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Photochemistry Lecture 6 Chemical reactions of electronically excited molecules.

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Presentation on theme: "Photochemistry Lecture 6 Chemical reactions of electronically excited molecules."— Presentation transcript:

1 Photochemistry Lecture 6 Chemical reactions of electronically excited molecules

2 Factors affecting chemical behaviour following electronic excitation  Excess energy  Intrinsic reactivity of specific electronic arrangement – change of charge distribution  Efficiency of competing pathways for loss of electronic state  Change of geometry  Dipole moment  Redox characteristics  Acid base characteristics

3 Excited state reactions  Reactions of electronically excited states occur initially on a potential energy surface which is not the ground state of the system.  Reaction fastest if it can proceed in adiabatic manner – reactants and products correlate  Hence likely to have different transition state and primary products.  However potential surface crossings may also lead to ground state products  Photon excitation not equivalent in general to heating

4 Schematic of photochemical process Photochemical thermal

5 Effects of competing processes Slow phosphorescence and possibly slow T 1 - S 0 means that in many cases triplet state may have greatest role in photochemistry S0 S1T1

6 Geometry changes Biradical –almost tetrahedral Excimer like interaction between two rings.

7 Example of effect of geometry change in excited state :Isomerisation of stilbenes Ph-CH=CH-Ph trans cis

8 Change of dipole moment e.g., Formaldehyde S 0 state = 2.3D 1 (n*)S 1 state  = 1.6D 4-Nitroaniline S 0 state  = 6D S 1 state = 14D Indicate major changes in charge distribution (charge transfer) on excitation

9 Acid-base behaviour  Phenols – pK a of excited singlet state up to 6 units smaller  Amino-aromatics less basic in excited state  Aromatic carboxylic acids pK a up to 8 units higher in excited state  Triplet states typically similar pK a to ground state (zwitterionic character surpressed due to spin correlation)

10 Forster cycle to calculate pK A

11 Forster cycle Shift of absorption and fluorescence spectra

12 Photochemically Induced Bimolecular Reactions  Additions  Reductions by H atom extraction or electron transfer  Oxygenations  Substitutions

13 Addition reactions  Unsaturated molecule uses its weakened - bond to form two new  bonds

14 Substitution  Nucleophilic substitution at aromatic ring shows opposite trends to ground state  e.g. electron withdrawing groups activate meta positions, electron-donating groups activate ortho and para positions.

15 Redox characteristics  Electronically excited states are stronger reducing agents and stronger oxidising agents than the ground state

16 Photoreduction Photoreduction of carbonyl compounds - Half filled n orbital on oxygen in excited state acts as strong electron acceptor ZH = H atom donor e.g., alcohols, ethers

17 Electron transfer  In high polarity solvents, first step of photochemical process may involve electron transfer and ion pair formation  Electron transfer takes place within intermediate complex  Non-adiabatic process – effectively a change of electronic state within the complex.

18 Marcus electron transfer theory Solvent molecules in fluctuation – constant change in energy of donor-acceptor complex At critical solvent configuration D*A complex has same free energy as D + A - Gibbs energy of activation – Free energy required to reach this configuration D*A D+A-D+A-

19 Photosynthesis  Two parallel photosystems in plants PSI and PSII – chlorophyll protein complexes  Light absorption by harvesting chlorophyll molecules followed by fast energy and electron transfer processes  Electrons funnelled into reaction centre to cause net reduction of H 2 O to O 2 and conversion of NADP to NADPH, plus fixation of CO 2. nCO 2 + nH 2 O  (CH 2 O) n + nO 2 saccharides and polysaccharides

20 Absorption spectrum of chlorophyll in solution Green S1S1 S2S2

21 Photosyntheic bacteria  Photosynthetic bacteria 3 x 10 9 years  Higher plants0.5 x 10 9 years Rhodobacter sphaeroides highly studied model system Contains only one photosystem, and bacteriochlorophyll instead of chlorophyll as active pigment

22 Photosynthesis in bacteria  Step 1a: Light harvesting (absorption) by chlorophyll and auxilliary pigments.  Step 1b: Rapid multistep Forster energy transfer to reaction centre, “special pair” of chlorophyll a.  Step 2: Rapid ( ps) electron transfer to pheophytin  Step 3: Charge separation by electron transfer via quinones and further electron transfer  Steps 4 - x: Reduction processes at reaction centre  Recent studies of these processes by ps or fs flash photolysis

23 Bacterial Photosynthetic Reaction Centre

24 Electron transfer in photosynthesis

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