Room temperature ionic liquids (RTILs) are salts that are liquid at or below room temperature. The unique properties of RTILs such as low melting point, low vapor pressure, non-flammability and large electrochemical potential window have made them very promising in applications ranging from solvents for organic synthesis and electrolytes for energy storage devices to potentially novel electro-optic materials. Despite the broad utility of RTILs, understanding the fundamental interactions between constituents at the molecular scale, and the existence of long-range organization in these systems remains limited. Therefore, it is important to characterize the length scale of organization in RTILs because such order will bring with it the development of a variety of novel applications.We used picosecond laser technologies and time resolved spectroscopic approaches to gain insight into the existence and length scale of molecular organization in RTILs. We have used time correlated single photon counting (TCSPC) detection with a confocal microscope for spatially resolved excitation and time-resolved emission collection to measure the rotational diffusion dynamics of three structurally similar chromophores (anionic, cationic and neutral) as a function of distance from the silica support. The results reflected the existence of a charge density gradient induced in the RTIL by the charge present on the silica surface. Control experiments were also performed for 1) identical measurements in ethylene glycol and 2) capping the silica support with Me2SiCl2 to prove this property is unique to RTILs in contact with charged surfaces. Second, we have used fluorescence anisotropy decay imaging (FADI) with a confocal microscope to measure the rotational diffusion dynamics of cationic chromophore cresyl violet as a function of distance from the conductive oxide support, FTO (fluorine doped tin oxide) or ITO (indium doped tin oxide). Using this experimental configuration, control over the bias and current applied to the support can be achieved, allowing the reorientation dynamics of the charged chromophores to vary with the potential difference between and the current across the FTO or ITO support. The effects of water on the induced charge density gradient in the RTILs were also explored. Data from those measurements showed that when ca. 25,000 ppm or more of water is added to the RTIL, the induced charge density gradient persists but with apparently diminished amplitude.The results of this work have demonstrated a novel experimental method to study the local organization in RTILs. These findings represent an initial step for characterizing and modulating the long range order in RTILs, which will result in using this family of materials most effectively and providing a practical framework to better understand ionic liquids.
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