Solvation Probe Research Articles

Solvation Oscillations and Excited-State Dynamics of 2-Amino- and 2-Hydroxy-7-nitrofluorene and Its 2‘-deoxyriboside

Push-pull substituted fluorenes are considered for use as dynamic solvation probes in polynucleotides. Their fluorescence band is predicted (by simulations) to show weak spectral oscillations on the subpicosecond time scale depending on the nucleotide sequence. The oscillations reflect the local far-infrared spectrum of the environment around the probe molecule. A connection is provided by the continuum theory of polar solvation which, however, neglects molecular aspects. We examine the latter using acetonitrile solution as a test case. A collective librational solvent mode at 100 cm(-1) is observed with 2-amino-7-nitrofluorene, 2-dimethylamino-7-nitrofluorene, 2-hydroxy-7-nitrofluorene, and its 2'-deoxyriboside. Different strengths of the oscillation indicate that rotational friction of nearby acetonitrile molecules depends on the solute structure or that H bonding is involved in launching the librational coherence. Polar solvation in methanol is used for comparison. With hydroxynitrofluorenes, the observation window is limited by intersystem crossing for which rates are reported. A prominent excited-state absorption band of nitrofluorenes at 430 nm can be used to monitor polar solvation. Structural and electronic relaxation pathways are discussed with the help of quantum chemical calculations.

Structural transition in alcohol-water binary mixtures: A spectroscopic study

The strengthening of the hydrogen bonding (H-bond) network as well as transition from the tetrahedral-like water network to the zigzag chain structure of alcohol upon increasing the alcohol concentration in ethanol-water and tertiary butanol (TBA) — water mixtures have been studied by using both steady state and time resolved spectroscopy. Absorption and emission characteristics of coumarin 153 (C153), a widely used non-reactive solvation probe, have been monitored to investigate the structural transition in these binary mixtures. The effects of the hydrogen bond (H-bond) network with alcohol concentration are revealed by a minimum in the peak frequency of the absorption spectrum of C153 which occur at alcohol mole fraction ∼0·10 for water-ethanol and at ∼0·04 for water-TBA mixtures. These are the mole fractions around which several thermodynamic properties of these mixtures show anomalous change due to the enhancement of H-bonding network. While the strengthening of H-bond network is revealed by the absorption spectra, the emission characteristics show the typical non-ideal alcohol mole fraction dependence at all concentrations. The time resolved anisotropy decay of C153 has been found to be bi-exponential at all alcohol mole fractions. The sharp change in slopes of average rotational correlation time with alcohol mole fraction indicates the structural transition in the environment around the rotating solute. The changes in slopes occur at mole fraction ∼0·10 for TBA-water and at ∼0·2 for ethanol-water mixtures, which are believed to reflect alcohol mole fraction induced structural changes in these alcohol-water binary mixtures.

Formation of reverse micelles in supercritical carbon dioxide and its thermodynamics

The solubilization behavior of methyl orange as a solvation probe in multiple systems composed of supercritical carbon dioxide, surfactants and co-solvents, is studied. It is concluded that some surfactants, such as sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) and isooctyl phenol polyethoxylate (TX-10), could form reverse micelles in supercritical carbon dioxide under the action of butanol. The formation of reverse micelles is a spontaneous process thermodynamically. Specifically for the nonionic surfactant TX-10, the formation of reverse micelles is dependent on the entropy increase in the system, while for the anionic surfactant AOT, the micellization is mainly dominated by the increase in enthalpy at higher temperatures, but by the increase in entropy at lower temperatures.

Watching Na Atoms Solvate into (Na+,e-) Contact Pairs: Untangling the Ultrafast Charge-Transfer-to-Solvent Dynamics of Na- in Tetrahydrofuran (THF)

