Dynamic Stokes Shift Research Articles

Solvent-Controlled Excited State Relaxation Path of 4-Acetyl-4'-(dimethylamino)biphenyl.

Stationary and picosecond time-resolved fluorescence (TRF) and absorption spectra were compared in different aprotic solvents at various temperatures for 4-acetyl-4'-(dimethylamino)biphenyl (ADAB). A large value of the excited state dipole moment, 18-25 D, was estimated from the plot of solvatochromic shift. TRF spectra of ADAB recorded as a function of solvent polarity and temperature show unusual temporal evolution (shift and decay) of the fluorescence bands. In some cases, the dynamic Stokes shifts occur on a time scale much shorter than expected on the basis of literature data on solvent relaxation. In order to investigate variations in the energy of electronic transitions, oscillator strengths, and dipole moments upon changing the molecular geometry, quantum chemical modeling (DFT, TD-DFT, CIS) was performed for ADAB and its ground-state pretwisted derivative, 4-acetyl-4'-dimethylamino-2,2'-dimethylbiphenyl (ADAB-Me). Combination of spectroscopic data and computational results leads to the model of excited state relaxation which involves dynamic solvent-dependent interaction between two close-lying (1)nπ* and (1)ππ* excited electronic states.

Density relaxation and particle motion characteristics in a non-ionic deep eutectic solvent (acetamide + urea): Time-resolved fluorescence measurements and all-atom molecular dynamics simulations

Temperature dependent relaxation dynamics, particle motion characteristics, and heterogeneity aspects of deep eutectic solvents (DESs) made of acetamide (CH3CONH2) and urea (NH2CONH2) have been investigated by employing time-resolved fluorescence measurements and all-atom molecular dynamics simulations. Three different compositions (f) for the mixture [fCH3CONH2 + (1 - f)NH2CONH2] have been studied in a temperature range of 328-353 K which is ∼120-145 K above the measured glass transition temperatures (∼207 K) of these DESs but much lower than the individual melting temperature of either of the constituents. Steady state fluorescence emission measurements using probe solutes with sharply different lifetimes do not indicate any dependence on excitation wavelength in these metastable molten systems. Time-resolved fluorescence anisotropy measurements reveal near-hydrodynamic coupling between medium viscosity and rotation of a dissolved dipolar solute. Stokes shift dynamics have been found to be too fast to be detected by the time-resolution (∼70 ps) employed, suggesting extremely rapid medium polarization relaxation. All-atom simulations reveal Gaussian distribution for particle displacements and van Hove correlations, and significant overlap between non-Gaussian (α2) and new non-Gaussian (γ) heterogeneity parameters. In addition, no stretched exponential relaxations have been detected in the simulated wavenumber dependent acetamide dynamic structure factors. All these results are in sharp contrast to earlier observations for ionic deep eutectics with acetamide [Guchhait et al., J. Chem. Phys. 140, 104514 (2014)] and suggest a fundamental difference in interaction and dynamics between ionic and non-ionic deep eutectic solvent systems.

Solvent sorting in (mixed solvent + electrolyte) systems: Time-resolved fluorescence measurements and theory

In this manuscript we explore electrolyte-induced modification of preferential solvation of a dipolar solute dissolved in a binary mixture of polar solvents. Composition dependence of solvation characteristics at a fixed electrolyte concentration has been followed. Binary mixtures of two different polarities have been employed to understand the competition between solute-ion and solute-solvent interactions. Time-resolved fluorescence Stokes shift and anisotropy have been measured for coumarin 153 (C153) in moderately polar (ethyl acetate + 1-propanol) and strongly polar (acetonitrile + propylene carbonate) binary mixtures at various mixture compositions, and in the corresponding 1.0 M solutions of LiClO4. Both the mixtures show red shifts in C153 absorption and fluorescence emission upon increase of mole fraction of the less polar solvent component in presence of the electrolyte. In addition, measured average solvation times become slower and rotation times faster for the above change in the mixture composition. A semi-molecular theory based on solution density fluctuations has been developed and found to successfully capture the essential features of the measured Stokes shift dynamics of these complex multi-component mixtures. Dynamic anisotropy results have been analyzed by using both Stokes-Einstein-Debye (SED) and Dote-Kivelson-Schwartz (DKS) theories. The importance of local solvent structure around the dissolved solute has been stressed.

