Emission spectra model compounds

The most direct information on the state of cobalt has come from Mossbauer spectroscopy, applied in the emission mode. As explained in Chapter 5, such experiments are done with catalysts that contain the radioactive isotope 57Co as the source and a moving single-line absorber. Great advantages of this method are that the Co-Mo catalyst can be investigated under in situ conditions and the spectrum of cobalt can be correlated to the activity of the catalyst. One needs to be careful, however, because the Mossbauer spectrum one obtains is strictly speaking not that of cobalt, but that of its decay product, iron. The safest way to go is therefore to compare the spectra of the Co-Mo catalysts with those of model compounds for which the state of cobalt is known. This was the approach taken... 

The steady state absorption and fluorescence spectra of both dendrimer generations 1 and 2 are depicted in Fig. 2. The former are merely superpositions of the absorption spectra of both chromophores involved. In the fluorescence, however, the peryleneimide part is almost completely quenched compared to the model compound. Instead, the fluorescence at wavelengths longer than 650 nm almost completely resembles the emission spectrum of the terrylene-diimide model compound 3. This feature is a strong indication that within these dendrimers the excitation energy is efficiently transferred from the peryleneimide to the terrylenediimide. 

In the example discussed above, the heterotriad consists of a photosensitizer and an electron donor. In the following example, a ruthenium polypyridyl sensitizer is combined with an electron acceptor, in this case a rhodium(lll) polypyridyl center [15]. The structure of this dyad is shown in Figure 6.21 above. The absorption characteristics of the dyad are such that only the ruthenium moiety absorbs in the visible part of the spectrum. Irradiation of a solution containing this ruthenium complex with visible light results in selective excitation of the Ru(ll) center and in an emission with a A.max of 620 nm. This emission occurs from the ruthenium-polypyridyl-based triplet MLCT level, the lifetime of which is about 30 ns. This lifetime is very short when compared with the value of 700 ns obtained for the model compound [Ru(dcbpy)2dmbpy)], which does not contain a rhodium center. Detailed solution studies have shown that this rather short lifetime can be explained by fast oxidative quenching by the Rh center as shown in the following equation ... 

Model compound and difference spectral studies have been carried out on tyrosine and related compounds by Wetlaufer (1956), Laskowski (1957), Edelhoch (1958), Chervenka (1959), Bigelow and Geschwind (1960), Yanari and Bovey (1960), and Foss (1961) (see also Section VI,Z)). Studies of the fluorescence of tyrosine—activation spectrum, fluorescence emission spectrum, and quantum yield—have been reported by Teale and Weber (1957). The pH-dependence of the fluorescence of tyrosine and related compounds was studied by White (1959) fluorescence-polarization studies from the same laboratory were reported by Weber (1960a) for simple compounds, and for proteins (Weber, 1960b). Teale (1960) has carried out extensive studies on the fluorescence characteristics of a score of proteins. 

A Spex Fluorolog 212 spectrophotometer was used for recording the emission and excitation spectra of the polyimide films and the model compounds. The slit width used for the films was 2 mm and for the model compounds was 1 mm. Excitation and emission spectra were subsequently normalized with respect to the lamp intensity fluctuations by dividing each spectrum by that obtained with a Rhodamine-B standard solution. Absorption spectra were obtained with... 

Fluorescence spectra from a polystyrene film photolyzed in vacuum are shown in Figure 2. Similar but less intense spectra were observed in films irradiated in air. The Product I responsible for this spectrum was partially extractable from the film with methanol the fluorescence spectrum of the extract is shown also in Figure 2. Comparison of these spectra with those of a wide variety of reasonable model compounds suggests that Product I is related to 1,3-diphenyl-l,3-butadiene since the spectral match with 1,4-diphenyl-l,3-butadiene, shown in Figure 2, is quite close. Product I spectra were obtained also from the residual films after extraction, indicating that the diene moiety may form part of a photolyzed chain as well as exist as a short-chain fragment. Fluorescence spectra that could be related to higher polyenes were not detected in the vacuum exposures. In air exposures, however, the prompt emission spectra from films did exhibit a weak shoulder superimposed on the... 

Among the most commonly used fluorescent probes for biochemical and biological systems are l-dimethylamino-5-sulfonamidonaphthalenes (DNS derivatives) (SCHEME IV). The emission spectrum of the model compound l-dimethylamino-5-(S-isobutyroyl-aminoethyl)sulfonamidonaph-thalene (IB DNS) in absolute methanol is characterized by one band with an emission maximum at 538 nm which corresponds to the excited state of IB DNS, with a nonprotonized dimethylamino group. The emission spectrum was independent on the exciting radiation for wavelengths from 250 to 380 nm. 

The emission behaviors of [2.2]paracyclophane-based jc-stacked polymers were investigated in detail. Almost all the polymers exhibited photoluminescence in solution with high photoluminescence quantum efficiencies (Opl). For example, the relative OpL values of polymers 13 and 16 were calculated to be 0.82 and 0.70, respectively. The photoluminescence spectra of 13 and the stacked model compound 19 in CHCI3 (1. Ox 10 M) were obtained upon excitation at the absorption maximum (Fig. 5). In polymer 13, the emission spectrum was observed in the visible blue region with a peak top wavelength of 430 nm. It is noteworthy that the... 

Spectrum of the unstacked model compound displays a vibronic progression with the 0-0 peak at 338 nm. The spectra of the two stacked analogs exhibit broad featureless emission peaks that are significantly red shifted, with a maximum at 412 nm for the pseudopara analog and 401 nm for the psendoortho compound. 

Accordingly, the spectra of the cyclophanes with shorter stacked segments (PV, PV2) resemble those of exdmers and are distinct from those of the corresponding unstacked analogs, whereas the spectra of compounds with the longer stacked PV3 segments closely resemble those of the unstacked model./>/>-CP[(PV2)V]2 appears to represent a transition between these two cases, with a broad excimer-like emission spectrum at room temperature and a vibronic emission at low temperature. 

The first model compound [19] to be synthesised (see scheme) shared all the properties of the luciferins - most noticeably its acidity (pKa 8.3), forming a yellow anion in DMSO/base. This anion oxidised spontaneously and the mechanism shown predicted in all important respects that now accepted for the natural system. The blue emission spectrum was... 

Mi CO). The first metal-metal bond to be characterized (35) is the formally single Mn-Mn bond in Mi CO). This compound has often been used as the model for developing electronic structure theories (1.18.36.37). Extremely efficient photofragmentation is responsible for the structureless electronic spectrum and the lack of emission following excitation of this molecule. This spectroscopic deficiency necessitates photofragmentation studies to obtain data to verify theoretical models. Most of the photochemical experiments in the past explored the reactions of the lowest excited singlet state in the near ultraviolet. 

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