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Photochemistry of coordination compounds

Absorption band. Electronic transition. Photochemistry of coordination compounds.

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Photochemistry of coordination compounds

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  1. Absorption band Electronic transition Photochemistry of coordination compounds The excited states are treated in the "localised MO approximation": the transition is considered to involve two predominant orbitals, the electron being promoted from OM1 to OM2, ignoring more or less the other orbitals What are the most important excited states, as far as electron transfer is concerned?

  2. eg* t2g 1. Ligand-Field bands/states, often referred to as LF, d-d or MC (for metal-centered ) These states are dissociative, because an antibonding orbital is populated by the electronic transition. Two classical examples: a) Ti(H2O)63+ : the electronic configuration of Ti(III) is d1. This implies that, to generate the excited state, the electron be taken from a t2gorbital so as to populate an eg* level, which is strongly antibonding. b) Rh(NH3)63+ : the electronic configuration of Rh(III) is d6. To generate the excited state, the electron has also to be taken from a t2gorbital to populate an eg* level (strongly antibonding).

  3. As a consequence, Rh(NH3)63+ is rapidly aquated in water, under UV light irradiation, whereas it is thermally very stable. The excited state is 1014 times more reactive than the ground state! h Rh(NH3)63+ + H2O Rh(NH3)5(H2O)3+ + NH3 In conclusion, LF states are not adapted to photosynthesis and photoredox reactions in general. On the other hand, if we want to exchange ligands under the action of light, they could be particularly useful ( light-driven machines and motors)

  4. 2. Charge-transfer states: MLCT and LMCT LMCT: electron-rich ligand and high oxidation state for the metal [CoIII(NH3)5Br]2+ IrIVCl62- MLCT: electron-poor ligand and relatively low oxidation state for the metal. These states are the most important ones in relation to electron transfer and mimics of the natural photosynthesis by transition metal complexes: ReI(bipy)(CO)3Cl, Mo(bipy)(CO)4, Ru(bipy)32+ For these three complexes, the transition is a metal-to-ligand charge transfer: dof the metal-P*of the bipy ligand. The ligand must have relatively low-lying orbitals available

  5. eg* * t2g A paradoxical statement: an excited state is generally a better oxidant and a better reductant than the ground state a d6 metal centre with a -accepting ligand energy level of the free electron 0 h Ground State Excited State

  6. eg* * t2g EA, the electronic affinity, is directly related to the oxidizing character of the molecule, whereas the ionisation potential, IP, is roughly inversely proportional to its reducing power. energy level of the free electron 0 EA EA IP IP h Ground State Excited State

  7. The prototype: Ru(bipy)32+ L or  This complex is chiral. It can be regarded as a triple-stranded helix

  8. h Ru(bipy)32+ Ground State *Ru(bipy) 32+ MLCT Excited State visible Ru(bipy)32+ has unique properties, related to its light absorbing properties, nature and lifetime of excited state, emission properties and ability to transfer an electron or accept it in its excited state. In addition, it is a very robust complex, both thermally and photochemically. Absorption: lmax = 452 nm in H2O ; e13 500 (deep red-orange complex in solution)

  9. h Ru(bipy)32+ *Ru(bipy) 32+ ~ Ru(III) (bipy-.)(bipy)22+ 1MLCT/ 3MLCT visible Emission: lmax ~ 605 nm in H2O (red emission) em ~ 0.05 (emission quantum yield) ° ~ 600 ns in water to ~ 1s in organic solvents. This means that the excited state has a very long lifetime, which will allow, in particular, bimolecular reactions with various reagents. The excited state is a triplet Metal-to-Ligand Charge Transfer state (3MLCT). In its excited state, the complex is best described as Ru(III) and bipy-.(forone of the three bipy ligands): oxidant reductant

  10. *Ru(bipy) 32+ is thus an interesting electron transfer reagent: it can act as an electron donor (reductant) or as an electron acceptor (oxidant) Ru(III) (bipy-.)(bipy)22+ e- -e- h e- -e- " Ru(bipy) 3+ " Ru(bipy) 32+ Ru(bipy) 33+ formally Ru(I) but, in fact, Ru(II)

  11. *Ru(bipy) 32+ Ru(bipy) 32+ + h E0-0 ~ 2.1 eV h605 nm the redox potentials of the various couples involving the excited state can be deduced from the electrochemical properties of the ground state and the energy level of the excited state (E0-0 )

  12. h h EMISSION ABSORPTION 1MLCT excited state 3d-d state 3MLCT E0-0 ~ 2.1 eV ground state

