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.

Thermal lens in a liquid sample with focal length controllable by bulk temperature

“…Selection of a mixture of DMSO and water was dictated by observations of strong effect of temperature on the energy of hydrogen bonds that 4-AP forms with e.g. water molecules in such mixtures [27,28]. Thus, we expected that temperature may most significantly affect f in such a solution of 4-AP, and indeed, it was confirmed for all thermal lens media we selected.…”

Non-Gaussian statistics of binding/unbinding events and the energetics of electron transfer reactions

“…(22) with pK ð2Þ B and b 2 g 2 D considered as fitting parameters. The emission slope k À1 B ðoX2=oTÞ P ¼ cP;2 of AP in toluene is 16.5 [26] and the best-fit parameters are pK ð2Þ B ¼ 0:95 and b 2 g 2 D ¼ À149.…”

“…Since the combination of strong binding to charged/polar surface residues with much softer binding to hydrophobic patches is a universal property of hydrated biopolymers [32], such an extension might provide a general theoretical framework for describing the energetics of redox proteins. [26] and the line shows the fit to Eq. (22) with pK ð2Þ B and b 2 g 2 D considered as fitting parameters.…”

“…As an example of application of this equation, we show in Fig. 5 the data for the temperature slope of the emission frequency, hm em ¼ X 2 , of 4-aminophthalimide (AP) recordered by Wetzler et al [26] as a function of ethanol concentration in the ethanol-toluene solvent mixture. Ethanol is known to form hydrogen bonds with three sites of AP.…”

“…Ethanol is known to form hydrogen bonds with three sites of AP. These bonds are significantly stronger in the excited state compared to the ground state [26,27] (g 2 > 0 and D < 0 in Eq. (22)).…”