The solute-solvent interaction dictates the equilibrium structure of ionic assemblies in solution... more The solute-solvent interaction dictates the equilibrium structure of ionic assemblies in solution, ranging from free ions, ion pairs, ion-pair dimers, and ionpair tetramers to nanoparticles and ionic crystals. We use two-dimensional infrared (2DIR) spectroscopic measurements of the antisymmetric CN stretch of thiocyanate (NCS -) to study the equilibrium chemical exchange between the spectrally distinct ion pair and the ion-pair dimer in benzonitrile. We observe this chemical exchange to occur with a 36 ( 4 ps time constant. Our measurement indicates that 2DIR will provide a useful tool for investigating the dynamics of ion assembly in solution.
We have used time-resolved multidimensional vibrational spectroscopy, generally termed 2DIR spect... more We have used time-resolved multidimensional vibrational spectroscopy, generally termed 2DIR spectroscopy, to study the equilibrium dynamics of ligand exchange in an aqueous solution containing 3.4 M Mg(ClO 4 ) 2 and 1.2 M NaSCN. The sensitivity of the CN stretching frequency of thiocyanate (SCN -) to contact ion pair formation with Mg 2+ ions generates distinct spectroscopic signatures for the MgNCS + contact ion pair and the free SCN -. We have utilized 2DIR spectroscopy to successfully resolve the interconversion between these thiocyanate configurations and measured the MgNCS + contact ion pair dissociation time constant to be 52 ( 10 ps. We attribute the observed dynamics to perchlorate-thiocyanate anion exchange in the first solvation shell of the Mg 2+ cation. Magnesium ions in this concentrated ionic solution will be coordinated by water molecules, as well as perchlorate and thiocyanate ions. While prior studies have observed microsecond residence times for water ligands in the first coordination sphere of Mg 2+ , our study represents the first experimental observation of anion exchange in the first solvent shell of the Mg 2+ cation. We have also used orientational relaxation and spectral diffusion dynamics to quantify the dynamical distinctions between the free anion and the anion in the contact ion pair.
The dynamics of hydrogen bond (H-bond) formation and dissociation depend intimately on the dynami... more The dynamics of hydrogen bond (H-bond) formation and dissociation depend intimately on the dynamics of water rotation. We have used polarization resolved ultrafast two-dimensional infrared (2DIR) spectroscopy to investigate the rotational dynamics of deuterated hydroxyl groups (OD) in a solution of 6M NaClO 4 dissolved in isotopically mixed water. Aqueous 6M NaClO 4 has two peaks in the OD stretching region, one associated with hydroxyl groups that donate a H-bond to another water molecule (OD W ) and one associated with hydroxyl groups that donate a H-bond to a perchlorate anion (OD P ). Two-dimensional IR spectroscopy temporally resolves the equilibrium inter conversion of these spectrally distinct H-bond configurations, while polarization-selective 2DIR allows us to access the orientational motions associated with this chemical exchange. We have developed a general jump-exchange kinetic theory to model angular jumps associated with chemical exchange events. We use this to model polarization-selective 2DIR spectra and pump-probe anisotropy measurements. We determine the H-bond exchange induced jump angle to be 49 ± 5 • and the H-bond exchange rate to be 6 ± 1 ps. Additionally, the separation of the 2DIR signal into contributions that have or have not undergone H-bond exchange allows us to directly determine the orientational dynamics of the OD W and the OD P configurations without contributions from the exchanged population. This proves to be important because the orientational relaxation dynamics of the populations that have undergone a H-bond exchange differ significantly from the populations that remain in one H-bond configuration. We have determined the slow orientational relaxation time constant to be 6.0 ± 1 ps for the OD W configuration and 8.3 ± 1 ps for the OD P configuration. We conclude from these measurements that the orientational dynamics of hydroxyl groups in distinct H-bond configurations do differ, but not significantly.
We have spectrally resolved the intraband transient absorption of photogenerated excitons to quan... more We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.
Aqueous ionic solutions lubricate the chemical machinery of the environment and life. Understandi... more Aqueous ionic solutions lubricate the chemical machinery of the environment and life. Understanding the impact of ions on the properties of aqueous solutions and how these modified properties influence chemical and conformational dynamics remains an important and elusive objective of physical chemistry research Here we discuss recent advances in our understanding that have been derived from ultrafast vibrational spectroscopy and molecular dynamics simulations.
