Picosecond water dynamics adjacent to charged paramagnetic ions measured by magnetic relaxation dispersion

Measurements of water-proton spin-lattice relaxation rate constants as a function of magnetic field strength [magnetic relaxation dispersion (MRD)] in aqueous solutions of paramagnetic solutes reveal a peak in the MRD profile. These previously unobserved peaks require that the time correlation functions describing the water-proton-electron dipolar coupling have a periodic contribution. In aqueous solutions of iron(III) ion the peak corresponds to a frequency of 8.7 cm-1, which the authors ascribe to the motion of water participating in the second coordination sphere of the triply charged solute ion. Similar peaks of weaker intensity in the same time range are observed for aqueous solutions of chromium(III) chloride as well as for ion pairs formed by ammonium ion with trioxalatochromate(III) ion. The widths of the dispersion peaks are consistent with a lifetime for the periodic motion in the range of 5 ps or longer.     

Picosecond relaxation path of ethyl violet

Steady-state and time-resolved spectroscopy of a triphenylmethane dye (ethyl violet) exhibiting a fast electronic relaxation is studied. Transient transmissions were probed with a supercontinuum in the 1-20 ps range and fluorescence decays were monitored with a streak camera. Delayed bleaching is observed in solution at room temperature when probing in the short wavelength edge of the main ground state absorption band (530–560 nm). The various shapes of the transmission curves measured at wavelengths where transient absorption, ground state bleaching or amplification dominates are explained by a model involving one absorbing intermediate state. The existence of two electronic transitions in the visible ground state absorption is confirmed as well as non-exponential fluorescence decay. The relaxation mechanism is discussed in terms of internal twisting with charge redistribution.     

Picosecond fluorescence relaxation kinetics from all-trans retinal

The fluorescence relaxation kinetics from all-trans retinal in polar and non-polar solvents have been investigated as a function of temperature. An activation energy of ≈1 keal/mole has been measured. Our results, in conjunction with previous picosecond absorption measurements made by Hochstrasser et al., strongly suggest that initially excited molecules to Franck-Condon states relax to three singlet excited states.     

Picosecond recovery dynamics of malachite green

Using sub-picosecond pulses from a mode-locked cw dye laser we have studied ultra-fast absorption recovery in the triphenylmethane dye malachite green after excitation to the first singlet. In methanol an exponential time constant of 2.1 ps is measured. As solvent viscosity is increased recovery becomes slower and a two component relaxation is evident.     

Picosecond photoelectron studies of cluster dynamics

Time-resolved photoelectron spectroscopy is proving to be a versatile tool for investigating dynamical properties of molecules and clusters. In this account we review our recent work involving studies of cluster reactivity by photoelectron spectroscopy. Two topics are discussed. The first, involves measurements of the rate of solvent reorganization and the magnitude of the solvent reorganization energy (i.e., the solute-solvent interaction). The solvent rearrangement is induced by an excited-state proton transfer in phenol-ammonia clusters. These studies are motivated by the opportunity to probe condensed-phase solvation effects on a molecular level in clusters. The second topic describes how the chemical properties of small clusters (e.g., dimers) provides detailed information regarding the collision complexes of bimolecular gas-phase reactions. We show an example of how the combination of photoelectron spectroscopy and ion fragmentation mass spectrometry can be used to determine barrier heights for bimolecular reactions.     

Picosecond polarization spectroscopy as a probe of intramolecular dynamics: rovibronic relaxation in the S1 state of trans-stilbene

The first application of picosecond polarization spectroscopy to probe gas phase intramolecular relaxation under collision free conditions is reported. This technique is a sensitive probe of molecular rotation and when applied to trans-stilbene vapor at 200°C an exponential decay in the anisotropy with a lifetime of 48 ± 8 ps is observed. The results are discussed in the context of recent work on IVR and rovibrational intramolecular processes.     

