Solvation & Solute Dynamics in Ionic Liquids Mark Maroncelli, Penn State

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1 Solvation & Solute Dynamics in Ionic Liquids Mark Maroncelli, Penn State Translation & Rotation 2. Solvation Dynamics 3. Reaction Kinetics & Heterogeneity 4. Effect of Nanostructure? Friction Ratio obs / SE SE Li NH 4 ionic solutes CH 4 neutral solutes Volume Ratio V U / V V S 0 S 1 t

2 (Room Temperature) Ionic Liquids [Pr 41 ][Tf 2 N] T f =3 C Cation Families: imidazolium Im n1 pyrrolidinium Pr n1 phosphonium P 1234 ammonium N 1234 Common Anions: BF 4-, PF 6-, CF 3 SO 3-, DCA - Tf 2 N - Lowering T f : charge delocalization size mismatch low symmetry conformational disorder Typical Properties: Low Volatility High Thermal Stability Intrinsically Conductive Wide Electrochemical Window Solubility/Phase Control Our Interests: Distinctive solvation/chemistry? New lessons to be learned?

3 1. The Simplest Dynamics: Solute Translation & Rotation Anne Kaintz Juan Carlos Araque Chris Rumble 3

4 Ion Self-Diffusion Coefficients & Friction Stokes-Einstein (Sutherland) Hydrodynamic Prediction: D SE kbt 6R Friction Ratio: obs SE D D SE obs SE friction on sphere of radius R measure of the coupling between the diffusor and solvent bath SE predictions are reasonably accurate for estimating ion self diffusion in ILs nearly identical situation exists in conventional solvents D obs / m 2 s -1 obs / SE data from Watanabe, Matsumoto, Martinelli,... Ion D in 32 ILs (298 K) cations anions 1: D SE / m 2-1 s Viscosity / cp.5.2 4

5 Diffusion of Aromatic Hydrocarbons in ILs PFG-NMR measurements of simple aromatic hydrocarbons 0.6 obs / SE vs. Solute/Solvent Size in a homologous series of ILs [Pr n1 ][Tf 2 N] n=3, 4, 6, 8, 10 Friction Ratio obs / SE departure from SE predictions increases with decreasing solute and increasing solvent size shape is also a factor Bz>Na>Py>An>Bp oblate ~prolate n=3 n= Volume Ratio V U / V V solute solvent Kaintz et al., J. Phys. Chem. B 117, (2013).

6 nonpolar solutes polar solutes N Neutral Solutes Diffusing in ILs literature data: 323 solute/solvent pairs, 37 solutes, 56 ILs small gaseous solutes: CO, CO 2, C 2 H 4, C 4 H 10,... large solutes: Fe F N N F N F =5 D Friction Ratio obs / SE SE >100 1 a( V U / V V ) nonpolar polar gas other Volume Ratio V U / V V lit. data from: Noble, Baltus, Scovazzo, Kimura, Hardacre, Compton, License, Halpiot, Lagroste, Hussey,... p marked departures from SE predictions primary determinant is U/V size ratio 6

7 Charged (1e) Solute Diffusion in ILs 10 1 (self diffusion in neat ILs) Friction Ratio obs / SE SE Li NH 4 ionic solutes CH 4 neutral solutes obs / SE ~1 for ions (helpful for interpretations) large difference in obs / SE for small ions compared to neutral solutes the solventberg limit welldocumented for Li analogous but weaker trends also seen in conventional solvents mismatch between solute and solvent in size or interactions responsible for marked departures from SE predictions Volume Ratio V U / V V 7

8 Transport Mechanisms from MD [Pr 41 ][Tf 2 N] CH 4 NH 4 Araque, Margulis et al., J. Phys. Chem. B 119, 7015 (2015). 8

9 Trajectory-Resolved Structure CH 4 NH 4 jumps are associated with low density (and low polarity = soft ) and caging with high density (and polarity = stiff ) regions of the liquid CH 4 explores both regions more effectively than NH 4 9

10 CH 4 blue = cation heads green = cation tails NH 4 (only anions depicted) CH 4 exchanges with nonpolar tails -- Coulomb lattice unperturbed NH 4 exchanges anions in its 1 st shell -- Coulomb lattice must rearrange 10

11 Solute & Ion Rotation Rotation time rot / s Fluorescence Measurements Im Pr NN P CF 3 O O aprotic protic /T / (cp K -1 ) stick slip ( L) rot 6V hyd L( L 1) k V vdw /V hyd from Dielectric Measurements Cat/An DCA - BF - 4 PF - 6 Im Im Recent NMR & Fluorescence Data V vdw /V hyd V vdw /V hyd B T Jin et al., JPCB 111, 7291 (2007) rotation of large solutes not unusual Im cations and other small molecules may show more interesting dynamics 11

