Excited-state Electron Transfer

The triplet excited state CAtj ) of Pt2(pop)4, like that of the complex Rh2(l,3-diisocyanopropane)4 is a viable chemical reagent for bimolecular reactions because of its long lifetime at ambient temperature, and its high quantum yield = 0.5) of formation. By contrast, the singlet excited state (U211) of Pt2(pop)4 is not 

The triplet state A2m, with a single electron in a high energy 2a g orbital, is a strong one-electron reductant in aqueous solution. The phosphorescence of Pt2(pop)4 is quenched by the oxidant l,r-bis(sulfoethyl)-4,4 -bis(pyridinium inner salt) (BSEP) to give Pt2(pop)4 and BSEP (Ref. 39)  

Similar reactions occur with nitric acid [Eq. (4.4)], and with vesicle-bound A -alkyl-A -methyl-4,4 -bipyridinium ions  

Both the quenching reaction and the bimolecular back electron transfer reaction occur at rates that are close to the diffusion limit. Nicotinamide and OsCl(NH3)5 are also reduced by Pt2(pop)4, thereby making the A2u state of Pt2(pop)4 a stronger reductant [EP (Pt2(pop)4 /Pt2 op)4 ] -1 V against NHE) than is the excited state Ru(bipy)3 [EP (Ru(bipy)3 /Ru(bipy)3 ] = -0.88 V against NHE) (see Chapter 5).  

The A2a state is also a strong one-electron oxidant. From the molecular orbital diagram shown in Fig. 4.7, it is apparent that the triplet excited state has a vacancy in the lu2u orbital that can accept a single electron from a reductant. Reductive quenching of the triplet excited state of Pt2(pop)4 occurs with a series of amines  

Micellar effects on electron transfer rates have also been examined. 

Electron transfer reactions of the excited states of metal ion complexes continue to attract much attention with interest in solar energy storage and the cleavage of H2O to H2 and 02. ° ° A useful review of the properties of excited-state polypyridine complexes has appeared. The much studied [ Ru(bipy)3] and related systems remain at the forefront of new work but other metal ion systems are attracting increasing interest (Table 1.3.) When a complex is photochemically activated to form ML, excited state 

Reductions of [Co(phen)3] and [Co(bipy)3] by [ Ru(bipy)3] are close to diffusion controlled,and calculated rate constants 

Rate constants for electron transfer quenching of substituted [ Ru(phen)3] derivatives by Eu show behavior consistent with Marcus theory with a self-exchange rate constant of 5 x 10 mol liter s for When Cu  

Initial electron transfer to orbitals on coordinated nitrobenzoate ligands is proposed for the excited state quenching of [ Ru(bipy)3] by o- and / -nitrobenzoate complexes of Co(III)(NH3)5. When quenching rate constants are analyzed in terms of the mechanism 

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Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

Boxer S G, Goldstein R A, Lockhart D J, Middendorf T R and Takiff L 1989 Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers J. Rhys. Chem. 93 8280-94... 

Excited state electron transfer. T. J. Meyer, Prog. Inorg. Chem., 1983, 30, 389-440 (104). 

Excited state electron transfer also needs electronic interaction between the two partners and obeys the same rules as electron transfer between ground state molecules (Marcus equation and related quantum mechanical elaborations [ 14]), taking into account that the excited state energy can be used, to a first approximation, as an extra free energy contribution for the occurrence of both oxidation and reduction processes [8]. 

PBE dendrons bearing a focal bipyridine moiety have been demonstrated to coordinate to Ru + cations, exhibiting luminescence from the metal cation core by the excitation of the dendron subunits [28-30]. The terminal peripheral unit was examined (e.g., phenyl, naphthyl, 4-f-butylphenyl) to control the luminescence. The Ru +-cored dendrimer complexes are thought to be photo/redox-active, and photophysical properties, electrochemical behavior, and excited-state electron-transfer reactions are reported. 

We now turn to an example of nonadiabatic chemistry where the nonadiabatic process starts on the ground state, and is followed by an excursion upward onto the excited state electron transfer (see references 2-5). 

Excited-state electron transfer represents one of the most fundamental pathways in chemical and biological processes. This, together with its prospects for application, elaborated in later sections, has been attracting considerable attention. As a result, many review papers and books have been published to address this issue [1-6]. 

For molecule that lacks excited-state electron transfer, upon excitation, the fluorescence quantum yield (, ) and the fluorescence lifetime (rllor) can be expressed as ... 

Chen KY, Hsieh CC, Cheng YM et al (2006) Tuning excited state electron transfer from an adiabatic to nonadiabatic type in donor-bridge-acceptor systems and the associated energy-transfer process. J Phys Chem A 110 12136-12144... 

The efficient on/off switching of fluorescence from substituted zinc porphyrin-ferrocene dyads 16a and 16b is achieved through redox control of the excited-state electron transfer quenching.26 This redox fluorescence switch is based on the switching of the excited-state electron transfer from the ferrocene to the zinc porphyrin through the use of the ferrocene/ferrocenium (Fc/Fc +) redox couple. 

Bimolecular excited state electron transfer reactions have been investigated extensively during the last decade (1-3). Electron transfer is favored thermodynamically when the excitation energy E of an initially excited molecule A exceeds the potential difference of the redox couples involved in the electron transfer process. 

Studies of such systems provided a better understanding of the mechanism of electron transfer processes in general. This reaction type is also the basis of almost any type of natural or artificial photosynthesis. Hence it is not surprising that many investigations have been devoted to excited state electron transfer reactions. On the contrary, the reversal of excited state electron transfer has found much less attention although it is certainly not less interesting. 

An electronic excited state of a metal complex is both a stronger reductant and oxidant than the ground state. Therefore, complexes with relatively long-lived excited states can participate in inter-molecular electron transfer reactions that are uphill for the corresponding ground state species. Such excited state electron transfer reactions often play key roles in multistep schemes for the conversion of light to chemical energy ( 1). 

The complexes react with oxygen to form metal oxides (eq. 18). This reaction is likely a radical trapping reaction but may involve excited state electron transfer. 

Meyer, Thomas J., Excited-State Electron Transfer. 30 389... 

These excited-state electron transfer reactions were mainly investigated using time-resolved spectroscopic techniques such as flash photolysis and flash fluorescence. The extensive work on the photochemistry of MLCT excited states is motivated by both the interest in basic science and the potential applications to many areas of chemistry, for example, biochemistry, solar energy, and conducting polymers.130 135... 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

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