1 / 35

Triplet Extinction Coefficients, Triplet Quantum Yields, and (mainly) Laser Flash Photolysis

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.

junius
Download Presentation

Triplet Extinction Coefficients, Triplet Quantum Yields, and (mainly) Laser Flash Photolysis

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. Competitive Kinetics out of Singlet State 1M + hn 1M*, kex 1M*  products, kpc 1M* 3M* + heat, kisc 1M* 1M + heat, kic 1M* 1M + hnf, kf kS = kpc + kisc + kic + kf FT = kisc/kS

  3. Competitive Kinetics out of Triplet State 3M*  products, k’pc 3M* 1M + heat, k’isc 3M* 1M + hnp, kp kT = k’pc + k’isc + kp

  4. Competitive Kinetics Intramolecular decay channels isc T p S0 Intermolecular decay channels T + Q  S0 + Q’

  5. Transient Absorption 3M** 3M* 3M* + hn’ 3M** k’ex

  6. Creation of Triplets (1) Intramolecular radiationless transitions 1M + hn 1M* kex 1M* 3M* + heat kisc (2) Intermolecular energy transfer 3M1* + 1M 1M1 + 3M* (3) Transfer from solvent triplets in radiolysis of benzene.

  7. Beer’s LawConnection betweenlight absorption and concentration DOD = log10 (I0/I) DOD = eT* [3M*] l DA (l) 0 IPMT time I0 I t laser    I0 (l) DA (l) = log10 I (l) time

  8. Spatial Overlap ofLaser and Monitoring Beam Cell Unexcited Excited Laser Laser Monitoring Light Beam Monitoring Light Beam Proper alignment: Sample is excited along the entire optical pathlength Improper alignment: Sample is not excited along the entire optical pathlength

  9. Placement ofMonitoring BeamRelative toIncident Laser [3M*] from Beer’s Law Cell Laser Extrapolated [3M*] for long cells Monitoring Light Beam Cell Wall Laser Entering Cell Wall Laser Exiting

  10. Triplet-Triplet Absorption Spectra of Organic Moleculesin 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

  11. Methods of DeterminingTriplet Extinction Coefficients • Energy Transfer Method • Singlet Depletion Method • Total Depletion Method • Relative Actinometry • Intensity Variation Method • Kinetic Method • Partial Saturation Method

  12. Energy Transfer (General) • Two compounds placed in a cell. • Compound R has a known triplet extinction coefficient. • Compound T has a triplet extinction 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.

  13. 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* ( DODT / DODR ) Note it doesn’t matter whether T or R is the triplet energy donor.

  14. 3R* + 1T  1R + 3T* kobs = ket [1T]0 [3R*] = [3R*]0 exp(kobst) [3T*] = [3R*]0 [3T*] = [3T*] {1  exp(kobst)} Initial Conditions [3R*]0 = 1 mM [1T]0 = 1 mM ket = 1 × 109 M-1 s-1

  15. Also a Fisc Method kobs = ket [1T]0 [3R*] = [3R*]0 exp(kobst) [3T*] = [3R*]0 [3T*] = [3T*] {1  exp(kobst)} If eT* and absorption of 3R* is mainly hidden under its ground-state absorption, Fisc(R) = [3T*] / # photons into R = (DODl) / (eT* photons)

  16. Kinetic Corrections (1) Need to account for unimolecular decay of the triplet donor: 3D* 1D kD 3D* + 1A 1D + 3A* ket The probability of transfer (Ptr) is no longer one, but Ptr = ket[1A] / (ket[1A] + kD) eA* = eD* ( DODA / DODD ) / Ptr

  17. 3D* + 1A  1D + 3A* [3D*] = [3D*]0 exp(kobst) kobs = kD + ket [1A]0 [3A*] = [3A*] {1  exp(kobst)} [3A*] = [3R*]0 Ptr Unimolecular 3D* decay kD = 0.5 × 106 s-1 Otherwise same initial conditions as before ket = 1 × 109 M-1 s-1 [1A]0 = 1 mM

  18. 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*]0ket[1A] / (kD + ket[1A] - kA) the maximum of this concentration profile is at tmax tmax = ln{kA/(ket[1A] + kD)} / (kA - ket[1A] - kD ) DODA = DODA(tmax) exp(kAtmax)

