Energy 488 absorption

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. 

Most cyanines show prominent, hypsochromic vibrational shoulders at shorter wavelength associated with the long-wavelength electronic transition (Fig. 2). These vibrational shoulders include one or two vibrational quanta (0 — 1 0 — 2 ), in addition to the absorption energy at Intensity... 

The first, and to this writing still only case of a ketone a-cleavage-recombi-nation sequence in the steroid field was reported by Butenandt, who found that 17-ketones epimerize at C-13. Ultraviolet irradiation of either stereoisomer produces an equilibrium mixture in which the thermodynamically more favored 13a-compound cf. (15)] with cw-fusion of rings C and D predominates at room temperature. As ultraviolet absorption energies and intensities of the two isomeric ketones are practically identical, the equilibrium composition depends largely on the rate of the competing recombination process from (14). For further examples of the photoisomerization at C-13 of 17-ketosteroids, see ref. 8, 12, 15 and 43. 

Weakly absorbed does not mean that the adsorption energy e is small in comparison to k T, since entropic effects are important rather, it means e - ej ed, ed being the absorption energy at the threshold [2]. 

Sufficient data are extant for three sets of charge transfer complex association constants and one set of charge transfer absorption energies. The sets studied are reported in Table XV. Results of the correlations with eq. (2) are given in Table XVI. All the sets studied gave significant results. An improvement in the correlation for set 15-2 occurs on the exclusion of the values for X = Ac and X = COaMe (set 15-2BA). 

The composition of the electrical effect in the case of the 1,4-benzo-quinones seems to depend upon the degree of substitution. The association constants and charge transfer absorption energies of the 2-substituted-l,4-benzo-quinone-hexamethylbenzene complexes (sets 15-2 and 154) show values of pj of 55 and 44 respectively. Values of p are set forth in Table XVII. 

Poly(l,4-naphthylenevinylene) (106) is accessible via the Wessling polymerization procedure. Lenz, Karasz, Wegner et al. have published the synthesis of PNV 106, starting from l,4-bis(chloromethyl)naphthalene [127, 128]. The poly(l,4-naphthylenevinylene) (106) displays an optical absorption energy of 2.05 eV, slightly red-shifted by about 0,3 eV relative to the parent PPV 60-system, due to the electronic effect of the annelated benzene ring. 

As mentioned above, poly(9,10-anthrylenevinylene) is not accessible by means of the Wessling polymerization procedure. Well defined oligo(9,10-anthrylene-vinylene)s (111) were synthesized via Horner-type coupling [132]. Extrapolation of the optical absorption energies in the 111 series against the corres-... 

In these dye-functionalized dendrimers, light absorbed by the numerous peripheral coumarin-2 units is funneled to the coumarin-343 core with remarkably high efficiency (toluene solution 98% for the first three generations 93% for compound 8). Given the large transition moments and the good overlap between donor emission and acceptor absorption, energy transfer takes place by Forster mechanism [34]. 

Indeed, the charge-transfer absorption energies of dibromine complexes with various arenes [65] and alkenes [45] both show clear correlations with the ionization potentials of various donors (Fig. 5). 

However, in contrast to XPS, electrons are promoted into bound or continuum states and still provide partial screening to the excited atom, so that these absorption energies are less sensitive to relaxation effects. Shifts in absorption energies are thus often more easily detected than those in XPS BEs. Moreover, XANES is a bulk-sensitive technique, and avoids the ambiguities associated with XPS in attributing trends to a bulk vs. surface phenomenon. 

The XPS and XANES investigations of MnP-type binary phosphides MP and mixed-metal phosphides M XM XP discussed above have revealed some of the factors that influence BEs and absorption energies. In particular, a charge redistribution can take place within the metal-metal bonding network when two different... 

XPS has typically been regarded primarily as a surface characterization technique. Indeed, angle-resolved XPS studies can be very informative in revealing the surface structure of solids, as demonstrated for the oxidation of Hf(Sio.sAso.5)As. However, with proper sample preparation, the electronic structure of the bulk solid can be obtained. A useful adjunct to XPS is X-ray absorption spectroscopy, which probes the bulk of the solid. If trends in the XPS BEs parallel those in absorption energies, then we can be reasonably confident that they represent the intrinsic properties of the solid. Features in XANES spectra such as pre-edge and absorption edge intensities can also provide qualitative information about the occupation of electronic states. 

The electronic properties and thus the absorption bands of a polymer depend on the conjugation length Z. The absorption energy decreases as Z increases reaching a limiting value of 16 000 cm l as Z approaches infinity. If the polymer solution consists of a distribution of conjugation lengths (i.e. chromophores), photoselection of the different chromophores within the... 

The Raman modes yC=C and v =C depend on chain length in a manner which parallels the absorption energy dependence on chain length (25). The contribution of each chromophore to the observed Raman peak is determined by the RS intensity for incident light at energy The RS peak position is then a weighted average of all... 

---