Solvent Relaxation Dynamics Research Articles

Femtosecond Excited State Studies of the Two-Center Three-Electron Bond Driven Twisted Internal Charge Transfer Dynamics in 1,8-Bis(dimethylamino)naphthalene

Femtosecond fluorescence upconversion and transient absorption experiments have been performed to monitor the photoinduced electronic, geometry, and solvent relaxation dynamics of 1,8-bis(dimethylamino)naphthalene dissolved in methylcyclohexane or n-hexane, n-dodecane, dichloromethane, and acetonitrile. The data have been analyzed by using a sequential global analysis method that gives rise to species associated difference spectra. The spectral features in these spectra and their dynamic behavior enable us to associate them with specific processes occurring in the molecule. The experiments show that the internal charge-transfer lpi* state is populated after internal conversion from the 1La state. In the lpi state the molecule is concluded to be subject to a large-amplitude motion, thereby confirming our previous predictions that internal charge transfer in this state is accompanied by the formation of a two-center three-electron bond between the two nitrogen atoms. Solvent relaxation and vibrational cooling in the lpi* state cannot be separated in polar solvents, but in apolar solvents a distinct vibrational cooling process in the lpi* state is discerned. The spectral and dynamic characteristics of the final species created in the experiments are shown to correspond well with what has been determined before for the relaxed emissive lpi state.

Effect of Water, Methanol, and Acetonitrile on Solvent Relaxation and Rotational Relaxation of Coumarin 153 in Neat 1-Hexyl-3-methylimidazolium Hexafluorophosphate

The dynamics of solvent relaxation in ionic liquid (IL)-water, IL-methanol, and IL-acetonitrile mixtures have been investigated using steady state and picosecond time-resolved fluorescence spectroscopy. We have used Coumarin 153 (C-153) and 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF(6)]) as fluorescence probe and IL, respectively. The steady-state emission spectra showed that the gradual addition of cosolvents increases the polarity of the mixtures. In neat [hmim][PF(6)] and all IL-cosolvent mixtures, solvation occurs in two well-separated time regimes within the time resolution of our instrument. A substantial portion of the solvation has been missed due to the limited time resolution of our instrument. The gradual addition of cosolvents decreases the viscosity of the medium and consequently solvation time also decreases. The decrease in solvation time is more pronounced on addition of acetonitrile compared to water and methanol. The rotational relaxation time of the probe is also decreasing with gradual addition of the cosolvents. The decrease in viscosity of the solution is responsible for the decrease in the rotational relaxation time of the probe molecule.

Solvation dynamics in triton-X-100 and triton-X-165 micelles: effect of micellar size and hydration.

Dynamic Stokes' shift measurements using coumarin 153 as the fluorescence probe have been carried out to study solvation dynamics in two nonionic micelles, viz., triton-X-100 (TX-100) and triton-X-165 (TX-165). In both the micelles, the solvent relaxation dynamics is biexponential in nature. While the fast solvation time tau(s1) is seen to be almost similar for both the micelles, the slow solvation time tau(s2) is found to be appreciably smaller in TX-165 than in TX-100 micelle. Dynamic light scattering measurements indicate that the TX-165 micelles are substantially smaller in size than that of TX-100. Assuming similar core size for both the micelles, as expected from the similar chemical structures of the nonpolar ends for both the surfactants, the Palisade layer is also indicated to be substantially thinner for TX-165 micelles than that of TX-100. The aggregation number of TX-165 micelles is also found to be substantially smaller than that of TX-100 micelles. Fluorescence spectral studies of C153 dye in the two micelles indicate that the Palisade layer of TX-165 micelles is more polar than that of TX-100 micelles. Fluorescence anisotropy measurements indicate that the microviscosity in the Palisade layer of TX-165 micelles is also lower than that of TX-100 micelles. Based on these results it is inferred that the structure of the Palisade layer of TX-165 micelles is quite loose and have higher degree hydration in comparison to that of TX-100 micelles. Due to these structural differences in the Palisade layers of TX-165 and TX-100 micelles the solvation dynamics is faster in the former micelles than in the latter. It has been further inferred that in the present systems the collective response of the water molecules at somewhat away from the probes is responsible for the faster component of the solvation time, which does not reflect much of the structural changes of the micellar Palisade layer. On the contrary, the slower solvation time component, which is mainly due to the single particle response arising from water molecules adjacent to the probe in the micellar Palisade layer, is largely affected by the structural changes in the micellar Palisade layer.

