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

Fluorescence following Excited-State Protonation of Riboflavin at N(5)

Excited-state protonation of riboflavin in the oxidized form is studied in water. In the −1 < pH < 2 range, neutral and N(1)-protonated riboflavin coexist in the electronic ground state. Transient absorption shows that the protonated form converts to the ground state in <40 fs after optical excitation. Broadband fluorescence upconversion is therefore used to monitor solvation and protonation of the neutral species in the excited singlet state exclusively. A weak fluorescence band around 660 nm is assigned to the product of protonation at N(5). Its radiative rate and quantum yield relative to neutral riboflavin are estimated. Protonation rates agree with proton diffusion times for H⁺ concentrations below 5 M but increase at higher acidities, where the average proton distance is below the diameter of the riboflavin molecule

Conductivity and Solvation Dynamics in Ionic Liquids

It was shown recently that a simple dielectric continuum model predicts the integral solvation time of a dipolar solute ⟨τ_solv⟩ 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(⟨τ_solv⟩/ps) = 4.37 – 0.92 ln (σ₀/S m^–1) in 34 common ionic liquids

Complete Solvation Response of Coumarin 153 in Ionic Liquids

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

Resonance Femtosecond-Stimulated Raman Spectroscopy without Actinic Excitation Showing Low-Frequency Vibrational Activity in the S₂ State of All-Trans β‑Carotene

Raman scattering with stimulating femtosecond probe pulses (FSR) was used to observe vibrational activity of all-trans β-carotene in <i>n</i>-hexane. The short-lived excited electronic state S₂ was accessed in two ways: (i) by transient FSR after an actinic pulse to populate the S₂ state, exploiting resonance from an S_<i>x</i> ← S₂ transition, and (ii) by FSR without actinic excitation, using S₂ ↔ S₀ resonance exclusively and narrow-band Raman/broad-band femtosecond probe pulses only. The two approaches have nonlinear optical susceptibilities χ⁽⁵⁾ and χ⁽³⁾, respectively. Both methods show low-frequency bands of the S₂ state at 200, 400, and ∼600 cm^–1, which are reported for the first time. With (ii) the intensities of low-frequency vibrational resonances in S₂ are larger compared to those in S₀, implying strong anharmonicities/mode mixing in the excited state. In principle, for short-lived electronic states, the χ⁽³⁾ method should allow the best characterization of low-frequency modes

Solvation Dynamics in a Prototypical Ionic Liquid + Dipolar Aprotic Liquid Mixture: 1‑Butyl-3-methylimidazolium Tetrafluoroborate + Acetonitrile

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 (<i>D</i>) of Im₄₁⁺ and CH₃CN as well as C153 rotation times (τ) are approximately related to solution viscosity (η) as <i>D</i>, τ ∝ η^<i>p</i> with values of <i>p</i> = −0.88, −0.77, and +0.90, respectively. Spectral/solvation response functions (<i>S</i>_ν(<i>t</i>)) 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 (⟨τ_solv⟩ = ∫₀^∞<i>S</i>_ν(<i>t</i>) d<i>t</i>) follow a power law on viscosity for mixture compositions 0.2 ≤ <i>x</i>_IL ≤ 1 with <i>p</i> = 0.79. With recent broad-band dielectric measurements [<i>J</i>. 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 <i>x</i>_IL ∼ 1 predicted values of ⟨τ_solv⟩ are smaller than those observed by a factor of 2–3, but the two become approximately equal at <i>x</i>_IL = 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

Observing the Hydration Layer of Trehalose with a Linked Molecular Terahertz Probe

The terahertz (THz) absorption bands of biomolecular hydration layers are generally swamped by absorption from bulk water. Using the disaccharide trehalose, we show that this limitation can be overcome by attaching a molecular probe. By time-resolving the fluorescence shift of the probe, a local THz spectrum is obtained. From the dependence on temperature and H₂O/D₂O exchange, it is concluded that the trehalose hydration layer is being observed. The region of dynamic water perturbation by the disaccharide encompasses the probe and is therefore larger than the first two solvation layers

Perpendicular State of an Electronically Excited Stilbene: Observation by Femtosecond-Stimulated Raman Spectroscopy

In the photoisomerization path of stilbene, a perpendicular state P on the S₁ potential energy surface is expected just before internal conversion through a conical intersection S₁/S₀. For decades the observation of P was thwarted by a short lifetime τ_P in combination with slow population flow over a barrier. But these limitations can be overcome by ethylenic substitution. Following optical excitation of <i>trans</i>-1,1′-dicyanostilbene, P is populated significantly (τ_P = 27 ps in <i>n</i>-hexane) and monitored by an exited-state absorption band at 370 nm. Here we report stimulated Raman lines of P. The strongest, at 1558 cm^–1, is attributed to stretching vibrations of the phenyl rings. Transient electronic states, resonance conditions, and corresponding Raman signals are discussed

Effect of a Tertiary Butyl Group on Polar Solvation Dynamics in Aqueous Solution: Femtosecond Fluorescence Spectroscopy

We monitor the time-dependent Stokes shift (TDSS) of fluorescence from the zwitterionic probe <i>N</i>-methyl-6-oxyquinolinium betaine in water. A spectral relaxation time τ_solv = 0.57 ps (at 20.5 °C) is attributed to a solvation process involving water in the hydration layer. In this article we show that a tertiary butyl group, when attached to the chromophore, slows the dynamics to τ_solv = 0.76 ps and increases the corresponding activation energy by 5 kJ/mol. In a companion paper (10.1021/acs.jpcb.7b05039), simulations suggest that the observed slow-down indicates coupling of solute vibrations to hydration water. Thus, a new angle on a thoroughly researched topic, solvation dynamics, has been opened