Rotational Relaxation Dynamics Research Articles

Solvent and rotational relaxation of Coumarin-153 in a micellar solution of a room-temperature ionic liquid, 1-butyl-3-methylimidazolium octyl sulfate, in ethylammonium nitrate

We have investigated the micelle formation by room-temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium octyl sulfate (bmimOs) with another RTIL, ethyl ammonium nitrate (EAN). In addition, the effect of micelle formation on the solvation and rotational relaxation dynamics have been observed using steady-state and picoseconds time-resolved spectroscopy. Both the solvent and rotational relaxations of Coumarin-153 are retarded in the micelle compared to that of neat EAN. The increase in average solvation time on going from neat EAN to bmimOs-EAN micelle is very small compared to the increase in solvation time on going from pure water to water containing micelle.

Vibrational dynamics of metal cyanides

Time-resolved IR spectroscopy has been used to characterize vibrational and rotational relaxation dynamics of the C–N stretching bands for aqueous molecular cyanides, Au ( CN ) 2 - , Ni ( CN ) 4 2 - , Pt ( CN ) 4 2 - , Co ( CN ) 6 3 - , Mn ( CN ) 6 3 - , and Ru ( CN ) 6 4 - . The spectra and dynamics of Ru ( CN ) 6 4 - resemble those previously reported for ferrocyanide with a relatively short (<10 ps) vibrational energy relaxation (VER) time. VER times are longer (>30 ps) for Au ( CN ) 2 - , Ni ( CN ) 4 2 - , Pt ( CN ) 4 2 - and Co ( CN ) 6 3 - . Mn ( CN ) 6 3 - is an intermediate case with a VER time of 15 ps in water. These VER dynamics extend and reinforce the trends for metal cyanide CN vibrational band frequencies and intensities.

Effect of sphere to rod transition on the probe microenvironment in sodium dodecyl sulphate micelles: A time resolved fluorescence anisotropy study

The effect of different hydrotropic salts on the microenvironment at the anionic head group region of sodium dodecyl sulphate (SDS) micelle has been studied through time-resolved fluorescence anisotropy measurements of a solubilized probe, coumarin-153 (C153). The organic cations of the hydrotropic salts used in this study, i.e. aniline hydrochloride (AHC) and o-, m- and p-toluidine hydrochlorides (OTHC, MTHC and PTHC, respectively), differ in their charge to size ratio and hydrophobicity. Present study utilizes the sensitivity of the fluorescence technique to understand the changes in the micropolarity and microviscosity experienced by the fluorescent probe, C153, solubilized in the micellar Stern layer, on addition of different hydrotropic salts. Significant changes are observed in the rotational relaxation dynamics of the probe with increasing concentration of the salts. The changes in the rotational relaxation dynamics clearly reflect the sphere to rod transition in the SDS micelles and correspond nicely with the reported results from dynamic light scattering measurements. The growth behavior of SDS micelles is found to be sensitive to the hydrophobicity of the organic cations. The charge to size ratio of the organic cations also indicated to play a role in inducing the sphere to rod transition in the SDS micelles. The interesting observation made from this study is that the sphere to rod transition of SDS micelles is largely facilitated by the presence of the hydrotropic salts and such a transition is successfully indicated by the simple fluorescence anisotropy measurements of a probe in the micelle carried out in the presence of different hydrotropic salts.

Photophysics and rotational relaxation dynamics of cationic phenazinium dyes in anionic reverse micelles: Effect of methyl substitution