Stokes shift dynamics in (non-dipolar ionic liquid + dipolar solvent) binary mixtures: a semi-molecular theory.

A semi-molecular theory for studying composition dependent Stokes shift dynamics of a dipolar solute in binary mixtures of (non-dipolar ionic liquid + common dipolar solvent) is developed here. The theory provides microscopic expressions for solvation response functions in terms of static and dynamic structure factors of the mixture components and solute-solvent static correlations. In addition, the theory provides a framework for examining the interrelationship between the time dependent solvation response in and frequency dependent dielectric relaxation of a binary mixture containing electrolyte. Subsequently, the theory has been applied to predict ionic liquid (IL) mole fraction dependent dynamic Stokes shift magnitude and solvation energy relaxation for a dipolar solute, C153, in binary mixtures of an ionic liquid, trihexyltetradecylphosphonium chloride ([P(14,666)][Cl]) with a common dipolar solvent, methanol (MeOH). In the absence of suitable experimental data, necessary input parameters have been obtained from approximate methods. Dynamic shifts calculated for these mixtures exhibit a linear increase with IL mole fraction for the most part of the mixture composition, stressing the importance of solute-IL dipole-ion interaction. Average solvation rates, on the other hand, show a nonlinear IL mole fraction dependence which is qualitatively similar to what has been observed for such binary mixtures with imidazolium (dipolar) ILs. These predictions should be re-examined in suitable experiments.

Understanding THz and IR signals beneath time-resolved fluorescence from excited-state ab initio dynamics.

The detailed interpretation of time-resolved spectroscopic signals in terms of the molecular rearrangement during a photoreaction or a photophysical event is one of the most important challenges of both experimental and theoretical chemistry. Here we simulate a time-resolved fluorescence spectrum of a dye in aqueous solution, the N-methyl-6-oxyquinolinium betaine, and analyze it in terms of far IR and THz frequency contributions, providing a direct connection to specific molecular motions. To obtain this result, we build up an innovative and general approach based on excited state ab-initio molecular dynamics and a wavelet-based time-dependent frequency analysis of nonstationary signals. We obtain a nice agreement with key parameters of the solvent dynamics, such as the total Stokes shift and the Stokes shift relaxation times. As an important finding, we observe a strong change of specific solute-solvent interactions upon the electronic excitation, with the migration of about 1.5 water molecules from the first solvation shell toward the bulk. In spite of this event, the Stokes shift dynamics is ruled by collective solvent motions in the THz and far IR, which guide and modulate the strong rearrangement of the dye microsolvation. By the relaxation of THz and IR contributions to the emission signal, we can follow and understand in detail the molecularity of the process. The protocol presented here is, in principle, transferable to other time-resolved spectroscopic techniques.

The Effect of Pre-solvation in the Ground State on Photoinduced Electron Transfer in Ionic Liquids

Photoinduced intramolecular charge transfer (CT) dynamics of the asymmetric and symmetric molecules, 4-(9-anthryl)-N,N-dimethylaniline (ADMA) and 9,9′-bianthryl (BA), was investigated by means of time-resolved fluorescence (TRF) spectroscopy and compared with the solvation dynamics of coumarin 153 (C153) in several ionic liquids (ILs). The normalized dynamic Stokes shift (DSS) function for the CT state emission of ADMA in the sub-ns to ns time scale is almost the same as those of BA and C153 in most of the ILs studied, in spite of the much larger Stokes shift of ADMA compared to those of BA and C153. On the other hand, the contribution from ultrafast DSS within 30 ps, which was undetectable with the time resolution in the present study, was estimated to be smaller for BA than that for ADMA. Meanwhile, in phosphonium IL, the DSS in sub-ns to ns time range was solute dependent. This peculiarity in phosphonium IL is discussed from the viewpoint of the hierarchy of the solvation dynamics.

