Ruthenium photo-induced electron transfer

Molecular engineering of ruthenium complexes that can act as panchromatic CT sensitizers for Ti02-based solar cells presents a challenging task as several requirements have to be fulfilled by the dye, which are very difficult to be met simultaneously. The lowest unoccupied molecular orbitals (LUMOs) and the highest occupied molecular orbitals (HOMOs) have to be maintained at levels where photo-induced electron transfer into the Ti02 conduction band and regeneration... 

PHOTO-INDUCED ELECTRON TRANSFER REACTIONS IN POLYMER-BOUND RUTHENIUM BIPYRIDYL COMPLEXES... 

Figure 11.6 (a) The triplet excitation transfer between the ruthenium(II) donor and the osmium(II) acceptor in [Ru(bpy)3] +-(ph)7-[Os(bpy)3] + (b) photo-induced electron transfer from Ru(II) to Rh(III) across varying spacer lengths. ... 

However, the luminescence measurements show quenching of fluorescence in the trimer, which is attributed to a photo-induced electron transfer from the axial ruthenium(II) porphyrin to the excited state of the basal tin porphyrin. Not only ruthenium(II) porphyrins, but also rhodium(III) porphyrins can easily be incorporated into the arrays with the same strategy [33]. Again, the isonicotinic acid is first reacted with the bis-hydroxy tin porphyrin to give the bis-isonicotinic acid complex. Addition of two equivalents of rhodium(III) porphyrin readily yields the trimeric array of the composition Rh-Sn-Rh. The X-ray structure of this complex, which is shown in Fig. 34c, shows that the ligands on the tin center (carboxylates) are in an off-direction which is close to orthogonal to the porphyrin plane, and the three porphyrins adopt a near coplanar arrangement. The tin porphyrin is tilted by about 8.6° with respect to the rhodium(III) porphyrins. 

Sabbatini and Balzani (1985) have reported a study of the photo-induced electron-transfer processes involving (2,2,1) Eu cryptates. Both Eu(II) and Eu(III) cryptates are suitable as redox quenchers of excited-state molecules, e.g., ruthenium complexes ... 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium(II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polyproline chain to a [Co(NH3)5] + or [(-NH-py)Ru (NH3)5] acceptor. Chains composed of from zero to six cis-prolines have been examined. The apparent distance dependence of the electron-transfer rate constant, corrected for variations in the solvent reorganizational energy, seems to exhibit two types of distance dependence, 0.7-1A for short chains and /3 a0.3 A for long chains. A very detailed theoretical analysis of electron transfer in the complexes with four proline linkers has indicated that the electronic coupling is sensitive to conformational variations within the proline chain. ... 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium (II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polypro-... 

Gratzel and coworkers have used a ruthenium (II) complex linked to Ti02 by means of a Ais-carboxylato-bipyridine ligand to construct an efficient photocell.Many details of the functioning of this photocell are not well understood, but it is based on efficient photo-induced injection of electrons, from the MLCT excited state, into the semiconductor. Femtosecond pump and probe experiments have demonstrated that at least some of the photoelectrons are injected within 50 ps of the MLCT excitation.This is one of the fastest electron-transfer rates yet measured it apparently occurs in competition with vibrational relaxation of the excited state. 

Molecular dyads of ruthenium(ii)- or osmium(ii)-bis(terpyridine) chromophores and expanded pyridinium acceptors have been used to demonstrate the effect of the bridge and the metal ions to the photophysical properties of linear systems. In particular, via ultrafast transient absorption spectroscopy, an equilibration between MLCT and photo-induced charge-separated excited states has been observed demonstrating that intramolecular photoinduced electron transfers can occur within multicomponent systems in spite of driving forces virtually approaching zero. ... 

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