Photophysics of calixarenes bearing two or four dansyl fluorophores: charge, proton and energy transfers

Abstract

The photophysical properties of calixarenes bearing two and four dansyl derivatives (Calix-DANS2 and Calix-DANS4) were investigated in a CH₃CN/H₂O mixture (60 ∶ 40 v/v). The carboxydansyl (DANS1) was used as a reference compound. The pH titration of DANS1 permits determination of two pK_A values (2.4 and 6.3). The charge-transfer character of this type of fluorophore is discussed. The protonated dansyl group LH₂⁺ undergoes photoinduced proton transfer. The photophysical characterization (absorption spectra, emission spectra, quantum yield and fluorescence decays) of Calix-DANS2 and Calix-DANS4 showed that nonradiative energy transfer occurs from a dansyl fluorophore in the basic form to another one in the neutral form. The transfer efficiencies are 0.1 and 0.5 for Calix-DANS2 and Calix-DANS4, respectively. Fluorescence data analysis allowed us to determine the fractions of dansyl fluorophores in the basic and neutral forms.