Solvent-Polarity Tuning Excited-State Charge Coupled Proton-Transfer Reaction inp-N,N-Ditolylaminosalicylaldehydes

Detailed insight into the excitation behavior for charge versus proton transfer in p-N,N-ditolylaminosalicylaldehyde (Ia) has been gained via luminescence spectroscopy and femtosecond dynamics. In cyclohexane, following an ultrafast rate (2.0 10 12 s -1 ) of excited-state intramolecular proton transfer (ESIPT), fast equilibrium takes place between normal (N*) and tautomer excited states (T*), resulting in dual fluorescence maximized at 450 and 540 nm, respectively, with a common population decay rate of 360 ps -1 . The normal emission exhibits drastic solvent-polarity dependence and has been concluded to originate from a chargetransfer species incorporating excited-state intramolecular charge transfer from ditolylamine to carbonyl oxygen. In dipolar solvents, competitive rates between ESIPT and solvent relaxation were observed, and the solvated charge-transfer state is thermodynamically more favorable, so that the T* f N* reverse proton transfer takes places. Supplementary support was provided by the corresponding experiments for the methoxy derivative of Ia as well as other relevant analogues. The results shed light on detailed proton/charge transfer coupled dynamics as well as the associated solvent-relaxation dynamics at an early time domain.

Dynamics of solvent relaxation in room temperature ionic liquids

In this Letter, the steady state and time resolved emission behaviour of two solvatochromic Coumarin dyes (Coumarin 153 and Coumarin 152) in two room temperature ionic liquids ([bmim][PF 6] and [hmim][PF 6]) are reported. The solvent relaxation in these ionic liquids is biphasic and retards drastically compared to conventional solvents of similar polarity. The motion of the ions is responsible for solvation in these ionic liquids. The rotational relaxation of both the dyes is also retarded in these ionic liquids in comparison to that of conventional solvents. The high viscosity of these ionic liquids is possibly responsible for the slow rotational relaxation.

Effects of Solubilized Water on the Relaxation Dynamics Surrounding 6-Propionyl-2-(N,N-dimethylamino)naphthalene Dissolved in 1-Butyl-3-methylimidazolium Hexafluorophosphate at 298 K

We report on the picosecond time-resolved fluorescence of 6-propionyl-2-(N,N-dimethylamino)naphthalene (PRODAN) dissolved in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) at 298 K as a function of solubilized water in the [bmim][PF6] phase. The observed solvent relaxation dynamics can be described by three components with apparent relaxation times that occur over a large time regime ( 10 ns). The average relaxation dynamics become faster as the water concentration in the [bmim][PF6] phase increases. Libration and vibration, ion ballistic motion, ion local basin exploration, and ion basin hopping, ion diffusion, and/or the ultrafast relaxation from water (or other small molecules/impurities) are suggested as possible reasons for the yet unquantified sub-15-ps dynamics. The sub-nanosecond dynamics are consistent with [PF6] anion relaxation. This process was found to be water-dependent, slowing as the amount of solubilized water in the [bmim][PF6] phase increased. We speculate that this slowing arises from the formation of 1:2 H-bonded [PF6]‚‚‚HOH‚‚‚[PF6] complexes. The nanosecond dynamics are consistent with the cation, decreasing slightly with an increase in the amount of solubilized water. We suggest that the decrease in this relaxation time arises from a decrease in the bulk viscosity on adding water.

