Solvation and electron transfer in ionic liquids
Open Access
- Author:
- Liang, Min
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 29, 2012
- Committee Members:
- Mark Maroncelli, Dissertation Advisor/Co-Advisor
Mark Maroncelli, Committee Chair/Co-Chair
James Bernhard Anderson, Committee Member
John B Asbury, Committee Member
John H Golbeck, Committee Member - Keywords:
- Solvation dynamics
ionic liquids
bimolecular electron transfer - Abstract:
- The complete solvation response of coumarin 153 (C153) in 21 neat ionic liquids (ILs) has been determined over the time range from 100 fs to 20 ns by combining broad-band fluorescence upconversion (FLUPS) and time-correlated single photon counting (TCSPC) measurements. The 80 fs time resolution of FLUPS provides accurate results for the fast dynamics and the long time window (20 ns) of TCSPC enables observation of the slow portion of the dynamics. These complete solvation response functions are compared to the solvation dynamics predicted by a simple dielectric continuum model. This dielectric continuum model works well in conventional solvents. However, the dynamics predicted in most ionic liquids are systematically faster than those observed, on average by a factor of 3–5. To bridge the knowledge gap between neat ILs and conventional solvents, the solvation dynamics of C153 in the mixture of a simple ionic liquid [Im41][BF4] and the prototypical dipolar solvent acetonitrile has been studied. This mixture was chosen because it is expected to exhibit the simplest behavior without much preferential solvation is not expected to complicate interpretation due to the similar ‘polarity’ of acetonitrile and [Im41][BF4]. The solvation energies of C153 in this mixture were determined and the complete solvation dynamics of C153 were measured and compared to simple dielectric continuum predictions. In addition, the rotational dynamics of C153 in the mixtures of [Im41][BF4] and acetonitrile were studied. The rotation times <rot> of C153 vary with viscosity in the manner <rot> p with p= 0.9. Steady-state and picosecond time-resolved emission spectroscopy are used to monitor the bimolecular electron transfer reaction between the electron acceptor 9,10-dicyanoanthracene in its S1 state and the donor N,N-dimethylaniline in a variety of ionic liquids and several conventional solvents. Detailed study of this quenching reaction was undertaken in order to better understand why rates reported for similar diffusion-limited reactions in ionic liquids sometimes appear much higher than expected given the viscous nature of these liquids. Consistent with previous studies, Stern-Volmer analyses of steady-state and lifetime data provide effective quenching rate constants kq, which are often 10 to100-fold larger than simple predictions for diffusion-limited rate constants kD in ionic liquids. Similar departures from kD are also observed in conventional organic solvents having comparably high viscosities, indicating that this behavior is not unique to ionic liquids. A more complete analysis of the quenching data using a model combining approximate solution of the spherically symmetric diffusion equation with a Marcus-type description of electron transfer reveals the reasons for frequent observation of kq kD. The primary cause is that the high viscosities typical of ionic liquids emphasize the transient component of diffusion-limited reactions, which renders the interpretation of rate constants derived from Stern-Volmer analyses ambiguous. Using a more appropriate description of the quenching process enables satisfactory fits of data in both ionic liquid and conventional solvents using a single set of physically reasonable electron transfer parameters. Doing so requires diffusion coefficients in ionic liquids to exceed hydrodynamic predictions by significant factors, typically in the range of 3-10. Direct NMR measurements of solute diffusion confirm this enhanced diffusion in ionic liquids.