Electron and Energy Transfer Chapter 7 (pp.383-481) Principles of Molecular Photochemistry An Introduction Spin t„ Dynamics j- Molecular Dynamic: -P, Radicals Nicholas J. Turro V. Ramamurthy J.C. Scaiano MODERN MOLECULAR PHOTOCHEMISTRY OF ORGANIC MOLECULES Nicholas J. Turro V. Ramamurthy J.C. Scaiano MODERN MOLECULAR PHOTOCHEMISTRY OF ORGANIC MOLECULES fey ^ Nicholji J. Tuto V' V Ramamurlty J.C Scauno Survival Strategy: Photosynthesis Electron Transfer-Phenomenon hv -► *sens *sens + R -* sens* + R sens*" + R*+ -► R + sens r-- P*+ P*+ + sens* -► P + sens • + Electron Transfer-Orbitals Electron transfer lu Electron transfer rR(eD)] HO M R" Electron and Hole hole No hole: filled Hole and Electron Transfer Electron transfer [*R(eD)] R M" In the physics literature, a half-filled HO is considered a "positive hole" in the electronic framework of a molecule; *R is viewed as simultaneously possessing both a positive hole (one electron in the half-filled HO) and one electron in the half-filled LU. Hole transfer [*R(eA)] R- M The term electron transfer is usually employed for single electron transfer involving LUs of the donor and acceptor; the term hole transfer is usually employed for electron transfer involving the HOs of the donor and acceptor electron transfer • is it difficult to achieve photo-induced electron transfer between two molecules? requirements for charge separation there should be a thermodynamic driving force AG < 0 rate should be high enough to compete with excited state deactivation absorption DA* etecferoK transfer D+A" fluorescence radiationless decay processes requirements for charge separation DA* there should be a thermodynamic driving force AG < 0 rate should be high enough to compete with excited state deactivation absorption electron transfer D+A- can we estimate the energy of the y D+A~ state? relative energies of CS-states © + ©^© + © Two neutral fragments Two (radical) ions • simple energy estimation requires: • Redox potentials • Solvation energies • Coulomb interaction Electron Addition and Removal is Easier in the Excited State than in the Ground State Reduction Oxidation "Vacuum" -Q- --- I T EA(R) ! IP(R) ! HO ♦d- Ground Ground state, R state, R Electron affinity Ionization potential Excited states of diamagnetic molecules with closed shell ground states are always better oxidizing and reducing agents than their corresponding ground states Ground state (gas phase) Excited state (gas phase) Excited state In solution D + A —► D*+ + A* AGet = (IP)D-(EA)A *D + A —► D#+ + A#_ *AG = (IP)D - (EA)A - E*D AG, = E»l(D) - Eg (A) - E (A) + b£CmlmMc Rehm-Weller Equation Gas Phase to Solution The free energy of electron transfer processes in solution can be estimated by two different approaches: >The value of AG for the gas phase reaction is calculated using IP and EA and then corrected to take into account the solvation energies for all the participants (i.e. , *D, *A, D+ and A~ in the electron transfer reaction. >The electrochemical potentials for the oxidations E° (DVD) and reductions E° (A/A") in solution are measured and then employed to calculate AG directly for the solution electron transfer. >The key electrochemical parameters are more commonly available or can be determined using standard electrochemical techniques, such as cyclic voltammetry; as a result, the second approach is most commonly used. redox potentials 0 + q—^0 + © a good Donor has a low oxidation potential a good Acceptor has a low reduction potential in the ground state AGredox > 0 Caution Be careful about the sign It is very important to note that by convention in electrochemistry, both E° (D/D*+) and E° (A/A—) are expressed as reductions fl>+/D and A/A—). Both reactions are expressed as A + e—»A— and D*+ + e^D. Because of this convention, one must pay careful attention to the signs of E° (D*+/D) and E° (A/A—) when computing the overall value of AG. Be careful about the reference electrode Another important point in using