4′-N,N-Diethylamino-3-hydroxyflavone (DEAHF) exhibits dual fluorescence in most solvents as a result of a rapid excited-state intramolecular proton transfer reaction. The high sensitivity of its dual emission to solvent polarity and hydrogen bonding make DEAHF of interest as a ratiometric fluorescence sensor. In addition, prior work has suggested that the rate of this proton transfer should depend on solvent relaxation in an unusual manner. It has been proposed that rapid solvation of the initially excited reactant should retard reaction. The present work tests this idea by using femtosecond Kerr-gated emission spectroscopy to measure the reaction kinetics of DEAHF in mixtures of propylene carbonate (PC) + acetonitrile (ACN). This mixture was chosen to maintain constant solvent polarity and thereby constant reaction energies while varying solvation times ∼10-fold with composition. The reaction kinetics observed in these mixtures are multiexponential, consisting of resolvable components of ∼2 and ∼30 ps and a small fraction of reaction faster than detectable by the 400 fs resolution of the experiment. Average reaction times increase by approximately a factor of 2 as a function of ACN mole fraction, primarily as a result of changes to the slower kinetic component. This trend is opposite to the composition dependence of solvation times, thereby supporting the unusual role of polar solvation dynamics in this proton transfer. In n-alkane solvents, where electrostatic coupling is minimized, frictional properties of the solvent do not influence reaction rates.

Temperature-dependent 2H longitudinal spin relaxation times (T1) of dilute benzene-d6 in 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) and two deuterated variants of the 1-ethyl-3-methylimidazolium cation (Im21+-d1 and Im21+-d6) in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Im21][Tf2N]), measured at multiple Larmor frequencies, were used to probe rotational dynamics in ionic liquids. Rotational correlation times significantly faster than predicted by slip hydrodynamic calculations were observed for both solutes. Molecular dynamics simulations of these systems enabled extraction of more information about the rotational dynamics from the NMR data than rotation times alone. The multifrequency 2H T1(T) data could be fit to within uncertainties over a broad region about the T1 minimum using models of the relevant rotational time correlation functions and their viscosity/temperature dependence derived from simulation. Such simulation-guided fitting provided confidence in the semiquantitative accuracy of the simulation models and enabled interpretation of NMR measurements to higher viscosities than previously possible. Simulations of the benzene system were therefore used to explore the nature of solute rotation in ionic liquids and how it might differ from rotation in conventional solvents. Whereas “spinning” about the C6 axis of benzene senses similarly weak solvent friction in both types of solvents, “tumbling” (rotations about in-plane axes) differs significantly in conventional solvents and ionic liquids. In the sluggish environment provided by ionic liquids, orientational caging and the presence of rare but influential large-amplitude (180°) jumps about in-plane axes lead to rotations being markedly nondiffusive, especially below room temperature.

The branched ionic liquids (ILs) 1-(iso-alkyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(N – 2)mCN-1C1im][NTf2] with N = 3–7) were synthesized and their physicochemical properties characterized and compared with the properties of linear ILs 1-(n-alkyl)-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([CNC1im][NTf2] with N = 3–7). For N = 4–7, the density of the branched IL [(N – 2)mCN–1C1im][NTf2] is the same as that of its linear analogue [CNC1im][NTf2] within the standard uncertainty of the measurements. In the case of the N = 3 [1mC2C1im][NTf2]/[C3C1im][NTf2] pair, the density of the branched IL is 0.13% higher than that of the linear IL. For a branched/linear IL pair with a given N, the glass transition temperature Tg, melting temperature Tm, and viscosity η are higher for the branched IL than for the linear IL. [2mC3C1im][NTf2] is an exception in that its Tm is lower than that of [C4C1im][NTf2]. Moreover, the viscosity of [2mC3C1im][NTf2] is anomalously higher than what would be predicted based on the trend of the other branched ILs. These trends in the viscosities of the linear and branched ILs are consistent with recent molecular dynamics simulations. Thermal gravimetric analysis indicates that linear ILs are thermally more stable than branched ILs. Pulsed-gradient spin–echo (PGSE) NMR diffusion measurements show that the self-diffusion coefficients of the ions vary inversely with the viscosities according to the Stokes–Einstein (SE) equation. The hydrodynamic radii of the cations and anions of linear ILs calculated from the SE equation however are consistently higher than those of the corresponding branched ILs.

