Rotational dynamics of nondipolar probes in alkane–alkanol mixtures: Microscopic friction on hydrogen bonding and nonhydrogen bonding solute molecules

Experimental and Theoretical Rotational Diffusion Studies of 7DM4M1M1,8, N-2(1H)-one and 7A4T2H1B-2-one in Series of Alcohol Solvents: Stoke's-Einstein-Debye and Alavi-Waldeck Models

Rotational diffusion studies of two solutes 7-(dimethylamino)-4-methoxy-1-methyl-1,8-naphthyridin-2(1H)-one (7DM4M1M1,8, N-2(1H)-one) and 7-amino-4-(trifluoromethyl)-2H-1-benzopyran-2-one (7A4T2H1B-2-one) having equal volumes but different chemical natures are studied in series of alcohol solvents at 303 K using steady-state methods. HOMO-LUMO, Electron density, Molecular electrostatic potential (MEP), etc., are obtained from computational calculations using Gaussian 09 software. Rotational reorientation times of 7DM4M1M1,8, N-2(1H)-one solute molecule is found to be less than 7A4T2H1B-2-one solute molecule indicates it rotates slowly in chosen solvents. Stoke's-Einstein-Debye (SED) model with stick boundary conditions for the 7A4T2H1B-2-one solute molecule is modeled to describe mechanical friction. Polar solutes along with mechanical friction also experience dielectric friction. Both these frictions being non-separable, the Alavi-Waldeck (AW) model is studied for dielectric friction contribution to the total friction solute experiences in solvents. AW model effectively explains the observed dielectric friction in alcohol solvents.

An Experimental and Theoretical Test of Dielectric Friction Models Using Rotational Diffusion of 7-Diethylamino-2-H-1-Benzopyran-2-One in Non-associative Solvents

The rotational re-orientations times of the 7-DHB dye molecule have been examined in non-associative solvents (DMSO and Octanenitrile) by varying the temperature, by employing the Steady-State Fluorescence Depolarisation and Time-Correlated Single Photon Counting (TCSPC) techniques. Rotational re-orientations time values in DMSO are found larger by a factor of 1.136 than octanenitrile, which indicates that 7-DHB laser dye is experiencing higher friction in DMSO than octanenitrile. To determine mechanical friction Stokes Einstein's Debye theory (SED) -with a stick, slip boundary conditions parameters are used and found an interesting super slip trend. Point dipole models as Nee-Zwanzig (NZ) and van der Zwan-Hynes (ZH) fail to explain experimental dielectric friction observed trends. Alavi-Waldeck model successfully explains the observed dielectric friction trend in non-associative solvents.

Rotational Diffusion of Medium Sized 7-[Diethylamino]-2H-1-Benzopyran-2-One Molecule in Alcohols: Study of Temperature and Solvent Viscosity Effect

The rotational re-orientations times of the 7-[diethylamino]-2H-1-benzopyran-2-one (7-DHB) dye molecule have been examined in ethanol and octanol solvents when macroscopic solvent viscosity parameter is varied by varying the temperature, by employing the steady-state fluorescence depolarisation and Time-Correlated Single Photon Counting (TCSPC) techniques. Experimental observation shows that 7-DHB probe is experiencing higher friction in octanol compared to ethanol and rotates slower by a factor of 7.3. The hydrodynamic Stokes Einstein's Debye theory (SED) with a stick, slip boundary conditions parameters, quasi-hydrodynamic models (Dote-Kivelson-Schwartz and Geirer-Wirtz) were used to determine mechanical friction and found an interesting towards super slip trend. Dielectric frictional theories of point dipole, Nee-Zwanzig and van der Zwan-Hynes both models fail to describe experimentally observe dielectric friction trends. Evidently, both hydrodynamic and dielectric models failed to explain the examined behavior, even in the qualitative way in alcohols.

