Solvent Relaxation Processes Research Articles

Synthesis, photophysical properties, and in-silico nonlinear optics of 3-hydroxy-N-phenyl-2-naphthamide and its acetate, acrylate, and benzoate derivative

In this study, we report the successful growth of single crystals of 3-hydroxynaphthalene-2-carboxanilides (NAS) with a monoclinic structure in the P21/c space group and space group number of 14 for the first time. We have developed a yield and time-effective modified synthetic procedure using a synthetic reactor by replacing the traditional microwave method. Additionally, acylated derivatives of NAS, including novel acryloyl derivative (NAS AC), are synthesized with a modified procedure, replacing pyridine with triethylamine.We investigate the structural, photophysical, and quantum chemical properties of N-(4-acetylphenyl)-2-oxo-2H-chromene-3-carboxamide (NAS) and its acylated derivatives (NAS AC, NAS ACT, NAS BZ). Photophysical studies reveal significant Stokes shifts observed in different solvents, including time-enhanced fluorescence in DMSO, attributed to solvent relaxation processes. Natural bond orbital (NBO) analysis, molecular electrostatic potential (MEP) maps, and frontier molecular orbital (FMO) provide insights into electronic interactions and asymmetric charge distribution. Low-lying LUMO values of these compounds (-2.31 eV to -1.95 eV) support candidature for the electron acceptor applications. Nonlinear optical properties were thoroughly investigated, with NAS displaying the highest static first-order hyperpolarizability (17.89 × 10−30 esu) and NAS BZ exhibiting the highest second-order hyperpolarizability (113.09 × 10−36 esu), indicating significant nonlinear optical responses. Thermal stability (TGA and DTA) highlights the significant range of temperatures for the NLO applications.

Photophysics of Acyl- and Ester-DBD Dyes: Quadrupole-Induced Solvent Relaxation Investigated by Transient Absorption Spectroscopy.

A new generation of wavelength-tunable, fluorescent dyes, so-called DBD ([1,3]dioxolo[4,5- f][1,3]benzodioxole) dyes, were developed a few years ago, and they showed great potential as probes, for example, for fluorescence microscopy. However, their photophysics is not fully explored and leaves open questions regarding their large fluorescence Stokes shifts and sensitivity to solvent conditions of differently substituted DBD dyes. To improve the understanding of the influence of the substitution pattern of the DBD dyes on their respective photophysics, transient absorption spectroscopy (TAS) was used, that is, a pump-probe experiment on the femtosecond timescale. TAS allows measurements of excited states, ground state recovery, solvent relaxation, and fluorescence properties on time scales of up to several nanoseconds. Two different DBD dye samples were investigated: acyl- and ester-substituted DBD dyes. Experiments were carried out in solvents with different polarities using different excitation energies and at different viscosities. Based on the experimental data and theoretical calculations, we were able to determine the conformational changes of the molecule due to electronic excitation and were able to investigate solvent relaxation processes for both types of DBD dyes. By generalizing the theory for quadrupole-induced solvent relaxation developed by Togashi et al., we derived quadrupole moments of both molecules in the ground and excited state. Our data showed differences in the binding of polar solvent molecules to the dyes depending on the substituent on the DBD dye. In the case of water as the solvent, an additional efficient quenching process in the electronically excited state was revealed, which was indicated by the observation of solvated electrons in the TAS signals.

Orthogonal sampling in free-energy calculations of residue mutations in a tripeptide: TI versus λ-LEUS.

