Spectroscopic Determination of Pressure-Induced Shifts in Inclusion Complexation Equilibria

Abstract

The effect of modest hydrostatic pressure (<350 bar) on condensed-phase equilibrium processes has been largely overlooked, due in large part to the small compressibility of these phases relative to gases or supercritical fluids. Although the bulk properties of condensed phases are not significantly modified by pressure in this modest regime, the solvation processes driving inclusion complexation may be appreciably affected. In this paper, we examine this hypothesis using steady-state fluorescence spectroscopy to determine the pressure dependence of association constants. The widely used host molecule, β-cyclodextrin, provides an incompressible hydrophobic cavity into which structurally analogous fluorescent probes are encapsulated. By comparing the unique pressure dependencies of these equilibria, the importance of local site solvation and rim interactions in influencing the pressure dependence is demonstrated. The structurally analogous complexes chosen for these studies are expected to have similar pressure-dependent behavior based on comparable solvation structures. However, pressure-induced changes in the association constant for these two analogs are quite distinct, with differences in K(c) ranging from clearly pressure dependent (-14%) to pressure independent over 338 bar. Additional solvation perturbations are observed in the pressure dependence of the quantum efficiency for both complexes (-7.3% and -9.4%). Thus, pressure-induced perturbation in the fluorescence properties of the complex need not be accompanied by simultaneous changes in the complexation equilibrium. Finally, these pressure-induced changes in complexation selectivity are important for all measurements conducted under variable pressure conditions, including liquid chromatography and process monitoring.