Photochemical properties of the

However, examination of the spectral properties of the starting ketones and isolated photoproducts indicated that the product distribution was determined by the photochemical properties of the molecules rather than their relative thermodynamic stabilities. These workers proposed that these... 

In Chapter 2 we discussed a number of techniques used to study the various photophysical and photochemical processes occurring in anthracene and similar molecules. In that discussion we were primarily interested in the singlet state. In this chapter we will discuss some of the techniques available for studying the photophysical and photochemical properties of the triplet state. Most of our discussion will be directed to the photochemistry of simple ketones. 

Photodyncimics of metalloporphyrins have been extensively investigated on account of its importance in the understanding of photosynthesis and other processes of biological importance ( ). Particular atten-sion has been paid to the reason why the excited metalloporphyrins possess unique characteristics from the viewpoint of redox (2-4), energy transfer ( ), and other photodynamical processes (6,7). In comparison with the considerable knowledge accumulated on the photochemical properties of the lowest excited states, little has been known on the S2 - Sq fluorescence and Si Sq internal conversion processes which can also be regarded as unusual characters of metalloporphyrins. 

TABLE 3.3 Physicochemical and photochemical properties of the photoactive dyes (adapted from Brovko et at, 2009)... 

The photochemical stabihty of the molecules is characterized by the quantum yield of photodecomposition, (P = N/Q [69], where N and Q are the numbers of decomposed molecifles and absorbed photons, respectively. The photochemical properties of the fluorene derivatives were investigated in different organic solvents (hexane, CH2CI2, ACN, and polyTHF) at room temperature by the absorption and fluorescence methods and comprehensively described [70-72]. These methods are based on measurements of the temporal changes in the steady-state absorption and fluorescence spectra during irradiation. For the absorption method, the quantum yield of the photodecomposition under one-photon excitation, c >ipa, can be obtained by the equation [73] ... 

By using high intensity flash lamps and laser sources, photophysical and photochemical properties of the triplet states can be studied. These sources also help to study emission from upper excited state. 

Within their scaffolds, POPAM dendrimers also possess amine groups which can undergo protonation or coordination with metal ions. The spectroscopic and photochemical properties of the first to the fourth POPAM generation with peripheral fluorescing naphthylsulfonamide groups were compared with those of reference compounds A (N-methylnaphthalenesulfonamide) and B (N-(3-di-methylaminopropyl)-2-naphthalene-l-sulfonamide) [22] (Fig. 5.19). 

The photophysical and photochemical properties of the binuclear [Au2 (dppf)(C=CR)2] (R = Ph and fBu) were studied [82]. The complexes were non-emissive in the solid state even at 77 K, but emission bands at 410 nm in dichloromethane could be observed. The photoreaction between [Au2(dppf)(C=CPh)2] and the dichloromethane solvent was probed, which led to the formation of the organic C-C coupled product, PhC=CC=CPh, and the inorganic [Au2(dppf)Cl2] complex. 

Liquid crystalline polymers and molecular organic crystals are examples of supramolecular arrays in which long-range order plays an important role in determining the photophysical and photochemical properties of the system. Two chapters in the present volume explore the properties of liquid crystalline polymers and molecular crystals, with an emphasis on understanding how the structure of the supramolecular organization guides the course of the photochemical reactivity and photophysical properties of the component molecular units. 

E. Rabinowitch, The photogalvanic effect I. The photochemical properties of the thionine-iron system, J. Chem. Phys., 8, 551-559 (1940). 

McElwee White and coworkers investigated the luminescence and photochemical properties of the molybdenum and... 

Indeed, incorporation of two chromophores in one molecule separated by a certain number of atoms in such a way that there are no Important interactions in the ground state can substantially change the photochemical properties of the respective chromophores. Several processes from very short-lived excited states, which otherwise might have remained uncovered, are observed in such systems. On excitation of one of the chromophores (C) of a nonconjugated blchromophoric system several processes can occur (Scheme 1). 

Electrostatic attraction between charged coordination compounds and oppositely charged counter ions in solution leads to ion pairing. The spectroscopic and photochemical properties of the ion pairs may markedly differ from those of the individual components. In some cases ion pair charge-transfer (IPCT) optical transitions may be observed and ion pairs may undergo energy transfer and photoinduced electron transfer. 