With the large dye molecules employed in typical studies of solvation dynamics, it is often difficult to separate the intramolecular relaxation of the dye from the relaxation associated with dynamic solvation. One way to avoid this difficulty is to study solvation dynamics using an atom as the solvation probe; because atoms have only electronic degrees of freedom, all of the observed spectroscopic dynamics must result from motions of the solvent. In this paper, we use ultrafast transient absorption spectroscopy to investigate the solvation dynamics of newly created sodium atoms that are formed following the charge transfer to solvent (CTTS) ejection of an electron from sodium anions (sodide) in liquid tetrahydrofuran (THF). Because the absorption spectra of the sodide reactant, the sodium atom, and the solvated electron products overlap, we first examined the dynamics of the ejected CTTS electron in the infrared to build a detailed model of the CTTS process that allowed us to subtract the spectroscopic contributions of the sodide bleach and the solvated electron and cleanly reveal the spectroscopy of the solvated atom. We find that the neutral sodium species created following CTTS excitation of sodide initially absorbs near 590 nm, the position of the gas-phase sodium D-line, suggesting that it only weakly interacts with the surrounding solvent. We then see a fast solvation process that causes a red-shift of the sodium atom's spectrum in approximately 230 fs, a time scale that matches well with the results of MD simulations of solvation dynamics in liquid THF. After the fast solvation is complete, the neutral sodium atoms undergo a chemical reaction that takes place in approximately 740 fs, as indicated by the observation of an isosbestic point and the creation of a species with a new spectrum. The spectrum of the species created after the reaction then red-shifts on a approximately 10-ps time scale to become the equilibrium spectrum of the THF-solvated sodium atom, which is known from radiation chemistry experiments to absorb near approximately 900 nm. There has been considerable debate as to whether this 900-nm absorbing species is better thought of as a solvated atom or a sodium cation:solvated electron contact pair, (Na+,e-). The fact that we observe the initially created neutral Na atom undergoing a chemical reaction to ultimately become the 900-nm absorbing species suggests that it is better assigned as (Na+,e-). The approximately 10-ps solvation time we observe for this species is an order of magnitude slower than any other solvation process previously observed in liquid THF, suggesting that this species interacts differently with the solvent than the large molecules that are typically used as solvation probes. Together, all of the results allow us to build the most detailed picture to date of the CTTS process of Na- in THF as well as to directly observe the solvation dynamics associated with single sodium atoms in solution.

Ultrafast Excited-State Dynamics of DNA Fluorescent Intercalators: New Insight into the Fluorescence Enhancement Mechanism

The excited-state dynamics of the DNA bisintercalator YOYO-1 and of two derivatives has been investigated using ultrafast fluorescence up-conversion and time-correlated single photon counting. The free dyes in water exist in two forms: nonaggregated dyes and intramolecular H-type aggregates, the latter form being only very weakly fluorescent because of excitonic interaction. The excited-state dynamics of the nonaggregated dyes is dominated by a nonradiative decay with a time constant of the order of 5 ps associated with large amplitude motion around the monomethine bridge of the cyanine chromophores. The strong fluorescence enhancement observed upon binding of the dyes to DNA is due to both the inhibition of this nonradiative deactivation of the nonaggregated dyes and the dissociation of the aggregates and thus to the disruption of the excitonic interaction. However, the interaction between the two chromophoric moieties in DNA is sufficient to enable ultrafast hopping of the excitation energy as revealed by the decay of the fluorescence anisotropy. Finally, these dyes act as solvation probes since a dynamic fluorescence Stokes shift was observed both in bulk water and in DNA. Very similar time scales were found in bulk water and in DNA.

Photophysics of trans-4-(Dimethylamino)-4‘-cyanostilbene and Its Use as a Solvation Probe

Electronic structure calculations, steady-state electronic spectroscopy, and femtosecond time-resolved emission spectroscopy are used to examine the photophysics of trans-4-(dimethylamino)-4'-cyanostilbene (DCS) and its solvent dependence. Semiempirical AM1/CI calculations suggest that an anilino TICT state is a potential candidate for the emissive state of DCS in polar solvents. But observation of large and solvent-independent absorption and emission transition moments in a number of solvents (M(abs) = 6.7 +/- 0.4 D and M(em) = 7.6 +/- 0.8 D) rule out the involvement of any such state, which would have a vanishingly small transition moment. The absorption and steady-state emission spectra of DCS evolve in a systematic manner with solvent polarity, approximately as would be expected for a single, highly polar excited state. Attempts to fit the solvatochromism of DCS using standard dielectric continuum models are only partially successful when values of the solute dipole moments suggested by independent measurements are assumed. The shapes of the absorption and emission spectra of DCS change systematically with solvent polarity in a manner that is semiquantitatively reproduced using a coupled-state model of the spectroscopy. Kerr-gate emission measurements show that the emission dynamics of DCS down to subpicosecond times reflect only solvent relaxation, rather than any more complicated electronic state kinetics. The spectral response functions measured with DCS are well correlated to those previously reported for the solvation probe coumarin 153, indicating DCS to be a useful alternative probe of solvation dynamics.

Hydration dynamics of a protein in the presence of urea and sodium dodecyl sulfate

The antagonistic effects of unfolding of a protein (lysozyme) by urea and refolding by sodium dodecyl sulfate (SDS), have been studied by solvation dynamics and circular dichroism. Efficient fluorescence resonance energy transfer from tryptophan to coumarin 153 (C153) indicates that the solvation probe, C153 is located near tryptophan 62 and 108 of lysozyme. The average solvation time of C153 bound to lysozyme in 7 M urea and 3 mM SDS is quite close to that in the native state of lysozyme while in 28 mM SDS and 7 M urea the average solvation time is nearly three times slower.