Picosecond spectroscopy using continuous analyzing light. electronic relaxation in pinacyanol

A comparison is made between different methods of performing picosecond spectroscopy with a tunable analyzing light source. A method is presented in which low-intensity continuous analyzing light from a He-Ne laser is used. In our first experiment with this method the electronic relaxation of pinacyanol in ethanol at different temperatures has been measured.     

Picosecond dynamics and microheterogenity of water+dioxane mixtures

Dielectric spectra from 200 MHz up to 3 THz were determined to study the fast dynamics of dilute water+1,4-dioxane. epsilon(nu) could be fitted by a collision induced oscillator at high frequencies plus two Debye relaxations in the microwave region. Isotope substitution was used to assign water and dioxane modes. The presence of the cooperative hydrogen-bond network relaxation down to a water mole fraction of 0.005 suggests a microheterogeneous structure of the mixtures even at low water content. The collision mode of dioxane at approximately 2 THz grows upon water addition, revealing the presence of H2O molecules in dioxane-rich domains.     

Picosecond studies on the photodissociation dynamics of aromatic endoperoxides

Studies on various aromatic endoperoxides (POs) reveal a different photodynamic behavior despite similar excitation conditions and despite similar chromophore structures. Using picosecond laser pump-probe technique it was found that heterocoerdianthrone (HECD), dissolved in dichloromethane, is produced from photocycloreversion of its endoperoxide with a time constant of τ=40±10 ps. Since the lifetime of the photoreactive S₃ state is less than 3 ps, a two-step mechanism is expected. Photocleavage of the endoperoxide of anthradichromene (ADCPO) happens in 55±15 ps, whereas the endoperoxides of dimethylhomöocoerdianthrone (HOCDPO) and dimethoxyhomöocoerdianthrone (DMHDPO) photodissociate much faster. For their photodissociation we can state an upper time limit of 5 ps. The results of polarization and solvent dependent experiments demonstrate that the observed rise of signal is directly correlated with the formation of the parent compound in the ground state. The dynamics of the photodissociation of HECPO does not depend on polarity or viscosity of solvent. Therefore, an oxciplex configuration or an intermediate zwitterion cannot be involved in the photocycloreversion of aromatic endoperoxides. The model of a biradical mechanism must be claimed instead.     

Picosecond dynamics of large-amplitude motions of triphenylmethane molecules in alcohol solution

The relaxation dynamics in the excited states of crystal violet and ethyl violet in alcohol solutions were investigated by picosecond absorption recovery measurements. The experimental results were compared with the model for isomerization in solution proposed by Skinner and Wolynes. The relaxation rate as a function of viscosity displays the turnover behaviour predicted by this model to occur at very low friction.     

Intramolecular relaxation dynamics in semiflexible dendrimers

The intramolecular relaxation dynamics of semiflexible dendrimers in dilute solutions are theoretically investigated in the framework of optimized Rouse-Zimm formalism. Semiflexibility is implemented by modeling topological restrictions on the bond directions and orientations of the respective bond-vectors. Based on our recently developed approach for semiflexible dendrimers [A. Kumar and P. Biswas, Macromolecules 43, 7378 (2010)], the mechanical and dielectric relaxation moduli are studied as functions of local flexibility parameters and branching topology. It is rather interesting to observe that semiflexibility affects the local modes of G'(ω) and Δε'(ω), which have lower relaxation rate with increasing bond restrictions, while the collective modes with small relaxation rate remain almost constant. The relaxation dynamics of the flexible dendrimer is similar to that of the semiflexible dendrimer with unrestricted bond orientations (Φ = 0) and is flanked by the compressed (Φ = 30°) and expanded (Φ = 150°) conformations, respectively. The effect of semiflexibility is typically reflected in the intermediate frequency regime. The expanded conformations of semiflexible dendrimers display a power-law behavior in the intermediate frequency regime for both loss and storage modulus resembling fractal structures, while the compressed and unrestricted bond orientation conformations exhibit an approximately logarithmic dependence. The power-law exponent is found to be similar to the flexible dendrimers with excluded volume interactions. Thus, by tuning Φ, a spectrum of dynamic relaxation pattern is obtained spanning a broad range of conformations from a power-law fractal network to a non-fractal one. In certain limits, this highly generalized model captures the characteristics of flexible dendrimers and also resembles La Ferla's model semiflexible dendrimers. The influence of hydrodynamic interactions reduces the dynamical range and the width of the intermediate domain by decreasing the smaller relaxation rates and increasing the higher relaxation rates correspondingly.     