electrochemical data is that one must employ the standard electrode to which the values of E° (D+/D) and E° (A/A-) refer. Both the standard hydrogen electrode and the standard calomel electrode and silver electrode are commonly used as standards. So, care must be taken to know which is being used and not to mix data from the two standards unless appropriate corrections are made. driving force for charge separation DA* D+A if the redox potentials are measured in the solvent of interest and the ions are fully separated: • if not, correct for electrostatic interaction and solvation Coulombicterm = N* 3315 er (in kcal/mol wittin A) o E o E CD ■*-> O n E o o O 10 0.1 0.01 0 6 8 10 Distance (Á) Coulomb's Law Benzene (2.27) Acetone (20.6) Acetonitrile (35.9) Water (80.2) 12 AGeř = E-(D) - E[f2(Ä) - EM) + A£„,, Resource for Electrochemical Redox Potentials HANDBOOK OF PHOTOCHEMISTRY THIRD EDITION Marco Montalti Ubcrto Credfx I m a Prodi \l Teresa Gandolli nilh jnfrTayl)- +1.165 [6701] -1.29 [6701] 5 Anthracene, 9.10-dunethyl- +0.95 [6401] 6 Anthracene, 9.10-mphenyl- +1.22 [7701] -1.94 [7701] 7 Anthracene, 9-methyl- +0.96 [6301] -1.97* [6201] 8 Anthracene, 9-phenyl- - -1.86 [7001] 9 Azukne +0.71 [6301] -1.65* [6201] 10 Benz(a]antracene +1.18 [6301] Table 7b-7 Halfwave Redox Potentials of Nitriles. No. Compound £»(A'A0 (V vs. Ag electrode) DMF* £^A'A0 (Vvs. SCE) MeCN 1 Anthracene. 9-cyano- 2 Anthracene. 9.10-dicyano- 3 Benzene. l^^no-3.5-dmitro- 4 Benzene, l-cyano-4-nitro- 5 Benzene, 1.2-dicyano- -1.58b -0.98b -0.96 -125 -2.12 reactants products CN CN CN Naphthalene (S.,) 1,4-Dicyanobenzene (SQ) CN Radical ions EDVD = +1 60 V EA/A-=-164V E(SJ = 3.94 eV = 90.9 kcal mol -1 AG°=^D°/D-^A°/A--^-0-2 AG0 = 36.9 - (-37.8) - 90.9 - 0.2 = -16.4 kcal mol k (electron transfer) = 1.8 x1010 M"1 s"1 Red. Pot: 0.89 V (-) Oxn. Pot: 1.5 V (+) Free energy of activation expressed in terms of the free energy of reaction (AG) and free energy of activation (A6#) kel = k0 exp ( ——r AGef = - ^ (A) - £.(A) + &ECoulombic Rehm-Weller Equation Dependence of the electron transfer rate on the driving force AG0 and the free energy of activation AG* D. Rehm and A. Weller, Isr. J. Chem., 8, 259,1970 ag° (kcal mof1) A. Weiler Increasing exothermicity-► Rehm-Weller Plot The value of ke+ reaches a plateau value of ~ 2 x 1010 M-1s-1 after an exothermicity of ~ -10 kcal mol-1 and the value of ke+ remains the diffusion controlled value to the highest negative values of achievable. More Rehm-Weller Plots logkq 10- Figure 1. Plot of the logarithm of the rale constant vs. E\/2(Q/Q+) for the quenching of Cr(bpy)j3+ by aromatic amines (O), methoxybenzenes (•). and aliphatic amines (▲). V. Balzcmi,et. al., JACS, 100, 7219,1978 Figure 1. Plot of log k^*6 vs. quencher reduction potential, E\/2[Q/ C. R. Brock, T. J. Myers and b. G. Whitten, et. al., JACS, 97, 2909,1975 .2 I .4 .5 6 .7 • » 10 Figure 4. Dependence of quenching rate constant on quencher redox potential for several metallocyanide complexes. H. Toma and C. Creutz , Inorganic Chemistry, 16, 545,1977 Figure 3. Plot of the logarithm of the rate constant vs. fi/iCQ/Q*) for the quenching of Rutbpyli2* by aromatic amines. Figure 4. Plot of the logarithm of the rate constant vs. £, /:(Q/Q+) for the quenching of [r(Me2phen)2Cl2+ by aromatic amines (6). methoxyben-7enes (•). and aliphatic amines (A). Mechanism of eT: Libby Model W. F. Libby, J. Phys. Chem., 56, 863, 1952; J. Chem. Phys., 38, 420, 1963; The Nobel Prize in Chemistry 1960 was awarded to Willard F. Libby "for his method to use carbon-14 for age determination in archaeology, geology, geophysics, and other branches of science". Libby Model R#+(solvated) + R(solvatcd) R(solvated) + R#+(solvated) [*Fe(H20)6]2+ + [Fe(H20)6]^ - [*Fe(H20)6]3+ + [Fe(H20)6]2+ The electron jump from R (*Fe2+) to R'+ (Fe3+) is analogous to the electron jump from a HO to a LU that leads to formation of an electronically excited state. The electron jump is expected to occur "vertically" and to follow the Franck-Condon principle; the geometry of the products formed by an electron transfer would be the same as the geometry of the reactants. Two types of reorganization occur after the et: (1) an electronic and vibrational reorganization, termed internal molecular reorganization: and (2) a solvent reorganization associated with the solvent reorientation to accommodate the new electronic structures termed external solvent reorganization. D A <- 5 d 0 0 Libby Model R'+Csolvated) + R(solvatcd) R(solvated) + R#+(solvated) [*Fe(H20)6]2+ + [Fc(H20)6]* [*Fe(H20)6]3+ + [Fe(H20)6] 2+ & CD C CD CD CD Electron jump Solvent reorganization <2 0 <3 Solvent molecules oriented around R+ Solvent molecules random around R Solvent 8+.8-molecules Evolution of Marcus model R. A. Marcus, J. Chem. Phys., 24, 966,1956. Rcaclants Products f VI// u. AG0 \ / j_ \y Reaction Coordinate Weller Model f \ \ / / CD r— \\\ i_ CD \ \\ / / CD CD \ \\/ / LL \ Libby model Marcus model ket = A exp-(AG*/RT> The Marcus model Reorganization h-H JF h<-H Relaxation -► Reaction coordinate 0 4 Solvent molecules oriented around R+ Solvent molecules random around R Solvent 8+^5-molecules ^ R. A. Marcus, J. Chem. Phys., 24, 966,1956. R. A. Marcus and N. Sutin, Biochemica et Biophysica Acta, 811, 265,1985. R. A. Marcus, Electron transfer Reactions in Chemistry: Theory and Experiment, (tJobel Lecture) Angew. Chem. Int. Ed.,32, WW, 1993. ^ . , R. A. Marcus Rates are expected: > to be slow for weakly exothermic reactions, ^ to increase to a maximum for moderately exothermic reactions, and then X AG0 is even more negative and AG* becomes positive • The dot traces the energy of the transition again (!) state as AG° becomes more negative -AjG0 < A = A -Aj-G0 > A The predicted existence of such an "inverted region"'was controversial from the time the theory was proposed in 1956 until experimental evidence of it was found for a set of photoredox reactions (Closs & Miller, 1986). Marcus prediction vs Weller1 s experiments exergonic AG endergonic Electron Transfer Involves Two Steps D*+A < *' > D*-A < *ff I D+ A" *-d k-cs The experimental rate constant is limited by the diffusion rate constant in the solvent, it effectively hides the Marcus inverted region. On the right section of the plot the reaction is endothermic and the prediction of the Marcus equation is followed. The Rehm-Weller equation does not make allowance for an inverted region. Experimental conditions to observe the Marcus 11 inverted region'1? (A-D)* (A--D+) -3-2-1 0 1 2 3 4 5 For most donor-acceptor (DA) systems the inverted region is obscured by the diffusion limit. Suppress the need for diffusion. 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 -AG0 - X is lower than the diffusion limit. Effect of Free Energy on Rates of Electron Transfer Between Molecules in Glass at 77 ° K function of cxothcrmicity at KT6 s expressed as relative Franck-Condon factors (see eq 4, 10, 11, and 12). The line was calculated by using eq 4. J. R. Miller, J. V. Beitz, and R. K. Huddleston, J. Am. Chem. Soc, 106, 5057, 1984. ■ Pioneering 1984 Study by Miller and Closs Definitively Proved the Existence of the Inverse Region -AG (eV\ G. C\oss and J. R. Miller, Science, 240, 440-447 (1988) J. R. Miller, L. T. Calcaterra and G. L. Closs, J. Am. Chem. Soc, 106, 3047-3049 (1984) 27 : ■ i TPPNQ-a InTP"«J3-J. 7n7P°3C-3 7r.7PPf,Q-3 2n7PWQ-e •■' ZnTPPNQ-T 7PP9C-T ZrT?PBC-3 m :n7=Pt,a-B \ ZmPPBC-7 - TP°3C-B TPPKO-fl \ ZnTPPfitt-B ■ ZrTPPNS- —r-0.5 —I 2.0 -AG IeV) Figure 2. Plot of rate constant vs. exothermichy for the reaction '*P-Q — P+ - Q" and for P+ - Q" — P - Q. where P - porphyrin and Q = quinone. The B and T after the name of the compounds indicate data obtained in butyronitrile or in toluene, respectively. The maximum uncertainty in any given rate constant is ±20%. 