1. International Symposium on Molecular Spectroscopy
ULTRAFAST INFRARED STUDIES OF THE VIBRATIONAL RELAXATION DYNAMICS OF ANIONS IN IONIC LIQUIDS
Kevin Dahl, Gerald M. Sando, and Jeffrey C. Owrutsky Chemistry Division U.S. Naval Research Laboratory
International Symposium on Molecular Spectroscopy

2. Introduction: Ionic Liquids
Novel materials for chemistry
Low-temperature molten salts
Wide variety of cation/anion pairs will make ILs
Understanding of microscopic solvent properties of ILs N 1 C 5 4 3 2 +
[BMIM]+ C N S -
[NCS]- N +
[BM2IM]+ N C -
[N(CN)2]- F B -
[BF4]-
IR-inactive (2000 cm-1)

3. Introduction: IR Background Work
Vibrational spectroscopy and dynamics of small anions (N3-, NCS-, and N(CN)2-)
“Measure” solvent interaction
Similar solvent dependence of spectroscopy and dynamics leads to strong correlation
Compare ILs to molecular solvents

4. Experimental Concept IR Spectra Energy Levels Transient Experiment
Static (FTIR)
Transient
DA(n) at Dt
Excited state
absorption
(+DA)
Ground state
bleach (- DA)
IR Spectra
Energy Levels
V=0
V=1
V=2
Vibrational
Energy
Relaxation
(VER)
DA(Dt) at n
k1=1/T1
Transient Experiment
Experiment
pump
probe Dt
DA(n,Dt)
Probe population dynamics in the ground or excited state.

5. Experimental Considerations
Time-resolved IR pump – IR probe spectrometer
<200 fs, >4 μJ pump at ~ 5 μm

6. Solution-Phase Spectroscopy: N3-
Spectra in ILs similar to molecular solvents
ILs behavior closest to aprotic solvents (i.e. DMSO)
NCS- has no shift  solvent- dependent width

7. Solution-Phase Spectroscopy: N(CN)2-
Spectra in ILs similar to molecular solvents
Behavior closest to aprotic solvents (i.e. DMSO)
Three N(CN)2- bands behave in similar manner
Majority/minority anion behavior is identical

8. Anion Solution-Phase Dynamics
Transients similar to bulk molecular solvents (~10 ps)
N3- and NCS- transients fit by single exponential
Bleach recovery and absorption decay are identical
Strange behavior from N(CN)2- transients – persistent

9. Rate-Shift Correlations
Trend established by molecular solvents describes IL solvents (N3- and N(CN)2-)
ILs behavior most similar to aprotic solvents (i.e. DMSO)
Majority/minority anion behavior identical
NCS- dynamics scale with solvent polarity

10. Rotational Dynamics f T k V h t = Interest in reorientation in ILs
Stokes-Einstein-Debye (SED) prediction:
Poor resolution of rotational data (T1 << TR) in ILs
Other studies* of rotation in ILs show adherence to SED predictions f T k V B
rot h t =
Shirota, H., et al., J. Chem. Phys. 122, (2005).
*From:
Cang, H., et al., J. Chem. Phys. 119, p (2003).

11. Conclusions Acknowledgements
Small anion probes effective for investigating IL solutions
Spectroscopy and dynamics of ILs are similar to molecular solvents
ILs function as highly-polar, (weakly)-hydrogen bonding liquids
Acknowledgements
$ – Office of Naval Research (NRL)
NRC Postdoctoral Associateship – KD
ASEE Postdoctoral Fellowship – GMS
Doug Fox, Tom Sutto, Andrew Purdy (NRL)

12. Bulk Solution Spectroscopy: NCS-
NCS- has strong spectral solvent-dependence
IL spectra similar to bulk solvents (no shift!)
Comparison of [BM2IM] and [BMIM] cations – H-bonding?

13. Spectroscopy and Dynamics Summary