Binary and ternary complexes containing alpha-cyclodextrin and bromonaphthalene derivatives: a note of caution in interpreting UV absorption spectral data.

Abstract

Ultraviolet absorption spectra, NMR spectra, and phosphorescence measurements were used to confirm that alpha-cyclodextrin (CD) and 2-bromo-6-beta-D-glucopyranosidylnaphthalene (BGN) form only a binary complex and to characterize its properties. The binding constant for the CD.BGN complex was found to be 886 +/- 24 M(-1) and 770 +/- 110 M(-1) from NMR and UV absorbance measurements, respectively. Comparison of spectral properties revealed the CD.BGN complex to be binary and complexes containing CD and n-alkoxy (n-alkanoloxy) derivatives of 2-bromonaphthalene (N) to be of higher order, notably ternary. A red shift was observed in the UV absorption spectra of the CD(2).N complexes. The absence of a hydroxyl hydrogen atom on the naphthalene ring of N molecules made it impossible for hydrogen bond formation to a glucosidic oxygen in the CD cavity to be the cause of the red shift. The similar red shifts reported herein and for the ternary complexes of CD with 2-naphthol and 2-bromo-6-hydroxynaphthalene (BOHN) indicated that hydrogen bond formation between the hydroxyl hydrogen and glucosidic oxygen atom might not be the cause of the red shift for the latter guest molecules, as has been proposed previously. This result emphasizes the caution necessary in using UV absorption spectral data as evidence for hydrogen bond formation in molecular complexes containing CD.