Chromophores, electronic absorption

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

TABLE 7.9 Electronic Absorption Bands for Representative Chromophores... 

APPENDIX 8 P0LAR0GRAPHIC HALF-WAVE POTENTIALS 835 APPENDIX 9 RESONANCE LINES FOR ATOMIC ABSORPTION 837 APPENDIX 10 ELECTRONIC ABSORPTION CHARACTERISTICS OF SOME COMMON CHROMOPHORES 838... 

Resonance Raman (RR) spectroscopy provides information about the vibrational characteristics of a chromophore, for example, a metal center, within the complex environment of a protein. In RR spectra, those vibrational transitions are observed selectively that are coupled to electronic transitions. In iron sulfur proteins, this technique has been used to resolve the complex electronic absorption spectra and to identify both vibrational and electronic transitions. 

We give here only a brief summary of the electronic absorption spectrum of the diene chromophore which has been extensively treated by Gross and Schnepp5 with reference to a- and /1-phellandrene. 

The broad emission and low-fluorescence quantum yield of PPS suggested a distribution of trapping sites in the Si skeleton, which were also considered responsible for the lower-than-expected conductivity. The far-IR spectrum of PPS suggested the existence of cyclohexasilane rings connected by linear chains.361,362 Subsequent investigations by Irie et al. on the electronic absorption spectra of radical ions of poly(alkylsilyne)s were taken to indicate the presence of various cyclic silicon species, in corroboration of this conclusion.363 The large Stokes shift and broadness of the fluorescence emission indicate a range of fluorophore structures, different from the chromophore structures. This is... 

Polymers with chromophores exhibiting mt transitions (e.g., C=0) exhibit weaker UV-absorption and these groups together with unsaturated carbon-carbon bonds which develop during radiation damage can be detected by electronic absorption spectroscopy. 

A few typical examples having electronic absorption bands for various representive chromophores are provided in the following Table 21 1 ... 

Last but not least, ageing and destruction processes can be monitored in polymers under application, and structural and quantitative analysis of unknown additives (stabilizers etc.) is possible in commercial polymers using UV-vis spectroscopy. Advantage can be taken here of the fact that the position of an electronic absorption in unsaturated systems depends only weakly on the surroimd-ing medium. Even though UV-vis spectroscopy is not very specific in the absorption band, it is highly sensitive and therefore much better than NMR or IR spectroscopy to detect small amounts of chromophors. 

This section discusses some simple chemical reactions which convert a chiral nonracemic compound containing no electron absorption band in an easily accessible spectral range into a derivative with absorption in either the visible or quartz ultraviolet region. This is a useful operation if a reliable correlation exists between absolute configuration (conformation) and chiroptical properties. A collection of useful chromophoric systems is found in reference 167. 

In addition to the absorption corresponding to the anionic part (tetraphenylborate, iodide, etc.), the electronic absorption spectra of phosphonium salts are typical of those of the independent chromophoric group present in the cationic part108. However, as the phosphorus atom is positively charged, its 3d orbitals become able to overlap with any nearby n orbital this d-n interactions leads to bathochromic and hyperchromic... 

No zirconium(III) complexes with oxygen donor ligands have been isolated. However, the electronic absorption spectra of aqueous solutions of Zrl3 have been interpreted in terms of the formation of aqua complexes (equation 4).29 The spectrum of a freshly prepared solution of Zrl3 exhibits a band at 24 400 cm-1, which decays over a period of 40 minutes, and a shoulder at 22000 cm-1, which decays more rapidly. The 24400 cm-1 band has been assigned to [Zr(H20)6]3+, and the 22000 cm-1 shoulder has been attributed to an unstable intermediate iodo-aqua complex. If it is assumed that the absorption band of [Zr(H20)6]3+ is due to the 2T 2Ee ligand-field transition, the value of A is 24 400 cm. This corresponds to a A value of 20 300 cm-1 for [Ti(H20)6]3+ 30 and 17 400 cm-1 for the octahedral ZrCl6 chromophore in zirconium(III) chloride.25... 