The dynamics of mechanical relaxation in polyisobutylene

The published data on the mechanical response of a set of samples prepared from a single batch of polyisobutylene have been examined and three relaxation/retardation processes have been characterized in the temperature region above the glass transition temperature. It is shown that the time/frequency dependences can be explained in terms of the cooperative theory of relaxation/retardation. It is also shown that the values of the fractional power-law indices of relaxation/retardation, which are a characteristic feature of the cooperative approach, are independent of the technique used in their observation.     

Picosecond acoustic transmission measurements. II. Probing high frequency structural relaxation in supercooled glycerol

The high frequency acoustic response of liquids is measured in a manner directly analogous to conventional ultrasonic measurements. Two thin metal films act as acoustic transducer and receiver for a liquid layer between them. Pulsed optical excitation generates high bandwidth wave packets in the transducer, and these are detected in the receiver after damping and dispersion by the liquid. This initial measurement probes structural relaxation dynamics of glycerol in the frequency range 2-20 GHz, for temperatures between 235 and 291 K. The analysis presented here demonstrates the presence of excess relaxation, not accounted for by either the alpha or beta relaxation of the mode-coupling theory, and suggests the presence of constant loss in the susceptibility spectrum of supercooled glycerol.     

Picosecond fluorescence studies of polypeptide dynamics: fluorescence anisotropies and lifetimes

Fluorescence lifetimes and anisotropies for single tryptophan-containing polypeptide hormones ACTH and glucagon, and a series of their fragments are reported. The anisotropy data are discussed in the context of the theory of Perico and Guenza (J. Chem. Phys. 84 (1986) 510). A persistence length of about 7 to 10 residues is obtained for the mobilities in the two hormones. The theory is able to account for chain length and probe location effects, but the calculated time dependence of the anisotropy does not fit the experimental curve well at short times.     

Picosecond dynamics of ligand interconversion in the primary docking site of heme proteins

We have used femtosecond IR spectroscopy to probe interconversion dynamics of ligand in the primary docking site of heme proteins under physiological conditions. The docking site, fashioned with highly conserved amino acid residues, modulates ligand-binding activity by mediating the passage of ligand to and from the active binding site. Ligands in two states of the docking site interconvert on the picosecond time scale, and the rates are about 4 times slower in hemoglobin than that in myoglobin. The accurate interconversion rates on the time scale readily accessible by MD simulations can be used to refine computer simulations, which could in turn provide a detailed mechanistic picture of ligand binding in heme proteins.     

Molecular relaxation dynamics of self-assembled monolayers

Dielectric relaxation spectroscopy is used to quantify molecular motion in alkylsilane SAMs coated on porous glass over a broad temperature range, -30 to -150 degrees C. Systematic measurements using SAMs with variable coating densities allow us to determine the effect of monolayer disorder on molecular mobility in thin molecular films. A relaxation process with an activation energy of approximately 25 kJ/mol is found to dominate dynamics of SAM-chain segments near the substrate. By introducing polar CN groups at the ends of the chain, we show that the relaxation process in the monolayer canopy can be isolated and studied. This approach can be generalized to other substituent polar groups to probe localized relaxation dynamics in surface-grafted monolayer films.     