In solution, the photochemical properties of the bis-pyridyl polymer (py/Ru 5/1) are related to those of analogous monomers in solution (32). The photochemical loss of pyridine from Ru-22+ a high efficiency reaction which has proven to be... 

Again it is noted that this method, in common with many others discussed above, requires the addition of a well-characterized probe molecule. Only a few studies have exploited the intrinsic luminescence properties of the polymer. The authors of some chemiluminescence studies described later attempted to use the changes in intrinsic photophysical and photochemical properties of the polymers as they cure to probe the viscosity and phase changes in the system. 

The reaction enthalpies for the photo-reduction of ketones and azoalkanes in n,ji states by the model hydrogen donors methanol and dimethylamine have been calculated. These data, together with a consideration of excited-state electrophilicity and the antibonding character of transition states, have been used to rationalize the nucleophilic character of singlet n,ji excited azoalkanes and the electrophilic character of correspondingly excited ketones. A theoretical model for the photo-reduction of carbonyl compounds, based on the assumption that hydrogen transfer involves electron transfer followed by proton transfer, has been developed. The model was able to describe the available kinetic data for the photo-reduction of o-benzoquinones and fluorenone in the presence of para substituted iV,iV-dimethylanilines and those for the photo-reduction of p-chloro-anil and 2,6-dichloro-l,4-benzoquinones in the presence of polymethylben-zenes. The photochemical properties of the triplet ji, n state of 5,12-naph-... 

The gaseous products act as a driving force of ablation and some indications were found suggesting that the photochemical properties of the dopants influence the ablation characteristics. Encouraged by these results the synthesis of polymers containing the -N=N-X- chromophore in the polymer main chain was started. The first polymer containing the -N=N-N- chromophore in the polymer chain is shown in Scheme 1, structure A. 

Norrish Type I cleavage of benzylketones occurs very readily to yield benzyl and acyl radicals. In competition with other processes, these can couple by attack of the acyl radical upon the aromatic nucleus of the benzyl radical to give as a product an acyl toluene derivative. Turro has examined the photochemical properties of the 2-phenylalkanones (326) in solution and complexed with cyclodextrins. In solution the major fate of the Type I biradicals formed is intr2unolecular recombination by attack of the acyl radical at the para position of the benzyl radical to generate (327). However, complexatlon of (326) within the cyclodextrin cavity inhibits formation of (327) and promotes disproportionation of the blradicals to give (328). Turro has also examined the photochemistry of dlbenzylketone Included in zeolites.Recombination of the Type I radicals in this case can produce a mixture of (2-methyl-phenyl) benzylketone and (4-methylphenyl)benzylketone decarbonylation of the acyl radical competes and leads to the isolation of 1,2-dlphenylethane. It is found that the relative yields of the products... 

The spectroscopic and photochemical properties of the synthetic carotenoid, locked-15,15 -cA-spheroidene, were studied by absorption, fluorescence, CD, fast transient absorption and EPR spectroscopies in solution and after incorporation into the RC of Rb. sphaeroides R-26.1. High performance liquid chromatography (HPLC) purification of the synthetic molecule reveal the presence of several Ai-cis geometric isomers in addition to the mono-c/x isomer of locked-15,15 -c/x-spheroidene. In solution, the absorption spectrum of the purified mono-cA sample was red-shifted and showed a large c/x-peak at 351 nm compared to unlocked all-spheroidene. Spectroscopic studies of the purified locked-15,15 -mono-c/x molecule in solution revealed a more stable manifold of excited states compared to the unlocked spheroidene. Molecular modeling and semi-empirical calculations revealed that geometric isomerization and structural factors affect the room temperature spectra. RCs of Rb. sphaeroides R-26.1 in which the locked-15,15 -c/x-spheroidene was incorporated showed no difference in either the spectroscopic properties or photochemistry compared to RCs in which unlocked spheroidene was incorporated or to Rb. sphaeroides wild type strain 2.4.1 RCs which naturally contain spheroidene. The data indicate that the natural selection of a c/x-isomer of spheroidene for incorporation into native RCs of Rb. sphaeroides wild type strain 2.4.1 was probably more determined by the structure or assembly of the RC protein than by any special quality of the c/x-isomer of the carotenoid that would affect its ability to accept triplet energy from the primary donor or to carry out photoprotection. 

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