Stretches of guanines in DNA and RNA can fold into guanine quadruplex structures (GQSs). These structures protect telomeres in DNA and regulate gene expression in RNA. GQSs have an intrinsic fluorescence that is sensitive to different parameters, including loop sequence and length. However, the dependence of GQS fluorescence on solution and sequence parameters and the origin of this fluorescence are poorly understood. Herein we examine effects of dangling nucleotides and cosolute conditions on GQS fluorescence using both steady-state and time-resolved fluorescence spectroscopy. The quantum yield of dGGGTGGGTGGGTGGG, termed “dG3T”, is found to be modest at ∼2 × 10–3. Nevertheless, dG3T and its variants are significantly brighter than the common nucleic acid fluorophore 2-aminopurine (2AP) largely due to their sizable extinction coefficients. Dangling 5′-end nucleotides generally reduce emission and blue-shift the resultant spectrum, whereas dangling 3′-end nucleotides slightly enhance fluorescence, particularly on the red side of the emission band. Time-resolved fluorescence decays are broadly distributed in time and require three exponential components for accurate fits. Time-resolved emission spectra suggest the presence of two emitting populations centered at ∼330 and ∼390 nm, with the redder component being a well-defined long-lived (∼1 ns) entity. Insights into GQS fluorescence obtained here should be useful in designing brighter intrinsic RNA and DNA quadruplexes for use in label-free biotechnological applications.

Ionic liquids with electron-donating anions are used to investigate rates and mechanisms of photoinduced bimolecular electron transfer to the photoexcited acceptor 9,10-dicyanoanthracene (9,10-DCNA). The set of five cyano anion ILs studied comprises the 1-ethyl-3-methylimidazolium cation paired with each of these five anions: selenocyanate, thiocyanate, dicyanamide, tricyanomethanide, and tetracyanoborate. Measurements with these anions dilute in acetonitrile and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide show that the selenocyanate and tricyanomethanide anions are strong quenchers of the 9,10-DCNA fluorescence, thiocyanate is a moderately strong quencher, dicyanamide is a weak quencher, and no quenching is observed for tetracyanoborate. Quenching rates are obtained from both time-resolved fluorescence transients and time-integrated spectra. Application of a Smoluchowski diffusion-and-reaction model showed that the complex kinetics observed can be fit using only two adjustable parameters, D and V0, where D is the relative diffusion coefficient between donor and acceptor and V0 is the value of the electronic coupling at donor–acceptor contact.

Solvation energies, rotation times, and 100 fs to 20 ns solvation response functions of the solute coumarin 153 (C153) in mixtures of 1-butyl-3-methylimidazolium tetrafluoroborate ([Im41][BF4]) + acetonitrile (CH3CN) at room temperature (20.5 °C) are reported. Available density, shear viscosity, and electrical conductivity data at 25 °C are also collected and parametrized, and new data on refractive indices and component diffusion coefficients presented. Solvation free energies and reorganization energies associated with the S0 ↔ S1 transition of C153 are slightly (≤15%) larger in neat [Im41][BF4] than in CH3CN. No clear evidence for preferential solvation of C153 in these mixtures is found. Composition-dependent diffusion coefficients (D) of Im41+ and CH3CN as well as C153 rotation times (τ) are approximately related to solution viscosity (η) as D, τ ∝ ηp with values of p = −0.88, −0.77, and +0.90, respectively. Spectral/solvation response functions (Sν(t)) are bimodal at all compositions, consisting of a subpicosecond fast component followed by a broadly distributed slower component extending over ps-ns times. Integral solvation times (⟨τsolv⟩ = ∫0∞Sν(t) dt) follow a power law on viscosity for mixture compositions 0.2 ≤ xIL ≤ 1 with p = 0.79. With recent broad-band dielectric measurements [J. Phys. Chem. B 2012, 116, 7509] as input, a simple dielectric continuum model provides predictions for solvation response functions that correctly capture the distinctive bimodal character of the observed response. At xIL ∼ 1 predicted values of ⟨τsolv⟩ are smaller than those observed by a factor of 2–3, but the two become approximately equal at xIL = 0.2. Predictions of a recent semimolecular theory [ J. Phys. Chem. B 2011, 115, 4011] are less accurate, being uniformly slower than the observed solvation dynamics.

