Excimer emission chain conformation effect

It has been demonstrated that the excimer emission intensity from chromophores incorporated into the vinyl polymer chain have been correlated with a change in the effective volume of random polymer coil in solution, and the volume of random polymer coil was correlated with viscosity, which is dependent on temperature. But a temperature change of a polymer solution does not always lead to expansion or contraction of the chain coil it can cause conformation change to a more or less packed polymer chain and to a more or less excimer forming conformation. Both of these structural changes in a polymer chain can affect the fluorescence, but each polymer needs to be considered individually. 

In spite of the large spacing of the fluorescent residues, an intense excimer emission was observed (48). The In/L ratio increased with concentration beyond a sharply defined point, corresponding presumably to the onset of chain interpenetration. However, the lifetime of monomer emission was concentration independent and it was concluded that a cooperative effect of evenly spaced chromophores favored chromophore dimer formation even in the ground state. The high intensity of excimer emission observed even in very dilute solutions seemed to imply a folded chain conformation. 

The hypochromicity of Ad-Q-Ad and poly-VAd corresponds to the formation of the excimer which has also been concluded from the results of the fluorescence emission spectra. The stereomodel of Ad-Q-Ad shows that the conformation in which two adenine rings are situated plane-parallel and, thus, the formation of the excimer is possible. From these considerations, it can be inferred that there exist two kinds of interactions between adenine residues exhibiting either plane-to-plane or coplanar arrangements. Both interactions can be expected to be valid for poly-VAd which includes two kinds of interactions along the polymer chain corresponding to Ad-Q-Ad and Ad-Q-Ad. This effect may give rise to high hypochromicity in the spectra of the polymer. 

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