Effect of β-Cyclodextrin on the Excited State Properties of 3-Substituted Indole Derivatives

The effect of β-cyclodextrin on the absorption and fluorescence spectra of 3-R-indoles (R = CH3 or (CH2)nCOOH with n = 1, 2 and 3) was studied. The inclusion in β-cyclodextrin barely changed the UV spectra, while significant variations were produced in the emission spectra, particularly regarding the fluorescence quantum yields. These changes, which are pH dependent, allow the determination of the association equilibrium constants (kAssoc) under different sets of conditions. For the derivatives with R = (CH2)nCOOH, the kAssoc at pH = 2 increased linearly with the number of CH2 groups. The changes in fluorescence quantum yield are higher for the neutral than for the ionized compounds due to greater changes in the nonradiative rates of the neutral species when it is un-ionized.