Inclusion of 2,4- and 2,5-Diphenyloxazole by γ-Cyclodextrin in Aqueous Solution

Abstract

Abstract In aqueous solution, the interactions of 2,4- and 2,5-diphenyloxazole (2,4- and 2,5-DPO) with β- and γ-cyclodextrin (β- and γ-CD) have been examined by means of absorption and fluorescence spectroscopies. γ-CD forms a 2:1 host–guest inclusion complex with 2,4-DPO, whereas β-CD forms a 1:1 inclusion complex with 2,4-DPO. Equilibrium constants for the formation of these inclusion complexes have been evaluated. Upon the addition of alcohol to 2,4-DPO solution containing γ-CD, the 2:1 γ-CD–2,4-DPO inclusion complex accommodates alcohol to form a 2:2:1 γ-CD–alcohol–2,4-DPO inclusion complex. The equilibrium constant for the formation of the 2:2:1 inclusion complex increases as the alkyl chain of the alcohol is lengthened from 1-pentanol to 1-heptanol. A similar trend in the magnitude of the equilibrium constant has been observed for diols (1,9-nonanediol and 1,10-decanediol). In contrast to 2,4-DPO, 2,5-DPO forms a 1:1 γ-CD–2,5-DPO inclusion complex at a low concentration of 2,5-DPO. At a high concentration of 2,5-DPO, excimer fluorescence of 2,5-DPO has been observed in the presence of γ-CD. From simulations of excimer fluorescence intensity as a function of γ-CD concentration, the excimer fluorescence is ascribed to both a 1:2 γ-CD–2,5-DPO inclusion complex and a 2:2 γ-CD–2,5-DPO inclusion complex.