We present here, a detailed photophysical and rotational relaxation dynamical study of three structurally analogous cationic dyes, namely, phenosafranin (PSF), safranin-T (ST), and safranin-O (SO), carried out in well characterized, monodispersed biomimicking anionic reverse micellar nanocavities composed of sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/heptane with increasing water contents. The dyes belong to the phenazinium family and they differ in terms of methyl substitution on the planar phenazinium skeleton. The objective of the present study is to investigate the modification in the photophysical and dynamical behavior of the dyes with the change in the size of the water pool of the reverse micelle and thereby to explore the role of methyl substitution. Steady state and time resolved emission and anisotropy studies have been exploited for the purpose. The dyes are found to exhibit a marked decrease in the fluorescence anisotropy with increasing water/surfactant mole ratio (w), i.e., the water pool size in the reverse micellar core, implying that overall motional restriction experienced by the molecules are decreased with increasing hydration. Some of the depth dependent fluorescence parameters such as fluorescence maximum, fluorescence anisotropy (r) have been monitored for exploring the microenvironment around the probes in the reverse micelles. Fluorescence studies suggest that at low w values, the probes do not penetrate into the reverse micellar core; rather it binds at the interfacial region. Estimates of the micropolarity at the binding sites of the probe molecule have been determined as a function of w. Finally, dynamic studies reveal that both the lifetime and rotational relaxation time decrease with an increase in w for all the three probes, the extent of the decrease being more for PSF than ST and SO. This indicates a stronger binding of the reverse micelle with ST and SO compared to that with PSF which is rationalized in terms of an increase in the hydrophobicity of the former two dyes because of the methyl substitution on the phenazinium moiety.

Rotational relaxation dynamics of a β-carboline analogue in cyclodextrin nanocavity: How does the cavity size barricade the molecular rotation?

In the present work the rotational dynamics of a polarity sensitive biologically active β-carboline derivative, 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ), have been discussed in three native cyclodextrin environments using ultrafast time-resolved fluorescence technique. AODIQ exhibited bi-exponential anisotropy decay in the cyclodextrin environments. The rotational motion of the probe was interpreted on the basis of two independent motion; local (internal motion of the guest within the aggregate) and global (overall rotational tumbling of the complexes). It is found that the cyclodextrin environment causes significant retardation of rotational motion of the probe. Experimental results reveal that relative host–guest size determines the character of the intermolecular host–guest dynamics.

To Probe the Interaction of Methanol and Acetonitrile with the Ionic Liquid N,N,N-Trimethyl-N-propyl Ammonium Bis(trifluoromethanesulfonyl) Imide at Different Temperatures by Solvation Dynamics Study

The dynamics of solvent and rotational relaxation of coumarin 153 (C-153) has been studied in neat N,N,N-trimethyl-N-propyl ammonium bis(trifluoromethanesulfonyl) imide ([N(3111)][Tf(2)N]) and its mixtures with polar solvents, namely, methanol and acetonitrile at three different temperatures from 294 to 303 K. Both the solvent and rotational relaxation dynamics of C-153 in neat [N(3111)][Tf(2)N] are linearly correlated with the bulk viscosity at different temperatures. The solvent relaxation time and rotational relaxation time of C-153 decrease with gradual addition of cosolvents in [N(3111)][Tf(2)N]. The gradual addition of cosolvent decreases the viscosity of the medium, and consequently, the solvation and rotational relaxation time also decrease. The decrease of solvation time is more pronounced on addition of acetonitrile compared to methanol.

Photophysics and rotational relaxation dynamics of a β-carboline based fluorophore in cationic alkyltrimethylammonium bromide micelles

Photophysics and rotational relaxation dynamics of a β-carboline analog, 3-acetyl-4-oxo-6,7-dihydro-12 H-indolo-[2,3-a] quinolizine (AODIQ) have been investigated in cationic alkyltrimethylammonium bromide (nTAB) micelles using steady-state and time-resolved fluorometric techniques. The study reveals modification of its photophysics by the conjugate effect of polarity and rigidity of the micellar environments with varying alkyl chain lengths of the surfactants. Furthermore, it suggests that the fluorophore resides at the micelle–water interfacial domain. Contrary to the single exponential nature of the fluorescence anisotropy decay of AODIQ in aqueous medium, the decay is found to be biexponential in all the micellar environments studied. The enhancements in the steady-state anisotropy and rotational relaxation time in the micellar media compared to that in pure aqueous solution reflect that the fluorophore resides in a motionally restricted environment introduced by the cationic micelles. The rotational correlation time increases marginally with an increase in the surfactant chain length. The rotational relaxation of AODIQ in the micellar environments has been discussed in the light of the two-step and wobbling in a cone model. The model helps in evaluating different rotational parameters and in ascertaining the location of the fluorophore in the micellar media. This technique provides valuable information regarding the rotational relaxations of the fluorophore within an organized assembly. When the lifetime measurements and orientational relaxation measurements are combined, significant inferences can be made regarding the partitioning of the probe in different regions of the micelles.