Characterization of a novel intrinsic luminescent room-temperature ionic liquid based on [P6,6,6,14 ][ANS

Intrinsically luminescent room-temperature ionic liquids (RTILs) can be prepared by combining a luminescent anion (more common) or cation with appropriate counter ions, rendering new luminescent soft materials. These RTILs are still new, and many of their photochemical properties are not well known. A novel intrinsic luminescent RTIL based on the 8-anilinonaphthalene-1-sulfonate ([ANS]) anion combined with the trihexyltetradecylphosphonium ([P6,6,6,14 ]) cation was prepared and characterized by spectroscopic techniques. Detailed photophysical studies highlight the influence of the ionic liquid environment on the ANS fluorescence, which together with rheological and (1) H NMR experiments illustrate the effects of both the viscosity and electrostatic interactions between the ions. This material is liquid at room temperature and possesses a glass transition temperature (Tg ) of 230.4 K. The fluorescence is not highly sensitive to factors such as temperature, but owing to its high viscosity, dynamic Stokes shift measurements reveal very slow components for the IL relaxation.

Selective Probing of Reverse Micelle Interfacial Layer upon Silver Nanoparticle Formation using Dynamic Stokes Shift Measurements

Nanoparticle synthesis within reverse micelle core has exceptional control over size and morphology. The aqueous core (water pool) acts as a nanoreactor supporting chemical reduction of metal salts, while a layer of surfactant molecules (interfacial layer) casing the water pool serves as a template to halt the growth up to a specific size. Because of the soft nature of the interfacial layer (template), the grown nanoparticles may severely influence the structural organization of the original reverse micelle. Here, using a site-selective hydrophilic probe, coumarin 343 (C343), we have characterized the interfacial layer of water/sodium dioctylsulfosuccinate (AOT)/n-heptane reverse micelles before and after silver nanoparticle (AgNP) formation by combination of steady-state absorption, excitation, emission spectroscopy, time-resolved fluorescence anisotropy, and dynamic Stokes shift measurements. We found that both solvation dynamics and rotational dynamics inside the interfacial region of the reverse micel...

Composition Dependent Stokes Shift Dynamics in Binary Mixtures of 1-Butyl-3-methylimidazolium Tetrafluoroborate with Water and Acetonitrile: Quantitative Comparison between Theory and Complete Measurements

Here we predict, using a semimolecular theory, the Stokes shift dynamics of a dipolar solute in binary mixtures of an ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), with water (H2O) and acetonitrile (CH3CN), and compare with the experimental results. The latter are from the recent measurements that combined broad-band fluorescence up-conversion (FLUPS) with time-correlated single photon counting (TCSPC) techniques and used coumarin 153 (C153) as a solute probe. Nine different compositions of ([Bmim][BF4] + H2O) and ([Bmim][BF4] + CH3CN) binary mixtures are considered for the extensive comparison between theory and experiments. Two separate model calculations have been performed using the available experimental frequency dependent dielectric function, ε(ω). These calculations semiquantitatively reproduce the experimentally observed (i) IL mole fraction dependence of dynamic Stokes shifts in these mixtures, (ii) composition dependence of average fast, slow, and solvation times, (iii) viscosity dependence of slow times, and (iv) the nonlinear dependence of average solvation times on experimental inverse conductivity. Variations of the calculated dynamics on water dipole moment values (gas phase or liquid phase) and sensitivity to different measurements of ε(ω) for ([Bmim][BF4] + H2O) mixtures are examined. In addition, the importance of the missing contribution to experimental ε(ω) from high frequency collective solvent intermolecular modes for generating the experimentally observed sub-picosecond solvation response in these (IL + polar solvent) binary mixtures has been explored.

Coherent Oscillations in the PC577 Cryptophyte Antenna Occur in the Excited Electronic State

Transient absorption spectroscopy is a useful measurement for investigating ultrafast dynamics in molecules. We have developed a transient absorption spectrometer that utilizes balanced and fast detection methods to suppress noise and maintain high temporal and spectral resolution. We use the spectrometer to investigate the ultrafast dynamics in a photosynthetic pigment-protein complex, the phycobiliprotein PC577 isolated from the cryptophyte alga Hemiselmis pacifica CCMP706. We analyze coherent oscillations in the transient absorption data and attribute them to vibrational coherences. Analysis of the dynamic Stokes shift and motion of the wave packet on the potential-energy surface indicate that the coherences arise from vibrational wave packets in the excited electronic state of the protein.