  13. Electrochemical properties of Ru(bipy)32+ and *Ru(bipy)32+ E (redox potential), in Volts RuIII/RuII +1.26 *RuII/RuI +0.84 For *Ru(bipy) 32+ in its 3MLCT state: E0-0 E0-0 ~ 2.1 eV 0 E0-0 RuIII/*RuII -0.86 RuII/RuI -1.28

  14. hn RuII *RuII kQ *RuII + A RuIII + A-. *Ru(bipy) 32+ is thus able to react with an electron donor (D) or an electron acceptor (A) : quenching of luminescence : quenching of the excited state by an electron transfer reaction: kQ is the quenching rate constant; in this particular case, we have an oxidative quenching (the complex is oxidized). kQ is oftenreferred to as keT (electron transfer)or kCS, CS meaning "charge separation" . The value of this rate constant can be large (typically 108-109 mol L-1s-1). It is nevertheless limited by diffusion (~ 1010 in usual solvents). What should be done to circumvent this difficulty if we want a really fast electron transfer from the excited state?

  15. kQ *RuII + A RuIII + A-. kCR or kb RuIII + A-. RuII + A Once the CS state has been formed (forward electron transfer), recombination of the charges (backward eT) can also be very fast, and sometimes even faster than the forward reaction: These rate constants can be roughly predicted from Marcus theory

  16. = non radiative InterSystem Crossing (yield=1) • E0-0 ~ 2.1 eV • = 600 ns em = 0.04 h' em h abs -0.86 +0.84 E0-0 ~ 2.1 eV = redox properties = radiative (1MLCT)*Ru(bipy) 32+ max = 452 nm max = 14 000 *Ru(bipy) 32+ 3MLCT -1.28 V +1.26 V RuIII(bipy) 3+ RuII(bipy) 32+ RuI(bipy) 3+

  17. Other complexes of interest, whose excited states are relatively long-lived and which can participate in electron transfer processes in the ground state and in the excited state (3MLCT excited state): Re(bipy)(CO)3Cl or Re(bipy)(CO)3L+ , L = py, CH3CN, etc.. Absorption centred around 400 nm; E0-0 ~ 2.3 eV Cu(dpp)2+ and related tetrahedral complexes (d10) MLCT dpp = 2,9-diphenyl-1,10-phenanthroline

  18. kQ eT RuIII + OH- *RuII + A RuII + 1/4 O2 + 1/2 H2O RuIII + A-. A-. + H2O A + 1/2 H2 + OH- hn 1/2 H2O 1/2 H2 + 1/4 O2 Photochemical splitting of water to H2 and O2 : approaches based on RuII(bipy) 32+ Hypothetical sequence of reactions: hn RuII *RuII overall reaction:

  19. Does it really work as described on the previous slide?

  20. h H2 H2O D+. A RuII *RuII RuIII H2O D A-. O2 A = MV2+, RhIII, CoIII, etc… D = sacrificial agent, which is unable to oxidize water once oxidized to D+., but which reacts rapidly with RuIII in its reduced state D ( amine: EDTA, TOA, …)

  21. Energy transfer: two important mechanisms: D D* D* + A D + A* D : energy donor A : energy acceptor Förster (dipole-dipole interaction or coulombic mechanism) Dexter (double exchange of electrons : collisional mechanism)

  22. Förster : the oscillating dipole of D* creates an electrostatic field, which induces the transition A A* when D* deactivates D* A* D A energy transfer

  23. Constant x em kF = JF n4 r6 Förster's mechanism does not imply that the two reagents collide in order to exchange energy : the process can occur at large distance (up to 60-70 Å) kF : rate constant of the energy transfer reaction em : emission quantum yield of the donor D*  excited state lifetime of the donor D* n: refraction index of the solvent r : centre-to-centre distance between D* and A JF : overlap integral between the emission spectrum of D and the absorption spectrum of A

  24. Dexter's mechanism requires that the two reagents D* and A exchange a particle (electron) in the course of the energy transfer process: they thus have to come to close contact. This mechanism is operative at short distances only D* A D A*

  25. Dexter's mechanism: exchange of electrons implies orbitals overlapping; the distance between D and A will thus have a great importance kD = constant x exp(-r) • = attenuation factor r = edge-to-edge distance between the donor and the acceptor Very fast decay of the energy transfer rate with the distance; this reflects the nature of the process, similar to electron transfer