Photophysical Studies on the Noncovalent Interaction of Thioflavin T with Cucurbit[n]uril Macrocycles

Noncovalent interaction of Thioflavin T (ThT) with versatile macrocyclic host molecules, namely, cucurbit[7]uril (CB7) and cucurbit[5]uril (CB5), has been investigated in aqueous solutions by photophysical methods. Steady-state and time-resolved fluorescence studies illustrate significant enhancements/modifications in the ThT fluorescence yield, lifetime, and spectral features on interaction with the CBs and are assigned due to the formation of 1:1 and 2:1 complexes between the CBs and the ThT. The high binding constant values for the 1:1 complex (K(1) approximately 10(5) M(-1)) indicate the strong ion-dipole interaction between the host and guest molecules, whereas the 2:1 complex formation is mainly driven by weaker forces like hydrophobic interaction as evident from the lower binding constants (K(2) approximately 10(3) M(-1)). From the characteristic differences in the photophysical properties of the CB7-ThT and CB5-ThT complexes, it has been adjudged that ThT forms an inclusion complex with CB7 whereas with CB5, the interaction is through an exclusion complex formation. These contentions have been further verified by the rotational relaxation dynamics, NMR, and quantum chemical calculations on CB-ThT systems. The present results have also been compared with those reported for the dye in the presence of cyclodextrin hosts.

Microenvironment in the Corona Region of Triblock Copolymer Micelles: Temperature Dependent Solvation and Rotational Relaxation Dynamics of Coumarin Dyes

Dynamic Stokes' shift and fluorescence anisotropy measurements using coumarin-153 (C153) and coumarin-151 (C151) as the fluorescence probes have been carried out in aqueous poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 (P123) and poly(ethylene oxide)100-poly(propylene oxide)70-poly(ethylene oxide)100 (F127) block copolymer micelles with an aim to understand the water structures and dynamics in the micellar corona region. It has been established that the probes reside in the micellar corona region. It is indicated that the corona regions of P123 and F127 micelles are relatively less hydrated than the Palisade layers of neutral micelles like Triton-X-100 and Brij-35. From the appraisal of total Stokes' shift values for the probes in the two block copolymer micelles, it is inferred that the F127 micelle is more hydrated than the P123 micelle. It is observed that the dynamic Stokes' shift values for both of the probes remain more or less similar at all the temperatures studied in the P123 micelle. For C153 in F127, however, the observed Stokes' shift is seen to decrease quite sharply with temperature, though it remains quite similar for C151. Moreover, the fraction of the unobserved initial dynamic Stokes' shift is appreciably higher for both the probes in the F127 micelle compared to that in P123. Over the studied temperature range of 293-313 K, the spectral shift correlation function is described adequately by a bi-exponential function. Rotational relaxation times for C153 in both the micelles show a kind of transition at around 303 K. These results have been rationalized assuming collapse of the poly(ethylene oxide) (PEO) blocks and formation of water clusters in the corona region due to dehydration of poly(ethylene oxide) blocks with an increase in temperature. A dissimilar probe location has been inferred for the differences in the results with C153 and C151 probes in F127. Comparison of the microviscosity and the hydration of the block copolymer micelles has also been made with those of the other commonly used neutral micelles, for a better understanding of the results in the block copolymer micelles.

Photophysical Properties and Rotational Relaxation Dynamics of Neutral Red Bound to β-Cyclodextrin

Photophysical properties and the rotational relaxation dynamics of a phenazine based dye, neutral red, have been investigated in aqueous solutions in the presence of β-cyclodextrin (β-CD) using ground-state absorption and steady-state and time-resolved fluorescence measurements. It has been observed that while the neutral form (NR) of the dye forms an inclusion complex with β-CD, its protonated form (NRH+) does not. In the presence of β-CD (10 mM), the pKa value of the dye is estimated to be about 6.06 ± 0.05, much lower than the pKa value of 6.81 ± 0.05 measured in aqueous solution. Both photophysical properties and the rotational relaxation dynamics of NR undergo substantial changes in the presence of β-CD. Steady-state and time-resolved fluorescence studies suggest simultaneous formation of 1:1 and 1:2 complexes of NR/β-CD in aqueous solutions. Time-resolved fluorescence anisotropy measurements confirm that only NR forms an inclusion complex with β-CD and the NR/β-CD complex rotates as a whole with a r...

