3 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
The Nature of the Dielectric Response of Methanol Revealed by the Terahertz Kerr Effect
The
dielectric response of liquids in the terahertz (THz) and sub-THz
frequency range arises from low-energy collective molecular motions,
which are often strongly influenced by intermolecular interactions.
To shed light on the microscopic origin of the THz dielectric response
of the simplest alcohol, methanol, we resonantly excite this liquid
with an intense THz electric-field pulse and monitor the relaxation
of the induced optical birefringence. We find a unipolar THz-Kerr-effect
signal which, in contrast to aprotic polar liquids, shows a weak coupling
between the THz electric field and the permanent molecular dipole
moment of the liquid. We assign this weak coupling to the restricted
translational rather than rotational nature of the excited mode. Our
approach opens a new avenue to the assignment of the dielectric spectrum
of liquids to a microscopic origin
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