Naphthalene complexation by β-cyclodextrin:: Influence of added short chain branched and linear alcohols

Naphthalene forms 1 : 1 complexes with beta-cyclodextrin (beta-CD) in water. The binding constant is 377 +/- 35 M-1. Addition of linear or branched alcohols causes a reduction in the apparent strength of naphthalene binding (K-app) compared to the value in the absence of additives. For example, 1% 1-pentanol reduces K-app to 184 +/- 31 M-1. Branching does not alter K-app much for a given number of carbon atoms, e.g., it is 113 +/- 9 M-1 for 2-pentanol and 116 +/- 8 M-1 for 3-pentanol. The exception to this is tert- butanol for which K-app is 577 +/- 40 M-1. The variation in K-app as a function of [1-pentanol] yields values for the individual equilibrium constants contributing to K-app. This reveals that a ternary complex forms involving naphthalene, the CD and 1-pentanol. The constant for formation of the ternary complex is 99 +/- 29 M-2. NaI quenching of naphthalene fluorescence indicates that the CD cavity partially protects the naphthalene excited state from this water phase quencher. Interestingly, the Stern-Volmer constant is lower in the presence of 1-pentanol than in its absence, although there should be more unbound (and therefore more NaI accessible) naphthalene in the former system than in the latter. These apparently contradictory results are discussed in terms of ternary complex formation.