Irradiation polyenes, conjugated

Solvent cast, oriented PVC films exposed to monochromatic radiation also underwent dehydrochlorination when exposed to radiation in the wavelength range of X. > 340 nm. The optical density of polymer changes as conjugated polyene sequences are produced by photodehydrochlorination. Action spectrum of the change in optical density per incident photon showed [83] a logarithmic dependence on irradiation wavelength for X > 254 nm. 

As shown in Figure 1.13, irradiation of an alkene or conjugated polyene system promotes an electron from its ground state HOMO to the ground state LUMO, which then becomes the highest occupied molecular orbital in the excited state, for example, W3 of butadiene becomes HOMO upon excitation of an electron from W2 to W3 on irradiation. 

When poly(vinyl chloride) is exposed to UV irradiation it turns yellow and subsequently a deep red-brown colour. This discoloration occurs rapidly, due to the formation of conjugated polyene structures. Simultaneously, large... 

The electronically excited conjugated polyenes behaves as radical on irradiation, which leads to a number of products by the recombination of radicals and also lead to cyclisation. For example- cyclisation of hex-1, 3, 5-triene. 

The intense discoloration which developed rapidly upon UV exposure reveals the high photosensitivity of C-PVC that is even more pronounced than for PVC itself, as shown by the UV-visible absorption spectra of figure 5. After 15 minutes of irradiation, large amounts of polyenes have already accumulated in C-PVC, with sequence lengths up to 20 conjugated double bonds, while PVC is hardly affected after that short exposure. 

On the other hand, photochemical addition of alcohols to conjugated dienes, observed in the case of cholestadieiie-3,5, is probably related to the reactions observed when vitamin A acetate is irradiated in methyl alcohol. It is also possible that the photoisomerization of acyclic trienes to allenes is related to the as yet incompletely studied dimerization reactions of larger polyenes. 

We noted previously that photochemical electrocyclic reactions take a different stereochemical course than their thermal counterparts, and we can now explain this difference. Ultraviolet irradiation of a polyene causes an excitation of one electron from the ground-state HOMO to the ground-state LUMO. For example, irradiation of a conjugated diene excites an electron from il/2 to and irradiation of a conjugated triene excites an electron from i/f j to (/ 4 (Figure 30.6). 

Photo-irradiation of [Pd3 Ar(CH = CH)4Ar 2][BAr ]2 results in an unusual photochemical face inversion of the multihapto-coordinated p-olefin ligands on a metal-metal chain. This process originates from the unique electronic structure of the polyene sandwich compounds as well as from the 7i-stacking of the pTi-conjugated ligands (Scheme 4S)P... 

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