Excited state reductive

The estimated excited-state reduction potential E° (Au " ) of 2.2 V (versus NHE) suggests that the excited state of [Au(C N N-dpp)Cl] is a powerful oxidant. 

The diphenylphenanthroline complex 23 is emissive in fluid solutions, with quantum yields of ca. 10-4 and lifetimes of 0.4-0.7 ps. The estimated excited state reduction potential of 2.2 V (vs. normal hydrogen electrode (NHE)) suggested that the complex is a strong photooxidant, which was demonstrated with the formation of the 1,4-dimethoxybenzene radical cation (DMB+) upon UV-visible irradiation of an MeCN solution of the complex with DMB.22... 

A similar line of reasoning can be adopted to discuss excited state reduction. 

In order for injection of an electron from the excited state of the dye species into the conduction band of a semiconductor (as described by Equation (2.39)) to occur, the oxidation potential of the dye excited state (A+ / A ) must be more negative than the conduction band potential of the semiconductor. Conversely, photoinduced hole injection from the excited dye into the semiconductor valence band (Equation (2.40)) requires the excited-state reduction potential of the sensitizer (A /A-) to be more positive than the valence band potential. 

V, while the excited-state oxidation potential (Pt+/ ) could be varied from — 1.60 to —1.17 V. Consistent with the assignment of the excited state in Pt(diimine)(dithiolate) complexes as 3[Pt(excited-state oxidation potential, whereas variation of the dithiolate influenced (Pt / ) most markedly. Parenthetically, Base and Grinstaff (110) reported that the related complex Pt(dpphen)(l,2-dithiolato-l,2-dicarba-Goso-dodecaborane) is a strong excited-state oxidant, on the basis of a 1,09-V excited-state reduction potential estimated as in Fig. 4 from spectroscopic and electrochemical data. 

Similar to other d -d systems, the drnuclear iridium(I) complex [Ir(/x-pz)(COD)]2 (23) showed spin-allowed and spin-forbidden (da — pa) absorption bands at 498 and 585 nm, respectively. Under ambient conditions, the complex displayed fluorescence at 564 nm and phosphorescence at 687 nm, which were assigned to singlet and triplet excited states of (da — pa) character. The triplet excited state of the complex was a powerful reductant with an excited-state reduction potential E° (Ir2+ ) of-1.81 V vs. SSCE. Facile electron transfer reactions occurred between the excited complex and methyl viologen and other pyridinium acceptors. The absence of an inverted effect for the forward electron transfer reactions, and the presence of such inverted behavior for the back-electron-transfer reactions were observed and explained. ... 

Our initial interest in these systems was stimulated by observations of their photochemical electron-transfer reactivity (6.12). From spectroscopic and electrochemical studies, the 3(da pa) excited state is predicted to be a powerful reductant, with E (M2 /3M2 ) estimated to range from -0.8 to -2.0 V vs SSCE in CH3CN. That this state is a powerful reductant has been confirmed by investigation of the electron-transfer quenching of 3M2 by a series of pyridinium acceptors with varying reduction potentials ( X For several binudear complexes, the excited-state reduction potenfial cannot be calculated accurately due to the irreversibility of the ground-state electrochemistry but it can be estimated from bimolecular electron-transfer quenching experiments. 

For systems that are powerful excited-state reductants, photoreduction of alkyl halides is observed (6.16). This reaction was initially interpreted to be an outer-sphere electron transfer to form the radical anion, which rapidly decomposes to yield R- and X . Subsequent thermal reactions yield the observed products, an SrnI mechanism (Figure 3a). While such a mechanism, SrnI, appears plausible for a metal complex with E°(M2 /3M2 ) < -1.5 V (SSCE), it seems unlikely for complexes with E°(M2 /3M2 ) > -1.0 V (SSCE). Reduction potentials for alkyl halides of interest are generally more negative than -1.5 V (SSCE) (1/7). Alkyl halide photoreduction is observed for binudear d complexes whose excited-state reduction potentials are more positive than -1.0 V (SSCE) in CH3CN. 

Appropriate modification of the ESR spectrometer and generation of free radicals by flash photolysis enables time-resolved (TR) ESR spectroscopy [22]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They can be enhanced as much as one-hundredfold and appear as absorption, emission, or a combination of both. Effects of this type are a result of chemically induced dynamic electron polarization (CIDEP) these spectra indicate the intermediacy of radicals whose sublevel populations deviate substantially from equilibrium populations. Significantly, the splitting pattern characteristic of the spin-density distribution of the intermediate remains unaffected thus, the CIDEP enhancement not only facilitates the detection of short-lived radicals at low concentrations, but also aids their identification. Time-resolved ESR techniques cannot be expected to be of much use for electron-transfer reactions from alkanes, because their oxidation potentials are prohibitively high. Even branched alkanes have oxidation potentials well above the excited-state reduction potential of typical photo-... 

The parent cyclopropane system does not, in fact, readily undergo electron transfer in solution apparently, the excited state reduction potentials of most sensitizers are too low (Table 3). However, introducing simple alkyl substituents increases the donor capacity of the cyclopropane system. This is aptly shown by the (gas-phase) ionization potential of 1,1-dimethylcyclopropane (9.0 eV) compared with that of cyclopropane (9.87 eV). PET from a series of methyl-substituted cyclopropanes to photoexcited chloranil was probed in solution. These experiments failed to provide evidence for electron transfer from cis- or rrans-1,2-dimethylcyclopropane. On the other hand, 1,1,2-trimethyl- and 1,1,2,2-tetramethylcyclopropane were oxidized [108, 109]. 

Table 3. Excited state reduction potentials of selected electron acceptors.

Now, the excited state oxidation potential is related to the potential of the ground-state ligand-localized redox couple and the excited state reduction potential is related to the potential of the ground-state metal-localized redox couple, see Figure 5. These relations are very logical since oxidation of an MLCT-excited polypyridine complex actually amounts to oxidation of the reduced polypyridine ligand N,N . Similarly, reduction of an MLCT-excited polypyridine complex corresponds to re-... 

When connected to other components of a supramolecular assembly or to a semiconductor electrode through a polypyridine ligand, a metal-polypyridine unit is kinetically especially suited to inject an electron from its MLCT excited state, that is to act as an excited state reductant. Ultrafast rates can be reached. 

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