Comprehensive Studies on Dual Excitation Behavior of Double Proton versus Charge Transfer in 4-(N-Substituted amino)-1H-pyrrolo[2,3-b]pyridines

Comprehensive spectroscopic and dynamical studies on the dual excitation behavior of proton vs charge transfer for 4-(dimethylamino)-1H-pyrrolo[2,3-b]pyridine (DPP) and its related derivatives are reported. In cyclohexane, DPP dimer and/or dual hydrogen-bonded complex are formed with association constants Ka as high as ∼4.2 × 103 and 5.2 × 104 M-1 (e.g., the DPP/acetic acid complex) at 298 K, respectively, which upon electronic excitation undergo ultrafast rate (≫6.7 × 1010 s-1) of double-proton transfer, resulting in a unique tautomer emission. Dual fluorescence was observed in polar, aprotic solvents, in which the large Stokes shifted emission band originates from the charge-transfer species incorporating a dimethylamine and pyridine ring as electron donor and acceptor, respectively. Detailed solvent−polarity and temperature-dependent studies in combination with theoretical approaches have been performed to determine the excited-state charge-transfer properties such as dipole moment, orbital configuration, etc. Supplementary support for the dual charge/proton-transfer behavior was provided by the comparative spectroscopy and dynamics of various DPP-related derivatives. Further time-resolved measurements conclude that dual emissions share a common Franck−Condon excited state but undergo two independent relaxation channels. In protic solvents, such as ethanol, following fast solvent relaxation dynamics, the excited charge-transfer state further undergoes a solvent (i.e. alcohol) assisted proton-transfer reaction. The charge versus proton-transfer emission can be distinguished via the temporal spectral evolution. The results demonstrate DPP to be a unique model among 7-azaindole analogues in which the interplay between charge and proton-transfer reactions is operative in the excited state.

Electron Transfer Kinetics of Tris(1,10-phenanthroline)ruthenium(II) Electrooxidation in Aprotic Solvents

Double layer and solvent effects on the kinetics of the one-electron oxidation of Ru(phen)32+ (where phen = 1,10-phenanthroline) at platinum and gold ultramicroelectrodes were studied by ac-voltammetry. The standard rate constant of Ru(phen)32+ electrooxidation is independent of both electrode material and concentration of supporting electrolyte, indicating the absence of double-layer effects on the kinetics of electron transfer. The small variation of the formal redox potential of the Ru(phen)32+/3+ system with change of the supporting electrolyte concentration demonstrates the absence of a significant influence of ion pairing on the reaction rate. Therefore, the electrode process of Ru(phen)32+ oxidation is found to be ideally suitable for the study of solvent effects on electron transfer kinetics. The solvent dependence of the electron transfer rate constant was interpreted within the context of contemporary theory. The charge transfer process was found to be perfectly adiabatic. Since the crossing rate of the energy barrier is controlled by the dynamics of solvent relaxation, the rate of the electrode reaction primarily depends on the solvent's longitudinal relaxation time or, more approximately, on the viscosity of the solution.

Spectroscopy and photophysics of tropolone in condensed media

A systematic study of the spectroscopy and photophysics of tropolone in condensed media has been undertaken by measuring its UV–visible absorption, emission and emission–excitation spectra in a number of solvents of varying structure, at temperatures between 77 K and 295 K, and by measuring its quantum yields of emission, ϕem, and time-resolved emission decays as a function of temperature, T, and excitation wavelength, λex, in each solvent. In weakly interacting solvents, such as 3-methylpentane and perfluoro-2-n-butyltetrahydrofuran, the only species yielding significant fluorescence is the S1, 1(π,π*) state of the intramolecularly hydrogen-bonded neutral molecule. In acidic aqueous solution at pH <3, the emitting state is also 1(π,π*) S1, but the intramolecular hydrogen bond has been disrupted by the solvent. In basic aqueous solution at pH >8 the emitting species is the corresponding excited anion. In some solvents, emission from photochemical products prevents reliable measurements from being made at room temperature. Contrary to previous reports, no emission from solvent-stabilised 1(n,π*) excited states is observed. In glass-forming media the fluorescence quantum yields and the lifetimes of the emitting species increase in a sigmoidal fashion as the temperaure decreases, and are a factor of more than 100 greater at 77 K than at room temperature. The fluorescence quantum yields also decrease with increasing excitation energy in the UV, an effect which is most pronounced in more weakly interacting solvents at temperatures near room temperature. A mechanism is proposed. Both the spectra and the excited state temporal decays indicate that emission from short-lived, vibrationally unrelaxed S1 constitutes the majority of the fluorescence at room temperature, a small fraction of the total emission at 77 K, and an increasing fraction of the total emission with decreasing excitation wavelength. The variations of ϕem and lifetime with temperature are attributed to the effects of temperature-dependent solvent relaxation dynamics on the non-radiative decay, based on qualitative correlations between the observed parameters describing the effects in different media and known characteristics of the media, including their glass transition temperatures and polarity.

