Dyes, excited-state

Keywords Cyanine dyes Excited state absorption Polymethine dyes Pump-probe Two-photon absorption Z-scan... 

We have investigated the photocurrent behavior of multilayers of a Chi a-DPL (molar ratio 1/1) mixture on platinum in an aqueous electrolyte without added redox agents (80). Cathodic photocurrents with quantum efficiencies in the order of 10- were obtained with films consisting of a sufficient number of monolayers. The photocurrent was increased in acidic solutions. However, no appreciable photocurrent was observed with a single monolayer coated on platinum. The latter fact most probably results from minimal rectifying property of the metal surface and/or an efficient energy quenching of dye excited states by free electrons in... 

Photophysical energy transfer between the initially populated dye excited state (which is itself incapable of initiation) and other chromophores which, if populated, would yield free radical initiating species, is generally disfavored energetically. Nonclassical endothermic energy transfer processes are required. Alternatively, other processes which avoid typical energy transfer restrictions must be devised. Exciplex mediated electron transfer processes represent one such alternative. 

Other carboxylate-dye interactions have been reported. Ethylenediamine tetracarboxylic acid (EDTA) and its salts are well known reductants for a variety of dyes (54,55). The amino-acid N-phenylglycine can be photooxidized and induce polymer formation (26,56,57). Studies of the efficiency of photopolymerization of acrylate monomers by MB/N-phenylglycine combinations as a function of the pH of the medium suggest that either the amino group or the free carboxylate can act as an electron donor for the dye excited state, but that the amine functional-lity is the more efficient coinitiator (10). Davidson and coworkers (58) have shown that ketocarboxylic acids are photode-carboxylated by electron transfer quenching of dye triplet states under anaerobic conditions. Superoxide formation can occur when oxygen is present. 

Aryldiazosulfones of general structure Ar-N=N-S02Ar were shown to interact with excited dyes to yield aryldlazonium ions and arylsulfinate ions via a sensitized dissociation (9b) as shown in eq. 31. The sulfinate product can interact with another dye excited state to produce a sulfinyl radical capable of initiation in the conventional way (43.44). 

For some dyes the diazonium product may also act as a source of radical via photooxidation of the dye excited state (see Section III). 

Just as the oxidizing power of a dye increases on excitation, so does the reducing power. However, examples of the use of dye excited states to initiate photopolymerization via dye photooxidation are less common than photoreducible dye sensitization. 

Early experiments by a number of workers demonstrated that dye excited states could function as electron donors (69,92-94). Lindquist (69) showed that the triplet state of fluorescein was quenched by oxygen in basic media with the formation of superoxide anion, by ferric ion in acid media to form ferrous ion, and by peroxydisulfate ions in alkaline solutions. The photo-... 

Macrae and Wright (96) demonstrated that visible light irradiation of xanthene dyes (eosin, erythrosin, rhodamine B, or RB) in ethanolic solutions of 4-(N,N-diethylamino)benzene-diazonium chloride (as the zinc chloride double salt) resulted in decomposition of the diazonium salt. Electron transfer from the dye excited state(s) to the diazonium salt was postulated and dye-diazonium salt ion pair formation in the ground state was shown to be important. Similar dyes and diazonium salts were claimed by Cerwonka (97) in a photopolymerization process in which vinyl monomers (vinylpyrrolidone, bis(acrylamide)) were crosslinked by visible light. Initiation occurs by the sequence of reactions in eqs. 40-42 ... 

Alternatively, charge injection into the semiconductor can involve the reductive or oxidative quenching of the dye excited state by a redox-active species (a supersensitizer (S)), followed by thermal interfacial electron transfer ... 

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. 

Fulfillment of the Marcus equation requirements for the primary process, e.g., for the process of the quenching of the dye excited state. 

Charge injection is slow enough compared with the vibrational relaxation of the dye excited state (k/ kj). In this event, electron transfer would be able to take place only from the lowest excited state v — 0) (Eq. (35)), and the injection quantum yield would be simply controlled by the kinetic competition between the electron injection (Eq. (35)) and the decay of the excited state (Eq. (36)) ... 

Earlier studies on dye-sensitized Ti02 reported nanosecond time constants for the injection kinetics [16, 40-42]. These results were obtained indirectly from the measurement of the injection quantum yield and implicitly assumed that the interfacial electron transfer reaction was competing only with the decay of the dye excited state. Other studies were based on the same assumption but used measurements of the dye fluorescence lifetime, which provided picosecond-femtosecond time resolution [43-45]. Direct time-resolved observation of the buildup of the optical absorption due to the oxidized dye species S+ has been employed in more recent studies [46-51]. This appears to be a more reliable way of monitoring the charge injection process as it does not require any initial assumption on the sensitizing mechanism. 

The fastest kinetic phase of electron injection in c/j-[Ru (dcbpy)2(NCS)2]-sensitized nanocrystalline titanium dioxide films apparently takes place in the femtosecond regime. Besides, the vibrational relaxation of the dye excited state is expected to occur typically within 0.4-1 ps k 10 s ) [57, 58]. Observed injec-... 

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.

Compound Id behaves similarly, but the overall conversion efficiency is significantly lower, i. e. 0.022%. As a result, although no itt-conjugation between the perylene dye and the tin dioxide nanoparticles exists, electron transfer actually occurs, probably through a bridge-assisted mechanism as previously proposed for dye-excited states weakly coupled to semi-conducting particles. o On the basis of this mechanism, the lower cell efficiency found for Id-modified electrodes could be due to the longer hexylene linker in... 

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.

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