Charge polyenes

Raman spectra of the polyene 54 and its anion radical and dianion have been reported [156], Inversion of bond alternation was suggested for the dianion. Absorption bands of the radical anion and dianion at 1275 cm 1 was considered as a marker of negatively charged polyenes. 

Fig. 7-15 Density of states of charged polyene chain showing the mid gap (charged soliton lattice) and the metallic levels (poison lattice). After Reference [232], reproduced with permission.

The charge distribution may be symmetrical when the system possesses similar nitrogen atoms and identical or unsymmetrical nuclei approaching more or less a polyene-like bond state when the nuclei are of different... 

The perturbation of the PMD symmetry is accompanied by a decrease in the charge alternation and by the appearance of bond alternation from one end group to another. The bond alternation ampHtude has been revealed to be proportional to the asymmetry degree, which can be calculated as the difference of topological indexes = 4>gj — 4>gg. The effect is maximum if Tgj > 45° and Tgg < 45°. If A4>j2 = 90°, the ideal polyene state is... 

Since highly basic heterocycles accommodate the positive charge readily and maintain the aromatic nature of the dimethylarnino benzene group, the resulting bond alternation induces a polyene character to the dye chromophore, and the absorption is shifted accordingly to a shorter wavelength. 

Monocyclic conjugated polyenes are referred to as annulenes, and there exists ample experimental evidence to support the conclusions based on application of HMO theory to neutral and charged annulenes. The relationship between stability and structure in cyclic conjugated systems will be explored more fully in Chapter 9. 

Previous theoretical studies of isolated and doped PA have mostly dealt with geometrical structures and charge and spin wavess. A few have dealt with the vibrational spectrumS2i = using force fields derived from fits to experimental infrared and Raman spectra of smaller polyenes or from scaled force constants obtained from semi-empirical SCF calculations on these systems. Very little the-... 

The autoinhibiting effect which was apparent in the later stages, particularly for thicker films, was attributed to the formation of charge transfer complexes between HC1 and polyenes. These complexes were assumed to be photochemically inert as far as further dehydrochlorination of the polymer was concerned but may be involved in the reverse process of re-addition of HC1 to the polyenes. The balance between the two effects depends on the ease of diffusion of HC1 out of the film. 

For 10-fold 13C labelled retinal, it has been shown that the differences between chemical shifts for polyene chain carbons of the chromophore in its native environment and detergent-solubilised system were small67 Analysis of the environment of the Schiff base has supported the model of stabilisation based on the protonation by a complex counterion. Three factors were responsible for the excessive positive charge in polyene (i) electronegative nitrogen, (ii) protonation and (iii) counterion strength. 

Charge transfer during diphenyl-polyene sorption in acidic ZSM-5 zeolite a primordial reaction for catalysis processes... 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

Keywords ZSM-5, diphenyl polyene, ionization, charge separation, spectroscopy. 

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