Five-ring chromophores

A comparison of the absorption and emission spectra of Ooct-OPV5 with those of the fully conjugated, similarly substituted polymer Ooct-PPV shows that the absorption and luminescence maxima of the five-ring model compound are only slightly blue-shifted relative to those of the polymer (see Fig. 16-11). Hence, the 

Optical properties of these oligomers have nearly converged to those of the polymer at the five-ring level. The luminescence quantum yields of oct-OPV5 and Ooct-OPV5 in solution are nearly 90%, more than twice that of the polymers. 

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The epoxyketone 3 is a versatile precursor for a variety of tricycloundecane systems having a p,y-unsaturated carbonyl chromophore, which are not so readily accessible. The contiguous epoxy ketone functionality and the double bond present in the five-membered ring provide opportunities for further manipulation. Adduct 3 may be transformed into a variety of molecular frameworks such as linearly fused cis anti cis tricyclopentanoids, protoilludanes, and marasmanes in a stereoselective fashion after suitable chemical and photochemical manipulation.13... 

The magnesium porphyrin radical, Mg(tetraphenylporphyrin)C104 (136), has been used as a model for the structural and stereochemical consequences of loss of an electron in photosynthetic chromophores.521 The primary photosynthetic reaction in plants and bacteria consists of a transfer of an electron, in the picosecond time domain, from the chlorophyll phototrap to nearby acceptors yielding chlorophyll -cation radicals. The structure of (136) shows a five-coordinated Mg2+ cation which is not quite symmetrically sited in the porphyrin ring but has metal-ligand distances similar to those found in the previous structures. The perchlorate anion is tightly bound (Mg—O = 2.01 A) in a monodentate mode. It was concluded that the porphyrin can act as an electron sink and that no major effects are found in the bond lengths, or on the stereochemistry, of the macrocycle. 

The photocycloaddition of vinylogous amides is not limited to substrates in which the chromophore is constrained in a five- or six-membered ring, and several groups have recently described the photocycloaddition of acyclic vinylogous amides. Tietze and coworkers reported that irradiation of 92 gives the bicyclic product 95 in quantitative yield (Scheme 22)43. The authors proposed that the observed product 95 could be the result of cycloaddition from the tautomeric form of the chromophore, followed by fragmentation of the resulting cycloadduct 93 to the imino-aldehyde 94, which condensed to the observed product. 

A comparison of similar substitution products such as the methyl and phenyl compounds shows that the four-membered ring seems to be a stronger chromophore than the five- and six-membered rings. In our present knowledge only cyclic systems (and probably also cage structures) seem to be able to form fluorescent polymers in a matrix of oxygen. 

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