Excimer fluorescence and structures of the inclusion complexes of beta-cyclodextrin with naphthalene and its derivatives

Fluorescence of the inclusion complexes with different compositions formed by naphthalene-h(8), naphthalene-d(8), 2,7-dimethylnaphthalene (DMN), and 2 -benzylnaphthalene (BN) with beta -cyclodextrin (beta -CD) in water was stu died. Two types of fluorescence are observed, monomer (MF) and excimer (EF) fluorescence. The excimer fluorescence of the 2 : 2 complex emitted by agg regated light-dispersing crystals forming a precipitate, whereas is the MF is observed in the transparent solution. When naphthalene and beta -CD are present in equimolar concentrations, EF predominates far the resulting comp lexes. A proposed structure of the inclusion complexes was derived from MND O/PM3 semiempirical quantum-chemical calculations. The EF is caused by the structure of the complex, in which both naphthalene molecules are separated by a distance of 4.7 A; they lie in parallel orientation to each other, wh ereby one ring is displaced from the other by one-fourth of the length of t he naphthalene ring. The complexes of 2,7-DMN and 2-benzylnaphthalene with beta -CD do nor exhibit EF. For the 2. 2 complex of 2,7-DMN with beta -CD, this is due to the fact that the aromatic fragments are removed too far fro m one another. 2-Benzylnaphthalene is unable to form an inclusion complex w ith beta -CD, in whose structure the aromatic fragments inside the cavity c ould be arranged in parallel planes; instead, it forms a 1:2 complex with b eta -CD.