  19. Kinetics involving decay of both triplets Unimolecular 3D* decay 3D*  1D kD = 0.5 × 106 s-1 Unimolecular 3A* decay 3A*  1A kA = 0.5 × 106 s-1 Energy Transfer 3D* + 1A  1D + 3A* ket = 1 × 109 M-1 s-1 [1A]0 = 1 mM

  20. 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)

  21. Energy TransferAdvantages 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.

  22. Singlet Depletion • By Kasha’s Rule, after the excited singlets have decayed, only the lowest triplet state and the ground state should be present. • Any ground state molecules that are missing should be in the lowest triplet state. • In other words, the missing concentration of ground states should be the same as the triplet concentration. • At a wavelength where they both absorb DOD = (eT* eS) [3M*] l

  23. Singlet Depletion Step 1 Assuming that there is a wavelength region (l1) where the ground state absorbs and the triplet doesn’t DODS(l1) = eS [3M*] l 0 DA (l1) “bleaching” time I0 I t laser    I0 (l1) DA (l) = log10 I (l1) time

  24. Singlet Depletion Step 2 Go to a wavelength region (l2) where the ground state doesn’t absorb DODT(l2) = eT* [3M*] l DA (l2) 0 time I0 I t laser    I0 (l2) DA (l2) = log10 I (l2) time

  25. Singlet DepletionAdvantages and Disadvantages • The main problem is the assumption in Step 1: that the chosen wavelength l1 is in a region where the triplet does not absorb. • There are methods for attempting to compensate for this, but they involve further assumptions. • The main advantage of singlet depletion is that it is free from kinetic considerations.

  26. Total Depletion Method • Assumes that increasing the intensity of the pulse complete conversion of a small ground state conversion to the triplet state is possible if the intersystem crossing is not negligibly small. • Then the concentration of triplet is equal to the initial ground state concentration. [3M*] = [1M]

  27. Total DepletionKinetic Derivation d[1M]/dt = -2303eSIp(t)FT[1M] d[3M*]/dt = +2303eSIp(t)FT[1M] where the excitation rate constant is kex = 2303 eSIp(t) note its intensity dependence [3M*] = [1M]0(1 - exp{-2303eSIpFTt})

  28. Total Depletion • When a three-state model is used, namely including the excited singlet state, then it was found that 95% conversion could occur only if • tSFTGp/2 where Gp is the laser pulse width • This is difficult to satisfy for most lasers

  29. Total DepletionAdvantages and Disadvantages • Principal advantage is that it offers a simple direct estimate of the triplet concentration • However, even though the approach to total depletion is inferred from a saturation in the DOD, the curve can saturate for other reasons • Multiphotonic processes, e.g. biphotonic ionization can come into play at high laser intensities • Excited state absorption can also invalidate the simple kinetic equations

  30. Relative Actinometry • This is a two cell experiment. • In one cell there is a compound of unknown eT*(l1), but with a known intersystem crossing yield FT(T) • In the other cell there is a compound of known eR*(l2), and also with a known intersystem crossing yield of FT(R)

  31. Relative Actinometry • If the optical densities at the respective wavelengths are the same, then the number of photons absorbed by each cell is exactly the same and • This is a consequence of Beer’s Law • The monitor beam must also be fixed relative to the cell and the laser eT*(l1) = { DODTFT(R) / DODRFT(T) }eR*(l2)

  32. Relative ActinometryAdvantages and Disadvantages • Disadvantage is that both triplet quantum yields must be known • However, it is more often used to measure intersystem crossing quantum yields once both triplet extinction coefficients are known

  33. Relative Actinometry and Fisc Rearranging formula from one of the preceding slides DODT (l1) eR*(l2) FT(T) = FT(R) DODR (l2) eT*(l1) This is one of the most popular ways to measure triplet yields Need two extinction coefficients and the reference triplet yield

  34. Partial Saturation Method DOD = a(1  exp{bIp}) a = (eT* eS)[1M]0l b = 2303eStFT t is length of pulse

  35. Partial Saturation MethodAdvantages and Disadvantages • This has the same conceptual foundation as the Total Depletion Method • However, the fitting parameters a and b can be obtained without total saturation being reached • It has this advantage over the Total Depletion Method • The disadvantage is that high laser intensities must be used to reach the region where the plots of DOD vs Ip becomes nonlinear.

More Related