High Pressure Studies on Excited Intramolecular Charge-Transfer Reactions and Solvation Dynamics.

The effect of solvent relaxation on the intramolecular charge-transfer (ICT) reactions in the excited state has been studied in alcohol solvents by high pressure method. To get insight into the solvent relaxation dynamics, the time-dependent fluorescence Stokes shift of coumarin 153 was measured at high pressures. The time scale of solvent relaxation thus determined agreed well with the longest component of longitudinal relaxation time of solvent which has been determined from low temperature dielectric measurements.

Electronic and Solvent Relaxation Dynamics of a Photoexcited Aqueous Halide

The details of the electronic and solvent relaxation dynamics following two-photon excitation of an aqueous halide ion are studied via nonadiabatic quantum molecular dynamics simulation. It is found that the branching ratio at very early times (<50 fs) between two channels, a minor channel involving direct electron detachment to a spatially separated solvent void and a dominant channel characterized by delayed adiabatic detachment following a cascade through excited electronic states of the ion, is determined by the effect of solvent dynamics on the values of the ionic and void electronic energies, as well as the relative small matrix elements for tunneling into void states. Solvent dynamics is also found to be important in controlling the rate of electron transfer which leads to geminate recombination of proximal electron−halogen atom pairs. The sensitivity of this recombination rate to the relative energy levels of the unoccupied valence electron hole on the solvated halogen and that of the hydrated ele...

Kinetics of the electroreduction of tris(acetylacetonato)chromium(III) in nonaqueous solvents

Rate constants for the one-electron cathodic reduction of tris(acetylacetonato)chromium(III) were determined by pulse polarographic and cyclic voltammetric techniques at 23°C in 10 organic solvents (acetonitrile, propylene carbonate, γ-butyrolactone, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulphoxide, N,N-diethylformamide, hexamethylphosphoric triamide, N-methylformamide and formamide). It has been shown that the observed solvent dependence of the rate constant is mostly caused by solvent relaxation dynamics, but the donor—acceptor interactions between solvent and solute should not be neglected.

Photochemistry of an unsymmetrical polymethine-cyanine dye; solute—solvent interactions and relaxation dynamics of LDS 751

The photophysical and photochemical properties of the LDS 751 or styryl 8 molecule have been investigated in various solvents. Our results indicate that the photostability of this laser dye is mostly due to an efficient S1 → S0 non-radiative internal conversion process which is faster than the radiative process. This radiationless relaxation is governed by a viscosity-dependent process, tentatively assigned to the free rotor effect between the polymethinic chain and the end groups due to the single bond character of the dominant benzenoid structure. Moreover, intersystem crossing to the triplet state and trans-cis photoisomerization are totally inefficient. The shape (relatively small molar extinction coefficient and large bandwidth) and the blue shift of the absorption spectrum in polar solvents show that this unsymmetrical polymethine-cyanine dye is highly polar in the ground state with the cationic positive charge mainly localized on the benzothiazole nitrogen and inducing a large orientation polarization of the surrounding solvent molecules. The absorption transition involves a much less polar Franck—Condon excited state and a charge transfer from the dimethylanilino nucleus to the benzothiazole nucleus. The shape of the fluorescence spectrum, similar to that of symmetrical polymethine-cyanines and almost independent of solvent polarity, confirms the unpolar character of the first singlet excited state. The sub-picosecond analysis of the time-dependent fluorescence Stokes shift thus probes the relaxation of solvent molecules freed from solute—solvent interactions after the Franck—Condon absorption transition.

