Intramolecular excimer formation in a tripodal polyamine receptor containing three naphthalene fluorophores

A comprehensive investigation on the energetics and dynamics of a new fluorescent sensor constituted by a tripodal polyamine receptor containing three naphthalene fluorophores, compound L, is reported. The influence of external factors such as the solvent, hydrogen ion concentration, and temperature in the photophysics of the sensor is discussed. The temperature dependence of monomer/excimer interconversion of L revealed an average percentage relative sensitivity of 4.5%/°C thus portending its use as a temperature sensor. The activation energy for excimer formation (E₁ = 12 kJ mol^-1) and dissociation (E_-1 = 57 kJ mol^-1), entropy change (ΔS = -128 J K^-1 mol^-1), and the binding energy of the excimer (ΔH = 45 kJ mol^-1) were obtained in water at acidic pH values and ethanol (E₁ = 15 kJ mol^-1, E_-1 = 40 kJ mol^-1, ΔS = -61 J K^-1 mol^-1, and ΔH = 25 kJ mol^-1). The dependence of the kinetic and thermodynamic parameters on the dielectric constant of the medium and on the degree of protonation of the polyamine chain was interpreted in terms of the excimer destabilization provoked by the electrostatic repulsion between the positively charged chains.