  26. Zn H2 Generally, transition metal complexes are prompted to undergo energy transfer processes according to Dexter mechanism (exp-r) whereas porphyrins, in their singlet excited state, react according to Förster's mechanism (1/r6) OsII RuII Ru(bipy)32+ derivative Os(bipy)32+ derivative PZn PH2

  27. kel D + A D+ + A- Marcus theory is too complex to be treated in less than a few hours…Sorry! kel(r) = nn(r) el(r) n(r) nn(r) : nuclear vibration frequency el(r) : electronic transmission factor: electronic coupling which contains, in particular, the distance between D and A and the nature of the chemical functions located in between D and A n(r) : nuclear factor: thermodynamics of the process and reorganization parameter

  28. The electronic factor  •  • el is proportional to HAB2, HAB being thecoupling matrix element or the coupling Hamiltonian. • HAB = Ho exp(-r) • = attenuation factor, in Å-1, if r is expressed in Å. The nature of the bridge will determine the coupling and it is quantified by  If ~ 0.1 or 0.2 Å-1, the electronic coupling is very strong and the electron can travel over very large distances (aromatic or conjugated bridges between D and A). If ~ 0.8 or 1 Å-1, D and A are only very slightly coupled and the electron transfer rate will fall off rapidly (a few Å); The rate of electron transfer in DNA has recently been the subject of intense debates…

  29. G≠ The nuclear factor : n = exp(- G≠ / RT) G≠ = "classical" activation barrier R reagents (D + A) P products ( D+ +A-)

  30. R P  Marcus quadratic equation: G≠ = /4 ( 1 + G° /  )2 G° = free energy difference between the reagents and the products. For a spontaneous reaction, G° is negative. = reorganization parameter, which measures the energy cost that the system would have to pay to undergo the distorsion leading to P from R,without electron transfer. For a self-exchange process, G° = 0. The energy curves are at the same level

  31. In general, G° ≠ 0 : R P  G≠ G° contains 2 distinct terms: in ,the internal reorganization parameter( internal coordination sphere), and out , which takes into account the solvent molecules, the counterions, etc…

  32. Let us increase the driving force (IG°I = - G°) of the electron transfer reaction: R P R P the activation barrier decreases, which is in agreement with our intuition: very generally, if we increase the driving force of a reaction, we expect it to become faster

  33. R R P P G≠ = 0 We continue to increase the driving force (- G°) so as to reach the "activation-less" situation; in this case, - G° = 

  34. R R P P The "inverted" region: when the reaction is more and more favourable from a thermodynamic viewpoint, the rate of electron transfer decreases! This is, of course, countreintuitive. To any "classical" chemist, the rate of any reaction is expected to increase with the driving force. G≠ = 0 G≠ ≠ 0

  35. G≠ ≠ 0 R R P P G≠ increases!

  36. hn e- e- PC D2 D1 A1 A2 electron e- e- Charge Separation long-range and multistep electron transfer hole Artificial Photosynthesis

  37. Rhodopseudomonas viridis Photosynthetic Reaction Centre Deisenhoffer et al. (D) (A1) (A3) (A2)

  38. Photosynthetic Reaction Centre

  39. hn hn the "molecular triad" approach PC A1 A2 strict geometrical control is essential PC D A classical work: Porphyrins: Mataga, Gust-Moore-Moore, Wasielewsky 2) Ruthenium complexes: Meyer

  40. hn e- e- RuII D2 D1 A1 A2 electron e- e- Charge Separation Ruthenium(II) as the central photoactive species hole Fast forward electron transfer reactions Slow recombination reactions

  41. A D In a bis-terpyridine complex, it is easy to identify an axis on which donor (D) and acceptor (A) groups can to be attached The situation is very different for a tris-bipy complex : a ready-to-functionalize axis is difficult to identify Ru(bipy)32+

  42. N 3 2 1 Synthesis : Collin et al., 1989 Photochemistry : Barigelletti, De Cola, Flamigni, Balzani, 1991-1994 Review article : Chambron, Coudret, Collin, Guillerez, Barigelletti, Balzani, De Cola, Flamigni, CHEM. REV., 1994

  43. multicomponent systems combining transition metals and porphyrins

  44. Fabrice Odobel Photochemistry: A. Harriman

  45. a n i r i d i u m ( I I I ) - b i s - t e r p y c o m p l e x a s c e n t r a l s p e c i e s 3 + M M I r 1 2 Isabelle Dixon & Jean-Paul Collin Photochemistry: Lucia Flamigni

  46. Zn

  47. Zn

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