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

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 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