Sensitizers electronically excited, oxidation

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. 

Photo-oxidation of LCV by the Pyrene-Bonded Film. The surface properties and structure of these systems should be related to the sensitizer efficiency of pyrenyl groups. Pyrene is a good sensitizer for the oxidative color formation of LCV. The singlet excited state of Py acts as an electron acceptor bringing about one electron oxidation of LCV+. The unit processes are considered as... 

A more economic way to circumvent the BET pathway is co-sensitiza-tion as illustrated in Sch. 3 [2,15]. In this strategy the substrate is not directly oxidized (or reduced) by the electronically excited sensitizer but by the radical ion of the co-sensitizer (ETb). This thermal electron transfer is not affected by back electron transfer since (i) sensitizer and substrate radical ions are separated from each other (back electron transfer from Sens - to D +) and (ii) a back electron shift from C to D + is thermodynamically unfavourable. As a consequences co-sensitized reactions often proceed with higher efficiencies and different selectivities if compared with simple sensitized PET reactions. 

For this purpose an electron transfer across the bilayer boundary must be accomplished (14). The schematic of our system is presented in Figure 3. In this system an amphiphilic Ru-complex is incorporated Into the membrane wall. An electron donor, EDTA, is entrapped in the inner compartment of the vesicle, and heptylviolo-gen (Hv2+) as electron acceptor is Introduced into the outer phase. Upon illumination an electron transfer process across the vesicle walls is initiated and the reduced acceptor (HVf) is produced. The different steps involved in this overall reaction are presented in Figure 3. The excited sensitizer transfers an electron to HV2+ in the primary event. The oxidized sensitizer thus produced oxidizes a Ru located at the inner surface of the vesicle and thereby the separation of the intermediate photoproducts is assisted (14). The further oxidation of EDTA regenerates the sensitizer and consequently the separation of the reduced species, HVi, from the oxidized product is achieved. In this system the basic principle of a vectorial electron transfer across a membrane is demonstrated. However, the quantum yield for the reaction is rather low (0 4 X 10 ). 

An alternative reductive quenching mechanism for the supersensitizer sensitization of metal oxide semiconductors has been explored in considerable detail by Kirsch-De Mesmaeker and co-workers [78-82]. These researchers found that unlike Ru(bpy)3 +, sensitizers such as Ru(tap)3 + did not inject electrons into the semiconductor from the excited state, yet an anodic photocurrent was observed when an electron donor such as hydroquinone (H2Q) was present in the electrolyte. The sensitization mechanism they proposed is distinctly different from trapping the oxidized sensitizer described above, and is more similar to that proposed for photogalvanic cells. In the presence of H2Q, excited state intermolecular electron transfer yields Ru(tap)3+. The photogenerated Ru(tap)3+ is a strong reductant (-0.76 V vs. SCE) that injects an electron into the Sn02 conduction band which is measured as a photocurrent (Figure 14). 

Figure 17 shows an energetic scheme for the electron transfer processes taking place at the dye-sensitized heterojunction of such a device. Electron injection from the sensitizer s excited state into the conduction band of Ti02 is followed by regeneration of the dye by hole injection into the hole transport material (HTM). Conduction-band electrons in the metal oxide, as well as holes in the organic medium, are then transported by electronic conduction to the anode and the cathode, respectively. Pulsed picosecond laser photolysis has shown that the hole injection from the oxidized dye sensitizer [Ru (dcbpy)2(NCS)2]" into the spiro-MeOTAD... 

Figure 13. Schematic outline of a dye-sensitized photovoltaic cell, showing the electron energy levels in the different phases. The system consists of a semiconducting nanocrystalline Ti02 film onto which a Ru-complex is adsorbed as a dye and a conductive counterelectrode, while the electrolyte contains an I /Ij redox couple. The cell voltage observed under illumination corresponds to the difference, AF, between the quasi-Fermi level of Ti02 and the electrochemical potential of the electrolyte. S, S, and S+ designate, respectively, the sensitizer, the electronically excited sensitizer, and the oxidized sensitizer. See text for details. Adapted from [69], A Flagfeldt and M. Gratzel, Chem Rev. 95, 49 (1995). 1995, American Chemical Society.

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