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

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)

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

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.

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

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

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

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

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

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, 184512 (2005). *From: Cang, H., et al., J. Chem. Phys. 119, p. 13017 (2003).

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)

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?

Spectroscopy and Dynamics Summary