In a recent article (Bieler et al., J. Chem. Theory Comput. 2014, 10, 3006), we introduced a combination of λ-dynamics and local-elevation umbrella-sampling termed λ-LEUS to calculate free-energy changes associated with alchemical processes using molecular dynamics simulations. This method was suggested to be more efficient than thermodynamic integration (TI), because the dynamical variation of the alchemical variable λ opens up pathways to circumvent barriers in the orthogonal space (defined by the N - 1 degrees of freedom that are not subjected to the sampling enhancement), a feature λ-LEUS shares with Hamiltonian replica-exchange (HR) approaches. However, the mutation considered, hydroquinone to benzene in water, was no real challenge in terms of orthogonal-space properties, which were restricted to solvent-relaxation processes. In the present article, we revisit the comparison between TI and λ-LEUS considering non-trivial mutations of the central residue X of a KXK tripeptide in water (with X = G, E, K, S, F, or Y). Side-chain interactions that may include salt bridges, hydrogen bonds or steric clashes lead to slow relaxation in the orthogonal space, mainly in the two-dimensional subspace spanned by the central φ and ψ dihedral angles of the peptide. The efficiency enhancement afforded by λ-LEUS is confirmed in this more complex test system and can be attributed explicitly to the improved sampling of the orthogonal space. The sensitivity of the results to the nontrivial choices of a mass parameter and of a thermostat coupling time for the alchemical variable is also investigated, resulting in recommended ranges of 50 to 100 u nm(2) and 0.2 to 0.5 ps, respectively.

From intra- to inter-molecular hydrogen bonds with the surroundings: steady-state and time-resolved behaviours.

We report on the photodynamics of 2-(2'-hydroxyphenyl)benzoxazole (HBO), compared to its amino derivatives, 6-amino-2-(2'-hydroxypheny)benzoxazole (6A-HBO) and 5-amino-2-(2'-hydroxypheny)benzoxazole (5A-HBO) in N,N-dimethylformamide (DMF) solutions. HBO at S0 shows a reversible deprotonation reaction leading to the production of anionic forms. However, for 6A-HBO and 5A-HBO, DMF containing KOH is necessary to produce the anions. Excited HBO in DMF exhibits intra- as well as inter-molecular proton transfer (ESIPT and ESPT) reactions. With excitation at 330 nm, we observed the open-enol, anti-enol and keto forms with different emission and lifetimes (620 ps, 1.5 ns, and 74 ps, respectively), while with the excitation at 433 nm, only the anionic species emission was detected (3.7 ns). Contrary to HBO, 6A-HBO and 5A-HBO do not exhibit any proton transfer process, and only the emissions of the open-enol charge-transferred forms (open-ECT) were observed, which are comparable to those of their methylated derivatives (6A-MBO and 5A-MBO). Femtosecond studies of 6A-MBO and 6A-HBO in DMF indicate that an intramolecular charge-transfer (ICT) reaction (∼80 fs) and solvent relaxation process (2 ps) take place at S1. Remarkably, the photoinduced breaking of the intramolecular hydrogen bond of 6A-HBO and the formation of an intermolecular hydrogen bond with DMF molecules occurs in 80 ps, while for 5A-HBO, this process occurs in less than 10 ps. In this study, we have demonstrated that the presence and position of the amino group in the HBO framework change both the S0 and S1 behaviours of the intramolecular H-bonds; a result which might be useful for the design and better understanding of supramolecular systems based on intra- and intermolecular H-bonds.

Effect of nanosize micelles of ionic and neutral surfactants on the photophysics of protonated 6-methoxyquinoline: Time-resolved fluorescence study

The excited state dynamic studies have been carried out to investigate the effects of micellar surface charge on the photophysics of protonated 6-methoxyquinoline (6MQ+) in anionic, sodium dodecylsulphate (SDS), cationic, cetyltrimethylammonium bromide (CTAB) and neutral, triton X-100 (TX100) surfactant at premicellar, micellar and postmicellar concentrations in aqueous phase at room temperature. At premicellar concentrations of SDS, there is a slight decrease in emission intensity and at micellar and postmicellar concentrations, increase in emission intensity and blue shift of spectrum has been observed. The blue shift in fluorescence spectrum and slight increase in quantum yield are attributed to incorporation of solute molecule to the micelles. Edge excitation red shift (EERS) in fluorescence maximum of 6MQ+ has been observed in all the surfactant solutions studied. The EERS has been ascribed in terms of solvent relaxation process. In SDS surfactant system, due to heterogeneous restricted motion of solvent molecules, the solvent viscosity increases which results in an increase in net magnitude of EERS. The fluorescence decay components of 6MQ+ fit with multi exponential functions in all the micellar systems studied. The location of the probe molecule in micellar systems is justified by a variety of spectral parameters such as refractive index, dielectric constant, ET (30), EERS, average fluorescence decay time, radiative and non radiative rate constants, and rotational relaxation time.

