Optical Activity (CD and CPL) as a Probe of Ion Pairing and Solution Structure of Macrocycle Complexes

Abstract

The nature, significance and methods of evaluating ion pairing effects are reviewed, particularly with respect to the complexes of crown ethers and related macrocycles. The application of chiroptical techniques, specifically circular dichroism (CD) and circularly polarized luminescence (CPL), to the investigation of ion pairing and related solution structural phenomena in complexes of chiral crown ethers with alkali, alkaline earth, transition metal and lanthanide ions is described. The use of induced optical activity of crown substituent π → π* transitions, cation d → d and f → f transitions and anion n → π* transitions is illustrated. Emphasis is placed on recent results of the use of NO 3 - and NO 3 - n → π* CD as a probe of ion pairing in alkali and alkaline earth complexes of four chiral derivatives of 18-crown-6 in chloroform and on related studies in various solvents of the f → f CPL of europium(III) nitrate, Perchlorate and chloride complexes of the same macrocycles. Structural deductions based on the chiroptical results are correlated with total luminescence, vibrational spectroscopy, conductance and crystal structure data, as appropriate.