Solvation Dynamics of a Probe Covalently Bound to a Protein and in an AOT Microemulsion: 4-(N-Bromoacetylamino)-Phthalimide

4-(N-bromoacetylamino)-phthalimide (I) is used as a new solvation probe for protein and microemulsions. The photophysics of the probe 4-(N-bromoacetylamino)-phthalimide (I) is dramatically different from that of the parent compound, 4-aminophthalimide (4-AP). The solvation dynamics of I in an AOT microemulsion is similar to that of 4-AP in microemulsions. Solvation dynamics in the vicinity of a protein glutaminyl-tRNA synthetase (GlnRS) is studied by covalently attaching I to the protein. The solvation dynamics of the protein-bound probe is described by a very fast component of 40 ps and another of 580 ps.

Solvation dynamics and probe rotation in glass-forming liquids

Abstract We have measured optical depolarization and solvation dynamics using triplet state molecular probes in various glass-forming organic liquids of low molecular weight. Optical depolarization yields the single particle rotational correlation function r ( t ) of the guest molecules, while mechanical solvation dynamics reflect the time resolved shear modulus G ( t ) specifically of those host molecules which surround the probe. Using the ratio m guest / m host of molecular weights as an indicator for the relative sizes, it is observed that large m guest / m host values lead to much slower and more exponential probe rotation compared with the structural relaxation of the environment. As m guest approaches m host , the time dependencies of r ( t ) and G ( t ) become very similar. It appears that a larger and relatively immobile guest molecule has little effect on the otherwise cooperative dynamics of the surrounding solvent.

Derivatives of Bis(2,2′-bipyridyl)dicyanoiron(II) with Long Alkyl Chains − Versatile Solvatochromic Probes that Form Metalloaggregates in Water-Rich Media

We report on the preparation, characterisation, and solvatochromic behaviour of four new [FeL2(CN)2] complexes, where L is the ligand 4,4′-diisobutyl-2,2′-bipyridine, 4,4′-di-n-pentyl-2,2′-bipyridine, 4,4′-di-n-heptyl-2,2′-bipyridine, or 4-n-heptyl-4′-methyl-2,2′-bipyridine. The solvatochromic properties of these complexes show that the hydrophobicity imposed by the alkyl chains controls the response of the complexes towards the solvent, and that they are ideal to use as solvation probes in heterogeneous/organised media. An interesting result emerging from this work is the demonstration of the self-aggregating properties of the complexes cis-[Fe(Pe2bpy)2(CN)2], cis-[Fe(He2bpy)2(CN)2], and cis-[Fe(Hebpy)2(CN)2] which are, without doubt, metallosurfactants.

Bis-(4-methyl-4′-pentyl-2,2′-bipyridyl)(dicyano)iron(II): a versatile solvatochromic solvation probe

We report the preparation, characterisation, and solvatochromic behaviour of bis-(4-methyl-4′-pentyl-2,2′-bipyridyl)(dicyano)iron(II). This is the first compound whose solubility characteristics permit its use as a solvatochromic indicator over the complete range of solvents from water to paraffins.

Electron transfer in a donor-substituted acridinium dye: evidence for dynamical solvent control

Time-resolved emission is used to explore the intramolecular excited-state electron transfer in a donor-substituted acridinium dye in a variety of solvents at room temperature. Emission decays are predominantly bi-exponential, with the faster of the two time constants being a good measure of the charge transfer time. In 14 high-polarity solvents this time is correlated to the solvation times previously measured with the solvation probe Coumarin 153 via an apparent power law, τ LE→CT≈8.4〈 τ〉 solv 0.65. In solvents less polar than acetone the times observed deviate widely from this relationship, probably due to the effects of ion pairing.

Cooperativity and heterogeneity of the dynamics in nano-confined liquids

We have measured the time resolved Stokes shift of the probe molecule quinoxaline in glass-forming solvents of different polarities under the condition of geometrical confinement, for which this solvation dynamics technique is particularly suitable. While solvation probes the local dielectric relaxation in polar liquids, it is associated with the local mechanical responses in non-polar systems. The effect of porous glasses on the relaxations of supercooled liquids is strongly dependent on the surface chemistry and can be rationalized on the basis of the cooperativity concept. We also present evidence for the heterogeneous nature of relaxation dynamics in nano-confined liquids. In a further experiment, in which the chromophores are selectively positioned at the liquid/solid interface, we observe that the relaxation in the immediate vicinity of the glass surface can be qualitatively similar to the bulk behaviour.