3D cross-correlative matrix temperature detection and non-invasive thermal mapping based on a molecular probe

Cryogenic temperature detection underlies the understanding of important physical phenomena and is crucial for material science and space exploration. Simple, facile, and accurate assessment of temperature, especially for surface or body thermal mapping, is one of the challenges that must be addressed when developing new materials or technologies for thermometers. Here, we introduce a temperature detection strategy based on a 3D cross-correlative matrix derived from fluorescence response patterns of an organic probe in three solvation envelopes. The intermolecular interactions, such as excimer or exciplex formation, intramolecular charge transfer and solvent relaxation processes, between the probe and molecule surroundings, are attributed to the unique temperature-dependent optical behaviors in different solutions. The precise temperature detection originated from apparent variations in fluorescence intensity and wavelength upon temperature change in these solvents. This enabled the construction of a comprehensive temperature sensor array, which gives a unique visible color pattern of signals as a read-output discernible by the naked eye. Furthermore, as a proof-of-concept, a new strategy for facile and non-destructive thermal mapping of a crystal surface is presented.

Solvation and Association of 3:1 Electrolytes in N,N-Dimethylformamide

A detailed study has been made of the solvation and ion association of the trifluoromethanesulfonate (Tf(-)) salts of aluminum(III), scandium(III), and lanthanum(III) in N,N-dimethylformamide (DMF) at 25 °C using dielectric relaxation spectroscopy over the frequency range of 0.1 ≲ ν/GHz ≤ 89. The spectra of all solutions exhibited either two (for ScTf3 and LaTf3) or three (for AlTf3) relaxation processes, a dominant mode centered at ∼15 GHz due to the solvent and one or (for AlTf3 solutions) two solute-related processes at lower frequencies (νmax ≲ 2 GHz). Effective solvation numbers, Zib, calculated from the solvent relaxation process indicated that all three cations were strongly solvated by DMF with Zib(0) values at infinite dilution in the order (Al(3+) ≈ Sc(3+) ≈ 10) < (La(3+) ≈ 13), consistent with at least partial formation of a second solvation shell around each cation. One solute-related mode for each set of salt solutions was assigned to the rotational diffusion of solvent-shared ion pairs (SIPs) of 1:1 stoicheometry; the additional slower process for AlTf3 solutions in DMF was attributed to the presence of double-solvent-separated IPs. The overall association constants at infinite dilution for the 1:1 IPs, KA°(MTf(2+)), were significant, but as expected from Debye-Hückel considerations, the KA values decreased rapidly with increasing solute concentration.

Ultrafast excited state dynamics of 1-nitropyrene: Effect of H-bonding

Abstract Ultrafast excited state dynamics of 1-nitropyrene has been carried out using femtosecond fluorescence upconversion technique. Detailed time-resolved emission measurements were performed in protic and aprotic solvents of similar polarity. It is seen that although the singlet–triplet energy gap in polar solvents is quite large, the excited state decay shows intersystem crossing in sub-picosecond time scale as observed for nonpolar solvents with isoenergetic singlet and triplet state. The appearance of the sub-picosecond decay component in polar solvents has been explained on the basis of the dynamical nature of the solvent relaxation process. Further, the contribution of the sub-picosecond decay component is much more for the solvent with slower solvent relaxation time. Time-resolved area normalized emission spectra (TRANES) show the presence of two emissive species for 1-nitropyrene in aprotic solvent while only one emissive species has been observed in protic solvent. The additional emissive species in aprotic solvent has been assigned to the structurally relaxed excited singlet state formed due to the rotation of the nitro group around the pyrene moiety. However, such rotation of the nitro group is prevented in protic solvent due to the formation of H-bonded complex between the solute and the solvent. Formation of H-bonding between protic solvent and nitropyrene is also confirmed by quantum chemical calculations.