The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate des... more The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate despite extensive experimental and theoretical studies. We have applied polarization-selective multidimensional vibrational spectroscopy to investigate the Hbond exchange mechanism in aqueous NaClO 4 solution. The results show that a water molecule shifts its donated H-bonds between water and perchlorate acceptors by means of large, prompt angular rotation. Using a jump-exchange kinetic model, we extract an average jump angle of 49±4°, in qualitative agreement with the jump angle observed in molecular dynamics simulations of the same aqueous NaClO 4 solution.
Stimulated Raman scattering (SRS) microscopy has opened up a wide range of biochemical imaging ap... more Stimulated Raman scattering (SRS) microscopy has opened up a wide range of biochemical imaging applications by probing a particular Raman-active molecule vibrational mode in the specimen. However, the original implementation with picosecond pulse excitation can only realize rapid chemical mapping with a single Raman band. Here we present a novel SRS microscopic technique using a grating-based pulse shaper for excitation and a grating-based spectrograph for detection to achieve simultaneous multicolor SRS imaging with high sensitivity and high acquisition speeds. In particular, we use a linear combination of the measured CH2 and CH3 stretching signals to map the distributions of protein and lipid contents simultaneously.
Stimulated Raman scattering (SRS) microscopy allows label-free chemical imaging based on vibratio... more Stimulated Raman scattering (SRS) microscopy allows label-free chemical imaging based on vibrational spectroscopy. Narrowband excitation with picosecond lasers creates the highest signal levels and enables imaging speeds up to video-rate, but it sacrifices chemical specificity in samples with overlapping bands compared to broadband (multiplex) excitation. We develop a rapidly tunable picosecond optical parametric oscillator with an electro-optical tunable Lyot filter, and demonstrate multicolor SRS microscopy with synchronized line-by-line wavelength tuning to avoid spectral artifacts due to sample movement. We show sensitive imaging of three different kinds of polymer beads and live HeLa cells with moving intracellular lipid droplets.
The solute-solvent interaction dictates the equilibrium structure of ionic assemblies in solution... more The solute-solvent interaction dictates the equilibrium structure of ionic assemblies in solution, ranging from free ions, ion pairs, ion-pair dimers, and ionpair tetramers to nanoparticles and ionic crystals. We use two-dimensional infrared (2DIR) spectroscopic measurements of the antisymmetric CN stretch of thiocyanate (NCS -) to study the equilibrium chemical exchange between the spectrally distinct ion pair and the ion-pair dimer in benzonitrile. We observe this chemical exchange to occur with a 36 ( 4 ps time constant. Our measurement indicates that 2DIR will provide a useful tool for investigating the dynamics of ion assembly in solution.
We have used time-resolved multidimensional vibrational spectroscopy, generally termed 2DIR spect... more We have used time-resolved multidimensional vibrational spectroscopy, generally termed 2DIR spectroscopy, to study the equilibrium dynamics of ligand exchange in an aqueous solution containing 3.4 M Mg(ClO 4 ) 2 and 1.2 M NaSCN. The sensitivity of the CN stretching frequency of thiocyanate (SCN -) to contact ion pair formation with Mg 2+ ions generates distinct spectroscopic signatures for the MgNCS + contact ion pair and the free SCN -. We have utilized 2DIR spectroscopy to successfully resolve the interconversion between these thiocyanate configurations and measured the MgNCS + contact ion pair dissociation time constant to be 52 ( 10 ps. We attribute the observed dynamics to perchlorate-thiocyanate anion exchange in the first solvation shell of the Mg 2+ cation. Magnesium ions in this concentrated ionic solution will be coordinated by water molecules, as well as perchlorate and thiocyanate ions. While prior studies have observed microsecond residence times for water ligands in the first coordination sphere of Mg 2+ , our study represents the first experimental observation of anion exchange in the first solvent shell of the Mg 2+ cation. We have also used orientational relaxation and spectral diffusion dynamics to quantify the dynamical distinctions between the free anion and the anion in the contact ion pair.