Spatial uniformity of the β-relaxation in D-sorbitol

Signatures of the slow β-relaxation of D-sorbitol as observed by macroscopic dielectric relaxation and solvation dynamics over a wide range of temperatures are compared. The latter technique senses the local dielectric relaxation in the immediate vicinity of a chromophore, which is present only at very low concentrations. The conformity of locally sensitized and macroscopically averaged results for the glassy state indicates that the secondary relaxation is a spatially uniform feature. If the β-process were spatially confined it should not be detectable by solvation probes.

Solvent isotope effects as a probe of general catalysis and solvation in phosphoryl transfer

Phosphoryl transfer to methanol from tris(p-nitrophenyl) phosphate (PNNN), methyl bis(p-nitrophenyl) phosphate (PMNN), and dimethyl p-nitrophenyl phosphate (PMMN) exhibits general base catalysis by acetate ion but no detectable catalysis by acetic acid. For PNNN, acetate catalysis produces normal solvent isotope effects kROH/kROD of 1.68 ± 0.01 at high ionic strength (0.475) and 1.77 ± 0.04 at low ionic strength (0.048). A linear proton inventory indicates most simply that the isotope effect arises from a one-proton catalytic bridge in the transition state, although this model cannot strongly be distinguished from a generalized solvation effect. Reactions of methoxide ions produce slight inverse isotope effects kROD/kROH of 1.1–1.2, far smaller than the inverseeffect of about 2.5 expected for complete and uncompensated desolvation of the reactant-state methoxide ion. The transition state is thus stabilized by substantial interaction with the solvent. The proton inventory for the least reactive substrate PMMN (relative rate constant 1) is suggestive of transition-state stabilization by a combination of one-proton catalytic bridge(s) and distributed sites, while the proton inventory for the most reactive substrate PNNN (relative rate constant 1388) suggests only generalized transition-state solvation (many distributed sites); the proton inventory for PMNN, a substrate of intermediate reactivity (relative rate constant 60), suggests an intermediate situation. The data are consistent with a model in which transition states with exterior concentrations of charge favor stabilization of the charge by isotope-fractionating one-proton bridges, while transition states with distributed charge favor stabilization of the charge by many distributed sites. Key words: phosphoryl transfer, proton inventories, solvent isotope effects.

The interplay of dielectric and mechanical relaxation in solvation dynamics

Quantum molecular dynamics simulations are used to explore the possible coupling between dielectric solvation, the response of the solvent to a change in charge distribution of the solute, and mechanical solvation, the response of the solvent to a change in solute size or shape. The hydrated electron is chosen as a solvation probe, due to its large increase in spatial extent upon photoexcitation and significant contraction in size upon non-adiabatic relaxation. The strong displacement of translational solvent modes upon excitation hampers the effectiveness of individual solvent molecule rotations in providing relaxation, decreasing the relative amplitude of the inertial response. Following non-adiabatic relaxation, solvent molecules can freely translate and reorient, leading to rapid, effective initial solvation. These results suggest that in many situations where solutes undergo changes in both charge distribution and size, solvation can become rate-limited by the relatively slow viscoelastic solvent response.

Rate of Decarboxylation of Benzisoxazole-3-Carboxylate Ions as a Probe of Solvation in Biological and Other Media

Rate of Decarboxylation of Benzisoxazole-3-Carboxylate Ions as a Probe of Solvation in Biological and Other Media

Effects of fluorine substitution on the solvatochromic behaviour of an iron(II)–cyanide Schiff base complex

The complex [Fe(CN)2(LL)2], in which LL is the diimine Schiff base derived from 2-acetylpyridine and 4-fluoro-3-methylaniline, is, like other dicyanobisdiimineiron(II) complexes, strongly solvatochromic. Wavelengths of its lowest-energy charge-transfer (MLCT) absorption are reported in water and a range of non-aqueous solvents, in several series of binary aqueous solvent mixtures, in micellar systems, in microemulsions and in aqueous salt solutions. The observed shifts are discussed in terms of solvation in the respective media; this complex appears to be a particularly useful probe of solvation and selective solvation.

Ultraviolet femtosecond fluorescence spectroscopy: techniques and applications

Because of important improvements made to the UV-region femtosecond emission apparatus at the University of Minnesota, it is now possible to record the UV fluorescence of organic molecules with an effective resolution reaching ~50 fsec and a useful signal-to-noise ratio. Applications of this technology to two fundamental processes in chemistry (solvation and electron transfer) are briefly described. Molecular examples include (i) the solvation probes Coumarin 152 and Coumarin 343 and (ii) the electron-transfer molecules 9,9′-bianthryl and 4-(9-anthryl)-N,N′-dimethylaniline.

A systematic entropy relationship for the general-base catalysis of the deprotonation of a carbon acid. A quantitative probe of transition-state solvation

A systematic entropy relationship for the general-base catalysis of the deprotonation of a carbon acid. A quantitative probe of transition-state solvation