Steady state and time-resolved fluorescence spectroscopy of quinine sulfate dication in ionic and neutral micelles: Effect of micellar charge on photophysics

Steady state and time-resolved fluorescence studies have been carried out to investigate the effects of micellar surface charge on the photophysics of a well known fluorescent molecule quinine sulfate dication (QSD) in cationic, cetyltrimethylammonium bromide (CTAB), anionic, sodium dodecylsulphate (SDS) and neutral, triton X-100 (TX100) surfactants at concentrations above the critical micelle concentrations (c.m.c) in aqueous phase. Edge excitation red shift (EERS) in fluorescence maximum of QSD has been observed in all the surfactant solutions studied. The magnitude of observed EERS is less in anionic SDS surfactant solution compared to the EERS in CTAB and TX100 surfactants. The magnitude of EERS in CTAB and TX100 is almost the same as in bulk water solution. The EERS has been ascribed in terms of solvent relaxation process. The observed multi-exponential decay of fluorescence is due to the different locations of QSD in micellar systems. In SDS surfactant system, due to heterogeneous restricted motion of solvent molecules the solvent relaxation rate decreases which results in a decrease in net magnitude of EERS and fluorescence decay components fit in three exponentials. Following the two step and wobbling in a cone model for the analysis of the temporal fluorescence anisotropy decay of QSD in SDS micelles allows determination of restriction on the motion of fluorophore. Further, we have shown that the extraordinary capability to sense the surrounding environment makes QSD molecule very efficient for surface and interface studies and can also be used as a probe to investigate the mobility of solvent molecules around the excited molecules.

Ultrafast Photochemistry in Liquids

Ultrafast photochemical processes can occur in parallel with the relaxation of the optically populated excited state toward equilibrium. The latter involves both intra- and intermolecular modes, namely vibrational and solvent coordinates, and takes place on timescales ranging from a few tens of femtoseconds to up to hundreds of picoseconds, depending on the system. As a consequence, the reaction dynamics can substantially differ from those usually measured with slower photoinduced processes occurring from equilibrated excited states. For example, the decay of the excited-state population may become strongly nonexponential and depend on the excitation wavelength, contrary to the Kasha and Vavilov rules. In this article, we first give a brief account of our current understanding of vibrational and solvent relaxation processes. We then present an overview of important classes of ultrafast photochemical reactions, namely electron and proton transfer as well as isomerization, and illustrate with several examples how nonequilibrium effects can affect their dynamics.

Photoionization and Time-Dependent Stokes Shift of Coumarin 307 in Soft Matter: Solvation and Radical-Ion Pair Recombination Dynamics

Photoionization, fluorescence time-dependent Stokes shift (TDSS), and rotational dynamics of coumarin 307 (C307) have been investigated in soft matter system such as micelles using time-resolved transient absorption and fluorescence spectroscopy. Photoionization of C307 leads to the formation of coumarin radical cation and hydrated electron, which were characterized by their respective transient absorption. The photoionization yields are significantly higher in anionic sodium dodecyl sulfate (SDS) micelle than in cationic cetyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) micelles, indicating the influence of micellar surface charge on the efficient separation of radical cation-hydrated electron pair. The CTAB micelle favors the recombination of radical cation and hydrated electron leading to the formation of triplet state of C307, which causes a decrease in the photoionization yield. C307 exhibits TDSS in all micelles; the time evolution and the magnitude of the TDSS depend on nature of the micelle. In TX-100 micelles, the decay of the TDSS exhibits ultraslow component (165 ns) and is affected by the presence of electron scavengers. The ultraslow component in TX-100 micelle originates from the recombination of radical cation-hydrated electron, which results in the formation of twisted intramolecular charge transfer (TICT) state; such formation of TICT state was not observed in SDS and CTAB micelles. To the best of our knowledge, this is the first report where the radical-ion pair recombination dynamics is probed using TDSS in combination with time-resolved transient absorption studies. The activation energy for the solvent relaxation and radical-ion pair (solvent separated) recombination process was found to be 6.1 and 3.0 kcal mol(-1), respectively. Temperature effect on TDSS in TX-100 micelles confirmed the increase in the water hydration, and size of the micelle influences the relative contribution of the solvation and radical-ion pair recombination dynamics toward the total TDSS. We propose that TDSS observed in neutral micelles and reported in other biomolecules such as proteins by the 7-amino coumarin probe is not only due to the solvation dynamics alone but also due to the radical-ion pair recombination dynamics.

