Polyenes absorption spectra

The ultraviolet/visible absorption spectrum of a polyene shows an intense absorption band and an extremely weak absorption band which is located below the strong absorption band, as described in the following section. This spectral pattern is a general property of linear polyenes of all chain lengths independent of local symmetry and/or the presence of cis bonds. This is the reason why in the literature on polyenes the labels 1 kg for So, 2 kg for Si and 1 feu for Si are used even in cases where Cih symmetry is not realized. The ordering that the 2 kg excited state is located below the 1 feu excited state is peculiar to linear polyenes. 

Han and Elsenbaumer also note that the BP initially formed can react with neutral polymer to form two distinctly different polarons via interchain electron transfer. After twenty-four hours, our optical spectra are unchanged, and have measurable ESR activity. However, in contrast to alkoxy-PPV polymer, we do not observe a typical polaronic absorption spectrum, but rather one almost identical to the bipolaron obtained from SbCl5 doping of 10-5 M solutions in CH2CI2. A possible interpretation is one which allows for P and BP states coexisting in dynamic equilibrium, with the bipolaron dominating the optical absorption. The absorption characteristics of the protonically doped polyenes are shown in Table II compared to the same samples doped with SbCl5. 

The vacuum UV absorption spectrum of 2-vinylfuran has been recorded and interpreted. The electronically excited molecule was found to behave like a linear polyene rather than like furan <2004JCP10972>. 

Most current interest in u.v. spectra is concerned with retinal and its cis-trans isomers. Studies at 77 K include theoretical calculations, fluorescence, and the effect of protonation on the corresponding Schiff base. The singlet-triplet absorption spectrum of all-trans-retinol has been measured. Calculations on the absorption-emission spectra of rhodopsin suggest that there is little dependence on the angular twist in the polyene chain. Pulse radiolysis has made it... 

Essential features of the electronic spectra of linear polyenes have already been mentioned in Chapter 1. The HOMO LUMO transition is shifted to longer wavelengths as the number of conjugated double bonds increases, and this is easy to rationalize in the FEMO model. However, the next transition that is to be expected from the simple MO model is only allowed if the molecule does not have a center of symmetry that is, if it is not in the all-trans configuration. (Cf. Example 1.7.) The absorption spectrum of /S-caro-... 

Many linear polyenes exhibit anomalous fluorescence behaviour in the sense that the fluorescence rate constant kf calculated from the absorption spectrum (Equation 2.11) is much smaller than that determined by lifetime and quantum yield measurements (Equation 3.33). In 1972, Hudson and Kohler reported high-resolution absorption and emission spectra of all-trart.v-l, 8-diphenylocta-l, 3,5,7-tetraene at low temperature, which proved that the lowest excited singlet state Si was not that reached by the strongly allowed 7t,Jt transition (/ 1.5) that is predicted by MO theory and observed at 410nm.3" Rather, very weak (f 0.06), structured absorption that had been hidden under the tail of the jt,jt -absorption in solution spectra was detected at slightly longer wavelengths. 

Change in the dipole moment and polarizability upon electronic excitation was determined for various linear polyenes by adopting electric field-induced changes in the optical absorption spectrum [70]. Polyenes studied (Exhibit 2) were diphenylbutadiene (DPB 7), diphenylhexatriene (DPH 8), diphenyloctate-traene (DPO 9), diphenyldecapentaene (DPD 10), and all-tranj-retinal (3). Exhibit 2 describes the experimental values determined. It is proposed that (based on the results obtained) the excited state dipole moments determined on these polyene systems have a role to play in the mechanism of trans-cis photoisomerization. 

Fig. 47. Absorption spectrum of thermally degraded polyvinylchloride (Vestolit SK 55), polyvinylbromide and a copolymer vinyl chloride—isobutene in tetrahydrofuran. The absorption maxima correspond to polyene sequences with n = 4, 5, 6, etc. [ 184 ].

Color Development. The absorption spectrum in the visible region of PVC subjected to heat degradation shows several maxima that are related to the degree of conjugation of the polyenes formed by dehydrochlorination. Bengough and Varma (29) have studied the... 

Petek H, Bell AJ, Choi YS, Yoshihara K, Tounge BA and Christensen RL (1995) One- and two-photon fluorescence excitation spectra of the 2 Ag states of linear tetraenes in free jet expansions. J Chem Phys 102 4726 739 Ramasesha S and Zoos ZG (1984) Correlated states in linear polyenes, radicals, and ions Exact PPP transition moments and spin densities. J Chem Phys 80 3278-3287 Raubach RA and Guzzo AV (1973) Singlet-triplet absorption spectrum of uH-trans-retinal. J Phys Chem 75 983-984 Ricci M, Bradforth SE, Jimenez R and Fleming G (1996) Internal conversion and energy transfer dynamics of spheroidene in solution and in the LH-1 and LH-2 light-harvesting complexes. Chem Phys Lett 259 381-390... 

Figure 1.2.10 Polyene-type spectrum of sorbaldehyde showing only one broad band for the 7t-system and—aromatic-type spectrum of benzene. An intense absorption band at low wavelength (high energy) is followed by several less intense ( forbidden ) absorptions at longer wavelengths. Broadening of abso tion bands usually either indicates more flexible or distorted or aggregated chromophore. Such a broadening of the bands leads to smaller extinction coefficients, whereas the area of the absorption bands remains about constant for a given chromophore. This area is related to the so-called transition moment, which can be approximated by the product of extinction coefficient and linewidth.

In the ground electronic state of (16.9), the highest occupied and lowest vacant tt MOs have j = jMc 5 c + respectively HMO theory predicts the longest wavelength band of the electronic absorption spectrum of a conjugated polyene to occur at... 

Understanding of the UV absorption spectrum of polyenes has been increased... 

UV spectroscopy also allows identification of subdivisions within the five polyene groups. Methylation adjacent to the polyene chromophore results in a bathochromic shift of the absorption maxima of about 6 nm. The presence of a ketone group adjacent to the chromophore radically alters the absorption spectrum, e.g. the carbonyl pentaene flavomycoin possesses only two absorption peaks, while the carbonyl hexaene dermostatin possesses a single peak [24—26]. 

Schaffner [145] showed that addition of cholesterol or other 3 3-OH, A-5 sterols to aqueous ethanolic solutions of polyene antibiotics produced changes in the UV absorption spectrum suggestive of molecular interaction. In aqueous... 

In this study we have investigated the mechanism of the solvent induced shift in the absorption spectrum of two carotenoids, i.e. spheroidene and trans- 3-carotene. Carotenoids belong to the class of polyenes and they are essentially hydrophobic. In vivo the carotenoids are non-covalently bound to specific pigment-protein complexes. Our working model is the red-shifted spectra of carotenoids in vivo is mainly due to dispersive interactions of the carotenoid and the surrounding, and consequently the shift is due to the polarizability of the surrounding medium. The measurements reported here support this model. 

---