Complexation (cucurbit[6]uril-pyrene): Thermodynamicand spectroscopic properties

Abstract

The influence of the macrocyclic compound cucurbit[6]uril (CB6) on the photophysical properties of the fluorophore pyrene (PYR) has been studied. Guest–host interaction was observed by UV–visible spectroscopy and spectrofluorimetry. The fluorescence of PYR was significantly increased in the presence of CB6. The binding equilibrium constants for the complex with 1:1 stoichiometry were determined in HCOOH 55% w/v. The values of the association constants, KA, and the fluorescence quantum yield ratios between complexed and free substrate, ϕPYR–CB6/ϕPYR, at different temperatures were (3.1±0.9)×102 M−1 and (5.1±0.2), (3.6±0.5)×102 M−1 and (5.9±0.1), (4.8±0.7)×102 M−1 and (5.5±0.1) at 15.0 °C, 25.0 °C and 40.0 °C, respectively. The enthalpic and entropic contributions to the complexation process were determined, yielding ΔS=(92±3) J mol−1 K−1 and ΔH=(13±1) kJ mol−1. From these results it can be concluded that the complex formation is mainly driven by the entropic term. The forces involved in the complexation are interpreted from the sign and magnitude of the thermodynamic parameters obtained. The partial inclusion of PYR or the formation of a suspended complex is proposed in base of all the data. The interaction is also demonstrated in the solid state by differential scanning calorimetric (DSC) measurements.