Excited state redox potentials

The excited state redox potential of a sensitizer plays an important process. An approximate value of the excited state redox potential potentials of the ground state couples and the zero-zero excitation Equations (13) and (14). The zero-zero energy can be obtained from of the sensitizer 38 role in the electron transfer can be extracted from the energy (E0 0) according to the 77 K emission spectrum... 

Cyclic voltammetry is an excellent tool to explore electrochemical reactions and to extract thermodynamic as well as kinetic information. Cyclic voltammetric data of complexes in solution show waves corresponding to successive oxidation and reduction processes. In the localized orbital approximation of ruthenium(II) polypyridyl complexes, these processes are viewed as MC and LC, respectively. Electrochemical and luminescence data are useful for calculating excited state redox potentials of sensitizers, an important piece of information from the point of view of determining whether charge injection into Ti02 is favorable. 

From the discussion of the Marcus theory above and equations (20) and (21), we see that the experimental data needed to judge the feasibility of ET steps involving spin traps and spin adducts are the redox potentials and A values of the ST +/ST and ST/ST - couples, as well as those for hydroxylamine derivatives related to the operation of reactions (4) or (5). The electroactivity of the spin adducts themselves is also of interest since it must somehow be related to their lifetimes in a redox-active environment. Moreover, the excited-state redox potentials (of ST /ST and ST,+/ST ) are also necessary for the understanding of photo-ET processes of spin traps. 

The luminescent properties of [Os (N)(NH3)4]X3 (X = C1, CF3S03) and (Ph4As)2[Os -(N)(CN)5] have also been reported (Table 3). The complexes are emissive and have long excited state lifetimes both in the solid state and in fluid solutions at room temperature (Aen,= 550nm in MeCN). [Os(N)(NH3)4]Cl3 is a powerful one-electron oxidant in the excited state, with an excited state redox potential of 2.1 V vs. NHE. The emitting state has been suggested to be d y) dj f] in origin. 

Excited state redox potentials may be different from those of the ground state values. Redox reactions can be initiated on electronic excitation against the electrochemical gradients. Electron transfer in the excited state may be reversed in the ground state. An important example is photosynthesis in plants. 

Figure 2.15 Schematic orbital diagram illustrating the relationship between ionization potential (IP) and electron affinity (EA) for the ground and excited states of a molecule and the corresponding ground- and excited-states redox potentials...

Figure 5. Simplified thermochemical analysis to estimate excited-state redox potentials.

Based on the above analysis validating Fig. 4 for estimating excited-state redox potentials, values of YPt / ) and (Pt+/ t ) were estimated for both series of Pt(diimine)(dithiolate) complexes studied by Cummings and Eisenberg (109) using electrochemical data and emission energy maxima at 77 K to estimate The results are summarized in Table III. It was found that ligand variation... 

Combining Eqs 12, 13, and 17 yields new expressions for excited state redox potentials [28] ... 

Figure 5. Relationship between ground- and excited state redox potentials. The upper diagram is valid generally, regardless of the excited state character. The lower diagram is applicable only to predominantly localized MLCT excited states of those complexes whose first oxidation and reduction are predominantly metal- and ligand-localized, respectively. See Figure 2 and Eqs 12, 13, 18, and 19.

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