The dynamics of hydrogen bond (H-bond) formation and dissociation depend intimately on the dynami... more The dynamics of hydrogen bond (H-bond) formation and dissociation depend intimately on the dynamics of water rotation. We have used polarization resolved ultrafast two-dimensional infrared (2DIR) spectroscopy to investigate the rotational dynamics of deuterated hydroxyl groups (OD) in a solution of 6M NaClO 4 dissolved in isotopically mixed water. Aqueous 6M NaClO 4 has two peaks in the OD stretching region, one associated with hydroxyl groups that donate a H-bond to another water molecule (OD W ) and one associated with hydroxyl groups that donate a H-bond to a perchlorate anion (OD P ). Two-dimensional IR spectroscopy temporally resolves the equilibrium inter conversion of these spectrally distinct H-bond configurations, while polarization-selective 2DIR allows us to access the orientational motions associated with this chemical exchange. We have developed a general jump-exchange kinetic theory to model angular jumps associated with chemical exchange events. We use this to model polarization-selective 2DIR spectra and pump-probe anisotropy measurements. We determine the H-bond exchange induced jump angle to be 49 ± 5 • and the H-bond exchange rate to be 6 ± 1 ps. Additionally, the separation of the 2DIR signal into contributions that have or have not undergone H-bond exchange allows us to directly determine the orientational dynamics of the OD W and the OD P configurations without contributions from the exchanged population. This proves to be important because the orientational relaxation dynamics of the populations that have undergone a H-bond exchange differ significantly from the populations that remain in one H-bond configuration. We have determined the slow orientational relaxation time constant to be 6.0 ± 1 ps for the OD W configuration and 8.3 ± 1 ps for the OD P configuration. We conclude from these measurements that the orientational dynamics of hydroxyl groups in distinct H-bond configurations do differ, but not significantly.
We have spectrally resolved the intraband transient absorption of photogenerated excitons to quan... more We have spectrally resolved the intraband transient absorption of photogenerated excitons to quantify the exciton population dynamics in colloidal PbSe quantum dots (QDs). These measurements demonstrate that the spectral distribution, as well as the amplitude, of the transient spectrum depends on the number of excitons excited in a QD. To accurately quantify the average number of excitons per QD, the transient spectrum must be spectrally integrated. With spectral integration, we observe efficient multiple exciton generation in colloidal PbSe QDs.
Aqueous ionic solutions lubricate the chemical machinery of the environment and life. Understandi... more Aqueous ionic solutions lubricate the chemical machinery of the environment and life. Understanding the impact of ions on the properties of aqueous solutions and how these modified properties influence chemical and conformational dynamics remains an important and elusive objective of physical chemistry research Here we discuss recent advances in our understanding that have been derived from ultrafast vibrational spectroscopy and molecular dynamics simulations.
The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate des... more The mechanism for hydrogen bond (H-bond) switching in solution has remained subject to debate despite extensive experimental and theoretical studies. We have applied polarization-selective multidimensional vibrational spectroscopy to investigate the Hbond exchange mechanism in aqueous NaClO 4 solution. The results show that a water molecule shifts its donated H-bonds between water and perchlorate acceptors by means of large, prompt angular rotation. Using a jump-exchange kinetic model, we extract an average jump angle of 49±4°, in qualitative agreement with the jump angle observed in molecular dynamics simulations of the same aqueous NaClO 4 solution.
Stimulated Raman scattering (SRS) microscopy has opened up a wide range of biochemical imaging ap... more Stimulated Raman scattering (SRS) microscopy has opened up a wide range of biochemical imaging applications by probing a particular Raman-active molecule vibrational mode in the specimen. However, the original implementation with picosecond pulse excitation can only realize rapid chemical mapping with a single Raman band. Here we present a novel SRS microscopic technique using a grating-based pulse shaper for excitation and a grating-based spectrograph for detection to achieve simultaneous multicolor SRS imaging with high sensitivity and high acquisition speeds. In particular, we use a linear combination of the measured CH2 and CH3 stretching signals to map the distributions of protein and lipid contents simultaneously.
Stimulated Raman scattering (SRS) microscopy allows label-free chemical imaging based on vibratio... more Stimulated Raman scattering (SRS) microscopy allows label-free chemical imaging based on vibrational spectroscopy. Narrowband excitation with picosecond lasers creates the highest signal levels and enables imaging speeds up to video-rate, but it sacrifices chemical specificity in samples with overlapping bands compared to broadband (multiplex) excitation. We develop a rapidly tunable picosecond optical parametric oscillator with an electro-optical tunable Lyot filter, and demonstrate multicolor SRS microscopy with synchronized line-by-line wavelength tuning to avoid spectral artifacts due to sample movement. We show sensitive imaging of three different kinds of polymer beads and live HeLa cells with moving intracellular lipid droplets.
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Papers by Minbiao Ji