Free-Space and Intermolecular Interaction Effects on the Local-Chain Rotational Relaxation Dynamics in Dye + Polymer Lasing Materials

Dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC) are combined to analyze the secondary (β) side-chain relaxation mode of poly(methyl methacrylate) (PMMA) in conventional solid-state laser matrices. The response of undoped PMMA is compared with that of mixtures with fluorescent dyes: the highly polar and ionic rhodamine 6G/Cl- and pyrromethene 567 dyes and the neutral and apolar substituted perylene and perylimides. Variations in characteristic features of the β relaxation, such as the spectral position and the relative strength of the thermocurrent and dielectric loss bands as well as the absolute values and the width of the distribution in apparent activation energies (Ea), are exploited to isolate different types of intermolecular and intramolecular guest−host interactions. The results are interpreted in view of opposing effects on the relaxation characteristics of the pendant groups. In the presence of the perylene/perylimide dyes the β process is mainly influenced by variations in the local free space: by increasing the size of the dye molecule, the (re)orientation of the ester carbonyl side groups is more effectively hindered (i.e., increased Ea and antiplasticization). In contrast, in the xanthene/pyrromethene + PMMA mixtures, dipole−dipole and electrostatic interactions and charge-transfer reactions may be considered to control the dynamics of the β relaxation mode. The complementary character of the two dielectric techniques and possible relationships between the dielectric results and optical characteristics of each blend are discussed.

Orientational and Vibrational Relaxation Dynamics of Perylene and 1-Methylperylene in Aldehydes and Ketones

We report on the rotational diffusion and vibrational population relaxation dynamics of the probe molecules perylene and 1-methylperylene in selected aliphatic aldehyde and ketone solvents. The reorientation data demonstrate that, for both solvent systems, there is a change in the nature of the solvent−solute interactions with both probe molecules as the hydrodynamic volume of the solvent molecules approaches that of the probes. The solvent-dependent change in behavior occurs for different solvent aliphatic chain lengths in the two systems. Vibrational population relaxation measurements for two probe molecule vibrational modes reveal efficient intermolecular coupling in all cases. The data point to the role of the solvent aldehyde proton in mediating solvent−solvent and solvent−solute interactions, leading to chromophore dynamics reminiscent of those seen in the n-alcohols. Reorientation of the two probes in the ketones yields results that are more consistent with those seen in the alkanes, suggesting the...

Rotational Relaxation in a Nondipolar Supercritical Fluid: Toluene in CO2

Studies of rotational relaxation dynamics provide particular insight into local solution structures and consequently into the interactions between species in a solution. We report here the results of molecular dynamics simulations describing a neat CO2 supercritical fluid and an infinitely dilute solution of toluene in supercritical CO2. Over a period of 0.1−0.2 ps, the rotation of the near-critical solvent molecules is relatively unhindered, becoming purely diffusive only on a time scale that is long compared with the decay of the orientational correlations. As expected, the rotational relaxation rate of a toluene molecule is found to increase with increasing solvent density, although the simulation results imply some anomalous behavior near the critical point that may be associated with the appearance of long-range spatial correlations. We also show that a system consisting of a nonpolar toluene analogue experiences an isotropic rotational friction environment, unlike the anisotropic environment in which a real toluene molecule is found when dissolved in supercritical CO2.

