenching of BP (2 mM) by Transient absorption spectra following triplet quenching of BP (2 mM) by CC66HH55-S-CH-S-CH

22-COO-COO--NN++RR44 (10 mM) after 1 (10 mM) after 1 s and 150 s and 150 s delays after the flash in s delays after the flash in MeCNMeCN solution. solution. IInsetnsetss: kinetic traces on the nanosecond: kinetic traces on the nanosecond and and microsecond time scalemicrosecond time scaless

C OH N

RR

R

R

C OH

C O

+ H+

RN

R

R

RC O

RN

R

R

R

(Hofmann elimination)

RN

R

R

R

S

CH2

CO O

S

CH2

PTAASBP

RN

R

R

R

C OC O S

CH2

CO O

CO2

HS + HG

HS + HG

• Spectra Physics INDI, 266, 355, 532Spectra Physics INDI, 266, 355, 532 nm, nm, 1010 Hz, 6-8Hz, 6-8 ns, 450 mJ @ 1064ns, 450 mJ @ 1064 nmnm

• Si photodiode, 2Si photodiode, 2 ns rise-timens rise-time

• flow cell + temperature controlled flow cell + temperature controlled holderholder

• fibre coupled 150fibre coupled 150 W Xe lamp (Applied W Xe lamp (Applied Photophysics) with pulser, 500Photophysics) with pulser, 500 s plateu s plateu (or alternatively 175(or alternatively 175 W Xe Cermax CW W Xe Cermax CW lamp)lamp)

• Acton Spectra Pro SP-2155 Acton Spectra Pro SP-2155 monochromator with dual grating turretmonochromator with dual grating turret

• Hamamatsu R955 PMT + SRS PS-310 Hamamatsu R955 PMT + SRS PS-310 power supplypower supply

• LeCroy WR 6100A DSOLeCroy WR 6100A DSO

• PC (GPIB, NI-DAQ, LabView)PC (GPIB, NI-DAQ, LabView)

• opto-mechanics Standaopto-mechanics Standa

Nanosecond Nanosecond flash photolysis flash photolysis

HS + HG

Instrumentation

)(

)(logOD

tI

tI

signal

ref

Femtosecond transient absorption spectrometerFemtosecond transient absorption spectrometer

Pump-Probe Femtosecond Pump-Probe Femtosecond LaserLaser at Notre Dame at Notre Dame UniversityUniversity

NDRL femto labNDRL femto lab

• time resolution < 100 fstime resolution < 100 fs

• sensitivity better than OD=0.005 sensitivity better than OD=0.005

• excitation: tunable Ti:Sapphire laser excitation: tunable Ti:Sapphire laser (750-840 nm at fundamental)(750-840 nm at fundamental)

• detection: time-gated CCD cameradetection: time-gated CCD camera

• SHG (375-420 nm)SHG (375-420 nm)

• THG (250-280 nm)THG (250-280 nm)

Femtosecond transient Femtosecond transient absorption spectrometer: absorption spectrometer:

AMU Center for Ultrafast Laser SpectroscopyAMU Center for Ultrafast Laser Spectroscopy

AMU Physics DepartmentAMU Physics DepartmentPicosecond Transient AbsorptionPicosecond Transient Absorption

Sub-nanosecond emission spectrometer IBH System 5000Sub-nanosecond emission spectrometer IBH System 5000• excitation: nanoLEDs (295, 370, 408, 474 nm)excitation: nanoLEDs (295, 370, 408, 474 nm)• FWHM 200 psFWHM 200 ps• detection: PMT operated in TCSPC modedetection: PMT operated in TCSPC mode• PC based MCA: 6 ps/channel (50 ns time window / 8196 channels)PC based MCA: 6 ps/channel (50 ns time window / 8196 channels)• emission and fluorescence anisotropy measurements emission and fluorescence anisotropy measurements

• excitation: tunable Ti:Sapphire laser excitation: tunable Ti:Sapphire laser (720-1000 nm) pumped by Argon-Ion (720-1000 nm) pumped by Argon-Ion laserlaser

• detection: PMT (IRF 200 ps) or MCP detection: PMT (IRF 200 ps) or MCP (IRF 25 ps) operated in TCSPC mode(IRF 25 ps) operated in TCSPC mode

• SHG (360-500 nm)SHG (360-500 nm)

• THG (240-330 nm)THG (240-330 nm)

• FWHM 1.5 psFWHM 1.5 ps

Picosecond emission Picosecond emission spectrometer spectrometer (TCSPC):(TCSPC):

AMU Center for AMU Center for Ultrafast Laser Ultrafast Laser SpectroscopySpectroscopy

Long Lifetime SampleLong Lifetime Sample

Triplet-Triplet Absorption Spectra of Triplet-Triplet Absorption Spectra of Organic MoleculesOrganic Molecules

in Condensed Phasesin Condensed Phases

Ian Carmichael and Gordon L. HugIan Carmichael and Gordon L. Hug

Journal of Physical and Chemical Reference Data Journal of Physical and Chemical Reference Data 15, 1-150 (1986)15, 1-150 (1986)

http://www.rcdc.nd.edu/compilations/Tta/tta.pdfhttp://www.rcdc.nd.edu/compilations/Tta/tta.pdf

Methods of DeterminingMethods of DeterminingTriplet Triplet AbsorptionAbsorption Coefficients Coefficients

• Energy Transfer MethodEnergy Transfer Method

• Singlet Depletion MethodSinglet Depletion Method

• Total Depletion MethodTotal Depletion Method

• Relative ActinometryRelative Actinometry

Energy Transfer (General)Energy Transfer (General)

• Two compounds placed in a cell.Two compounds placed in a cell.

