Survival Strategy: Photosynthesis

Transcription

1 Energy and Electron Transfer

2 Survival Strategy: Photosynthesis

3 Light Energy Harvested by Plants 6 CO H 2 O + light energy C 6 H 12 O O 2

4 Importance of Photosynthesis Provides energy for plants Provides energy for animals that eat plants Provides energy for animals that eat animals that ate plants Provides energy for organisms that break down all of the above Provides the energy for most ecosystems on earth

5 Photosynthesis and Solar Energy The Nobel Prize in Chemistry 1961 Joseph Priestley M. Calvin The Nobel Prize in Chemistry 1988 The Nobel Prize in Chemistry 1992 J. Deisenhofer R. Huber H. Michel R. Marcus

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7 k diff D* + A {D*A} {D*A} {DA*} k -diff {DA*} D + A* 8RT k DIF 8RT 3000

8 Possibilities 1 D* + A D + 1 A* 1 D* + A D + 3 A* 3 D* + A D + 1 A* 3 D* + A D + 3 A*

9 Energy Requirement Allowed E D > E A Forbidden E D < E A

10 Mechanisms Radiative Energy Transfer Tiil Trivial ET Non-Radiative Energy Transfer Resonance ET Exchange ET

11 Trivial energy transfer (radiative energy transfer) *D A A *D A B no electronic interaction between D* and A D* emits a quantum of light which is absorbed by A A physical encounter between A and D* is not required, the photon must only be emitted in an appropriate direction and the medium must be transparent in order to allow transmission.

12 Non-Radiative Energy Transfer-1 Exchange Energy Transfer Dexter Energy Transfer Collisional Energy Transfer

13 Exchange Energy Transfer k ET (exchange) = KJ exp( 2r DA /L) where K is related to the specific orbital interactions such as the dependence of orbital overlap to the instantaneous orientations of *D and A. J is the normalized spectral overlap integral, where normalized means that both the emission intensity (I D ) and extinction coefficient ( A )have been adjusted to unit area on the wavenumber scale. It is important that J, by being normalized does not depend on the actual magnitude of A. r DA radii, L is the donor-acceptor separation relative to their van der Waals

14 Non-Radiative Energy Transfer-2 Dipole-Dipole Energy Transfer Coulombic Energy Transfer Resonance Energy Transfer Förster Energy Transfer Förster Resonance Energy Transfer (FRET)

15 Förster Resonance Energy Transfer (FRET) A Transmitter-Antenna Receiver-Antenna Mechanism E (*D D) = E (A *A) k ET (Dipole - dipole) E 2 2 D A D A 6 R DA R DA

16 Differences between Förster (dipole-dipole interaction) and Dexter (electron exchange) energy transfer processes The rate of dipole-induced energy transfer decreases as R 6 whereas the rate of exchange-induced transfer decreases as exp (2r/L). ( ) Quantitatively, this means that k ET (exchange) drops to negligibly small values (relative to the donor lifetime) as the intermolecular (edge-to-edge) distance increases more than on the order of one or two molecular diameters (5-10Å) The rate of dipole-induced transfer (Forster ET) depends on the oscillator strength th of the *D D and da *A radiative transitions, but the rate of the exchange-induced transfer is independent of the oscillator strength of the *D D and A *A transitions

17 D* + A D + A* k ET total D* A H e D Electron exchange k ET (Dipole-dipole) E 2 D A 3 D A 6 R DA R DA A* 2 + D* A 2 H D A* c Electron dipole-dipole interactions Distance dependence, when it can be measured accurately, is a basis for distinguishing energy transfer that occurs by dipole dipole interactions from electron exchange interactions, since the latter generally falls off exponentially with the separation R DA

18 Energy Transfer: A Spectroscopic Ruler L. Stryer and R. Hauhland, PNAS, 58, 719 (1967)

19 Making Use of Förster Resonance Energy Transfer

20 Spin in Energy Transfer 1 D* + A D + 1 A* 1 D* + A D + 3 A* 3 D* + A D + 1 A* 3 D* + A D + 3 A*

21 Spin Allowed Energy Transfer Processes 1 D* + A D + 1 A* Forster 3 D* + A D + 3 A* Dexter

22 A Theory of Sensitized Luminescence in Solids, D. L. Dexter, J. Chem. Phys. 21, 836 (1953) Transfer mechanisms of electronic excitation, Th. Forster, Discussions Faraday Soc. 27, 7, (1959)

23 Comparison of Electron Transfer and Energy Transfer

24 Photoinduced electron transfer Photoinduced Electron Transfer Charge Separation

25 G et = (IP)D (EA)A * G = (IP) D (EA)A E*D Electron Addition and Removal is Easier in the Excited State than in the Ground State

26 G E ox (D) E red (A) E (A) E et 1/2 1/2 exc Coulombic

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28 Free energy of activation expressed in terms of the free energy of reaction ( G) and free energy of activation ( G # ) ( for a hypothetical iso-energetic self-exchange reaction. D* + A D.+ + A. G E ox (D) E red (A) E (A) E et 1/2 1/2 * Coulombic Rehm-Weller Equation

29 Dependence of the electron transfer rate on the driving force G 0 and the free energy of activation G D. Rehm and A. Weller, Isr. J. Chem., 8, 259, 1970 A. Weller Rehm-Weller Plot The value of k et reaches a plateau value of ~ 2 x M -1 s -1 after an exothermicity of ~ -10 kcal mol -1 and the value of k et remains the diffusion controlled value to the highest negative values of achievable.

30 R. A. Marcus, J. Chem. Phys., 24, 966, R. A. Marcus, Electron transfer Reactions in Chemistry: Theory and Experiment, (Nobel Lecture) Angew. Chem. Int. Ed.,32, 1111, R. A. Marcus and N. Sutin, Biochemica et Biophysica Acta, 811, 265, Rates are expected: R. A. Marcus to be slow for weakly exothermic reactions, to increase to a maximum for moderately exothermic reactions, and then to decrease with increasing exothermicity for highly exothermic et reactions.

31 The re-emergence of the activation barrier ( G ) at large negative G 0 values

32 Experimental conditions to observe the Marcus inverted region? For most donor-acceptor (DA) systems the inverted region is obscured by the diffusion limit. This can be circumvented by: freezing the donor-acceptor distribution (glassy medium) covalently linking the donor and the acceptor lowering the donor-acceptor interaction (electronic coupling V) so that the maximum rate for - G 0 = is lower than the so that the maximum rate for G = is lower than the diffusion limit.

33 G. Closs and J. R. Miller, Science, 240, (1988) G. Closs

34 The Nobel Prize in Chemistry 1992 The Nobel Prize in Chemistry 1983 was awarded to Henry Taube "for his work on the mechanisms of electron transfer reactions, especially in metal complexes". The Nobel Prize in Chemistry 1992 was awarded to Rudolph A. Marcus "for his contributions to the theory of electron transfer reactions in chemical systems".

35 Triplet Sensitization

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37 Electron transfer sensitizer

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