Interaction of 4-hydroxybiphenyl with cyclodextrins: Effect of complex structure on spectroscopic and photophysical properties

The structures, spectroscopic and photophysical properties of the inclusion complexes between 4-OH-biphenyl and cyclodextrins (CD) in water were studi ed theoretically and experimentally. The complex structures were predicted using a dynamic Monte Carlo procedure including solvation effects and analy zed via computed and experimentally determined circular dichroism. The inte rpretation of the induced circular dichroism spectra indicated the formatio n of stable 2:2 complexes between the probe and alpha-CD, while 1:1 structu res account for the circular dichroism induced by beta- and gamma-CD. Fluor escence emission quantum yields and lifetimes, measured as a function of th e CD concentration, confirmed the formation of this higher order complex wi th alpha-CD. Prototropic equilibration in the singlet excited state was fou nd to be depressed in 2:2 complexes due to the hydrophobic environment of t he OH groups, while it remained unperturbed in 1:1 complexes, where the sub stituent is exposed to the aqueous environ-ment. Triplet-triplet absorption and triplet quenching data supported this interpretation. The photophysica l properties of both the 1:1 and the 2:2 complexes are characterized by a s ignificant reduction of the nonradiative decay rates.