Enantioselective room temperature phosphorescence detection of non-phosphorescent analytes based on interaction with...

Menthol (MT) induces strong room temperature phosphorescence (RTP) of 1-bromonaphthalene (1BrN) in aqueous beta-cyclodextrin (beta-CD) suspensions, even under non-deoxygenated conditions. Interestingly, (-)-MT and (+)-MT enantiomers give rise to different phosphorescence intensities, the difference being 19+/-3%. It is argued that the signal can be mainly ascribed to the formation of ternary complexes beta-CD/1BrN/MT which show different RTP lifetimes, i.e. 4.28+/-0.06 and 3.71+/-0.06ms for (-)-MT and (+)-MT, respectively. Most probably, the stereochemical structure of (-)-MT provides a better protection of 1BrN against quenching by oxygen than (+)-MT. This interpretation is in line with the observation that under deoxygenated conditions the phosphorescence intensity difference for the two complexes becomes very small, i.e. only about 4%. The lifetime difference under aerated conditions enables the direct determination of the MT stereochemistry. For mixtures, in view of the 0.06ms uncertainty in the lifetime, enantiomeric purity can be determined down to 10%. Furthermore, in the case of MT the concentration of the least abundant enantiomer should be at least 3x10(-4)M, since otherwise complex dissociation would obscure the lifetime difference.