1.0 2.0 -A6°(eV)- n BPCj OBPC3 0 EJPC5 Ok -0* -08 -12 Fm Energy teV) 0.5 2 3 -'SGi'p/eV Fig. 10. The dependence of lhe CR rate constant kn of geminate ion pairs produced by fluorescence quenching reaction on the free energy gap - AfjJJ, in acelonitrile solution. -AG (eV) Figure 8. Plot of intracomplex electron-transfer rate between cyt c and cyt b6 as a function of free energy. Solid line is fit to Marcus' theory, X = O.Bv. AG° eV Figure 8. AC0 dependence of kjk^ for backward ET between Ru-(bpz)j+ and the cation radicals of the aromatic donors. The line is drawn for easy viewing. 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". The re-emergence of the activation barrier (A G*) at large negative AG0 values A AG° = 0 -AG° < A, -AG° = A, -AG° > A, AG0 is even more negative and AG* becomes positive • The dot traces the energy of the transition again (!) state as AG0 becomes more negative 2jt 3/2 (V)2 exp -(AG +A)2/4A Classical Marcus What is X ? Total reorganization energy is composed of the solvation component a,outer (or a,sol) and the inner or internal component, a,inner (or a,int) ^ ~ ^inner + ^outer The solvent component is usually described in terms of dielectric continuum theory / ^outer ^ 1 1 1 + 2rD 2rA rAD ) ( \ 1 1 where rA and rD are the atomic radii of A and D, respectively, s is the dielectric constant of the medium that responds to the electronic polarization (e is the square of the refractive index), and ss is the static dielectric constant or relative permittivity corresponding to the solvent dipole. I W 0 \ D A Eis ^ * 1 ^ 9 % v. The intramolecular component is most generally expressed as a summation over all vibrational modes of the reactant state and product state which undergo change during the electron transfer reaction. The AqA is the displacement caused by the electron transfer reaction A,. inner f-fl \ Aqt What is V ? Marcus Theory Classical Marcus equation Avoided crossing of potential energy surfaces In order for electron transfer to occur, an overlap between the populated orbital of the donor and the empty orbital of the acceptor is necessary in the activated complex. This electronic interaction involves a split of electronic energy levels. [D/A] / HdaI\[D+-A-] VI Hda A If the electronic states are of the same symmetry, breakage of the Frank-Condon principle at the saddle point causes in turn a split of potential energy levels and avoided crossing of the two curves. As a result, two potential surfaces are created that are separated at the configuration of the activated complex by an energy gap 2|Hda|. |Hda| is a matrix element for electronic coupling between the donor and the acceptor. What is V ? Adiabatic Strong coupling Adiabatic Nonadiabatic Weak coupling V Non-adiabatic Forward and back electron transfers have different AG0and therefore different rates CS and CR AG D+/A- A-etG° nuclear configuration In practical applications of photo-induced ET reactions, charge separation has to be maintained during a period of time sufficient for further redox reactions to take place. Ideally, forward ET involving an excited state has to be as fast as possible, while back electron transfer during which charges recombine has to be slow. Such an ideal situation is achieved when the energetics of the system implies activation-less forward ET and very exoergic back ET process situated in the inverted Marcus region. Forward and backward electron transfer rates are not the same: Charge separation Easier to observe Marcus inversion during back electron transfer process "A + D kdif [A", D*+] sep A*" + D kbet -AGget (eV) Increasing exothermicity A + D Scavenger Scavengering of free radical ions Caution: The internal reorganization energy and the electronic coupling V are generally not the same for charge separation and recombination. As a result, the charge separation and recombination rates in the same donor-acceptor system usually do not belong to the same Marcus curve. Dynamics of Bimolecular Photoinduced Electron-Transfer Reactions, I. R. Gould and S. Farid, Acc. Chem. Res. 1996, 29, 522-528. Back electron transfer A Generation