Rigid chain polymers fluorescence emission

Ethanol swells cellulose and some dye molecules stay entrapped into the natural polymer chains and in close contact with the substrate. The use of dichloromethane, a solvent that does not swell microcrystalline cellulose, provides samples that exhibit a smaller fluorescence quantum yield. This is consistent with a larger degree of mobility (and also formation of nonplanar and less emissive conformers) of the cyanines adsorbed on the surface of the solid substrate, while entrapment provides more rigid, planar, and emissive fluorophors. 

An interesting result was also obtained (10) in a study of polyamides with stilbene residues in the chain backbone. In that case,an increasing polymer concentration led to a decreasing fluorescence intensity. This effect can be understood since the trans-cis isomerization and the emission from the excited stilbene moiety compete with each other. Thus, an increasing concentration of chain molecules which hinders the photoisomerization favors the fluorescence. This effect is observed long before the systems becomes glassy and it is, therefore, distinct from the enhancement of fluorescence in rigid media in dyes whose emission is quenched by internal motions of the excited molecules (11). 

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