Structures and excited state properties of 2-and 3-hydroxybiphenyl complexed with cyclodextrins

The dual fluorescence of 2- and 3-hydroxybiphenyl in water, due to intermolecular proton transfer of the excited molecules to the solvent, was found to be specifically affected by the addition of alpha-, beta- and gamma-cyclodextrins. The extent of the effect depends on the nature of the cyclodextrin and is the result of the formation of inclusion complexes, in which proton transfer is partially depressed. The structure of the complexes was determined by conformational calculations applying a dynamic Monte Carlo approach. The computed conformations were verified by comparing experimentally determined and calculated induced circular dichroism (icd) spectra. The fluorescence characteristics of the inclusion compounds were also well accounted for by the calculated complex structures. All investigated complexes were found to adopt a 1:1 configuration, with the exception of the 3-hydroxybiphenyl-alpha-cyclodextrin system, for which both icd and conformational calculations indicated the formation of 2:2 complexes. This 2:2 structure was, however, not appreciably reflected in the triplet state properties of the system, indicating that these complexes are dynamically much less stable than their counterparts in the 4-hydroxybiphenyl-alpha-cyclodextrin system. The role of bulk and co-included water molecules on observed fluorescence properties is discussed.