TERNARY COMPLEXES OF 4H-1-BENZOPYRAN-4-THIONE WITH BETA-CYCLODEXTRIN AND ALCOHOLS - DRAMATIC EFFECT OF COMPLEX-FORMATION ON FLUORESCENCE AND PHOSPHORESCENCE INTENSITY

The effect of coinclusion of alcohol (ROH) on the stoichiometry, stabi lity, spectral, as well as photophysical properties of beta-cyclodextr in (beta-CD) complexes of 4H-1-benzopyran-4-thione (BPT) was studied. Although the binary complex was of 1:1 stoichiometry, the formation of ternary complexes of 1:2:2 BPT:ROH:beta-CD stoichiometry was observed . The formation of the ternary complexes is revealed by a dramatic enh ancement of fluorescence from the second excited singlet state and pho sphorescence occurring from the two lowest thermally equilibrated trip let states. The increase in intensities of these emissions is discusse d in terms of shielding the emissive molecule from bulk water molecule s and expulsion (perhaps still partial) of water molecules coincluded with the BPT molecule in the binary complex. Even the coinclusion with a molecule that acts in solution as a very efficient quencher of BPT phosphorescence led to a very strong phosphorescence enhancement, whic h indicates that there is a spatial separation of reactive centers of the two coincluded molecules. The possible reasons for the changes in the shape in the phosphorescence spectrum and the appearance of vibrat ional structure in the S-2 band in the absorption spectrum are also di scussed. The stability constants of the ternary complexes determined f rom absorption, fluorescence, and phosphorescence measurements depende d strongly on the molecular size and structure of a coincluded alcohol .