Dyes, excited-state reduction potentials

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. 

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... 

The efficiency of charge injection depends on the absorption spectra of the dye, quantum yield for the formation of the excited state of interest and excited state redox potentials of the dye appropriate with respect to location of band edges. That is, the excited state of the dye should be a strong reductant to have its potential more negative (cathodic) with respect to the conduction band edge [EO(S /S" )] < Ecb(SC)]. 

We can conclude from these thermodynamic considerations that it is possible to estimate the redox potentials of excited molecules, if we know the equilibrium redox potentials for the molecules in the ground state, as well for reduction as for oxidation, and add or subtract from these redox potentials the excitation energy AE of the lowest singlet or triplet state. For most dye molecules the reduction redox potential is experimentally more easily accessible than the oxidation redox potential. In such cases we have found that an estimation can be made by assuming that the ionisation energy of the dye molecule in crystalline state is similar to the ionisation energy in a polar solvent and gives an approximate value for the absolute redox potential. Such estimations are especially useful for a comparison of molecules with similar structure. 

Ultrafast injection of carriers into the UO2 substrate suggests that overall cell efficiency is not limited by this process but by intervening transfer mechanisms (e.g., trapped state populations reducing quantum yield) or longer time scale electron-dye recombination rates (typically taking microseconds). For example, it was found that the absorbed photon to current efficiency (APCE) is considerably reduced for V compared to IV under identical cell conditions (27,55). While V has a 350 mV lower electrochemical reduction potential than IV (and hence a red-shifted absorption spectrum), it is unclear why the redder absorbing dye (V) does not inject as efficiently (56). Recent visible excitation with broadband... 

Such compounds as polynuclear aromatics, heteroaromatics, ketones, quinones and dyes can serve as donors. Both excited singlet and triplet-states of these products can be involved in the PET. Diaryliodonium salts, triarylsulfonium salts, phosphonium salts, ammonium salts, pyrylium and thiapyrylium salts possess enough thermal stability and corresponding reduction potential to function as electron acceptors (R X+). In order to select suitable photoinitiator systems based on compounds discussed, the Weller-Eq. (5) can be employed. 

The formal reduction potentials, E°, for a sensitizer, S, that are most relevant to dye sensitization correspond to the ground, reduced and excited states. The first two can be directly measured by electrochemical techniques, such as cyclic voltammetry, and often in situ at the sensitized electrode of interest [7]. The excited state reduc-... 

The intense colors in 2,2 bipyridyl complexes of iron, ruthenium, and osmium are due to excitation of an electron from metal t2g orbitals to the empty jr -orbitals of the conjugated 2,2 bipyridyl. The photoexcitation of this MLCT excited state can lead to emission. However, not all complexes are luminescent because of the different competing deactivation pathways. This aspect is beyond the scope of this chapter the interested reader can refer to a number of publications on this subject [16-20]. The other potential deactivation pathways for the excited dye are donation of an electron (called oxidative quenching, Eq. 2) or the capture of an electron (reductive quenching, Eq. 3) or transfer of its energy to other molecules or... 

A theoretical study of the excited states of stilbene and stilbenoid donor-acceptor dye systems as potential laser was performed [4]. Semiempirical calculations within the CNDO/S framework were used to characterize the nature of the phantom-singlet excited state P (double-bond twisted geometry) of stilbene and stilbenoid donor-acceptor dye systems including 4-(dimethylamino)styrylpyridylmethylium and DCM laser dyes. It was shown that for stilbene, a slight geometric symmetry reduction is... 

The mechanism of spectral sensitisation has been shown to involve the direct transfer of an electron from the excited state of the dye into the AgX conduction band (see Fig. 11.8). In confirmation of this mechanism, excellent correlation has been demonstrated between the sensitisation capability of dyes and their electrochemical reduction potential. For the most efficient dyes, the quantum yield of the electron transfer step is close to 1.0. Using picosecond laser pulse excitation to measure the fluorescence decay rates of dyes adsorbed to AgX crystals, it has been shown that the electron transfer is very fast, with rate constants in the region of 10 s having been reported [24]. Once an electron has been transferred into the AgX conduction band, the latent image formation process ensues more or less as it does following direct absorption by AgX itself. The difference in the case of the dye-sensitised process is that the positive holes are trapped at the dye molecules. 

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