Liquid solutions and liquid-liquid interfaces are the media for numerous chemical reactions, where the local environments are conducive to interactions and exchanges. As a means of broadening our understanding of the molecular organization in liquid solutions and in proximity to phase boundaries, we report on the rotational diffusion dynamics of probe molecules in increasingly complex systems. Neat liquids represent the simplest systems interrogated. In our first study, we measure the induced orientational anisotropy function, R(t), using time-correlated single photon counting of two chromophores, resorufin and 6-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoic acid (NBDHA) in water and in N-octyl-2-pyrrolidone (NOP). Our data show distinctly different anisotropy decays in aqueous solution compared to NOP solutions, and taken in the context of the Chuang and Eisenthal model, this indicates a change in effective rotor shape swept out by the solute's rotation. The similarity in the behavior of the two different chromophores in these solvent systems points to solvent-solvent interactions and local organization as the dominant factors in mediating motional dynamics. By adding varying concentrations of electrolyte to aqueous and NOP systems, the rotational diffusion dynamics of the anionic chromophore, resorufin, are altered as a result of interaction with electrolyte cations. In aqueous solutions containing lithium perchlorate (LiClO4), resorufin exhibits a single exponential anisotropy decay as was the case in pure water, however, reorientation times are dependent on electrolyte concentration. In contrast to the observed behavior of resorufin in pure NOP, where bi-exponential decay occurs, we observe a single exponential anisotropy decay for resorufin in NOP with the addition of non-aqueous electrolyte. For resorufin in NOP, the reorientation time constant increases with increasing electrolyte concentration, consistent with formation of a complex between the resorufin anion and the electrolyte cation. Elucidating the local organization near phase boundaries compared to bulk solutions represents a more complex problem to interrogate. We use a TCSPC confocal imaging instrument to obtain depth-resolved fluorescence lifetime and anisotropy decay data for lissamine rhodamine B sulfonyl chloride (LRSC) in NOP saturated with water and in ethylene glycol (EG) supported on a glass surface. The fluorescence anisotropy data of LRSC in NOP and EG as a function of distance from NOP|glass and EG|glass interfaces reveal a gradient spanning tens of micrometers from the NOP|glass interface into the NOP phase, and no corresponding gradient in EG. From these data, we assert there is a compositional heterogeneity in the form of water nano-droplets in the NOP phase, where water molecules solvate LRSC. NOP is miscible with small amounts of water allowing for the presence of water inclusions in solution, while phase separation occurs for sufficiently high water concentrations. By forming a liquid-liquid interface composed of water and NOP supported on glass, we observe both compositional and dielectric gradients. We report on spatially resolved fluorescence lifetime and anisotropy decay time constant data of LRSC in the NOP phase near the liquid-liquid (NOP|water) interface and the liquid-solid (NOP|glass) interface. These data reveal a micron-scale position-dependent molecular environment characterized by an anisotropy decay gradient as a function of distance from the NOP|glass interface. In addition, the presence of a fluorescence lifetime gradient normal to the NOP|water interface indicates the existence of a gradient in the concentration of water nano-droplets.
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