triplet extinction coefficients, triplet quantum yields, and (mainly) laser flash photolysis
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Triplet Extinction Coefficients, Triplet Quantum Yields, and (mainly) Laser Flash Photolysis. http://150.254.84.227/HUG. This weekend. http://www.zfch.amu.edu.pl. Competitive Kinetics out of Singlet State. 1 M + h n 1 M*, k ex. 1 M* products, k pc. 1 M* 3 M* + heat, k isc. - PowerPoint PPT PresentationTRANSCRIPT
Triplet Extinction Coefficients,Triplet Extinction Coefficients,Triplet Quantum Yields,Triplet Quantum Yields,
and (mainly)and (mainly)Laser Flash PhotolysisLaser Flash Photolysis
http://150.254.84.227/HUG
http://www.zfch.amu.edu.pl This weekend
Competitive Kinetics out of Singlet StateCompetitive Kinetics out of Singlet State
1M + h 1M*, kex
1M* products, kpc
1M* 3M* + heat, kisc
1M* 1M + heat, kic
1M* 1M + hf, kf
kS = kpc + kisc + kic + kf
T = kisc/kS
Competitive Kinetics out of Triplet StateCompetitive Kinetics out of Triplet State
3M* products, k’pc
3M* 1M + heat, k’isc
3M* 1M + hp, kp
kT = k’pc + k’isc + kp
Competitive KineticsCompetitive Kinetics
p
iscT
S0
Intramolecular decay channels
Intermolecular decay channels
T + Q S0 + Q’
]T][Q[]T[]T[]T[qpisc kkk
dtd
tkkk ]Q[exp]T[]T[ qpisc0
Transient AbsorptionTransient Absorption
S0
S1
S2
T1
Tn
A
TATA
PF
IC
IC
ISC
ISCIC
ISC
3M* + h’ 3M** k’ex
3M**
3M*
Creation of TripletsCreation of Triplets
3M1* + 1M 1M1 + 3M*
(1) Intramolecular radiationless transitions
(2) Intermolecular energy transfer
(3) Transfer from solvent triplets in radiolysis of benzene.
1M + h 1M* kex
1M* 3M* + heat kisc
Beer’s LawBeer’s LawConnection betweenConnection between
light absorption and concentrationlight absorption and concentration
OD = T* [3M*] l
OD = log10 (I0/I)
A = log100
IPMT
II0
time
A
t laser
0
time
Spatial Overlap ofSpatial Overlap ofLaser and Monitoring BeamLaser and Monitoring Beam
Laser
MonitoringLight Beam
Cell
Laser
MonitoringLight Beam
UnexcitedExcited
Proper alignment:Sample is excited along theentire optical pathlength
Improper alignment:Sample is not excited along theentire optical pathlength
Placement ofPlacement ofMonitoring BeamMonitoring Beam
Relative toRelative toIncident LaserIncident Laser
Laser
MonitoringLight Beam
Cell
[3M*] fromBeer’s Law
Cell WallLaserEntering
Cell WallLaserExiting
Extrapolated[3M*] for longcells
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 DeterminingMethods of DeterminingTriplet Extinction CoefficientsTriplet Extinction Coefficients
Energy Transfer MethodEnergy Transfer Method Singlet Depletion MethodSinglet Depletion Method Total Depletion MethodTotal Depletion Method Relative ActinometryRelative Actinometry Intensity Variation MethodIntensity Variation Method Kinetic MethodKinetic Method Partial Saturation MethodPartial Saturation Method
Energy Transfer (General)Energy Transfer (General)
Two compounds placed in a cell.Two compounds placed in a cell. Compound R has a known triplet extinction Compound R has a known triplet extinction
coefficient.coefficient. Compound T has a triplet extinction coefficient to Compound T has a triplet extinction coefficient to
be determined.be 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 Thus triplet energy of one can be transferred to
the other.the other.
Energy Transfer (General)Energy Transfer (General)
If the lifetimes of both triplets are long in the If the lifetimes of both triplets are long in the absence of the other molecule, thenabsence of the other molecule, then
One donor triplet should yield one acceptor One donor triplet should yield one acceptor triplet.triplet.