Excited-state solvation dynamics of meso-tetrakis(4-sulfonatophenyl) porphyrin in solid imidazolium-sulfonate-based ionic liquids

The steady-state absorption and fluorescence emission positions of diprotonated meso-tetrakis(4-sulfonatophenyl) porphyrin (H4TPPS2−) are dependent on the polarity of the selected two solid ionic liquids (ILs) and are red-shifted with the increase of cation size. The solvation dynamics process of H4TPPS2− in these ILs occurs on two well-separated time scales. The short components with 121.2–128.6 ps arise from the local motion of the ion-pairs in close proximity to the porphine core, and the long components with 1056.6–1261.8 ps are due to the collective translation motions of the ion-pairs. The dynamic Stokes shifts and the relaxation times increase with the increasing cation size of the ILs.

Construction of energy transfer pathways self-assembled from DNA-templated stacks of anthracene

We describe optical properties of anthracene stacks formed from single-component self-assembly of thymidylic acid-appended anthracene 2,6-bis[5-(3′-thymidylic acid)pentyloxy] anthracene (TACT) and the binary self-assembly of TACT and complementary 20-meric oligoadenylic acid (TACT/dA20) in an aqueous buffer. UV–Vis and emission spectra for the single-component self-assembly of TACT and the binary self-assembly of TACT/dA20 were very consistent with stacked acene moieties in both self-assemblies. Interestingly, time-resolved fluorescence spectra from anthracene stacks exhibited very different features of the single-component and binary self-assemblies. In the single-component self-assembly of TACT, a dynamic Stokes shift (DSS) and relatively short fluorescence lifetime (τ=0.35ns) observed at around 450nm suggested that the anthracene moieties were flexible. Moreover, a broad emission at 530nm suggested the formation of an excited dimer (excimer). In the binary self-assembly of TACT/dA20, we detected a broad, red-shifted emission component at 534nm with a lifetime (τ=0.4ns) shorter than that observed in the TACT single-component self-assembly. Combining these results with the emission spectrum of the binary self-assembly of TACT/5′-HEX dA20, we concluded that the energy transfer pathway was constructed by columnar anthracene stacks formed from the DNA-templated self-assembly of TACT.

Stokes shift dynamics of ionic liquids: Solute probe dependence, and effects of self-motion, dielectric relaxation frequency window, and collective intermolecular solvent modes

In this paper we have used a semi-molecular theory for investigating the probe dependence of Stokes shift dynamics in room temperature ionic liquids (ILs) by considering three different but well-known dipolar solvation probes--coumarin 153, trans-4-dimethylamino-4(')-cyanostilbene, and 4-aminophthalimide. In addition, effects on polar solvation energy relaxation in ILs of solute motion, frequency coverage (frequency window) accessed by dielectric relaxation measurements and collective IL intermolecular modes (CIMs) at tera-hertz range have been explored. Eleven different ILs have been considered for the above theoretical study. Calculated results show better agreement with the recent (fluorescence up-conversion (FLUPS) + time-correlated single photon counting (TCSPC)) experimental results, particularly at short times, when the CIM contribution to the frequency dependent dielectric function (ɛ(ω)) is included. This is done via assigning the missing dispersion in an experimental ɛ(ω) to an IL intermolecular mode at 30 cm(-1). No significant probe dependence has been observed for solvation energy relaxation although the magnitude of dynamic Stokes shift varies with the dipole moment of the excited solute. Calculations using experimental ɛ(ω) measured with broader frequency window generate solvation response functions closer to experiments. However, average solvation rates predicted by using different ɛ(ω) for the same IL do not differ appreciably, implying over-all validity of these dielectric relaxation measurements. Results presented here indicate that inclusion of solvent molecularity via wavenumber dependent static correlations and ion dynamic structure factor relaxation improves significantly the comparison between theory and experiments over the continuum model predictions for polar solvation dynamics in these solvents.