of excited states D+# + A- khet > *D + A When back electron transfer to the ground state D#+ + A- -> D + A AG°e+ large is in the Marcus inverted region it is inhibited. the formation of the excited products D#+ + A- -> *D + A AG°*ct small may be kinetically preferred because of the smaller A6°*et Excited state production through back electron transfer D*/A* could be D+°/A^ could be -► Excited-state Charge-transfer Nuclear coordinate formation recombination Bioapplications, Light emitting diodes (TV, Computern, Cell phone screens) The Nobel Prize in Physics 2014 "for the invention of efficient blue light-emitting diodes which has enabled bright and energy-saving white light sources." Isamu Akasaki Hiroshi Amano Shuji Nakamura Charge separation and Photosynthesis light energy transfer to Special Pair light harvesting system SP*-Ph-QA-QB three consecutive electron transfer steps excitation excitation SP+-*Ph-QA-QB SP-Ph-QA-QB SP-PIi-Qa-Qb SP-Ph-QA-QB creation of a proton gradient Fig. 2. Representation of the first evejits in photosyjithesis. Light harvesting, followed by energy transfer to the special pair, and subsequently by three electron transfer steps. The charge separated state is used to created a transmembrane proton gradient. Charge Separation: Diads, Triads and Tetrads Molecular Mimicry of Photosynthetic Energy and Electron Transfer, D. Gust, T. A. Moore, and A. L. Moore, Acc. Chem. Res., 1993, 26, 198 ZnP-H2P*f-C60"' j Devens Gust Moore, and A. L. Moore, Acc. Chem. Res., 2009, 42, 1890 Cascade electron transfer in a tetrad Ground State D(S0) CSS D-A D-A CR -► D (SO Singlet ion pair Singlet state 3CS (3D-A) CR f D-A) T, 3(Ď-+,A-) -> ^A)' spin forbidden 3(Ď-+,A-) -> 3(D,A)' spin allowed b*3 or A*3 formed Making triplets from photo-generated charges: observations, mechanisms and theory, D. J. Gibbons, A. Farawar, P. Mazzell, S. Leroy-Lhez and René M. Williams, Photochem. Photobiol. Sci., 2020,19, 136 Role of Spin: Triplet ion pairs have longer lifetime Benzophenone as triplet sensitizer COOMe COOMe COOMe COOMe OMe DMN[3]DCME Sensitiser 3.26 -- 1BP- 2.96 ■- Energy/ eV 0 355 nm exc. (3.49 eV) D-bridge-A - 3.78 2.65 Energy/ eV 0 Electron Transfer at a Distance No need for Donor A Acceptor Orbitals to Overlap A CN Electron transfer through a bonds 8 AGe!= -0.32 eV ket = 4.0 x 07 s"1 9 AGe/= -0.05 eV kel= 1.0 XO6 s"1 10 On the role of spin correlation in the formation, decay, and detection of long-lived, intramolecular charge-transfer states, Jan W. Verhoeven, J. Photochem. Photobio. C: Photochem. Rev., 2006, 7, 40-60 Investigating Long-Range Electron-Transfer Processes with Rigid, Covalently Linked Donor-( Norbornylogous Bridge)-Acceptor Systems, M. Paddon-Row, Acc. Chem. Res. 1994, 27, 18-25 Long distance electron transfer and the distance dependence of the coupling element VDA Difference between super-exchange and molecular wire > a en u 2! UJ superexchange mechanism molecular wire B A D Decreasing a -N B D ' □ —A —A superexchange: = A exp(-/3r) e * Me Met) 23(13) ,CN VCN ELECTRON TRANSFER molecular wire: polaron e.g. polyacetylene bridge ELECTRON TRANSPORT VACANT SOLVENT ORBITALS FILLED SOLVENT ORBITALS Through-bond interactions in donor-acceptor systems separated by solvent or by covalently bound spacers. Although the available orbitals of the solvent or spacer lie at energies incompatible with intermediate states participating in mediated electron transfer, their presence provides an electronic perturbation of the donor and acceptor orbitals and enhanced electron transfer rates compared with the interactions occurring over the same spatial separation with an intervening vacuum. 