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42375-AC6
Dynamics and Interactions in Ionic Liquid
This ACS-PRF grant enabled us to jump-start a research program in the physical chemistry of ionic liquids that has shown rapid progress, a substantial productivity, and a high level of national and international recognition. Though we had preliminary data reported in the grant proposal submitted in spring 2004, there had been no publications ready at that time.
Highlights of the research work were in three primary areas. First, as proposed, we carried out extensive studies of the temperature-dependent solvation dynamics and local friction in ionic liquids. The standard coumarin 153 solvatochromic molecule was used to probe the dynamics using time-resolved fluorescence spectroscopy. A series of four ionic liquids having non-aromatic cations were studied; each had the bis(trifluoromethylsulfonyl)imide anion in common.
Notable features resulting from this work included observation of non-Arrhenius temperature-dependence that was well fit using the Vogel-Fulcher-Tammann model. Though stretched-exponential models provided acceptable fits to the solvation dynamics correlation functions as a few temperatures, in general these models did not work overall, and three- and four-exponential models were required to fit the data. In the fluorescence anisotropy experiments, lifetimes both shorter and longer than would be predicted by a simple hydrodynamic model were observed, providing insights into the complexity of the local structure of the liquid surrounding the probe.
A second research accomplishment was the creation and spectroscopic characterization of a unique model system, an ionic liquid with a homologous neutral binary liquid. The ionic liquid was comprised of N-methoxyethylpyridinium cation paired with the dicyanamide anion. Methoxyethylbenzene was the isoelectronic and isostructural homolog of the pyridinium cation, while dicyanomethane was the isoelectronic homolog of the dicyanamide anion. The ionic liquid viscosity was more than 30-fold higher than for the isoelectronic neutral homolog, and its density was also 20% higher. These macroscopic factors pointed to stronger intermolecular interactions being present in the ionic liquid, which were indeed observed using the intermolecular vibrational spectrum obtained using femtosecond nonlinear optical spectroscopy.
The third major research area was to develop and characterize a novel series of ionic liquids where the imidazolium cations had alkylsilyl or alkylsiloxy substituents instead of the common alkyl groups. Lower glass transition temperatures and room-temperature viscosities were observed for each of the new ionic liquids in this series. The dynamics were characterized by both femtosecond nonlinear optical spectroscopy as well as by NMR diffusion studies using the pulsed-gradient spin echo method.
Other highlights that resulted from the ACS-PRF funded research project included:
• Five major research papers in ionic liquids, plus the grant partially supported work in two other areas (dynamics in polymer aggregates, and fluorescence probing of s.s. DNA hairpins).
• Researchers included undergrad. student and faculty from an undergrad. institution (R. Lopato and Prof. S. H. Chung, William Paterson University of New Jersey), Brookhaven National Laboratory (Dr. A. Funston and Dr. J. F. Wishart), a faculty member from a minority-serving research university (Prof. S. G. Greenbaum, Hunter College, CUNY), as well as postdoctoral, and undergraduate and graduate students at Rutgers University (B. J. Lee, T. A. Fadeeva, and H. Shirota).
• The ionic liquids research program initiated using this ACS-PRF grant developed sufficiently to be awarded a regular NSF Physical Chemistry 3-year program grant from 2007-2010.
• The P.I. co-organized a symposium on the "Physical Chemistry of Ionic Liquids" at the 232nd A.C.S. National Meeting, Sept. 10-14, 2006, in San Francisco. The Symposium had eight half-day sessions, with over 48 speakers and numerous posters.
• The P.I. shared the Guest Editor duties for assembling a special issue on the "Physical Chemistry of Ionic Liquids" for J. Phys. Chem. B, Vol. 111, Issue 18 (May 10, 2007) with 46 invited articles. Three of the articles included in the special issue resulted from work directly supported by the ACS-PRF grant.
• The P.I. was invited to join the International Organizing Committee for the Second International Congress on Ionic Liquids (COIL-2), held August 5-10, 2007 in Yokohama, Japan.
• One of the postdoctoral workers supported in part by this grant, Dr. Hideaki Shirota, has obtained a tenured faculty position at Chiba University in Japan.
• The P.I. was invited to write an article for a special issue on Ionic Liquids for Accounts of Chemical Research, which is now accepted for publication.
