Inclusion Complexes of 6-Bromo-2-Naphthol (Guest) and α-Cyclodextrin (Host): Thermodynamics of the Binary Complex and First-Reported Dynamics of a Triplet-State Guest/Host2 Complex

Abstract

The thermodynamics properties of the 1 guest−1 host ground-state complex were determined from near UV absorbance measurements as a function of temperature. Entropy changes for formation of the binary and ternary complexes suggest that the brominated end of the guest molecule is attached to the first host molecule, and the hydroxyl end is encapsulated by the second host. The enthalpy of stabilization is approximately twice as great for the ternary complex as for the binary complex in the ground state. A molecular kinetics mechanism is presented that is used to determine some of the dynamics properties of the 1 guest−2 host complex from measurements of the pulsed laser-induced phosphorescence that emanates from the triplet state of 6-bromo-2-naphthol. The rate of dissociation of the triplet-state ternary complex is governed by a large Arrhenius activation energy (82 ± 1 kJ/mol), and the dissociation rate constant (k-d = 8400 s-1 at 25 °C) is more than an order of magnitude smaller than that in typical binary complexes of cyclodextrins. It is suggested that the apparent increased stability of the ternary complex over the binary complex is due to general van der Waals interactions and/or hydrogen bonding between host molecules.