Electron-transfer photoinduced

Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. This process is involved in many organic photochemical reactions. It plays a major role in photosynthesis and in artificial systems for the conversion of solar energy based on photoinduced charge separation. Fluorescence quenching experiments provide a useful insight into the electron transfer processes occurring in these systems. 

HOMO — excited HOMO electron-poor -f- oxidised reduced 

The oxidative and reductive properties of molecules can be enhanced in the excited state. Oxidative and reductive electron transfer processes according to the following reactions  

In the gas phase, the variations in standard free enthalpy AG° for the above reactions can be expressed using the redox potentials E° and the excitation energy AEoo, i.e. the difference in energy between the lowest vibrational levels of the excited state and the ground state  

These equations are called Rehm-Weller equations. If the redox potentials are expressed in volts, AG° is then given in volts. Conversion into J mol-1 requires multiplication by the Faraday constant (F = 96500 C mol-1). 

In solution, two terms need to be added to take into account the solvation effect (enthalpic term AHsoiv) and the Coulombic energy of the formed ion pair  

Photoinduced electron transfer scenarios. A. Example where the LUMO of the excited state is above the LUMO of the ground state of the acceptor. B. Example where the HOMO of the excited state is below the HOMO of the grou nd state of the donor. 

The free energy change for an electron transfer reaction is governed by the redox potentials of the reactants, and the relation between them under standard conditions is given by 

Since photoinduced electron transfer reactions almost exclusively involve a single electron process, the value of n in this equation is unity. For a reductive quenching process [Eq. (1.37)], the free energy charge is given by 

Photoelectron transfer reactions can be considered to occur by a mechanism that involves a pair of encounter complexes. Thus, the reductive quenching reaction shown in Eq. (1.37) can be expanded to include both a first- and a second-encounter complex. This first-encounter complex is formed between the excited state M and the quencher Q. Electron transfer then occurs within this complex to give the ionic second-encounter complex, which then finally dissociates to give the separated product ions 

Fig ure 1.8. Rate constant plot for a bimolecular electron transfer reaction. 

In the most common scheme (Fig 12a), photoinduced electron transfer in a two-component supermolecule ( dyad ) involves (i) excitation of a photosensitizer molecular component (P), (ii) electron transfer from the excited photosensitizer to an electron acceptor component (A)(a process often called charge separation ), (iii) back electron transfer from the reduced acceptor to the oxidized photosensitizer (often designated as charge recombination , not shown in the figure) [17,82]. The practical consequences of this sequence of processes may vary from system to system. Quenching of the excited photosensitizer is always observed (usually from emission intensity and lifetime measurements). The formation and disappearance of the charge separated state can in principle be monitored by fast spectroscopic techniques. The possibility of observation depends on both instrumental factors (sensitivity and time resolution) and on kinetic 

The dyad designed to this purpose is shown in Fig 12b [83]. It is thereafter indicated as Ru(NN)3 -Rh(NN)3 , where Ru(NN)3 represents (4,4 -dimethyl-2,2 -bipyridine)-bis(4,7-dimethyl-1,10-phenan-throline)- ruthenium(II) and Rh(NN)3 represents tris(4,4 -dimethyl-2,2 -bipyridine)rhodium(III). The Ru(NN)3 unit is designed to play the role of P and the Rh(NN)3 unit that of A in Fig. 12a. Upon excitation of the Ru(NN)3 unit in the dyad (eq 18), the typical MLCT emission of this unit is strongly quenched with respect to that of a free Ru(NN)3 model, indicating the occurrence of efficient excited-state electron transfer (eq 19). 

An approximate value for the rate constant of this process ( 1-2x10 s 1 in methanol) can be measured from the emission decay (multiexponential because of some conformational freedom). Laser flash photolysis at 440 nm, which corresponds to excitation of the Ru(NN)3 chromophore (eq 18), fails to show any transient accumulation of the electron transfer product, simply proving that the back electron transfer process (eq 20), is faster than the forward reaction (eq 19) [83]. 

Although this was not the main purpose in the design of the dyad, interesting results can also be obtained by exciting the Rh(NN)3 component (eq 21) using near ultraviolet radiation. Upon laser excitation at 298 nm, transient formation and decay of the electron transfer product is observed. This experiment shows that another excited-state electron transfer process (eq 22) can take place in this system (rate constant, 3x10 s ). Because of the fast generation of the electron transfer product, the 

The electron transfer quenching processes (eqs 19 and 22) seem to be completely suppressed when the dyad is embedded in 77 K alcoholic glasses). Thus, upon excitation of the Ru(NN)3 component at 77 K, the unquenched MLCT luminescence is observed. On the other hand, excitation of the Rh(NN)3 component at 77 K gives now rise to an energy transfer process (eq 23), as observed by monitoring the Ru-based MLCT sensitized emission [83]. This energy transfer process, also shown 

Semiconducting electrodes offer the intriguing possibility to enhance the rate of an electron-transfer reaction by photoexcitation. There are actually two different effects Either charge carriers in the electrode or the redox couple can be excited. We give examples for both mechanisms. 

