Abstract
An absorption and fluorescence spectral study has been carried out for four molecules: 2-[(p-amino)phenyl]-3,3-dimethyl-3H-indole (1), 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-3H-indole (2), 2-[(p-amino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (3), and 2-[(p-dimethylamino)phenyl]-3,3-dimethyl-5-carboethoxy-3H-indole (4) in cetyltrimethylammonium bromide (CTAB) and water solutions. Arrhenius plots for the nonradiative decay processes competing with fluorescence are nonlinear in CTAB. This is ascribed to the fact that the probes are locked into a site where the twisting of the phenyl moiety is nearly inhibited in CTAB at room temperature. The binding constants increase in an increasing order of the molecular size (exponential growth) thereby showing that hydrophobic effects are the driving force behind the solubilization of these molecules in CTAB. Calculation of transfer free energies has confirmed the importance of the hydrophobic effect compared to the Coulombic interactions. The neutral-monocation equilibrium is lowered in presence of CTAB.
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