0 LU LU Electron transfer *D Hole transfer keT = k0exp[-ß(RDA-R0)] Smaller the ß the larger the keT The propagation of electronic coupling along the bridging material (aka "superexchange" leads to exponential distance dependence. The initial coupling into the bridge depends on the energy gap between the relevant orbitals of the donor (acceptor) and the bridge, AE^b and AEBa , as well as stereoelectronic factors. ket = k0exp[-P(RDA - R0)] 0spacer (A1) How does the rate of electron transfer change with increasing distance between the two groups? Distance dependance of electron transfer (3 for polyphenyl P = 0.32 A'1 Excited state lifetimes of the quenched unit and electron transfer rate constants x(ps) kel (S1) Osn(phH)2Osm <5 >2xlOn Osn(phH)3Osin 8 1.2 xlO11 Osn(phH)4Osm 34 2.9 xlO10 Osn(ph)3Osin 17 5.8 xlO10 Osn(ph)5Osm 340 2.7 xlO9 Osn(ph)7Osm 2900 9.4 xlO7 ket = k0exp[-P(RDA <3b Sh R= hexyl Electron transfer through a bonds ket = k0exp[-(3(RDA - R0)] Long distance electron transfer in proteins Wenger\ Leigh, Villahermosa, Gray, Winkler, Science, 2005; 307, 99-102. ET rates far rune l-ks-labeted cyt b^.. denvattves, seven follow the exponentialdrstance dependency of Eqn(1), whereas m two cases slower rates trian predicted by Eqn (1) have been measured (6*]. Published with permission of Science. Activationless electron tunneling through various media: vacuum (black, t> = 2.9-4.0 k l), MTHF glass (violet, f> = 1.57-1.67 A'1), aqueous glass (cyan, f> = 1.55-1.65 and toluene glass (green, (b = 1.18-1.28 10 Ä 20 Ä 30 Ä distance ©2005 by National Academy of Sciences Gray H B , and Winkler J R PNAS 2005;102:3534-3539 PNAS Long distance electron transfer in general Wenger, Leigh, Villahermosa, Gray, Winkler, Science, 2005; 307, 99-102. G. Mc Lendon, Acc. Chem. Res. 1988, 27, 160-167 DNA-Mediated Photoelectron Transfer Reactions Jacqueline K. Barton,* Challa V. Kumar, and Nicholas J. Turro* Department of Chemistry, Columbia University New York, New York 10027 Received April 7, 1986 J. Am. Chem. Soc. 1986, 108, 6391-6393 Accelerated Electron Transfer Between Metal Complexes Mediated by DNA Michael D. Purugganan, Challa V. Kumar, Nicholas J. Turro,* Jacqueline K. Barton* N. J. Turro Science, 1988, 247,1645-1649 =*o exp(-^r) (3<0.2/A° e transfer > 200 A DNA is a Molecular Wire Schematic representation of a DNA duplex with a tethered rhodium photooxidant containing six 5'-GG-3' guanine doublets up to 200 A from the metallo-intercalator binding site. Oxidative damage at each of the guanine doublet sites, as a result of photoexcitation of the rhodium intercalator, has been demonstrated. Long distance electron transfer in DNA Long-range photoinduced electron transfer through a DNA helix C.J. Murphy, M.R. Arkin, N.D. Ghatlia, S.H. Bossmann, N.J. Turro and J.K. Barton, Science, 1993, 262, 5136 DNA Is Not a Molecular Wire: Protein-like Electron-Transfer Predicted for an Extended it-Electron System S. Priyadarshy, S. M. Risserand D. N. Beratan J. Phys. Chem. 1996,100, 17678-17682 DNA: insulator or wire? D. N.Beratan, S. Priyadarshy and S. M.Risser Chemistry & Biology 1997, 4, 3-8 Electron transfer across a molecular wall Electron Plot of ket vs AG0 for different donor-acceptor pairs obtained from transient absorption spectroscopy when donor molecules are enclosed within octa acid cavity. R. Marcus J. Miller G. Closs H. Gray J. K. Barton N. J. Turro M. Wasielewski F. D. Lewis T. Meyer B. Giese G. Schuster S. Farid I. Gould N. Mataga J. W. Verhoven M. N. Paddon-Row D. Gust ®W1LEY-VCH Vinccn/o ßal/ani (Ed ) Electron Transfer in Chemistry Volume I: lYinciples.Theories. Method".. and Techniques BalzaniV.(Ed.) Electron Transfer in Chemistry 5 Volume Set WILEY-VCH Verlag GmbH, Weinheim, Germany, 2001.-3970 p Volume I Principles and Theories Volume II Organic Molecules Organometallic and Inorganic Molecules Volume III Biological Systems Artificial Supramolecular Systems Volume IV Catalysis of Electron Transfer Heterogeneous Systems Gas-phase Systems Volume V Molecular-Level Electronics Imaging and Information Energy and the Environment Wii i > »1 ~ iw.ci DE GBUYTEB Visible Light Photocatalysis in Organic Chemistry Burkhard König CHEMICAL PHOTOCATA 2nd edition r- C^C Science of Synthesis Photocatalysis in Organic Synthesis Volume Editor B. König §Thieme