Calculation of nanocolloidal liquid time scales by molecular dynamics simulations

Molecular dynamics, MD, simulations have been used to calculate the translational and rotational relaxation dynamics of model atomistically rough spherical nanocolloidal particles in solution at infinite dilution by immersing a single Lennard-Jones cluster in a molecularly discrete solvent. Key time scales characterizing colloidal particle dynamical relaxation were computed from time correlation functions. For translational motion these were τv, the colloidal velocity relaxation time, τf, the hydrodynamic relaxation time and the time scale for significant particle displacement, τd. We show that τv ≃ τf when the relative mass density of the colloidal particle divided by the bulk density of the solvent is ca. ρ* = 20, in agreement with theoretical predictions. Preliminary evidence from the velocity autocorrelation functions, VACF, of the nanocolloidal particle also supports the theoretical treatments that the transition from the Liouville to Fokker—Planck description (evident by exponential decay in the VACF) is determined by both the colloidal particle mass and size. We calculated the relaxation times for angular velocity relaxation, τw and reorientation, τu and found them to scale reasonably well with the relaxation time for the free rotor, for size dependence but not so well for mass dependence. The angular velocity correlation function of 13 atom clusters departed from Langevin (exponential) relaxation also for ρ* < 20. The rotational self-diffusion coefficient was also non-classical in this range.

Orientational and Vibrational Relaxation Dynamics of Perylene and 1-Methylperylene in n-Alcohols: Probing the Balance between van der Waals and Hydrogen-Bonding Interactions

We report on the rotational diffusion and vibrational population relaxation dynamics of perylene and 1-methylperylene in the primary normal alcohols methanol through n-decanol. The rotational diffusion dynamics of the two chromophores are the same to within the experimental uncertainty for n-propanol through n-decanol. We observe a double exponential decay of the induced orientational anisotropy for 1-methylperylene in all of the alcohols and for perylene in n-propanol through n-decanol. These data are consistent with previous literature reports on perylene in high-viscosity solvents and on 1-methylperylene in n-alkanes longer than n-octane. These data also reveal a substantial difference in the behavior of perylene in n-alkanes and n-alkanols. Both chromophores reorient as oblate rotors in the n-alkanols, with the aspect ratio of the oblate ellipsoid describing their motion depending on the solvent aliphatic chain length. The vibrational population relaxation dynamics of the two probe molecules differ significantly, not only because of the difference in the nature of the solvent−solute coupling but also because of subtle differences in the organization of the solvent around the two chromophores. These data reflect the importance of solvent self-association in determining the local environments of these two chromophores.

Solvent polarity indicators: Flexible styrylpyridinium and quinolinium fluorescence probes for medium free-volume

Fluorescence properties are studied for four structurally related flexible styrylpyridinium and quinolinium dyes of electron donor-acceptor type in glycerol at different temperatures. The response of the fluorescence quantum yield ( φ f) to viscosity-temperature changes ( n( T) function) is a sensitive function of dye structure. Probe rotational relaxation dynamics are accurately described in terms of the free-volume hypothesis. An intriguing gauge of the interplay between average medium microviscosity/free-volume and some molecular properties such as rotor size and flexibility is developed for potential analytical application as fluorescence probes for medium viscosity.

Comparison of Solvent and Probe Reorientation in Polyacrylamide Gels

Raman spectroscopy and time-resolved fluorescence anisotropy decay were used to measure the rotational relaxation dynamics of acetonitrile and the dianion polyphenyl 2 (PP2) in polyacrylamide (PAA) gels in a water−acetonitrile solvent mixture. In the neat solvent mixture the acetonitrile molecule rotates, underlying a reduced friction in comparison to its dynamics in pure acetonitrile. The rotational behavior of PP2 in the mixture is well described by the Stokes−Einstein−Debye equation. In the PAA gels the rotational relaxation of acetonitrile is only weakly influenced by the gel network even at high polymer concentrations. In contrast, the larger probe molecule PP2 reflects a significant retardation of the reorientational motion with increasing PAA concentration. We suggest that this behavior results from the different environments experienced by the probe and solvent molecules.

The influence of wave vector dependent dielectric properties on rotational friction. Rotational diffusion of phenoxazine dyes

The rotational diffusion of three mechanically similar phenoxazine dyes possessing distinct electrical properties was studied in isopropanol. The results, along with previously presented results from other polar solvents, were analyzed in terms of continuum theories for rotational dielectric friction. It was found that a continuum based theory can account for the observed rotational relaxation dynamics, but only with realistic modeling of the solute charge distribution (not a point dipole), and by accounting for both frequency and wave vector dependences of the solvent dielectric properties.