In the future, we may be able to harvest solar energy in parts of the world where there is no natural photosynthesis, in a way that is even more efficient than natural photosynthesis. The primary reaction in photosynthesis is a photoinduced electron transfer (PIET) reaction. 

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Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

Sekiguchi S, Kobori Y, Akiyama K and Tero-Kubota S 1998 Marcus free energy dependence of the sign of exchange interactions in radical ion pairs generated by photoinduced electron transfer reactions J. Am. Chem. Soc. 120 1325-6... 

Levstein P R and van Willigen H 1991 Photoinduced electron transfer from porphyrins to quinones in micellar systems an FT-EPR study Chem. Phys. Lett. 187 415-22... 

Walker G 0, Barbara P F, Doom S K, Dong Y and Hupp J T 1991 Ultrafast measurements on direct photoinduced electron transfer in a mixed-valence complex J. Rhys. Chem. 95 5712-15... 

Imahori H and Sakata Y 1997 Donor-linked fullerenes photoinduced electron transfer and its potential application Adv. Mater. 9 537-46... 

Williams R M, Koeberg M, Lawson J M, An Y-Z, Rubin Y, Paddon-Row M N and Verhoeven J W 1996 Photoinduced electron transfer to Cgg across extended 3- and 11 a-bond hydrocarbon bridges creation of a long-lived charge-separated state J. Org. Chem. 61 5055-62... 

Imahori H, Hagiwara K, Aoki M, Akiyama T, Taniguchi S, Okada T, Shirakawa M and Sakata Y 1996 Linkage and solvent dependence of photoinduced electron transfer in porphyrin-Cgg dyads J. Am. Chem. Soc. 118 11 771-82... 

Kuciauskas D, Lin S, Seely G R, Moore A L, Moore T A, Gust D, Drovetskaya T, Reed C A and Boyd P D W 1996 Energy and photoinduced electron transfer in porphyrin-fullerene dyads J. Phys. Chem. 100 15 926-32... 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

Fox M A and Chanon M (eds) 1988 Photoinduced Electron Transfer 4 vois (NewYork Eisevier)... 

Photochemical technology has been developed so as to increasingly exploit inorganic and organometaUic photochemistries (2,7), recognizing the importance of photoinduced electron transfer as the phenomenological basis of a majority of commercially successful photochemical technologies (5,8). 

The area of photoinduced electron transfer in LB films has been estabUshed (75). The abiUty to place electron donor and electron acceptor moieties in precise distances allowed the detailed studies of electron-transfer mechanism and provided experimental support for theories (76). This research has been driven by the goal of understanding the elemental processes of photosynthesis. Electron transfer is, however, an elementary process in appHcations such as photoconductivity (77—79), molecular rectification (79—84), etc. 

A review of the role of thiols as electron donors in photoinduced electron-transfer reactions has been compiled (49). 

Photoinduced electron transfer reactions in supramolecularmodel systems based on metalloporphyrins 97YGK557. 

Ultrafast photoinduced electron transfer in semiconducting polymers mixed with controlled amounts of acceptors this phenomenon has opened the way to a variety of applications including high-sensitivity plastic photodiodes, and efficient plastic solar cells ... 

Sub-picosecond photoinduced absorption studies were employed to demonstrate the speed of the photoinduced electron transfer. Upon addition of C(M to P30T, the P1A spectrum, decay kinetics, and intensity dependence all change dramatically 36J. Already at 1 ps after photoexcitation by a 100 fs pump pulse at... 

MEH-PPV and P3MBET, were used. As a measure of the efficiency of the photo-induced charge transfer, the degree of luminescence quenching and the ratio of the charged photoexcitation bands to the neutral photoexcitation bands were taken. These two numbers are plotted in Figure 15-15 versus the electrochemical reduction potential. A maximum in the photoinduced electron transfer was determined for Cbo. 

Aminopyridines can be perfluoroalkylated in a photoinduced electron transfer process. A charge transfer complex between the heterocycle and polyfluoroalkyl iodide, observable by NMR, is photolytically stimulated... 

Photoinduced Electron Transfer in Amphiphilic Polyelectrolyte Systems... 

This review article attempts to summarize and discuss recent developments in the studies of photoinduced electron transfer in functionalized polyelectrolyte systems. The rates of photoinduced forward and thermal back electron transfers are dramatically changed when photoactive chromophores are incorporated into polyelectrolytes by covalent bonding. The origins of such changes are discussed in terms of the interfacial electrostatic potential on the molecular surface of the polyelectrolyte as well as the microphase structure formed by amphiphilic polyelectrolytes. The promise of tailored amphiphilic polyelectrolytes for designing efficient photoinduced charge separation systems is afso discussed. 

Effects of Hydrophobic Compartmentalization of Photoinduced Electron Transfer... 

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