Photoinduced intramolecular electron transfer reactions within donor-acceptor linked compounds in solution and within donor-acceptor ion-pairs in crystal

Intramolecular electron transfer (ET) processes within donor-acceptor linked compounds in solution and donor-acceptor ion-pairs in crystal have been investigated by means of laser photolysis kinetic spectroscopy. An excited Ru(II)-moiety of donor-acceptor compounds undergoes intramolecular electron-transfer to either ruthenium(III) ion, rhodium(III) ion or a cobalt(III) ion, followed by back ET to regenerate the original reactant. An Arrhenius plot of the ET rate gave a straight line with an intercept (frequency factor) and a slope (activation energy) for the photoinduced ET and the back ET. Mixed-valence isomer states produced via photoinitiated ET rapidly decayed via back ET. A common and large frequency factor observed for Ru(II)-Rh(III) compounds is accounted for in terms of solvent-relaxation dynamics. For the back ET in the Ru(II)-Co(III) compounds, the frequency factors are reduced because of negative entropy change. ET within donor-acceptor ion-pair of Ru(bpy)23 and Co(CN)36 in crystal took place very rapidly compared with in water.

Hammett reaction constants for the electroreduction of substituted chloro- and bromobenzenes: Effect of solvent dynamics on the two-electron irreversible reductive cleavage

The half-wave potentials, corresponding to irreversible two-electron transfers, were determined for the electroreduction of substituted chloro- and bromobenzenes in a number of organic solvents. In each solvent they obey the Hammett equation, and the reaction constants ϱ, found for each series are solvent dependent. Since the first electron transfer is the rate-determining step for the overall process and the irreversible half-wave potentials depend on the heterogeneous rate constants, it is shown that the ϱ values obtained are linear functions of the logarithms of the solvent longitudinal relaxation times. This is the first example indicating the role of solvent relaxation dynamics in irreversible electrode processes accompanied by bond cleavage. Differences in the Hammett behaviour obtained and that described by us previously for the reduction of substituted iodobenzenes are discussed on the basis of a mechanism of electrode processes.

Solvent relaxation dynamics and electron transfer

In this paper we explore the effects of medium dynamics on activationless and inverted-region electron transfer (ET), when medium-induced dynamics is slow on the time scale of the electronic processes. ET dynamics, with electron-nuclear coupling to the medium modes, was characterized in terms of incoherent population decay of vibronic states in the initial donor-acceptor manifold, which is characterized by nonadiabatic, energy ( E)-dependent, microscopic ET rates, k( E). These k( E)'s are determined by average Franck-Condon densities (AFDs), which were evaluated by quantum and classical formalisms, with model calculations being performed for multimode harmonic systems with displaced potential surfaces. In spite of the intrinsic limitations of the classical AFDs, which do not account for mode specificity and nuclear tunneling effects, the classical Franck-Condon factors provide a good description of the E dependence of the microscopic ET rates. For activationless ET we show that k( E)∝( E+ nϵ) − 1 2 , where nϵ is the zero point energy, implying a weak energy dependence of k( E). Accordingly, the averaged experimental activationless ET rates exhibit a weak variation between the limits of slow medium-induced relaxation and that of fast medium-induced dynamics. Subsequently, the theory of k( E) was extended to include the effects of ET-induced excitations of high-frequency intramolecular vibrational modes, providing a unified description of the weak E dependence of k( E) in the activationless and inverted regions. We predict that for activationless and inverted-region ET the experimental ET rates are only weakly dependent on the characteristics of medium relaxation dynamics, and can be appreciably higher than the solvent-controlled values (i.e., the reciprocal values of the medium relaxation time induced by a constant charge distribution). Our analysis provides an adequate explanation for recent experimental observations of ultrafast ( k=(1 ps) −1–(100 fs) −1) activationless and inverted-region ET, which apparently violate the predictions of solvent-controlled ET theory.

