Electron transfer from MLCT excited states

Importantly, it was found [80-82, 311] that interfacial electron transfer from MLCT-excited Ru polypyridine complexes to Ti02 is an ultrafast process, completed in 25-150 fs This groundbreaking discovery implies that the search for new sensitizers need not to be limited to complexes with long-lived excited states. Indeed, [Fe(4,4 -(COOH)2-bpy)2(CN)2], whose MLCT excited state lifetime is only ca 330 ps, was found [304] to act as a sensitizer in a Ti02-based solar cell. In fact, even the classical Gratzel cell [36, 77, 78] would not operate as well as it does, were the interfacial electron transfer not ultrafast, since the [Ru(4,4 -(COOH)2-bpy)2-(NCS)2] sensitizer has an inherent excited state lifetime of only 50 ns. 

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. 

G.A. Crosby and co-workers finally found that the energies of the emissions of several related complexes correlated with energies of the MLCT absorption bands and not with energies predicted for the LF transitions. They concluded that the emissions from all of these complexes (Table 7) occur from MLCT excited states (164-166). Recently, however, Houten and Watts found that the emission lifetime and quantum yield of Ru(bipy) are strongly affected by deuteration of the solvent while only slightly increased by deuteration of the bound bipyridines (167). They have thus proposed that the emitting excited state of Ru(bipy)3 has some CTTS character, which may be important in electron transfer quenching reactions of this complex (Section III-D-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. 

In this Section, two types of photo-electron transfer processes with the MLCT state of complexes will be successively discussed. We will first introduce the direct photo-electron transfer from a DNA base to the excited complex Sect. 4.3.1. Afterwards we will coiKider the electron transfer between an excited... 

On the basis of the reduction potential of Rh(phen) (Eo = — 0.75 V/SCE) and of its nn energy (2.75 eV), Rh(phen)3 in the nn state is expected to be a very powerful oxidising agent (with a reduction potential of 2.0 V/SCE [133]), making it a stronger oxidant than the MLCT states of the Ru(II) complexes discussed above. Electron transfer from aromatic amines [134] or di-and tri-methoxybenzenes [135] to excited Rh(III) polypyridyl complexes have indeed been observed. 

The reaction mechanism was proposed to involve electron transfer from the d-7i metal to ligand charge transfer excited state (MLCT transition) to the chlorinated solvent,... 

The effect of the FC term on ICT and MLCT-based chemosensors appears when the electron transfer rate constant is generalized within the context of nonradiative decay theory [191-193], MLCT excited states are produced directly upon excitation whereas ICT states are produced by a surface crossing from an initially prepared localized excited state (see Fig. 9). Return of the system from the charge transfer excited state to ground state has the overall form of an electron transfer recombination problem that is described by the inverted Marcus curve of Fig. 13. As described by the FC term of Eq. (5), the rate constant for... 

A variety of spectroscopic methods has been used to determine the nature of the MLCT excited state in the /ac-XRe(CO)3L system. Time-resolved resonance Raman measurements of /ac-XRe(CO)3(bpy) (X = Cl or Br) have provided clear support for the Re -a- n (bpy) assignment of the lowest energy excited state [44], Intense excited-state Raman lines have been observed that are associated with the radical anion of bpy, and the amount of charge transferred from Re to bpy in the lowest energy excited state has been estimated to be 0.84 [45], Fast time-resolved infrared spectroscopy has been used to obtain the vibrational spectrum of the electronically excited states of/ac-ClRe(CO)3(bpy) and the closely related/ac-XRe(CO)3 (4,4 -bpy)2 (X = Cl or Br) complexes. In each... 

Scheme 9 illustrates the sequence of events that occur when these Ru(II)-pyridinium type 1 dyads (16) are photoexcited. Visible light excitation produces the MLCT excited state, 17. Forward ET occurs via transfer of an electron from the bipyridine acceptor ligand to the covalently linked pyridinium acceptor to produce charge separated state 18, which features a d5 Ru(III) ion linked to the reduced pyridinium acceptor. Finally, back ET occurs by transfer of the odd electron from the pyridinium radical to the Ru(III) center. 

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