Diffusion coefficients of a variety of dilute solutes in the series of 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imides ([Prn1][Tf2N], n = 3, 4, 6, 8, and 10), trihexyltetracedecylphosphonium bis(trifluoromethanesulfonyl)imide [P14,666][Tf2N], and assorted imidazolium ionic liquids are measured using pulsed field gradient 1H NMR. These data, combined with available literature data, are used to try to uncover the solute and solvent characteristics most important in determining tracer diffusion rates. Discussion is framed in terms of departures from simple hydrodynamic predictions for translational friction using the ratio ζobs/ζSE, where ζobs is the observed friction, determined from the measured diffusion coefficient D via ζobs = kBT/D, and ζSE = 6πηR is the Stokes friction on a sphere of radius R (determined from the solute van der Waals volume) in a solvent with viscosity η. In the case of neutral solutes, the primary determinant of whether hydrodynamic predictions are accurate is the relative size of solute versus solvent molecules. A single correlation, albeit with considerable scatter, is found between ζobs/ζSE and the ratio of solute-to-solvent van der Waals volumes, ζobs/ζSE = {1 + a(VU/VV)−p}, with constants a = 1.93 and p = 1.88. In the case of small solutes, the observed friction is over 100-fold smaller than predictions of hydrodynamic models. The dipole moment of the solute has little effect on the friction, whereas solute charge has a marked effect. For monovalent solutes of size comparable to or smaller than the solvent ions, the observed friction is comparable to or even greater than what is predicted by hydrodynamics. These general trends are shown to be quite similar to what is observed for tracer diffusion in conventional solvents.

The photophysics of trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) was investigated by electronic structure calculations and steady-state and time-resolved emission spectroscopy in a wide range of solvents, including temperature and pressure dependence. DMASBT undergoes a facile photoinduced trans–cis isomerization similar to the reaction of trans-stilbene. The cis isomer has a lifetime of ∼1 ps and does not contribute appreciably to the steady-state emission spectrum. The absorption spectrum of the cis form overlaps that of the trans form such that considerable care is needed in determining correct emission quantum yields. The approximate equality of absorption and emission transition moments of DMASBT in all solvents indicates that absorption and emission involve a single excited state with high radiative rate. The low quantum yields of DMASBT in low-viscosity solvents reflect emission lifetimes in the 20–50 ps range. The nonradiative rates of DMASBT, which are assumed to measure the rate of isomerization in S1, depend upon both solvent viscosity and polarity. A modified Kramers analysis, which allows for a polarity-dependent barrier height, provides a satisfactory description of these rates but only if it is assumed that friction in alcohol solvents is more weakly dependent upon viscosity than in other types of solvents.

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.

The dynamic Stokes shift of coumarin 153, measured with a combination of broad-band fluorescence upconversion (80 fs resolution) and time-correlated single photon counting (to 20 ns), is used to determine the complete solvation response of 21 imidazolium, pyrrolidinium, and assorted other ionic liquids. The response functions so obtained show a clearly bimodal character consisting of a subpicosecond component, which accounts for 10–40% of the response, and a much slower component relaxing over a broad range of times. The times associated with the fast component correlate with ion mass, confirming its origins in inertial solvent motions. Consistent with many previous studies, the slower component is correlated to solvent viscosity, indicating that its origins lie in diffusive, structural reorganization of the solvent. Comparisons of observed response functions to the predictions of a simple dielectric continuum model show that, as in dipolar solvents, solvation and dielectric relaxation involve closely related molecular dynamics. However, in contrast to dipolar solvents, dielectric continuum predictions systematically underestimate solvation times by factors of at least 2–4.

It was shown recently that a simple dielectric continuum model predicts the integral solvation time of a dipolar solute ⟨τsolv⟩ to be inversely proportional to the electrical conductivity σ0 of an ionic solvent or solution. In this Letter, we provide a more general derivation of this connection and show that available data on coumarin 153 (C153) in ionic liquids generally support this prediction. The relationship between solvation time and conductivity can be expressed by ln(⟨τsolv⟩/ps) = 4.37 – 0.92 ln (σ0/S m–1) in 34 common ionic liquids.Keywords: conductivity; coumarin 153; dielectric continuum theory; ionic liquids; room-temperature ionic liquids; solvation dynamics;