12 2. More Complicated Dynamics: The Solvation Response Min Liang Xin-Xing Zhang 12

13 The Spectral / Solvation Response S 1 t Dynamic Stokes Shift of Emission Spectra time Energy S 0 Solvation Coordinate (t) peak frequency basic property of polar liquids determines friction on charge motion S ( t) ( ) ( t) (0) ( ) spectral or solvation response function 13

14 S(t) in Ionic Liquids Relative Intensity upconversion (FLUPS) & time-correlated single photon counting (TCSPC) experiments combined to capture the full response over 80 fs 40 ns Emission Spectra in [Im 41 ][PF 6 ] TCSPC 20 ns FLUPS 100 fs Frequency / 10 3 cm -1 t Spectral Response S (t) S(t) in CH 3 CN & IL IL 300x CH 3 CN Time / s ~100-fold slower than dipolar solvents ( also 100-fold larger) strongly bimodal decay broadly distributed in time N CF 3 O O 14

15 Dissecting the Solvent Response #1 S ( t) f G exp{ t } (1 f )exp{ ( t / ) G G } S(t) Gaussian Frequency G / ps Time / s [Im n1 ][Tf 2 N] [Pr n1 ][Tf 2 N] IRF # ( M1 ) N=21 R=.92 Inverse Reduced Mass m u / #1 Zhang et al., JPCB 117, 4291 (2013). Slow Time str / ps # M Viscosity / cp [Im n1 ][Tf 2 N] [Pr ][Tf n1 2 N] mechanism is primarily solvent translation #1: 30-40% inertial motions displacements of 1 st shell ions #2: the rest is overdamped & highly coupled ion motions subtle displacements of ions over <1 15

16 Solvation & Dielectric Relaxation Solvation Dynamics h Dielectric Response E () P () dielectric continuum predictions are qualitatively correct in ILs but too fast by factors of 3-4 similar results in IL dipolar solvent mixtures Solvation Response S(t) Some S(t) Comparisons Pred. #1 #2 [Im 41 ][DCA] 2x B1 B2 [Im 41 ][PF 6 ] Obs. 5x B N* Time / ps [N 3000 ][NO 3 ] 3x B* W* [S 333 ][Tf 2 N] 4x B W Zhang et al., JPCB 117, 4291 (2013). 16

17 Why Continuum Predictions are too Fast molecular solvation entails charge redistribution in response to nonuniform fields; basic dynamics captured in qq(k,t) of the neat solvent Charge Density Correlation Function ( k, t) ( k ) ( k, t) 1.0 qq q q (k=2/ =ion size.) qq (k,t) dipole solvation 0.8 k k=0.1 Å -1 () captures only k=0 (=) dynamics solvation involves entire spectrum k=0- solvent response is slower at k>0 thus dielectric predictions are too fast Time / ps Maroncelli et al., Faraday Discuss. 154, 409 (2012). 17

18 Solvation & Conductivity Solvation Time solv / ps The D.C. model predicts integral solvation times solv to be inversely proportional to conductivity solv ( S( t) dt 4 0 continuum prediction 1 2 Im Pr N P misc Resistivity -1 / m ) C153 in 34 Ionic Liquids 0 Tf 2 N - c misc ~const. as predicted, solv and 0 are strongly correlated prediction is an average of 3-fold too fast ( is easy) The () 0 Connection: 4 0 ˆ ( ) ˆ corr ( ) i expts. report ˆ( ) ˆ ( ) i 4 divergence removed conductivity & permittivity reflect identical dynamics J J (t) Zhang et al., JPC Lett. 4, 1205 (2013). M M (t) 18

19 3. Even More Complicated: Reaction Kinetics Min Liang Lillian Li Minako Kondo 19

20 1. Intramolecular Charge Transfer Biphenyl Acridinium (BPAc ) Reaction and Solvation Times LE e CT 10 3 Relative Intensity N TR Emission in [N 3111 ][Tf 2 N] N 2.5 ns t=0 CT LE et / ps 10 2 [N ip311 ][Tf 2 N] 10 1 [Im 21 ][Tf 2 N] [Im 41 ][PF 6 ] [N 3111 ][Tf 2 N] 1:1 [P 14,666 ][Tf 2 N] solv / ps reaction in ILs continues trend established by dipolar solvents Frequency / 10 3 cm -1 Li et al., JPCB 115, 6592 (2011). 20

