Sensitizer, photo-excitation

Dye-sensitized photo excitation (eg with TTP, below) Photosensitizer + hv — - Photosensitizer ... 

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. 

Ratiometric luminescent probes make a smart use of the excitation wavelength effect on the emission intensity for extended optosensor performance. For example, the fluorescence from 8-hydroxy-l,3,5-pyrenetrisulfonate (HPTS) and other pH-sensitive dyes in water comes only from its (photo)excited basic form, but the absorption spectra of HPTS and PTS (pAa 7.3) differ considerably (Figure 3). 

HPTS is a highly water-soluble, pH-sensitive dye with a pK of 7.5 in aqueous solution [8], When in alkaline medium, pH > 7.5, acid-base equilibrium is totally displaced toward the anion form (3sPyO ) of the dye. The electronic character of 3sPyO remains unchanged after photo-excitation, and corresponds to a singlet-excited state [9], Fluorescence from this state undergoes a fast 0.4 ps Stokes shift and has a maximum at 515 nm and a lifetime of 5.3 0.1 ns [10],... 

Sensitized photo-oxidation reactions are an example of such photoactivated catalysis (Section 4.5) in which C would be a dye which absorbs light and forms excited oxygen molecules (P) through energy transfer these excited oxygen molecules then lead to various oxidation reactions. There are other important examples of photoactivated catalysis in photoelectrochemistry (section 4.5.1). 

When 14C-benzoin (19) or its methyl ether (20) is used as photosensitizer for polymerizations, more of the sensitizer is incorporated in polymer than can possibly be accounted for by the initiation process. The reactions have the characteristics of mono-radical polymerizations and separate experiments with thermal initiators have shown that transfer to the carbonyl compound is of little importance. It appears that photo-excited states of these compounds, but not the ground states, can engage... 

The details of the operating principles of the dye-sensitized solar cell are given in Fig. 2. The photo excitation of the metal-to-ligand charge transfer (MLCT) of the adsorbed sensitizer (Eq. 1) leads to injection of electrons into the conduction band of the oxide (Eq. 2). The oxidized dye is subsequently reduced by electron donation from an electrolyte containing the iodide/triiodide redox system (Eq. 3). The injected electron flows through the semiconductor network to arrive at the back contact and then through the external load to the counter... 

Moreover, the remarkable sensitization of the photoconductivity of MEH-PPV by C o implies that the initial photo-excitations are mobile charged polarons (see... 

Reiser s and Frazer published an important series of papers beginning in 1965. They were the first to observe the low-temperature UV-Vis spectrum of triplet phenylnitrene 49. A later studyof low-temperature glassy matrices revealed an additional long-wavelength band in the spectrum of 49, reported earlier, showing that it was contaminated by the presence of ketenimine 51. It was found that 49 is extremely light sensitive and, upon photo excitation at 77 K, 49 rapidly isomerizes to the isomeric 51. ... 

More information on the initial attack of the photoexcited sensitizer on the alcohol and about the subsequent behavior of the sensitizer is given by Wells in a comprehensive series of kinetic investigations. It was shown that deactivation of the photo-excited sensitizer proceeds in two ways only, as shown in reactions (9) and (10). 

The views of Bamford and Dewar - on the initiation of the sensitized photo-oxidation are closer to current thought. They envisage the initial reaction, in the absence of moisture, as occiu-ring between the cellulose (/B — H) and the excited dye (D ). 

His investigations on the auto-oxidation of alcohols - have been extended by Wells to various a- and iS-n-aldopyranoses, methyl a- and /3-D-aldopyranosides, and polysaccharides. The author is indebted to Dr. C. F. Wells for making his results available for this article prior to their publication.During the auto-oxidation of alcohols in aqueous solutions photosensitized by sodium 9,10-dihydro-9,10-dioxo-2-anthracene-sulfonate, the energy of the photo-excited, quinonoid sensitizer is degraded by the following two processes. 

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