Hydrogen bonding dynamics of cyclodextrin–water solutions by depolarized light scattering

AbstractDepolarized light scattering measurements performed on aqueous solutions of α‐cyclodextrin are used for investigating the hydration properties of this natural cyclic oligosaccharide, which is well known for its ability to form inclusion complexes with a large variety of guest hydrophobic molecules. The spectral analysis allows the characterization of the solvent relaxation process assigned to hydration water, which is found to be seven to eight times slower than the bulk. As a structural information, the number of water molecules involved in the hydration process is also estimated at about 46. These results are a key step for understanding the role played by water in determining the properties of different forms of cyclodextrins and their derivatives, in turn related to complexation ability of these macrocycles. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Dielectric Relaxation Study of the Ion Solvation and Association of NaCF3SO3, Mg(CF3SO3)2, and Ba(ClO4)2inN,N-Dimethylformamide

Solutions of sodium trifluoromethanesulfonate, magnesium trifluoromethanesulfonate, and barium perchlorate in N,N-dimethylformamide (DMF) have been investigated using broadband dielectric relaxation spectroscopy at 25 °C. All spectra were dominated by a solvent relaxation process centered at ∼15 GHz but also exhibited one (for NaCF(3)SO(3)) or two (for the 2:1 salts) low-amplitude processes, centered at frequencies below 2 GHz, that could be attributed to the presence of ion pairs. Effective solvation numbers calculated from the solvent relaxation amplitudes indicated strong solvation of all three cations, with evidence for the formation of a second solvation sheath for Mg(2+) and possibly Ba(2+). Detailed analysis of the solute-related processes showed that solvent-shared ion pairs (SIPs) were formed in NaCF(3)SO(3) solutions in DMF. The data for Mg(CF(3)SO(3))(2) and Ba(ClO(4))(2) solutions were not definitive but, consistent with the solvation evidence, favored the presence of double solvent-separated ion pairs and SIPs. Overall association constants, K(A), were small for all three salts in DMF and increased in the order: NaCF(3)SO(3)<Ba(ClO(4))(2)<Mg(CF(3)SO(3))(2).

Spectroscopic and Photophysical Properties of ZnTPP in a Room Temperature Ionic Liquid

The steady-state absorption and emission spectra and the time-resolved Soret- and Q-band excited fluorescence profiles of the model metalloporphyrin, ZnTPP, have been measured in a highly purified sample of the common room temperature ionic liquid, [bmim][PF₆]. S₂-S₀ emission resulting from Soret-band excitation behaves in a manner completely consistent with that of molecular solvents of the same polarizability. The ionic nature of the solvent and its slow solvation relaxation times have no significant effect on the nature of the radiationless decay of the S₂ state, which decays quantitatively to S₁ at a population decay rate that is consistent with the weak coupling case of radiationless transition theory (energy gap law). The ratio of the intensities of the Qα:Qβ (0-0:1-0) bands is consistent with the solvatochromic shift correlation data obtained for molecular solvents. The temporal S₁ fluorescence decay profiles measured at a single emission wavelength are biexponential; the longer-lived major component is similar to that observed for ZnTPP in molecular solvents, and the minor shorter-lived component is attributed to solvent relaxation processes on a nanosecond time scale.

Numerical studies of the membrane fluorescent dyes dynamics in ground and excited states

Fluorescence methods are widely used in studies of biological and model membranes. The dynamics of membrane fluorescent markers in their ground and excited electronic states and correlations with their molecular surrounding within the fully hydrated phospholipid bilayer are still not well understood. In the present work, Quantum Mechanical (QM) calculations and Molecular Dynamics (MD) simulations are used to characterize location and interactions of two membrane polarity probes (Prodan; 6-propionyl-2-dimethylaminonaphthalene and its derivative Laurdan; 2-dimethylamino-6-lauroylnaphthalene) with the dioleoylphosphatidylcholine (DOPC) lipid bilayer model. MD simulations with fluorophores in ground and excited states are found to be a useful tool to analyze the fluorescent dye dynamics and their immediate vicinity. The results of QM calculations and MD simulations are in excellent agreement with available experimental data. The calculation shows that the two amphiphilic dyes initially placed in bulk water diffuse within 10 ns towards their final location in the lipid bilayer. Analysis of solvent relaxation process in the aqueous phase occurs on the picoseconds timescale whereas it takes nanoseconds at the lipid/water interface. Four different relaxation time constants, corresponding to different relaxation processes, where observed when the dyes were embedded into the membrane.

