Modified β-cylclodextrins as enhancers of fluorescence emission of carbazole alkaloid derivatives

Experimental conditions for formation of inclusion complexes between carbazole and ellipticine and several cyclodextrins (CDs): β-cyclodextrin (β-CD), hydroxypropyl β-cyclodextrin (HPβ-CD), (2,6-di-O-methyl)-β-cyclodextrin (DMβ-CD), (2,3,6-tri-O-methyl)-β-cyclodextrin (TMβ-CD) and γ-cyclodextrin (γ-CD), are described. A notable increase of fluorescence intensity of the host molecules is observed upon complexation, attributed to protection of the excited state within the cyclodextrin cavity. This allows the quantitation of both compounds in aqueous media in concentration levels of 10−7 M to 10−6 M. Inclusion also affects the deprotonation equilibrium of the pyrrolic nitrogen of both compounds. Thus, β-CD facilitates the deprotonation of carbazole while HPβ-CD, DMβ-CD, TMβ-CD and γ-CD make it more difficult. Similar observations were made for ellipticine, although its behaviour was more complex because of the coexistence of two ionization equilibria owing to the presence of a basic pyridine-like nitrogen. Finally, the effect of cyclodextrins on fluorescence quenching caused by heavy halogen atoms was also studied. In the case of carbazole, a static quenching was observed, which is prevented by modified β-CDs. In the case of ellipticine, the quenching effect was related to acid-base equilibria, and therefore it has been studied under acidic (λex = 305 nm, λem = 520 nm) and basic (λex = 292 nm, λem = 430 nm) conditions.