Photoinduced ET and back-ET in bimetallated compounds of Ru(II)-Rh(III) and Ru(II)-Co(III)

Intramolecular electron transfer processes in bimetallated donor-acceptor compounds have been investigated by means of laser photolysis kinetic spectroscopy. An excited Ru(II)-moiety of donor-acceptor compounds undergoes intramolecular electrontransfer to either a rhodium(III) ion or a cobalt(III) ion, followed by back-electron transfer, an Arrhenius plot of the electron-transfer-rate gave a straight line of intercept (frequency factor) and slope (activation energy) for the photoinduced electron transfers and the back electron transfers. A common and large frequency factor observed for Ru(H)-Rh(III) compounds is accounted for in terms of solvent-relaxation dynamics. The activation energy observed consists of outersphere rearrangement energy depending on the metal ion-metal ion distance. For the photoinduced electron transfers and subsequent back-electron transfers in the Ru(II)-Co(III) compounds, the electron-transfer-rates are reduced because of weak electronic coupling, large rearrangement energy and negative entropy change.

Solvent effects on the kinetics of heterogeneous electron transfer processes. The TMPD/TMPD ·+ redox couple

Solvent effects on the heterogeneous electron exchange between N, N, N′, N′-tetramethyl- p-phenylendiamine (TMPD) and its monocation radical (TMPD ·+) have been studied by chronocoulometry at a platinum electrode in perchlorate solutions in a wide range of aprotic and hydrogen-bonded solvents. Formal potentials, diffusion coefficients, anodic transfer coefficients and formal rate constants have been evaluated. The variation of the formal potential with the solvent can be interpreted by determining the solvent—solvent—solute interactions using Taft theory. Correlations between predicted and experimental formal potentials for the Taft model were an improvement over those obtained using the Gutmann donor number approach. A weak correlation between the anodic transfer coefficient and the Taft β parameter was also found. The variations in the rate constants were analyzed on the basis of current electron-transfer models using a statistical method to separate the effects of the longitudinal relaxation time τ L from those of the solvent permittivity parameter γ. It has been shown that the dynamics of solvent relaxation affect the heterogeneous electron-transfer rate in this case, but the rate constants depend on τ −0.53 instead of τ −1, as theory predicts. The degree of reaction adiabaticity and the relative sizes of the inner- and outer-sphere components of the Gibbs energy of activation were considered.

Kinetic parameters for heterogeneous electron transfer to tris(acetylacetonato)manganese(III) and tris(acetylacetonato)iron(III) in aproptic solvents

Rate constants for the one-electron heterogeneous electron transfer between a mercury electrode and tris(acetylacetonato)manganese(III) and tris(acetylacetonato)iron(III) (acetylacetonato=2,4-pentanedione) have been determined by ac admittance voltammetry at different temperatures in seven aprotic solvents. It has been shown that a major portion of the observed solvent dependence of the experimental activation enthalpy for both reactions is caused by solvent relaxation dynamics. An analogous conclusion has also been reached on the basis of the solvent dependence of the rate constant. The factors which contribute to the activation enthalpy, especially the outer-sphere contribution, have been examined in detail. Estimates of the latter quantity on the basis of the Born model and mean spherical approximation have been made and are discussed with respect to the experimental results.

Equilibrium and nonequilibrium solvation and solute electronic structure. II. Strong coupling limit

The formulation developed in the preceding paper [H. J. Kim and J. T. Hynes, J. Chem. Phys. 93, 5194 (1990)] is applied to describe the electronic structure and spectroscopic features of a model symmetric electron–donor–acceptor solute system D−A⇌DA− in solution in the strong coupling limit. In this limit, the electronic coupling is sufficiently strong to overcome the localizing influence of the solvent polarization, and two stable delocalized solute electronic states are found in the presence of either nonequilibrium or equilibrium solvation. The nonlinear influence of the equilibrated solvent electronic polarization and of exchange contributions to the solute electronic distribution incorporated in the theory lead to several consequences absent in standard descriptions. Among these are the necessity of two solvent coordinates to describe the system, and the prediction of solvent-dependent spectral shifts and the appearance of solvent relaxation dynamics after a Franck–Condon transition between the delocalized electronic states. Estimates of the magnitude of these new effects are provided, and the possibility for their experimental observation is briefly discussed.