ULTRAFAST INFRARED STUDIES OF THE VIBRATIONAL RELAXATION DYNAMICS

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 N 1 C 5 • Low-temperature molten salts + C 2 N 3 N C S - C 4 [NCS]- [BMIM]+ + - N • Wide variety of cation/anion pairs will make ILs N N F N N - B F F F IR-inactive (2000 cm-1) C [N(CN)2]- [BM 2 IM]+ • Understanding of microscopic solvent properties of ILs C [BF 4]-

Introduction: IR Background Work • Vibrational spectroscopy and dynamics of small anions (N 3 -, 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 V=2 Static (FTIR) Excited state absorption (+DA) Transient DA(n) at Dt Ground state bleach (- DA) DA(Dt) DA(n, Dt) probe pump V=0 Vibrational Energy Relaxation (VER) Transient Experiment Dt V=1 at n k 1=1/T 1

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

Solution-Phase Spectroscopy: N 3 • Spectra in ILs similar to molecular solvents • ILs behavior closest to aprotic solvents (i. e. DMSO) • NCS- has no shift solventdependent 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) • N 3 - 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 (N 3 - 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 • Interest in reorientation in ILs • Stokes-Einstein-Debye (SED) prediction: t rot *From: Vh = f k BT Shirota, H. , et al. , J. Chem. Phys. 122, 184512 (2005). Cang, H. , et al. , J. Chem. Phys. 119, p. 13017 (2003). • Poor resolution of rotational data (T 1 << TR) in ILs • Other studies* of rotation in ILs show adherence to SED predictions

Conclusions • 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 [BM 2 IM] and [BMIM] cations – H-bonding?

Spectroscopy and Dynamics Summary