Light Scattering Spectra of Water in Trehalose Aqueous Solutions: Evidence for Two Different Solvent Relaxation Processes

Light scattering spectra on aqueous solutions of trehalose were recorded in a wide frequency range combining the use of a double monochromator and a multipass Fabry-Perot interferometer. Experimental results indicate the presence of a slow relaxation mode related to the solute dynamics, which is clearly separated from the solvent one. The spectral analysis reveals the existence of two separate solvent relaxation processes assigned to hydrating and bulk water molecules. The picosecond dynamics of water molecules directly interacting with the solute (proximal water) is consistently delayed with the corresponding relaxation time increase is about 5-6 times compared to the bulk. The slowing down induced by the sugar on the water dynamics mainly involves a restricted hydration layer constituted of 16-18 water molecules. These results improve our knowledge about the influence of carbohydrates on the fast rearrangement dynamics of water and may serve as a model to gain important insight on basic solvation properties of other biorelevant systems in aqueous media.

Quantitative Distinction between Competing Intramolecular Bond Twisting and Solvent Relaxation Dynamics: An Ultrafast Study

Often an intramolecular relaxation process takes place in a time scale similar to that of the solvent relaxation process. Under these circumstances the dynamic Stokes' shift of the probe can be modulated by the combined effect of these two relaxation processes. In the present article we have studied ultrafast solvent relaxation using three different coumarin dyes and proposed a methodology for the quantitative separation of the dynamics of two competing processes, namely, solvent relaxation and bond twisting, that take place simultaneously in the present systems.

Extrinsic fluorescent dyes as tools for protein characterization.

Noncovalent, extrinsic fluorescent dyes are applied in various fields of protein analysis, e.g. to characterize folding intermediates, measure surface hydrophobicity, and detect aggregation or fibrillation. The main underlying mechanisms, which explain the fluorescence properties of many extrinsic dyes, are solvent relaxation processes and (twisted) intramolecular charge transfer reactions, which are affected by the environment and by interactions of the dyes with proteins. In recent time, the use of extrinsic fluorescent dyes such as ANS, Bis-ANS, Nile Red, Thioflavin T and others has increased, because of their versatility, sensitivity and suitability for high-throughput screening. The intention of this review is to give an overview of available extrinsic dyes, explain their spectral properties, and show illustrative examples of their various applications in protein characterization.

Temperature-dependent Time-resolved Fluorescence Study of Cinchonine Alkaloid Dication

Photo induced excited state dynamical processes of cinchonine alkaloid dication (C(++)) have been studied over a wide range of temperature using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The temperature-dependent fluorescence studies of C(++) clearly indicate the existence of two distinct emitting species having their own characteristic decay rates. The shorter-lived species shows a usual temperature dependence with increasing non-radiative deactivation at higher temperatures, while the longer-lived species show features resembling to the excited state solvent relaxation process with a large solvent relaxation time (tau(r) approximately 6 ns). The species emitting in the lower energy side, having longer decay time is found to be more sensitive towards chloride ion quenching and has a charge transfer character. Further, concentration quenching with decrease in tau(r) of long lived species shows the possibility of energy migration along with solvent relaxation in C(++).

Edge excitation red shift and energy migration in quinine bisulphate dication

Photoinduced excited state dynamical processes in quinine sulphate dication (QSD) have been studied over a wide range of solute concentrations using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The edge excitation red shift (EERS) of emission maximum, emission wavelength dependence of fluorescence lifetimes and the time dependence of emission maximum are known to occur due to the solvent relaxation process. With increase in solute concentration, the emission spectrum shifts towards the lower frequencies accompanied with decrease in fluorescence intensity, however, absorption spectrum remains unchanged. A decrease in EERS, fluorescence lifetimes, time dependent fluorescence Stokes shift (TDFSS), fluorescence polarization and the solvent relaxation time ( τ r) is observed with the increase in solute concentration. The process of energy migration among the QSD ions along with solvent relaxation has been found responsible for the above experimental findings.