In an ideal experimentIn an ideal experiment
T* = R* ( ODT / ODR )
Note it doesn’t matter whether T or R is the triplet energy donor.
33R* + R* + 11T T 11R + R + 33T*T*
ket = 1 × 109 M-1 s-1
[3R*]0 = 1 M[1T]0 = 1 mM
kobs = ket [1T]0[3R*] = [3R*]0 exp(kobs t)[3T*] = [3T*] {1 exp(kobs t)}
Initial Conditions
[3T*] = [3R*]0
Also a Also a iscisc Method Methodkobs = ket [1T]0[3R*] = [3R*]0 exp(kobs t)
[3T*] = [3T*] {1 exp(kobs t)} [3T*] = [3R*]0
If T* andabsorption of 3R* ismainly hidden underits ground-stateabsorption,
isc(R) =
[3T*] / # photons into R
= (OD l) / (T* photons)
Kinetic CorrectionsKinetic Corrections
(1) Need to account for unimolecular decay of the triplet donor:
3D* 1D kD
3D* + 1A 1D + 3A* ket
Ptr = ket[1A] / (ket[1A] + kD)
The probability of transfer (Ptr) is no longer one, but
A* = D* ( ODA / ODD ) / Ptr
33D* + D* + 11A A 11D + D + 33A*A*
kobs = kD + ket [1A]0[3D*] = [3D*]0 exp(kobs t)[3A*] = [3A*] {1 exp(kobs t)} [3A*] = [3R*]0 Ptr
kD = 0.5 × 106 s-1
ket = 1 × 109 M-1 s-1
[1A]0 = 1 mM
Unimolecular 3D* decay
Otherwise same initialconditions as before
Kinetic CorrectionsKinetic Corrections
(2) May need to account for the unimolecular decay
3A* 1A kA
if the rise time of 3A* is masked by its decay. Thenthe 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 tripletsKinetics involving decay of both triplets
kD = 0.5 × 106 s-1
ket = 1 × 109 M-1 s-1
[1A]0 = 1 mM
Unimolecular 3D* decay
kA = 0.5 × 106 s-1
Unimolecular 3A* decay
33D* + D* + 11A A 11D + D + 33A*A*
33D* D* 11DD
33A* A* 11AA
Energy Transfer
Uncertainty in Probability of TransferUncertainty 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 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.
Singlet DepletionSinglet Depletion
By Kasha’s Rule, after the excited singlets have By Kasha’s Rule, after the excited singlets have decayed, only the lowest triplet state and the decayed, only the lowest triplet state and the ground state should be present.ground state should be present.
Any ground state molecules that are missing Any ground state molecules that are missing should be in the lowest triplet state.should be in the lowest triplet state.
In other words, the missing concentration of In other words, the missing concentration of ground states should be the same as the triplet ground states should be the same as the triplet concentration.concentration.
At a wavelength where they both absorbAt a wavelength where they both absorbOD = (T* S) [3M*] l
Singlet DepletionSinglet Depletion
Assuming that there is a wavelength region (1) wherethe ground state absorbs and the triplet doesn’t
ODS(1) = S [3M*] l
A = log100
II0
time
A
t laser
0
time
“bleaching”
Step 1
Singlet DepletionSinglet DepletionStep 2
Go to a wavelength region (2) where the ground statedoesn’t absorb
ODT(2) = T* [3M*] l
A = log100
II0
time
A
t laser
0
time
Singlet DepletionSinglet DepletionAdvantages and DisadvantagesAdvantages and Disadvantages
The main problem is the assumption in Step 1: The main problem is the assumption in Step 1: that the chosen wavelength that the chosen wavelength 11 is in a region is in a region where the triplet does not absorb.where the triplet does not absorb.
There are methods for attempting to compensate There are methods for attempting to compensate for this, but they involve further assumptions.for this, but they involve further assumptions.
The main advantage of singlet depletion is that it The main advantage of singlet depletion is that it is free from kinetic considerations.is free from kinetic considerations.
Total Depletion MethodTotal Depletion Method
Assumes that increasing the intensity of the pulse Assumes that increasing the intensity of the pulse complete conversion of a small ground state complete conversion of a small ground state conversion to the triplet state is possible if the conversion to the triplet state is possible if the intersystem crossing is not negligibly small.intersystem crossing is not negligibly small.
