9 research outputs found
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 S<sub>1</sub> → S<sub>0</sub> emission band or of a vibronic line
shape is measured with ∼80 fs, 10 cm<sup>–1</sup> resolution.
Polar solutes in acetonitrile and acetone, when excited into S<sub>1</sub> 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<sup>–1</sup> 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<sup>+</sup> 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<sub>solv</sub>⟩ to be inversely proportional to the electrical
conductivity σ<sub>0</sub> 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<sub>solv</sub>⟩/ps) = 4.37 – 0.92 ln (σ<sub>0</sub>/S m<sup>–1</sup>) 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 S<sub>2</sub> 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
S<sub>2</sub> was accessed in two ways: (i) by transient FSR after
an actinic pulse to populate the S<sub>2</sub> state, exploiting resonance
from an S<sub><i>x</i></sub> ← S<sub>2</sub> transition,
and (ii) by FSR without actinic excitation, using S<sub>2</sub> ↔
S<sub>0</sub> resonance exclusively and narrow-band Raman/broad-band
femtosecond probe pulses only. The two approaches have nonlinear optical
susceptibilities χ<sup>(5)</sup> and χ<sup>(3)</sup>,
respectively. Both methods show low-frequency bands of the S<sub>2</sub> state at 200, 400, and ∼600 cm<sup>–1</sup>, which
are reported for the first time. With (ii) the intensities of low-frequency
vibrational resonances in S<sub>2</sub> are larger compared to those
in S<sub>0</sub>, implying strong anharmonicities/mode mixing in the
excited state. In principle, for short-lived electronic states, the
χ<sup>(3)</sup> 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 ([Im<sub>41</sub>][BF<sub>4</sub>]) + acetonitrile (CH<sub>3</sub>CN) 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 S<sub>0</sub> ↔ S<sub>1</sub> transition of C153 are
slightly (≤15%) larger in neat [Im<sub>41</sub>][BF<sub>4</sub>] than in CH<sub>3</sub>CN. No clear evidence for preferential solvation
of C153 in these mixtures is found. Composition-dependent diffusion
coefficients (<i>D</i>) of Im<sub>41</sub><sup>+</sup> and
CH<sub>3</sub>CN as well as C153 rotation times (τ) are approximately
related to solution viscosity (η) as <i>D</i>, τ
∝ η<sup><i>p</i></sup> with values of <i>p</i> = −0.88, −0.77, and +0.90, respectively.
Spectral/solvation response functions (<i>S</i><sub>ν</sub>(<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<sub>solv</sub>⟩ = ∫<sub>0</sub><sup>∞</sup><i>S</i><sub>ν</sub>(<i>t</i>) d<i>t</i>) follow
a power law on viscosity for mixture compositions 0.2 ≤ <i>x</i><sub>IL</sub> ≤ 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><sub>IL</sub> ∼ 1 predicted values of ⟨τ<sub>solv</sub>⟩ are smaller than those observed by a factor of 2–3,
but the two become approximately equal at <i>x</i><sub>IL</sub> = 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 H<sub>2</sub>O/D<sub>2</sub>O 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 S<sub>1</sub> potential energy surface is expected just before
internal conversion through a conical intersection S<sub>1</sub>/S<sub>0</sub>. For decades the observation of P was thwarted by a short
lifetime τ<sub>P</sub> 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<sub>P</sub> = 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<sup>–1</sup>, 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<sub>solv</sub> = 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<sub>solv</sub> = 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