Steady-state and picosecond time-resolved emission spectroscopy are used to monitor the bimolecular electron transfer reaction between the electron acceptor 9,10-dicyanoanthracene in its S1 state and the donor N,N-dimethylaniline in a variety of ionic liquids and several conventional solvents. Detailed study of this quenching reaction was undertaken in order to better understand why rates reported for similar diffusion-limited reactions in ionic liquids sometimes appear much higher than expected given the viscous nature of these liquids. Consistent with previous studies, Stern–Volmer analyses of steady-state and lifetime data provide effective quenching rate constants kq, which are often 10–100-fold larger than simple predictions for diffusion-limited rate constants kD in ionic liquids. Similar departures from kD are also observed in conventional organic solvents having comparably high viscosities, indicating that this behavior is not unique to ionic liquids. A more complete analysis of the quenching data using a model combining approximate solution of the spherically symmetric diffusion equation with a Marcus-type description of electron transfer reveals the reasons for frequent observation of kq ≫ kD. The primary cause is that the high viscosities typical of ionic liquids emphasize the transient component of diffusion-limited reactions, which renders the interpretation of rate constants derived from Stern–Volmer analyses ambiguous. Using a more appropriate description of the quenching process enables satisfactory fits of data in both ionic liquid and conventional solvents using a single set of physically reasonable electron transfer parameters. Doing so requires diffusion coefficients in ionic liquids to exceed hydrodynamic predictions by significant factors, typically in the range of 3–10. Direct, NMR measurements of solute diffusion confirm this enhanced diffusion in ionic liquids.

Equilibrium and nonequilibrium molecular dynamics simulations of solvation and solvation dynamics of a variety of solutes have been performed in the coarse-grained ionic liquid model ILM2 (Roy, D.; Maroncelli, M. J. Phys. Chem. B2010, 114, 12629). Some comparisons are made between ionic and dipolar solvation using parallel simulations in CH3CN. Despite the fact that the multipolar character of electrostatic interactions and their spatial extent differ in the two solvents, solvation energies are equal to within about 10% in ILM2 and CH3CN. This near equality also holds with reduced accuracy in the case of reorganization energies. Solvation energies of spherical solutes in ILM2 and its variants can be correlated as a function of solute and solvent size using a Born-type expression with an effective cavity size. Solvation time correlation functions in ILM2 exhibit a subpicosecond inertial component followed by a broadly distributed component related to solvent viscosity, comparable to what has been observed in experiment. Direct comparison of simulation to experiment using the solute coumarin 153 (C153) shows general agreement on the time scales and character of the fast and slow components, but the amplitude of the fast component is overestimated by the simulations. Solute motion can significantly increase the speed of solvation, even in the case of large solutes such as C153. Good agreement is found between linear response estimates and the nonequilibrium dynamics associated with electronic excitation of C153. In contrast, perturbations involving changes of a full electron charge in atomic solutes lead to local heating which greatly hastens solvation compared to linear response predictions. The mechanism of charge solvation in atomic solutes is examined in some detail. It is found that ion translation dominates the inertial dynamics. The rotational contribution only becomes comparable to the translation contribution in the tail of the response. Adjustments of ion positions over distances of ∼30% of their diameters are all that is required to relax the solvation energy in these systems.

The photochemistry of the rotor probe 9-(2-carboxy-2-cyanovinyl)julolidine (CCVJ) was studied to elucidate a curious effect of fluid flow previously reported. The apparent sensitivity to fluid motion observed in CCVJ but not in the closely related molecule 9-(dicyanovinyl)julolidine (DCVJ) is found to be an indirect effect of a photoisomerization reaction. The results presented here demonstrate that it is this isomerization, rather than the commonly assumed TICT process, that confers viscosity-sensing ability on these fluorophores. In micromolar solutions in hydroxylic solvents CCVJ exists primarily in the carboxylate form. Only the E isomer of this anion is initially present in solutions prepared from the solid, but in room light such solutions rapidly achieve a photostationary state in which the E isomer and an essentially nonfluorescent Z isomer exist in comparable concentrations. The Z isomer is metastable in S0 such that in the absence of light the solution reverts slowly to pure E. Unlike DCVJ where only a single isomer is possible, the production of long-lived photoproducts in CCVJ and other asymmetrically substituted styryenyl probes complicates their fluorescence response. Considerable care is needed when such fluorphores are used as steady-state sensors of environmental fluidity are used.

