Polyenes electronic spectra

The free-electron model can also be applied to the electronic spectrum of butadiene and other linear polyenes. The lowest unoccupied molecular orbital (LUMO) in butadiene corresponds to the = 3 particle-in-a-box state. Neglecting electron-electron interaction, the longest-wavelength (lowest-energy) electronic transition should occur from n — 2, the highest occupied molecular orbital (HOMO), as shown below ... 

According to the Woodward-Hoffmann rule [6, 7], conjugate polyenes with 4n and 4n+2 n electrons undergo cychzations in conrotatory and disrotatory fashions under the thermal conditions, respectively. Recently, novel cycloisomerizations were found to be catalyzed by Lewis acid and to afford bicychc products [39] as photochemical reactions do [40]. The new finding supports the mechanistic spectrum of chemical reactions. 

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

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>. 

Electronic Absorption Spectroscopy. Theoretical considerations of the light absorption spectra of /3-carotene (189) and related polyenes have been presented.A method has been devised for describing the shapes of the absorption spectra of polyenes such as -carotene and retinyl acetate as lognormal distribution curves.A new absorption band is seen in the spectrum of jS-carotene acting as electron donor in charge-transfer complexes.The triplet-triplet... 

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-... 

As a typical example of polyene spectroscopy, absorption and fluorescence spectra of frons,frans-l,3,5,7-octatetraene in hexane at 23 °C are shown in Figure 3. An absorption band with several peaks is observed in Figure 3a. It should be noted that the positions of electronic absorption bands strongly depend on solvents . This absorption band is dipole allowed, because the molar absorption coefficient of this band is very large (Table 1). This band is attributed to the transition from the 1 kg ground state to the 1 feu excited state (tt-tt transition). Although the absorption peaks are due to vibrational transitions, a precise vibrational analysis cannot be made because of the broad band widths. The position of the observed emission spectrum (Figure 3b) shows a considerable red shift in... 

In the singlet manifold, carotenoids have, like all polyenes, an unusual electronic structure The hrst excited state (Si) has the same symmetry, A, as the ground state, and thus one-photon transitions from So to Si are forbidden. In other words, the Si state does not appear in the absorption (or emission) spectrum of carotenoids (with more than 9 double bonds), which is dominated by the very strong So S2 (B ) transition. Carotenoids also possess a state of symmetry, which may lie near S2, though evidence for the spectroscopic observation of this state remains controversial [132-135]. Finally, some unusual carotenoids with polar substituents, such as peridinin, may also have low-lying charge transfer states [42, 136, 137]. 

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. 

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