Rosindone revisited: a computational and photophysical study of 7-phenylbenzo[a]phenazine-5(7H)-one (PBP)

A reasonable gram quantity of the crystalline red dye 7-phenylbenzo[a]phenazine-5(7H)-one (PBP) was synthesised by the condensation of N-phenylbezene-1,2-diamine with 2-hydroxynaphthalene-1,4-dione in acetic acid (58% yield). The molecular structure of the dye, as determined by single-crystal X-ray crystallography, reveals a near planar phenazinone-like core, and an N-phenyl group twisted out of this plane by around 85°. The CO bond length of 1.241(2) Å is consistent with double bond character, which supports minor ground state zwitterionic character to the compound. The wavelength maximum for the observed partially structured low-energy absorption band is relatively insensitive to changes in the solvent polarity and polarizability. TD-DFT calculations predict that the long wavelength absorption envelope originates from localised π-π* transitions with no contribution from an n-π* state. The fluorescence quantum yield and singlet lifetime of the dye in MeCN are 0.12 and 3.0 ns, respectively. Fluorescence maxima are slightly sensitive to the solvent and changes in the Stokes shifts for a small series of alkanols were fitted to the Lippert-Mataga equation to afford a change in dipole moment of 8 ± 2 D. Calculations also reveal that full rotation of the N-phenyl group is severely restricted in the ground state (ΔE_GS = 29 kcal mol^-1) and in the first-excited singlet state (ΔE_ES = 34 kcal mol^-1). The rocking back and forth of the phenyl group distorts the phenazinone-like backbone as it becomes co-planar and a minor solvent viscosity effect was observed in hydrogen bonding alkanol solvents.

Probing the interactions of structurally similar but chemically distinguishable organic solutes with 1-ethyl-3-methylimidazolium alkyl sulfate (alkyl = ethyl, hexyl and octyl) ionic liquids through fluorescence, NMR and fluorescence correlation spectroscopy (FCS) studies

Structurally similar but chemically distinguishable solutes provide idea about intermolecular interactions in ionic liquids.

Does addition of an electrolyte influence the rotational diffusion of nondipolar solutes in a protic ionic liquid?

Rotational diffusion of two structurally similar nondipolar solutes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been examined in ethylammonium nitrate-lithium nitrate (EAN-LiNO3) mixtures to understand the influence of added electrolyte on the local environment experienced by the solute molecules. The measured reorientation times of both DMDPP and DPP in EAN-LiNO3 mixtures fall within the broad limits set by the hydrodynamic slip and stick boundary conditions. The hydrogen bond accepting DMDPP and the hydrogen bond donating DPP experience specific interactions with the cation and anion of the ionic liquid, respectively. Addition of LiNO3 (0.1 and 0.2 mole fraction) to EAN induces only viscosity related effects on the rotational diffusion of the two nondipolar solutes. These observations suggest that the local environment experienced by DMDPP and DPP in EAN is not altered upon the addition of LiNO3. Our results are consistent with the structural details available in the literature for EAN-LiNO3 mixtures.

ESR Spin Probes in Ionic Liquids

The spin probes 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), and 2,2,6,6-tetramethyl-4-trimethylammoniumpiperidine-1-oxylIodide (CAT-1) are examined in a number of ionic liquids based on substituted imidazolium cations and tetrafluoroborate and hexafluorophosphate anions, respectively. The reorientation correlation times tau(R) of the spin probes in these systems have been determined by complete spectra simulation and, for rapid reortientation, by analysis of the intensities of the hyperfine lines of the electron spin resonance (ESR) spectra. A comparison of the results with those from the model system glycerol/water and selected organic solvents is made. Additions of diamagnetic and paramagnetic ions allow the conclusion that salt effects and spin exchange are present, and that both are superimposed by motional effects. Specific interactions in the ionic liquids, as well as between the spin-probe molecules and the constituents of the ionic liquids are reflected in the spectra of the spin probes, depending on their molecular structure.

