Effects on the photophysical properties of naphthylamine derivatives upon their inclusion in cyclodextrin nanocavities

Abstract

In this study we report on steady state absorption, emission, and time resolved fluorescence emission studies of 2-naphthylamine-6-sulfonate (2NA6S) and 1-naphthylamine-4-sulfonate (1NA4S) compounds in neutral aqueous solution and when added to different cyclodextrin derivatives (CDs) such as α-, β-, γ-, methyl-β- and 2-(hydroxypropyl) β-cyclodextrin. Steady state measurements reveal that 2NA6S and 1NA4S form 1:1 inclusion complexes with the studied cyclodextrin derivatives. Steady state absorption measurements show a considerable change in 2NA6S spectrum upon addition to β, methyl β- and 2-(hydroxypropyl) β-cyclodextrin with association constants, Ka, of 70.0, 160.0 and 20.0 M−1, respectively, while no change was observed upon the addition of α-CD or γ-CD to its aqueous solution. Addition of the studied CDs to 1NA4S did not show any changes to its absorption spectrum, indicating a very weak interaction in the ground state in this case. In contrast to the weak changes observed in the absorption spectra of the host compounds, changes in their corresponding steady state fluorescence emission spectra were much pronounced with an association constants, Kf, of 20.0, 220.0, 42.0, 550.0, and 110.0 M−1 when 2NA6S was included in α-, β-, γ-, methyl-β- and 2-(hydroxypropyl)-β-cyclodextrin, and 57.0, 420.0 and 110.0 M−1 when 1NA4S was included in β, methyl β- and 2-(hydroxypropyl) β-cyclodextrin, respectively. Fluorescence decay traces of 2NA6S and 1NA4S were found to fit well with biexponential decay function in the absence and presence of the studied CDs. Fluorescence decay changes were very noticeable in case of 2NA6S inclusion in β-CD with an association constant very close to that obtained from steady state fluorescence measurements. 1H NMR measurements have been used to confirm the inclusion of 2NA6S and the partial inclusion of 1NA4S in the cyclodextrin derivatives.