Ultrafast Time-Resolved Broadband Fluorescence Studies of the Benzene-Tetracyanoethylene Complex: Solvation, Vibrational Relaxation, and Charge Recombination Dynamics

The charge-transfer (CT) state relaxation dynamics of the benzene-tetracyanoethylene (BZ-TCNE) complex was studied with broadband ultrafast time-resolved fluorescence spectroscopy implemented by optical Kerr gating in three solvents of different polarities. The CT state of the BZ-TCNE complex is reached via femtosecond laser excitation, and the subsequent temporal evolutions of the fluorescence spectra were measured. Analyses of various time-dependent spectral properties revealed rapid relaxations along solvent and vibrational coordinates in competition with charge recombination (CR). By comparing the results in solvents of different polarities, we partially separated solvation and vibrational relaxation dynamics and explored the solvent-dependent CR dynamics. Time-dependent dynamic fluorescence Stokes shift (TDFSS) measurements unveiled the solvation and vibrational relaxation contributions to the observed spectral relaxation. The biphasic and slow time scales of the vibrational contributions identified in TDFSS suggested nonstatistical and hindered intramolecular vibrational-energy redistribution that can be attributed to the unique structural properties of EDA complexes. The slowest spectral relaxation of 10-15 ps identified in TDFSS was ascribed to relaxation of the BZ(+)-TCNE(-) intermolecular vibrations, which is equivalent to a structural relaxation from the initial Franck-Condon configuration to the equilibrium CT-state structure. The time scales of vibrational relaxation indicate that a fraction of the CT-state population undergoes CR reactions before complete vibrational/structural equilibrium is achieved. In carbon tetrachloride, a nonexponential temporal profile was observed and attributed to vibrational nonequilibrium CR. In dichloromethane, polar solvation greatly accelerates CR reactions, and a slower reaction-field-induced structural relaxation gives rise to a pronounced biexponential decay. The equilibrium CR time constants of the BZ-TCNE CT state are 29 ps, 150 ps, and 68 ps in dichloromethane, carbon tetrachloride, and cyclohexane, respectively.

Excess Dynamic Stokes Shift of Molecular Probes in Solution

The solvation dynamics of molecular probes is studied by broad-band fluorescence upconversion. The time-dependent position of the S1 → S0 emission band or of a vibronic line shape is measured with ~80 fs, 10 cm(-1) resolution. Polar solutes in acetonitrile and acetone, when excited into S1 with excess vibrational energy, show a dynamic Stokes shift which extends to the red beyond the quasistationary state. Equilibrium is then reached by a slower blue shift on a 10 ps time scale. In methanol, excess vibrational energy as large as ~14,000 cm(-1) shows no such effect. Nonpolar solutes exhibit an excess red shift of the emission band in both polar and nonpolar solvents even upon excitation near the vibronic origin. The observed dynamics are discussed in terms of transient heating of the excited chromophore, conformational change, and changes of the molecular cavity size. For solvation studies the optical excitation should be chosen close to the band origin.

Dielectric Relaxation and Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Protic Liquid Mixture: 1-Butyl-3-Methylimidazolium Tetrafluoroborate + Water

Dielectric and solvation data on mixtures of 1-butyl-3-methylimidazilium tetrafluoroborate ([Im41][BF4]) + water are reported and used to examine the utility of dielectric solvation models. Dielectric permittivity and loss spectra (25 °C) were recorded over the frequency range 200 MHz to 89 GHz at 17 compositions and fit to a 4-Debye form. Dynamic Stokes shift measurements on the solute coumarin 153 (C153), made by combining fluorescence upconversion (80 fs resolution) and time-correlated single photon counting data (20 ns range), were used to determine the solvation response at 7 compositions (20.5 °C). All properties measured here were found to depend upon mixture composition in a simple continuous manner, especially when viewed in terms of volume fraction. Solvation response functions predicted by a simple dielectric continuum model are similar to but ∼7-fold faster than the spectral response functions measured with C153. The solvation data are in better agreement with the recently published predictions of a semimolecular model of Biswas and co-workers [J. Phys. Chem. B 2011, 115, 4011], but these latter predictions are systematically slow by a factor of ∼3.