21 2. Isomerization DMASBT N S N Non-Exponential Kinetics [N ip311 ][Tf 2 N] Observed Time obs / ps An Empirical Correlation ln a bln( / T) cf ( ) Calculated Time clc / ps Relative Intensity CH 3 CN 298 K 279 K Time / ps rates in ILs correlate with those in dipolar solvents kinetics more complex Kondo & Maroncelli, JPCB 117, (2013). 21

22 REE & Kinetic Heterogeneity S N N Energy Red Edge Excitation S 1 S 0 p() rxn / ps 100 Solvation Coordinate ex / 10 3 cm -1 heterogeneous kinetics should be the norm for ps processes given the slow solvation / structural relaxation times of ILs Reaction Time vs exc abs em ~ 50% em / 10 3 cm -1

23 Other Examples LE N C N DMABN CT N * DEAHF T * - Relative Emission Intensity TICT Reaction of DMABN LE exc CT Wavelength / nm exc /nm Samanata & Co., JPCB 114, 1967 (2010). Relative Emission Intensity ESPT in DEAHF exc N * T * Wavelength / nm exc /nm Kimura et al., JPCB 114, (2010); JPCB 117, 6759 (2013). 23

24 Possible Interpretations Distinct Local Dynamics reaction senses dynamic heterogeneity the fact that local packing effects cause different regions to have different fluidities Hurley et al., Phys. Rev. E 52, 1694 (1995) Distinct Local Energies Nanoscopic Domains different solute-solvent interaction energies affect PES for reaction solutes are distributed among nanoscopic domains of different polarity and/or fluidity S1 Lopes & Padua, J. Phys. Chem. B 110, 3330 (2006). 24

25 4. Aside: Where s the Effect of Nanoscopic Structure? Jacob Schesser 25

26 Solvation in [P 14,666 ][X] ILs Snapshot of [P 14,666 ][Tf 2 N] Simulation Viscosity Correlation Solvation Time solv / ps x Im Pr N misc Tf 2 N - misc Viscosity / cp Resistivity Correlation Shimizu, J. Mol. Struct. 946, 70 (2010). blue=, red=-, grey=alkane Solvation Time solv / ps continuum prediction Im Pr N P misc Resistivity -1 / m Tf 2 N - misc 26

27 Further Recent Attempts - anthracene & bianthryl in [Im n1 ][Tf 2 N] (n=2-12), & [P 14,666 ][Tf 2 N]n-hexane Anthracene Absorption Spectra Anthracene Solvatochromic Shifts Absorbance n n Frequency in 74 organic solvents: gas = 4100 cm -1, = 170 cm -1 n n Observed - gas / cm n-c 5 P 14,666 n-c 6 MeOH THF n-c 16 DMSO Im n1 n= Predicted - gas / cm -1 27

28 Bianthryl ET Reaction Times Emission Intensity TR Emission Spectra CT Frequency [P 14,666 ][Tf 2 N] n-c 6 x C6 =0.8 t LE green=n-c 6 red=p 14,666 blue=tf 2 N - ET Reaction Time / ps LE e - CT ET Reaction Times ILs mixtures M. Liang, preliminary results C153 Solvation Time / ps 28

29 Summary Solute Translation & Rotation: D ~ T/ as expected by hydrodynamic models, but Stokes-Einstein predictions can be wrong by factors of 100 or more (at room T) diffusion of small neutral versus charged solutes is markedly different rotations of small solutes can be far from hydrodynamic expectations Solvation Dynamics: solvation is highly bimodal: ~1/3 sub-ps and 2/3 relax over 1 ps- 10 ns unlike dipolar solvents, translational solvent motions dominate S(t) and () reflect the same underlying molecular dynamics but dielectric continuum predictions too fast by a factor >4 solvation closely related to conductivity ( solv 1/ ) Fast Reaction Kinetics: slow solvation/structural relaxation in ILs leads to heterogeneous reaction kinetics for fast reactions (<1 ns) origins of heterogeneity unclear (especially relevance of nanostructure) 29

30 Acknowledgements Humboldt University Penn State University University of Iowa Other Contributors Sergei Arzhantsev Hui Jin Durba Roy Minako Kondo Lillian Li Jacob Schesser* Xin-Xing Zhang Min Liang Anne Kaintz Juan Carlos Araque Collaborators: Claudio Margulis, University of Iowa Ed Castner, Rutgers University Gary Baker, University of Missouri Nikolaus Ernsting, Humboldt University Chris Rumble* Brian Conway Mike Shadeck Marissa Saladin 30