Excited state oxidative

Based on extensive screening of hundreds of ruthenium complexes, it was discovered that the sensitizer s excited state oxidation potential should be negative of at least —0.9 V vs. SCE, in order to inject electrons efficiently into the Ti02 conduction band. The ground state oxidation potential should be about 0.5 V vs. SCE, in order to be regenerated rapidly via electron donation from the electrolyte (iodide/triiodide redox system) or a hole conductor. A significant decrease in electron injection efficiencies will occur if the excited and ground state redox potentials are lower than these values. 

The adsorbed sensitizers in the excited state inject an electron into the conduction band of the semiconductor substrate, provided that the excited state oxidation potential is above that of the conduction band. The excitation of the sensitizer involves transfer of an electron from the metal t2g orbital to the 7r orbital of the ligand, and the photo-excited sensitizer can inject an electron from a singlet or a triplet electronically excited state, or from a vibrationally hot excited state. The electrochemical and photophysical properties of both the ground and the excited states of the dye play an important role in the CT dynamics at the semiconductor interface. 

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 ... 

Turning to chemically unsymmetrical systems, recall that the situation is complicated by the appearance in the equations of the internal energy difference between the ground and excited state, oxidation state isomers, (eq 9). 

An interesting use of Re(bpy)(CO)3CN as an excited state oxidant of fluo-rotyrosines (substituted at a number of positions) was published emphasizing the driving force dependence of the reaction using a Marcus analysis (see... 

Although direct excited-state electron transfer from 2PA dyes to monomer is successful for polymerizing acrylates and depositing silver, few other materials can be patterned in the same way for effective initiation, the reduction potential for the monomer, V2(M/M- ), needs to be greater, i.e. less negative, than the excited-state oxidation potential for the initiator, E /2(M+/M ), which can be estimated from... 

Both, strained and unsaturated organic molecules are known to form cation radicals as a result of electron transfer to photoexdted sensitizers (excited-state oxidants). The resulting cation radical-anion radical pairs can undergo a variety of reactions, including back electron transfer, nucleophilic attack on to the cation radical, electrophilic attack on the anion radical, reduction of anion radical, and addition of anion radical to the cation radical. This concept has been nicely demonstrated by Gassman et al. [103, 104], using the photoinduced electron-transfer cydization of y,8-unsatu-rated carboxylic add 232 to y-ladones 233 and 234 as an example (see Scheme 8.65). 

Among the isocyanide complexes, Ru(bpy) (CNMe)42+ stands out as a remarkable Ru(II) polypyridine photosensitizer in view of (i) its long lifetime in fluid solution, and (ii) its very high excited-state oxidizing power ( e1/2 red v vs SCE)... 

Although only rarely luminescent in ambient fluid solutions, square-planar transition metal bis(dithiolene) complexes do display significant and varied photochemical reactivity. Much of the photoreactivity described above for dianionic bis(dithiolene) complexes involves excited-state oxidation and often leads to radical formation. In addition, the excited states of these complexes are receiving attention for their potential as materials for optical (15), nonlinear optical (10-13), and electrooptical (16) devices. The relevance of this work to those applications is addressed in other parts of chapter 8 in this volume (87b). 

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(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. 

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-... 

Polypyridine complexes containing a phen ligand or its derivatives (4,7-Ph2-phen), dppz, tap, hat or other extended polypyridines or polyazines intercalate into DNA strands at specific sites. If the DNA-bound polypyridine complex is a sufficiently strong excited-state oxidant, a photoinduced guanine oxidation takes place with a concomitant DNA strand cleavage. This reaction has indeed been observed for [Ru(tap)3] + or [Ru(hat)3] + and some of their derivatives [355]. Photoexcited [Rh(phi)2(bpy)] + oxidizes DNA by an H-atom abstraction from a sugar moiety. 

Electron injection from MLCT-excited Ru-polypyridine complexes are used to investigate electron transfer along DNA strands, that is to decide whether DNA can behave as a molecular wire [358-360]. In these studies, derivatives of [Ru(phen)2(dppz)] + act as excited-state electron donors and [Rh (phi)2(bpy)] + as a ground-state electron acceptor. Both complexes are anchored at different DNA sites and the rate of Ru —> Rh photoinduced electron transfer is measured. In another study [361], a [Ru (bpy)2(im)(NH2-)] + unit attached to a terminal ribose of a DNA duplex acted as an excited-state oxidant toward a [Ru (NH3)4(py)(NH2-)] " unit attached at the other end. 

The reconstitution of chemically modified heme with apo-Mb has been well established [173-176]. The one-electron reduction potential of ferryl-Mb (Fe -heme) was reported as 0.896 V relative to the NHE [177] which is less positive than the one-electron oxidation potential of [Ru(bpy)3] + (1.25 V). Thus, generation of ferryl-Mb by electron transfer from the met-Mb (ferric state) moiety to the Ru + moiety might be thermodynamically favorable. An appropriate amount of a sacrificial electron acceptor such as [Co(NH)3Cl] + which can quench the Ru + excited state oxidatively was, however, required to produce the Ru + state in competition with the reductive quenching of the Ru + excited state by the met-Mb [178, 179]. No direct electron transfer from the met-Mb (ferric sate) moiety to the Ru + excited state occurs, because this process is thermodynamically disfavored [178, 179]. 

The simple model described by eq. 11.5 should serve as a useful starting point to test non-adiabatic interfacial ET theory. The predicted dependence on electronic coupling, density of semiconductor states, adsorbate potential and reorganisation energy can be tested experimentally. As shown in Fig. 11.6, the calculated injection rate increases the further the adsorbate excited-state oxidation potential lies above the conduction band edge. The variation is slow high above the band edge, but... 

Photoexcitation [Eq. (1)] is followed by an excited state oxidation process in the encounter with a suitable oxidant [Ox, Eq. (3)], which, of course, must not be able to oxidize Cu(I)N4 in the ground state. Such a reaction competes with luminescence [Eq. (2)], which is therefore a useful tool to evaluate whether reaction 3 takes place. The reduction potential of the Cu(II)N4/ Cu(I)N,1 couple can be evaluated by the following equation [35] ... 

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