Excited sensitizers

Energy Transfer. In addition to either emitting a photon or decaying nonradiatively to the ground state, an excited sensitizer ion may also transfer energy to another center either radiatively or nonradiatively, as illustrated in Figure 4. 

A principal appHcation for photomedicine is the photodynamic treatment of cancer. Photochemical and clinical aspects of this topic have been reviewed (10,11). Direct irradiation of tumors coupled with adininistration of a sensitizer is used to effect necrosis of the malignancy. In this process, an excited state sensitizer interacts with dissolved in vivo to effect conversion of the oxygen from its triplet ground state to an excited singlet state, which is highly cytotoxic. In principle, excited sensitizers in either the singlet or the triplet state can effect this conversion of molecular oxygen (8). In... 

Another common loss process results from electron—hole recombination. In this process, the photoexcited electron in the LUMO falls back into the HOMO rather than transferring into the conduction band. This inefficiency can be mitigated by using supersensitizing molecules which donate an electron to the HOMO of the excited sensitizing dye, thereby precluding electron—hole recombination. In optimally sensitized commercial products, dyes... 

According to the electron-transfer mechanism of spectral sensitization (92,93), the transfer of an electron from the excited sensitizer molecule to the silver haHde and the injection of photoelectrons into the conduction band ate the primary processes. Thus, the lowest vacant level of the sensitizer dye is situated higher than the bottom of the conduction band. The regeneration of the sensitizer is possible by reactions of the positive hole to form radical dications (94). If the highest filled level of the dye is situated below the top of the valence band, desensitization occurs because of hole production. 

S = excited sensitizer, R = chain-transfer agent radical, M = monomer, M = monomer radical, and P = polymer. 

Through energy transfer of an excited sensitizer molecule (S) to either a monomer (M) or foreign molecule (A) resulting in the formation of species capable of initiation (e.g., radical) ... 

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

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. 

The kinetics of three redox processes have been studied for sensitized Ti02 systems where the sensitizers are [Ru(dicarboxy-bpy)2(CN)2], [Ru(dicarboxy-bpy)2(SCN)2], [Os(dicarboxy-bpy)2(CN)2], and [Os(dicar-boxy-bpy)2(SCN)2] (30). The Ru(II) complexes display characteristic excited-state spectra in methanol solution and decay back to the ground state with lifetimes of about 200 ns. For the Os(II) complexes in solution the excited states decay much more rapidly (< 10ns). On the other hand, when these complexes are adsorbed on Ti02 excitation leads to the prompt conversion to the M(III) oxidation state, as indicated by transient visible absorption spectra. These results imply that electron injection from all four of the excited sensitizers into the Ti02 occurs rapidly (< 10 ns). 

A proposed mechanism taking place when an excited sensitizer (HS ) interacts with an energy acceptor can be described by the key energy-transfer steps depicted in the following scheme ... 

Sensitized PET reactions are often very slow and have low quantum yields due to dominating back-electron transfer. In these cases, the addition of cosubtrates (e.g., biphenyl or phenanthrene to DCA- or DCN-sensitized reactions) is useful. The use of such an additive is called cosensitization. In these reactions, the substrate is not oxidized (or reduced) by the excited sensitizer but by the radical ion of the cosensitizer (ET, ). This is a thermal electron-transfer step without the problems of back-electron transfer. The key step is the primary PET process (ETJ in which the cosensitizer radical ion is formed. The main characteristic of cosensitization systems is the high quantum yield of the free-radical ion (e.g., overall quantum yield is high and the reaction is fast (Scheme 7). 

Schenck, the main contributor to the field of photosensitized oxygenation reactions, strongly advocated the view that the interaction between an electronically excited sensitizer and oxygen should result in a (chemical) addition reaction rather than in a (physical) energy-transfer reaction.7,26,60-62... 

Comparison of the behavior of free (mono) radicals with that of electronically excited sensitizers has led to the assumption that the excited sensitizer, Sensrad, is best described as a phototropic-isomeric diradical. Thus, photodimerization and photodehydrogenation, exhibited by certain sensitizers in the absence of oxygen, reflect radical-combination and hydrogen-abstraction reactions. Furthermore, fluorescein (a photosensitizer) becomes paramagnetic when excited... 

In the absence of photodecomposition reactions, the quantum yield for electron injection from the excited sensitizer to the semiconductor is given by Equation 17.11. 

Many compounds sensitize biomolecules to damage by UVA (320-380 nm) and visible light. Two general mechanisms of sensitization are encountered. The Type I mechanism involves electron or hydrogen transfer from the target molecule to the photosensitizer in its triplet state. If 02 is present, this can be reduced to 02 by the reduced sensitizer. In the Type II mechanism, the excited sensitizer is quenched by 02, which is excited to the singlet state (typically A"g) and attacks the target molecule. Photosensitization is exploited in photodynamic therapy (PDT) for the destruction of cancerous or other unwanted cells. 

The inherent electronic nature of semiconductor metal oxides can direcdy interact with molecular excited states in a manner not energetically possible with insulators. More specifically, an excited sensitizer, S, may transfer an electron to the semiconductor forming a charge separated pair [Eq. (1)] ... 

Two main models are usually discussed for the mechanism of the spectral sensitization. The excitation of the sensitizer by absorbed light and electron transfer from the excited sensitizer to the semiconductor is the first model. The alternative mechanism consists of the transfer of the excitation energy from the sensitizer to the semiconductor. This energy is used for photogeneration of the charge carriers in the sensitized photoconductor. In the first case the excited singlet level of the sensitizers has to be located above the conduction band of the semiconductor for realization of the electron transfer. For hole transfer the basic sensitizer level has to be located lower than the valence band of the sensitized photoconductor. The energy transfer mechanism does not need a special mutual location of the semiconductor and sensitizer levels. 

Two main mechanisms were proposed for the supersensitization effect. One is the hole-trapping mechanism in which the electron from SS fills the hole in the highest occupied molecular orbital (HOMO) of the excited sensitizing dye, since the HUMO level of SS is chosen to be higher than that of the sensitizer (Fig. 5) [2,10,11]. The resultant ionic state gives up an electron to the conduction band of silver halide with much higher quantum yield. 

Two types of electron transfer sensitization can be identified. In one the excited sensitizer S functions as an electron acceptor. In the presence of a suitable electron donor D, reduction of S produces the donor cation radical D+, which... 

A novel synthesis of 5,6-dihydro-4//-1,2-oxazines (20) is presented via the photo-induced cyclization of y. d-unsaturated oximes (21) see Scheme 4. Irradiation of (21) in the presence of 9,10-dicyanoanthraccnc (DCA) led to the heterocycle (20) only. The proposed mechanism proceeds via the radical cation (22), generated by single-electron transfer (SET) from the oxime (21) to the excited sensitizer (DCA. Cyclization of (22) affords the oxazine (20) after proton transfer to the DCA radical anion (DCA ) and H abstraction.61... 

Direct excitation of C=C bonds to the T i state can be achieved via transfer of triplet excitation from electronically excited sensitizers. Selected singlet and triplet energies... 

The quenching rate constants (kq) (7 j0 of the excited sensitizer by PVS° and DQS0 in the Si02 colloid are 1.5xl09 and 4xl08 M-1.s-1... 

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