The rates of excited-state intramolecular electron transfer in 9-(4-biphenyl)-10-methylacridinium (BPAc+), crystal violet lactone (CVL), and bianthryl have been measured in a variety of ionic liquids using time-correlated single-photon counting. All three of these reactions had previously been studied in conventional dipolar solvents and their reaction rates shown to be controlled by solvation dynamics. The main focus of this work is to ask whether the same relationships between reaction and solvation times already established in dipolar solvents also apply in ionic liquids. In BPAc+, where reaction conforms to a simple two-state kinetic scheme and reaction rates are easily measured, the result is a clear “yes”. In the case of bianthryl, whose spectra reflect the more complex kinetics of a barrierless process, the answer is also yes. In contrast to other recent studies of bianthryl, the present results demonstrate that the same equality between (integral) reaction times and solvation times observed in conventional solvents also applies in ionic liquids. Finally, the case of CVL is less clear due to the greater uncertainty associated with the data afforded by this weak fluorophore, combined with a lack of data in conventional solvents having large solvation times. But the CVL reaction can also be reasonably interpreted as exhibiting a common behavior in dipolar and ionic solvents.

Steady-state and picosecond time-resolved emission experiments are used to examine the excited-state charge transfer reaction of crystal violet lactone (CVL) in aprotic solvents. Solvatochromic analysis using a dielectric continuum model suggests dipole moments of 9−12 D for the initially excited (LE) state and ∼24 D for the charge-transfer (CT) state. Intensities of steady-state emission as well as kinetic data provide free energies for the LE → CT reaction that range from +12 kJ/mol in nonpolar solvents to −10 kJ/mol in highly polar solvents at 25 °C. Reaction rates constants, which lie in the range of 10−100 ns−1 in most solvents, depend on both solvent polarity and solvent friction. In highly polar solvents, rates are correlated to solvation times in a manner that indicates that the reaction is a solvent-controlled electron transfer on an adiabatic potential surface having a modest barrier.

Computer simulations provide insight into the molecular-level details responsible for the unique properties of ionic liquids. Due to the sluggish dynamics and nanostructured nature of many ionic liquids, coarse-grained models are an important complement to fully atomistic simulations because they enable simulation of much larger system sizes and much longer times, which are often of interest. This paper reports a four-site, coarse-grained model for studying ionic liquids and their solutions. It is intended to be a generic model representative of common ionic liquids currently in use, but it is parametrized to fit the properties of 1-butyl-3-methylimidazolium hexafluorophosphate, [Im41][PF6]. The present model is a variant of one introduced in J. Phys. Chem. B 114, 8410 (2010). Reduction of ion charges to ±0.78e and fine-tuning Lennard-Jones parameters from the original model leads to a remarkable improvement in the realism of the model and surprisingly good agreement between simulation and experiment for a variety of static and dynamic properties of [Im41][PF6]. This idealized model should prove valuable for studies of solute-based dynamics and other phenomena occurring on nanosecond and longer time scales, which are not feasible with all-atom simulations.

Electronic structure calculations are used to examine the nuclear motions responsible for ultrafast internal conversion in the benzylidene malononitriles DMN (4-N,N-dimethylaminobenzylidenemalononitrile) and JDMN (julolidinemalononitrile). Gas-phase B3LYP and RI-CC2 calculations using triple-ζ valence polarized basis sets reproduce the structural features measured in the crystalline state and the solution-phase dipole moments of these molecules in their ground states with reasonable accuracy. Most properties of the vertical S0 → S1 transition, which is well separated from other transitions, are also reasonably reproduced by both types of calculation. The large change in dipole moment (8−9 D) upon excitation is grossly underestimated by TDDFT calculations, despite the fact that such calculations predict the transition energies to within experimental uncertainties. Exploration of the S1 potential energy surface of DMN using DFT, RI-CC2, and CASSCF methods indicates that the internal conversion pathway is double-bond isomerization, not the TICT process often assumed. Preliminary classical molecular dynamics simulations of DMN in acetonitrile using the ab initio S1 surface support this assignment.

Subpicosecond time-resolved fluorescence of trans-4-dimethylamino-4′-cyanostilbene (DCS) is used to measure solvation dynamics in the gas-expanded liquid (GXL) system CH3CN + CO2 at 25 °C along the liquid−vapor coexistence curve. These measurements are supplemented by measurements of the steady-state solvatochromism of DCS and of its rotation and isomerization times. Molecular dynamics computer simulations and a semiempirical spectral model that reproduces the observed solvatochromism in this system are used to interpret the experimental results. Simulations indicate that at compositions of xCO2 > 0.5, the cybotactic region surrounding DCS is enriched in CH3CN molecules, and the extent of this enrichment is greater in S1 than that in S0. Solvation dynamics are dominated by the CH3CN component. These dynamics are biphasic, consisting of a subpicosecond inertial component, followed by a slower picosecond component, related to the redistribution of CH3CN molecules between the cybotactic region and the bulk solvent.