Dynamic Solvation of Aminophthalimides in Solvent Mixtures

Abstract

The solvatochromism and thermochromism of 4-aminophthalimide and 4-amino-N-methylphthalimide were studied by absorption and steady state and time-resolved fluorescence emission in solvent mixtures of toluene−ethanol and toluene−acetonitrile in the temperature range 5−70 °C. The wavelengths of the maximum of absorption and of fluorescence emission shift to the red with the increase of the proportion of the polar component in the mixture. The greater affinity for the polar component of the mixture of the excited state compared to the ground state enhances preferential solvation, which is the origin of this red shift. On the other hand, a spectral shift to the blue is found upon temperature increase in solvent mixtures. This fact can be explained by considering that association of the polar component with the excited-state solute is exothermic and decreases with temperature. The appointed solvation change is attained by diffusion-controlled exchange of solvent molecules between the bulk and the solvation sphere. This process leads to a time-dependent emission spectrum in the nanosecond time domain. In this work a kinetic scheme is developed to describe this exchange and explain the time-dependent fluorescence emission spectra. The description is based on a Langmuir type association of the solvent molecules with the solute. The solvation equilibrium is attained by stepwise solvent exchange. The kinetic data and the spectral information are integrated in a thermodynamic cycle that can describe the solvation of excited and ground states at any solvent composition.