Ultrafast relaxation dynamics of perchlorinated cycloheptatriene in solution

The photochemistry of perchlorinated cycloheptatriene (CHTCl(8)) has been studied by means of ultrafast pump-probe, transient anisotropy and continuous UV-irradiation experiments in various solvents as well as by DFT calculations. After UV-excitation to the 1A' '-state, two competing reactions occur--a [1,7]-sigmatropic chlorine migration via two ultrafast internal conversions and a [4,5]-electrocyclization forming octachlorobicylo[3.2.0]hepta-[2,6]-diene. The first reaction has been studied by excitation with a 263 nm femtosecond-laser pulse. Pump-probe experiments reveal a first, solvent-independent time constant, tau1(CHTCl(8)) = 140 fs, that can be associated with the electronic relaxation of the 2A'-1A' ' transition, while a second one, tau2(CHTCl(8)), ranges from 0.9 to 1.8 ps depending on the polarity of the solvent. This finding is consistent with a [1,7]-chlorine migration during the 1A'-2A' transition where the migrating chlorine atom is partly negatively charged. The charge separation has also been confirmed by DFT calculations. Transient anisotropy measurements result in a time zero value of r(0) = 0.35 after deconvolution and a decay constant of tau1(a) = 120 fs, which can be explained by vibrational motions of CHTCl(8) in the electronically excited states, 1A' ' and 2A'. After continuous UV-irradiation of CHTCl(8), octachlorobicylo[3.2.0]hepta-[2,6]-diene is primarily formed with a solvent-dependent yield. From these investigations, we suggest a relaxation mechanism for CHTCl(8) after photoexcitation that is comparable to cycloheptatriene.     

Energetics and dynamics in MbCN: CN--vibrational relaxation from molecular dynamics simulations

The dynamics of the cyanide anion bound to sperm-whale myoglobin is investigated using atomistic simulations. With density-functional theory, a 2D potential energy surface for the cyanide-heme complex is calculated. Two deep minima with a stabilization energy of approximately 50 kcal/mol corresponding to two different binding orientations (Fe-CN and Fe-NC) of the ligand are found. The Fe-CN conformation is favored over Fe-NC by several kcal/mol. Mixed quantum mechanics/molecular mechanics calculations show that the binding orientation affects the bond strength of the ligand, with a significantly different bond length and a 25 cm-1 shift in the fundamental CN-frequency. For the molecular dynamics (MD) simulations, a 3-center fluctuating charge model for the Fe-CN unit is developed that captures polarization and ligand-metal charge transfer. Stability arguments based on the energetics around the active site and the CN- frequency shifts suggest that the Fe-CN conformation with epsilon-protonation of His epsilon 64 are most likely, which is in agreement with experiment. Both equilibrium and nonequilibrium MD simulations are carried out to investigate the relaxation time scale and possible relaxation pathways in bound MbCN. The nonequilibrium MD simulations with a vibrationally excited ligand reveal that vibrational relaxation takes place on a time scale of hundreds of picoseconds within the active site. This finding supports the hypothesis that the experimentally observed relaxation rate (3.6 ps) reflects the repopulation of the electronic ground state.     

Picosecond dynamics of proton transfer of a 7-hydroxyflavylium salt in aqueous-organic solvent mixtures

The intermediacy of the geminate base-proton pair (A*···H(+)) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH(+)* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base-proton pair A*···H(+). The geminate pair becomes detectable because of the increase in the recombination rate constant, k(rec), of (A*···H(+)) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH(+)*, k(d), and recombination, k(rec), of A*···H(+)) from the diffusion controlled rates (dissociation, k(diss), and formation, k(diff)[H(+)], of A*···H(+)), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k(d) and k(rec)) as well as on proton diffusion/migration (k(diss) and k(diff)).     

Picosecond time-resolved infrared spectroscopic investigation of excited state dynamics in a PtII diimine chromophore

This is the first report describing the use of picosecond time-resolved infrared (TRIR) spectroscopy to probe a d8 metal chromophore, Pt(4,4'-(CO2Et)(2)-2,2'-bpy)Cl2: monitoring changes in the v(CO) vibrations allows for an assignment of the lowest excited state to an MLCT with an 8.7 ps lifetime.