Dynamics of glass-forming liquids. XI. Fluctuating environments by dielectric spectroscopy

The dielectric relaxation of a 1 wt % mixture of di-n-butylether in 3-methylpentane has been measured across a range of eight decades, in which the characteristic relaxation time varies from 5 s to 50 ns. Each loss spectrum is a superposition of the dispersive solvent peak and a Debye peak which is one decade slower and readily assigned to the larger and more dipolar solute molecules. Fluctuating environments or rate exchange is made responsible for the Debye nature of probe rotation, implying that the environmental relaxation times fluctuate on time scales which are faster than the rotational correlation decay of the probe molecule. Within the experimental range from 2.2 s to 42 ns regarding the mean alpha-relaxation time, the results are consistent with the exchange time matching the upper limit of structural relaxation times or two to three times their average value. As T(g) is approached, no indication for a variation in exchange behavior or for slower environmental fluctuations is found.

Do organic solutes experience specific interactions with ionic liquids?

In an attempt to understand the nature of interactions between organic solutes and room temperature ionic liquids, temperature-dependent rotational relaxation of two structurally similar nondipolar solutes--2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP)--has been examined in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim+][PF6(-)]). Even with the ionic liquid, where the cation and the anion are strongly associated, the solute DPP experiences specific interactions, which is evident from its reorientation times that are 50%-60% longer in relation to DMDPP. It has been noticed that the reorientation times of both the solutes are faster in [bmim+][PF6(-)] than in glycerol, which is also a strongly associated solvent and whose viscosity is similar to the ionic liquid. This observation has been explained by taking into consideration the relative sizes of the solvents. By comparing the ratios of the reorientation times of DPP to DMDPP, in [bmim+][PF6(-)] and glycerol, it has been deduced that the strengths of the interaction between DPP-[bmim+][PF6(-)] and DPP-glycerol are the same.

Molecular Rotation as a Tool for Exploring Specific Solute-Solvent Interactions

Solute-solvent interactions play an important role in determining the physicochemical properties of liquids and solutions. As a consequence, understanding these interactions has been one of the long-standing problems in physical chemistry. This Minireview describes our approach towards attaining this goal, which is to investigate rotational relaxation of a pair of closely related, medium-sized nondipolar solutes in a set of appropriately chosen solvents. Our studies indicate that solute-solvent hydrogen bonding significantly hinders solute rotation. We have also examined the role of solvent size both in the absence and presence of specific interactions and it has been observed that the size of the solvent has a bearing on solute rotation especially in the absence of specific interactions. Our results point to the fact that only strong solute-solvent hydrogen bonds have the ability to impede the rotation of the solute molecule because, in such a scenario, hydrogen-bonding dynamics and rotational dynamics transpire on comparable time scales. This aspect has been substantiated by measuring the reorientation times of the chosen solutes in solvents such as ethanol and trifluoroethanol, which have distinct hydrogen-bond donating and accepting abilities, and correlating them with solute-solvent interaction strengths. As an alternative treatment, it has been shown that specific interactions between the solute and the solvent can be modeled as dielectric friction with the extended charge distribution model. This approach is not unrealistic considering the fact that specific as well as non-specific interactions are electrostatic by nature and the differences between them are subtle.

How Critical Micelle Temperature Influences Rotational Diffusion of Hydrophobic Probes Solubilized in Aqueous Triblock Copolymer Solutions

Rotational diffusion of two structurally similar hydrophobic probes, 2,5-dimethyl-1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DMDPP) and 1,4-dioxo-3,6-diphenylpyrrolo[3,4-c]pyrrole (DPP), has been examined in aqueous solutions of poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20 triblock copolymer as a function of temperature. These studies have been carried out to explore the influence of critical micelle temperature (cmt) on probe dynamics. It has been observed that, below cmt, the anisotropy decays can be adequately described by single-exponential functions with one time constant each for DMDPP and DPP. However, above cmt, biexponential functions with two time constants are needed to satisfactorily fit the anisotropy decays. Another important observation is that both the probes rotate more rapidly below the critical micelle temperature. The dynamics of the probe molecules are akin to that in a homogeneous solution below cmt, whereas above cmt, the rotational diffusion of the probes has been accounted by the two-step model, which is usually employed to explain the results in micelles. A comparison between the microviscosities of these micelles with other nonionic micelles such as Triton X-100 and Brij-35 reveals that the internal environment of the micelles formed with the triblock copolymer is less fluid.