• Compound R has a known triplet Compound R has a known triplet absorptionabsorption coefficient.coefficient.

• Compound T has a triplet Compound T has a triplet absorptionabsorption coefficient to be coefficient to be determined.determined.

• Ideally, the triplet with the higher energy can be Ideally, the triplet with the higher energy can be populated.populated.

• Thus triplet energy of one can be transferred to the Thus triplet energy of one can be transferred to the other.other.

Energy Transfer (General)Energy Transfer (General)

• If the lifetimes of both triplets are long in the absence If the lifetimes of both triplets are long in the absence of the other molecule, thenof the other molecule, then

• One donor triplet should yield one acceptor triplet.One donor triplet should yield one acceptor triplet.

• In an ideal experimentIn an ideal experiment

TT* = * = RR* ( * ( ODODTT / / ODODRR ) )

Note it doesn’t matter whether T or R is the triplet energy donor.Note it doesn’t matter whether T or R is the triplet energy donor.

33R* + R* + 11T T 11R + R + 33T*T*

kketet = 1 × 10 = 1 × 1099 M M-1-1 s s-1-1

[[33R*]R*]00 = 1 = 1 MM

[[11T]T]00 = 1 mM = 1 mM

kkobsobs = = kketet [ [11T]T]00[[33R*] = [R*] = [33R*]R*]00 exp( exp(kkobsobs tt))

[[33T*] = [T*] = [33T*]T*] {1 {1 exp( exp(kkobsobs tt)})}

Initial ConditionsInitial Conditions

[[33T*]T*] = [ = [33R*]R*]00

Kinetic CorrectionsKinetic Corrections

(1) Need to account for unimolecular decay of the triplet donor:(1) Need to account for unimolecular decay of the triplet donor:

33D* D* 11DD kkDD

33D* + D* + 11A A 11D + D + 33A*A* kketet

PPtrtr = = kketet[[11A] / (A] / (kketet[[11A] + A] + kkDD))

The probability of transfer (PThe probability of transfer (Ptrtr) is no longer one, but) is no longer one, but

AA* = * = DD* ( * ( ODODAA / / ODODDD ) / P ) / Ptrtr

33D* + D* + 11A A 11D + D + 33A*A*

kkobsobs = = kkDD + + kketet [ [11A]A]00[[33D*] = [D*] = [33D*]D*]00 exp( exp(kkobsobs tt))

[[33A*] = [A*] = [33A*]A*] {1 {1 exp( exp(kkobsobs tt)})}[[33A*]A*] = [ = [33R*]R*]00 P Ptrtr

kkDD = 0.5 × 10 = 0.5 × 1066 s s-1-1

kketet = 1 × 10 = 1 × 1099 M M-1-1 s s-1-1

[[11A]A]00 = 1 mM = 1 mM

Unimolecular Unimolecular 33D* decayD* decay

Otherwise same initialOtherwise same initialconditions as beforeconditions as before

Kinetic CorrectionsKinetic Corrections

(2) May need to account for the unimolecular decay(2) May need to account for the unimolecular decay

33A* A* 11AA kkAA

if the rise time of if the rise time of 33A* is masked by its decay. ThenA* is masked by its decay. Thenthe growth-and decay scheme can be solved asthe growth-and decay scheme can be solved as

[[33A*] =W {exp(-A*] =W {exp(-kkAAtt) - exp(-) - exp(-kketet[[11A]A]tt--kkDDtt)})}

W =[W =[33D*]D*]00 kketet[[11A] / (A] / (kkDD + + kketet[[11A] - A] - kkAA))

the maximum of this concentration profile is at the maximum of this concentration profile is at ttmaxmax

ttmaxmax = ln{ = ln{kkAA/(/(kketet[[11A] + A] + kkDD)} / ()} / (kkAA - - kketet[[11A] - A] - kkDD ) )

ODODAA = = ODODAA((ttmaxmax) exp() exp(kkAAttmaxmax))

Kinetics involving decay of both tripletsKinetics involving decay of both triplets

kkDD = 0.5 × 10 = 0.5 × 1066 s s-1-1

kketet = 1 × 10 = 1 × 1099 M M-1-1 s s-1-1

[[11A]A]00 = 1 mM = 1 mM

Unimolecular Unimolecular 33D* decayD* decay

kkAA = 0.5 × 10 = 0.5 × 1066 s s-1-1

Unimolecular Unimolecular 33A* decayA* decay

33D* + D* + 11A A 11D + D + 33A*A*

33D* D* 11DD

33A* A* 11AA

Energy TransferEnergy Transfer

Uncertainty in Probability of TransferUncertainty in Probability of Transfer

If there is a dark reaction for bimolecular deactivation of If there is a dark reaction for bimolecular deactivation of

33D* + D* + 11A A 11D + D + 11A,A, kkDADA

then the true probability of transfer is then the true probability of transfer is

PPtrtr = = kketet[[11A] / (A] / (kkDADA[[11A] + A] + kketet[[11A] + A] + kkDD))

Energy TransferEnergy TransferAdvantages and DisadvantagesAdvantages and Disadvantages

• The big advantage is over the next method which The big advantage is over the next method which depends on whether the triplet-triplet absorption depends on whether the triplet-triplet absorption overlaps the ground state absorption.overlaps the ground state absorption.

• The big disadvantage is the uncertainty in the The big disadvantage is the uncertainty in the probability of transfer.probability of transfer.