Fluorescence conjugated chains

Hemicya.nines, These half-cyanine dyes may be represented by stmcture (48). They may be considered as cyanines in which a benzene ring has been inserted into the conjugated chain. Hemicyanines provide some bright fluorescent red dyes for polyacrylonitrile. 

The studies at solid electrodes showed that the introduction of delocalized conjugated units and aromatic groups in a molecular monolayer affects the ET rate (32b). The SECM has been used to compare the ET rate across C-16 and similarly sized dipalmytoyl phosphocholine (DPHC) (Fig. 13). The latter phospholipid has a fra .s,/ra/7.y,fram-l, 6-diphenyl-l,3,5-hexatrieiie chain. This prototype of polyene systems has been previously used as a fluorescent probe (45). Its nonpolar hydrocarbon structure is compatible with membranes and can serve as an electron relay in a blocking monolayer film. In addition to its conjugated chain, DPHC also contains one saturated C-16 alkyl chain, which is easily incorporated into the C-16 monolayer. 

Although the 2Ag state is optically-forbidden , efficient B to 2Ag internal conversion causes fluorescence fromthe 2Ag state when the conjugated chain is short enough, en the conjugated chain becomes longer, the energy gap between the B and 2 A states increases, and then, the crossover from the 2A fluorescence to the B fluorescence takes place. Therefore, the detection of the 2A fluorescence becomes extremely difficult when the number of the conjugated double bonds (n) exceeds 9. 

Fluorescence spectroscopy can be applied, in principle, to carotenoids having a longer conjugated chain. However, the most serious problem is the identification of the (0-0) origin, because the 2A fluorescence becomes broader with increasing chain... 

The optical properties of polymer conjugated hydrocarbons are of increasing importance due to the possibility to induce an electrically stimulated luminescence. Because the fluorescence is depended from the conjugation chain length the information of absorption dependence from the chain length is necessary. 

Many chemical compounds have been described in the Hterature as fluorescent, and since the 1950s intensive research has yielded many fluorescent compounds that provide a suitable whitening effect however, only a small number of these compounds have found practical uses. Collectively these materials are aromatic or heterocycHc compounds many of them contain condensed ring systems. An important feature of these compounds is the presence of an unintermpted chain of conjugated double bonds, the number of which is dependent on substituents as well as the planarity of the fluorescent part of the molecule. Almost all of these compounds ate derivatives of stilbene [588-59-0] or 4,4 -diaminostilbene biphenyl 5-membeted heterocycles such as triazoles, oxazoles, imidazoles, etc or 6-membeted heterocycles, eg, coumarins, naphthaUmide, t-triazine, etc. 

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