Ab Initio Study on an Excited-State Intramolecular Proton-Transfer Reaction in Ionic Liquid

An excited-state intramolecular proton transfer (ESIPT) reaction of 4'-N,N-dimethylamino-3-hydroxyflavone in room temperature ionic liquid is theoretically investigated using RISM-SCF-SEDD, which is a hybrid method of molecular liquid theory and ab initio molecular orbital theory. The photo-excitation and proton-transfer processes are computed by considering the solvent fluctuation. The calculated absorption and emission energy are in good agreement with the experiments. The changes in the dipole moment indicate that the drastic solvation relaxation is accompanied by the excitation and an ESIPT process, which is consistent with the remarkable dynamic Stokes shift observed in the experiments. We calculated the nonequilibrium free-energy contour as a function of the proton coordinate and the solvation coordinate. We conclude that although immediately after the excitation the barrier height of the ESIPT process is relatively small, the barrier becomes larger as the solvation relaxation to the excited normal state proceeds. The solvation relaxation process is also investigated on the basis of microscopic solvation structure obtained by RISM calculations.

Surface hopping modeling of two-dimensional spectra

Recently, two-dimensional (2D) electronic spectroscopy has become an important tool to unravel the excited state properties of complex molecular assemblies, such as biological light harvesting systems. In this work, we propose a method for simulating 2D electronic spectra based on a surface hopping approach. This approach self-consistently describes the interaction between photoactive chromophores and the environment, which allows us to reproduce a spectrally observable dynamic Stokes shift. Through an application to a dimer, the method is shown to also account for correct thermal equilibration of quantum populations, something that is of great importance for processes in the electronic domain. The resulting 2D spectra are found to nicely agree with hierarchy of equations of motion calculations. Contrary to the latter, our method is unrestricted in describing the interaction between the chromophores and the environment, and we expect it to be applicable to a wide variety of molecular systems.

Solute and Solvent Dynamics in Confined Equal-Sized Aqueous Environments of Charged and Neutral Reverse Micelles: A Combined Dynamic Fluorescence and All-Atom Molecular Dynamics Simulation Study

Here a combined dynamic fluorescence and all-atom molecular dynamics simulation study of aqueous pool-size dependent solvation energy and rotational relaxations of a neutral dipolar solute, C153, trapped in AOT (charged) and IGPAL (neutral) reverse micelles (RMs) at 298 K, is described. RMs in simulations have been represented by a reduced model where SPC/E water molecules interact with a trapped C153 that possesses realistic charge distributions for both ground and excited states. In large aqueous pools, measured average solvation and rotation rates are smaller for the neutral RMs than those in charged ones. Interestingly, while the measured average solvation and rotation rates increase with pool size for the charged RMs, the average rotation rates for the neutral RMs exhibit a reverse dependence. Simulations have qualitatively reproduced this experimental trend and suggested interfacial location for the solute for all cases. The origin for the subnanosecond Stokes shift dynamics has been investigated and solute-interface interaction contribution quantified. Simulated layer-wise translational and rotational diffusions of water molecules re-examine the validity of the core-shell model and provide a resolution to a debate regarding the origin of the subnanosecond solvation component in dynamic Stokes shift measurements with aqueous RMs but not detected in ultrafast IR measurements.

Picosecond Solvation Dynamics of Coumarin153 in Bis(1-methyl-1H-imidazol-3-ium-3-yl)dihydroborate Cation Containing Room Temperature Ionic Liquid and Ionic Liquid-DMF Mixtures

Abstract: Steady state and time-resolved fluorescence behavior of coumarin153(C153) in bis(1-methyl-1H-imidazol-3-ium-3-yl)dihydroborate cation containing room temperature ionic liquid and its mixture with dimethylformamide (DMF) has been investigated. Density functional calculations on the present ionic liquid have been carried out to have ground state structural information of this system. C-H···N and C-H···O hydrogen bonding interactions between cationic and anionic moiety of the present ionic liquid has been observed. Steady state absorption and emission spectral profiles of C153 are found not to be influenced by the polar cosolvent. Time-resolved fluorescence anisotropy experiments show that the rotational motion of the probe becomes faster in presence of DMF. During time dependent dynamic Stokes shift measurements in ionic liquid-DMF mixtures, the average solvation time is found to decrease with the addition of DMF to the ionic liquid. The decrease in both average solvation and rotational time of probe molecule upon gradual addition of polar organic co-solvent is attributed to the lowering of bulk viscosity of the medium.