Stilbene absorption spectra

Kira and coworkers25 found that in deaerated DMSO solution of frans-stilbene both the solute cation and anion are produced and the anions are eliminated by aeration. Since they found26 that the absorption spectra of the anthracene cation and anion are quite similar, they suggested25 that the absorption spectrum observed by Hayon for anthracene solution in DMSO is a superposition of the spectra of the solute cation and anion. This observation casts a serious question on the yield of solvated electrons found by Hayon23. 

Figure 1.7 Electronic absorption spectrum of frans-stilbene.

The utility of such an approach is demonstrated on the molecular level with the case of benzylideneaniline 6-XXXV (R-H), which occupied spectroscopists for nearly three decades (Haselbach and Heilbronner 1968, and references therein). This material is isoelectronic with azobenzene 6-XXXVI and stilbene 6-XXXVII, but its solution absorption spectrum differs significantly from them (Fig. 6.18). 

The same tendency was observed in the cases of stylene,stilbene( 6) and cinnamoyloxy group( 6). The calculated value of Ti is 21700 cm". The So-)Ti absorption spectrum in EtI gives only tail near the calculated value. The emission spectrum in EPA gave such a phosphorescence as was shown in Fig.3. The Intensity Increases with the irradiation time. Therefore, it is clear that the emission spectrum is that of the photodecomposed product, which is estimated to be aniline from the similarity of the phosphorescence of the authentic sample. 

A very similar situation has been observed in Similar but less well studied indications for metastable products have been obtained for the DHP s formed from the m-substituted stilbenes Some observations about time-dependent changes in the absorption spectrum of 1 could well be due to such processes ... 

The UV-absorption spectrum of frans-stilbene exhibits two main bands with maxima near 280 nm (A-band) and 230nm (B-band) [22, 54, 78, 79], The A-band of ris-stilbene is attributed to a (n, n ) transition [80], As compared to frans-stilbene it is blue-shifted, much broader, and has a considerably lower molar absorption coefficient (ec) [24], For the B-band, cc is greater than e, (Figure 1) the origin of the absorption transition in ris-stilbene is red-shifted relative to the same transition in frans-stilbene [81], Two-photon excitation spectra (< 5 x 104cm ), together with theoretical calculations, indicate seven excited singlet states [38]. 

The extended conjugated stilbene moiety of 1 has been examined using UV spectroscopy. This chromophore typically presents a UV absorption spectrum with a A,max of approximately 310-320 nm. However, it has been noted that for resveratrol oligomers in which the ethylene bridge has been reduced, such as that seen in (-)-a-viniferin (8b) [100], kobophenol A (9) [43], and tricuspidatol A (71) [89], the A,max is shifted to a lower wavelength of approximately 280-290 nm. 

Isomerization. Finally, some investigations of unstable radical anions will be discussed. Radical anions of ds-stilbene rapidly isomerize to radical anions of trans-stilbene therefore, their spectra or relative electron affinities cannot be studied by conventional techniques. Flashing a solution of perylene radical anions mixed with cis-stilbene yields cis-stilbene radical anions, which persist for 200-300 jls before the ejected electrons are recaptured by the perylene formed by the photolysis. The difference spectrum of the photolyzed solution is shown in Figure 12. The concentration of the ds-stilbene radical anions formed can be determined by the degree of bleaching the absorption peak of the perylene radical anions. The absorption spectrum of cis-stilbene radical anions can also be determined in this way (19). Similar experiments, results of which are shown in Figure 13, permit the determination of the absorption spectrum of trans-stilbene radical anions. Because this spectrum has been established by other methods, the reliability of the flash-photolytic method can be determined by comparing the differ-... 

Figure 1.11), and a pseudo-stilbene type with a (7t,7t ) state as the lowest-excited state (see also Figure 1.13). The assignment of an azo molecule or an azobenzene-containing macromolecule or system to one of these classes can be made by a simple inspection of the absorption spectrum. The spectroscopic properties of these types of azo compounds will be covered in Sections 1,3 to 1.5 of this chapter.

Upon pulse radiolysis of traws-stilbene (t-St) solutions in THE, the radical anion of trans-stilbene was demonstrated to be formed by the reaction of electrons with St (reaction with the rate constant ks = (1.16 0.03) x 10 dm /(mol s)) [86]. The transient absorption spectrum observed with Xmax 500 and 720 nm was attributed to the unassociated radical anion St" . This species reacted with the countercation of THE formed upon radiolysis and with radiolytically generated radicals. Addition of sodium tetrahydridoaluminate (NAH) resulted in the radical anion being associated with Na as a contact ion pair. In the presence of the lithium salt, formation of solvent-separated ion pairs has been detected. 

Ruddle and Wilson [66] developed two further stages in this procedure for extending the use of UV spectrophotometry in the characterisation of these compounds. They consist of (a) measuring the UV absorption spectrum of the stabiliser solution after reaction with solid nickel peroxide and (b) remeasuring it after making the reaction products alkaline. Cook [67] obtained the substituted stilbene quinone after... 

These polynuclear gold(I) phosphine complexes have been found to possess rich photochemistry. The phosphorescence of 49 is found to be quenched by a series of energy acceptors such as trans- and cis-stilbene, styrene, hept-l-ene, and cyclohexene [129], The transient absorption difference spectrum of a degassed acetonitrile solution of 49 and irans-stilbene displays absorption bands at about 365 and 390 nm, typical of the triplet excited state absorption spectrum of trans-stilbene, indicating the energy transfer nature of the quenching mechanism, Be-... 

Figure 10.3 Room temperature (295 K) So Si absorption spectrum of rraws-stilbene computed using harmonic approximation by time-independent (upper panel [7]) and time-dependent (lower panel [63]) methods. Both computational results are compared with the experimental absorption spectmm measured at 295 K in cyclohexane by Mathies and co-workers [77]. (From J. Tatchen and E. Pollack, J. Chem. Phys. 2008, 128, 164303. Copyright 2008. Reprinted with permission of the American Institute of Physics.)...

Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and accurate method of quantitative analysis. Care must be taken when interpreting the spectra of stilbene-type compounds, since turns to cis isomerisation is promoted by ultraviolet radiation. Usually, however, a control spectrum of the turns isomer can be obtained before the compound undergoes any analytically significant isomerisation. FBAs are often marketed on the basis of strength comparisons determined by ultraviolet spectroscopy. 

Cis-stilbene (Zl) also undergoes a conrotatory cyclization reaction into trans-4a,4b-dihydrophenanthrene (DHP, 2), a short living, not isolated product with an absorption in the visible spectrum at 450 nm. In the absence of an oxidizing agent DHP will return to the starting material, by both a thermal and a photochemical ring opening reaction. 

Thus, we can assume that two types of stilbene structures contribute to the chromophoric systems of kraft lignin. If further double bonds or carbonyl groups are conjugated with the stilbene as in XIII, the resulting structure is likely to have absorption within the visible region of the spectrum. 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

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