At low enough temperatures vibrational fine structure of aromatic chromophores may be well resolved, especially if they are embedded in a suitable matrix such as argon or N2, which is deposited on a transparent surface at 15 K. This matrix isolation spectroscopy77166 may reveal differences in spectra of conformers or, as in Fig. 23-16, of tautomers. In the latter example the IR spectra of the well-known amino-oxo and amino-hydroxy tautomers of cytosine can both be seen in the matrix isolation IR spectrum. Figure 23-16 is an IR spectrum, but at low temperatures electronic absorption spectra may display sharp vibrational structure. For example, aromatic hydrocarbons dissolved in n-heptane or n-octane and frozen often have absorption spectra, and therefore fluorescence excitation spectra, which often consist of very narrow lines. A laser can be tuned to excite only one line in the absorption spectrum. For example, in the spectrum of the carcinogen ll-methylbenz(a)anthrene in frozen octane three major transitions arise because there are three different environments for the molecule. Excitation of these lines separately yields distinctly different emission spectra.77 Likewise, in complex mixtures of different hydrocarbons emission can be excited from each one at will and can be used for estimation of amounts. Other related methods of energy-... 

For both cis- and Irans-dianthrylethylenes, the degree of deconjugation, which parallels the degree of deviation from coplanarity of the ethylene and anthracene ir-systems, is borne out in the shape of the electronic absorption spectra. Thus, the long-wavelength absorption of cis-dianthrylethylene 38a is characterized by the fine structure pattern which is typical of the anthracene chromophore, while the bathochromically shifted spectrum of the transisomer 39a is virtually structureless (see Figure 7). Substitution of the ethylene double bond is absorption spectroscopically noticeable for the cis-isomers 38b-f by distortion of the anthracene absorption. The absorption... 

The name lepidopterene refers to the hydrocarbon 113 (L) whose butterflylike molecular shape was first revealed by X-ray diffraction analysis [129,130]. The structured electronic absorption spectra of lepidopterenes around 270 nm closely resemble that of 9,10-dihydroanthracene (see Figure 31). However, in terms of excited state properties, lepidopterenes have very little in common with 9,10-dihydroanthracene, which in solution fluoresces with a quantum yield of 0.16. By contrast, photoexcitation of lepidopterenes leads mainly to intramolecular exciplexes of 7i-chromophorically substituted anthracenes in an adiabatic process, for which both the molecular topology... 

Effect of Conjugation on Electronic Absorption by the Benzene Chromophore... 

The ultimate limit of the transparency of a dye above its first electronic absorption is uncertain, but an order of magnitude improvement is not unreasonable in asymmetric chromophores, i.e. to levels of 10 per 100,000 of molar absorptivity. At this level, and assuming an ADPM dye concentration of 0.045 M (the same as observed for ADPM SHG with FBB by EFISH), a dye/host material would show a loss of 4.5 dB/cm. However, because of the tremendous increase in nfi for this material, it is likely to be 1 mm (or less) in thickness for a total loss of only 0.45 dB. This would be in the range of practical devices. 

The total absorption spectrum appears as the simple addition of the individual absorbance of all chromophores in the CC. The trace is reduced to the vibrational wave packet overlap in the electronic ground and excited state (averaged with respect to the chromophore electronic ground-state vibrational equilibrium, described by the density operator Rmg Hma = Tm + Uma). 

Reactions analogous to these are observed with the polymers that contain metal-metal bonds along their backbones. Because the metal-metal bond chromophore absorbs in the visible region, the photochemical reactions of the polymers can be conveniently monitored by electronic absorption spectroscopy. The quantum yields for the reactions are in the range 0.1 to 0.6, depending on the specific polymer and the M-M bond.14 Sample reactions of the polymers are shown in equations 20-22. 

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