Dyes, excited-state oxidation potentials

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. 

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... 

Pastore M, Fantacci S, De Angelis F (2010) Ab Initio determination of ground and excited state oxidation potentials of organic chromophores for dye-sensitized solar cells. J Phys ChemC 114(51) 22742-22750... 

Examples of striking differences in redox potentials between ground and first triplet states are observed in the oxidizing powers of methylene blue and thionine. In the ground state, the dyes cannot oxidize Fe2+ to Fe +. But when excited in the presence of Fe +, the colour of the dye is bleached. The colour returns on removal of the excitation source. In the long lived triplet state, oxidation potential of the dye increases and the ferrous iron is oxidized to ferric state, the dye itself being reduced to the leuco form. 

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. 

Figure 15. Energetics of the charge recombination following electron injection (/ i) from a dye excited state S into the conduction band of a semiconductor. Thermalization and/or trapping of injected electrons (Mh) takes place prior to the interfacial electron back transfer to the dye oxidized state S (/cb). The reaction free energy associated to the latter process depends upon the population of the electronic states in the solid and can be distributed over a broad range of values. Numerical potential data shown are those of the c/s-[Ru (dcbpy)2(NCS)2] Ti02 system.

Fig. 2 Potential energy diagram of DSSC. For net forward electron transfer the oxidation potential of the dye-excited state, D /D+ should be higher than the semiconductor conduction band, and the redox potential of the hole conductor should be higher than the dye-ground state potential D/D+. Absorption of light by the dye causes charge separation across the interface, resulting in a splitting of the electrochemical potential and hence a photovoltage.

The semiconductor nanocrystallites work as electron acceptors from the photoexcited dye molecules, and the electron transfer as sensitization is influenced by electrostatic and chemical interactions between semiconductor surface and adsorbed dye molecules, e.g., correlation between oxidation potential of excited state of the adsorbed dye and potential of the conduction band level of the semiconductor, energetic and geometric overlapping integral between LUMO of dye molecule and the density of state distribution of the conduction band of semiconductor, and geometrical and molecular orbital change of the dye on the... 

The electron injection from the excited states of the dyes to the conduction band of the electrode is predominately affected by the oxidation potential of the dye molecules as well the distribution of the LUMO over the whole molecule. Tuning the redox potentials and the LUMO distribution by functionalizing the aromatic ring asymmetrically with different groups at different positions seems a promising way towards a better sensitizer. 

The response of oxide semiconductor photoelectrodes (as anodes) to solar radiation can be enhanced by chemisorbed dyes. The dye must be selected such that its ground state red-ox potential lies within the band gap of the semiconductor while that of the excited state lies above the conduction band edge (see Gratzel,... 

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