Potential excited-state reduction

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... 

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. ... 

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-... 

Table 5. Structures, singlet energies (E ), reduction potentials (Erdn), and excited state reduction potentials (E rdn) of cationic organic acceptors .

Clark and Sutin studied the photosensitization of rutile Ti02 single crystals in dilute aqueous solution by Ru(bpy)3 + and two closely related Ru sensitizers that have different excited state reduction potentials, Ru[4,7-(CH3)2-phen]3 +, where 4,7-(CH3)2-phen is 4,4 -dimethyl-l,10-phenanthroline, and Ru(5-Cl-phen)3 + [63]. 

The excited-state reduction potential, °( Cr3+/Cr2+), can be estimated using an analysis similar to Hess s law of heat summation (Fig. 8.5). Using the emission maximum (730 nm) in the luminescence spectrum and converting units yields an excited-state energy of 164 kJ mol-1 for [Cr(phen)3]3+. That means that relaxation of the 2E excited state to the ground state involves AG° = 164 kJ mol-1 or a one-electron electro-... 

Series of reactions used to estimate the excited-state reduction potential, 0( Cr3+/Cr)2+. 

MLCT Excited States on Ti02 Experimental studies of MLCT states on Ti02 (and other semiconductors) are few mainly because of rapid interfacial charge separation that shortens their lifetimes considerably. Some aspects of MLCT excited states anchored to nanocrystalline Ti02 thin films are now becoming available through studies where the semiconductor acceptor states lie above (toward the vacuum level) the excited-state reduction potential of the sensitizer such that excited-state electron transfer from the thexi state is unfavorable. 

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