Then the concentration of triplet is equal to the Then the concentration of triplet is equal to the initial ground state concentration.initial ground state concentration.
[3M*] = [1M]
Total DepletionTotal DepletionKinetic DerivationKinetic Derivation
d[1M]/dt = -2303SIp(t)T[1M]
d[3M*]/dt = +2303SIp(t)T[1M]
kex = 2303 S Ip(t)where the excitation rate constant is
note its intensity dependence
[3M*] = [1M]0(1 - exp{-2303SIpTt})
Total DepletionTotal Depletion
When a three-state model is used, namely When a three-state model is used, namely including the excited singlet state, then it was including the excited singlet state, then it was found that 95% conversion could occur only iffound that 95% conversion could occur only if
S Tp/2 where p is the laser pulse width This is difficult to satisfy for most lasersThis is difficult to satisfy for most lasers
Total DepletionTotal DepletionAdvantages and DisadvantagesAdvantages and Disadvantages
Principal advantage is that it offers a simple Principal advantage is that it offers a simple direct estimate of the triplet concentrationdirect estimate of the triplet concentration
However, even though the approach to total However, even though the approach to total depletion is inferred from a saturation in the depletion is inferred from a saturation in the OD, OD, the curve can saturate for other reasonsthe curve can saturate for other reasons
Multiphotonic processes, e.g. biphotonic Multiphotonic processes, e.g. biphotonic ionization can come into play at high laser ionization can come into play at high laser intensitiesintensities
Excited state absorption can also invalidate the Excited state absorption can also invalidate the simple kinetic equationssimple kinetic equations
Relative ActinometryRelative Actinometry
This is a two cell experiment.This is a two cell experiment. In one cell there is a compound of unknown In one cell there is a compound of unknown
TT*(*(11), but with a known intersystem crossing ), but with a known intersystem crossing yield yield TT(T)(T)
In the other cell there is a compound of known In the other cell there is a compound of known RR*(*(22), and also with a known intersystem ), and also with a known intersystem crossing yield of crossing yield of TT(R)(R)
Relative ActinometryRelative Actinometry
If the optical densities at the respective If the optical densities at the respective wavelengths are the same, then the number of wavelengths are the same, then the number of photons absorbed by each cell is exactly the photons absorbed by each cell is exactly the same andsame and
This is a consequence of Beer’s LawThis is a consequence of Beer’s Law The monitor beam must also be fixed relative to The monitor beam must also be fixed relative to
the cell and the laserthe cell and the laser
T*(1) = { ODT T(R) / ODR T(T) }R*(2)
Relative ActinometryRelative ActinometryAdvantages and DisadvantagesAdvantages and Disadvantages
Disadvantage is that both triplet quantum yields Disadvantage is that both triplet quantum yields must be knownmust be known
However, it is more often used to measure However, it is more often used to measure intersystem crossing quantum yields once both intersystem crossing quantum yields once both triplet extinction coefficients are knowntriplet extinction coefficients are known
Relative Actinometry and Relative Actinometry and iscisc
T() = T(R)ODT (1) R*(2)
ODR (2) T*(1)
Rearranging formula from one of the preceding slides
This is one of the most popular ways to measure triplet yields
Need two extinction coefficients and the reference triplet yield
Partial Saturation MethodPartial Saturation Method
OD = a(1 exp{bIp})
a = (T* S)[1M]0l
b = 2303StT
t is length of pulse
Partial Saturation MethodPartial Saturation MethodAdvantages and DisadvantagesAdvantages and Disadvantages
This has the same conceptual foundation as the This has the same conceptual foundation as the Total Depletion MethodTotal Depletion Method
However, the fitting parameters a and b can be However, the fitting parameters a and b can be obtained without total saturation being reachedobtained without total saturation being reached
It has this advantage over the Total Depletion It has this advantage over the Total Depletion MethodMethod
The disadvantage is that high laser intensities The disadvantage is that high laser intensities must be used to reach the region where the plots must be used to reach the region where the plots of of OD vs OD vs IIpp becomes nonlinear. becomes nonlinear.