Chromophores sensitizer

Indirect photolysis is most important for compounds that absorb little or no sunlight. Light absorption by chromophores (sensitizers) other than the compound of interest begin the... 

The use of lanthanides are common for optical purposes because of their narrow and sharp bands, and distinguishable long lifetimes, accomparied by low transition probabilities due to the forbidden nature of the transitions [10-13]. Thus chromophoric sensitization of ligand to metal has been subjected to numerous theoretical and experimental investigations [14—16]. However, only limited classes of organic-lanthanide complexes have been developed and shown to display nonlinear processes [17-19]. Common nonlinear processes from lanthanide complexes include harmonic generation, photon up-conversion and multiphoton absorption induced emission. 

Chromophores (Ch) are transformed after absorption of the actinic solar radiation in excited singlet ( Ch ) and triplet (3Ch ) states (Rabek, 1996) (Eq. 3-11). Excited chromophores sensitize the formation of macroalkyls from the matrix polymer (Eq. 3-12a) and singlet oxygen from the ground state oxygen (Eq. 3-12b) and accelerate homolysis of POOH via an exciplex (Eq. 3-12c), Reaction scheme 3-3. 

A number of interesting inorganic-organic dyads have been recently investigated that, rather than chromophore-quencher, could be called chromophore-sensitizer ... 

Absorption spectroscopy Indicated no strong coupling between the chromophores. Sensitization, determination of quantum yield of fluorescence (Op = 0.2), in comparison with the hydrogenated model show that the reaction occurs with high efficiency (Oj- O- ) from the first triplet excited state only. 

Luminescence of organic chromophores sensitized by energy transfer from singlet oxygen has also been reported for many years. " Several mechanisms have been suggested for this luminescence. 

This result could be obtained through the relatively high density of photons resulting from the macromolecular structure which combines eight Ln cations and thirty-two chromophoric sensitizers. ... 

In the case of chromophore sensitized emission, the efficiency of energy transfer to Loa will depend upon the separation between the lanthanide and the chromophore and upon the spectral overlap between the chromophore donor state and the... 

Rylenes XXV represent a further interesting class of NIR-dyes [YUA 13,SCH 08]. The substituents Rl and R2 influence the solubility of these neutral compounds. Nevertheless, these chromophores sensitively change their absorption spectra upon change of the surrounding, which was discussed as a result of H-aggregation [YUA 13]. 

The vast majority of single-molecule optical experiments employ one-photon excited spontaneous fluorescence as the spectroscopic observable because of its relative simplicity and inlierently high sensitivity. Many molecules fluoresce with quantum yields near unity, and spontaneous fluorescence lifetimes for chromophores with large oscillator strengths are a few nanoseconds, implying that with a sufficiently intense excitation source a single... 

Sensitivity levels more typical of kinetic studies are of the order of lO molecules cm . A schematic diagram of an apparatus for kinetic LIF measurements is shown in figure C3.I.8. A limitation of this approach is that only relative concentrations are easily measured, in contrast to absorjDtion measurements, which yield absolute concentrations. Another important limitation is that not all molecules have measurable fluorescence, as radiationless transitions can be the dominant decay route for electronic excitation in polyatomic molecules. However, the latter situation can also be an advantage in complex molecules, such as proteins, where a lack of background fluorescence allow s the selective introduction of fluorescent chromophores as probes for kinetic studies. (Tryptophan is the only strongly fluorescent amino acid naturally present in proteins, for instance.)... 

In view of the chromophoric character of the elemental iodine itself, many colorimetric methods have been proposed for the deterrnination of inorganic iodine (88—92). These methods use the visible portion of the spectmm in reading iodine concentrations. In the visible range the extinction coefficient for iodine is not high enough to be used for minute quantities of iodine in water and other solvents (93). Higher sensitivities have been reported for elemental iodine in potassium iodide solutions in the ultraviolet (93,94). 

Recently photorefractivity in photoconductive polymers has been demonstrated (92—94). The second-order nonlinearity is obtained by poling the polymer doped with a nonlinear chromophore. Such a polymer may or may not be a good photoconductor. Usually sensitizers have to be added to enhance the charge-generation efficiency. The sensitizer function of fuUerene in a photorefractive polymer has been demonstrated (93). 

Many spectral-sensitizing dyes can be classified according to molecular stmctures (228). The stmctural part of a dye molecule that enables the molecule to absorb visible or infrared radiation is called a chromophore. The resonance stmcture for three common chromophores is shown. 

Spectroscopic methods such as uv and fluorescence have rehed on the polyene chromophore of vitamin A as a basis for analysis. Indirectly, the classical Carr-Price colorimetric test also exploits this feature and measures the amount of a transient blue complex at 620 nm which is formed when vitamin A is dehydrated in the presence of Lewis acids. For uv measurements of retinol, retinyl acetate, and retinyl palmitate, analysis is done at 325 nm. More sensitive measurements can be obtained by fluorescence. Excitation is done at 325 nm and emission at 470 nm. Although useful, all of these methods suffer from the fact that the method is not specific and any compound which has spectral characteristics similar to vitamin A will assay like the vitamin... 

The color and constitution of cyanine dyes may be understood through detailed consideration of their component parts, ie, chromophoric systems, terminal groups, and solvent sensitivity of the dyes. Resonance theories have been developed to accommodate significant trends very successfully. For an experienced dye chemist, these are useful in the design of dyes with a specified color, band shape, or solvent sensitivity. More recendy, quantitative values for reversible oxidation—reduction potentials have allowed more complete correlation of these dye properties with organic substituent constants. 

More recent research provides reversible oxidation-reduction potential data (17). These allow the derivation of better stmcture-activity relationships in both photographic sensitization and other systems where electron-transfer sensitizers are important (see Dyes, sensitizing). Data for an extensive series of cyanine dyes are pubflshed, as obtained by second harmonic a-c voltammetry (17). A recent "quantitative stmcture-activity relationship" (QSAR) (34) shows that Brooker deviations for the heterocycHc nuclei (discussed above) can provide estimates of the oxidation potentials within 0.05 V. An oxidation potential plus a dye s absorption energy provide reduction potential estimates. Different regression equations were used for dyes with one-, three-, five-methine carbons in the chromophore. Also noted in Ref. 34 are previous correlations relating Brooker deviations for many heterocycHc nuclei to the piC (for protonation/decolorization) for carbocyanine dyes the piC is thus inversely related to oxidation potential values. 

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